Reaction Scope of Methyl 1,2,3-Triazine-5-carboxylate with Amidines and the Impact of C4/C6 Substitution

Cysteine-Specific Multifaceted Bioconjugation of Peptides and Proteins Using 5-Substituted 1,2,3-Triazines

Peptide and protein postmodification have gained significant attention due to their extensive impact on biomolecule engineering and drug discovery, of which cysteine-specific modification strategies are prominent due to their inherent nucleophilicity and low abundance. Herein, the study introduces a novel approach utilizing multifunctional 5-substituted 1,2,3-triazine derivatives to achieve multifaceted bioconjugation targeting cysteine-containing peptides and proteins. On the one hand, this represents an inaugural instance of employing 1,2,3-triazine in biomolecular-specific modification within a physiological solution. On the other hand, as a powerful combination of precision modification and biorthogonality, this strategy allows for the one-pot dual-orthogonal functionalization of biomolecules utilizing the aldehyde group generated simultaneously. 1,2,3-Triazine derivatives with diverse functional groups allow conjugation to peptides or proteins, while bi-triazines enable peptide cyclization and dimerization. The examination of the stability of bi-triazines revealed their potential for reversible peptide modification. This work establishes a comprehensive platform for identifying cysteine-selective modifications, providing new avenues for peptide-based drug development, protein bioconjugation, and chemical biology research.

Inverse Electron Demand Diels-Alder-Type Heterocycle Syntheses with 1,2,3-Triazine 1-Oxides: Expanded Versatility

1,2,3-Triazine 1-oxides are remarkably effective substrates for inverse electron demand Diels-Alder reactions. Formed from vinyldiazoacetates via reaction with tert-butyl nitrite, these stable heterocyclic compounds undergo clean nucleophilic addition with amidines to form pyrimidines, with β-ketocarbonyl compounds and related nitrile derivatives to form polysubstituted pyridines and with 3/5-aminopyrazoles to form pyrazolo[1,5-a]pyrimidines, in high yield. These practical reactions are rapid at room temperature, are base catalyzed, and offer a diversity of structural modifications.

Cross-Coupling Reactions of 5-Bromo-1,2,3-triazine

An efficient Pd catalyzed cross-coupling method for 5-bromo-1,2,3-triazine is described. Optimization of the reaction conditions allowed for the preparation of a representative scope of (hetero)aryl-1,2,3-triazines (20 examples, up to 97% yield). The reaction scope was evaluated using a data science enabled boronic acid chemical space to assess the generality of the method. Additionally, diversification of the resulting products enabled the preparation of pyrimidines and pyridines in yields of up to 80% and in only two steps.

Unified Access to Pyrimidines and Quinazolines Enabled by N-N Cleaving Carbon Atom Insertion

Given the ubiquity of heterocycles in biologically active molecules, transformations with the capacity to modify such molecular skeletons with modularity remain highly desirable. Ring expansions that enable interconversion of privileged heterocyclic motifs are especially interesting in this regard. As such, the known mechanisms for ring expansion and contraction determine the classes of heterocycle amenable to skeletal editing. Herein, we report a reaction that selectively cleaves the N–N bond of pyrazole and indazole cores to afford pyrimidines and quinazolines, respectively. This chlorodiazirine-mediated reaction provides a unified route to a related pair of heterocycles that are otherwise typically prepared by divergent approaches. Mechanistic experiments and DFT calculations support a pathway involving pyrazolium ylide fragmentation followed by cyclization of the ring-opened diazahexatriene intermediate to yield the new diazine core. Beyond enabling access to valuable heteroarenes from easily prepared starting materials, we demonstrate the synthetic utility of skeletal editing in the synthesis of a Rosuvastatin analog as well as in an aryl vector-adjusting direct scaffold hop.

Mechanistic Insights into the Reaction of Amidines with 1,2,3-Triazines and 1,2,3,5-Tetrazines

1,2,3-Triazines and 1,2,3,5-tetrazines react rapidly, efficiently, and selectively with amidines to form pyrimidines/1,3,5-triazines, exhibiting an orthogonal reactivity with 1,2,4,5-tetrazine-based conjugation chemistry. Whereas the mechanism of the reaction of the isomeric 1,2,4-triazines and 1,2,4,5-tetrazines with alkenes is well understood, the mechanism of the 1,2,3-triazine/1,2,3,5-tetrazine-amidine reaction as well as its intrinsic reactivity remains underexplored. By using ¹⁵N-labeling, kinetic investigations, and kinetic isotope effect studies, complemented by extensive computational investigations, we show that this reaction proceeds through an addition/N₂ elimination/cyclization pathway, rather than the generally expected concerted or stepwise Diels-Alder/retro Diels-Alder sequence. The rate-limiting step in this transformation is the initial nucleophilic attack of an amidine on azine C4, with a subsequent energetically favored N₂ elimination step compared with a disfavored stepwise formation of a Diels-Alder cycloadduct. The proposed reaction mechanism is in agreement with experimental and computational results, which explains the observed reactivity of 1,2,3-triazines and 1,2,3,5-tetrazines with amidines.