β- and γ-lactams by nickel powder mediated 4-exo or 5-endo radical cyclisations. A concise construction of the mesembrine skeleton

N-Alkenyl trichloroacetamides, upon treatment with nickel powder and acetic acid in refluxing 2-propanol undergo reversible 4-exo radical cyclisation. The cyclised radical can be trapped in different ways leading to β-lactams. When the trap is omitted or not efficient enough, unusual irreversible 5-endo cyclisation occurs affording functionalised five-membered lactams. Synthesis of bicyclic γ-lactams has also been examined providing in few steps an access to the Sceletium alkaloids skeleton.