STEREOSPECIFICITY FOR THE ZINC BOROHYDRIDE REDUCTION OF ALPHA-ARYLOXY-BETA-HYDROXY KETONES

Several lignin model precursors have been submitted to zinc borohydride reductions and their resultant stereochemistries determined by NMR spectroscopy. Benzoyl carbonyls of alpha-aryloxy-beta-hydroxy systems were reduced without significant stereoselectivity to produce both threo and erythro isomers. However, protection of the beta-hydroxyl with either an acyl, alkyl, or silyl group and subsequent reduction gave erythro-specificities of up to 97%. A mechanism where competition for zinc cation complexation between the beta-hydroxyl and the alpha-aryloxy substituent is invoked to explain the observed results. Protection of the beta-hydroxyl prevents its complexation with the zinc cation; complexation occurs solely with the alpha-aryloxy substituent, affording the erythro-isomers.