Rearrangements of Quebrachamine and Aspidospermine

2-Didehydroaspidospermidine (27), on deamination with nitrous acid, gives a hemiacetal 250, an unexceptional result that can be explained by any one of three mechanisms (194). 

Pyrolysis of (-)-l,2-didehydroaspidospermidine (27) at 200°C affords (-)-aspidospermidine (251) and (-)-ebumamenine (252). This is interpreted as proceeding via a duneric species, which undergoes two 1,5-sigma-tropic shifts to give an intermediate 253 that then fragments to (-)-aspidospermidine (251) and (-)-ebumamenine (252) (195). At much higher temperatures, under flow thermolysis conditions, several products are formed. At 580 C the only product is vincane (254), which presumably 

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