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

Abstract

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.

1. Introduction

Triazolopyrimidines, which are analogues of purines, are the subject of research for chemists and biologists due to their wide range of pharmacological activities, antimicrobial, antimalarial, cardiac stimulant, antifungal, anti-HBV, anticancer, analgesic, antipyretic, anti-inflammatory, namely antihypertensive, leishmanicidal and potential herbicidal action [1,2,3,4,5,6,7,8]. Thus, triazolopyrimidines are one of the promising structural fragments for new methods of new potential drug synthesis.

Several synthetic methods for the preparation of triazolo[4,3-a]pyrimidine derivatives are described in the literature, consisting of interaction with subsequent cyclization of 3-ethoxycarbonyl-2-hydrazinylpyrimidines [9] (Scheme 1) or 2-hydrazinylpyrimidines [10,11] with various reagents (carbon disulfide, ethyl chloroformate, triethylorthoformate, acetic anhydride) (Scheme 2).

Another way to prepare of triazolo[4,3-a]pyrimidines is the dipolar 1,3-addition of nitrile imide (formed in situ from hydrazonoyl halide and triethylamine) to 1,2,3,4-tetrahydropyrimidin-2-thione at the C=S bond and Smiles rearrangement with loss of hydrogen sulfide (Scheme 3) [12,13].

Recently, our scientific group has shown that 2-arylhydrazone derivatives of thiazolo[3,2-a]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.

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[3,2-a]pyrimidine molecule under the action of the reducing system—NaBH₄/V₂O₅ were discussed.

2. 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-d₆. 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.

CHNS elemental analysis was carried out using a high-temperature one-/two-reactor analyzer (oxidation tube and reduction tube) EuroEA3028-HT-OM “Eurovector SpA. Synthesis of 6-acetyl-5-(4-bromophenyl)-7-methyl-5H-thiazolo[3,2-a]pyrimidin-3(2H)-one hydrochloride 2b.

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. ¹H NMR (400 MHz, DMSO-d₆, 25 °C) δH ppm: 2.23 (s, 3H, CH₃), 2.35 (s, 3H, CH₃), 4.12–4.13 (m, 2H, SCH₂), 5.99 (s, 1H, CH-Ar), 7.20–7.22(m, 2H, CH (Ar)), 7.53–7.55 (m, 2H, CH (Ar)). ¹³C NMR (100 MHz, DMSO-d₆, 25 °C) δC ppm: 23.93, 31.36, 33.11, 54.24, 116.63, 122.16, 130.29, 132.10, 139.96, 171.52, 196.71. IR (KBr, cm⁻¹): 2965 (CH₂); 1744 (C=O); 1650 (C=O). Anal. Calcd. for C₁₅H₁₃BrN₂O₂S, %: C 49.33, H 3.59, Br 21.88; N 7.67; O 8.76, S 8.78. Found C 49.31; H 3.54; Br 21.91; N 7.69; S 8.80.

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.

6-Acetyl-7-methyl-5-phenyl-2-(2-phenylhydrazineylidene)-5H-thiazolo[3,2-a]pyrimidin-3(2H)-one 3a. Yield 76%, orange powder, mp 245–247 °C. ¹H NMR (400 MHz, DMSO-d₆, 25 °C) δH ppm: 2.26 (s, 3H, CH₃), 2.38 (s, 3H, CH₃), 6.17 (s, 1H, CH-Ph), 6.98–7.00 (m, 1H, CH (Ph)), 7.22–7.23 (m, 2H, CH (Ph)), 7.30–7.37 (m, 7H, CH (Ph)), 10.92 (s, 1H, NH). ¹³C NMR (100 MHz, DMSO-d₆, 25 °C) δC ppm: 23.73, 31.26, 54.75, 114.69, 118.52, 120.71, 123.04, 128.08, 129.06, 129.27, 129.83, 140.39, 143.46, 149.76, 153.73, 160.89, 197.31. IR (KBr, cm⁻¹): 3222, 3189 (NH); 1731 (C=O); 1621 (C=O); 1514 (C-C(Ph)). MS (ESI), m/z, [M+H]⁺: calcd. for C₁₆H₁₆BrN₂O₃S⁺: 391,47; found: 391,23. Anal. Calcd. for C₂₁H₁₉N₄O₂S, %: C 64.43; H 4.89; N 14.31; O 8.17, S 8.19. Found C 64.45; H 4.86; N 14.34; S 8.19.

6-Acetyl-5-(4-bromophenyl)-7-methyl-2-(2-phenylhydrazineylidene)-5H-thiazolo[3,2-a]pyrimidin-3(2H)-one 3b. Yield 69%, orange powder, mp 236–238 °C. ¹H NMR (400 MHz, DMSO-d₆, 25 °C) δH ppm: 2.27 (s, 3H, CH₃), 2.38 (s, 3H, CH₃), 6.13 (s, 1H, CH-Ar), 6.98–7.01 (m, 1H, CH (Ar)), 7.19–7.33 (m, 6H, CH (Ar)), 7.49–7.51 (m, 1H, CH (Ar)), 7.55–7.56 (m, 1H, CH (Ar)), 10.24, 10.29, 10.53, 10.61, 10.96 (five s, 1H, NH). ¹³C NMR (100 MHz, DMSO-d₆, 25 °C) δC ppm: 23.86, 31.39, 54.19, 114.71, 118.16, 120.57, 122.29, 123.07, 129.83, 130.38, 132.17, 139.70, 143.44, 150.24, 160.89, 197.14. IR (KBr, cm⁻¹): 3235 (NH); 1706 (C=O); 1642 (C=O); 1544 (C-C(Ph)). MS (ESI), m/z, [M+H]⁺: calcd. for C₂₁H₁₈BrN₄O₂S⁺: 470,36; found: 471,16. Anal. Calcd. for C₁₁H₁₇BrN₄O₂S, %: C 53.74; H 3.65; Br 17.02; N 11.94; O 6.82; S 6.83. Found C 53.71; H 3.62; Br 17.06; N 11.95; S 6.81.

6-Acetyl-2-(2-(2-methoxyphenyl)hydrazineylidene)-7-methyl-5-phenyl-5H-thiazolo[3,2-a]pyrimidin-3(2H)-one 3c. Yield 75%, orange powder, mp 210–212 °C. ¹H NMR (400 MHz, DMSO-d₆, 25 °C) δH ppm: 2.25 (s, 3H, CH₃), 2.36, 2.37 (two s, 3H, CH₃), 3.86, 3.88 (two s, 3H, CH₃), 6.16 (s, 1H, CH-Ph), 6.91–6.95 (m, 1H, CH (Ar)), 6.99–7.08 (m, 2H, CH (Ar)), 7.27–7.40 (m, 6H, CH (Ar)), 10.24, 12.18 (two s, 1H, NH). ¹³C NMR (100 MHz, DMSO-d₆, 25 °C) δC ppm: 24.26, 31.74, 55.23, 56.75, 112.46, 112.75, 113.17, 116.71, 118.98, 122.12, 124.33, 124.61, 128.60, 129.58, 129.66, 129.80, 129.90, 132.95, 140.97, 148.63, 150.34, 154.78, 161.63, 197.80. IR (KBr, cm⁻¹): 3269 (NH); 1729 (C=O); 1633 (C=O); 1525 (C-C(Ph)). MS (ESI), m/z, [M+H]⁺: calcd. for C₂₂H₂₁N₄O₃S⁺: 421,50; found: 421,26. Anal. Calcd. for C₂₂H₂₀N₄O₃S, %: C 62.84; H 4.79; N 13.32; O 11.41; S 7.62. Found C 62.86; H 4.77; N 13.35; S 7.56.

Synthesis 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 4a. Vanadium(V) oxide (0.2 g, 1 mmol) and sodium borohydride (0.3 g, 7 mmol) were added to thiazolo[3,2-a]pyrimidine 2-phenylhydrazone derivative 3a (0.4 g, 1 mmol) dissolved in ethanol (5 mL). Next, the reaction mixture was stirred at room temperature for 72 h. The mixture was filtered, and the filtrate was diluted with water. The precipitate that formed was filtered off and recrystallized from ethanol. Yield 44%, white powder, mp 165–166 °C. ¹H NMR (400 MHz, CDCl₃, 25 °C) δH ppm: 0.75 (d, J = 6.9 Hz, 3H, CH(OH)CH₃), 1.17 (d, J = 6.0 Hz, 3H, CH₃), 1.99–2.05 (m, 1H, CH-6), 3.60–3.65 (m, 1H, CH-N), 4.31–4.36 (m, 1H, CH-OH), 4.39 (d, J = 5.9 Hz, 2H, CH₂OH), 5.15 (d, J = 4.6 Hz, 1H, CHOH), 5.63 (t, J = 6.0 Hz, 1H, CH₂OH), 7.01–7.07 (m, 1H, CH (Ph)), 7.12–7.16 (m, 1H, CH (Ph)), 7.21–7.25 (m, 2H, CH (Ph)), 7.43–7.45 (m, 2H, CH (Ph)), 8.17–8.19 (m, 2H, CH (Ph)). ¹³C NMR (100 MHz, CDCl₃, 25 °C) δC ppm: 16.0, 22.7, 47.1, 48.6, 54.7, 55.7, 64.3, 117.7, 123.5, 126.7, 128.1, 129.0, 129.2, 139.9, 143.0, 147.6, 148.2. IR (KBr, cm⁻¹): 3348 (OH); 1631 (C=N); 1593 (C-C(Ph)). Anal. Calcd. for C₂₁H₂₄N₄O₂, %: C 69.21; H 6.64; N 15.37; O 8.78. Found C 69.27; H 6.53; N 15.23.

Crystals of 3a suitable for X-ray diffraction study were obtained by slow evaporation of ethanol solution (20 mL) containing 0.02 mol of the dissolved compound after 5 days.

X-ray diffraction analysis of 3a was performed on a Bruker D8 QUEST automatic three-circle 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 F² 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. Crystallographic data for compound 3a.

Compound	3a (from Ethanol)
Molecular formula	C21H18N4O2S
Formula	C21H18N4O2S
Formula Weight	286.35
Crystal System	monoclinic
Space group	P21/n
Cell parameters	a = 9.5262(8) Å, b = 12.1416(12) Å, c = 16.0525(16) Å; α = 90°, β = 90.248(4)°, γ = 90°.
V [Å3]	1856.67 Å3
Z and Z′	4 and 0
D(calc) [g/cm3]	1.397
λ (Å)	(MoKα) 0.71073
μ [/mm]	0.200
F(000)	816
Theta Min-Max [Deg]	2.103–29.998°
Reflections measured	58800
Independent reflections	5398
Observed reflections [I > 2σ(I)]	3596
Goodness of fit	1.035
R [I > 2σ(I)]	R1 = 0.0492, wR2 = 0.1118
R (all reflections)	R1 = 0.0936, wR2 = 0.1255
Max. and Min. Resd. Dens. [e/Å−3]	0.339 and −0.376 e Å−3
Depositor number in CCDC	2252794

.

3. Results and Discussion

2-Arylhydrazinylidenethiazolo[3,2-a]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 ¹H and ¹³C 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 sp³-hybridized C⁵ 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 R- and 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.

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 ¹³C NMR spectrum shift were upfield and appeared at 47.1 and 48.6 ppm, respectively. In the ¹H 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 ¹H and ¹³C 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. Relative energies of diastereomer 4.

C1 C5 C6 C7	Ph (at C5)	CH3-CH(OH)- (at C1)	CH3- (at C7)	E (kJ/mol)
RRRR/SSSS	e	e	e	115.05
RRRS/SSSR	e	e	a	103.53
RRSR/SSRS	a	e	a	112.76
RRSS/SSRR	a	e	e	113.23
RSRR/SRSS	a	e	e	99.50
RSRS/SRSR	a	e	a	118.68
RSSR/SRRS	e	e	a	111.95
RSSS/SRRR	e	e	e	113.43

e—pseudo-equatorial, a—pseudo-axial.

.

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.

4. Conclusions

In this study, new 6-acetyl-2-arylhydrazone derivatives of thiazolo[3,2-a]pyrimidine were obtained. The structure of 6-acetyl-7-methyl-5-phenyl-2-(2-phenylhydrazineylidene)-5H-thiazolo[3,2-a]pyrimidine-3(2H)-one was confirmed by X-ray diffraction. It was shown that a zigzag heterochiral chain of hydrogen-bonded molecules was formed in the crystalline phase. The reaction of the 6-acetyl-2-phenylhydrazone derivative of thiazolo[3,2-a]pyrimidine with a seven-fold excess of sodium borohydride in the presence of vanadium(V) oxide led to the diastereoselective formation of 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.