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 B-lactams. When the trap is omitted or not efficient enough, unusual irreversible 5-endo cyclisation occurs affording functionalised five-membered lactams. Synthesis of bicyclic gamma-lactams has also been examined providing in few steps an access to the Sceletium alkaloids skeleton. (C) 1998 Elsevier Science Ltd. All rights reserved.