Exploiting the Reactivity of Destabilized Pyrrolylketene for the Stereoselective Synthesis of β-Lactams

Heteroarylketenes have recently attracted considerable attention as reactive intermediates in the synthesis of β-lactams due to their structural diversity and relevance in pharmaceutical chemistry (Chen et al., J. Am. Chem. Soc. 2021; Mann et al., SynOpen 2024; Alcaide & Almendros, Curr. Med. Chem. 2004).

In this work, stereoselective synthesis of novel β-lactams via in situ generation of a highly destabilized and nonconjugated pyrrolylketene intermediate from N-pyrrolylpropanoic acid was reported. Unlike typical aryl ketenes, this system lacks resonance stabilization, which enhances its reactivity toward aromatic imines under mild conditions.

The [2+2] cycloaddition proceeded efficiently, affording β-lactams predominantly as single trans-isomers. The stereochemistry of the products was confirmed via ¹H NMR, ¹³C NMR, and IR analysis. NOE experiments were employed to elucidate the relative stereochemistry of the trans-β-lactams. The observed selectivity supports a zwitterionic intermediate mechanism, allowing precise control over stereochemical outcome. Importantly, the approach tolerates various imine substrates and may serve as a basis for rapid access to azetidinone scaffolds of potential biological interest.

These findings broaden the scope of synthetic approaches to β-lactam synthesis and highlight the unique reactivity profile of destabilized heteroarylketenes as privileged intermediates.

This abstract is based on a previously published study (Synlett 2013, 24, 1937–1940) with emphasis on its stereoselective features and synthetic utility in β-lactam synthesis. Co-authors (Dr. M.R. Islami and Dr. M. Kalantari) of the original publication have been fully acknowledged.