5-(2-Fluorophenyl)-1H-Pyrrole-3-Carboxylic Acid Ethyl Ester

In a multi-kilogram campaign targeting a selective VEGFR-2/PDGFRβ bispecific inhibitor, 5-(2-fluorophenyl)-1H-pyrrole-3-carboxylic acid ethyl ester is employed not as a peripheral synthon but as the central pyrrole core that engages the DFG-out conformation of the kinase hinge region. The ester is first saponified to the carboxylic acid using LiOH·H₂O (1.2 eq.) in THF/water (3:1 vol) at 0–5 °C over 4 h, a protocol selected over NaOH to avoid competitive N-deprotonation of the pyrrole nitrogen that leads to oligomeric by-products, a failure mode documented across multiple production batches on a 500 L enamel-lined jacketed reactor equipped with a retreat-curve impeller. Following acidification and isolation, the acid is converted to the corresponding acid chloride with oxalyl chloride (1.5 eq.) and catalytic DMF (0.02 eq.) in dichloromethane at −5 °C, then quenched into a solution of the elaborated aniline in the presence of N-methylmorpholine (2.5 eq.) to regenerate the free amine. The amide coupling stream is held at 0 °C for 1 h and then warmed to 20 °C over 2 h, achieving a crude assay yield of 91–93% by HPLC. Critical process parameters (CPP) identified during process qualification include the moisture content of the acid chloride solution (specification ≤ 200 ppm) and the hold time of the acid chloride prior to amidation (< 30 min), as the acyl chloride slowly undergoes ring-dechlorination at the pyrrole 2-position under acidic conditions, generating a des-fluoro-phenyl impurity tracked at RRT 0.82. Regulatory conduct complies with ICH Q7, Chapter 11 (process validation) and the starting-material designation principles of ICH Q11, with the isolated intermediate filed under a Type II DMF. The final API, a bis-arylamide fumarate salt, demonstrates IC₅₀ 3.8 nM against VEGFR-2 kinase domains when the pyrrole-3-carboxamide moiety remains unsubstituted at N1, confirming that the ethyl ester precursor is not merely a cost-saving surrogate but a deliberate protection strategy to preserve N1-H during downstream Suzuki diversification.

What Modifications Enable SDHI Fungicide Lead Optimization?

The ethyl ester serves as a precursor to 5-(2-fluorophenyl)-1H-pyrrole-3-carboxylic acid amides that mimic the pharmacophore of succinate dehydrogenase inhibitors (SDHI) active against Botrytis cinerea and Zymoseptoria tritici. The ester is reacted with aqueous methylamine (40% w/w, 2.5 eq.) in ethanol under reflux (78 °C) for 12 h; after vacuum distillation of the solvent, the resulting N-methylamide is crystallized from isopropanol/water (7:3) to yield off-white needles with a melting point of 164–166 °C and purity 99.2% by qNMR. Foliar rainfastness trials have shown that a formulation containing the N-cyclopropyl analog exhibits EC₅₀ 0.45 μg/mL against SDH isolated from B. cinerea, but soil column leaching studies (OECD 312) revealed a K_oc of 1450 L/kg, indicating very low mobility, a finding that triggered a 10-month chronic Daphnia magna reproduction assessment under OECD 211. Registration-grade technical material is isolated by spray drying a 20% aqueous slurry of the active ingredient with an inlet temperature of 180 °C to achieve a moisture content ≤ 1.5%, followed by air-jet milling to a volume median diameter D[v,0.5] of 3.5–4.0 μm. The formulated product, a water-dispersible granule (WG) containing 50% w/w active, incorporates a naphthalenesulfonate dispersant (6%) and a lignosulfonate binder (4%), and is manufactured on a high-shear mixer-granulator before fluid-bed drying at 55 °C. Compliance encompasses FAO Specification 59/WG (March 2021 revision) for physical stability, EPA 40 CFR Part 158 data requirements for biochemical pesticides, and the analytical enforcement methods listed in SANCO/3030/99 rev.5 for EU residue definition. Scale-up batches exceeding 100 kg have identified an exothermic decomposition onset at 225 °C via differential scanning calorimetry, necessitating an upper temperature limit for micronization of 40 °C to prevent autocatalytic runaway.

Vacuum-Deposited Hole-Transporting Layers: Fluorine-Induced HOMO Stabilization

When the ethyl ester is elaborated via Suzuki-Miyaura cross-coupling with 4-(diphenylamino)phenylboronic acid pinacol ester (1.05 eq.), Pd(PPh₃)₄ (0.04 eq.), and K₂CO₃ (3.0 eq.) in toluene/ethanol/water (5:1:1) at 85 °C for 18 h, the resultant D-π-A triarylamine-pyrrole adduct possesses a HOMO of −5.38 eV (measured by AC-2 photoelectron spectroscopy in air) and a triplet energy of 2.71 eV. In a bottom-emission OLED stack with ITO/MoO₃ (10 nm)/hole-transport layer (40 nm)/emissive layer (30 nm)/TPBi (60 nm)/LiF/Al, the material doped into 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) at a concentration of 10 wt% delivers a current efficiency of 72.3 cd/A and external quantum efficiency 22.4% at a luminance of 1000 cd/m². The processing window is severely confined: elevating the doping ratio above 15 wt% induces a 40% drop in photoluminescence quantum yield due to excimer formation, while concentrations below 6 wt% result in incomplete energy transfer from the host and a shoulder emission at 430 nm originating from CBP. Deposition is performed in an Angstrom Engineering EvoVac system with a base pressure of 5×10⁻⁷ Torr; the source temperature for the pyrrole dopant is held at 285–295 °C, and the rate is locked at 0.5–0.8 Å/s using a quartz crystal monitor calibrated against a spectroscopic ellipsometry step-height standard. Post-deposition encapsulation is completed with a UV-epoxy and getter-attached glass lid inside a nitrogen glovebox (H₂O < 0.1 ppm). The device conforms to IEC 62368-1:2023 for audio/video electronic safety, EU RoHS Directive 2011/65/EU (the pyrrole fluorination ensures no intentionally added brominated flame retardants), and the material purity is verified by inductively coupled plasma mass spectrometry for transition metals, with specifications for Pd and Cu residual content at ≤ 5 ppm and ≤ 2 ppm, respectively, to prevent luminescence quenching.

In the fabrication of polymer dots (Pdots) for two-photon fluorescence lifetime imaging microscopy (2P-FLIM) at 800 nm excitation, the ester is reduced with LiAlH₄ (1.1 eq.) in anhydrous THF at 0–5 °C to give 5-(2-fluorophenyl)-1H-pyrrole-3-methanol, which is subsequently converted to a methacrylate-terminated monomer using methacryloyl chloride (1.2 eq.) in the presence of triethylamine. The functionalized monomer is copolymerized with styrene and acrylic acid (15 mol% of the fluorescent pyrrole unit) via mini-emulsion polymerization in a high-pressure homogenizer (Microfluidizer M-110P, 15,000 psi, three passes), using sodium dodecyl sulfate (2 mM) and hexadecane (0.5 wt%) as co-stabilizer. The resulting Pdots exhibit a hydrodynamic diameter of 28 ± 4 nm (dynamic light scattering, Zetasizer Ultra) and a peak emission at 520 nm with a quantum yield of 0.68 in PBS buffer. Processing pitfalls include aggregation above 40 °C during the solvent evaporation step, which is mitigated by a controlled nitrogen purge at 25 °C and a rotary evaporator bath temperature maintained at ≤ 30 °C. Pdot dispersions intended for in vitro live-cell uptake studies must pass the endotoxin limit (< 0.5 EU/mL) per ISO 10993-11:2017 and are sterile-filtered through a 0.22 μm PVDF membrane. The Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH, EC 1907/2006) dossier for the monomer intermediate includes a registered tonnage band of 1–10 tonnes/year and a CSR covering life-cycle stages at an activated-sludge sewage treatment plant. The terminal product type is a ready-to-use colloidal dispersion packaged in amber vials under argon, labeled for research use only but prepared under a quality system aligned with ISO 13485:2016 for laboratory-scale diagnostics.

Why Does the Ethyl Ester Function as a System Suitability Standard for HPLC Method Development?

Quantitative trace analysis of the parent acid and the des-fluoro impurity in drug substance release testing relies on the labeled ethyl ester as a resolution standard due to its retention time proximity to the main peak (tR 12.3 min vs. 11.8 min for the acid on a Waters XBridge C18 column, 150 × 4.6 mm, 3.5 μm, with mobile phase acetonitrile/0.1% trifluoroacetic acid 60:40 at 1.0 mL/min). The system suitability solution is prepared at a concentration of 0.1 mg/mL in diluent, and a 10 μL injection must produce a resolution factor Rs ≥ 2.5 between the ester and the acid, and a tailing factor T ≤ 1.8 at 10% peak height, satisfying Ph. Eur. monograph 2.2.46 chromatographic separation techniques. The standard is purified by preparative HPLC on a 50 mm diameter column to a certified purity of 99.8% (area normalization, 210 nm) and is accompanied by a certificate of analysis reporting mass balance with water content by Karl Fischer titration (< 0.1%), residual solvents tested per USP <467>, and identity confirmed via ¹H, ¹³C, ¹⁹F NMR and high-resolution mass spectrometry (ESI+, m/z 248.0845 [M+H]⁺, Δ <1.5 ppm). Method validation follows ICH Q2(R1) guidelines for linearity (r² ≥ 0.999 over 0.05–0.15 mg/mL), accuracy (recovery 98–102%), and intermediate precision (RSD ≤ 2.0% across six independent runs). The absence of a commercial reference standard from any pharmacopoeia forces reliance on this calibrated in-house primary standard, which is prepared in accordance with ISO 17025:2017, Section 7.2.1.4 for metrological traceability via quantitative NMR against certified benzoic acid. Terminal product is a freeze-dried, vacuum-sealed 100 mg vial intended for use within the quality control laboratory.

Radical copolymerization of styrene with a methacryloyl-ester derivative of the pyrrole alcohol introduces a fluorescent repeat unit that remains covalently anchored in the polymer backbone, eliminating migration-based blooming phenomena observed with low-molecular-weight additives in polyolefin films. The grafting monomer is synthesized from the ethanolamine-derived amide and methacrylic anhydride; it is incorporated into the polystyrene chain at a loading of 2–5 mol% during a suspension polymerization conducted in a 100 L Pfaudler vessel with a water-to-monomer ratio of 3:1, using poly(vinyl alcohol) (0.5% w/w to water) as suspending agent and azobisisobutyronitrile (0.3 mol%) as initiator at 80 °C. The resulting beads possess a weight-average molecular weight of 180 kDa (GPC-MALS) and a glass transition temperature raised by 7 °C (Tg 108 °C vs. 101 °C for polystyrene homopolymer) as measured by differential scanning calorimetry at 10 K/min. Pelletization on a Coperion ZSK-26 twin-screw extruder with L/D 40 requires processing temperatures of 170–190–210–220 °C across zones 1–4 and a screw speed of 300 rpm; the die pressure must be maintained below 35 bar to avoid shear-induced degradation of the pyrrole chromophore, which is evidenced by a yellowing index (YI D1925) increase of 2.5 units when residence time exceeds 120 s. Compliance with REACH, Annex XVII, and migration testing under EU 10/2011 for food contact materials is verified via total organic carbon analysis after 10-day simulant D (3% acetic acid) exposure at 40 °C, yielding migration values ≤ 0.05 mg/kg. The product is an internally dyed polystyrene masterbatch (20% active in GPPS carrier) destined for injection-molding thick-section optical lenses, where the fluorinated pyrrole unit provides intrinsic blue fluorescence without additional colorant. The reported shift in emission maximum from 515 nm (in THF solution) to 528 nm (in the solid matrix) is attributed to energy transfer to excimer-like traps when the pyrrole repeat-unit fraction exceeds 4 mol%, defining the upper formulation limit.