2-Pyrrolecarboxylic Acid
|
HS Code |
671640 |
| Chemical Formula | C5H5NO2 |
| Molar Mass | 111.10 g/mol |
| Appearance | Solid |
| Odor | Characteristic |
| Solubility In Water | Slightly soluble |
| Solubility In Organic Solvents | Soluble in some organic solvents |
| Melting Point | 137 - 140 °C |
| Boiling Point | Decomposes before boiling |
| Acidity Pka | ~4.4 (approximate value) |
As an accredited 2-Pyrrolecarboxylic Acid factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 2 - Pyrrolecarboxylic Acid packaged in 1 - kg containers for chemical use. |
| Shipping | 2 - Pyrrolecarboxylic Acid is shipped in well - sealed, corrosion - resistant containers. These are carefully packed to prevent breakage. Shipments follow strict chemical transport regulations to ensure safe delivery. |
| Storage | 2 - Pyrrolecarboxylic acid should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly - sealed container to prevent moisture absorption and exposure to air, which could potentially lead to degradation. Store it separately from incompatible substances like strong oxidizing agents. Ideal storage temperature is around room temperature, but avoid areas with large temperature fluctuations. |
Building the triazine-fused pyrrolo[2,1-f][1,2,4]triazine core—critical to the intracellular delivery of the nucleotide analogue antiviral pro-drug Remdesivir—begins with 2-pyrrolecarboxylic acid as the primary annulation substrate. Industry compliance follows ICH Q7, Section 19.4 (validation of API starting materials) and supplier qualification under FDA 21 CFR 211.84, with additional genotoxic impurity control aligned to ICH M7(R2). In a validated multi-kilo route, the carboxylic acid is activated in a mixed-solvent system (anhydrous THF/DMF, 4:1 v/v) at -15 °C ± 2 °C using isobutyl chloroformate (1.05 molar equivalents) in the presence of N-methylmorpholine to form a moisture-sensitive mixed anhydride. This intermediate undergoes immediate cyclocondensation with a cyanoamidine hydrochloride salt; the stoichiometric ratio relative to the amidine free base is tightly maintained at 0.98–1.02 eq. to suppress regioisomeric N-acylation, a persistent impurity that resists removal by simple recrystallization and requires preparative HPLC intervention if formed above 0.15% area. Downstream manufacturing deploys a three-step telescoped process in a multi-purpose glass-lined reactor train (≤5000 L, Pfaudler AWL 12000) equipped with tempered jacket cooling and Hastelloy C-22 wetted centrifugal separator (Heinkel HX 1200). Process analytical technology based on in-line ReactIR monitors the disappearance of the mixed anhydride carbonyl stretch (1815 cm⁻¹), preventing over-hydrolysis that would generate N-methyl morpholine hydrochloride accumulation and triazine ring-opening during the subsequent amidine displacement. The crude triazine ester is telescoped into the displacement step without isolation, reducing operator exposure to potent intermediates and cutting cycle time by 8 hours compared to a stop-and-isolate campaign. Final crystallisation from IPA:water (3:2) followed by jet-milling (Sturtevant Micronizer, polycarbonate liner) delivers the active pharmaceutical ingredient with D₉₀ ≤ 15 µm. The terminal dosage form is a sterile, lyophilized powder for injection (100 mg/vial), reconstituted for hospital-based intravenous infusion; excipient selection is restricted to sulfobutylether-β-cyclodextrin and pH-adjusting HCl/NaOH to avoid incompatibility with the phosphorylated prodrug moiety.What limitations arise from unmodified polypyrrole in amperometric biosensors, and how does a 5-substituted pyrrole-2-carboxylate comonomer resolve them?Electropolymerized polypyrrole (PPy) films deposited on indium tin oxide-coated polyethylene terephthalate flex circuits suffer from two linked failure modes in continuous glucose monitoring strips: anion-exchange-driven mechanical delamination when cycled in phosphate-buffered saline, and non-specific protein fouling that reduces amperometric signal by 35–50% within 6 hours. Copolymerisation of 2-pyrrolecarboxylic acid—introduced directly as the 2-carboxyl-substituted monomer—at a feed ratio of 7–20 mol% (relative to pyrrole) into the electroformed film introduces pendant carboxyl anchoring sites that simultaneously crosslink the matrix and provide covalent enzyme tethering points. The carboxyl groups are activated post-polymerisation by immersion in 50 mM MES buffer pH 5.5 containing 2 mM EDC and 5 mM NHS for 90 minutes at 4 °C, enabling direct conjugation to the amine residues of glucose oxidase without a separate glutaraldehyde cross-linking bath. Biocompatibility evaluation of the finished sensor strip follows ISO 10993-5:2009 (MEM elution cytotoxicity) and ISO 10993-10:2010 (intracutaneous irritation). The polymerisation itself is conducted galvanostatically at 0.50 mA/cm² in a flow-through cell with aqueous 0.1 M sodium polystyrenesulfonate (NaPSS, Mw 70,000) supporting electrolyte and 0.05 M total monomer concentration; film thickness is coulometrically controlled to 55 mC/cm², delivering a dry-state thickness of 180–220 nm. Post-conjugation, the coated trace is dip-rinsed in deionized water, dried with filtered nitrogen, and assembled into a die-cut medical-grade PET spacer laminate with a 0.6 µL capillary chamber. Sterilisation by ethylene oxide at 55 °C with 30% relative humidity is validated per ISO 11135:2014. The terminal product is a single-use enzymatic amperometric biosensor test strip for capillary whole blood, yielding a linear measurement range of 20–600 mg/dL and a total test time of 5 seconds.SEI instability at potentials ≥4.40 V against LiNi₀.₈₂Mn₀.₁₀Co₀.₀₈O₂ (NMC811) cathodes remains the primary capacity-fade pathway for silicon-graphite pouch cells tested at 45 °C and a charge cutoff of 4.35 V. Introducing 2-pyrrolecarboxylic acid as a film-forming additive into the baseline electrolyte (1.0 M LiPF₆ in EC:EMC:DMC 3:3:4 vol%) at a concentration of 0.8–2.0 wt% generates a multifunctional polycarboxylate-containing passivation layer during the first formation cycle. Cyclic voltammetry on a glassy carbon working electrode (scan rate 1 mV/s, Li/Li⁺ reference) reveals an irreversible oxidation onset at 4.35 V, roughly 200 mV below the carbonate solvent decomposition edge, confirming preferential sacrificial oxidation. This preferential reactivity suppresses electrolyte consumption and transition-metal dissolution from the cathode; post-mortem ICP-OES analysis of the anode shows nickel content reduced from 480 ppm (baseline) to 125 ppm (additive-containing) after 800 deep-discharge cycles. Compliance with hazardous goods transportation standards for the assembled cell is verified per UN 38.3 Section 38.3.4.8 (overcharge test), and cell-level safety evaluation follows IEC 62660-3:2022 (puncture and crush). Wetting and distribution of the additive are critical: the compound is pre-dissolved in EMC at 25 wt% under argon with molecular sieve dehydration (H₂O < 5 ppm by Karl Fischer), then metered by a magnetic-drive gear pump into the main electrolyte blending vessel held under nitrogen blanket (O₂ < 3 ppm). Formation follows a multi-step protocol: 0.02C constant-current to 3.85 V, tap-charge to 4.20 V, aging at 55 °C for 24 hours, degassing and resealing in a -65 kPa vacuum chamber. The following table documents capacity retention across varying additive loadings in single-layer pouch cells (NMC811 | Si-C, 220 mAh design capacity) after cycling at 1C charge / 1C discharge, 3.0–4.35 V, 45 °C:
When the pyrrole ring participates in a bis-heterocyclization toward pyrrolo[1,2-a]pyrrole analgesics2-Pyrrolecarboxylic acid enters a divergent intermediate platform for non-steroidal pyrrolo[1,2-a]pyrrole anti-inflammatory agents via a decarboxylative Friedel-Crafts acylation route, a synthetic strategy documented in multiple DMF dossiers for over-the-counter ketorolac-like compounds. The regulatory framework applicable to the manufacturing of this advanced intermediate follows ICH Q3C (R8) residual solvent classification and EMA/CHMP/ICH/135/95 for genotoxic impurity control, with specific alert structures (acyl halides, anhydrides) monitored as potential PMI (potential mutagenic impurity) classes. In cGMP synthesis, the acid is first converted to a thioester derivative by reaction with 2-mercaptopyridine N-oxide (1.15 molar equivalents) in anhydrous dichloromethane under DCC (1.10 eq.) coupling at 0–5 °C. The isolated thioester is critical because direct activation of the carboxylic acid to an acid chloride leads to pyrrole ring chlorination at the 5-position at temperatures above -10 °C. The thioester then engages a samarium diiodide-mediated reductive coupling with a substituted aryl vinyl ketone; strict control of the SmI₂ addition rate (0.1 M solution in THF, 2.50 eq.) and internal temperature (-30 °C ± 3 °C) is mandatory to preclude uncontrolled radical propagation and intractable tar formation. During scaling from 20 L to 1000 L glass-lined equipment, process development identified that exotherm overshoot to -22 °C increased tar content from 2% to 18% and required a subsequent activated-carbon treatment that attenuated overall yield by 12 points. The crude tetracyclic dihydropyrrolo-pyrrole is directly crystallized from MTBE:n-heptane (1:4 v/v) at -10 °C, bypassing a silica-gel plug and satisfying a residual Pd-content specification of < 5 ppm (ICP-MS) required for oral-dosage APIs. Terminal micronisation is performed by air-jet milling (Sturtevant Micronizer) with a polycarbonate liner and nitrogen carrier gas to achieve D₉₀ < 10 µm, enabling direct compression with microcrystalline cellulose (Avicel PH-102) and sodium stearyl fumarate lubricant; magnesium stearate is avoided due to its documented incompatibility with the pyrrolo ring, which undergoes ring-opening under shear in the presence of divalent cations. The finished dosage form is an immediate-release oral tablet at 10 mg label strength, packaged in PVC/PVDC/aluminium blisters to limit moisture ingress below 25% RH equilibrium.The lipophilic halogenated pyrrole motif found in the broad-spectrum miticide/insecticide Chlorfenapyr is assembled from a 2-pyrrolecarboxylic acid-derived 2-aryl-5-(trifluoromethyl)pyrrole-3-carbonitrile intermediate. Manufacturing activities performed within OECD member-state boundaries are conducted under the framework of OECD Principles of Good Laboratory Practice (ENV/MC/CHEM(98)17) and must conform to the FAO/WHO Manual on Pesticide Specification equivalence procedures for technical-grade active ingredients (≥940 g/kg purity). The synthetic sequence starts with Fischer esterification of the acid in absolute ethanol saturated with anhydrous HCl gas, achieving 99.2% conversion after 8-hour reflux; the resulting ethyl ester is neutralized with aqueous NaHCO₃ and distilled at 100 °C / 15 mbar through a wiped-film evaporator (UIC KDL 5) to remove water and color bodies. The dry ester then undergoes a Hantzsch-type heterocyclization with trifluoroacetaldehyde methyl hemiacetal in glacial acetic acid at 80 °C in the presence of ammonium acetate; an optimized 2.05 molar equivalents of the tri-fluoro building block is required because substoichiometric loading leads to aldol-condensed linear byproducts that precipitate on the condenser surfaces of the continuous distillation head, forcing a shutdown of the rectification column after 3–4 batches. After quenching into ice-water and toluene extraction, the resulting trifluoromethylpyrrole intermediate is subjected to regioselective bromination using 1.10 eq. bromine in 1,2-dichlorobenzene at 80–85 °C under irradiation from a 500 W UV lamp (365 nm peak). Temperature control during bromination is critical: exotherms above 95 °C produce a dibrominated impurity that co-crystallises with the desired 4-bromo-aryl-5-(trifluoromethyl)pyrrole-3-carbonitrile and requires a 5-plate DMF/water recrystallization to remove. The purified nitrile is hydrogenated over Raney Nickel at 5 bar H₂, 40 °C in isopropanol to yield the corresponding amine key intermediate for the final-stage diazotization and coupling. Downstream formulation of the technical material into an emulsifiable concentrate (240 g/L chlorfenapyr) passes through a high-shear rotor-stator pre-mix (IKA Ultra-Turrax) followed by a horizontal media mill (NETZSCH MiniCer) operating with 0.3 mm yttria-stabilized zirconia beads at a tip speed of 10 m/s to achieve D₅₀ < 1.8 µm. The commercial end-use product is a 240 g/L EC formulation, applied via ground boom sprayer to fruiting vegetables and ornamentals for control of Tetranychus spp. and lepidopteran pests.A Mixed-Ligand Zirconium-Pyrrole-2-Carboxylate MOF for Post-Combustion Carbon CaptureMetal-organic frameworks assembled with 2-pyrrolecarboxylic acid as a heteroatom-functionalized, non-linear co-ligand demonstrate enhanced CO₂/N₂ selectivity and hydrostatic stability that surpasses that of unfunctionalized UiO-type sorbents when exposed to humid flue gas at 45 °C. Industrial qualification of such shaped adsorbents for fixed-bed gas separation refers to ISO 17284:2014 (performance testing of packed-bed adsorbers) and pressure-vessel structural integrity codes under ASME BPVC Section VIII, Division 1. The framework designated Zr-PyrCA-505 is synthesized solvothermally: ZrCl₄ (1.00 mmol) and a mixed linker set consisting of 2-pyrrolecarboxylic acid (2.00 mmol, 66.7 mol% of total carboxylate ligand) and terephthalic acid (1.00 mmol) are dissolved in anhydrous DMF (20 mL) with formic acid (4.0 mL) as a crystal-growth modulator, then sealed in a Teflon-lined stainless-steel autoclave and held at 120 °C for 24 hours. The fine modulation by formic acid limits crystallite size to 80–120 nm, which reduces mass-transfer resistance in subsequent pelletization. Following centrifugal recovery, the microcrystalline powder undergoes solvent exchange with methanol in a Soxhlet extractor for 48 hours, followed by activation under dynamic vacuum (< 10⁻³ mbar) at 150 °C for 12 hours. The activated powder is shaped by pan-granulation with an aqueous polyvinyl alcohol binder (5.0 wt% on dry solids) into 0.6–1.0 mm spherical pellets; binder calcination post-granulation is avoided because heating above 220 °C decarbonylates the pyrrole-2-carboxylate ligand, causing framework collapse that reduces BET surface area from 1050 m²/g to 310 m²/g. Single-column breakthrough testing on a 15/85 v/v CO₂/N₂ mixture at 1 bar, 298 K and a gas-hourly space velocity of 600 h⁻¹ yields a dynamic CO₂ uptake of 2.14 mmol/g and an equilibrium selectivity factor of 52 over N₂, with the heel bound water content (0.8 wt%) not degrading performance below 90% of dry adsorption capacity across 120 adsorption-desorption cycles. The terminal installation for which this material is designed is a vacuum-pressure swing adsorption skid processing 50,000 Nm³/h of blast furnace off-gas, reducing CO₂ concentration from 22% to 3% before the purified stream enters the primary gas grid. |
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In the context of heterocyclic building blocks employed in active pharmaceutical ingredient (API) synthesis, 2-pyrrolecarboxylic acid (CAS 634-97-9; synonym: pyrrole-2-carboxylic acid) is supplied in crystalline form with a documented tendency to sublime when exposed to dynamic vacuum at temperatures exceeding 80 °C. Production-scale batches are typically isolated via aqueous acidic precipitation from alkaline hydrolysates of the corresponding nitrile, a route that introduces trace sodium and chloride contaminants. Residual sodium levels determined by inductively coupled plasma mass spectrometry (ICP-MS) in accordance with USP <233> are routinely controlled to < 50 ppm. The product is stored in polyethylene-lined fibre drums under nitrogen overlay, as exposure to relative humidity above 60 % RH at 25 °C leads to surface hydration with a measurable increase in titratable water content of 0.3–0.5 wt% within 48 hours. This hygroscopic drift mandates pre-drying under vacuum (50 mbar, 40 °C) for 12 hours prior to use in moisture-sensitive transformations such as HATU-mediated amide couplings in anhydrous dimethylformamide.
The pharmaceutical intermediate grade (designated 2-PCA-PHI) is released against a specification panel anchored to orthogonal chromatographic and titrimetric methods. Purity by high-performance liquid chromatography (HPLC) with diode-array detection at 254 nm, using a C18 column (150 mm × 4.6 mm, 5 µm) and an acetonitrile/0.1 % phosphoric acid gradient, is set at ≥ 99.0 area%. The single maximum impurity is capped at ≤ 0.3 area%, with any unspecified impurity limited to ≤ 0.10 area%. Principal process-related impurities monitored include pyrrole-2-carbonitrile (≤ 0.2 %) and unreacted pyrrole (≤ 0.1 %). Water content is measured by coulometric Karl Fischer titration according to ASTM E203-16, with a specification of ≤ 0.5 %. Loss on drying at 105 °C for 3 hours does not exceed 0.5 %. Residue on ignition (sulfated ash) is specified at ≤ 0.1 % per Ph. Eur. 2.4.14. Heavy metals, expressed as lead, are restricted to ≤ 10 ppm by a semi-quantitative method aligned with USP <231>, though validated ICP-MS protocols are increasingly substituted for regulatory submissions. Melting range, determined by differential scanning calorimetry (DSC) at a scan rate of 10 K/min under nitrogen, spans 204–208 °C with decomposition onset near 210 °C. The typical assay against a certified reference standard by non-aqueous titration with tetrabutylammonium hydroxide yields 99.5–100.5 % on the anhydrous basis.
For in-process control and final release testing, a robust gradient method is configured on a 4.6 mm ID column packed with endcapped octadecylsilyl silica of 3 µm particle size. Mobile phase A is 0.1 % phosphoric acid, and mobile phase B is acetonitrile. The gradient profile runs from 5 % B to 80 % B over 25 minutes, with a hold at 80 % B for 5 minutes before re-equilibration. Injection volume is 10 µL, column temperature is maintained at 30 °C, and flow rate is 1.0 mL/min. Under these conditions, 2-pyrrolecarboxylic acid elutes at approximately 8.2 minutes, well resolved from the pyrrole peak at 4.7 minutes and the nitrile impurity at 12.3 minutes. System suitability requires a resolution of ≥ 3.0 between the main peak and a spiked pyrrole standard. Tailing factor for the principal peak is monitored and must fall between 0.9 and 1.5.
| Parameter | Research Grade | Pharmaceutical Intermediate Grade | Test Method |
|---|---|---|---|
| Assay (anhydrous basis) | ≥ 97.0 % | ≥ 99.0 % | Non-aqueous titration (TBAH) |
| Water (Karl Fischer) | ≤ 1.0 % | ≤ 0.5 % | ASTM E203-16 |
| Sulfated ash | ≤ 0.3 % | ≤ 0.1 % | Ph. Eur. 2.4.14 |
| Single max. impurity | ≤ 1.0 area% | ≤ 0.3 area% | HPLC (Ph. Eur. 2.2.29) |
| Melting range (DSC) | 202–209 °C | 204–208 °C | DSC, 10 K/min, N₂ |
| Residual solvents (GC-HS) | Not specified | Ethanol ≤ 500 ppm | USP <467> |
Pilot-scale batches processed in 200 L glass-lined reactors have demonstrated a particle size distribution with a D₅₀ of 120–180 µm when crystallized under controlled cooling rates of 0.5 K/min from 80 °C to 5 °C. This morphology is favorable for dissolution in polar aprotic solvents; full dissolution in dimethyl sulfoxide at 100 g/L is achieved with stirring at 25 °C within 15 minutes. Milling to a D₉₀ below 50 µm is offered as a bespoke option for customers operating continuous flow hydrogenation platforms where particle size directly influences column back-pressure fluctuations. In such fine-powder form, the material presents a dust explosion hazard (Kst value not established; combustible dust classification per OSHA standard 29 CFR 1910.1200 applies), requiring grounding and inert gas blanketing during milling and transfer operations.
A known operational limitation emerges when the compound is recrystallized from water at temperatures exceeding 90 °C in the presence of residual acid. Decarboxylation to pyrrole is accelerated, with a half-life of approximately 4 hours in refluxing aqueous 0.5 M HCl. This pathway necessitates strict pH control during hot aqueous work-up; the liquor is neutralized with sodium bicarbonate to pH 5–6 prior to cooling to optimize yield and suppress the pyrrole impurity that otherwise partitions into the crystalline product at levels of 0.5–1.0 %.
2-Pyrrolecarboxylic acid differs fundamentally from its 3-carboxy isomer in the orientation of the carboxyl group relative to the heterocyclic nitrogen. In the 2-position, the carboxylic acid exerts a strong electron-withdrawing inductive and resonance effect that deactivates the ring toward electrophilic substitution at the 5-position while simultaneously enhancing the acidity of the N–H proton. The pKₐ of the carboxyl group is 3.80 (potentiometric, 25 °C, 0.1 M KCl), compared with 4.10 for 3-pyrrolecarboxylic acid. The N–H deprotonation occurs above pH 15 for both isomers, but the heightened acidity of the 2-carboxy derivative facilitates selective N-alkylation under phase-transfer conditions using mild bases such as potassium carbonate in acetone at reflux. The 3-isomer, by contrast, requires stronger bases (sodium hydride in DMF) to achieve comparable alkylation rates, a difference that has been exploited in the synthesis of substituted indole scaffolds where N-protection selectivity is paramount.
The directing effect of the 2-carboxy group also modifies the oxidative addition step in palladium-catalyzed direct arylation. With aryl bromides and a palladium(II) acetate/tricyclohexylphosphine catalyst system in pivalic acid, C–H activation occurs preferentially at the 5-position of the 2-carboxy pyrrole, whereas the 3-isomer yields a mixture of 2- and 5-arylated products under identical conditions. This regioselectivity is lost if the carboxyl group is esterified, as the methyl ester directs metallation to the 3-position via a competing chelation pathway. Scaling such reactions to 50 L vessel size has required careful catalyst loading optimization: at 0.5 mol% Pd, conversion stalls at 70–80 % after 16 hours due to catalyst deactivation by coordination to the pyrrole nitrogen. Increasing the loading to 1.5 mol% restores complete conversion, though this adds cost and mandates a rigorous residual palladium scavenging step (trimercaptotriazine-functionalized silica, down to < 10 ppm Pd) to meet ICH Q3D oral permitted daily exposure limits.
| Property | 2-Pyrrolecarboxylic Acid | 3-Pyrrolecarboxylic Acid |
|---|---|---|
| CAS RN | 634-97-9 | 931-03-3 |
| Melting range (DSC, °C) | 204–208 | 140–143 |
| pKₐ (COOH, 25 °C, aq. KCl) | 3.80 | 4.10 |
| Preferred site for electrophilic bromination | 5-position | 2- & 5- positions (mixture) |
| Solubility in water (g/L, 20 °C) | ~12 | ~25 |
| Rate of thermal decarboxylation (rel.) | Faster (proximity to ring N) | Slower |
| N-alkylation efficiency (K₂CO₃, acetone) | High | Low; requires NaH/DMF |
| IR carbonyl stretch (KBr, cm⁻¹) | 1665 | 1680 |
The solubility differential between the two isomers has a direct consequence on work-up during multi-step telescoped processes. In a representative synthesis of a pyrrole-2-carboxamide kinase inhibitor intermediate, the crude post-coupling mixture is quenched into water at 0–5 °C. The low aqueous solubility of 2-pyrrolecarboxylic acid (~12 g/L at 20 °C) allows direct isolation by filtration, whereas the 3-isomer, with roughly twice the solubility, would require two extractions with ethyl acetate, increasing cycle time and solvent consumption. This aspect frequently drives the selection of the 2-carboxy scaffold in process chemistry when a crystalline acid intermediate can streamline the downstream synthetic sequence.
Published data on the long-term stability of 2-pyrrolecarboxylic acid in solution is limited; however, accelerated aging studies (ICH Q1A, 40 °C/75 % RH, open container) show that discoloration to a light beige tint occurs after 4 weeks, accompanied by a purity drop of 0.5–0.8 area% with an increase in oligomeric species detected at relative retention times of 1.8–2.2. The product is therefore supplied in sealed containers with desiccant packs and is recommended for use within 12 months when stored at 15–25 °C in the original unopened packaging. For kilo-scale laboratory synthesis, anhydrous formulations of 2-pyrrolecarboxylic acid are available as a lyophilized powder from selected vendors, packaged in septum-capped amber glass bottles under argon; this presentation is particularly suited to users operating Glovebox systems where moisture-sensitive Grignard reagent formation is required.
In contrast to unsubstituted pyrrole, which polymerizes readily upon exposure to acid or air, 2-pyrrolecarboxylic acid is considerably more bench-stable owing to the electron-deficient character of the ring. This enables its use as a surrogate for pyrrole in Sonogashira alkynylation sequences where free pyrrole would undergo competing oxidative coupling. The carboxy group can be subsequently removed via protodecarboxylation using a copper(I) oxide catalyst in quinoline at 180 °C, releasing pyrrole in situ for further functionalization — a strategy documented in the synthesis of lamellarin alkaloid cores.
Avoid combining 2-pyrrolecarboxylic acid with strong oxidizing agents, as contact with concentrated nitric acid leads to exothermic nitration and decomposition. In neutralization with strong bases, the heat of neutralization in water is approximately –55 kJ/mol; scale-up of neutralization steps should be carried out with jacket cooling capable of maintaining the internal temperature below 30 °C to avoid decarboxylation side reactions. Pre-weighed, ground-joint compatible containers are employed for shipping single-use aliquots of 1 kg and 5 kg to minimize repetitive opening of bulk containers in production suites.
