Heterocyclic skeletons are widespread among natural products as well as bioactive molecules. Cycloaddition reaction has created new opportunities to access heterocycles of great complexity due to its advantage of multiple bond formation in a single step. Here, we describe an unprecedented formal [5 + 2] cycloaddition of nitrone with o-carboryne to afford carborane-fused seven-membered heterocycles. Experimental and theoretical data suggest that a sequence of [3 + 2] cycloaddition, N-O bond cleavage, oxygen migration and rearomatization is involved in this unprecedented reaction. In this process, the nitrone moieties serve as five-atom coupling partners with both heteroatoms being incorporated into the framework of the final products. This new methodology also offers new insight into the chemistry of nitrones.