Formation of an Endoperoxide upon Chromium-Catalyzed Allylic Oxidation of a Triterpene by Oxygen

The
chromium-catalyzed allylic oxidation of triterpene 1 with
O2 and N-hydroxyphthalimide (NHPI,
5 equiv) formed endoperoxide 2 in 76% yield at ambient
temperature. Unlike standard allylic oxidations, this oxidation is
catalytic in chromium because oxygen, not the chromium reagent, is
the oxidant. This oxidation is sensitive to the precise structure
of the substrate. The endoperoxide is only formed if ring A is unsaturated
and ring C contains an enone. A mechanism is proposed that involves
the coupling of two stabilized radicals on rings A and C to form endoperoxide 2.