A Lewis Acid Promoted Asymmetric Umpolung Reaction with Chiral N ‐Sulfinyl Imines as the Electrophiles

An new asymmetric umpolung reaction has been developed by reacting N ‐sulfinyl imines with 2‐lithio‐2‐phenyl‐1,3‐dithiane. The reaction was conducted at between –20 and–25 °C in THF in the presence of Et 2 AlCl as the Lewis acid promoter. Excellent diastereoselectivities (up to >95 % de ) and chemical yields (64–95 %) have been achieved for nine substrates with all individual isomers separated and characterized. The absolute structure of the chiral products has been unambiguously determined by synthetic conversions to a known sample. 2‐Lithio‐2‐phenyl‐1,3‐dithiane was found to be much less reactive than its 2‐methyl counterpart, which was reported very recently. All individual isomers have been readily separated by column chromatography. The absolute structure of the chiral products has been unambiguously determined by conversion into a known compound. This method provides an easy access to enantiomerically pure α‐amino ketones. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)