Synthesis of pyrrolidines and tetrahydro-1H-azepines from 4-aryl-1-benzoyl(ethoxycarbonyl)methyl-1-methyl-1,2,3,6-tetrahydropyridinium halides

4-Aryl-1,2,3,6-tetrahydropyridinium quaternary salts which have a benzoylmethyl or ethoxycarbonylmethyl group on atom N-1 generate N-ylides when heated in the presence of NaH and they can rearrange in situ with contraction or expansion of the six-membered heterocycle to give substituted pyrrolidines (as a result of a [2,3]-sigmatropic rearrangement) or 1H-tetrahydroazepine derivatives (via Stevens rearrangement). The presence of an aryl substituent at position C-4 in the tetrahydropyridine ring allows to avoid the formation of elimination products and changes the direction of the reaction towards the preparation of the tetrahydroazepines.