Patent Application
20090124820
1. Field of Invention
This invention relates to C1-symmetric bisphosphine ligands and corresponding catalysts, and to their use in asymmetric syntheses, including the enantioselective hydrogenation of prochiral olefins to prepare pharmaceutically useful compounds, including (S)-(+)-3-(aminomethyl)-5-methyl-hexanoic acid,
which is commonly known as pregabalin.
2. Discussion
Chiral phosphine ligands have played a significant role in the development of novel transition metal catalyzed asymmetric reactions to produce enantiomeric excess of compounds with desired activities. The first successful attempts at asymmetric hydrogenation of eneamide substrates were accomplished in the late 1970s using chiral bisphosphines as transition metal ligands. See, e.g., B. D. Vineyard et al., J. Am. Chem. Soc. 99(18):5946-52 (1977); W. S. Knowles et al., J. Am. Chem. Soc. 97(9):2567-68 (1975). Since these first published reports, there has been an explosion of research related to the synthesis of new chiral bisphosphine ligands for asymmetric hydrogenations and other chiral catalytic transformations. See I. Ojima, ed., Catalytic Asymmetric Synthesis (1993); D. J. Ager, ed., Handbook of Chiral Chemicals (1999).
Some of the most efficient and broadly useful ligands developed for asymmetric hydrogenation include BPE ligands (e.g., (R,R)-Et-BPE or (+)-1,2-bis((2R,5R)-2,5-diethylphospholano)ethane); DuPhos ligands (e.g., (R,R)-Me-DUPHOS or (−)-1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene); and Bis P* ligand ((S,S)-1,2-bis(t-butylmethylphosphino)ethane). See, e.g., M. J. Burk, Chemtracts 11(11):787-802 (1998); M. J. Burk et al., Angew Chem., Int. Ed. 37(13/14):1931-33 (1998); M. J. Burk, et al., J. Org. Chem. 63(18):6084-85 (1998); M. J. Burk et al., J. Am. Chem. Soc. 120(18):4345-53 (1998); M. J. Burk et al., J. Am. Chem. Soc. 117(15):4423-24 (1995); M. J. Burk et al., J. Am. Chem. Soc. 115(22):10125-38 (1993); W. A. Nugent et al., Science 259(5094):479-83 (1993); M. J. Burk et al., Tetrahedron: Asymmetry 2(7):569-92 (1991); M. J. Burk, J. Am. Chem. Soc. 113(22):8518-19 (1991); T. Imamoto et al., J. Am. Chem. Soc. 120(7):1635-36 (1998); G. Zhu et al., J. Am. Chem. Soc. 119(7):1799-800 (1997).
The success of BPE, DUPHOS, BisP* and related ligands in asymmetric hydrogenation reactions has been attributed, among other factors, to rigidity in their C2-symmetric structure. As shown in
Researchers have recently described C1-symmetric bisphosphine ligands and corresponding catalysts, which are useful in asymmetric transformations, including enantioselective hydrogenation reactions. See, e.g., commonly assigned U.S. Patent Application No. 2002/0143214 A1, published Oct. 3, 2002, and commonly assigned U.S. Patent Application No. 2003/0073868, published Apr. 17, 2003, the complete disclosures of which are herein incorporated by reference for all purposes. As shown in
Pregabalin, (S)-(+)-3-aminomethyl-5-methyl-hexanoic acid, binds to the alpha-2-delta (α2δ) subunit of a calcium channel, and is related to the endogenous inhibitory neurotransmitter γ-aminobutyric acid (GABA), which is involved in the regulation of brain neuronal activity. Pregabalin exhibits anti-seizure activity, as described in U.S. Pat. No. 5,563,175 to R. B. Silverman et al., and is thought to be useful for treating, among other conditions, pain, physiological conditions associated with psychomotor stimulants, inflammation, gastrointestinal damage, alcoholism, insomnia, and various psychiatric disorders, including mania and bipolar disorder. See, respectively, U.S. Pat. No. 6,242,488 to L. Bueno et al., U.S. Pat. No. 6,326,374 to L. Magnus & C. A. Segal, and U.S. Pat. No. 6,001,876 to L. Singh; U.S. Pat. No. 6,194,459 to H. C. Akunne et al.; U.S. Pat. No. 6,329,429 to D. Schrier et al.; U.S. Pat. No. 6,127,418 to L. Bueno et al.; U.S. Pat. No. 6,426,368 to L. Bueno et al.; U.S. Pat. No. 6,306,910 to L. Magnus & C. A. Segal; and U.S. Pat. No. 6,359,005 to A. C. Pande, which are herein incorporated by reference in their entirety and for all purposes.
Pregabalin has been prepared in various ways. Typically, a racemic mixture of 3-(aminomethyl)-5-methyl-hexanoic acid is synthesized and subsequently resolved into its R- and S-enantiomers. Such methods may employ an azide intermediate (e.g., U.S. Pat. No. 5,563,175 to R. B. Silverman et al.), a malonate intermediate (e.g., U.S. Pat. No. 6,046,353 to T. M. Grote et al., U.S. Pat. No. 5,840,956 to T. M. Grote et al., and U.S. Pat. No. 5,637,767 to T. M. Grote et al.), or Hofman synthesis (U.S. Pat. No. 5,629,447 to B. K. Huckabee & D. M. Sobieray, and U.S. Pat. No. 5,616,793 to B. K. Huckabee & D. M. Sobieray). In each of these methods, the racemate is reacted with a chiral acid (a resolving agent) to form a pair of diastereoisomeric salts, which are separated by known techniques, such as fractional crystallization and chromatography. These methods thus involve significant processing beyond the preparation of the racemate, which along with the resolving agent, adds to production costs. Moreover, the undesired R-enantiomer is frequently discarded since it cannot be efficiently recycled, thereby reducing the effective throughput of the process by 50%.
In addition, pregabalin has been synthesized directly using a chiral auxiliary, (4R,5S)-4-methyl-5-phenyl-2-oxazolidinone. See, e.g., U.S. Pat. Nos. 6,359,169, 6,028,214, 5,847,151, 5,710,304, 5,684,189, 5,608,090, and 5,599,973, all to Silverman et al. Although these methods provide pregabalin in high enantiomeric purity, they are less desirable for large-scale synthesis because they employ costly reagents (e.g., the chiral auxiliary) that are difficult to handle, as well as special cryogenic equipment to reach required operating temperatures, which can be as low as −78° C.
U.S. Patent Application 2003/0212290 A1 describes a method of making pregabalin via asymmetric hydrogenation of a cyano-substituted olefin to produce a chiral cyano precursor of (S)-3-(aminomethyl)-5-methylhexanoic acid. The cyano precursor is subsequently reduced to yield pregabalin. The application discloses the use of various C2-symmetric bisphosphine ligands, including (R,R)-Me-DUPHOS, which result in substantial enrichment of pregabalin over (R)-3-(aminomethyl)-5-methylhexanoic acid.
Although the method disclosed in U.S. Patent Application 2003/0212290 A1 represents a commercially viable method for preparing pregabalin, further improvements would be desirable for many reasons. C2-symmetric bisphosphine ligands, including the proprietary ligand (R,R)-Me-DUPHOS, are often difficult to prepare because they possess two chiral centers, which adds to their cost. Furthermore, although the chiral catalysts disclosed in U.S. Patent Application 2003/0212290 A1 generate the cyano precursor of pregabalin in good enantiomeric excess (in some cases, equal to about 95% ee or greater), higher enantioselectivity (equal to about 98% ee or greater) would be beneficial. Additionally, chiral catalysts capable of being used at higher substrate-to-catalyst ratios (s/c) would be beneficial since they would permit, for a given catalyst loading or substrate concentration, higher substrate concentrations or lower catalyst loadings. Higher substrate concentrations would result in increased process throughput and therefore lower unit production costs. Similarly, lower catalyst loadings would result in substantially lower unit production costs.
The present invention provides materials and methods for preparing pregabalin (Formula 1) and structurally related compounds. The claimed methods employ novel chiral catalysts, each of which comprises a C1-symmetric bisphosphine ligand bound to a transition metal (e.g., rhodium) through phosphorus atoms. The claimed invention provides many advantageous over existing methods for preparing pregabalin and similar compounds. For example, the C1-symmetric bisphosphine ligands have a single stereogenic center, which should make the ligands and their corresponding chiral catalysts relatively inexpensive to prepare. Moreover, and as indicated in the examples below, the claimed invention can generate a chiral cyano precursor of pregabalin with higher enantioselectivity (about 98% ee or greater) than known methods. As also shown in the examples below, the novel chiral catalysts may be used at higher substrate-to-catalyst ratios (s/c) than known catalysts, which should lead to substantially lower unit production costs.
One aspect of the present invention provides a method of making a desired enantiomer of a compound of Formula 2,
or a pharmaceutically acceptable complex, salt, solvate or hydrate thereof. In Formula 2,
with hydrogen in the presence of a chiral catalyst to yield the compound of Formula 2; and (b) optionally converting the compound of Formula 2 into a pharmaceutically acceptable complex, salt, solvate or hydrate. Substituents R1, R2, R3, R4, and X in Formula 3 are as defined in Formula 2. The chiral catalyst comprises a chiral ligand bound to a transition metal through phosphorus atoms, and has a structure represented by Formula 4,
Generally, the method may be used to produce the desired enantiomer of the compound of Formula 2 with an ee of about 95% or greater, and in some cases, with an ee of about 99% or greater. Useful prochiral substrates include 3-cyano-5-methyl-hex-3-ennoic acid or base addition salts thereof, such as 3-cyano-5-methyl-hex-3-enoate t-butyl-ammonium salt. Other useful prochiral substrates include those in which Y is a Group 1 (alkali) metal ion, a Group 2 (alkaline earth) metal ion, a primary ammonium ion, or a secondary ammonium ion.
A particularly useful chiral catalyst includes the chiral ligand of Formula 4, which is bound to rhodium through the phosphorus atoms. Another particularly useful chiral catalyst includes an enantiomer of the bisphosphine ligand of Formula 4, which has a structure represented by Formula 5,
and an ee of about 95% or greater. An especially useful chiral catalyst includes an enantiomer of the bisphosphine ligand of Formula 4 having a structure represented by Formula 5 and ee of about 99% or greater.
Another aspect of the present invention provides a method of making pregabalin or (S)-(+)-3-(aminomethyl)-5-methyl-hexanoic acid (Formula 1) or a pharmaceutically acceptable complex, salt, solvate or hydrate thereof. The method includes the steps of (a) reacting a compound of Formula 6,
its corresponding Z-isomer, or a mixture thereof, with H2 (hydrogen) in the presence of a chiral catalyst to yield a compound of Formula 7,
wherein R5 is a carboxy group or CO2—Y, Y is a cation, and the chiral catalyst comprises a chiral ligand (Formula 4) bound to a transition metal through phosphorus atoms; (b) reducing a cyano moiety of the compound of Formula 7 to yield a compound of Formula 8,
(c) optionally treating the compound of Formula 8 with an acid to yield pregabalin; and (d) optionally converting the compound of Formula 8 or Formula 1 to a pharmaceutically acceptable complex, salt, solvate or hydrate.
The method may be used to produce pregabalin having an ee of about 95% or greater, or having an ee of about 99% or greater, and in some cases having an ee of about 99.9% or greater. Useful prochiral substrates (Formula 6) include a base addition salt of 3-cyano-5-methyl-hex-3-enoic acid, such as 3-cyano-5-methyl-hex-3-enoate t-butyl-ammonium salt. Other useful prochiral substrates include those in which Y in Formula 6 is a Group 1 metal ion, a Group 2 metal ion, a primary ammonium ion, or a secondary ammonium ion. A particularly useful chiral catalyst includes the chiral ligand of Formula 4, which is bound to rhodium through the phosphorus atoms. Another particularly useful chiral catalyst includes an enantiomer of the bisphosphine ligand of Formula 4, which has a structure represented by Formula 5 (above), and an ee of about 95% or greater. An especially useful chiral catalyst includes an enantiomer of the bisphosphine ligand of Formula 4 having a structure represented by Formula 5 and ee of about 99% or greater.
Still another aspect of the present invention provides a method of making a desired enantiomer of the compound of Formula 4. The method includes the steps of (a) reacting a compound of Formula 9,
with a compound of Formula 10,
to yield a compound of Formula 11,
in which the compound of Formula 9 is treated with a base prior to reaction with the compound of Formula 10, X is a leaving group, and R6 is BH3, sulfur or oxygen; (b) reacting the compound of Formula 11 with a borane, with sulfur, or with oxygen to yield a compound of Formula 12,
wherein R7 is the same as or different than R6 and is BH3, sulfur, or oxygen; and (c) removing R6 and R7 from the compound of Formula 12 to yield the compound of Formula 4.
The claimed method is particularly useful for making the R-enantiomer of the compound of Formula 5, having an ee of about 80%, about 90%, about 95% or about 99% or greater. Typically, the compound of Formula 12 is resolved into separate enantiomers before removal of R6 and R7. Substituents R6 and R7 may be removed many different ways depending on the nature of the particular substituents. For instance, when R6 and R7 are each BH3, they may be removed by reacting a compound of Formula 13,
with an amine or an acid to yield the compound of Formula 4. Thus, for example, the compound of Formula 13 may be reacted with HBF4.Me2O, followed by base hydrolysis to yield the compound of Formula 4. Similarly, the compound of Formula 13 may be treated with DABCO, TMEDA, DBU, or Et2NH, or combinations thereof to remove R6 and R7.
When both substituents are sulfur atoms, R6 and R7 may be removed using various techniques. One method includes the steps of (a) reacting a compound of Formula 14,
with R8OTf to yield a compound of Formula 15,
in which R8 is a C1-4 alkyl; (b) reacting the compound of Formula 15 with a borohydride to yield the compound of Formula 13; and (c) reacting the compound of Formula 13 with an amine or an acid to yield the compound of Formula 4. A particularly useful R8 substituent is methyl and a particularly useful borohydride is LiBH4.
Another method includes steps (a) and (b) above, and further includes the steps of (c) reacting the compound of Formula 13 with HCl to yield a compound of Formula 15,
and
(d) reacting the compound of Formula 16 with an amine or an acid to yield the compound of Formula 4. When both substituents are sulfur or oxygen, R6 and R7 may also be removed by treating the compound of Formula 12 with a reducing agent, including a perchloropolysilane such as hexachlorodisilane.
Yet another aspect of the present invention provides a method of making a catalyst or pre-catalyst comprised of a chiral ligand bound to a transition metal through phosphorus atoms, the chiral ligand having a structure represented by Formula 4. The method includes the steps of (a) removing both R9 substituents from a compound of Formula 17,
to yield a compound of Formula 4, wherein R9 is BH3, sulfur, or oxygen; and (b) binding the compound of Formula 4 to a transition metal (e.g., rhodium). Step (b) may include reacting the compound of Formula 4 with a complex of Formula 18,
[Rh(L1)m(L2)n]Ap, 18
in which
A further aspect of the present invention provides compounds of Formula 19,
in which R10 and R11 are independently BH3, BH2Cl, sulfur, oxygen, C1-4 alkylthio, or absent, and subject to the proviso that R10 and R11 are not both BH3.
Useful compounds of Formula 19 include those in which R10 and R11 are absent and those having R-absolute stereochemical configuration with an ee of about 95% or with an ee of 99% or greater. Other useful compounds of Formula 19 include those in which R10 and R11 are the same, and are each oxygen, sulfur or C1-4 alkylthio, and those having R-absolute stereochemical configuration with an ee of about 95% or greater or with an ee of about 99% or greater. Useful compounds represented by Formula 19 thus include:
An additional aspect of the present invention provides a catalyst or pre-catalyst comprising a chiral ligand bound to a transition metal through phosphorus atoms. The chiral ligand has a structure represented by Formula 4.
A particularly useful chiral catalyst or pre-catalyst includes rhodium bound to a bisphosphine ligand having a structure represented by Formula 5. Other useful chiral catalysts or pre-catalysts include the bisphosphine ligand having a structure represented by Formula 5 and an ee of about 95% or greater. An especially useful chiral catalyst includes the bisphosphine ligand having a structure represented by Formula 5 and ee of about 99% or greater. The catalyst or pre-catalyst may further include one or more dienes (e.g., COD) or halogen anions (e.g., Cl−) bound to the transition metal, and may include a counterion, such as OTf−, PF6−, BF4−, SbF6−, or ClO4−, or mixtures thereof.
A further aspect of the present invention provides method of making a desired enantiomer of a compound of Formula 32,
or a pharmaceutically acceptable complex, salt, solvate or hydrate thereof. The method comprises the steps of (a) reacting a compound of Formula 33,
with hydrogen in the presence of a chiral catalyst to yield the compound of Formula 32; and (b) optionally converting the compound of Formula 32 into a pharmaceutically acceptable complex, salt, solvate or hydrate. Substituents R1, R2, R3, R4, and X in Formula 32 and Formula 33 are as defined in Formula 2; the chiral catalyst comprises a chiral ligand bound to a transition metal through phosphorus atoms, the chiral ligand having a structure represented by Formula 4, above. Useful compounds of Formula 32 include optically active β-amino acids that, like pregabalin, bind to the α2δ subunit of a calcium channel. These compounds, including their pharmaceutically acceptable complexes, salts, solvates and hydrates, are useful for treating pain, fibromyalgia, and a variety of psychiatric and sleep disorders. See, e.g., U.S. Patent Application No. 2003/0195251 A1 to Barta et al., the complete disclosure of which is herein incorporated by reference.
The scope of the present invention includes all pharmaceutically acceptable complexes, salts, solvates, hydrates, polymorphs, esters, amides, and prodrugs of the claimed and disclosed compounds, including compounds of Formula 1, 2, 8, and 32.
Unless otherwise indicated, this disclosure uses definitions provided below. Some of the definitions and formulae may include a dash (“-”) to indicate a bond between atoms or a point of attachment to a named or unnamed atom or group of atoms. Other definitions and formulae may include an equal sign (“=”) or an identity sign (“≡”) to indicate a double bond or a triple bond, respectively. Certain formulae may also include one or more asterisks (“*”) to indicate stereogenic (chiral) centers, although the absence of asterisks does not indicate that the compound lacks one or more stereocenters. Such formulae may refer to the racemate or to individual enantiomers or diastereomers, which may or may not be substantially pure. Some formulae may also include a crossed double bond or a double either bond, , to indicate a Z-isomer, an E-isomer, or a mixture of Z and E isomers.
“Substituted” groups are those in which one or more hydrogen atoms have been replaced with one or more non-hydrogen atoms or groups, provided that valence requirements are met and that a chemically stable compound results from the substitution.
“Alkyl” refers to straight chain and branched saturated hydrocarbon groups, generally having a specified number of carbon atoms (i.e., C1-6 alkyl refers to an alkyl group having 1, 2, 3, 4, 5, or 6 carbon atoms). Examples of alkyl groups include, without limitation, methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, i-butyl, t-butyl, pent-1-yl, pent-2-yl, pent-3-yl, 3-methylbut-1-yl, 3-methylbut-2-yl, 2-methylbut-2-yl, 2,2,2-trimethyleth-1-yl, n-hexyl, and the like.
“Alkenyl” refers to straight chain and branched hydrocarbon groups having one or more unsaturated carbon-carbon bonds, and generally having a specified number of carbon atoms. Examples of alkenyl groups include, without limitation, ethenyl, 1-propen-1-yl, 1-propen-2-yl, 2-propen-1-yl, 1-buten-1-yl, 1-buten-2-yl, 3-buten-1-yl, 3-buten-2-yl, 2-buten-1-yl, 2-buten-2-yl, 2-methyl-1-propen-1-yl, 2-methyl-2-propen-1-yl, 1,3-butadien-1-yl, 1,3-butadien-2-yl, and the like.
“Alkynyl” refers to straight chain or branched hydrocarbon groups having one or more triple carbon-carbon bonds, and generally having a specified number of carbon atoms. Examples of alkynyl groups include, without limitation, ethynyl, 1-propyn-1-yl, 2-propyn-1-yl, 1-butyn-1-yl, 3-butyn-1-yl, 3-butyn-2-yl, 2-butyn-1-yl, and the like.
“Alkanediyl” refers to divalent straight chain and branched saturated hydrocarbon groups, generally having a specified number of carbon atoms. Examples include, without limitation, methylene, 1,2-ethanediyl, 1,3-propanediyl, 1,4-butanediyl, 1,5-pentanediyl, 1,6-hexanediyl, and the like.
“Alkanoyl” and “alkanoylamino” refer, respectively, to alkyl-C(O)— and alkyl-C(O)—NH—, where alkyl is defined above, and generally includes a specified number of carbon atoms, including the carbonyl carbon. Examples of alkanoyl groups include, without limitation, formyl, acetyl, propionyl, butyryl, pentanoyl, hexanoyl, and the like.
“Alkenoyl” and “alkynoyl” refer, respectively, to alkenyl-C(O)— and alkynyl-C(O)—, where alkenyl and alkynyl are defined above. References to alkenoyl and alkynoyl generally include a specified number of carbon atoms, excluding the carbonyl carbon. Examples of alkenoyl groups include, without limitation, propenoyl, 2-methylpropenoyl, 2-butenoyl, 3-butenoyl, 2-methyl-2-butenoyl, 2-methyl-3-butenoyl, 3-methyl-3-butenoyl, 2-pentenoyl, 3-pentenoyl, 4-pentenoyl, and the like. Examples of alkynoyl groups include, without limitation, propynoyl, 2-butynoyl, 3-butynoyl, 2-pentynoyl, 3-pentynoyl, 4-pentynoyl, and the like.
“Alkoxy,” “alkoxycarbonyl,” and “alkoxycarbonylamino” refer, respectively, to alkyl-O—, alkenyl-O, and alkynyl-O, to alkyl-O—C(O)—, alkenyl-O—C(O)—, alkynyl-O—C(O)—, and to alkyl-O—C(O)—NH—, alkenyl-O—C(O)—NH—, alkynyl-O—C(O)—NH—, where alkyl, alkenyl, and alkynyl are defined above. Examples of alkoxy groups include, without limitation, methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, s-butoxy, t-butoxy, n-pentoxy, s-pentoxy, and the like. Examples of alkoxycarbonyl groups include, without limitation, methoxycarbonyl, ethoxycarbonyl, n-propoxycarbonyl, i-propoxycarbonyl, n-butoxycarbonyl, s-butoxycarbonyl, t-butoxycarbonyl, n-pentoxycarbonyl, s-pentoxycarbonyl, and the like.
“Alkylamino,” “alkylaminocarbonyl,” “dialkylaminocarbonyl,” “alkylsulfonyl” “sulfonylaminoalkyl,” and “alkylsulfonylaminocarbonyl” refer, respectively, to alkyl-NH—, alkyl-NH—C(O)—, alkyl2-N—C(O)—, alkyl-S(O2)—, HS(O2)—NH-alkyl-, and alkyl-S(O)—NH—C(O)—, where alkyl is defined above.
“Aminoalkyl” and “cyanoalkyl” refer, respectively, to NH2-alkyl and N≡C-alkyl, where alkyl is defined above.
“Halo,” “halogen” and “halogeno” may be used interchangeably, and refer to fluoro, chloro, bromo, and iodo.
“Haloalkyl,” “haloalkenyl,” “haloalkynyl,” “haloalkanoyl,” “haloalkenoyl,” “haloalkynoyl,” “haloalkoxy,” and “haloalkoxycarbonyl” refer, respectively, to alkyl, alkenyl, alkynyl, alkanoyl, alkenoyl, alkynoyl, alkoxy, and alkoxycarbonyl groups substituted with one or more halogen atoms, where alkyl, alkenyl, alkynyl, alkanoyl, alkenoyl, alkynoyl, alkoxy, and alkoxycarbonyl are defined above. Examples of haloalkyl groups include, without limitation, trifluoromethyl, trichloromethyl, pentafluoroethyl, pentachloroethyl, and the like.
“Hydroxyalkyl” and “oxoalkyl” refer, respectively, to HO-alkyl and O=alkyl, where alkyl is defined above. Examples of hydroxyalkyl and oxoalkyl groups, include, without limitation, hydroxymethyl, hydroxyethyl, 3-hydroxypropyl, oxomethyl, oxoethyl, 3-oxopropyl, and the like.
“Cycloalkyl” refers to saturated monocyclic and bicyclic hydrocarbon rings, generally having a specified number of carbon atoms that comprise the ring (i.e., C3-7 cycloalkyl refers to a cycloalkyl group having 3, 4, 5, 6 or 7 carbon atoms as ring members). The cycloalkyl may be attached to a parent group or to a substrate at any ring atom, unless such attachment would violate valence requirements. Likewise, any of the ring members may include one or more non-hydrogen substituents unless such substitution would violate valence requirements. Useful substituents include, without limitation, alkyl, alkenyl, alkynyl, alkanoyl, alkenoyl, alkynoyl, alkylamino, alkylaminocarbonyl, dialkylaminocarbonyl, alkylsulfonyl, sulfonylaminoalkyl, alkylsulfonylaminocarbonyl, alkoxy, alkoxycarbonyl, alkoxycarbonylamino, aminoalkyl, cyanoalkyl, hydroxyalkyl, oxoalkyl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloalkanoyl, haloalkenoyl, haloalkynoyl, haloalkoxy, haloalkoxycarbonyl, as defined above, and hydroxy, mercapto, nitro, and amino.
Examples of monocyclic cycloalkyl groups include, without limitation, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and the like. Examples of bicyclic cycloalkyl groups include, without limitation, bicyclo[1.1.0]butyl, bicyclo[1.1.1]pentyl, bicyclo[2.1.0]pentyl, bicyclo[2.1.1]hexyl, bicyclo[3.1.0]hexyl, bicyclo[2.2.1]heptyl, bicyclo[3.2.0]heptyl, bicyclo[3.1.1]heptyl, bicyclo[4.1.0]heptyl, bicyclo[2.2.2]octyl, bicyclo[3.2.1]octyl, bicyclo[4.1.1]octyl, bicyclo[3.3.0]octyl, bicyclo[4.2.0]octyl, bicyclo[3.3.1]nonyl, bicyclo[4.2.1]nonyl, bicyclo[4.3.0]nonyl, bicyclo[3.3.2]decyl, bicyclo[4.2.2]decyl, bicyclo[4.3.1]decyl, bicyclo[4.4.0]decyl, bicyclo[3.3.3]undecyl, bicyclo[4.3.2]undecyl, bicyclo[4.3.3]dodecyl, and the like.
“Cycloalkenyl” refers monocyclic and bicyclic hydrocarbon rings having one or more unsaturated carbon-carbon bonds and generally having a specified number of carbon atoms that comprise the ring (i.e., C3-7 cycloalkenyl refers to a cycloalkenyl group having 3, 4, 5, 6 or 7 carbon atoms as ring members). The cycloalkenyl may be attached to a parent group or to a substrate at any ring atom, unless such attachment would violate valence requirements. Likewise, any of the ring members may include one or more non-hydrogen substituents unless such substitution would violate valence requirements. Useful substituents include, without limitation, alkyl, alkenyl, alkynyl, alkanoyl, alkenoyl, alkynoyl, alkylamino, alkylaminocarbonyl, dialkylaminocarbonyl, alkylsulfonyl, sulfonylaminoalkyl, alkylsulfonylaminocarbonyl, alkoxy, alkoxycarbonyl, alkoxycarbonylamino, aminoalkyl, cyanoalkyl, hydroxyalkyl, oxoalkyl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloalkanoyl, haloalkenoyl, haloalkynoyl, haloalkoxy, haloalkoxycarbonyl, as defined above, and hydroxy, mercapto, nitro, and amino.
“Cycloalkanoyl” and “cycloalkenoyl” refer to cycloalkyl-C(O)— and cycloalkenyl-C(O)—, respectively, where cycloalkyl and cycloalkenyl are defined above. References to cycloalkanoyl and cycloalkenoyl generally include a specified number of carbon atoms, excluding the carbonyl carbon. Examples of cycloalkanoyl groups include, without limitation, cyclopropanoyl, cyclobutanoyl, cyclopentanoyl, cyclohexanoyl, cycloheptanoyl, 1-cyclobutenoyl, 2-cyclobutenoyl, 1-cyclopentenoyl, 2-cyclopentenoyl, 3-cyclopentenoyl, 1-cyclohexenoyl, 2-cyclohexenoyl, 3-cyclohexenoyl, and the like.
“Cycloalkoxy” and “cycloalkoxycarbonyl” refer, respectively, to cycloalkyl-O— and cycloalkenyl-O and to cycloalkyl-O—C(O)— and cycloalkenyl-O—C(O)—, where cycloalkyl and cycloalkenyl are defined above. References to cycloalkoxy and cycloalkoxycarbonyl generally include a specified number of carbon atoms, excluding the carbonyl carbon. Examples of cycloalkoxy groups include, without limitation, cyclopropoxy, cyclobutoxy, cyclopentoxy, cyclohexoxy, 1-cyclobutenoxy, 2-cyclobutenoxy, 1-cyclopentenoxy, 2-cyclopentenoxy, 3-cyclopentenoxy, 1-cyclohexenoxy, 2-cyclohexenoxy, 3-cyclohexenoxy, and the like. Examples of cycloalkoxycarbonyl groups include, without limitation, cyclopropoxycarbonyl, cyclobutoxycarbonyl, cyclopentoxycarbonyl, cyclohexoxycarbonyl, 1-cyclobutenoxycarbonyl, 2-cyclobutenoxycarbonyl, 1-cyclopentenoxycarbonyl, 2-cyclopentenoxycarbonyl, 3-cyclopentenoxycarbonyl, 1-cyclohexenoxycarbonyl, 2-cyclohexenoxycarbonyl, 3-cyclohexenoxycarbonyl, and the like.
“Aryl” and “arylene” refer to monovalent and divalent aromatic groups, respectively, including 5- and 6-membered monocyclic aromatic groups that contain 0 to 4 heteroatoms independently selected from nitrogen, oxygen, and sulfur. Examples of monocyclic aryl groups include, without limitation, phenyl, pyrrolyl, furanyl, thiopheneyl, thiazolyl, isothiazolyl, imidazolyl, triazolyl, tetrazolyl, pyrazolyl, oxazolyl, isooxazolyl, pyridinyl, pyrazinyl, pyridazinyl, pyrimidinyl, and the like. Aryl and arylene groups also include bicyclic groups, tricyclic groups, etc., including fused 5- and 6-membered rings described above. Examples of multicyclic aryl groups include, without limitation, naphthyl, biphenyl, anthracenyl, pyrenyl, carbazolyl, benzoxazolyl, benzodioxazolyl, benzothiazolyl, benzoimidazolyl, benzothiopheneyl, quinolinyl, isoquinolinyl, indolyl, benzofuranyl, purinyl, indolizinyl, and the like. They aryl and arylene groups may be attached to a parent group or to a substrate at any ring atom, unless such attachment would violate valence requirements. Likewise, any of the carbon or nitrogen ring members may include a non-hydrogen substituent unless such substitution would violate valence requirements. Useful substituents include, without limitation, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, alkanoyl, alkenoyl, alkynoyl, cycloalkanoyl, cycloalkenoyl, alkylamino, alkylaminocarbonyl, dialkylaminocarbonyl, alkylsulfonyl, sulfonylaminoalkyl, alkylsulfonylaminocarbonyl, alkoxy, cycloalkoxy, alkoxycarbonyl, cycloalkoxycarbonyl, alkoxycarbonylamino, aminoalkyl, cyanoalkyl, hydroxyalkyl, oxoalkyl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloalkanoyl, haloalkenoyl, haloalkynoyl, haloalkoxy, haloalkoxycarbonyl, as defined above, and hydroxy, mercapto, nitro, and amino.
“Heterocycle” and “heterocyclyl” refer to saturated, partially unsaturated, or unsaturated monocyclic or bicyclic rings having from 5 to 7 or from 7 to 11 ring members, respectively. These groups have ring members made up of carbon atoms and from 1 to 4 heteroatoms that are independently nitrogen, oxygen or sulfur, and may include any bicyclic group in which any of the above-defined monocyclic heterocycles are fused to a benzene ring. The nitrogen and sulfur heteroatoms may optionally be oxidized. The heterocyclic ring may be attached to a parent group or to a substrate at any heteroatom or carbon atom unless such attachment would violate valence requirements. Likewise, any of the carbon or nitrogen ring members may include a non-hydrogen substituent unless such substitution would violate valence requirements. Useful substituents include, without limitation, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, alkanoyl, alkenoyl, alkynoyl, cycloalkanoyl, cycloalkenoyl, alkylamino, alkylaminocarbonyl, dialkylaminocarbonyl, alkylsulfonyl, sulfonylaminoalkyl, alkylsulfonylaminocarbonyl, alkoxy, cycloalkoxy, alkoxycarbonyl, cycloalkoxycarbonyl, alkoxycarbonylamino, aminoalkyl, cyanoalkyl, hydroxyalkyl, oxoalkyl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloalkanoyl, haloalkenoyl, haloalkynoyl, haloalkoxy, haloalkoxycarbonyl, as defined above, and hydroxy, mercapto, nitro, and amino.
Examples of heterocycles include, without limitation, acridinyl, azocinyl, benzimidazolyl, benzofuranyl, benzothiofuranyl, benzothiophenyl, benzoxazolyl, benzthiazolyl, benztriazolyl, benztetrazolyl, benzisoxazolyl, benzisothiazolyl, benzimidazolinyl, carbazolyl, 4aH-carbazolyl, carbolinyl, chromanyl, chromenyl, cinnolinyl, decahydroquinolinyl, 2H, 6H-1,5,2-dithiazinyl, dihydrofuro[2,3-b]tetrahydrofuran, furanyl, furazanyl, imidazolidinyl, imidazolinyl, imidazolyl, 1H-indazolyl, indolenyl, indolinyl, indolizinyl, indolyl, 3H-indolyl, isobenzofuranyl, isochromanyl, isoindazolyl, isoindolinyl, isoindolyl, isoquinolinyl, isothiazolyl, isoxazolyl, morpholinyl, naphthyridinyl, octahydroisoquinolinyl, oxadiazolyl, 1,2,3-oxadiazolyl, 1,2,4-oxadiazolyl, 1,2,5-oxadiazolyl, 1,3,4-oxadiazolyl, oxazolidinyl, oxazolyl, oxazolidinyl, pyrimidinyl, phenanthridinyl, phenanthrolinyl, phenazinyl, phenothiazinyl, phenoxathiinyl, phenoxazinyl, phthalazinyl, piperazinyl, piperidinyl, pteridinyl, purinyl, pyranyl, pyrazinyl, pyrazolidinyl, pyrazolinyl, pyrazolyl, pyridazinyl, pyridooxazole, pyridoimidazole, pyridothiazole, pyridinyl, pyridyl, pyrimidinyl, pyrrolidinyl, pyrrolinyl, 2H-pyrrolyl, pyrrolyl, quinazolinyl, quinolinyl, 4H-quinolizinyl, quinoxalinyl, quinuclidinyl, tetrahydrofuranyl, tetrahydroisoquinolinyl, tetrahydroquinolinyl, 6H-1,2,5-thiadiazinyl, 1,2,3-thiadiazolyl, 1,2,4-thiadiazolyl, 1,2,5-thiadiazolyl, 1,3,4-thiadiazolyl, thianthrenyl, thiazolyl, thienyl, thienothiazolyl, thienooxazolyl, thienoimidazolyl, thiophenyl, triazinyl, 1,2,3-triazolyl, 1,2,4-triazolyl, 1,2,5-triazolyl, 1,3,4-triazolyl, and xanthenyl.
“Heteroaryl” and “heteroarylene” refer, respectively, to monovalent and divalent heterocycles or heterocyclyl groups, as defined above, which are aromatic. Heteroaryl and heteroarylene groups represent a subset of aryl and arylene groups, respectively.
“Arylalkyl” and “heteroarylalkyl” refer, respectively, to aryl-alkyl and heteroaryl-alkyl, where aryl, heteroaryl, and alkyl are defined above. Examples include, without limitation, benzyl, fluorenylmethyl, imidazol-2-yl-methyl, and the like.
“Arylalkanoyl,” “heteroarylalkanoyl,” “arylalkenoyl,” “heteroarylalkenoyl,” “arylalkynoyl,” and “heteroarylalkynoyl” refers, respectively, to aryl-alkanoyl, heteroaryl-alkanoyl, aryl-alkenoyl, heteroaryl-alkenoyl, aryl-alkynoyl, and heteroaryl-alkynoyl, where aryl, heteroaryl, alkanoyl, alkenoyl, and alkynoyl are defined above. Examples include, without limitation, benzoyl, benzylcarbonyl, fluorenoyl, fluorenylmethylcarbonyl, imidazol-2-oyl, imidazol-2-yl-methylcarbonyl, phenylethenecarbonyl, 1-phenylethenecarbonyl, 1-phenyl-propenecarbonyl, 2-phenyl-propenecarbonyl, 3-phenyl-propenecarbonyl, imidazol-2-yl-ethenecarbonyl, 1-(imidazol-2-yl)-ethenecarbonyl, 1-(imidazol-2-yl)-propenecarbonyl, 2-(imidazol-2-yl)-propenecarbonyl, 3-(imidazol-2-yl)-propenecarbonyl, phenylethynecarbonyl, phenylpropynecarbonyl, (imidazol-2-yl)-ethynecarbonyl, (imidazol-2-yl)-propynecarbonyl, and the like.
“Arylalkoxy” and “heteroarylalkoxy” refer, respectively, to aryl-alkoxy and heteroaryl-alkoxy, where aryl, heteroaryl, and alkoxy are defined above. Examples include, without limitation, benzyloxy, fluorenylmethyloxy, imidazol-2-yl-methyloxy, and the like.
“Aryloxy” and “heteroaryloxy” refer, respectively, to aryl-O— and heteroaryl-O—, where aryl and heteroaryl are defined above. Examples include, without limitation, phenoxy, imidazol-2-yloxy, and the like.
“Aryloxycarbonyl,” “heteroaryloxycarbonyl,” “arylalkoxycarbonyl,” and “heteroarylalkoxycarbonyl” refer, respectively, to aryloxy-C(O)—, heteroaryloxy-C(O)—, arylalkoxy-C(O)—, and heteroarylalkoxy-C(O)—, where aryloxy, heteroaryloxy, arylalkoxy, and heteroarylalkoxy are defined above. Examples include, without limitation, phenoxycarbonyl, imidazol-2-yloxycarbonyl, benzyloxycarbonyl, fluorenylmethyloxycarbonyl, imidazol-2-yl-methyloxycarbonyl, and the like.
“Leaving group” refers to any group that leaves a molecule during a fragmentation process, including substitution reactions, elimination reactions, and addition-elimination reactions. Leaving groups may be nucleofugal, in which the group leaves with a pair of electrons that formerly served as the bond between the leaving group and the molecule, or may be electrofugal, in which the group leaves without the pair of electrons. The ability of a nucleofugal leaving group to leave depends on its base strength, with the strongest bases being the poorest leaving groups. Common nucleofugal leaving groups include nitrogen (e.g., from diazonium salts); sulfonates, including alkylsulfonates (e.g., mesylate), fluoroalkylsulfonates (e.g., triflate, hexaflate, nonaflate, and tresylate), and arylsulfonates (e.g., tosylate, brosylate, closylate, and nosylate). Others include carbonates, halide ions, carboxylate anions, phenolate ions, and alkoxides. Some stronger bases, such as NH2− and OH− can be made better leaving groups by treatment with an acid. Common electrofugal leaving groups include the proton, CO2, and metals.
“Enantiomeric excess” or “ee” is a measure, for a given sample, of the excess of one enantiomer over a racemic sample of a chiral compound and is expressed as a percentage. Enantiomeric excess is defined as 100×(er−1)/(er+1), where “er” is the ratio of the more abundant enantiomer to the less abundant enantiomer.
“Enantioselectivity” refers to a given reaction (e.g., hydrogenation) that yields more of one enantiomer than another.
“High level of enantioselectivity” refers to a given reaction that yields product with an ee of at least about 80%.
“Enantiomerically enriched” refers to a sample of a chiral compound, which has more of one enantiomer than another. The degree of enrichment is measured by er or ee.
“Substantially pure enantiomer” or “substantially enantiopure” refers to a sample of an enantiomer having an ee of about 90% or greater.
“Enantiomerically pure” or “enantiopure” refers to a sample of an enantiomer having an ee of about 99.9% or greater.
“Opposite enantiomer” refers to a molecule that is a non-superimposable mirror image of a reference molecule, which may be obtained by inverting all of the stereogenic centers of the reference molecule. For example, if the reference molecule has S absolute stereochemical configuration, then the opposite enantiomer has R absolute stereochemical configuration. Likewise, if the reference molecule has S,S absolute stereochemical configuration, then the opposite enantiomer has R,R stereochemical configuration, and so on.
“Pre-catalyst” or “catalyst precursor” refer to a compound or set of compounds that are converted into a catalyst prior to use.
“Pharmaceutically acceptable” refers to substances, which are within the scope of sound medical judgment, suitable for use in contact with the tissues of patients without undue toxicity, irritation, allergic response, and the like, commensurate with a reasonable benefit-to-risk ratio, and effective for their intended use.
“Treating” refers to reversing, alleviating, inhibiting the progress of, or preventing a disorder or condition to which such term applies, or to preventing one or more symptoms of such disorder or condition.
“Treatment” refers to the act of “treating” as defined immediately above.
“About” or “approximately,” when used in connection with a measurable numerical variable, refers to the indicated value of the variable and to all values of the variable that are within the experimental error of the indicated value (e.g., within the 95% confidence interval for the mean) or within ±10 percent of the indicated value, whichever is greater.
“Solvate” refers to a molecular complex comprising a disclosed or claimed compound (e.g., pregabalin) and a stoichiometric or non-stoichiometric amount of one or more solvent molecules (e.g., EtOH).
“Hydrate” refers to a solvate comprising a disclosed or claimed compound (e.g., pregabalin) and a stoichiometric or non-stoichiometric amount of water.
“Pharmaceutically acceptable esters, amides, and prodrugs” refer to acid or base addition salts, esters, amides, zwitterionic forms, where possible, and prodrugs of claimed and disclosed compounds. Examples of pharmaceutically acceptable, non-toxic esters include, without limitation, C1-6 alkyl esters, C5-7 cycloalkyl esters, and arylalkyl esters of claimed and disclosed compounds, where alkyl, cycloalkyl, and aryl are defined above. Such esters may be prepared by conventional methods, as described, for example, in M. B. Smith and J. March, March's Advanced Organic Chemistry (5th Ed. 2001).
Examples of pharmaceutically acceptable, non-toxic amides include, without limitation, those derived from ammonia, primary C1-6 alkyl amines, and secondary C1-6 dialkyl or heterocyclyl amines of claimed and disclosed compounds, where alkyl and heterocyclyl are defined above. Such amides may be prepared by conventional methods, as described, for example, in March's Advanced Organic Chemistry.
“Prodrugs” refer to compounds having little or no pharmacological activity that can, when metabolized in vivo, undergo conversion to claimed or disclosed compounds having desired activity. For a discussion of prodrugs, see T. Higuchi and V. Stella, “Pro-drugs as Novel Delivery Systems,” ACS Symposium Series 14 (1975), E. B. Roche (ed.), Bioreversible Carriers in Drug Design (1987), and H. Bundgaar, Design of Prodrugs (1985).
Table 1 lists abbreviations used throughout the specification.
In some of the reaction schemes and examples below, certain compounds can be prepared using protecting groups, which prevent undesirable chemical reaction at otherwise reactive sites. Protecting groups may also be used to enhance solubility or otherwise modify physical properties of a compound. For a discussion of protecting group strategies, a description of materials and methods for installing and removing protecting groups, and a compilation of useful protecting groups for common functional groups, including amines, carboxylic acids, alcohols, ketones, aldehydes, and the like, see T. W. Greene and P. G. Wuts, Protecting Groups in Organic Chemistry (1999) and P. Kocienski, Protective Groups (2000), which are herein incorporated by reference in their entirety for all purposes.
In addition, some of the schemes and examples below may omit details of common reactions, including oxidations, reductions, and so on, which are known to persons of ordinary skill in the art of organic chemistry. The details of such reactions can be found in a number of treatises, including Richard Larock, Comprehensive Organic Transformations (1999), and the multi-volume series edited by Michael B. Smith and others, Compendium of Organic Synthetic Methods (1974-2003). Generally, starting materials and reagents may be obtained from commercial sources or known procedures.
The present invention provides materials and methods for preparing chiral compounds represented by Formula 2, above, including pharmaceutically acceptable salts, esters, amides, or prodrugs thereof. In Formula 2, the chiral compounds have at least one stereogenic center, as indicated by the “*”, and includes substituents R1, R2, R3, R4, and X, which are defined above. Useful compounds represented by Formula 2 include those in which R1 is amino, amino-C1-6 alkyl, cyano or cyano-C1-6 alkyl; R2 is C1-6 alkoxycarbonyl or carboxy; X is —CH2— or a bond; and R3 and R4 are independently hydrogen atom or C1-6 alkyl. Particularly useful compounds include α-amino acids, β-amino acids, and γ-amino acids in which R1 is amino or aminomethyl; R2 is carboxy; X is a bond or —CH2—; and R3 and R4 are independently hydrogen atom or C1-6 alkyl. Especially useful compounds thus include (S)-3-cyano-5-methyl-hexanoic acid, and (S)-(+)-3-(aminomethyl)-5-methyl-hexanoic acid, Formula 1, which is known as pregabalin.
Scheme I illustrates a method of preparing a desired enantiomer of the compound of Formula 2. The enantioselective synthesis includes the steps of (a) reacting a prochiral substrate (olefin) of Formula 3, with hydrogen in the presence of a chiral catalyst and organic solvent to yield the compound of Formula 2; and (b) optionally converting the compound of Formula 2 into a pharmaceutically acceptable salt, ester, amide, or prodrug. Substituents R1, R2, R3, R4, and X in Formula 3 are as defined in Formula 2. More generally, and unless stated otherwise, when a particular substituent identifier (R1, R2, R3, etc.) is defined for the first time in connection with a formula, the same substituent identifier, when used in a subsequent formula, will have the same definition as in the earlier formula. Thus, for example, if R20 in a first formula is hydrogen, halogeno, or C1-6 alkyl, then unless stated differently or otherwise clear from the context of the text, R20 in a second formula is also hydrogen, halogeno, or C1-6 alkyl.
Useful prochiral substrates of Formula 3 include individual Z- or E-isomers or a mixture of Z- and E-isomers. Useful prochiral substrates further include compounds of Formula 3 in which R1 is amino, amino-C1-6 alkyl, cyano or cyano-C1-6 alkyl; R2 is C1-6 alkoxycarbonyl, carboxy or —CO2—Y; X is —CH2— or a bond; R3 and R4 are independently hydrogen atom or C1-6 alkyl; and Y is a cation. Other useful compounds include α-, β-, and γ-cyano acids in which R1 is cyano or cyanomethyl; R2 is carboxy or —CO2—Y; X is a bond or CH2—; R3 and R4 are independently hydrogen atom or C1-6 alkyl; and Y is a Group 1 (alkali) metal ion, a Group 2 (alkaline earth) metal ion, a primary ammonium ion, or a secondary ammonium ion. Particularly useful compounds of Formula 3 include 3-cyano-5-methyl-hex-3-ennoic acid or base addition salts thereof, such as 3-cyano-5-methyl-hex-3-enoate t-butyl-ammonium salt. The prochiral substrates may be obtained from commercial sources or may be derived from known methods.
The chiral catalyst comprises a chiral ligand bound to a transition metal (i.e., Group 3-Group 12 metals) through phosphorus atoms, and has a structure represented by Formula 4 or Formula 5 (or its mirror image), as noted above. An especially useful chiral catalyst includes the bisphosphine ligand of Formula 5 having an ee of about 95% or greater or, preferably, having an ee of about 99% or greater. Useful transition metals include rhodium, ruthenium, and iridium. Of these, rhodium is especially useful.
The reaction shown in Scheme I may employ a chiral catalyst precursor or pre-catalyst. A catalyst precursor or pre-catalyst is a compound or set of compounds, which are converted into the chiral catalyst prior to use. Catalyst precursors typically comprise a transition metal (e.g., rhodium) complexed with the bisphosphine ligand (e.g., Formula 4) and a diene (e.g., norbornadiene, COD, (2-methylallyl)2, etc.), a halide (Cl or Br) or a diene and a halide, in the presence of a counterion, A−, such as OTf−, PF6−, BF4−, SbF6−, ClO4−, etc. Thus, for example, a catalyst precursor comprised of the complex, [(bisphosphine ligand)Rh(COD)]+A− may be converted to a chiral catalyst by hydrogenating the diene (COD) in MeOH to yield [(bisphosphine ligand)Rh(MeOH)2]+A−. MeOH is subsequently displaced by the prochiral olefin (Formula 3), which undergoes enantioselective hydrogenation to the desired chiral compound (Formula 2). Thus, for example, a useful chiral catalyst precursor includes (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate
Depending on which enantiomer of the chiral catalyst is used, the asymmetric hydrogenation generates an enantiomeric excess (ee) of an (R)-enantiomer or (S)-enantiomer of Formula 2. Although the amount of the desired enantiomer produced will depend on the reactions conditions (temperature, H2 pressure, catalyst loading, solvent), an ee of the desired enantiomer of about 80% or greater is desirable; an ee of about 90% or greater is more desirable; and an ee of about 95% is still more desirable. Especially useful asymmetric hydrogenations are those in which the ee of the desired enantiomer is about 99% or greater. For the purposes of this disclosure, a desired enantiomer of Formula 2 is considered to be substantially pure if it has an ee of about 90% or greater.
For a given chiral catalyst and prochiral substrate, the molar ratio of the substrate and catalyst (s/c) may depend on, among other things, H2 pressure, reaction temperature, and solvent. Usually, the substrate-to-catalyst ratio exceeds about 10:1 or 20:1, and substrate-to-catalyst ratios of about 100:1 or 200:1 are common. Although the chiral catalyst may be recycled, higher substrate-to-catalyst ratios are useful. For example, substrate-to-catalyst ratios of about 1000:1, 10,000/1, and 20,000:1, or greater, would be useful. The asymmetric hydrogenation is typically carried out at about RT or above, and under about 0.1 MPa (1 atm) or more of H2. The temperature of the reaction mixture may range from about 20° C. to about 80° C., and the H2 pressure may range from about 0.1 MPa to about 5 Mpa or higher, but more typically, ranges from about 0.3 Mpa to about 3 Mpa. The combination of temperature, H2 pressure, and substrate-to-catalyst ratio is generally selected to provide substantially complete conversion (i.e., about 95 wt % or higher) of the prochiral olefin within about 24 h. Generally, increasing the H2 pressure increases the enantioselectivity.
A variety of organic solvents may be used in the asymmetric hydrogenation, including protic solvents, such as MeOH, EtOH, and i-PrOH. Other useful solvents include aprotic polar solvents, such as THF, MeCl2, and acetone, or aromatic solvents, such as toluene, trifluorotoluene, and chlorobenzene. The enantioselective hydrogenation may employ a single solvent, or may employ a mixture of solvents, such as MeOH and THF.
As shown in Scheme II, the disclosed asymmetric hydrogenation is useful for preparing pregabalin or (S)-(+)-3-(aminomethyl)-5-methyl-hexanoic acid (Formula 1). The method may be used to produce pregabalin having an ee of about 95% or greater, or having an ee of about 99% or greater, and in some cases having an ee of about 99.9% or greater. The method includes the enantioselective hydrogenation of the compound of Formula 6 using a chiral catalyst to yield a chiral cyano precursor of pregabalin (Formula 7). The chiral cyano precursor is subsequently reduced and optionally treated with an acid to yield pregabalin. In Formula 6-8, substituent R5 can be carboxy group or —CO2—Y, where Y is a cation.
Useful prochiral substrates (Formula 6) include a base addition salt of 3-cyano-5-methyl-hex-3-enoic acid, such as 3-cyano-5-methyl-hex-3-enoate t-butyl-ammonium salt. Other useful prochiral substrates include those in which Y in Formula 6 is a Group 1 metal ion, a Group 2 metal ion, a primary ammonium ion, or a secondary ammonium ion. The prochiral substrate may be obtained from commercial sources or may be derived from known methods. For a discussion of the preparation of useful prochiral substrates and the reduction of chiral cyano pregabalin precursors, see, for example, commonly assigned U.S. Patent Application No. 2003/0212290 A1, published Nov. 13, 2003, the complete disclosure of which is herein incorporated by reference for all purposes.
Scheme III shows a method for preparing the chiral ligand of Formula 4. The method may be used to prepare either the R-enantiomer (Formula 5) or the S-enantiomer, each having an ee of about 80%, 90%, 95%, or 99% or greater. As shown in Scheme III, the method includes reacting a first monophosphine (Formula 9) with a second monophosphine (Formula 10) to yield a first bisphosphine intermediate (Formula 11), in which the first monophosphine is treated with a base prior to reaction, X is a leaving group (e.g., halogeno such as chloro), and R6 is typically BH3, but can also be sulfur or oxygen. The method further includes reacting the first bisphosphine intermediate (Formula 11) with a borane or with sulfur or oxygen to yield a second bisphosphine intermediate (Formula 12), in which R7 is the same as or different than R6 and is BH3, sulfur, or oxygen. Substituents R6 and R7 are subsequently removed to yield the chiral bisphosphine ligand of Formula 4. Though not shown in Scheme III, the second bisphosphine intermediate (Formula 12) is resolved into separate enantiomers before or after removal of R6 and R7.
Substituents R6 and R7 may be removed many different ways depending on the nature of the particular substituents. For instance, when R6 and R7 are each BH3 (Formula 13), they may be removed by reacting the second bisphosphine intermediate with an amine or an acid to yield the compound of Formula 4. Thus, for example, the compound of Formula 13 may be reacted with HBF4.Me2O, followed by base hydrolysis to yield the compound of Formula 4. Similarly, the compound of Formula 13 may be treated with DABCO, TMEDA, DBU, or Et2NH, or combinations thereof to remove R6 and R7. See, for example, H. Bisset et al., Tetrahedron Letters 34(28):4523-26 (1993); see also, commonly assigned U.S. Patent Application No. 2003/0143214 A1, published Oct. 3, 2002, and commonly assigned U.S. Patent Application No. 2003/0073868, published Apr. 17, 2003, the complete disclosures of which are herein incorporated by reference for all purposes.
When both substituents are sulfur atoms (Formula 14), R6 and R7 may be removed using techniques shown in Scheme IV. One of the methods includes the steps of (a) reacting the compound of Formula 14 with R8OTf to yield a compound of Formula 15, in which R8 is a C1-4 alkyl (e.g., methyl); (b) reacting the compound of Formula 15 with a borohydride (e.g., LiBH4) to yield the compound of Formula 13; and (c) reacting the compound of Formula 13 with an amine or an acid to yield the compound of Formula 4. Another method includes steps (a) and (b) above, and further includes the steps of (c) reacting the compound of Formula 13 with HCl, which is dispersed in a polar aprotic solvent, to yield a compound of Formula 15, and (d) reacting the compound of Formula 16 with an amine or an acid to yield the compound of Formula 4.
When both substituents are sulfur or oxygen, R6 and R7 may also be removed by treating the compound of Formula 12 with a reducing agent, including a perchloropolysilane such as hexachlorodisilane. For a discussion of the use of a perchloropolysilane for stereospecific deoxygenation of an acyclic phosphine oxide, see K. Naumann et al., J. Amer. Chem. Soc. 91(25):7012-23 (1969), which is herein incorporated by reference in its entirety and for all purposes.
As noted above in connection Scheme I, the methods used to convert the prochiral substrates of Formula 3 or Formula 6 to the desired enantiomers of Formula 1 or Formula 7, employ chiral catalysts or catalyst precursors, which are converted to the chiral catalysts prior to use. The catalyst or pre-catalysts are comprised of the chiral ligand of Formula 4 or Formula 5 (or its mirror-image) bound to a transition metal (e.g., Rh) through phosphorus atoms.
The catalyst or pre-catalyst may be prepared using the method shown in Scheme V. The method includes the steps of (a) removing substituents R9 to yield a compound of Formula 4, in which R9 is BH3, sulfur, or oxygen; and (b) binding the compound of Formula 4 to a transition metal (e.g., rhodium). Step (b) generally includes reacting the compound of Formula 4 with a complex of Formula 18, in which ligands L1 and L2 are, respectively, a diene or anionic ligand as defined above, A is a negatively-charged counterion as defined above, and m, n, and p are, respectively, an integer from 0 to 2, inclusive, an integer from 0 to 4, inclusive, and a positive odd integer, such that 4×m+2×n+p=9. The pre-catalyst may provide certain advantages over either the free ligand (Formula 4) or the chiral catalyst, such as improved stability during storage, ease of handling (e.g., a solid rather than a liquid), and the like.
Generally, the chemical transformations described throughout the specification may be carried out using substantially stoichiometric amounts of reactants, though certain reactions may benefit from using an excess of one or more of the reactants. Additionally, many of the reactions disclosed throughout the specification may be carried out at about RT, including the asymmetric hydrogenation of the compounds of Formula 2 and Formula 6, but particular reactions may require the use of higher temperatures (e.g., reflux conditions) or lower temperatures, depending on reaction kinetics, yields, and the like. Many of the chemical transformations may also employ one or more compatible solvents, which may influence the reaction rate and yield. Depending on the nature of the reactants, the one or more solvents may be polar protic solvents, polar aprotic solvents, non-polar solvents, or some combination. Any reference in the disclosure to a stoichiometric range, a temperature range, a pH range, etc., includes the indicated endpoints.
The desired (S)- or (R)-enantiomers of any of the compounds disclosed herein may be further enriched through classical resolution, chiral chromatography, or recrystallization. For example, the compounds of Formula 1 or Formula 2 may be reacted with an enantiomerically-pure compound (e.g., acid or base) to yield a pair of diastereoisomers, each composed of a single enantiomer, which are separated via, say, fractional recrystallization or chromatography. The desired enantiomer is subsequently regenerated from the appropriate diastereoisomer. Additionally, the desired enantiomer often may be further enriched by recrystallization in a suitable solvent when it is it available in sufficient quantity (e.g., typically not much less than about 85% ee, and in some cases, not much less than about 90% ee).
Many of the compounds described in this disclosure, including those represented by Formula 1, 2, 8, and 32 are capable of forming pharmaceutically acceptable salts. These salts include, without limitation, acid addition salts (including diacids) and base salts. Pharmaceutically acceptable acid addition salts include nontoxic salts derived from inorganic acids such as hydrochloric, nitric, phosphoric, sulfuric, hydrobromic, hydroiodic, hydrofluoric, phosphorous, and the like, as well nontoxic salts derived from organic acids, such as aliphatic mono- and dicarboxylic acids, phenyl-substituted alkanoic acids, hydroxy alkanoic acids, alkanedioic acids, aromatic acids, aliphatic and aromatic sulfonic acids, etc. Such salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, nitrate, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide, iodide, acetate, trifluoroacetate, propionate, caprylate, isobutyrate, oxalate, malonate, succinate, suberate, sebacate, fumarate, maleate, mandelate, benzoate, chlorobenzoate, methylbenzoate, dinitrobenzoate, phthalate, benzenesulfonate, toluenesulfonate, phenylacetate, citrate, lactate, malate, tartrate, methanesulfonate, and the like.
Pharmaceutically acceptable base salts include nontoxic salts derived from bases, including metal cations, such as an alkali or alkaline earth metal cation, as well as amines. Examples of suitable metal cations include, without limitation, sodium cations (Na+), potassium cations (K+), magnesium cations (Mg2+), calcium cations (Ca2+), and the like. Examples of suitable amines include, without limitation, N,N′-dibenzylethylenediamine, chloroprocaine, choline, diethanolamine, dicyclohexylamine, ethylenediamine, N-methylglucamine, and procaine. For a discussion of useful acid addition and base salts, see S. M. Berge et al., “Pharmaceutical Salts,” 66 J. of Pharm. Sci., 1-19 (1977); see also Stahl and Wermuth, Handbook of Pharmaceutical Salts: Properties, Selection, and Use (2002).
One may prepare a pharmaceutically acceptable acid addition salt (or base salt) by contacting a compound's free base (or free acid) with a sufficient amount of a desired acid (or base) to produce a nontoxic salt. One may then isolate the salt by filtration if it precipitates from solution, or by evaporation to recover the salt. One may also regenerate the free base (or free acid) by contacting the acid addition salt with a base (or the base salt with an acid). The degree of ionization in the resulting salt may vary from completely ionized to almost non-ionized.
Claimed and disclosed compounds may exist in both unsolvated and solvated forms and as other types of complexes besides salts. Useful complexes include clathrates or drug-host inclusion complexes where the drug and host are present in stoichiometric or non-stoichiometric amounts. Useful complexes may also contain two or more organic, inorganic, or organic and inorganic components in stoichiometric or non-stoichiometric amounts. The resulting complexes may be ionized, partially ionized, or non-ionized. For a review of such complexes, see J. K. Haleblian, J. Pharm. Sci. 64(8):1269-88 (1975).
Useful forms of the claimed and disclosed compounds, including compounds represented by Formula 1, 2, 8 and 32, include all polymorphs and crystal habits, as well as stereoisomers (geometric isomers, enantiomers, and diastereomers), which may be pure, substantially pure, enriched, or racemic. Useful forms of the claimed and disclosed compounds also include tautomeric forms, where possible.
Additionally, certain compounds of this disclosure, including those represented by Formula 1, 2, 8 and 32, may exist as an unsolvated form or as a solvated form, including hydrated forms. Pharmaceutically acceptable solvates include hydrates and solvates in which the crystallization solvent may be isotopically substituted, e.g. D2O, d6-acetone, d6-DMSO, etc. Unless expressly noted, all references to the free base, the free acid, zwitterion, or the unsolvated form of a compound also includes the corresponding acid addition salt, base salt or solvated form of the compound.
The disclosed compounds also include all pharmaceutically acceptable isotopic variations, in which at least one atom is replaced by an atom having the same atomic number, but an atomic mass different from the atomic mass usually found in nature. Examples of isotopes suitable for inclusion in the disclosed compounds include, without limitation, isotopes of hydrogen, such as 2H and 3H; isotopes of carbon, such as 13C and 14C; isotopes of nitrogen, such as 15N; isotopes of oxygen, such as 17O and 18O; isotopes of phosphorus, such as 31P and 32P; isotopes of sulfur, such as 35S; isotopes of fluorine, such as 18F; and isotopes of chlorine, such as 36Cl. Use of isotopic variations (e.g., deuterium, 2H) may afford certain therapeutic advantages resulting from greater metabolic stability, for example, increased in vivo half-life or reduced dosage requirements. Additionally, certain isotopic variations of the disclosed compounds may incorporate a radioactive isotope (e.g., tritium, 3H, or 14C), which may be useful in drug and/or substrate tissue distribution studies.
The following examples are intended to be illustrative and non-limiting, and represent specific embodiments of the present invention.
All reactions and manipulations were performed under nitrogen in standard laboratory glassware. Asymmetric hydrogenation was performed in a nitrogen-filled glovebox. THF (anhydrous, 99.9%), ACN (anhydrous, 99.8%), diethyl ether (anhydrous, 99.8%), MeOH (anhydrous, 99.8%), and MeCl2 (anhydrous, 99.8%) were obtained from ALDRICH. Bis(1,5-cyclooctadiene)rhodium (I) tetrafluoroborate was synthesized according to a procedure in T. G. Schenk et al., Inorg. Chem. 24:2334 (1985). Hydrogen gas was used from a lecture bottle supplied by SPECIALTY GAS. Hydrogenations were performed in a Griffin-Worden pressure vessel supplied by KIMBLE/KONTES.
400 MHz 1H NMR, 100 MHz 13C NMR, and 162 MHz 31P NMR spectra were obtained on a VARIAN INOVA400 spectrometer equipped with an Auto Switchable 4-Nuclei PFG probe, two RF channels, and a SMS-100 sample changer by ZYMARK. Spectra were generally acquired near RT, and standard autolock, autoshim and autogain routines were employed. Samples were usually spun at 20 Hz for 1D experiments. 1H NMR spectra were acquired using 45-degree tip angle pulses, 1.0 s recycle delay, and 16 scans at a resolution of 0.25 Hz/point. The acquisition window was typically 8000 Hz from +18 to −2 ppm (Reference TMS at 0 ppm), and processing was with 0.2 Hz line broadening. Typical acquisition time was 80 s. Regular 13C NMR spectra were acquired using 45-degree tip angle pulses, 2.0 s recycle delay, and 2048 scans at a resolution of 1 Hz/point. Spectral width was typically 25 KHz from +235 to −15 ppm (Reference TMS at 0 ppm). Proton decoupling was applied continuously, and 2 Hz line broadening was applied during processing. Typical acquisition time was 102 min. 31P NMR spectra were acquired using 45-degree tip angle pulses, 1.0 s recycle delay, and 64 scans at a resolution of 2 Hz/point. Spectral width was typically 48 KHz from +200 to −100 ppm (Reference 85% Phosphoric Acid at 0 ppm). Proton decoupling was applied continuously, and 2 Hz line broadening was applied during processing. Typical acquisition time was 1.5 min.
Mass Spectrometry was performed on a MICROMASS Platform LC system operating under MassLynx and OpenLynx open access software. The LC was equipped with an HP1100 quaternary LC system and a GILSON 215 liquid handler as an autosampler. Data were acquired under atmospheric pressure chemical ionization with 80:20 ACN/water as the solvent. Temperatures: probe was 450° C., source was 150° C. Corona discharge was 3500 V for positive ion and 3200 V for negative ion.
High Performance Liquid Chromatography (HPLC) was performed on a series 1100 AGILENT TECHNOLOGIES instrument equipped with a manual injector, quaternary pump, and a UV detector. The LC was PC controlled using HP Chemstation Plus Software. Normal Phase chiral HPLC was performed using a Chiracel OJ column supplied by CHIRAL TECHNOLOGIES.
Gas Chromatography (GC) was performed on a 110 volt VARIAN STAR 3400 equipped with an FID detector with electrometer, a model 1061 packed/530 μm ID flash injector, a model 1077 split/splitless capillary injector, a relay board that monitors four external events, and an inboard printer/plotter. Gas chromatography was performed using 40 m×0.25 mm CHIRALDEX G-TA or B-TA columns supplied by ADVANCED SEPARATION TECHNOLOGIES, INC. or on a 25 m×0.25 mm coating CHIRASIL-L-VAL column supplied by CHROMPACK.
A solution of t-butyl-dimethyl-phosphine borane (Formula 20) (20.1 g, 152 mmole) in THF (50 mL) was stirred at 0° C. To the solution was added s-BuLi (104 mL, 145 mmole) over a 20 min period while maintaining the reaction temperature below 20° C. Following the addition, the solution turned slightly cloudy and orange. The reaction was stirred for one hour at 0° C. The solution was subsequently transferred over a 20 min period, via a cannula, to a pre-cooled solution of di-t-butylchlorophosphine (25 g, 138 mmole) in THF (50 mL) at 0° C., which turned red immediately upon addition. The temperature was maintained below 20° C. during the transfer. Following addition, the reaction was stirred at 0° C. for 2 h. To this solution was added BH3*Me2S (14.4 mL, 152 mmole) over 10 min while maintaining the reaction temperature below 20° C. The reaction was stirred for 1 h, after which it was poured onto 100 g of ice in 1N HCl (100 mL) and stirred for 30 min. The aqueous solution was extracted with EtOAc (2×100 mL) and the combined organic layers were dried over MgSO4 and filtered. Volatiles were then removed on a rotary evaporator. The residue was recrystallized from hot heptane to yield the titled compound (racemate) as a white crystalline solid. The solid weighed 25 g (63%); mp=150-152° C.; 1H NMR (400 MHz, CDCl3) δ 1.88 (t, J=12 Hz, 2H), 1.56 (d, J=10 Hz, 3H), 1.33 (d, J=13 Hz, 9H), 1.27 (d, J=13 Hz, 9H), 1.19 (d, J=13 Hz, 9H), 0.61 (br q, 6H); 13C NMR (100 MHz, CDCl3) δ 34.29 (d, J=25 Hz), 33.41 (d, J=25 Hz), 30.00 (d, 25 Hz), 28.30 (s), 27.89 (s), 25.21 (s), 9.12 (dd, J=21 and 15 Hz), 6.52 (d, J=32 Hz); 31P NMR (162 MHz, CDCl3) δ 49.70-48.15 (m), 33.03-31.56 (m). Anal Calc'd for C14H38B2P2: C, 57.98; H, 13.21. Found: C, 57.64; H, 13.01.
The (R)-(−)- and (S)-(+)-enantiomers (Formula 21 and 22, respectively) of (2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-diborane (Formula 13) were separated by HPLC using a chiral preparatory column and conditions noted in Table 2 below. Since a preparatory-scale RI detector was unavailable, RI detection could not be used to monitor the retention times of the enantiomers. Instead, the solvent was fractionated using a fraction collector and the individual fractions were assayed by HPLC using a chiral analytical column and conditions provided in Table 2. Retention times for the R- and S-enantiomers were 6.8 min, [α]24D=−5.5° (c 0.5, MeOH), and 8.2 min, respectively.
(R)-(−)-(2-{[(di-t-Butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-diborane (Formula 21, 290 mg, 1.0 mmol) and DABCO (135 mg, 1.2 mmol) were dissolved in degassed toluene (10 mL) at 20° C. The solution was stirred for 4 h at 80° C. The solvent was removed in vacuo and the resulting residue was extracted with hexane (3×20 mL). The combined organic extracts were concentrated and dried producing (R)-2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane (Formula 5, 228 mg, 87%) as colorless oil. 1H NMR (400 MHz, CDCl3) δ 1.47-1.41 (m, 2H), 1.09 (d, J=11 Hz, 9H), 1.03 (d, J=11 Hz, 9H), 0.94 (d, J=11 Hz, 9H), 0.93 (d, J=3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 7.44 (dd, J=19 and 6 Hz), 16.09 (dd, J=32 and 25 Hz), 26.63 (d, J=14 Hz), 27.95 (dd, J=23 and 3 Hz), 29.73 (d, J=14 Hz), 30.16 (dd, J=13 and 4 Hz), 31.70 (dd, J=23 and 9 Hz), 32.16 (dd, J=23 and 3 Hz); 31P NMR (162 MHz, CDCl3) δ −13.66 (br m), 18.35 (br m).
A solution of (R)-2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane (Formula 5, 66 mg, 0.25 mmol) in THF (5 mL) was added drop wise to a solution of [Rh(COD)2]BF4 (102 mg, 0.25 mmol) in MeOH (10 mL) at 20° C. with stirring. After addition, the reaction mixture was stirred for 1 h and solvent was removed in vacuo to provide a red solid. Recrystallization of product from warm THF provided (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23, 89 mg, 64%) as a red crystalline product. [α]24D=+52.4° (c 0.9, MeOH); 1H NMR (400 MHz, CDCl3) δ 5.63-5.51 (m, 2H), 5.11 (br s, 2H), 3.48-3.328 (m, 1H), 3.14 (dt, J=17 and 10 Hz, 1H), 2.49-2.25 (m, 4H), 2.21-2.09 (m, 4H), 1.69 (d, J=9 Hz, 3H), 1.39 (d, J=14 Hz, 9H), 1.33 (d, J=14 Hz, 9H), 1.13 (d, J=16 Hz, 9H); 13C NMR (100 MHz, CDCl3) δ 100.20 (dd, J=9 and 6 Hz), 97.70 (dd, J=9 and 6 Hz), 92.95 (t, J=8 Hz), 92.27 (d, J=8 Hz), 37.68 (m), 36.04 (d, J=9 Hz), 32.54 (m), 31.48 (s), 30.94 (s), 30.09 (d, J=5 Hz), 29.81 (d, J=5 Hz), 29.32 (s), 29.16 (s), 26.57 (d, J=5 Hz), 9.58 (d, J=21 Hz); 31P NMR (162 MHz, CDCl3) δ −3.97 (dd, J=126 and 56 Hz), −29.36 (dd, J=126 and 56 Hz). Anal Calc'd for C21H42B1F4P2Rh1: C, 46.18; H, 7.75. Found: C, 45.66; H, 7.19.
Table 3 lists substrates (Formula 3), ee, and absolute stereochemical configuration of chiral products (Formula 2) prepared via asymmetric hydrogenation using chiral catalyst precursor, (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23). For each entry in Table 3, the catalyst precursor (0.01 mmole) was dissolved in degassed MeOH (1 mL) in a Griffin-Worden pressure vessel equipped with the attachments necessary to connect to a lecture bottle. The substrate (1 mmole) was first dissolved in MeOH (4 mL) and then delivered to the catalyst-MeOH solution via syringe. The vessel was sealed and pressurized to 50 psi H2. The time to the completion of reaction was determined by the cessation of H2 gas uptake.
For each of the reactions shown in Table 3, enantiomeric excess was determined via chiral GC or chiral HPLC. Table 4 provides details of the ee methodology. To determine ee's for N-acetylalanine (Example 5) and N-acetylphenylalanine (Example 6), each compound was treated with trimethylsilyldiazomethane to convert it to its corresponding methyl ester, which was analyzed as provided in Example 7 or Example 8, respectively. Absolute stereochemical configuration was determined by comparing the signs of optical rotation with those of literature values: (S)—N-acetylalanine methyl ester [α]20D=−91.7° (c 2, H2O), J. P. Wolf III & C. Neimann, Biochemistry 2:493 (1963); (S)—N-acetylphenylalanine methyl ester [α]20D=+16.4° (c 2, MeOH), B. D. Vineyard et al., J. Am. Chem. Soc. 99:5946 (1997); (S)—N-acetylcyclohexylglycine methyl ester [α]20D=−4.6° (c=0.13, EtOH), M. J. Burk et al., J. Am. Chem. Soc. 117:9375 (1995).
Table 5 lists catalyst (or catalyst precursor), substrate concentration (in MeOH, w/w %), s/c, reaction temperature, H2 pressure, time to completion, and ee for the preparation of (S)-3-cyano-5-methyl-hexanoic acid t-butylammonium salt (Formula 25) via asymmetric hydrogenation of 3-cyano-5-methyl-hex-3-enoic acid t-butylammonium salt (Formula 24). For each entry in Table 5, the substrate (Formula 24, 100 g, 442 mmole) was weighed into a hydrogenation bottle in air. The hydrogenation bottle was then transferred to a glovebox ([O2]<5 ppm). To the substrate was added degassed MeOH (500 mL) with stirring to dissolve the substrate. The requisite amount of catalyst precursor—either (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23) or (R,R)—Rh-Me-DuPhos—was added to the substrate solution. The hydrogenation vessel was sealed and pressurized to 50 psi H2 and stirred vigorously with a TEFLON® coated magnet. The pressure of the reaction was maintained at 50 psi H2. The time to the completion of reaction was measured by the cessation of H2 gas uptake.
To determine enantiomeric excess, the chiral pregabalin precursors (Formula 25 and its mirror image) were acidified in-situ with 1 N HCl. The organic components were extracted into MeCl2. After drying over MgSO4, the volatiles were removed in vacuo. The carboxylic acids were treated with trimethylsilyldiazomethane to convert them to their corresponding methyl esters, which were subsequently analyzed using capillary GC (Astec GTA (30 m), 140° C., isothermal, R t1=8.8 min, S t2=9.5 min). Absolute Configurations of the chiral pregabalin precursors were determined by comparing the order of elution to an authenticated sample having S-configuration.
Dichloro-t-butyl-phosphine (Formula 26, 10.0 g, 62.9 mmol) was dissolved in THF (100 mL) under N2 blanket and the resulting solution was cooled to 0° C. MeMgBr (16.5 g, 138 mmol) was added via syringe over a 10 min period. The addition was exothermic. The reaction was warmed to RT and then sulfur (2.22 g, 69.2 mmol) was added in one portion with generation of heat. After stirring for 1 h, the reaction was subjected to a standard aqueous work-up. Recrystallization of the product from heptane yielded 2-(dimethyl-phosphinothioyl)-2-methyl-propane (Formula 27, 8.0 g, 85% yield).
A flask was charged with diisopropylamine (74.2 g, 102.8 mL, mmol) and THF (100 mL) and cooled to −10° C. under argon. To the solution was added n-BuLi (44.8 g, 280 mL, 700 mmol) via a dropping funnel while maintaining the temperature below 0° C. To the resulting LDA solution was added, under argon and via a dropping funnel, a solution of 2-(dimethyl-phosphinothioyl)-2-methyl-propane (Formula 27, 50.07 g, 333.3 mmol) dissolved in THF (300 mL). During the addition, the temperature stayed below −5° C. To this solution was added, under argon and via a dropping funnel, a solution of chloro-di-t-butylphosphine (60.2 g, 333 mmol) dissolved in THF (80 mL) during which the temperature stayed below −3° C. The reaction mixture was stirred for 1 h at −10° C. and was quenched under argon with 6 N HCl (290 mL) while maintaining the temperature below −5° C. After the addition the pH was about 2. Sulfur (11.8 g, 367 mmol) was added in one portion and the reaction mixture was stirred overnight without cooling. The organic layer was separated and then washed with 6 N HCl and then with distilled H2O. The aqueous layer was extracted with EtOAc. The organic layers were combined and washed with brine, dried over MgSO4, filtered, and stripped in vacuo. The residue was slurried at 40° C. in IPA (60 mL) and cooled to 5° C. The solid was collected and washed three times with EPA and then dried in vacuo at RT overnight. The procedure yielded 2-[(di-t-butyl-phosphinothioylmethyl)-methyl-phosphinothioyl]-2-methyl-propane (Formula 14) as a white solid (64.6 g, 59% yield).
The R- and S-enantiomers (Formula 28 and 29, respectively) of 2-[(di-t-butyl-phosphinothioylmethyl)-methyl-phosphinothioyl]-2-methyl-propane (Formula 14) were separated by HPLC using a chiral preparatory column and conditions noted in Table 5 below. As noted in Table 5, HPLC was also used to check chiral purity and chemical purity.
(S)-2-[(di-t-butyl-phosphinothioylmethyl)-methyl-phosphinothioyl]-2-methyl-propane (Formula 29, 5.10 g, 15.6 mmol) was dissolved in 1,2-dichloroethane (50 mL). Methyl triflate (7.69 g, 46.9 mmol) was added to the solution. The reaction mixture was blanketed under argon and stirred at RT. After 10 min MS showed only mono-methylated product. The reaction was stirred overnight whereupon a precipitate had formed (di-methylated product). The solid was collected, washed three times with 1,2-dichloroethane and dried in a vacuum oven at RT to yield, after drying, (S)-(di-t-butyl-methylthio-phosphoniumyl-methyl)-t-butyl-methyl-methylthio-phosphonium di-triflate (Formula 30) as a white solid (6.90 g, 67% yield).
(S)-(Di-t-butyl-methylthio-phosphoniumyl-methyl)-t-butyl-methyl-methylthio-phosphonium di-triflate (2.005 g, 3.063 mmol) was slurried in THF (25 mL). Using an ice bath, the reaction mixture was cooled to 0° C. under argon. LiBH4 (0.400 g, 18.4 mmol) was added via dropping funnel while maintaining the temperature below 5° C. Gas evolution was observed during the addition. After the addition, the ice bath was removed and the reaction was stirred overnight at RT. 1H NMR showed that some starting material remained. Additional LiBH4 (3 mL) was added. No gas evolution or exotherm was observed. The reaction mixture was stirred overnight whereupon it was deemed complete via 1H NMR. The reaction solution was cooled in an ice bath and quenched with 1 N HCl (15 mL). Vigorous evolution of gas was observed. EtOAc was added with stirring. The organic layer was separated and washed with 1 N HCl and H2O. The aqueous layer was extracted with EtOAc. The combined organic layers were washed with brine, dried over MgSO4, filtered, and removed in vacuo to yield (R)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-diborane (Formula 21, 0.492 g, 55% yield). Enantiomeric excess was determined using the analytical procedure described in Table 2, above: ee≧98.7%; mp=150-152° C.; Anal Calc'd for C14H38B2P2: C, 57.98; H, 13.21. Found: C, 57.64; H, 13.01.
(R)-(2-{[(Di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-diborane (Formula 21, 0.200 g, 0.690 mmol) was placed in a thick-walled tube equipped with a #15 ACE thread. To the tube was added 2M HCl (0.438 g, 12 mmol) dispersed in ethyl ether (6 mL). Argon was blown over the headspace and the tube was sealed with a #15 ACE plug equipped with a TEFLON® gasket. The contents of the tube were heated to 85° C. for 12 h and then cooled to RT, yielding (R)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-di-(chloroborane) (Formula 31) as a white solid. Since the reaction evolves H2 gas, care was taken to prevent over pressurization of the tube during and after reaction. The solvent was removed via pipette and the solids were triturated with ethyl ether three times. The solids were dried under vacuum to yield a white solid product (0.222 g, 90% yield). Because the titled compound is hygroscopic, contact with air was avoided, and the product was stored under vacuum or in a glovebox until use.
(R)-(2-{[(Di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-di-(chloroborane) (Formula 31, 179 mg, 0.5 mmol) was dissolved in MeOH (5 mL) and cooled to 0° C. To this solution was added drop wise Et3N (505 mg, 5.0 mmol). After addition, the mixture was warmed to 20° C. and stirred for 30 min. MeOH was removed in vacuo and the residue extracted with hexane (3×20 mL). The organic layers were combined, filtered, and concentrated to produce (R)-2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane (Formula 5, 66 mg). 31P & 1H NMR showed small impurity peaks. The chiral ligand (Formula 5) was dissolved in THF (5 mL) and added drop wise to a solution of [Rh(COD)2]BF4 (102 mg, 0.25 mmol) in MeOH (10 mL) at RT with stirring. After addition, the reaction mixture was stirred for 1 h. Solvent was removed in vacuo to provide a red solid. Recrystallization of the solid from warm THF provided a red crystalline product. The crystals were washed with 5:1 hexane/diethyl ether and dried in vacuo to produce (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23, 89 mg, 48% yield from 31). [α]24D=+52.4° (c 0.9, MeOH); Anal Calc'd for C21H42B1F4P2Rh1: C, 46.18; H, 7.75. Found: C, 45.66; H, 7.19.
Hexachlorodisilane (2.0 g, 7.5 mmol) was added via syringe to a solution of (S)-2-[(di-t-butyl-phosphinothioylmethyl)-methyl-phosphinothioyl]-2-methyl-propane (Formula 29, 0.5 g, 1.5 mmol) in degassed toluene (5 mL). The solution was heated with stirring at 80° C. for 3 h after which a yellow precipitate had formed. The mixture was then cooled to 0° C. and quenched by slowly adding 6.5 N NaOH aq (8 mL) with stirring while maintaining the temperature of the reaction below 70° C. After NaOH addition, the mixture was stirred for 1 h at 50° C. until the reaction mixture turned clear. The organic phase was separated in a separatory funnel and the aqueous phase was washed with diethyl ether (2×15 mL). The organic layers were combined and dried over MgSO4, filtered, and concentrated in vacuo to provide (R)-2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane (Formula 5) as a colorless oil (0.25 g, 64% yield). The free phosphine was used directly in the rhodium catalyst formation step (Example 22) without further purification. The preparation of the free phosphine (Formula 5) has been scaled up to 2.2 g of starting material (Formula 29), 5.0 g of starting material, and 10.0 g of starting material, resulting in yields of 82%, 80%, and 98%, respectively.
A solution of (R)-2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane (Formula 5, 0.32 g, 1.2 mmol) in degassed THF (5 mL) was added drop wise at a rate of 1 mL/min and at RT to a solution of [Rh(COD)2]BF4 (0.49 g, 1.2 mmol) in degassed MeOH (10 mL) with stirring. The color changed from brown to red. After the addition, the mixture was stirred for 1 h and was concentrated in vacuo. The residue was stirred with degassed THF (5 mL) and then filtered. The filtrate was washed with 1:1 diethyl ether/THF (2×5 mL) and then dried in vacuo producing an orange dusty solid, (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23, 0.5 g, 75% yield). The preparation of rhodium complex (Formula 23) has been scaled up to 1.51 g of starting material (Formula 5), 3.27 g of starting material, and 8.15 g of starting material, resulting in yields of 87%, 92%, and 91%, respectively.
Table 6 lists substrates (Formula 33) and their double bond stereochemical configuration, hydrogen pressure, solvent, ee, and absolute stereochemical configuration of chiral products (Formula 32) prepared via asymmetric hydrogenation using chiral catalyst precursor, (S)-(+)-(2-{[(di-t-butyl-phosphanyl)-methyl]-methyl-phosphanyl}-2-methyl-propane)-(1,5-cyclooctadiene) rhodium (I) tetrafluoroborate (Formula 23). For each of the entries in Table 6, the catalyst precursor (Examples 23-45, 0.005 mmol; Example 46, 0.025 mmol) and substrate (0.50 mmol, 0.2 M) were dissolved in solvent (2.5 mL) in a Griffin-Worden pressure vessel, which was sealed and pressurized to the desired pressure of H2. The mixture was vigorously stirred with a PTFE coated magnet at 25° C. until H2 uptake ceased (less than 15 min for Examples 23-45; 6 h for Example 46, as indicated by capillary GC). The H2 pressure in the bottle was subsequently released, and the reaction mixture was analyzed via chiral GC to provide the percent conversion to product and enantiomeric excess.
Each of the Z- and E-β-acetamido-β-substituted acrylates (Formula 33) was obtained from an appropriate β-keto ester. A solution of the requisite β-keto ester (24 mmol) and NH4OAc (9.2 g, 120 mmol) in MeOH (30 mL) was stirred at 20° C. for 3 d. After evaporating the solvent, chloroform (50 mL) was added to the residue to give a white solid, which was filtered and washed with chloroform (2×50 mL). The combined filtrate was washed with water and brine, and dried over sodium sulfate. Evaporating the solvent provided a β-amino-β-substituted acrylate. To a solution of the β-amino-β-substituted acrylate in THF (24 mL) was added pyridine (12 mL) and anhydrous acetic anhydride (36 mL). The mixture was refluxed for 18 h. The mixture was subsequently cooled to RT and the volatiles were evaporated. The resulting residue was dissolved in EtOAc (40 mL) to give a solution, which was washed with water (20 mL), 1 N HCl (20 mL), 1 M KH2PO4 (20 mL), saturated NaHCO3 (20 mL), and brine (30 mL). The solution was dried over sodium sulfate and residual solvent was evaporated under reduced pressure. Fast chromatography on silica gel with 5:1 and 3:1 hexane/ethyl acetate mobile phases, respectively, provided Z- and E-isomers of the β-acetamido-β-substituted acrylate, which were confirmed by comparison of 1H NMR data.
Table 7 provides details of the methodology used to determine the stereochemical configuration of products from the reactions shown in Table 6. Enantiomeric excess (ee) was determined via chiral GC using a helium carrier gas at 20 psi. In Table 7, “Column A” refers to CP Chirasil-Dex CB (30 m×0.25 mm) and “Column B” refers to ChiralDex-gamma-TA (25 m×0.25 mm). Racemic products were prepared by hydrogenation of corresponding enamines catalyzed by 10% Pd/C in MeOH under 50 psi of H2 at RT for 2 h.
Absolute stereochemical configurations were determined by comparing the signs of optical rotation with literature values given in G. Zhu et al., J. Org. Chem. 64:6907-10 (1999): methyl 3-acetamidobutanoate, [α]D20=+8.09 (c 1.24, MeOH), lit. +21.4 (c 1.4, CHCl3); ethyl 3-acetamidohexanoate, [α]D20=+18.26 (c 1.02, MeOH), lit., ethyl ester, +42.8 (c 1.86, CHCl3); ethyl 3-acetamido-4-methypentanoate, [α]D20=+9.05 (c 1.15, MeOH), lit., ethyl ester, +52.8 (c 1.2, CHCl3); ethyl 3-acetamido-5-methylhexanoate, [α]D20=+24.44 (c 0.95, MeOH), lit. +44.6 (c 1.56, CHCl3); methyl 3-acetamido-4,4-dimethylpentanoate, [α]D 20=+4.86 (c 0.93, MeOH), lit. no report; ethyl 3-acetamido-3-phenylpropanoate, [α]D20=47.66 (c 0.91, MeOH), lit. −40.5 (c 2.15, MeOH).
It should be noted that, as used in this specification and the appended claims, singular articles such as “a,” “an,” and “the,” may refer to one object or to a plurality of objects unless the context clearly indicates otherwise. Thus, for example, reference to a composition containing “a compound” may include a single compound or two or more compounds.
It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reading the above description. The scope of the invention should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patents, patent applications and publications, are incorporated herein by reference in their entirety and for all purposes.
1. A method of making a desired enantiomer of a compound of Formula 2,
or a pharmaceutically acceptable complex, salt, solvate or hydrate thereof, in which
with hydrogen in the presence of a chiral catalyst to yield the compound of Formula 2; and
and wherein R1, R2, R3, R4, and X in Formula 3 are as defined in Formula 2.
This application claims the benefit of U.S. Provisional Application No. 60/552,586, filed Mar. 12, 2004, and U.S. Provisional Application No. 60/586,512, filed Jul. 9, 2004, the complete disclosures of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60552586 | Mar 2004 | US | |
| 60586512 | Jul 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11078228 | Mar 2005 | US |
| Child | 12353450 | US |
