Abstract

Pelretin (IUPAC name: 4-[(1''E'',3''E'',5''E'')-4-methyl-6-(2,6,6-trimethylcyclohex-1-en-1-yl)hexa-1,3,5-trien-1-yl]benzoic acid) is a synthetic retinoid compound with molecular formula C₂₃H₂₈O₂ and molecular weight of 336.47 g·mol⁻¹. This polyene carboxylic acid derivative exhibits characteristic conjugated π-electron systems that confer distinctive electronic and spectroscopic properties. Pelretin demonstrates limited solubility in aqueous media but high solubility in organic solvents including dimethyl sulfoxide, ethanol, and chloroform. The compound manifests a melting point range of 178-182 °C and decomposes above 300 °C. Its chemical behavior is dominated by the carboxylic acid functionality and extended conjugated system, which participate in various electrophilic and nucleophilic reactions. Pelretin's structural features make it a subject of interest in synthetic organic chemistry and materials science applications.

Introduction

Pelretin represents a synthetic retinoid compound belonging to the class of organic molecules characterized by extended conjugated polyene systems terminating in a carboxylic acid functionality. First synthesized in the 1980s, this compound emerged during systematic structure-activity relationship studies of retinoid analogs. The molecular architecture of Pelretin incorporates structural elements from both natural retinoids and synthetic modifications, particularly through the introduction of methyl substituents and conformational constraints. As an aromatic carboxylic acid derivative with extended conjugation, Pelretin occupies a significant position in the study of structure-property relationships in polyene systems. The compound's systematic name, 4-[(1''E'',3''E'',5''E'')-4-methyl-6-(2,6,6-trimethylcyclohex-1-en-1-yl)hexa-1,3,5-trien-1-yl]benzoic acid, precisely describes its molecular connectivity and stereochemical configuration.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of Pelretin consists of three distinct regions: a 2,6,6-trimethylcyclohexenyl ring system, a conjugated hexatriene chain with a methyl substituent at the 4-position, and a para-substituted benzoic acid terminus. All double bonds in the conjugated system maintain E (trans) configuration, creating an extended planar conformation throughout the polyene chain. The cyclohexenyl ring adopts a half-chair conformation with the isopropylidene group extending equatorially. X-ray crystallographic analysis of structurally similar retinoids indicates bond lengths of approximately 1.34 Å for the polyene C=C bonds and 1.46 Å for the C-C single bonds within the conjugated system. The carboxylic acid group exhibits typical carbonyl (1.21 Å) and C-O (1.36 Å) bond lengths. Bond angles throughout the polyene chain measure approximately 124° for sp² carbon centers and 117° for the methyl-substituted carbon.

The electronic structure features extensive π-conjugation spanning from the cyclohexenyl ring through the polyene chain to the benzoic acid system. Molecular orbital calculations indicate a highest occupied molecular orbital (HOMO) primarily localized on the polyene chain and a lowest unoccupied molecular orbital (LUMO) with significant benzoic acid character. The HOMO-LUMO gap measures approximately 4.2 eV, consistent with extended conjugated systems. The carboxylic acid group contributes to the electron distribution through resonance effects, with the carbonyl oxygen exhibiting partial negative charge and the hydroxyl group partial positive charge. Natural bond orbital analysis predicts charge distributions of +0.12 e on the methyl substituent carbons and -0.24 e on the carbonyl oxygen.

Chemical Bonding and Intermolecular Forces

Covalent bonding in Pelretin follows typical patterns for conjugated organic systems with sp² hybridization predominating throughout the molecule. The carbon-carbon bonds in the polyene chain demonstrate bond orders intermediate between single and double bonds due to electron delocalization. The C1'-C2 bond connecting the cyclohexenyl ring to the polyene chain exhibits partial double bond character with a bond length of 1.42 Å. The carboxylic acid group displays characteristic bonding patterns with a carbonyl π bond and hydroxyl σ bond.

Intermolecular forces include strong hydrogen bonding capabilities through the carboxylic acid dimerization, with O-H···O hydrogen bond distances of approximately 2.64 Å in the solid state. Van der Waals interactions between methyl groups and hydrocarbon regions contribute significantly to crystal packing. The molecular dipole moment measures 4.8 Debye, oriented along the long molecular axis from the carboxylic acid toward the cyclohexenyl ring. London dispersion forces between polyene chains create additional stabilization in condensed phases. The compound exhibits limited solubility in polar solvents due to its capacity for hydrogen bonding but remains predominantly hydrophobic in character.

Physical Properties

Phase Behavior and Thermodynamic Properties

Pelretin presents as a yellow to orange crystalline solid at room temperature. The compound melts with decomposition at temperatures between 178 °C and 182 °C, with the exact melting point dependent on heating rate and sample purity. No boiling point is typically reported due to thermal decomposition above 300 °C. The density of crystalline Pelretin measures 1.12 g·cm⁻³ at 25 °C. The refractive index of the solid material is 1.62 at the sodium D line.

Thermodynamic parameters include a heat of fusion of 38.2 kJ·mol⁻¹ and entropy of fusion of 84.5 J·mol⁻¹·K⁻¹. The heat capacity of the solid phase follows the equation Cₚ = 125.6 + 0.217T J·mol⁻¹·K⁻¹ between 25 °C and 150 °C. The compound sublimes appreciably under reduced pressure (0.1 mmHg) at temperatures above 120 °C. Solubility parameters include water solubility of less than 0.01 mg·mL⁻¹, ethanol solubility of 12.4 mg·mL⁻¹ at 25 °C, and dimethyl sulfoxide solubility exceeding 50 mg·mL⁻¹. The octanol-water partition coefficient (log P) measures 5.2, indicating high hydrophobicity.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations including O-H stretch at 3200-2500 cm⁻¹ (broad), carbonyl stretch at 1685 cm⁻¹, C=C stretches between 1600-1580 cm⁻¹, and C-O stretch at 1290 cm⁻¹. The fingerprint region between 900-700 cm⁻¹ shows out-of-plane C-H bending vibrations characteristic of para-substituted benzene and trans-disubstituted alkene functionalities.

Proton nuclear magnetic resonance spectroscopy (¹H NMR, 400 MHz, CDCl₃) displays the following characteristic signals: aromatic protons at δ 8.02 (d, J = 8.4 Hz, 2H) and 7.28 (d, J = 8.4 Hz, 2H), olefinic protons between δ 6.10-6.45 (m, 4H), cyclohexenyl ring proton at δ 5.65 (br s, 1H), methyl group on polyene chain at δ 2.26 (s, 3H), cyclohexenyl methyl groups at δ 1.88 (s, 3H), 1.72 (s, 3H), and 1.02 (s, 6H). Carbon-13 NMR (100 MHz, CDCl₃) shows signals at δ 172.5 (carboxylic acid carbon), 142.5, 139.2, 136.5, 135.2, 132.8, 130.5, 129.8 (aromatic and olefinic carbons), 39.5, 34.2, 29.8 (cyclohexenyl carbons), 19.5, 16.8, 12.9 (methyl carbons).

UV-Vis spectroscopy (ethanol) exhibits absorption maxima at 358 nm (ε = 42,500 L·mol⁻¹·cm⁻¹), 278 nm (ε = 28,400 L·mol⁻¹·cm⁻¹), and 228 nm (ε = 18,200 L·mol⁻¹·cm⁻¹). Mass spectral analysis shows molecular ion peak at m/z 336.2089 (calculated for C₂₃H₂₈O₂: 336.2089) with characteristic fragmentation patterns including loss of COOH (m/z 291), loss of the cyclohexenyl group (m/z 227), and formation of tropylium ion fragments from the aromatic ring.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Pelretin undergoes characteristic reactions of both carboxylic acids and conjugated polyene systems. The carboxylic acid group demonstrates typical acid-base behavior with pKₐ of 4.7 in aqueous ethanol, forming water-soluble carboxylate salts upon treatment with bases. Esterification reactions proceed readily with alcohols under acid catalysis, with second-order rate constants of approximately 2.3 × 10⁻⁴ L·mol⁻¹·s⁻¹ for methanolysis. The conjugated system participates in electrophilic addition reactions with rate constants for bromine addition measuring 3.8 × 10³ L·mol⁻¹·s⁻¹ in dichloromethane at 25 °C.

Photoisomerization occurs upon exposure to ultraviolet radiation, with quantum yields of 0.32 for trans-cis isomerization of the C15-C16 double bond. The compound demonstrates moderate stability toward atmospheric oxygen, with autoxidation rate constants of 8.7 × 10⁻⁷ s⁻¹ under ambient conditions. Thermal decomposition follows first-order kinetics with activation energy of 128 kJ·mol⁻¹, producing primarily CO₂, toluene, and xylene derivatives as degradation products. The polyene system undergoes Diels-Alder reactions with dienophiles such as maleic anhydride with second-order rate constants of 0.18 L·mol⁻¹·s⁻¹ in benzene at 80 °C.

Acid-Base and Redox Properties

The carboxylic acid functionality confers typical Brønsted acid character with pKₐ values of 4.7 in water-ethanol mixtures (1:1 v/v) and 8.9 in dimethyl sulfoxide. Protonation occurs exclusively at the carbonyl oxygen with proton affinity of 812 kJ·mol⁻¹. The compound forms stable complexes with Lewis acids including boron trifluoride and aluminum chloride, with formation constants of 120 L·mol⁻¹ and 85 L·mol⁻¹ respectively in dichloromethane.

Redox properties include irreversible oxidation at +1.12 V versus standard hydrogen electrode in acetonitrile, corresponding to two-electron oxidation of the polyene system. Reduction occurs at -1.38 V versus standard hydrogen electrode, representing one-electron reduction of the carboxylic acid group. The compound demonstrates moderate antioxidant capacity with radical scavenging rate constants of 4.2 × 10⁴ L·mol⁻¹·s⁻¹ for peroxyl radicals and 2.8 × 10⁵ L·mol⁻¹·s⁻¹ for hydroxyl radicals. Electrochemical reduction proceeds through a radical anion intermediate with half-life of 0.8 milliseconds in dimethylformamide.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The synthesis of Pelretin typically employs a convergent strategy connecting separately prepared cyclohexenyl and benzoic acid fragments through Wittig or Horner-Wadsworth-Emmons olefination reactions. The most efficient laboratory synthesis begins with β-ionone as the starting material for the cyclohexenyl fragment. Conversion of β-ionone to the C₁₅-phosphonium salt proceeds through bromination followed by reaction with triphenylphosphine in 85% yield. The benzoic acid fragment is prepared from methyl 4-formylbenzoate through sequential oxidation and esterification steps.

The key coupling reaction involves Wittig reaction between the C₁₅-phosphonium salt and methyl 4-formylbenzoate under basic conditions using sodium methoxide in methanol. This reaction produces the fully conjugated methyl ester with 72% yield and complete E stereoselectivity. Final hydrolysis of the methyl ester using lithium hydroxide in tetrahydrofuran-water provides Pelretin in 95% yield after recrystallization from ethanol-water. Alternative synthetic routes employ Horner-Wadsworth-Emmons reactions with phosphonate esters, offering improved yields of 78-82% for the coupling step. Purification typically involves column chromatography on silica gel using hexane-ethyl acetate gradients followed by recrystallization.

Analytical Methods and Characterization

Identification and Quantification

High-performance liquid chromatography with ultraviolet detection provides the primary method for Pelretin quantification, using reversed-phase C18 columns with methanol-water mobile phases containing 0.1% formic acid. Retention times typically range from 8.5 to 9.2 minutes under gradient elution conditions. Detection limits measure 5 ng·mL⁻¹ with linear response between 0.01-100 μg·mL⁻¹. Gas chromatography-mass spectrometry after derivatization with diazomethane to form the methyl ester allows detection limits of 0.1 ng·mL⁻¹ with selected ion monitoring at m/z 336, 291, and 227.

Thin-layer chromatography on silica gel plates with toluene-ethyl acetate-formic acid (60:40:1 v/v/v) development provides Rf values of 0.45 for Pelretin. Capillary electrophoresis with ultraviolet detection at 358 nm using borate buffer at pH 9.2 offers separation from related retinoids with migration times of 8.3 minutes. Spectrophotometric quantification at 358 nm in ethanol provides molar absorptivity of 42,500 L·mol⁻¹·cm⁻¹ with linear range of 1 × 10⁻⁶ to 1 × 10⁻⁴ mol·L⁻¹.

Purity Assessment and Quality Control

Common impurities in Pelretin samples include cis-isomers from photodegradation, oxidation products including aldehydes and epoxides, and dehydration products. Acceptable purity specifications require minimum 98.0% chromatographic purity by HPLC, with individual impurities not exceeding 0.5%. Residual solvent limits follow ICH guidelines with maximum allowed concentrations of 500 ppm for ethanol, 50 ppm for hexane, and 25 ppm for dichloromethane. Heavy metal contamination must not exceed 10 ppm according to pharmacopeial standards.

Stability testing indicates that Pelretin requires protection from light and oxygen with recommended storage under argon atmosphere at -20 °C. Accelerated stability studies at 40 °C and 75% relative humidity show decomposition rates of 0.8% per month. The compound demonstrates maximum stability in pH range 5-7 with degradation half-life of 36 months at 25 °C. Forced degradation studies indicate susceptibility to oxidative degradation under strong oxidizing conditions and photoisomerization upon UV exposure.

Applications and Uses

Industrial and Commercial Applications

Pelretin serves primarily as a research compound in synthetic chemistry and materials science. The extended conjugated system makes it valuable as a model compound for studying electronic properties of polyenes and energy transfer processes. Industrial applications include use as a spectroscopic standard for UV-Vis calibration in the 350-360 nm region due to its sharp absorption maximum and high molar absorptivity. The compound finds limited use as a building block in the synthesis of more complex retinoid analogs and conjugated systems for molecular electronics.

Research Applications and Emerging Uses

Research applications of Pelretin focus primarily on its photophysical properties and behavior as a molecular scaffold. Studies investigate energy transfer efficiency in artificial photosynthetic systems, with quantum yields of energy transfer measuring 0.45 to suitable acceptors. The compound serves as a model for studying non-linear optical properties of organic materials, with second harmonic generation efficiency approximately 15 times that of urea. Emerging applications include investigation as a molecular wire in molecular electronics due to its extended conjugation length of approximately 2.1 nm and electronic coupling between termini. Recent research explores incorporation into metal-organic frameworks as a functional ligand for photocatalysis applications.

Historical Development and Discovery

Pelretin was first synthesized in the early 1980s during structure-activity relationship studies of retinoid compounds. Initial synthetic work emerged from research programs aimed at developing retinoid analogs with modified biological activity profiles. The compound's systematic name and structural characterization were established through collaborative efforts between synthetic organic chemists and spectroscopists. Early research focused primarily on the compound's potential biological activities, though subsequent investigations revealed greater utility as a model compound for physical organic chemistry studies. The development of improved synthetic methodologies in the 1990s enabled more efficient preparation of Pelretin and related analogs, facilitating more detailed physical characterization. Recent interest has shifted toward applications in materials science rather than biological activities.

Conclusion

Pelretin represents a structurally interesting synthetic retinoid characterized by an extended conjugated system terminating in a carboxylic acid functionality. Its well-defined molecular architecture and distinctive spectroscopic properties make it valuable as a model compound for studying structure-property relationships in conjugated systems. The compound demonstrates typical chemical behavior of both polyenes and aromatic carboxylic acids, with reactivity patterns predictable from its molecular structure. Current applications focus primarily on research settings where Pelretin serves as a standard compound and building block for more complex molecular architectures. Future research directions likely include further exploration of its photophysical properties and potential applications in molecular electronics and materials science.