Proline-Catalyzed One-Step Asymmetric Synthesis of 5-Hydroxy-(2E)-hexenal from Acetaldehyde

Abstract
For the first time, the l-proline-catalyzed direct asymmetric self-aldolization of acetaldehyde is described affording (+)-(5S)-hydroxy-(2E)-hexenal 2 with ee's ranging from 57 to 90%. Further transformations of 2 into synthetically valuable building blocks are presented. A mechanism for the formation of 2 is proposed.