Resolution of chiral amines

Abstract

Chiral amines are resolved by selectively reacting an enantiomer of the amine with an alkyl ester in the presence of an enantioselective lipase enzyme to produce an amide of that enantiomer and separating it from the unreacted enantiomer, the alkyl group of the ester being an isoalkyl group. Isobutyl and especially isopropyl groups are preferred.

Description

This application is the national phase of international application PCT/C7B98/03769 filed Dec. 9, 1998 which designated the U.S.

THIS INVENTION relates to the resolution of chiral amines.

It is known for example from WO95/08636, Kitaguchi et al (J.Am. Chem. Soc., 1989, 111, 3094-3095), Gotor et al (J.Chem. Soc. Perkin Trans. 1, 1993, 2453-2456), Ohmer et al (Enzyme and Microbial Technology, 1996, 19, 328-331) and Sanchez et al (Tetrahedron Asymmetry, 1997, 8, 37-40) and Chiou et al, Bio-organic & Medicinal Chemistry Letters Vol. 7, No 4, pp 433-436 (1977 to resolve racemic amines by acylating one enantiomer by reaction with an alkyl ester in the presence of an enantioselective enzyme as catalyst However, the results obtained have in many cases been disappointing.

We have found that such reactions of attractive stereospecificity occur if the alkyl group of the ester is an isoalkyl group preferably an isopropyl group.

The invention therefore comprises a process of resolution of chiral amines which comprises selectively reacting one enantiomer of the amine with an alkyl ester in the presence of a enantioselective lipase enzyme to produce an amide of one enantiomer and separating it from an unreacted enantiomer optionally after further reaction charactersed in that the alkyl group of the ester is an isoalkyl group and preferably an isopropyl group. A lipase is an enzyme capable of catalysing the esterification of aliphatic adds with glycerol and the hydrolysis of esters of glycerol and aliphatic adds.

Either or both enantiomers may be recovered. The untreated enantiomer may be recovered as such. The reacted enantiomer may be converted to the original amine enantiomer suitably by hydrolysis. It may of course be utilised as the amide if desired. Suitably such hydrolysis may be carried out using as catalyst an amidase of the same stereospecificity and/or hydrolysing any unwanted stereoisomer present using an amidase of opposite stereospecificity, separating the unwanted amide and hydrolysing that, thus providing a second stage of resolution and enhancing the enantiomeric excess of the product, but if the first stage provides sufficient specificity a non selective hydrolysis may be employed.

The acid component of the ester may have 1 to 10 for example 1 to 5 carbon atoms. It is preferably of formula RCOOH in which R is a hydrocarbyl group, for example an aryl group such as a phenyl, naphthyl or benzyl group, an alkyl or cydoalkyl group or a chloroor bromo substituted derivative thereof, the substitution being preferably on a carbon atom adjacent to the C═O group or one next to it It may suitably be an unsubstituted alkyl group suitably having 1 to 4 carbon atoms as these are often of moderate cost tend not to be involved in unwanted side reactions and tend not to be aggressive to metal reaction vessels.

The process is suitable for the resolution of primary and secondary amines for example amines of formula

in which R 1 and R 2 are alkyl, cycloalkyl, alkenyl or alkynyl, or an aryl group or such a group, which is substituted with for example NO 2 , SO 3 H, COOR 4 , Cl, Br, F, I, OH, SO, SO 2 , CN, alkoxy and in the case of aryl substitution NH 2 in which R 1 and R 2 are different and R 3 is H, alkyl, cycloalkyl, alkenyl, alkynyl, or an aryl group or such a group which is substituted with for example, NO 2 , SO 3 H, COOH, Cl, Br, F, I, OH, SO, SO 2 , CN and R 4 is alkyl, cycloalkyl, alkenyl, alkynyl or an aryl group optionally substituted as described above. The process is suitable for the resolution of amino acids and their esters.

The amine preferably has the formula

in which R 4 is an alkyl group having for example1 to 12 and preferably 1 to 6 carbon atoms and R 5 is an aryl preferably a naphthyl group, an alkyl group or cycloalkyl group in each case optionally substituted by one or more alkoxy, hydroxy, halogen and/or —CN group or in the case of aryl groups, amine groups, groups which preferably have at most 12 and more preferably at most 6 carbon atoms in total in all of the said substituents.

The amount of lipase present is preferably 10 to 50% by weight of the amine. The lipase is preferably supported on a solid support to enable it to be removed mechanically, for example by filtration or centrifugation, after reaction.

Separation of the amide of the reacted amine from unreacted amine may be accomplished by known methods, for example, distillation or crystallisation.

The reaction may be carried out in the presence of a solvent which may be the ester, an ether (for example methyl tert butyl ether, dimethoxy ethane or tetrahydrofuran) or a hydrocarbon, for example toluene or an alkane or cycloalkane having 5 to 10 carbon atoms or a halogenated hydrocarbon solvent. It is preferably free from —OH and NH 2 groups.

The reaction may be carried out at 20-60° C. for example at 20-40° C. At least one mole of the ester should be provided per two moles of the amine so as to permit stoichiometric reaction of an enantiomer, but it is preferred that an excess be provided. The excess should normally be sufficient to provide preferably at least 90% and more preferably at least 95% for example 99% reaction of the most reactive enantiomer. In judging the appropriate excess, conditions should not be such as to cause unacceptable conversion of the less reactive enantiomer, and if a very high selectivity for the more reactive enantiomer is needed it may be preferred to convert only part thereof, thus requiring little or no excess; indeed operation with less than the stoichiometrc amount may be desirable in some cases.

The step of converting the amine to the amide is preferably carried out in the substantial absence of water and other hydroxy compounds.

EXAMPLE 1

2-Amino-3, 3-dimethylbutane (125 mg) was added to 3 ml of acyl donor (e.g. ethyl acetate or isopropyl acetate) and incubated at 28° C. in the presence of 60 mg of immobilised lipase from Candida antarctica (NOVO SP 435). The extent of reaction was followed by quantitative gas chromatography on a Perkin Elmer 8500 system fitted with a J&W 30 m×0.32 mm fused silica capilliary GC The gas chromatograph was operated with helium carrier gas at 5.5×104 Pa (8 psi) using a temperature gradient starting at 80° C. and rising to 200° C. at a rate of 20° min −1 followed by 6 minutes held at 200° C. Under such conditions 2-amino-3, 3-dimethylbutane was eluted with a retention time of 3.0 minutes and the corresponding acetamide at 6.1 minutes. The enantiomeric purity of unreacted 2-amino-3, dimethylbutane (derivatised as the N-butyramide) and the acetamide product of the aminolysis reaction was determined by chiral phase gas chromatography using a 25 m×0.32 mm Chrompack CP-Chirasil-Dex CB column. The gas chromatograph was operated with a helium carrier gas at 5.5×104 Pa (8 psi) and at 20° C. Under such conditions the two enantiomers of the acetamide derivative of 2-amino-3, 3-dimethylbutane were eluted with retention times of 4.78 minutes (S) and 4.95 minutes (R) and the two enantiomers of the butyramide derivative 8.38 minutes (S) and 8.51 minutes (R). The results are summarised in Table 1.

TABLE 1
Effect of structural changes to the alcohol component of the acyl donor
on the enantioselectivity of the resolution of 2-amino-3,3-dimethylbutane
by Candida antarctica lipase.
			E.e.
			unreacted
	Reac-		amine (as	E.e.
Acyl	tion	Conversion	butyramide)	product	Enantiomeric
donor	time (h)	(%)	(%)	(%)	ratio (E)
Ethyl	216	39.2	50	78	13
acetate
Isopropyl	216	46.5	83	95	104
acetate

EXAMPLE 2

1-(1-Naphthyl) ethylamine (100 mg) was added to 5 ml of acyl donor e.g. ethyl acetate or isopropyl acetate) and incubated at 28° C. in the presence of 50 mg of immobilised Candida antarctica lipase (NOVO SP 435). The extent of conversion was determined from measurements of the enantiomeric excess of the unreacted amine (derivatised as its butyramide) and product acetamide using the mathematical expression described by Chen et. al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). Enantiomeric excess measurements were made by chiral phase HPLC on a Hewlett Packard HP 1050 system fitted with a 250 mm×4.6 mm Daicel Chiralcel OD column. The column was eluted isocratically with a mixture of 92.5% hexane and 7.5% ethanol in 1 ml min −1 . Compounds were detected by UV absorbance at 254 mm. The retention times of the unreacted amine (derivatised as its butyramide) were 7.65 minutes (R) and 15.2 minutes (S) and the product acetamide 8.9 minutes (R) and 17.9 minutes (S). The results are summarised in Table 2.

TABLE 2
Effect of structural changes to the alcohol component of the acyl donor
on the enantioselectivity of the resolution of 1-(1-naphthyl) ethylamine
by Candida antarctica lipase.
			E.e.
			unreacted
	Reac-		amine (as	E.e.
Acyl	tion	Conversion	butyramide)	product	Enantiomeric
donor	time (h)	(%)	(%)	(%)	ratio (E)
Ethyl	70	52.5	66	60	8
acetate
Isopropyl	66	48.9	88	92	70
acetate

EXAMPLE 3

Racemic 1-(1-naphthyl)ethylamine (100 mg) was added to 5 ml of acyl donor (see Table 3) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC on a Hewlett Packard HP1050 system using a Daicel Chiralcel OD analytical column (250 mm×4.6 mm) eluted with hexanelethanol (92.5:7.5) at a flow rate of 1 ml min −1 . Compounds were detected by UV absorbance at 254nm. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 7.9 minutes (R) and 15.0 minutes (S) and the product acetamide 9.3 minutes (R) and 17.9 minutes (S). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et. al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 3.

TABLE 3
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine
by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	120	30	6	13	1
acetate
Ethyl	120	43	59	79	15
acetate
n-Propyl	120	40	54	81	16
acetate
n-Butyl	120	32	40	77	18
acetate
Isopropyl	72	45	78	95	100
acetate
Isobutyl	72	50	86	88	37
acetate
Isoamyl	72	44	68	88	28
acetate

EXAMPLE 4

Racemic 1,2,3,4-tetrahydro1-naphthylamine (100 mg) was added to 5 ml of acyl donor (see Table 4) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC on a Perkin Elmer 8700 system using a Chrompack CP-Chirasil-Dex CB column (25 m×0.32 mm). The gas chromatograph was operated isothermally at 175° C. with helium carrier gas at 5.5×104 Pa (8 psi) Compounds were detected by flame ionisation. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 23.8 minutes (S) and 25.8 minutes (R) and the product acetamide 14.4 minutes (S) and 15.9 minutes (R). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et. al (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 4.

TABLE 4
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 1,2,3,4-tetrahydro-
1-naphthylamine by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	72	57	16	12	1
acetate
Ethyl	72	54	82	69	14
acetate
n-Propyl	72	52	78	74	14
acetate
n-Butyl	72	26	14	42	3
acetate
Isopropyl	72	50	98	97	458
acetate
Isobutyl	72	53	95	83	43
acetate
Isoamyl	72	46	70	81	21
acetate

EXAMPLE 5

Racemic 2-amino3,3-dimethylbutane (100 mg) was added to 5 ml of acyl donor (see Table 5) and incubated at ambient temperature in the presence of 40 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC on a Perkin Elmer 8700 system using a Chrompack CP-Chirasil-Dex CB column (25 m×0.32 mm). The chromatograph was operated isothermally at 120° C. with helium carrier gas at 5.5×104 Pa (8 psi). Compounds were detected by flame ionisation. The retention times of the two enantiomers of the unreacted amine (derivatised as its butyramide) were 10.3 minutes (S) and 10.5 minutes (R) and the product acetamide 5.6 minutes (S) and 5.9 minutes (R). The extent of conversion and enantiomeric ratio was determined from measurements of the enantiomeric excess of unreacted amine and product acetamide using the mathematical expression described by Chen et al. (J. Am. Chem. Soc., 1982, Vol 104, pp 7294-7299). The results are summarised in Table 5.

TABLE 5
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 2-amino-3,3-dimethylbutane
by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	168	29	2	5	1
acetate
Ethyl	168	34	36	69	8
acetate
n-Propyl	168	26	24	69	7
acetate
n-Butyl	168	17	13	62	5
acetate
Isopropyl	168	41	67	98	109
acetate
Isobutyl	168	33	44	88	27
acetate
Isoamyl	168	25	26	79	10
acetate

EXAMPLE 6

Racemic 1-(1-naphthyoethylamine (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 6) and incubated at ambient temperature in the presence of 20 mg of Chirazyme L2 (immobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC as described in Example 3. The results are summarised in Table 6.

TABLE 6
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine
in dimethoxyethane by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	168	26	276	77	10
acetate
Ethyl	168	37	53	91	33
acetate
n-Propyl	168	34	47	92	35
acetate
n-Butyl	168	36	52	94	43
acetate
n-amyl	168	22	26	93	32
acetate
Isopropyl	168	44	78	>98	650
acetate
Isobutyl	168	40	64	95	95
acetate
Isoamyl	168	35	51	94	61
acetate

EXAMPLE 7

Racemic 1,2,3,4-tetrahydro-1-naphthylamine (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 7) and incubated at ambient temperature in the presence of 20 mg of Chirayme L2 (ummobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 4. The results are summarised in Table 7.

TABLE 7
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 1,2,3,4-tetrahydro-
1-naphthylamine in dimethoxyethane by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	120	41	49	69	9
acetate
Ethyl	120	47	85	95	129
acetate
n-Propyl	120	47	83	94	79
acetate
n-Butyl	120	42	68	95	65
acetate
n-Amyl	120	34	46	90	28
acetate
Isopropyl	120	50	96	98	194
acetate
Isobutyl	120	50	97	96	278
acetate
Isoamyl	120	42	69	96	86
acetate

EXAMPLE 8

Racemic 2-amino-3,3-dimethylbutane (100 mg) was added to a solution of 4 ml of dimethoxyethane and 1 ml of acyl donor (see Table 8) and incubated at ambient temperature in the presence of 40 mg of Chirazyme L2 Cimmobilised Candida antarctica lipase). At intervals 0.5 ml samples were removed and diluted to 1 ml with dichloromethane. The unreacted amine was converted to its corresponding butyramide by the addition of 10 μl of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 5. The results are summansed in Table 8.

TABLE 8
Effect of structural changes to the alcohol component of the acyl donor
on the enantiospecificity of the resolution of 2-amino-3,3-dimethylbutane
in dimethoxyethane by Chirazyme L2 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Methyl	336	22	17	59	5
acetate
Ethyl	336	29	37	90	29
acetate
n-Propyl	336	27	34	91	33
acetate
n-Butyl	336	25	30	91	26
acetate
n-Amyl	336	17	18	86	19
acetate
Isopropyl	336	36	56	>98	791
acetate
Isobutyl	336	30	41	96	68
acetate
Isoamyl	336	27	35	94	51
acetate

EXAMPLE 9

Racemic 1-(1-naphthyoethylamine (100 mg) was added to 5 ml of acyl donor (see Table 9) and incubated at ambient temperature in the presence of 50 mg of Chirazyme L6 (Pseudomonas species lipase). At intervals 0.2 ml samples were removed and diluted to 1 ml with a solution of hexanelethanol (92.5:7.5). The unreacted amine was converted to its corresponding butyramide by the addition of 4 μl of butyric anhydride. Each sample was analysed by chiral phase HPLC as described in Example 3. The results are summarsed in Table 9.

TABLE 9
Effect of structural changes to the alcohol component of the acyl donor
on the Enantiospecificity of the resolution of 1-(1-naphthyl)ethylamine by
Chirazyme L6 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Ethyl	266	21	12	47	3
acetate
Isopropyl	266	12	12	87	18
acetate

EXAMPLE 10

Racemic 1,2,3,4-tetrahydro-1-naphthylamine (100 mg) was added to 5 ml of acyl donor (see Table 10) and incubated at ambient temperature in the presence of 50 mg of Chirazyme L6 (Pseudomonas species lipase). At intervals 0.2 ml samples were removed and diluted to 1 μl with MTBE. The unreacted amine was converted to its corresponding butyramide by the addition of 6 ml of butyric anhydride. Each sample was analysed by chiral phase GC as described in Example 4. The results are summarised in Table 10.

TABLE 10
Effect of structural changes to the alcohol component of the acyl donor
on the Enantiospecificity of the resolution of 1,2,3,4-tetrahydro-
1-naphthylamine by Chirazyme L6 lipase.
				E.e. of
			E.e. of	product
	Time	Conversion	unreacted	acetamide	Enantiomeric
Acyl donor	(h)	(%)	amine (%)	(%)	ratio (E)
Ethyl	244	43	18	24	2
acetate
Isopropyl	244	36	50	90	28
acetate
Chirazyme is a trade mark of Boehringer Mannheim GmbH
Chiralcel is a trade mark of Daicel Chemical Industries Limited

Claims

1. A process of resolution of chiral amines which comprises selectively reacting an enantiomer of the amine with an alkyl ester in the presence of a enantioselective lipase enzyme to produce an amide of that enantiomer and separating it from an unreacted enantiomer optionally after further reaction characterised in that the acid component of the ester has 1 to 10 carbon atoms and the parent acid is of formula RCOOH in which R is a hydrocarbyl group and the alkyl group of the ester is an isoalkyl group.

2. A process as claimed in claim 1 in which the isoalkyl group is an isobutyl or isopropyl group.

3. A process as claimed in claim 1 or 2 in which the unreacted enantiomer is recovered as such.

4. A process as claimed in claims 1 or 2 in which the reacted enantiomer is converted to the original amine enantiomer by hydrolysis.

5. A process as claimed in claim 1 or 2 in which the reacted enantiomer is utilised in its amide form.

6. A process as claimed in claim 1 or claim 2 in which the hydrocarbyl group R is an unsubstituted alkyl group having 1 to 4 carbon atoms.

7. A process as claimed in claim 1 or claim 2 in which 10 to 50% by weight of lipase is present based on the amine.

8. A process as claimed in claim 1 or claim 2 in which the lipase is supported on a solid support.

9. A process as claimed in claim 1 or claim 2 which is carried out in the presence of a solvent which is an ester, ether or hydrocarbon or a halogenated hydrocarbon which is free from OH and NH2 groups.

10. A process as claimed in claim 1 or claim 2 which is carried out at a temperature of 20 to 60° C.

11. A process as claimed in claim 1 or claim 2 in which the amine has the formula in which R1 and R 2 are, independently, an alkyl, cycloalkyl, alkenyl, alkynyl, or aryl group, said group being unsubstituted or substituted with a substituent selected from the group consisting of NO2, SO3H, COOR4, Cl, Br, F, I, OH, SO, SO2, CN, alkoxy and, in the case of aryl groups, NH2 in which R1 and R2are different;R3 is H, an alkyl, cycloalkyl, alkenyl, alkynyl, or aryl group, said group being unsubstituted or substituted with a substituent selected from the group consisting of NO2, SO3H, COOR4, Cl, Br, F, I, OH, SO, SO2, CN and alkoxy; and R4 is an alkyl, cycloalkyl, alkenyl, alkynyl or aryl group optionally substituted by one or more NO2, SO3H, COOR3, Cl, Br, F, I, OH, SO, SO2, CN or alkoxy groups.

12. A process as claimed in claim 11 in which the amine has the formula in which R4 is an alkyl group having from 1 to 12 carbon atoms and R5 is an aryl, alkyl or cycloalkyl group, R5 being optionally substituted by one or more substituents selected from the group consisting of alkoxy, hydroxy, halogen, cyano, and when R5 is aryl, amine groups.

13. A process according to claim 12 in which R4 is an alkyl group having from 1 to 6 carbon atoms.

14. A process according to claim 12 in which R5 is either unsubstituted or substituted by substituents having at most 6 carbons in total in all of the said substituents.

15. A process according to claim 12 in which the hydrocarbyl group R is an unsubstituted alkyl group having 1 to 4 carbon atoms.

16. A process according to claim 15 in which R5 is unsubstituted.

17. A process according to claim 16 in which R4 is an alkyl group having from 1 to 6 carbon atoms.