2,2- and 2,6-Diarylpiperidines by Aryl Migration within Lithiated Urea Derivatives of Tetrahydropyridines

2-Aryltetrahydropyridines formed by anionic cyclization or ring-closing metathesis were converted to their N'-aryl urea derivatives. Depending on the position of the unsaturation within the tetrahydropyridine ring, metalation by deprotonative lithiation or carbolithiation led to migration of the N'-aryl substituent to the 2- or 6-position via intramolecular nucleophilic attack of a benzylic organolithium on the aryl ring. The products are a range of 2,2-, 2,2,3-, and 2,6-polysubstituted piperidine derivatives. Related chemistry was observed in pyrroline homologues.