Ethyl 2-(3-Formyl-4-Hydroxyphenyl)-4-Methylthiazole-5-Carboxylate

Ethyl 2-(3-formyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate (molecular formula C₁₄H₁₃NO₄S, relative molecular mass 291.32 g mol⁻¹) is designated as Febuxostat Impurity F within the European Pharmacopoeia (Ph. Eur. 10.0, monograph 2997) and equivalent compendia. Its structural motif—a 4-methyl-1,3-thiazole ring esterified at the 5-position and substituted at the 2-position with a 3-formyl-4-hydroxyphenyl moiety—places it as a specified, process-related impurity arising during the final condensation or esterification steps of febuxostat active pharmaceutical ingredient synthesis. The substance is provided as a reference standard for liquid chromatographic system suitability verification, specifically to resolve the impurity from the API peak and from the earlier-eluting cyano-hydroxy impurity (Impurity C) in the reversed-phase HPLC assay detailed in the monograph. Handling protocols mandate storage in amber sealed vials under inert gas at −20 °C, with sub-aliquoting into single-use containers recommended after initial opening to suppress oxidation to the corresponding carboxylic acid (Impurity E).

What Structural Features Govern Its Chromatographic Retention Relative to the API?

The reversed-phase retention behaviour of the impurity under the official Ph. Eur. conditions is governed by the interplay of the ethyl ester at the 5-position of the thiazole ring and the 3-formyl-4-hydroxy substitution on the pendant phenyl ring. The chromatographic method prescribed in monograph 2997 employs a stainless steel column (0.15 m × 4.6 mm) packed with octadecylsilane bonded silica (particle size 5 µm, pore size 10 nm) thermostatted at 30 °C (±0.5 °C). Mobile phase A is a phosphate buffer prepared by dissolving 2.72 g L⁻¹ potassium dihydrogen phosphate in water, adjusted to pH 2.5 with dilute phosphoric acid and filtered through a 0.45 µm membrane; mobile phase B is acetonitrile. A multi-step gradient proceeds from an initial 75:25 (A:B) to 30:70 over 35 min at a flow rate of 1.0 mL min⁻¹, with UV detection at 230 nm—the absorbance maximum common to the thiazole chromophore and the conjugated formyl group. The ethyl ester imparts a substantially higher lipophilicity than the carboxylic acid congener (Impurity E), shifting the retention time to a later window. Febuxostat itself bears an isobutoxy ether at the 4-position, which strengthens hydrophobic interaction with the stationary phase; the lack of that ether in Impurity F results in an earlier relative retention time (RRT) of approximately 1.15 relative to the principal API peak. Resolution between Impurity F and febuxostat is specified as not less than (NLT) 3.0, while the separation from Impurity C (the 3-cyano-4-hydroxy ethyl ester, RRT 0.85) is controlled with a resolution requirement NLT 1.5. Traces of water in the diluent promote reversible hydration of the aldehyde, creating a small secondary peak that can compromise integration precision. The monograph therefore specifies a diluent of acetonitrile:water 50:50 (v/v) and mandates storage of the diluted system suitability solution at 2–8 °C for a maximum of 24 h. Published data on the equilibrium constant of hydrate formation for this specific aldehyde is limited; however, organic-rich diluent suppresses the equilibrium sufficiently to maintain peak area relative standard deviation (RSD) below 5.0% across six replicate injections at the 0.10% specification limit, meeting system suitability criteria aligned with ICH Q2(R1). Stainless steel frits and inline filters must be passivated or replaced with PEEK components where routine contact with mobile phases of pH 2.5 occurs, as iron leaching from corroded surfaces generates late-eluting metal-complex peaks that interfere at 230 nm. Column equilibration for not less than 45 min (≥15 column volumes) prior to the sequence is essential to stabilise the aldehyde peak shape; tailing factors for Impurity F typically fall below 1.5 when a high-purity, end-capped L1 packing is selected.

Specification Assignments Versus the Cyano-Containing Congeners

As a pharmacopoeial reference substance, the material is assigned a minimum purity (area %) by HPLC-UV under the same gradient conditions applied to the API. The typical acceptance criterion on the certificate of analysis is NLT 95.0%, determined by area normalisation. Full assay by mass balance is used in reference standard qualification; water content is determined by coulometric Karl Fischer titration (Ph. Eur. 2.5.12) with a limit of not more than (NMT) 0.5%, and residual process solvents—predominantly ethanol and ethyl acetate—are controlled by headspace gas chromatography (Ph. Eur. 2.4.24, system A) to NMT 0.1% each. A representative batch-specification profile is shown in the following table.

Parameter	Limit	Analytical Technique
Purity	NLT 95.0% (area %)	Ph. Eur. 2.2.29, C18 gradient, 230 nm
Water	NMT 0.5%	Ph. Eur. 2.5.12
Residual solvents (ethanol, EtOAc)	NMT 0.1% each	Ph. Eur. 2.4.24
Identity	Retention time ±2% vs. working standard	HPLC-DAD, Ph. Eur. 2.2.29

Comparison with the principal process-related and degradant impurities underscores the analytical selectivity demand. Febuxostat Impurities C, E, and F share the 2-(4-hydroxyphenyl)-4-methylthiazole core but differ in the phenyl-ring substituent at the 3-position and the functional group at the 5-position of the thiazole. The table below contrasts the critical structural and chromatographic fingerprints.

Compound	3,4-Phenyl Substituents	5-Thiazole Group	Key Chromatographic Markers
Febuxostat	3-CN, 4-OCH2CH(CH3)2	COOH	Main peak; UV λmax 230, 315 nm
Impurity C	3-CN, 4-OH	COOEt	RRT ≈ 0.85; UV λmax 230, 315 nm
Impurity E	3-CHO, 4-OH	COOH	RRT ≈ 0.60; UV λmax 230, 280, 340 nm
Impurity F (title compound)	3-CHO, 4-OH	COOEt	RRT ≈ 1.15; UV λmax 230, 280, 340 nm

The formyl-hydroxy pattern differentiates Impurities E and F from the cyano-containing congeners; the aldehyde introduces a bathochromic shift detectable at 340 nm, enabling selective peak tracking. The ethyl ester on Impurity F suppresses ionisation in the acidic mobile phase, whereas the carboxylic acid of Impurity E remains partially ionised, leading to earlier elution and asymmetry. Because the pharmacopoeial method does not employ an ion-pair reagent, resolution of the acid impurity E from the solvent front is marginal; hence, the monograph acceptance criterion for Impurity F relies primarily on its separation from the API and from Impurity C, both of which are neutral in the pH 2.5 environment. In forced degradation studies conducted under ICH Q1A(R2) oxidative conditions, febuxostat API generates Impurity F along with the corresponding carboxylic acid Impurity E; the ratio of the two depends on dissolved oxygen and peroxide concentration. Spiking experiments at the 0.05–0.15% level confirm the isolated aldehyde peak does not co-elute with the 3-cyano-4-isobutoxy impurity or the des-isobutyl intermediate. Mobile phases containing primary amine modifiers (e.g., triethylamine) must be strictly avoided during method development or transfer, as Schiff base formation produces a late-eluting adduct exhibiting a λ_max shift to 350 nm. Analysis of such adducts by LC-MS frequently yields a pseudomolecular ion [M+H]⁺ corresponding to the addition product plus water loss, which can be misidentified as an unknown process impurity.

When Formyl-Hydroxy Is Preferable to the Cyano Precursor in Synthetic Pathway Tracing

Impurity F is often synthesised deliberately by controlled partial reduction of ethyl 2-(3-cyano-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate (Impurity C) for use as a spike marker in batch genealogy investigations. Reduction with diisobutylaluminium hydride (DIBAL-H, 1.2 eq) in anhydrous toluene at −78 °C under argon, followed by quenching with 2 M hydrochloric acid, yields the aldehyde with selectivity over the alcohol (estimated >90% conversion at 4 h reaction time). Purification by flash chromatography on silica gel (particle size 40–63 µm, hexane:ethyl acetate 3:1 v/v) furnishes a solid with a purity suitable for secondary reference standard construction. The aldehyde proton (singlet, δ 9.85–9.90 ppm in DMSO-d₆) and the formyl carbon (191–193 ppm in ¹³C NMR) act as unambiguous forensic tags; their presence in a retention time-shifted peak during analysis of production batches indicates a reduction-pathway intermediate rather than a direct esterification side-product. Thermal characterisation of the isolated impurity by differential scanning calorimetry (heating rate 10 K min⁻¹, nitrogen purge 50 mL min⁻¹) exhibits a single melting endotherm; published data for this specific configuration, drawn from certificates of analysis of accredited reference material producers, places the onset in the interval 165–175 °C, with a peak maximum near 172 °C. The narrow range (1–2 °C) of the endotherm for batches purified to >98% by preparative HPLC confirms polymorphic homogeneity useful for identity cross-check. Operational boundaries for formulators handling the neat solid: the compound is stable for 36 months in sealed, light-resistant primary containers stored at −20 °C. Opening the container in a standard laboratory atmosphere (relative humidity 50–70%) initiates measurable surface oxidation to Impurity E within 72 h, as tracked by the appearance of a shoulder eluting at the RRT of the acid. Pre-weighing into single-use, septum-capped vials under a dry nitrogen blanket is therefore standard procedure in quality-control dispensaries. The compound is incompatible with primary amines and strong bases; contamination with ammonium acetate or triethylamine during mobile-phase preparation generates imine adducts that increase back-pressure on analytical columns due to precipitation in the high-aqueous segment of the gradient. No specific acute toxicity data are included in the safety data sheets of commercial suppliers; handling in accordance with the principles of GLP and with the assumption of skin-sensitising potential is advised. Disposal of waste solutions must comply with local regulations for aldehyde-containing organic streams.