Photoinduced molecular transformations. Part 143. (2 + 2) Photoaddition of 3-hydroxy-1-benzopyran-2-one, 3-benzyloxycarbonyloxy-1-benzopyran-2-one, and their 8-methoxy derivatives, with alkenes and formation of 1,2-disubstituted 1,2-dihydrofuro[2,3-c][1]benzopyran-4-ones by way of β-scission of cyclobutanoxyl radicals generated from the resulting [2 + 2] photoadducts

Abstract

Direct photoaddition of 3-hydroxy-1-benzopyran-2-one with 2,3-dimethylbut-2-ene in tert-butyl alcohol gave tetrahydro-2a-hydroxycyclobuta[c][1]benzopyran-3-one (63%) arising from a (2 + 2) photoaddition while photoaddition of 3-(benzyloxycarbonyloxy)-1-benzopyran-2-one and its 8-methoxy derivative with cyclopentene gave 65:35 and 40 : 60 mixtures of cis-cisoid-cis-(65%) and cis-transoid-cis photoadducts (75%). Removal of the protecting groups of these adducts by hydrogenolysis gave the corresponding cis-cisoid-cis cyclobutanols (37 and 24%, respectively).Similar (2 + 2) photoadditions of the protected 3-hydroxy-1-benzopyran-2-one and its 8-methoxy derivative with cyclohexene gave a mixture of cis-cisoid-cis and cis-transoid-cis adducts, respectively. Removal of the protecting groups from each adduct gave 85 : 15 and 70 : 30 mixtures of the cis-cisoid-cis(86%) and cis-transoid-cis cyclobutanols (70%).Photolysis of the hypoiodites generated in situ from these cyclobutanols induced regioselective β-scissions of the corresponding cyclobutanoxyl radicals to give furo[2,3-c][1] benzopyran-4-ones (20–34%) together with 4-(trans-2-iodocycloalkyl)-1-benzopyran-2-ones (7–25%). The pathways leading to the formation of the products arising from the β-scission are discussed.