Organocatalytic syn-aldol reactions of hydroxy ketones with (S)-isoserinal: Asymmetric synthesis of 6-deoxy-1,5-iminohexitols and related compounds

Abstract

An improved and convenient preparation of protected (S)-isoserinal on a large scale is reported. This key intermediate was reacted through organocatalyzed aldol reaction or Wittig based chain extension and functionalization to give enantiopure 1,5,6-trideoxy-1,5-imino-hexitols such as 10a (L-manno) and 10b (D-gluco). These two compounds are of interest as glycosidase inhibitors. The elaborated organocatalytic process includes diastereoselective syn aldol reaction of (S)-isoserinal hydrate and hydroxyacetone or 1-hydroxy-2-octanone and is promoted by various amino acid-based catalysts. Diastereoselectivities of up to 8:1 were achieved, thus establishing a new, efficient synthetic route to these important carbohydrate mimics. A novel protocol for the preparation of 1,5,6-trideoxy-1,5-imino-L- mannitol and 1,5,6-trideoxy-1,5-imino-D-glucitol is reported. The key steps include organocatalyzed syn-selective direct aldol reaction of hydroxyacetone and CBz-protected isoserinal hydrate, followed by reductive amination/ cyclization. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.