In this article the utility of water-compatible amino-acid-based catalysts was explored in the development of diastereo- and enantioselective direct aldol reactions of a broad range of substrates. Chiral C(2)-symmetrical proline- and valine-based amides and their Zn(II) complexes were designed for use as efficient and flexible chiral catalysts for enantioselective aldol reactions in water, on water, and in the presence of water. The presence of 5 mol % of the prolinamide-based catalyst affords asymmetric intermolecular aldol reactions between unmodified ketones and various aldehydes to give anti products with excellent enantioselectivities. We also demonstrate aldol reactions of more demanding substrates with high affinity to water (i.e., acetone and formaldehyde). Newly designed serine-based organocatalyst promoted aldol reaction of hydroxyacetone leading to syn-diols. For presented catalytic systems organic solvent-free conditions are also acceptable, making the elaborated methodology interesting from a green chemistry perspectives.
Although stereoselective transformations in living systems occur in an aqueous environment, it was only recently that a breakthrough was achieved in the use of water as a solvent or co-solvent in asymmetric synthesis. While few years ago, only a few practitioners studied the subject, now organic reactions in water have become one of the most exciting research areas, not only because of green chemistry perspectives. Both organometallic and organocatalytic processes in aqueous systems are of great interest and are under intensive investigation. This chapter will focus on recent advances in performing enantioselective reactions in aqueous media. The quest to identify water-compatible catalysts has evoked an intense search for new exciting possibilities. Here, we summarize recent efforts towards asymmetric transformations performed in water or in the presenceof water, with a focus on homogeneous solutions.
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