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.