AbstractTreatment of N‐1‐(2‐chloro‐2‐methylpropylidene) amines with potassium cyanate in methanol afforded 1‐substituted‐4‐methoxy‐5,5‐dimethyl‐2‐imidazolidinones, which were easily converted into the non‐methoxylated derivatives, i.e. 1‐substituted‐5,5‐dimethyl‐2‐imidazolidinones, by reaction with lithium aluminium hydride in diethyl ether. The structure was confirmed by independent synthesis of the latter heterocycles and their isomeric structure (4,4‐dimethyl derivative). The formation of the 2‐imidazolidinones, starting from α‐chloroaldimines and potassium cyanate, involved a rearrangement of the imino‐nitrogen atom, whic