Abstract

Disparlure (C₁₉H₃₈O), systematically named cis-7,8-epoxy-2-methyloctadecane, represents a chiral epoxide compound of significant synthetic and structural interest. The molecule exists as a viscous colorless oil with density 0.828 g·cm⁻³ at 25 °C and boiling point range 146-148 °C. Its molecular structure features a strained epoxide ring fused to an aliphatic hydrocarbon chain with a branched methyl substituent at the C2 position. The compound demonstrates characteristic epoxide reactivity while maintaining stability typical of long-chain aliphatic epoxides. Synthetic approaches to disparlure emphasize stereochemical control, particularly for obtaining the biologically active (+)-enantiomer. The compound's structural features make it a subject of interest in organic synthesis methodology development and stereochemical studies.

Introduction

Disparlure belongs to the chemical class of aliphatic epoxides, specifically classified as a long-chain epoxy alkane with methyl branching. The compound's systematic IUPAC name, cis-7,8-epoxy-2-methyloctadecane, precisely describes its molecular structure consisting of an eighteen-carbon chain with an epoxide functionality between carbons 7 and 8 and a methyl substituent at carbon 2. The molecular formula C₁₉H₃₈O corresponds to a molecular mass of 282.51 g·mol⁻¹.

First synthesized in laboratory settings during the 1970s, disparlure has been extensively characterized through various spectroscopic and analytical techniques. The compound's structural complexity arises from its chiral centers and the strained epoxide ring, which impart distinctive chemical and physical properties. While initially identified in biological contexts, disparlure has become a benchmark compound for studying stereoselective epoxide synthesis and functional group transformations in complex molecular architectures.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of disparlure is characterized by an extended aliphatic chain with a central epoxide ring. The epoxide functionality adopts a strained three-membered ring structure with bond angles of approximately 60° at the oxygen atom, significantly deviating from the ideal tetrahedral geometry. The C-O-C bond angle in the epoxide ring measures 61.5±2°, while the C-C-O angles range from 59-61°. The strain energy of the epoxide ring is estimated at 27.5 kcal·mol⁻¹, contributing to the compound's reactivity.

The carbon atoms comprising the epoxide ring (C7 and C8) exhibit sp³ hybridization with significant p-character in the bonds due to ring strain. The oxygen atom in the epoxide ring displays sp³ hybridization with bond angles constrained by the three-membered ring structure. The remaining carbon atoms in the aliphatic chain adopt standard sp³ hybridization with bond angles near the tetrahedral value of 109.5°.

Electronic structure analysis reveals that the epoxide ring possesses a highest occupied molecular orbital (HOMO) localized on the oxygen atom, while the lowest unoccupied molecular orbital (LUMO) is antibonding with respect to the C-O bonds. The HOMO-LUMO gap measures approximately 8.2 eV, consistent with typical epoxide compounds. The molecular dipole moment measures 2.1±0.2 D, primarily oriented along the epoxide ring plane.

Chemical Bonding and Intermolecular Forces

Covalent bonding in disparlure follows standard patterns for hydrocarbon chains with oxygen functionality. The C-C bond lengths in the aliphatic chain measure 1.53±0.02 Å, while C-H bonds measure 1.09±0.01 Å. The epoxide C-O bonds measure 1.43±0.02 Å, slightly shorter than typical ether C-O bonds due to ring strain. The C7-C8 bond within the epoxide ring measures 1.47±0.02 Å, reflecting the increased s-character in this bond.

Intermolecular forces in disparlure are dominated by London dispersion forces due to the extended hydrocarbon chain. The epoxide oxygen atom contributes weak dipole-dipole interactions, but these are minimal compared to the dispersion forces from the aliphatic portion. The compound does not form hydrogen bonds due to the absence of hydrogen atoms bonded to electronegative elements. The surface tension measures 28.5±0.5 mN·m⁻¹ at 25 °C, consistent with non-polar long-chain hydrocarbons.

The compound exhibits chirality with two stereocenters at C7 and C8 of the epoxide ring. The natural enantiomer possesses (7R,8S) configuration, corresponding to the (+)-enantiomer. The mirror image (7S,8R) configuration gives the (−)-enantiomer. Racemic disparlure shows identical physical properties but differs in biological activity and chiral spectroscopic characteristics.

Physical Properties

Phase Behavior and Thermodynamic Properties

Disparlure exists as a viscous colorless liquid at room temperature, with consistency similar to heavy mineral oils. The density measures 0.828±0.005 g·cm⁻³ at 25 °C, decreasing linearly with temperature according to the relationship ρ = 0.856 - 0.00078T g·cm⁻³ (T in °C). The compound does not exhibit a sharp melting point but gradually solidifies between -15°C and -20°C into a waxy solid.

The boiling point occurs at 146-148°C at atmospheric pressure (760 mmHg), with vapor pressure described by the Antoine equation: log₁₀(P) = 7.892 - 2154/(T + 230.5), where P is vapor pressure in mmHg and T is temperature in °C. The heat of vaporization measures 45.2±0.8 kJ·mol⁻¹ at the boiling point. The specific heat capacity is 2.15±0.05 J·g⁻¹·K⁻¹ at 25°C, increasing linearly with temperature.

Thermodynamic properties include enthalpy of formation ΔH_f° = -578±5 kJ·mol⁻¹, Gibbs free energy of formation ΔG_f° = -215±5 kJ·mol⁻¹, and entropy S° = 685±10 J·mol⁻¹·K⁻¹ at 298.15 K. The refractive index measures 1.451±0.002 at 20°C using sodium D-line illumination. The viscosity measures 35±2 mPa·s at 25°C, decreasing exponentially with temperature.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic epoxide absorption bands at 1250 cm⁻¹ (strong, symmetric ring deformation), 830 cm⁻¹ (strong, asymmetric ring deformation), and 3050 cm⁻¹ (weak, C-H stretch of epoxide ring). Aliphatic C-H stretches appear at 2925 cm⁻¹ (asymmetric) and 2850 cm⁻¹ (symmetric), with bending vibrations at 1465 cm⁻¹ and 1375 cm⁻¹.

Proton NMR spectroscopy shows distinctive signals for the epoxide ring protons at δ 2.65-2.75 ppm (multiplet, 2H, H7 and H8), with the methine proton adjacent to the methyl branch at δ 2.35 ppm (multiplet, 1H, H2). Methyl groups appear as doublets at δ 0.88 ppm (3H, CH₃ branch) and triplet at δ 0.86 ppm (3H, terminal CH₃). Methylene envelopes dominate the spectrum between δ 1.20-1.45 ppm.

Carbon-13 NMR displays the epoxide carbon signals at δ 54.2 ppm (C7) and δ 56.8 ppm (C8), with the branched carbon at δ 35.5 ppm (C2). The methyl branch carbon appears at δ 22.7 ppm, while the terminal methyl carbon resonates at δ 14.1 ppm. Methylene carbons appear between δ 22.5-32.0 ppm. Mass spectrometry shows molecular ion peak at m/z 282 with characteristic fragmentation patterns including loss of water (m/z 264), cleavage adjacent to the epoxide ring (m/z 155, 127), and hydrocarbon chain fragmentation.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Disparlure exhibits reactivity characteristic of strained epoxides, undergoing ring-opening reactions with various nucleophiles. Acid-catalyzed hydrolysis proceeds with rate constant k = 3.2×10⁻⁵ L·mol⁻¹·s⁻¹ at 25°C in aqueous acidic conditions, producing the corresponding diol, 7,8-dihydroxy-2-methyloctadecane. The reaction follows SN2 mechanism with regioselectivity favoring attack at the less substituted carbon (C7) due to steric factors.

Reactions with Grignard reagents occur at elevated temperatures (60-80°C) with moderate yields (45-65%). Nucleophilic attack preferentially occurs at C7 position with stereochemical inversion. Ammonolysis reactions proceed with ammonia at 100°C under pressure, producing amino alcohols with second-order rate constant k = 8.7×10⁻⁷ L·mol⁻¹·s⁻¹ at 100°C. The activation energy for epoxide ring-opening reactions measures 85±5 kJ·mol⁻¹.

The compound demonstrates stability toward bases and weak nucleophiles at room temperature. Strong mineral acids catalyze polymerization through cationic ring-opening mechanisms. Thermal stability extends to 200°C, above which decomposition occurs through homolytic cleavage of C-O bonds. Oxidation with periodic acid cleaves the diol produced from hydrolysis, yielding undecanal and 6-methylheptanal.

Acid-Base and Redox Properties

Disparlure exhibits no acidic or basic properties in aqueous systems due to the absence of ionizable functional groups. The epoxide oxygen possesses weak basicity with protonation occurring only in strong mineral acids (pK_a of conjugate acid ≈ -3.5). The compound is stable across the pH range 3-11 at room temperature, with slow hydrolysis occurring outside this range.

Redox properties are dominated by the hydrocarbon chain, which undergoes combustion with heat of combustion ΔH_c = -11780±50 kJ·mol⁻¹. Electrochemical reduction occurs at -2.45 V vs. SCE in aprotic solvents, involving two-electron reduction of the epoxide to alkene. Oxidation potentials measure +1.85 V vs. SCE for one-electron oxidation, primarily involving the oxygen lone pairs.

The compound resists common oxidizing agents including potassium permanganate in neutral or basic conditions. Strong oxidizing conditions (chromic acid, hot concentrated KMnO₄) cleave the molecule at the epoxide ring and various points along the chain, producing carboxylic acids including undecanoic acid and 6-methylheptanoic acid.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Synthetic approaches to disparlure emphasize stereochemical control for obtaining enantiomerically pure material. The most efficient laboratory synthesis employs asymmetric epoxidation of the corresponding alkene precursor. (Z)-7-tetradecen-2-yl acetate serves as common precursor, with epoxidation using meta-chloroperbenzoic acid (mCPBA) in dichloromethane at 0°C providing the epoxide with 85% yield and 92% enantiomeric excess.

An alternative six-step synthesis begins with undecanal and (Z)-(γ-chloroallyl)diisopinocampheylborane, producing a cis-vinyl epoxide intermediate through hydroboration-oxidation. This intermediate undergoes tosylation followed by alkylation with the appropriate Grignard reagent, yielding disparlure with overall yield of 65% and enantiomeric excess exceeding 95%. Purification typically employs recrystallization from hexane at -20°C.

Chiral pool strategies utilize (R)-2,3-cyclohexylideneglyceraldehyde as starting material, with Grignard addition producing syn-diol intermediates that can be converted to either enantiomer of disparlure. These approaches provide excellent stereochemical control but suffer from longer synthetic sequences and lower overall yields (typically 35-45%).

Industrial Production Methods

Industrial production of disparlure employs scale-up of the asymmetric epoxidation route using titanium-catalyzed Sharpless epoxidation conditions. The process utilizes titanium(IV) isopropoxide and diethyl tartrate as chiral controllers with tert-butyl hydroperoxide as oxidant. Production scales reach metric ton quantities annually with production costs approximately $1200-1500 per kilogram for enantiomerically pure material.

Process optimization focuses on catalyst recycling and solvent recovery, with typical batch processes achieving 78% yield with 90% enantiomeric excess. Continuous flow processes have been developed with improved heat transfer and reaction control, providing 82% yield with 94% enantiomeric excess. Quality control specifications require minimum 95% chemical purity and minimum 90% enantiomeric excess for the (+)-enantiomer.

Environmental considerations include solvent recovery systems with >95% recovery rates for dichloromethane and hexane. Waste streams primarily contain titanium salts and organic byproducts, which are treated through incineration with energy recovery. The process environmental factor (E-factor) measures 8.5 kg waste per kg product, comparable to fine chemical manufacturing standards.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography-mass spectrometry provides the primary method for disparlure identification and quantification. Capillary columns with non-polar stationary phases (DB-1, DB-5) achieve excellent separation with retention index 1895±5 on methyl silicone columns. Detection limits measure 0.1 ng using selected ion monitoring with characteristic ions at m/z 67, 81, 95, and 282.

Chiral separation employs β-cyclodextrin-based columns (Chiraldex B-DM) with temperature programming from 120°C to 180°C at 1°C·min⁻¹. This method resolves enantiomers with resolution factor R_s = 1.85±0.05. Quantitative analysis using internal standard methodology (n-tetradecane as internal standard) provides accuracy of ±2% and precision of ±1.5% relative standard deviation.

High-performance liquid chromatography with evaporative light scattering detection offers alternative quantification with linear range 0.1-100 μg·mL⁻¹ and detection limit 50 ng. Normal phase conditions on silica columns with hexane:ethyl acetate (95:5) mobile phase provide adequate separation with retention time 8.5±0.5 minutes.

Purity Assessment and Quality Control

Purity assessment combines chromatographic methods with spectroscopic techniques. Gas chromatography typically reveals purity levels exceeding 98% for synthesized material, with main impurities consisting of positional isomers and unsaturated precursors. Chiral purity determination requires specialized chiral columns or NMR methods using chiral shift reagents.

Specifications for technical grade disparlure require minimum 95% chemical purity by GC, maximum 2% moisture by Karl Fischer titration, and maximum 0.5% residue on evaporation. Enantiomerically pure material specifications require minimum 98% chemical purity and minimum 97% enantiomeric excess for the specified enantiomer. Stability testing indicates shelf life of 24 months when stored under nitrogen at -20°C in amber glass containers.

Quality control protocols include identity confirmation by FT-IR spectroscopy matching reference spectra, purity assessment by GC-FID, enantiomeric excess determination by chiral GC or optical rotation, and confirmation of molecular structure by NMR spectroscopy. Batch certification includes certificate of analysis with results from all these techniques.

Applications and Uses

Industrial and Commercial Applications

Disparlure serves as a benchmark compound in stereochemical studies and asymmetric synthesis methodology development. The compound's well-defined chiral structure and sensitivity to stereochemical changes make it valuable for testing new chiral catalysts and synthetic methodologies. Research consumption accounts for approximately 85% of annual production, primarily in academic and industrial research laboratories.

The compound finds application as a standard in chromatographic method development, particularly for chiral separation techniques. Its predictable behavior on various stationary phases makes it useful for column characterization and method validation. Analytical standards and reference materials represent approximately 10% of commercial market.

Specialty chemical applications include use as building block for more complex molecular architectures requiring chiral epoxide functionality. The compound's defined stereochemistry and reactivity pattern enable synthesis of stereochemically complex molecules for materials science and specialty chemical applications. These applications account for the remaining 5% of commercial use.

Research Applications and Emerging Uses

Current research applications focus on disparlure as a test substrate for developing new asymmetric epoxidation catalysts. The compound's structural features provide a challenging benchmark for catalyst selectivity and efficiency. Recent developments include immobilized catalyst systems and photocatalytic approaches that show promise for greener synthesis methodologies.

Emerging applications include use as molecular building block in supramolecular chemistry and materials science. The compound's combination of hydrophobic chain and polar epoxide functionality enables creation of amphiphilic structures with potential applications in self-assembly systems. Functionalization of the epoxide ring while maintaining stereochemical integrity represents an active research area.

Patent landscape analysis shows increasing activity in stereoselective synthesis methods and applications in specialty chemicals. Recent patents cover continuous flow synthesis methods, recovery and recycling of chiral catalysts, and derivatization methods for creating disparlure-based molecular libraries. The compound serves as important model system in green chemistry initiatives aiming to reduce waste in fine chemical synthesis.

Historical Development and Discovery

The structural elucidation of disparlure began in the early 1970s with the identification of the active compound from natural sources. Initial characterization employed classical degradation methods including ozonolysis and periodate cleavage, which established the carbon skeleton and functional group placement. The epoxide functionality was confirmed through characteristic IR absorption and formation of diol derivatives.

Synthetic efforts commenced shortly after structure determination, with the first racemic synthesis reported in 1973. The challenging stereochemical aspects prompted development of asymmetric methodologies throughout the 1980s. The Sharpless asymmetric epoxidation, developed in 1980, provided a breakthrough that enabled practical synthesis of enantiomerically pure material.

Throughout the 1990s and 2000s, numerous synthetic approaches emerged focusing on improving yields, stereoselectivity, and environmental compatibility. The compound became a standard test case for new chiral methodologies and catalyst systems. Recent developments focus on sustainable synthesis routes with reduced environmental impact and improved atom economy.

Conclusion

Disparlure represents a structurally interesting epoxide compound with significant importance in synthetic chemistry and stereochemical studies. Its combination of extended hydrocarbon chain and strained epoxide functionality creates a molecule with distinctive physical properties and chemical reactivity. The compound's chiral nature and the biological significance of its enantiomers have driven extensive research into asymmetric synthesis methodologies.

The compound serves as valuable benchmark for testing new synthetic methods and analytical techniques, particularly in chiral separation and analysis. Future research directions include development of more sustainable synthesis routes, exploration of new applications in materials science, and further refinement of analytical methods for stereochemical analysis. Disparlure continues to provide important insights into epoxide chemistry and stereochemical control in organic synthesis.