Prolinamides Bearing Thiourea Groups as Catalysts for Asymmetric Aldol Reactions

Organocatalysts based on a-amino acid amides (proline, valine, and threonine) and chiral diamines bearing thiourea groups were synthesized and their activities for aldol reactions were studied. The catalyst based on (S)-proline and (1S,2S)-diphenylethylenediamine proved to be an excellent catalyst, providing the products of reactions between ketones and aromatic aldehydes in high to quantitative yields and with high stereoselectivities (up to 98: 2 dr and 99% ee). The presence of an acid additive (4-nitrobenzoic acid) has a considerable impact on both yield and enantioselectivity.