Syntheses of permethylated derivatives of pinastric acid and gomphidic acid, pulvinic acid pigments of lichen and fungi

Abstract

The application of 2-aryl O-methyltetronic acids (7) and (20) to the total synthesis of permethylated derivatives of pulvinic acid pigments found in Lepraria flava and Gomphidius glutinosus is described. Metallation of the tetronic acid (7), followed by treatment with methyl benzoylformate and dehydration of the intermediate carbinol led to O-methyl pinastric acid (9), identical with the O-methyl derivative of natural pinastric acid from L. flava. In a similar manner the tetronic acid (11) and methyl 4-methoxybenzoylformate gave O-methylisopinastric acid (12), and condensation between (20) and methyl 4-methoxybenzoylformate led to permethylated gomphidic acid (21). Comparison of ¹H n.m.r. shift data for (21) with those reported for gomphidic acid from G. glutinosus established structure (2b) for the natural metabolite.