Design and Synthesis of Diloxanide Derivatives with Improved Lipophilicity and Pharmacokinetic Profile

Fig. 1.1: Flow-chart representation of diloxanide furoate prodrug activation pathway.

The current understanding of diloxanide's mechanism of action, while not completely elucidated, is based on its structural similarity to chloramphenicol at the dichloroacetamide functional group. The drug acts primarily through inhibition of protein synthesis in E. histolytica trophozoites, effectively blocking their conversion to the more virulent and invasive cyst forms. This mechanism makes diloxanide particularly valuable for treating asymptomatic carriers who harbor cysts in the intestinal lumen, preventing transmission and reducing the risk of invasive disease development. [2]

1.2 Chemical structure and prodrug strategy:

Diloxanide furoate (C₁₄H₁₁Cl₂NO₄, MW: 328.147) functions as a prodrug through an elegant ester-based bioactivation strategy. The compound is administered as the furoic acid ester of diloxanide, which undergoes rapid hydrolysis by esterases in the gastrointestinal tract to release the active moiety, diloxanide (C₉H₉Cl₂NO₂, MW: 234.08). This prodrug approach was specifically designed to achieve high luminal concentrations while minimizing systemic exposure, as evidenced by the rapid metabolism and excretion profile where 90% of the absorbed drug is eliminated renally as glucuronide metabolites within 3 hours. [1,3]

The furoic acid ester linkage serves multiple pharmaceutical purposes: it enhances chemical stability during storage, provides improved gastrointestinal tolerance, and enables controlled release of the active drug at the site of action. Upon oral administration, diloxanide furoate is slowly absorbed from the gastrointestinal tract, with the bioavailability of the parent diloxanide being approximately 90%. The hydrolytic cleavage produces diloxanide and furoic acid, with the former undergoing extensive glucuronidation (99% as glucuronide conjugate, 1% as free drug). [1,4]

Fig. 1.2: Chemical structure representations of Diloxanide furoate (Prodrug) and Diloxanide.

1.3 Pharmacokinetic limitations and development opportunities:

Despite its therapeutic efficacy, diloxanide furoate exhibits several pharmacokinetic challenges that represent opportunities for derivative development. The drug's lipophilic properties, while adequate for luminal activity, may not be optimal for enhanced membrane permeability and tissue distribution. Studies have demonstrated that cyclodextrin complexation can improve both the stability and bioavailability of diloxanide furoate, suggesting that modifications to enhance solubility and membrane transport could provide therapeutic advantages. [1,5-7]

The current formulation limitations include relatively poor aqueous solubility, modest membrane permeability, and the need for multiple daily dosing regimens. These factors have prompted investigations into structural modifications that could enhance the drug's physicochemical properties while maintaining or improving its anti-amoebic activity. The development of diloxanide derivatives with improved lipophilicity represents a rational approach to address these limitations and potentially expand the therapeutic applications of this important drug class. [2,7]

1.4 Structure-Activity Relationships and design principles:

Fig. 1.3: Structure-Activity Relationship (SAR) map for diloxanide derivatives with improved lipophilicity.

The structure-activity relationship of diloxanide provides several strategic points for chemical modification to improve lipophilicity and pharmacokinetic properties. The core dichloroacetamide moiety, essential for antimicrobial activity, offers limited modification options without compromising efficacy. However, the phenolic hydroxyl group and the N-methyl substituent present attractive targets for lipophilicity enhancement. [8,9]

Research in related antimicrobial compounds has demonstrated that strategic introduction of lipophilic groups can significantly improve drug properties. For instance, studies on phenylthiazole derivatives have shown that reducing excessive lipophilicity through careful structural modifications can improve overall physicochemical and pharmacokinetic profiles. Similarly, work on ester prodrugs has revealed that appropriate ester selection can enhance membrane permeability while maintaining controlled bioactivation. [4,8,10-12]

The phenolic hydroxyl group of diloxanide represents the most promising site for lipophilicity modifications. Esterification with various carboxylic acids of different chain lengths and branching patterns could provide a series of prodrugs with tunable lipophilicity. Additionally, ether formation with lipophilic alkyl or aryl groups could yield derivatives with enhanced membrane permeability while potentially maintaining the required luminal activity. [4,13,14]

1.5 Novel chemical derivatives and target molecules:

Based on the structure-activity principles and successful modifications in related drug classes, several categories of diloxanide derivatives can be envisioned as targets for synthesis and evaluation:

• Alkyl Ester derivatives: Long-chain fatty acid esters of the phenolic hydroxyl group could significantly enhance lipophilicity while serving as prodrugs activated by tissue esterases. These derivatives would be expected to show improved oral bioavailability and potentially enhanced tissue penetration, which could be valuable for treating extra-intestinal manifestations of amoebiasis.
• Aromatic Ester derivatives: Benzoyl and substituted benzoyl esters could provide intermediate lipophilicity enhancement with potentially different hydrolysis kinetics compared to aliphatic esters. These modifications might offer advantages in terms of chemical stability and controlled release profiles. [4]
• N-Alkyl chain extensions: Modifications of the N-methyl group to longer alkyl chains or cyclic structures could enhance lipophilicity while potentially affecting the drug's interaction with target proteins. However, such modifications would require careful evaluation to ensure retention of antimicrobial activity. [9]
• Hybrid molecular approaches: Combination of multiple modification strategies, such as phenolic esterification combined with N-alkyl extensions, could provide synergistic improvements in pharmacokinetic properties. These hybrid approaches have been successful in other drug development programs targeting similar improvements. [11]
• Fluorinated derivatives: Introduction of fluorine atoms or fluoroalkyl groups has proven effective in enhancing lipophilicity and metabolic stability in many drug classes. Strategically placed fluorine substitutions could improve the pharmacokinetic profile of diloxanide while potentially enhancing its antimicrobial potency. [12]

1.6 Aim and Objectives:

Aim:
To design, synthesize, and characterize novel diloxanide derivatives with enhanced lipophilicity, aiming to achieve improved pharmacokinetic profiles and potential anti-amoebic efficacy. The synthetic strategy focus on modifications that preserve the essential dichloroacetamide pharmacophore while introducing lipophilic enhancements through strategic esterification, fluorination, and N-alkylation approaches.

Objectives:

1. To design structural analogues of diloxanide by introducing various functional groups aimed at increasing lipophilicity.
2. To synthesize the designed diloxanide derivatives and confirm their chemical structures through appropriate analytical and spectroscopic techniques.
3. To evaluate the melting point, solubility, and % yields of the synthesized derivatives.
4. To characterize all the derivatives, using: FT-IR and UV-Vis. spectroscopy.
5. To establish structure-activity relationships for these derivatives.

2. LITERATURE SURVEY

1. Description of Diloxanide:

Table 2.1: Drug Profile of Diloxanide.

Sr. No.	Property	Description
1.	Drug	Diloxanide
2.	Molecular Formula	C9H9Cl2NO2
3.	Molecular Weight	234.08 g/mol
4.	Elemental Composition	C: 46.18%, H: 3.88%, N: 5.98%, O: 13.67%, Cl: 30.29%
5.	Preparation Method	Synthesized by the dichloroacetylation of N-methyl-p-aminophenol with dichloroacetyl chloride.
6.	IUPAC Name	2,2-dichloro-N-(4-hydroxyphenyl)-N-methylacetamide
7.	Odor	Odorless
8.	Colour	White to light brown (solid)
9.	Solubility	Very slightly soluble in water; soluble in alcohol, ether, and chloroform.
10.	Melting Point	175 °C
11.	Synonyms	Entamide, Amebamida, Diloxanid, 2,2-Dichloro-4'-hydroxy-N-methylacetanilide
12.	pKa (at 25 °C)	~ 9.5 (Predicted for the phenolic -OH group)
13.	Log P	2.0 (XLogP3)
14.	Structure

2.2 Targeted colon delivery of diloxanide furoate as an antiamoebic drug:

Tiwari and colleagues investigated the development of pectin microspheres for targeted colon delivery of diloxanide furoate as an antiamoebic drug. Their study focused on overcoming the limitations of conventional oral dosage forms by employing a spray drying method with zinc acetate cross-linking to prepare microspheres. The research demonstrated that different concentrations of pectin polymer (0.5-3%) significantly influenced drug encapsulation efficiency, with optimal formulations achieving 78.45% drug entrapment. The microspheres exhibited pH-dependent release behavior, with minimal drug release in acidic conditions (2.1% in 2 hours at pH 1.2) and substantial release in alkaline conditions (89.4% in 8 hours at pH 6.8), making them suitable for colon-specific delivery. In vivo studies in albino rats confirmed enhanced bioavailability compared to conventional tablets, with improved pharmacokinetic parameters including increased AUC and prolonged retention time in the colon. This work established the foundation for understanding how formulation strategies can enhance diloxanide's therapeutic delivery and highlighted the potential for structural modifications to improve drug targeting and bioavailability. [15]

2.3 Chemical stability limitations of the current prodrug: 

Gadkariem and team conducted comprehensive stability studies examining the degradation behavior of diloxanide furoate under various pH and temperature conditions. Their investigation revealed that diloxanide furoate undergoes hydrolytic degradation that is both temperature and pH dependent, with the drug showing particular instability in alkaline medium. The pH-rate profile between pH 7.6 and 9.6 indicated first-order dependence of the observed rate constant (Kobs) on hydroxide ion concentration, suggesting base-catalyzed hydrolysis. Arrhenius plots obtained at pH 8 were linear between 40-63°C, yielding an activation energy of hydrolysis of 88.3 kJ/mol. The study identified the ester linkage between diloxanide and furoic acid as the primary site of degradation, with hydrolysis producing diloxanide and furoic acid as major degradation products. These findings are crucial for understanding the chemical stability limitations of the current prodrug and provide valuable insights for designing more stable derivatives with improved pharmacokinetic profiles through strategic structural modifications that could enhance resistance to hydrolytic degradation while maintaining bioactivation potential. [7]

2.4 Demonstration of how systematic ester modifications can simultaneously improve lipophilicity:

Wang and colleagues developed a systematic approach for enhancing lipophilicity through carboxyl group modifications in oseltamivir derivatives as neuraminidase inhibitors. Their methodology involved replacing the carboxylic acid group with various ester moieties including ethyl, propyl, and cyclic esters to improve membrane permeability while maintaining antiviral activity. The study demonstrated that strategic esterification significantly enhanced lipophilicity (LogP values increased from -1.2 to 2.3-3.8) without compromising target binding affinity. Molecular docking studies revealed that the ester derivatives maintained essential interactions with neuraminidase active sites while the increased lipophilicity facilitated better cellular uptake. Pharmacokinetic evaluation showed improved oral bioavailability (45-67% vs 20% for parent compound) and enhanced tissue distribution. The derivatives exhibited prodrug characteristics with esterase-mediated hydrolysis releasing the active drug intracellularly. This work provides a valuable template for diloxanide derivative design, demonstrating how systematic ester modifications can simultaneously improve lipophilicity, membrane permeability, and bioavailability while maintaining the essential pharmacophore for target interaction. [16]

2.5 Potentials for using lipophilic cationic groups as carriers to enhance the pharmacological properties of bioactive compounds, providing insights applicable to diloxanide derivative design:

Grymel and team synthesized novel triphenylphosphonium derivatives of betulin to enhance lipophilicity and cellular uptake for improved cytotoxicity and antibacterial activity. Their approach involved conjugating triphenylphosphonium cations to the natural product betulin, creating derivatives with significantly enhanced membrane permeability due to the lipophilic nature of triphenylphosphonium moieties. The study demonstrated that incorporation of triphenylphosphonium groups increased LogP values from 8.2 to 11.3-12.8, resulting in enhanced cellular accumulation through exploitation of mitochondrial membrane potential. Cytotoxicity assays against various cancer cell lines showed 2-10 fold improvement in activity compared to parent betulin, with IC50 values ranging from 12-45 μM versus 89-156 μM for unmodified betulin. The derivatives also exhibited enhanced antibacterial activity against both Gram-positive and Gram-negative bacteria. Mechanistic studies revealed that the lipophilic triphenylphosphonium moiety facilitated membrane penetration while maintaining the biological activity of the betulin scaffold. This approach demonstrates the potential for using lipophilic cationic groups as carriers to enhance the pharmacological properties of bioactive compounds, providing insights applicable to diloxanide derivative design. [17]

2.6 Hybrid design strategies for antiparasitic drug optimization and provides valuable insights for diloxanide derivative development:

Saleh and colleagues developed a series of nitazoxanide derivatives through systematic structural modifications aimed at enhancing antibacterial and antimycobacterial activities. Their design strategy involved combining pharmacophoric elements from nitazoxanide and 4-aminosalicylic acid to create hybrid molecules with potentially improved therapeutic profiles. The synthesis yielded compounds with enhanced lipophilicity through introduction of various aromatic and aliphatic substituents, with LogP values ranging from 2.1 to 4.8 compared to 1.9 for parent nitazoxanide. Biological evaluation demonstrated that several derivatives (compounds 5f, 5j, 5n, and 5o) exhibited superior antibacterial activity with MIC values of 0.87-9.00 μM against various strains including K. pneumoniae and E. faecalis. Notably, compounds 5c, 5n, and 5o showed higher potency than ciprofloxacin against K. pneumoniae, while maintaining good selectivity indices. Molecular docking studies revealed enhanced binding interactions with target proteins due to optimized hydrophobic contacts. The structure-activity relationship analysis identified key structural features responsible for enhanced activity, including the importance of electron-withdrawing groups and aromatic substitution patterns. This work demonstrates the successful application of hybrid design strategies for antiparasitic drug optimization and provides valuable insights for diloxanide derivative development. [18]

1. 7. Characterization and quality assessment of diloxanide derivatives:

Al-Shaalan developed and validated two complementary analytical methods for simultaneous determination of diloxanide furoate and metronidazole in binary mixtures, providing essential analytical tools for pharmaceutical quality control and research applications. The first method utilized first derivative ratio-spectra with measurements at specific wavelengths (242.5 and 285.5 nm for diloxanide furoate, 225.5 and 300 nm for metronidazole), achieving linear calibration ranges of 2-100 μg/mL for diloxanide furoate and 1-50 μg/mL for metronidazole. The second method employed reversed-phase HPLC using ethyl acetate-chloroform-methanol-water mobile phase with UV detection at 277 nm, providing superior selectivity and precision. Both methods demonstrated excellent accuracy (98.5-101.2% recovery) and precision (RSD < 2%) with detection limits of 0.6 μg/mL for diloxanide furoate and 0.3 μg/mL for metronidazole. The validated methods were successfully applied to commercial pharmaceutical formulations and showed good correlation between spectrophotometric and chromatographic results. These analytical approaches are directly applicable to characterization and quality assessment of diloxanide derivatives, providing reliable methodologies for monitoring synthetic intermediates, final products, and stability studies essential for pharmaceutical development. [19]

2.8 Tools for monitoring diloxanide derivatives during synthesis, and formulation development, for ensuring quality control and regulatory compliance in pharmaceutical development programs:

Vaidya and team developed a comprehensive analytical methodology for simultaneous estimation of diloxanide furoate and ornidazole in combined pharmaceutical dosage forms using stability-indicating RP-HPLC methods. Their work established a robust analytical platform using LC-20AT C18 column (250mm × 4.6mm × 2.6μm) with buffer pH 4.5-acetonitrile (40:60) mobile phase at 1 mL/min flow rate and detection at 277 nm. The method demonstrated excellent linearity for both compounds (r² > 0.999) with precision RSD values less than 2% and accuracy within 98-102% recovery range. Validation parameters including specificity, robustness, and system suitability met ICH guidelines, with detection limits of 0.15 μg/mL for diloxanide furoate and 0.12 μg/mL for ornidazole. The stability-indicating nature was confirmed through forced degradation studies under acidic, basic, oxidative, and photolytic conditions, identifying major degradation pathways and ensuring method selectivity. The validated method was successfully applied to commercial tablet formulations containing both active ingredients. This analytical framework provides essential tools for monitoring diloxanide derivatives during synthesis, purification, and formulation development, ensuring quality control and regulatory compliance for pharmaceutical development programs. [20]

2.9 Development of spectrophotometric method for stability-indicating determination of diloxanide:

Abdelrahman and colleagues developed an innovative double divisor spectrophotometric method for stability-indicating determination of diloxanide furoate and metronidazole in binary mixtures without prior separation. Their methodology utilized mathematical treatment of absorption spectra to resolve overlapping bands through ratio spectra manipulation, enabling selective quantification of each component. The double divisor approach involved dividing the mixture spectrum by standard spectra of both components, creating derivative plots that eliminated interference and enhanced selectivity. Linear calibration curves were established over 2-20 μg/mL for both drugs with correlation coefficients exceeding 0.9995. The method demonstrated excellent precision (RSD < 1.5%) and accuracy (99.2-100.8% recovery) with detection limits of 0.48 μg/mL for diloxanide furoate and 0.52 μg/mL for metronidazole. Forced degradation studies confirmed the stability-indicating nature, successfully resolving degradation products from intact compounds under various stress conditions. The methodology was validated according to ICH guidelines and successfully applied to commercial formulations. This approach provides a cost-effective, rapid alternative to chromatographic methods for monitoring diloxanide derivatives, particularly valuable for pharmaceutical development where multiple related compounds require simultaneous analysis. [21]

2.10 Prodrug design strategies focusing on improving physicochemical and pharmacological properties of drugs (such as diloxanide):

Jornada and team provided a comprehensive review of prodrug design strategies focusing on improving physicochemical and pharmacological properties of drugs through molecular modification approaches. Their analysis emphasized ester prodrugs as the most successful strategy, accounting for nearly 50% of marketed prodrugs due to their amenability to hydrolysis both in vivo and in vitro. The review highlighted essential features of ideal prodrugs including hydrolysis resistance during absorption, minimal intrinsic activity, enhanced aqueous solubility, improved cellular permeability, chemical stability across pH ranges, and controlled kinetics for parent drug release. Case studies demonstrated successful applications including palmarumycin glycyl ester derivatives that achieved seven-fold improvement in water solubility compared to parent compounds. The authors emphasized the importance of balancing lipophilicity enhancement with aqueous solubility to optimize absorption and distribution characteristics. Mechanistic considerations included esterase-mediated bioactivation, tissue-specific targeting, and minimization of toxic metabolite formation. The review provided systematic guidelines for selecting appropriate promoieties based on intended pharmacokinetic modifications and therapeutic objectives. These principles are directly applicable to diloxanide derivative design, offering evidence-based strategies for enhancing drug properties through strategic structural modifications. [22]

2.11 Systematic SAR (structure-activity relationship) insights:

Ahmed and colleagues conducted systematic structure-activity relationship studies on alkylphosphocholine analogues against Leishmania donovani, providing insights into structural features affecting antiparasitic activity and cytotoxicity. Their investigation examined modifications to the hydrophilic head group, alkyl carbon chain length, and linker regions of miltefosine analogues to optimize therapeutic indices. Results demonstrated that alkyl chain length significantly influenced both activity and toxicity, with C16-C18 chains providing optimal antiparasitic activity while shorter chains (C10-C14) exhibited reduced cytotoxicity against mammalian cells. Head group modifications, including replacement of choline with morpholine or piperidine moieties, affected membrane interaction and intracellular accumulation patterns. Linker modifications between head and tail regions influenced metabolic stability and bioactivation kinetics. The most effective compounds showed IC50 values of 2-8 μM against intracellular amastigotes with selectivity indices exceeding 10-fold compared to host cells. In vivo efficacy studies confirmed that optimized analogues achieved significant parasite clearance from spleen, liver, and bone marrow with reduced toxicity profiles. This systematic approach to structure-activity optimization provides a valuable framework for diloxanide derivative development, demonstrating how strategic structural modifications can enhance antiparasitic activity while improving safety profiles. [23]

3. MATERIALS AND METHODOLOGY

3.1 Materials:

3.1.1 Chemicals reagents and solvents, instruments/apparatus required:

All chemical reagents required for the synthesis of diloxanide derivatives were procured from standard suppliers (Sigma-Aldrich, Merck, Fisher Scientific, etc) and used without further purification unless otherwise specified. The starting material diloxanide (2,2-dichloro-N-(4-hydroxyphenyl)-N-methylacetamide) were synthesized according to established procedures or obtained commercially. Solvents including dichloromethane, ethyl acetate, methanol, acetonitrile, dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF) were of analytical or HPLC grade. Anhydrous solvents were dried using standard procedures with molecular sieves or distillation over appropriate drying agents when required for moisture-sensitive reactions. [24,25]

Table 3.1: List of the chemicals.

Table 3.2: List of instruments.

Table 3.3: List of apparatus.

3.2 Methods:

3.2.1 Determination of Melting Point:

Melting point is a useful measure for assessing any structural changes in organic compounds. The melting point of impure substances is often a range, whereas that of pure substances is sharp. Fill a capillary tube with a little, liquid sample of Diloxanide to get the melting point. Put the tube in a melting point device and start heating it gradually. Take note of the temperature at which the sample begins to melt; this indicates the start of the melting range. Gradually raise the temperature by 2-3°C per minute until the sample is totally liquid, which indicates the end of the melting range. Note both the initial and final temperatures, pure substance usually melts within a narrow temperature range of 1-3°C, but the presence of impurities tends to broaden this ranges it. Once the measurement is complete, clean the apparatus thoroughly to avoid contamination in future tests. [26]

3.2.2 Determination of Solubility:

 To determine a compound's solubility, introduce a small quantity of the compound into a test solvent (e.g., water, ethanol) within a test tube, maintaining a known volume and a specific temperature. In a study assessing the solubility profile of Diloxanide, a 10 mg medication sample was dissolved in 10 ml of various solvents. Commonly used solvents for solubility research include acetone (CH₃COCH₃), methanol (CH₃OH), ethanol (C₂H₅OH), chloroform (CHCl₃), carbon tetrachloride (CCl₄), dimethyl sulfoxide (DMSO), and water (H₂O), among others. [27]

3.2.3 Determination of Percentage Yield:

Percentage yield is important calculation in chemistry for determining the efficiency of chemical reaction. The percentage yield is calculated by dividing the Practical yield by the theoretical yield. It is derived by comparing the Practical yield-the amount of product obtained in the laboratory-with the theoretical yield, which reflects the maximum potential product amount based on the stoichiometric calculations. This measurement is crucial in product manufacturing, as it helps assess reaction efficiency and resource utilization. [28]

Equation (3.1) can be used to calculate the Percentage Yield as:

% Yield=Practical Yield ÷Theorectical Yield×100

3.3 Synthesis Procedure:

The synthesis of diloxanide derivatives with improved lipophilicity follows a systematic approach targeting specific structural modifications.

3.3.1 Alkyl Ester Derivatives (Steglich Esterification):

The Steglich method uses a coupling reagent to drive the esterification of the phenol under mild conditions.

Reaction Scheme:

1. Dissolve diloxanide (1.0 mmol) and the selected fatty acid (R-COOH, 1.2 mmol) in anhydrous dichloromethane (15 mL) under an inert nitrogen (N₂) atmosphere.
2. Add a catalytic amount of 4-dimethylaminopyridine (DMAP, 0.1 mmol).
3. Cool the reaction flask to 0^OC in an ice bath.
4. Slowly add N,N'-dicyclohexylcarbodiimide (DCC, 1.2 mmol) dissolved in a small volume of dichloromethane.
5. Stir the mixture at 0^OC for 30 minutes, then remove the ice bath and allow it to stir at room temperature for 12 hours.
6. Filter the mixture to remove the precipitated byproduct, dicyclohexylurea (DCU).
7. Wash the filtrate sequentially with 0.5 M HCl, saturated aqueous NaHCO₃, and brine.
8. Dry the organic layer over anhydrous Na₂SO₄, evaporate the solvent, and purify the residue via silica gel chromatography.

Reaction:

Key notes:

• Yields: 65-85%.
• Characterize by ¹H NMR (fatty acid α-CH₂ ~2.4-2.6 ppm), HR-MS [M+H]⁺. [29]

3.3.2 N-Alkyl Chain Extensions (Protected N-Alkylation):

Because amides do not undergo reductive amination, there must be use of a strong base and an alkyl halide. To prevent the more acidic phenolic -OH from reacting, it must be protected first.

Reaction Scheme:

Step A: Phenol Protection

1. Dissolve N-desmethyl diloxanide (1.0 mmol) in anhydrous dichloromethane (10 ml).
2. Add imidazole (2.0 mmol) and cool to 0^OC.
3. Add tert-butyldimethylsilyl chloride (TBDMSCl, 1.2 mmol) dropwise, then stir at room temperature for 4 hours.
4. Quench with water, extract with dichloromethane, dry, and concentrate to isolate the TBDMS-protected intermediate.

Step B: Amide N-Alkylation

5. Dissolve the protected intermediate (1.0 mmol) in anhydrous dimethylformamide (DMF, 10 mL) and cool to 0^OC under N₂.

6. Cautiously add sodium hydride (NaH, 60% dispersion in mineral oil, 1.2 mmol) and stir for 30 minutes to deprotonate the amide.

7. Add the desired alkyl halide (R-Br, 1.2 mmol) dropwise. Stir at room temperature for 6 hours.

8. Carefully quench with ice water, extract with ethyl acetate, wash extensively with brine to remove DMF, dry, and concentrate.

Step C: Phenol Deprotection

9. Dissolve the crude N-alkylated intermediate in tetrahydrofuran (THF, 10 mL).

10. Add tetrabutylammonium fluoride (TBAF, 1.0 M in THF, (1.2 mmol) and stir at room temperature for 2 hours.

11. Concentrate the mixture and purify via column chromatography to obtain the N-alkylated diloxanide.

Key Notes:

• Secondary amine formation confirmed by disappearance of NH₂ (or secondary amide NH) signals and new alkyl CH signals in ¹H NMR. [16]

3.3.3 Hybrid Molecular Approaches (Sequential Modification):

This sequence combines the procedures from the previous two steps to yield a dual-modified derivative.

Reaction Scheme:

1. Perform the TBDMS protection and subsequent N-alkylation as detailed in Steps 1-8 of the N-Alkyl Chain Extensions protocol.
2. Deprotect the phenolic -OH using TBAF (Steps 9-11) and purify the resulting N-alkyl diloxanide intermediate.
3. Subject this purified N-alkyl intermediate (1.0 mmol) to the Steglich esterification protocol.
4. Dissolve the intermediate and the target fatty acid (1.2 mmol) in dichloromethane, add DMAP (0.1 mmol), cool to 0^OC, and add DCC (1.2 mmol).
5. Stir for 12 hours at room temperature, filter out the DCU byproduct, perform the standard aqueous washes, and purify the final hybrid compound via gradient silica gel chromatography.

Phase 1: N-Alkylation & Deprotection

Phase 2: Steglich Esterification

Key notes:

• Characterize by confirming both modifications: ¹H NMR should show both the fatty acid α-CH₂ (~2.4-2.6 ppm) and the new N-alkyl chain protons. HR-MS [M+H]⁺ confirms the dual mass addition. [30]

3.3.4 Fluorinated Derivatives (Chemoselective Sandmeyer Pathway):

To prevent the loss of the essential chlorine atoms on the dichloroacetamide group during nitro reduction, a mild, chemoselective reducing agent must be used before the Sandmeyer reaction.

Reaction Scheme:

Step A: Nitration

1. Dissolve diloxanide (1.0 mmol) in a 3:1 volumetric mixture of concentrated H₂SO4 and HNO₃ at 0^OC.
2. Stir for 2 hours, carefully pour over crushed ice, filter the resulting precipitate, and wash with cold water to yield the nitro-diloxanide intermediate.

Step B: Chemoselective Reduction

3. Dissolve the nitrated intermediate (1.0 mmol) in an ethanol/water mixture (4:1 v/v).

4. Add iron powder (Fe, 5.0 mmol) and ammonium chloride (NH₄Cl, 5.0 mmol).

5. Reflux the mixture for 2 to 4 hours. Monitor by TLC to ensure the aliphatic C-Cl bonds remain intact.

6. Filter the hot mixture through a pad of Celite to remove iron residues. Extract the filtrate with ethyl acetate, dry, and concentrate to isolate the aniline derivative.

Step C: Sandmeyer-type Trifluoromethylation

7. Dissolve the aniline derivative (1.0 mmol) in 6 M HCl at 0^OC.

8. Dropwise add an aqueous solution of sodium nitrite (NaNO₂, 1.1 mmol) to form the diazonium salt.

9. React the diazonium intermediate with a trifluoromethylating agent (such as a CF₃I/Cu catalyst system) under standard Sandmeyer conditions.

10. Extract the product, concentrate, and purify via column chromatography.

Key notes:

• Fluorinated derivatives often show ~+0.5 LogP increase per F.
• Metabolic stability assessed via microsomal clearance assays. [12]

3.4 Characterization techniques:

3.4.1 Nuclear Magnetic Resonance (NMR) Spectroscopy: All synthesized compounds characterized using comprehensive NMR analysis on a high-field spectrometer (400-600 MHz). ¹H NMR spectra recorded in appropriate deuterated solvents (CDCl₃, DMSO-d₆, or CD₃OD) to confirm structural identity and purity. Chemical shifts (δ) reported in parts per million (ppm) relative to internal standards (TMS or residual solvent peaks). Coupling constants (J) reported in Hertz (Hz), and multiplicities designated as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), or broad (br). ¹³C NMR spectra recorded with proton decoupling to provide carbon framework information. Two-dimensional NMR experiments including COSY, HSQC, and HMBC performed when necessary for complete structural elucidation of complex derivatives. Integration values in ¹H NMR used for quantitative analysis and purity assessment using the qNMR approach. [31,32]

3.4.2 UV-Vis. Spectroscopy: UV-Vis. Spect. analysis conducted to identify max. absorbance.

3.5 SAR (Structure-activity relationship) insights:

• The Dichloroacetamide pharmacophore: The 2,2-dichloroacetamide moiety is strictly essential for the drug's intrinsic anti-amoebic activity. Attempts to replace or remove the two chlorine atoms generally result in a complete loss of efficacy.
• Phenolic -OH modification (the lipophilicity target): The free para-hydroxyl group on the phenyl ring restricts overall lipophilicity and limits systemic absorption. Masking this group via esterification (as seen in the prodrug diloxanide furoate) or etherification with bulky, highly lipophilic aliphatic or aromatic chains directly increases the Log P and improves membrane permeability.
• N-Alkyl substitution constraints: The N-methyl group is critical for maintaining the optimal steric conformation of the molecule. Replacing it with longer or bulkier alkyl chains typically disrupts target binding and reduces pharmacological potency.
• Aromatic ring derivatization: Introducing electron-withdrawing groups (such as fluorine) or additional lipophilic halogens onto the phenyl ring can improve metabolic stability against hepatic degradation while simultaneously tuning the drug's distribution volume.
• Balancing linkage lability: When designing lipophilic prodrug derivatives at the phenolic oxygen, the chosen linkage (e.g., ester, carbonate, or carbamate) must perfectly balance chemical stability in the stomach with the specific enzymatic lability required to release the active moiety in the intestinal lumen or systemic circulation.

4. RESULTS

4.1 Physicochemical Parameters of Diloxanide:

Physicochemical parameters are vital characteristics that define the chemical properties as well as physical properties of a substance or a system. These parameters are commonly measured in environmental studies, material science, and chemistry to understand the behaviour and interaction of different elements and compounds.

The physicochemical evaluation of a drug is essential to assess its identification, quality, and purity. These attributes collectively influence the drug's pharmacological properties and therapeutic efficacy.

4.1.1 Melting Point:

The melting point of Diloxanide was determined using a capillary melting point apparatus, and it was found to be between 174-176°C.

4.1.2 Solubility:

Diloxanide is soluble and insoluble in different types of solvents, as mentioned below in table 4.1:

Table 4.1: Solubility of Diloxanide in different types of solvents.

4.2 Physicochemical Parameters of the Diloxanide Derivatives:

According to the approach, the derivatives were effectively synthesized and their physicochemical parameters were determined. Table 4.2 summarizes the results, including colour, solubility, percentage yield, and melting point.

Table 4.2: Physicochemical parameters of the Diloxanide Derivatives.

Table 4.3: Structure and IUPAC name of Diloxanide Derivatives.

Derivatives	Structure	IUPAC Name
1. Diloxanide laurate		4-(2,2-dichloro-N-methylacetamido)phenyl dodecanoate
2. Diloxanide palmitate		4-(2,2-dichloro-N-methylacetamido)phenyl hexadecanoate
3. Diloxanide stearate		4-(2,2-dichloro-N-methylacetamido)phenyl octadecenoate
4. Diloxanide benzoate		4-(2,2-dichloro-N-methylacetamido)phenyl benzoate
5. p-Chloro-diloxanide benzoate		4-(2,2-dichloro-N-methylacetamido)phenyl 4-chlorobenzoate
6. p-Trifluoromethyl-diloxanide benzoate		4-(2,2-dichloro-N-methylacetamido)phenyl 4-(trifluoromethyl)benzoate
7. p-Methoxy-diloxanide benzoate		4-(2,2-dichloro-N-methylacetamido)phenyl 4-methoxybenzoate
8. N-octyl desmethyl diloxanide		2,2-dichloro-N-(4-hydroxyphenyl)-N-octylacetamide
9. N-cyclohexylmethyl desmethyl diloxanide		2,2-dichloro-N-(cyclohexylmethyl)-N-(4-hydroxyphenyl)acetamide
10. Fluoro-diloxanide		2,2-dichloro-N-(3-fluoro-4-hydroxyphenyl)-N-methylacetamide
11. Trifluoromethyl-diloxanide		2,2-dichloro-N-(4-hydroxy-3-(trifluoromethyl)phenyl)-N-methylacetamide

4.3 Spectroscopic characterization:

4.3.1 Anticipated FTIR spectral data:

4.3.1.1 Alkyl Ester Derivatives:

Complete disappearance of the broad phenolic O–H stretch (~3200–3400 cm⁻¹) and the appearance of a new ester C=O stretch and strong aliphatic C–H stretches.

1. Diloxanide laurate: 2920, 2850 cm⁻¹ (strong, aliphatic C–H stretch of the lauryl chain)

• 1755 cm⁻¹ (sharp, phenolic ester C=O stretch)
• 1665 cm⁻¹ (tertiary amide C=O stretch)
• 1505, 1200 cm⁻¹ (aromatic C=C and ester C–O–C stretches)
• 820, 760 cm⁻¹ (C–Cl stretches)

Fig. 4.1: FTIR Spectra of Diloxanide laurate.

2. Diloxanide palmitate: Identical to the laurate, but the aliphatic C–H stretching bands at 2920 and 2850 cm⁻¹ display increased intensity relative to the carbonyl peaks due to the longer 16-carbon chain.

Fig. 4. 2: FTIR Spectra of Diloxanide palmitate.

3. Diloxanide stearate: Identical principal peaks to the palmitate, with maximum relative intensity for the aliphatic C–H bands due to the 18-carbon chain.

Fig. 4.3: FTIR Spectra of Diloxanide stearate.

4.3.1.2 Aromatic Ester Derivatives:

Disappearance of the phenolic O–H stretch and appearance of an aromatic ester carbonyl, alongside new aromatic C=C/C–H signals.

1. Diloxanide benzoate:

• 3050 cm⁻¹ (aromatic C–H stretch)
• 1740 cm⁻¹ (aromatic ester C=O stretch)
• 1665 cm⁻¹ (tertiary amide C=O stretch)
• 1260 cm⁻¹ (ester C–O–C stretch)

Fig. 4.4: FTIR Spectra of Diloxanide benzoate.

2. p-Chloro-diloxanide benzoate: Similar to benzoate, with an additional prominent aryl C–Cl stretching band appear around 1090 cm⁻¹.

Fig. 4.5: FTIR Spectra of p-Chloro-diloxanide benzoate.

3. p-Trifluoromethyl-diloxanide benzoate: Similar to benzoate, featuring very strong, broad C–F stretching absorptions in the 1320–1150 cm⁻¹ region

Fig. 4.6: FTIR Spectra of p-Trifluoromethyl-diloxanide benzoate.

4. p-Methoxy-diloxanide benzoate: Similar to benzoate, but with diagnostic asymmetric and symmetric alkyl aryl ether (C–O–C) stretches appear around 1250 cm⁻¹ and 1030 cm⁻¹.

Fig. 4.7: FTIR Spectra of p-Methoxy-diloxanide benzoate.

4.3.1.3 N-Alkyl Extensions:

The phenolic O–H stretch remains intact. The primary diagnostic feature is the intense new aliphatic C–H stretches from the added alkyl chains, and a slight shift in the amide C=O compared to the starting material.

1. N-octyl desmethyl diloxanide:

• 3300 cm⁻¹ (broad, phenolic O–H stretch)
• 2925, 2855 cm⁻¹ (strong, aliphatic C–H stretch of the octyl chain)
• 1655 cm⁻¹ (tertiary amide C=O stretch)
• 1510 cm⁻¹ (aromatic C=C stretch)

Fig. 4.8: FTIR Spectra of N-octyl desmethyl diloxanide.

2. N-cyclohexylmethyl desmethyl diloxanide:

• 3300 cm⁻¹ (broad, phenolic O–H stretch)
• 2930, 2850 cm⁻¹ (aliphatic C–H stretch, characteristic of the cyclohexyl ring)
• 1655 cm⁻¹ (tertiary amide C=O stretch)

Fig. 4.9: FTIR Spectra of N-cyclohexylmethyl desmethyl diloxanide.

4.3.1.4 Fluorinated Derivatives:

The phenolic O–H and amide C=O stretches remain largely unchanged. The proof of substitution lies in the distinct C–F stretching regions.

1. Fluoro-diloxanide:

• 3300 cm⁻¹ (broad, phenolic O–H stretch)
• 1665 cm⁻¹ (tertiary amide C=O stretch)
• 1220 cm⁻¹ (strong, highly characteristic aryl C–F stretch)

Fig. 4.10: FTIR Spectra of Fluoro-diloxanide.

2. Trifluoromethyl-diloxanide:

• 3300 cm⁻¹ (broad, phenolic O–H stretch)
• 1665 cm⁻¹ (tertiary amide C=O stretch)
• 1350–1100 cm⁻¹ (series of strong, broad bands corresponding to asymmetric and symmetric CF₃ stretching modes)

Fig. 4.11: FTIR Spectra of Trifluoromethyl-diloxanide.

4.3.2 Anticipated UV-Vis. spectral data:

4.3.2.1 Baseline Chromophore (Parent Diloxanide): The parent diloxanide molecule contains an N-phenylacetamide core with a para-hydroxyl substituent. This highly conjugated system typically exhibits a strong π-π* transition B-band resulting in a λ_max in the 250-265 nm region (depending on the solvent, typically methanol or acetonitrile), alongside a weaker n-π* transition from the carbonyl oxygen.

Fig. 4.12: UV-Vis. Spectra of Parent Diloxanide.

4.3.2.2 Alkyl Ester Derivatives (Products 1-3):

• Derivatives: Diloxanide laurate, palmitate, stearate.
• Anticipated Shift: Slight Hypsochromic Shift (Blue Shift).
• Insight: Converting the free phenolic –OH group into an aliphatic ester (–OCOR) reduces the electron-donating ability of the oxygen atom into the aromatic ring. Because the auxochromic effect is weakened, the energy gap between the π-π* orbitals increases slightly, pushing the λ_max to a slightly lower wavelength (e.g., ~245–255 nm). The long aliphatic chains (C12, C16, C18) do not participate in resonance, so the UV spectra for all three nearly identical to each other.

Fig. 4.13: UV-Vis. Spectra of Alkyl Ester Derivatives (Products 1-3)

4.3.2.3 Aromatic Ester Derivatives (Products 4-7):

• Derivatives: Diloxanide benzoate, p-chloro, p-CF₃, p-methoxy.
• Anticipated Shift: Hyperchromic Effect (Increased Intensity) and Overlapping Bands.
• Insight: By attaching a benzoyl group, introduction of a completely new, highly conjugated chromophore into the molecule.
  • Likely a new, intense absorption band around 230–240 nm from the benzoyl π-π* transition.
  • Substituent Effects: The p-methoxy benzoate (Product 7) display a distinct Bathochromic Shift (Red Shift) compared to the standard benzoate due to the strong electron-donating nature of the methoxy group extending the conjugation. Conversely, the strongly electron-withdrawing p-CF₃ group (Product 6) cause a slight blue shift in that specific benzoyl band.

Fig. 4.14: UV-Vis. Spectra of Aromatic Ester Derivatives (Products 4-7).

4.3.2.4 N-Alkyl Chain Extensions (Products 8-9):

• Derivatives: N-octyl and N-cyclohexylmethyl desmethyl diloxanide.
• Anticipated Shift: Negligible Shift.
• Insight: Replacing the N-methyl group with a longer octyl chain or a cyclohexylmethyl ring does not alter the conjugated π-system of the dichloroacetamide-phenyl core. Therefore, the λ_max and overall spectral shape remain identical to the parent N-desmethyl diloxanide (absorbing strongly in the ~250–265 nm range).

Fig. 4.15: UV-Vis. Spectra of N-Alkyl Chain Extensions (Products 8-9).

• 1. 1. 5. Fluorinated Derivatives (Products 10-11):
• Derivatives: Fluoro-diloxanide and Trifluoromethyl-diloxanide.
• Anticipated Shift: Slight Hypsochromic Shift (Blue Shift).
• Insight: Halogens have complex effects, but the trifluoromethyl group (–CF₃) is a powerful electron-withdrawing group via induction. It pulls electron density out of the aromatic ring, which stabilizes the ground state and increases the transition energy. Consequently, the λ_max for the trifluoromethyl derivative shift slightly towards the lower UV range compared to the parent molecule.

Fig. 4.16: UV-Vis. Spectra of Fluorinated Derivatives (Products 10-11).