Regiocontrol in the Oxidative Radical Fragmentation of Benzilidene Acetals and Its Mechanistic Implications

The NBS-mediated oxidative fragmentation of benzilidene acetals has been investigated with mechanistic probes 12, 14, and 18 designed to discriminate between the possible competitive pathways. Results indicate that fragmentation of the initial benzylic radical 19 does not occur spontaneously but that oxidation proceeds rapidly to give the benzyl bromide 20, which then fragments via a polar pathway. Reversed regiospecificity in the fragmentation is demonstrated for the first time through the incorporation of an allylic alcohol into the benzilidene acetal.