Synthesis and structural study of [2.n](2,5)pyridinophanes

[2.n](2,5)Pyridinophanes possessing a cyclobutane ring and dioxaoligomethylene chain as tethers were efficiently prepared by intramolecular [2 + 2] photocycloaddition of vinylpyridine derivatives using a 400-W high-pressure mercury lamp through a Pyrex filter. They were of cis-configuration with respect to the cyclobutane ring, which was proved by the specific methine proton signals at δ 3.95–4.14. If the dioxaoligomethylene chain is short, the reaction affords only two exo,syn- and anti-isomers among three possible ones. On the other hand, if the chain is long enough, it gives three possible isomers interconverting at equilibrium. It is concluded that [2.n](2,5)pyridinophanes are rigid if n=4, 5 but flexible if n⩾6.