Photosensitized Diels–Alder reactions of N-arylimines: synthesis of tetrahydroquinoline derivatives

Irradiation (λ>345 nm) of catalytic amounts of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) in a CH2Cl2 solution of N-aryl imines and α-methylstyrene or styrene produced the corresponding [4+2] cycloaddition products, tetrahydroquinoline derivatives, in good yield. The reaction was controlled by the relative oxidation potentials of the dienophile and the diene.