Synthetic studies towards the mulberry Diels–Alder adducts: H-bond accelerated cycloadditions of chalcones
作者:Sompong Boonsri、Christian Gunawan、Elizabeth H. Krenske、Mark A. Rizzacasa
DOI:10.1039/c2ob25115a
日期:——
The methyl ether derivatives 2, 4 and 6 of the mulberry DielsâAlder 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 DielsâAlder reactivity. Density functional theory calculations show that the OH group lowers the barrier for the DielsâAlder reaction by 2â3 kcal molâ1 compared with OMe. The acceleration by the OH group is traced to two transition-state effects: a stronger dieneâchalcone interaction and better planarity of the arylâdiene unit.
桑树Diels-Alder加合物白果草素(1)、桑酮C(3)和桑酮J(5)的甲基醚衍生物2、4和6分别通过查耳酮与脱氢 prenyl diene之间的热[4 + 2]环加成反应合成。研究发现,查耳酮上形成氢键的邻位羟基取代基对于Diels-Alder反应性至关重要。密度泛函理论计算表明,与甲氧基相比,羟基可将Diels-Alder反应的能垒降低2-3千卡/摩尔。羟基的加速作用归因于两种过渡态效应:更强的二烯-查耳酮相互作用和更好的芳基-二烯单元平面性。