Synthesis, effect of substituents on the regiochemistry and equilibrium studies of tetrazolo[1,5-a]pyrimidine/2-azidopyrimidines

Synthesis and Study of Steering of Azido-tetrazole Behavior in Tetrazolo[1,5-c]pyrimidin-5-amine-Based Energetic Materials

Tetrazoles and their derivatives are essential for compound synthesis due to their versatility, effectiveness, stability in air, and cost-efficiency. This has stimulated interest in developing techniques for their production. In this work, four compounds, tetrazolo[1,5-c]pyrimidin-5-amine (1), N-(4-azidopyrimidin-2-yl)nitramide (2), tetrazolo[1,5-c]pyrimidin-5(6H)-one (3), and tetrazolo[1,5-a]pyrimidin-5-amine (4), were obtained from commercially available reagents and straightforward synthetic methodologies. These new compounds were characterized by infrared (IR), 13C, and 1H NMR spectroscopy, differential scanning calorimetry (DSC), and single-crystal X-ray diffraction. The solvent, temperature, and electron-donating group (EDG) factors that were responsible for the steering of azido-tetrazole equilibrium in all compounds were also studied. In addition, the detonation performance of the target compounds was calculated by using heats of formation (HOFs) and crystal densities. Hirshfeld surface analysis was used to examine the intermolecular interactions of the four synthesized compounds. The results show that the excellent properties of 1-4 are triggered by ionic bonds, hydrogen bonds, and π-π stacking interactions, indicating that these compounds have the potential to be used in the development of high-performance energetic materials. Additionally, DFT analysis is in support of experimental results, which proved the effect of different factors that can influence the azido-tetrazole equilibrium in the synthesized pyrimidine derivatives in the solution.

SnAr Reactions of 2,4-Diazidopyrido[3,2-d]pyrimidine and Azide-Tetrazole Equilibrium Studies of the Obtained 5-Substituted Tetrazolo[1,5-a]pyrido[2,3-e]pyrimidines

A straightforward method for the synthesis of 5-substituted tetrazolo[1,5-a]pyrido[2,3-e]pyrimidines from 2,4-diazidopyrido[3,2-d]pyrimidine in SnAr reactions with N-, O-, and S- nucleophiles has been developed. The various N- and S-substituted products were obtained with yields from 47% to 98%, but the substitution with O-nucleophiles gave lower yields (20-32%). Furthermore, the fused tetrazolo[1,5-a]pyrimidine derivatives can be regarded as 2-azidopyrimidines and functionalized in copper(I)-catalyzed azide-alkyne dipolar cycloaddition (CuAAC) and Staudinger reactions due to the presence of a sufficient concentration of the reactive azide tautomer in solution. In total, seven products were fully characterized by their single crystal X-ray studies, while five of them were representatives of the tetrazolo[1,5-a]pyrido[2,3-e]pyrimidine heterocyclic system. Equilibrium constants and thermodynamic values were determined using variable temperature ¹H NMR and are in agreement of favoring the tetrazole tautomeric form (ΔG₂₉₈ = -3.33 to -7.52 (kJ/mol), ΔH = -19.92 to -48.02 (kJ/mol) and ΔS = -43.74 to -143.27 (J/mol·K)). The key starting material 2,4-diazidopyrido[3,2-d]pyrimidine presents a high degree of tautomerization in different solvents.

15N Chemical Shifts and JNN-Couplings as Diagnostic Tools for Determination of the Azide-Tetrazole Equilibrium in Tetrazoloazines

Selectively ¹⁵N-labeled tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines bearing one, two, or three ¹⁵N labels were synthesized. The synthesized compounds were studied by ¹H, ¹³C, and ¹⁵N NMR spectroscopy in DMSO and TFA solutions, where the azide-tetrazole equilibrium can lead to the formation of two tetrazole (T, T') isomers and one azide (A) isomer for each compound. Incorporation of the ¹⁵N-label(s) leads to the appearance of ¹⁵N-¹⁵N coupling constants (J_NN), which can be easily measured via simple 1D ¹⁵N NMR spectra, even at natural abundance between labeled and unlabeled ¹⁵N atoms. The chemical shifts for the ¹⁵N nuclei in the azole moiety are very sensitive to the ring opening and azide formation, thus providing information about the azido-tetrazole equilibrium. At the same time, the ^1-2J_NN couplings between ¹⁵N-labeled atoms in the azole and azine fragments unambiguously determine the fusion type between tetrazole and azine rings in the cyclic isomers T and T'. Thus, combined analysis of ¹⁵N chemical shifts and J_NN values in selectively isotope-enriched compounds provides an effective diagnostic tool for direct structural determination of tetrazole isomers and azide form in solution. This method was found to be the most simple and efficient way to study the azido-tetrazole equilibrium.

Synthesis and antiviral activity of nonannulated tetrazolylpyrimidines

Nonannulated tetrazolylpyrimidines in the structure of which the heterocyclic fragments are separated by hydrazinocarbonylmethyl, methylpyrazolyl groups or a sulfur atom were synthesized. Some of these compounds showed moderate in vitro activity against H1N1 subtype of influenza A virus. The selectivity index of the anti-influenza action of {5-[(4,6-dimethylpyrimidin-2-yl)sulfanyl]-1H-tetrazol-1-yl}acetic acid, which has very low cytotoxicity, was twice as high as the selectivity index of the reference drug rimantadine.

Synthesis of (E)-5-Arylvinyl-7-methyltetrazolo[1,5-a]pyrimidines

Abstract

A three-component reaction of 5-aminotetrazole with aromatic aldehydes and acetylacetone under solvent- and catalyst-free conditions at a temperature of 150–160°С proceeds with the formation of (E)-5-arylvinyl-7-methyltetrazolo[1,5-a]pyrimidines. 5,7-Dimethyltetrazolo[1,5-a]pyrimidine is formed as a side-product of the reaction.