Radical cyclisations of methylenecyclopropyl azetidinones-synthesis of novel tricyclic beta-lactams

Addition of lithium bis(methylenecyclopropyl)cuprates to acetoxy azetidinon es gives methylenecyclopropyl azetidinones, which can be converted to vario us radical cyclisation precursors. Attempted 4-exo cyclisation of 3 led onl y to reduced product, while cyclisation of 5, using CuCl/bipy, gave a carba cephem, via a 5-exo cyclisation, but in low yield. Cyclisation of 6 and 7, however, gave novel tricyclic beta -lactams, as the result of 7-endo cyclis ation, in good yield, and a cyclisation of bromide 23 led to the tricyclic beta -lactam 24, via a radical cascade sequence. (C) 2000 Elsevier Science Ltd. All rights reserved.