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 acid; 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.