Direct Organocatalytic Asymmetric Aldol Reactions of α-Amino Aldehydes:  Expedient Syntheses of Highly Enantiomerically Enriched <i>a</i><i>nti</i>-β-Hydroxy-α-amino Acids

A simple and efficient method for the synthesis of highly enantiomerically enriched β-hydroxy-α-amino acid derivatives has been developed. Direct asymmetric aldol reactions of a glycine aldehyde (aminoacetaldehyde) derivative have been performed under organocatalysis using l-proline or (<i>S</i>)-5-pyrrolidine-2-yl-1<i>H</i>-tetrazole. The reactions afforded <i>anti</i>-β-hydroxy-α-amino aldehydes in good yield with high diastereoselectivity (dr up to >100:1) and high enantioselectivity (up to >99.5% ee), which were easily transformed into β-hydroxy-α-amino acid derivatives.