2,4-Diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile

Abstract

5H-Chromeno[2,3-b]pyridines are important compounds with industrial, biological, and medicinal properties. In this short note, the multicomponent reaction of salicylaldehyde, malononitrile dimer, and 2-phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one in dimethyl sulfoxide at ambient temperature was investigated to give 2,4-diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile with good yield. The structure of the previously unknown chromeno[2,3-b]pyridine derivative was confirmed by elemental analysis, mass, nuclear magnetic resonance, and infrared spectroscopy data. The ADME (absorption, distribution, metabolism, and excretion) properties were also assessed.

1. Introduction

The assessment of absorption, distribution, metabolism, and excretion (ADME) properties is a necessary and responsible approach to drug discovery and design that aids in optimizing the safety of the hit compound [1]. Early estimation of ADME in the discovery phase drastically reduces the fraction of pharmacokinetics-related failures in clinical phases [2]. The high-throughput and low-cost nature of ADME prediction models allow for a more streamlined drug development process [3].

Chromeno[2,3-b]pyridine fragment is known as a privileged medicinal scaffold [4]. Its derivatives are important compounds with industrial, biological, and medicinal properties [5]. Chromeno[2,3-b]pyridines have a wide spectrum of pharmacological activity, and this moiety was found in two commercial anti-inflammatory drugs: Amlexanox [6] and Pranoprofen [7] (Figure 1).

The trifluoromethyl pyrazole fragment is a privileged medicinal scaffold as well. Many examples of bioactive fluorinated pyrazoles have emerged in recent years, including the non-steroidal anti-inflammatory drug celecoxib (Celebrex) [8] and the herbicide fluazolate [9] (Figure 2).

The incorporation of both of these fragments may lead to new properties. Thus, the synthesis of a compound containing both of these fragments is of prominent interest.

2. Results and Discussion

2.1. Multicomponent Synthesis of 2,4-Diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile 4

We previously carried out a multicomponent transformation of salicylaldehydes, 2-aminoprop-1-ene-1,1,3-tricarbonitrile (malononitrile dimer), and 5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one into 2,4-diamino-5-(5-hydroxy-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitriles by two different methods [10] (Scheme 1). However, the use of these reaction systems did not allow for the introduction of the N-phenyl-substituted C-H acid derivative into the reaction.

We developed the multicomponent synthesis of chromeno[2,3-b]pyridines in dimethyl sulfoxide (DMSO) [11,12,13]. This method produced chromeno[2,3-b]pyridines, which were previously unknown and unavailable via other methods (methods A and B [10]). Salicylaldehydes, malononitrile dimer, and malonic acid or dimethyl malonate were then transformed into 2-(2,4-diamino-3-cyano-5H-chromeno[2,3-b]-pyridin-5-yl)malonic acids or dimethyl 2-(2,4-diamino-3-cyano-5H-chromeno[2,3-b]-pyridin-5-yl)malonate [11,12]. Then, similarly, 2,4-Diamino-5-(nitromethyl)5H-chromeno[2,3-b]pyridine3-carbonitrile was also synthesized [13].

Now, we wish to report our results in the facile multicomponent transformation of salicylaldehyde (1), 2-aminoprop-1-ene-1,1,3-tricarbonitrile (2), and 2-phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one (3) into the previously unknown 2,4-diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile (4) in DMSO at ambient temperature (23 °C, 24 h), as shown in Scheme 2.

After the reaction was completed, water was poured into the DMSO solution, and the pure chromeno[2,3-b]pyridine (4) was precipitated. The yield of compound 4 was 84%.

The BFI (bond-forming index) [14] of the process is as high as four, as four new non-hydrogen bonds were formed in one synthetic transformation, namely two C-C bonds, one C-N bond, and one C-O bond.

The structure of the new chromeno[2,3-b]pyridine (4) was confirmed by spectral methods such as ¹H and ¹³C NMR, IR spectroscopy and mass spectrometry data, and elemental analysis data (see Supplementary Materials). The NMR spectrum corresponds to similar known structures [10]. There is also a 2D NMR assignment for chromeno[2,3-b]pyridine aromatics [15].

Taking into consideration our previous results on the determination of intermediates [16] and ¹H NMR monitoring data of similar multicomponent processes [11,17], the following mechanism for the transformation of salicylaldehyde (1), malononitrile dimer (2), and 2-phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one (3) was proposed, as shown in Scheme 3.

The first stage of the multicomponent transformation was a Knoevenagel condensation with the formation of unsaturated adduct 5 and the release of a hydroxide anion [18]. This hydroxide anion catalyzed a Pinner cyclization of adduct 5 into intermediate 6. The Michael addition of 2-phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one (3) then led to the formation of anion A. In the last stage, tautomerization, Pinner-type cyclization, and another tautomerization with protonation led to the final 2,4-diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile (4).

2.2. ADME Prediction

The ADME of the synthesized chromeno[2,3-b]pyridine (4) was predicted using an online resource [19,20].

The bioavailability radar of chromeno[2,3-b]pyridine (4) is shown in Figure 3. Bioavailability radar allows for rapid assessment of drug similarity parameters [21]. Six physicochemical properties are taken into account: lipophilicity, size, polarity, solubility, flexibility, and saturation [22]. The pink area is the optimal range for each property. The synthesized compound (4) has a low boundary value of polarity but within the normal range of flexibility, as well as low saturation (Figure 3). Thus, compound 4 corresponds to Lipinski’s rule (Table 1) but has limited oral bioavailability.

The BOILED-Egg is a graphical display of two important parameters, i.e., passive gastrointestinal absorption (HIA) and brain access (BBB) [23]. The yolk is the physicochemical space for highly probable BBB permeation, and the white is the physicochemical space for highly probable HIA absorption. The two spaces are not mutually exclusive; the outside gray region stands for low absorption and limited brain penetration of the molecule. The point is supposed to be blue if the molecule is actively effluxed by P-glycoprotein (PGP+) and red (in this case) if it is a non-substrate of P-glycoprotein (PGP−) [24]. Figure 4 shows that the synthesized compound (4) is predicted not to be absorbed and not brain penetrant (outside the Egg), as well as not subject to active efflux (red dot).

Some of the calculated ADME parameters are presented in Table 1. It also follows from the calculations that compound 4 complies with the Lipinski [25], Ghose [26], Veber [27], and Muegge [28] rules.

Table 1. Calculated ADME parameters of synthesized chromeno[2,3-b]pyridine (4).

Parameter	Value
Fraction Csp3	0.09
Num. rotatable bonds	3
Topological polar surface area	136.00 Å2
Consensus Log Po/w (Lipophilicity)	3.55
Log S (ESOL) [29]	−5.72
Water solubility	8.88 × 10−4 mg/mL; 1.91 × 10−6 mol/L
Class	Moderately soluble
Gastrointestinal absorption	Low
BBB permeant	No
P-gp substrate	No
Log Kp (skin permeation)	−6.09 cm/s
Bioavailability score	0.55

Table 1. Calculated ADME parameters of synthesized chromeno[2,3-b]pyridine (4).

Parameter	Value
Fraction Csp3	0.09
Num. rotatable bonds	3
Topological polar surface area	136.00 Å2
Consensus Log Po/w (Lipophilicity)	3.55
Log S (ESOL) [29]	−5.72
Water solubility	8.88 × 10−4 mg/mL; 1.91 × 10−6 mol/L
Class	Moderately soluble
Gastrointestinal absorption	Low
BBB permeant	No
P-gp substrate	No
Log Kp (skin permeation)	−6.09 cm/s
Bioavailability score	0.55

Based on the above data, it can be concluded that pyridine may be a potential drug.

3. Materials and Methods

3.1. General Methods

All solvents and reagents were used as received from commercial sources without further purification (except the reagents described below). 2-Aminoprop-1-ene-1,1,3-tricarbonitrile (2) (malononitrile dimer) was synthesized by dimerization of malononitrile in an alkaline medium [30]. 2-Phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one (3) was obtained from phenylhydrazine and ethyl 4,4,4-trifluoroacetoacetate according to the literature [31].

The melting point was measured with a Gallenkamp melting-point apparatus (Gallenkamp & Co., Ltd., London, UK). ¹H and ¹³C NMR spectra were registered in DMSO-d₆ with a Bruker AM300 spectrometer (Bruker Corporation, Billerica, MA, USA) at ambient temperature. The IR spectrum was recorded with a Bruker ALPHA-T FT-IR spectrometer (Bruker Corporation, Billerica, MA, USA) in KBr pellets. The MS spectrum (EI = 70 eV) was recorded with a Kratos MS-30 spectrometer (Kratos Analytical Ltd., Manchester, UK). For elemental analysis, a 2400 elemental analyzer (Perkin Elmer Inc., Waltham, MA, USA) was applied.

The ADME prediction of the synthesized molecule (4) was carried out using online resources [19,20].

3.2. Multicomponent Synthesis of 2,4-Diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile 4

Salicylic aldehyde (1) (0.122 g, 1 mmol), 2-aminoprop-1-ene-1,1,3-tricarbonitrile (2) (0.132 g, 1 mmol), and 2-phenyl-5-(trifluoromethyl)-2,4-dihydro-3H-pyrazol-3-one (3) (0.228 g, 1 mmol) were stirred in 5 mL of DMSO for 24 h at ambient temperature (23 °C). After the process was finished, 10 mL of water was added to the reaction mixture. The precipitate was filtered, washed with well-chilled ethanol (3 mL × 2), and dried to isolate pure 2,4-diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chro-meno[2,3-b]pyridine-3-carbonitrile (4).

2,4-Diamino-5-(5-hydroxy-1-phenyl-3-(trifluoromethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile (4). Yellowish powder; yield 84% (0.390 g); mp = 202–203 °C (decomp.) (from DMSO-H₂O); FTIR (KBr) cm⁻¹: 3557 (NH₂), 3437 (NH₂), 3342 (NH₂), 3207 (NH₂), 3069 (O-H), 2219 (C≡N), 1691 (C=N), 1647 (C=C Ar), 1600 (C-C Ar), 1536 (C=C Ar), 1494 (C=C Ar), 1473 (C=C Ar), 1454 (C=C Ar), 1280 (C-F), 1146 (C-F), 1120 (C-F). ¹H NMR (300 MHz, DMSO-d₆): δ 5.36 (s, 1H, CH), 6.02 (br s, 2H, NH₂), 6.51 (br s, 2H, NH₂), 7.07 (t, ³J = 8.1 Hz, 2H, 2 CH Ar s.ald.), 7.19-7.30 (m, 2H, 2 CH Ph), 7.43 (t, ³J = 7.2 Hz, 1H, CH Ph), 7.55 (t, ³J = 7.2 Hz, 2H, 2 CH Ph), 7.74 (d, ³J = 8.1 Hz, 2H, 2 CH Ar s.ald.), 11.52-12.78 (br s, 1H, OH) ppm; ¹³C NMR (75 MHz, DMSO-d₆): δ 27.8 (C(5)H), 71.2 (C(3)-CN), 88.0 (C(4a)), 106.8 (C(4’)), 116.7 (C(9)H Ar), 116.8 (CN), 121.2 (q, ¹J = 269.5 Hz, 1C, CF₃), 122.9 (C(7)H Ar), 123.1 (2C, o-CH from Ph), 124.3 (p-CH from Ph), 128.0 (C(6)H Ar), 128.8 (C(8)H Ar), 129.7 (2C, m-CH from Ph), 137.2 (q, ²J = 37.1 Hz, 1C, C(3’)-CF₃), 138.0 (2C, C(5a) and N(1’)-C), 149.8 (C(9a)), 150.9 (C(4)-NH₂), 157.4 (C(2)-NH₂), 158.9 (C(5’)-OH), 159.9 (C(1a)) ppm; MS (m/z, relative intensity %): 464 [M]⁺ (3), 444 [M − HF]⁺ (43), 277 [M − C₈H₆F₃N₂]⁺ (5), 237 [M − C₁₀H₆F₃N₂O]⁺ (100), 228 [C₁₀H₇F₃N₂O]⁺ (47), 77 [C₆H₅]⁺ (31); Anal. calcd. for C₂₃H₁₅F₃N₆O₂: C, 59.48; H, 3.26; N, 18.10%; found: C, 59.40; H, 3.36; N, 18.05%.

¹H and ¹³C NMR, IR and MS spectra for compound 4 are presented in the Supplementary Materials.

4. Conclusions

In conclusion, the title compound, 2,4-diamino-5-(5-hydroxy-1-phenyl-3-(trifluo-romethyl)-1H-pyrazol-4-yl)-5H-chromeno[2,3-b]pyridine-3-carbonitrile, was synthesized in good yield using a facile and efficient multicomponent reaction. During the study, we used simple equipment and available starting materials. The newly synthesized compound was characterized by spectroscopic techniques (NMR, IR, and MS-EI) and elemental analysis. According to ADME parameters, the resulting chromo[2,3-b]pyridine contains two pharmacologically promising fragments and has a chance of being useful as a drug.