Acylsilanes biotransformations

Enantioselective reduction of the prochiral cyclic acylsilane 42 with growing cells of the yeast Kloeckera corticis (ATCC 20109) yielded the optically active reduction product (R)-43 (Scheme 8)53. On a preparative scale, the l-silacyclohexan-2-ol (R)-43 was isolated in 60% yield with an enantiomeric purity of 92% ee. Repeated recrystallization of the biotransformation product from n-hexane raised the enantiomeric purity to 99% ee. 

Scheme 15)62. After terminating the reaction at a conversion of 38% (relative to total amount of substrate rac-78), the product (S)-43 was separated from the nonreacted substrate by column chromatography on silica gel and isolated on a preparative scale in 71% yield (relative to total amount of converted rac-78) with an enantiomeric purity of 95% ee. Recrystallization led to an improvement of the enantiomeric purity by up to >98% ee. The biotransformation product (S)-43 is the antipode of compound (/ )-43 which was obtained by enantioselective microbial reduction of the acylsilane 42 (see Scheme 8)53. The nonreacted substrate (/ )-78 was isolated in 81% yield (relative to total amount of nonconverted rac-78) with an enantiomeric purity of 57% ee. For further enantioselective enzymatic hydrolyses of racemic organosilicon esters, with the carbon atom as the center of chirality, see References 63 and 64. 

All biotransformations mentioned above represent enantioselective reductions. In contrast, the reduction of the cyclic chiral acylsilane vac-225 with growing cells of Kloeckera corticis (ATCC 20109) into a 1 1 mixture of (S,R)-226 and (jR,S)-226 is an example of a diastereoselective conversion (yield of reduction product 95%, diaster-eomeric excess 90% de)19,283. 

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