Costunolide (I) displays anomalous ultraviolet absorption (Xmax 214.5 nm, ~11841) * which has been attributed to the transannular interaction of the double bonds in the medium 192 ring . Since the oxidative cyclization of an olefinic system is largely dependent upon its steric and conformational characteristics3, the unique conformation4 of the substrate (I) favours facile cyclization. When treated with N-bromosuccinimide (NBS) in aqueous acetone, costunolide (I) furnished the bromohydrin (II) and the bromolactones (III, IV) in ** 50, 18 and 23% yields, respectively * Ultraviolet spectra were recorded in ethanol on Unicam Spectrophotometers, models SP.700 and SP.800. ** Briefly reported by T.C.J. in connection with another paper presented at the 158th National Meeting of the American Chemical Society, New York (see reference 8). *** All reported yields refer to the weights of the crystalline lactones isolated. * In light of our recent work5 on the reversibility in the germacranolide-Cope reaction, an earlier reportlo on acid catalyzed cyclization of saussurea lactone (VI) at elevated temperature would presumably involve the substrate (XI).
** Full details of this reaction on substrates (V) and, (VI) will be reported in due course.