Stereoselective Reduction of 2-Hydroxy Ketones towards syn- and anti-1,2-Diols

Stereoselective reduction of 2-hydroxy ketones should in principle give access to syn- and anti-1,2-diols. anti-1,2-Diols are accessible in a highly selective way using zinc borohydride [Zn(BH4)2] under chelation control (dr>20:1). Diastereoselective reduction of unprotected or even protected 2-hydroxy ketones towards syn-1,2-diols could be achieved only with moderate selectivity of dr≤5:1. Even when using sterically demanding protecting groups and/or polymer-supported borohydride reagents high selectivity could not be achieved. A new ionic liquid-dependent borohydride reduction method, although highly attractive with respect to reaction engineering, resulted in only moderate to good selectivity. An efficient two-step biocatalytic method for the synthesis of syn-1,2-diols is described. The method relies on the whole-cell Pichia glucozyma-catalyzed stereoselective reduction of the unprotected (R)-2-hydroxy ketones (dr>10:1). The latter are accessible through thiamine diphosphate-dependent enzyme-catalyzed synthesis starting from simple aldehydes. Thus, biocatalytic transformations enable a process which is hardly accessible through present non-enzymatic methods.