Transition-metal-free skeletal editing of five- and six-membered N-heterocycles

N-Heterocycles represent ubiquitous structural motifs in pharmaceuticals, natural products, and functional materials. Recently, skeletal editing of these N-heterocycles has garnered considerable attention in organic and medicinal chemistry, owing to its capacity to directly reconstruct molecular skeletons, thereby inducing remarkable alterations in chemical reactivity and biological activity. Despite significant advancements achieved in this field, the development of novel transition-metal-free skeletal editing strategies remains a formidable challenge. In view of the lack of a comprehensive review focusing on the skeletal editing of N-heterocycles under transition-metal-free conditions, this review provides a comprehensive overview of the major progress made in the transition-metal-free skeletal editing of five- and six-membered N-heterocycles, encompassing ring expansion, ring contraction, and atom-swap transformations.