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

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 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.

Crystal Structure of 4,13-Dimethyl-3,7,10,14,19,22,25-heptaoxapentacyclo-[25.0.01,6.06,26.011,16.011,18]heptacosa-4,11-diene-2,15-dione

“…We have previously reported on the intermolecular photocycloaddition reactions of 2-pyrones with olefins, [1][2][3] and the intramolecular photodimerization reactions of 4,4′-polymethylenedioxy-di-2-pyrones 4 to clarify the reactivity involving the study of peri-, site, and stereoselectivities. As a part of an application of the cycloaddition reactions, we have planned to synthesize a macrocyclic compound possessing two cyclobutane rings using double photocycloaddition reactions of 6,6′-dimethyl-4,4′-dimethylenediox-di-2-pyrone (1) with di(ethylene glycol) divinyl ether (2).…”

Crystal Structure of Photocycloadduct between 4-Methoxy-6-methyl-2-pyrone and Maleimide

“…As a part of a continuing program involving the study of peri-, site-, and stereoselective photocycloaddition reactions of 2-pyrones with alkenes [1][2][3] , a solid-state photoreaction of 4-methoxy-6-methyl-2-pyrone with maleimide was investigated. 4-Methoxy-6-methyl-2-pyrone (0.14 g) and maleimide (0.097 g) were dissolved in acetonitrile (5 ml) and the solvent was evaporated spontaneously in a chemical hood to give crystals(A).…”

Crystal Structure of Methyl endo-8-Cyano-exo-8-methyl-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylate

“…1 We previously reported on the photoreaction of methyl 2-pyrone-5-carboxylate (1) with methacrylonitrile to give a [4 + 2] cycloadduct as a major product together with [2 + 2] cycloadducts. 2 Since the usual analytical and spectroscopic techniques do not permit one to identify the [4 + 2] cycloadduct (endo-8-CN (2) or exo-8-CN (3)) [Scheme 1], an X-ray diffraction study of the product was carried out to determine the structure. A colorless prism crystal with dimensions of 0.3 × 0.2 × 0.1 mm was obtained from a benzene solution at room temperature, and used for diffraction experiments.…”