Photoisomerization of 2 H ,6 H ‐thiin‐3‐one 1‐oxides to 3 H ,7 H ‐[1,2]oxathiepin‐4‐ones

Oxidation of 2 H , 6 H ‐thiin‐3‐ones 1a – c with 3‐chloroperbenzoic acid affords the corresponding 1‐oxides 2a – c . On irradiation (350 nm) in either benzene or MeCN, these cyclic sulfoxides 2 isomerize to 3 H , 7 H ‐1,2‐oxathiepin‐4‐ones 3 . The tetramethyl derivative 3a is isolated by flash chromatography at −10°, but, at higher temperatures, it undergoes ring contraction and H 2 O elimination to give 4,4‐dimethyl‐2(2‐methylprop‐2enylidene)thietan‐3‐one ( 4 ). Diemthyloxathiepinones 3b and 3c undergo ring contraction in MeOH to afford 1‐(4‐methylthiophen‐2‐yl)ethanone ( 5 ) and two diastereoisomeric 4,4‐dimethyl‐2‐methoxy‐2‐(1‐methoxyethyl)thietan‐3‐ones ( 6 and 7 , respectively).