A Novel Tandem Michael Addition/Meerwein-Ponndorf-Verley Reduction: Asymmetric Reduction of Acyclic α,β-Unsaturated Ketones Using a Chiral Mercapto Alcohol.

The introduction of a thiol group into a chiral alcohol reagent for asymmetric Meerwein−Ponndorf−Verley (MPV) reductions allows asymmetric reduction of α,β-unsaturated ketones to secondary alcohols and allylic alcohols via a novel tandem Michael addition/MPV reduction. The reaction of acyclic α,β-unsaturated ketones 1 and an optically active 1,3-mercapto alcohol (−)-2 using dimethylaluminum chloride afforded the MPV reduction products 3 diastereoselectively in very high yields (up to 96%). Mechanistic studies elucidated (1) the structure of the chelation complex D with (−)-2 and Me2AlCl, (2) an asymmetric 1,7-hydride shift (intramolecular MPV reduction), and (3) dynamic kinetic resolution via reversible Michael addition. Subsequent reductive desulfurization of the MPV products 3 with a modified Raney nickel system led to the highly enantioselective reduction of α,β-unsaturated ketones to saturated secondary alcohols in 96−98% ee. β-Elimination of the corresponding sulfoxides gave the allylic alcohols in 8...