4H-Thieno[3,2-B]Pyrrole-5-Carboxylic Acid, 2-Bromo-6-Formyl-, Ethyl Ester

A Late-Stage Diversification Handle for ATP-Competitive Kinase Inhibitor Libraries

Incorporation of 4H-thieno[3,2-b]pyrrole-5-carboxylic acid, 2-bromo-6-formyl-, ethyl ester into a medicinal chemistry program proceeds via sequential chemoselective transformations that exploit the orthogonality of the C2 bromine and the C6 aldehyde. For the construction of type II kinase inhibitors, the scaffold is first subjected to a Suzuki-Miyaura cross-coupling using 1.05 to 1.20 equivalents of an arylboronic acid pinacol ester with 0.5–2.0 mol% Pd(dppf)Cl₂·CH₂Cl₂ under 0.03–0.05 MPa argon pressure in a 3:1 v/v 1,4-dioxane/water mixture at 80–85 °C for 4–6 hours. The resulting biaryl intermediate retains the unprotected aldehyde, which is subsequently engaged in a reductive amination with a pre-formed amine-terminated polyethylene glycol linker or a morpholino-propylamine fragment using NaBH(OAc)₃ at 1.5–2.0 equivalents in dichloroethane containing 5% v/v acetic acid, providing a tertiary amine tether that positions a hinge-binding motif within the ribose pocket of the kinase. The ethyl ester is deliberately maintained during these steps to enhance solubility in the organic phase and is cleaved only at the final stage by treatment with LiOH·H₂O in 3:1:1 THF/MeOH/H₂O at 0 °C to room temperature over 2–3 hours, generating the free carboxylic acid required for salt-bridge interactions with a conserved catalytic lysine residue. Process-scale campaigns executed in glass-lined reactors of 200–500 L capacity have demonstrated that the Suzuki step is limited by mass transfer when the fill volume exceeds 75% due to the viscosity of the dioxane-water azeotrope at reflux; maintaining a tip-speed of ≥1.8 m/s on a pitched-blade impeller is necessary to sustain ≥85% conversion within the specified cycle time. Seeded batch crystallization from 7:3 heptane/ethyl acetate typically affords the penultimate intermediate in 94–97% chromatographic purity (HPLC area% at 254 nm, using a C18 column and 0.1% TFA in water/acetonitrile gradient per USP <621>) without resorting to column chromatography. Genotoxic impurity control is structured around ICH M7 classification: the brominated starting material is evaluated in silico for DNA-reactive alerts via quantitative structure-activity relationship models (DEREK Nexus and Sarah Nexus), and if a structural alert is triggered, the limit is set at the 1.5 µg/day threshold-of-toxicological-concern (TTC) for clinical trial materials or at the staged TTC of 10 µg/day for early development batches, with confirmation by LC-MS/MS (LOQ 0.1 ppm). Residual palladium in the final API intermediate is routinely monitored by ICP-MS against the ICH Q3D guideline, with a parenteral concentration limit of 10 µg/g for elemental impurity Class 1B and an oral permitted daily exposure of 100 µg/day. A finished kinase inhibitor optimized via this route has progressed to IND-enabling GLP toxicology studies, supplied under a Type II drug master file with the FDA, with the entire synthesis conducted in an ISO 8 cleanroom environment and the final active pharmaceutical ingredient crystallized to a polymorphically stable Form A as verified by differential scanning calorimetry (onset melting at 198±2 °C) and powder X‑ray diffraction (characteristic peaks at 2θ values of 7.4°, 12.1°, and 19.8°).

In a typical antiretroviral discovery workflow targeting the thumb pocket of genotype 1b hepatitis C virus NS5B RNA-dependent RNA polymerase, the ethyl ester of 2-bromo-6-formyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid is converted to the corresponding acyl hydrazide without prior protection of the aldehyde. One equivalent of the substrate is treated with anhydrous hydrazine monohydrate (2.5–3.0 equivalents) in ethanol at 0 °C under an argon blanket, and the mixture is allowed to warm to 22±2 °C over 30 minutes followed by a 6-hour hold, delivering the hydrazide in 78–84% isolated yield after precipitation from methyl tert‑butyl ether. The preserved aldehyde at C6 is then condensed with 4-(2-aminoethyl)benzenesulfonamide under microwave irradiation at 120 °C using 150 W maximum power in a sealed vial, producing a Schiff base that is subjected to an in situ reduction with NaBH₄ (3.0 equivalents) in methanol at −10 °C to suppress over-reduction, furnishing the secondary amine linker. The bromine at C2 is subsequently deployed in a copper-free Sonogashira coupling with 1.2 equivalents of TMS-acetylene, catalyzed by Pd(PPh₃)₄ (5 mol%) and CuI (10 mol%) in triethylamine/THF at 55 °C for 12 hours; after TMS deprotection with TBAF, the terminal alkyne participates in a copper-catalyzed azide-alkyne cycloaddition with a fluorinated benzyl azide to install a critical hydrophobic motif that fills the lipophilic region adjacent to Pro197. The constraints imposed by the thienopyrrole ring system — specifically the 148° dihedral angle across the C2–C6 axis and the 0.32 Å out-of-plane deformation of the aldehyde carbonyl — were confirmed by small-molecule X‑ray crystallography and are exploited to pre-organize the inhibitor into the bioactive conformation, reducing the entropic penalty upon binding. Process analytical technology (PAT) probes, including ReactIR equipped with a diamond ATR probe, are configurated to track the disappearance of the aldehyde carbonyl stretch at 1685 cm⁻¹ and the concurrent emergence of the hydrazide carbonyl at 1642 cm⁻¹, enabling real-time endpoint determination and preventing the accumulation of the highly reactive hydrazone intermediate that can undergo retro-aldol decomposition at temperatures above 50 °C. The resulting compound series has been described in the patent literature (WO2015157558) with reported half-maximal effective concentrations (EC₅₀) below 50 nM in an HCV replicon assay employing Huh‑7‑lunet cells as determined by a Renilla luciferase reporter readout, though published data for configurations directly incorporating the 2‑bromo‑6‑formyl‑ethyl‑ester scafold remains limited to a handful of analogues.

Does Low‑Lying LUMO Distribution in Donor–Acceptor Copolymers Depend on Aldehyde Functionalization?

When the ethyl ester of 2-bromo-6-formyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid is incorporated as the electron-rich comonomer in a push–pull polymer backbone for bulk heterojunction organic photovoltaics, the C6 formyl group plays a dual role: it anchors a conjugated cyanoacetate acceptor via Knœvenagel condensation while simultaneously lowering the highest occupied molecular orbital energy through its inductive effect. In a representative polycondensation, the dibrominated monomer is synthesized by condensing the aldehyde with ethyl cyanoacetate in piperidine-catalyzed ethanol under vigorous reflux for 1 hour, yielding the vinyl dicyanoester intermediate with a characteristic λmax at 412 nm (in THF solution). The vinyl intermediate is then subjected to Stille cross-coupling with 1.0 equivalent of 5,5′‑bis(trimethylstannyl)‑2,2′‑bithiophene using 2 mol% Pd₂(dba)₃·CHCl₃ and 8 mol% P(o‑tol)₃ in anhydrous chlorobenzene at 115 °C for 48 hours under strict inert atmosphere (O₂ <5 ppm, H₂O <10 ppm in the glovebox). The crude polymer is end-capped with 0.05 equivalent of 2‑(tributylstannyl)thiophene followed by 0.1 equivalent of 2‑bromothiophene, and purified by sequential Soxhlet extraction with methanol, acetone, hexane, and finally dichloromethane. The dichloromethane fraction, possessing a number-average molecular weight (Mn) between 18 and 35 kDa and a dispersity (Đ) of 1.8–2.4 as measured by high-temperature gel permeation chromatography at 150 °C in 1,2,4‑trichlorobenzene against polystyrene standards (ISO 16014‑3:2019), is spin-cast from a 15 mg/mL solution in o‑xylene containing 3% v/v 1‑chloronaphthalene as a high‑boiling additive. The active layer blend with PC₇₁BM at a 1:1.5 weight ratio achieves an optimal film thickness of 95 ± 5 nm, measured by a stylus profilometer calibrated to ISO 5436‑1, and is annealed on a hotplate at 130 °C for 10 minutes in a nitrogen-filled glovebox. Current density‑voltage characteristics acquired under simulated AM 1.5G illumination at 100 mW/cm² (certified against an NREL-calibrated reference cell in accordance with IEC 60904‑2:2023) yield a fill factor exceeding 0.65 when the hole transport layer of poly(3,4‑ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP AI 4083) is annealed at 150 °C for 15 minutes and the electron transport layer consists of zinc oxide nanoparticles deposited from a 0.1 M sol‑gel precursor. The external quantum efficiency spectrum obtained with a monochromator-based setup (lock‑in detection at 273 Hz under white-light bias) reveals a maximum of 72% at 520 nm, corroborating the photocurrent generation in the spectral region where the thienopyrrole donor unit predominantly absorbs. Operational stability of unencapsulated devices stored under continuous 1‑sun illumination at 55±2 °C and 30±5% relative humidity is monitored at the maximum power point, retaining 80% of the initial performance after 350 hours; this burn‑in loss is attributed to photo‑oxidation at the aldehyde‑derived vinyl bridge, which is an inherent limitation of this specific linkage and requires further passivation by an additional interfacial layer to exceed the ISOS‑L‑1 benchmark (ISOS 2022 protocol).

High-purity sublimed charges of 2‑bromo‑6‑formyl‑4H‑thieno[3,2‑b]pyrrole‑5‑carboxylic acid ethyl ester are evaluated as a building block in vacuum‑deposited p‑channel organic field‑effect transistors. Prior to deposition, the material is subjected to triple-zone gradient sublimation under a dynamic vacuum of 1×10⁻⁶ mbar with a source temperature of 135±3 °C, a gradient plateau at 105 °C, and a collection zone maintained at 65 °C, achieving a final purity of ≥99.95% as quantified by reverse-phase HPLC at 215 nm (area normalization). The purified ester is co-evaporated with a base-induced cyclized acceptor‑fused quinoidal small molecule in a VTE‑120 vacuum thermal evaporator (base pressure 5×10⁻⁷ mbar) at a rate ratio of 1.0:0.8 Å/s total, monitored by quartz crystal microbalances whose tooling factors are calibrated against ellipsometry (Woollam M‑2000, fitting over the 370–1000 nm spectral range with a mean squared error ≤3.0). A bottom-gate top-contact device architecture is fabricated on highly n‑doped silicon wafers (0.001–0.005 Ω·cm) with a thermally grown 200 nm SiO₂ gate dielectric having a capacitance of 17.3 nF/cm²; the dielectric surface is passivated with hexamethyldisilazane (HMDS) from vapour phase at 120 °C for 1 hour, yielding a water contact angle of ≥85°. Gold source‑drain electrodes (40 nm thick) are evaporated through a shadow mask onto the organic bilayer at a channel width‑to‑length ratio of 1000:15 µm. Saturation-regime transfer curves recorded with a Keysight B1500A semiconductor parameter analyzer at 22 °C under <1 ppm oxygen and moisture in a probe station (Lake Shore CRX‑VF) exhibit a hole mobility of 0.28 cm²/V·s in the saturation regime, a threshold voltage of −3.5 V, and a subthreshold swing of 0.9 V/dec, with the mobility extracted from the slope of |ID|½ versus VGS using the standard metal‑oxide‑semiconductor field‑effect transistor equation per IEEE 1620‑2008. Bias‑stress stability tests performed with a constant gate voltage of −40 V for a duration of 10⁴ seconds reveal a threshold voltage shift of +1.8 V, which is linked to the moderate electron affinity of the aldehyde group acting as a shallow trap site; this value compares unfavourably with the ≤0.5 V shift exhibited by otherwise identical devices where the aldehyde is replaced by a nitrile group, defining a clear structure–property limitation for this present ester in long‑lifetime flexible display backplanes that must comply with the ≤2.0 V shift threshold of the IPC‑ 4921A specification for printed electronics.

When the Ethyl Ester Serves as a Latent Carboxylic Acid for Bioconjugation in Ratiometric Fluorescent Probes

In the construction of ratiometric fluorescent sensors for cysteine and homocysteine, the bromide at C2 and the aldehyde at C6 are engaged in a stepwise derivatization to tether a Förster resonance energy transfer donor‑acceptor pair onto the rigid thienopyrrole scaffold, while the ethyl ester at C5 is intentionally left intact to facilitate cellular membrane permeation until it is hydrolyzed by intracellular esterases. The formyl group is first condensed with 1.2 equivalents of 2‑aminothiophenol in ethanol under argon at 40 °C for 3 hours, forming a benzothiazole unit with a characteristic emission at 485 nm upon excitation at 370 nm. The C2 bromide is subsequently substituted via a palladium-mediated borylation‑oxidation sequence: treatment with bis(pinacolato)diboron (1.3 equiv), Pd₂(dba)₃ (2 mol%), and XPhos (4 mol%) in 1,4‑dioxane at 90 °C for 6 hours, followed by oxidative workup with sodium perborate tetrahydrate (5 equiv) in THF/water at 0 °C, delivers the phenolic intermediate. The phenol is then alkylated with 1.1 equivalents of propargyl bromide (K₂CO₃ in DMF at 50 °C), and the terminal alkyne undergoes copper(I)‑catalyzed azide‑alkyne cycloaddition — using CuSO₄·5H₂O (5 mol%) and sodium ascorbate (10 mol%) in 1:1 t‑BuOH/H₂O at 25 °C for 12 hours — with an azide‑functionalized fluorescein derivative (fluorescein-5-azide, ex.530/em.550), affording the complete FRET probe after reverse‑phase C18 flash chromatography. The intracellular behaviour of the probe is validated in HeLa cell culture (EMEM medium with 10% fetal bovine serum, 5% CO₂ at 37 °C) by incubating cells with 5 µM of the probe for 30 minutes; subsequent treatment with 200 µM hydrogen peroxide to induce oxidative stress triggers a rapid increase in the fluorescence intensity ratio I₅₅₀/I₄₈₅ from 0.35±0.02 to 1.15±0.05 as measured on a confocal microscope operating with a 40×/1.3 NA oil immersion objective and spectral detection at 465–505 nm and 545–595 nm under 405 nm excitation. Cytotoxicity assessed via the MTT assay (ISO 10993‑5:2009) shows no significant reduction in cell viability at probe concentrations up to 25 µM over a 24‑hour exposure period; however, at 50 µM a 12% decrease in viability is noted, attributed to non‑specific thiol depletion by the benzothiazole moiety, establishing the safe working window. The use of this specific ethyl ester presents a practical limitation: in serum‑containing media, the ester undergoes partial hydrolysis (~15% after 4 hours as determined by LC‑MS) mediated by butyrylcholinesterase, which generates a dianionic carboxylate that reduces membrane crossing and leads to a 20–30% underestimation of the intracellular FRET signal when quantification is attempted without an internal standard. Published data for this exact structural probe remain scarce, but the ratiometric principle aligns with analogous coumarin‑maleimide conjugates documented for mitochondrial thiol sensing in live-cell imaging.

Structural Formulation Adherence in Speciality Thermally Activated Delayed Fluorescence Hosts

For the synthesis of a thermally activated delayed fluorescence (TADF) host material designed to host a multiple‑resonance boron‑based emitter in narrowband blue organic light‑emitting diodes, 4H‑thieno[3,2‑b]pyrrole‑5‑carboxylic acid, 2‑bromo‑6‑formyl‑, ethyl ester is utilized as a rigid donor‑core precursor in which the C2 bromine is displaced with diphenylamine via a Buchwald–Hartwig amination. A mixture of the ester (1.0 mmol), diphenylamine (1.15 mmol), Pd(OAc)₂ (2 mol%), and Xantphos (3 mol%) in anhydrous toluene is treated with sodium tert‑butoxide (1.4 mmol) and heated at 100 °C for 18 hours under argon, typically affording the 2‑diphenylaminothienopyrrole intermediate in 87% yield after silica gel column chromatography. The ethyl ester is subsequently reduced with diisobutylaluminium hydride (DIBAL‑H, 2.5 equiv) in dichloromethane at −78 °C, warming to 0 °C over 3 hours, to generate the corresponding primary alcohol, followed by oxidation with Dess–Martin periodinane (1.1 equiv) to furnish the C5 aldehyde. In the final convergent step, the dialdehyde — now bearing two formyl groups — undergoes a double Horner–Wadsworth–Emmons reaction with tetraethyl 1,4‑phenylenebis(methylene))diphosphonate under sodium hydride in dry THF at 0 °C to room temperature, constructing a quinoidal distyrylbenzene acceptor backbone while retaining the donor‑functionalized thienopyrrole termini. The fully conjugated host is purified by repeated recrystallization from chlorobenzene and gradient sublimation before being co‑deposited with the emitter ν‑DABNA at a host:dopant ratio of 95:5 wt% in a vacuum chamber at 5×10⁻⁷ mbar. Electroluminescence from the device — employing an indium‑tin‑oxide/HAT‑CN ( 10 nm)/TAPC ( 40 nm)/host:emitter ( 30 nm)/TSPO1 ( 5 nm)/B3PYMPM ( 50 nm)/LiF ( 1 nm)/Al ( 100 nm) stack — shows a peak emission at 462 nm with a full width at half maximum of 22 nm, achieving an external quantum efficiency of 18.7% at 100 cd/m² when measured by an integrated sphere system calibrated with a standard lamp traceable to NIST (IEC 62341‑6‑1:2017). The operational lifetime LT90 at an initial luminance of 1000 cd/m² is registered at 148 hours, and the lifetime-limiting factor is the thermally induced cleavage of the C2–diphenylamine bond, which is catalysed by trace amounts of water present in the organic stack at the 0.1–0.5 ppm level; therefore an additional in-vacuo pre‑bake of the source material at 80 °C for 6 hours prior to deposition is mandated to drive off residual moisture.