The First Direct and Enantioselective Cross-Aldol Reaction of Aldehydes

The first enantioselective catalytic direct cross-aldol reaction that employs nonequivalent aldehydes has been accomplished using proline as the reaction catalyst. Structural variation in both the aldol donor (R_1 = Me, n-Bu, Bn, 91 to >99%) and aldol acceptor (R_2 = I-Pr, I-Bu, c-C6H11, Et, Ph, 97−99% ee) are possible while maintaining high reaction efficiency (75−88% yield). Significantly, this new aldol variant allows facile enantioselective access to a broad range of β-hydroxy aldehydes which are valuable intermediates in polyketide syntheses.