Oxidative Aromatization of 4,7-Dihydro-6-nitroazolo[1,5-a]pyrimidines: Synthetic Possibilities and Limitations, Mechanism of Destruction, and the Theoretical and Experimental Substantiation

The reaction tolerance of the multicomponent process between 3-aminoazoles, 1-morpholino-2-nitroalkenes, and aldehydes was studied. The main patterns of this reaction have been established. Conditions for the oxidation of 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines were selected. Previous claims that the 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines could not be aromatised have now been refuted. Compounds with an electron-donor substituent at position seven undergo decomposition during oxidation. The phenomenon was explained based on experimental data, electro-chemical experiment, and quantum-chemical calculation. The mechanism of oxidative degradation has been proposed.

A special place is occupied by 6-nitroazolo[1,5-a]pyrimidines [30]. They are an even more specific class of heterocycles that have been explored much less thoroughly, although due to their nitro group, the variety of their properties is just as wide.
Thus, in this work we study the possibility of oxidation of 4,7-dihydro-6-nitroazolo [1,5-a]pyrimidines, while saving the nitro-group, and the reaction tolerance of the multicomponent reaction and oxidation process (Scheme 1d). Previously we discovered the multicomponent reaction between aminoazoles, aromatic aldehydes, and 1-morpholino-2-nitroalkenes, which proceeds through different mechanisms depending on catalysis with Lewis [44] or Brønsted acids [45] (Scheme 1a,b). Formed that way, 4,7-dihydro-6-nitroazolo [1,5-a]pyrimidines have been obtained by the condensation of corresponding azoles with sodium salts of malonic dialdehyde [46] and the subsequent nucleophilic addition of π-extended (hetero)aromatic systems [43,47,48] (Scheme 1c). However, that approach has the following few disadvantages: a two-step synthesis procedure, an exclusively nucleophilic variant of structural modifications, and the limitation of possible modification positions. Moreover, the inability to aromatize 4,7-dihydro-6-nitroazolo [1,5-a]pyrimidines was established [49]. The method of "reductive autoaromatization" is used for oxidation, where the spontaneous aromatization of the heterocyclic system occurs during the reduction of the nitro group (Scheme 1c).
Thus, in this work we study the possibility of oxidation of 4,7-dihydro-6-nitroazolo[1,5a]pyrimidines, while saving the nitro-group, and the reaction tolerance of the multicomponent reaction and oxidation process (Scheme 1d).

Results
To study the substrate scope, we carried out the multicomponent process by changing one of the initial reagents, while two were unchanged. Using 1-morpholino-2-nitropropylene 2b, benzaldehyde 3g with varying 3-aminoazoles 1 and molecular fragments in their structure in the medium BF3·OEt2-n-butanol, we obtained a series of 4,7-dihydro-6-nitro-7-phenylazolo[1,5-a]pyrimidines 4 in a moderate to good yield (Scheme 2).
The next goal of the work was to examine the obtained 4,7-dihydro-5-methyl-6-nitro-7-phenylazolo[1,5-a]pyrimidines 4. Using compound 4c as an example, we studied the effect of the nature of solvents and oxidants on the yield of the model reaction product. The results are shown in Table 1.
We found that the studied compound 4c decomposes under the action of the strong inorganic oxidizer ( Thereby, by using diacetoxyiodobenzene in acetic acid at a corresponding temperature, we obtained a number of new 6-nitro-5-methyl-7-phenylazolo[1,5-a]pyrimidines 5 in moderate to high yields (Scheme 3).

Results
To study the substrate scope, we carried out the multicomponent process by changing one of the initial reagents, while two were unchanged. Using 1-morpholino-2-nitropropylene 2b, benzaldehyde 3g with varying 3-aminoazoles 1 and molecular fragments in their structure in the medium BF 3 ·OEt 2 -n-butanol, we obtained a series of 4,7-dihydro-6-nitro-7phenylazolo[1,5-a]pyrimidines 4 in a moderate to good yield (Scheme 2).
The next goal of the work was to examine the obtained 4,7-dihydro-5-methyl-6-nitro-7-phenylazolo[1,5-a]pyrimidines 4. Using compound 4c as an example, we studied the effect of the nature of solvents and oxidants on the yield of the model reaction product. The results are shown in Table 1.
We found that the studied compound 4c decomposes under the action of the strong inorganic oxidizer ( Thereby, by using diacetoxyiodobenzene in acetic acid at a corresponding temperature, we obtained a number of new 6-nitro-5-methyl-7-phenylazolo[1,5-a]pyrimidines 5 in moderate to high yields (Scheme 3).
Thus, in the first part of the work, the possibility of the three-component reaction of 1-morpholino-2-nitropropylene 2a, benzaldehyde 3g, and 3-aminoazoles 1 was established with an assessment of the last reaction method. In addition, the oxidative aromatization of type 4 dihydroazolopyrimidines, which was previously considered impossible, was realized. Scheme 2. Preparation of 4,7-dihydro-6-nitro-5-methyl-7-phenylazolo[1,5-a]pyrimidines. The oxidation of triazolo-and tetrazolopyrimidines 4e-i occurs under harsher conditions than the pyrazolo-derivatives 4a-d; the process requires both a higher temperature and a longer reaction time. This is apparently due to the increasing π-deficient properties of five-membered heterocycles with the increasing numbers of N-atoms. A feature of the tetrazolopyrimidine 5i is the formation of the azido-tetrazole tautomerism product 2-azido-5-nitro-6-methyl-4-phenylpyrimidine 5i′, which was supported by IR spectroscopy data (see Supplementary Materials).
Thus, in the first part of the work, the possibility of the three-component reaction of 1-morpholino-2-nitropropylene 2a, benzaldehyde 3g, and 3-aminoazoles 1 was established with an assessment of the last reaction method. In addition, the oxidative aromatization of type 4 dihydroazolopyrimidines, which was previously considered impossible, was realized. The oxidation of triazolo-and tetrazolopyrimidines 4e-i occurs under harsher conditions than the pyrazolo-derivatives 4a-d; the process requires both a higher temperature and a longer reaction time. This is apparently due to the increasing π-deficient properties of five-membered heterocycles with the increasing numbers of N-atoms. A feature of the tetrazolopyrimidine 5i is the formation of the azido-tetrazole tautomerism product 2-azido-5-nitro-6-methyl-4-phenylpyrimidine 5i , which was supported by IR spectroscopy data (see Supplementary Materials).
Thus, in the first part of the work, the possibility of the three-component reaction of 1-morpholino-2-nitropropylene 2a, benzaldehyde 3g, and 3-aminoazoles 1 was established with an assessment of the last reaction method. In addition, the oxidative aromatization of type 4 dihydroazolopyrimidines, which was previously considered impossible, was realized. The next stage of the study was the change the 1-morplholino-2-nitroalkene component 2. The number of synthetically available nitroalkenes 2 is limited, and all of them turned out to be reactive in the preparation of dihydronitro derivatives 6, which, in turn, were successfully dehydrogenated (Scheme 4).
Finally, we examined the effect of the structure of the third component, aldehydes, on the formation of 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines 8 and their subsequent oxidation. The object of study was to use benzaldehyde 3g and its derivatives with electrondonating 3c,d and electron-acceptor 3b groups, polycyclic anthracenecarbaldehyde 3e, heterocyclic thophenecarbaldehyde 3f, and propanal 3a. The use of the developed reaction conditions allowed us to obtain 7-R-5-methyl-6-nitropyrazolo[1,5-a]pyrimidines 8 in moderate to good yields (Scheme 5). Finally, we examined the effect of the structure of the third component, aldehydes, on the formation of 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines 8 and their subsequent oxidation. The object of study was to use benzaldehyde 3g and its derivatives with electron-donating 3c,d and electron-acceptor 3b groups, polycyclic anthracenecarbaldehyde 3e, heterocyclic thophenecarbaldehyde 3f, and propanal 3a. The use of the developed reaction conditions allowed us to obtain 7-R-5-methyl-6-nitropyrazolo [1,5-a] During the oxidation, we discovered that the interaction of heterocycle 8d with phenyliodozodiacetate, even at room temperature, leads to the resinification of the reaction mass. At the same time, the oxidation of heterocycles 8c and 8e was also accompanied by the formation of side products, but we have succeeded in isolating the desired compounds in yields of 24 and 30%, respectively. The results are presented in Scheme 6.
It was previously noted that it is impossible to obtain an aromatic system for similar structures, since attempts at oxidation have led either to unsuccessful results or to a mixture of unidentifiable products. However, results of this work complement the existing precedent since oxidation is complicated only in the case of compounds with an electrondonor substituent at position seven. During the oxidation of nitropyrazolopyrimdines 9e, besides the main product 9e, side products 9-acetoxy-anthracene 10 and anthraquinone 11 were also isolated using the flash chromatography method. These compounds were characterized in the composition of the mixture (Scheme 7). During the oxidation, we discovered that the interaction of heterocycle 8d with phenyliodozodiacetate, even at room temperature, leads to the resinification of the reaction mass. At the same time, the oxidation of heterocycles 8c and 8e was also accompanied by the formation of side products, but we have succeeded in isolating the desired compounds in yields of 24 and 30%, respectively. The results are presented in Scheme 6.
It was previously noted that it is impossible to obtain an aromatic system for similar structures, since attempts at oxidation have led either to unsuccessful results or to a mixture of unidentifiable products. However, results of this work complement the existing precedent since oxidation is complicated only in the case of compounds with an electrondonor substituent at position seven. During the oxidation of nitropyrazolopyrimdines 9e, besides the main product 9e, side products 9-acetoxy-anthracene 10 and anthraquinone 11 were also isolated using the flash chromatography method. These compounds were characterized in the composition of the mixture (Scheme 7). To elucidate the experimental data, we performed a quantum chemical calculation of the compounds 5a, 8a-e to obtain the charge distribution over the molecular structure. Figure 2 demonstrates the spin distribution of the highest occupied molecular orbital (HOMO) for compounds 8b and 8e. Other data are shown in the Supplementary Materials. To elucidate the experimental data, we performed a quantum chemical calculation of the compounds 5a, 8a-e to obtain the charge distribution over the molecular structure. Figure 2 demonstrates the spin distribution of the highest occupied molecular orbital (HOMO) for compounds 8b and 8e. Other data are shown in the Supplementary Materials. To elucidate the experimental data, we performed a quantum chemical calculation of the compounds 5a, 8a-e to obtain the charge distribution over the molecular structure. Figure 2 demonstrates the spin distribution of the highest occupied molecular orbital (HOMO) for compounds 8b and 8e. Other data are shown in the Supplementary Materials. Indeed, the calculated data demonstrate that in compounds with an electron donating substituent 8e, the substituent is involved in the electron density distribution, which can have a serious influence on the direction of oxidation. Therefore, in these structures, the covalent bond between the C-7 atom and the substituent fissions and the nonstoichiometric formation of oxidation side products occurs.
Furthermore, the results of cyclic voltammetry also indicate the different behavior of compound 8 under oxidation conditions ( Figure 3). Thus, for compound 8b, one irreversible peak of two-electron oxidation is observed. In turn, compound 8e is characterized by two peaks, apparently, of one-electron oxidation. The data for other compounds are given in the Supplementary Materials.  Indeed, the calculated data demonstrate that in compounds with an electron donating substituent 8e, the substituent is involved in the electron density distribution, which can have a serious influence on the direction of oxidation. Therefore, in these structures, the covalent bond between the C-7 atom and the substituent fissions and the nonstoichiometric formation of oxidation side products occurs.
Furthermore, the results of cyclic voltammetry also indicate the different behavior of compound 8 under oxidation conditions ( Figure 3). Thus, for compound 8b, one irreversible peak of two-electron oxidation is observed. In turn, compound 8e is characterized by two peaks, apparently, of one-electron oxidation. The data for other compounds are given in the Supplementary Materials. Indeed, the calculated data demonstrate that in compounds with an electron donating substituent 8e, the substituent is involved in the electron density distribution, which can have a serious influence on the direction of oxidation. Therefore, in these structures, the covalent bond between the C-7 atom and the substituent fissions and the nonstoichiometric formation of oxidation side products occurs.
Furthermore, the results of cyclic voltammetry also indicate the different behavior of compound 8 under oxidation conditions ( Figure 3). Thus, for compound 8b, one irreversible peak of two-electron oxidation is observed. In turn, compound 8e is characterized by two peaks, apparently, of one-electron oxidation. The data for other compounds are given in the Supplementary Materials.  Based on the results above and the literature's data [50][51][52], it can be assumed that initially, for both compounds 8b and 8e, single electron transfer (SET) occurs with the formation of an acetate-anion and a heterocyclic cation-radical 13 (Scheme 8). Then, the tautomeric transformation, the deprotonation of structure 14, and a shift of the hydrogen radical with the formation of radical particle 15 occur. Next, depending on the nature of the substituent, oxidation proceeds either with one more single electron transfer for compounds with an electron withdrawing substituent at position seven, or with the migration of a radical to a substituent for compounds with an electron donating substituent. Apparently, the intermediate 21 interacts with an oxygen molecule with the formation of carbonyl compound 23. Further intramolecular transformations lead to the homolytic fission of the covalent bond between the C-7 atom and the substituent in structure 24. The aromatic radical is either oxidized to anthraquinone 11 or is attacked by the acetoxy-radical from phenyliodozomonoacetate 18. The heterocyclic radical 25, apparently, decomposes nonstoichometrically. Based on the results above and the literature's data [50][51][52], it can be assumed that initially, for both compounds 8b and 8e, single electron transfer (SET) occurs with the formation of an acetate-anion and a heterocyclic cation-radical 13 (Scheme 8). Then, the tautomeric transformation, the deprotonation of structure 14, and a shift of the hydrogen radical with the formation of radical particle 15 occur. Next, depending on the nature of the substituent, oxidation proceeds either with one more single electron transfer for compounds with an electron withdrawing substituent at position seven, or with the migration of a radical to a substituent for compounds with an electron donating substituent. Apparently, the intermediate 21 interacts with an oxygen molecule with the formation of carbonyl compound 23. Further intramolecular transformations lead to the homolytic fission of the covalent bond between the C-7 atom and the substituent in structure 24. The aromatic radical is either oxidized to anthraquinone 11 or is attacked by the acetoxy-radical from phenyliodozomonoacetate 18. The heterocyclic radical 25, apparently, decomposes nonstoichometrically. Scheme 8. Plausible mechanism of oxidation of 6-nitro-5-methyl-7-R-azolo[1,5-a]pyrimidines 8.

Materials and Methods
Unless stated otherwise, all solvents and commercially available reactants/reagents were used as received. Non-commercial starting materials were prepared as described below or according to the literature's procedures. One-dimensional 1 H and 13 С NMR Scheme 8. Plausible mechanism of oxidation of 6-nitro-5-methyl-7-R-azolo[1,5-a]pyrimidines 8.

Materials and Methods
Unless stated otherwise, all solvents and commercially available reactants/reagents were used as received. Non-commercial starting materials were prepared as described below or according to the literature's procedures. One-dimensional 1 H and 13 C NMR spectra, as well as two-dimensional 1 H-13 C HMBC experiments were acquired on a Bruker DRX-400 instrument (400 and 101 MHz, respectively) or a Bruker Avance NEO 600 instrument (600 and 151 MHz, respectively), equipped with a Prodigy broadband gradient cryoprobe, utilizing DMSO-d 6 and CDCl 3 as the solvent and TMS as the internal standard. IR spectra were recorder on a Bruker Alpha FTIR spectrometer equipped with a ZnSe ATR accessory. Mass spectra were recorded on a Shimadzu GCMS-QP2010 Ultra mass spectrometer, using EI method of ionization (70 eV). Elemental analysis was performed on a PerkinElmer 2400 CHN analyzer. The reaction progress was controlled by TLC on Silufol UV-254 plates, eluent-CH 3 Cl. Melting points were determined on a Stuart SMP3 apparatus at the heating rate of 25 • C/min. 1-Morpholino-2-nitroethylenes 2 were prepared according to a literature procedure [53].
Density functional theory (DFT) simulation via Vienna Ab initio Simulation Package (VASP) [54] was performed for structural optimization of the molecules and obtaining of the charge distribution. The length of the supercell of every molecule was set to 22.5 Å and the cut-off energy of plane wave basis set was set to 750 eV. Structural optimization was performed using generalized gradient approximation (GGA) by Pedrew, Burke, and Ernzerhof (PBE) [55] for exchange-correlation potential. For all chemical species, projected augmented wave (PAW) pseudopotentials were used. The structures were optimized using two algorithms: conjugate gradient algorithm was implemented for initial optimization and after that RMM-DIIS (residual minimization scheme, direct inversion in the iterative subspace) algorithm was used for final optimization. The stopping criteria for optimization was |F i | ≤ 0.01 eV/Å, where i = x, y, z, i.e., absolute value of each force component acting on nuclei should be not greater than 10 meV/Å.
For PBE-optimized structures, a hybrid PBE0 simulation was performed to obtain charge distribution, for accounting of long-range interactions Grimme DFT-D3 corrections [56] were used. Since all the performed calculation were spin polarized, it was possible to extract spin density from the charge distribution. For the interpretation of the obtained data, the program VESTA [57] was used.

Conclusions
Thus, we can conclude that 6-nitroazolo[1,5-a]pyrimidines, even containing an electron donating substituent at position seven, are capable of oxidation with the formation of an aromatic structure. On the other hand, during oxidation, it is necessary to take into consideration the features of these structures since the reaction is often complicated by a side process. However, there is no reason to suppose that the same compounds cannot be obtained by any alternative synthetic approach that excludes the destructive nature of key intermediates. Nevertheless, the obtained experimental and theoretical data correlate well with each other, which indicates the possibility of using this set of methods to study the oxidation reactions of this class of organic compounds.