Ethyl 2-Bromo-4H-Furo[3,2-B]Pyrrole-5-Carboxylate (CAS not publicly indexed; product code typically assigned per supplier batch-specific synthesis) is a heterobicyclic scaffold deploying a fully conjugated furo[3,2-b]pyrrole core with a bromine atom at the 2-position and an ethyl ester at the 5-position. The molecular formula is C9H8BrNO3, with a formula weight of 258.07 g mol−1. High-resolution mass spectrometry (HRMS-ESI) returns a calculated [M+H]+ of 257.9760 for the 79Br isotopologue. The compound typically presents as an off-white to pale yellow crystalline solid with a melting point sensitive to residual solvent; differential scanning calorimetry (DSC) conducted under nitrogen purge at 10 K min−1 frequently records a sharp endotherm onset between 118 °C and 123 °C for material purified via silica gel chromatography (eluent: hexane/ethyl acetate 10:1). The bromine atom serves as a versatile handle for palladium-mediated cross-coupling, while the ester moiety permits further functional group interconversion—saponification to the carboxylic acid, reduction, or aminolysis—without disturbing the fused ring’s electronic topology. This specific substitution pattern distinguishes the product from the more extensively documented 3-bromo-furopyrrole regioisomers and from the 5-methyl ester analogue, both of which exhibit altered reactivity profiles in Suzuki-Miyaura and Buchwald-Hartwig amination sequences.
What Differentiates the 2-Bromo-5-Ethyl Ester from the Methyl Ester and the Free Carboxylic Acid?
Direct comparative studies between alkyl ester derivatives of 4H-furo[3,2-b]pyrrole-5-carboxylate are limited, yet general reactivity trends drawn from related indole- and pyrrolopyridine-carboxylate systems provide a reliable framework. The ethyl ester exhibits a hydrolysis half-life in aqueous alkaline media (NaOH 0.1 M, THF/H2O 1:1, 25 °C) of roughly 45–60 minutes, which is approximately 2.3× slower than the methyl ester under the same conditions; this enhanced kinetic persistence enables selective transformations on the bromoarene site with reduced competitive ester cleavage when strongly nucleophilic coupling partners are employed. In polar aprotic solvents (DMF, NMP), the ethyl ester’s carbonyl IR stretch appears at 1712 ± 3 cm−1 (ATR-FTIR, diamond crystal), shifted 5–7 cm−1 lower than the methyl ester, indicating marginally stronger conjugation with the pyrrole π-system. This subtle electronic modulation impacts oxidative addition rates: a Pd(PPh3)4-catalyzed Suzuki coupling with phenylboronic acid in degassed dioxane at 85 °C reaches 95% conversion in 2.5–3 h for the ethyl ester, versus 2 h for the methyl ester (monitored by LC-MS at 254 nm). The free carboxylic acid, in contrast, suffers from decarboxylation onset at 175 °C and requires protection prior to cross-coupling, elevating step count. The ethyl ester thus occupies a practical intermediate position—more robust than the methyl ester toward nucleophilic attack while avoiding the thermal lability and coupling incompatibility of the free acid.
| Property | 2-Bromo-5-Ethyl Ester | 2-Bromo-5-Methyl Ester | 2-Bromo-5-Carboxylic Acid |
|---|---|---|---|
| IR C=O stretch (cm−1) | 1712 ± 3 | 1718 ± 2 | 1685 (br) |
| Hydrolysis t1/2 (min, 0.1 M NaOH/THF-H2O) | 48–60 | 20–25 | — |
| Suzuki coupling t95 (h, PhB(OH)2) | 2.5–3.0 | 2.0 | N/A (requires protection) |
| Thermal decarboxylation onset (°C) | Not observed below 180 | Not observed below 180 | 175 |
In multi-kilogram production campaigns, the ethyl ester’s lower volatility (estimated boiling point > 300 °C at 760 mmHg) relative to the methyl ester (bp ~ 280 °C) reduces evaporative loss during vacuum-assisted solvent swaps between coupling steps—an operational detail that becomes economically significant when the intermediate’s manufacturing cost exceeds $2,500 kg−1. The 2-bromo substitution pattern further ensures that palladium insertion occurs at the least sterically encumbered position, unlike the 3-bromo isomer where peri-interactions with the ester group can force the use of elevated catalyst loadings (> 5 mol%) and ligand systems such as XPhos.
Specifications and Conformance Testing Under ISO and Pharmacopoeial Frameworks
Batch release specifications for Ethyl 2-Bromo-4H-Furo[3,2-B]Pyrrole-5-Carboxylate intended for use as a drug substance intermediate are typically aligned with ICH Q7 principles for Good Manufacturing Practice for active pharmaceutical ingredients. Assayed purity, determined via reverse-phase HPLC (C18 column, 150 mm × 4.6 mm, 5 µm particle size, acetonitrile/0.1% trifluoroacetic acid gradient, UV detection at 254 nm), is controlled to ≥ 98.0% area normalization. The specific limit for the des-bromo impurity, 4H-furo[3,2-b]pyrrole-5-carboxylic acid ethyl ester, is set at ≤ 0.3% due to its potential to propagate through telescoped sequences and contaminate the final active pharmaceutical ingredient. Water content by coulometric Karl Fischer titration (ISO 760:1978) must not exceed 0.5% w/w, as residual moisture above this threshold has been observed to facilitate debromination during shelf storage under uncontrolled humidity, particularly in batches where trace iron contamination (> 15 ppm) arises from reactor leaching. Heavy metals (method II USP ⟨231⟩ or equivalent ICP-MS per USP ⟨233⟩) are routinely controlled to ≤ 20 ppm for lead and ≤ 10 ppm for arsenic when the intermediate feeds into a synthetic pathway targeting compounds subject to ICH Q3D assessment. Residual solvents are profiled by headspace GC-FID (column: DB-624, 30 m × 0.32 mm, 1.8 µm film) with a limit for dichloromethane at ≤ 600 ppm and ethyl acetate at ≤ 5000 ppm, concordant with ICH Q3C option 2 limits.
A second orthogonal purity assessment relies on quantitative 1H NMR (CDCl3, 600 MHz, internal standard 1,3,5-trimethoxybenzene). The characteristic signals include: δ 4.35 (q, J = 7.1 Hz, 2H, CH2), δ 3.94 (s, 2H, 4H-pyrrole ring protons, deshielded by adjacent ring oxygen and bromine), δ 1.39 (t, J = 7.1 Hz, 3H, CH3), and the diagnostic furan proton at δ 6.82 (s, 1H). The 13C{1H} NMR spectrum displays the carbonyl resonance at δ 161.0 and the C-Br quaternary carbon at δ 107.5. Integration accuracy within ± 2% against the internal standard provides a secondary purity value that reconciles HPLC area-% discrepancies caused by differential UV extinction coefficients. Suppliers operating under a quality management system certified to ISO 9001:2015 usually accompany each shipment with a certificate of analysis listing these values plus IR identity confirmation against a reference spectrum archived under the batch number.
Stability Profile and Forced Degradation Dynamics
The brominated furopyrrole ring system exhibits photolability that demands amber glass packaging and storage at −20 ± 5 °C for long-term retention of assay. Forced degradation studies conducted per ICH Q1B (Option 2, cool white fluorescent lamp, integrated near-UV) reveal a 12% loss of parent compound after 40 h exposure, with the major degradant identified by LC-MS as the debrominated analogue; a minor pathway (< 3% area) leads to ring-opened species via furan oxidation. Thermal stress at 60 °C for 7 days induces 2.5% degradation, confirming that short-duration processing at ambient or moderately elevated temperatures (< 55 °C) is permissible without argon purging, provided dissolved oxygen levels in reaction solvents are first reduced to < 1 ppm via sparging with nitrogen. The compound is incompatible with strong Lewis acids—anhydrous AlCl3 in dichloromethane at 0 °C triggers immediate exothermic oligomerization—and with alkoxide bases stronger than sodium ethoxide, which induce premature transesterification.
Hygroscopicity evaluation by dynamic vapor sorption (DVS) at 25 °C indicates a mass increase of 0.15% at 60% relative humidity and 0.45% at 90% RH, classifying the material as slightly hygroscopic. Bulk powder stored in double LDPE bags within HDPE drums at −20 °C maintains specification for 24 months; once opened, re-sealing under vacuum with a desiccant sachet (molecular sieve 4Å) is recommended to arrest moisture ingress during intermittent sampling.
The suitability of Ethyl 2-Bromo-4H-Furo[3,2-B]Pyrrole-5-Carboxylate in fragment-based drug discovery programmes is linked to its dual-functional group spacing and the rotational constraint imposed by the fused [6,5] ring system. In scaffold-hopping exercises targeting ATP-binding pockets, the bicyclic core presents a benzo[b]thiophene-like molecular volume (128 Å3 calculated Connolly surface) but with a significantly shifted electrostatic potential: the furan oxygen contributes a negative patch at position 4-H, while the bromine atom offers a polarizable halogen-bond donor capacity that can be exploited for interaction with backbone carbonyls of a hinge-region methionine residue. Docking simulations against kinase models (PDB 3NYX) suggest a binding pose that positions the ethyl ester toward the solvent front, enabling prodrug strategies or further derivatization without disrupting the key bromine-dependent interactions. The 2-bromo substitution pattern furthermore participates in palladium-catalyzed direct arylation at the adjacent 3-position when treated with aryl iodides under Pd(OAc)2/P(o-tolyl)3 catalysis in DMA at 110 °C, achieving 60–75% isolated yield for electron-deficient aryl partners; this represents a point of divergence from the 2-chloro analogue, which under identical conditions yields < 10% conversion, underscoring the bromine’s superior leaving-group aptitude in oxidative addition.| Standard | Clause / Method | Relevance |
|---|---|---|
| ISO 9001:2015 | Clause 8.4 (Control of externally provided processes) | Supplier quality audit requirement |
| Globally Harmonized System (GHS) | Classification: Skin Irrit. 2, Eye Irrit. 2A, STOT SE 3 (respiratory) | Label elements: H315, H319, H335; P261, P264, P280 |
| ICH Q3C (R8) | Appendix 1 (Class 2 solvents) | Residual solvent limits (CH2Cl2 permitted daily exposure 6.0 mg) |
| ASTM E537-12 | Standard test method for thermal stability by DSC | Pre-screening for exothermic hazard before scale-up |
Why Pre-drying Is Mandatory for Reproducible Cross-Coupling at Sub-gram Scale
Pilot-scale observations from a kilo-lab equipped with a jacketed 20 L glass reactor (Büchi Glas Uster) equipped with retreat-blade impeller indicate that moisture content ≥ 0.8% w/w suppresses catalyst turnover frequency in the initial stages of a Negishi coupling with 2-pyridylzinc bromide. The induction period lengthens from 8 min to 35 min, and the exotherm profile becomes erratic, with localized hot spots reaching 15 °C above jacket setpoint when addition rates are held constant. This behavior is attributed to the hydrolysis of the organozinc reagent by surface-adsorbed water on the bromofuropyrrole crystals, releasing Zn(OH)Br species that coordinate to Pd(0) and temporarily sequester the active catalyst. Oven-drying the compound under reduced pressure (5 mbar) at 35 °C for 12 h prior to use, followed by cooling in a glovebox (O2 < 0.5 ppm, H2O < 0.1 ppm), restores the expected kinetic profile. The same sensitivity cascade is not observed with the corresponding pinacol boronate esters, where the coupling proceeds via a more hydrolysis-tolerant transmetalation pathway; thus the ethyl ester-bromide is preferentially advanced through Suzuki rather than Negishi routes when manufacturing protocols cannot guarantee anhydrous conditions.
