Synthesis of Triazolyl Derivatives Based on Thiazolo[3,2-a]pyrimidine Propargyl Ethers ^†

Abstract

This work is devoted to the synthesis of triazolyl derivatives based on propargyl ethers of the thiazolo[3,2-a]pyrimidine series by [3+2]-cycloaddition and the study of their structure in solution and crystalline phase. The formation of homochiral chains in the crystalline phase is attributed to the establishment of a halogen bond between the bromine atom and the nitrogen atom of the nitrile group. Additionally, the generation of a racemic dimer is linked to the formation of a halogen bond between the bromine atom and the nitrogen atom of the triazolyl fragment.

1. Introduction

Today, the investigation and creation of chemical compounds with antitumor properties represent a significant and crucial undertaking within the field of chemistry. It is estimated that 56.8% of drugs contain a heterocyclic backbone [1]. This class of organic compounds is prevalent among pharmaceuticals due to their capacity to simultaneously function as hydrogen bond donors and acceptors, enabling efficient interaction with target enzymes and receptors. In addition, heterocycles can alter the lipophilicity of drug molecules, thereby conferring the requisite pharmacokinetic and pharmaceutical properties [2].

2-Arylmethylidenthiazolo[3,2-a]pyrimidine derivatives exhibit high biological and pharmacological activity, making them promising molecules with significant potential as antitumor agents. For example, compound A, illustrated in Figure 1, was observed to exhibit enhanced cytotoxic activity and selectivity towards M-Hela and HuTu 80 cancer cell lines in comparison to the comparator drug, Sorafenib [3].

Additionally, heterocyclic compounds based on 1,2,3-triazole demonstrated notable efficacy in the creation of synthetic frameworks with pronounced anti-HIV, anti-cancer, and antibacterial activity [4]. Compound B was identified as an NMDA receptor antagonist [5], while compound C displayed antitumor activity (see Figure 1) [4].

In this context, it is worthwhile to examine the structure and biological properties of compounds that contain both a thiazolo[3,2-a]pyrimidine and a 1,2,3-triazole fragment.

2. Results and Discussion

Triazolyl derivatives on the thiazolo[3,2-a]pyrimidine platform 6 were synthesized following the scheme presented in Figure 2. In the first stage, a three-component Biginelli condensation involving appropriate 4-brombenzaldehyde, thiourea, and acetoacetic ether in a molar ratio of 1:1.5:1 led to the preparation of 1,2,3,4-tetrahydropyrimidine-2-thione 1 [6]. The next step was the preparation of thiazolo[3,2-a]pyrimidine 2 by interaction of 1,2,3,4-tetrahydropyrimidine-2-thione 1 with excess ethyl chloroacetate [7]. The resulting compound was then used as a precursor for the synthesis of 4-hydroxybenzylidenethiazolo[3,2-a]pyrimidine 3. The desired product was obtained by filtration of the reaction mixture followed by recrystallization from ethanol solution in high yield (88%).

Next, the obtained 4-hydroxybenzylidenethiazolo[3,2-a]pyrimidine 3 was introduced into Mitsunobu reaction with propargyl alcohol to give propargyl ester 4. The final step was the click reaction between the propargyl derivative on the thiazolo[3,2-a]pyrimidine platform 4 and 4-(azidomethyl)benzonitrile 5. Thus, ethyl (Z)-7-methyl-3-oxo-5-(4-bromophenyl)-2-(4-((1-((1-(4-cyanobenzyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate 6 was obtained in high yield (83%). The structure of the obtained compound was confirmed by a complex of physicochemical methods of analysis. Also, the structure of this compound was confirmed by X-ray diffraction analysis (see Figure 3).

In a previous study, our research group investigated the supramolecular organization in the crystalline phase of 2-arylmethylidene derivatives of thiazolopyrimidine. The development of approaches for the control of supramolecular synthons to form chiral supramolecular structures in the crystalline phase in order to achieve chiral discrimination has attracted considerable interest in our research group [8,9]. In order to examine the supramolecular organization of the novel triazolyl derivatives of thiazolopyrimidine, crystalline samples were obtained through the slow evaporation of various solvent mixtures (ethyl acetate/methanol and DMF/methanol) at a 1:1 volume ratio. The driving force in the formation of supramolecular organization in both cases was Br…N halogen bonding, but the nature of this bonding differed.

It was found that in the case of crystal I, racemic dimers were formed in which a halogen bond between the bromine atom and the nitrogen atom of the triazole moiety was realized (d_Br1…N9 = 3.272 Å, ∑(Br + N) = 3.4 Å) (see Figure 4). However, in the case of crystal II, homochiral chains are formed in which the halogen bond between the bromine atom of the thiazolpyrimidine moiety and the nitrogen atom of the nitrile fragment is realized (d_Br1…N12 = 3.321 Å, ∑(Br + N) = 3.4 Å) (see Figure 5).

3. Materials and Methods

3.1. Synthesis and Characterisation

All reagents (Acros Organics (Geel, Belgium), Alfa Aesar (Haverhill, MA, USA)) were used without further purification. The ethyl 4-(4-bromophenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate 1 [10], ethyl 5-(4-bromophenyl)-7-methyl-3-oxo-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate 2 [11], ethyl (Z)-5-(4-bromophenyl)-2-(4-hydroxybenzylidene)-7-methyl-3-oxo-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate 3 [3] and 4-(azidomethyl)benzonitrile 5 [12] were synthesized according to reported methods.

NMR experiments were performed on Bruker Avance instruments (Billerica, MA, USA) with an operating frequency of 400 MHz for shooting ¹H NMR spectra. Chemical shifts were determined relative to the signals of residual protons of the DMSO-d₆ solvent.

IR spectra in KBr tablets were recorded on a Bruker Vector-22.

Electrospray ionization (ESI) mass spectra were obtained using a Bruker AmaZon Xion trap mass spectrometer. Melting points were determined on a BOETIUS melting table with an RNMK 05 imaging device (VEB MLW Analytik, Dresden, Germany, 100x zoom).

3.1.1. General Method for Compounds 4 and 6 Preparation

To 1 mmol of 4-hydroxybenzylidene derivative of thiazolo[3,2-a]pyrimidine 3 and 1.5 mmol of triphenylphosphine dissolved in 20 mL toluene was added 10 mmol of propargyl alcohol. Then 1.5 mmol of diethylazodicarboxylate was added dropwise to the reaction mixture under argon atmosphere without allowing heating в тeчeниe 12 чacoв. The solvent was removed under reduced pressure and a precipitate was formed when cold methanol was added to the residue. The product was filtered off, washed with cold methanol and recrystallized from ethanol.

Ethyl (Z)-5-(4-bromophenyl)-7-methyl-3-oxo-2-(4-(prop-2-yn-1-yloxy)benzylidene)-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate 4. Yield 80%, yellow powder, mp 160–162 °C. IR (KBr, cm⁻¹): 3272 (≡C–H); 2128 (C≡C); 1704 (C=O); 1545 (C=N); 749 (C–S). ¹H NMR (400 MHz, DMSO-d₆, 25°C) δ_H ppm: 1.13 (t, 3H, J = 7.0 Hz, CH₃CH₂O), 2.39 (s, 3H, CH₃), 3.61 (t, 1H, J = 2.4 Hz, HC≡C-), 4.01–4.07 (m, 2H, CH₃CH₂O), 4.89–4.90 (d, 2H, J = 2.4 Hz, C₆H₄-OCH₂), 6.02 (s, 1H, -CH-Ar), 7.14–7.17 (m, 2H, CH (Ar)), 7.26–7.28 (m, 2H, CH (Ar)), 7.54–7.60 (m, 4H, CH (Ar)), 7.76 (s, 1H, C=CH). MS (ESI), m/z: [M+H]⁺: calcd. for C₂₆H₂₁BrN₂O₄S⁺: 538,43; found: 537,07. Anal. Calcd. for C₂₆H₂₁BrN₂O₄S, %: C 58.11; H 3.93; Br 14.87; N 5.21; O 11.91; S 5.97. Found C 58.12; H 3.98; Br 14.82; N 5.18; O 11.94; S 5.96 (see Figures S1–S3).

To 1 mmol of 4-(prop-2-in-1-yloxy)benzylidenethiazolo[3,2-a]pyrimidine 4 and 1.2 mmol of copper iodide (I) dissolved in 5 mL of toluene and 1.5 mL of triethylamine, 2 mmol of azide 5 was added dropwise. Peaкцию пpoвoдили в тeчeниe 6 чacoв. The solvent was evaporated under reduced pressure. The product obtained was filtered off, washed with ethyl acetate and recrystallized from ethanol.

Ethyl (Z)-5-(4-bromophenyl)-2-(4-((1-(4-cyanobenzyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)-7-methyl-3-oxo-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate 6. Yield 83%, brown powder, mp 193–195 °C. IR (KBr, cm⁻¹): 2231 (C≡N); 1709 (C=O); 1596, 1509 (N=N); 1542 (C=N); 752 (C–S). ¹H NMR (400 MHz, DMSO-d₆, 25°C) δ_H ppm: 1.13 (t, 3H, J = 7.1 Hz, CH₃CH₂O), 2.39 (s, 3H, CH₃), 4.02–4.07 (m, 2H, CH₃CH₂O), 5.25 (s, 2H, C₆H₄-OCH₂), 5.74 (s, 2H, NCH₂), 6.03 (s, 1H, -CH-Ar), 7.20–7.28 (m, 4H, CH (Ar)), 7.44–7.47 (m, 2H, CH (Ar)), 7.54–7.59 (m, 4H, CH (Ar)), 7.76 (s, 1H, CH (triazole)), 7.84–7.86 (m, 2H, CH (Ar)), 8.36 (s, 1H, C=CH). MS (ESI), m/z: [M+H]⁺: calcd. for C₃₄H₂₇BrN₆O₄S⁺: 696.59; found: 697.05. Anal. Calcd. for C₃₄H₂₇BrN₆O₄S, %: C 58.71; H 3.91; Br 11.49; N 12.08; O 9.20; S 4.61. Found C 58.69; H 3.93; Br 11.47; N 12.10; O 9.18; S 4.63 (see Figures S4–S6).

3.1.2. Crystallization Conditions

Crystals I of compound 6 suitable for X-ray diffraction study were obtained by slow evaporation of a methanol (7.5 mL)/ethyl acetate (7.5 mL) solution containing 0.02 mol of the dissolved compound after 3 days.

Crystals II of compound 6, suitable for X-ray diffraction study, were obtained by slow evaporation of a methanol (7.5 mL)/DMFA (7.5 mL) solution containing 0.02 mol of the dissolved compound after 6 days.

3.1.3. Single-Crystal X-Ray Diffraction

Single-crystal X-ray diffraction (SC XRD) study was performed on a Bruker D8 QUEST automated three-circle diffractometer (Bruker, Germany) with a PHOTON III area detector and an IμS DIAMOND microfocus X-ray tube (Incoatec, Germany) at a temperature of 100(2) K for crystal I and 120(2) K for crystal II: λ (Mo Kα) = 0.71073 Å, ω/ϕ-scanning mode with a step of 0.5°. The APEX3 software package was used to index the diffraction data and to determine and refine the unit cell parameters. Numerical absorption correction based on the crystal shape, additional spherical absorption correction, and systematic error correction were performed using the SADABS–2016/2 software (Bruker, 2016/2) [13]. Structures were solved by direct methods using the SHELXT-2018/3 program [14] and refined by full-matrix least-squares on F² using the SHELXL-2018/3 program [15]. Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms of methyl groups were inserted using the rotation of the group with idealized bond angles; the remaining hydrogen atoms were refined using a riding model. Most calculations were performed using the WinGX-2021.3 software package [16]. Crystallographic data for structures are listed in

Table 1. Crystallographic data and X-ray experimental parameters for the single crystals I and II.

Compound	I	II
Empirical formula	C34H27BrN6O4S
Formula weight	695.58
Radiation, wavelength	Mo Kα, 0.71073 Å
Temperature, Kº	100(2)	120(2)
Crystal system	Monoclinic	Monoclinic
Space group	P2/c (No. 13)	C2/c (No. 15)
Unit cell dimensions: a, b, c, Å; β, °	24.489(4), 9.6102(16), 14.361(2); 93.022(5)	45.095(6), 9.6154(12), 14.5927(18); 105.111(4)
Volume, Å3	3375.1(9)	6108.6(13)
Z/Z’	4/1	8/1
Calculated density, g cm−3	1.369	1.513
Absorption coefficient, mm−1	1.326	1.465
F(000)	1424	2848
θ range for data collection, °	2.119–25.998	2.538–26.999
Index ranges	−30 ≤ h ≤ 30, −11 ≤ k ≤ 11, −17 ≤ l ≤ 17	−57 ≤ h ≤ 57, −12 ≤ k ≤ 12, −18 ≤ l ≤ 18
Reflections collected/Independent reflections (Rint)	91,666/6654 (0.0968)	69,808/6673 (0.1170)
Rσ	0.0373	0.0591
Tmax/Tmin	0.7460/0.6761	0.7460/0.5679
Observed data [I > 2σ(I)]	6040	5180
Data/restraints/parameters	6654/0/399	6673/0/417
Goodness-of-fit on F2	1.203	1.133
Final R indices [I > 2σ(I)]	R1 = 0.0974, wR2 = 0.2028	R1 = 0.0629, wR2 = 0.1148
R indices (all data)	R1 = 0.1042, wR2 = 0.2062	R1 = 0.0868, wR2 = 0.1225
Largest diff. peak and hole, e Å−3	1.361 and −1.070	0.448 and −0.415
CCDC number	2,383,564	2,383,565

.

4. Conclusions

In this work ethyl (Z)-5-(4-bromophenyl)-2-(4-((1-(4-cyanobenzyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)-7-methyl-3-oxo-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidine-6-carboxylate was first obtained. The structure of the obtained compounds was confirmed by a complex of physicochemical methods of analysis. Two crystalline samples of the newly synthesized derivative were obtained using different solvent systems. The formation of homochiral chains in the crystalline phase is explained by the formation of a halogen bond between the bromine atom and the nitrogen atom of the nitrile group. In addition, the formation of the racemic dimer is associated with the formation of a halogen bond between the bromine atom and the nitrogen atom of the triazolyl fragment.