[2+2] Photodimerization of (Z)-4-styrylpyridine through a cation–π interaction: formation of cis–cis–trans dimers

The cis–cis–trans cyclobutanes are prepared by photodimerization of (Z)-4-styrylpyridine. A catalytic amount of HCl plays a key role in enhancing the [2+2] photocyclization reactions between (Z)- and (E)-4-styrylpyridines to give r-cct and r-ctc cyclobutane dimers through a cation–π interaction.