Retracted Article: Synthesis and characterization of novel silica-coated magnetic nanoparticles with tags of ionic liquid. Application in the synthesis of polyhydroquinolines

Abstract

A novel, green and recoverable silica-coated magnetic nanoparticle immobilized ionic liquid, was designed, synthesized and fully characterized using IR, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetry (TG) and vibrating sample magnetometer (VSM) analysis. The resulting nanomagnetic catalyst has been successfully used for the condensation of various aromatic aldehydes, dimedone, β-ketoester and ammonium acetate under solvent free and mild conditions to yield polyhydroquinoline derivatives in good to excellent yields. The described catalyst was recycled and reused without any significant changes in the yield and the reaction time.

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Introduction

In recent times, the design, synthesis and application of magnetically recoverable catalysts have received immense attention as they are a fascinating choice to enhance the efficiency of heterogeneous catalyst separation from the reaction mixture by applying a simple magnet.¹ On the other hand, a rational design of ILs allows for the creation of additional functionalities in the IL structure to influence either their physical or chemical properties. Some synthesized ILs have been defined as task-specific ionic liquids (TSILs),² in which a wide range of Brønsted and Lewis acidic or basic ILs have been used as recyclable acid or base catalysts.³ The combination of functionalized ILs and catalytically active nanoparticles (NPs) provides suitable heterogeneous systems where different bond cleavage and formation processes can be induced by IL functionality.⁴ Although, ionic liquids show promising applications in the knowledge-based development of green and sustainable methodologies; recent technological cycle analysis revealed that not all ILs are intrinsically green so many of them were found to be toxic or even highly toxic towards cells and living organisms.⁵ The main aim of future strategies is to improve significantly the important drawbacks and hazards associated with traditional ILs, replacing them with safer and more efficient alternatives. Furthermore, recent investigations have revealed that magnetic nanoparticles can act as excellent supports for ionic liquids (ILs) due to their high stability, easy synthesis and applicability, large surface area and facile separation, and low toxicity and price.⁶ Having the above background in mind, we have tried to synthesize a new nanocatalyst by the immobilization of an ionic liquid on a support of silica-coated magnetic nanoparticles (MNPs).

One-pot multi-component reactions are promising designs in novel combinatorial chemistry due to some advantages such as their ability to synthesize the desired products with high atom economy by the reaction of three or more starting materials in one step. Additionally, this technical skill improves the synthetic efficiency and simplicity of the formation of complex molecules with respect to conventional organic reactions.^7,8

Lately, the production of polyhydroquinoline kernels has been a subject of huge attraction for synthetic and medicinal chemists due to their application in biological activities such as Ca²⁺ channel blockers,⁹ also they can be used in a variety of pharmaceutical and biological applications including bronchodilator, hepatoprotective, vasodilator, anti-tumor, anti-atherosclerotic and anti-diabetic agents for the treatment of cardiovascular diseases including hypertension.^10,11 The photocatalytic oxidation of polyhydroquinolines to their corresponding pyridine derivatives establishes the principal metabolic pathway in biological systems.¹² Due to the therapeutic importance of these compounds, organic chemists have introduced several procedures for the synthesis of polyhydroquinolines using different catalysts as follows: [pyridine-SO₃H]Cl,¹³ [Dsim]HSO₄,¹⁴ TiO₂ NPs,¹⁵ SnO₂ NPs,¹⁶ [2-MPyH]OTf,¹⁷ Ni-nanoparticles,¹⁸ Baker’s yeast,¹⁹ L-proline,²⁰ and [bmim]BF₄.²¹ Also, these interesting compounds have been synthesized by conventional heating,²² refluxing in acetic acid,²³ microwave irradiation²⁴ and ultrasound.²⁵ Although, several catalysts and different methods have been reported for the synthesis of polyhydroquinolines, surveying a newer recyclable and reusable catalytic environmentally compatible approach is still the great demand of the day and a challenging mission to methodologists.

Results and discussion

In continuation of our studies on the synthesis of solid acids,²⁶ inorganic acidic salts,²⁷ and nanomagnetic catalysts,²⁸ the knowledge-based development of task specific ionic liquids (TSILs),²⁹ and the synthesis of nanoparticles with tags of ionic liquid (NPs@IL),³⁰ we found that the desirable structural diversity of ILs with special properties could be achieved via the design and synthesis of novel cationic cores with suitable anionic counterparts. With this aim, we decided to join all of the above mentioned research areas to design, synthesize and apply a novel ionic liquid immobilized on silica coated Fe₃O₄ magnetic nanoparticles {Fe₃O₄@SiO₂@(CH₂)₃Im}C(CN)₃ as a novel heterogeneous catalyst for the preparation of polyhydroquinoline derivatives using a good range of aromatic aldehydes, ethyl acetoacetate, ammonium acetate and dimedone under benign and solvent-free conditions (Schemes 1 and 2).

Scheme 1 Preparation of a novel silica-coated magnetic nanoparticle immobilized ionic liquid.

Scheme 2 Synthesis of polyhydroquinoline derivatives under solvent-free and mild conditions.

Characterization of the novel catalyst

The structure of {Fe₃O₄@SiO₂@(CH₂)₃Im}C(CN)₃ was fully confirmed using FT-IR, TG, XRD, TEM, SEM and VSM analysis.

In the IR spectrum of the new catalyst, the broad peak in the region of about 3100–3650 cm⁻¹ can be ascribed to the overlapping of O–H stretching groups (corresponds to uncoated O–H groups) and N–H stretching group on the imidazolium ring. Furthermore, the peak discerned at 2050 cm⁻¹ is attributed to the nitrile stretching mode (Fig. 1).

Fig. 1 The IR spectrum of {Fe₃O₄@SiO₂@(CH₂)₃Im}C(CN)₃.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the prepared catalyst were recorded and are illustrated in Fig. 2a–d. The investigation of the SEM images reveals that the size of the particles is under the 50 nm. Also, the images display some larger particles that can be attributed to the overlapping or aggregating of the smaller particles. Also, the transmission electron microscopy (TEM) analysis supported the observations obtained from SEM and confirmed the morphology of the new catalyst.

Fig. 2 (a–d) The scanning electron microscopy (a and b) and the transmission electron microscopy (c and d) images of the catalyst.

As depicted in Fig. 3a–e, the XRD pattern of the prepared catalyst in comparison with different stepwise synthesized compounds was studied in a domain of about 2–90 degrees. The XRD pattern of the Fe₃O₄ nanoparticles (Fig. 3a) presents peaks at 2θ = 14.80, 30.10, 35.50, 43.10, 53.00, 57.00, 62.80, 70.50 and 73.90 that can be attributed to the corresponding diffraction lines of the cubic spinel phase (110), (220), (311), (400), (331), (422), (511), (440) and (531) respectively (JCPDS card no. 85-1436). The XRD pattern of silica coated magnetite nanoparticles (Fig. 3b) illustrates a broad peak at about 2θ = 20–30. The addition of different layers in the synthetic pathway of the new catalyst to the Fe₃O₄ nanoparticle surface changed the XRD patterns with preservation of all the above mentioned peaks, confirming the formation of the target compound. It can be inferred from the XRD pattern that the catalyst (Fig. 3e) has a crystalline nature with diffraction lines at 2θ ≈ 21.30°, 30.50°, 35.25°, 35.56°, 35.90°, 56.92° and 62.90°. Also, the XRD data, including the 2θ, peak width, size of the particles and inter planar distance were extracted and are presented in Table 1. As shown in Table 1, the size of the particles according to the Scherrer equation is in the nanometer range of about 9.69–46.40. With great importance, the calculated results from the XRD patterns are in a good agreement with the scanning electron microscopy and transmission electron microscopy analysis.

Fig. 3 (a–e) XRD pattern of the catalyst in comparison with the different stepwise synthesized materials.

Table 1 XRD data for the new catalyst

Entry	2θ	Peak width [FWHM] (degree)	Size [nm]	Inter planer distance [nm]
1	21.30	0.26	31.09	0.416646
2	30.50	0.85	9.69	0.292741
3	35.25	0.23	36.25	0.254305
4	35.56	0.28	29.80	0.252159
5	35.90	0.18	46.40	0.249848
6	56.92	0.58	15.58	0.161580
7	62.90	0.40	23.28	0.147579

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Energy dispersive X-ray (EDX) analysis of the new catalyst presents all expected elements including iron, silicon, oxygen, carbon and nitrogen as indicated in Fig. 4. Also, according to the SEM coupled EDX data the catalyst is composed of only C (0.98), N (3.92), Fe (54.08), O (17.75), and Si (23.26), which is exposed in the elemental analysis.

Fig. 4 Energy-dispersive X-ray (EDX) spectra of the catalyst.

The magnetic properties of {Fe₃O₄@SiO₂@(CH₂)₃Im}C(CN)₃ as a catalyst have been investigated and are depicted in Fig. 5, in comparison with Fe₃O₄ nanoparticles, using vibrating sample magnetometer (VSM) analysis. It can be concluded from the magnetization curves that the saturation of the prepared catalyst diminished to 43 emu g⁻¹ from 70 emu g⁻¹ for the Fe₃O₄ nanoparticles which can be attributed to different layers coated onto the surface of the Fe₃O₄ nanoparticles.

Fig. 5 The vibrating sample magnetometer (VSM) magnetization curves of the novel catalyst in comparison with the Fe₃O₄ nanoparticles.

In another study, the stability of the novel catalyst was investigated with thermogravimetry (TG) and he curve is portrayed in Fig. 6. The weight loss from room temperature to about 120 °C can be assigned to the desorbed water or other organic solvents which were employed for the preparation steps of the catalyst. Also, the weight loss from 120 to 440 °C can be ascribed to the tags of ionic liquid. Finally, the weight loss after 440 °C is due to the decomposition of the catalyst.

Fig. 6 Thermogravimetry (TG) curve of the prepared catalyst.

Synthesis of the polyhydroquinoline derivatives using the new catalyst (Scheme 2)

Firstly, in order to find the best experimental conditions for the synthesis of polyhydroquinoline derivatives, the reaction of benzaldehyde 1a, dimedone 2, ethyl acetoacetate 3 and ammonium acetate 4 was selected as a model reaction (Scheme 3). The obtained results for the different loads of the novel catalyst, temperatures and solvents are depicted in Table 2.

Scheme 3 The optimized conditions for the synthesis of the polyhydroquinoline derivatives.

Table 2 Optimization of reaction conditions for the synthesis of polyhydroquinolines

Entry	Solvent	Load of catalyst (mg)	Temperature (°C)	Time (min)	Yield^a (%)
a Refers to isolated yields.
1	—	5	r.t.	70	20
2	—	5	60	35	76
3	—	5	80	25	90
4	—	5	100	30	88
5	—	—	80	70	35
6	—	3	80	35	50
7	—	4	80	25	85
8	—	7	80	30	89
9	EtOH	6	Reflux	120	70
10	H2O	5	Reflux	90	45
11	CHCl3	5	Reflux	90	35
12	THF	5	Reflux	120	68
13	CH3CN	5	Reflux	70	50

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The results depicted in Table 2 show that the optimized conditions for the model reaction were obtained when the reaction was carried out in the presence of 5 mg of MNPs@IL catalyst at 80 °C under solvent-free conditions (Table 2, entry 3). Also, to find the best ratio of ammonium acetate to other starting materials, we tested the model reaction with different amounts of ammonium acetate (1, 1.5 and 2 mmol). The obtained results show that using 1.5 mmol of ammonium acetate is the best choice (25 min with a yield of 90%).

In order to confirm the applicability and efficiency of the new protocol for polyhydroquinoline synthesis, a good scope of arylaldehydes (including those bearing electron-withdrawing and electron-releasing groups and halogens) were reacted with dimedone and ethyl acetoacetate as β-dicarbonyl compounds and ammonium acetate using MNPs@IL as the catalyst under the optimized reaction conditions. As depicted in Table 3, all starting materials reacted with each other to afford the target molecules in good to excellent yields in short reaction times.

Table 3 Synthesis of the hexahydroquinoline derivatives using a good range of arylaldehydes in the presence of MNPs@IL under solvent free conditions

Entry	R	Product	Time (min)	Yield^a (%)	Mp (°C) found [Lit.]^ref.
a Refers to isolated yields.
1	H	5a	25	90	224–226 [220–224]^32
2	4-Me	5b	30	89	263–265 [259–262]^33
3	3-OH	5i	25	90	230–232 [218–220]^34
4	2,4-Cl2	5c	25	78	242–244 [241–243]^35
5	4-NO2	5d	40	80	241–242 [245–248]^33
6	4-OMe	5e	30	87	255–257 [256–258]^33
7	4-Cl	5f	20	92	242–244 [244–246]^33
8	3-NO2	5g	45	80	182–184 [179–181]^33
9	4-F	5h	25	91	193–195 [184–186]^35
10	3,4-(OMe)2	5j	20	92	204–206 [202–204]^33
11	3-OEt-4-OH	5k	20	91	190–192 [205–206]^36
12	4-OH	5l	25	90	233–235 [234–236]^33
13	4-CN	5m	30	81	223–225 [199–202]^37
14	4-N(Me)2	5n	25	95	235–237 [230–232]^34

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Additionally, the reusability of the catalyst was investigated upon the reaction between 4-hydroxy benzaldehyde, dimedone, ethyl acetoacetate and ammonium acetate. After completion of the reaction, ethanol was added to the mixture and heated to dissolve the desired product and unreacted starting materials. Then, the catalyst was collected using a magnet, washed with ethanol, dried and reused for the next attempt. The catalyst was reused for eight runs. The obtained data show that the catalytic activity of the new catalyst was restored without any significant changes in the yield and the reaction time (Fig. 7).

Fig. 7 Reusability of the MNPs@IL catalyst in the reaction of 4-hydroxy benzaldehyde, dimedone, ethyl acetoacetate and ammonium acetate.

In order to explore the competence of the new protocol for hexahydroquinoline synthesis in comparison with the other reported procedures in the literature, herein, we have compared our obtained results with some previous reported methods as outlined in Table 4. As depicted in Table 4, the other previously reported protocols suffer from one or more drawbacks like longer reaction times, using hazardous organic solvents and transition metal catalysts and low yields of products.

Table 4 Comparison of novel nano magnetic catalyst and other systems for synthesis of compound 5a

Entry	Reaction conditions	Time (min)	Yield (%)	Lit.
1	Hf(NPf2)4 (1 mol%), C10F18, 60 °C	180	95	38
2	GuHCl (10 mol%), r.t.	180	98	39
3	Mont. supported Ni^0-nanoparticle, r.t., no solvent	15	95	40
4	(bzacen)MnCl (2.5 mol%), EtOH, reflux	20	90	41
5	Nano-Fe3O4 (5 mol%), solvent-free, 50 °C	6	89	26
6	Nafion-H®, PEG 400-water (60 : 40), 50 °C	90	96	22
7	Co3O4–CNTs (0.03 g), EtOH, 50 °C, sonication	15	97	42
8	Mn@PMO-IL (1 mol%), solvent-free, 80 °C	20	95	43
9	Fe3O4-SA-PPCA (10 mg), EtOH, 50 °C	120	97	44
10	Nanocat Fe–Ce (100 mg), EtOH, r.t.	20	95	45
This work	{Fe3O4@SiO2@(CH2)3Im}C(CN)3, solvent-free, 80 °C	25	90	—

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It is important to notice that the obtained data for run 1 in Fig. 7 and the same reaction in Table 3 (entry 12) are equivalent, which is evidence of the reproducibility of the reaction. Also, other proof of the reproducibility of the presented procedure can be found in Table 2 (optimization of reaction conditions), where the resulting data for the reaction of benzaldehyde, dimedone, ethyl acetoacetate and ammonium acetate in both Table 2 (entry 3) and Table 3 (entry 1) are similar.

In a plausible mechanism, we believe that the novel catalyst acts as a promoter for the reaction as follows: at first dimedone is converted to its active enol form by the catalyst and undergoes reaction with the activated arylaldehydes to afford a Knoevenagel intermediate I. On the other hand, ammonia (resulting from ammonium acetate) and the β-ketoester (activated by MNPs@IL) yield enamine II. Subsequently, the reaction between these two intermediates gives the corresponding intermediate III. Tautomerization of intermediate III affords intermediate IV. Afterward, the intramolecular nucleophilic attack of the NH₂ group to the activated carbonyl group leads on intermediate IV which forms its ring closed adduct V. At the end, the dehydration of intermediate V yields the hexahydroquinoline derivatives 5a–n (Scheme 4).

Scheme 4 Plausible mechanism for the synthesis of target molecules in the presence of {Fe₃O₄@SiO₂@(CH₂)₃Im}C(CN)₃.

Conclusion

In summary, in this research, we have described a new green, recoverable nanomagnetic supported ionic liquid which was fully characterized using IR, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and TG, DTG and vibrating sample magnetometer (VSM) analysis. The applicability of the catalyst was tested for the synthesis of hexahydroquinoline derivatives (5a–n) by the condensation of dimedone, ethyl acetoacetate, ammonium acetate as a source of nitrogen and a good range of arylaldehydes under benign, green and solvent-free conditions. The promising points of this work are the environmentally mild conditions, reusability of the catalyst, short reaction time, high yield and easy work-up.

Experimental

General

All chemicals were purchased from Fluka and Merck chemical companies. The known products were identified by comparison of their physical properties and spectral data with their reported authentic samples in the literature. The reaction progress and purity of the compounds were checked using TLC performed with silica gel SIL G/UV 254 plates. The ¹H NMR (400 or 300) and ¹³C NMR (100 or 75 MHz) spectra were recorded on a Bruker spectrometer (δ in ppm). Melting points were recorded on Buchi B-545 apparatus in open capillary tubes.

General procedure for the preparation of novel silica-coated magnetic nanoparticles with tags of ionic liquid (Scheme 1)

Fe₃O₄ nanoparticles A were produced according to the reported literature³¹ and afterward, the resulting nanoparticles were coated with a layer of silica through reaction with tetraethyl orthosilicate (TEOS). The subsequent step is the silanization of the silica-coated magnetite nanoparticles with 3-chloropropyltrimethoxysilane to afford compound C. Then, imidazole (6 mmol) was added to compound C and refluxed for 12 h in toluene to give compound D. At the end, tricyanomethane (6 mmol) was added to compound D in toluene and the mixture was stirred for 12 h to yield compound E as a silica-coated magnetic nanoparticle with tags of ionic liquid (Scheme 1).

General procedure for the synthesis of hexahydroquinolines (Scheme 2)

To a test tube containing a mixture of aromatic aldehydes (1 mmol), dimedone (1 mmol, 0.14 g), ethyl acetoacetate (1 mmol, 0.13 g) and ammonium acetate (1.5 mmol, 0.11 g) was added MNPs@IL as a catalyst (5 mg). Then, the resulting mixture was stirred intensively for an appropriate time in an oil bath at 80–85 °C under solvent-free conditions. After completion of the reaction, as monitored using TLC with a mixture of n-hexane and ethylacetate as the eluent, the reaction mixture was quenched to room temperature. Afterward, for the isolation of the catalyst, boiling ethanol was added to the vessel and the catalyst was separated using a magnet. Finally, the pure products were recrystallized from ethanol with good to excellent yields (Scheme 2 and Table 2).

Spectral data

Ethyl 2,7,7-trimethyl-5-oxo-4-phenyl-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate (5a)

White powder, melting point = 224–226 °C.

FT-IR (KBr): ν (cm⁻¹) = 3290, 3082, 2962, 1699, 1611, 1485, 1381, 1212, 1072, 837.

¹H NMR (DMSO-d₆, 300 MHz): δ (ppm) = 0.84 (s, 3H, CH₃), 1.00 (s, 3H, CH₃), 1.13 (t, 3H, CH₃, J = 7.2 Hz), 1.98 (d, 1H, J = 16.2 Hz), 2.17 (d, 1H, J = 16.2 Hz), 2.29 (s, 3H, CH₃), 2.32 (s, 1H), 2.42 (d, 1H, J = 17.1 Hz), 3.97 (q, 2H, J = 7.2 Hz), 4.86 (s, 1H, CH), 7.04–7.21 (m, 5H, Ph), 9.08 (s, 1H, NH).

¹³C NMR (DMSO-d₆, 75 MHz): δ (ppm) = 14.6, 18.8, 26.9, 29.6, 32.6, 36.3, 50.7, 59.5, 104.1, 110.4, 126.1, 127.1, 128.2, 145.5, 148.1, 150.0, 167.3, 194.7.

Ethyl 4-(4-chlorophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate (5f)

Pale yellow powder, melting point = 242–244 °C.

FT-IR (KBr): ν (cm⁻¹) = 3275, 3078, 2959, 1707, 1605, 1489, 1382, 1214, 1108, 844.

¹H NMR (DMSO-d₆, 400 MHz): δ (ppm) = 0.83 (s, 3H, CH₃), 1.00 (s, 3H, CH₃), 1.12 (t, 3H, CH₃, J = 6.8 Hz), 1.98 (d, 1H, J = 16.0 Hz), 2.17 (d, 1H, J = 16.0 Hz), 2.26 (s, 1H), 2.30 (s, 3H, CH₃), 2.42 (d, 1H, J = 17.2 Hz), 3.97 (q, 2H, J = 6.8 Hz), 4.84 (s, 1H, CH), 7.16 (d, 2H, Ph, J = 8.4 Hz), 7.25 (d, 2H, Ph, J = 8.4 Hz), 9.13 (s, 1H, NH).

¹³C NMR (DMSO-d₆, 100 MHz): δ (ppm) = 14.1, 18.2, 26.4, 29.0, 32.1, 35.6, 50.1, 59.1, 103.0, 109.6, 127.7, 129.3, 130.2, 145.4, 146.5, 149.5, 166.6, 194.3.

Ethyl 4-(3-ethoxy-4-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate (5k)

White powder, melting point = 190–192 °C.

FT-IR (KBr): ν (cm⁻¹) = 3443, 3280, 3078, 2959, 1688, 1615, 1491, 1380, 1217, 1073, 866.

¹H NMR (DMSO-d₆, 400 MHz): δ (ppm) = 0.87 (s, 3H, CH₃), 1.01 (s, 3H, CH₃), 1.15 (t, 3H, CH₃, J = 7.2 Hz), 1.29 (t, 3H, CH₃, J = 7.2 Hz), 1.98 (d, 1H, J = 16.4 Hz), 2.17 (d, 1H, J = 16.4 Hz), 2.26 (s, 3H, CH₃), 2.3 (s, 1H), 2.40 (d, 1H, J = 17.2 Hz), 3.86–3.94 (m, 2H), 3.98 (q, 2H, J = 7.2 Hz), 4.74 (s, 1H, CH), 6.50 (dd, 1H, Ph, J = 2 Hz), 6.59 (d, 1H, Ph, J = 8 Hz), 6.69 (d, 1H, Ph, J = 2 Hz), 8.56 (s, 1H, OH), 8.99 (s, 1H, NH).

¹³C NMR (DMSO-d₆, 100 MHz): δ (ppm) = 14.2, 14.7, 18.1, 26.3, 29.2, 32.1, 34.9, 50.3, 59.0, 64.0, 104.0, 110.1, 113.5, 114.9, 119.6, 139.0, 144.3, 144.7, 145.7, 149.1, 167.0, 194.4.

Ethyl 4-(4-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate (5l)

Pale yellow powder, melting point = 233–235 °C.

FT-IR (KBr): ν (cm⁻¹) = 3418, 3281, 3079, 2960, 1687, 1614, 1485, 1380, 1221, 1086, 855.

¹H NMR (DMSO-d₆, 400 MHz): δ (ppm) = 0.86 (s, 3H, CH₃), 1.00 (s, 3H, CH₃), 1.14 (t, 3H, CH₃, J = 7.2 Hz), 1.97 (d, 1H, J = 16.0 Hz), 2.16 (d, 1H, J = 16.0 Hz), 2.26 (s, 3H, CH₃), 2.3 (s, 1H), 2.40 (d, 1H, J = 16.8 Hz), 3.97 (q, 2H, J = 7.2 Hz), 4.74 (s, 1H, CH), 6.56 (d, 2H, Ph, J = 8.4 Hz), 6.93 (d, 2H, Ph, J = 8.4 Hz), 8.98 (s, 1H, OH), 9.07 (s, 1H, NH).

¹³C NMR (DMSO-d₆, 100 MHz): δ (ppm) = 14.1, 18.1, 26.4, 29.1, 32.1, 34.7, 50.3, 58.9, 104.0, 110.2, 114.4, 128.3, 138.4, 144.3, 149.0, 155.1, 167.0, 194.3.

Acknowledgements

We thank Bu-Ali Sina University and Iran National Science Foundation (INSF) for financial support (Grant Number: 94002177) to our research group.

Notes and references

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