Chemoenzymatic synthesis of both enantiomers of 3-hydroxy- and 3-amino-3-phenylpropanoic acid

Ethyl (S)-3-hydroxy-3-phenylpropionate (S)-2 was obtained by the asymmetric reduction of ethyl 3-phenyl-3-oxopropionate 1 with the yeast Saccharomyces cerevisiae (ATCC 9080). The kinetic resolution of racemic ethyl 2-acetoxy-3-phenyl-propionate rac-3 with the same microorganism, gave after hydrolysis ethyl (R)- and (S)-3-hydroxy-3-phenylpropionates (R)-2 and (S)-2 which were converted by a straightforward series of reactions to the enantiomers of 3-amino-3-phenyl-propionic acids (S)-6 and (R)-6. The asymmetric reduction and hydrolytic kinetic resolution were also tested with several other whole cell systems under a variety of conditions.     

Synthesis of optically active β′-hydroxy-β-enaminoketones via enzymatic resolution of carbinols derived from 3,5-disubstituted isoxazoles

The preparation of several enantiomerically pure β′-hydroxy-β-enaminoketones from the corresponding isoxazolic carbinols, which have been obtained by enzymatic kinetic resolution of the racemic β-hydroxyisoxazoles catalyzed by lipases, is described. The enzymatic transesterification of racemic (±)-5-(2-hydroxypropyl)-3-methylisoxazole 3a, and racemic (±)-5-(2-hydroxy-2-p-tolylethyl)-3-methylisoxazole 3d, has been studied with respect to the influence of experimental variables such as the used enzyme, the acylating agent or the solvent on the enantioselectivity of the reaction. After the reductive cleavage of the isoxazolic ring of the enantiopure carbinols, (R)- and (S)-2-amino-4-oxo-2-hepten-6-ol, (R)- and (S)-5, and (R)-2-amino-6-p-tolyl-4-oxo-2-hexen-6-ol, (R)-7 with an enantiomeric excess >98% were obtained.     

New chemical and chemo-enzymatic routes for the synthesis of (RS)- and (S)-enciprazine

The chemo-enzymatic synthesis of racemic and enantiopure (RS)- and (S)-enciprazine 1, a non-benzodiazepine anxiolytic drug, is described herein. The synthesis started from 1-(2-methoxyphenyl) piperazine 3, which was treated with 2-(chloromethyl) oxirane (RS)-4 using lithium bromide to afford a racemic alcohol, 1-chloro-3-(4-(2-methoxyphenyl) piperazin-1-yl) propan-2-ol (RS)-6 in 85% yield. Intermediate (S)-6 was synthesized from racemic alcohol (RS)-6 using Candida rugosa lipase (CRL) with vinyl acetate as the acyl donor. Various reaction parameters such as temperature, time, substrate, enzyme concentration, and the effect of the reaction medium on the conversion and enantiomeric excess for the transesterification of (RS)-6 by CRL were optimized. It was observed that 10 mM of (RS)-6, 50 mg/mL of CRL in 4.0 mL of toluene with vinyl acetate (5.4 mmol) as acyl donor at 30 °C gave good conversion (C = 49.4%) and enantiomeric excess (ee_P = 98.4% and ee_S = 96%) after 9 h of reaction. Compound (S)-6 is a key intermediate for the synthesis of enantiopure (S)-1. The (RS)- and (S)-enciprazine drug 1 was synthesized by treating (RS)- and (S)-6 with 3,4,5-trimethoxyphenol 5 using MeCN as a solvent and K₂CO₃ as a base.     

Synthesis of diarylheptanoids, (5S)-5-acetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-3-heptanone and (3S,5S)-3,5-diacetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)heptane

The total syntheses of the first examples of diarylheptanoid natural products (5S)-5-acetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-3-heptanone 1, and (3S,5S)-3,5-diacetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)heptane 2 isolated from the rhizomes of Zingiber officinale were accomplished using Sharpless epoxidation and cross-metathesis reactions as the key steps.     

Enamine chemistry—XIX: Preparation and alkylation of cyclic and non-cyclic enamino-thiones

Enamino-thiones of the general type ArC(S)CH:CHNR₂,3(Ar=phenyl p-methoxyphenyl and p-bromophenyl NR₂= pyrrolidinyl piperidinol were prepared by reacting the corresponding enaminones 2 with 2,4-bis-(4-methoxyphenyl) 1,3,2,4-dithiadiphosphetane 2,4-disultide 1. The compounds 3 were reacted with methyl iodide and ethyl iodide to give exclusively S-alkylated iminium iodides 4 and 5, in quantitative overall yields N phenyl-2,3,5,6,7,8-hexahydro-4-qumolinethione, 8, and 2-methyl-N phenyl-2, 3,5,6,7,8-hexahydro-4-quinolmethione 9, were alkylated with methyl iodide ethyl iodide and benzyl bromide giving S-alkylated hexahvdro quinolinium halides. 10 and 11, respectively in high yields. The stereochemistry of the compounds of types 4 and 5 is discussed in detail.     

The effect of vinyl esters on the enantioselectivity of the lipase-catalysed transesterification of alcohols

The enantioselectivity of the lipase from Pseudomonas cepacia (PCL) in the transesterification of 2-phenyl-1-propanol 1 was studied using a series of vinyl 3-arylpropanoates as acyl donors. The most enantioselective transesterification reaction of the alcohol was attained by using vinyl 3-(p-iodophenyl)- or 3-(p-trifluoromethylphenyl)propanoates, with enantiomer ratios, E, of 116 and 138, respectively. Vinyl 3-phenylpropanoate was also effective for the resolution of 1 mediated by lipases from P. fluorescens and porcine pancreas and for the PCL-catalysed transesterification of several 2-phenyl-1-alkanols. The enantiomeric resolution of 1 was practically carried out by the first enantioselective transesterification using PCL and vinyl 3-(p-iodophenyl)propanoate to afford (R)-1 and then the enantioselective hydrolysis of the resultant ester to afford (S)-1.     

Total synthesis of optically active liverwort sesquiterpenes,trifarienols A and B,using phenylethylamine as a chiral auxiliary

The imine of (rac)-2,3-dimethylcyclohexanone 10a with (S)-(−)-phenylethylamine was reacted with methyl acrylate to yield methyl (1′S,6′R)-3-(1′,6′-dimethyl-2′-oxocyclohexyl)propanoate 4a in 26% (97% ee) after hydrolysis. When (2RS,3R)-2,3-dimethylcyclohexanone 10b was used, the same product 4b was obtained in 59% yield (>99.5% ee) after hydrolysis. When (2RS,3R)-2,3-dimethylcyclohexanone 10b and (R)-(+)-phenylethylamine were used, the reaction underwent in only 5% yield, the products being 4c, 12, 13, 14, and 15. Thus, the reaction of 3b with methyl acrylate is a matched case, while that of 3c is a mismatched case. These phenomena are explained by the nonbonded interaction of methyl acrylate with chiral phenylethylamine and the methyl group at the 6′-position of the cyclohexanone ring in the transition state. The propanoate product 4b was successfully transformed into liverwort sesquiterpene (+)-trifarienol A 1 and (−)-trifarienol B 2 in 10 steps. We have developed an HPLC method to determine the ees of 2,2-disubstituted and 2,2,3-trisubstituted cyclohexanones using the corresponding pentafluorophenyl esters.     

Asymmetric intramolecular Pd(II)-catalysed amidocarbonylation of unsaturated amino alcohols

The first example of an asymmetric carbonylative bicyclisation of racemic N-protected 1-amino-pent-4-ene-3-ols (±)-1 catalysed by palladium(II) with chiral bis(oxazoline) ligands was investigated. The kinetic resolution of (±)-1 in the presence of chiral catalyst, p-benzoquinone in acetic acid under carbon monoxide atmosphere afforded enantiomerically enriched derivatives of 2-oxa-6-azabicyclo[3.3.0]octan-3-ones (R,R)-2 and (S,S)-2, respectively.     

Arthrobacter sp.: a lipase of choice for the kinetic resolution of racemic arylazetidinone precursors of taxanoid side chains

The native strain of Arthrobacter sp. (MTCC 5125) bearing a lipase has been found to be the most effective in the kinetic resolution of racemic arylazetidinones for producing cis-(3R,4S)-3-acetoxy-1-(4-methoxyphenyl)-4-phenyl-2-azetidinone, cis-(3R,4S)-3-acetoxy-1-(4-methoxyphenyl)-4-(2-furanyl)-2-azetidinone, cis-(3R,4S)-3-acetoxy-1-(4-methoxyphenyl)-4-(2-thienyl)-2-azetidinone, and cis-3-acetoxy-4-(t-butyl)-2-azetidinone products. The resolved compounds, which were obtained in high enantiopurity are important intermediates of amino acid side chains of paclitaxel as well as a new generation of taxanoids. The use of co-solvents dramatically improved the resolution efficacy of the lipase.     

Dynamic kinetic resolution of racemic N-phthalyl amino acids using (S)-α-methylpantolactone as the chiral auxiliary

A dynamic kinetic resolution of racemic N-phthalyl amino acids by stereoselective esterification was examined using (S)-α-methylpantolactone as the chiral auxiliary. The reaction of various racemic N-phthalyl amino acids with this chiral alcohol in the presence of both DCC and DMAP afforded predominantly the (S,S)-esters in nearly quantitative yield.     

The influence of microwave irradiation on lipase-catalyzed kinetic resolution of racemic secondary alcohols

The influence of microwave irradiation on the Novozyme 435^® (Candida antarctica lipase) catalyzed kinetic resolution of secondary alcohols with different functional groups was studied in comparison to the use of conventional heating at 60 °C. p-Chlorophenyl acetate was used as an acyl donor and toluene as the solvent. (±)-1-Phenyl-1-propanol 1, (±)-1-(4-bromophenyl)-propan-1-ol 3, (±)-1-phenylbut-3-en-1-ol 5 and (±)-3-bromo-2-(2-hydroxypropyl)-1,4-dimethoxynaphthalene 7 were successfully resolved into their (S)-alcohols and (R)-esters, respectively, in good enantiomeric excess. Resolution of (±)-ethyl-5-(4-methoxybenyloxy)-3-hydroxypentanoate 9 afforded its (R)-alcohol and (S)-ester using this method. In addition, microwave-assisted lipase transesterification of meso-symmetric diol 11 effected desymmetrization to ester 12 with high enantiomeric excess. In all cases studied, the conversion value for the microwave-assisted lipase kinetic resolution of secondary alcohols was higher than that obtained using conventional heating.     

Stereospecific enzymatic hydrolysis of racemic epoxide: a process for making chiral epoxide

Among various microbial cultures evaluated, Rhodotorula glutinis SC 16293 and Aspergillus niger SC 16311 catalyzed the stereospecific hydrolysis of the racemic epoxide, RS-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 1 to the corresponding R-diol, R-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-ethane-1,2-diol, 3. The S-epoxide, S-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 2 remained unreacted in the reaction mixture. A reaction yield of 45–50% (theoretical maximum yield is 50%) and an enantiomeric excess (ee) of >95% were obtained for unreacted S-epoxide 2 using each culture. Addition of 10% methyl tert-butyl ether to an aqueous reaction mixture during hydrolysis by R. glutinis improved the ee of the unreacted S-epoxide 2 to >99% (yield 48%) and that of the R-diol 3 to 79%. Unlike R. glutinis, hydrolysis of racemic epoxide 1 in the presence of 10% methyl tert-butyl ether by A. niger showed an adverse effect and gave S-epoxide 2 in 54% yield and 49% ee.     

Resolution of ortho- and meta-substituted 1-phenylethylamines with isopropylidene glycerol hydrogen phthalate

The hydrogen phthalate of isopropylidene glycerol 1, previously reported as an efficient resolving agent of p-substituted 1-phenylethylamines, was also found to resolve selected o- and m-isomers. In particular, the (S)-enantiomers of 1-(2-methylphenyl)ethylamine 2, 1-(3-methylphenyl)ethylamine 3, 1-(2-chlorophenyl)ethylamine 4 and 1-(3-methoxyphenyl)ethylamine 5 were obtained in good yields and very high enantiomeric excess (e.e.) by selective crystallization of the respective salts with (S)- or (R)-1. The e.e.s of the resolved substrates were determined by chiral HPLC analysis. The (S)-configuration of (−)-3 was established according to Raban's procedure. Optical rotations of non-racemic free amines 2 and 3 are reported. The success of the resolutions presented and of the precedent ones using 1 indicate that the position of the substituent on the 1-phenylethylamine framework does not affect the resolution, showing the uncommon versatility of 1 in the resolution of monosubstituted 1-phenylethylamines.     

Deracemisation of aromatic β-hydroxy esters using immobilised whole cells of Candida parapsilosis ATCC 7330 and determination of absolute configuration by 1H NMR

Deracemisation of aryl and substituted aryl β-hydroxy esters using immobilised whole cells of Candida parapsilosis ATCC 7330 yields the corresponding (S)-enantiomer in >99% enantiomeric excess and good yield (up to 68%). The absolute configuration of ethyl 3-(2,4-dichlorophenyl)-3-hydroxy propanoate and ethyl 3-hydroxy-5-phenyl-pent-4-enoate as determined by ¹H NMR using MTPA chloride was found to be ‘S’. The chemical shifts of the methoxy groups of the two diastereomeric MTPA esters were used as diagnostic signals to determine the absolute configuration.     

Design,synthesis, resolution,and stereochemical assignment of a conformationally rigid chiral ligand derived from anthracene and a dienophile containing a pyridine moiety

The conformationally rigid chiral ligand, trans-12-(pyridin-2-yl)-9,10-dihydro-9,10-ethanoanthracene-11-carboxylic acid ethyl ester, 1, was designed and synthesized in racemic form. Both isomers were successfully obtained in enantiomerically pure form through classical resolution using l-(+)-tartaric acid in acetonitrile. The nature of the diastereomeric complex formed in this resolution was elucidated using single crystal X-ray crystallographic studies. The absolute configuration of (+)-1 was unambiguously assigned as (11S,12S) by single crystal structural analysis of salt 5 formed from (+)-1 and l-(+)-tartaric acid.     

Total,asymmetric synthesis of ethyl d-ido-4-heptulosuronate derivatives starting from diethyl 4-oxopimelate

Bromination of diethyl 4-oxopimelate, followed by double elimination of HBr and ketalization provided diethyl (E,E)-4,4-(ethylidenedioxy)hepta-2,5-dienedioate 4. Sharpless asymmetric dihydroxylation of 4 produced diethyl (2S,3S)-4,4-(ethylidenedioxy)-2,3-dihydroxyhept-5-enedioate (+)-5, with 78% e.e. The corresponding tetrol could not be obtained in one step. Silylation of (+)-5 and a second asymmetric dihydroxylation, followed by silylation led to 20% of meso-diester 9 and 60% of diethyl (2S,3S,5S,6S)-2,3,5,6-tetrakis[(t-butyl)dimethylsilyloxy]-4,4-(ethylidenedioxy)heptanedioate (−)-10. Reductive desymmetrization of (−)-10 with DIBAL-H furnished, after selective oxidation, ethyl (2S,3S,5S,6S)-2,3,5,6-tetrakis-[(t-butyl)dimethylsilyloxy]-4,4-(ethylidenedioxy)-7-oxoheptanoate (+)-13 which was then converted into ethyl 1,2,3,6-O-tetraacetyl-4,4-ethylidenedioxy-α- and β-d-ido-heptapyranuronate (−)-15α,β and into the corresponding 3-(α-d-pyranosyl)propene (−)-16.     

Resolution of racemic cis-1-amino-2-indanol by diastereomeric salt formation with (S)-2-phenylpropionic acid

Resolution of racemic cis-1-amino-2-indanol 1, a key intermediate for the synthesis of indinavir, is reported. The conditions were optimized for an industrial-scale resolution of racemic cis-1 using (S)-2-phenylpropionic acid 6 as the resolving agent and ethanol as the solvent. The less-soluble diastereomeric salt, (1R,2S)-1·(S)-6, was obtained in 35% yield with 99% de (E >69%) by crystallization. Resolving agent 6 was efficiently recovered from the salt and the mother liquor.     

Synthesis of the (1S,2S)- and (1R,2S)-stereoisomers of the respective E- and Z-isomers of ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoate using yeast whole cell bioreduction of the parent ketones

Saccharomyces cerevisiae, strain DBM 2115, was successfully employed in the reduction of the separated Z- and E-isomers of ethyl 4-[(2-oxocyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 1 and 2, in order to prepare the (1S,2S)- and (1R,2S)-enantiomers of the corresponding ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 3–6. The products were obtained with the required absolute configuration: (1S,2S)-3 (ee = 98%; yield 48%), (1R,2S)-4 (ee = >99%; yield 45%), (1S,2S)-5 (ee = 98.5%; yield 47%), and (1R,2S)-6 (ee = >99%; chemical yield 44%).     

Biocatalytic oxidation of thiophosphoryl compounds: a new chemo-enzymatic approach to enantiomeric insecticidal thionophosphates and their oxons

The biocatalytic oxidation of racemic O-S-dimethyl O-p-nitrophenyl phosphorodithioate 5 catalyzed by chloroperoxidase from Caldariomyces fumago resulted in the formation of the (?)-(S)-enantiomer of the corresponding oxon 4 and unoxidized substrate 5 with a (+)-(R)-configuration. Both compounds were obtained with very high enantiomeric excesses, 99.6% and 97%, respectively. The thionation reaction of the resulting (?)-(S)-oxon 4 with Lawesson’s reagent gave (?)-(S)-phosphorodithioate 5 with full stereoselectivity, while the oxidation of unreacted substrate (+)-(R)-5 with iodoxybenzene afforded oxon (+)-(R)-4 with 94.9% ee.     

Non-thiazolidinedione antihyperglycaemic agents. Part 4: Synthesis of (±)-, (R)-(+)- and (S)-(−)-enantiomers of 2-oxy-3-arylpropanoic acids

The synthesis of a new series of potent 2-oxy-3-arylpropanoic acid antihyperglycaemic agents in both racemic and non-racemic form is described. Resolution of racemic acids 1 is accomplished via amide formation with either (S)-2-phenylglycinol or (S)-4-benzyloxazolidin-2-one as complementary resolving agents.