Synthesis of Pyridinium Moiety Containing Triazolyl Purines

: Pyridinium salts of 2-piperidinyl-6-triazolylpurine derivatives were obtained by the introduction of pyridinium moieties into the propane-1,3-diol fragment at the N (9) position of purine to enhance the solubility of 2-amino-6-triazolylpurine derivatives in water. Target structures were obtained using the tosylation of hydroxyl groups of 2-(6-(4-(4-methoxyphenyl)-1 H -1,2,3-triazol-1-yl)- 2-(piperidin-1-yl)-9 H -purin-9-yl)propane-1,3-diol, the subsequent introduction of pyridine, and ion exchange. The compounds were characterized using 1 H-and 13 C-NMR spectra, FTIR, UV–Vis, and HRMS data.

Many purine derivatives also possess fluorescent properties and are widely used in studies of biological and biochemical processes in cells and nucleic acids [12][13][14] due to the fact that a purine moiety works as a building block in DNA and RNA syntheses.Such compounds should usually have low cytotoxicity and are soluble in water.Many derivatization approaches are used to increase the solubility of purine derivatives in aqueous media, for example, introducing an amine-containing moiety to the purine N9 position [15], making purine derivatives co-crystals with benzenetricarboxylic acids [16] or obtaining purine-based ionic liquids [17].
In addition, a variety of reported novel fluorescent purine derivatives are also exploited as pH sensors [18], photocatalysts [19] and metal ion sensors [20].
2-Piperidinyl-6-triazolylpurine derivatives are known as push-pull chromophores that possess photophysical properties [21][22][23].In our study, we decided to enhance their solubility, especially in water, and synthesize pyridinium salts of 2-piperidinyl-6-triazolylpurine derivatives, and then examine how much the fluorescence is quenched upon introduction of pyridine moieties in the structure.There are a few examples in the literature that report that the introduction of pyridinium moieties in organic structures can increase the solubility [24,25] and also quench the fluorescence [26,27].

Results and Discussion
Starting material 1 was prepared according to the literature [21].Then, two hydroxyl groups of propane-1,3-diol at the N9 position of purine derivative 1 were tosylated using TsCl, DMAP, and Et 3 N in DCM, resulting in product 2 with a 76% yield (Scheme 1).compound 2 in pyridine was heated in the pressure vial at 120 °C for 1 h.After workup, compound 3 was acquired with a 72% yield with no need for further purification.Although pyridinium salt 3 was soluble in water, it still possessed bulky and aromatic tosylate anions that potentially could limit the solubility and impact fluorescence.Therefore, the tosylate anions were exchanged to chlorides using ion exchange resin, and product 4 was obtained in an 84% yield.Next, the solubility of compounds 1-4 in water was determined by using qNMR (1-0.21mg/mL, 2-0.19 mg/mL, 3-57 mg/mL, 4-133 mg/mL).Absorption and emission spectra were measured for compound 3 in H2O, DMSO, and DCM (Figure 1) and for compound 4 in MeCN, MeOH, H2O, DMSO, and DCM (Figure 2).Compound 3 exhibited absorption maxima at 359−362 nm and emission maxima at 428−565 nm (Table 1).Compound 4 exhibited absorption maxima at 360−363 nm and emission maxima at 452−463 nm (Table 1).Purine derivative 3 showed a blue shift in DCM and red shift in DMSO for emission spectra compared to derivative 4, probably due to the presence of tosylate ions (Figures 1 and 2).The quantum yields for 10 −4 M solutions of both compounds were below the detection range (<0.5%).In contrast, starting material 1 was reported to have a 98% QY in DMSO solution at 10 −5 M concentration [21].Thus, upon derivatization of propane-1,3-diol fragments at the purine N( 9) position with pyridinium moieties, the solubility of compounds in water was greatly enhanced, but the fluorescence was practically quenched.To make the triazolyl purine derivative's pyridinium tosylate salt 3, the solution of compound 2 in pyridine was heated in the pressure vial at 120 • C for 1 h.After workup, compound 3 was acquired with a 72% yield with no need for further purification.Although pyridinium salt 3 was soluble in water, it still possessed bulky and aromatic tosylate anions that potentially could limit the solubility and impact fluorescence.Therefore, the tosylate anions were exchanged to chlorides using ion exchange resin, and product 4 was obtained in an 84% yield.Next, the solubility of compounds 1-4 in water was determined by using qNMR (1-0.21mg/mL, 2-0.19 mg/mL, 3-57 mg/mL, 4-133 mg/mL).
Absorption and emission spectra were measured for compound 3 in H 2 O, DMSO, and DCM (Figure 1) and for compound 4 in MeCN, MeOH, H 2 O, DMSO, and DCM (Figure 2).Compound 3 exhibited absorption maxima at 359-362 nm and emission maxima at 428-565 nm (Table 1).Compound 4 exhibited absorption maxima at 360-363 nm and emission maxima at 452-463 nm (Table 1).Purine derivative 3 showed a blue shift in DCM and red shift in DMSO for emission spectra compared to derivative 4, probably due to the presence of tosylate ions (Figures 1 and 2).The quantum yields for 10 −4 M solutions of both compounds were below the detection range (<0.5%).In contrast, starting material 1 was reported to have a 98% QY in DMSO solution at 10 −5 M concentration [21].Thus, upon derivatization of propane-1,3-diol fragments at the purine N(9) position with pyridinium moieties, the solubility of compounds in water was greatly enhanced, but the fluorescence was practically quenched.To make the triazolyl purine derivative's pyridinium tosylate salt 3, the solution compound 2 in pyridine was heated in the pressure vial at 120 °C for 1 h.After work compound 3 was acquired with a 72% yield with no need for further purification.A hough pyridinium salt 3 was soluble in water, it still possessed bulky and aromatic tos ate anions that potentially could limit the solubility and impact fluorescence.Therefo the tosylate anions were exchanged to chlorides using ion exchange resin, and produc was obtained in an 84% yield.Next, the solubility of compounds 1-4 in water was de mined by using qNMR (1-0.21mg/mL, 2-0.19 mg/mL, 3-57 mg/mL, 4-133 mg/mL Scheme 1. Synthesis of pyridinium moiety containing triazolyl purine derivatives 3 and 4. Absorption and emission spectra were measured for compound 3 in H2O, DMS and DCM (Figure 1) and for compound 4 in MeCN, MeOH, H2O, DMSO, and DCM (F ure 2).Compound 3 exhibited absorption maxima at 359−362 nm and emission maxima 428−565 nm (Table 1).Compound 4 exhibited absorption maxima at 360−363 nm and em sion maxima at 452−463 nm (Table 1).Purine derivative 3 showed a blue shift in DCM a red shift in DMSO for emission spectra compared to derivative 4, probably due to presence of tosylate ions (Figures 1 and 2).The quantum yields for 10 −4 M solutions of b compounds were below the detection range (<0.5%).In contrast, starting material 1 w reported to have a 98% QY in DMSO solution at 10 −5 M concentration [21].Thus, up derivatization of propane-1,3-diol fragments at the purine N( 9) position with pyridini moieties, the solubility of compounds in water was greatly enhanced, but the fluoresce was practically quenched.

Materials and Method
NMR spectra were recorded on Bruker Avance 300 and Bruker Avance 500 spectrometers. 1 H-NMR spectra were recorded at 300 or 500 MHz with internal references from nondeuterated solvents (δ = 7.26 for CDCl3, δ = 2.50 for DMSO-d6) at 20° or 80 °C. 13C-NMR spectra were recorded at 75.5 or 125.7 MHz with internal references from nondeuterated solvents (δ = 77.16for CDCl3, δ = 39.52 for DMSO-d6) at 20° or 80 °C.Coupling constants are reported in Hz, chemical shifts of signals are given in ppm, and standard abbreviations are used for multiplicity assignments.Fourier transform infrared (FTIR) spectra were recorded using a Thermo Scientific Nicolet™ iSTM50 (Thermo Fisher, Waltham, MA, USA) spectrometer in the Attenuated Total Reflectance (ATR) mode.Spectra were obtained over a range of wavenumbers from 400 to 4000 cm −1 , co-adding 64 scans at 4 cm −1 resolution.Before every measurement, a background spectrum was taken and deducted from the sample spectrum.An Orbitrap Exploris 120 (Thermo Scientific, Waltham, MA, USA) mass spectrometer was used for high-resolution mass spectra (ESI).UV-Vis absorption spectra recorded with a PerkinElmer Lambda 35 spectrometer.Emission spectra and quantum yields for solutions were recorded using a QuantaMaster 40 steady-state spectrofluorometer (Photon Technology International, Inc., Birmingham, NJ, USA) equipped with a 6-inch integrating sphere by LabSphere (North Sutton, NH, USA), using the software package provided by the manufacturer.

Materials and Method
NMR spectra were recorded on Bruker Avance 300 and Bruker Avance 500 spectrometers. 1 H-NMR spectra were recorded at 300 or 500 MHz with internal references from nondeuterated solvents (δ = 7.26 for CDCl 3 , δ = 2.50 for DMSO-d 6 ) at 20 • or 80 • C. 13 C-NMR spectra were recorded at 75.5 or 125.7 MHz with internal references from nondeuterated solvents (δ = 77.16for CDCl 3 , δ = 39.52 for DMSO-d 6 ) at 20 • or 80 • C. Coupling constants are reported in Hz, chemical shifts of signals are given in ppm, and standard abbreviations are used for multiplicity assignments.Fourier transform infrared (FTIR) spectra were recorded using a Thermo Scientific Nicolet™ iSTM50 (Thermo Fisher, Waltham, MA, USA) spectrometer in the Attenuated Total Reflectance (ATR) mode.Spectra were obtained over a range of wavenumbers from 400 to 4000 cm −1 , co-adding 64 scans at 4 cm −1 resolution.Before every measurement, a background spectrum was taken and deducted from the sample spectrum.An Orbitrap Exploris 120 (Thermo Scientific, Waltham, MA, USA) mass spectrometer was used for high-resolution mass spectra (ESI).UV-Vis absorption spectra were recorded with a PerkinElmer Lambda 35 spectrometer.Emission spectra and quantum yields for solutions were recorded using a QuantaMaster 40 steady-state spectrofluorometer (Photon Technology International, Inc., Birmingham, NJ, USA) equipped with a 6-inch integrating sphere by LabSphere (North Sutton, NH, USA), using the software package provided by the manufacturer.
In the pressure vial, compound 2 (120 mg, 0.158 mmol) in pyridine (7 mL) was heated at 120 • C temperature for 1 h.The reaction mixture was evaporated, then dissolved in DCM (20 mL) and washed with cold water (2 × 30 mL) and cold brine (30 mL).The organic phase was concentrated in a vacuum, giving product 3 (104 mg, 72%) as a yellow oil.

Table 1 .
Photophysical properties of compounds 3 and 4 in various solvents.

Table 1 .
Photophysical properties of compounds and 4 in various solvents.