8‐Phenyl‐10,10a‐dihydropyrido[1,2‐a]azepines by 1,7‐Electrocyclization of Conjugated Pyridinium Ylides—Rationalization of the Periselectivity

Base treatment of the pyridinium bromides 11 a–e gives rise to the formation of the dihydropyridoazepines 14 a–e as the only monomolecular products. The reaction takes place by initial deprotonation to the ylides 12, which undergo 8π-electrocyclization affording the seven-membered-ring systems; no products of a dipolar 6π-cyclization were detected. On the basis of quantum mechanical calculations a rationalization of the periselectivity of the electrocyclization process is given.