Annelation of ethyl propiolate with ethyl pipecolate.

In conjunction with studies of the alkaloid slaframinel we undertook a search for routes to A2 derivatives of l-oxooctahydroindolizines. The cycloaddition of ethyl propiolate to ethyl pipecolate (1) was formally attractive for this purpose although an earlier attempt by Winterfeldt and Dillinger to effect a similar condensation of dimethyl acetylenedicarboxylate with methyl ethylaminoacetate had met with failure, only an uncyclized Michael adduct being obtained.2,3 Nevertheless, we were encouraged by molecular model studies which suggested that the geometry of the zwitterionic intermediate resulting from addition of 1 to the acetylenic ester would be particularly conducive to annelation.