INTRODUCTION:

A purine is a heterocyclic aromatic organic compound that consists of a pyrimidine ring fused to an imidazole ring. Purine gives its name to the wider class of molecules, purines, which include substituted purines and their tautomers, are the most widely occurring nitrogen-containing heterocycle in nature.[1] Purines are found in high concentration in meat and meat products, especially internal organs such as liver and kidney. In general, plant-based diets are low in purines.[2] Examples of high-purine sources include: sweetbreads, anchovies, sardines, liver, beef kidneys, brains, meat extracts (e.g., Oxo, Bovril), herring, mackerel, scallops, game meats, beer (from the yeast) and gravy.

Purines and pyrimidines make up the two groups of nitrogenous bases, including the two groups of nucleotide bases. Two of the four deoxyribonucleotides (deoxyadenosine and deoxyguanosine) and two of the four ribonucleotides (adenosine, or AMP, and guanosine, or GMP), the respective building blocks of DNA and RNA, are purines. In order to form DNA and RNA, both purines and pyrimidines are needed by the cell in approximately equal quantities. Both purine and pyrimidine are self-inhibiting and activating. When purines are formed, they inhibit the enzymes required for more purine formation. This self-inhibition occurs as they also activate the enzymes needed for pyrimidine formation. Pyrimidine simultaneously self-inhibits and activates purine in similar manner. Because of this, there is nearly an equal amount of both substances in the cell at all times.[4]

Fig 1: Structure of Purine.

Since purine is very useful molecule for biological life as it is basic building blocks of DNA and RNA, the modification of purine structure shows various biological activity it shown in fig. 2

Fig 2: Modification of Purines shows various biological activity.

The substituted purines has gained more attention in this decades, its due to its biological importance. Substituted purines continue to attract attention as biologically active compounds, molecular tools, and probes for investigating biological systems. Because of their similarity to adenosine, many 9-substituted adenines interact with adenosine receptors, showing important biological activity including antiviral and cytostatic effects. Although 7-substituted adenines are not so readily available, some of them were also shown to have significant activity against viruses.

6- Chloropurines is very suitable substrate for making substituted purines for their further biological evaluations. Since 6-Chloropurines is commercially available and its cheap, its make easy for chemist to start with this. For this purpose ongoing synthesis of heterocyclic compounds in our laboratory, we planned to synthesis the substituted purine derivatives from commercially available 6-Chloropurine with boronic acid.

RESULTS AND DISCUSSION:

As we have shown in above graphical abstract, our synthetic journey of 6-aryl purines started with commercially available 6-chloropurine. In 6-chloropurine the free nitrogen atom needs to be protect to do further reaction on it. For that purpose we took compound 1 which on reaction with isopropyl iodide (2) in presence of NaH as a base in DMF at room temperature gave compound A with 70% yield. Subsequently, similar kind of reaction condition was performed with different protecting partner like ethyl iodide(2) and benzyl bromide (4) to give respective protected products A1 and A3 with 75% and 77 % yield. To perform Suzuki coupling with boronic acid the protection of free nitrogen of purine required.

After the completion of preparation of starting material the next task was, coupling reaction with different commercially available boronic acids.

Fig 3: Protection of Purines with different Protection Group.

The synthesis of 6-arylpurines, started with the coupling reaction between the 6-chloropurine and boronic acid this is C-C bond forming reaction called Suzuki coupling reaction. The versatile C-C bond forming reaction between aryl iodide and boronic acid gives C-C bond forming products.

By keeping in mind the Suzuki reaction, we build our strategy to perform the Suzuki coupling reaction between 6-chloropurine and boronic acid in water under palladium catalysis. We started our synthetic journey by taking simple 6-chloropurine (A) and phenyl boronic acid (B) in round bottom flask under nitrogen atmosphere Pd(PPh3) ( 5 mol%), Na₂CO₃ (2 equiv.) and the mixture of toluene and water was added, under reflux condenser the reaction mixture was heated 12 h. The completion of the reaction was checked by TLC.

The complete reaction scheme is shown in fig: 4, the 6-arylpurine derivatives is shown in (C). We changed different boronic acid to check the feasibility and scope of boronic acid. For that purpose the phenyl boronic acid (C1) gave 88% yield, which shown the optimization of the reaction. Subsequently the Electron withdrawing as well as donating boronic acid was used and all worked well. Apart from normal aryl boronic acid heteroaryl boronic acid like furan, indole, thiophenes and substituted thiophenes also worked well.

Like indole (C2) gave 74% yield, furan (C3) gave 70% yield and electron withdrawing group on boronic acid like acetyl (C4) obtained with 82% yield. The thiophenes like (C6), (C7) and (C8) gaves 90%, 66% and 60% yields. The donating group like methoxy and methyl on phenyl boronic acid worked well and gave 62% and 90% yields. To check the protecting group tolerance in the reaction condition we used phenyl protecting group (C11) which worked well and gave 74%.So this protocol allows ample opportunity to modify further substituted purines and can be change many diverse group.

The below Fig. 4 shows the standard reaction condition as well as prepared substituted purine derivatives.

These prepared purine derivatives were subjected for biological screening TB and shown very good to moderated results, which also summarized below given table. All the synthesized derivatives were characterized by NMR, mass and IR analysis and all are confirmed well.

Fig 4 : Synthesis of 6-arylpurine derivatives.

Biological Study:

These all the synthesized 6-arylpurines derivatives were screened for biological activity, since its novel compounds and based on literature observations it can be very potential drug candidates for drug discovery programme. The purine derivatives were tested for inhibition of Mycobacterium tuberculosis in invitro MABA assay. The results are summarized in table 1.

In-vitro Mycobacterium tuberculosis MABA assay

A growth control containing no antibiotic and a sterile control were also prepared on each plate. Sterile water was added to all perimeter wells to avoid evaporation during the incubation. The plate was covered, sealed in plastic bags and incubated at 37 °C in normal atmosphere. After 7 days incubation, 30 µl of alamar blue solution was added to each well, and the plate was re-incubated overnight. A change in colour from blue (oxidised state) to pink (reduced) indicated the growth of bacteria, and the MIC was defined as the lowest concentration of compound that prevented this change in colour.

The biological activity against TB cell lines shows that compound (C2) is shown 12.5 MIC and subsequently the compound (C9), (C10) and (C12) shown good MIC against TB cell lines. This is the very good starting results for future drug discovery programme and other number of purine substituted derivatives can be made and that can be checked for other cell lines.

Table 1: MIC results of purine derivatives against TB cells in (µM).

Entry	Compound	MIC (µM)
1	C1	100
2	C2	12.5
3	C3	15.6
4	C4	50.2
5	C5	66.2
6	C6	76.2
7	C7	32.4
8	C8	22.3
9	C9	6.25
10	C10	8.18
11	C11	9.15
12	C12	7.15

CONCLUSIONS:

In summary, the ecofriendly protocol was developed and applied for the synthesis of 6-arylpurine derivatives in water. Purine derivatives showed significant inhibition of Mycobacterium tuberculosis. These analogues are chemically tractable and hence provides ample opportunities for further modification to obtain potent ant tuberculosis agents. The isolated yield of the purine derivatives is excellent, so gram scale synthesis possible. The scope for boronic acids were shown and it can be further extend to get diverse purine derivatives.

EXPERIMENTAL SECTION:

Unless otherwise stated, all commercial reagents and solvents were used without additional purification. Analytical thin layer chromatography (TLC) was performed on pre-coated silica gel 60 F₂₅₄ plates. Visualization on TLC was achieved by the use of UV light (254 nm). Column chromatography was undertaken on silica gel (100‒200 mesh) using a proper eluent system. NMR spectra were recorded in chloroform-d and DMSO-d₆at 300 or 400 or 500 MHz for ¹H NMR spectra and 75 MHz or 100 or 125 MHz for ¹³C NMR spectra. Chemical shifts were quoted in parts per million (ppm) referenced to the appropriate solvent peak or 0.0 ppm for tetramethylsilane. The following abbreviations were used to describe peak splitting patterns when appropriate: br = broad, s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, dd = doublet of doublet, td = triplet of doublet, m = multiplet. Coupling constants, J, were reported in hertz unit (Hz). For ¹³C NMR chemical shifts were reported in ppm referenced to the center of a triplet at 77.0 ppm of chloroform-d and 40.0 ppm center for DMSO-d_6.

General procedure for preparation of compounds:

1) Procedure for compound A :

A stirred suspension of sodium hydride (60%, in mineral oil, 620 mg, 16.61 mmol) in DMF (30 ml) was treated with 6-chloro-9H-purine 13 (2.0 g, 12.94 mmol) under N₂ atmosphere at room temperature for 2 h. Isopropyl iodide (1.03 g, 16.54 mmol) was added to the reaction mixture at 0 ⁰C, and the resultant was stirred at room temperature for 12 h, and quenched with aqueous NH₄Cl (25 ml). The mixture was poured into water (100 ml), and extracted with EtOAc (200 ml). The combined organic layer was washed with water and brine (50 ml), and dried over Na₂SO₄, and concentrated in vacuo to leave a yellowish oil residue which was purified by column chromatography on SiO₂ with ethyl acetate and pet. Ether to (50:50, Rf = 0.5) to give title compound as white solid. Further the compound was confirmed by ¹H,¹³C NMR and elemental mass analysis.

2) Procedure for compound A1:

A stirred suspension of sodium hydride (60%, in mineral oil, 620 mg, 16.61 mmol) in DMF (30 ml) was treated with 6-chloro-9H-purine (2.0 g, 12.94 mmol) under N₂ atmosphere at room temperature for 2 h. Ethyl iodide (1.03 g, 16.54 mmol) was added to the reaction mixture at 0 ⁰C, and the resultant was stirred at room temperature for 12 h, and quenched with aqueous NH₄Cl (25 ml). The mixture was poured into water (100 ml), and extracted with EtOAc (200 ml). The combined organic layer was washed with water and brine (50 ml), and dried over Na₂SO₄, and concentrated in vacuo to leave a yellowish oil residue which was purified by column chromatography on SiO₂ with ethyl acetate and pet. Ether to (40:60, Rf = 0.45) to give title compound as white solid. Further the compound was confirmed by ¹H,¹³C NMR and elemental mass analysis.

3) Procedure for compound A3:

A stirred suspension of sodium hydride (60%, in mineral oil, 620 mg, 16.61 mmol) in DMF (30 ml) was treated with 6-chloro-9H-purine (2.0 g, 12.94 mmol) under N₂ atmosphere at room temperature for 2 h. Benzyl bromide (1.03 g, 16.54 mmol) was added to the reaction mixture at 0 ⁰C, and the resultant was stirred at room temperature for 12 h, and quenched with aqueous NH₄Cl (25 ml). The mixture was poured into water (100 ml), and extracted with EtOAc (200 ml). The combined organic layer was washed with water and brine (50 ml), and dried over Na₂SO₄, and concentrated in vacuo to leave a yellowish oil residue which was purified by column chromatography on SiO₂ with ethyl acetate and pet. Ether to (50:50, Rf = 0.6) to give title compound as white solid. Further the compound was confirmed by ¹H,¹³C NMR and elemental mass analysis.

4) General Procedure for Suzuki coupling reaction (C):

To a solution of 6-Chlro purine (0.316 g, 2 mmol) in toluene (7 mL), (1.5 mL), and H₂O (7 mL) was added Na₂CO₃ (1.569 g, 14.8 mmol) followed by Pd(PPh₃)₄ (0.069 g, 0.06 mmol) and phenylboronic acid (2.6 mmol) under argon in a 50 mL two-necked flask. The reaction mixture was refluxed for 12 h, and then cooled to room temperature. To the reaction mixture was added aqueous NH₄Cl (15 mL), extracted by Ethyl acetate for three times, dried over Na₂SO₄, and evaporated in vacuum to afford the crude product, which was further purified by flash chromatography on silica gel with n-hexane/EtOAc( 60: 40, Rf = 0.65) to give quantitative yield of corresponding 9-ethyl-6-phenyl-9H-purine as white solid, Which was further characterized by NMR and elemental mass analysis.

Spectral Data for all Synthesized Compounds:

1) 9-ethyl-6-phenyl-9H-purine (C1)

White solid, Mp 122- 124 °C, Mass : Cal. 224.2, Observe. 224.3

¹H NMR (300 MHz, CDCl₃) δ 8.97 (s, 1H), 8.79 – 8.60 (m, 2H), 8.08 (s, 1H), 7.57 – 7.39 (m, 3H), 4.31 (q, J = 7.3 Hz, 2H), 1.53 (t, J = 7.3 Hz, 3H).

¹³C NMR (100 MHz, CDCl₃) δ 158.9, 151.8, 142.7, 140.2, 139.9, 131.9, 131.0, 130.7, 128.0, 95.8, 47.5, 22.5.

2) 6-(benzo[b]thiophen-3-yl)-9-isopropyl-9H-purine (C2) :

White solid, Mp 177- 179 °C, Mass : Cal. 294.2, Observe. 294.3

¹H NMR (300 MHz, CDCl₃) δ 9.25 (s, 1H), 9.15 (d, J = 8.2 Hz, 1H), 9.04 (s, 1H), 8.14 (s, 1H), 7.87 (d, J = 8.0 Hz, 1H), 7.42 (dd, J = 19.9, 7.2 Hz, 2H), 4.95 (s, 1H), 1.64 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 158.6, 151.8, 143.9, 142.7, 139.1, 139.0, 138.5, 132.1, 131.3, 128.9, 128.1, 127.2, 126.9, 96.3, 47.6, 22.6.

3) 6-(furan-2-yl)-9-isopropyl-9H-purine (C3):

Brown solid, Mp 116-118 °C, Mass : Cal. 228.3, Observe. 228.6.

¹H NMR (300 MHz, CDCl₃) δ 9.04 – 8.87 (m, 1H), 8.29 (d, J = 5.0 Hz, 1H), 8.17 (s, 1H), 7.46 (dd, J = 5.0, 3.0 Hz, 1H), 5.16 – 4.79 (m, 1H), 1.67 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 165.2, 157.9, 151.9, 151.8, 143.9, 143.2, 141.2, 132.1, 131.7, 131.0, 129.1, 95.5, 52.5, 47.8, 22.5.

4) methyl 4-(9-isopropyl-9H-purin-6-yl)benzoate (C4) :

Orange solid, Mp 116-118 °C, Mass : Cal. 296.3, Observe. 296.7.

¹H NMR (300 MHz, CDCl₃) δ 9.00 (s, 1H), 8.81 (d, J = 7.9 Hz, 2H), 8.16 (d, J = 6.9 Hz, 3H), 7.19 (s, 1H), 5.12 – 4.73 (m, 1H), 4.36 (q, J = 7.0 Hz, 2H), 1.63 (d, J = 6.5 Hz, 6H), 1.48 – 1.28 (t, 3H).

¹³C NMR (75 MHz, CDCl₃) δ 166.4, 157.9, 152.3, 152.0, 145.2, 142.1, 141.0, 139.6, 137.1, 131.5, 127.4, 120.6, 96.3, 60.7, 44.0, 31.9, 19.9, 14.3, 13.5.

5) 6-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-9-isopropyl-9H-purine (C5) :

Yellow solid, Mp 147-149 °C, Mass : Cal. 296.3, Observe. 296.7.

¹H NMR (400 MHz, CDCl₃) δ 9.02 (s, 1H), 8.82 (d, J = 8.5 Hz, 2H), 8.20 (s, 1H), 7.68 (d, J = 8.3 Hz, 2H), 5.94 (s, 1H), 5.18 – 4.88 (m, 1H), 4.24 – 4.02 (m, 4H), 1.68 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 196.8, 158.0, 151.9, 151.8, 143.1, 140.8, 140.6, 136.8, 131.9, 130.7, 129.8, 102.6, 47.7, 26.5, 22.5.

6) 9-isopropyl-6-(5-methylthiophen-2-yl)-9H-purine (C6) :

White solid, Mp 122-124 °C, Mass : Cal. 258.3, Observe. 258.5.

¹H NMR (300 MHz, CDCl₃) δ 8.79 (s, 1H), 8.41 (d, J = 3.4 Hz, 1H), 8.07 (s, 1H), 6.86 (s, 1H), 4.88 (dt, J = 13.5, 6.7 Hz, 1H), 2.53 (s, 3H), 1.59 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 196.8, 158.0, 151.9, 151.8, 143.1, 140.8, 140.6, 136.8, 131.9, 130.7, 129.8, 102.6, 47.7, 26.5, 22.5.

7) 1-(5-(9-isopropyl-9H-purin-6-yl)thiophen-2-yl)ethanone (C7) :

White solid, Mp 167-169 °C, Mass : Cal. 286.1, Observe. 286.5.

¹H NMR (300 MHz, CDCl₃) δ 8.79 (s, 1H), 8.41 (d, J = 3.4 Hz, 1H), 8.07 (s, 1H), 6.86 (s, 1H), 4.88 (dt, J = 13.5, 6.7 Hz, 1H), 2.53 (s, 3H), 1.59 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 196.8, 158.0, 151.9, 151.8, 143.1, 140.8, 140.6, 136.8, 131.9, 130.7, 129.8, 102.6, 47.7, 26.5., 20.2.

8) 9-isopropyl-6-(thiophen-2-yl)-9H-purine (C8) :

White solid, Mp 141-143 °C, Mass : Cal. 244.1, Observe. 244.5.

¹H NMR (300 MHz, CDCl₃) δ 8.79 (s, 1H), 8.41 (d, J = 3.4 Hz, 2H), 8.07 (s, 1H), 6.86 (s, 1H), 4.88 (dt, J = 13.5, 6.7 Hz, 1H), 1.59 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 196.8, 158.0, 151.9, 151.8, 143.1, 140.8, 140.6, 136.8, 131.9, 130.7, 129.8, 102.6, 47.7, 26.5.

9) 9-isopropyl-6-(m-tolyl)-9H-purine (C9) :

White solid, Mp 152-154 °C, Mass : Cal. 252.2, Observe. 252.4.

¹H NMR (300 MHz, CDCl₃) δ 8.93 (s, 1H), 8.63 (d, J = 4.0 Hz, 2H), 8.22 (s, 1H), 7.80 (d, J = 4.0 Hz, 2H), 4.98 (dt, J = 13.6, 6.8 Hz, 1H), 2.63 (s, 3H), 1.69 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 157.1, 151.9, 151.7, 142.8, 136.6, 133.6, 132.4, 131.7, 130.8, 127.4, 122.0, 47.6, 22.6.

10) 9-isopropyl-6-(3-methoxyphenyl)-9H-purine (C10) :

White solid, Mp 111-114 °C, Mass : Cal. 268.2 Observe. 268.3.

¹H NMR (300 MHz, CDCl₃) δ 8.93 (s, 1H), 8.63 (d, J = 4.0 Hz, 2H), 8.22 (s, 1H), 7.80 (d, J = 4.0 Hz, 2H), 4.98 (dt, J = 13.6, 6.8 Hz, 1H), 3.5 (s, 3H), 1.69 (d, J = 6.8 Hz, 6H).

¹³C NMR (75 MHz, CDCl₃) δ 157.1, 151.9, 151.7, 142.8, 136.6, 133.6, 132.4, 131.7, 130.8, 127.4, 122.0, 47.6, 21.7

11) 9-benzyl-6-phenyl-9H-purine (C11) :

Pink solid, Mp 130-132 °C, Mass : Cal. 286.2 Observe. 286.3.

¹H NMR (300 MHz, CDCl₃) δ 8.72 (s, 1H), 8.04 (s, 1H), 7.40 – 7.20 (m, 5H), 6.8- 6.10 (m, 5H), 5.39 (s, 2H).

¹³C NMR (75 MHz, CDCl₃) δ 158.9, 151.7, 151.2, 142.0, 141.8, 141.3, 134.4, 132.7, 131.0, 130.8, 129.7, 129.2, 127.7, 127.2, 126.4, 47.2.

12) 9-isopropyl-6-phenyl-9H-purine (C12) :

White solid, Mp 182-184 °C, Mass : Cal. 238.2 Observe. 240.3.

¹H NMR (300 MHz, CDCl₃) δ 9.05 (s, 1H), 8.79 (dd, J = 8.0, 1.6 Hz, 2H), 8.22 (s, 1H), 7.69 – 7.48 (m, 3H), 5.19 – 4.86 (m, 1H), 1.70 (d, J = 6.8 Hz, 6H).

¹³C NMR (100 MHz, CDCl₃) δ 158.9, 151.8, 142.7, 140.2, 139.9, 131.9, 131.0, 130.7, 128.0, 95.8, 47.5, 22.5;

In-vitro Mycobacterium tuberculosis MABA assay

The inoculum was prepared from fresh LJ medium re-suspended in 7H9-S medium (7H9 broth, 0.1% casitone, 0.5% glycerol, albumin, dextrose, supplemented oleic acid, and catalase [OADC]), adjusted to a McFarland tube No. 1, and diluted 1:20; 100 µl was used as inoculum. Each drug stock solution was thawed and diluted in 7H9-S at four-fold the final highest concentration tested. Serial two-fold dilutions of each drug were prepared directly in a sterile 96-well microtiter plate using 100 µl 7H9-S. A growth control containing no antibiotic and a sterile control were also prepared on each plate. Sterile water was added to all perimeter wells to avoid evaporation during the incubation. The plate was covered, sealed in plastic bags and incubated at 37 °C in normal atmosphere. After 7 days incubation, 30 µl of alamar blue solution was added to each well, and the plate was re-incubated overnight. A change in colour from blue (oxidised state) to pink (reduced) indicated the growth of bacteria, and the MIC was defined as the lowest concentration of compound that prevented this change in colour.

ACKNOWLEDGMENT:

Authors are thankful to management and principal for providing infrastructural facilities and encouragement.

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Received on 01.12.2017 Modified on 15.12.2017

Asian J. Research Chem. 2017; 10(6):845-851.

DOI: 10.5958/0974-4150.2017.00141.9

Kalimoddin I. Momin1, Abhay S. Bondge2, Vikas B. Surawanshi3,Jairaj K. Dawale4*

INTRODUCTION:

Kalimoddin I. Momin¹, Abhay S. Bondge², Vikas B. Surawanshi³,Jairaj K. Dawale⁴*