Photosensitized cycloaddition of 4,6‐dimethyl‐2‐pyrone (1) with methacrylonitrile (3b) afforded two types of [2 + 2]cycloadducts, 4b and 6b, across the C5‐C6 and C3‐C4 double bonds in 1, respectively. Photosensitized reactions of 1 with dimethyl maleate and dimethyl cyclobutene‐1,2‐dicarboxylate gave [2 + 2]cycload‐ducts 4d, 4e across the C5‐C6 double bond, in addition to [4 + 2]cycloadduct 9d or bicyclo[4.2.0]octadiene 10e. The photoreactions of methyl 2‐pyrone‐5‐carboxylate (2) with 3b and 2‐chloroacrylonitrile (3c) gave [4 + 2]cycloadducts 5b, 5c in addition to [2 + 2]cycloadducts 11b and 11c across the C5‐C6 double bond in 2. The photocycloaddition mechanism was explained from results calculated by means of PM3‐CI method. Namely, the site‐ and/or regio‐selective products, 4, 5, 8, 9 and 10 were thought to come from the same site‐selective radical intermediates in the case of electron‐poor olefins. Pyrolysis and/or hydrolysis of the cycload‐ducts 4e, 5b, 5c gave 5,6‐dihydro‐2‐pyrone 12 or benzene derivatives.

Novel Transformations of Two Kinds of Chlorinated Photo (2 + 2) Cycloadducts of 2-Pyrone-5-carboxylate.-DBU treatment of the photoproducts (III) leads to the compounds (V) via dehydrochlorination, retro-aldol reaction and lactonization. Treatment of the photoproduct (IVb) with DBU yields the compound (VII) via dehydrochlorination. -(SHIMO, T.; DATE, K.; SOMEKAWA, K.; J. Heterocycl. Chem. 29 (1992) 2, 387-390; Dep. Appl. Chem. Chem. Eng., Fac. Eng., Kagoshima Univ., Korimoto, Kagoshima 890, Japan; EN)

Novel reactions of 7,7‐dichloro‐ and 7,7,8‐trichloro‐3‐oxo‐2‐oxabicyclo[4.2.0]oct‐4‐ene‐6‐carboxylates 5 with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in alcohol gave pyrano[4,3‐b]pyran‐2,5‐diones 8 via (Z)‐(2H‐pyran‐2‐on‐3‐yl)butenoates 7. On the other hand, the same treatment of 7,7,8‐trichloro‐2‐oxo‐3‐oxabicyclo‐[4.2.0]oct‐4‐ene‐5‐carboxylate 6b afforded 2‐oxo‐3‐oxabicyclo[4.2.0]oct‐4,7‐diene‐5‐carboxylate 14 via cyclobutene formation and SN2′ displacement by attack of the alkoxy anion.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.