One-pot, telescoped synthesis of N-aryl-5-aminopyrazoles from anilines in environmentally benign conditions

Abstract

An efficient synthetic approach to synthesize N-aryl-5-aminopyrazoles from anilines via a one-pot, telescoped reaction performed in entirely aqueous conditions has been developed. This protocol provides a rapid, convenient method to prepare N-aryl-5-aminopyrazoles, useful building blocks for the synthesis of several bicyclic nitrogen heterocycles, by avoiding the isolation of the toxic intermediate arylhydrazines and the use of a metallic reductant.

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Introduction

Among the structurally simple heterocycles, 5-aminopyrazole certainly occupies a relevant position because of its biological and medicinal properties as well as its synthetic versatility. Two books^1,2 edited in 1964 and 1967 and two recent reviews^3,4 bear witness to the interest in this privileged structure. 5-Aminopyrazoles are present in many bioactive compounds targeting neuropeptide Y receptor 5 (NPY5),⁵ corticotrophin-releasing factor-1 (CRF-1) receptor,⁶ kinases such as Aurora-, polo-like (PLK) and cyclin-dependent (CDK)-kinases,^7,8 adenosine A₁ receptor,⁹ and alpha-7 nicotinic acetylcholine receptors (α7nAChR).¹⁰ Furthermore, 5-aminopyrazoles represent useful building blocks for the preparation of several fused nitrogen heterocycles (Fig. 1).^11-17

Fig. 1 5-Aminopyrazole as precursor for synthesis of fused pyrazoles.

As part of a project aimed at finding novel non-steroidal Farnesoid X Receptor (FXR) ligands we recently reported the discovery of pyrazolo[3,4-e][1,4]thiazepin-7-one derivatives as a novel class of FXR agonists.¹⁸ During our initial hit-to-lead optimization work that led us to the identification of the derivatives 1 and 2 (Fig. 2) exhibiting low micromolar range of potency (10-fold higher than the initial hit) and full efficacy, we were in need to prepare in an efficient way a variety of 1-aryl-substituted 3-methyl-5-aminopyrazoles 3 to utilize together with an aryl aldehyde 4 and 2-mercaptopropanoic acid (5) in the condensation reaction affording pyrazolo[3,4-e][1,4]thiazepin-7-one core (Scheme 1).

Fig. 2 Pyrazolo[3,4-e][1,4]thiazepin-7-one-based FXR agonists.

Scheme 1 Multicomponent synthesis of pyrazolo[3,4-e][1,4]thiazepin-7-ones.

The preparation of 3 appeared to be the limiting step towards a quick development of a structure–activity relationship scheme. Indeed, N-aryl-3-methyl-5-aminopyrazoles 3 were commonly prepared in three steps, that are the condensation of 3-aminobut-2-enenitrile (8) with the appropriate arylhydrazine 7,¹⁹ in turn obtained from the corresponding aniline 6, via diazonium salt and reduction operated by stannous chloride (Scheme 2).²⁰

Scheme 2 Three-step procedure for the preparation of N-aryl-3-methyl-5-aminopyrazoles 3 from anilines 6.

This three-step procedure suffers from the burden of some drawbacks. First, the reduction step required an excess of reducing agent and as a consequence tin wastes were produced in significant quantity working in a gram scale; second, the work-up of the arylhydrazines 7 was a tedious and cumbersome process, requiring the treatment with excess alkali and the extraction by organic solvent of the free bases, which were in most cases unstable and had to be used immediately for the next step. Furthermore, by considering that both the reduction- and condensation steps required acidic conditions, the isolation of 7 appeared redundant.

By considering that telescoping two or more steps into a one-pot reaction is a powerful tool to enhance synthetic economy and efficiency,²¹ we report herein a practical one-pot protocol allowing us to obtain N-aryl-5-aminopyrazoles 3 from anilines 6 without the isolation of the arylhydrazines 7, by using a natural organic reducing agent, such as L-ascorbic acid, in entirely aqueous conditions.

Results and discussion

Despite being known since 1939 that L-ascorbic acid could react with diazonium salts to give arylhydrazines,²² only recently this reduction has found two synthetic applications.²³ A discussion about the mechanism of this reaction has been also reported.²⁴ L-Ascorbic acid, unlike than commonly used SnCl₂, is an environmentally benign reagent and as the latter works in aqueous acidic conditions, which are also the proper ones for the condensation step of the arylhydrazine 7 with 3-aminobut-2-enenitrile (8).

These considerations moved us to design an environmentally benign, one-pot process to manufacture in an efficient way N-aryl-5 aminopyrazoles 3.

At first, 2-chloroaniline (6a) was used to set the reaction conditions. According to Norris' paper,^23a diazonium salt was formed by treatment of 6a with 1.2 equiv. of sodium nitrite in concentrated hydrochloric acid at ∼0 °C. Subsequent reaction with L-ascorbic acid (1 equiv.) was initiated at ∼0 °C and ended at room temperature ∼25 °C. This resulted in the formation of the oxalic acid intermediate 9a, which was isolated as crude wet solid and directly treated with 8 in aqueous hydrochloric acid at 80 °C. After 5 hours, the reaction mixture was worked-up and 3-methyl-1-(2-chlorophenyl)-1H-pyrazol-5-amine (3a) was isolated in 55% (from 6a) by chromatography (Scheme 3). Encouraged by this result we tested the possibility to avoid the isolation of the oxalic acid intermediate 9a.

Scheme 3 Synthesis of 1-(2-chlorophenyl)-3-methyl-1H-pyrazol-5-amine (3a) from 2-chloroaniline (6a) by three-step procedure involving L-ascorbic acid as reducing agent.

Thus, after the formation of the diazonium salt and the subsequent addition of L-ascorbic acid (∼0 °C), the cooling-bath was removed and the heating of the reaction mixture started. When the internal temperature reached 45–50 °C, 8 was added to the reaction mixture and the temperature increased up to 90 °C. After 5 hours the reaction was worked-up and the residue purified by chromatography obtaining 3a in 70% yield. Longer reaction time did not improve the reaction yield. This result was really satisfactory if compared with the tin-mediated three-step procedure we followed previously, that gave a 40% overall yield.^18a Furthermore, this outcome was obtained in metal-free conditions, minimizing human exposure to the toxic hydrazine²⁵ and saving time and solvent for the work-up of this intermediate.

The scope of the protocol was then investigated starting from a set of variously functionalized anilines 6b–t (Table 1). Although most of the tested anilines successfully produced the corresponding desired N-aryl-5-aminopyrazoles 3 in moderate to good isolated yields, in few cases we obtained a complex mixture, which did not contain any of the desired product. According to the data shown in Table 1 we may assume that the position of the substituent was not influential for the outcome of the reaction: indeed, 3-chloro- (6b) and 4-chloro-aniline (6c) gave the corresponding 5-aminopyrazoles 3b and 3c with yield comparable to that obtained for 3a (entries 1–3). A similar trend was observed in the series of aminobenzoic acids (6g–i, entries 7–9) and in the case of 2- and 4-nitroaniline, (6e) and (6f), respectively (entries 5 and 6). On the contrary, the results suggested that the electronic density of the corresponding aniline might have some influence on this one-pot reaction. Anilines having an electron-withdrawing group were preferred substrates to those with an electron-donating group (Table 2, entries 1–20). Thus, when the reaction was carried out on 2-methoxyaniline (6q) (entry 17) the yield of the desired product significantly decreased. Anilines possessing methyl-, ethyl- or hydroxy-group even failed to react. (Entries 12, 13 and 16), as well as (4-aminophenyl)acetic acid (6o), whereas aniline without any substituent (entry 11) gave moderate yield. Starting from 2-aminobiphenyl (6n) only traces of the desired 5-aminopyrazole was obtained (entry 14). The negative influence of the electron-releasing groups on the reaction's outcome was nullified by the concomitant presence of an electron-withdrawing group on the aniline (entries 19 and 20). For instance, 45% and 50% yield of the desired product was isolated upon reacting 2-methyl-6-nitroaniline (6s) and 2-nitroaniline (6e), respectively. This example also suggested that steric hindrance did not influence the reaction outcome.

Table 1 Scope of the anilines^a

Entry	Aniline	R	Yield^b (%)	Yield^b (%)
a Reactions were performed on 8 mmol aniline with 1.2 equiv. sodium nitrite, 1.0 equiv. ascorbic acid and 1.0 equiv. 3-aminobut-2-enenitrile (8).b Isolated yield.c Mixture of unidentified products.d SnCl2 in telescoped conditions was used.
1	6a	2-Cl	70
2	6b	3-Cl	65
3	6c	4-Cl	68^c
4	6d	2-Br	67
5	6e	2-NO2	50
6	6f	4-NO2	48
7	6g	2-COOH	65
8	6h	3-COOH	45
9	6i	4-COOH	65
10	6j	2-SO2NH2	61
11	6k	H	36
12	6l	2-CH3	—^c	68^d
13	6m	2-C2H5	—^c
14	6n	2-C6H5	Traces
15	6o	4-CH2COOH	—	65^d
16	6p	2-OH	^c
17	6q	2-OCH3	20
18	6r	3-SO2CH3	25
19	6s	2-CH3, 6-NO2	45
20	6t	2-OCH3, 4-NO2	42

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Table 2 Scope of the nitriles^a

Entry	Nitrile	Product	Yield^b (%)
a Reactions were performed on 8 mmol aniline with 1.2 equiv. sodium nitrite, 1.0 equiv. ascorbic acid and 1.0 equiv. nitrile.b Isolated yield.
1		12	11%
2		13	19%

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The reaction's failure observed in the case of the anilines 6l–p can be ascribed to the step involving the interaction between diazonium salt and ascorbic acid. Indeed, both o-toluidine (6l) and (4-aminophenyl)acetic acid (6o) succeeded in the formation of 3l^18a and 3o, respectively, using SnCl₂ as reducing agent in telescoped conditions (entries 12 and 15). In other words, it is possible to make use of the advantages of the one-pot telescoped procedure independently from the substituent on the aniline.

In an attempt to explain the reactivity differences of the different diazonium salts towards ascorbic acid, a computational study was carried out by the aid of quantum mechanical calculations on the intermediate diazonium salts. All the molecules were designed using the Maestro 9.4 graphical interface and then submitted to the optimization protocol of Jaguar v 8.0 present in the Schrodinger Suite 2013.^26,27 Inspired by a previous theoretical paper on benzenediazonium salt,²⁸ the molecular optimization run was performed at the local Møller-Plesset second-order perturbation level (LMP2)²⁹ together with the augmented cc-pVDZ (augcc-pVDZ) basis set.³⁰ The starting idea was to carefully check the differences in the partial charges of the nitrogens of the diazonium moiety, expecting a correlation between working (6a–k, 6q–t) and not working anilines (6l–p). Unexpectedly, quantum mechanics calculation showed differences in terms of the orbital symmetry and frontier molecular orbital (FMO) theory³¹ rather than in the partial charges. Indeed, the LUMO energy of the reacting diazonium salts displayed an average value of −95.11 ± 5.89 kcal mol⁻¹ with respect to that of the not reacting ones endowed with values of −86.57 ± 0.66 kcal mol⁻¹. The lower the LUMO value of the diazonium salt is, the higher the reactivity of the species should be, as it was already pointed out by a recent theoretical study on the reaction of carbon nanotube with aryl diazonium salts.³² However it should be noted that although our results are in line with that observation, we did not find a strict correlation between reaction yield and LUMO energy. Most probably other factors should be considered and further studies are needed to carefully understand the experimental results.

To further examine the scope and limitation of the telescoped procedure, we studied the reaction of 2-chloroaniline (6a) with the commercial available nitriles 10 and 11. As shown in Table 2, the procedure succeeded in the formation of the corresponding 1-(2-chlorophenyl)-1H-pyrazol-5-amines 12 and 13, although lower yields than in the case of the reaction with 8 were obtained.

Conclusions

In summary, we have developed an efficient and rapid protocol for the preparation of N-aryl-5-aminopyrazoles, privileged structures and useful building blocks in medicinal chemistry. The three-step synthesis was realized in one-pot aqueous conditions, using a natural organic reducing agent, such as L-ascorbic acid. The simple telescoped procedure, in particular, minimizes human exposure to the toxic hydrazines and allows saving time, solvent and energy. Given the high yield, “greenness”, and possibility of scaling-up, the reaction has considerable potential for adoption by the pharma and fine chemical industries. The detailed mechanistic study and further optimization of the reaction conditions are currently underway and the results will be reported in due time.

Experimental section

One-pot telescoped procedure using L-ascorbic acid (for 3a–k, 3q–t, 12 and 13)

A solution of sodium nitrite (0.66 g, 9.6 mmol) in water (1.3 mL) was added dropwise to a mechanically stirred suspension of the aniline (8.0 mmol) in 12 M hydrochloric acid (4.5 mL) kept in the range −2–0 °C. After stirring for 1 h at the same temperature, a solution of ascorbic acid (1.41 g, 8.0 mmol) in water (6.8 mL) was added slowly at −2–0 °C. The ice-bath was removed and the heating of the reaction mixture was started. When the internal temperature was 45–50 °C, 8.0 mmol of the appropriate nitrile was added (3-aminobut-2-enenitrile (8) for 3a–k and 3q–t, ethyl 2-cyano-3-ethoxyacrylate (10) for 12, and 3-oxo-3-phenylpropanenitrile (11) for 13) and the heating continued up to 90 °C. After 6–7 h, the reaction was worked-up and the residue purified by chromatography. N-Aryl-5 aminopyrazoles 3h and 3i were directly isolated from the reaction mixture by filtration.

One-pot telescoped procedure using SnCl₂ (for 3l and 3o)

A solution of sodium nitrite (1 equiv.) in water (0.5 mL mmoL⁻¹) was added dropwise to a stirred solution of o-toluidine (1 equiv.) in 12 M hydrochloric acid (0.5 mL mmoL⁻¹) kept in the range of −2–0 °C. After stirring for 1 h at the same temperature, a solution of stannous chloride (3 equiv.) in HCl (0.3 mL mmoL⁻¹) was added slowly at −2–0 °C. After stirring for 1 h at the same temperature, the ice bath was removed and the heating of the reaction was started. When the internal temperature was 45–50 °C, 3-aminobut-2-enenitrile (8, 1 equiv.) was added and the heating continued up to 90 °C. After 6–7 h, the reaction was worked-up and the desired compound purified by chromatography (3l) or directly isolated from the reaction mixture by filtration (6o). The aqueous phase was made alkaline up to pH 9.00 with aqueous sodium hydroxide solution and the product extracted with EtOAc.

References

1. R. H. Wiley and P. Wiley, in Pyrazolones, Pyrazolidones and Derivatives, John Wiley and Sons, New York, 1964.
2. L. C. Behr, R. Fusco and C. H. Jarboe, in The Chemistry of Heterocyclic Compounds, Pyrazoles, Pyrazolines, Pyrazolidines, Indazoles and Condensed Rings, ed. A. Weissberger, Interscience Publishers, New York, 1967.
3. R. Aggarwal, V. Kumar, R. Kumar and S. P. Singh, Beilstein J. Org. Chem., 2011, 7, 179–197.
4. T. M. A. Elmaati and F. M. El-Taweel, J. Heterocycl. Chem., 2004, 41, 109–134.
5. C. P. Kordik, C. Luo, B. C. Zanoni, S. L. Dax, J. J. McNally, T. W. Lovenberg, S. J. Wilson and A. B. Reitz, Bioorg. Med. Chem. Lett., 2011, 11, 2283–2286.
6. A. Nakazato and S. Okuyama, Drugs Future, 1999, 24, 1089–1098.
7. L. T. Pierce, M. M. Cahill and F. O. McCarthy, Tetrahedron, 2011, 67, 4601–4611.
8. T. A. Carter, L. M. Wodicka, N. P. Shah, A. M. Velasco, M. A. Fabian, D. K. Treiber, Z. V. Milanov, C. E. Atteridge, W. H. Biggs, P. T. Edeen, M. Floyd, J. M. Ford, R. M. Grotzfeld, S. Herrgard, D. E. Insko, S. A. Mehta, H. K. Patel, W. Pao, C. L. Sawyers, H. Varmus, P. P. Zarrinkar and D. J. Lockhart, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 11011–11016.
9. N. Griebenow, L. Bärfacker, H. Meier, D. Schneider, N. Teusch, K. Lustig, R. Kast and P. Kolkhof, Bioorg. Med. Chem. Lett., 2010, 20, 5891–5894.
10. R. Zanaletti, L. Bettinetti, C. Castaldo, G. Cocconcelli, T. Comery, J. Dunlop, G. Gaviraghi, C. Ghiron, S. N. Haydar, F. Jow, L. Maccari, I. Micco, A. Nencini, C. Scali, E. Turlizzi and M. Valacchi, J. Med. Chem., 2012, 55, 4806–4823.
11. V. A. Chebanov, V. E. Saraev, S. M. Desenko, V. N. Chernenko, I. V. Knyazeva, U. Groth, T. N. Glasnov and C. O. Kappe, J. Org. Chem., 2008, 73, 5110–5118.
12. S. V. Ryabukhin, D. S. Granat, A. S. Plaskon, A. N. Shivanyuk, A. A. Tolmachev and Y. M. Volovenko, ACS Comb. Sci., 2012, 14, 465–470.
13. K. Liubchak, A. Tolmachev and K. Nazarenko, J. Org. Chem., 2012, 77, 3365–3372.
14. S. L. Bogza, K. I. Kobrakov, A. A. Malienko, I. F. Perepichka, S. Y. Sujkov, M. R. Bryce, S. B. Lyubchik, A. S. Batsanov and N. M. Bogdan, Org. Biomol. Chem., 2005, 3, 932–940.
15. (a) H. Hu, L. Song, Q. Fang, J. Zheng, Z. Meng and Y. Luo, Molecules, 2011, 16, 1878–1887; (b) J. D. Ratajczyk and L. R. Swett, J. Heterocycl. Chem., 1975, 12, 517–522; (c) S. Lee and S. B. Park, Org. Lett., 2009, 11, 5214–5217.
16. (a) A. Colombo, J. Frigola, J. Parés and B. Andaluz, J. Heterocycl. Chem., 1989, 26, 949–955; (b) H. A. DeWald, S. Lobbestael and B. P. H. Poschel, J. Med. Chem., 1981, 24, 982–987.
17. L. R. Swett, J. D. Ratajczyk, C. W. Nordeen and G. H. Aynilian, J. Heterocycl. Chem., 1975, 12, 1137–1142.
18. (a) M. Marinozzi, A. Carotti, E. Sansone, A. Macchiarulo, E. Rosatelli, R. Sardella, B. Natalini, G. Rizzo, L. Adorini, D. Passeri, F. De Franco, M. Pruzanski and R. Pellicciari, Bioorg. Med. Chem., 2012, 20, 3429–3445; (b) M. Marinozzi, A. Carotti, R. Sardella, F. Buonerba, F. Ianni, B. Natalini, D. Passeri, G. Rizzo and R. Pellicciari, Bioorg. Med. Chem., 2013, 21, 3780–3789.
19. N. L. Nam, I. I. Grandberg and V. I. Sorokin, Chem. Heterocycl. Compd., 2000, 36, 281–283.
20. M. Hunsberger, E. R. Shaw, J. Fugger, R. Ketcham and D. Lednicer, J. Org. Chem., 1956, 21, 394–399.
21. W. Zhao and F.-E. Chen, Curr. Org. Synth., 2012, 9, 873–897.
22. R. Weidenhagen, H. Wegner, K. H. Lung and L. Nordstrom, Chem. Ber., 1939, 72B, 2010–2020.
23. (a) T. Norris, C. Bezze, S. Zambelli Franz and M. Stivanello, Org. Process Res. Dev., 2009, 13, 354–357; (b) C. P. Ashcroft, P. Hellier, A. Pettman and S. Watkinson, Org. Process Res. Dev., 2011, 15, 98–103.
24. D. L. Browne, I. R. Baxendale and S. V. Ley, Tetrahedron, 2011, 67, 10296–10303.
25. J. H. Powell and P. M. Gannett, J. Environ. Pathol., Toxicol. Oncol., 2002, 21, 1–31.
26. A. D. Bochevarov, E. Harder, T. F. Hughes, J. R. Greenwood, D. A. Braden, D. M. Philipp, D. Rinaldo, M. D. Halls, J. Zhang and R. A. Friesner, Int. J. Quantum Chem., 2013, 113, 2110–2142.
27. Schrödinger, LLC, New York, NY, 2013.
28. Z. Wu and R. Glaser, J. Am. Chem. Soc., 2004, 126, 10632–10639.
29. (a) C. Møller and M. S. Plesset, Phys. Rev., 1934, 46, 618–622; (b) P. Pulay and S. Saebø, Theor. Chim. Acta, 1986, 69, 357–368; (c) S. Saebø and P. Pulay, Annu. Rev. Phys. Chem., 1993, 44, 213–236; (d) S. Saebø, W. Tong and P. Pulay, J. Chem. Phys., 1993, 98, 2170–2175.
30. (a) T. H. Dunning Jr, J. Chem. Phys., 1989, 90, 1007–1023; (b) R. A. Kendall, T. H. Dunning Jr and R. J. Harrison, J. Chem. Phys., 1992, 96, 6796–6806; (c) D. E. Woon and T. H. Dunning Jr, J. Chem. Phys., 1993, 98, 1358–1371; (d) D. E. Woon and T. H. Dunning Jr, J. Chem. Phys., 1994, 100, 2975–2988.
31. H. Wang, Y. Wang, K. L. Han and X. J. Peng, J. Org. Chem., 2005, 70, 4910–4917.
32. H. Wang and J. Xu, Chem. Phys. Lett., 2009, 477, 176–178.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and copies of ¹H- and ¹³C NMR spectra for all compounds. See DOI: 10.1039/c3ra47541g

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