Triazines are an important class of six-membered aromatic heterocycles possessing three nitrogen atoms, resulting in three types of regio-isomers: 1,2,4-triazines (a-triazines), 1,2,3-triazines (v-triazines), and 1,3,5-triazines (s-triazines). Notably, the application of triazines as cyclic aza-dienes in inverse electron-demand Diels–Alder (IEDDA) cycloaddition reactions has been established as a unique and powerful method in N-heterocycle synthesis, natural product preparation, and bioorthogonal chemistry. In this review, we comprehensively summarize the advances in the construction of these triazines via annulation and ring-expansion reactions, especially emphasizing recent developments and challenges. The synthetic transformations of triazines are focused on IEDDA cycloaddition reactions, which have allowed access to a wide scope of heterocycles, including pyridines, carbolines, azepines, pyridazines, pyrazines, and pyrimidines. The utilization of triazine IEDDA reactions as key steps in natural product synthesis is also discussed. More importantly, a particular attention is paid on the bioorthogonal application of triazines in fast click ligation with various strained alkenes and alkynes, which opens a new opportunity for studying biomolecules in chemical biology.

A spiroing reactivity: A series of novel binol‐derived P‐spiro quaternary phosphonium salts were designed, prepared, and used for the first highly enantioselective amination of benzofuranones (see scheme; binol=1,1′‐2‐binaphthol, Bn=benzyl). An unprecedented mechanism involving the π–π interactions between the substrate and the catalyst was proposed as the primary binding mode on the basis of molecular modelling and DFT calculations.

A highly regioselective cycloaddition procedure of electron-deficient allenes with trifluorodiazoethane (CF3CHN2) is described. In absence of bases, the reaction proceeded smoothly to give 5-(trifluoromethyl)pyrazolines, whereas the utility of Et3N led to the formation of 3-(trifluoromethyl)pyrazoles.