Synthesis and Structure of 6-Acetyl-2-Arylhydrazone Derivatives of Thiazolo [ 3 , 2-a ] Pyrimidine

Triazolo[4,3-a]pyrimidine is one of the promising structural fragments for the development of drugs, including anticancer drugs. This work is devoted to the synthesis of a number of new 2-arylhydrazone derivatives of thiazolo[3,2-a]pyrimidine, which are synthetic precursors for triazolo[4,3-a]pyrimidines. The crystal structure of 6-acetyl-7-methyl-5-phenyl-2-(2-phenylhydrazineylidene)-5H-thiazolo[3,2-a]pyrimidin-3(2H)-one was established by SCXRD. In the reduction reaction of the compound, the following system was used: vanadium(V) oxide, and sodium borohydride in ethanol at room temperature, which led to the formation of only one pair of diastereomers (1R*)-1-((5S*,6R*,7R*)-(1-(hydroxymethyl)-7-methyl-1,5-diphenyl-1,5,6,7-tetrahydro[1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol.

Recently, our scientific group has shown that 2-arylhydrazone derivatives of thiazolo[3,2a]pyrimidine can be transformed into 1,5-dihydrotriazolo[4,3-a]pyrimidines (Scheme 4) in the presence of a new reducing system-vanadium(V) oxide and a fourfold excess of sodium borohydride at room temperature [14,15]. This method includes the hydrogenation of a hydrozone moiety at the first stage and subsequent rearrangement with hydrogen sulfide elimination. It is a promising method for triazolo[4,3-a]pyrimidine derivative synthesis containing a hydroxymethylene substituent. Scheme 2. Cyclization2-hydrazinylpyrimidines with various reagents (carbon disu chloroformate, triethylorthoformate, acetic anhydride).
Another way to prepare of triazolo[4,3-a]pyrimidines is the dipolar 1 nitrile imide (formed in situ from hydrazonoyl halide and triethylamine) t hydropyrimidin-2-thione at the C=S bond and Smiles rearrangement with gen sulfide (Scheme 3) [12,13]. Recently, our scientific group has shown that 2-arylhydrazone deriv zolo[3,2-a]pyrimidine can be transformed into 1,5-dihydrotriazolo[4,3-(Scheme 4) in the presence of a new reducing system-vanadium(V) oxide excess of sodium borohydride at room temperature [14,15]. This method in drogenation of a hydrozone moiety at the first stage and subsequent rearra hydrogen sulfide elimination. It is a promising method for triazolo[4,3-a]p rivative synthesis containing a hydroxymethylene substituent.
In the present work, the synthesis of new 2-arylhydrazone thiazolo[3,2-a]pyrimidine derivatives containing an acetyl group at C6, the crystal structure of the 2-phenylhydrazone derivative, and unique and diastereoselective reduction of the 2-phenylhydrazonethiazolo[3,2a]pyrimidine molecule under the action of the reducing system-NaBH 4 /V 2 O 5 were discussed.

Materials and Methods
NMR experiments were performed on Bruker Avance 500 (Saarbrucken, Germany). Chemical shifts were determined relative to the signals of residual protons of the DMSO-d 6 . Electrospray ionization (ESI) mass spectra were obtained using a Bruker AmaZon X ion trap mass spectrometer. IR spectra in KBr tablets were recorded on a Bruker Vector-22.
The method of halogens determination is based on the combustion at 1200 • C of organic compound in oxygen in the presence of a platinum catalyst; the combustion products are adsorbed by the alkali and the halides formed were determined by mercurimetric titration with diphenylcarbazone as an indicator.
1,2,3,4-Tetrahydripyrimidine-2-thion 1b (0.3 g, 1 mmol) was mixed with ethyl chloroacetate (5.4 mL, 5 mmol) without solvent. The mixture was stirred at a temperature of 120 • C for 1 h, then cooled to room temperature; ethyl acetate (20 mL) was added and precipitate was filtered out followed by washing with ethyl acetate and recrystallization from ethyl alcohol. Yield 87%, yellow powder, mp 238-240 • C. 1  General Method for the Preparation of Compounds 3a-c. Sodium nitrite cold solution (1 mmol) in water (3 mL) was added drop by drop to an aromatic amine hydrochloride (1 mmol) suspension in water (5 mL) with stirring at 0-5 • C for 1 h. The resultant solution of aryldiazonium chloride (1 mmol) was added drop by drop with stirring at 0-5 • C to a cold solution of the corresponding thiazolo[3,2-a]pyrimidine 2a,b (1 mmol) and sodium acetate (1.1 mmol) in ethyl alcohol (10 mL). The mixture was stirred at room temperature for 2 h. Next, the reaction mixture was diluted with water, and the crude precipitate was collected by filtration, washed with water, and crystallized from ethyl alcohol.
X-ray diffraction analysis of 3a was performed on a Bruker D8 QUEST automatic threecircle diffractometer with a PHOTON III two-dimensional detector and an IµS DIAMOND microfocus X-ray tube (λ[Mo Kα] = 0.71073 Å) at 100 (2) K. Data collection and processing of diffraction data were performed using the APEX3 software package.
Structure 3a was solved by the direct method using the SHELXT program [16] and refined by the full-matrix least-squares method over F 2 using the SHELXL program [17]. All calculations were performed in the WinGX software package [18]. The calculation of the geometry of molecules and intermolecular interactions in crystals was carried out using the PLATON program [19], and the drawings of molecules were done using the ORTEP-3 [18] and MERCURY [20] programs.
Non-hydrogen atoms were refined in the anisotropic approximation. The positions of the hydrogen atoms H(O) were determined using difference Fourier maps, and these at-oms were refined isotropically. The remaining hydrogen atoms were placed in geometrically calculated positions and included in the refinement in the "riding" model. The crystallographic data of structure 7 were deposited at the Cambridge Crystallographic Data Center, and the registration numbers and the most important characteristics are given in Table 1.

Results and Discussion
2-Arylhydrazinylidenethiazolo[3,2-а]pyrimidin-3-one 3a-c were synthesized according to Scheme 5. The first step was a three-component Biginelli condensation between acetylacetone, thiourea and an aromatic aldehyde (benzaldehyde or 4-bromobenzaldehyde) carried out in boiling acetonitrile in the presence of catalytic amounts of molecular iodine [21]. The obtained 1,2,3,4-tetrahydropyrimidine-2-thiones 1a,b were involved in the reaction of sulfur atom alkylation by ethyl chloroacetate followed by cyclization with the formation of thiazolo[3,2-a]pyrimidine-3-one 2a,b [22,23]. Finally, the interaction of CH-active derivatives 2a,b with aryldiazonium salts upon cooling to 0-5 °C gave the target derivatives 3a-c in good yields (69-76%). The structure of compounds 2a and 3a-c was established by 1 H and 13 C NMR IR-, and mass-spectra (see Figures S1-S15). The structure of derivative 3a was additionally confirmed by SCXRD (Table 1). According to SCXRD data, the bicyclic tiazolo[3,2-a]pyrimidine fragment was almost flat (Figure 1a). The six-membered cycle assumed a sofa conformation. The sp 3 -hybridized C 5 carbon atom deviates slightly from the plane formed by the other five atoms of the pyrimidine ring. The acetyl group was located in the plane of the bicyclic thiazolopyrimidine fragment. The formation of hydrogen bonds between the oxygen of the acetyl group and the N-H hydrazone fragment (dO…N = 2.953(1) Å, φ = 172.07(4)°) was observed in the crystal (Figure 1b). It is interesting to note that the established hydrogen interaction leads to the formation of zigzag heterochiral chains in the crystalline phase (Figure 1c). Heterochiral chains consisting of alternating R-and S- The structure of compounds 2a and 3a-c was established by 1 H and 13 C NMR IR-, and mass-spectra (see Figures S1-S15). The structure of derivative 3a was additionally confirmed by SCXRD (Table 1). According to SCXRD data, the bicyclic tiazolo[3,2-a]pyrimidine fragment was almost flat (Figure 1a). The six-membered cycle assumed a sofa conformation. The sp 3 -hybridized C 5 carbon atom deviates slightly from the plane formed by the other five atoms of the pyrimidine ring. The acetyl group was located in the plane of the bicyclic thiazolopyrimidine fragment. The formation of hydrogen bonds between the oxygen of the acetyl group and the N-H hydrazone fragment (d O. . .N = 2.953(1) Å, ϕ = 172.07(4) • ) was observed in the crystal (Figure 1b). It is interesting to note that the established hydrogen interaction leads to the formation of zigzag heterochiral chains in the crystalline phase ( Figure 1c). Heterochiral chains consisting of alternating Rand S-isomers were arranged parallel to each other due to π-stacking (Figure 1c). Thus, it was found that hydrogen bonding determines the crystal packing of 3a. It should be noted that, as was shown in our previous work [24], the formation of the Z-isomer was observed both in solution and in the crystalline phase.
Organics 2023, 4, FOR PEER REVIEW 6 isomers were arranged parallel to each other due to π-stacking (Figure 1c). Thus, it was found that hydrogen bonding determines the crystal packing of 3a. It should be noted that, as was shown in our previous work [24], the formation of the Z-isomer was observed both in solution and in the crystalline phase. The obtained 2-arylhidrazone derivatives 3a-c were involved in the reaction with reducing system-vanadium(V) oxide and a fourfold excess of sodium borohydride at room temperature [14]. However, the complicated mixture of hard-to-separate substances which we obtained instead yielded 1,5-dihydrotriazolo[4,3-a]pyrimidines (Scheme 4). The temperature decrease up to 0-5 °C did not affect the reaction. The individual product was isolated in the case of the increase in sodium borohydride in excess of seven equivalents. In these experimental conditions, compound 4-1-(1-(hydroxymethyl)-7-methyl-1,5-diphenyl-1,5,6,7-tetrahydro[1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol was isolated in 44% yield (Scheme 6). Thus, it was found that the hydroxymethylene derivative of triazolo[4,3-a]pyrimidine was formed in agreement with [14]. However, the reaction was complicated by the hydrogenation of the conjugated C=C-C=O system due to a large excess of sodium borohydride. Scheme 6. Reduction of 2-arylhydrazonothiazolopyrimidine derivative 3a. The obtained 2-arylhidrazone derivatives 3a-c were involved in the reaction with reducing system-vanadium(V) oxide and a fourfold excess of sodium borohydride at room temperature [14]. However, the complicated mixture of hard-to-separate substances which we obtained instead yielded 1,5-dihydrotriazolo[4,3-a]pyrimidines (Scheme 4). The temperature decrease up to 0-5 • C did not affect the reaction. The individual product was isolated in the case of the increase in sodium borohydride in excess of seven equivalents. In these experimental conditions, compound 4-1-(1-(hydroxymethyl)-7-methyl-1,5-diphenyl-1,5,6,7-tetrahydro[1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol was isolated in 44% yield (Scheme 6). Thus, it was found that the hydroxymethylene derivative of triazolo[4,3-a]pyrimidine was formed in agreement with [14]. However, the reaction was complicated by the hydrogenation of the conjugated C=C-C=O system due to a large excess of sodium borohydride.
The signals of the C6 and C7 atoms of triazolo[4,3-a]pyrimidine 4 in the 13 C NMR spectrum shift were upfield and appeared at 47.1 and 48.6 ppm, respectively. In the 1 H NMR spectrum, the signals of hydrogen atoms at C6 and C7 resonated at 1.99-2.05 ppm and 3.60-3.65 ppm as multiplets. Additionally, in the carbon spectrum, there was no signal of the carbonyl group carbon in the region of 197.3 ppm, but a signal of the methine carbon atom was found in the region of 64.3 ppm, which in the proton spectrum corresponds to a proton signal in the form of a multiplet in the region of 4.31-4.36 ppm (see Figures S16-S18).
Four carbon atoms are asymmetric in compound 4; therefore, the formation of a mixture of diastereomers is possible. The only set of signals in the 1 H and 13 C NMR spectra indicated the formation of one diastereomer. Obviously, 2D NMRs and SCXRD are the best ways to establish the compound configuration. Unfortunately, our attempts to prepare a single crystal of compound 4 failed. The low solubility of 4 in most organic solvents did not allow recording the 2D NOESY spectrum. For this reason, molecular mechanics calculations using the MMFF94s force field were performed to estimate the thermodynamic stability of all possible stereoisomers. The calculated data are presented in Table 2. According to these data, (1R*)-1-((5S*, 6R*,7R*)-(1-(hydroxymethyl)-7-methyl-1,5diphenyl-1,5,6,7-tetrahydro [1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol ( Figure 2) was relatively more stable other diastereomers. So, at the thermodynamic reaction control, the formation of this stereoisomer is preferable. On the other hand, the phenyl substituent located in a pseudo-axial position (Figure 1a) blocks the approach of the reagent from one side of the pyrimidine ring and leads to cis-orientation of substituents at C5 and C6 carbon atoms. The orientation of substituents at C7 and C6 carbon atoms can be assigned to the hydrogen trans-addition to carbon-carbon double bonds in the case of reduction by sodium borohydride. So, the formation of (1R*)-1-((5S*, 6R*,7R*)-(1-(hydroxymethyl)-7methyl-1,5-diphenyl-1,5,6,7-tetrahydro [1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol can be explained from a kinetic reaction control point of view as well.
Organics 2023, 4, FOR PEER REVIEW 7 The signals of the C6 and C7 atoms of triazolo[4,3-a]pyrimidine 4 in the 13 C NMR spectrum shift were upfield and appeared at 47.1 and 48.6 ppm, respectively. In the 1 H NMR spectrum, the signals of hydrogen atoms at C6 and C7 resonated at 1.99-2.05 ppm and 3.60-3.65 ppm as multiplets. Additionally, in the carbon spectrum, there was no signal of the carbonyl group carbon in the region of 197.3 ppm, but a signal of the methine carbon atom was found in the region of 64.3 ppm, which in the proton spectrum corresponds to a proton signal in the form of a multiplet in the region of 4.31-4.36 ppm (see Figures S16-S18).
Four carbon atoms are asymmetric in compound 4; therefore, the formation of a mixture of diastereomers is possible. The only set of signals in the 1 Н and 13 С NMR spectra indicated the formation of one diastereomer. Obviously, 2D NMRs and SCXRD are the best ways to establish the compound configuration. Unfortunately, our attempts to prepare a single crystal of compound 4 failed. The low solubility of 4 in most organic solvents did not allow recording the 2D NOESY spectrum. For this reason, molecular mechanics calculations using the MMFF94s force field were performed to estimate the thermodynamic stability of all possible stereoisomers. The calculated data are presented in Table 2. According to these data, (1R*)-1-((5S*, 6R*,7R*)-(1-(hydroxymethyl)-7-methyl-1,5-diphenyl-1,5,6,7-tetrahydro[1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol ( Figure 2) was relatively more stable other diastereomers. So, at the thermodynamic reaction control, the formation of this stereoisomer is preferable. On the other hand, the phenyl substituent located in a pseudo-axial position (Figure 1a) blocks the approach of the reagent from one side of the pyrimidine ring and leads to cis-orientation of substituents at C5 and C6 carbon atoms. The orientation of substituents at C7 and C6 carbon atoms can be assigned to the hydrogen trans-addition to carbon-carbon double bonds in the case of reduction by sodium borohydride. So, the formation of (1R*)-1-((5S*, 6R*,7R*)-(1-(hydroxymethyl)-7-methyl-1,5-diphenyl-1,5,6,7-tetrahydro[1,2,4]triazolo[4,3-a]pyrimidin-6-yl)ethan-1-ol can be explained from a kinetic reaction control point of view as well.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/org4030031/s1, Figure S1 Funding: This work was funded by financial support from a government assignment for the Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences (122011800132-5).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are contained within the article or in Supplementary Materials, or are available on request from the corresponding author Artem Agarkov.