Analysis by tritium n.m.r. has indicated that the incorporation or the pro-S methyl group of ‘chiral methyl valine’[as (1)] into the C-3′ exocyclic methylene group of cephalosporin C (5) in Cephalosporium acremonium occurs with at least 70% stereospecificity overall; in contrast, the tritium n.m.r. spectra obtained after the incorporation of pro-R methyl groups of opposite configuration into the C-2 methylene group of cephalosporin C were virtually identical, indicating that at some stage there is a common stereochemically identical intermediate.
