Studies of the syntheses of heterocyclic compounds containing benzopyrone. Part 3. Synthesis of 2-methyl-4H,5H-pyrano[3,2-c][1]benzopyran-4-one, the basic skeleton in citromycetin

Abstract

The synthesis of 2-methyl-4H,5H-pyrano[3,2-c][1]benzopyrano-4-one (1b), the basic skeleton in citromycetin, is described. The alcohol (6a), chosen as the starting material, was oxidized to the dione (6c) which, after methylenation, was treated with concentrated hydrochloric acid–methanol (1 :100) at ambient temperature to afford the pyrone (9a) regioselectively. Hydrogenation and bromine substitution of the pyrone (9a) gave the bromide (9e), which was converted into the benzopyranone (1b) with aqueous sodium hydrogen carbonate.