Synthesis of N-bridgehead fused bicyclic beta-lactams through organometal-mediated transformations of 1,2-dialkenylaziridines

N-Bridgehead fused bicyclic beta-lactams have been synthesized through sequential organometal-mediated transformations of 1,2-dialkenylaziridines, namely Co-2(CO)(8)-catalysed carbonylation and ring-closing metathesis (RCM). When firstly subjected to RCM conditions, either with Schrock-Hoveyda or Grubbs' catalyst, 1,2-dialkenylaziridines did not afford the expected bicyclic N-bridgehead products. In contrast, however, cis-1,2-dialkenylazirldines were amenable to carbonylative ring expansion, affording trans-1,4-dialkenyl-beta-lactams which could be subsequently metathesized to the corresponding fused bicyclic compounds using Grubbs' catalyst. Unexpectedly, carbonylation of trans-1,2-dialkenylaziridines occurred with the unprecedented rearrangement to the 5,6-dihydro-4H-[1,3]oxazine skeleton.