Photoinduced molecular transformations. Part 106. The formation of cyclic anhydrides via regioselective β-scission of alkoxyl radicals generated from 5-and 6-membered α-hydroxy cyclic ketones

Abstract

The irradiation of the hypoiodites of 5- or 6-membered cyclic α-ketols in benzene containing mercury(II) oxide and iodine (each 3 equiv.) resulted in the formation of the corresponding cyclic anhydrides arising from the insertion of an oxygen. A novel formation of a methylenedioxy group arising from an intramolecular hydrogen abstraction, on the other hand, was found when a steroidal α,α-dimethoxy alcohol hypoiodite was irradiated in benzene. An ¹⁸O labelling study of the formation of the cyclic anhydride on photolyses of 17β-hydroxy-4β-methoxy-5α-androstan-16-one hypoiodite generated in situ by an excess of mercury(II) oxide and iodine in benzene indicated that the heavy oxygen of Hg ¹⁸O is incorporated as the ring oxygen of the anhydride. On the basis of this result, a pathway involving a regioselective β-scission of the alkoxyl radical is proposed as leading to formation of the cyclic anhydride.