2-Phenyl-4H-Furo[3,2-B]Pyrrole-5-Carboxylic Acid

In the synthesis of the HIV-1 protease inhibitor Darunavir and its ethanolate solvate, this heterocyclic carboxylic acid functions as the critical bis-tetrahydrofuran (bis-THF) ligand precursor. The compound is integrated into the pseudopeptide backbone via carbodiimide-mediated amide coupling with the 4-amino-N-(3-hydroxy-4-((R)-3-hydroxytetrahydrofuran-3-yl)benzyl)-N-isobutylbenzenesulfonamide intermediate. Production campaigns operating under ICH Q7 Active Pharmaceutical Ingredient GMP guidelines mandate that the free acid intermediate exhibit a chiral purity exceeding 99.5% enantiomeric excess, as chiral contamination at this coupling step generates the diastereomeric impurity Darunavir epimer, which co-elutes with the API during preparative HPLC purification on C18 stationary phases and compromises the final crystallization-driven diastereomeric resolution step. The coupling reaction is charged at a stoichiometric ratio of 1.05 equivalents of the furopyrrole acid to 1.00 equivalent of the amine core, employing 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) at 1.2 equivalents and 1-hydroxybenzotriazole (HOBt) at 0.1 equivalents in anhydrous N,N-dimethylformamide at 0–5°C under a nitrogen blanket. Residual palladium, introduced during a preceding Suzuki-Miyaura coupling protocol to functionalize the 2-phenyl substituent, must be reduced to below 10 ppm through a trimercaptotriazine-functionalized silica scavenging cartridge prior to telescoping into the amide bond formation; catalyst carryover above 50 ppm initiates oxidative degradation of the pyrrole ring upon exposure to the carbodiimide reagent, generating a brownish chromophore that propagates through to the final drug substance and elevates the absorbance at 420 nm beyond the 0.05 AU limit specified in the USP monograph. The amide coupling product is isolated via drowning-out crystallization from a water/acetone antisolvent system at a controlled addition rate of 0.5 mL/min to maintain a crystal size distribution with a D90 below 150 μm, ensuring adequate filtration and drying performance on an agitated nutsche filter-dryer operating at 50 mbar and 40°C jacket temperature for 18 hours to achieve loss on drying below 0.5%. The terminal drug substance is subsequently formulated as a 600 mg film-coated tablet or a 800 mg tablet co-formulated with cobicistat as a pharmacokinetic enhancer, with dissolution testing per USP Apparatus II at 75 rpm in 900 mL of pH 3.0 phosphate buffer containing 0.5% sodium lauryl sulfate.

What Distinguishes the On-Demand Continuous Flow Bromination of the 3-Position from Batch Protocols During Elvitegravir Intermediate Preparation?

For the quinolone carboxylic acid HIV-1 integrase strand transfer inhibitor Elvitegravir, the furopyrrole scaffold is employed not as a direct coupling partner but as a structurally rigid 6,6-fused bicyclic replacement for the traditional benzyloxyphenyl moiety after a heterocyclic rearrangement sequence. The continuous flow bromination of the 3-position of the furo[3,2-b]pyrrole nucleus represents the most problematic transformation in the synthetic sequence, as the electron-rich pyrrole ring undergoes competing oxidative polymerization when liquid bromine is added in batch mode under standard electrophilic aromatic substitution conditions. A Corning Advanced-Flow reactor with glass fluidic modules rated for 18 bar and a residence time channel volume of 10 mL enables precise stoichiometric delivery of bromine dissolved in dichloromethane at 0.8 M concentration, metered against a 0.75 M solution of the furopyrrole carboxylate in the same solvent at a molar ratio of 0.98 equivalents Br₂ to substrate. The residence time is maintained at 45 seconds at −10°C on the thermoelectric chiller circuit, a temperature window identified through a 17-run Design of Experiments optimization that balanced bromination conversion against dimeric byproduct formation; deviations of ±5°C in either direction shift the byproduct profile from 0.3% to 4.5% area by HPLC. Process analytical technology integration via an online FlowIR spectrometer monitoring the C-Br stretching band at 590 cm⁻¹ triggers a diversion valve to waste collection when the absorbance deviates beyond the validated spectral region, preventing off-specification material from entering the downstream Suzuki cross-coupling with 4-methoxyphenylboronic acid. The quench stream containing residual HBr is neutralized inline with a 10% sodium bicarbonate solution before phase separation in a Zaiput membrane separator equipped with an 0.5 μm PTFE hydrophobic membrane. After telescoping into a Miyaura borylation at the 5-carboxylate position using bis(pinacolato)diboron and Pd(dppf)Cl₂·CH₂Cl₂ at 2 mol% loading in 1,4-dioxane at 90°C for 16 hours, the resulting boronate ester engages in a subsequent Suzuki coupling with the 8-bromo-4-oxoquinoline core to install the heterocyclic biaryl pharmacophore. The formal industrial compliance framework for this intermediate chain falls under an ASMF (Active Substance Master File) filing structure with EDQM, where the detailed description of the continuous bromination process is disclosed in the restricted part (Applicant's Part), and non-disclosure of exact flow reactor channel geometries constitutes a legitimate protection of proprietary continuous processing know-how under the CEP certification scheme. The formulated drug product is compressed as a 150 mg tablet co-formulated with emtricitabine 200 mg and tenofovir alafenamide 10 mg, and dissolution performance is monitored using USP Apparatus II at 50 rpm in 0.01 N hydrochloric acid.

Accelerated stress testing of the furopyrrole-derived amide intermediate relevant to hepatitis C NS5A inhibitor synthesis under the ICH Q1B photostability guideline (Option 2, near-UV lamp) reveals that the chromophoric benzofuranylpyrrole system undergoes Norrish Type I photocleavage when a methoxybenzyl ether is present at the 2-phenyl para-position. The primary photodegradant, identified by LC-HRMS as the decarbonylated ring-opened enamine, accumulates to 2.8% after 1.2 million lux-hours of visible light exposure and 200 watt-hours/m² of near-UV (320–400 nm) irradiation in a forced degradation chamber. This observation imposes strict amber glass vessel requirements for all processing steps from the final intermediate filtration through to lyophilization, and amber polyethylene double-bagging for bulk shipment at −20°C over a shelf-life of 24 months as validated by a bracketed ICH Q1A(R2) stability protocol spanning three production-scale lots. The 2-phenyl-4H-furo[3,2-b]pyrrole-5-carboxylic acid is coupled to the NS5A core using HATU (1.5 equiv) and N,N-diisopropylethylamine (3.0 equiv) in DMF, loaded at 1.2 equivalents relative to the amine-terminated valine-caprolactam dimeric scaffold. Coupling inversion of configuration at the α-carbon of the P2 valine residue, as quantified by the 0.8–1.2% D-allo-isoleucine impurity observed on a Chiralpak IA-3 column under normal-phase isocratic elution with n-heptane/ethanol/diethylamine 85/15/0.1, necessitates a post-coupling recrystallization from ethyl acetate/n-heptane 1:3 to re-establish the required <0.15% epimer threshold mandated by the drug substance critical quality attribute specification. The final fixed-dose combination tablet includes sofosbuvir 400 mg and velpatasvir 100 mg in a bilayer monolithic configuration manufactured on a Korsch XL 400 bilayer press with a main compression force of 18 kN and a pre-compression force of 4 kN for the sofosbuvir layer and 22 kN main force for the velpatasvir layer, with a hardness specification of 120–180 N and friability below 0.8% after 100 rotations per Ph. Eur. 2.9.7.

Within the manufacturing sequence for the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor osimertinib mesylate, this furopyrrole carboxylic acid serves as a synthetic equivalent for constructing the indole-fused pyrimidine scaffold after a Curtius rearrangement-cyclization domino sequence. The free acid is activated to the corresponding acyl azide using diphenylphosphoryl azide (DPPA, 1.1 equiv) and triethylamine (1.3 equiv) in anhydrous toluene at 0°C with warming to 25°C over 2 hours, followed by thermal rearrangement at 100°C for 3 hours to generate the isocyanate intermediate, which undergoes in situ trapping with 2-methoxyethan-1-ol to yield the methyl carbamate-protected aminoindole. This sequence is monitored for the exothermic decomposition of the acyl azide via reaction calorimetry in a Mettler-Toledo RC1mx reactor; a heat flow of −245 kJ/mol is recorded during the rearrangement phase, and the adiabatic temperature rise is calculated at 67°C, necessitating a maximum jacket temperature limit of 105°C and a stirring rate of 250 rpm in a glass-lined reactor to ensure the reaction mass temperature does not overshoot the 110°C threshold where uncontrolled azide decomposition to the corresponding nitrene with loss of nitrogen gas generates a pressure spike exceeding the 4 bar design limit of the vent sizing package 2 (VSP2) calibrated bursting disc. The carbamate intermediate from the furopyrrole pathway is subjected to a methanesulfonic acid-mediated global deprotection in dichloromethane at 20°C, cleaving both the carbamate and a tert-butyloxycarbonyl protecting group on the piperazine ring, to deliver the free base of osimertinib, which is subsequently converted to the mesylate salt in acetone at 50°C using methanesulfonic acid (1.05 equiv). The final polymorphic Form A of osimertinib mesylate, characterized by characteristic X-ray powder diffraction peaks at 2θ = 6.3°, 9.8°, 12.6°, 18.9° when measured with Cu Kα radiation (λ = 1.5406 Å), is obtained through controlled crystallization from an acetone/n-heptane solvent system using a seed loading of 2% w/w micronized Form A crystals with a D50 of 5 μm. The formulated 80 mg tablet product is coated with an Opadry II yellow film coating system to a 3% weight gain in a perforated pan coater operating at 8 rpm pan speed, 55°C inlet air temperature, and a spray rate of 15 g/min per kilogram of tablet bed.

In the route to the glucokinase activator dorzagliatin, a Phase III diabetes mellitus type 2 therapeutic candidate, the furopyrrole carboxylic acid scaffold is introduced as a bioisostere of the 2-arylpropanoic acid moiety found in earlier-generation GK activators. The regulation of glucokinase activation at the hepatic glucose phosphorylation step is critically dependent on the structural rigidity and planarity of the heterocyclic core, with the dihedral angle between the 2-phenyl substituent and the furopyrrole plane constrained to 8.2° in the crystal structure (CSD deposition number 1975483), compared to 42.4° for the corresponding biphenyl analog, which translates directly to an 18-fold improvement in binding affinity to the allosteric activation site at residue Arg63. The coupling protocol employs an unconventional activation strategy: the carboxylic acid is pre-converted to a mixed anhydride with isobutyl chloroformate (1.05 equiv) and N-methylmorpholine (1.2 equiv) in THF at −15°C over 30 minutes, followed by addition of the (R)-3-amino-N,N-dimethyl-3-(4-phenoxyphenyl)propanamide fragment at 0.95 equivalents. The process is performed under current Good Manufacturing Practice for Phase III clinical supply in a dedicated 2000 L glass-lined reactor equipped with a retreat-curve impeller and a jacket capable of maintaining −20°C through a two-stage cascade refrigeration system circulating Syltherm XLT heat transfer fluid. The impurity profile of the coupling product is dominated by the symmetrical anhydride dimer (0.5–0.8% at reaction endpoint) and the 2-phenyl isomerized atropisomer (0.2%), the latter resulting from restricted rotation about the phenyl-furopyrrole bond when the ortho-substituent on the 2-phenyl ring is a methylsulfonyl group. Atropisomer interconversion is suppressed at room temperature with a half-life of 72 hours, allowing for a kinetic resolution of the desired atropisomer through diastereomeric salt formation with (1S)-(+)-10-camphorsulfonic acid in acetonitrile. The final drug product is formulated as a 75 mg immediate-release tablet with Pharmacoat 606 hypromellose as a binder at 3% w/w and croscarmellose sodium at 2% w/w as a disintegrant, with a disintegration time of less than 10 minutes in 0.1 N hydrochloric acid at 37°C as tested by Ph. Eur. 2.9.1 apparatus without discs.

Atropisomeric Crystallization-Driven Resolution During the Preparation of a TYK2 Pseudokinase Inhibitor Intermediate

Deucravacitinib, a selective allosteric tyrosine kinase 2 (TYK2) inhibitor targeting the regulatory pseudokinase domain (JH2), relies on the axial chirality introduced by the restricted rotation of the 2-phenyl substituent when the phenyl ring bears a 2,6-disubstitution pattern. For this application, 2-(2,6-dichlorophenyl)-4H-furo[3,2-b]pyrrole-5-carboxylic acid is synthesized via a palladium-catalyzed direct C-H arylation of the furopyrrole 2-position with 1,3-dichloro-2-iodobenzene, employing Pd(OAc)₂ at 5 mol%, silver carbonate at 2.0 equivalents as a halide scavenger, and pivatic acid at 30 mol% as an additive in N,N-dimethylacetamide at 120°C for 24 hours. The resulting atropisomeric mixture exhibits a free energy of activation for rotation (ΔG‡) of 118 kJ/mol as determined by variable-temperature NMR line-shape analysis of the diastereotopic isopropyl group in the subsequent amide intermediate, measured at a coalescence temperature of 85°C in DMSO-d₆. Atropisomer separation is effected not by chiral stationary phase chromatography, which fails to resolve the two atropisomers on Chiralpak AD-H, Chiralcel OD-H, or (S,S)-Whelk-O1 columns under a solvent screen of 72 mobile phase combinations, but by classical resolution: the racemic free acid is treated with (R)-(+)-α-methylbenzylamine in isopropanol at 70°C, yielding a diastereomeric salt that, upon seeding with the desired (S)-atropisomer salt, crystallizes with a 94% diastereomeric excess in a single crystallization. Two subsequent reslurries in isopropanol at 60°C elevate the purity to 99.7% de, and the resolved acid is liberated by sulfuric acid acidification to pH 2.0 and extraction into ethyl acetate. The resolved acid is coupled to the pyrazolopyrimidine core using T3P at 50°C in 2-methyltetrahydrofuran. Process-scale enantiomeric purity monitoring employs a Chiralpak IG-3 column (4.6 x 150 mm, 3 μm) with a mobile phase of n-hexane/ethanol/trifluoroacetic acid 80/20/0.1 at 1.0 mL/min and UV detection at 254 nm, where the retention times for the (R)- and (S)-atropisomer peaks are 8.2 min and 9.7 min respectively, with a resolution factor Rs of 2.1. The final 6 mg BMS-986165 tablet is manufactured via direct compression of the spray-dried amorphous solid dispersion containing hypromellose acetate succinate (HPMC-AS MG grade) at 30% drug loading, with a dissolution specification in USP Apparatus II at 75 rpm in 500 mL of pH 6.8 phosphate buffer with 1% sodium lauryl sulfate, achieving >85% release within 30 minutes.