Three-membered ring heterocydes

Etliyleneimine (aziridine, azacydopropane) is the smallest cyclic imine consisting of a three-membered iV-heterocydic ring (n... 

In this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocydes. The crudal consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkene (see p. 2). Various aromatidty criteria apply to heteroarenes, and, as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocydes with one oxygen atom is furan. 

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