α-Haloenol Acetates: Versatile Reactants for Oxetan-2-one, Azetidin-2-one and Isoxazolidin-5-one Synthesis

New ketene equivalents, namely α‐haloenol acetates, are investigated as both nucleophilic and electrophilic reactants in a tandem aldol–lactonization reaction. Diethylaluminum ethoxide proves to be an efficient promoter for the aldol reaction with a wide range of substrates, including inter alia, aldehydes, ketones, imines, nitrones and oximes, leading to oxetan‐2‐ones, azetidin‐2‐ones and isoxazolidin‐5‐ones. The resultant heterocyclic adducts are common structural elements in numerous compounds of interest as well as key intermediates in the preparation of other functionalities. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)