5-(2-Fluorophenyl)-1-(3-Pyridinylsulfonyl)-1H-Pyrrole-3-Carb

When sourcing advanced pharmaceutical intermediates for the potassium-competitive acid blocker (P-CAB) therapeutic class, the 5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrole-3-carboxylic acid scaffold frequently serves as the practical bifurcation point between the early-stage pyrrole build and the late-stage C-3 aminomethyl side-chain installation. Commercial-scale synthesis of vonoprazan fumarate originally described in Takeda’s manufacturing patent family relies on the corresponding 3-carbaldehyde for reductive amination with methylamine; however, process chemistry groups operating under tight CapEx constraints have adapted the carboxylic acid congener as a storage-stable solid with indefinite ambient shelf life that avoids the aldehyde’s tendency toward air oxidation and dimerisation. A representative kilo-lab protocol charges 1.0 kg (2.96 mol) of the dry acid into a nitrogen-inerted, jacketed 20-L glass-lined reactor equipped with a cascade PID temperature controller. Anhydrous tetrahydrofuran (8.0 L, water content <100 ppm by Karl Fischer) is added, followed by 0.35 kg (2.96 mol) of oxalyl chloride dosed via a PTFE diaphragm pump at a rate maintaining the internal temperature below 5 °C. Catalytic N,N-dimethylformamide (2.2 mL, 0.02 mol) triggers vigorous gas evolution; the off-gas is scrubbed through a 20% w/w aqueous sodium hydroxide trap connected to a rupture-disc-protected vent line. After 4 h of post-dosing agitation at 20–25 °C, the resulting acyl chloride solution is pressure-transferred into a second 20-L reactor containing a pre-cooled (−10 °C) solution of methylamine (0.52 kg, 16.7 mol, 5.6 eq.) in tetrahydrofuran (6.0 L). The stoichiometric excess of methylamine is deliberately maintained to suppress bis-acylation; batch records from multiple toll manufacturers indicate that dropping the amine ratio below 4.5 eq. elevates the dimeric amide impurity to 0.8–1.2 area%, which cannot be adequately purged by a single reslurry. The resulting N-methylamide intermediate is isolated by drowning the quenched reaction mixture into 50 L of ice-cold water, filtration through a 5-µm polypropylene cloth, and drying in a conical vacuum dryer at 50 °C under 10 mbar until loss on drying <0.5%. Reduction to the free base vonoprazan is accomplished with lithium aluminium hydride (2.4 eq. relative to the amide) in refluxing tetrahydrofuran; this step remains the rate-limiting safety concern because the hydrogen evolution upon quenching requires a stirred, nitrogen-swept quench vessel with a 1.5× designed headspace factor and the use of saturated Rochelle salt to break the aluminium emulsion without localised overheating. After conversion to the fumarate salt in 2-propanol, the final active pharmaceutical ingredient (API) routinely meets the residual carboxylic acid limit of ≤0.10% when measured by a validated HPLC method employing a 5-µm C18 column (250 × 4.6 mm), 0.1% v/v trifluoroacetic acid in water–acetonitrile gradient, and UV detection at 237 nm, in accordance with ICH Q3A thresholds for unspecified impurities.

What Regulatory Thresholds Govern the Use of This Intermediate in CEP Applications?

Manufacturers filing a Certificate of Suitability to the European Pharmacopoeia for vonoprazan fumarate monohydrate must demonstrate that the free carboxylic acid is controlled as a non-mutagenic process-related impurity. The current Ph.Eur. individual monograph under elaboration defines an acceptance criterion of ≤0.15% for any single unspecified impurity, whereas the carboxylic acid intermediate—when unambiguously detected in the API by spiking experiments—is typically bracketed under the same limit. Reference standards of the acid with a chromatographic purity of ≥99.7% (by area normalisation at 237 nm) and a water content of <0.2% are required for system suitability testing; the standard solution is prepared at a concentration of 1.5 µg/mL in a diluent of water–acetonitrile (50:50 v/v) containing 0.1% formic acid to suppress peak tailing caused by silanol interactions on the column. In the context of a Type II drug master file submitted to the US FDA, the sponsor typically quantifies the carboxylic acid in the API using a validated LC-MS/MS method with a limit of quantification (LOQ) of 0.02 ppm relative to the API matrix; this necessitates the use of a deuterated internal standard of the acid, synthesised via base-catalysed exchange in D₂O–THF at the α-position of the pyrrole ring, to correct for ion-suppression effects observed during electrospray ionisation in positive-ion mode.

Direct application of the isolated carboxylic acid as an analytical reference marker extends beyond the API itself. Pharmacopoeial impurity reference standards intended for peak identification in the drug product—vonoprazan fumarate 10 mg and 20 mg film-coated tablets—require the acid standard to be dissolved in a mixture of 0.05 M phosphate buffer (pH 7.0) and methanol (60:40 v/v) at a concentration identical to the reporting threshold of the finished product specification, commonly 0.10% of the label claim. Inter-laboratory collaborative trials organised by the manufacturer demonstrate that the acid exhibits a relative response factor of 1.14 against vonoprazan free base at the detection wavelength, a value that must be entered into the chromatography data system to avoid under-reporting of the impurity content by approximately 12%, as documented in the transfer report filed under ICH Q9 quality risk management.

Amide Coupling Pathways to Novel P-CAB Analogues

The carboxylic acid serves as the primary building block for exploratory amide libraries designed to evaluate the steric and electronic tolerance at the pyrrole C-3 position. Unlike the reductive amination route leading to vonoprazan, the carboxylic acid can be activated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl, 1.2 eq.) and 1-hydroxybenzotriazole hydrate (HOBt·H₂O, 1.2 eq.) in anhydrous N,N-dimethylformamide at 0–5 °C for 30 min, followed by addition of a structurally diverse amine nucleophile (1.0 eq.). This protocol is compatible with amine substrates bearing additional Lewis-basic pyridine or piperazine moieties without observable cross-reactivity at the sulfonamide linkage, provided the pH of the aqueous workup is maintained between 6.8 and 7.2 to avoid premature hydrolysis of the sulfonamide under acidic conditions. Medicinal chemistry groups have documented that amides derived from N-Boc-piperazine under these conditions achieve isolated yields of 78–85% after flash chromatography on silica gel 60 (eluent: dichloromethane–methanol 95:5 v/v), and that subsequent Boc deprotection with 4 M HCl in 1,4-dioxane proceeds cleanly to afford the secondary amine hydrochloride without detectable desulfonylation. The terminal pharmaceutical forms arising from these intermediates are candidate compounds evaluated in in vitro H⁺,K⁺-ATPase enzyme inhibition assays using porcine gastric vesicles according to an internally validated protocol, and the most promising leads are further characterised by single-crystal X-ray diffraction of the corresponding fumarate salt to confirm absolute configuration and intermolecular hydrogen-bonding networks relevant to formulation stability.

When synthesising glucuronide conjugates for metabolite identification studies, the carboxylic acid intermediate is directly used as the aglycone substrate in an in vitro enzymatic reaction with UDP-glucuronic acid trisodium salt (UDPGA, 5 mM) and alamethicin-activated human liver microsomes (1.0 mg protein/mL) in Tris-HCl buffer (50 mM, pH 7.4) containing magnesium chloride (5 mM) and 25 µg/mL alamethicin. The incubation is conducted at 37 °C for 120 min with gentle orbital shaking; the reaction is terminated by addition of an equal volume of ice-cold acetonitrile containing 0.2% formic acid. The resulting acyl glucuronide is isolated from the supernatant by semi-preparative HPLC on a 10-µm C18 column using a gradient of 10–60% acetonitrile in aqueous 0.1% acetic acid over 30 min. The purified conjugate, characterised by a characteristic anomeric proton doublet at δ 5.53 ppm (J = 7.8 Hz) in D₂O-based ¹H NMR, serves as the bioanalytical reference standard for quantifying systemic exposure of the phase II metabolite during pharmacokinetic studies in Sprague-Dawley rats, where a validated LC-ESI-MS/MS method achieves a lower limit of quantification of 0.5 ng/mL in plasma.

When the Carboxylate Serves as a Reactive Handle for Process-Related Impurity Synthesis

Regulatory starting material risk assessments mandated by ICH Q11 frequently require the unambiguous structural confirmation and toxicological qualification of late-eluting dimeric or over-reacted impurities that originate from the carboxylic acid during large-scale manufacture. The acid can be deliberately dimerised by treatment with 1.0 eq. of oxalyl chloride without adding a trapping amine; the resulting symmetrical anhydride, precipitated by drowning into ice-cold methyl tert-butyl ether, displays a diagnostic IR carbonyl stretching band at 1818 cm⁻¹ (symmetrical stretching) and an HPLC retention time roughly 1.6-fold that of the parent acid under the pharmacopoeial monograph conditions. This impurity standard is co-injected with API samples to confirm that the anhydride is not present above the identification threshold of 0.10% in any production-scale batch. Similarly, the N-methylamide reduction intermediate may undergo retro-Mannich fragmentation if the lithium aluminium hydride reduction temperature accidentally exceeds 60 °C; the resulting 5-(2-fluorophenyl)-1H-pyrrole-3-carbaldehyde can re-oxidise in air to reform the carboxylic acid, creating a false-positive signal for residual starting material. Quality control chemists therefore validate a gradient HPLC method with diode-array detection that can baseline-resolve the carboxylic acid, the symmetrical anhydride, and the 3-carbaldehyde degradation product within a 45-minute analysis window, using a 3.5-µm endcapped phenyl-hexyl column maintained at 35 °C.

Manufacturing deviation investigations at a multi-tonne contract manufacturing site identified a recurring metal contamination fingerprint when the acid-chloride formation step was executed in a non-dedicated 316L stainless steel reactor that had previously been used for a chlorination campaign. Trace iron(III) residues catalysed electrophilic ring chlorination at the electron-rich pyrrole C-4 position, generating a chlorinated analogue that co-eluted with the target amide intermediate during preparative thin-layer chromatography. The contamination was eliminated by switching to a glass-lined vessel and implementing a 0.2% w/w citric acid pre-wash of the reactor before each batch, a corrective action that became a permanent feature of the master batch record. The isolated chlorinated impurity, synthesised in 2-g quantity using N-chlorosuccinimide in acetonitrile at ambient temperature, was fully characterised by high-resolution mass spectrometry ([M+H]⁺ calculated 405.0280, found 405.0283) and supplied as a retention time marker to all receiving laboratories.

When the carboxylic acid is deployed as a custom synthesis building block for other gastric acid secretion inhibitors under preclinical development, the carboxylate is frequently converted to a methyl ester using thionyl chloride in methanol (3.0 eq. of SOCl₂, 0 °C to reflux, 6 h) in 95% crude yield. The ester serves as a crystalline, HPLC-friendly intermediate for subsequent N-sulfonamide functionalisation tolerance screening. Crystallographic data confirm that the methyl ester crystallises in the monoclinic space group P2₁/c with unit cell parameters a = 8.2340(5) Å, b = 13.5612(8) Å, c = 15.0272(9) Å, and a calculated density of 1.456 g·cm⁻³, a property exploited for polymorph screening via vapour diffusion against n-heptane.

The limits for key residual solvents and catalysts in the carboxylic acid employed as a registered starting material are codified in the below matrix, aligned with current ICH M7 and ICH Q3C options.