NEW SYNTHESIS OF 28-HOMOBRASSINOLIDE FROM STIGMASTEROL

An efficient and short synthesis of 28-homobrassinolide (6) has been a chieved in 12 steps with 15.6% overall yield from stigmasterol (8). Sa lient features of this synthesis are: (a) use of tetradecyltrimethylam monium permanganate for chemoselective cis-dihydroxylation of the 2,22 E-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 o f the 22,23-epoxide 12 to the bromohydrin 13 with LiBr and Amberlyst-1 5 resin under mild conditions and in excellent yield.