<p>1,3-Oxazinanes are prized motifs found in bioactive heterocycles, but their synthesis is challenging owing to the lack of reliable methodologies. An unorthodox yet elegant approach to access 1,3-oxazinanes is to design a transformation that selectively inserts a carbon and nitrogen into readily available oxetane building blocks. However, despite progress in two-component skeletal expansions, the corresponding multicomponent reactions utilizing two distinct inserting entities remain elusive. Here we report that dual-atom insertion into oxetanes using various nitrogen and carbon sources can be achieved with a boron catalyst. The method streamlines the preparation of bioactive 1,3-oxazinanes and is amenable to late-stage editing to create multiheteroatom cyclic molecules. Mechanistic studies reveal a cascade pathway in which an in situ-generated frustrated Lewis pair enables ring deconstruction and reconstruction.</p><p></p>