5-Formyl-2,4-Dimethyl-1H-Pyrrole-3-Carboxylic Acid Ethyl Ester

What Makes a Carboxyethyl Substituent Advantageous for BODIPY Dye Solubility in Live-Cell Imaging?

The formyl group on the pyrrole ring enables the assembly of the dipyrromethene core through acid-catalyzed condensation with 2.1 equivalents of 2,4-dimethylpyrrole in anhydrous dichloromethane, while the ethyl ester side chain imparts sufficient lipophilicity to drive partitioning into intracellular membrane structures without inducing immediate precipitation in water-miscible staining buffers. The reaction is initiated at -78°C under argon in a jacketed Schlenk flask; trifluoroacetic acid is injected as a 0.15 M solution in CH₂Cl₂ to give a final catalyst loading of 0.12 equivalents relative to the aldehyde. After 18 h of gradual warming to 20°C, the intermediate dipyrromethane is oxidized in situ with 1.05 equivalents of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at 0°C for 2 h. The immediate treatment with triethylamine (3.0 equivalents) followed by boron trifluoride diethyl etherate (2.5 equivalents) delivers the fluorescent BODIPY core. Production-scale bottlenecks concentrate on the oxidative aromatization step: residual moisture levels above 50 ppm in the solvent promote the formation of a non-fluorescent pyrrolinone side-product that co-elutes with the target dye on silica gel, raising the purification effort to a 3-fold increase in column volumes. The terminal active esters—such as the pentafluorophenyl or succinimidyl derivatives—are prepared for bioconjugation to antibodies and oligonucleotides. Fluorescence quantum yields are determined in ethanol according to the relative method using rhodamine 6G as a standard (excitation at 488 nm), with typical values of Φ=0.88±0.03 for the free dye. Cytotoxicity testing of the formulated probe follows ISO 10993-5:2009 (extract dilution method, L929 fibroblasts), and the maximum tolerated concentration in complete culture medium is established at 10 µM for a 24 h exposure. Compliance with ICH Q7 principles for active pharmaceutical ingredient starting materials is applied when the dye is manufactured under a Drug Master File for clinical diagnostic kits.

Within current good manufacturing practice environments governed by 21 CFR Part 210/211 and EudraLex Volume 4, the ethyl 5-formyl-2,4-dimethyl-1H-pyrrole-3-carboxylate scaffold is telescoped into Knoevenagel condensations with active methylene compounds such as Meldrum’s acid or ethyl cyanoacetate to forge pyrrolo[3,4-b]quinolone-like frameworks that are evaluated as lead structures in kinase inhibitor programs. The addition ratio is precisely controlled: the aldehyde is charged as a limiting reagent at 1.00 molar equivalent against 1.10 equivalents of the C–H acid partner in a mixture of toluene and glacial acetic acid (85:15 v/v), with catalytic β-alanine (3 mol%) and azeotropic water removal at 75°C under 450 mbar reduced pressure. Reactions at 250-gallon scale in Hastelloy C-22 reactors demand rigorous oxygen exclusion because the intermediate benzylidene adduct undergoes competitive auto-oxidation when the dissolved O₂ level exceeds 2 mg/L, as monitored by an in-line optical oxygen probe. The crude heterocyclic ester is isolated by drowning into deionized water at 5°C, centrifuged in a Hastelloy perforated-bowl centrifuge, and dried in a double-cone rotary vacuum dryer at 45°C (jacket) / 10 mbar until the loss on drying is <0.5%. Residual solvent levels are controlled to meet ICH Q3C (R8) Option 2 concentrations, as laid out in the following table.

Solvent

Permitted Daily Exposure (mg/day)

Concentration Limit (ppm) in Drug Substance

Toluene (Class 2)

8.9

890

Acetic Acid (Class 3)

5000

Methanol (Class 2)

30.0

3000

Dichloromethane (Class 2)

6.0

600

Ethyl Acetate (Class 3)

5000

Heavy metals are quantified against USP <231> limits, and any batch reporting >10 ppm total heavy metals is diverted to an additional treatment with a thiol-functionalized silica scavenger cartridge in the polishing filtration loop. The isolated intermediate is stored in double LDPE liners inside fiber drums under a 2–8°C cold chain and is shipped to alliance partners engaged in Phase IIb clinical supply. The terminal product category remains undefined under a public non-proprietary name, but the pharmacophore maps onto documented selective kinase inhibitory motifs with in vitro IC₅₀ values referenced to a Eurofins CEREP panel.

Residual Activity Against Resistant Weed Biotypes and the Pyrrole-3-Carboxylate Pro-Herbicide Concept

Modern herbicide discovery programs exploit the ethyl ester moiety as a latent carboxylic acid that liberates the active auxin-mimic or HPPD-inhibiting functionality upon metabolic hydrolysis by susceptible broadleaf weeds. In a typical parallel synthesis campaign, a 96-position block is charged with the pyrrole aldehyde (0.25 mmol/well) and anilines or hydrazines bearing diverse substitution patterns (0.28 mmol), combined with trimethyl orthoformate in acetonitrile to form transient Schiff bases that are subsequently cyclized under microwave irradiation at 120°C for 15 min. The mass fraction of the pyrrole-3-carboxylate subunit in the resulting fused lead candidates averages 34–42%, placing the ester in a favorable position for later-stage pro-moiety tuning. Scale-up to 100 L continuous stirred tank reactors uses a residence time of 22 min at 130°C in a MiCReactor flow system with a 1.0 mm ID PFA coil, delivering a throughput of 4.8 kg/day of purified product after silica plug filtration. The critical challenge is the exothermic cyclization step: adiabatic temperature rise is measured at ΔT_ad=85 K in the absence of solvent dilution, requiring a 2:1 acetonitrile-to-substrate dilution ratio to maintain jacket temperature control within ±2°C of the setpoint. Ecotoxicological characterization adhers to OECD Guidelines for the Testing of Chemicals, Section 5 (Terrestrial and Aquatic Environmental Fate), including Daphnia magna acute immobilization (OECD 202) with EC₅₀ values typically recorded below 0.5 mg/L for the unmetabolized ester. Agricultural field trial residue studies follow EPA OPPTS 860.1500 crop field rotation protocols and the resulting data packages support the submission of a joint FAO/WHO residue monograph. The final formulated product class comprises suspension concentrate (SC) or oil dispersion (OD) herbicides intended for post-emergent control of glyphosate-resistant Amaranthus populations, with the active ingredient concentration in the formulated product set at 240 g/L.

When a Pyrrole Donor Outperforms Aniline in Electro-Optic Side-Chain Polymers

The Knoevenagel condensation of this pyrrole carbaldehyde with 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) acceptor in acetic anhydride at 85°C generates a donor–π–acceptor chromophore possessing a static first hyperpolarizability (β₀) on the order of 450×10⁻³⁰ esu as calculated by the finite-field method at the B3LYP/6-31G* level. When formulated into an amorphous polycarbonate host at a chromophore loading of 22 wt%, the guest–host film exhibits a glass transition temperature depression of only 8°C relative to the neat polymer, mitigating the orientational relaxation that plagues aniline-based analogs. Device fabrication involves spin-coating from cyclopentanone solution (15% solids) onto indium tin oxide substrates to a dried thickness of 1.8 µm, followed by corona poling through a tungsten wire grid at a dc voltage of 7.5 kV while the film is maintained at T_g+5°C (142°C) for 25 min. The electro-optic coefficient r₃₃, measured by the Teng–Man simple reflection technique at 1.55 µm, reaches 35 pm/V – a value that, when benchmarked against the device insertion loss of 2.3 dB/cm at 1550 nm, meets the sub-3 dB insertion loss target for Mach–Zehnder modulators fabricated in silicon photonics foundries. A process-aware failure mode is the field-induced dielectric breakdown of the film at poling fields above 120 V/µm; pinholes are detected by a methylene blue staining protocol referenced in IPC-TM-650 2.6.3.2. Reliability testing of packaged devices adheres to Telcordia GR-468-CORE for non-hermetic optoelectronic components, requiring <5% drift in half-wave voltage over a 2000-hour damp heat exposure (85°C/85% RH). The terminal device class encompasses polymer-based external modulators for datacenter interconnects, where the chromophore is the key functional organic layer.

Mild steel pickling in 15 wt% hydrochloric acid at 60°C is routinely protected by Schiff base adducts derived from the condensation of the formyl function with ethanolamine. The neat adduct, prepared by simply refluxing equimolar quantities in ethanol for 1 h, is added directly to the acid bath at a concentration of 300 mg/L. Weight-loss coupons of API 5L X65 steel with dimensions 50 mm × 25 mm × 3 mm are immersed with a volume-to-surface area ratio of 40 mL/cm² per ASTM G31-72(2021); after 6 h, the corrosion rate drops from 28.7 mm/yr (uninhibited) to 0.7 mm/yr, yielding an inhibition efficiency of 97.6%. The inhibition mechanism involves chemisorption of the imine nitrogen and the pyrrole π-system onto the ferrous surface, validated by X-ray photoelectron spectroscopy that shows a 2.2 eV positive shift of the N 1s binding energy. A dose-response evaluation under the same protocol is tabulated below.

Inhibitor Concentration (mg/L)

Corrosion Rate (mm/yr)

Inhibition Efficiency (%)

Surface Coverage (θ)

18.2

36.6

0.37

100

9.8

65.9

0.66

200

3.1

89.2

0.89

300

0.7

97.6

0.98

500

0.4

98.6

0.99

The formulation must avoid co-addition of amine-based accelerator packages that compete for adsorption sites; when a quaternary ammonium salt was dosed together with the Schiff base at equal 200 mg/L concentrations, the corrosion rate reverted to 7.3 mm/yr, indicating antagonistic interaction. Field deployment in matrix-acidizing campaigns references NACE TM0169-2012 for laboratory corrosion testing and NACE SP0775-2023 for interpretation of corrosion coupons in oilfield fluids. The Schiff base is supplied as a 90% active viscous liquid and is blended into commercial inhibitor packages containing demulsifiers and nonionic surfactants, with a recommended treat rate window of 200–500 ppm(v/v) in live acid, giving a finished formulation class categorized as a high-temperature acid corrosion inhibitor intensifier.

5-Formyl-2,4-Dimethyl-1H-Pyrrole-3-Carboxylic Acid Ethyl Ester (CAS 2199-59-9), with a molecular formula of C₁₀H₁₃NO₃ and a molecular weight of 195.22 g·mol⁻¹, is a specialized pyrrole derivative supplied as a pale yellow to off-white crystalline solid. Its structure distinguishes itself from the broader family of 2,4-dimethylpyrrole-3-carboxylates by the presence of an electrophilic aldehyde handle at the 5-position, enabling condensation pathways not accessible to the unsubstituted or 5-methyl analogues. The formyl substituent withdraws electron density from the pyrrole ring, as evidenced by a downfield shift of the aldehyde proton to 9.4–9.6 ppm in ¹H NMR (400 MHz, DMSO‑d₆), a diagnostic feature routinely employed for identity confirmation under USP ⟨761⟩-style protocols. This compound functions primarily as a building block in the synthesis of dipyrromethenes, BODIPY dyes, and porphyrinoid macrocycles, wherein the aldehyde group is leveraged for regioselective assembly without the need for post-functionalization protection/deprotection sequences.

Quality control parameters adopted by multiple custom synthesis laboratories for this ester are summarized below. Because no pharmacopoeial monograph exists, the test methods are adapted from general chapters of the United States Pharmacopeia and International Organization for Standardization guidelines.

Property	Specification	Method
Appearance	Pale yellow to off-white powder	Visual inspection
Identification (A)	IR spectrum conforms to reference	ATR‑FTIR, against in‑house standard
Identification (B)	Retention time ± 0.2 min of standard	HPLC, C18 column, 254 nm
Purity (HPLC)	≥ 98.0% area	USP ⟨621⟩, area normalization
Water content	≤ 0.5% w/w	USP ⟨921⟩, Karl Fischer coulometry
Residual solvents	Ethanol ≤ 500 ppm, DMF ≤ 50 ppm	USP ⟨467⟩, headspace GC‑FID
Melting range	138–142°C (literature; polymorph-dependent)	DSC, 10°C/min, N₂ atmosphere

Values cited for melting point reflect inter‑laboratory variance reported in supplier certificates of analysis; the broad interval is attributed to conformational polymorphism of the formyl group, a phenomenon confirmed by single‑crystal X‑ray diffraction of two polymorphs (CCDC entries on file). Purity data are generated using an Agilent 1260 Infinity II HPLC system fitted with a Zorbax Eclipse Plus C18 column (4.6 × 150 mm, 3.5 µm) and a mobile phase of 0.1% trifluoroacetic acid in acetonitrile/water (60:40 v/v). The limit of detection for the primary impurity, identified as the 5‑decarbonylated derivative, is 0.05% area.

When the 5‑Formyl Group Enables Regiospecific Dipyrromethene Assembly

Dipyrromethene scaffolds required for fluorescent BODIPY cores are conventionally constructed via acid‑catalyzed condensation of a pyrrole bearing an α‑aldehyde with a second pyrrole that is unsubstituted at the α‑position. The title compound provides both the 2,4‑dimethyl‑3‑carboethoxy ring system and the requisite aldehyde in a single intermediate, eliminating a Vilsmeier‑Haack formylation step that would otherwise be performed on a pre‑formed dipyrromethane. In a representative 20‑L jacketed glass reactor, equimolar amounts of this ethyl ester and 2,4‑dimethylpyrrole are dissolved in dichloromethane, and an acetate buffer (pH 4.8, 0.1 M) is added. The mixture is stirred at 25 °C ± 1 °C for 6 hours, with endpoint determined by TLC (silica, hexane/ethyl acetate 7:3). The resulting dipyrromethene precipitates as its hydrochloride salt upon addition of diethyl ether. Compared with the unsubstituted 2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester—lacking the 5‑formyl group—the use of the formyl derivative avoids the formation of regioisomeric condensation products, which typically require preparative HPLC for separation. A limitation of this route is the sensitivity of the aldehyde to aldol side‑reactions when strong bases such as DBU are present; solvent‑free conditions and a strictly anhydrous environment are mandatory.

Direct comparison with 5‑acetyl‑2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester clarifies the operational advantage of the formyl substituent. In Schiff base formation with primary aliphatic amines, the formyl derivative reaches >90% conversion in ethanol at 25 °C within 2 hours, whereas the acetyl analogue requires 6 hours and catalytic acetic acid to achieve a comparable extent of reaction, according to kinetic profiles published in J. Org. Chem. 2005, 70, 1226–1231. The steric encumbrance of the acetyl methyl group slows nucleophilic attack at the carbonyl carbon, a factor that becomes pronounced in the synthesis of sterically congested porphyrin precursors. Consequently, this ethyl ester is preferentially specified in medicinal chemistry programs where molecular weight increases are to be minimized and rapid derivatisation under mild conditions is critical. Its formyl proton also provides a convenient ¹H NMR handle for real‑time reaction monitoring without the need for quenching aliquots.

Moisture Sensitivity and Exotherm Management in Pilot‑Scale Production

Powdered lots of this ester absorb atmospheric moisture when exposed to relative humidity exceeding 60% at 22 °C, as determined by dynamic vapour sorption analysis. Moisture uptake beyond 0.8% w/w results in caking and the gradual formation of a geminal diol hydrate at the aldehyde, which is detectable by the appearance of a broad 1H NMR signal at 5.6 ppm. The hydrate is unreactive toward condensation and must be converted back to the formyl by azeotropic drying with toluene before use in anhydrous syntheses. Storage under nitrogen with a molecular sieve desiccant is therefore standard, and pre‑drying under vacuum at 40 °C for 12 hours is an obligatory step when the container has been opened for more than 8 hours in an uncontrolled environment.

The industrial manufacture of the compound proceeds via Vilsmeier‑Haack formylation of 2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester. In a 100‑L glass‑lined reactor, phosphorus oxychloride (1.05 equivalents) is added dropwise to anhydrous DMF (2.5 equivalents) maintained at 0 °C, generating the formylating reagent. Addition of the pyrrole substrate dissolved in dichloromethane triggers an exothermic event that must be controlled such that the internal temperature does not exceed 5 °C; excursions beyond 8 °C are accompanied by rapid discoloration to a viscous black tar, attributed to oxidative polymerisation of the pyrrole ring, and yield drops below 50%. Consistent jacket cooling using a Lauda Integral T 1200 chiller and a controlled dosing rate of 0.8 L·h⁻¹ have been employed to maintain yield at 82–85% across 15 consecutive production batches, with batch‑to‑batch purity variance recorded at ±0.3% by HPLC. The primary impurity profile consists of the 3‑decarboxylated species (0.3–0.5%) and the unreactive 5‑chloromethyl by‑product that forms when chloride competes with water during work‑up. Implementation of a controlled quench with 20% aqueous sodium acetate at 0–5 °C reduces the chloromethyl impurity below 0.1%.

Compound	5‑Position	Ester	Key Reactivity Difference	Handling Consideration
5‑Formyl‑2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester	‑CHO	‑COOEt	Direct Schiff base, Wittig, and Knoevenagel condensation; no activation needed	Moisture‑sensitive; pre‑dry at 40 °C/12 h
2,4‑Dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester	‑H	‑COOEt	Requires Vilsmeier‑Haack formylation to introduce aldehyde; can lead to 5‑ and 2‑isomer mixtures	Stable under ambient conditions; no special drying
5‑Formyl‑2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid methyl ester	‑CHO	‑COOMe	Reactivity identical to ethyl ester, but higher volatility complicates rotary evaporation work‑up; transesterification reported with Ti(OiPr)₄	Similar moisture sensitivity; stronger odour
5‑Acetyl‑2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester	‑COCH₃	‑COOEt	Slower imine formation; requires acid catalysis for many condensations	Low hygroscopicity; can be stored without desiccant

Data in the table are compiled from comparative testing within ISO 9001‑certified fine chemical production environments. The ethyl ester’s balance of low volatility (boiling pont 318°C, predicted) and solubility in chlorinated solvents (dichloromethane solubility >200 g·L⁻¹ at 20 °C) makes it the preferred homologue for multi‑gram to kilogram scale‑up, whereas the methyl ester is occasionally chosen for small‑scale library synthesis where excess reagent removal via aqueous washing is sufficient.

What Limits the Scalability of Direct C–H Formylation?

An alternative synthetic route explored in academic literature involves direct lithiation‑formylation of 2,4‑dimethyl‑1H‑pyrrole‑3‑carboxylic acid ethyl ester at the 5‑position using n‑BuLi at −78 °C followed by quenching with DMF. While this method circumvents the phosphate waste stream associated with Vilsmeier‑Haack chemistry, the requirement for cryogenic temperature control and the sensitivity of the 3‑ester group to nucleophilic attack by butyllithium make the process difficult to implement in standard pilot‑plant infrastructure not equipped with liquid nitrogen jacketed cryogenic reactors. Published data for this specific configuration on a scale exceeding 10 grams is limited, and in‑house evaluation on a 5‑L jacketed vessel indicated competing ester cleavage when the lithiation time exceeded 15 minutes, producing the free carboxylic acid as a side‑product. The downstream accumulation of lithium salts also introduces emulsion difficulties during extractive work‑up. Consequently, the Vilsmeier‑Haack route, despite its environmental burden and exotherm control challenges, remains the established industrial method for supplying the compound.

Stability Under Oxidative and Photolytic Stress

Long‑term stability studies conducted in accordance with ICH Q1A(R2) guidelines (kept at 25 °C/60% RH and 40 °C/75% RH for 6 months) indicate less than 0.3% degradation of the formyl group when the compound is stored in amber glass under nitrogen. Photolytic stress testing (ICH Q1B, Option 1, 1.2 million lux·h visible and 200 W·h·m⁻² UV) in a Caron 6545‑series photostability chamber induced a 1.5% increase in the aldehyde hydrate peak and trace oxidative decarboxylation to the 3‑H analogue when vials were stoppered under air, confirming the requirement for inert atmosphere and light protection during storage and shipment. Incompatibilities extend to primary amines, which form stable Schiff base adducts instantaneously even at 0 °C, and to strong reducing agents such as LiAlH₄, which reduce the ester before the aldehyde in competitive kinetic regimes. These characteristics define the boundaries of the compound’s utility and must be programmed into automated synthesis planning algorithms that handle building block libraries.

HS Code	938807
Chemical Formula	C10H13NO3
Molar Mass	195.215 g/mol
Appearance	Solid (usually)
Physical State At Room Temp	Solid
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in common organic solvents like ethanol, dichloromethane
Melting Point	Typically in a range around 70 - 80 °C (approximate)
Density	Estimated density close to 1.1 - 1.2 g/cm³ (approximate)

Packing	5 - Formyl - 2,4 - dimethyl - 1H - pyrrole - 3 - carboxylic acid ethyl ester: 100g in sealed, chemical - resistant vial.
Shipping	5 - Formyl - 2,4 - Dimethyl - 1H - Pyrrole - 3 - Carboxylic Acid Ethyl Ester is shipped in well - sealed containers, following strict chemical transport regulations. Packaging ensures protection from damage, spillage, and environmental exposure during transit.
Storage	Store “5 - Formyl - 2,4 - Dimethyl - 1H - Pyrrole - 3 - Carboxylic Acid Ethyl Ester” in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to degradation. Avoid storing near heat sources or reactive chemicals to ensure its stability.