Vicinol diols

With benzaldehyde 144 or halogenated derivatives (Cl, F) as acceptors the yeast-PDC-catalyzed addition proceeds with almost complete stereoselectivity to furnish the corresponding (R)-configurated 1-hydroxy-1-phenylpropanones 145 [447]. For practical reasons, whole yeast cells are most often used as the catalyst, with only small loss of enantioselectivity [423,424]. The conversion of benzaldehyde in particular has gained industrial importance because the acyloin is an important precursor for the synthesis of L-(-)-ephedrine [448]. Otherwise, the substrate tolerance is remarkably broad for aromatic aldehydes on the laboratory scale, however, yields of acyloins are usually low because of the prior or consequent reductive metabolism of aldehyde substrate and product, giving rise to considerable quantities of alcohol 146 and vicinol diols 147, respectively [423,424,449], The range of structural variability covers both higher a-oxo-acids (e.g. -butyrate, -valerate) as the donor component, as well as a,/J-un-saturated aldehydes (e.g. cinnamaldehyde 148) as the acceptor [450]. 

Ring opening of 3-cyclohexyl-2,3-epoxypropan-l-ol (39) with sodium azide over H-X gave an 85% yield of the isomeric azido diols, 40 and 41, with the vicinol diol, 40, the almost exclusive product. With other zeolites and clays, the yields were lower but the regioselectivity still favored the formation of 40 (Eqn. 22.39). 6... 

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