Electrocyclic rearrangements, equilibration

The synthesis of a number of pyrrole derivatives has been effected by the thermal conversion of 3-vinyl-2H-aziiines to butadienylnitrenes followed by 1,5-electrocyclization (75JA4682). For example, the thermal transformations observed on thermolysis of 2H-azirine 28 were rationalized in terms of an equilibration of the 2H-azirine with a butadienylnitrene, which subsequently rearranged to the final products. The rearrangement of 28 to pyrrole 30 was envisaged as occurring by an electrocyclic reaction followed by a 1,5-sigma tropic ethoxycarbonyl shift and subsequent tautomerization... 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

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