https://doi.org/10.1351/goldbook.PT07451
Intermolecular photochemical process leading to carbo- or hetero- cyclic ring systems by formation of two new single bonds, either by a concerted process (e.g., a concerted addition) or by multi-step processes (e.g., the Paternò–Büchi reaction).
Note: triplet state photocycloadditions are two-step processes involving the formation of diradical intermediates.