Regioselective ring-opening reactions of unsymmetric azetidines are systematically summarized and reviewed in this paper. Ring-opening reactions of unsymmetric azetidines include nucleophilic ring opening, Stevens rearrangement and ring expansion, and elimination reactions. Nucleophilic ring-opening reactions are major ring opening reactions of azetidines. The regioselectivity in ring-opening reactions is closely related to the structure of substituents of azetidines. Azetidines are relatively stable and often require Lewis acid catalysis or converted into their quaternary ammonium salts when they undergo ring-opening reactions. Thus, their ring-opening reactions are more susceptible to electronic effects. Azetidines and azetidiniums with 2-unsaturated substituents often undergo cleavage of the C-N bond between the nitrogen atom and carbon atom with the unsaturated groups, because the unsaturated groups such as aryl, 1-alkenyl, cyano, carboxylate, and carboxamide groups possess the conjugative effects with their adjacent carbon atom, stabilizing the generated transition states or intermediates in the ring-opening reactions. So that their C-N bond is more easily broken. In nucleophilic ring opening reactions of azetidines, nucleophiles generally attack arylmethylic, allylic, cyano, acyl, carboxylate, and carboxamide-attached nitrogen-adjacent carbon atoms, controlled by the electronic effect. However, sterically bulky or strong nucleophiles usually attack the less substituted nitrogen adjacent carbon atom of 2-alkylazetidines and azetidiniums in their ring opening reactions, controlled by steric hindrance. The structure of products in intramolecular nucleophilic ring opening reactions of azetidines is controlled by the ring size in the reaction processes, the ring-opening reactions favorably undergo through three-membered ring, five-membered ring, six-membered ring and seven-membered ring processes. Azetidines are a class of very important nitrogen-containing heterocyclic compounds. We can better understand and utilize this kind of reactions by summarizing and analyzing the ring-opening reactions and their regioselectivity of unsymmetric azetidines. The desired compounds can be prepared efficiently after predicting and controlling the regioselectivity in ring-opening reactions of azetidines. We hope that the summarized conclusions can promote the development and application of ring-opening reactions of azetidines in organic synthesis.

Contents
1 Nucleophilic ring-opening reactions of azetidines
1.1 Nucleophiles in group C
1.2 Nucleophiles in group N
1.3 Nucleophiles in group O
1.4 Halogen nucleophiles
1.5 Hydride nucleophiles
2 Stevens rearrangement and ring enlargement of azetidines
2.1 Base-catalyzed Stevens rearrangement and ring enlargment
2.2 Stevens rearrangement and ring enlargement of azetidines and carbenes
3 Eliminating ring-opening reactions
3.1 Transition metal-catalyzed eliminating ring-opening reactions
3.2 Sterically hindered strong base-promoted eliminating ring-opening reactions
3.3 Thermal elimination of azetidines
4 Miscellaneous ring-opening reactions of azetidines
4.1 Reductive ring-opening reactions
4.2 Cleaving ring-opening reactions
5 Conclusion