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

Synthesizing ATP-Competitive Kinase Inhibitors — The Pyrrolo[2,3-d]pyrimidine Scaffold

Construction of the pyrrolo[2,3-d]pyrimidine heterocycle, a privileged pharmacophore in Janus kinase (JAK), Bruton’s tyrosine kinase (BTK), and epidermal growth factor receptor (EGFR) inhibitor programs, proceeds via condensation of ethyl 5-formyl-1H-pyrrole-2-carboxylate with a suitably activated pyrimidine precursor bearing a leaving group at the 4-position. In a representative kilogram-scale campaign compliant with ICH Q7 Good Manufacturing Practice for Active Pharmaceutical Ingredients, the aldehyde-ester is charged at 1.08–1.15 molar equivalents relative to the pyrimidin-4-amine component under anhydrous acetonitrile in a glass-lined reactor with jacket temperature held at -15 °C to suppress exothermic imine formation. This load corresponds to approximately 24–29% of the total solid batch mass before work-up, and the pyrrole fragment accounts for 38–46% of the final API molecular weight once the pendant carboxylate is hydrolysed and further derivatised. Residual ethyl acetate and ethanol introduced with the starting material are controlled to ≤5000 ppm and ≤500 ppm respectively according to ICH Q3C(R8) Class 3 and Class 2 limits; palladium catalyst scavengers are applied during subsequent Suzuki coupling steps to maintain Pd content below 10 µg/g as verified by inductively coupled plasma mass spectrometry per USP 〈232〉/〈233〉. Downstream unit operations encompass condensation at 0–5 °C for 2 h, cyclisation by heating to gentle reflux (82 °C) for 16 h, solvent swap to toluene, washing with 5% w/w aqueous citric acid to remove unreacted amine, and drying over anhydrous sodium sulfate. The crude product is purified by flash chromatography on silica gel (230–400 mesh, isocratic dichloromethane:methanol 98:2), and the combined fractions are concentrated and crystallised from methyl tert-butyl ether/n-heptane to yield a white to off-white crystalline solid with HPLC purity ≥99.2% (C18 column, 1.0 mL/min, detection at 254 nm). Polymorph control is achieved through seeded cooling crystallisation at a cooling rate of 0.2 °C/min within 55–-5 °C range. Terminal dosage forms include film-coated tablets of strengths 5 mg, 15 mg, and 100 mg, with dissolution tested against USP 〈711〉 Apparatus II at 50 rpm in 0.1 M HCl.

What Limits Batch Homogeneity in Asymmetric Porphyrin Syntheses Using Mixed Aldehydes?

Photoactive A₃B-type porphyrins intended for photodynamic therapy require regiospecific placement of hydrophilic and hydrophobic groups to avoid π-stacking aggregation in physiological media. Ethyl 5-formyl-1H-pyrrole-2-carboxylate functions as a pre-formed pyrrole building block that contributes both an aldehyde for macrocycle formation and an ester handle later converted to a carboxylate for improved aqueous solubility of the photosensitiser. During statistical condensation under Adler-Longo or Lindsey conditions, the pyrrole aldehyde is introduced to the mixed-aldehyde pool at 0.18–0.25 molar equivalents relative to the total aldehydric substrates, a proportion that biases formation of the desired mono-substituted porphyrin while limiting symmetrical tetrapyrrole by-products; under optimised Lindsey procedures with BF₃·Et₂O (2.5 mol% relative to total aldehyde) in dichloromethane at 0.22 M concentration, the target porphyrin is isolated in 4–9% chromatographic yield after DDQ oxidation. Compliance requirements for the photosensitiser intermediate manufactured under contract for a parenteral drug product include conformance with ISO 10993-1:2018 biological evaluation of medical devices when the lyophilised powder is reconstituted for intravenous administration, adherence to USP 〈788〉 particulate matter in injections (light obscuration method, limits for particles ≥10 µm and ≥25 µm), and absence of visible foreign matter per USP 〈790〉. Downstream processing involves quenching the reaction with triethylamine, filtering the DDQ by-product hydroquinone through a short neutral alumina plug, and subjecting the oligomeric residue to size-exclusion chromatography on Bio-Beads S-X1 resin with tetrahydrofuran as eluent. The collected A₃B fraction is further purified via preparative HPLC on a C4 column (acetonitrile/water with 0.1% trifluoroacetic acid) and lyophilised to obtain a dark violet amorphous powder; residual palladium, if a Heck coupling was employed to install a vinyl substituent, is verified at <5 µg/g by graphite furnace atomic absorption spectroscopy. The terminally formulated drug product, typically a 15 mg single-dose vial of freeze-dried solid for reconstitution in 5 mL of saline, displays a singlet oxygen quantum yield ΦΔ > 0.72 in deuterated methanol referenced against Rose Bengal and is indicated for vascular-targeted photodynamic therapy in neovascular age-related macular degeneration.

Non-fullerene acceptors incorporating a 5-formylpyrrole-2-carboxylate ester moiety as the terminal electron-withdrawing block exhibit LUMO levels below -3.9 eV, a prerequisite for efficient exciton dissociation when paired with a wide-bandgap polymer donor. The aldehyde group undergoes Knoevenagel condensation with 3-ethylrhodanine or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile end-capping reagents under piperidine catalysis. For solvent-based synthesis in a kilo-lab setting, the pyrrole ester is loaded at a 2.2–2.5 molar excess relative to the active methylene compound, which amounts to 36–44% of the total solid reagent mass; this excess compensates for aldehyde hydration in hot toluene and ensures conversion above 97% as monitored by thin-layer chromatography. Regulatory gatekeeping for commercial organic photovoltaic materials in the European Union rests on REACH registration of the non-fullerene acceptor substance and compliance with RoHS Directive 2011/65/EU for any cadmium or lead residues below 100 ppm. Device fabrication in a pilot roll-to-roll line occurs in a cleanroom satisfying ISO 14644-1 Class 6 (≤35,200 particles/m³ at ≥0.5 µm), and the active layer coating solution is filtered through a 0.2 µm polytetrafluoroethylene membrane. Post-reaction purification commences with silica gel column chromatography (dichloromethane:hexane gradient from 1:1 to 4:1) followed by repetitive recrystallisation from chloroform/acetone; the final polishing step is temperature-gradient vacuum sublimation at 10⁻⁶ mbar, where the crucible temperature is ramped from 270 °C to 320 °C over 6 h. The sublimed material achieves ≥99.95% purity by HPLC–mass spectrometry with a single dominant peak. In the active layer ink, the purified acceptor is blended with a donor polymer such as PM6 at a weight ratio of 1:1.3 to 1:1.6, along with 0.5–1.0 wt% 1,8-diiodooctane as a processing additive, and coated via slot-die on polyethylene terephthalate substrate under 40–50% relative humidity. Power conversion efficiency is certified under IEC 60904-3 AM1.5G illumination (1000 W/m²) and measured with a calibrated silicon reference cell. The finished goods are flexible photovoltaic modules laminated with a weatherable barrier film, validated for damp heat resistance according to IEC 61215-2:2021 with leakage current below 50 µA at 1000 V, intended for indoor light-harvesting applications powering IoT sensor nodes.

When a Pyrrolotriazine Nucleobase Must Be Built from a Singly Functionalized Pyrrole

The pyrrolo[2,1-f][1,2,4]triazin-4-amine core present in remdesivir and its parent nucleoside GS-441524 can be assembled from an acyclic precursor via annulation with aminoguanidine bicarbonate, a route that demands a pyrrole component bearing an aldehyde at the α-position. Ethyl 5-formyl-1H-pyrrole-2-carboxylate fulfills this role, providing the intact pyrrole ring with the necessary oxidation state for triazine construction. Under the prevailing process safety and quality framework, the synthesis is conducted in dedicated stainless steel reactors compliant with ICH Q7 Chapter 5 on production equipment, and 21 CFR 211.65 on equipment construction and maintenance, while residual Class 2 solvent N,N-dimethylformamide is limited to <880 ppm in the isolated intermediate according to ICH Q3C Table 2. The stoichiometry of the key cyclocondensation is tightly controlled: the pyrrole aldehyde-ester is used at precisely 1.00 equivalent relative to aminoguanidine bicarbonate because any excess of the aldehyde fosters irreversible by-product formation through Schiff base oligomerisation, and this substrate accounts for about 17–22% of the total batch mass input. Once the triazine ring is formed, the pyrrole fragment represents 31–34% of the molecular weight of GS-441524 intermediate, with the ethyl ester group retained until a late-stage enzymatic hydrolysis step. Process execution on a 50–200 L scale involves charging the pyrrole ester, aminoguanidine bicarbonate, and trimethyl orthoformate in N-methylpyrrolidone, sealing the autoclave, and heating to an internal temperature of 160 ± 3 °C for 52–65 h; a pressure of 3–5 bar is generated, requiring a burst disc rated at 12 bar. After cooling to 60 °C, the mixture is diluted with deionised water, and the crude solid is collected by centrifugation, washed repeatedly with water and methyl tert-butyl ether, and dried at 45 °C under vacuum for 24 h. Further purification employs preparative medium-pressure liquid chromatography on C18-functionalised silica with a step gradient of acetonitrile in 0.5% aqueous formic acid, pooling fractions of >99.0% purity. Crystallisation from 2-propanol/water (65:35) yields a white crystalline solid with HPLC purity of 99.6% and total aerobic microbial count <10 CFU/g. The terminal pharmaceutical presentation is a 100 mg lyophilised powder for concentrate for solution for infusion in a 30 mL Type I borosilicate glass vial closed with a bromobutyl rubber stopper, verified for container closure integrity by helium leak testing per USP 〈1207.1〉 at a reject limit of <6.0 × 10⁻⁶ mbar·L/s.

Total synthesis of Lamellarin D sulfate, a potent inhibitor of topoisomerase I and mitochondrial function currently under preclinical evaluation, relies on a convergent [3+2] cycloaddition strategy in which a 1,4-dicarbonyl equivalent is required to construct the central pyrrole ring regioselectively. Ethyl 5-formyl-1H-pyrrole-2-carboxylate acts as a masked synthon: the aldehyde is protected as a dithiolane or directly employed in an aldol-type condensation, while the ethyl ester at the 2-position provides a handle for later functionalisation into the pendant carboxylate or sulfamate present in the natural product. Syntheses carried out at a minority aseptic kilo-laboratory under a quality system aligned with OECD Series on Principles of Good Laboratory Practice (ENV/MC/CHEM(98)17) apply residual solvent monitoring per USP 〈467〉 Procedure A, with particular attention to limiting dichloromethane to <600 ppm and 1,4-dioxane to <380 ppm in the advanced intermediate. During the pivotal intramolecular aldol condensation that forges the fused pentacyclic core, the pyrrole aldehyde is introduced at 0.88–0.93 equivalents relative to the dihydroisoquinoline hydrochloride to avoid biscationic dimerisation; this stoichiometric limitation necessitates a slow addition rate ≤1.5 mL/min via a syringe pump into a slurry of the nucleophile and potassium tert‑butoxide in tetrahydrofuran at -40 °C. The pyrrole starting material constitutes roughly 20–25% of the total reactant mass for this step. Downstream processing involves quenching with saturated ammonium chloride, extraction with ethyl acetate, drying over magnesium sulfate, and filtration through a short pad of Celite. The crude lamellarin ester is purified by flash chromatography on triethylamine-deactivated silica gel with a hexane/ethyl acetate gradient, and the pooled fractions are concentrated under reduced pressure below 30 °C to minimise retro-Diels-Alder degradation. Final deprotection of the ethyl ester is achieved with lithium hydroxide in tetrahydrofuran/water (3:1) at 0 °C, followed by ion-pair extraction and lyophilisation to afford the carboxylate as an amorphous beige powder of >97.5% purity (HPLC at 280 nm). The product is configured as a research-grade standard for in vivo xenograft studies and is not intended for human administration at this stage.

Electrically conductive primers for solventborne epoxy coatings based on dispersions of polypyrrole nanoparticles suffer from rapid re-agglomeration and poor storage stability unless the particle surface is covalently modified with a steric stabiliser that is compatible with the resin system. Grafting ethyl 5-formyl-1H-pyrrole-2-carboxylate to plasma-activated polypyrrole via imine linkage achieves this goal: the aldehyde condenses with primary amine groups introduced by ammonia plasma treatment, while the pendant ethyl ester groups extend into the surrounding medium and disrupt interparticle hydrogen bonding. The modified pigment falls under the requirement of REACH registration as a chemically modified substance, and the formulated primer must demonstrate performance in accordance with ISO 12944-5:2019 for high-durability protective systems on structural steel, with cyclic aging per ISO 11997-1:2017 and blister evaluation per ASTM D714-13 blister size and frequency reference standards. The grafting density achieved under optimised conditions — amination at 50 W radio frequency for 90 s followed by reflux in anhydrous toluene with 0.8–1.2 g of pyrrole aldehyde-ester per gram of nanopigment — amounts to 5–8 wt% covalently bound organic functionality, as quantified by thermogravimetric analysis in nitrogen atmosphere up to 600 °C. When the functionalised polypyrrole is incorporated into a two-component bisphenol-A epoxy/polyamidoamine primer at a loading of 1.9–2.3 wt% relative to total resin solids, the dispersion is pre-dispersed with a high-speed dissolver operating at a tip speed of 16–19 m/s for 20 min, then passed twice through a triple-roll mill with a 5 µm front-gap setting to achieve a fineness of grind below 12 µm as determined by the Hegman gauge method per ISO 1524:2020. The catalysed mixture maintains a pot life of 80–95 min at 23 °C, allowing single-coat application by airless spray at 200 bar to grit-blasted carbon steel panels (surface preparation grade Sa 2½, ISO 8501-1) and forced drying at 60 °C for 45 min. The resultant dry film, typically 150–180 µm thick, exhibits 2400–2800 h resistance in neutral salt spray (ASTM B117-19) when scribed and overcoated with an aliphatic acrylic polyurethane topcoat, with under-film creep limited to <2.0 mm from scribe. This conductive primer technology targets application on storage tank floors and offshore platform splash zones where static dissipation and barrier protection must coexist.