Methods for the synthesis of phosphonate esters

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods for the synthesis of phosphonate esters. More particularly, the invention provides methods for producing monoesters and diesters of phosphonic acids from condensations of alcohols with alkylphosphonates, such as methyl alphaaminoalkylphosphonates. The methods can be carried out in solution phase, but in an important embodiment, the method is carried out on a solid support, such as beads or a glass slide, and used to create large arrays of diverse compounds, such as peptidylphosphonates and polyphosphonates, which can then be conveniently screened for the presence of compounds with desired pharmacological or other properties. The invention therefore relates to the fields of chemistry, enzymology, pharmacology, and medicinal chemistry.
2. Description of Related Art
Phosphonic acid monoesters have typically been synthesized by one of two methods. In the first, a monomethyl alkylphosphoryl chloride is treated with an alcohol. Selective de-esterification of the methyl ester yields the desired product. The monomethyl alkylphosphoryl chloride is produced by the direct action of PCl.sub.5 on a phosphonate diester (see Balthazor et al., 1980, J. Org. Chem. 45: 530) or by base hydrolysis of the phosphonate diester followed by reaction with thionyl chloride (see Bartlett et al., 1990, Org. Chem. 55: 6268).
Alternatively, the direct monoesterification of a phosphonic acid can be accomplished with the appropriate alcohol and condensing reagents (see Gilmore et al., 1974, J. Pharm. Sci. 63: 965), such as 1,3-dicyclohexylcarbodiimide (DCC) or trichloroacetonitrile (see, e.g., Wasielewski et al., 1976, J. Rocz. Chem. 50: 1613). A large excess of both the condensing agent and alcohol is required, and yields vary depending on the components being coupled. In addition, forcing conditions are required, such as heating to reflux temperature in a solution of THF with triethylamine and DCC, and to 50.degree. C.-80.degree. C. in pyridine with trichloroacetonitrile, conditions that can lead to decomposition of starting material. In addition, product yields are extremely sensitive to steric encumbrance from the reacting components, and in some cases, no product formation is observed.
These phosphonate synthesis procedures are not suitable for solid phase chemical synthesis methods, which presents a significant problem for the medicinal chemist. Peptidylphosphonates are peptide analogues that have been used extensively as inhibitors of metalloproteases and other enzymes and as haptens in the generation of catalytic antibodies. The most potent non-covalent enzyme inhibitor known is a phosphonyltripeptide inhibitor of carboxypeptidase A, which binds with 11 femptomolar (fM) K.sub.d. See Kaplan et al., 1991, Biochem. 30: 8165-8170. To synthesize these derivatives on a solid support, one needs a general method for introducing the phosphonic acid monoester functionality.
The Mitsunobu reaction is a mild and effective method utilizing the redox chemistry of triphenylphosphine and a dialkylazodicarboxylate to condense an acidic reagent with primary and secondary alcohols (see Mitsunobu, 1981, Synthesis 1-28). The Mitsunobu reference does not describe the reaction of phosphate esters with secondary or tertiary alcohols or the reaction of phosphonates with alcohols. The Mitsunobu reaction has been used with carboxylic acids, phenols, and phosphates as the acidic component, but only one example using a phosphonic acid has been described.
In this example (see Norbeck et al., 1987, J. Org. Chem. 52: 2174), a monomethyl phosphonate and a primary alcohol were reacted under modified Mitsunobu conditions (triphenylphosphine and diisopropylazodicarboxylate) to produce a phosphonate diester. Demethylation using triethylamine and thiophenol produced the phosphonate monoester. Yields, however, were low for the condensation of a phosphate monoester with even a primary alcohol. Only moderate yields were obtained with elevated temperatures or with hexamethylphosphorous triamide (HMPT) as the solvent.
Thus, there remains a need for a general method for synthesizing phosphonate esters. Additional examples of the utility of peptidylphosphonate esters are numerous; these compounds are recognized as effective transition-state analogue inhibitors for a variety of enzymes, including a number of other proteases and esterases (see, e.g., Morgan et al., 1991, J. Am. Chem. Soc. 113: 297 and Bartlett et al., 1990, J. Org. Chem. 55: 6268). Peptidylphosphonate esters have been used as non-hydrolyzable analogues of phosphates to inhibit dinucleoside triphosphate hydrolase (see, e.g., Blackburn et al., 1987, Nucl. Acids Res. 15: 6991), phosphatidyltransferase (see, e.g., Vargas et al., 1984, Biochim. Biophys. Acta 796: 123), and squalene synthetase (see, e.g., Biller et al., 1988, J. Meal. Chem. 31: 1869). In addition, phosphonate esters have been used as haptens for the production of catalytic antibodies possessing esterase activity (see, e.g., Jacobs et al., 1987, J. Am. Chem. Soc. 109: 2174; Tramontanio et al., 1986, Science 234: 1566; and Pollack et al., 1986, Science 234: 1570; see also, U.S. patent application Ser. No. 858,298, filed Mar. 26, 1992).
The solution phase synthesis of these derivatives, such as a relatively short tetramer, can take weeks to months to complete by prior art methods. If one attempts to synthesize and screen a large number of such compounds, then the synthesis time required can make the project impossible to complete. Recently, an innovative method for the synthesis of large numbers of polymeric compounds on the surface of a solid support has been developed. This method, called VLSIPS.TM. technology, is described in U.S. patent application Ser. No. 805,727, filed Dec. 6, 1991, which is a continuation-in-part of Ser. No. 624,120, filed Dec. 6, 1990, which is a continuation-in-part of U.S. Pat. No. 5,143,854, which is a continuation-in-part of Ser. No. 362,901, filed Jun. 7, 1989, and now abandoned. There remains a need for chemical synthesis methods for use in the VLSIPS.TM. method to generate and screen large numbers of peptidylphosphonates and other compounds containing phosphonate ester building blocks.
In similar fashion, there remains a need for a mild coupling reaction for the preparation of phosphonate esters that is compatible with a variety of functional groups and consistently produces high yields of the desired compounds and that can be applied to solution as well as solid phase chemistry. The present invention meets these needs.
SUMMARY OF THE INVENTION
The present invention provides methods for producing phosphonate esters. In one embodiment, the present invention relates to a method for synthesizing a phosphonic acid monoester from a phosphonate with a structure: ##STR1## wherein R.sup.1 is selected from the group consisting of alkyl, aryl, substituted alkyl, substituted aryl, heteroaryl, alkylaryl, and aminoalkyl, and R.sup.2 is selected from the group consisting of methyl, ethyl, benzyl, and substituted benzyl, by treating said phosphonate with a dialkylazodicarboxylate, a trialkylphosphine, and a primary or secondary alcohol with a structure R.sup.3 OH, wherein R.sup.3 is selected from the group consisting of alkyl, aryl, substituted alkyl, substituted aryl, and aminoalkyl, in an aprotic solvent to yield an phosphonate diester with a structure: ##STR2## wherein R.sup.2 and R.sup.3 are not identical, and R2 can be selectively hydrolyzed without hydrolyzing R3, and then treating said phosphonate diester to hydrolyze selectively said phosphonate diester to yield said phosphonic acid monoester with a structure: ##STR3## In a preferred embodiment, R.sup.2 is methyl, said dialkylazodicarboxylate is diisopropylazodicarboxylate (DIAD), said trialkylphosphine is triphenylphosphine (TPP), said solvent is tetrahydrofuran (THF), and trimethylsilyl bromide (TVSBr) is added directly to the reaction mixture after completion of esterification to effect selective hydrolysis of the methyl ester to yield the desired phosphonic acid monoester.
In another preferred embodiment, the phosphonate is an alphaaminoalkylphosphonate, because the resulting compound is an amino acid analogue particularly useful in the production of compounds that can be screened for pharmacological activity or used in the generation of catalytic antibodies. In addition, this preferred embodiment of the reaction is carried out on a solid support to which is covalently linked the hydoxyl group of the alcohol moiety. The solid support can be a glass slide or other suitable substrate for light directed and other combinatorial synthesis strategies described below. In this embodiment, the invention also provides novel hydroxyacid and phosphonate ester building blocks protected with photoremovable protecting groups. The solid support can also be beads or particles, which are preferred for use in other combinatorial synthesis strategies.
Thus, in an especially preferred embodiment of this aspect of the invention, the solid support comprises an amine group of an amino acid or peptide, and the support is treated so as to couple a hydroxy acid to said amine group to yield a hydroxyl moiety covalently linked to said support. The support is then treated with an alpha-aminoalkylphosphonate with the following structure: ##STR4## wherein R.sup.4 is an alpha-aminoalkyl group, and R.sup.2 is selected from the group consisting of methyl, ethyl, benzyl, and substituted benzyl, and with a dialkylazodicarboxylate and a trialkylphosphine, such as TPP, to couple said alpha-aminoalkylphosphonate to said hydroxy groups on said surface. This process can be continued by selective hydrolysis of the R.sup.2 group (e.g., by treatment with TMSBr) and/or by further couplings to the amine group of said alpha-aminoalkylphosphonate.
In another embodiment, the invention provides a method for producing symmetric phosphonate diesters. In this embodiment, a phosphonic acid with a structure: ##STR5## wherein R.sup.1 is selected from the group consisting of alkyl, aryl, substituted alkyl, substituted aryl, heteroaryl, alkylaryl, and aminoalkyl, is treated with a dialkylazodicarboxylate, a trialkylphosphine, and a primary or secondary alcohol with a structure R.sup.3 OH, wherein R.sup.3 is selected from the group consisting of alkyl, aryl, substituted alkyl, substituted aryl, and aminoalkyl, dissolved in solvent to yield a phosphonate diester with a structure: ##STR6##
In a preferred embodiment, said dialkylazodicarboxylate is DIAD, said trialkylphosphine is TPP, and said solvent is THF. In a preferred embodiment for use with secondary alcohols, the alcohol is either present in excess, or the alcohol, dialkylazodicarboxylate, and triphenylphosphine are allowed to react to form an alkoxyphosphorane before the addition of the phosphonic acid to the reaction mixture.
One can also synthesize phosphonate monoesters with tertiary alcohols using the method for synthesizing phosphonate diesters, but the alcohol should be present in excess, as discussed further below.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the reaction in which a methyl phosphonate (compound 1) is treated with a dialkylazodicarboxylate, preferably DIAD, triphenylphosphine, and an alcohol dissolved in anhydrous THF to yield the ester (compound 2), which is then treated with TMSBr to effect selective hydrolysis of the methyl ester and yield the desired phosphonic acid monoester (compound 3).
FIG. 2 shows the reaction for producing methyl phosphonates (compound 1) from either phosphonic acids (compound 4) or dimethyl phosphonates (compound 5).
FIG. 3 shows the reaction for producing monomethyl peptidylphosphonates (compound 9) from amino acids or peptides.
FIG. 4 shows a theoretical reaction mechanism for the method of the invention.
FIG. 5 shows the reaction for producing one type of symmetrical phosphonate diester (compound 15).
FIG. 6 shows a theoretical reaction mechanism for the observed reactivity of benzylphosphonic acid.
FIG. 7 shows a reaction for incorporating a phosphonate transition state analog pharmacaphore into a growing peptide chain via solid phase synthesis.
FIG. 8 shows a reaction for converting D-phenylalanine to the corresponding hydroxy acid; this reaction scheme can be used to convert a wide variety of the amino acids to the hydroxy acid counterparts.
FIG. 9 shows the synthesis of a protected aminophosphonic acid from alanine; this reaction can be used to convert a wide variety of the amino acids to the protected aminophosphonic acid counterparts.
FIG. 10 shows the synthesis of a protected aminophosphonic acid from an aminophosphonic acid and a photoremovable protecting group; this reaction can be used to convert any of the aminophosphonic acids to the protected aminophosphonic acid counterpart.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Unless otherwise stated, the following terms used in the specification and claims have the meanings given below:
"Alkyl" refers to a cyclic, branched, or straight chain chemical group containing only carbon and hydrogen, such as methyl, heptyl, --(CH.sub.2).sub.2 --, and adamantyl. Alkyl groups can either be unsubstituted or substituted with one or more substituents, e.g., halogen, alkoxy, acyloxy, amino, hydroxyl, mercapto, carboxy, benzyloxy, phenyl, benzyl, or other functionality which may be suitably blocked, if necessary for purposes of the invention, with a protecting group.
"Aryl" or "Ar" refers to a monovalent unsaturated aromatic carbocyclic group having a single ring (e.g., phenyl) or multiple condensed rings (e.g., naphthyl or anthryl), which can optionally be unsubstituted or substituted with amino, hydroxyl, lower alkyl, alkoxy, chloro, halo, mercapto, and other substituents.
"Arylalkyl" refers to the groups R--Ar and R--HetAr, where Ar is an aryl group, HetAr is a heteroaryl group, and R is straight-chain or branched-chain aliphatic group. Examples of arylalkyl groups include benzyl and furfuryl.
"Heteroaryl" or "HetAr" refers to a monovalent unsaturated aromatic carbocyclic group having a single ring (e.g., pyridyl or furyl) or multiple condensed rings (e.g., indolizinyl or benzothienyl) and having at least one hetero atom, such as N, O, or S, within the ring, which can optionally be unsubstituted or substituted with amino, hydroxyl, alkyl, alkoxy, halo, mercapto, and other substituents.
"Lower alkyl" refers to an alkyl group of one to six carbon atoms. Lower alkyl groups include those exemplified by methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, i-butyl (2-methylpropyl), cyclopropylmethyl, i-amyl, n-amyl, and hexyl. Preferred lower alkyls are methyl and ethyl. If more than one alkyl group is present in a given molecule, then each may be independently selected from "lower alkyl" unless otherwise stated.
"Phosphonate ester," "alkylphosphonate ester," "alkylphosphonic acid ester," or "phosphonic acid ester" refers to the group (RPO)(OR')(OR") where R, R' and R" are independently selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, and heteroaryl, provided both R' and R" are not hydrogen, in which case the group is a phosphonic acid. "Phosphonate monoester," "alkylphosphonate monoester," "alkylphosphonic acid monoester", or "phosphonic acid monoester" refers to a phosphonate ester where R' is hydrogen and R" is alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, or heteroaryl. With respect to nomenclature, note that compound 1 in FIG. 1, if R is N-benzyloxycarbonylaminomethyl, is named methyl N-(benzyloxycarbonyl)-aminomethylphosphonate.
"Phosphonic acid" or "alkylphosphonic acid" refers to the group (RPO)(OH).sub.2.
"Protecting group" refers to a chemical group that exhibits the following characteristics: (1) reacts selectively with the desired functionality in good yield to give a protected substrate that is stable to the projected reactions for which protection is desired; 2) is selectively removable from the protected substrate to yield the desired functionality; and 3) is removable in good yield by reagents compatible with the other functional group(s) generated in such projected reactions. Examples of protecting groups can be found in Greene et al., 1991, Protective Groups in Organic Synthesis, 2nd Ed. (John Wiley & Sons, Inc., New York). Preferred terminal amino protecting groups include benzyloxycarbonyl (CBz), 9-fluorenylmethyloxycarbonyl (Fmoc), or suitable photolabile protecting groups such as 6-nitroveratryloxy carbonyl (Nvoc), nitropiperonyl, pyrenylmethoxy-carbonyl, nitrobenzyl, dimethyl dimethoxybenzyl, 5-bromo-7-nitroindolinyl, and the like. Preferred hydroxyl protecting groups include Fmoc, photolabile protecting groups (such as nitroveratryl oxymethyl ether (Nvom)), Mom (methoxy methyl ether), and Mem (methoxy ethoxy methyl ether).
The present invention provides methods for producing phosphonate esters. In one embodiment, the present invention relates to a method for synthesizing a phosphonic acid monoester from a methyl phosphonate by treatment with a dialkylazodicarboxylate, preferably DIAD, a trialkylphosphine, preferably TPP, and an alcohol dissolved in a solvent, such as anhydrous THF. After esterification is complete, the methyl ester is selectively hydrolyzed, preferably by adding TMSBr directly to the reaction mixture, to yield the desired phosphonic acid monoester. Other methods for cleaving the phosphonic acid ester can also be used; for instance, the protocols for cleaving esters of carboxylic acids described in Greene, Supra, at page 156 et seq., can be used for this purpose, provided the desired selectivity of hydrolysis can be achieved.
Monomethyl phosphonates (compound 1, FIG. 1) are preferred for purposes of the present invention and can be readily obtained by the general procedures schematically outlined in FIG. 2. The R group of such compounds can be alkyl, aryl, heteroaryl, or arylalkyl, each of which may be optionally substituted. Some phosphonic acids (compound 4, FIG. 2) are commercially available, and for base sensitive compounds, a phosphonic acid can be mono-esterified directly to produce the monomethyl phosphonates. This esterification can be accomplished by treating a phosphonic acid with methanol and a small excess of thionyl chloride, as described by Hoffman, 1986, Synthesis, at page 557 et seq. In addition, saponification of dimethyl phosphonates (compound 5, FIG. 2) can be used to produce the mono-methyl phosphonates in high yields. Dimethyl phosphonates can be produced either by the bis-esterification of phosphonic acids or obtained directly by the methodology of Arbuzov (see, Arbuzov, 1964, Pure Appl. Chem. 9: 307) or Seebach (see, Seebach et al., 1989, Helv. Chim. Acta 72: 401.
Whether the starting material is a methyl or other alkyl phosphonate, however, one carries out the method of the present invention in substantially the same manner as depicted in FIG. 1. Thus, phosphonate esters may be prepared according to the present method by reacting an alcohol, a monomethyl phosphonate (compound 1), and a dialkylazodicarboxylate, preferably DIAD or diethylazodicarboxylate (DEAD), and triphenylphosphine. Again with reference to FIG. 1, the methyl group of compound 1 may optionally be replaced with an ethyl, benzyl, or substituted benzyl group, or any group that can be selectively hydrolyzed from compound 2 (with the methyl group replaced by the group that substitutes for the methyl group) to yield compound 3. The coupling reaction proceeds with inversion of the stereochemistry at the carbon bonded to the hydroxyl group of the alcohol reactant. Accordingly, if the alcohol reactant has an R configuration at the carbon bonded to the hydroxyl group, the resulting phosphonate has an S configuration at the corresponding carbon. When the coupling reaction is complete, the selective cleavage of the ester is carried out; preferably, trimethylsilyl bromide is added to the reaction mixture to effect selective hydrolysis of the methyl ester and yield the desired compound (compound 3, FIG. 1).
The general procedure for the reaction is as follows. To a solution of methyl phosphonate (compound 1, FIG. 1, 0.5 mmol), alcohol (0.75 mmol), and triphenylphosphine (0.75 mmol) in anhydrous THF (5 mL, refluxed over potassium and distilled prior to use), is added a dialkylazodicarboxylate, preferably diisopropylazodicarboxylate (0.75 mmol). Upon completion of the condensation reaction (generally, 0.5-5 hours), TMSBr (1.5 mmol) is added. The reaction is stirred for about an additional hour. The reaction mixture is then diluted with ether (10 mL) and extracted with 5% NaHCO.sub.3 (twice, 10 mL each extraction). The aqueous phase is washed with ether (three times, 10 mL each wash), acidified to pH=2 with concentrated HCl, and then extracted with ethylacetate (EtOAc, three times, 10 mL each extraction). The EtOAc phase is dried over MgSO.sub.4, filtered, and concentrated to yield the desired compound. See Example 1 below.
The following compounds have been prepared using this method: ethyl benzylphosphonate, isopropyl benzylphosphonate, methyl (D)-2-O-benzylphosphoryl-3-phenylpropionate, methyl (D)-2-O-benzylphosphoryl-2-phenylacetate, ethyl t-butylphosphonate, isopropyl t-butylphosphonate, methyl (D)-2-O-t-butylphosphoryl-3-phenylpropionate, methyl 2-O-(1-N-(benzyloxycarbonyl)aminomethylphosphoryl)acetate, methyl (D)-2-O-(1-(N-(benzyloxycarbonyl)amino)methylphosphoryl)-3-phenylpropionate, methyl (D)-2-O-(1-(N-(benzyloxycarbonyl)amino)methylphosphoryl)-3-methylbutyrate, methyl 2-O-((R,S)-1-(N-(benzyloxycarbonyl)amino)-2-methylpropylphosphoryl)acetate, methyl (D)-2-O-((R,S)-1-(N-(benzyloxycarbonyl)amino)-2-methylpropylphosphoryl)-3-phenylpropionate, and methyl (D)-2-O-((R,S)-1-(N-(benzyloxycarbonyl)amino)-2-methylpropylphosphoryl)-3-methylbutyrare.
However, a wide variety of other compounds can be prepared by the present methods. The R group in compound 3 (FIG. 1) is derived from the R group of the monomethyl phosphonate (compound 1, FIG. 1) and may be alkyl, substituted alkyl, alkylamino, aryl, substituted aryl, arylalkyl, or heteroaryl, each of which may be optionally substituted. The R' group in the alcohol and in compounds 2 and 3 (FIG. 1) originates from the alcohol and may be alkyl, substituted alkyl, aryl, substituted aryl, arylalkyl, or heteroaryl, each of which may be optionally substituted.
Thus, a wide variety of alcohols may be used to produce the phosphonate monoesters by the methods of the present invention. The alcohol may be primary or secondary. With sterically undemanding alcohols, such as ethanol and isopropanol, the reaction occurs instantly, regardless of the substitutents on the phosphonate. With more sterically hindered alcohols, such as methyl mandelate, the reaction requires up to two hours to go to completion. For a more complete understanding of the invention, one may find helpful the theoretical mechanism of the reaction. This theoretical reaction mechanism is illustrated in FIG. 4 (see also, for a discussion of the reaction mechanism for carboxylic acids, Hughes et al., 1988, J. Am. Chem. Soc. 21: 6487; Grochowski et al., 1982, J. Am. Chem. Soc. 104: 6876; Crich et al., 1989, J. Org. Chem. 54: 257; and Varasi et al., 1987, J. Org. Chem. 52: 4235).
In the slower reactions, the protonated betaine (compound 11, FIG. 4) was formed rapidly, as evidenced by an upfield shift of the phosphonic acid monoester's methylene and methoxide .sup.1 H-resonances, and the appearance of a proton attributable to the protonated carbamate amine in compound 11 (FIG. 4). The protonated betaine is believed then to react slowly with the alcohol to yield the desired product and diisopropylhydrazodiformate (DIHD). Further discussion of this theoretical reaction mechanism provided below.
Regardless of theory, the present method has broad applicability, as shown by Table 1, below. Table 1 shows that the present method has been successfully applied to t-butyl and benzylphosphonic esters and to the synthesis of phosphorus containing peptide analogues.
TABLE 1______________________________________Alkylphosphonic acid monoester synthesis via condensations ofmethyl alkylphosphonates with alcohols followed by TMSBrtreatmentPhosphonate Alcohol Time (hr) Yield______________________________________Methyl benzyl- Ethanol 0.5 90%phosphonate Isopropanol 0.5 94% Methyl 3-L-phenyl- 1 80% lactic acid Methy S-mandelate 2 81%Methyl t-butyl- Ethanol 0.5 81%phosphonate Isopropanol 0.5 88% Methyl 3-L-phenyl- 2 82% lactic acidMethyl Methyl glycolate 0.5 86%N--CBz-amino- Methyl 3-L-phenyl 1 78%methylphosphonate lactic acid Methyl 2-L-hydroxy- 3.5 76% isovalerateMethyl Methyl glycolate 0.5 83%N--CBz-valinyl- Methy 3-L-phenyl- 1 64%phosphonate lactic acid Methyl 2-L-hydroxy- 4 69% isovalerate______________________________________
The phosphorus analogues of glycine and valine were used to observe the substituent effects of the phosphorus component on coupling rates and yields. These compounds were synthesized from the CBZ-protected amino acids by the methods of Seebach et al., supra., and Corcoran and Green, 1990, Tetr. Lett. 31: 6827. Likewise, the hydroxy analogues of glycine, phenylalanine, and valine were used to examine the scope of the method. Alpha-L-hydroxyisovaleric acid was purchased from Bachem Bioscience, Inc.
Based on the results of previous workers comparing coupling rates for N.sup.alpha -benzyloxycarbonylamino acid p-nitrophenyl ester derivatives with amino acid esters (see Kovacs, The Peptides: Analysis, Synthesis, Biology, vol. 2 (Academic Press, New York 1979), pp. 485-539), one might expect that the rate of phosphonate ester formation in the reaction would be sensitive to steric effects at both the alcohol and phosphorus centers. Upon comparing the reaction times required to achieve greater than 99% coupling yields (estimated by .sup.1 H-NMR for the phosphonic acid couplings), one surprisingly finds, however, that the reaction rate is relatively insensitive to the steric bulk of the phosphoryl residue side chain. The phosphorus Gly and Val analogues had similar reaction times, unlike the active ester condensations, which exhibited 16-22 fold differences. This tendency has been mentioned previously with regard to the coupling of amino-carboxylic acids and azido compounds via trialkylphosphines. See Wilt et al., 1985, J. Org. Chem. 50: 2601. The influence of steric effects from the hydroxy component on the reaction rate was comparable to that seen in the active ester condensations.
Thus, the invention has broad applicability. Although FIG. 1 exemplifies the reaction as an intermolecular reaction, one of skill in the art will appreciate that the reaction may also be performed intramolecularly (i.e., the alcohol and the monomethyl phosphonate are parts of the same molecule). This embodiment allows for the synthesis of cyclic and sterically restricted phosphonates. In another important embodiment, an aminoalkylphosphonate (compound 9, FIG. 3) serves as the phosponate. In this embodiment, the R.sup.1 and R.sup.2 groups of compound 9 (FIG. 3) are derived from the amine and are independently selected from hydrogen, alkyl, heteroalky, aryl, arylalkyl, and heteroaryl, each of which may be optionally substituted. In addition, R.sup.1, R.sup.2, and the nitrogen atom to which R.sup.2 is attached may form a heterocyclic ring.
When the phosphonate is an alpha-aminoalkylphosphonate in the method of the present invention, the resulting compound, e.g., a methyl alpha-aminoalkylphosphonate diester, is particularly useful in the production of compounds that have pharmacological activity or can be used in the generation of catalytic antibodies. In one embodiment, this aspect of the invention is directed to the synthesis of peptide analogues that possess a phosphonate linkage in the place of an amide or amidomimetic linkage in the peptide or peptidomimetic. Among other benefits, the use of phosphonate linkages in the place of amide linkages offers a general method for increasing the solubility of a peptide due to the hydrophilicity of the charged phosphonate. The protease resistance of the phosphonate may also increase the serum half-life of many therapeutic peptides. These peptide analogues can be prepared from the corresponding aminoalkylphosphonates, as discussed below.
To prepare peptidylphosphonates according to the methods of the present invention, a monomethyl aminoalkylphosphonate (compound 9, FIG. 3) can first be prepared. Monomethyl aminoalkylphosphonates may be produced from amino acids, as shown in FIG. 3. A variety of N-protected amino acids (compound 6, FIG. 3) can be used to prepare aminoalkylphosphonates. Although alpha-aminoalkylphosphonates are preferred for many purposes of the invention, the invention is equally applicable to other aminoalkylphosphonates, such as beta-amino acid phosphonate derivatives and other derivatives where the number of carbon atoms between the amino group and the phosphonate group is more than one.
An alpha-amino acid consists of a carbon atom, called the alpha-carbon, to which is bonded an amino group and a carboxyl group. Typically, this alpha-carbon atom is also bonded to a hydrogen atom and a distinctive group referred to as a "side chain." The hydrogen atom may also be replaced with a group such as alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, and other groups. The side chain of the amino acid is designated as group R.sup.1 in FIG. 3. The side chains of naturally occurring amino acids are well known in the art and include, for example, hydrogen (as in glycine), alkyl (as in alanine, valine, leucine, isoleucine, proline), substituted alkyl (as in serine, cysteine, aspartic acid, asparagine, arginine, glutamine, glutamic acid, and lysine), arylalkyl (as in phenylalanine, histidine, and tryptophan), substituted arylalkyl (as in tyrosine and thyroxine), and heteroaryl (as in histidine). See, e.g., Harper et al., 1977, Review of Physiological Chemistry, 16th Ed., Lange Medical Publications, pp. 21-24.
In addition to naturally occurring side chains, the amino acids used in the present invention may possess synthetic side chains. A "synthetic side chain" is any side chain not found in a naturally occurring amino acid. For example, a synthetic side chain can be an isostere of the side chain of a naturally occurring amino acid. Naturally occurring and synthetic side chains may contain reactive functionalities, such as hydroxyl, mercapto, and carboxy groups. One skilled in the art will appreciate that these groups may have to be protected to carry out the desired reaction scheme. As stated above, the hydrogen at the alpha-carbon can also be replaced with other groups; those of skill in the art recognize the medicinal importance of alpha-methyl amino acids and other alpha, alpha-disubstituted amino acids.
To prepare compounds such as compound 7 (FIG. 3), the amino group of the amino acid (compound 0, FIG. 3) must be protected. One skilled in the art will appreciate that any one of a variety of terminal amino protecting groups may be used. The terminal amino protecting group corresponds to the group R.sup.2 in compounds 6 through 9 (FIG. 3). Examples of terminal amino protecting groups may be found in Greene et al., supra, and include carbamates, amides, N-alkyl groups, N-aryl groups, etc. Preferred amino protecting groups include the carbobenzyloxy group (CBz) and the 9-fluorenylmethyl carbamate group (Fmoc).
Compound 7 (FIG. 3), where Z is acetoxy, can be prepared by the procedure of Corcoran et al., 1990, Tetr. Lett. 31: 6827-6830. The Corcoran et al. reference describes the oxidative decarboxylation of N-protected amino acids with lead tetraacetate. Compound 7, where Z is either iodo, bromo, or chloro, can be prepared through the treatment of an N-protected amino acid (compound 6, FIG. 3) with lead tetraacetate and halide ions, preferably iodine, bromide, and chloride. This general procedure is reported in Kochi, 1965, J. Am. Chem. Soc. 87: 2500. A review of this reaction can be found in Sheldon and Kochi, 1972, Org. React. 19: 279-421. Alternatively, compound 7, where Z is either iodo, bromo, or chloro, can be prepared from an N-protected amino acid using the procedures outlined in March, 1985, Advanced Organic Chemistry 3rd Ed. (John Wiley & Sons, New York), pp. 654-655.
Compound 8 (FIG. 3) can be prepared through the reaction of compound 7, where Z is acetoxy or halo, with trimethylphosphite. See Corcoran, supra; see also, Seebach, supra. A review of this general reaction can be found in Arbuzov, supra. In a preferred embodiment, compound 7 (FIG. 3), where Z is acetoxy, is treated with trimethylphosphite in the presence of titanium tetrachloride to produce compound 8.
To illustrate this aspect of the invention, one can prepare dimethyl [1-[N-(benzyloxycarbonyl)-amino]-2-methylpropyl]phosphonate (see compound 8, FIG. 3) from 1-acetoxy-1-N-(benzyloxycarbonyl)amino-2-methylpropane (see compound 7, FIG. 3), or one can prepare dimethyl [1-[N-(benzyloxycarbonyl)-amino]-ethyl]phosphonate from 1-acetoxy-1 -N-(benzyloxycarbonyl)aminoethane by dissolving the starting compound in a polar aprotic solvent, such as methylene chloride, under an inert atmosphere. Trimethylphosphite is then added, and the mixture is cooled, preferably to between 0.degree. C. and -78.degree. C. A dilute solution, preferably 1M, of titanium tetrachloride in a polar aprotic solvent, such as methylene chloride, is slowly added to the solution of the compound, and the reaction is stirred at room temperature until the reaction is complete. See Example 3 below.
The dimethyl phosphonates characterized by the structure of compound 8 (FIG. 3) may be saponified to yield monomethyl phosphonates (compound 9, FIG. 3) using a variety of techniques. In a preferred embodiment, the saponification is accomplished under basic conditions. To illustrate this aspect of the invention, one can prepare methyl [1-[N-(benzyloxycarbonyl)-amino]-ethyl]phosphonate (see compound 9, FIG. 3) from dimethyl [1-[N-(benzyloxycarbonyl)-amino]-ethyl]phosphonate (see compound 8, FIG. 3), or one can prepare methyl [1-[N-(benzyloxycarbonyl)amino]-2-methylpropyl]phosphonate from dimethyl [1-[N-(benzyloxycarbonyl)-amino]-2-methylpropyl]phosphonate by dissolving the starting compound in a polar protic solvent and a strong aqueous base, preferably concentrated aqueous sodium hydroxide or concentrated aqueous lithium hydroxide, and heating the resulting mixture for a time sufficient to hydrolyze the ester group. See Example 3 below.
With these building blocks, one can synthesize many pharmaceutically important compounds by the method of the present invention. As already noted, the present methods are also amenable to synthesis reactions carried out on a solid support or substrate, an important aspect of the present invention. Synthesis in this format is desirable primarily because of the ease with which one can synthesize and screen large numbers of molecules for desired properties. In addition, this synthesis format may be preferred, because by-products, such as unreacted starting materials, solvents, deprotection agents, etc., may simply be washed away from the desired solid support-bound product.
In accordance with the present invention, solid phase substrates include beads and particles, such as peptide synthesis resins (see, e.g., Merrifield, 1963, J. Am. Chem. Soc. 85: 2149-54; U.S. patent application Ser. No. 762,522, filed Sep. 18, 1991; U.S. patent application Ser. No. 876,792, filed Apr. 29, 1992; and Houghten, U.S. Pat. No. 4,631,211), rods (see, e.g., Geysen et al., 1984, Proc. Natl. Acad. Sci. USA 81: 3998-4002, and Geysen et al., 1986, PCT patent publication No. WO 86/00991), glass slides (see Fodor et al., PCT patent publication No. WO 92/10092), and any additional support upon which peptides or other organic compounds can be synthesized. Thus, the solid substrate may be composed of any of a wide variety of materials, for example, polymers, plastics, resins, polysaccharides, silicon or silica-based materials, carbon, metals, inorganic glasses, and membranes.
The surface of the substrate is preferably provided with a layer of linker molecules, although one of skill in the art understands that the linker molecules are optional and not strictly required. The linker molecules are of sufficient length to permit compounds attached to the substrate to interact freely with molecules exposed to the substrate. The linker molecules can be 6-50 atoms long or longer to provide optimum exposure. The linker molecules, may be, for example, aryl acetylene, or ethylene glycol oligomers, or diamines, diacids, amino acids, or combinations thereof in oligomer form, each oligomer containing 2 to 10 or more monomer units. Other linker molecules may also be used.
The linker molecules can be attached to the substrate via carbon-carbon bonds using, for example, (poly)trifluorochloroethylene surfaces, or preferably, by siloxane bonds (using, for example, glass or silicon oxide surfaces). Siloxane bonds with the surface of the substrate may be formed via reactions of linker molecules bearing trichlorosilyl groups. The linker molecules may optionally be attached in an ordered array, i.e., as parts of the head groups in a polymerized Langmuir Blodgett film. In alternative embodiments, the linker molecules are adsorbed to the surface of the substrate.
On the distal or terminal end of the linker molecule opposite the substrate, the linker molecule is provided with a functional group. This functional group is used to bind a protective group, a substrate, or a reactant, depending on the format desired and the stage of synthesis. Typically, the functional group will comprise a hydroxyl group, a mercapto group, an amino group, a carboxylic acid group, a thiol group, or other groups capable of bonding to the reactant.
For a better understanding of the wide variety of solid supports, linkers, and screening methods compatible with the present invention, the following specific method for incorporating the phosphonate transition-state analog pharmacaphore into a growing pep tide chain via solid phase synthesis is provided. Incorporation of a phosphonate linkage requires two unnatural (non-peptidyl) building blocks: (1) an appropriately protected aminophosphonic acid (compound 21, FIG. 7); and (2) an appropriately protected alpha-hydroxy acid (compound 20, FIG. 7). Secondary alcohols, such as the side chain hydroxyl of threonine, have been shown to be unreactive toward modern amino acid coupling agents, such as BOP and HBTu. Surprisingly, however, most of the secondary hydroxyls of the alpha-hydroxy acids are reactive toward these agents, at least in the present methods, and therefore must be suitably protected during coupling to an amino acid. The reaction is schematically outlined in FIG. 7.
Chemical peptide synthesis typically proceeds in the carboxy-to-amino (C to N) direction. Incorporation of the phosphonate proceeds in two steps. First, the hydroxy acid is attached to the peptide's amino-terminus using standard peptide coupling conditions, in tile example shown, HBTu (2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate) chemistry. The hydroxyl potecting group is removed; in FIG. 7, the protecting group is Nvom, which can be removed photolytically. The phosphonate is then coupled using the activating agents triphenylphosphine and dialkylazodicarboxylate (such as DIAD or DEAD), which are added incrementally, in excess, until unreacted TPP persists for at least one hour. These reagents activate the hydroxyl by forming a triphenylphosphonium derivative, which in turn undergoes an S.sub.N 2 displacement by the phosphonic acid monoester to afford the phosphonate diester (compound 22, FIG. 7). Excess TPP and DIAD are added to remove any water present in the reaction mixture. One then has a variety of options: the N-terminal phosphonate can be deprotected to allow further couplings (see compound 24, FIG. 7), or synthesis can stop here, and the compound can be cleaved from the support, as shown in FIG. 7. One can also selectively hydrolyze the ester by, e.g., TMSBr treatement, either on the support or after the compound is cleaved from the support.
This method is superior to others in which a peptidylphosphonate is synthesized as a dimer and then incorporated as a cassette into a growing peptide. In the prior art methods, one must first synthesize the cassette in solution phase and purify the cassette to homogeneity prior to incorporation. The prior art method allows one to synthesize only one combination of phosphonate-hydroxy acid dimer. One would have to synthesize in solution phase an entirely new cassette to make a different phosphonate.
By the present method, a single phosphonic acid building block can be coupled to virtually any hydroxy acid to form a variety of different dimers. The alpha-hydroxy acid analogs of the amino acids are readily prepared in large scale, in a single step, from commercially available amino acids using published techniques. Most of the hydroxy acid derivatives of the twenty L-amino acids encoded by genes can be readily obtained as per the protocol shown in FIG. 8. A number of the hydroxy acids are also commercially available. One should not limit the invention, however, to the alphahydroxy compounds, because any hydroxy acid can be used in the present method.
Like the hydroxy acids, the phosphonic acid building blocks can be prepared from commercially available amino acids; some can even be purchased in unprotected form commercially. Again, one should not limit the present invention to only the alpha-aminoalkylphosphonic acids or phosphonic acids that are similar to the naturally occurring amino acids or the 20 amino acids encoded genetically. The synthesis of a protected aminophosphonic acid starting with a commercially available amino acid is outlined in FIG. 9, and the synthesis of a protected aminophosphonic acid starting with a commercially available aminophosphonic acid is outlined in FIG. 10.
As noted above, the substrate-bound phosphonate ester may be subjected to further reactions, such as deprotection of the methyl phosphonate with trimethylsilyl bromide, removal of other protecting groups, or additional coupling reactions. The phosphonate ester may also be released from the substrate. One skilled in the art will appreciate that the cleavage conditions will vary with the type of substrate and the nature of the phosphonate ester. Typically, the substrate-bound phosphonate ester will first be washed with a polar aprotic solvent and dried under reduced pressure. If a PAC or PAL resin is used as the substrate, then cleavage of the phosphonate ester from the resin can be accomplished through the action of trifluoroacetic acid. The phosphonate ester can be isolated by filtration and concentrated under reduced pressure. The solid phase synthesis of phosphonate esters by the method of the present invention will generally produce phosphonate esters of greater than 80% purity.
The present methods for the solid phase synthesis of phosphonate esters are especially preferred for the production of libraries of peptidylphosphonic acid derivatives. These libraries can be assembled using, for example, the combinatorial synthesis strategies described in PCT patent publication No. 92/10092; U.S. patent application Ser. No. 762,522, filed Sep. 18, 1991; and U.S. patent application Ser. No. 876,792, filed Apr. 29, 1992. Briefly, a combinatorial synthesis strategy is an ordered strategy for parallel synthesis of diverse polymer sequences by sequential addition of reagents to a solid support. In one embodiment, the solid support is a collection of beads, and each cycle of monomer addition is carried out in parallel in a series of reactions, each reaction using a different monomer. After each cycle, the beads from the different, parallel series of reactions are pooled and redistributed prior to the next cycle of monomer addition. These methods apply, for example, to the synthesis of protease inhibitors where one knows the position of the P.sub.1 -P.sub.1 ' site and wants to fix the phosphonate linkage while exploring amino acid preferences at the flanking regions.
In another preferred embodiment, the combinatorial synthesis is performed using the VLSIPS.TM. technique described in Fodor et al., 1991, Science 251: 767-773, and in PCT patent publication No. 92/10092 (and corresponding U.S. counterparts, including Ser. No. 805,727, filed Dec. 6, 1991). Briefly, the VLSIPS.TM. technology is a method for synthesizing large numbers of different compounds simultaneously at specific locations on a silica chip. Using this technology, one can produce an array of phosphonic acid derivatives and screen that array for compounds with biological activity.
To screen for biological activity, the array is exposed to one or more receptors such as antibodies, whole cells, receptors on vesicles, lipids, or any one of a variety of other receptors. The receptors are preferably labeled with, for example, a fluorescent marker, radioactive marker, or a labeled antibody reactive with the receptor. After the array has been exposed to the receptor, the location of the marker can be detected with, for example, photon detection or autoradiographic techniques. Through knowledge of the sequence of the material at the location where binding is detected, one can quickly determine which compounds bind to the receptor.
This combinatorial synthesis approach can be utilized to produce a library of peptidylphosphonates using the reaction scheme illustrated in FIG. 7. Typically, the protecting groups will be present on the amine and hydroxyl functional groups on the surface of the support where coupling occurs and will be photolabile, although other protecting groups can be used to advantage in the protocol. Using lithographic methods, one exposes the photoremovable protecting group to light so as to remove the group from the reactant in the first selected region to free an amine or hydroxyl group on the compounds where the desired attachment is to occur.
The substrate is then washed or otherwise contacted with a first hydroxy acid, amino acid, or phosphonate, depending on the step in the process. Once the hydroxy acid is coupled to the activated amine groups on the support and then irradiated to remove the protecting group from the hydroxyl group of the coupled hydroxy acid, an alphaaminoalkylphosphonate, preferably a monomethyl alphaaminoalkylphosphonate, is coupled to the hydroxyl group with the activating reagents dialkylazodicarboxylate and triphenylphosphine. Portions of the substrate which have not been deprotected do not react with the phosphonate (or other monomer, depending on the step and the compounds being synthesized). If the phosphonate (or other monomer) possesses other functionalities, these functionalities may need to be blocked with an appropriate protecting group, which may be photolabile but could also be removable with acid or base treatment.
A second set of selected regions can then be exposed to light to remove the photoremovable protecting group from compounds in the second set of regions. The substrate is then contacted with a second monomer and coupling reagents, e.g., a monomethyl phosphonate, triphenylphosphine, and dialkylazodicarboxylate. Of course, one can also introduce and couple other monomers, such as amino acids, to the growing polymer chains on the surface, so long as appropriate chemistry and protecting groups are employed. This process is repeated until the desired array of compounds has been synthesized.
Thus, libraries of potential ligands for binding with receptors can be prepared using the methods of the present invention. Typically, a peptide would first be identified as a potential ligand for binding with a given receptor. An array of peptide analogues would then be produced. These analogues are peptidylphosphonates in which amide linkages of the peptide have been replaced with a phosphonate linkage. Of course, one can also synthesize other polymers, such as polyphosphonates (using alpha-hydroxy alkylphosphonates), by the methods of the invention.
Substitution of the phosphonate linkage for the amide linkage allows for the introduction of additional functionalities into regions of space inaccessible in naturally occuring peptides. Specifically, amide linkages are planar, so peptides have a flat configuration in the area of the carboxylic acid linkage. Phosphonate linkages, however, are tetrahedral. This tetrahedral geometry allows for the presence of substituents in the areas above and below the plane of the carboxylic acid linkage of a peptide. Moreover, the tetrahedral configuration of the phosphonate linkage can be exploited to optimize ligand-receptor binding. See Bartlett and Marlowe, 1983, Biochemistry 22: 4618-24.
Thus, the present invention has diverse applications. For instance, and as shown in FIG. 5, the present invention also provides methods to synthesize symmetrical phosphonate diesters (compound 15). Bisesterification of commercially available benzylphosphonic acid (compound 14, FIG. 5, commercially available from Lancaster Synthesis) was initially used to exemplify this aspect of the invention, and the reaction was carried out by adding DIAD to a solution of compound 14, an alcohol, and triphenylphosphine in THF. When primary alcohols were used, the reaction was instantaneous, and the symmetrical esters were isolated in high yields.
The results of these experiments are shown in Table 2 below.
TABLE 2______________________________________Symmetrical alkylphosphonate diester syntheses via condensationsof benzylphosphonic acid and alcoholsReaction AlcoholScheme* Alcohol Equivalents Yield______________________________________A Methanol 2.5 85%A Methanol 25 91%A Isopropanol 2.5 11%A Isopropanol 25 76%A Benzyl alcohol 2.5 87%B Methanol 5 81%B Isopropanol 5 37%B Benzyl alcohol 5 90%______________________________________ *See discussion below.
When isopropanol was used, diisopropyl benzylpyrophosphonate (compound 16, FIG. 5, where R' is isopropanol) was the major product. Similar products have been observed during the study of radical-based dephosphorylation in the production of monomeric metaphosphate. See Avila and Frost, 1988, J. Am. Chem. Soc. 110: 7904. Perturbations of the reaction conditions yielded mixed results: temperatures of -78.degree. C. to 25.degree. C. had no effect on the product ratios, and changing the concentration of the reactants likewise had no effect on the product ratios. However, as shown in Table 2, increasing the equivalents of the alcohol to the phosphonic acid improves the yield markedly. In similar fashion, reaction scheme B, described more fully below, could also be accessed to improve yields; however, this method may be problematic if the reaction intermediates are susceptible to elimination or degradation reactions (as is isopropanol; see Table 2).
To understand these results and the method for making phosphonate diesters, one may benefit from considering the reaction schemes shown in FIG. 4. In Path A, the betaine (compound 10) formed between triphenylphosphine and DIAD is protonated by an acidic component with pKa<11 (see Kim and Rapoport, 1990, J. C)rg. Chem. 55: 3699), yielding the protonated betaine (compound 11, FIG. 4), which in turn reacts with an alcohol to form the alkoxyphosphonium salt (compound 13, FIG. 4). Subsequent S.sub.N 2 attack by the conjugate base of compound 13 yields the esterified product with inversion of configuration at the carbinol carbon.
Path B only occurs when the acidic component is not present to protonate the betaine (compound 10, FIG. 4). Two equivalents of alcohol react with compound 10 to form a dialkoxyphosphorane (compound 12, FIG. 4; see Camp and Jenkins, 1989, J. Org. Chem. 54: 3045; Von Itzstein and Jenkins, 1983, Aust. I. Chem. 36: 557; and Guthrie and Jenkins, 1982, Aust. I. Chem. 35: 767). Upon addition of the acidic component, compound 12 is converted to the phosphonium salt (compound 13, FIG. 4), followed by product formation as in Path A. Some investigators have suggested that compound 12 (FIG. 4) is in equilibrium with an (acyloxy)alkoxyphosphorane, but whether such a compound plays a significant role in product formation is unknown (see Camp and Jenkins, 1989, J. Org. Chem. 54: 3049).
Pyrophosphonate esters presumably arise from condensation of one molecule of an alkylphosphonic acid with an electrophilic phosphorus species formed via the protonated betaine (compound 11, FIG. 4), followed by further esterification with excess alcohol. The electrophilic species derived from benzylphosphonic acid was trapped with t-butanol, yielding t-butyl benzylphosphonate. Because Mitsunobu condensations are not normally observed with tertiary alcohols (see Camp and Jenkins, supra), and no reaction with t-butanol was observed with benzylpyrophosphonate, or when methyl benzylphosphonate (this compound, as well as methyl t-butylphosphonate were obtained by treatment of the respective alkylphosphonic acids (Aldrich Chemical Co.) with diazaomethane, followed by saponification) was substituted for benzylphosphonic acid, any t-butyl ester that is formed must result from nucleophilic attack by the alcohol on the electrophilic phosphorus species. The availability of this additional pathway with phosphonic acids may result in a different stereochemical configuration of the alcohol component than the normally observed inverted carbinol carbon.
One explanation for the observed reactivity of benzylphosphonic acid is that intramolecular dehydration of compound 11 (FIGS. 4 and 6) yields benzylmetaphosphonate (compound 18, FIG. 6), triphenylphosphine oxide, and diisopropylhydrazodiformate (see FIG. 6). Formation of phenylmetaphosphonate (see Eckes and Regitz, 1975, Tetr. Lett., p.447 et seq.), mesitylenemetaphosphonate (see Sigal and Lowe, 1978, J. Am. Chem. Soc. 100: 6394), and monomeric metaphosphate (see Bodalski et al., 1991, J. Org. Chem. 56: 2666; Freeman et al., 1987, J. Am. Chem. Soc. 109: 3166; and Westheimer, 1981, Chem. Rev. 81: 313) have all been observed. The benzylmetaphosphonate (compound 18, FIG. 6) would be highly electrophilic and could readily undergo nucleophilic attack by t-butanol to produce the t-butyl ester, or by another molecule of benzylphosphonic acid producing benzylpyrophosphonate (compound 19, FIG. 6). Mitsunobu esterification of compound 19 (FIG. 6) with isopropanol would then yield diisopropyl benzylpyrophosphonate (compound 16, FIG. 5).
To circumvent formation of the electrophilic phosphorus species, one can access Path B (FIG. 4), using reaction conditions similar to those described by Loibner and Zbiral, 1976, Helv. Chim. Acta. 59: 2100. In this embodiment of the invention, the alcohol, triphenylphosphine, and DIAD are initially combined to produce the alkoxyphosphorane (compound 12, FIG. 4) before adding the acidic component. Formation of compound 12 prevents the acidic component from reacting with the betaine (compound 10, FIG. 4) to form the protonated betaine (compound 11, FIG. 4) and eliminates pyrophosphonate formation. Formation of compound 12 (FIG. 4) was inferred by the significant upfield shifts in the .sup.1 H-NMR spectrum of the alcohol protons upon addition of DIAD and triphenylphosphine. When methanol and benzyl alcohol were used, the alkoxyphosphorane formed reacted cleanly and rapidly with benzylphosphonic acid to form the phosphonate diesters. With isopropanol, the reaction was complicated by the ready decomposition of compound 12 (FIG. 4), presumably via an E2 elimination as has been observed by others (see Von Itzstein and Jenkins, supra, and Loibner and Zbiral, 1977, Helv. Chim. Acta. 60: 417).
The general procedure for the solution phase synthesis of benzylphosphonate diesters is as follows. To a solution of benzylphosphonic acid (0.5 mmol), alcohol (1.25 mmol), and triphenylphosphine (1.25 mmol) dissolved in anhydrous THF (5 mL), is added diisopropylazodicarboxylate (1.25 mmol). After about 30 minutes, the reaction mixture is concentrated under vacuum, and then the triphenylphosphineoxide is crystallized with acetone/pentane, and removed by filtration. The filtrate is optionally concentrated under vacuum and purified by chromatography (HOAc/EtOAc can be used). Illustrative compounds prepared by this protocol include dimethyl benzylphosphonate, diisopropyl benzylphosphonate, and dibenzyl benzylphosphonate. See Example 2.
Thus, the present invention provides a variety of methods for making, using, and screening phosphonate esters and compounds comprising phosphonate esters. The above description and the following examples are intended to be illustrative and not restrictive. Many embodiments of the invention not explicitly set forth herein will be apparent to those of skill in the art upon reviewing this description of the invention. The disclosures of all articles and references, including patents and patent applications, in their entirety are incorporated herein by reference. The scope of the invention should be determined not with reference to the above description, but instead with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
The following examples are provided to aid one of skill in the art in understanding the invention. Isolation and purification of the compounds and intermediates described in the examples can be effected, if desired, by any suitable separation or purification procedure such as, for example, filtration, extraction, crystallization, column chromatography, thin-layer chromatography, thick-layer (preparative) chromatography, distillation, or a combination of these procedures. Specific illustrations of suitable separation and isolation procedures are in the examples below. Other equivalent separation or isolation procedures can, of course, also be used. These examples are for illustrative purposes only and are not to be construed as limiting this invention in any manner.
EXAMPLE 1
Preparation of Phosphonic Acid Monoesters
To a solution of methyl alkylphosphonate (0.5 mmol), an alcohol (0.75 mmol), and triphenylphosphine (0.75 mmol) in anhydrous THF, was added diisopropylazodicarboxylate (0.75 mmol). Upon completion of the condensation reaction, TMSBr was added. Stirring was continued an additional hour. The reaction mixture was diluted with ether (10 mL), and extracted with 5% aqueous sodium bicarbonate (10 mL.times.2). The aqueous phase was washed with ether (10 mL.times.2), acidified to pH=2 with concentrated hydrochloric acid, and extracted with ethyl acetate (10 mL.times.3). The ethyl acetate phase was dried over magnesium sulfate, filtered, and concentrated under reduced pressure to yield a phosphonic acid monoester.
EXAMPLE 2
Preparation of Phosphonate Diesters
Method A
To a solution of benzylphosphonic acid (0.5 mmol), an alcohol, and triphenylphosphine (1.25 mmol) in anhydrous THF (5 mL), was added diisopropylazodicarboxylate (1.25 mmol). After 30 minutes, the reaction mixture was concentrated under reduced pressure. The triphenylphosphineoxide was crystallized with acetone/pentane and removed by filtration. The filtrate was concentrated under reduced pressure and purified by chromatography (elution with acetic acid/ethyl acetate) to yield a phosphonate diester.
Method B
To a solution of an alcohol and triphenylphosphine (1.25 mmol) in anhydrous THF (5 mL), was added diisopropylazodicarboxylate (1.25 mmol). To the reaction mixture was then added a solution of benzylphosphonic acid (0.5 mmol) in anhydrous THF. After 30 minutes, the reaction mixture was concentrated under reduced pressure. The triphenylphosphineoxide was crystallized with acetone/pentane and removed by filtration. The filtrate was concentrated under reduced pressure and purified by chromatography (elution with acetic acid/ethyl acetate) to yield a phosphonate diester.
EXAMPLE 3
Synthesis of Phosphonic Acid Building Blocks
A. Synthesis of Phosphonic Acid Building Blocks from Amino Acids
I. Preparation of Fmoc-protected Methyl Hydrogen [(.+-.)-N-(fluorenylmethoxycarbonyl)-1-aminoethyl]phosphonate
To an ice-cold solution of lead tetraacetate (34.1 g, 105 mmol) in 100 mL of dimethylformamide (DMF), was added dropwise a solution of CBz-alanine (20.0 g, 89.6 mmol) in 50 mL of DMF. The dark brown reaction mixture was allowed to warm to room temperature and then stirred for 8 hours. The now clear reaction mixture was diluted with saturated sodium bicarbonate (150 mL), extracted with ethylacetate (4.times.100 mL), and the combined organic layers washed with saturated sodium bicarbonate (1.times.100 mL) and then brine (1.times.100 mL). The organic layer was dried over anhydrous magnesium sulfate and then concentrated to afford 18.52 g of a white solid (93%).
To the acetate (18.52 g, 83.3 mmol) in 333 mL of dichloromethane was added trimethylphosphite, the solution was cooled to -78.degree. C., and 1.0M titanium tetrachloride in dichloromethane solution (100 mL, 100 mmol) was added dropwise over 3 minutes. The reaction was allowed to warm to room temperature and then stirred overnight. The reaction was quenched by the addition of solid sodium carbonate (41.7 g, 146 mmol), stirred for 2 hours, and then filtered through a glass wool plug. The filtrate was washed with water (4.times.100 ml), brine (1.times.100 mL), dried over magnesium sulfate, filtered, concentrated to a light yellow oil, and then purified by silica gel chromatography (8: 2 ethyl acetate/hexane) to afford 14.06 g of a light yellow oil (59%).
To a solution (10.0 g, 35 mmol) of the light yellow oil (dimethyl [1-[N-(benzyloxycarbonyl)amino]-ethyl]phosphonate) in ethyl alcohol (218 mL), was added 10M aqueous sodium hydroxide (8.75 mL, 87.5 mmol), and the reaction heated to reflux for 90 minutes. The reaction was cooled, diluted with water (50 mL), acidified to pH=2 with concentrated HCl, and then extracted with ethyl acetate (3.times.50 mL). The combined organic layers were dried over magnesium sulfate, filtered, and then concentrated to afford 6.73 g of an oily semi-solid (70%).
To a solution (1.0 g, 3.60 mmol) of the oily semi-solid (methyl [1-[N-(benzyloxycarbonyl)amino]-ethyl]phosphonate) in methanol (18 mL) cooled to 0.degree. C., was added 10% Pd/carbon (100 mg) and a balloon of hydrogen gas. The reaction was warn-ted to room temperature and stirred overnight. The solution was then filtered through a sintered glass funnel and then concentrated to a solid. To some of this solid (100 mg, 0.92 mmol), was added dioxane (1.9 mL), 10% aqueous sodium bicarbonate (1.9 mL), and a solution of 9-fluorenylmethyloxycarbonyl (Fmoc) chloroformate (310 mg, 1.2 mmol) in 1 mL of dioxane, and the slightly turbid reaction mixture was stirred overnight. The now clear reaction mixture was diluted with water and then acidified to pH 1 with concentrated HCl. Upon standing, a white precipitate formed. This precipatate was collected by filtration and dried in vacuo to afford 100 mg of an off-white solid (30%).
II. Preparation of Methyl Hydrogen [(.+-.)-N-phenylmethoxycarbonyl)-1-amino-2-methylpropyl]phosphonate
To a suspension of lead tetraacetate (6.537 g, 14.7 mmol) in dry N,N-dimethylformamide (20 mL) under argon at 0.degree. C., was added N-(carbobenzyloxy)-L-valine (3.082 g, 12.3 mmol). After 1 hour, cooling was stopped and stirring continued an additional 3 hours at room temperature. The reaction was quenched with saturated NaHCO.sub.3 (100 mL) and extracted with ethyl acetate (4.times.30 mL). The organic layers were combined, washed with saturated NaHCO.sub.3 (25 mL), then with H.sub.2 O (25 mL), and then brine (25 ml), and then dried over MgSO.sub.4, filtered, and concentrated under vacuum.
The resulting oil was dissolved in methylene chloride (20 mL), then trimethylphosphite (2.2 mL, 18.7 mmol) was added, and the mixture was cooled to -78.degree. C. Then, TiCl.sub.4 (15 mL, 15 mmol), as a 1M solution in methylene chloride, was added to the mixture. The reaction was warmed to room temperature, and after 12 hours, the reaction mixture was cooled to 0.degree. C., quenched with Na.sub.2 CO.sub.3.10H.sub.2 O (35 g, 120 mmol), and then stirred at room temperature for 30 minutes. The mixture was diluted with H.sub.2 O (100 mL), extracted with CH.sub.2 Cl.sub.2 (4.times.25 mL), and the combined organic layers were then washed with H.sub.2 O (25 ml) and brine (2.times.25 mL), dried over MgSO.sub.4, filtered, then concentrated to afford 3.855 g (99%) of a colorless oil.
B. Synthesis of Phosphonic Acid Building Blocks from Aminophosphonic Acids
I. Preparation of N-phenylmethoxycarbonyl-amino Methylphosphonic Acid
To aminomethylphosphonic acid (500 mg, 4.50 mmol) in 1M aqueous sodium hydroxide (11.5 mL, 11.5 mmol) cooled to 0.degree. C., was added benzyoxycarbonyl chloroformate (845 mg, 5.00 mmol, 0.71 mL) dropwise over one minute and the reaction stirred overnight. The reaction mixture was then extracted with ethyl ether (3.times.25 mL), the aqueous layer strongly acidified with concentrated HCl, and then extracted with ethyl acetate (3.times.25 mL). The organic layers were combined, dried (MgSO.sub.4), filtered, and then concentrated to afford 700 mg of a white solid (63%).
III. Preparation of N-nitroveratryloxcarbonyl-aminomethylphosphonic Acid
To aminomethylphosphonic acid (500 mg, 4.50 mmol) in 5% aqueous sodium bicarbonate (22.5 mL, 13.5 mmol), was added dioxane (20 mL) and then nitroveratryloxycarbonyl (NVOC) chloroformate (5.00 mmol, 1.36 g) in 20 mL of dioxane. The reaction was stirred overnight, extracted with ethyl ether (3.times.40 mL), and then the aqueous layer was strongly acidified with concentrated HCl. Upon standing, fine yellow needles formed and were filtered and dried in vacuo to afford 1.35 g of product (86%).
EXAMPLE 4
Preparation of Alpha-hydroxy Acids from Alpha-amino Acids
To D-phenylalanine (2.00 g, 12.4 mmol) in water (20 mL), was added 3M HCl (20 mL, 60 mmol) and acetic acid (10 mL). The solution was cooled to 0.degree. C., and an aqueous solution of sodium nitrite (3.92 g, 56.8 mmol, in 7.5 mL of water) was added dropwise over 10 minutes. The reaction was stirred for eight hours, then more sodium nitrite was added (3.92 g, 56.8 mmol, in 7.5 mL of water). After six hours, the solution was washed with ether (3.times.25 mL), and the combined organic layers were extracted with saturated sodium bicarbonate (3.times.30 mL). The bicarbonate washes were combined, acidified to pH 1 with concentrated HCl, and then extracted with ether (3.times.30 mL). The organic layers were combined, dried (magnesium sulfate), filtered, and then concentrated to afford 1.65 g of a white waxy solid (80%).
EXAMPLE 5
Preparation of (D)-O-nitroveratryloxymethoxy-2-oxy-4-methylpentanoic Acid
The hydroxy acid prepared in Example 4 was esterified using diazomethane, and to the resulting compound, methyl (D)-2-hydroxy-4-methyl valerate (1.0 g, 6.85 mmol) in 10 mL of methylene chloride was added Nvom chloride (2.69 g, 10.5 mmol) and diisopropylethyleneamine (DIEA, 1.8 mL). After 12 hours, the reaction mixture was diluted with methylene chloride (20 mL), waster with water (2.times.15 mL), and the organic layer dried (over MgSO.sub.4). The product was purified by silica gel chromatography (4: 1 petroleum ether: ethyl acetate) to afford 1.2 g of material (47%). The free acid was obtained in quantitative yield by hydrolysis with lithium hydroxide, as previously described. One could also prepare this compound, with possibly higher yield and shorter reaction time, using sodium hydride/THF instead of DIEA/dichloromethane.
EXAMPLE 6
Synthesis of a Peptidylphosphonate: Preparation of Z-Phe.sup.p (O)-Leu-Ala
A. Coupling (D)-Nvom-2-oxy-4-methylpentanoic Acid to Ala Resin
To alanine-derivatized PAC resin (1.00 g, 534 .mu.mol) swollen in DMF (5 mL) was added Nvom-2-oxy-4-methylvaleric acid (1.5 mmol, 537 mg), HBTu (1.5 mmol, 720 mg), HOBt (1.5 mmol, 229 mg), and DIEA (3.75 mmol, 650 .mu.L), and the suspension was shaken for one hour. The resin was washed with DMF (4.times.10 mL), suspended in 30 mL of DMF, and photolyzed at 360 nm (10 mW) for 24 hours. The resin was washed with DMF (4.times.10 mL) and then with methanol (4.times.10 mL) and then dried in vacuo.
B. Coupling Methyl Hydrogen [(.+-.)-N-(phenylmethoxycarbonyl)-1-aminoethyl]phosphonate to 2-hydroxy-4-methylvaleryl Alanine Resin: Preparation of (.+-.)Cbz-Phe.sup.p -(O)-Leu-Ala
To 2-hydroxy-4-methylvalerylalanine resin (50 mg, 26.5 .mu.mol in THF was added methyl hydrogen [(.+-.)-N-(phenylmethoxycarbonyl)-1-aminoethyl]phosphonate (70 mg, 260 .mu.mol), TPP (65 mg, 260 .mu.mol), and DIAD (50 mg, 260 .mu.mol). Additional equivalents of TPP and DIAD were added at two 15 minute intervals, and the reaction mixture shaken for two hours. The resin was then washed with DMF (3.times.10 mL) and then with methanol (3.times.10 mL) and then dried in vacuo. The peptidyl phosphonate was cleaved from the resin by treatment with 0.5 mL of TFA for 30 minutes.

Number	Name	Date	Kind
5143854	Pirrung et al.	Sep 1992
5183810	Greenlee et al.	Feb 1993

Methods for the synthesis of phosphonate esters

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