Abstract

3-Dehydroshikimic acid (C₇H₈O₅), systematically named (4S,5R)-4,5-dihydroxy-3-oxocyclohex-1-ene-1-carboxylic acid, represents a key intermediate in the shikimate pathway of aromatic amino acid biosynthesis. This cyclohexene derivative exhibits distinctive structural features including a conjugated enone system, carboxylic acid functionality, and two chiral hydroxyl groups. The compound demonstrates significant polarity with a calculated logP value of -1.2 ± 0.3 and water solubility exceeding 50 g/L at 25°C. Characteristic spectroscopic signatures include strong infrared absorption at 1715 cm⁻¹ (carbonyl stretch) and 1650 cm⁻¹ (conjugated C=C). 3-Dehydroshikimic acid serves as a crucial biosynthetic precursor to numerous aromatic compounds and finds applications in synthetic chemistry as a chiral building block. Its thermal stability extends to approximately 180°C before decomposition initiates.

Introduction

3-Dehydroshikimic acid belongs to the class of organic compounds known as cyclohexenecarboxylic acids, specifically classified as a hydroxy-oxo-cyclohexenecarboxylic acid derivative. The compound occupies a central position in biochemical pathways, serving as an intermediate in the shikimate pathway that connects carbohydrate metabolism to aromatic compound biosynthesis. First identified in the mid-20th century through studies of microbial metabolism, 3-dehydroshikimic acid has since been characterized extensively using modern analytical techniques. The compound's systematic name according to IUPAC nomenclature is (4S,5R)-4,5-dihydroxy-3-oxocyclohex-1-ene-1-carboxylic acid, reflecting its specific stereochemistry at carbon positions 4 and 5. Its molecular formula of C₇H₈O₅ corresponds to a molecular mass of 172.13 g/mol.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of 3-dehydroshikimic acid features a cyclohexene ring system with specific substituents arranged in three-dimensional space. The ring adopts a half-chair conformation with the carboxylic acid group at position 1, a carbonyl group at position 3, and hydroxyl groups at positions 4 and 5. X-ray crystallographic analysis reveals bond lengths of 1.23 Å for the C3=O carbonyl bond and 1.32 Å for the C1-C2 double bond. The C1-COOH bond measures 1.47 Å, typical for carbon-carbon single bonds adjacent to carboxylic acids. Bond angles at the carbonyl carbon (C3) approximate 120°, consistent with sp² hybridization. The hydroxyl groups at C4 and C5 maintain standard C-O bond lengths of 1.42 Å and O-H bonds of 0.97 Å. The molecule possesses two chiral centers at C4 and C5, with natural occurrences exclusively exhibiting the (4S,5R) configuration.

Chemical Bonding and Intermolecular Forces

3-Dehydroshikimic acid exhibits extensive conjugation throughout its molecular framework. The π-electron system extends from the carboxylic acid group through the C1-C2 double bond to the C3 carbonyl group, creating a conjugated enone system. This electronic delocalization results in bond length equalization, with the C2-C3 bond measuring 1.45 Å, intermediate between single and double bond character. The molecule demonstrates significant polarity with a calculated dipole moment of 4.2 Debye. Intermolecular forces include strong hydrogen bonding capacity through its carboxylic acid (donor and acceptor), carbonyl (acceptor), and hydroxyl (donor and acceptor) functionalities. The compound forms extensive hydrogen bonding networks in the solid state, with O···O distances ranging from 2.65 to 2.85 Å. Van der Waals interactions contribute to crystal packing, with closest non-hydrogen bonding contacts at approximately 3.2 Å.

Physical Properties

Phase Behavior and Thermodynamic Properties

3-Dehydroshikimic acid presents as a white to off-white crystalline solid at room temperature. The compound melts with decomposition at approximately 185°C, though precise determination proves challenging due to thermal instability. Crystallographic studies indicate the compound forms orthorhombic crystals with space group P2₁2₁2₁ and unit cell parameters a = 7.52 Å, b = 9.87 Å, c = 11.23 Å, α = β = γ = 90°. The experimental density measures 1.54 g/cm³ at 20°C. The compound exhibits high solubility in polar solvents including water (>50 g/L at 25°C), methanol (≈120 g/L), and ethanol (≈85 g/L), but limited solubility in non-polar solvents such as hexane (<0.1 g/L) and diethyl ether (≈2 g/L). The enthalpy of formation from elemental constituents is calculated as -825.4 kJ/mol using computational methods. The specific heat capacity at 25°C is estimated at 225 J/mol·K.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 1715 cm⁻¹ (carbonyl stretch of carboxylic acid), 1690 cm⁻¹ (ketone carbonyl stretch), 1650 cm⁻¹ (conjugated C=C stretch), and broad O-H stretching between 2500-3300 cm⁻¹. Proton NMR spectroscopy (400 MHz, D₂O) displays signals at δ 6.85 ppm (d, J = 2.0 Hz, H-2), 4.45 ppm (dd, J = 4.5, 10.0 Hz, H-4), 4.12 ppm (m, H-5), 2.95 ppm (dd, J = 4.5, 17.5 Hz, H-6_ax), and 2.65 ppm (dd, J = 10.0, 17.5 Hz, H-6_eq). Carbon-13 NMR shows resonances at δ 183.5 ppm (C-3 carbonyl), 175.2 ppm (carboxylic acid), 146.5 ppm (C-2), 137.8 ppm (C-1), 72.5 ppm (C-4), 68.2 ppm (C-5), and 37.5 ppm (C-6). UV-Vis spectroscopy demonstrates strong absorption at 245 nm (ε = 12,400 M⁻¹cm⁻¹) corresponding to the π→π* transition of the conjugated enone system. Mass spectrometric analysis shows a molecular ion peak at m/z 172.0372 (C₇H₈O₅⁺) with major fragment ions at m/z 154 (M-H₂O)⁺, 126 (M-H₂O-CO)⁺, and 98 (M-H₂O-CO-CO)⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

3-Dehydroshikimic acid demonstrates diverse reactivity patterns stemming from its multifunctional structure. The conjugated enone system undergoes Michael addition reactions with nucleophiles at the β-position (C-2), with second-order rate constants of approximately 0.15 M⁻¹s⁻¹ for reaction with thiols at pH 7.0 and 25°C. The carboxylic acid group exhibits typical acid-base behavior with a pKₐ of 3.8 ± 0.1, while the hydroxyl groups have pKₐ values estimated at 12.2 (C4-OH) and 13.5 (C5-OH) based on analogous compounds. The compound undergoes dehydration under acidic conditions (pH < 2) to form 3,4-didehydroshikimic acid with a rate constant of 2.3 × 10⁻⁴ s⁻¹ at 25°C. Under basic conditions (pH > 10), retro-aldol cleavage occurs slowly with a half-life of approximately 48 hours at 25°C. The carbonyl group at C-3 undergoes nucleophilic addition reactions, particularly with hydroxylamine and hydrazine derivatives, with second-order rate constants ranging from 0.5-5.0 M⁻¹s⁻¹ depending on the nucleophile.

Acid-Base and Redox Properties

The compound functions primarily as a monoprotic acid due to its carboxylic acid group, with the first proton dissociation constant pKₐ₁ = 3.8. The hydroxyl groups remain protonated under typical conditions, with pKₐ values above 12. The molecule exhibits buffering capacity in the pH range 2.8-4.8. Redox properties include electrochemical reduction of the carbonyl group at E₁/₂ = -1.25 V vs. SCE in aqueous solution at pH 7.0. The enone system can undergo both chemical and electrochemical reduction, with the two-electron reduction potential measured at -0.95 V. Oxidation occurs preferentially at the electron-rich double bond, with an oxidation potential of +0.85 V vs. SCE for the first one-electron transfer. The compound demonstrates reasonable stability in neutral aqueous solution (half-life > 30 days at 25°C) but decomposes rapidly under strongly acidic or basic conditions. Autoxidation occurs slowly in aerated solutions with a rate constant of 1.2 × 10⁻⁶ s⁻¹ at 25°C and pH 7.0.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of 3-dehydroshikimic acid typically proceeds through either chemical synthesis from simpler precursors or enzymatic preparation. Chemical synthesis approaches often begin with protected derivatives of shikimic acid, employing oxidation at C-3 using chromium-based oxidants or dimethyl sulfoxide-based oxidation systems. A representative procedure involves protection of shikimic acid hydroxyl groups as acetates, followed by oxidation with pyridinium chlorochromate in dichloromethane at 0°C, yielding the 3-oxo derivative after deprotection. Typical yields range from 45-60% over multiple steps. Alternative routes utilize microbial fermentation with genetically modified Escherichia coli strains that overproduce shikimate pathway intermediates. Enzymatic preparation employs 3-dehydroquinate dehydratase (EC 4.2.1.10) acting on 3-dehydroquinic acid as substrate, with reaction conditions typically involving pH 7.0-7.5, 25-37°C, and magnesium ion cofactors. Purification commonly employs ion-exchange chromatography or crystallization from aqueous ethanol, yielding material with >98% purity by HPLC analysis.

Analytical Methods and Characterization

Identification and Quantification

Analysis of 3-dehydroshikimic acid employs multiple chromatographic and spectroscopic techniques. Reverse-phase high-performance liquid chromatography with UV detection at 210 nm provides sensitive quantification with a detection limit of 0.5 μg/mL using C18 columns and mobile phases consisting of aqueous phosphoric acid (0.1%) and methanol (95:5 to 80:20 gradient). Capillary electrophoresis with UV detection at 200 nm offers alternative separation with baseline resolution from related cyclohexenecarboxylic acids. Gas chromatography-mass spectrometry requires derivatization, typically using trimethylsilylation with N,O-bis(trimethylsilyl)trifluoroacetamide, producing characteristic fragments at m/z 459 (M⁺), 369, and 147. Nuclear magnetic resonance spectroscopy serves as the definitive identification method, particularly utilizing ¹H-¹H COSY and HSQC experiments to establish connectivity patterns. Quantitative ¹³C NMR with internal standards provides absolute quantification without need for calibration curves, with typical precision of ±2%.

Purity Assessment and Quality Control

Purity assessment typically employs orthogonal methods including HPLC-UV, capillary electrophoresis, and NMR spectroscopy. Common impurities include shikimic acid (typically <0.5%), 3-dehydroquinic acid (<0.3%), and dehydration products (<0.2%). Karl Fischer titration determines water content, typically <0.5% w/w in dried samples. Residual solvent analysis by gas chromatography with flame ionization detection establishes compliance with ICH guidelines, with limits typically set at <500 ppm for Class 3 solvents. Elemental analysis confirms composition within ±0.3% of theoretical values (C: 48.84%, H: 4.68%, O: 46.48%). The compound demonstrates good stability when stored desiccated at -20°C, with decomposition rates <0.1% per year. For research applications, purity specifications typically require >98.5% by HPLC area percentage, with individual impurities <0.5%.

Applications and Uses

Industrial and Commercial Applications

3-Dehydroshikimic acid serves as a key intermediate in the industrial production of aromatic compounds through biotechnological processes. Engineered microbial strains utilizing the shikimate pathway convert carbohydrate feedstocks to 3-dehydroshikimic acid, which subsequently undergoes chemical or enzymatic transformation to value-added products. The compound functions as a precursor in the synthesis of gallic acid through chemical dehydration and rearrangement, with typical conversion yields exceeding 85% under optimized conditions. Industrial scale production employs fermentation processes with annual production volumes estimated at several hundred metric tons globally. The compound finds application as a chiral building block in asymmetric synthesis, particularly for the construction of functionalized cyclohexane systems with specific stereochemistry. Its enone functionality allows diverse chemical modifications including reductions, nucleophilic additions, and cycloadditions, making it valuable for fine chemical synthesis.

Research Applications and Emerging Uses

In research settings, 3-dehydroshikimic acid serves as a standard reference compound for studies of the shikimate pathway and its inhibition. The compound enables mechanistic investigations of enzyme catalysis, particularly for 3-dehydroquinate dehydratase and shikimate dehydrogenase, with kinetic isotope effects using deuterated analogs providing insight into enzymatic mechanisms. Materials science applications explore its use as a multifunctional monomer for polymer synthesis, with potential for creating biodegradable polymers with tailored properties. The compound's ability to form metal complexes through its carboxylate and carbonyl groups enables coordination chemistry applications, particularly with transition metals including copper(II) and iron(III). Emerging applications investigate its potential as a precursor for novel liquid crystal compounds, leveraging its rigid cyclohexene core and multiple functional groups for mesogen design. Patent literature indicates growing interest in electrochemical applications, particularly for energy storage materials derived from its redox-active quinone/hydroquinone system.

Historical Development and Discovery

The discovery of 3-dehydroshikimic acid emerged from mid-20th century investigations into aromatic amino acid biosynthesis in microorganisms. Initial reports in the 1950s identified the compound as an intermediate in the pathway from carbohydrates to aromatic compounds, with structural elucidation completed through chemical degradation and early spectroscopic methods. The 1960s brought confirmation of its role in the shikimate pathway through enzymatic studies demonstrating its formation from 3-dehydroquinic acid by 3-dehydroquinate dehydratase and conversion to shikimic acid by shikimate dehydrogenase. X-ray crystallographic determination of its molecular structure in the 1970s provided definitive confirmation of its stereochemistry and molecular conformation. The 1980s and 1990s witnessed development of synthetic routes to the compound and its derivatives, enabling more detailed chemical studies. Recent decades have seen increased interest in metabolic engineering approaches that optimize 3-dehydroshikimic acid production for industrial applications, with advances in analytical methodology enabling more precise characterization of its properties and behavior.

Conclusion

3-Dehydroshikimic acid represents a structurally interesting and chemically versatile cyclohexenecarboxylic acid derivative with significant importance in both biological and synthetic contexts. Its conjugated enone system, carboxylic acid functionality, and chiral hydroxyl groups create a multifunctional molecule with diverse reactivity patterns. The compound serves as a crucial intermediate in biosynthetic pathways and as a valuable building block for chemical synthesis. Well-established analytical methods enable precise characterization and quantification, while synthetic approaches provide material for research and applications. The compound's stability under appropriate conditions and predictable chemical behavior make it suitable for various industrial processes. Future research directions likely include further exploration of its coordination chemistry, development of novel synthetic transformations utilizing its functional groups, and optimization of production methods for industrial scale applications. The fundamental understanding of 3-dehydroshikimic acid's properties and behavior provides a foundation for continued innovation in its utilization across chemical disciplines.