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Site‐selective photocycloadditions of 2‐pyrones with electron‐poor olefins and the derivation from the cycloadducts

1993; Wiley; Volume: 30; Issue: 2 Linguagem: Inglês

10.1002/jhet.5570300223

1943-5193

Tetsuro Shimo, Shinichi Yamasaki, Kenichi Date, Hisako Uemura, Kenichi Somekawa,

Oxidative Organic Chemistry Reactions

Abstract Photosensitized cycloaddition of 4,6‐dimethyl‐2‐pyrone ( 1 ) with methacrylonitrile ( 3b ) afforded two types of [2 + 2]cycloadducts, 4b and 6b , across the C 5 ‐C 6 and C 3 ‐C 4 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 C 5 ‐C 6 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 C 5 ‐C 6 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.