Synthesis of Novel 4‐(2‐Oxoethylidene)azetidin‐2‐ones by a Lewis Acid Mediated Reaction of Acyldiazo Compounds

We report the synthesis of a class of 4‐(2‐oxoethylidene)azetidin‐2‐ones by a novel Lewis acid mediated reaction of acyldiazo compounds with 4‐acetoxyazetidin‐2‐ones. Using this approach, C‐3‐and C‐4‐substituted 4‐alkylideneazetidin‐2‐ones were obtained depending on the choice of starting azetidinone and α‐diazocarbonyl compound. The products reveal excellent Z diastereoselection of the C‐4 double bond. Variable amounts of E isomers are obtained depending on the nature of the C‐3 side chain, the Lewis acid, and the β‐lactam protection. VT NMR spectroscopic experiments and X‐ray structural analysis of crystalline derivatives demonstrate the presence of an intramolecular hydrogen bond in Z isomers, which drives the stereochemical outcome of the reaction. A possible mechanism for this novel reaction is proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)