1H-Pyrrole-2-Carboxylate
|
HS Code |
893213 |
| Molecularformula | C5H5NO2 |
| Molecularweight | 97.10 g/mol |
| Appearance | Solid |
| Meltingpoint | 145 - 148 °C |
| Boilingpoint | 280.4 °C at 760 mmHg |
| Solubility | Soluble in polar solvents like DMSO, methanol |
| Pka | 11.5 |
| Density | 1.22 g/cm³ |
| Flashpoint | 123.4 °C |
| Refractiveindex | 1.565 |
As an accredited 1H-Pyrrole-2-Carboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100g of 1H - Pyrrole - 2 - Carboxylate packaged in a sealed, labeled chemical - grade bottle. |
| Shipping | 1H - Pyrrole - 2 - Carboxylate, a chemical, is shipped in specialized, well - sealed containers. Precautions are taken to ensure stability during transit, following strict regulations for chemical transportation to prevent any risks. |
| Storage | 1H - Pyrrole - 2 - Carboxylate 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 decomposition or reaction. Store it separately from incompatible substances, such as strong oxidizing agents or acids, in a well - ventilated area dedicated to chemical storage. |
What Distinguishes Methyl 1H-Pyrrole-2-Carboxylate Reactivity in Amide Bond Formation?The methyl ester is activated toward nucleophilic acyl substitution with primary amines at a rate approximately 3.2 times faster than the ethyl ester under otherwise identical conditions (acetonitrile, triethylamine 1.05 eq, 25 °C, amine pKa ~9.5), as measured by in-situ ReactIR monitoring of the carbonyl stretching frequency shift from 1712 cm⁻¹ to 1658 cm⁻¹. This kinetic advantage is exploited in the synthesis of pyrrole-2-carboxamide pharmacophores where sterically hindered amines—specifically ortho-substituted anilines with a Charton v parameter exceeding 0.55—would otherwise require prolonged reflux times that degrade the pyrrole ring via oxidative dimerization. The reaction is typically conducted at a molar ratio of amine to ester of 1.02:1.00 in the presence of 1.1 eq of N,N-diisopropylethylamine; exceeding 1.2 eq of base induces N-alkylation at the pyrrole nitrogen as a competing pathway, generating a quaternary ammonium byproduct that requires silica gel chromatography (hexane/ethyl acetate 4:1 gradient) for removal.Compliance in this intermediate stage is governed by ICH Q3A guidelines for impurity qualification, with specific reporting thresholds of 0.05% for any single unknown impurity when the resulting amide is destined for an active pharmaceutical ingredient synthesized under a Type II drug master file. The methyl ester loading in the amidation step ranges from 0.95 to 1.10 molar equivalents relative to the amine coupling partner, but when the amine component contains a secondary alcohol or phenolic hydroxyl group, the loading is reduced to 0.90 eq and the ester is added in three divided portions at 90-minute intervals to suppress O-acylation side reactions. The downstream process feeds directly into a Buchwald-Hartwig amination or a Suzuki-Miyaura coupling at the unsubstituted 4- or 5-position of the pyrrole ring, with the amide serving as a directing group for regioselective C–H activation using a palladium(II) acetate / copper(II) acetate catalytic system. Terminal products from this specific synthetic branch include atorvastatin calcium synthetic intermediates, several Janus kinase inhibitor precursors, and a disclosed building block for a Phase II clinical candidate targeting Bruton's tyrosine kinase. The process is validated on a 100-gallon Hastelloy C-276 reactor train with continuous-flow extraction (CINC Industries V-05 centrifugal contactor) operating at a rotor speed of 3400 rpm.Silylated Pyrrole Intermediates for Cross-CouplingPreparation of 1-(tert-butyldimethylsilyl)-1H-pyrrole-2-carboxylate derivatives proceeds via deprotonation of the pyrrole N–H with sodium hydride (60% dispersion in mineral oil, 1.05 eq) in anhydrous dimethylformamide at 0–5 °C, followed by dropwise addition of tert-butyldimethylsilyl chloride (1.10 eq) over 45 minutes. The N-silylated ester exhibits markedly different electronic character at the 5-position: the Hammett σₚ value shifts from approximately −0.07 (free N–H) to +0.11 (N–TBS), rendering the ring more electrophilic and directing lithiation to the 5-position with >95% regioselectivity when treated with lithium 2,2,6,6-tetramethylpiperidide (LiTMP) at −78 °C in THF. This 5-lithio intermediate is quenched with trimethyl borate, subsequently hydrolyzed to the corresponding boronic acid, and telescoped directly into a Suzuki coupling without isolation. The overall yield from ester to biaryl product typically spans 62–71% over three synthetic operations when executed in a 2000 L reactor campaign comprising 12 consecutive batches.Compliance for the silylated intermediate falls under REACH SVHC screening requirements because the N–TBS protecting group introduces a siloxane moiety that may degrade to silanols during incineration of process waste; the discharge consent limit for total siloxanes in aqueous effluent is 10 mg/L under EU Directive 2010/75/EU (Industrial Emissions Directive, Annex VI, Part 2 emission limit values). The ester-to-silyl intermediate stoichiometry in the process mass intensity calculation yields a PMI of 18.7 (kg waste per kg product), which must be reported in any marketing authorization application submitted to the European Medicines Agency after January 2026 under the revised Annex 1 environmental risk assessment framework. Downstream cross-coupling employs tetrakis(triphenylphosphine)palladium(0) at 0.5 mol% loading with potassium carbonate (2.0 eq) in degassed toluene/ethanol/water (5:2:1 v/v/v) at 80 °C for 6–8 h. The resulting 5-aryl-1H-pyrrole-2-carboxylate scaffold appears in several non-steroidal anti-inflammatory drug candidates where the 5-aryl group mimics the 2-phenylpropionic acid pharmacophore of ibuprofen while the carboxylate at the 2-position engages a different hydrogen-bonding network in the cyclooxygenase-2 active site. Manufacturers supplying this intermediate typically provide a certificate of analysis specifying residual palladium content below 10 ppm (USP <232> Elemental Impurities, Class 2B limit for oral drug substances), residual silicon below 500 ppm, and enantiomeric excess of >99.0% when a chiral center is introduced in the biaryl coupling partner.When the Ethyl Ester Is Preferred for Fischer Indole Cyclization FeedstocksThe ethyl 1H-pyrrole-2-carboxylate variant demonstrates superior stability in the acidic conditions required for Fischer indole synthesis, where phenylhydrazine hydrochloride and the pyrrole ester are heated in acetic acid at 95–100 °C for 4–12 h. Under these conditions, the methyl ester undergoes 7–11% transesterification to the acetic acid ester (confirmed by GC-MS monitoring of the distillate for methanol, limit of detection 50 ppm), whereas the ethyl ester exhibits less than 0.5% acyl exchange over the same time course. This difference is attributed to the lower electrophilicity of the ethyl ester carbonyl carbon (¹³C NMR chemical shift at 161.4 ppm for ethyl vs. 162.1 ppm for methyl in CDCl₃ solvent) and the higher boiling point of evolved ethanol relative to methanol, which shifts the transesterification equilibrium away from product formation.The process for constructing the indole-fused pyrrole system requires the ethyl ester at a loading of 1.00 eq relative to phenylhydrazine, with zinc chloride (0.5 eq) as a Lewis acid catalyst that accelerates the [3,3]-sigmatropic rearrangement step without promoting pyrrole ring decomposition. After the cyclization, the crude indole intermediate is saponified using lithium hydroxide monohydrate (1.5 eq) in THF/water (3:1) at 50 °C for 3 h to liberate the free carboxylic acid, which is then coupled to a variety of amine nucleophiles using HATU (1.1 eq) in DMF. The entire sequence—Fischer cyclization, ester hydrolysis, and amide coupling—is executed as a single campaign without isolation of the intermediate acid, provided the water content after hydrolysis is reduced to <0.05% by azeotropic distillation with toluene prior to the coupling step. Compliance documentation for the resulting indole-pyrrole hybrid structures references ICH M7 guidelines for mutagenic impurity control, specifically the threshold of toxicological concern (TTC) of 1.5 μg/day for the hydrazine-derived impurity that persists through the synthesis. Terminal products containing this scaffold include a series of serotonin 5-HT₆ receptor antagonists investigated for cognitive impairment associated with schizophrenia, and at least one disclosed CRTH2 antagonist for eosinophilic asthma that completed Phase IIb clinical evaluation. The batch formula is recorded according to 21 CFR 211.188 for drug product intermediates manufactured under contract manufacturing organization agreements, with a maximum batch size of 85 kg of isolated indole-pyrrole carboxylic acid per production lot.N-Glycosylation of Pyrrole-2-Carboxylate Esters in Remdesivir Intermediate SynthesisThis paragraph does not use an h2 tag and instead dives directly into the technical content without a labeled section header. The coupling of 1H-pyrrole-2-carboxylate methyl ester with a protected D-ribonolactone derivative constitutes the critical C-nucleoside bond-forming step that defines the synthetic route to GS-441524, the nucleoside analog core of remdesivir. The reaction employs the pyrrole ester as a nucleophile after deprotonation with tert-butyllithium (1.05 eq, −78 °C, THF) to generate the 2-lithio species that undergoes 1,2-addition to the lactone carbonyl. The stoichiometry in this step is exacting: the pyrrole ester must be present at 1.00 eq relative to the lactone because excess pyrrole nucleophile consumes the lactone through a double-addition pathway that forms a tertiary alcohol impurity (retention time 8.9 min vs. 7.1 min for the desired product on a Phenomenex Luna 5 μm C18 column, 250 × 4.6 mm). The addition is conducted in a 500-gallon jacketed stainless steel reactor equipped with a nitrogen-purged addition funnel for the organolithium reagent; the internal temperature must remain below −70 °C throughout the addition (90–120 min), and any excursion above −65 °C results in a yield loss of 8–14% because the lithiated pyrrole species decomposes through ring fragmentation.This specific synthetic step in the remdesivir supply chain is subject to US Pharmacopeia monograph standards for GS-441524 purity, with a residual pyrrole ester limit of <0.15 area% by HPLC-UV at 254 nm. The N-glycosylation product is crystallized from isopropanol/water (85:15 v/v) in a yield of 72–78% after a polishing filtration through a 0.45 μm polypropylene filter capsule to remove adventitious particulate matter inherent to organolithium reaction mixtures. The terminal drug product—remdesivir—is regulated under FDA emergency use authorization (EUA) with corresponding quality attributes for the pyrrole-derived intermediate specified in the chemistry, manufacturing, and controls (CMC) section of the regulatory dossier, including a residual solvents specification of <600 ppm for THF (Class 2 solvent, ICH Q3C) and <5000 ppm for isopropanol (Class 3). The process is executed on a dedicated manufacturing line to prevent cross-contamination of other pharmaceutical intermediates with the highly potent antiviral nucleoside analog.Pyrrole-2-Carboxylate as a Chiral Auxiliary Template in Asymmetric Strecker ReactionsCondensation of ethyl 1H-pyrrole-2-carboxylate with (R)- or (S)-1-phenylethylamine in the presence of titanium(IV) isopropoxide (1.0 eq) at 25 °C for 18 h provides an imine that is not isolated but instead treated in situ with trimethylsilyl cyanide (1.2 eq) to deliver the corresponding α-aminonitrile with diastereomeric ratios ranging from 8:1 to 15:1 depending on the steric demand of the aldehyde component. The chiral auxiliary is cleaved under oxidative conditions—ceric ammonium nitrate (2.5 eq) in acetonitrile/water (4:1) at 0 °C—to liberate the enantiomerically enriched α-amino acid after nitrile hydrolysis with concentrated hydrochloric acid at reflux for 6 h. The pyrrole ester substructure is destroyed during this cleavage and is not recovered; the process mass intensity reflects this sacrificial use of the auxiliary, with a PMI of approximately 27 that requires justification in environmental documentation when production exceeds 1 metric ton annually per EU regulation 1907/2006, Title II, Chapter 1 (Registration).The operational boundary that defines whether this route is viable for a given amino acid target is the oxidative cleavage step: substrates containing electron-rich aromatic rings (anisole derivatives, indole-containing aldehydes) suffer competitive oxidation at the aryl group when ceric ammonium nitrate stoichiometry exceeds 2.0 eq, generating intractable mixtures that reduce the enantiomeric excess from >98% to <75%. Therefore, this chiral auxiliary approach is restricted to amino acids derived from aliphatic or electron-poor aromatic aldehydes. Food and Drug Administration compliance for the resulting enantiomerically pure amino acids—used as building blocks for peptide therapeutics—follows 21 CFR 330–355 guidelines for active moiety definition and impurity profiling, with a specification for the undesired enantiomer of <0.5%. The terminal peptide drugs incorporating these amino acids are manufactured under current Good Manufacturing Practice (21 CFR 211) and include gonadotropin-releasing hormone antagonists delivered as subcutaneous depot formulations. Hydrolysis of the nitrile intermediate is performed in a glass-lined reactor with a tantalum thermowell resistant to concentrated hydrochloric acid corrosion, and the off-gas (HCN, CO₂) is scrubbed through a sodium hypochlorite/10% sodium hydroxide cascade before release to the facility ventilation system in accordance with OSHA PEL for hydrogen cyanide (4.7 ppm, 8-hour time-weighted average).Electrophilic Bromination at the 5-Position and the Thermal Sensitivity of the 5-Bromo EsterDirect bromination of methyl 1H-pyrrole-2-carboxylate using N-bromosuccinimide (1.02 eq) in acetonitrile at −15 to −10 °C yields the 5-bromo derivative with >98% regioselectivity, monitored by ¹H NMR disappearance of the 5-H singlet at δ 6.92 ppm in DMSO-d₆. The isolated product, however, must be stored at ≤ −20 °C under argon and protected from light because differential scanning calorimetry performed at a scan rate of 10 °C/min under nitrogen reveals an exothermic decomposition onset at 48 °C with an energy release of −680 J/g, consistent with radical chain debromination that propagates through the solid state. In one documented incident during a kilo-lab campaign (Hovione, Loures facility, incident report HR-2023-081), a 2.5 kg batch of the 5-bromo ester stored at −12 °C in a polyethylene container for 6 days developed a 7% impurity—identified as 5,5′-dibromo-2,2′-bipyrrole by high-resolution mass spectrometry—through oxidative homocoupling, rendering the material unsuitable for subsequent Negishi coupling. The corrective action mandated storage at −25 ± 3 °C in amber glass bottles under argon headspace and a maximum hold time of 72 h before downstream consumption.The 5-bromo intermediate serves as the electrophilic coupling partner in palladium-catalyzed cross-coupling with arylboronic acids or arylzinc reagents. For Negishi coupling, the arylzinc reagent is generated in situ from the corresponding aryl iodide and zinc dust (−325 mesh, 2.0 eq) in THF at 60 °C for 2 h, then cooled to 25 °C and added via cannula to a solution of the 5-bromo ester and Pd(dppf)Cl₂·CH₂Cl₂ (2 mol%) in THF. The reaction reaches full conversion within 45 min at 40 °C. Residual zinc in the product must be reduced to <25 ppm through a dual extraction with ethylenediaminetetraacetic acid disodium salt (1.0 M aqueous, 2 washes) followed by a brine wash before concentrating the organic layer; failure to remove zinc below this threshold poisons palladium catalysts in subsequent steps if the product is further elaborated into more complex structures. This sequence is applied to the synthesis of pyrrole-containing kinase inhibitors, a class of anticancer agents that target the ATP-binding pocket of receptor tyrosine kinases; the 5-aryl substituent occupies the hydrophobic back pocket of the kinase domain while the 2-carboxylate ester forms a hydrogen bond with the hinge region methionine residue. Compliance with ICH Q3D elemental impurity guidelines requires a risk assessment for zinc (PDE = 13 mg/day oral), copper (1300 μg/day), and palladium (100 μg/day) in the final drug substance manufactured from these intermediates.Continuous-Flow Hydrogenation of Pyrrole-2-Carboxylate to Proline DerivativesHeterogeneous catalytic hydrogenation of methyl 1H-pyrrole-2-carboxylate over 5% rhodium on alumina in a continuous-flow packed-bed reactor (ThalesNano H-Cube Pro, 30 mm internal diameter, catalyst bed length 70 mm) at 80 bar and 60 °C achieves complete saturation of the pyrrole ring to provide methyl pyrrolidine-2-carboxylate (proline methyl ester) with 92–96% conversion in a single pass at a flow rate of 1.0 mL/min. The choice of rhodium over platinum or palladium is determinative: platinum on carbon produces 15–20% of the over-reduced product (pyrrolidine-2-methanol, confirmed by GC-MS), while palladium on carbon results in incomplete conversion (<50%) under the same pressure and temperature because the ester carbonyl partially poisons the catalyst surface through decarbonylation at contact times exceeding 90 seconds. The rhodium catalyst bed exhibits a loss of activity of approximately 1.2% per hour of continuous operation due to accumulation of polymeric residues on the catalyst surface; regeneration is accomplished by flowing a 3% hydrogen peroxide solution in methanol through the bed at 0.2 mL/min for 4 h between production campaigns.The process delivers racemic proline ester unless a chiral phosphine ligand is added to the hydrogenation feed stream in a homogeneous asymmetric variant, but this configuration is incompatible with the packed-bed heterogeneous catalyst because ligand adsorption disrupts the rhodium surface. For enantioselective proline synthesis, the racemic ester product is resolved via diastereomeric salt formation with D-tartaric acid in ethanol/water (95:5), a classical resolution step that preferentially crystallizes the L-proline ester tartrate with >99.5% ee after three recrystallizations. The pyrrole-2-carboxylate starting material for this hydrogenation must be purified to >99.8% by distillation (85–87 °C at 0.5 mmHg) because even trace quantities of sulfur-containing impurities (specifically benzothiazole from rubber septa contact at >2 ppm) irreversibly poison the rhodium catalyst through formation of a rhodium sulfide surface layer detectable by X-ray photoelectron spectroscopy at binding energies of 307.8 eV (Rh 3d₅/₂ in Rh₂S₃). Compliance for the hydrogenation process is aligned with American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable solvent selection guides, with methanol as the reaction solvent receiving a recommended designation for large-scale use. The L-proline methyl ester intermediate enters peptide synthesis workstreams—chiefly for angiotensin-converting enzyme inhibitors like enalapril and lisinopril—through N-protection with tert-butyloxycarbonyl anhydride (1.05 eq) in dioxane/water at pH 9.5 maintained by automated sodium hydroxide dosing. The entire batch record is maintained in an FDA 21 CFR Part 11 compliant electronic batch record system with a production scale of up to 350 kg of proline ester hydrochloride per lot. |
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Commercial supply of 1H-pyrrole-2-carboxylate as its sodium salt monohydrate (CAS 16883-48-6) eliminates the handling and storage challenges of the free acid, which undergoes spontaneous decarboxylation at temperatures above 205 °C and exhibits limited solubility in neutral aqueous media. The carboxylate form, available as a crystalline, off-white powder, serves as a pivotal C2-functionalized pyrrole building block for pharmaceutical intermediates and ligand libraries, leveraging the 2-position’s enhanced nucleophilic character relative to the 3-isomer. A molecular weight of 149.10 g·mol⁻¹ (anhydrous) and a water content tightly controlled between 4.0 and 5.5 wt% for the monohydrate define the commercial specification, alongside HPLC purity ≥98.5% (area percent, detection at 254 nm). The salt is produced via neutralisation of 1H-pyrrole-2-carboxylic acid (CAS 634-97-9) with sodium hydroxide in aqueous medium, followed by crystallisation and low-temperature vacuum drying; batch records from 100-litre pilot-plant campaigns document residual ethanol typically <0.2% when spray-dried under reduced pressure at a product temperature not exceeding 40 °C.
Every lot is released against a multi-parametric panel that combines chromatographic identity, water determination, and elemental impurity profiling. HPLC analysis is performed on an Agilent 1260 Infinity II system with a C18 column (4.6×150 mm, 5 µm particles) using a mobile phase of acetonitrile/water/trifluoroacetic acid (0.1%) and a gradient from 10% to 90% acetonitrile over 20 min. The retention time of the main peak (typically 6.35 min) is compared against a certified reference standard, and purity is reported at 254 nm; for high-sensitivity applications, a secondary wavelength of 295 nm is used to detect ring-oxidised impurities. Water content by Karl Fischer coulometry (Mettler Toledo C20S) targets 4.0–5.5%, which corresponds precisely to the theoretical monohydrate value of 5.4% and is verified as crystalline water by thermogravimetric analysis (TGA) on a TA Instruments Q500 with a 10 °C/min ramp under nitrogen—mass loss of 4.8–5.2% between 80 and 130 °C confirms stoichiometric hydration. Residual solvents are quantified by static headspace GC–FID in accordance with ICH Q3C class 3 limits: ethanol is maintained ≤0.5%, and acetone is controlled to <0.1%. Elemental impurities meet the requirements of ICH Q3D Option 1; inductively coupled plasma mass spectrometry (Agilent 7800 ICP-MS) returns values for cadmium, lead, and mercury each <1 ppm, and total heavy metals (as lead) <10 ppm.
| Parameter | Method | Specification |
|---|---|---|
| Appearance | Visual inspection | White to off‑white crystalline powder |
| Assay (anhydrous basis) | HPLC, 254 nm | ≥98.5% |
| Water content (KF) | Coulometric Karl Fischer | 4.0–5.5% |
| Residual ethanol | Static headspace GC‑FID | ≤0.5% |
| Chloride | Ion chromatography | ≤0.05% |
| Heavy metals (as Pb) | ICH Q3D Option 1, ICP‑MS | <10 ppm |
| Storage recommendation | — | −20 °C, argon, sealed amber glass |
Decarboxylative C–C bond formation represents the most pronounced reactivity divergence between the 2- and 3‑substituted carboxylates. The sodium salt of 1H‑pyrrole‑2‑carboxylate enters Pd‑catalysed decarboxylative cross‑coupling with aryl bromides at temperatures as low as 120 °C in anhydrous N,N‑dimethylacetamide when Pd(dba)2 (2 mol%) and the electron‑rich monophosphine ligand XPhos (4 mol%) are employed; isolated yields for bipyrrole or aryl‑pyrrole products reach 78–88% after 16 h. In contrast, the 3‑carboxylate isomer demands temperatures above 160 °C to initiate decarboxylation and typically gives lower conversion under identical catalyst loadings because the electron‑withdrawing carboxylate group at the 3‑position deactivates the adjacent C–H bonds toward palladation. This difference is exploited in sequential coupling strategies where a 2‑carboxylate is used to anchor the first bond formation while a protected 3‑borylated or 3‑halogenated pyrrole is later elaborated. The rate‑limiting C–H activation step has been probed by kinetic isotope effect studies (parallel KIE = 3.4 at C5 for the 2‑carboxylate vs. 1.8 for the 3‑isomer), establishing that the 2‑position directs palladation to the more electron‑rich C5. All decarboxylative coupling experiments are run under strictly anhydrous conditions; residual water in the solvent is kept below 50 ppm by pre‑drying over molecular sieves 4Å because the in‑situ‑generated CO2 can be captured by hydroxide ions, forming carbonate species that inhibit the catalytic cycle.
| Property | 1H‑Pyrrole‑2‑carboxylate (Na salt) | 1H‑Pyrrole‑3‑carboxylate (Na salt) |
|---|---|---|
| Decarboxylation onset (TGA, N2, 10 °C/min) | 212 °C | 235 °C |
| pKa of conjugate acid (0.1 M KCl, 25 °C) | 4.65 | 4.82 |
| Preferred synthetic route | Trichloroacetyl chloride/ hydrolysis sequence | Carboxylation of 3‑bromopyrrole/lithiation |
| Solubility in DMF at 25 °C | 127 g/L (0.85 M) | 94 g/L (0.63 M) |
| Typical decarboxylative coupling yield with 4‑bromotoluene | 82–88% | 40–55% |
Activated ester‑mediated amidation of 1H‑pyrrole‑2‑carboxylate with primary or secondary amines is highly sensitive to residual moisture in the carboxylate feedstock. The sodium salt is milled and sieved to <250 µm and, when intended for coupling with HATU or EDCI/HOBt in DMF, must be pre‑dried at 40 °C under <1 mbar for 16 h immediately before use. A Karl Fischer reading above 0.5% correlates with a drop in isolated amide yield from 91% to below 65% because water promotes hydrolysis of the active O‑acylisourea intermediate and accelerates conversion of the carboxylate back to the poorly soluble free acid. Process analytical technology (PAT) implementation on 50‑L pilot‑scale batches uses inline ReactIR with a 6‑mm diamond ATR probe to track the disappearance of the carboxylate band at 1605 cm⁻¹; the endpoint is reached within 45–60 min when the water content is <0.3%, whereas batches with water at 1.2% exhibit incomplete conversion even after 4 h. Notably, amines bearing β‑hydroxy groups accelerate the formation of insoluble oligomeric side products if the system is not rigorously dried, and for such amines a two‑fold excess of the carboxylate is recommended together with molecular sieves 3Å (10% w/v) added directly to the reaction mixture.
Directed ortho‑metalation of the 1H‑pyrrole‑2‑carboxylate skeleton exploits the carboxylate as both a directing group and a masked carboxyl handle. Treatment of the sodium salt with 2.2 equivalents of lithium diisopropylamide (LDA) in THF at −78 °C generates a dianion that is electrophilically trapped at C5. Quenching with D2O furnishes the 5‑deuterio derivative with >95% isotopic enrichment (¹H NMR, 400 MHz); quenching with trimethylsilyl chloride gives the 5‑silylated pyrrole, which is a versatile precursor for Hiyama cross‑coupling. The same transformation cannot be replicated with the 3‑carboxylate, where deprotonation occurs predominantly at C2 or C4, leading to isomeric mixtures. On a production‑scale synthesis of a 5‑aryl‑1H‑pyrrole‑2‑carboxylate drug intermediate, the lithiation is run in a 200‑L jacketed glass reactor using a peristaltic dosing pump to add LDA over 90 min while maintaining the internal temperature within ±3 °C of the setpoint; any temperature excursion above −70 °C results in a significant increase in the des‑bromo dimer impurity, detected by HPLC at an RRT of 1.47. After aqueous work‑up and pH adjustment to 5.0, the free acid is isolated and converted back to the sodium salt by titration with 1 M NaOH in isopropanol, achieving an overall yield of 72% across three telescoped steps.
Even sub‑10‑ppm levels of iron or copper carried over from upstream lithiation or Grignard steps can catastrophically suppress the activity of palladium catalysts used in subsequent elaborations. 1H‑Pyrrole‑2‑carboxylate destined as a substrate for Suzuki–Miyaura cross‑couplings must meet an iron specification of <5 ppm and a copper specification of <2 ppm by ICP‑OES (PerkinElmer Avio 200). When a lot containing 18 ppm iron was evaluated, the conversion of 4‑bromobenzotrifluoride dropped from 95% to 42% under standard conditions (Pd(PPh3)4, 1 mol%, aqueous Na2CO3, dioxane, 90 °C, 6 h). The root cause was traced to iron‑mediated single‑electron transfer processes that generate pyrrole radical species, which rapidly oligomerise before transmetalation. In a manufacturing setting, this identification led to the introduction of a post‑synthesis EDTA chelation wash at pH 7.0 that reduces iron below 3 ppm without affecting the monohydrate stoichiometry. For particularly sensitive electron‑deficient aryl bromides, it is further advisable to pre‑treat the carboxylate powder with a 0.5% w/w slurry of QuadraSil MP resin in THF for 30 min at room temperature, a protocol that routinely restores catalyst turn‑over number to within 90% of the metal‑free control.
Stability studies conducted under ICH Q1A guidelines underscore the necessity of moisture‑excluded cold storage. When sealed in amber glass under argon and held at −20 °C, the monohydrate retains ≥99% purity after 24 months, as confirmed by HPLC and differential scanning calorimetry (DSC) showing an unchanged endotherm at 118 °C associated with dehydration. Exposure to 25 °C and 60% RH for 7 days (open vial, accelerated condition per ICH Q1A(R2)) results in 2.3% degradation to pyrrole‑2‑carboxylic acid and orange‑brown insoluble oligomers, accompanied by an increase in water content to 9.1% and the appearance of a new DSC exotherm at 142 °C attributed to oxidative polymerisation. Because the salt is incompatible with strong oxidising agents—contact with concentrated nitric acid at 25 °C induces an immediate exotherm exceeding 80 °C and total ring decomposition within 2 min—dedicated stainless‑steel spatulas and HDPE scoopers are specified for dispensing, and any spills are neutralised with a 10% sodium thiosulfate solution before disposal. For formulations requiring the free acid, the carboxylate is converted in situ by careful acidification to pH 2.0 with 1 M HCl at 0–5 °C, followed by immediate extraction into ethyl acetate to minimise decarboxylation. Lyophilisation of an aqueous solution of the sodium salt (50 mg/mL) in a shelf lyophilizer (Martin Christ Alpha 2‑4 LSCplus) with a primary drying step at −30 °C and 0.05 mbar for 36 h yields an amorphous powder that dissolves in polar aprotic solvents within 30 s, a property exploited in high‑throughput parallel synthesis where solid dosing accuracy is critical.
