Synthesis, Characterization, and Cycloaddition Reactivity of a Monocyclic Aromatic 1,2,3,5-Tetrazine

Abstract

Herein we disclose the synthesis and full characterization of the first monocyclic aromatic 1,2,3,5-tetrazine, 4,6-diphenyl-1,2,3,5-tetrazine. Initial studies of its cycloaddition reactivity, mode, regioselectivity, and scope illustrate that it participates as the 4π-component of well-behaved inverse electron demand Diels−Alder reactions where it preferentially reacts with electron-rich or strained dienophiles. It was found to exhibit an intrinsic reactivity comparable to that of the isomeric 3,6diphenyl-1,2,4,5-tetrazine, display a single mode of cycloaddition with reaction only across C4/N1 (no N2/N5 cycloaddition observed), proceed with a predictable regioselectivity (dienophile most electron-rich atom attaches to C4), and manifest additional reactivity complementary to the isomeric 1,2,4,5-tetrazines. It not only exhibits a remarkable cycloaddition reactivity, surprisingly good stability (e.g., stable to chromatography, long-term storage, presence of H 2 O even as reaction co-solvent), and broad cycloaddition scope, but it also displays powerful orthogonal reactivity with the 1,2,4,5-tetrazines. Whereas the latter reacts at extraordinary cycloaddition rates with strained dienophiles (tetrazine ligation), the new and isomeric 1,2,3,5-tetrazine displays similarly remarkable cycloaddition rates and efficiencies with amidines (1,2,3,5-tetrazine/amidine ligation). The crossover reactivities (1,2,4,5-tetrazines with amidines and 1,2,3,5-tetrazines with strained dienophiles) are sufficiently low to indicate they may be capable of use concurrently without competitive reactions.

Calculation of magnetic response properties of tetrazines

“…NMR signals were detected. 8 Calculations yielded a rotational barrier of 31 kJ mol À1 implying that the phenyl groups rotate at room temperature. The interaction between the phenyl hydrogens in the ortho position and the nitrogens of the tetrazine ring as well as the common ring current stabilize the planar structure.…”

“…Novel methods for synthesizing heterocyclic aromatic molecules such as di-, tri-and tetrazines have recently been developed. [1][2][3][4][5][6][7][8] 1,3-Diazine (pyrimidine) is one of the more important azines, because its derivatives are of biological relevance as building blocks in the nucleobases of deoxyribonucleic acid (DNA). 9,10 Triazines have been widely studied because of their use in biological and industrial applications.…”

“…27 Wu and Boger recently synthesized 4,6diphenyl-1,2,3,5-tetrazine and found that is aromatic. 8 In this work, we investigate the aromatic properties of tetrazines by calculating magnetically induced current densities and induced magnetic elds of unsubstituted and substituted 1,2,3,5-tetrazines. The calculations yield qualitative and quantitative information about electron delocalization and the degree of aromaticity of the tetrazine ring.…”

Bioorthogonal Ligations and Cleavages in Chemical Biology

“…[137] In 2019, Boger's group reported the synthesis, characterization, and cycloaddition reactivity of a monocyclic aromatic 1,2,3,5-tetrazine and found that it showed good stability and rapid IEDDA rate with electron-rich or strain-containing dienophiles. [138] In addition, Murphy and Houk reported the cycloaddition reactions of tetrachlorocyclopentadiene ketal (142) with bicyclononyne (BCN) (85) or trans-cyclooctene (TCO) for bioorthogonal labeling (Figure 30). [139]…”

Metabolism of a Bioorthogonal PET Tracer Candidate [19F/18F]SiFA-Tetrazine in Mouse Liver Microsomes: Biotransformation Pathways and Defluorination Investigated by UHPLC-HRMS

“…Tetrazines are six-membered heterocyclic compounds, found as three structural isomers, containing a ring of four nitrogens and two carbons ( Figure 2 ). 29 , 30 The high reactivity of 1,2,4,5-tetrazine ( s -tetrazine) is attributed to the extreme electron deficiency of the nitrogen-rich ring. The fast reaction kinetics of IEDDA between 1,2,4,5-tetrazines and trans -cyclooctenes makes it the most efficient bioorthogonal reaction to date.…”