Optimisation, Scope and Limitations of Enantioselective Aldol Reactions of an S-Ketene Silyl Acetal with Aliphatic Aldehydes under (R)-BINOL-Titanium(IV) Catalysis Conditions

Abstract

The Mukaiyama aldol reaction between the functionalised aldehydes 5 and the S-ketene silyl acetal 2 catalysed by 1,1′-binaphthyl-derived chiral titanium(IV) complex 4 afforded the corresponding aldol products 6 in good yields and with good to excellent enantioselectivities. The chemical yield could further be enhanced, without loss of stereoselection, by addition of phenol and/or molecular sieves. The presented aldol reactions with aluminium, boron and ytterbium-BINOL catalysts demonstrate that only low chiral induction can be achieved. Aldol product 6a was converted into an α-lipoic acid precursor 8, thus providing a formal synthesis of this biologically active compound. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)