(2 S, 4 R)-N-Benzyloxycarbonyl-4-tosyloxyproline methyl ester, readily prepared from commercially available (2 S, 4 R)-N-benzyl-oxycarbonyl-4-hydroxyproline, was converted to (S)-3, 4-dehydroproline of high enantiomeric purity by a process which involved a highly regioselective phenylselenoxide elimination to introduce the olefinic function.
