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From the ethyl acetate extract of root bark of the Japanese cultivated mulberry tree (a variety of Morus alba L.), a novel chalcone derivative with a fused dihydrochalcone partial moiety was isolated and named kuwanon I. The structure was shown to be I on the basis of chemical and spectral data. Kuwanon I (I) is the first example which is regarded biogenetically as a Diels-Alder adduct of a prenylchalcone derivative and a dehydroprenylchalcone derivative. NMR variable temperature studies of I suggested that kuwanon I (I) exists as an equilibrium mixture of conformational isomers in solution.

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