Photoinduced Intramolecular formal [4 + 2] Cycloaddition of Aryl-Substituted o-Vinylstyryl-2-oxazoles To Form Benzo[f]quinoline Derivatives: Experimental Results and Theoretical Interpretation.

A new approach to benzo[f]quinoline derivatives has been found by an effective formal [4 + 2] photocycloaddition process from novel aryl-substituted o-vinylstyryl-2-oxazoles. All of the o-vinylstyryl-2-oxazoles were synthesized by a multicomponent Wittig reaction from the diphosphonium salt of α,α'-o-xylene dibromide, formaldehyde, and 5-tolyl-, 4-phenyl-5-methyl-, and 4,5-diphenyloxazole-2-carbaldehydes. TD-DFT calculations revealed that the intramolecular photocyclization in 2-(2-vinylstyryl)oxazoles to form benzo[f]quinoline derivatives proceeds on the S1 PES via a stepwise pathway, namely by 10π followed by 6π ring closure. On that path the existence of an S0/S1 conical intersection was indicated. The reactivity of the photocyclization steps depends on the substitution pattern at positions 4 and 5 of the oxazole ring, where the aryl group in position 5 deactivates the reaction.