HETEROCYCLIC PHOTOCHEMISTRY .1. PHOTOTRANSPOSITIONS IN HYDROXYPYRYLIUM CATIONS - PERMUTATION PATTERN-ANALYSIS AND MECHANISTIC STUDIES

A number of dialkyl-4-pyrones have been irradiated (as the corresponding 4-hydroxypyrylium cations) in sulfuric acid and the transposed 2-pyrones have been identified. A permutation pattern analysis of these and literature data shows that the photorearrangement follows a P4 permutation pattern, thus defining the fate of all the ring atoms. This conclusion is supported by a total-labeling investigation : a study of the rearrangement of a l4C-labeled 4-pyrone confirmed that the alkyl migrations are intramolecular, the alkyl groups remaining bonded to the ring atoms during the rearrangement. The irradiation of 3, 5-dimethyl-4-pyrone in sulfuric acid led to the isolation of a new intermediate, the cyclic sulfate 22, which in sulfuric acid is converted thermally or photochemically to the 2-pyrone 8. In aqueous sulfuric acid, 2-acylfurans are also found as irradiation products. It is concluded that the excited 4-hydroxypyrylium cations relax to hydroxyoxabicyclohexenyl cations (30) as primary ground-state intermediates. A minor pathway of isomerization via a P8 permutation pattern is also discussed. The photointcrconversion of 2-hydroxypyrylium cations (e.g., eq 8) is shown to occur by a P8 permutation pattern. © 1979, American Chemical Society. All rights reserved.