Enzymatic and Microbial Reductions

Enzymes are an increasingly available and important tool in the arsenal of the synthetic chemist. Enzymatic reductions are often straightforward and highly stereoselective. There are now many enzymatic transformations that are compatible with the use of organic solvents.6l5 Other solvents can be used as well, illustrated by the enzyme alcohol dehydrogenase from Geotrichum candidum, which is active in supercritical carbon dioxide.6i6 Prelog studied the reduction of ketones with several enzymatic systems. Reduction of 

Oishi encountered problems in the reduction of aryl P-keto esters such as the furan derivative 604, where baker s yeast was unable to reduce the ketone moiety. 22 oishi examined other selected yeasts including 

Kloeckera saturnus, which converted 604 in 47% yield to a 47 53 mixture of the syn- (605) and anti- (606) diastereomers in 87% ee and 53% ee respectivelyAnother yeast (Saccharomyces delbrueckii) gave 43% reduction with a syn/anti ratio of 39 61. Reduction with 5. delbrueckii gave 99% ee of 605 and 94% ee of 

22 oishi examined the selectivity for reduction of 607 with a variety of organisms, as shown in Table 4.17. In all cases, the organism produced an excess of either the syn- or the anti-diastereomer (608 or 609). The chemical yields were poor in some cases and the reduction proceed with modest to good diastereoselectivity in other cases. The enantioselectivity of the reaction was good to excellent with 60-99% ee 

Other organisms have been used to reduce 3-keto esters. Indeed, either the (R)- or the (S)- alcohol can be obtained. Bernardi et al. showed that 393 was reduced to (S)-alcohol 599 in 96% yield, 7 93 (R/S)- with G. candidum. The reaction medium is quite important. Nakamura et al. found, for example, that reduction of 

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