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Starting from achiral closo-carborane and using natural amino acid as chiral auxiliary, individual (RP)- and (SP)-enantiomers of C-monosubstituted nido-carboranes were obtained.

(R)-2-Phenoxypropanoate ester was efficient chiral resolving agent in acylative KR of racemic cyclic alkylamines; highest selectivity was observed for 2-methylpiperidine with predominant formation of (R,R)-amide, which was confirmed by DFT modelling.

A comparative study of the kinetic resolution (KR) of racemic 6-substituted 2-methyl-1,2,3,4-tetrahydroquinolines with acyl chlorides of (S)-naproxen, N-phthaloyl-(S)-leucine, and (R)-O-phenyllactic acid was carried out. The selectivity factors in the KR of racemic amines with acyl chlorides of (S)-naproxen and (R)-O-phenyllactic acid were shown to be approximately the same and higher than those for the KR of N-phthaloyl-(S)-leucyl chloride. The reasons for the stereodifferentiation in the KR of racemic tetrahydroquinaldines containing groups of different electronic properties by acyl chlorides of three different chiral acids were explained using the DFT method. The conditions for stabilizing π—π interactions of aromatic fragments of the reagents, which do not occur in the same form in the transition state and lead to the minor diastereoisomeric product, are created in the transition state of the faster acylation reaction with (S)-naproxen and (R)-O-phenyllactic acyl chlorides. In the case of the KR of 2-methyl-1,2,3,4-tetrahydroquinoline and 2-methyl-6-methoxy-1,2,3,4-tetrahydroquinoline with N-phthaloyl-(S)-leucyl chloride, the acylation diastereoselectivity is determined, most likely, by conformational factors. The individual (S)-enantiomer of 2-methyl-6-methoxy-1,2,3,4-tetrahydroquinoline of high optical purity was synthesized using the KR of the racemate with (S)-naproxen acyl chloride.

The possibility for separating enantiomers of a number of practically significant 2-aryloxycarboxylic acids was studied by normal- and reversed-phase HPLC on popular chiral stationary phases. The best separation parameters were achieved on the chiral phases with the polysaccharide base Chiralcel OD-H and Chiralpack AD under the normal-phase HPLC conditions. The (S)- and (R)-enantiomers of 2-(1-naphthyloxy)- and 2-(2-iodophenoxy)propionic acids with enantiomeric excess ee >99% were isolated using preparative chiral HPLC.

A method for the synthesis of novel purine conjugates with 7,8-difluoro-3-methyl-3,4-dihydro-2H-1,4-benzoxazine containing fragments of ω-amino acids with different lengths of the polymethylene chain as a linker has been developed. It was found in experiments in vitro that the obtained compounds are active against the herpes simplex virus type 1, including the acyclovir-resistant strain.

A number of novel amides were synthesized by coupling of 6-[(9H-purin-6-yl)amino]hexanoic acid to heterocyclic amines. The antiviral activity of the obtained compounds, as well as of purine conjugates in which 7,8-difluoro-3-methyl-3,4-dihydro-2H-1,4-benzoxazine is linked to position 6 of purine through a fragment of ω-amino acids with varying lengths of polymethylene chains against influenza A and B viruses was studied in vitro. Purine derivatives have been shown to have moderate activity against influenza A (H1N1) virus. The antiinfluenza activity and cytotoxicity of conjugates with 7,8-difluoro-3-methyl-3,4-dihydro-2H-1,4-benzoxazine depend on the length of the linker fragment.

The deboronation of enantiomers of planar-chiral benzyl (3-formamido-1,2-dicarba-closo-dodecaboran-1-yl)acetate gave the individual (RP)- and (SP)-enantiomers (ee > 98%) of a new amino acid containing a nido-carborane fragment, (3-ammonio-7,8-dicarba-nido-undecaboran-7-yl)acetic acid. Chiral HPLC methods for the analysis of enantiomeric purity of this compound was developed.

1-Substituted (2S,4S)-4-amino-5-oxoprolines have been obtained by nucleophilic substitution of bromine in dimethyl (2S,4RS)-4-bromo-N-phthaloylglutamate followed by isolation of the predominant (2S,4S)-diastereomer and removal of protecting groups. The psychotropic activity of the obtained compounds has been studied after a single administration in experimental animals. 4-Amino-1-(4-bromophenyl)-5-oxoproline and especially 4-amino-1-(4-aminophenyl)-5-oxoproline showed pronounced anxiolytic activity in the Elevated plus maze test. (2S,4S)-4-Amino-5-oxoprolines containing 4-methylphenyl, 4-bromophenyl, 4‑aminophenyl, and 2,3-dimethyl-5-oxo-1-phenyl-3-pyrazolin-4-yl substituents at the 1-position exhibited nootropic activity: they improved the formation and retention of the memory trace in the Passive avoidance test and the Active avoidance test (Extrapolation escape test).

A series of pyrimidine conjugates containing a fragment of racemic 7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine and its (S)-enantiomer attached via a 6-aminohexanoyl fragment were synthesized by the reaction of nucleophilic substitution of chlorine in various chloropyrimidines. The structures of the synthesized compounds were confirmed by1H,19F, and 13 C NMR spectral data. Enantiomeric purity of optically active derivatives was confirmed by chiral HPLC. Antiviral evaluation of the synthesized compounds has shown that the replacement of purine with a pyrimidine fragment leads to a decrease in the anti-herpesvirus activity compared to the lead compound, purine conjugate. The studied compounds did not exhibit significant activity against influenza A (H1N1) virus. © 2022 by the authors. Licensee MDPI, Basel, Switzerland. Russian Science Foundation, RSF: 19-13-00231-P Funding: This research was funded by the Russian Science Foundation, grant number 19-13-00231-P.

Recent publications on the key preparation methods of the enantiomers of 2-aryloxy carboxylic acids are summarized and comparative analysis of the methods is given. The information is arranged according to the type of the starting compound, being classified into syntheses from enantiomerically pure chiral precursors and syntheses from prochiral precursors, which imply generation of an asymmetric centre in the substrate molecule. Data on the chemical resolution of racemic mixtures of the title compounds are addressed in a separate Section. Attention is focused on the preparation of practically valuable 2-aryloxy acids. Examples of biologically active derivatives of 2-aryloxy carboxylic acids are given. The bibliography includes 121 references.

New conjugates of purine and 2-aminopurine with several α- and ω-amino acids have been synthesized following two approaches based on the condensation and nucleophilic substitution reactions. The enantiomeric purity of the isolated compounds has been confirmed by reversed-phase HPLC using a chiral stationary phase to demonstrate the absence of racemization during the synthesis. The conjugates are inactive against Mycobacterium tuberculosis H37Rv.

A comparative study of the kinetic resolution of racemic derivatives of 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine using racemic 2-aryloxypropionyl chlorides was performed. It was found that the acylation of racemic amines with racemic 2-(1-naphthyloxy)propionyl chloride leads to amides enriched with (3R*,2′R*)-diastereomers, while the acylation with 2-phenoxypropionyl chloride gives predominantly (3R*,2′S*)-amides. Quantum chemical modeling of the process of kinetic resolution at the COSMO-CH2Cl2-B3LYP-D3-gCP/def2-TZVP//B3LYP-D3-gCP/def2-SVP level of theory was performed. The computational results are in a good agreement with the experimental data.

A comparative study of the kinetic resolution of racemic 3-tert-butyl-3,4-dihydro-2H-[1,4]-benzoxazine through the acylation by various chiral acyl chlorides was carried out. A preparative method for obtaining enantiomerically pure (S)-3-tert-butyl-3,4-dihydro-2H-[1,4]benzoxazine can be based on the acylation with N-phthaloyl-(S)-phenylalanyl or (R)-2-phenoxypropionyl chlorides, followed by the acid hydrolysis of the corresponding diastereomerically pure amides.

The enantiomerically pure N- and C-protected derivatives of planar-chiral (3-amino-1,2-dicarba-closo-dodecaboran-1-yl)acetic acid (>99% ee) have first been obtained in good yields via a combination of HPLC separation and synthetic procedures. Assignment of absolute configuration of the carborane fragments has been made based on the X-ray diffraction data of diastereoisomeric derivatives of (S)-amino acids.

The synthesis of purine conjugates with natural amino acids is one of the promising directions in search for novel therapeutic agents, including antimycobacterial agents. The purpose of this study was to synthesize N-(purin-6-yl)dipeptides containing the terminal fragment of (S)-glutamic acid. To obtain the target compounds, two synthetic routes were tested. The first of them is based on coupling of N-(purin-6-yl)-(S)-amino acids to dimethyl (S)-glutamate in the presence of carbodiimide coupling agent followed by the removal of ester groups. However, it turned out that this coupling process was accompanied by racemization of the chiral center of N-(purin-6-yl)-α-amino acids and in all cases led to mixtures of (S,S)- and (R,S)-diastereomers (6:4). Individual (S,S)-diastereomers were obtained using an alternative approach based on the nucleophilic substitution of chlorine in 6-chloropurine or 2-amino-6-chloropurine with corresponding dipeptides as nucleophiles. The enantiomeric purity of the target compounds was confirmed by chiral HPLC. To test the assumption that racemization of the chiral center of N-(purin-6-yl)-α-amino acids occurs with the participation of nitrogen atoms of the imidazole ring via the stage of formation of a chirally labile intermediate, we obtained such structural analogs of N-(purin-6-yl)-(S)-alanine as N-(9-benzylpurin-6-yl)-(S)-alanine and N-(7-deazapurin-6-yl)-(S)-alanine. It was found that coupling of these compounds to dimethyl (S)-glutamate was also accompanied by racemization. This indicates that the imidazole fragment does not play a crucial role in this process. When testing the antimycobacterial activity of some of the obtained compounds, conjugates with moderate activity against the laboratory Mycobacterium tuberculosis H37Rv strain (MIC 3.1–6.25 μg/mL) were identified. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, AT part of Springer Nature. The work was financially supported by the Russian Science Foundation (Grant 19-13-00231). We are grateful to Dr. Marionella A. Kravchenko [Ural Research Institute of Phthisiopulmonology (National Medical Research Center, Ministry of Healthcare of the Russian Federation, Ekaterinburg, Russia)] for conducting biological trials. Analytical studies were carried out using equipment of the Centre for Joint Use ?Spectroscopy and Analysis of Organic Compounds? at the Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences.

Using purine as a scaffold, the methods for preparation of novel 2-aminopurine and purine derivatives substituted at position C6 by the fragments of natural amino acids, short peptides, andN-heterocycles, including enantiopure ones, have been proposed. The methods for determination of the enantiomeric purity of the obtained chiral compounds have been developed. Conjugates exhibiting high antimycobacterial or antiherpesvirus activity against both laboratory and multidrug-resistant strains were revealed among the obtained compounds. © 2020 De Gruyter. All rights reserved. Russian Science Foundation, RSF: 19-13-00231 Funding : The reviewed studies were financially supported by the Russian Science Foundation (project no. 19-13-00231).

Stereoselectivity in the mutual kinetic resolution (KR) of racemic 3,4-dihydro-3-methyl-2H-[1,4]benzoxazines and racemic 2-aryloxy propionyl chlorides was studied. Based on the results obtained, preparative methods for single enantiomers of a series of 2-aryloxy propionic acids via acylative KR of their racemates with enantiopure (S)-3,4-dihydro-3-methyl-2H-[1,4]benzoxazines have been proposed. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The work was financially supported by the Russian Science Foundation (grant 19‐13‐00231, in the part of studying mutual kinetic resolution) and the Russian Foundation for Basic Research (grant 18‐33‐00027_mol_a, in the part of preparation of enantiomers of 2‐aryloxy propionic acids). Analytical studies were carried out using equipment of the Centre for Joint Use “Spectroscopy and Analysis of Organic Compounds” at the Postovsky Institute of Organic Synthesis, Russian Academy of Sciences (Ural Branch)

New (S)-homocysteine derivatives containing a meta-carborane fragment were synthesized. m-Carboranyl-(S)-homocysteine sulfoxide was obtained as a mixture of diastereoisomers. The reduction of the side-chain carboxy group of N-tert-butoxycarbonyl-(S)-aspartic acid α-tert-butyl ester with sodium tetrahydridoborate was not accompanied by racemization.

Reversed-phase HPLC on chiral stationary phases was used to study the influence of the structure of purine and 2-aminopurine conjugates with chiral heterocyclic amines on the retention times. The optimum conditions for separation of enantiomers were found. The best parameters of separation of stereoisomers of all the compounds examined were achieved on a (S,S)-Whelk-O1 column in weakly alkaline aqueous methanol systems.

The review summarizes the data published over the last 10 – 15 years concerning the key groups of purine derivatives with antituberculosis activity. The structures of purines containing heteroatoms (S, O, N), fragments of heterocycles, amino acids and peptides, in the 6-position, as well as of purine nucleosides are presented. The possible targets for the action of such compounds and structure – activity relationship are discussed. Particular attention is paid to the most active compounds, which are of considerable interest as a basis for the development of efficient antituberculosis drugs. The bibliography includes 99 references.

Stereoselective acylation of racemic 3-methyl-3,4-dihydro-2Н-1,4-benzoxazine and its 7,8-difluoro-substituted analog with racemic 2-alkoxyacyl chlorides was stidied. The reactions of 3-methyl-3,4-dihydro-2H-1,4-benzoxazines with 2-methoxyisopentanoyl chloride were found to be more selective (selectivity factor s 31–32) compared to the acylation with other studied propanoyl chlorides (s 18–21). This fact was probably caused by the significant steric hindrance due to the isopropyl substituent in acyl chloride compared to the methyl group in reagents derived from propanoic acid.

A series of novel (S)-amino acid derivatives bearing planar-chiral (3-acylamino-1,2-dicarba-closo-dodecaboran-1-yl)acetyl substituents was synthesized. Carboranyl amino esters were obtained in a diastereomerically pure form (according to chiral HPLC). Single crystals of individual diastereomers were studied by optical microscopy, X-ray diffraction analysis and piezoresponse force microscopy. The piezoelectric activity of some compounds was shown to exceed that of most known organic piezoelectrics.

Novel purin-6-yl, 2-aminopurin-6-yl and N-(purin-6-yl)glycyl derivatives of (S)-lysine and (S)-ornithine with free α-functional groups, as well as compounds containing N9-(2-hydroxy-ethoxy)methyl substituent in the purine core have been synthesized and tested for their inhibitory effect against Mycobacterium tuberculosis strains. Among the synthesized conjugates, one compound exhibiting a significant level of antimycobacterial activity has been found.

Studies of the antimycobacterial activity of 2-amino-6-chloropurine and its N-acyl derivatives against laboratory (Mycobacterium tuberculosis H37Rv, M. avium, M. terrae) and clinical Mycobacterium tuberculosis strains (MDR-TB) identified compounds with high tuberculostatic activity (MIC from 0.35 – 1.5 μg/ml). The cytotoxic activities of the compounds with antimycobacterial activity against human embryo fibroblasts were studied in the MTT test, which showed that the study compounds were essentially nontoxic (IC50 > 50 μM). Further chemical modification may yield compounds with potential for creating drugs for the treatment of tuberculosis. With the aim of identifying the possible mechanism of the tuberculostatic activity, the ability of the study compounds to inhibit mycobacterial serine-threonine protein kinases (STPK) was assessed. The tuberculostatic activity of 2-amino-6-chloropurine derivatives was not linked with their inhibition of STPK.

The acylative kinetic resolution of racemic heterocyclic amines such as 3,4-dihydro-3-methyl-2H-[1,4]benzoxazines, 3,4-dihydro-3-methyl-2H-[1,4]benzothiazine, 2-methyl-1,2,3,4-tetrahydro-quinolines and 2-methylindoline with enantiopure (R)-2-phenoxypropionyl chloride has been studied. It has been found that acylation of 3,4-dihydro-3-methyl-2H-[1,4]benzothiazine proceeds with the best stereoselectively when compared with other racemic amines. An efficient method for the preparation of (S)-3,4-dihydro-3-methyl-2H-[1,4]benzothiazine (99.4% ee) via a kinetic resolution protocol was developed. The possibility of recycling (R)-2-phenoxypropionic acid has been demonstrated.

Novel purine and 2-aminopurine conjugates containing fragments of (S)-aspartic and (S)-glutamic acids with free α-functional groups attached via a 1,2-ethylenediamine linker at 6-position of the purine core, as well as their analogues with N9-(2-hydroxyethoxy)methyl substituent have been synthesized. Among the conjugates obtained, compounds exhibiting a significant antimycobacterial activity have been found.

Derivatives of dicarba-closo-dodecaboranes and dicarba-nido-undecaboranes (carboranes) are of increasing interest for modern medicinal chemistry and materials science. The combination of carborane residues and amino acid fragments in one structure can serve as a basis for obtaining promising medicinal agents (primarily, for boron neutron capture therapy), as well as for design the materials with unique properties. For the first time, this review summarizes and analyzes information on the methods of preparation and possible applications of compounds combining in their structure amino acid and peptide residues and carborane clusters, their biological and physicochemical properties. Special attention is paid to the preparation of enantiopure conjugates, the chirality of which is due to both the presence of an asymmetric carbon atom in amino acid fragments and the planar chirality of the carborane cluster. We believe that this review provides comprehensive information about carborane-containing amino acids and peptides and will give a new impetus to the in-depth fundamental research in this field.

Objective: Evaluation of antitumor activity of a novel alkylnitrosoureas derivative Ormustine (an alkylnitrosocarbamoyl L-ornithine) in mouse lymphoid leukemia models. Materials and methods Antitumor activity of Ormustine has been evaluated in B6D2F1 mice with ascites form of leukemia (L1210, L1210/arenosa, L1210/citrullin and P388) and the solid (P388) form. In this study we used preparations from the alkylnitrosourea group: Ormustine, Aranoza and Lizomustine. Treatment of animals was started 24 hours after inoculation of leukemia intraperitoneally, and 48 hours after inoculation subcutaneous P388. Drugs in a wide range of doses were administrated once intravenously. The follow up period of the animals continued until their death. Criteria of antitumor effect were increasing life expectancy and cure. Evaluation criteria of antitumor effect was the increase in life of experimental mice compared to control ones. Results. Antitumor activity of a novel alkylnitrosoureas derivative, Ormustine has been studied in vivo on the growth of transplanted lymphoid leukemia, such as L1210 (ascites version) and P388 (ascites and solid tumor). Effective dose of single intravenous injection Ormustine against lymphoid leukemia L1210 and P388 was 125 mg/kg. The drug effectively inhibited growth of experimental leukemia. The significant part of the mice with limfoleukemia has been cured. We have also established the single intravenous therapeutic dose of Ormustine on L1210 and Р388 leukemia - 125mg/kg of body weight. Conclusion. The data obtained characterizes Ormustin as a promissing anticancer drug.

A series of ribo- and deoxyribonucleosides bearing 2-aminopurine as a nucleobase with 7,8-difluoro- 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine (conjugated directly or through an aminohexanoyl spacer) was synthesized using an enzymatic transglycosylation reaction. Nucleosides 3-6 were resistant to deamination under action of adenosine deaminase (ADA) Escherichia coli and ADA from calf intestine. The antiviral activity of the modified nucleosides was evaluated against herpes simplex virus type 1 (HSV-1, strain L2). It has been shown that at sub-toxic concentrations, nucleoside (S)-4-[2-amino-9-(β-D-ribofuranosyl)-purin-6-yl]-7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine exhibit significant antiviral activity (SI > 32) on the model of HSV-1 in vitro, including an acyclovir-resistant virus strain (HSV-1, strain L2/R).

A series of novel N-(purin-6-yl)- and N-(2-aminopurin-6-yl) derivatives of heterocyclic amines was synthesized. It was found that preferred preparative method for N-(purin-6-yl) derivatives of heterocyclic amines is interaction of 6-chloropurine with heterocyclic amines without any catalyst; whereas N-(2-aminopurin-6-yl) derivatives were obtained in the moderate-to-high yields while heating 2-amino-6-chloropurine and heterocyclic amines in water in the presence of sulfuric acid.

In the search for novel efficient chiral resolving agents, we have studied the diastereoselective acylation of 3,4-dihydro-3-methyl-2 H -[1,4]benzoxazine and 7,8-difluoro-3,4-dihydro-3-methyl-2 H -[1,4]benzoxazine with acyl chlorides of 2-phenoxy carboxylic acids (2-phenoxypropionic, 2-phenoxyisovaleric, and 2-phenoxy-3-phenylpropionic) in toluene and dichloromethane at different temperatures. HPLC analysis of the acylation products showed the predominant formation of ( R ∗ , S ∗ )-diastereoisomeric amides in all cases. It was found that acylation with the studied acyl chlorides was highly stereoselective; 2-phenoxyisovaleryl chloride demonstrated the highest stereoselectivity (selectivity factor s up to 499). A model for the diastereoisomeric transition states in the interaction between racemic amine and acyl chloride is proposed.

It has been found that acyl chlorides of (S)-naproxen, N-phthaloyl-(S)-leucine, and N-tosyl-(S)-proline are efficient chiral resolving agents for the acylative kinetic resolution of racemic 3-methyl- and 3-phenyl-3,4-dihydro-3-methyl-2H-[1,4]benzothiazines. Based on the experimental results, a preparative protocol comprising the acylative kinetic resolution followed by isolation of a single (major) diastereoisomeric amide and subsequent hydrolysis of amide bond was proposed. Using this approach with various chiral resolving agents, (S)- and (R)-enantiomers of 3,4-dihydro-3-methyl-2H-[1,4]benzothiazine (ee >99%) were obtained. The oxidation of the corresponding diastereoisomers of the amides followed by deacylation led to the (S)- and (R)-enantiomers of 3,4-dihydro-3-methyl-2H-[1,4]benzothiazine-1,1-dioxide (ee >97%). This method proved to be less suitable for the preparation of the (S)-enantiomer of 3,4-dihydro-3-phenyl-2H-[1,4]benzothiazine (ee up to 93%) and its sulfone (ee 82%) which were both obtained in low yields. The loss of enantioselectivity for (S)-3,4-dihydro-3-phenyl-2H-[1,4]benzothiazine and its sulfone occurred during hydrolysis of the corresponding diastereoisomerically pure amides.

Chemoenzymatic transglycosylation of 2-aminopurine conjugates with enantiomerically pure 7,8-difluoro-3-methyl-3,4-dihydro-2H-[1,4]benzoxazines under the action of recombinant E. coli purine nucleoside phosphorylase afforded the corresponding arabinofuranosides, yields of the target compounds being dependent on both the structure and configuration of the fragment of heterocyclic amine attached at C-6 position of purine moiety.

Novel conjugates of purine and 2-aminopurine linked with heterocyclic amines, including chiral derivatives of 3,4-dihydro-2 H -[1,4]benzoxazine, 3,4-dihydro-2 H -[1,4]benzothiazine and 1,2,3,4-tetrahydroquinoline, by 6-aminohexanoyl fragment at the 6-position of purine moiety were obtained. For this purpose, replacement of the chlorine atom in 2-amino-6-chloropurine or 6-chloropurine by direct nucleophilic substitution reaction with 6-aminohexanamides or the coupling of 6-(purin-6-ylamino)-6-hexanoic acid with nitrogen heterocycles were used.

A comparative study on the kinetic resolution of racemic amines (2,3-dihydro-4H-1,4-benzoxazine and 1,2,3,4-tetrahydroquinoline derivatives) via diastereoselective acylation with N-tosyl-(S)-prolyl chloride and its structural analogs was performed. The effect of resolving agent structure on the stereoselectivity of heterocyclic amine acylation was examined. The highest stereoselectivity was achieved in the case of acylation with acyl chlorides bearing a conformationally restricted pyrrolidine ring and an aromatic substituent in the protecting group at the nitrogen atom.

The kinetic resolution of racemic heterocyclic amines via acylation with N-tosyl-(S)-prolyl chloride was systematically investigated. It was established that racemic mixtures of aromatic amines could be resolved with high efficiency, while the acylation of 2- and 3-methylpiperidines occurred with low diastereoselectivity. A method for the preparation of enantiomerically pure (3R)-7,8-difluoro-3-methyl-3,4-dihydro-2H-[1,4]benzoxazine was developed.

New strains of Rhodococcus erythropolis 25 and Microbacterium paraoxydans 20-11c actinobacteria were used to obtain (S)-7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine (99.6% ee) by enantioselective microbial hydrolysis of the corresponding racemic N-acetyl derivative.