Photochemical synthesis of tricyclic β-lactams and their isomerization to β-thiolactones

The photochemistry of cyclic N-(α,β-unsaturated carbonyl)-thionocarbamates and -dithiocarbamates has been studied. Photolysis of a benzene solution of 3-(methacryloyl)thiazolidine-2-thione gave 4-methyl-2,9-dithia-6-azatricyclo[4.3.0.01.4]nonan-5-one (a tricyclic-β-lactam)(30%) and 2-(3′-methyl-2′-oxothietan-3′-yl)-2-thiazoline (a β-thiolactone)(30%). Irradiation of 3-(2′-methylbut-2-enoyl)thiazolidine-2-thione, 3-(methacryloyl)oxazolidine-2-thione, and 3-(2′-methylbut-2-enoyl)oxazolidine-2-thione gave the corresponding β-lactams (33–67%), whereas that of the six-membered compounds, 3-(methacryloyl)- and 3-(2′-methylbut-2-enoyl)tetrahydro-1,3-oxazine-2-thione, led to β-thiolactones (33–66%). The ring transformation of this new ring system (thietane-fused penams and oxapenams) to β-thiolactones was confirmed by the fact that, on pyrolysis, the β-lactams gave the corresponding β-thiolactones in quantitative yield.