An intermediate containing the ACE ring system of quassimarin was prepared. The isopropylidene malonate 8 reacted with diene 2 to afford two Diels-Alder adducts. The major adduct was converted into lactone 11 by a sequence involving epoxidation followed by acid-mediated epoxide opening and lactonization.
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Kraus et al. (1986) studied this question.
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