Synthetic studies towards the mulberry Diels–Alder adducts: H-bond accelerated cycloadditions of chalcones

The methyl ether derivatives 2, 4 and 6 of the mulberry Diels–Alder adducts chalcomoracin (1) and mulberrofuran C (3) and kuwanon J (5) respectively have been synthesized by a thermal [4 + 2]-cycloaddition reaction between a chalcone and dehydroprenyl diene. A H-bonded ortho OH substituent on the chalcone was found to be essential for Diels–Alder reactivity. Density functional theory calculations show that the OH group lowers the barrier for the Diels–Alder reaction by 2–3 kcal mol⁻¹ compared with OMe. The acceleration by the OH group is traced to two transition-state effects: a stronger diene–chalcone interaction and better planarity of the aryl–diene unit.