Synthesis of five-membered lactams by α-carbamoyl radical cyclisations

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Abstract
Free-radical cyclisation of N-allyl-α-carbamoyl radicals generated by tributyltin radical-mediated cleavage of carbon–chlorine or carbon–sulphur bonds has been studied in some detail. The radicals substituted at the α position by methylthio (phenylthio), chloro, methyl, phenyl, methoxy, or acetoxy groups underwent smooth cyclisation in a 5-exo-trig manner to give five-membered lactams in which the trans-isomer predominated. In contrast, the N- or α-unsubstituted, or α-sulphonyl-substituted, radicals gave predominantly or exclusively the reduction products. Cyclisation of the N-β-methylallyl system proceeded again regioselectively to give the five-membered lactam.