N-Arylpyrrolidin-3-one is a key starting material for the synthesis of a number of fused heterocycles, among them certain analogs of the anticancer drug methotrexate [1,2]. Pyrrolidine derivatives which bear an alkyloxime substituent in position 4 and an aminoalkyl substituent in position 3 of the pyrrolidine ring were used to produce a series of fluoroquinolone antibacterials (e.g., gemifloxacin, norfloxacin) and also used as bax inhibitors [3][4][5][6]. Some of the 3-pyrrolidinone derivatives were used as inhibitors of HIV-1 replication [7]. We are reporting here the synthesis of some new fused heterocycles having a pyrrolidine moiety, such as pyrrolo [3,4-c]isoxazoles, pyrrolo[2,3-d][1,2,3]triazoles, triazolo[4,5-c]pyridazine, and dipyrrolo[3,2-b:3',4'-d]pyran derivatives.The starting pyrrolidin-3-ones (considered as heterocyclic β-oxonitriles) 1a,b were prepared from primary aromatic amines, acrylonitrile, and ethyl bromoacetate according to the methods reported [8-10]. We investigated their reaction with NH 2 OH·HCl in refluxing ethanol containing sodium acetate. The reaction furnished the expected oxime derivatives [11,12] 2a,b (Scheme 1), but all attempts to cyclize these oximes into the pyrroloisoxazole derivatives 3a,b were unsuccessful. We assumed that oximes 2a,b were present completely in the Z-form relative to the pyrrolidine ring, in which the OH group was oriented away from the cyano group. The Z-form of 2a,b was stable under thermal, acidic, or basic conditions, therefore, the transformation of 2a,b into 3a,b proved to be difficult. We envisioned that if we introduce a bulky substituent in position 2 of the 3-pyrrolidinone 1a,b, the geometry of the resulted oxime would be changed. Accordingly, we investigated the reaction of 1a with aryldiazonium salts under mild basic conditions (ethanol and sodium acetate); the reaction __________________________________________________________________________________________