Solid-state photochromism and photoreactivity of o- and p-anisaldehydes. Remarkable stabilization of o-xylylenols

reaction: see text] All of the crystalline o-anisaldehyde derivatives 1 and 3 change to brick-red color upon brief exposure to UV-vis radiation. The red color, attributed to (E)-xylylenol, is remarkably persistent for hours (ca. 10 h) in the case of 1c; such a long lifetime for the reactive o-xylylenols is unprecedented. In contrast, the p-anisaldehydes 2 undergo cyclization. Solid-state photolysis of 2b affords the benzocyclobutenol 7b regioselectively, which is not accessible from solution-phase photolysis.