Annelated tropones. Part 3. Photochemistry of some cyclohepta[b]-thiophen-4-ones

Abstract

On irradiation, methanolic solutions of the cyclohepta[b]thiophen-4-ones (5)–(7) give dimers, shown by spectral evidence to be [_π 4 +_π2] dimers, formed by reaction of the 5,6-bond of one thienotropone molecule with the 5,7-diene system of another. The dimers are of the head-to-head type and have a trans-ring junction. Irradiation of 2,6-dimethylcyclohepta[b]thiophen-4-one (8) leads to an intramolecular electrocyclic disrotatory reaction, giving a tricyclic ketone (14). The syntheses of the parent cycloheptathiophenone (5) and of the 7-methyl derivative (8) are described, and that of 7,8-dihydro-2-methylcyclohepta[b]thiophen-4-one (17).