Synthesis and Characterization of Piperazine-Linked Eugenol Derivative

Abstract

In the present work, we synthesize 2-(4-allyl-2-methoxyphenoxy)-1-(4-phenylpiperazin-1-yl)ethan-1-one, a semisynthetic derivative of the natural product eugenol. The compound was synthesized via a three-step synthetic pathway involving esterification, hydrolysis, and subsequent coupling with 4-phenylpiperazine, as confirmed by FTIR, ¹H NMR, ¹³C NMR, and mass spectrometric data.

1. Introduction

Natural products have long been used as therapeutic agents and continue to contribute significantly to drug discovery. The structurally complex molecules found in natural products provide unique chemical features that are difficult to replicate through synthetic methods. This creates new opportunities for biological modulation and drug development [1]. Semisynthetic derivatization of natural products, involving strategic functional modification of naturally occurring molecules, has led to the development of many semisynthetic drugs approved for clinical use. Frequently, semisynthetic modifications of natural products have significantly expanded the chemical space of bioactive molecules and altered other pharmacological parameters, such as potency, solubility, selectivity, and pharmacokinetic parameters [2,3].

In cancer therapy, assessments of drug approvals over the past several decades reveal that a substantial proportion of approved small-molecule drugs are either natural molecules or semisynthetic derivatives [4].

The incorporation of nitrogen-containing moieties, such as piperazine, represents a strategic approach in drug design and is frequently found in bioactive and clinically approved drugs. Structural modification of the piperazine scaffold has been extensively explored in natural product derivatives owing to its favorable physicochemical properties and its ability to enhance biological interactions [5,6].

Eugenol, the main phenylpropanoid component of clove oil, has been widely recognized as a versatile framework for chemical modification because of its broad range of pharmacological effects, including antimicrobial, anti-inflammatory, antioxidant, and anticancer activities. Numerous studies have explored structural modifications of eugenol to produce derivatives with improved or new biological activities, such as antibacterial, anti-inflammatory, and anti-proliferative effects, thereby demonstrating its potential as a useful scaffold in medicinal chemistry and drug discovery [7,8,9,10,11,12].

Given the biological relevance of eugenol and the frequent incorporation of piperazine moieties in clinically used drugs, the development of hybrid structures combining both motifs represents a promising approach to medicinally relevant compounds. A structurally related compound has been previously reported in the previous literature [13]; however, a detailed spectroscopic characterization using modern techniques such as ¹H NMR, ¹³C NMR, and ESI–MS was not available. Herein, we describe a refined synthetic procedure together with complete spectral characterization of a eugenol-derived piperazine compound. Recent studies on eugenol–amide hybrids further highlight the continued interest in amide-linked derivatives synthesized via alkylation-based approaches [14,15]. In contrast, the approach described here adopts a stepwise synthetic strategy, providing improved control over intermediate formation and enhanced reproducibility.

2. Results and Discussion

Compound 4 was synthesized in three steps, as outlined in Scheme 1. First, eugenol (1) was reacted with ethyl chloroacetate to introduce an ethyl acetate group. Potassium carbonate increased the nucleophilic power of the oxygen atom, forming the eugenol ester intermediate (2) [16]. The product was extracted, dried under reduced pressure, and used without further purification. The ester group was then hydrolyzed with aqueous sodium hydroxide, forming eugenol acid (3), using a slightly modified literature procedure [17,18]. Finally, the acid intermediate (3) was converted to amide (4) by activation with thionyl chloride in dichloromethane, followed by reaction with phenylpiperazine and trimethylamine. This approach follows a literature method for amide bond formation via in situ-generated acid chlorides [19].

The FTIR spectrum of the compound (4) showed a characteristic amide C=O stretching band at 1651 cm⁻¹, whereas the carbonyl group at 1715 cm⁻¹ in the eugenol acid intermediate had disappeared, confirming amide formation. Absorption bands at 1258 and 1145 cm⁻¹ were attributed to C-O-C and C-N stretching vibrations, indicating incorporation of the piperazine moiety. The ¹H NMR spectrum showed a singlet at δ 3.89 ppm (3H) for the methoxy group and a signal at δ 4.80 ppm (2H) for the methylene (–O–CH₂–). Signals confirmed the allyl moiety at δ 5.07–5.12 ppm (2H, CH₂=CH–) and δ 5.91–6.01 ppm (1H, –CH=), while aromatic protons resonated between δ 6.71 and 7.32 ppm. The benzylic methylene (Ar-CH₂-) appeared as a doublet at δ 3.36 ppm (2H). The piperazine moiety exhibited two distinct multiplets at δ 3.16–3.20 ppm and 3.78–3.85 ppm (each 4H), indicating magnetically nonequivalent CH₂ groups due to substitution on the nitrogen atoms, which disrupts the ring symmetry. In contrast, unsubstituted piperazine shows equivalent CH₂ signals due to rapid conformational averaging, whereas substitution generates two chemically distinct environments [20]. The ¹³C NMR spectrum showed the amide carbonyl at δ 166.7 ppm, aromatic carbons between δ 112.4–150.9 ppm, methoxy carbon at δ 55.8 ppm, and methylene carbon at δ 69.2 ppm. Allylic carbons appeared at δ 115.8 and 137.4 ppm. Notably, the piperazine ring showed four distinct signals at δ 42.1, 45.3, 49.3, and 49.8 ppm, confirming that all CH₂ carbons are chemically nonequivalent due to loss of symmetry upon substitution [20,21]. The ESI–MS spectrum showed a molecular ion peak at m/z 367.59 [M + H]⁺, consistent with the calculated molecular weight of 366.46 for C₂₂H₂₆N₂O₃, confirming the proposed structure.

3. Materials and Methods

All reagents and solvents were obtained from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. Reactions were monitored by thin-layer chromatography (TLC) on pre-coated silica gel plates (Merck Kieselgel 60 F254 (Merck KGaA, Darmstadt, Germany)), with visualization under UV light at 254 nm. Solvent evaporation and concentration were performed under reduced pressure using a Buchi-Rotavapor R-210 (BÜCHI Labortechnik AG, Flawil, Switzerland). Melting points and boiling points were determined using a Lab Junction Digital Melting Point/Boiling Point Apparatus (SKU: TI-LJ-46918) (Kingston Lab Solutions, Panchkula, Haryana, India) and are uncorrected. FT-IR spectra were recorded on a Bruker Alpha II spectrophotometer (Bruker Optics GmbH & Co. KG, Ettlingen, Germany). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer (Bruker BioSpin, Billerica, MA, USA) using CDCl₃ as solvent; chemical shifts (δ) are reported in ppm relative to residual solvent signals. Mass spectra were obtained on a Waters Quattro Micro API triple quadrupole mass spectrometer (Waters Corporation, Milford, MA, USA).

3.1. Synthesis of Ethyl [2-Methoxy-4-(prop-2-en-1-yl)phenoxy]acetate (Compound 2)

A mixture of 4-allyl-2-methoxyphenol (15.4 mL, 0.1 mol) and anhydrous potassium carbonate (20.7 g, 0.15 mol) was dissolved in dry acetone (50 mL) in a 250 mL round-bottom flask. Ethyl chloroacetate (21.39 mL, 0.2 mol) was added dropwise, and the reaction mixture was refluxed at 40 °C for 24 h. After completion, the solvent was removed under reduced pressure, and the residue was poured into ice-cold water. The product was extracted with diethyl ether, and the organic layer was dried over anhydrous sodium sulfate. Removal of the solvent under reduced pressure afforded compound 2. Compound 2 was previously reported in the literature [16].

Ethyl [2-methoxy-4-(prop-2-en-1-yl)phenoxy]acetate (Compound 2): Brown-colored liquid (b.p. 280–282 °C), yield: 19.77 g (79.0 mmol, 79.0%), TLC: R_f = 0.79 (n-hexane/ethyl acetate 7:3% v/v). FTIR (cm⁻¹): 1756 (C=O), 1259 (C-O-C), 1510 (C=C). ESI-MS (-ve) m/z: 249.89 [M − H]⁻.

3.2. Synthesis of 2-(4-Allyl-2-methoxyphenoxy)acetic Acid (Compound 3)

Ethyl [2-methoxy-4-(prop-2-en-1-yl)phenoxy]acetate (2) (2.52 g, 0.01 mol) was dissolved in 40 mL of aqueous sodium hydroxide (10% w/v) in a 250 mL round-bottom flask and refluxed at 80 °C for 2 h. Reaction progress was monitored by TLC using n-hexane/ethyl acetate (7:3 v/v) as the mobile phase. After completion, the reaction mixture was cooled to room temperature and poured into distilled water. The solution was acidified with dilute hydrochloric acid, resulting in the precipitation of a white solid. The solid was filtered, washed with water, and recrystallized from water to afford compound 3. Compound 3 was previously reported in the literature [17].

2-(4-allyl-2-methoxyphenoxy)acetic acid (compound 3): White solid (m.p. 90–92 °C), yield: 1.70 g (7.64 mmol, 76.4%), TLC: R_f = 0.25 (n-hexane/ethyl acetate 7:3% v/v). FTIR (cm⁻¹): 3522 (-OH), 1715 (C=O), 1637 (C=C). ESI-MS (-ve) m/z: 221.28 [M − H]⁻.

3.3. Synthesis of 2-(4-Allyl-2-methoxyphenoxy)-1-(4-phenylpiperazin-1-yl)ethan-1-one (Compound 4)

Triethylamine (0.303 mL, 3 mmol) was added at room temperature to a stirred solution of 2-(4-allyl-2-methoxyphenoxy)acetic acid (0.222 g, 1 mmol) and 4-phenylpiperazine (1.25 mmol) in dichloromethane. Thionyl chloride (0.119 mL, 1 mmol) was then added dropwise, and the reaction mixture was stirred at room temperature for 2 h. After completion of the reaction, the solvent was removed under reduced pressure. The residue was dissolved in dichloromethane and successively washed with 1 N hydrochloric acid (2 × 20 mL) and 1N sodium bicarbonate (2 × 20 mL). The organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel using n-hexane/ethyl acetate mixtures as eluents to afford compound 4.

2-(4-allyl-2-methoxyphenoxy)-1-(4-phenylpiperazin-1-yl)ethan-1-one (Compound 4):

White solid (m.p. 160–162 °C), yield: 0.267 g (0.729 mmol, 72.9%), TLC: R_f = 0.70 (n-hexane/ethyl acetate 7:3% v/v). FTIR (cm⁻¹): 1651 (C=O), 1504 (C=C), 1258 (C-N), 1145 (C-O-C). ¹H NMR (400 MHz, CDCl₃): 3.16–3.20 (m, 4H, -CH₂-piperazine), 3.36 (d, J = 8.0 Hz, 2H, Ar-CH₂-), 3.78–3.85 (m, 4H, -CH₂-piperazine), 3.89 (s, 3H, –OCH₃), 4.80 (s, 2H, –O–CH₂–), 5.07–5.12 (m, 2H, CH₂=CH−), 5.91–6.01 (m, 1H, –CH=), 6.71–6.75 (m, 2H, Ar-H), 6.90–6.94 (m, 4H, Ar-H), 7.28–7.32 (m, 2H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): 39.8 (-CH₂-Ar), 42.1, 45.3, 49.3, 49.8 (4 × CH₂, piperazine), 55.8 (Ar-OCH₃), 69.2 (–O–CH₂–), 115.8, 137.4 (–CH₂=CH–), 112.4, 114.4, 116.6, 120.5, 120.6, 129.2, 134.4, 145.5, 149.4, 150.9 (Ar-CH), 166.7 (–C=O, amide). ESI-MS (+ve): 367.59 [M + H]⁺.

4. Conclusions

A new piperazine-linked eugenol derivative was successfully synthesized in a multi-step synthetic route from eugenol. The target compound was obtained in good yield, and its structure was confirmed by FTIR, ¹H NMR, ¹³C NMR, and ESI-MS analyses. Spectroscopic data were fully consistent with the proposed molecular structure, confirming successful amide formation and piperazine incorporation. This work demonstrates the utility of eugenol as a versatile scaffold for semisynthetic modification. It provides a structurally well-characterized molecule that may serve as a useful reference for future medicinal chemistry and natural product-based derivatization studies.