Azetidiniums: Ring‐Expansion to Pyrrolidines, Piperidines, Azepanes, and Azocanes

The 4‐membered ring in bicyclic azetidiniums, e.g. 1‐azabicyclo[2.n.0]alkanes ( n = 1–4), can be efficiently transformed to larger rings. This minireview focuses on the ring‐expansion of these azetidiniums to five‐ and eight‐membered nitrogen containing heterocycles (pyrrolidines, piperidines, azepanes and azocanes). The ring‐expansion is induced by nucleophiles which can be internal nucleophiles (leaving groups), or external nucleophiles.