Marsformoxide A (IX) is a triterpene substance possessing an epoxide ring in the molecule, which was previously isolated by us from Marsdenia formosana MASAMUNE (Asclepiadaceae). Irradiation of marsformoxide A with a high pressure mercury lamp in acidic medium furnished D-friedours-11, 14-dien-3β-yl acetate (XI), and urs-9 (11), 12-dien-3β-yl acetate (XII), together with recovery of the starting material IX.
