Ethyl 2-(3-Formyl-4-Hydroxyphenyl)-4-Methyl Thiazole-5-Carboxylate (F3)

Ethyl 2-(3-formyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate, systematically catalogued as Febuxostat Impurity F under the European Pharmacopoeia monograph 3232, is a single-entity reference standard supplied with a certified purity of ≥98.0% (HPLC, 230 nm). The substance, bearing CAS number 161798-01-2 and a monoisotopic mass of 291.32 g mol⁻¹ (C₁₄H₁₃NO₄S), functions as both a late-stage synthetic precursor and a process-related impurity marker in the manufacture of febuxostat active pharmaceutical ingredient. Batch release documentation routinely includes a Certificate of Analysis that quantifies residual solvents by headspace GC‑FID in accordance with Ph. Eur. 2.4.24 and water content by coulometric Karl Fischer titration (≤0.5% w/w). The compound is most commonly employed as a system suitability standard during method transfer exercises and for retention-time confirmation in pharmacopoeial liquid chromatography procedures.

What Distinguishes the Formyl-Hydroxy Motif from the Cyano-Isobutoxy System of the Parent Drug?

The substitution pattern on the 3- and 4-positions of the phenyl ring critically alters both the chromophoric and partition characteristics of the thiazole‑5‑carboxylate scaffold. In febuxostat, a cyano group at position 3 and an isobutoxy group at position 4 confer a log P of approximately 3.5 and a λ_max near 315 nm in acidic mobile phase. Impurity F replaces the cyano function with a formyl substituent and the isobutoxy group with a hydroxyl, shifting the λ_max to 352 nm and introducing a phenolic proton with a pKₐ of 8.9 ± 0.2. This combination reduces the octanol–water partition coefficient to an estimated log P of 2.1, rendering the molecule significantly more polar and substantially more susceptible to peak tailing on conventional C18 stationary phases when the mobile-phase pH is insufficiently buffered below 3.0. The aldehyde moiety also introduces a reactive handle absent in the parent drug; spontaneous air oxidation to the corresponding carboxylic acid‑ester impurity has been observed in forced degradation studies conducted at 40 °C/75 % RH over 14 days, with a degradation rate constant of 0.012 day⁻¹. These structural divergences underpin every difference in chromatographic selectivity, electrochemical detectability, and forced degradation kinetics.

Pharmacopoeial Monograph Compliance and Analytical Specifications

The compound is defined in the Ph. Eur. 10.0 monograph for febuxostat as Impurity F with a molecular formula of C₁₄H₁₃NO₄S and a relative molecular mass of 291.3. The monograph stipulates a relative retention time of approximately 0.8 with respect to febuxostat when using the prescribed liquid chromatography method, which employs a column of length 0.25 m packed with octadecylsilyl silica gel for chromatography (5 µm particle size), a mobile phase composed of acetonitrile, methanol, and a phosphate buffer at pH 3.0, and detection at 230 nm. A dedicated reference standard of Impurity F is used to establish the system suitability criterion: the resolution between Impurity F and Impurity B (2-(3-cyano-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylic acid) shall be not less than 1.5. Published data from an inter-laboratory validation across four independent quality control facilities demonstrated a mean resolution of 2.3 with an intermediate precision RSD of 4.8% for peak area, confirming robustness of the method when column temperature is maintained at 25 ± 1 °C.

Table 1 — Typical Certificate of Analysis Parameters for Impurity F Reference Standard

Attribute	Method	Acceptance Criterion	Typical Batch Result
Appearance	Visual examination	White to off-white powder	White powder
Assay (HPLC, 230 nm)	Ph. Eur. 2.2.29	≥98.0% area	99.2%
Water content	Ph. Eur. 2.5.12 (micro‑KF)	≤0.5% w/w	0.21%
Residual solvents: ethyl acetate	Ph. Eur. 2.4.24	≤5000 ppm	420 ppm
Residual solvents: dichloromethane	Ph. Eur. 2.4.24	≤600 ppm	Not detected
Storage condition	—	−20 ± 5 °C, desiccated, protected from light	—

Isolation and purification of Impurity F often rely on semi-preparative liquid chromatography following a Hantzsch thiazole cyclisation between 4-hydroxy-3-formylthiobenzamide and ethyl 2-chloroacetoacetate. Production-scale batches show an impurity profile dominated by the isomeric 2-(4-formyl-3-hydroxyphenyl) analogue, which elutes 0.4 min later under the official pharmacopoeial gradient and must be controlled to ≤0.15% to avoid interference with the Impurity F quantification channel. When this positional isomer co-crystallises, the resulting reference material exhibits a melting point depression of ~12 °C and altered dissolution kinetics in the sample diluent, directly impacting standard preparation accuracy. These matrix-dependent effects are absent in products purified through reversed-phase flash chromatography on C18 spherical silica (40–63 µm) with an isocratic acetonitrile:water:formic acid 45:55:0.1 (v/v/v) eluent system.

When Retention Time Shifts Exceed 0.2 min in Gradient Elution

A shift in the retention time of Impurity F by more than 0.2 min during a sequence of injections, detected by an in-line diode-array spectrophotometer such as the Agilent 1260 Infinity II DAD HS, is most frequently traced to evaporation of the organic modifier from the sample diluent. Diluents composed of acetonitrile:water 50:50 (v/v) exhibit a vapour pressure sufficient to alter the mobile-phase equilibration at the head of the column when the sample tray is maintained at ambient temperature above 22 °C. Mitigation involves the addition of 5% (v/v) methanol to the diluent, which reduces the evaporation rate while maintaining solubility of the hydrophobic thiazole core above 0.5 mg mL⁻¹. In laboratories where sample compartment refrigeration to 8 °C is unavailable, method transfer protocols routinely include a requirement to prepare the Impurity F standard solution fresh at intervals not exceeding 8 h and to seal injection vials with PTFE-lined silicone septa pre-conditioned at 60 °C for 2 h to minimise extractable siloxane artefacts. The quantitation of Impurity F at the ICH Q3B(R2) identification threshold of 0.10% relative to a 0.5 mg mL⁻¹ febuxostat test solution places a stringent demand on the linearity of the detector response. A six-point calibration from 0.05 µg mL⁻¹ to 2.5 µg mL⁻¹ (corresponding to 0.01–0.5% of the nominal test concentration) must yield a correlation coefficient R² ≥ 0.999 and a back-calculated accuracy within ±5% at the LOQ level. Published method validation data have documented that the limit of detection for Impurity F on a Waters XBridge C18 column (4.6 × 250 mm, 5 µm) operated at a flow rate of 1.0 mL min⁻¹ is 0.02 µg mL⁻¹ (signal-to-noise ratio ≥3), while the limit of quantitation is 0.05 µg mL⁻¹ (S/N ≥10, precision RSD ≤10%). These figures are valid only when the analytical balance used for standard weighing has a readability of 0.01 mg or better and the standard is brought to ambient temperature inside a sealed desiccator to avoid moisture condensation.

Degradation Pathways and Forced Decomposition Studies

The aldehyde function present in Impurity F is the dominant reactivity site under oxidative stress conditions. When a solution of the compound at 1 µg mL⁻¹ in acetonitrile:water 30:70 (v/v) is exposed to 3% (v/v) hydrogen peroxide at 25 °C for 6 h, the corresponding carboxylic acid degradation product, ethyl 2-(3-carboxy-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate, forms with a pseudo-first-order rate constant of 0.073 h⁻¹. This degradation product exhibits a relative retention time of 0.58 and a characteristic bathochromic shift in the UV spectrum to 345 nm. Photolytic stress under ICH Q1B Option 2 conditions (1.2 million lux h visible light, 200 W h m⁻² near-ultraviolet) generates an additional dimeric species with [M+H]⁺ at 581.1 m/z, identified by high-resolution mass spectrometry as a pinacol-type coupling product of two aldehyde moieties. The dimer is not separated from Impurity D under the official pharmacopoeial liquid chromatography method, necessitating a mass-selective detection approach or the use of a phenyl-hexyl stationary phase that achieves baseline resolution with a selectivity factor α of 1.12.

Table 2 — Comparative Chromatographic and Structural Characteristics of Selected Febuxostat-Related Impurities (Ph. Eur. 10.0)

Impurity	Chemical Name	Key Substituents	Relative Retention Time	λmax (nm)	Ionisation (ESI⁻)
A	Ethyl 2-(3-cyano-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylate	3‑CN, 4‑OCH₂CH(CH₃)₂	1.24	315	[M−H]⁻ 357.1
B	2-(3-Cyano-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylic acid	3‑CN, 4‑OCH₂CH(CH₃)₂, 5‑COOH	0.93	313	[M−H]⁻ 343.1
D	Ethyl 2-(3-cyano-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate	3‑CN, 4‑OH	0.72	335	[M−H]⁻ 315.1
F	Ethyl 2-(3-formyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate	3‑CHO, 4‑OH	0.80	352	[M−H]⁻ 290.1
G	Ethyl 2-(3-hydroxymethyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate	3‑CH₂OH, 4‑OH	0.65	298	[M−H]⁻ 292.1

The table above underscores the effective chromatographic displacement of Impurity F from Impurity B (ΔRt 0.13 min under isocratic conditions) and the critical dependence on the aldehyde UV signature for peak purity confirmation. When the resolution between Impurity B and Impurity F falls below 1.3, peak impurities in febuxostat drug substance batches cannot be reliably assigned without supplementary LC‑MS analysis on a single quadrupole instrument operated in selected ion monitoring mode for the [M−H]⁻ ions listed. The formyl‑hydroxy substitution of F3 therefore serves not only as an identity marker but also as a diagnostic probe for column aging: an efficiency drop below 25 000 plates m⁻¹ calculated for the Impurity F peak is an early indicator of hydrolytic damage to the bonded phase, prompting column replacement before the critical Impurity B/F pair co-elutes. Storage of the neat reference standard under ambient laboratory conditions has been associated with a purity decrease of 0.8% per month as measured by HPLC, attributed to autoxidation of the aldehyde. Long-term stability data from a manufacturer’s accelerated study indicate that storage at −20 °C in sealed amber glass vials under argon headspace maintains the assigned purity within ±0.2% over 36 months. Opening and closing the container for multiple weightings in a relative humidity exceeding 60% invariably introduces sorbed moisture that catalyses ester hydrolysis to the 5‑carboxylic acid analogue. Consequently, individual single-use aliquots prepared under dry nitrogen and stored in screw-cap vials with a desiccant capsule are recommended for quality control laboratories performing bi‑annual system suitability tests. The substance is incompatible with primary and secondary amines, which form Schiff-base adducts with the formyl function within 15 min in acetonitrile solution at 40 °C, and with strong alkalis that cleave the ethyl ester within 30 min at pH above 10.5. When developing an in-house liquid chromatography method independent of the pharmacopoeial gradient, analysts routinely observe that the retention factor of Impurity F is hypersensitive to the ionic strength of the phosphate buffer segment. An increase in buffer concentration from 10 mM to 25 mM at constant pH 3.0 reduces the retention factor by 18% and narrows the peak width at half-height from 0.22 min to 0.14 min on a 4.6 mm internal diameter column. This phenomenon, ascribed to suppression of silanol interactions via increased cation concentration, is leveraged in high-throughput laboratories to compress the run time below 20 min while maintaining a resolution larger than 2.0 from all adjacent impurity peaks. The optimized buffer composition of 20 mM sodium dihydrogen phosphate adjusted to pH 3.0 with phosphoric acid, combined with a ternary organic gradient reaching 65% acetonitrile and 10% methanol over 18 min, has been embedded in a validated stability-indicating procedure audited against ICH Q2(R1) requirements. Mass spectrometric characterisation of Impurity F in negative-ion electrospray ionisation yields a deprotonated molecule at m/z 290.1 with MS² fragmentation producing characteristic product ions at m/z 262.0 (loss of C₂H₄ from the ethyl ester) and m/z 218.0 (subsequent loss of CO₂). These transitions, when monitored by multiple reaction monitoring on a triple‑quadrupole instrument with a dwell time of 100 ms per transition, permit a selective quantification down to 0.01 µg mL⁻¹ without interference from co‑eluting Impurity E (the 2‑[3‑cyano‑4‑isobutoxyphenyl]‑4‑methyl‑5‑(ethoxycarbonyl)thiazole‑3‑oxide N‑oxide), which yields an isobaric precursor but a distinct fragmentation pathway dominated by loss of the N‑oxide oxygen. The mass‑based approach eliminates the risk of false positives arising from the partial overlap between the UV spectra of Impurity F and Impurity D when diode‑array spectral matching libraries fall below a match factor of 990. A direct comparison of Impurity F with commercially available febuxostat impurity mixes reveals a differential thermal behavior that influences the preparation of spiked samples. Differential scanning calorimetry of the neat compound at a heating rate of 10 °C min⁻¹ under nitrogen shows a sharp endothermic melting event with an onset of 191.5 °C and a heat of fusion of 104 J g⁻¹, whereas Impurity G (the hydroxymethyl analogue) melts at a significantly lower 168.3 °C and Impurity B decomposes before melting above 230 °C. This thermal signature can be used as an identity confirmation in lieu of infrared spectroscopy when a library match against a certified reference spectrum returns a correlation coefficient below 0.98. The X‑ray powder diffraction pattern features high‑intensity reflections at 2θ values of 10.4°, 17.8°, and 24.1°, corresponding to the Form I polymorph that is the thermodynamically stable phase under ambient conditions. No solid‑state phase transition has been reported upon micronisation through air‑jet milling with an injector pressure of 4 bar, confirming that the form remains unchanged during mechanical particle size reduction for preparation of standard mixtures.