In the synthesis of the tyrosine kinase inhibitor sunitinib malate, the preparation of the 5-formyl-2,4-dimethyl-1H-pyrrole-3-carboxylic acid ethyl ester intermediate—a critical scaffold for the indolin-2-one pharmacophore—requires ethyl 2-methyl-1H-pyrrole-3-carboxylate as the starting pyrrole nucleus. Introduction of the formyl group proceeds via a Vilsmeier-Haack complex generated from phosphorus oxychloride and anhydrous N,N-dimethylformamide at a controlled temperature of 0–5 °C under nitrogen. Industrial batches employ a molar ratio of pyrrole ester to POCl3‑DMF adduct of 1:1.05–1.2, and the reaction mass is quenched into ice-cold aqueous sodium acetate to neutralise excess acid while preventing pyrrole ring degradation. The isolated 5-formyl-2-methylpyrrole-3-carboxylate is then subjected to selective N-methylation using dimethyl sulfate or methyl iodide in the presence of powdered potassium carbonate (2.5 equiv) in acetone at reflux, yielding the fully substituted pyrrole intermediate. Throughout this sequence, the regulatory framework of ICH Q7 (GMP for active pharmaceutical ingredients) dictates equipment qualification, process validation, and residual solvent control; specifically, the Class 2 solvent DMF content in the isolated intermediate is routinely reduced below 880 ppm by reslurrying in water–ethanol, conforming to ICH Q3C (R7) Option 1 limits. Additional compliance obligations arise under REACH (EC) No 1907/2006 when the substance is imported into the EU; it is typically registered as an intermediate under strictly controlled conditions (Article 18(4)), with a exposure scenario describing use in a sequentially closed, multi-step batch synthesis. Process-scale observation on glass-lined reactors (3000 L nominal capacity) indicates that premature hydrolysis of the Vilsmeier reagent above 8 °C triggers a self-condensation side reaction that elevates the dimeric impurity to >3.2 area% by HPLC, necessitating a reprocessing pass over silica gel. Downstream, the purified 5-formyl-2,4-dimethylpyrrole-3-carboxylate is condensed with tert-butyl acetoacetate under Knoevenagel conditions and subsequently cyclised with hydrazine hydrate to install the pyrazole ring, ultimately affording sunitinib base, which is converted to the malate salt for oral capsule formulation. Finished product quality specifications are anchored to USP-NF monographs for sunitinib malate, with enantiomeric purity >99.0 % and total related substances ≤0.5 %. The entire supply chain mandates that the pyrrole intermediate be accompanied by a transmissible technical dossier demonstrating absence of genotoxic impurities, as evaluated through an Ames test (OECD 471) and quantitative structure-activity relationship (QSAR) assessment per ICH M7(R1).How does ethyl 2-methylpyrrole-3-carboxylate contribute to reduced heavy-metal residues in arylpyrrole insecticides?Arylpyrrole acaricides and insecticides, such as chlorfenapyr, share a 2-aryl-5-(trifluoromethyl)pyrrole-3-carbonitrile core that is accessible from the ethyl 2-methylpyrrole-3-carboxylate platform through sequential bromination, palladium-catalyzed cross-coupling, and nitrile introduction. In the industrial route, the pyrrole ester is first regioselectively brominated at the 5-position using N-bromosuccinimide (1.08 equiv) in acetonitrile at 20–25 °C, yielding 5-bromo-2-methylpyrrole-3-carboxylate after aqueous work-up and crystallisation from heptane. The critical Suzuki-Miyaura coupling between this bromide and 4-chlorophenylboronic acid is executed with tetrakis(triphenylphosphine)palladium(0) as catalyst (0.002–0.005 mol equiv relative to bromide) and degassed aqueous K2CO3 (2.0 equiv) in a 1,4-dioxane–water biphasic mixture at 85 °C. Because the target technical-grade active ingredient must meet the heavy-metal limits prescribed in FAO Specification 59/TC (2021) for chlorfenapyr technical material, residual palladium content after work-up is monitored by inductively coupled plasma mass spectrometry (ICP-MS) and suppressed below 10 mg/kg through a mercapto-functionalised silica scavenger treatment. The table below reproduces the maximum permissible limits for elemental contaminants in accordance with that specification, together with the respective analytical reference methods used in batch release.| Element | Limit (mg/kg) | Analytical method | Standard designator |
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| Arsenic (As) | 5 | Hydride generation AAS | OECD 318 | | Cadmium (Cd) | 1 | Graphite furnace AAS | ISO 15586:2003 | | Mercury (Hg) | 1 | Cold vapour AAS | ISO 12846:2012 | | Lead (Pb) | 10 | ICP-OES | ISO 11885:2007 |
Following Suzuki coupling, the 2-(4-chlorophenyl)-5-methylpyrrole-3-carboxylate ester is converted to the 3-nitrile via amidation-dehydration employing thionyl chloride and DMF, and the 5-methyl group is exhaustively brominated and fluorinated with CF2Br2/SbF3 to install the trifluoromethyl moiety. Industrial-scale experience reveals that traces of water in the amidation step lead to formation of a recalcitrant 3-carboxamide dimer, forcing an intermediate drying stage under vacuum at 50 °C for ≥8 h before dehydration. Final chlorfenapyr technical is formulated as a suspension concentrate (SC) or emulsifiable concentrate (EC) for agricultural use. The process also remains subject to EU Regulation (EC) No 396/2005 for maximum residue limits (MRLs) and requires compliance with the Globally Harmonized System (GHS) for classification, with Signal Word “Warning” and H302/H410 statements.When tralopyril replaces cuprous oxide in self-polishing coatingsCopper-free antifouling systems built on the pyrrole-derivative biocide tralopyril (4-bromo-2-(4-chlorophenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile) begin with ethyl 2-methylpyrrole-3-carboxylate as the heterocycle precursor. The synthetic pathway starts with alkaline decarboxylation: the ester is saponified using potassium hydroxide (1.5 molar eq.) in ethylene glycol at 175–185 °C under an inert atmosphere, producing 2-methylpyrrole in crude yield exceeding 88 % after steam distillation. The decarboxylation gas evolution must be carefully vented; on manufacturing lines, a scrubber charged with 20 % aqueous NaOH captures CO2 and any entrained pyrrole, avoiding pressure build-up in glass-lined equipment. The 2-methylpyrrole is subsequently functionalized through a multi-step sequence—bromination, regioselective arylation, nitrile introduction, and trifluoromethylation—that does not involve organotin compounds, allowing the final biocide to meet the stringent heavy-metal residue thresholds stipulated in the IMO Antifouling System Convention (AFS 2001) and EU Biocidal Products Regulation (EU) No 528/2012. Once isolated to >98 % chemical purity, tralopyril is micronised to a median particle size D50 < 4 µm by air-jet milling and incorporated into self-polishing copolymer (SPC) rosin-silyl acrylate binder systems at a loading of 3.0–6.0 % w/w dry film weight, together with co-biocides such as zinc pyrithione. Performance validation according to ASTM D6903-07 (2020) using a rotating cylinder test documents a steady-state release rate below 5 µg cm⁻² day⁻¹ for a coating with a pigment volume concentration (PVC) of 34–38 %. A critical processing boundary emerges during twin-screw extrusion of the paint mass: when the jacket temperature exceeds 55 °C for more than 12 min, tralopyril undergoes partial decomposition to release hydrogen bromide, which accelerates hydrolysis of the silyl ester binder and collapses the polishing rate. Hence, extruder barrel temperature is maintained at 45–50 °C with L/D ratio 32:1, and the melt temperature is monitored by infrared thermography at the die face. Finished antifouling paints are supplied to shipyards and dry-dock facilities with a validated 60-month in-service performance life before recoating, and all biocidal products must carry a BPR authorisation number listed on the ECHA Article 95 list.Thermal decarboxylation of ethyl ester for roasted-note flavour chemicalsWhen ethyl 2-methylpyrrole-3-carboxylate is subjected to alkaline hydrolysis followed by acidification and thermal cleavage, the resulting 2-methylpyrrole functions as a key impact chemical for roasted, nutty, and cocoa-like flavour profiles. The two-step conversion begins with refluxing the ester in aqueous sodium hydroxide (2.2 molar equivalents of NaOH as a 10 % solution) for 4–6 h until saponification is complete, as confirmed by the absence of the carbonyl stretch at 1695 cm⁻¹ in FT-IR. After cooling, the reaction mixture is acidified to pH 3.0 with concentrated HCl, and the liberated 2-methylpyrrole-3-carboxylic acid is extracted into methyl tert-butyl ether. Drying over Na₂SO₄ and solvent removal afford the free acid, which is thermally decarboxylated at 190–200 °C in a wiped-film evaporator operating at 50–70 mbar to avoid charring. Distillation at atmospheric pressure cuts the 2-methylpyrrole fraction (boiling point 147–149 °C) with a recovery >85 %. The distilled product complies with the purity requirements of food-grade flavour substances: sensory evaluation detects a characteristic roasted, slightly smoky odour resembling coffee pyrazines, with no ammoniacal off-note. This pyrrole is employed as a building block in process flavourings produced by controlled Maillard reactions between reducing sugars and amino acids, typically incorporated at 1–20 mg/kg in the final foodstuff. The regulatory status of 2-methylpyrrole is established under FEMA GRAS No. 5125 and is included in the Union list of flavouring substances pursuant to Regulation (EC) No 1334/2008, with corresponding specifications for residual solvents aligning with FDA 21 CFR 172.515. Industrial process flavour houses require the pyrrole intermediate to be accompanied by a certificate of analysis demonstrating absence of N-nitrosamine contamination (limit of detection <0.5 µg/kg via LC-MS/MS), a concern arising from the decarboxylation step’s potential interaction with nitrosating agents present in air. The finished products are liquid or spray-dried encapsulated flavours marketed for coffee beverages, cocoa replacers, and savoury snack seasonings, all carrying an EU FL number and compliant with the respective EU purity criteria for chemically defined flavouring substances.BODIPY fluorophore assembly via Knoevenagel routeIn the field of functional fluorescent dyes, ethyl 2-methylpyrrole-3-carboxylate serves as a monomeric synthon for the construction of borondipyrromethene (BODIPY) chromophores, a class of probes valued for their high molar absorptivity and narrow emission bandwidths. The preparation employs a Knoevenagel-type condensation between two equivalents of the pyrrole ester and one equivalent of an aromatic aldehyde—typically 4-methoxybenzaldehyde or 4-formylphenylboronic acid—in anhydrous dichloromethane containing molecular sieves (4 Å). Piperidine is used as the organocatalyst at a loading of 2 mol% with respect to the aldehyde, and the reaction is stirred at 20–25 °C for 18–24 h under nitrogen until thin-layer chromatography (silica gel, ethyl acetate–hexane 1:4) confirms complete consumption of the aldehyde. The resulting dipyrromethane intermediate is oxidized in situ with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 0.95 equiv) at 0 °C over 30 min, then complexed with boron trifluoride diethyl etherate (3.0 equiv) in the presence of N,N-diisopropylethylamine at room temperature. After quenching with water, the crude product is purified by column chromatography on silica gel (mesh 230–400) eluting with dichloromethane, delivering the BODIPY dye as a dark red crystalline solid with HPLC purity >97.5 %. The absorption maximum (λabs) of the 3,5-dicarbethoxy-2-methyl-substituted scaffold typically falls in the range 498–508 nm in dichloromethane, with a molar extinction coefficient ε exceeding 80,000 M⁻¹ cm⁻¹. Since these compounds are manufactured for research and diagnostic use only (RUO), they are placed under the exemption provisions of REACH for substances used in scientific R&D in quantities below 1 tonne per annum; however, suppliers routinely screen for the absence of potential mutagenic aromatic amines arising from aldehyde impurities through Ames testing (OECD 471) on representative batches. Small-scale production reactors are typically borosilicate glass jacketed vessels, and the BF3 complexation stage requires that ambient relative humidity remain <40 % to avoid precipitation of boric acid side products that reduce fluorophore quantum yield. Terminal products are sold as vials of dye powder or pre-dissolved in spectro-grade solvents, applied in flow cytometry, fluorescence microscopy, and live-cell imaging assays, where laser excitation at 488 nm or 514 nm is standard. No medical or diagnostic label claims are attached.
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