HETEROCYCLES

Paper | Regular issue | Vol. 78, No. 8, 2009, pp. 2003-2012
Received, 21st February, 2009, Accepted, 9th April, 2009, Published online, 9th April, 2009.
DOI: 10.3987/COM-09-11686

■ Synthesis of Fused Pyrazolo[1,5-a]pyrimidine Derivatives under Microwave Irradiation and Ultrasound, as Ecofriendly Energy Sources

Khadijah M. Al-Zaydi*

Chemistry Department, Girl’s College of Education, King Abdul-Aziz University, Jeddah 21533 P.O.Box 50918, Saudi Arabia

Abstract

4-(4-Chlorophenylazo)-1H-pyrazole-3,5-diamine reacted with ethyl acetoacetate, benzylidenemalononitrile, ethyl propiolate and malononitrile under microwave irradiation to afford pyrazolo[1,5-a]pyrimidine derivatives in high yields. The structures of compounds were confirmed by 1H, 13C NMR, MS, elemental analyses and X-ray crystallography.

INTRODUCTION
Pyrazole compounds can provide privileged scaffolds for the generation of target compounds for drug discovery.1-6 Hence, the synthesis and study of pyrazolo-fused compounds have been of interest due to their wide variety of biological and pharmacological properties.1,4-6
The structural diversity and biological importance of pyrimidines have made them attractive targets for the synthesis over many years.7,8 Pyrazolopyrimidines and related fused heterocycles are of interest as potential bioactive molecules. They are known to exhibit pharmacological activities such as CNS depressant,9,10 neuroleptic,11 and tuberculostatic.12 Furthermore, pyrazolo[1,5-a]pyrimidines have recently been prepared as good inhibitors of KDR kinase.13
At the dawn of this century, green chemistry has become a major driving force for organic chemists to develop environmentally benign routs to a myriad of materials.14-16 Generally, application of microwaves and ultrasound in synthesis leads to energy conservation in addition to substantial reductions in reaction times, yield increases, solvent and waste minimization, formation of cleaner products and sometimes, to chemo-, regio- and stereoselectivity changes.17-19
It is well known that fused pyrazoles are synthesized, in general, by condensation of aminopyrazoles with 1,3-bifunctional reagents.20 Although several synthetic approaches for the preparation of fused pyrazoles, especially pyrazolo[1,5-a]pyrimidines, in this way have been described in the literature,21 to our knowledge, their synthesis under microwave and ultrasound irradiation has not been found. Considering the above and continuing our investigations on the application of microwave and ultrasound in organic synthesis,22-27 We wish to report in this paper an efficient and practical procedure for the synthesis of polyfunctionally substituted pyrazolo[1,5-a]pyrimidines via reaction of 4-(4-chlorophenylazo)-1H-pyrazole-3,5-diamine (1) with ethyl acetoacetate, benzylidenemalononitrile, ethyl propiolate and malononitrile under microwave and ultrasound irradiation. The regioorientation of reagents was determined by X-ray crystallography of the products.

RESULTS AND DISCUSSION
In this study, We first investigated the condensation of 4-(4-chlorophenylazo)-1H-pyrazole-3,5-diamine (1) with ethyl acetoacetate, as active methylene bifunctional reagent. Thus, compound 1 was prepared as described in literature.28 In fact, we found that the reaction of amino-pyrazoles 1 with ethyl acetoacetate (2) under 440 W microwave radiation smoothly produced a solid product of molecular formula C13H11ClN6O. Thus, this product could be formulated as pyrazolo[1,5-a]pyrimidin-5(4H)-one 3 or isomeric pyrazolo[1,5-a]-pyrimidin-7(4H)-one 4 (Scheme 1). The difficulties encountered to differentiate between the two isomeric structures 3 and 4 were unambiguously solved by X-ray crystallography29 which established the structure as 4,7-dihydro-pyrazolo[1,5-a]pyrimidin-7-one 4 (Figure 1). In order to verify the effect of microwave irradiation, in the absence of microwave, we have performed the reaction of aminopyrazoles 1 with 2 by refluxing in acetic acid for 4h till no starting material (aminopyrazoles 1) present as examined by TLC.

In order to construct new derivatives of the interesting heterocyclic systems of type 4, we next have examined the reaction of 1 with benzylidenemalononitrile (5) under microwave irradiation. Thus, reacting 1 with 5 under microwave irradiation (440 W, 5 min), gave 1:1 adduct whose structure was assumed to be 7 or 9 (Scheme 2). The structure of pyrazolo[1,5-a]pyrimidine derivative 7 was preferred over the possible isomer 9 based on its elemental analysis, spectral data and X-ray crystal structure29 (Figure 2). Formation of 7 is assumed to proceed via the addition of the exo-amino group to the α,β-unsaturated carbon in 5. The Michael adduct 6 cyclized spontaneously to yield the dihydropyrazolopyrimidine which aromatized, under these reaction conditions, to give final product 7 (Scheme 2).

More recently, it has been reported that the pyrazolo[1,5-a]pyrimidine derivative of type 12 was obtained upon long reflux of 4-phenylazo-1H-pyrazole-3,5-diamine with ethyl propiolate in pyridine.20 Encouraged by this and due to our interest to synthesize new derivatives of pyrazolo[1,5-a]pyrimidines, We next investigated the reaction of 1 with ethyl propiolate (10) under microwave irradiation. Similary, when a mixture of 1 and 10 was irradiated in a microwave oven in open vessel with a power of 440 W for 6 min, did not afford the required pyrazolopyrimidines 12 directly, but rather the aminopyrazoles 11 is obtained. The later compound 11 underwent intramolecular condensation, under the same reaction conditions as used for the synthesis of 11, via elimination of one molecule of ethanol, to afford the desired pyrazolo[1,5-a]pyrimidines 12 (Scheme 3).

Also, the reaction of 4-(4-chlorophenylazo)-1H-pyrazole-3,5-diamine (1) with malononitrile (13) was investigated under microwave irradiation it afforded products that were formulated as the pyrazolo[1,5-a]pyrimidine derivatives 14 (Scheme 4). The structure of compound 14 was established on the basis of their elemental analysis and spectral data. Thus, the structure 14 is supported by its mass spectrum which shows a molecular ion peak at m/z 302. The 1H NMR spectrum of the same compound exhibits two doublet signals at δ 7.22, 7.37 (J = 7 Hz) due to aromatic protons, a three D2O-exchangeable signals at δ 5.51, 12.10, 13.59 respectively due to NH2 protons, in addition to singlet signals at δ 6.15 due to pyrimidine proton at C-6 (Cf. experimental part).

The investigated reactions for the preparation of compounds 4, 7, 11, 12 and 14 were compared with the same reactions carried out under the classical conditions of thermal heating (heating the reagents in a solvent). The results are presented in Table 1. The reaction under microwaves or ultrasonic is substantially faster and gives higher yields. The method used can be considered as a “green chemistry” method.

CONCLUSION
We have synthesized a class of pyrazolo[1,5-a]pyrimidine derivatives under microwave, sonication and classical conditions(heating). In general, improvements in rates and yield of reactions are observed when reactions were carried out under microwave and sonication compared with classical condition. It should be noted, however, that activation occurs at different temperatures with these techniques and, therefore strict comparisons will require a balance between effectiveness and energy costs.

EXPERIMENTAL
All melting points were measured on a Gallenkamp electrothermal melting point apparatus and are uncorrected. The IR spectra were recorded on a Nicolet Magna 520FT IR spectrophotometer (KBr pellets). 1H and 13C NMR spectra were recorded on a Varian Gemini-400 and Bruker-DPX-400 (400 MHz for 1H and 100 MHz for 13C) spectrometers with DMSO as solvent and TMS as an internal standard. Chemical shifts are expressed in δ values (ppm). MS were measured on a Shimadzu GCMS-QP 1000 EX mass spectrometer at 70 eV. X-Ray crystallography was carried out on a Kappa CCD Enraf Nonius FR 590 diffractometer at National Research Center, Dokki, Cairo, Egypt. Microwave irradiation was carried out using the commercial microwave oven (SGO 1000 w) with a thermocouple to monitor the temperature inside the vessel. It was found that ≈ 105-110 °C. Ultrasound, microprcess controlled-2004, high intensity ultrasonic processor with temperature controller (750 W). The ultrasonic frequency of the cleaning bath used equal 25 KHz. The reaction temperature stabilized at 35-40 °C by addition or removal of water in ultrasonic bath to keep the required temperature. Elemental analyses were recorded on Perkin Elmer 2400 CHN Elemental analyzer flowchart. All reagents were of commercial quality or were purified before use and the organic solvents were of analytical grade or purified by standard procedures.
General procedure for the synthesis of pyrazolo[1,5-a]pyrimidines 4, 7 , 12 and 14.
Method (A): To a suspension of compound 1 (0.01 mol) in AcOH (30 mL), ethyl acetoacetate (2), benzylidenemalononitrile (5) or malononitrile (13) (0.01 mol) was added. The reaction mixture was heated under reflux for 4 h. After concentration and cooling to rt, a small amount of EtOH (2 mL) was added. Then, the resulting solid product was collected by filtration, dried and recrystallized from DMF to give compounds 4, 7 or 14, respectively. In the case of compound 12, a solution of compound 11 (0.01 mol) in AcOH was refluxed for 2 h and then worked up as described above to give 12.
Method (B): To a mixture of compound 1 (0.01 mol), ethyl acetoacetate (2), benzylidenemalononitrile (5) or malononitrile (13) (0.01 mol) in a pyrex conical flask, a few drops of AcOH was added. Then, it was irradiated at 440 W in the microwave oven for the appropriate time in 1 min intervals (Table 1) until completion of the reaction (monitored by TLC). The reaction mixture left to cool to rt. After addition of EtOH (10 mL), the resulting solid product so formed was collected by filtration and recrystallized from EtOH to give compounds 4, 7 or 14, respectively. In the case of compound 12, compound 11 (0.01 mol) was irradiated for 2 min, under the same reaction conditions, and then worked up as described above to give 12.
Method (C): To a mixture of compound 1 (0.01 mol), ethyl acetoacetate (2), benzylidenemalononitrile (5) or malononitrile (13) (0.01 mol) in a pyrex conical flask, 30 mL of AcOH was added. Then, it was heated under ultrasound irradiation at 40 ºC for the appropriate time (Table 1) until completion of the reaction (monitored by TLC). The reaction mixture left to cool to rt. The resulting solid product so formed was collected by filtration and recrystallized from EtOH to give compounds 4, 7 or 14, respectively. In the case of compound 12, compound 11 (0.01 mol) was irradiated with ultrasonic for 30 min, under the same reaction conditions, and then worked up as described above to give 12.
Synthesis of ethyl 3-(3-amino-4-((4-chlorophenyl)diazenyl)-1H-pyrazol-5-ylamino) acrylate (11):
Method (A): To a suspension of compound 1 (0.01 mol) in dioxane (30 mL), ethyl propiolate (10) (0.01 mol) was added. The reaction mixture was refluxed for 2 h and then left to cool to rt. The solid product so formed was filtered off and recrystallized from EtOH.
Method (B): To a mixture of compound 1 (0.01 mol) and ethyl propiolate (10) (0.01 mol) in a pyrex conical flask, a few drops of dioxane was added. The reaction mixture was placed in the microwave oven and irradiated at 440 W for 6 min. Then, it was left to cool to rt The solid product so formed was filtered off and recrystallized from EtOH.
Method (C): To a mixture of compound 1 (0.01 mol), and ethyl propiolate (10) (0.01 mol) in a pyrex conical flask, 20 mL of dioxane was added. Then, it was heated under ultrasound irradiation at 35-40 ºC for the 30 min (Table 1) until completion of the reaction (monitored by TLC). The reaction mixture left to cool to rt. The resulting solid product so formed was collected by filtration and recrystallized from EtOH.
2-Amino-3-(4-chlorophenylazo)-5-methyl-pyrazolo[1,5-a]pyrimidin-7(4H)-one (4). Yellow crystals; mp 284-286 °C (lit.,30 285-287 °C); IR (KBr) υ 3445, 3350, 3230 (NH, NH2), and 1640 (CO), 1605 (C=N) cm-1; 1H NMR (DMSO-d6) δ 2.23 (s, 3H, CH3), 5.57 (br, 2H, NH2, D2O exchangeable), 7.18 (d, J= 7 Hz, 2H, Ar-H), 7.27 (d, J= 7 Hz, 2H, Ar-H), 7.67 (s, 1H, CH at C6), 12.10 (s, 1H, NH, D2O exchangeable); 13C NMR (DMSO-d6) δ 162.80, 156.77, 150.89, 143.55, 133.11, 130.19, 126.23, 123.32, 105.7, 95.44, 24.11 ppm; MS m/z (%) 302 (M+); Anal. Calcd for C13H11Cl N6O (302.72): C, 51.58; H, 3.66; N, 27.76. Found: C, 51.45; H, 3.86; N, 27.57.
2,7-Diamino-3-(4-chlorophenylazo)-5-phenylpyrazolo[1,5-a]pyrimidin-6-carbonitrile (7).
Orange crystals from dioxane; mp 200 ºC; IR (KBr) υ 3440, 3332 (NH2), 3417,3300 (NH2) and 2220 (CN), 1612 (C=N) cm-1; 1H NMR (DMSO-d6) δ 5.47 (br, 2H, NH2, D2O exchangeable), 7.11-7.36 (m, 9H, Ar-H), 8.14 (br, 1H, NH2 D2O exchangeable) ppm; 13C NMR (DMSO-d6) δ 169.88, 161.08, 158.78, 150.71, 146.50, 142.11, 138.19, 129.46, 129.23, 126.23, 123.32, 112.61,118.34, 96.55, 70.93 ppm; MS m/z (%) 388 (M+); Anal. Calcd for C19H13ClN8 (388.10); C 58.69, H 3.37, N 28.82; Found C 58.55, H 3.35, N 28.92.
(2E)-Ethyl 3-(3-amino-4-((4-chlorophenyl)diazenyl)-1H-pyrazol-5-ylamino)acrylate (11).
Orange crystals; mp 279-281 °C (lit.,30 280-282 °C); IR (KBr) υ 3430, 3329, 3315, 3200 (NH, NH2) and 1742 (CO ester) cm-1; 1H NMR (DMSO-d6) δ 1.41 (t, 3H, J = 7 Hz, CH3), 3.78 (q, 2H, J = 7 Hz, CH2), 5.52 (br s, 2H, NH2), 7.17 (d, J= 8 Hz, 2H, Ar-H), 7.33 (d, J= 8 Hz, 2H, Ar-H), 7.56 (d, J= 16 Hz, 1H, =CHNH), 7.76 (d, J= 16 Hz, 1H, CH), 9.84 (s, H, NH), 11.10 (s, 1H, pyrazole NH). 13C NMR (DMSO-d6) δ 167.05, 153.20, 150.87, 149.81, 133.11, 130.19, 126.23, 123.32, 102.40, 79.87, 53.11, 17.25; MS m/z (%) 334 (M+); Anal. Calcd for C14H15ClN6O2 (334.09): C, 50.23; H, 4.52; N, 25.10; Found: C, 50.56; H, 4.39; N, 24.90.
2-Amino-3-(4-chlorophenylazo)pyrazolo[1,5-a]pyrimidin-7(4H)-one (12).
Yellow crystals; mp 240-241 ºC; IR (KBr) υ 3430, 3332, 3170 (NH, NH2) and 1640 (CO) cm-1, 1H NMR (DMSO-d6) δ 5.59 (br s, 2H, NH2, D2O exchangeable), 6.56 (d, J= 7 Hz, 1H, H-6), 7.16 (d, J= 7 Hz, 2H, Ar-H), 7.23 (d, J= 7 Hz, 2H, Ar-H), 7.98 (d, J= 7 Hz, 1H, H-5), 9.10 (s, 1H, NH, D2O exchangeable). 13C NMR (DMSO-d6) δ 160.2, 156.8, 150.9, 143.6, 133.1, 130.2, 126.2, 123.3, 112.6, 81.7; MS m/z (%) 288 (M+); Anal. Calcd for C12H9ClN6O (288.69): C, 49.92; H, 3.14; N, 29.11. Found: C, 49.85; H, 3.21; N, 29.37.
2,5,7-Triamino-3-(4-chlorophenylazo)pyrazolo[1,5-a]pyrimidine (14).
Yellow crystals; mp 284-286 °C; IR (KBr) υ 3410, 3350, 3320 3220, 3159, 3110 (3NH2), and 1607 (C=N) cm-1; 1H NMR (DMSO-d6) δ 5.51 (br, 2H, NH2, D2O exchangeable), 6.15 (s, 1H, CH at C6), 7.22 (d, J= 7 Hz, 2H, Ar-H), 7.37 (d, J= 7 Hz, 2H, Ar-H), 12.10, 13.59 (2s, 2H, 2NH2, D2O exchangeable); 13C NMR (DMSO-d6) δ 167.54, 155.12, 150.98, 136.11, 134.23, 130.22, 126.77, 123.32, 97.52, 90.12 ppm; MS m/z (%) 302 (M+); Anal. Calcd for C12H11ClN8 (302.08): C, 47.61; H, 3.66; N, 37.02. Found: C, 47.80; H, 3.57; N, 36.92.

X-RAY CRYSTALLOGRAPHY:
The crystal structure were solved and refind using maxus (nonius,Deflt and MacScience, Japan). Mo-Kα radiation (λ = 0.71073 Å) and a graphit monochromator were used for data collection.
Compound 4 was recrystalized as yellow crystal from dry EtOH. Chemical formula C13H11ClN6O, Mr=302.725, monoclinic, crystallize in space group C2/c, a = 22.1453(8), b = 21.7359(11), c = 15.5716(8) Å, T= 298 0K, V= 5694.9 (5) Å3, Z= 8, Dx = 0.706Mg m-3, θ range 1.018-18.00o, absorption coefficient μ = 0.14 mm-1.The unique reflections measured 7605, of which 2134 independent reflections and 1367 observed reflections with threshold expression I>3σ(I), Refinement parameters are Rint=0.061, R(all)= 0.093, wR(ref)= 0.139, wR(all)=0.143, S(ref) = 4.850, S(all)= 4.470, D/smax= 0.008, Drmax= 0.38 eÅ3, Drmin = -0.33 eÅ3. The chemical formula and ring labeling system is shown in Figure 1.

Compound 7 was recrystalized as orange crystal from dry EtOH. Chemical formula C19H13ClN8, Mr=388.10, monoclinic, crystallize in space group C2/c, a = 28.7991(9), b = 8.4375(3), c = 16.0136(7) Å, T= 298 0K, V= 3868.2 (2) Å3, Z= 8, Dx = 1.401Mg m-3, θ range 2.910-21.036o, absorption coefficient μ = 0.23 mm-1.The unique reflections measured 3988, of which 2205 independent reflections and 1631 observed reflections with threshold expression I>3σ(I), Refinement parameters are Rint=0.042, R(all)= 0.059, wR(ref)= 0.080, wR(all)=0.083, S(ref) = 2.436, S(all)= 2.341, D/smax= 0.014, Drmax= 0.30 eÅ3, Drmin = -0.45 eÅ3. The chemical formula and ring labeling system is shown in Figure 2.

References

1. J. Elguero, P. Goya, N. Najerovic, and A. M. S. Silva, Targets Heterocycl. Syst., 2002, 6, 52.
2. D. Dressen, A. W. Garofalo, J. Hawkinson, D. Hom, J. Jagodzinski, J. L. Marugg, M. L. Neitzel, M. A. Pleiss, B. Szoke, J. S. Tung, D. W. G. Wone, J. Wu, and H. Zhang, J. Med. Chem., 2007, 50, 5161. CrossRef
3. D. J. Wustrow, T. Capiris, R. Rubin, J. A. Knobelsdorf, H. Akunne, M. D. Davis, R. Mackenzie, T. A. Pugsley, K. T. Zoski, T. G. Heffner, and L. D.Wise, Bioorg. Med. Chem. Lett., 1998, 8, 2067. CrossRef
4. C. R. Hardy, Adv. Heterocycl. Chem., 1984, 36, 343. CrossRef
5. M. H. Elnagdi, M. R. H. Elmoghayar, and G. E. H. Elgemeie, Adv. Heterocycl. Chem., 1987, 41, 319. CrossRef
6. M. H. Elnagdi, M. R. H. Elmoghayar, and K. U. Sadek, Adv. Heterocycl. Chem., 1990, 48, 223. CrossRef
7. U. Girreser, D. Heber, and M. Schütt, Tetrahedron, 2004, 60, 11511. CrossRef
8. J. Quiroga, J. Trilleras, B. Insuasty, R. Abonia, M. Nogueras, and J. Cobo, Tetrahedron Lett., 2008, 49, 2689. CrossRef
9. M. Julino and M. F. G. Stevens, J. Chem. Soc., Perkin Trans. 1, 1998, 1677. CrossRef
10. M. Ibrahim Abdou, A. M. Saleh, and H. F. Zohdi, Molecules, 2004, 9, 109. CrossRef
11. R. Filler, Chem. Technol., 1974, 4, 752.
12. M. M. Ghorab, Z. H. Ismail, S. M. Abdel-Gawad, and A. Abdel Aziem, Heteroatom Chem., 2004, 15, 57. CrossRef
13. M. E. Fraley, W. F. Hoffman, R. S. Rubino, R. W. Hungate, A. J. Tebben, R. Z. Rutledge, R. C. McFall, W. R. Huckle, R. L. Kendall, K. E. Coll, and K. A. Thomas, Bioorg. Med. Chem. Lett., 2002, 12, 2767. CrossRef
14. P. Anastas and T. Williamson, Green Chemistry, Frontiers in Benign Chemical Synthesis and Procedure; Oxford Science Publications (1998).
15. D. C. Dittmer, Chem. Ind., 1997, 779.
16. J. Quiroga, J. Portilla, H. Serrano, R. Abonia, B. Insuasty, M. Nogueras, and J. Cobo, Tetrahedron Lett., 2007, 48, 1987. CrossRef
17. M. Neuschl, D. Bogdal, and M. Potacek, Molecules, 2007, 12, 49. CrossRef
18. C. O. Kappe, Angew. Chem. Int. Ed., 2004, 43, 6250. CrossRef
19. Y. Xu and Q.-X. Guo, Heterocycles, 2004, 63, 903. CrossRef
20. A. Elkholy, F. Al-Qalaf, and M. H. Elnagdi, Arkivoc, 2008, xiv, 124.
21. T. J. Mason, “Practical Sonochemistry”, Ellis Horwood Limited, New York (1991).
22. K. M. Al-Zaydi and M. H. Elnagdi, J. Korean Chem. Soc., 2003, 47, 591. CrossRef
23. K. M. Al-Zaydi, A. Al-Shamary, and M. H. Elnagdi, J. Chem. Res. (S), 2006, 508.
24. K. M. Al-Zaydi and R. M. Borik, Molecules, 2007, 12, 2061. CrossRef
25. K. M. Al-Zaydi, R. M. Borik, and M. H. Elnagdi, J. Heterocycl. Chem., 2007, 44, 1187. CrossRef
26. K. M. Al-Zaydi, R. M. Borik, and M. H. Elnagdi, Ultrasonics Sonochemistry, 2008, in press.
27. K. M. Al-Zaydi, Ultrasonics Sonochemistry, 2009, in press.
28. M. H. Elnagdi and S. O.Abdoula, J. Prakt. Chem., 1973, 315, 1009. CrossRef
29. Crystallographic data for the structures of compounds 4 and 9 reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publications Nos. CCDC 698612, 698614. These data can be obtained free of charge via www.ccdc.com.ac.uk/data_request/cif.
30. H. A. Elfahham, G. E. H. Elgemeie, Y. R. Ibraheim, and M. H. Elnagdi, Liebigs Ann. Chem., 1988, 819. CrossRef