Photoinduced Transformations. 73. Transformations of Five- (and Six-) Membered Cyclic Alcohols into Five- (and Six-) Membered Cyclic Ethers - A New Method of a Two-Step Transformation of Hydroxy Steroids into Oxasteroids

Article abstract of DOI:10.1021/jo00194a015

We set out to describe a two-step transformation of saturated hydroxy steroids into oxasteroids with an oxgen-containing ring of the same size as that of the starting ring.The irradiation of the hypoiodites generated in situ by means of the reaction of the hydroxy steroids with an excess of mercury(II)oxide and iodine gives novel formates arising from the successive reactions triggered by a β-scission of the corresponding alkoxy radical.These formates can readily be transformed into oxasteroids by a treatment with a complex metal hydride or methyllithium.The method is exemplified by the transformations of 1-, 3-, and 17-hydroxy steroids into the corresponding oxasteroids.Experiments in which we have used ¹⁸O labeled mercury(II)oxide as a source of I₂¹⁸O provide evidence that the oxygen atom in the oxasteroids is derived from the hydroxyl group of the starting alcohol and not from the oxygen of mercury(II)oxide.This suggests that the pathway of the formation of the formates involves (a) β-scission of the corresponding alkoxy radical to give a carbon centered radical intermediate with a carbonyl group, (b) its intramolecular combination with the carbonyl oxgen to form a tetrahydropyranyl radical or an oxepanyl radical, (c) its reaction with iodine oxide or *OI to generate a new hypoiodite, and (d) a regiospecific β-scission of the second alkoxy radical generated from it.