Thermorubin II: 1,3-Dihydroxy-9H-xanthones and 1,3-Dihydroxy-9H-xanthenes. New Methods of Synthesis

Two new efficient methods for the synthesis of 1,3-dihydroxy-9H-xanthones have been developed. The first, an adaptation of the classical method of Grover, Shah, and Shah, utilizes Eaton’s reagent (P₂O₅/CH₃SO₃H) in place of ZnCl₂/POCl₃ to effect the acylation step. The second, an entirely new method, is based on the Friedel-Crafts acylation of a 0,0,0-tris(trimethylsilyl)phloroglucinol by a 2-chlorobenzoyl chloride. Both methods give good yields. Improvements have been made in the Tanase synthesis of 1,3-dihydroxy-9H-xanthenes and the methyl ethers of the latter have been shown to undergo formylation at the 4-position. By contrast, related xanthones are known to undergo substitution at the 2-position. The structures in a series of 2-substituted 9H-xanthones including the previously known 2- and 4-methyl-1,3-dihydroxy-9H-xanthones have been examined by the use of an NMR shift reagent.