Asymmetric Synthesis of (2R,3R,4R)-3-Hydroxy-4-methylproline via Chromium(II) Chloride-mediated Coupling Reactions of (S)-Garner Aldehyde with Crotyl Bromide

Efficient synthesis of(2R,4R,4R)-3-hydroxy-4-methylproline (la), which is an antipode of the component of potent antifungal echinocandins, from (5)-Garner aldehyde (2a) has been established. The key step is chromium(II) chloride-mediated coupling reactions with crotyl bromide giving homoallyl alcohols (4a) and (4a') in a ratio of 83:17.