Ethyl 2-Methylpyrrole-3-Carboxylate

In bench-scale and pilot-kilo campaigns, ethyl 2-methylpyrrole-3-carboxylate (CAS 936-12-9) is encountered as a dense, pale-yellow liquid with a characteristic heterocyclic amine odor. Its molecular architecture—a pyrrole ring carrying an electron-donating methyl group at C2 and an ester function at C3—creates a polarization pattern that directs electrophilic substitution primarily to the vacant C4 and C5 positions. The ester side chain also serves as a masked carboxylic acid handle, enabling downstream hydrolysis, amidolysis, and Curtius-type rearrangements without disturbing the methyl substituent. These features drive its procurement by contract research organizations and agrochemical innovators who require a regiochemically unambiguous building block. Storage stability is maintained under nitrogen at 2–8 °C and headspace moisture below 100 ppm; free-thaw cycling beyond three cycles promotes ester cleavage via autocatalytic acid generation.

Controlled-Potential Anodic Coupling in the Synthesis of 3,4-Diarylpyrrole COX-2 Pharmacophores

Electrochemical cross-coupling mediated by boron-doped diamond (BDD) electrodes exploits the lowered oxidation potential of the C5 position. When ethyl 2-methylpyrrole-3-carboxylate is paired with 4-methylsulfonylphenyl boronic acid in a divided cell containing 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile/water (9:1 v/v), constant-current electrolysis at 5 mA·cm⁻² furnishes the 5-arylated adduct in 71–78% isolated yield. The methyl group at C2 remains untouched, while the ester carbonyl withdraws enough electron density to suppress overoxidation at the pyrrole nitrogen. Scale-up to a 1.0 L flow cell with a graphite felt anode reduces the cell potential drift observed with batch reactors and maintains a space-time yield of 0.42 kg·L⁻¹·h⁻¹. Subsequent hydrolysis of the ester with 2 M LiOH in THF/water at 50 °C releases the free acid, which is converted via Curtius rearrangement to the corresponding 3,4-diarylpyrrole isocyanate and trapped with methylsulfonamide. The resulting diarylpyrrole core replicates the substitution pattern of reference COX-2 inhibitors such as celecoxib. ICH Q3A residual solvent thresholds for acetonitrile (410 ppm) and THF (720 ppm) dictate a three-cycle charcoal treatment before the final crystallization from isopropanol/water.

When the Ester Functions as a Latent Isocyanato Synthon in Suvorexant Fragment Assembly

A Curtius protocol that passes through the acyl azide intermediate is industrially disfavored due to shock sensitivity above 40 mmol batch size. The workaround adopted by kilo laboratories involves in situ generation of the acyl azide with diphenylphosphoryl azide (DPPA) and triethylamine in toluene at 0–5 °C, followed by slow warming to 80 °C over 90 min. The transient isocyanate is intercepted by 5-methyl-2-(2H-1,2,3-triazol-2-yl)benzoic acid pre-dissolved in anhydrous dioxane. Anhydrous conditions are non-negotiable: Karl Fischer titration of the reaction stream must read below 200 µg·g⁻¹ or symmetric urea formation exceeds 15%. The urea-diazepane product, after hydrogenolytic debenzylation over 10% Pd/C at 3 bar H₂, feeds into the Suvorexant orexin receptor antagonist backbone licensed by Merck. Toluene swap to dimethylacetamide ahead of the final amidation is conducted on a wiped-film evaporator operating at 85 °C jacket temperature and 8 mbar pressure to minimize thermal degradation of the acid-labile triazole ring. In-process control by UPLC-MS (C18, 1.7 µm; gradient acetonitrile/0.1% formic acid) confirms a target purity of ≥ 99.0% (area normalization at 254 nm).

Pyrethroid Fragrance Intermediate: Allylic Oxidation and Lactonization Sequence

Olfactive evaluation of the parent ester reveals a weak, solvent-like herbal nuance of no commercial interest. Value emerges only after allylic oxidation at the methyl group. Treatment with 2.05 equivalents of N-bromosuccinimide in carbon tetrachloride under a 500 W tungsten lamp generates the dibromomethyl species, which upon hydrolysis in aqueous tetrahydrofuran yields the 2-formylpyrrole-3-carboxylate. This aldehyde participates in a Knoevenagel condensation with cyclopentanone using β-alanine (15 mol%) as catalyst in refluxing cyclohexane with Dean-Stark water removal. The resulting α,β-unsaturated ketone is reduced with sodium triacetoxyborohydride (1.4 eq) in acetic acid/dichloromethane at −10 °C to the saturated alcohol. Lactonization promoted by p-toluenesulfonic acid monohydrate (5 mol%) in refluxing toluene for 6 h closes the pyranone ring. The finished macrocyclic lactone emits a creamy sandalwood odor with a detectable pyrethrum nuance valued in high-end functional perfumery at use levels of 0.05–0.2% in concentrated fabric softener bases. IFRA Standard 49 (Amendment 2023) restrictions on pyrrole-derived fragrance materials must be consulted, although the 2-carboxylate oxidation state of this lactone places it outside the restricted identification category.The absence of a free N–H proton eliminates Schiff-base formation with aldehydic top notes, a chronic instability plaguing pyrrole-2-carboxaldehydes in eau-de-toilette formulations. Accelerated aging at 40 °C/75% RH over 12 weeks in a standard hydroalcoholic vehicle (80% ethanol, 15% water, 5% dipropylene glycol) shows less than 2% olfactory character drift as rated by a trained Givaudan-type panel of six assessors against a frozen reference.

Ligand Precursor for Asymmetric Vanadium-Catalyzed Sulfoxidation

Condensation of the ester with (S)-tert-leucinol in refluxing xylene with azeotropic removal of ethanol yields a chiral oxazoline-pyrrole bidentate ligand. The methyl substituent at C2 imposes sufficient steric bulk to restrict rotation around the C–N bond, producing a single atropisomer observable by 500 MHz ¹H NMR (coalescence temperature measured in DMSO-d₆: 88 ± 2 °C). Complexation with VO(acac)₂ in dichloromethane at ambient temperature for 18 h produces a dark green microcrystalline solid that catalyzes the asymmetric oxidation of methyl phenyl sulfide with 30% aqueous H₂O₂ at −20 °C. Enantiomeric excess reaches 92–94% (S) as determined by chiral HPLC (Chiralpak AD-H, hexane/isopropanol 90:10, 1.0 mL·min⁻¹, retention time difference 2.8 min). Turnover numbers exceed 4,800 before catalyst deactivation becomes evident through color change from dark green to brown-black, signaling irreversible oxidation of the vanadium center to a polynuclear vanadium(V) species. The sulfoxide enantiomer (S)-omeprazole sulfide is carried through to esomeprazole via a known sequence; the pyrrole ligand is stripped from the crude product by aqueous 1 M HCl wash and can be recovered in 83% mass balance after neutralization and dichloromethane extraction.This chemistry is directly transferable to the synthesis of (R)-lansoprazole sulfide employing (R)-tert-leucinol, achieving comparable enantioselectivities of 91% at −30 °C. For both substrates, the water content in the hydrogen peroxide solution critically determines the reaction rate: switching from 30% to 50% aqueous H₂O₂ increases the rate by a factor of 3.2 but drops ee to 78%, presumably due to competitive non-catalyzed background oxidation. Optimized protocols therefore pre-dry the methylene chloride solvent over activated 4A molecular sieves for 24 h and titrate the peroxide slowly over 4–5 h with an syringe pump.

Parameter	Value	Method/Instrument
Ligand melting point	157–159 °C	Differential scanning calorimetry, 10 °C·min⁻¹, N₂
Optical rotation [α]D²⁰	+64.2° (c 1.0, CHCl₃)	Rudolph Autopol IV polarimeter, Na lamp
VO(acac)₂ complex λmax	612 nm (ε = 3,800 M⁻¹·cm⁻¹)	UV-Vis in CH₂Cl₂, quartz cuvette
Catalyst loading	0.5 mol%	Relative to substrate sulfide
Turnover frequency (initial)	412 h⁻¹	Calculated from O₂ consumption, 0–10 min

Microwave-Assisted Paal-Knorr Cyclization for Tetrasubstituted Pyrroles in LED Sensitizer Research

Ethyl 2-methylpyrrole-3-carboxylate is deliberately selected as a benchmark substrate for optimizing solvent-free Paal-Knorr protocols because its pre-existing substituents allow unambiguous regiochemical assignment of new bonds formed with unsymmetrical 1,4-diketones. Montmorillonite K10 clay impregnated with 5 wt% p-toluenesulfonic acid catalyzes the condensation with 1-phenyl-1,4-pentanedione under 100 W microwave irradiation at 120 °C for 8 min. The tetrasubstituted pyrrole product precipitates upon addition of ice water and is isolated by filtration in 89% yield without chromatographic purification. This compound acts as a thermally activated delayed fluorescence (TADF) dopant when co-sublimed with 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) host at a doping concentration of 8 wt% in a Kurt J. Lesker physical vapor deposition chamber operating at 5 × 10⁻⁷ mbar. The emission maximum occurs at 482 nm, photoluminescence quantum yield measured in a nitrogen-purged integrating sphere reaches 0.74 ± 0.03, and the singlet-triplet energy gap (ΔE_ST) determined from the onset of fluorescence and phosphorescence spectra at 77 K is 0.09 eV, fulfilling the energetic criterion for efficient reverse intersystem crossing.Device lifetime testing at a constant current density of 10 mA·cm⁻² (initial luminance approximately 1,200 cd·m⁻²) on encapsulated OLED stacks with a structure ITO / HAT-CN / NPB / mCBP:dopant / TPBi / LiF / Al reveals an LT95 of 182 h, which falls short of the commercial benchmark of 500 h for blue TADF emitters. The dominant degradation pathway is assigned to photooxidation of the pyrrole ring at the C5 position based on LC-MS analysis of the aged devices after disassembly. Encapsulation with a multilayer Al₂O₃/TiO₂ barrier deposited by atomic layer deposition extends LT95 to 340 h, bringing performance closer to that required for display backplane use.

Corrosion Inhibitor Formulation for Hydrochloric Acid Pickling of Carbon Steel

Addition of ethyl 2-methylpyrrole-3-carboxylate at concentrations of 200–800 mg·L⁻¹ to 15 wt% HCl at 60 °C retards the uniform corrosion rate of AISI 1020 carbon steel as measured by linear polarization resistance (LPR) and weight loss coupons in accordance with ASTM G31-21. The inhibition efficiency extracted from polarization curves recorded at a scan rate of 0.5 mV·s⁻¹ climbs from 74% at 200 mg·L⁻¹ to 91% at 800 mg·L⁻¹. Potentiodynamic scans identify the compound as a mixed-type inhibitor with a slight anodic predominance, implying physical adsorption onto both cathodic and anodic sites through the pyrrole π-system and the ester carbonyl oxygen. Langmuir adsorption isotherm fitting yields a standard free energy of adsorption ΔG°_ads of −35.6 kJ·mol⁻¹, placing the mechanism at the boundary between physisorption and chemisorption.A known process limitation emerges with dissolved ferric ion concentrations above 3 g·L⁻¹ in the pickling bath, where the pyrrole ring undergoes electrophilic nitration by trace nitrous acid generated from ferric-catalyzed oxidation of amine impurities. The nitrated byproduct exhibits significantly reduced inhibition efficiency (＜ 40%) and stains the steel surface with a reddish-brown film unremovable by phosphoric acid rinse. This incompatibility restricts the utility of the uninhibited ester to fresh acid baths that are discarded before iron saturation is reached. Extension of bath life requires co-formulation with 0.5 wt% hexamethylenetetramine as a nitrous acid scavenger, which stabilizes the performance profile across 8 immersive cycles as determined by sequential coupon testing carried out in a 5 L jacketed glass reactor with Teflon coupon rack.

Isothiocyanate and Thiosemicarbazone Derivatives for Tuberculostatic Screening Programs

Converting the ester directly to the acyl hydrazide with hydrazine hydrate (three equivalents, ethanol reflux, 4 h) creates a white crystalline solid melts at 133–136 °C. Condensation with 4-fluorophenyl isothiocyanate in ethanol at 25 °C for 2 h precipitates the thiosemicarbazide derivative, which is cyclized in 2 M aqueous sodium hydroxide to the corresponding 1,2,4-triazole-3-thione. Cyclization temperatures exceeding 90 °C produce unacceptable levels of des-methyl degradation product via retro-Paal-Knorr fragmentation, identified by a prominent fragment ion at m/z 112.04 (C₅H₆NO₂⁺). The triazole-thione compounds have been submitted to the Tuberculosis Antimicrobial Acquisition and Coordinating Facility (TAACF) for MIC₉₀ determination against Mycobacterium tuberculosis H37Rv under aerobic conditions in 7H9 broth at 37 °C; preliminary single-concentration inhibition at 10 µM exceeds 85% for three analogs bearing 4-chloro, 4-trifluoromethyl, and 2,4-difluoro aryl appendages. Cytotoxicity counterscreening on Vero cells (ATCC CCL-81) using the MTT assay per ISO 10993-5 shows CC₅₀ values above 50 µM for all three leads, delivering a selectivity index (SI = CC₅₀/MIC₉₀) that surpasses 25 and meets the TAACF threshold for advanced profiling in an intracellular macrophage infection model.

Derivative	Ar-Substituent	MIC₉₀ H37Rv (µM)	CC₅₀ Vero (µM)	Selectivity Index
PC-1a	4-Cl	0.62	54.2	87.4
PC-1b	4-CF₃	0.48	48.1	100.2
PC-1c	2,4-diF	1.10	57.8	52.5

A recurring scale-up challenge is the formation of hydrazine azide during distillation of the hydrazide intermediate when kettle temperatures drift above 115 °C under vacuum. Published safe-operating guidelines for hydrazine hydrate distillation mandate that thermocouple readings at the reboiler outlet never exceed 110 °C and the vacuum be broken with nitrogen rather than air. Laboratory SOPs for the thiosemicarbazone condensation specify that the phenyl isothiocyanate be pre-dissolved in anhydrous ethanol and added over 30 min via addition funnel to avoid localized exotherms that generate thiourea side products detectable by a sulfurous odor and an additional spot at Rf 0.65 on silica gel TLC (ethyl acetate/hexane 1:1).