Carbocyclization in natural products. III. Brominative cyclization of dehydrosaussurea lactone and costunolide

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Abstract
Treatment of dehydrosaussurea lactone 4 and costunolide 1 with N-bromosuccinimide (NBS) in aqueous acetone at room temperature furnished bromolactones 6, 7, and 8. Structural evidence for these bromolactones rests upon spectral data and transformation to the hydrogenated bromolactones 9, 10, and 11. The latter were found identical with the corresponding products obtained by the brominative cyclization of dihydrocostunolide 2.