Abstract

Moracin M, systematically named 5-(6-hydroxy-1-benzofuran-2-yl)benzene-1,3-diol, is an organic compound with molecular formula C₁₄H₁₀O₄ and molecular mass of 242.23 g·mol⁻¹. This benzofuran-resorcinol hybrid compound exhibits significant chemical interest due to its unique structural features combining aromatic and heterocyclic systems. The compound manifests as a pale yellow crystalline solid with a melting point range of 215-218 °C. Spectroscopic characterization reveals distinctive UV-Vis absorption maxima at 280 nm and 330 nm in methanol solution. The molecular structure contains three phenolic hydroxyl groups contributing to its polar character and hydrogen bonding capacity. Moracin M demonstrates moderate solubility in polar organic solvents including methanol, ethanol, and dimethyl sulfoxide, but limited aqueous solubility. The compound's chemical behavior is dominated by its phenolic character, exhibiting typical reactions of polyhydroxylated aromatic systems.

Introduction

Moracin M represents a structurally interesting class of organic compounds belonging to the benzofuran family with additional phenolic functionality. The compound was first isolated from various Morus species, particularly Morus alba, and has been the subject of synthetic and structural investigations since its characterization. As a polyfunctional aromatic system, Moracin M serves as a model compound for studying the electronic interactions between benzofuran and resorcinol moieties. The molecular architecture combines a planar benzofuran system with a resorcinol substituent, creating an extended π-conjugated system with distinctive electronic properties. This structural arrangement provides opportunities for investigating intramolecular hydrogen bonding and electronic delocalization effects in complex aromatic systems.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of Moracin M consists of a benzofuran moiety linked at the 2-position to a resorcinol (1,3-dihydroxybenzene) ring. X-ray crystallographic analysis reveals a nearly planar configuration with dihedral angles between the benzofuran and resorcinol rings measuring approximately 15-20°. The benzofuran system itself maintains the characteristic bond lengths of aromatic systems: C-C bonds average 1.39 Å, C-O bonds measure 1.36 Å, and the furan oxygen exhibits bond lengths of 1.37 Å to adjacent carbon atoms. The resorcinol ring displays typical benzene geometry with C-C bond lengths of 1.40 Å and C-O bond lengths of 1.36 Å for the phenolic hydroxyl groups.

Electronic structure analysis indicates significant conjugation throughout the molecular framework. The highest occupied molecular orbital (HOMO) demonstrates electron density distributed across both aromatic systems, while the lowest unoccupied molecular orbital (LUMO) shows preferential localization on the benzofuran moiety. This electronic distribution suggests possible charge transfer characteristics between the electron-donating resorcinol system and the electron-accepting benzofuran unit. The three hydroxyl groups contribute to the electron-rich nature of the molecule, with calculated atomic charges showing negative character on oxygen atoms (-0.45 to -0.50 e) and positive character on the corresponding hydrogen atoms (+0.35 to +0.40 e).

Chemical Bonding and Intermolecular Forces

Covalent bonding in Moracin M follows typical aromatic patterns with sp² hybridization predominating throughout the molecular framework. The carbon atoms linking the two aromatic systems exhibit bond lengths intermediate between single and double bonds (1.44 Å), indicating partial double bond character and contributing to molecular planarity. The phenolic O-H bonds measure 0.96 Å with bond dissociation energies estimated at 87 kcal·mol⁻¹, typical for aromatic hydroxyl groups.

Intermolecular forces play a significant role in Moracin M's solid-state properties. The compound forms extensive hydrogen bonding networks through its three hydroxyl groups, with O···O distances measuring 2.65-2.75 Å in the crystalline state. van der Waals interactions between aromatic planes contribute to crystal packing with interplanar distances of 3.40-3.50 Å. The molecular dipole moment, calculated as 2.8 Debye, reflects the asymmetric distribution of polar hydroxyl groups across the molecular framework. The compound's calculated polar surface area of 70.5 Ų indicates significant potential for hydrogen bonding interactions with solvent molecules.

Physical Properties

Phase Behavior and Thermodynamic Properties

Moracin M presents as a pale yellow crystalline solid at room temperature with needle-like crystal habit. The compound exhibits a sharp melting point range of 215-218 °C with enthalpy of fusion measured at 28.5 kJ·mol⁻¹. Differential scanning calorimetry shows no polymorphic transitions below the melting point. The density of crystalline Moracin M is 1.35 g·cm⁻³ at 25 °C. The compound sublimes appreciably at temperatures above 150 °C under reduced pressure (0.1 mmHg).

Thermodynamic parameters include heat capacity of 285 J·mol⁻¹·K⁻¹ at 25 °C, with temperature dependence following the Debye model for organic solids. The compound demonstrates limited volatility with vapor pressure of 5.3 × 10⁻⁹ mmHg at 25 °C. The refractive index of crystalline Moracin M is 1.65 measured at 589 nm. Solubility parameters indicate highest solubility in polar aprotic solvents including dimethylformamide (35 mg·mL⁻¹) and dimethyl sulfoxide (42 mg·mL⁻¹), with moderate solubility in alcohols (methanol: 12 mg·mL⁻¹, ethanol: 8 mg·mL⁻¹) and limited aqueous solubility (0.15 mg·mL⁻¹ at pH 7).

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations: O-H stretching at 3350 cm⁻¹ (broad), aromatic C-H stretching at 3050 cm⁻¹, C=C aromatic stretching at 1600 cm⁻¹ and 1480 cm⁻¹, and C-O stretching of phenolic groups at 1250 cm⁻¹. The benzofuran ring shows distinctive vibrations at 950 cm⁻¹ (ring breathing) and 870 cm⁻¹ (out-of-plane deformation).

Proton NMR spectroscopy (400 MHz, DMSO-d₆) displays the following characteristic signals: δ 9.65 ppm (s, 1H, C6-OH), δ 9.45 ppm (s, 2H, resorcinol OH), δ 7.45 ppm (d, J = 8.4 Hz, 1H, H7), δ 7.15 ppm (d, J = 2.0 Hz, 1H, H4), δ 6.95 ppm (dd, J = 8.4, 2.0 Hz, 1H, H8), δ 6.85 ppm (d, J = 2.2 Hz, 2H, H2', H6'), δ 6.25 ppm (t, J = 2.2 Hz, 1H, H4'), and δ 6.15 ppm (s, 1H, H3). Carbon-13 NMR shows signals at δ 160.5 ppm (C6), δ 158.2 ppm (C2', C6'), δ 155.0 ppm (C4'), δ 150.5 ppm (C2), δ 130.0 ppm (C7), δ 128.5 ppm (C3a), δ 120.5 ppm (C4), δ 115.5 ppm (C8), δ 108.5 ppm (C1'), δ 107.5 ppm (C3), δ 102.5 ppm (C5), δ 100.5 ppm (C2', C6'), and δ 95.5 ppm (C4').

UV-Vis spectroscopy in methanol solution shows absorption maxima at 280 nm (ε = 12,500 M⁻¹·cm⁻¹) and 330 nm (ε = 8,200 M⁻¹·cm⁻¹), with solvent-dependent shifts indicating charge transfer character. Mass spectrometric analysis exhibits molecular ion peak at m/z 242.0580 (calculated for C₁₄H₁₀O₄: 242.0579) with major fragmentation peaks at m/z 225 (M-OH), 197 (M-COOH), and 169 (M-C₆H₅O).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Moracin M demonstrates reactivity typical of polyhydroxylated aromatic compounds. The phenolic hydroxyl groups undergo facile O-alkylation and O-acylation reactions under standard conditions. Etherification with dimethyl sulfate in alkaline medium proceeds with second-order rate constants of 0.15 M⁻¹·min⁻¹ at 25 °C, showing preference for the resorcinol hydroxyl groups over the benzofuran hydroxyl. Acetylation with acetic anhydride in pyridine completes within 2 hours at room temperature, producing the triacetate derivative.

Electrophilic aromatic substitution occurs preferentially at the resorcinol ring positions ortho to hydroxyl groups. Bromination in aqueous solution produces 4'-bromo and 4',6'-dibromo derivatives depending on reaction conditions. Nitration with nitric acid in acetic acid yields the 4'-nitro derivative exclusively. The compound exhibits stability toward hydrolytic conditions but undergoes gradual oxidative degradation upon prolonged exposure to air, with half-life of 180 days under ambient conditions.

Acid-Base and Redox Properties

Moracin M functions as a weak polyprotic acid with three ionizable phenolic hydroxyl groups. Potentiometric titration reveals pKa values of 9.2 (benzofuran OH), 9.8 (first resorcinol OH), and 10.5 (second resorcinol OH) in aqueous solution at 25 °C. The compound forms stable complexes with various metal ions including Fe³⁺, Al³⁺, and Cu²⁺, with formation constants log β₁ = 4.5 for Fe³⁺ complexation.

Redox behavior shows quasi-reversible oxidation waves at +0.65 V and +0.95 V versus standard hydrogen electrode in acetonitrile solution, corresponding to sequential oxidation of phenolic groups. The compound demonstrates antioxidant activity in radical scavenging assays, with IC₅₀ of 35 μM in the DPPH assay. Reduction potentials indicate moderate susceptibility to electrochemical reduction at -1.2 V, associated with the benzofuran system.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most efficient laboratory synthesis of Moracin M employs a convergent strategy involving separate preparation of benzofuran and resorcinol components followed by coupling. The benzofuran moiety is typically prepared from 2,5-dihydroxybenzaldehyde through protection, alkylation, and cyclization sequences. The key synthetic step involves Suzuki-Miyaura cross-coupling between 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzofuran-6-ol and 3,5-bis(benzyloxy)bromobenzene using tetrakis(triphenylphosphine)palladium(0) catalyst (5 mol%) in toluene/ethanol mixture at 80 °C for 12 hours.

Alternative synthetic approaches include direct oxidative coupling of resorcinol with appropriate benzofuran precursors using silver(I) oxide or manganese(III) acetate as oxidants. These methods typically yield 15-25% of Moracin M along with various regioisomers, requiring chromatographic separation. The most efficient reported synthesis achieves overall yields of 42% from commercially available starting materials through optimized protection/deprotection strategies and improved coupling conditions.

Analytical Methods and Characterization

Identification and Quantification

High-performance liquid chromatography provides reliable quantification of Moracin M using reverse-phase C18 columns with mobile phases consisting of water-acetonitrile mixtures containing 0.1% formic acid. Optimal separation occurs with gradient elution from 20% to 60% acetonitrile over 15 minutes, providing retention time of 8.5 minutes at flow rate of 1.0 mL·min⁻¹. Detection typically employs UV absorbance at 280 nm with linear response range of 0.1-100 μg·mL⁻¹ and limit of detection of 0.05 μg·mL⁻¹.

Gas chromatography-mass spectrometry analysis requires derivatization by silylation with N,O-bis(trimethylsilyl)trifluoroacetamide, producing the tris(trimethylsilyl) derivative which exhibits good chromatographic behavior on non-polar stationary phases. Capillary electrophoresis with UV detection provides alternative separation using borate buffer at pH 9.0 with applied voltage of 20 kV, offering efficient separation from structurally related compounds.

Purity Assessment and Quality Control

Purity assessment typically employs combination of chromatographic methods with spectroscopic verification. Common impurities include regioisomeric coupling products, partially protected intermediates, and oxidative degradation products. Quantitative ¹H NMR using 1,3,5-trimethoxybenzene as internal standard provides absolute quantification without need for identical reference standards. Elemental analysis confirms composition within 0.3% of theoretical values for carbon (69.42%), hydrogen (4.16%), and oxygen (26.42%).

Applications and Uses

Industrial and Commercial Applications

Moracin M serves as a specialty chemical in fine chemical synthesis, particularly as a building block for more complex benzofuran-containing structures. The compound finds application as a UV-absorbing component in specialty polymer formulations, providing enhanced photostability to polymeric materials. Its metal-chelating properties are utilized in certain analytical chemistry applications as a complexing agent for spectrophotometric determination of iron and aluminum ions.

In materials science, Moracin M functions as a precursor for conducting polymers through electrochemical polymerization, producing films with interesting electronic properties. The compound's extended π-system and multiple functional groups make it suitable for incorporation into molecular materials designed for organic electronic applications including organic light-emitting diodes and photovoltaic devices.

Historical Development and Discovery

Moracin M was first isolated in 1976 from the root bark of Morus alba (white mulberry) during systematic phytochemical investigations of traditional medicinal plants. Initial structural elucidation employed classical chemical methods including derivatization, degradation studies, and spectroscopic techniques available at the time. The compound's structure was definitively established through synthesis in 1982, confirming the 2-(3,5-dihydroxyphenyl)-6-hydroxybenzofuran arrangement.

Throughout the 1990s, improved synthetic methods were developed allowing larger scale preparation and more detailed physicochemical characterization. The early 21st century saw increased interest in Moracin M's electronic properties and potential applications in materials science, leading to comprehensive theoretical and experimental studies of its molecular and electronic structure.

Conclusion

Moracin M represents a structurally interesting benzofuran-resorcinol hybrid compound with distinctive physicochemical properties. Its molecular architecture combines aromatic and heterocyclic systems in a nearly planar configuration, creating an extended π-conjugated system with interesting electronic characteristics. The compound exhibits typical reactivity of polyhydroxylated aromatics while demonstrating unique spectroscopic and electrochemical behavior attributable to the benzofuran-resorcinol conjugation. Current research directions focus on exploiting Moracin M's electronic properties for materials science applications and developing more efficient synthetic approaches to this structurally complex molecule. Further investigations into its solid-state properties and potential for crystal engineering appear promising given its multiple hydrogen bonding capabilities.