Azapenams: An Efficient Approach to a Novel Class of ß-Lactams¹

The Michael adduct of (diphenylmethylene)amine to methyl 2-chloro-2-cyclopropylideneacetate 1 is readily converted to the azidoester 2; the imino group is hydrolyzed and the primary amino group protected with a benzyloxy- or menthyloxycarbonyl group. The azide is reduced with hydrogen sulfide and the second amino group then transformed to the formamidines 5, which cyclize upon heating to the imidazolines 6 in high yields. Irradiation of (dibenzylaminocarbene)pentacarbonylchromium 7 in the presence of 6 then yields azapenams 8 in moderate to good yield as a single diastereomers. With an appropriate combination of protecting groups in the starting materials, the carboxylate function in 8b-Bn can be deprotected selectively to give the free acid 8b-H.