Diastereoselective Synthesis of (2S,3S,4S)-3-Hydroxy-4-methylproline, a Common Constituent of Several Antifungal Cyclopeptides

(2S,3S,4S)-3-Hydroxy-4-methylproline 2 was synthesized from unsaturated γ-lactams 4 and 5 derived from (S)-pyroglutaminol. Starting from 5, haplophilic effect in the hydrogenation of a 4-exo-methylenic intermediate improved the diastereoselectivity of the synthesis, which was achieved in 19% yield.