Reaction of the cholanic acid derivative (1) with thionyl chloride and pyridine gave, after quenching with methanol, the α-sulphine ester (3); treatment with acetic anhydride–sulphuric acid afforded the keto ester (5) which, on exposure to air in the presence of copper(II) ions and base, gave the 20-keto pregnane derivative (9) in excellent overall yield.
D. H. R. Barton, J. Wozniak and S. Z. Zard, J. Chem. Soc., Chem. Commun., 1987, 1383 DOI: 10.1039/C39870001383
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