Smooth generation of 3H-2-benzopyran-3-ones and their Diels–Alder reactions with olefinic dienophiles

Abstract

The generation of methoxy-substituted 3H-2-benzopyran-3-ones from o-acylphenylacetic acid derivatives in acetic anhydride at 140 °C and their in situ Diets–Alder reactions being inefficient in the synthesis of some carbocyclic compounds, alternative reagents have been used. It is shown that, e.g. 1,3-dicyclohexylcarbodiimide (DCC) with 2-hydroxypyridine, disuccinimidyl carbonate (DSC), 2-ethoxy-N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) of 2-isobutoxy-N-isobutoxycarbonyl-1,2-dihydroquinoline (IIDQ) are much more efficient reagents.

Using these alternative reagents the formation of side-products from the benzopyranone, e.g. chrysene, can be avoided; with the modified DCC-method the 6,7-dimethoxy-1 -methyl-2-benzopyran3-one intermediate 8a can be identified spectroscopically. An exemplified procedure with IIDQ is proposed, opening new prespectives for Diets–Alder reactions of unstable or less reactive dienophiles with problematic benzopyranones.