Transition Metal/Base-Catalyzed Aldol Reactions of Isocyanoacetic Acid Derivatives with Prochiral Ketones, a Straightforward Approach to Stereochemically Defined β,β-Disubstituted-β-hydroxy-α-amino Acids. 1 Scope and Limitations

A systematic study of the transition metal/base-catalyzed aldol reactions of methyl isocyanoacetate with a wide range of prochiral ketones, giving rise to the 4-(methoxycarbonyl)-5,5-disubstituted-2-oxazolines, has been made. The diastereoselectivity of these reactions was found to be influenced by a mix of several factors, including steric characteristic of the substituents, nature of the catalyst, electrostatic, and electron donor-acceptor type interactions. The former factor, a stereochemical discrimination between the substituents at the ketone carbon, was shown to be the most pronounced in controlling of the stereochemical outcome, which could be markedly improved with a proper choice of the catalyst. In particular, for the reactions of methyl isocyanoacetate with polyhaloalkyl aryl(alkyl) ketones, high diastereoselectivity (80-98% de) was achieved, thus allowing for straightforward and generalized access to the corresponding (2R*,3R*)-β,β-disubstituted-β-hydroxy-α-amino carboxylic acid.