Synthesis of (±)-thienamycin based on a new approach to β-lactams via 4-exo-trig cyclisation of carbamoylcobalt salophens

A series of N-propenyl substituted N-benzylcarbamoylcobalt(III) salophens, i.e.11, 20 and 30, have been prepared, and have been shown to be useful precursors in a new approach to β-lactams (viz.12-16, 21 and 31), via 4-exo-trig modes of cyclisation of the corresponding intermediate carbamoyl radicals. A new formal synthesis of (±)-thienamycin 1 from the trans-disubstituted azetidin-2-one 31 produced in one step by heating the carbamoylcobalt salophen 30 in toluene, is also described.