Reaction-manifold diversion enables general access to oxa-β-lactam chemical space

Oxa-β-lactams (1,2-oxazetidin-3-ones) constitute the formal oxygen analogues of β-lactams yet remain largely absent from chemical and biological studies because general synthetic access has been lacking, historically limited to specialized cycloaddition and precursor-encoded ring-contraction strategies. We report that incorporation of aminooxyacetic acid into the Ugi multicomponent reaction diverts the canonical reaction manifold and selectively furnishes oxa-β-lactams under mild, room-temperature conditions from simple carbonyl compounds and isocyanides. Automated nanoscale high-throughput experimentation enabled mapping of the accessible reaction space and identification of productive substrate classes, which translated to preparative synthesis of structurally diverse, fully characterized derivatives, including single-crystal X-ray structures. Structural analysis revealed pronounced pyramidalization and reduced coplanarity within the amide substructure, suggesting an electronically perturbed, strain-activated amide motif with potential relevance for covalent ligand design. This possibility was supported by preliminary high-throughput mass spectrometric screening, which identified covalent engagement of a mutant NPM1 W288C target by selected members of the scaffold class. In situ NMR kinetics support a pathway involving oxime formation, isocyanide insertion, and intramolecular O-acyl capture leading to four-membered ring closure, consistent with interception of the Ugi intermediate rather than formation of classical products. Together, this transformation establishes a general entry into oxa-β-lactam chemical space and suggests that these underexplored heterocycles may constitute a new class of covalent modifier scaffolds.