Methyl 1H-Pyrrole-2-Carboxylate

Catalytic hydrogenation of methyl 1H-pyrrole-2-carboxylate over a 5% Pd/C catalyst (water-wet, 50% moisture) in a stirred Hastelloy C-22 autoclave yields methyl pyrrolidine-2-carboxylate — the direct chiral pool precursor to the L-proline moiety present in angiotensin-converting enzyme (ACE) inhibitors. The process operates at a hydrogen pressure of 3.0–5.0 bar and a jacket temperature maintained between 25 °C and 45 °C to suppress pyrrole ring hydrogenolysis and maintain enantiomeric integrity when a chiral phosphine ligand is employed in the asymmetric variant. A typical batch charge is 800–1,200 kg of methyl 1H-pyrrole-2-carboxylate in methanol at a substrate-to-catalyst mass ratio of 10:1, with hydrogen uptake monitored by an in-line mass flowmeter calibrated per ISO 9300. The exotherm is controlled through staged gas introduction; failure to keep the peak temperature below 50 °C increases the formation of N-methylpyrrolidine by-products, which are difficult to reject during downstream crystallization. Pre-treatment with activated carbon at 60 °C for 2 h prior to hydrogenation is mandatory when the input ester contains > 5 ppm residual sulfur species that poison the palladium surface. After catalyst removal by cross-flow filtration through a 0.2 μm ceramic membrane, the hydrogenated ester is saponified with 2 M NaOH to afford the free proline, which is isolated as the hydrochloride salt by isoelectric precipitation. Compliance is maintained under ICH Q7 Section 12 (Cleaning Validation) and ICH Q11 Section 3.2 (Starting Material Selection). The final amino-acid derivative is incorporated at 0.85–1.10 kg per kilogram of captopril API via a mixed anhydride coupling with thioacetic acid, with residual solvent levels (methanol, ethyl acetate) below the limits prescribed in USP 467 Procedure A. Finished dosage forms include oral tablets — captopril, enalapril maleate, lisinopril dihydrate — and fixed-dose combinations with hydrochlorothiazide.

What Role Does Methyl 1H-Pyrrole-2-Carboxylate Play in the Asymmetric Synthesis of (S)-α-Ethyl-2-oxo-1-pyrrolidineacetamide?

In the enantioselective route to levetiracetam, the hydrogenated pyrrolidine ester obtained from asymmetric reduction of the pyrrole-2-carboxylate serves as the stereogenic center. Asymmetric hydrogenation employs a ruthenium-(R)-BINAP catalyst system in methanol at 60–80 °C and 8–12 MPa hydrogen pressure in a high-pressure Hastelloy reactor equipped with a magnetic-drive agitator, achieving enantiomeric excess values routinely above 95%, provided the substrate is dry and free of chloride ions above 50 ppm. Post-reduction, the (S)-pyrrolidine-2-carboxylic acid methyl ester hydrochloride is crystallized from methanol/MTBE (1:3 v/v) and reacted with α-bromobutyramide in the presence of potassium carbonate in acetonitrile at 50 °C to yield the α-ethyl-2-oxo-1-pyrrolidineacetamide intermediate. The ring-closing step requires strict exclusion of moisture; Karl Fischer titration of the reaction mixture must read < 0.05% water to avoid lactam ring-opening. A final de-esterification or deamidation step furnishes levetiracetam base, which is purified by recrystallization from 2-propanol. Typically, 1.75–1.95 kg of methyl 1H-pyrrole-2-carboxylate is consumed per kilogram of levetiracetam API, accounting for a 35–40% overall yield across the asymmetric hydrogenation and five subsequent transformations. Relevant quality standards include ICH Q3D elemental impurity thresholds (palladium ≤ 10 μg/g, ruthenium ≤ 10 μg/g), residual solvent compliance under USP 467 Option 2, and polymorphic control verified by X-ray powder diffraction per Ph. Eur. 2.2.33. The final product is micronized to a particle size distribution D90 ≤ 30 μm for immediate-release tablet compression, marketed under the trade name Keppra® and its generics.

In a cooled (-15 °C) dichloromethane solution containing 0.1 equivalents of trifluoroacetic acid, methyl 1H-pyrrole-2-carboxylate condenses with an arylaldehyde in a 2:2.2 molar ratio to afford a dipyrromethane intermediate, the immediate precursor to the BODIPY fluorophore. Oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and subsequent complexation with boron trifluoride diethyl etherate generates the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene core. The condensation is performed under an argon blanket at -15 °C to 0 °C with reaction progression monitored by thin-layer chromatography (silica gel GF₂₅₄, hexane:ethyl acetate 7:3). The intermediate dipyrromethane is notably air-sensitive; exposure to atmospheric oxygen for > 20 min leads to irreversible oligomerization, so the oxidation step is executed in the same vessel by dropwise addition of DDQ dissolved in THF. After stirring for 2 h at room temperature, triethylamine (3 eq) is added, followed by BF₃·OEt₂ (3 eq), and the mixture is heated to 50 °C for 4 h to complete the chelation. The crude BODIPY is purified via flash chromatography (silica, gradient CH₂Cl₂: MeOH) to achieve an HPLC purity ≥ 98% at λₐᵪ = 498 nm (ASTM E2719). In in-vitro diagnostics, the carboxylate ester on the BODIPY core is activated as an NHS ester and conjugated to antibodies or oligonucleotides; typical incorporation in a flow-cytometry calibration microsphere is 2–10 μg of fluorophore per gram of polystyrene beads. Manufacturing of fluorescent conjugates falls under ISO 13485:2016 Section 7.3 (Design and Development) and, if supplied for clinical diagnostics, must comply with IVDR (EU) 2017/746 Annex I for analytical performance. Final end-products include CD4/CD8 lymphocyte enumeration kits, high-resolution size-calibrated fluorescent microspheres for confocal microscopy, and real-time PCR probes bearing a carboxy-BODIPY quencher.

When treated with acetic anhydride and anhydrous aluminium chloride in carbon disulfide at 10–15 °C, methyl 1H-pyrrole-2-carboxylate undergoes electrophilic acetylation at the 2-position to produce 2-acetylpyrrole — a potent roasted, nutty, popcorn-like aroma compound that forms the key impact note in bread crust, coffee, and malt flavourings. An industrial charge under nitrogen typically loads 500 kg of the pyrrole ester, 420 kg of acetic anhydride, and 1,100 kg of ACS-grade AlCl₃ into a glass-lined double-jacketed reactor with a gate-type agitator, maintaining a setpoint of 12 °C to minimise polysubstitution. The reaction mass is quenched onto crushed ice, maintaining the internal temperature below 25 °C, and the organic layer is separated and washed with 5% sodium bicarbonate until neutral. The product is fractionally distilled under vacuum (10 mmHg, overhead at 118–120 °C) through a 1 m packed column to achieve a gas-chromatographic purity exceeding 99% area count (GC-FID per ASTM E2887, column DB-Wax 30 m × 0.25 mm × 0.25 μm). The acetylation step requires strictly anhydrous conditions; water contamination above 0.1% of the batch mass triggers formation of an unstirrable AlCl₃ hydrate slurry that halts production and demands mechanical cleaning. The final 2-acetylpyrrole is incorporated into compounded flavour systems at 0.05–0.5% w/w, and the typical use level in finished foodstuffs ranges from 10 ppm to 50 ppm, as regulated by EU Regulation 1334/2008/EC and FEMA GRAS No. 3202. Compliance with the IFRA Standard 49th Amendment Category 1 for leave-on applications requires additional testing for Dermal Sensitization QRA (Quantitative Risk Assessment). The methyl ester starting material must meet a purity ≥ 99.5% (GC) to avoid off-odor notes from sulfur- or nitrogen-containing precursors. End consumer products include bakery dry-mixes, microwavable popcorn seasoning, instant coffee retronasal enhancers, and meat analogue flavour bases.

Phenylpyrrole Fungicide Scarfold: Fludioxonil and Fenpiclonil Precursor Chemistry

The pyrrole-2-carboxylate methyl ester is the central building block for phenylpyrrole non-systemic fungicides, most notably fludioxonil [4-(2,2-difluoro-1,3-benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile] and fenpiclonil [4-(2,3-dichlorophenyl)-1H-pyrrole-3-carbonitrile]. In the fludioxonil process, the methyl ester is first N-protected with a trimethylsilylethoxymethyl (SEM) group, brominated at the 4- and 5-positions with N-bromosuccinimide in dimethylformamide at 0–5 °C, and then subjected to a sequential Suzuki-Miyaura cross-coupling with 2,2-difluoro-1,3-benzodioxole-4-boronic acid using Pd(PPh₃)₄ (2 mol%) and aqueous sodium carbonate in a toluene/ethanol mixture at 80 °C for 12 h. Copper(I) cyanide-mediated cyanation of the remaining bromide in N-methyl-2-pyrrolidone at 130 °C introduces the nitrile, and a final deprotection under acidic conditions releases fludioxonil, which is recrystallized from heptane/ethyl acetate to a purity > 98% by HPLC (CIPAC Method 359/TC/M/1). The mass intensity of this linear sequence drives a consumption factor of approximately 0.90–1.05 kg methyl 1H-pyrrole-2-carboxylate per kg of fludioxonil technical. The key process bottleneck lies in the bromination regioselectivity: unless the SEM protection is complete (confirmed by ¹H NMR absence of the N–H signal), dibromination occurs at the 2- and 5-positions, creating non-compliant impurities. The coupling step is sensitive to palladium metal leaching, which is controlled by post-reaction chelation with trimercaptotriazine silica scavenger, bringing residual Pd below 20 μg/g. Regulatory compliance integrates FAO Specification 580/TC (fludioxonil technical concentrate), EPA 40 CFR 180.516 tolerance limits in cereal grains, and CIPAC 1/A for suspension concentrate formulations. Fludioxonil is primarily sold as a 480 g/L SC formulation for seed treatment, with co-formulants such as metalaxyl-M or difenoconazole, applied at rates of 2.5–10 g a.i./100 kg seed on wheat and soybeans, and as a wettable powder (50% ai) for foliar application on vines and pome fruit.

Corrosion Inhibitor for Carbon Steel Exposed to Hydrochloric Acid Pickling Solutions

Methyl 1H-pyrrole-2-carboxylate functions as a mixed-type corrosion inhibitor for cold-rolled low-carbon steel (SAE 1008) in 1–3 M hydrochloric acid pickling baths commonly used for oxide-scale removal downstream of hot rolling. Weight-loss tests according to ASTM G31-72 (reapproved 2019) demonstrate that at an inhibitor loading of 100 mg/L and a temperature of 30 °C, the corrosion rate of freshly ground steel panels drops from 12.4 mm/yr to below 0.9 mm/yr, corresponding to an inhibition efficiency > 92% after 6 h immersion. Potentiodynamic polarization scans (ASTM G5, scan rate 0.166 mV/s, three-electrode flat cell with Ag/AgCl reference) reveal a parallel shift of both cathodic and anodic Tafel slopes, confirming the mixed inhibition mechanism through Langmuir adsorption of the pyrrole π-electron system onto the ferritic surface. Electrochemical impedance spectroscopy at open circuit potential (frequency range 100 kHz to 0.01 Hz, perturbation amplitude 10 mV) shows a two-fold increase in charge-transfer resistance and the appearance of a low-frequency inductive loop indicative of adsorbed intermediate species. The formulation is limited by solubility; concentrations above 300 mg/L cause precipitation of a greenish iron-pyrrole complex that raises turbidity beyond 10 NTU and can foul heat exchangers in recirculating acid systems. No pre-drying of the inhibitor is required, although bulk storage compatibility demands that moisture ingress be kept below 0.2% to prevent ester hydrolysis and off-gassing of methanol. While no dedicated ISO standard exists for this specific ester, compliance is demonstrated through OECD 301B (ready biodegradability, 28-day window) and REACH Annex VII (ECHA endpoint summaries). Industrial consumption rates are typically 50–200 g of inhibitor per ton of pickling solution, and the treated acid batch can be reused for up to 8 turns before metal accumulation mandates disposal. Finished products are marketed as additive packs for steel coil pickling lines and oilfield acidizing fluids, blended with non-ionic surfactants and acetylenic alcohols to achieve synergistic inhibition at temperatures up to 60 °C.