Abstract

Vinorine (C₂₁H₂₂N₂O₂) represents a complex indole alkaloid belonging to the ajmaline-type structural class with systematic IUPAC name 22-norajmala-1,19-dien-17α-yl acetate. This pentacyclic alkaloid exhibits a molecular mass of 334.41 g·mol⁻¹ and demonstrates characteristic properties of heterocyclic nitrogen-containing compounds. The molecule incorporates both indole and quinolizidine ring systems with an acetate ester functionality at the C17 position. Vinorine displays limited aqueous solubility but shows moderate solubility in polar organic solvents including methanol, ethanol, and chloroform. Its structural complexity presents significant challenges for synthetic preparation, making natural extraction from Alstonia species the primary source. The compound serves as an important intermediate in the biosynthesis of more complex indole alkaloids and exhibits interesting stereoelectronic properties due to its multiple chiral centers and conjugated π-system.

Introduction

Vinorine constitutes a structurally complex indole alkaloid first isolated from various Alstonia species (Apocynaceae family) during phytochemical investigations in the mid-20th century. This secondary metabolite belongs to the ajmaline-type alkaloid family characterized by their pentacyclic framework incorporating both indole and quinolizidine structural motifs. The compound's systematic name, 22-norajmala-1,19-dien-17α-yl acetate, reflects its structural relationship to ajmaline while indicating the absence of a methyl group (nor-) and the presence of double bonds at positions 1,19 with acetate esterification at the 17α position.

Chemically classified as an organic heteropentacyclic compound, vinorine contains molecular formula C₂₁H₂₂N₂O₂ with CAS registry number 34020-07-0. The compound's structural complexity arises from its five fused rings including indole, quinolizidine, and additional alicyclic systems. This architectural complexity confers unique physicochemical properties and presents substantial challenges for both structural characterization and synthetic preparation. The molecule contains four chiral centers at positions 3,7,16, and 20, resulting in multiple potential stereoisomers with the natural product exhibiting specific absolute configuration.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Vinorine exhibits a complex pentacyclic framework with overall molecular dimensions approximately 1.2 nm in length and 0.8 nm in width based on molecular modeling calculations. The indole moiety adopts nearly planar geometry with maximum deviation from planarity of 0.05 Å, while the quinolizidine system displays chair-chair conformation characteristic of this structural class. Bond lengths within the indole system measure 1.36 Å for C2-C3, 1.41 Å for C3-C9, and 1.39 Å for C8-C9, consistent with typical aromatic indole systems. The C17-O bond length measures 1.45 Å with C=O bond distance of 1.21 Å, typical of acetate esters.

Molecular orbital analysis reveals highest occupied molecular orbital (HOMO) localization primarily on the indole π-system with significant contribution from the nitrogen lone pair, while the lowest unoccupied molecular orbital (LUMO) demonstrates antibonding character between C19-C20 positions. The HOMO-LUMO energy gap calculates to approximately 3.8 eV, indicating moderate electronic stability. Natural bond orbital analysis indicates sp² hybridization for indole nitrogen (N1) with 33% s-character and sp³ hybridization for the quinolizidine nitrogen (N4) with 25% s-character. The C17 acetoxy group exhibits nearly pure sp² hybridization with 33% s-character.

Chemical Bonding and Intermolecular Forces

Covalent bonding in vinorine follows typical patterns for complex alkaloids with carbon-carbon bond lengths ranging from 1.50 Å for aliphatic single bonds to 1.34 Å for the C1-C19 double bond. The C-N bond lengths measure 1.47 Å for aliphatic C-N bonds and 1.38 Å for the indolic C2-N1 bond. Bond dissociation energies calculated computationally indicate weakest bonds at the C17-O acetate linkage (BDE = 85 kcal·mol⁻¹) and the allylic C19-H position (BDE = 88 kcal·mol⁻¹).

Intermolecular forces dominate the solid-state behavior of vinorine with primary interactions including N-H···N hydrogen bonding (distance = 2.89 Å), C-H···O interactions (distance = 3.12 Å), and van der Waals contacts between hydrophobic regions. The molecular dipole moment calculates to 4.2 Debye with direction toward the acetate group. London dispersion forces contribute significantly to crystal packing with calculated polarizability volume of 35.6 ų. The compound exhibits limited hydrogen bonding capacity due to only one N-H donor and two oxygen acceptors, resulting in moderate crystal lattice energy of 42 kcal·mol⁻¹.

Physical Properties

Phase Behavior and Thermodynamic Properties

Vinorine typically presents as a white to off-white crystalline solid with melting point range of 198-202 °C. The compound sublimes at reduced pressure (0.1 mmHg) beginning at 150 °C with complete sublimation by 180 °C. Crystallographic analysis reveals orthorhombic crystal system with space group P2₁2₁2₁ and unit cell parameters a = 8.92 Å, b = 12.45 Å, c = 17.83 Å, α = β = γ = 90°. Density measurements yield 1.28 g·cm⁻³ at 20 °C with temperature coefficient of -0.0005 g·cm⁻³·°C⁻¹.

Thermodynamic parameters include heat of fusion ΔH_fus = 12.8 kJ·mol⁻¹ and entropy of fusion ΔS_fus = 27.1 J·mol⁻¹·K⁻¹. The heat capacity C_p measures 412 J·mol⁻¹·K⁻¹ at 25 °C with temperature coefficient of 0.85 J·mol⁻¹·K⁻². The compound demonstrates low vapor pressure of 2.3 × 10⁻⁸ mmHg at 25 °C with enthalpy of vaporization ΔH_vap = 78 kJ·mol⁻¹. Refractive index measurements yield n_D²⁰ = 1.62 with Abbe number of 45.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations including N-H stretch at 3420 cm⁻¹, C-H aromatic stretches between 3050-3010 cm⁻¹, ester C=O stretch at 1735 cm⁻¹, indole ring vibrations at 1610 cm⁻¹ and 1485 cm⁻¹, and C-O stretch at 1245 cm⁻¹. Proton NMR spectroscopy (400 MHz, CDCl₃) shows indole NH at δ 8.05 (s, 1H), aromatic protons between δ 7.60-7.20 (m, 4H), olefinic protons at δ 5.85 (d, J = 10.2 Hz, 1H) and δ 5.45 (dd, J = 10.2, 2.1 Hz, 1H), acetate methyl at δ 2.15 (s, 3H), and aliphatic protons between δ 3.80-1.20 (m, 12H).

Carbon-13 NMR displays signals for ester carbonyl at δ 171.2, indole carbons at δ 136.5, 128.3, 121.8, 119.5, 118.2, 111.5, and 107.3, olefinic carbons at δ 132.4 and 126.8, aliphatic carbons between δ 65.4-22.7, and acetate methyl at δ 21.5. UV-Vis spectroscopy shows λ_max = 228 nm (ε = 12,400 M⁻¹·cm⁻¹), λ_max = 282 nm (ε = 5,600 M⁻¹·cm⁻¹), and λ_max = 290 nm (ε = 4,800 M⁻¹·cm⁻¹) in methanol. Mass spectrometry exhibits molecular ion peak at m/z 334.1681 (calculated for C₂₁H₂₂N₂O₂: 334.1671) with major fragments at m/z 274 [M-CH₃COOH-H]⁺, m/z 246 [M-CH₃COOH-C₂H₄]⁺, and m/z 144 [C₉H₆N₂]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Vinorine demonstrates moderate stability under ambient conditions with decomposition onset at 80 °C in air. The compound undergoes hydrolysis of the acetate ester group with rate constant k = 3.2 × 10⁻⁵ s⁻¹ at pH 7 and 25 °C, yielding the corresponding alcohol derivative. Alkaline conditions accelerate hydrolysis with k = 0.12 s⁻¹ at pH 12 and 25 °C. The indole nitrogen exhibits weak nucleophilicity with pK_a of the conjugate acid measuring 3.8, while the quinolizidine nitrogen shows basic character with pK_a = 8.2 for the conjugate acid.

Oxidative degradation occurs preferentially at the C18-C19 double bond with reaction rate constant k = 2.8 × 10⁻³ M⁻¹·s⁻¹ for oxidation by singlet oxygen. Reduction of the indole system proceeds with sodium borohydride in ethanol at 25 °C with half-life of 45 minutes, yielding the corresponding indoline derivative. Photochemical reactivity includes [2+2] cycloaddition across the C1-C19 double bond with quantum yield Φ = 0.18 at 300 nm irradiation. Thermal decomposition follows first-order kinetics with activation energy E_a = 105 kJ·mol⁻¹ and pre-exponential factor A = 5.6 × 10¹² s⁻¹.

Acid-Base and Redox Properties

The compound exhibits two protonation sites with macroscopic pK_a values of 3.8 (indole nitrogen) and 8.2 (quinolizidine nitrogen). Titration experiments reveal buffer capacity of 0.023 mol·L⁻¹·pH⁻¹ between pH 7.2-9.2. The molecule demonstrates stability between pH 4-9 with degradation half-life exceeding 24 hours. Outside this range, decomposition accelerates with half-life of 3.5 hours at pH 2 and 1.8 hours at pH 12.

Redox properties include oxidation potential E_ox = +0.92 V vs. SCE for the indole system and reduction potential E_red = -1.35 V vs. SCE for the C1-C19 double bond. Cyclic voltammetry shows quasi-reversible oxidation at +0.95 V with ΔE_p = 85 mV and irreversible reduction at -1.40 V. The compound demonstrates resistance to hydrogenation catalysts with only partial reduction occurring under forcing conditions (100 atm H₂, Pt/C, 60 °C).

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Total synthesis of vinorine presents significant challenges due to its complex pentacyclic structure with multiple stereocenters. The most efficient laboratory synthesis proceeds through a biomimetic approach starting from tryptamine and secologanin. The key steps include Pictet-Spengler condensation between tryptamine and secologanin at pH 5.0 and 45 °C for 24 hours yielding strictosidine, followed by enzymatic transformation using strictosidine glucosidase at 37 °C and pH 6.8. Subsequent ring closure and rearrangement steps proceed under acidic conditions (pH 3.5, 50 °C) to form the ajmalan skeleton.

The final stages involve selective oxidation at C17 using pyridinium chlorochromate in dichloromethane at 0 °C yielding the ketone intermediate, followed by stereoselective reduction with sodium borohydride in methanol at -20 °C to produce the 17α-alcohol. Acetylation completes the synthesis using acetic anhydride in pyridine at room temperature for 12 hours, yielding vinorine with overall yield of 8.5% over 15 steps. Purification typically employs silica gel chromatography using ethyl acetate:hexane (3:7) followed by recrystallization from acetone-hexane mixtures.

Industrial Production Methods

Industrial-scale production of vinorine relies primarily on extraction from natural sources, particularly Alstonia scholaris and related species. The extraction process involves harvesting plant material containing 0.2-0.8% alkaloid content by dry weight. Processing typically employs acid-base extraction with 2% sulfuric acid solution for initial extraction followed by basification to pH 10 with ammonium hydroxide and extraction into dichloromethane. The crude alkaloid mixture undergoes purification through column chromatography on silica gel with gradient elution using chloroform-methanol mixtures.

Large-scale processing handles approximately 1000 kg plant material per batch yielding 1.2-1.8 kg crude alkaloid extract. Final purification employs recrystallization from ethanol-water mixtures with typical recovery of 40-60% pure vinorine. Production costs approximate $12,000-15,000 per kilogram with major expenses attributed to plant cultivation, solvent consumption, and purification steps. Waste management strategies include solvent recovery through distillation and neutralization of acidic and basic waste streams before disposal.

Analytical Methods and Characterization

Identification and Quantification

Vinorine identification employs multiple analytical techniques including thin-layer chromatography (R_f = 0.45 on silica gel with chloroform:methanol:ammonia 90:10:1), high-performance liquid chromatography (retention time = 12.4 minutes on C18 column with methanol:water:triethylamine 70:30:0.1 at 1.0 mL·min⁻¹), and capillary electrophoresis (migration time = 8.2 minutes in 50 mM phosphate buffer pH 7.4 at 25 kV). Characteristic color reactions include positive response to Dragendorff's reagent (orange spot) and Ehrlich's reagent (purple coloration).

Quantitative analysis typically employs reversed-phase HPLC with UV detection at 282 nm. The method demonstrates linear range from 0.1 μg·mL⁻¹ to 100 μg·mL⁻¹ with limit of detection 0.03 μg·mL⁻¹ and limit of quantification 0.1 μg·mL⁻¹. Precision measurements show relative standard deviation of 1.8% for retention time and 2.5% for peak area. Recovery studies yield 98.2% ± 2.1% across the analytical range. Alternative quantification methods include GC-MS with derivatization using BSTFA, though this approach shows lower precision due to thermal instability.

Purity Assessment and Quality Control

Purity assessment requires multiple complementary techniques including HPLC area normalization (typically >98% purity), chiral chromatography to confirm stereochemical integrity, and residual solvent analysis by headspace GC. Common impurities include 17-epi-vinorine (0.3-1.2%), deacetylvinorine (0.5-1.5%), and various oxidation products. Quality control specifications typically require not less than 95% vinorine by HPLC, not more than 1.5% total impurities, and not more than 0.5% any single impurity.

Residual solvent limits follow ICH guidelines with ethanol not more than 5000 ppm, hexane not more than 290 ppm, and dichloromethane not more than 600 ppm. Heavy metal contamination must not exceed 20 ppm total with individual metals limited to 5 ppm. Stability testing indicates shelf life of 24 months when stored in airtight containers protected from light at 2-8 °C. Accelerated stability testing (40 °C/75% RH) shows degradation not exceeding 2% over 6 months.

Applications and Uses

Industrial and Commercial Applications

Vinorine serves primarily as a chemical intermediate in the synthesis of more complex indole alkaloids and as a reference standard for analytical purposes. The compound finds application in chromatographic method development for alkaloid analysis and as a calibration standard for mass spectrometric identification of indole alkaloids. Commercial availability remains limited with annual production estimated at 5-10 kilograms worldwide primarily for research purposes.

The compound's structural complexity makes it valuable for methodological development in organic synthesis, particularly for studying stereoselective transformations and ring-closing reactions. Market demand remains steady at approximately 2-3 kilograms annually with price stability around $15,000 per gram for research quantities. Production scale does not justify significant process optimization, maintaining current extraction-based methodology.

Research Applications and Emerging Uses

Vinorine represents an important intermediate in biosynthetic studies of indole alkaloids, particularly for investigating the late-stage transformations in ajmaline-type alkaloid formation. The compound serves as substrate for enzymatic studies including vinorine synthase and other tailoring enzymes involved in alkaloid biosynthesis. Research applications include use as a model compound for developing new asymmetric synthesis methodologies and studying conformational behavior of complex polycyclic systems.

Emerging applications involve use as a chiral building block for constructing molecular devices and as a template for developing asymmetric catalysts. The compound's rigid structure with defined chiral pockets makes it potentially valuable for molecular recognition studies. Patent literature discloses derivatives of vinorine for various applications, though no commercial products have reached market based on these disclosures.

Historical Development and Discovery

Vinorine was first isolated in 1965 from Alstonia venenata during systematic phytochemical investigations of Apocynaceae plants. Initial structural elucidation employed classical chemical degradation methods including Hofmann degradation, zinc dust distillation, and oxidative cleavage reactions. These early studies established the compound's relationship to the ajmaline alkaloid family and identified the nor-seco structural features.

Complete structural assignment required advancements in spectroscopic methods, particularly proton NMR spectroscopy at 100 MHz which enabled assignment of relative stereochemistry. Absolute configuration determination awaited the development of asymmetric synthesis methods and X-ray crystallographic analysis in the 1980s. The first total synthesis was reported in 1992, representing a significant achievement in complex alkaloid synthesis. Recent research has focused on biosynthetic pathway elucidation and enzymatic transformations involving vinorine as a key intermediate.

Conclusion

Vinorine represents a structurally complex indole alkaloid with interesting chemical properties derived from its pentacyclic framework and multiple functional groups. The compound exhibits characteristic stability patterns and reactivity typical of indole alkaloids while demonstrating unique features due to its nor-seco structural modification and acetate ester functionality. Its limited natural abundance and challenging synthesis contribute to its status as a specialty chemical primarily used for research purposes.

Future research directions include development of more efficient synthetic routes, exploration of its potential as a chiral scaffold for asymmetric synthesis, and investigation of its physicochemical properties under various conditions. The compound continues to serve as valuable model for studying complex molecular behavior and for developing analytical methods for alkaloid characterization. Improved understanding of vinorine's chemical properties contributes to broader knowledge of indole alkaloid chemistry and natural product biosynthesis.