The problem of synthesizing bispyrylium salts

Abstract

Potentially useful methods of synthesizing bispyrylium salts are reviewed. It is found that alkaline condensation of aromatic dialdehydes with aliphatic-aromatic ketones gives tetraketones, converted into bispyrylium salts by the action of hydride ion acceptors. Contrary to expectation, condensation of 1, 1, 2, 2-tetraacetylethane with resorcinol in acid solution gives a monopyrylium salt. Acetylation of 2, 4-dimethylpentane-2, 4-diol with acetic anhydride in the presence of perchloric acid led to the isolation of 2, 4, 6-trimethylpyrylium perchlorate, in addition to the expected methylenebispyrylium salt. Reaction of 2, 4-diphenyl-6-methylpyrylium perchlorate with terephthaldehyde gives a low yield of the product formed by condensation at both carbonyl groups.