Chronic Actinic Dermatitis to Sesquiterpene Lactones: [2+2] Photoreaction Toward Thymidine of (+) and (−) α-Methylene-Hexahydrobenzofuranone with a cis Ring Junction

(+) and (−) α-methylene-hexahydrobenzofuranone derivatives with a stereochemically pure cis ring junction were used as models of sesquiterpene lactones to study their photoreactivity toward thymidine. After 313 nm irradiation of a deoxygenated acetone solution of lactone models and thymidine, six 2+2] photoadducts were isolated for each enantiomer and fully characterized by a combination of NMR experiments. A common syn regioselectivity and exo stereoselectivity were observed for photoadducts. This high photoreactivity of α-methylene-γ-butyrolactone ring toward thymidine could be an explanation of the progressive evolution of allergic contact dermatitis toward chronic actinic dermatitis.