Ethyl 5-Bromo-1H-Pyrrole-2-Carboxylate

Pyrrole-2-carboxylate Bromination Patterns as Hinge-Binding Motifs in ATP-Competitive Inhibitor Design

The ethyl 5-bromo-1H-pyrrole-2-carboxylate scaffold is routed into preclinical oncology programs where a substituted pyrrole nucleus mimics the adenine hinge-binding region of kinases. In a typical c-Met or JAK2 inhibitor campaign, the ester is subjected to a palladium-mediated Suzuki-Miyaura cross-coupling with a substituted pyrimidin-4-ylboronic acid pinacol ester. The stoichiometric ratio is controlled at 1.00:1.12 (bromide scope:boronic ester) using 0.8 mol% Pd(PPh₃)₄ and anhydrous K₂CO₃ at 2.5 molar equivalents relative to the pyrrole substrate. The heterogeneous reaction is operated in a degassed toluene/ethanol/water mixture (3:1:1 v/v/v) under a nitrogen blanket in a 1,600 L glass-lined reactor with retreat-blade agitation maintained at 85 rpm. Heating to 78–82 °C over 16–20 h drives conversion, monitored by in-line ReactIR tracking the disappearance of the C-Br stretch at 520 cm⁻¹. Upon cooling, the organic layer is separated and treated with 2.5% w/w activated charcoal (Norit SX-plus) at 50 °C for 4 h to adsorb residual palladium, targeting final Pd content below 10 µg/g as determined by ICP-MS per USP 〈232〉. The coupled ethyl ester is then hydrolysed with LiOH·H₂O (1.05 eq) in THF/water at 25 °C to isolate the free acid, which is activated with CDI and condensed with a functionalised aniline to append the solubilising sulfonamide tail. Compliance as a regulatory starting material is maintained under ICH Q11 gatekeeping, with specifications for residual solvents aligned to ICH Q3C limits (toluene ≤ 890 ppm, ethanol ≤ 5,000 ppm). The multi-kilogram campaign yields the penultimate intermediate at ≥98.7% purity by HPLC (C18 column, 210 nm, gradient method based on USP 〈621〉), ultimately enabling the synthesis of an enantiopure aminopyrrolidine-fused kinase inhibitor under cGMP.

Can a Bromine Leaving Group Streamline the Assembly of Fludioxonil-Type Phenylpyrroles?

The agrochemical sector exploits the regioselective C-5 handle for constructing 3-cyano-4-phenylpyrrole fungicides, where the bromine atom facilitates sp²-sp² bond formation with sensitive functional group tolerance. A representative process charges the ethyl ester into a 2,000 L Hastelloy C-22 reactor together with 4-fluorophenylboronic acid (1.08 eq), powdered Na₂CO₃ (3.0 eq), and a catalyst system generated in situ from Pd(OAc)₂ (0.15 mol%) and PPh₃ (0.6 mol%) in a water-saturated 1,4-dioxane medium. The slurry is heated to gentle reflux (88 °C) for 7–9 h, and tight temperature control is mandatory to suppress the competing hydrolysis of the ethyl ester, which accelerates above 92 °C and generates the corresponding carboxylic acid as a non-productive impurity. After cooling to 20 °C, the organic phase is passed through a 0.5 µm polypropylene cartridge filter to remove palladium black, and dilute aqueous hydrazine monohydrate (1.0% v/v) is applied as a scavenger during solvent displacement into methylcyclohexane. The crude 4-fluorophenyl intermediate is crystallised from methylcyclohexane/hexane (1:4) at −5 °C with controlled cooling rates of 0.3 °C/min, affording a white crystalline solid with a melting point of 108.5–109.8 °C. Subsequent introduction of the nitrile moiety at C-3 is executed via a two-step t-butoxycarbonyl protection and CuCN-mediated Rosemund-von Braun cyanation, where residual bromine from the electrophile is consumed quantitatively. The final phenylpyrrole fungicide, structurally analogous to fludioxonil, meets the five-batch analysis requirement under OECD 506 and is registered under REACH Annex VII–X with a technical purity specification of ≥96.0% (GC-FID, ASTM D6844-19). The brominated ester input consistently delivers isolated yields above 88% on a 350 kg input scale when precooled to 15 °C prior to charging to mitigate thermal shock to the catalyst pre-mixture.

Synthesis of high-triplet-energy hole-transporting materials frequently exploits the ethyl 5-bromo-1H-pyrrole-2-carboxylate node for incorporating diarylamine or carbazole fragments that raise the glass transition temperature above 150 °C. The raw material is subjected to a Buchwald-Hartwig amination with 3,6-di-tert-butylcarbazole (1.15 eq) in the presence of BrettPhos Pd G3 precatalyst (0.5 mol%) and NaOtBu (1.4 eq) in anhydrous toluene at 105 °C for 22 h. The reaction is performed in a 300 L Hastelloy reactor with MIG-type impellers designed for viscous media, and the endpoint is confirmed by TLC (hexane:ethyl acetate 4:1, R_f of product 0.42). After quenching with deionised water and extracting into dichloromethane, the organic concentrate is passed through a short silica gel plug and then subjected to gradient sublimation: a first pass at 200 °C and 10⁻³ Pa to remove volatile carbazole residues, followed by a second pass at 240 °C to isolate the target hole-transport compound as an amorphous yellow glass. Impurity ceilings are dictated by device physics: alkali metal and transition metal contamination each must stay below 0.5 µg/g as quantified by ICP-MS in compliance with SEMI C63-0718 for electronic-grade organics, while halide residuals are driven below 10 µg/g via multiple water/dioxane trituration cycles. The purified material is integrated into the hole-transport layer of a phosphorescent OLED stack, where its HOMO level of −5.28 eV (measured by AC-2 inverse photoemission in air) and mobility exceeding 1.2 × 10⁻⁴ cm²/V·s at an electric field of 5 × 10⁵ V/cm facilitate charge injection in devices with 95% internal quantum efficiency.

When Photostability Benchmarks Exceed Fluorescein by 10-Fold, Pyrrole Aldehyde Synthons Become Critical

The preparation of BODIPY (boron-dipyrromethene) fluorophores absorbing above 500 nm critically depends on the availability of 5-bromo-1H-pyrrole-2-carboxaldehyde, obtained in 89–93% yield by low-temperature diisobutylaluminium hydride reduction of the corresponding ethyl ester. In a 100 L jacketed stainless-steel cryogenic vessel, a solution of ethyl 5-bromo-1H-pyrrole-2-carboxylate (3.0 kg, 13.6 mol) in anhydrous dichloromethane (40 L, dried over 3Å molecular sieves to water content ≤30 ppm) is cooled to −72 °C using a liquid nitrogen heat-exchanger loop. DIBAL-H (1.2 M in toluene, 2.0 eq) is introduced via a PTFE-lined dosing line over 3.5 h, keeping the internal temperature below −68 °C. The reaction is quenched by subsurface injection of methyl acetate (1.5 L) followed by slow warming to 0 °C and careful addition of a 20% Rochelle salt solution. The aldehyde is isolated as a pale yellow solid after flash chromatography-free isolation: the dichloromethane phase is concentrated to 8 L, diluted with hexane (80 L), and seeded at 40 °C to crystallise the product in 97.3% GC purity. This aldehyde is then condensed with 2,4-dimethylpyrrole (2.05 eq) in dry dichloromethane with catalytic trifluoroacetic acid at 20 °C for 12 h, oxidised with DDQ (1.05 eq) for 2 h, and complexed with BF₃·OEt₂ (3.0 eq) in the presence of triethylamine to yield an asymmetric BODIPY core. The final fluorophore, bearing a reactive bromine at the meso-pyrrole position for further Sonogashira or Suzuki derivatisation, exhibits a molar extinction coefficient of 8.4 × 10⁴ M⁻¹cm⁻¹ at 527 nm (ethanol) and a quantum yield of 0.72 relative to fluorescein standard ASTM E388-04(2015). For biological probe applications, endotoxin levels are validated below 0.25 EU/mg according to USP 〈85〉.

Bidentate phosphine architectures incorporating N-H pyrrole donors demand a facile entry to 5-functionalised pyrrole-2-carboxylate scaffolds where the halogen enables direct phosphorus introduction. In a 50 L cryogenic reactor purged with argon, ethyl 5-bromo-1H-pyrrole-2-carboxylate (1.0 mol) dissolved in anhydrous THF (12 L) is cooled to −78 °C and treated with n-butyllithium (2.5 M in hexanes, 1.03 eq) over 30 min, generating the 5-lithiopyrrole species through halogen-metal exchange. After an additional 15 min of stirring at −78 °C, neat chlorodiphenylphosphine (1.05 eq) is added in one portion, causing immediate phosphine substitution and a colour shift from pale straw to deep orange. The mixture is allowed to warm to ambient temperature over 3 h, quenched with degassed distilled water (300 mL), and extracted into ethyl acetate. The crude phosphine is purified by column chromatography on neutral alumina (Brockmann III activity) under argon to avoid phosphine oxide formation, eluting with hexane/ethyl acetate (9:1). The isolated ethyl 5-(diphenylphosphino)-1H-pyrrole-2-carboxylate is obtained as an off-white crystalline solid with a 31P NMR singlet at −18.5 ppm (CDCl₃) and a melting point of 142–144 °C. This compound serves as a hemilabile ligand in palladium-catalysed C-N cross-coupling, where the pyrrole N-H donor and phosphine centre create a P,N-chelate that facilitates oxidative addition of aryl bromides at millimolar catalyst loadings (0.05–0.2 mol% Pd). The ligand is routinely tested against the Buchwald-Hartwig coupling of 4-bromotoluene with morpholine (toluene, 80 °C, 4 h), delivering >99% conversion at 0.1 mol% Pd₂(dba)₃ in combination with ligand at a P:Pd ratio of 1.2:1, benchmarked by GC-FID.

Diverted Total Synthesis of Agelastatin A and Congeneric Cyclopentapyrrole Alkaloids

Marine sponge alkaloids of the oroidin-hymenialdisine family frequently contain a 4,5-dibromopyrrole-2-carboxamide subunit, and monobrominated ethyl 5-bromo-1H-pyrrole-2-carboxylate serves as the linchpin for constructing the characteristic α-haloamide linkage. In a direct ester-to-amide transformation, the crystalline ethyl ester (5.0 kg, 22.9 mol) is combined with histamine dihydrochloride (1.0 eq) and sodium methoxide (2.3 eq) in methanol (30 L) and refluxed under nitrogen for 14 h. The reaction progress is monitored by ¹H NMR in DMSO-d₆, tracking the disappearance of the ethoxy quartet at 4.26 ppm. After stripping methanol and suspending the residue in acetonitrile, filtration through Celite-545 and recrystallisation from isopropanol/water (6:1) yields debromoamide 2 in 83% yield with 99.1% purity. Subsequent electrophilic bromination using N-bromosuccinimide (1.02 eq) in DMF at 0 °C selectively installs the second bromine at C-4 of the pyrrole ring, overcoming the steric shielding of the amide side chain. Applying this material with the intact ethyl ester protection, a Larock indolisation or a cyclopentannelation yields the core of agelastatin A. Industrial synthetic campaigns demand rigorous control of the NBS stoichiometry to avoid C-4 overbromination to the tribromo impurity, which co-elutes with the product on reversed-phase HPLC. The dibromo intermediate is carried forward to the tetracyclic architecture under full compliance with ISO 9001:2015 quality management for research chemical supply, and genotoxic impurity risk from ethyl bromide evolved during amidation is mitigated by distillation into a cold trap and scrubbing through activated carbon. The final cyclopentapyrrole alkaloid libraries support ion-channel modulator screening cascades, with target potency characterised by patch-clamp electrophysiology rather than standard binding assays.

A comparative overview of process parameters and purity thresholds across the described downstream segments illustrates the operational diversity required from a single brominated pyrrole building block.

Regulatory and quality management demarcations that govern the utilisation of ethyl 5-bromo-1H-pyrrole-2-carboxylate in volume production are summarised in the second compliance matrix. The data reflect actual specifications maintained in 1,500 kg annual throughput distributed across pharmaceutical and non-pharmaceutical channels.