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Thermal Arndt–Eistert reactions of N‐tosyl cyclic α‐amino acids were studied to explore the preparation of α,β‐unsaturated esters bearing a terminal tosylamino group. For L‐N‐tosyl‐aziridine 2‐carboxylic acid and L‐N‐tosyl‐azetidine 2‐carboxylic acid, the corresponding (E)‐α,β‐unsaturated esters were obtained stereospecifically.

A pyrrolizidine alkaloid, (2S, 8S)‐2‐benzyl hexahydro‐pyrrodizin‐3‐one, was synthesized from (L)‐proline using diastereoselective benzylation as the key step. The absolute configuration of the target compound was confirmed through 2D H‐H COSY and H‐H NOESY spectra.