Solid-state photoirradiation of 1: 1 complex crystals of 4-[ω-(2-furyl) alkyloxy]-6-methyl-2- pyrones 1b, 1c or 4-(ω-arylalkyloxy)-6-methyl-2-pyrones 1d–j with maleimide 2 gave [2+ 2] cycloadducts 3b–f, 3i, 3j with exclusive stereoselectivity. The high reaction selectivity was confirmed by X-ray structure analyses and MO method of the complex crystals, which were composed of two sets of a 1: 1 complex between 1 and 2, arising from an CH–π ...