Unexpected ring C enlargement of the aspidospermane skeleton

Treatment of (−)-16-chloro-1-dehydro-5-methoxy-vincadifformine 4 with Ac2O–pyridine led to the rearranged compound 6 through an unexpected ring C expansion. Upon reaction with dimethyl acetylenedicarboxylate, 4 provided the [2+2] cycloaddition derivative 9 but did not undergo the ring C enlargement.