Optically active compounds, microbial production

Intact microbial cells have also been used as biocatalyst for this particular type of bioconversion. Incubation of the racemic 2-acetoxy-l-silacyclohexane rac-(SiS,CR/SiR,CS)-81 with growing cells of the yeast Pichia pijperi (ATCC 20127) yielded the optically active l-silacyclohexan-2-ol (Si/t,CS)-70 (Scheme 16)66,67. Under preparative conditions, this biotransformation product was isolated as an almost enantiomerically pure compound (enantiomeric purity >96% ee) in ca 80% yield [relative to (Si/ ,CS )-81 in the racemic substrate]. 

Enantioselective enzymatic transesterifications have been used as a complementary method to enantioselective enzymatic ester hydrolyses. The first example of this particular type of biotransformation is the synthesis of the optically active 2-acetoxy-l-silacyclohexane (5 )-78 (Scheme 19). This compound was obtained by an enantioselective transesterification of the racemic l-silacyclohexan-2-ol rac-43 with triacetin (acetate source) in isooctane, catalyzed by a crude lipase preparation from Candida cylindracea (CCL, E.C. 3.1.1.3)62. After terminating the reaction at 52% conversion (relative to total amount of substrate rac-43), the product (S)-78 was separated from the nonreacted substrate by column chromatography on silica gel and isolated in 92% yield (relative to total amount of converted rac-43) with an enantiomeric purity of 95% ee. The remaining l-silacyclohexan-2-ol (/ )-43 was obtained in 76% yield (relative to total amount of nonconverted rac-43) with an enantiomeric purity of 96% ee. Repeated recrystallization of (R)-43 led to an improvement of enantiomeric purity by up to >98% ee. Compound (R)-43 has already earlier been prepared by an enantioselective microbial reduction of the l-silacyclohexan-2-one 42 (see Scheme 8)53. The l-silacyclohexan-2-ol (R)-43 is the antipode of compound (.S j-43 which was obtained by a kinetic enzymatic resolution of the racemic 2-acetoxy-l-silacyclohexane rac-78 (see Scheme 15)62. For further enantioselective enzymatic transesterifications of racemic organosilicon substrates, with a carbon atom as the center of chirality, see References 64 and 70-72. 

Another Important attribute of whole cell/enzyme systems is for the production of optlcally-actlve compounds. As early as the 1950s, the synthesis of optically active gamma- and delta-lactones by microbiological reduction was demonstrated (18). Production of optlcally-pure L-glutamate for use in the flavor enhancer MSG by microbial means is another testimonial to this potential of biotechnology. 

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