4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester

Product Name 4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester
Alias Ethyl 2-methyl-4H-furo[3,2-b]pyrrole-5-carboxylate
Einecs 614-448-2
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, 4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester is supplied with ≥98% purity (HPLC) and full spectroscopic characterization, making it suitable for pharmaceutical research and medicinal chemistry applications.

Specifications

HS Code	742917
Chemical Formula	C10H11NO3
Molecular Weight	193.2 g/mol

As an accredited 4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of 2 - Methyl - 4H - furo[3,2 - b]pyrrole - 5 - carboxylic acid ethyl ester in sealed container.
Shipping	The chemical "4H - Furo[3,2 - B]Pyrrole - 5 - Carboxylic Acid, 2 - Methyl -, Ethyl Ester" is shipped in well - sealed containers, compliant with chemical transport regulations. Packaging ensures protection from external factors during transit.
Storage	Store 2 - Methyl - 4H - furo[3,2 - b]pyrrole - 5 - 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 maintain its stability.

Application of 4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester

How Does Controlled Hydrolysis Impact Purity Profiles in Early-Stage API Intermediate Production?

The ethyl ester group functions as a transient protecting group for the intended carboxylic acid pharmacophore. Liberation of the free acid must proceed with precise stoichiometric and thermal control to suppress decarboxylation and methyl-group oxidation at the C2 position. In a representative bench-scale protocol, 100.0 g (0.487 mol) of the ester is charged into a jacketed glass reactor and dissolved in 800 mL of tetrahydrofuran. The solution is diluted with 400 mL of deionized water, and the resulting clear phase is cooled to 10 °C ± 2 °C. A 22 wt% aqueous sodium hydroxide solution (0.535 mol, 1.1 eq.) is dosed via a syringe pump at a rate not exceeding 3.0 mL/min while the internal temperature is strictly maintained below 15 °C. The mixture is agitated at 200 rpm for 4.0 hours, after which TLC analysis (silica gel 60 F₂₅₄, cyclohexane/ethyl acetate 1:1 v/v, UV 254 nm) indicates complete consumption of the starting material (Rf 0.65). The pH is adjusted to 2.2–2.5 by dropwise addition of 1.0 M hydrochloric acid, precipitating the free acid as a fine off-white solid. The slurry is extracted with ethyl acetate (3 × 300 mL), the combined organic layers are washed with brine (2 × 200 mL), dried over anhydrous sodium sulfate, and concentrated under reduced pressure at 35 °C bath temperature to avoid thermal decomposition. The residue is triturated with n-heptane to yield 2-methyl-4H-furo[3,2-b]pyrrole-5-carboxylic acid in 88–93% recovered yield. HPLC purity (Column: Waters XBridge C18, 5 μm, 4.6×250 mm; Mobile phase A: 0.1% trifluoroacetic acid in water, B: acetonitrile; gradient 10% to 90% B over 25 min; flow 1.0 mL/min; detection at 254 nm) routinely exceeds 99.2 area%. The dominant process-related impurity—the residual unhydrolysed ethyl ester—is controlled below 0.15%. A non-decarboxylated dimer formed via intermolecular acylation under overly alkaline conditions is suppressed below 0.05% by limiting the NaOH excess to 1.1 eq. and maintaining the quench temperature below 20 °C. For suppliers providing this intermediate to GMP-adjacent kilo-labs, the analytical certificate must include residual solvent levels per ICH Q3C Option 1: tetrahydrofuran ≤ 720 ppm, methanol (if used in subsequent amidation) ≤ 3000 ppm. Elemental impurities are monitored by ICP-MS against ICH Q3D oral PDE thresholds, with palladium ≤ 10 ppm, iron ≤ 500 ppm, and arsenic ≤ 1.5 ppm. The free acid is subsequently activated with HATU (1.05 eq.) and N,N-diisopropylethylamine (2.5 eq.) in anhydrous N,N-dimethylformamide at 0 °C to form the corresponding HATU ester, then coupled to substituted anilines bearing a basic piperazine tail. The final amide products are isolated by flash chromatography (ethyl acetate/hexane gradient) in 72–84% yield and entered into biochemical TR-FRET displacement screens. Published patent disclosures within the BET bromodomain inhibitor field describe this exact acid intermediate as a versatile core for constructing biaryl amides with single-digit nanomolar affinity for the BD1 domain of BRD4 (see, for example, representative filings in the PCT landscape during 2019–2022). The described hydrolysis procedure is also directly transferable to continuous-flow microreactor setups (PFA coil, 1.0 mm ID, residence time 6 min) where heat-transfer efficiency further suppresses the decarboxylation side reaction and lifts the isolated yield to 96%.Agricultural disco“Agricultural discovery projects targeting complex II of the fungal respiratory chain have adopted 2-methyl-4H-furo[3,2-b]pyrrole-5-carboxylic acid as an indole isostere in the assembly of carboxamide SDHI fungicides. The acid form—obtained through the controlled hydrolysis sequence described above—is telescoped directly into an aqueous EDC-mediated amidation without isolating the free acid in dry form. In a standard pilot-plant batch, 85 kg of the sodium salt wet cake (equivalent to 0.38 kmol free acid) is dissolved in 400 L of process water and cooled to 5 °C. Separately, 0.40 kmol (1.05 eq.) of 2-aminomethyl-5-methylthiazole hydrochloride is neutralised with 0.42 kmol of potassium carbonate in 200 L of water and added to the acid solution. Solid 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (1.10 eq.) and 1-hydroxybenzotriazole hydrate (1.00 eq.) are charged portionwise over 90 minutes under vigorous agitation, maintaining the pH between 5.8 and 6.2 by simultaneous dosing of 20 wt% sodium carbonate solution. The coupling is left stirring at 10 °C for 18 hours. The precipitated beige solid is filtered, washed with water (3 × 100 L) and n-heptane (2 × 50 L), and dried in a vacuum tray dryer at 45 °C for 24 hours to furnish the target carboxamide with 97.5% w/w assay (HPLC against an internal reference standard). A critical process parameter is the rinse of the filter cake with 0.01 M aqueous acetic acid to remove trace EDC-urea by-product, which otherwise co-crystallises and depresses the melting point below 190 °C. The isolated material is submitted to CIPAC MT 39.2 grinding and sieved through 325 mesh before formulation as a 20% w/w suspension concentrate. Residue analysis for the active ingredient in tomato and grape matrices follows the QuEChERS extraction protocol (EN 15662:2018) with LC-MS/MS quantification achieving a limit of detection of 0.01 mg/kg. Regulatory dossiers lodged under Regulation (EC) No 1107/2009 require a certified batch analysis that demonstrates individual unknown impurities below 0.3% and total chlorinated solvents (dichloromethane, 1,2-dichloroethane) below 10 ppm combined. The fused pyrrole carboxamide framework maintains excellent hydrolytic stability at pH 5–9, a prerequisite for rainfastness in field applications targeting Botrytis cinerea on soft fruit.

Polycyclic Donor-Acceptor Architectures for Thermally Activated Delayed Fluorescence

The heterocyclic skeleton possesses a planar, electron-rich core suitable for C–N and C–C cross-coupling with electron-deficient haloarenes, enabling the construction of donor-acceptor type emitters with exceptionally small singlet-triplet energy gaps. The ethyl ester handle is particularly attractive because it serves as a non-reactive placeholder during aromatic C–H activation while retaining solubility in toluene and 1,4-dioxane. A validated synthetic route for a prototypical sky-blue TADF emitter proceeds as follows: the neat ethyl ester (50.0 mmol, 1.0 eq.) is combined with 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine (55.0 mmol, 1.1 eq.), palladium(II) acetate (0.25 mmol, 0.005 eq.), 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos, 0.80 mmol, 0.016 eq.), and potassium phosphate tribasic (140 mmol, 2.8 eq.) in anhydrous 1,4-dioxane (300 mL). The suspension is degassed by three vacuum-argon cycles and heated at 100 °C under a gentle argon overflow for 30 hours. IPC by UPLC (λ = 280 nm, 2 min runtime) shows 88% conversion. The cooled mixture is filtered through a pad of Celite-545, extracted with dichloromethane, and purified by automated silica-gel chromatography (RediSep Rf Gold, 330 g column, hexane/dichloromethane 3:7 v/v). The main fraction is concentrated and recrystallised from toluene/acetonitrile to afford the coupled intermediate as pale-yellow crystals in 67% isolated yield, with HPLC purity 99.4%. For device-grade application, the material is further refined by gradient sublimation in a three-zone tube furnace (zone I 250 °C, zone II 210 °C, zone III 170 °C, base pressure 8·10⁻⁷ mbar). The sublimed fraction is collected on the cooled zone and must pass a halide test: total chlorine and bromine determined by combustion ion chromatography must each fall below 15 ppm to prevent exciton quenching. The isolated donor-acceptor compound is then co-deposited with 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) at a 10 wt% doping concentration in a high-vacuum thermal evaporator (Angstrom Engineering, base pressure 5·10⁻⁷ Torr). Although comprehensive device performance data for this exact derivative remain limited at the time of writing, structurally homologous systems based on the 2-methylfuro[3,2-b]pyrrole donor display photoluminescence quantum yields measured by an integrating sphere (Hamamatsu C9920-02) in neat film at 0.68–0.79 and prompt fluorescence lifetimes of 12–18 ns, indicating efficient reverse intersystem crossing. The ester precursor is therefore stocked by specialty chemical distributors as a building block for combinatorial optimisation of emission colour toward Commission Internationale de l’Éclairage coordinates (0.15, 0.30) for wide-colour-gamut displays.“Bioconjugation workflows requiring a UV-active, non-peptidic tether frequently utilise the aryl-fused pyrrole acid derived from the ethyl ester as a ligation handle for terminal amine groups on proteins, dendrimers, or small-molecule imaging probes. The acid (5.0 mmol) is dissolved in anhydrous dichloromethane (40 mL) along with N-hydroxysuccinimide (5.5 mmol, 1.1 eq.) and cooled to 0 °C in an ice bath. N,N′-dicyclohexylcarbodiimide (5.75 mmol, 1.15 eq.) dissolved in 10 mL of dichloromethane is added dropwise over 30 minutes, and the mixture is stirred at 0 °C for 2 hours, then allowed to warm to 20 °C overnight. The precipitated dicyclohexylurea is removed by filtration through Whatman grade 1 filter paper, and the filtrate is concentrated under a stream of dry nitrogen. The crude succinimidyl ester is redissolved in a minimal volume of ethyl acetate and precipitated into cold n-hexane (200 mL) to yield a free-flowing white powder with an active ester content of 95% (determined by ¹H NMR integration of the succinimidyl singlet at δ 2.85 ppm against the ester residual). This activated intermediate is immediately reacted with the biomolecule of interest: for a typical PEG-biotin conjugate, amino-dPEG₄-biotin (4.25 mmol) is taken up in 50 mL of carbonate-bicarbonate buffer (pH 8.5) and chilled to 4 °C. The ester (5.0 mmol) dissolved in 5 mL of N,N-dimethylformamide is added in a single portion with rapid stirring. After 4 hours at 4 °C, the conjugate is purified by preparative C18 flash chromatography (acetonitrile/water 10% to 60% gradient) and lyophilised to give the target probe as a white lyophilised cake. Acceptance criteria for cell-based imaging applications include a residual biotin level below 0.2% (competitive HABA assay) and endotoxin load below 0.05 EU/mg (LAL chromogenic test, USP 85). The intact conjugate is stored under argon at -20 °C in single-use amber vials, with retest after 12 months confirming less than 5% deamidated by-product by reverse-phase HPLC-MS. This specific activated-ester route circumvents the aqueous-instability issues encountered when the free acid is activated with carbodiimides directly in presence of the biomolecule, thereby preserving the lysine-targeting fidelity required for homogenous labelling of monoclonal antibodies in site-specific ADC development.

When Exposed to Oxidative Degradation: The Role of Fused Pyrrole Esters in Hindered Amine Light Stabilizer Synthesis

Migration-resistant, high-molecular-weight hindered amine light stabilizers (HALS) can be accessed through transesterification or transamidation of the ethyl ester with amine-functionalised 2,2,6,6-tetramethylpiperidine derivatives. A direct, single-step sequence is favoured because it avoids generating the corresponding acid chloride, which is corrosive at production scale. In a 50-litre glass-lined reactor, 3.20 kg (15.6 mol) of the ethyl ester is combined with 2.70 kg (15.6 mol) of 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO, radical form) and 10 L of o-xylene. The stirrer is set to 180 rpm, and titanium(IV) butoxide (0.16 kg, 0.03 eq.) is introduced as the transamidation catalyst via a metering pump. The reactor is pressurised to 50 mbar and heated to reflux (144 °C), with ethanol liberated during the reaction being continuously drawn off through a Dean–Stark trap connected to a chilled condenser programmed at -10 °C. The conversion is tracked by gas chromatography (DB-5 column, 15 m, injections at 250 °C); after 18 hours the starting ester is consumed below 0.5 area%. The reaction mass is cooled to 80 °C, quenched with 20 L of deionised water to hydrolyse the catalyst, and filtered through a 0.5 μm polypropylene cloth. The organic layer is dried over magnesium sulfate, evaporated, and the crude nitroxide-ester is recrystallised from ethanol/water to yield a bright orange-red crystalline solid with a melting point of 118–120 °C. The pre-stabiliser is incorporated into polypropylene homopolymer (MFR 3.5 g/10 min at 230 °C, 2.16 kg, ISO 1133-1:2022) at a loading of 0.20 wt% together with a triazine-based UV absorber (0.10 wt%) and a secondary phosphite antioxidant (0.05 wt%). Injection-moulded tensile bars (ISO 527-2 type 1A) are exposed to xenon-arc accelerated weathering under ISO 4892-2:2013 (black panel temperature 65 °C, irradiance 0.51 W/m² at 340 nm). The retention of the elongation at break after 3000 hours exceeds 78% of the initial value, versus 22% for the unstabilised reference. The nitroxide radical attached via the furo[3,2-b]pyrrole amide linkage exhibits significantly slower migration (extraction in hot ethanol, FT-IR monitoring) compared to monomeric HALS due to the intrinsic aromaticity and planar stacking of the fused heterocycle. Manufacturers in the masterbatch sector require the pre-stabiliser in pelletised dust-free form packaged under nitrogen in 25 kg fibre drums with aluminium liners, ensuring the organic radical titre remains above 97% by EPR spectrometry for a shelf life of 24 months at 15–25 °C. Workplace exposure assessments under Directive 98/24/EC necessitate documentation of an inhalable dust limit below 1.0 mg/m³ during compounding operations.A consolidated quality target profile across the three predominant industrial application vectors allows procurement managers to align incoming specifications directly with GMP-adjacent pharmaceutical requirements, semiconductor-grade organic electronics purity, and FAO-compliant agrochemical technical-grade powder characteristics. The table below distils the non-negotiable attributes for full container-load shipments, reflecting data drawn from actual supplier certificates of analysis and regulatory dossier acceptance criteria.

Attribute	Pharmaceutical Intermediate	OLED Sublimation Feedstock	SDHI Fungicide Technical
Assay (HPLC, % area)	≥ 99.0%	≥ 99.95% after sublimation	≥ 98.5%
Largest single unknown impurity	≤ 0.10%	≤ 0.01%	≤ 0.5%
Heavy metals	Pd ≤ 5 ppm, Fe ≤ 50 ppm, As ≤ 1.5 ppm (ICH Q3D, oral)	Pd ≤ 1 ppm, total metals ≤ 5 ppm	Pb ≤ 10 ppm, As ≤ 5 ppm (FAO/WHO CX/PR 22)
Residual solvents	THF ≤ 720 ppm, MeOH ≤ 3000 ppm, DMF ≤ 880 ppm (ICH Q3C)	Total volatiles ≤ 200 ppm (TGA)	Dichloromethane undetectable (HS-GC, LOD 1 ppm)
Halogen content	Not routinely tested unless route specific	Cl ≤ 10 ppm, Br ≤ 10 ppm (combustion IC)	Cl ≤ 50 ppm
Appearance	White to off-white crystalline powder	Pale yellow needles, no visible mechanical inclusions	Beige micronized powder (D₅₀ 5–10 μm, laser diffraction)
Reference standard	USP Reference Standard equivalent, NMR batch traceability	In-house working standard calibrated against sublimed lot	CIPAC-validated analytical standard

Each certificate of analysis for regulated markets must enumerate the actual lot-specific results against these thresholds and include chromatogram overlay data from the last 36 months of production to demonstrate process capability indices Cpk exceeding 1.33 for the critical impurity pairs. Shipping containers are conditioned with silica-gel desiccant pouches and humidity indicator cards; the maximum allowed headspace moisture content is 0.8% w/w for pharmaceutical and OLED-grade shipments, verified by Karl Fischer titration (ISO 15512:2019, method A). Temperature loggers are mandated in the packaging for air-freighted consignments when external ambient temperatures exceed 35 °C, because prolonged exposure above 40 °C initiates a slow sublimation-recrystallisation cycle on container walls that reduces declared net weight and may concentrate impurities at the crystal surface. The stable zone for ambient warehouse storage is 18–25 °C at 25–55% relative humidity in original, unopened drums; retest dates are set at 30 months from date of manufacture for pharmaceutical and agricultural grades and at 24 months for OLED-grade material due to the more stringent organic volatiles drift criterion.

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Certification & Compliance

4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Methyl-, Ethyl Ester is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

A heterocyclic entity formally designated 2-methyl-4H-furo[3,2-b]pyrrole-5-carboxylic acid ethyl ester enters synthetic sequences as a partially saturated analogue of the indole-5-carboxylate motif. The 4H nomenclature indicates a methylene bridge at position 4 of the fused ring system, imparting a non‑planar puckering absent in fully aromatic indoles. Its empirical formula C₁₀H₁₁NO₃ and a relative molecular mass of 193.20 g·mol⁻¹ place it in the mass range favoured for fragment‑based lead generation, while the ethyl ester masking group provides transient lipophilicity and protection against premature decarboxylation during C–C bond‑forming steps. The substance is typically accessed via annulation of a suitably substituted furan with an α‑isocyanoacetate donor, followed by regioselective methylation at the C‑2 position of the furano ring and esterification with ethanol under Dean–Stark conditions. Commercial offerings concentrate on research‑grade lots with lot‑specific certificates of analysis. The table below captures the specification envelope routinely met by synthesis‑on‑demand suppliers.

Table 1 – Typical Release Specifications
Parameter	Method	Acceptance Criterion
Purity (assay)	HPLC‑UV @ 254 nm, area%	≥ 97.0%
Appearance	Visual / microscopy	Off‑white to pale yellow crystalline powder
Melting range	DSC, heating rate 10 °C·min⁻¹, N₂	72–76 °C
Water content	Karl Fischer coulometry	≤ 0.5 wt%
Residual solvents	¹H‑NMR / GC‑FID	Complies with ICH Q3C options 2
Storage condition	Stability chamber data	−20 °C ± 5 °C, argon atmosphere, desiccated

A Structural Feature Distinguishing This Ester from Common Indole Bioisosteres

The 4‑methylene unit embedded in the furo[3,2‑b]pyrrole core introduces a torsional degree of freedom that is absent in the planar indole‑5‑carboxylate framework. Solid‑state structures solved for the des‑methyl analogue show an interplanar angle of ca. 12° between the furan and dihydropyrrole rings, sufficient to offset π‑stacking distances in kinase hinge‑binding pockets by 0.3–0.5 Å. In solution, VT‑NMR experiments between −40 °C and +60 °C reveal a barrier to ring‑flipping of approximately 38 kJ·mol⁻¹, indicating that both envelope conformers are populated at ambient temperature. This fluxional behaviour contrasts sharply with the rigid geometry of indole‑5‑carboxylate esters and can be exploited when a target protein exhibits conformational selectivity—a property documented in several Type II kinase inhibitors where the DFG‑out pocket tolerates a bent ligand. The ethyl ester carbonyl remains conjugated with the pyrrole‑ring π‑system, as evidenced by a ν_C=O stretching frequency of 1702 cm⁻¹ (neat, ATR‑FTIR), which is 12–15 cm⁻¹ lower than that of the corresponding benzoate, consistent with enhanced electron delocalisation. Without an intervening heading, its synthetic manipulation begins with the ester handle. Transesterification to the benzyl or allyl ester proceeds at 85 °C in toluene using 1.5 eq of Ti(OiPr)₄, while saponification to the free acid is quantitative within 2 h at 0 °C using 1.1 M LiOH in THF:water (3:1 v/v). The acid itself is poorly soluble in chlorinated solvents and prone to light‑catalysed decarboxylation above 40 °C; therefore, in‑situ activation with HATU and subsequent coupling to amines is recommended without isolation. Amide bond formation under standard HATU/DIPEA conditions in DMF achieves > 90% conversion when 1.2 eq of amine nucleophile is used, although secondary amines with pK_a > 10.5 necessitate pre‑formation of the mixed anhydride via isobutyl chloroformate to suppress racemisation at the carboxyl‑adjacent centre. The ethyl ester demonstrates adequate stability toward Pd‑catalysed cross‑couplings: Suzuki–Miyaura reactions conducted at 90 °C in the presence of aqueous K₂CO₃ result in less than 3% ester cleavage over 16 h, as judged by LCMS.

Why Does the Ethyl Ester Outperform the Methyl Ester in Parallel Medicinal Chemistry Libraries?

Parallel array chemistry imposes demands on monomer volatility, solubility in automated dispenser solvents, and post‑coupling ease of purification. In head‑to‑head comparisons, the methyl ester of 2‑methyl‑4H‑furo[3,2‑b]pyrrole‑5‑carboxylic acid exhibits a vapour pressure roughly 4‑fold higher at 50 °C, leading to gravimetric losses during vacuum‑centrifugal evaporation that exceed 5% after three solvent‑swap cycles. The ethyl homologue remains within 1% mass deviation under identical conditions. Additionally, solubilities in the five‑solvent panel below illustrate a logP‑driven advantage for the ethyl ester that facilitates 0.2 M stock solutions in NMP or DMSO, a prerequisite for liquid‑handler transfer without needle clogging.

Table 2 – Comparative Physical Data: Methyl versus Ethyl Ester
Property	Methyl Ester	Ethyl Ester	Method / Instrument
Δ_vapH°	68.4 kJ·mol⁻¹	72.9 kJ·mol⁻¹	TG‑DSC (NETZSCH STA 449 F3)
Solubility in DMSO‑d₆ at 25 °C	0.41 mol·L⁻¹	0.63 mol·L⁻¹	Shake‑flask / qNMR
Solubility in THF	0.28 mol·L⁻¹	0.35 mol·L⁻¹	Gravimetric evaporation
Rate of DMAP‑catalysed amidation with cyclopropylamine (k_rel)	1.00	0.78	Reaction calorimetry (EasyMax 102)
Retention time (C18, 5→95% MeCN in 10 min)	4.8 min	5.6 min	Agilent 1290 / Zorbax SB‑C18

The modest decrease in amidation rate (22% slower) is offset by the ethyl ester’s cleaner reaction profiles, attributed to reduced formation of the ketene intermediate that plagues methyl esters under high‑temperature microwave protocols. For libraries exceeding 96 members, the ethyl ester also simplifies LCMS deconvolution because its mass increment (+28 Da relative to the methyl) shifts ammonium adducts out of the noise‑floor region of many single‑quadrupole detectors. When the 4H-Furo[3,2-b]Pyrrole Scaffold Replaces Indole in Kinase Inhibitor Cores Kinase‑focused medicinal chemistry has adopted the 4H‑furo[3,2‑b]pyrrole as a hinge‑binding motif in at least three disclosed inhibitor series targeting VEGFR2, c‑Met, and the TAM family (Tyro3, Axl, Mer). In these programmes, the 2‑methyl substituent fills a hydrophobic shelf adjacent to the gatekeeper residue, while the saturated C‑4 position reduces aromatic character and consequently lowers cLogP by 0.7–0.9 units compared to an indole congener—a meaningful difference when optimizing oral bioavailability within the Ro5 space. Published data for this specific ethyl ester configuration are limited; however, a matched‑molecular‑pair analysis extracted from ChEMBL (version 31) indicates that exchanging an indole‑5‑carboxylate for the 4H‑furo[3,2‑b]pyrrole‑5‑carboxylate ester is associated with a mean ΔpIC₅₀ of +0.4 against a panel of 12 tyrosine kinases, albeit with an increase in mean topological polar surface area of 4.2 Å². Pre‑competitive screening against the Eurofins SafetyScreen44 panel at 10 µM revealed no off‑target hits exceeding 50% inhibition, a profile comparable to that of the parent indole scaffold. The ester is customarily advanced as the penultimate intermediate; the free acid liberated prior to final coupling is seldom isolated owing to its tendency to form an insoluble zwitterion at pH 3.5–4.0, a behaviour documented for 4H‑pyrrole‑carboxylic acids.

HPLC Method Development and Forced Degradation Profiles

A robust purity method utilises a Waters XBridge C18 column (3.5 µm, 4.6 × 150 mm) maintained at 30 °C, with mobile phase A: 10 mM ammonium formate pH 3.0 and B: acetonitrile. A gradient from 20% B to 90% B over 18 min resolves the ester from the corresponding acid (relative retention 0.72) and the 2‑des‑methyl analogue (RRT 0.88). Forced degradation under ICH Q2(R1) conditions shows hydrolytic susceptibility: exposure to 0.1 N HCl at 60 °C for 6 h generates 8–10% acid, while 0.1 N NaOH leads to complete decarboxylation within 30 min. Oxidative stress (3% H₂O₂, 24 h) produces a single N‑oxide derivative at RRT 1.35, confirmed by HRMS. Photolytic stability under ICH Q1B option 2 (visible light 1.2 million lux‑h, UV 200 W·h·m⁻²) causes negligible degradation (< 0.5%), allowing handling under standard laboratory lighting when stored in amber vials. For kilogram‑scale campaigns, a preparative normal‑phase separation on Chiralpak AD‑H can optionally resolve enantiomers arising from the non‑planar ring system if a single atropisomer is desired; the enantiomers exhibit a separation factor α of 1.18 in 15% isopropanol/hexane. The racemate is employed in most discovery libraries. Routine safe‑handling protocols mandate nitrile gloves and safety spectacles; the substance is not classified as acutely toxic (LD₅₀ rat oral > 2000 mg·kg⁻¹, calculated per GHS additivity principles), but its behaviour as a potential skin sensitiser has not been fully characterised. Upon receipt, the material should be equilibrated to ambient temperature under argon before opening to avoid moisture condensation. Split into single‑use aliquots of 100–500 mg in flame‑dried borosilicate vials and return immediately to −20 °C storage. Do not store in solutions containing > 5% DMSO for more than 48 h, as slow ester hydrolysis catalysed by acidic impurities in the solvent has been detected by ¹H‑NMR. When transferring to reactor systems, confirm that all contact surfaces are passivated; stainless‑steel needle tips can promote de‑esterification through Lewis‑acid‑mediated pathways at temperatures above 60 °C, an incompatibility noted during process intensification attempts in microfluidic continuous‑flow setups. No explosive, oxidising, or self‑heating properties have been observed in standard DSC screening up to 300 °C.