Studies in terpenoid biosynthesis. Part 39. The stereochemistry of the relationship between substrate and oxidant in the hydroxylation of aphidicolin at C-18 by Cephalosporium aphidicola

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Abstract
The preparation of some 3α, 18-oxetane analogues of intermediates in the biosynthesis of aphidicolin is described. Their stereoselective labelling at C-18 with deuterium is reported. Biotransformation studies using these substrates with Cephalosporium aphidicola have shown that the 18-pro-R-hydrogen is removed in hydroxylation at this centre suggesting a Re stereochemical relationship between the substrate and oxidant for the normal hydroxylation at C-18. The X-ray crystallographic structure of 3α,18-oxetane 6 is presented and compared with that of aphidicolin 17-nor-16-ketone 13.