Aziridines as Versatile Intermediates for Aza-Heterocycles via Ring Expansions

Aziridine, a three-membered nitrogen-containing heterocyclic ring, is a versatile building block in organic synthesis. Due to high ring strain, it tends to undergo ring opening reactions under the influence of nucleophilic reagents. Facile construction of four- to seven-membered aza-heterocyclic ring systems is a challenging task for synthetic organic chemists. Fortunately, this small scaffold (aziridine) is an ideal starting material for conversion into larger nitrogenated heterocycles. This review presents some illustrative and contemporary examples to demonstrate the synthetic utility and efficiency of the ring-expansion strategies of the aziridines towards a wide range of small to medium-sized aza-heterocyclic moieties of pharmaceutical importance.