Photocycloaddition of arylethenes to 2-substituted-1,4-naphthoquinones and reactions of the cyclobutane adduct isomers

Abstract

The photoreactions of 2-chloro-, 2-bromo-, 2,3-dichloro- and 2,3-bromo-1,4-naphthoquinones with arylethenes are dependent on the addend structure. Cyclobutane adducts are formed from styrene, 1,1-diphenylethene undergoes photosubstitution to give photolabile 1,1-diphenyl-1,3-diene derivatives, and spiro-oxetanes are the main products with trans-stilbene. 2-Acetoxy-1,4-naphthoquinone undergoes efficient (2π + 2π) photocycloaddition to isobutene, styrene and 1,1-diphenylethene but trans-stilbene yields the spiro-oxetane, 3′,4′-diphenyl-3-acetoxyspiro[naphthalene-1,2′-oxetan]-4(1H)-one, 34 regiospecifically. The isobutene adduct, 1,1-dimethyl-8a-acetoxy-1,2,2a,8a-tetradihydrocyclobuta[b]naphthalene-3,8-dione, 27 reacts readily with potassium tert-butoxide to yield 1,1-dimethyl-1,2-dihydrocyclobuta[b]naphthalene-3,8-dione 11 and is hydrolysed under acid conditions to the corresponding alcohol. In contrast the acetoxycyclobutanes from styrene and 1,1-diphenylethene give only the quinone dimers on base treatment, and under the acid conditions of its formation, the alcohol from the styrene adducts rearranges to 1-hydroxy-11-exo-phenyltricyclo[7.2.1.0]dodeca-2,4,6-triene-8,12-dione 39 while the adduct from the latter arylethene undergoes formal loss of hydrogen to give the red photolabile 3-(2′,2′-diphenylethenyl)-2-hydroxy-1,4-naphthoquinone 42.