Reinvestigation of the stevens rearrangement of 1‐benzyl‐1,3,4‐trimethyl‐1,2,5,6‐tetrahydropyridinium salts. II. Synthesis of 2‐aryl‐3‐isopropenyl‐1,3‐dimethylpyrrolidines

Abstract

Abstractcis‐2‐Aryl‐3‐isopropenyl‐1,3‐dimethylpyrrolidines Ha and IIb have been synthesized by an unambiguous way, thus confirming the structure of the methylene derivatives obtained as by‐products in the Stevens rearrangement of 1‐benzyl‐1,3,4‐trimethyl‐1,2,5,6‐tetrahydropyridinium salts Ia and Ib. The synthesis is based on the acid‐induced intramolecular cyclization between an iminium salt and the α‐position of a ketal group. Thus, condensation between amino ketal XXI, prepared via Gabriel synthesis from 5‐chloro‐3‐methyl‐2‐pentanone, and the appropriate aldehyde afforded imines XXI. Their treatment with dry hydrogen chloride followed by acid hydrolysis and methylation gave 3‐acetylpyrrolidines IV, which were transformed into the isopropenyl derivatives II by reaction with methyl‐lithium and further dehydration.