Synthesis of carbacephems from serine

Carbacephems have been synthesized from D-serine by two routes involving construction first of the six-membered ring followed by cyclization to give the bicyclic beta-lactam. In one route, alkylation of a lactim ether was accomplished with Ni(Acac)(2) as a catalyst. The desired R stereochemistry at the carbon corresponding to C-6 of the cephem was obtained by stereospecific hydrogenation of a vinylogous carbamate. The second route involved a stereospecific Michael cyclization to give the same C-6 stereochemistry. Closure of a piperidyl beta-amino acid intermediate common to both routes was accomplished using a modified Mukaiyama reagent found to be superior in our system to the traditional reagent. The resulting carbacephem core was stereospecifically substituted at C-7 with an ethyl or amino functionality. The ethylated intermediate was transformed into a stable enol triflate useful for the further elaboration of biologically important carbacephems.