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

Abstract

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. © 2012 The Royal Society of Chemi..