PHOTOCHEMICALLY INDUCED CYCLIZATION OF BETA-KETO SULFIDES TO CYCLOALKANONES

On UV irradiation, beta-keto sulfides ArSCH(2)COPh Ar = C6F5, C6Cl5) cleaved to radicals ArS. and PhCOCH(2). which, in one case (Ar = C6F5), were trapped by 1,1-diphenylethylene as the adduct Pt-OCOCH2CH2C(Ph)(2)SC6F5. With diethyl ether as the solvent the major product was the monothioacetal ArSCH(Me)OEt. The keto sulfide 4-MeSO(2)C(6)H(4)SCH(2)COPh behaved similarly on irradiation in tetrahydrofuran. Irradiation of the unsaturated beta-keto sulfides CH2=CH[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 beta-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-ethoxycarbonyf-3-methyl-1,2,3,4-tetrahydronaphthalen-2-one and 6-methylthiochroman-3-one, respectively, in high yield.