Crinatusins, Bioactive Diels–Alder Adducts from Cyathocalyx crinatus

From the stems of Cyathocalyx crinatus, a rain forest plant in Borneo, six new Diels–Alder adducts, named crinatusins A1, A2, B1, B2, C1, and C2, together with a known chalcone derivative, 4′,6′-dihydroxy-3′,5′-dimethyl-2′-methoxychalcone, have been obtained. Their structures were determined on the basis of spectroscopic data and chemical methods. Crinatusins showed a lethal toxicity against brine shrimp.