AbstractAuthentic samples of 1,3,3‐trimethyl‐2‐(3,4,5‐trimethoxyphenyl)‐4‐methylenepiperidine (Ia) and 2‐(p‐chlorophenyl)‐1,3,3‐trimethyl‐4‐methylenepiperidine (Ib) are prepared by Mannich condensation between 4‐methyl‐1‐methylamino‐3‐pentanone hydrochloride (VI) and an aromatic aldehyde, followed by a Wittig reaction on the resulting 4‐piperidone. Comparing the physical and spectroscopic properties of Ia and Ib with those of the methylene derivatives IIa and IIb obtained as by‐products in the Stevens rearrangement of 1‐benzyl‐1,3,4‐trimethyl‐1,2,5,6‐tetrahydropyridinium salts IIIa and IIIb, respectively, it is shown that the assignment previously made for IIa and IIb is incorrect. Spectroscopic analysis (ir,1H nmr,13C nmr, ms) of these compounds and of its hydrogenation products VIII allows the structural and stereochemical assignment of 11a ascis‐3‐isopropenyl‐1,3‐dimethyl‐2‐(3,4,5‐trimethoxyphenyl)pyrrolidine and of IIb ascis‐2‐(p‐chlorophenyl)‐3‐isopropenyl‐1,3‐dimethylpyrrolidine. The formation of these rearrangem