Regioselective Photocycloaddition of Pyridine Derivatives to Electron-Rich Alkenes

Irradiation of a benzene solution of 3-cyano-2,6-dimethoxypyridine in the presence of ethyl vinyl
ether (EVE) gave 1:1 photoadducts, 3-cyano-5-ethoxy-2,8-dimethoxy-4,5-dihydroazocine, in good
yields, whose structure was established by X-ray single-crystal analysis. The photoadduct was
produced via cycloaddition between the C3−C4 position of the pyridine derivatives and an alkene
chromophore. On the other hand, 3-cyano-2,6-dimethoxy-4-methylpyridine cycloadds to EVE at
the C2−C3 position of the pyridine ring upon irradiation. The difference is explained on the basis
of the steric effect.