Base-catalyzed autoxidation of a triterpene ketone, friedelin, in the presence of potassium t-butoxide gave four products, and their structures and formation mechanism were investigated. The common intermediate to these compounds was suggested to be 4-hydroperoxyfriedelan-3-one, not friedelane-2,3-dione. 3-Oxafriedel-1-ene-2-carboxylic acid was a dehydration product of the main product, 2-hydroxy-