1-Methoxycarbonylpyrrolizin-3-one and related compounds.

Flash vacuum pyrolysis (FVP) of dimethyl E- or Z-pyrrol-2-ylbut-2-enedioate at 700 degrees C gave 1-methoxycarbonylpyrrolizin-3-one 1. The sequence involves E- to Z-isomerisation (if necessary), elimination of methanol and cyclisation; the elimination step is rate determining. The pyrrolizinone 1 is stable at low temperatures, but at room temperature dimerises spontaneously across the 1,2-bond to give a mixture of trans- and cis- cyclobutanes 2 and 3, respectively. In this process 1 behaves as a captodative alkene. The dimerisation can be reversed at >100 degrees C. Related pyrrolizinones such as the esters 14 and 22 and the amide 15 are stable to dimerisation. The diacid 12 and the amide 10 do not cyclise to pyrrolizinones under FVP conditions, but instead give the anhydride 19 and the maleimide 18, respectively; at high furnace temperatures, 2-ethynylpyrrole 17 is obtained from 12 and from 19.