Studies on the synthesis of heterocyclic compounds containing benzopyrone. Part 4. Synthesis of 4,10-dihydro-3-hydroxy-3-methyl-1H,3H-pyrano[4,3-b][1] benzopyran-10-one, the basic skeleton in fulvic acid

Abstract

The synthesis of 4,5-dihydro-3-hydroxy-3-methyl-lH,3H-pyrano[4,3-b][1]benzopyran-10-one (1b), the basic skeleton in fulvic acid, is described. The acetal (4), chosen as a common intermediate for syntheses of the basic skeletons in fungal metabolites such as fulvic acid and citromycetin, was cyclized into the dihydropyrone (7) with 5% HCl-tetrahydrofuran (1 : 2) regioselectively. Debenzylation followed by cyclization of the boron complex of (7) gave the tricyclic pyrone [2b], which was converted into the benzopyranone (1b) with 5% HCl–(1 : 1)