New synthesis of 28-homobrassinolide from stigmasterol

An efficient and short synthesis of 28-homobrassinolide (6) has been achieved in 12 steps with 15.6% overall yield from stigmasterol (8). Salient features of this synthesis are: (a) use of tetradecyltrimethylammonium permanganate for chemoselective cis-dihydroxylation of the 2,22E-dien-6-one 9, avoiding the use of OsO4; (b) one-step epoxidation of the (22E)-olefin and Baeyer-Villiger oxidation of the B-ring ketone 11 with trifluoroperoxyacetic add; and (c) regioselective ring-opening of the 22,23-epoxide 12 to the bromohydrin 13 with LiBr and Amberlyst-15 resin under mild conditions and in excellent yield.