Accessing Functionalized Tetrazines as Click Chemistry Tools: A Synthesis Guide for Chemists and Chemical Biologists

Abstract

Since the first report that 1,2,4,5-tetrazines undergo a bioorthogonal reaction with dienophiles in the form of an inverse electron-demand Diels-Alder reaction, the demand for high-yielding synthetic approaches towards them grew steadily. Despite this significant interest, tetrazines were predominantly accessed via the Pinner synthesis or other Pinner-like reactions, significantly limiting available substrates. In particular, the synthesis of unsymmetrically substituted s-tetrazines for the selective conjugation to another species presented a major challenge. To tackle these challenges, new and innovative high-yielding transformations have been developed to widen the scope of accessible symmetric and unsymmetric tetrazines. For instance, Ni(II), Zn(II), and sulphur-catalysed reactions between two nitriles and hydrazine were developed, which provide access to a wide range of (un-)symmetric aryl s-tetrazines. Also, amidines and orthoesters give tetrazines with alkyl substituents, whereas the usage of CH2Cl2 yields valuable H-monosubstituted tetrazines. Methods using thiocarbohydrazides, terminal fluoroolefins, oxetane esters, or tosyl hydrazones are among the recent additions. Due to the high interest in tetrazine click-chemistry approaches, some of which reach beyond inverse electron-demand Diels-Alder reactions, we expect this overview of synthetic routes to aid the exploration for further applications of tetrazines. Ultimately, we hope to provide a guide for chemists and chemical biologists for accessing functional s-tetrazines.