8-Phenyl-10,10a-dihydropyridol[1,2-a] azepines by 1,7-Electrocyclization of Conjugated Pyridinium Ylides - Rationalization of the periselectivity

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