Abstract

Pelanserin, systematically named 3-[3-(4-phenylpiperazin-1-yl)propyl]quinazoline-2,4(1H,3H)-dione (C₂₁H₂₄N₄O₂), represents a synthetic organic compound belonging to the quinazoline class with a molecular weight of 364.44 g·mol⁻¹. This heterocyclic compound exhibits a complex molecular architecture featuring both quinazoline-dione and phenylpiperazine moieties connected through a propyl linker. Pelanserin demonstrates characteristic physical properties including a crystalline solid state at standard temperature and pressure, moderate solubility in polar organic solvents, and distinct spectroscopic signatures. The compound's chemical behavior is governed by multiple nitrogen-containing functional groups that confer both basic and hydrogen-bonding capabilities. Its synthesis typically proceeds through nucleophilic substitution reactions between isatoic anhydride derivatives and appropriately functionalized piperazine precursors. Pelanserin serves as an important intermediate in medicinal chemistry research and represents a structurally interesting example of nitrogen-rich heterocyclic systems.

Introduction

Pelanserin (CAS Registry Number 2208-51-7) constitutes an organic compound of significant interest in synthetic and pharmaceutical chemistry. This nitrogen-containing heterocycle belongs to the quinazoline-dione structural class, characterized by a bicyclic 2,4-dioxoquinazoline system linked to a phenylpiperazine group through a three-carbon aliphatic chain. The compound's molecular formula, C₂₁H₂₄N₄O₂, reflects its substantial nitrogen content and complex heterocyclic nature. First synthesized in the mid-20th century during investigations into nitrogen-containing heterocycles, pelanserin represents a structurally sophisticated molecule that demonstrates the synthetic versatility of quinazoline chemistry. Its development coincided with growing interest in piperazine-containing compounds as potential pharmacologically active agents. The structural combination of electron-rich aromatic systems, multiple nitrogen centers, and flexible alkyl chains creates a molecule with interesting electronic properties and conformational flexibility. Pelanserin serves as a representative example of complex heterocyclic systems that bridge aromatic and aliphatic character while maintaining significant molecular rigidity through its bicyclic core.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of pelanserin features three distinct regions: the quinazoline-2,4-dione bicyclic system, the propyl linker, and the 4-phenylpiperazine moiety. The quinazoline-dione system adopts a nearly planar configuration with bond angles of approximately 120° around the nitrogen and carbon atoms, consistent with sp² hybridization. The carbonyl groups at positions 2 and 4 exhibit typical bond lengths of 1.22 Å for C=O bonds and 1.38 Å for the adjacent C-N bonds. The phenylpiperazine group displays chair conformation for the piperazine ring with the phenyl substituent in equatorial orientation. The propyl linker (CH₂-CH₂-CH₂) provides conformational flexibility with preferred antiperiplanar arrangements around the C-C bonds. Molecular orbital analysis reveals highest occupied molecular orbitals localized on the quinazoline π-system and nitrogen lone pairs, while the lowest unoccupied molecular orbitals show contributions from the carbonyl π* orbitals. The compound exhibits a calculated dipole moment of approximately 3.8 Debye, resulting from the asymmetric distribution of electron-donating and electron-withdrawing groups.

Chemical Bonding and Intermolecular Forces

Covalent bonding in pelanserin involves extensive π-conjugation within the quinazoline system, with bond orders indicating significant electron delocalization. The C₂=O and C₄=O bonds demonstrate typical carbonyl character with bond dissociation energies of approximately 85 kcal·mol⁻¹. The N-H bond in the quinazoline system exhibits a bond length of 1.01 Å with vibrational frequencies characteristic of secondary amides. Intermolecular forces include strong hydrogen bonding capabilities through both donor (N-H) and acceptor (carbonyl oxygen, piperazine nitrogen) sites. The compound forms characteristic hydrogen-bonded dimers through N-H···O=C interactions with typical H-bond distances of 1.9-2.1 Å. Van der Waals interactions contribute significantly to crystal packing, particularly through the phenyl ring systems. The molecule demonstrates moderate polarity with calculated log P values of approximately 2.1, indicating balanced hydrophilic-lipophilic character. London dispersion forces operate between the aromatic systems, with calculated polarizability volumes of 38.6 ų reflecting the compound's substantial electron cloud.

Physical Properties

Phase Behavior and Thermodynamic Properties

Pelanserin exists as a white to off-white crystalline solid at room temperature with a characteristic melting point range of 198-202 °C. The compound exhibits polymorphism with at least two crystalline forms identified, differentiated by their X-ray diffraction patterns and thermal behavior. The stable form demonstrates an enthalpy of fusion of 28.5 kJ·mol⁻¹ and entropy of fusion of 56.2 J·mol⁻¹·K⁻¹. Crystalline density measurements yield values of 1.24 g·cm⁻³ for the most common polymorph. The compound sublimes appreciably above 150 °C under reduced pressure (0.1 mmHg) with a sublimation enthalpy of 89 kJ·mol⁻¹. Solubility characteristics include moderate solubility in chlorinated solvents (dichloromethane: 15 mg·mL⁻¹), lower solubility in alcohols (methanol: 8 mg·mL⁻¹), and limited aqueous solubility (0.2 mg·mL⁻¹ at pH 7). The refractive index of crystalline pelanserin measures 1.62 at 589 nm, consistent with its aromatic character. Thermal gravimetric analysis shows decomposition commencing at approximately 250 °C under nitrogen atmosphere.

Spectroscopic Characteristics

Infrared spectroscopy of pelanserin reveals characteristic absorption bands at 3200 cm⁻¹ (N-H stretch), 1685 cm⁻¹ and 1660 cm⁻¹ (carbonyl stretches), and 1600 cm⁻¹ (aromatic C=C stretches). The fingerprint region between 1500-1000 cm⁻¹ shows multiple bands associated with C-N stretches and aromatic C-H bending vibrations. Proton NMR spectroscopy (400 MHz, DMSO-d₆) displays signals at δ 11.2 ppm (s, 1H, N-H), δ 7.2-8.1 ppm (m, 9H, aromatic), δ 3.8 ppm (t, 2H, N-CH₂), δ 3.2-3.5 ppm (m, 8H, piperazine), δ 2.4 ppm (t, 2H, CH₂-N), and δ 1.8 ppm (quin, 2H, CH₂). Carbon-13 NMR shows carbonyl signals at δ 162.0 and 159.5 ppm, aromatic carbons between δ 115-150 ppm, aliphatic carbons at δ 45-55 ppm (piperazine and N-CH₂), and the central methylene at δ 27.5 ppm. UV-Vis spectroscopy demonstrates absorption maxima at 245 nm (ε = 12,400 M⁻¹·cm⁻¹) and 315 nm (ε = 3,200 M⁻¹·cm⁻¹) in methanol solution. Mass spectrometric analysis shows a molecular ion peak at m/z 364.2 with characteristic fragmentation patterns including loss of the propyl chain (m/z 279.1) and cleavage of the piperazine ring (m/z 174.1).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Pelanserin demonstrates reactivity typical of N-alkylated quinazoline-diones and secondary amines. The quinazoline-2,4-dione system undergoes nucleophilic substitution at the carbonyl carbons under acidic conditions, with measured second-order rate constants of 2.3 × 10⁻³ M⁻¹·s⁻¹ for hydroxide ion attack at pH 12. The piperazine nitrogen atoms exhibit basic character with protonation occurring preferentially at the aliphatic nitrogen rather than the quinazoline system. Hydrolysis studies reveal relative stability at neutral pH (t₁/₂ > 1000 hours at 25 °C) but accelerated degradation under strongly acidic or basic conditions. Oxidation reactions proceed primarily at the piperazine nitrogens with hydrogen peroxide, yielding N-oxide derivatives with first-order kinetics (k = 4.7 × 10⁻⁵ s⁻¹ at 25 °C). The compound demonstrates thermal stability up to 200 °C with decomposition following first-order kinetics (Eₐ = 105 kJ·mol⁻¹). Photochemical degradation occurs under UV irradiation (λ = 254 nm) with quantum yield of 0.03 for ring-opening reactions. The propyl linker undergoes free radical halogenation at the central carbon with relative reactivity 0.8 compared to n-propane.

Acid-Base and Redox Properties

Pelanserin exhibits two ionization centers with measured pKₐ values of 3.2 (quinazoline N-H) and 8.7 (piperazine nitrogen). The isoelectric point occurs at pH 6.0, where the molecule exists predominantly as a zwitterion. Protonation studies show formation of monocations at acidic pH with the charge localized on the piperazine nitrogen. Buffer capacity calculations indicate maximum buffering around pH 3.2 and pH 8.7 with β values of 0.02 and 0.03, respectively. Redox properties include a one-electron oxidation potential of +1.12 V versus SCE, corresponding to removal of an electron from the quinazoline π-system. Reduction occurs in two one-electron steps at -1.35 V and -1.82 V versus SCE, associated with sequential addition of electrons to the carbonyl groups. The compound demonstrates stability toward common oxidizing agents including atmospheric oxygen but undergoes slow oxidation with strong oxidants like potassium permanganate. Cyclic voltammetry shows quasi-reversible redox behavior with peak separation of 85 mV for the first reduction wave.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The synthesis of pelanserin typically proceeds through a two-step sequence beginning with isatoic anhydride (1) as the quinazoline precursor. Reaction with 1-(3-aminopropyl)-4-phenylpiperazine (2) under phosgene or phosgene-equivalent conditions provides the target compound in moderate yields. Optimized conditions employ triphosgene (0.35 equivalents) in dichloromethane at 0-5 °C, followed by gradual warming to room temperature over 4 hours. The reaction proceeds through nucleophilic attack of the primary amine on the anhydride carbonyl, ring opening, and subsequent cyclization with loss of carbon dioxide. Typical isolated yields range from 55-65% after purification by recrystallization from ethanol-water mixtures. Alternative synthetic approaches include direct condensation of anthranilic acid derivatives with the appropriate amine using carbonyldiimidazole as activating agent, though this method gives lower yields (40-45%). Purification methods typically involve column chromatography on silica gel (eluent: ethyl acetate/methanol 9:1) followed by recrystallization. The final product characterization includes melting point determination, elemental analysis, and spectroscopic confirmation (IR, NMR, MS).

Analytical Methods and Characterization

Identification and Quantification

Analytical identification of pelanserin employs multiple complementary techniques. High-performance liquid chromatography with UV detection at 245 nm provides reliable quantification with retention times of 6.8 minutes on C18 columns using acetonitrile-water (65:35) mobile phase containing 0.1% trifluoroacetic acid. Method validation demonstrates linear response from 0.1-100 μg·mL⁻¹ with detection limit of 0.05 μg·mL⁻¹ and quantification limit of 0.15 μg·mL⁻¹. Gas chromatography-mass spectrometry employing capillary columns (DB-5MS) shows good separation with retention indices of 2850 relative to n-alkanes. Thin-layer chromatography on silica gel GF₂₅₄ plates with chloroform-methanol-ammonia (80:15:5) development gives Rf values of 0.45 with visualization under UV light at 254 nm. Spectrophotometric methods based on complex formation with cobalt thiocyanate allow quantification in the range of 1-20 μg·mL⁻¹ with molar absorptivity of 12,300 M⁻¹·cm⁻¹ at 625 nm. X-ray powder diffraction provides characteristic patterns with intense reflections at 2θ = 12.4°, 16.8°, 19.2°, and 22.6° for the most common crystalline form.

Purity Assessment and Quality Control

Purity assessment of pelanserin typically employs HPLC with peak area normalization, requiring minimum purity of 98.0% for research standards. Common impurities include starting materials (isatoic anhydride ≤0.2%, 1-(3-aminopropyl)-4-phenylpiperazine ≤0.3%), hydrolysis products (anthranilic acid derivatives ≤0.5%), and N-oxide derivatives (≤0.2%). Residual solvent content determined by gas chromatography must meet ICH guidelines with limits of 500 ppm for dichloromethane and 3000 ppm for ethanol. Elemental analysis requirements specify carbon 69.21±0.3%, hydrogen 6.64±0.2%, nitrogen 15.38±0.3%, and oxygen 8.78±0.3%. Loss on drying measures less than 0.5% after 2 hours at 105 °C. Heavy metal content determined by ICP-MS shows limits of less than 10 ppm for individual metals and less than 20 ppm total. Stability studies indicate shelf life of 36 months when stored protected from light at room temperature with acceptable degradation of less than 1.0% per year.

Applications and Uses

Industrial and Commercial Applications

Pelanserin serves primarily as a research chemical and synthetic intermediate in pharmaceutical development. Its structural features make it a valuable building block for the preparation of more complex molecules containing quinazoline and piperazine motifs. The compound finds application as a standard reference material in analytical chemistry laboratories for method development and validation, particularly in chromatographic techniques for nitrogen-containing heterocycles. Industrial uses include serving as a model compound for studying hydrogen bonding patterns in crystalline solids and as a template for molecular imprinting polymers. The compound's well-characterized spectroscopic properties make it useful for calibration purposes in UV-Vis and fluorescence spectroscopy. Production scales typically range from laboratory quantities (grams) to pilot plant scale (kilograms) with annual global production estimated at 50-100 kg. Manufacturing occurs primarily in specialized fine chemical facilities with strict quality control measures. Market pricing varies from $200-500 per gram for research quantities, reflecting its specialized nature and multi-step synthesis.

Research Applications and Emerging Uses

Research applications of pelanserin include its use as a model compound for studying electronic properties of extended π-systems with heteroatom incorporation. The molecule serves as a reference standard in computational chemistry for validating density functional theory methods for predicting NMR chemical shifts and vibrational frequencies of nitrogen-containing heterocycles. Recent investigations explore its potential as a ligand in coordination chemistry, forming complexes with transition metals through the piperazine nitrogen atoms and carbonyl oxygen centers. Emerging applications include use as a building block for metal-organic frameworks and as a template for designing molecularly imprinted polymers for selective recognition of similar heterocyclic compounds. The compound's chiral derivatives serve as auxiliaries in asymmetric synthesis, particularly for reactions requiring hydrogen-bond directed stereocontrol. Patent literature describes pelanserin derivatives as intermediates for preparing compounds with modified electronic properties for materials science applications, including organic semiconductors and light-emitting materials.

Historical Development and Discovery

The development of pelanserin emerged from mid-20th century research into nitrogen-containing heterocycles, particularly compounds combining quinazoline and piperazine systems. Initial synthetic work during the 1960s focused on creating molecules that might exhibit biological activity based on the known pharmacological properties of both structural components. The specific synthesis route using isatoic anhydride and substituted piperazines was first reported in patent literature around 1968, though detailed synthetic optimization continued through the 1970s. Structural characterization advanced significantly with the widespread availability of modern spectroscopic techniques in the 1980s, allowing complete assignment of all proton and carbon NMR signals. The compound's crystallization and X-ray structure determination in the early 1990s provided definitive confirmation of molecular geometry and solid-state arrangement. Throughout the 2000s, computational studies increasingly employed pelanserin as a test case for modeling medium-sized heterocyclic systems with multiple functional groups. Recent research has focused on developing more efficient synthetic routes and exploring derivatives with modified substitution patterns for materials applications.

Conclusion

Pelanserin represents a structurally interesting nitrogen-containing heterocycle that demonstrates the synthetic versatility and complex molecular behavior of quinazoline-piperazine hybrid systems. Its well-characterized physical and chemical properties provide a foundation for understanding similar compounds in this structural class. The molecule's combination of hydrogen bonding capability, moderate polarity, and conformational flexibility makes it particularly valuable as a model system for both experimental and computational studies. Current applications primarily focus on its role as a research chemical and synthetic intermediate, though emerging uses in materials science suggest expanding utility. Future research directions include development of more sustainable synthetic routes, investigation of its coordination chemistry with various metals, and exploration of its derivatives for specialized applications in molecular recognition and materials design. The compound continues to serve as an important reference point in heterocyclic chemistry and provides a platform for further structural modification and property investigation.