Remarkable interaction effects of molecular packing on site- and stereoselectivity in photocycloaddition of 2-pyrones with maleimide in the solid state

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 interaction between 2 and the aromatic rings of 1, and/or π–π stacking between the aromatic rings in addition to four kinds of hydrogen bonding between the ground state 2-pyrone moieties and 2.