Photo-induced transformations. Part 51. Photo- and thermally-induced rearrangements of hypoiodites of steroidal homoallyl alcohols. The formation of some oxygen heterocycles via photo- and thermally-induced rearrangements of 3-hydroxy-Δ5-steroid hypoiodites in the presence of mercury(II) oxide and iodine

Abstract

Hypoiodites of cholesterol and epicholesterol in benzene containing mercury(II) oxide and iodine underwent photo-induced rearrangement to give 3α,5-epoxy-6β- and -6α-iodo-A-homo-4-oxa-5α-cholestanes (3) and (4), together with 3-formyloxy-2-iodo-A-nor-2,3-secocholest-5-ene (2). Stereochemistry of the epoxide (3) was established by an X-ray crystallographic analysis. When the reaction of cholesterol hypoiodite was induced thermally at 55–60 °C, only the 6β-isomer, accompanied by cholest-5-en-3α-yl A-homo-4-oxacholest-5-en-3α-yl ether (20) and A-homo-4-oxacholest-5-en-3α-ol (21), is formed. Catalytic hydrogenolysis of iodo-epoxide (3) or (4) gave 3α,5-epoxy-A-homo-4-oxa-5β-cholestane (16) which was transformed into 2-acetyl-5-acetoxymethyl-4-oxa-5β-cholest-2-ene (29) upon treatment with boron trifluoride–ether–acetic anhydride in benzene.

In contrast, the newly synthesized hypoiodite of 3α,4,4-trimethylcholest-5-en-3β-ol, in benzene containing mercury(II) oxide and iodine, gave 2-acetyl-3-oxacholest-5-enes (26) and (27) together with 3α,5α-epoxy-A-homo-4-oxasteroids (24) and (25) on irradiation or thermolysis. The formation of the products (26) and (27) indicates the intervention of a common oxyl radical (E) in the rearrangements of 3-hydroxy-Δ⁵-steroid hypoiodites to products (3), (4), (24), (25), (26), and (27). The pathways of the rearrangements and the stereoselectivity of the reactions are discussed.