Reactivity of ()-4-Aryliden-5(4)-thiazolones: [2 + 2]-Photocycloaddition, Ring-Opening Reactions, and Influence of the Lewis Acid BF.

The irradiation of ()-2-phenyl-4-aryliden-5(4)-thiazolones with blue light (465 nm) in CHCl solution promotes [2 + 2]-photocycloaddition of the exocyclic C═C bonds and the formation of the dispirocyclobutanes . This reaction takes place with high stereoselectivity, given that the ε-isomer (1,3 head-to-tail syn coupling) is formed in more than 90% yield in most of the cases. However, irradiation of 5(4)-thiazolones with blue light (456 nm) in dry MeOH in the presence of BF·OEt leads to the monospirocyclobutanes with full stereoselectivity, also affording the ε-isomer.