Divergent Functionalizations of Azetidines and Unsaturated Azetidines

Azetidines are unusual heterocyclic motifs that comprise key components of biologically active molecules and provide desirable structural properties for drug discovery. While a variety of methods have been designed for the synthesis of azetidines, screening and optimization studies often require systematic and versatile strategies for varying structural characteristics. With this need in mind, this review surveys methods for the divergent preparation of substituted azetidines from starting materials that contain a 4-membered azacycle. The scope and tolerance of azetidine functionalization reactions are described including approaches such as lithiation and electrophilic trapping, nucleophilic displacement, and transition metal catalysis. To complement this discussion, opportunities for using unsaturated azetidines as precursors to functionalized saturated analogues are also examined as an emerging alternative tactic towards the diversity-oriented preparation of these strained heterocycles. The aim of this Minireview is to identify recent advances, as well as areas where further development is needed, to continue to inspire innovative solutions to the divergent synthesis of functionalized azetidines.