AbstractThe preparation of 3‐azetidinones with differentN‐substituents and their transformation to 3‐ethylideneazetidines has been studied in relation with a projected synthesis of 3‐ethylideneazetidine‐2‐carboxylic acid (= polyoximic acid;1). The stable crystalline 3‐azetidinone hydrochloride (18), obtained by hydrogenolysis of the known 1‐(diphenylmethyl)‐3‐azetidenone (4), is described for the first time, From this key intermediate, 3‐oxoazetidine‐derived amides, exemplified by benzamide12,urethanes (e.g.17), and ureas (e.g.20) can be prepared in good yield (Scheme 3). The olefination of 3‐azetidinones with alkylidene(triphenyl)phosphoranes is a perparatively useful process only for derivatives with a pyramidal N‐atom,e.g.the diphenylmethyl derivative4, and not for the amide12or the urethane17(Scheme 4). 3‐Alkylidene‐azetidines with an amide N‐atom should, therefore, be prepared by exchange of theN‐substituent after