Photoinduced molecular transformations. Part 136. Reactions of alkoxyl radicals generated from hypoiodites of 3β-hydroxy-7-oxo-Δ5-steroids. Synthesis of some functionalized oxasteroids

Abstract

The synthesis of several unsaturated 3-oxasteroids and 4a-homo-4-oxasteroids of cholestane series functionalized at their B-ring is described; 3β-hydroxycholest-5-en-7-one and its 4,4-dimethyl derivative were transformed into 3-oxacholest-5-en-7-one and its 4,4-dimethyl derivative in two or three steps involving a regioselective β-scission of the corresponding alkoxyl radicals (generated by irradiating their hypoiodites in benzene) as the key step. The formation of 2-iodo-7-oxo-4-nor-2,3-secocholest-5-en-3-yl formate by irradiation of the hypoiodite of 3β-hydroxycholest-5-en-7-one was accompanied by the formation of a product arising from a new ring contraction of the A-ring. The ring-contraction mechanism is discussed.