Direct Asymmetric Aldol Reaction Catalyzed by an Imidazolium-Tagged trans-4-Hydroxy-L-proline under Aqueous Biphasic Conditions

Imidazolium-tagged trans-4-hydroxy-L-proline, recently reported by us as an efficient catalyst for the asymmetric cross-aldol reaction in [bmim][Tf2N], gives much better results in terms of catalytic activity and stereochemical performance under aqueous biphasic conditions, providing aldols with anti/syn ratios up to 98:2 and ee (anti) up to 99%. The peculiar solubility properties of imidazolium-tagged trans-4-hydroxy-L-proline ensure an efficient separation of the product from the catalyst and the recycling of trans-4-hydroxy-L-proline for five times without appreciable loss of catalytic activity and stereochemical performance.