Photochemical reactions. Part V. Photoinduced cyclisations of 3-aroylchromones

Abstract

Whereas 3-benzoyl-2-methylchromone (1; R = H) does not undergo photocyclisation, its isomer, 3-(o-toluoyl)-chromone (2) produces a benzoxanthenone with ease; this implies that the electronic effects on the behaviour of the photochemical intermediates are delicately balanced. 2-Methyl-3-(o-toluoyl)chromone (5; R = H) also produces a benzoxanthenone on irradiation. Exploration of this last reaction with labelled substrates indicated that initial photoenolisation was followed by a rapid [1,7] sigmatropic hydrogen shift before cyclisation. Substituent effects on the process of photoenolisation are considered. The behaviour of the above chromones with base has been studied briefly.