Photo-induced transformations. Part 79. On the mechanism of the formation of oxa steroids via photo- and thermally-induced rearrangement of 3-hydroxy-Δ 5 -steroid hypoiodites in the presence of mercury( II ) oxide and iodine. An oxygen-18 labelling study.

An 18O labelling study of 3α,5-epoxy-4-oxa-A-homo-5α-cholestane and 3-oxacholest-5-ene formation, on photolysis and thermolysis of cholesterol hypoiodites and methyl-substituted cholest-3-en-3-ols generated in situ by an excess of mercury(II) oxide and iodine in benzene is described. Cholesterol, 4,4-dimethylcholest-5-en-3β-ol, and 3α,4,4-trimethylcholest-5-en-3β-ol were irradiated in benzene containing an excess of Hg18O and iodine and the extent of 18O incorporation into the oxa-steroids obtained from the reactions was determined by mass spectrometry. The results show that the oxygen of mercury(II) oxide is incorporated into the epoxy oxygens of the 3α,5-epoxy-A-homo-4-oxacholestanes and into the ring oxygen of one of the 3-oxacholest-5-enes; the extent of the incorporation of 18O is in the range 31–61%. The oxygen of mercury(II) oxide is not incorporated into 2-acetyl-3-oxacholest-5-enes. The results are used to rationalize the routes by which the oxa steroids are formed.