Photochemically induced cyclisation of β-keto sulfides to cycloalkanones

Abstract

On UV irradiation, β-keto sulfides ArSCH₂COPh (Ar = C₆F₅, C₆Cl₅) cleaved to radicals ArS^· and PhCOCH₂˙ which, in one case (Ar = C₆F₅), were trapped by 1, 1 -diphenylethylene as the adduct PhCOCH₂CH₂C(Ph)₂SC₆F₅. With diethyl ether as the solvent the major product was the monothioacetal ArSCH(Me)OEt. The keto sulfide 4-MeSO₂C₆H₄SCH₂COPh behaved similarly on irradiation in tetrahydrofuran. Irradiation of the unsaturated β-keto sulfides CH₂CH[CH₂]₂COCH₂SAr resulted in homolysis of the CH₂–S bond to give a 2-oxohex-5-enyl radical, which subsequently cyclised; 4-(pentafluorophenylsulfanyl)cyclohexanone (when Ar = C₆F₅) and cyclohexanone (when Ar =p-tolyl) were the major products. With β-keto sulfides containing an aryl substituent, ethyl 2-benzyl-2-methyl-3-oxo-4-(pentafluorophenylsulfanyl)butanoate and 1,3-bis(p-tolylsulfanyl)propanone, irradiation resulted in cyclisation with loss of the sulfanyl substituent (probably involving electron transfer) to give 3-ethoxycarbonyl-3-methyl-1,2,3,4-tetrahydronaphthalen-2-one and 6-methylthiochroman-3-one, respectively, in high yield.