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.