Peri-, Site- and Stereocontrolled Photocycloaddition of 4-Methoxy-6-methyl-2-pyrone with Maleimide Induced by the Hydrogen Bond and CT Stacking in the Solid State

Photoirradiation of a 1:1 complex crystal of 4-methoxy-6-methyl-2-pyrone (1) with maleimide (2) peri-, site- and stereospecifically gave 11-methoxy-7-methyl-4-aza-8-oxatricyclo[5.4.0.02,6]undec-10-en-3,5,9-trione (3) (endo-[2+2] adduct), which was different from the solution products. It was inferred to be effectively brought by two sets of the hydrogen bonding and CT stacking between 1 and 2.