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Direct Asymmetric Aldol ReactionCatalyzed by an Imidazolium-Tagged <i>trans</i>-4-Hydroxy-<scp>l</scp>-proline under Aqueous Biphasic Conditions
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2008
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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.