Efficient Synthesis of N‐Substituted 2,4‐Azepandione Ring System as an Active Intermediate for Heterocyclic Syntheses

An improved efficient synthesis for 2,4‐azepandiones (3, 8, and 14) could be achieved by a careful control of the reaction conditions to cyclize ethyl 4‐(N‐acetylarylamino) butanoate (1, 7, and 13), respectively. The ethyl 4‐arylamino butanoate (9 or 12) was prepared by stirring the ethyl 4‐bromobutanoate and substituted anilines at room temperature. Then, they were acetylated with acetyl chloride and triethylamine under the conditions that avoid the formation of 2‐pyrrolidinone derivatives 10. Due to the rapid decomposition of the acetylated product (7 or 13) to its starting material (9 or 12), the reaction mixture is directly transferred without workup to the next cyclization step. The azepandione synthesis is favored by using a weak base at low temperature, where it is in a competition with the other modes of ring closure. The structures of the new compounds were supported by correct analytical and spectral data.