HETEROCYCLIC PHOTOCHEMISTRY .2. 4-PYRONES - MECHANISTIC STUDY

The phototransposition of 4-pyrones into 2-pyrones in neutral solvents is shown to follow the sequence (4-pyrone)* →* oxabicyclohexenyl zwitterion (e.g., 14) → cyclopentadienone epoxide (e.g., 15) + hv → 2-pyrone. The oxabicyclohexenyl zwitterion derived from 3, 5-dimethyl-4-pyrone has been trapped as a [4 + 3] cycloadduct with furan and the corresponding cyclopentadienone epoxide has been isolated and shown to rearrange photochcmically to 2-pyrone. The zwitterion also reacts with hydroxylic solvents to give derivatives of cyclopentenone-4, 5-diols and of furfuraldehyde. The effect of substituents in the 4-pyrone and of solvents on the photochemical behavior is rationalized. © 1979, American Chemical Society. All rights reserved.