AbstractSynthesis and reactions of 4‐carboxy‐6‐alkyl‐2‐piperidones and 4‐carboxyl‐6‐alkyl‐2‐pyrrolidones are described. Upon heating, the substituted 4‐carboxy‐2‐piperidones underwent both polymerization to polysuccinimides and isomerization to pyrrolidone derivatives. No polymerization but isomerization to ethylidene succinimide was observed upon heating the substituted 4‐carboxy‐2‐pyrrolidone. Both polymerization and isomerization can be explained by