Abstract
Abstract It is one of the simplest and most convenient methods for the preparation of 2‐azetidinones (i.e., β‐lactams) by the [2+2] cycloaddition of ketenes and imines in a convergent and stereocontrolled manner. Therefore, it is generally known as the Staudinger reaction. Occasionally, this reaction is also referred to as Staudinger [2+2] cycloaddition. It has been reported that the Staudinger [2+2] cycloaddition is not a pericyclic reaction, but a stepwise reaction involving a nucleophilic attack of an imine on ketene to form a zwitterionic intermediate, which cyclizes conrotatorily to form the β‐lactam. The study finds that electron‐donating groups on ketenes and electron‐withdrawing groups on imines accelerate the direct ring closure, leading to the formation of cis ‐β‐lactams; in contrast, the electron‐withdrawing substituents on ketenes and electron‐donating substituents on imines stabilize the zwitterionic intermediate and retard the direct ring closure. This reaction has been extensively modified which results in an improved isolation yield of β‐lactam.
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