N-Vinylic α-chloro-α-thioacetamides were found to cyclize without a catalyst in two different manners depending upon the nature of the substituents at the terminus of the N-vinylic bond. Thus, bis (phenylthio)-substituted enamides 8a-c cyclized in a 4-exo-trig manner to give 4- methylene-β-lactams 10a-c, whereas mono (phenylthio)-, monophenyl-, diphenyl-, and dialkyl-substituted congeners 23a, b, 28, and 38 cyclized in a 5-endo-trig manner to give γ ...
