169. Photoisomerization of 2H,6H-Thiin-3-ones to 2-(Alk-1-enyl)thietan-3-ones

Article abstract of DOI:10.1002/hlca.19920750712

Reaction of 3-bromo-3-methylbutan-2-one (1) with mercapto-esters 2 affords 5-oxo-3-thiahexanoates 3 which cyclize to thiane-3,5-diones 4.Conversion of these dicarbonyl compounds to their ethyl enol ethers 5-7 followed by reduction with LiAlH4 gives 2H,6H-thiin-3-ones 8-10.On irradiation (350 nm) in either MeCN, benzene, or i-PrOH, these newly synthesized heterocycles isomerize efficiently to 2-(alk-1-enyl)thietan-3-ones 11-13.The rearrangement seems to proceed from an excited singlet state, as it is not quenched by naphthalene, and also occurs with the same efficiency in the presence of added alkene.A (9-S-3) sulfuranyl-alkyl biradical formed by bonding of C(α) of the enone C=C bond on sulfur is discussed as possible intermediate.