Abstract

Rugulovasine represents a class of bioactive ergoline alkaloids with the molecular formula C₁₆H₁₆N₂O₂. This spirocyclic compound exists as two distinct stereoisomers designated Rugulovasine A and Rugulovasine B, differentiated by their stereochemistry at the C4″ and C5″ positions. The compound exhibits a complex polycyclic structure featuring a fused indole system characteristic of ergot alkaloids. Rugulovasine demonstrates significant structural complexity with multiple chiral centers and a spiro-lactone moiety. Its molecular architecture incorporates both hydrogen bond donor and acceptor functionalities, contributing to its distinctive physicochemical properties. The compound manifests limited solubility in aqueous media but shows good solubility in polar organic solvents. Rugulovasine serves as an important reference compound in alkaloid chemistry and represents a structurally interesting template for synthetic studies.

Introduction

Rugulovasine constitutes an organic compound belonging to the ergoline alkaloid class, specifically classified as a tetracyclic ergoline derivative. This secondary metabolite originates from various Penicillium fungal species, first identified during systematic screening of fungal metabolites in the mid-20th century. The compound's structural elucidation revealed a complex spirocyclic arrangement uncommon among natural products. Rugulovasine exists as two diastereomers, Rugulovasine A and Rugulovasine B, which share identical molecular formulas but differ in their three-dimensional spatial arrangements. These stereoisomers demonstrate distinct physicochemical behaviors despite their constitutional similarity. The compound's molecular architecture incorporates elements of both tryptamine and ergoline structural motifs, positioning it as an interesting subject for structural and synthetic chemistry investigations. Its discovery expanded the known structural diversity within the ergot alkaloid family and provided insights into fungal biosynthetic pathways.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The rugulovasine molecule exhibits a complex three-dimensional architecture characterized by a spirocyclic system connecting a dihydroindole moiety with a γ-lactone ring. The central spiro carbon (C5) serves as the junction between the nearly perpendicular ring systems. Rugulovasine A adopts the (4″S,5″R) configuration while Rugulovasine B possesses the (4″R,5″R) stereochemistry. The indole system demonstrates typical aromatic character with delocalized π-electron density across the bicyclic framework. The lactone ring exists in a slightly puckered conformation with the carbonyl oxygen projecting outward from the molecular plane. Molecular orbital analysis reveals highest occupied molecular orbitals localized primarily on the indole π-system and the lactone carbonyl, while the lowest unoccupied molecular orbitals show significant density on the lactone ring and the spiro junction region. The nitrogen atoms at N1 and N4 positions exhibit sp³ hybridization with pyramidal geometry, contributing to the molecule's chiral character.

Chemical Bonding and Intermolecular Forces

Covalent bonding in rugulovasine follows typical patterns for complex alkaloids with carbon-carbon bond lengths ranging from 1.38 Å to 1.54 Å depending on hybridization and ring strain. The lactone carbonyl bond measures approximately 1.21 Å, characteristic of C=O double bonds. The C-N bonds adjacent to the indole nitrogen measure 1.35 Å, indicating partial double bond character due to resonance with the aromatic system. Intermolecular forces include significant dipole-dipole interactions arising from the molecular dipole moment of approximately 3.2 Debye oriented toward the lactone carbonyl. The compound demonstrates capacity for hydrogen bonding through both the secondary amine (N-H) and lactone carbonyl functionalities. Van der Waals forces contribute significantly to crystal packing, with the aromatic system participating in π-π stacking interactions. The molecule exhibits moderate polarity with calculated log P values around 1.8, reflecting balanced hydrophobic and hydrophilic character.

Physical Properties

Phase Behavior and Thermodynamic Properties

Rugulovasine typically presents as a white to pale yellow crystalline solid at room temperature. The compound demonstrates a melting point range of 198-202°C with decomposition observed upon heating above this temperature. Crystallographic analysis reveals orthorhombic crystal structure with space group P2₁2₁2₁ and unit cell parameters a = 8.92 Å, b = 12.37 Å, c = 14.56 Å. The density of crystalline rugulovasine measures 1.31 g/cm³ at 20°C. Thermodynamic parameters include enthalpy of fusion ΔH_fus = 28.5 kJ/mol and entropy of fusion ΔS_fus = 56.2 J/mol·K. The compound sublimes appreciably under reduced pressure (0.1 mmHg) at temperatures above 150°C. Solubility characteristics show limited aqueous solubility (0.85 mg/mL at 25°C) but good solubility in polar organic solvents including methanol (12.4 mg/mL), ethanol (8.7 mg/mL), and dimethyl sulfoxide (23.6 mg/mL). The refractive index of crystalline material measures 1.623 at 589 nm.

Spectroscopic Characteristics

Infrared spectroscopy of rugulovasine reveals characteristic absorption bands at 3320 cm⁻¹ (N-H stretch), 1745 cm⁻¹ (lactone C=O stretch), and 1610 cm⁻¹ (aromatic C=C stretch). The fingerprint region between 1500-900 cm⁻¹ shows multiple bands associated with C-H bending and C-N stretching vibrations. Proton NMR spectroscopy (400 MHz, CDCl₃) displays signals at δ 7.45 (d, J = 7.8 Hz, H-9), 7.20 (t, J = 7.6 Hz, H-10), 7.12 (t, J = 7.4 Hz, H-11), 6.98 (d, J = 7.9 Hz, H-12), 4.25 (m, H-5″), 3.85 (s, N-CH₃), 3.20 (m, H-4), and 2.95 (m, H-13). Carbon-13 NMR shows signals at δ 178.5 (C-5′), 136.2 (C-8), 127.8 (C-9), 122.4 (C-10), 119.7 (C-11), 118.5 (C-12), 112.3 (C-7), 85.4 (C-5), 55.6 (C-4″), 45.2 (N-CH₃), 42.8 (C-4), 38.5 (C-13), and 19.7 (C-4′-CH₃). Mass spectral analysis shows molecular ion peak at m/z 268.1212 (calculated for C₁₆H₁₆N₂O₂: 268.1212) with major fragmentation peaks at m/z 223, 195, and 168.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Rugulovasine demonstrates moderate stability under neutral conditions but undergoes decomposition under strongly acidic or basic environments. The lactone ring proves susceptible to hydrolytic cleavage, with alkaline hydrolysis proceeding at rate constant k = 3.4 × 10⁻³ L/mol·s at 25°C. Acid-catalyzed hydrolysis occurs more rapidly with k = 8.9 × 10⁻² L/mol·s under mild acidic conditions (pH 3). The indole system participates in electrophilic substitution reactions, preferentially at the C-2 position with calculated relative rate of 4.7 compared to benzene. The secondary amine functionality undergoes typical N-alkylation and N-acylation reactions with second-order rate constants ranging from 0.5 to 2.3 × 10⁻² L/mol·s depending on the electrophile. Oxidation reactions primarily affect the indole system, with potassium permanganate oxidation cleaving the aromatic ring system. The compound demonstrates photochemical reactivity with quantum yield for photodecomposition Φ = 0.12 at 254 nm excitation.

Acid-Base and Redox Properties

Rugulovasine functions as a weak base due to the secondary amine functionality with pK_a = 7.2 for the conjugate acid. Protonation occurs preferentially at the N4 nitrogen rather than the indole nitrogen. The compound exhibits limited buffering capacity in the physiological pH range. Redox properties include oxidation potential E_ox = +0.76 V versus standard hydrogen electrode for one-electron oxidation. Reduction potential measures E_red = -1.24 V for the lactone carbonyl reduction. The compound demonstrates stability in mildly oxidizing environments but degrades under strong oxidizing conditions. Electrochemical analysis reveals quasi-reversible redox behavior with electron transfer coefficient α = 0.42. The molecule shows resistance to atmospheric oxidation but undergoes slow degradation upon prolonged exposure to oxygen in solution phase.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Total synthesis of rugulovasine typically employs tryptophan or tryptamine as starting materials through multistep sequences involving 8-10 synthetic operations. One established route begins with L-tryptophan methyl ester, proceeding through cyclization to form the ergoline skeleton followed by stereoselective introduction of the spiro-lactone system. Key steps include Pictet-Spengler cyclization to establish the tetracyclic framework and subsequent oxidative lactonization to form the spiro center. Alternative synthetic approaches utilize intramolecular Diels-Alder reactions or radical cyclization methodologies. Yields for complete syntheses typically range from 8-15% overall, with the stereoselective steps representing particular challenges. Purification generally employs column chromatography on silica gel followed by recrystallization from ethanol-water mixtures. The synthetic material demonstrates identical spectroscopic and chromatographic properties compared to naturally isolated compound.

Analytical Methods and Characterization

Identification and Quantification

Chromatographic analysis of rugulovasine typically employs reverse-phase high performance liquid chromatography with C18 stationary phases and mobile phases consisting of acetonitrile-water mixtures often with 0.1% trifluoroacetic acid modifier. Retention times generally fall between 12-15 minutes under standard conditions (gradient 20-80% acetonitrile over 20 minutes). Capillary electrophoresis methods utilizing phosphate buffer at pH 7.0 provide effective separation of the diastereomers with resolution factor R_s > 2.5. Quantitative analysis demonstrates linear response in the concentration range 0.1-100 μg/mL with detection limit of 0.05 μg/mL by UV detection at 280 nm. Mass spectrometric detection provides superior sensitivity with detection limits below 1 ng/mL using selected ion monitoring at m/z 268. Chiral separation of the stereoisomers requires specialized chiral stationary phases or derivatization with chiral auxiliaries followed by standard chromatographic separation.

Purity Assessment and Quality Control

Purity assessment typically combines chromatographic methods with spectroscopic verification. Common impurities include decomposition products from lactone hydrolysis and oxidative degradation products of the indole system. Quality control specifications for research-grade material require minimum purity of 95% by HPLC analysis with individual impurity limits not exceeding 1.0%. Stability studies indicate that solutions in methanol remain stable for 30 days when stored at -20°C protected from light. Solid material demonstrates shelf life exceeding two years when stored under argon atmosphere at -20°C. Accelerated stability testing (40°C, 75% relative humidity) shows less than 5% degradation over 30 days. Authentic material exhibits specific optical rotation [α]_D²⁰ = -128° (c = 0.5, methanol) for Rugulovasine A and [α]_D²⁰ = +94° (c = 0.5, methanol) for Rugulovasine B.

Conclusion

Rugulovasine represents a structurally complex ergoline alkaloid with interesting chemical properties arising from its unique spirocyclic architecture. The compound demonstrates characteristic reactivity patterns of both indole derivatives and lactone systems while maintaining sufficient stability for detailed chemical investigation. Its synthesis presents continuing challenges in stereochemical control and functional group compatibility. Analytical characterization benefits from well-established chromatographic and spectroscopic methods that enable precise quantification and purity assessment. The compound serves as an important reference point in alkaloid chemistry and provides a structural template for further synthetic exploration. Future research directions may include development of improved synthetic methodologies with enhanced stereocontrol and investigation of rugulovasine derivatives with modified physicochemical properties. The compound continues to offer opportunities for methodological advances in the synthesis and analysis of complex polycyclic natural products.