Synthesis of Optically Active α-Hydroxy Carbonyl Compounds by the Catalytic, Enantioselective Oxidation of Silyl Enol Ethers and Ketene Acetals with (Salen)manganese(III) Complexes

Abstract

A set of silyl enol ethers and ketene acetals 1a−h with α- and/or β-phenyl as well as alkyl substituents of different steric bulk has been submitted to the enantioselective catalytic oxidation by chiral (salen)MnIII complexes 3. Highest conversions and best enantioselectivities have been obtained with bleach rather than iodosobenzene as oxygen source for the active oxo−metal species. With regard to substrate structure, ee values up to 89% have been achieved for enol ethers with short and unbranched alkyl substituents at the siloxy position. While β-phenyl groups are beneficial for enantiofacial control, phenyl substituents α to the siloxy functionality result in lower ee values, while the diphenyl-substituted derivative 1d displays the lowest stereoselectivity. The fact that β- versus α-phenyl substituents exhibit not only differences in the magnitude but also in the sense (opposite absolute product configuration) of the stereoselectivity may be utilized as a valuable mechanistic probe to assess steric an...