Photoinduced molecular transformations. Part 127. A new [2 + 2] photoaddition of 2-amino-1,4-naphthoquinone with vinylarenes and the synthesis of 2,3-dihydronaphtho[1,2-b]furan-4,5-diones and 2,3-dihydronaphtho[2,3-b]furan-4,9-diones by β-scission of alkoxyl radicals generated from the resulting photoadducts

Abstract

Irradiation of 2-amino-1,4-naphthoquinone and excess of vinylarenes such as styrene, its para-substituted derivatives, 2-vinylpyridine and 4-vinylpyridine in benzene with a Pyrex-filtered light at room temperature under nitrogen gave 1-aryl-1,2,2a,8b-tetrahydro-8b-hydroxycyclobuta[a]naphthalene-3,4-diones as a single product in 23–63% yields, respectively. These cyclobutanols arise from an unprecedented stereo- and regio-selective [2 + 2] photoaddition of an excited enol form of 2-amino-1,4-naphthoquinone with the vinylarenes.Irradiation of these cyclobutanols in benzene containing excess of mercury(II) oxide and iodine with Pyrex-filtered light then gave 2-aryl-2,3-dihydronaphtho[1,2-b]furan-4,5-diones and/or 2-aryl-2,3-dihydronaphtho[2,3-b]furan-4,9-diones (31–73% yields) arising from a regioselective β-scission of the alkoxyl radicals, followed by intramolecular cyclization of the resulting radical intermediates. The substituent at the para position of the 1-phenyl group was found to control the ratio of the two types of products: a methoxy group directs the formation of the 4,5-dione exclusively, while the p-cyano group directs the formation of the 4,9-dione exclusively. This considerable dependence of the products on the substituents suggests that at least some part of the formation of the dihydrofuran ring of the 4,5 and/or 4,9-diones takes place from the ionic intermediates arising owing to β-scission. 2,3-Dihydronaphtho[1,2-b]furan-4,5-dione was found to isomerize to 2,3-dihydro[2,3-b]furan-4,9-dione on silica gel.