Abstract

Phillyrin, systematically named (7α,7′β,8α,8′α)-3,3′,4′-trimethoxy-7,9′:7′,9-diepoxylignan-4-yl β-D-glucopyranoside, is a naturally occurring lignan glucoside with the molecular formula C₂₇H₃₄O₁₁ and molar mass of 534.55 g·mol⁻¹. This complex organic compound features a distinctive furofuran lignan core structure conjugated to a β-D-glucopyranoside moiety. Phillyrin demonstrates significant structural complexity with eight stereocenters and multiple ether linkages. The compound exhibits characteristic physical properties including limited solubility in aqueous media and moderate solubility in polar organic solvents. Its intricate molecular architecture presents challenges for synthetic preparation but offers opportunities for studying stereoselective glycosylation reactions. Phillyrin serves as a model compound for investigating the chemistry of complex lignan glycosides and their behavior under various analytical conditions.

Introduction

Phillyrin represents a structurally complex lignan glycoside belonging to the furofuran class of natural products. First isolated from Forsythia suspensa plants, this compound exemplifies the sophisticated molecular architectures found in plant secondary metabolites. The compound's systematic name, (7α,7′β,8α,8′α)-3,3′,4′-trimethoxy-7,9′:7′,9-diepoxylignan-4-yl β-D-glucopyranoside, precisely describes its stereochemical configuration and functional group arrangement. With CAS registry number 487-41-2, Phillyrin has been extensively characterized through modern spectroscopic techniques. The molecular structure incorporates both lipophilic lignan domains and hydrophilic carbohydrate components, creating an amphiphilic character that influences its physical properties and chemical behavior. This structural duality makes Phillyrin an interesting subject for studying structure-property relationships in complex natural products.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Phillyrin possesses a complex molecular architecture with eight stereocenters distributed across its furofuran lignan core and glucopyranoside moiety. The furofuran system consists of two fused tetrahydrofuran rings adopting a rigid, cage-like structure with defined stereochemistry at positions C7, C7', C8, and C8'. The glucopyranoside unit attaches at the C4 position through a β-glycosidic linkage, adopting the ^4C_1 chair conformation characteristic of D-glucose derivatives. Bond angles within the furofuran system approximate 109.5° for sp³ hybridized carbon atoms, while the glycosidic bond angle C1'-O-C4 measures approximately 117°. The methoxy substituents at positions 3, 3', and 4' adopt orientations that minimize steric interactions with adjacent functional groups. Electronic distribution throughout the molecule shows localized π systems in the aromatic rings with σ bonding frameworks in the aliphatic regions.

Chemical Bonding and Intermolecular Forces

Covalent bonding in Phillyrin follows typical patterns for organic compounds with carbon-carbon bond lengths of 1.54 Å for aliphatic single bonds and 1.40 Å for aromatic systems. Carbon-oxygen bonds vary from 1.43 Å for ether linkages to 1.36 Å for glycosidic bonds. The molecule exhibits multiple intermolecular forces including hydrogen bonding capacity through its hydroxyl groups (glucose moiety), dipole-dipole interactions from polar ether and glycosidic linkages, and van der Waals forces through its aromatic systems and aliphatic regions. The calculated dipole moment approximates 4.2 D, reflecting significant molecular polarity. The glucopyranoside unit provides hydrogen bond donor and acceptor sites, while the methoxy-substituted aromatic rings contribute to lipophilic character. This combination of functional groups creates amphiphilic properties that influence solubility behavior and chromatographic characteristics.

Physical Properties

Phase Behavior and Thermodynamic Properties

Phillyrin presents as a white to off-white crystalline solid at room temperature. The compound melts with decomposition at temperatures above 180°C, though reported melting points vary considerably depending on purity and heating rate. Differential scanning calorimetry shows endothermic events corresponding to dehydration around 110-120°C followed by melting and decomposition above 180°C. The density of crystalline Phillyrin approximates 1.35 g·cm⁻³ based on X-ray crystallographic data. Specific rotation measurements indicate [α]D²⁰ = -38.5° (c = 1.0, methanol), reflecting the compound's chiral nature. Solubility characteristics demonstrate moderate dissolution in polar organic solvents including methanol (12.4 mg·mL⁻¹), ethanol (8.7 mg·mL⁻¹), and dimethyl sulfoxide (23.6 mg·mL⁻¹), with limited aqueous solubility (0.8 mg·mL⁻¹) at 25°C. The octanol-water partition coefficient (log P) measures 1.42, indicating moderate lipophilicity.

Spectroscopic Characteristics

Infrared spectroscopy of Phillyrin shows characteristic absorption bands at 3395 cm⁻¹ (O-H stretch), 2935 cm⁻¹ (C-H stretch), 1605 cm⁻¹ and 1512 cm⁻¹ (aromatic C=C stretch), 1265 cm⁻¹ (aryl-O stretch), and 1075 cm⁻¹ (C-O-C glycosidic stretch). ^1H NMR spectroscopy (600 MHz, DMSO-d₆) reveals signals at δ 6.85 (d, J = 8.2 Hz, H-5), 6.75 (d, J = 1.8 Hz, H-2), 6.70 (dd, J = 8.2, 1.8 Hz, H-6), 6.65 (s, H-5' and H-6'), 4.85 (d, J = 7.2 Hz, H-1''), 4.25 (m, H-7 and H-7'), 3.85 (s, OCH₃), 3.80 (s, 2×OCH₃), and numerous aliphatic proton signals between δ 3.0-4.0. ^13C NMR spectroscopy (150 MHz, DMSO-d₆) displays characteristic signals at δ 152.5 (C-3), 148.2 (C-4'), 147.5 (C-3'), 134.5 (C-1), 132.8 (C-1'), 119.5 (C-6), 115.8 (C-5), 112.5 (C-2), 111.2 (C-5'), 108.5 (C-6'), 102.5 (C-1''), 87.5 (C-7), 86.2 (C-7'), 72.5-62.5 (glucose carbons), and 56.5-56.2 (OCH₃). Mass spectrometric analysis shows molecular ion peak at m/z 534.55 [M]⁺ with characteristic fragment ions at m/z 372.38 [M-glucose]⁺ and 327.32 [M-glucose-OCH₃]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Phillyrin demonstrates moderate chemical stability under neutral conditions but undergoes hydrolysis under acidic or basic conditions. Acid-catalyzed hydrolysis cleaves the glycosidic bond with pseudo-first order rate constant k = 3.2 × 10⁻⁴ s⁻¹ at pH 2.0 and 25°C, producing the aglycone phillygenin and D-glucose. Basic conditions above pH 9.0 promote demethylation of the aromatic methoxy groups with half-life of 45 minutes at pH 10.0 and 25°C. The compound exhibits resistance to enzymatic hydrolysis by β-glucosidases from some microbial sources but undergoes slow cleavage by specific plant-derived enzymes. Oxidation reactions occur preferentially at the phenolic positions following demethylation, with the C4' position showing greatest susceptibility to oxidative transformations. Hydrogenation under catalytic conditions reduces the furofuran ring system with concomitant saturation of aromatic rings, requiring harsh conditions (100 atm H₂, Pd/C, 80°C) due to steric constraints.

Acid-Base and Redox Properties

The phenolic hydroxyl group generated upon demethylation exhibits pKₐ = 9.8, indicating moderate acidity comparable to other para-substituted phenols. The sugar hydroxyl groups show typical alcohol pKₐ values ranging from 12.0-15.0. Redox properties include oxidation potential E° = +0.65 V versus standard hydrogen electrode for the phenolic oxidation couple. The compound demonstrates stability toward common oxidizing agents including molecular oxygen at ambient conditions but undergoes rapid oxidation in the presence of periodate or lead tetraacetate due to cleavage of glycol-like structures in the glucose moiety. Cyclic voltammetry shows irreversible oxidation waves at +0.72 V and +0.95 V corresponding to sequential oxidation processes.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Total synthesis of Phillyrin presents significant challenges due to the multiple stereocenters and complex ring system. The most successful approach involves convergent synthesis combining separately prepared lignan and glucose components. The furofuran lignan core synthesizes through stereoselective oxidative coupling of properly functionalized eugenol derivatives using silver(I) oxide or manganese(III) acetate as oxidants. Key steps include diastereoselective formation of the tetrahydrofuran rings through intramolecular nucleophilic displacement with reported yields of 65-72% for critical cyclization steps. Glycosylation employs protected D-glucose derivatives activated as trichloroacetimidates or bromides, with careful control of reaction conditions to ensure β-selectivity. Final deprotection steps utilize standard methods including hydrogenolysis for benzyl ether removal and acidic conditions for acetal cleavage. Overall yields for multistep syntheses typically range from 8-12% due to the complexity of stereochemical control and functional group compatibility issues.

Industrial Production Methods

Industrial-scale production of Phillyrin primarily utilizes extraction from Forsythia suspensa plants rather than synthetic approaches due to economic considerations. Extraction processes employ ethanol-water mixtures (70-80% ethanol) at elevated temperatures (60-80°C) with extraction yields of 2-3% based on dry plant material. Modern extraction techniques include ultrasound-assisted extraction that reduces processing time from 12 hours to 45 minutes while improving yield by 18-22%. Countercurrent chromatography provides efficient purification with typical recovery rates of 85-90% and final purity exceeding 98%. Process optimization focuses on solvent recycling, energy efficiency, and waste minimization to improve economic viability. Annual production estimates range from 500-800 kilograms worldwide, with China as the primary producer due to natural abundance of source plants.

Analytical Methods and Characterization

Identification and Quantification

High-performance liquid chromatography with ultraviolet detection serves as the primary method for Phillyrin quantification, using reversed-phase C18 columns with mobile phases consisting of methanol-water or acetonitrile-water mixtures. Typical retention times range from 12-15 minutes under gradient elution conditions. Detection wavelengths of 280 nm provide optimal sensitivity with limit of detection of 0.2 μg·mL⁻¹ and limit of quantification of 0.6 μg·mL⁻¹. Liquid chromatography-mass spectrometry employing electrospray ionization in negative mode shows deprotonated molecular ion [M-H]⁻ at m/z 533.54 with characteristic fragment ions at m/z 371.37 [M-H-glucose]⁻ and 326.31 [M-H-glucose-OCH₃]⁻. Capillary electrophoresis with ultraviolet detection offers alternative separation with migration times of 8-10 minutes using borate buffer at pH 9.2. Quantitative ^1H NMR spectroscopy using internal standards provides absolute quantification without need for compound-specific calibration curves.

Purity Assessment and Quality Control

Purity assessment employs orthogonal methods including HPLC with diode array detection to verify spectral homogeneity, chiral chromatography to confirm stereochemical integrity, and capillary electrophoresis to detect polar impurities. Common impurities include phillygenin (aglycone), demethylated derivatives, and oxidation products. Quality control specifications typically require minimum purity of 95% by HPLC area normalization, with limits for individual impurities not exceeding 1.0%. Residual solvent content monitored by gas chromatography must comply with ICH guidelines for Class 2 and Class 3 solvents. Water content determined by Karl Fischer titration typically measures less than 2.0% w/w for crystalline material. Stability studies indicate shelf life of 24 months when stored protected from light at -20°C, with degradation rates of less than 1.0% per year under recommended conditions.

Applications and Uses

Industrial and Commercial Applications

Phillyrin serves as a reference standard in analytical chemistry laboratories for quality control of Forsythia-based products and traditional medicine preparations. The compound finds application as a chiral template for asymmetric synthesis due to its rigid, well-defined stereochemistry. In material science, Phillyrin derivatives function as ligands for metal complexation and as building blocks for supramolecular assemblies exploiting its multiple hydrogen bonding sites. Specialty chemical applications include use as a model compound for studying glycosylation reactions and enzymatic transformations of complex natural products. Commercial availability remains limited with market prices approximately $350-500 per gram for research-grade material, reflecting the challenges in large-scale production.

Research Applications and Emerging Uses

Research applications focus on Phillyrin's potential as a molecular scaffold for drug discovery due to its complex three-dimensional structure and multiple functional groups. The compound serves as a substrate for studying enzyme kinetics of glycosidases and transferases, particularly those involved in lignan metabolism. Materials science research explores derivatives as components of liquid crystalline materials and as chiral auxiliaries in asymmetric catalysis. Emerging applications include use as a molecular probe for studying carbohydrate-protein interactions and as a template for developing synthetic methodologies for complex natural product synthesis. Patent literature describes derivatives as intermediates for preparing optically active compounds and as ligands for catalytic systems.

Historical Development and Discovery

Initial isolation of Phillyrin from Forsythia suspensa occurred in the early 20th century during phytochemical investigations of traditional medicinal plants. Structural elucidation progressed through the mid-20th century using classical degradation methods and spectroscopic techniques available at the time. The complete stereochemistry remained uncertain until the application of modern NMR spectroscopy and X-ray crystallography in the 1970s and 1980s. Synthetic efforts began in the 1990s with partial syntheses of the lignan aglycone, while complete total synthesis with correct stereochemistry was achieved only in the early 2000s. Analytical methodology development accelerated in the 2010s with improved chromatographic and spectroscopic techniques for quantification and characterization. Recent advances focus on biotechnological production using engineered microorganisms and plant cell cultures to address supply limitations.

Conclusion

Phillyrin represents a structurally complex lignan glycoside with interesting chemical properties stemming from its unique molecular architecture. The compound's combination of lipophilic lignan core and hydrophilic glucose moiety creates amphiphilic character that influences its physical behavior and chemical reactivity. Challenges in synthetic preparation have stimulated development of innovative methodologies for stereoselective synthesis and glycosylation. Analytical characterization benefits from advances in chromatographic and spectroscopic techniques that enable precise quantification and impurity profiling. Research applications continue to expand as the compound's potential as a molecular scaffold and chiral template becomes increasingly recognized. Future research directions likely include development of more efficient synthetic routes, exploration of novel derivatives with modified properties, and investigation of applications in materials science and asymmetric synthesis.