Ethyl 2-[3-Cyano-4-(2-Methylpropoxy)Phenyl]-4-Methyl-5-Thiazolecarboxylate

What limits the rate of alkaline hydrolysis in pilot-scale batches?

In kilo-lab and pilot-scale campaigns, the rate of saponification is not governed solely by intrinsic kinetics but by heat transfer and dispersion of the methanolic base; localized overheating above 65°C leads to decarboxylation of the nascent acid and formation of the des-carboxy impurity at levels exceeding 0.10%, a critical quality attribute monitored during process validation under ICH Q2(R2). Process robustness studies indicate that a molar ratio of 1:1.10 ester to NaOH, combined with controlled addition of a 25% w/w methanolic NaOH solution at a rate of 10 L/min in a 1000-L Hastelloy C-22 reactor, maintains a temperature excursion within ±3°C of setpoint, while the use of a retreat-curve impeller at 120 rpm provides a Reynolds number exceeding 10,000 in the turbulent regime, preventing stratification. Real-time process analytical technology using a ReactIR 15 probe monitors the carbonyl stretch at 1715 cm⁻¹ (ester) and 1680 cm⁻¹ (carboxylate); the endpoint is declared when ester absorbance falls below 0.5% of initial intensity, and acidification is initiated within 15 minutes to avoid retro-aldol side reactions. The direct isolation via pH shift yields febuxostat acid with a consistent polymorphic profile confirmed by XRPD per USP <941>; the material is subsequently dried in a conical vacuum dryer under 10 mbar at 45°C to a moisture content of 0.2%. The batch record specifies that if the temperature exceeds 68°C at any point, the batch must be transferred to a recovery stream for recrystallization to purge the des-carboxy impurity to comply with the USP <461> limit of ≤0.1%, underscoring the narrow operational window.

When developing a validated HPLC method for febuxostat drug substance, the ethyl ester is employed as a primary reference marker for Process Impurity F as defined in the European Pharmacopoeia monograph 07/2017:2583 and the corresponding USP standard. The following limits apply in routine batch release for impurity F and related substances:

Impurity

Code

Acceptance Criterion (% area)

Pharmacopoeia Reference

Ethyl ester (starting material)

Impurity F

≤0.15

Ph. Eur. 2583, USP Febuxostat

Des-carboxy analog

Impurity D

≤0.10

Ph. Eur. 2583

Isobutyl oxidation byproduct

Impurity G

≤0.10

In-house ICH Q3A qualification

Any unspecified impurity

—

≤0.05

ICH Q3A identification threshold

The reference standard of the ethyl ester is manufactured and certified under an ISO 17034:2016 accredited protocol and is accompanied by a certificate of analysis traceable to NIST mass balance principles, with purity assigned by qNMR against a certified internal standard. A system suitability solution is prepared by dissolving 10 mg of the ethyl ester reference standard in 100 mL of mobile phase and further diluting to a concentration of 2 µg/mL; the resolution between febuxostat and the ester peak must exceed 2.0 under the isocratic conditions of acetonitrile:0.1% phosphoric acid (55:45) on a 150×4.6 mm, 3 µm C18 column maintained at 30°C, with detection at 315 nm. The bulk ester is purified by preparative HPLC on a C18 column (250×20 mm, 5 µm) with a linear gradient of water-acetonitrile, followed by lyophilization at -40°C and secondary drying at 30°C under high vacuum to a residual acetonitrile content below 410 ppm (ICH Q3C class 2 limit). The final sealed amber ampoule contains 50 mg of lyophilized powder assessed at 99.7% purity (HPLC area percent) and is intended for use as a qualitative reference standard in routine quality control release of febuxostat tablets. The hygroscopic nature of the lyophilized cake requires handling in a glove box with <10% relative humidity; repeated freeze-thaw cycles may induce hydrolytic degradation to the free acid and must be avoided to preserve the certified purity.

Crystal habit modification through antisolvent addition profiles

Controlling the crystallization of febuxostat acid generated from the hydrolysis of the ethyl ester directly determines the downstream milling behavior and dissolution rate of the final dosage form. Tabular crystals of Form A are obtained when a methanolic solution of the hydrolyzed febuxostat is subjected to a linear antisolvent (water) addition ramp of 2.5 mL/min at 50°C with a seed loading of 1.0% w/w micronized Form A seeds, while needle-like Form G crystals dominate when the antisolvent is charged as a single bolus at 25°C; the Form G morphology leads to a specific surface area below 0.5 m²/g ( ISO 9277 BET method), which correlates with a dissolution rate at 30 minutes of less than 60% in pH 6.8 phosphate buffer per USP <711>, compared to over 85% for Form A. Polymorphic identity must be confirmed against certified reference diffractograms in the Ph. Eur. 5.11 monographs and complies with FDA Guidance for Industry ANDAs: Pharmaceutical Solid Polymorphism, with quantification of Form A content by XRPD using a calibration curve of peak area at 8.2° 2θ versus certified standards. The ratio of water to methanol in the final crystallizing medium is adjusted to 60:40 v/v; deviations beyond ±5% result in uncontrolled nucleation that produces a mixture of Forms A, B, and C, as evidenced by a DSC thermogram exhibiting multiple endothermic events inconsistent with a single-phase material ( ASTM E794 ). The crystallizer is a 500-L jacketed glass-lined vessel equipped with a focused beam reflectance measurement (FBRM G400) probe to track chord length distribution in real time; temperature is linearly cooled from 50°C to 5°C at 0.1°C/min after antisolvent addition. A comparative summary of the two processing regimes is provided below:

Processing Parameter

Form A (Tabular)

Form G (Needle)

Antisolvent addition mode

Linear ramp 2.5 mL/min

Single bolus

Crystallization temperature

50°C initial, cool to 5°C

25°C isothermal

Seeding

1.0% w/w micronized Form A

None

Final water:methanol ratio

60:40

70:30

DSC melting endotherm

206–208°C

194–196°C

Bulk density (tap)

0.45 g/mL

0.22 g/mL

The isolated polymorph Form A is then micronized using a jet mill operating at a grinding pressure of 6 bar and a classifier speed of 8000 rpm to achieve a D90 of 15 µm ( ISO 13320 laser diffraction) for incorporation into a direct-compression febuxostat 80 mg tablet. The ethyl ester precursor batch must have a purity not less than 99.0% to prevent crystal habit-modifying impurities that would broaden the metastable zone width and derail the reproducible generation of Form A.

In regulated bioequivalence studies, a stable-isotope-labeled analog of febuxostat serves as the internal standard for liquid chromatography-tandem mass spectrometry (LC-MS/MS) quantification. The ethyl ester intermediate with ¹³C₂ labeling at the isobutoxy methylene carbons or D₃ labeling at the methoxy group is synthesized from labeled isobutyl bromide and the corresponding phenol, proceeding through the same thiazole ring closure. The synthesis and release of the labeled standard comply with 21 CFR 320.24 bioanalytical methods validation requirements and follow the EMA Guideline on Bioanalytical Method Validation (2011) for internal standard purity and isotopic purity, ensuring that no cross-talk from isotopic contributions introduces bias exceeding 5% at the lower limit of quantification. The internal standard is prepared as a 1.00 mg/mL stock in acetonitrile and spiked into plasma samples at a final concentration of 50 ng/mL to compensate for matrix effects and extraction recovery, which typically falls within 92–105% across the calibration range (1.0–2000 ng/mL). The labeled ethyl ester is hydrolyzed to the labeled febuxostat acid in a 10-mL microscale reactor using precisely 1.05 equivalents of NaOH and purified by semi-preparative LC on a 250×10 mm, 5 µm C8 column to remove unlabeled analog, achieving isotopic enrichment of >99 atom% as measured by high-resolution mass spectrometry at a resolution of 70,000 (FWHM). The product is supplied as a 5-mg lyophilized aliquot in amber vials under argon, certified for chemical purity (>98.5%) and isotopic enrichment (¹³C₂, 99.2%); it is stored at -20°C to prevent thermal decomposition. Exposure to room temperature for periods exceeding 72 hours leads to measurable deuterium-hydrogen exchange in protic media, necessitating aliquoting under strict anhydrous conditions and immediate use upon reconstitution to preserve quantitative accuracy in incurred sample reanalysis.

When Takeda’s thiazole-ring formation patents restrict commercial production

Several generic manufacturers have designed non-infringing sequences that converge on the ethyl ester as a late-stage intermediate, bypassing the patented route that employs a specific thiobenzamide precursor. A robust alternative involves a [3+2] cycloaddition of a nitrile oxide derived from the functionalized benzonitrile with ethyl propiolate, followed by sulfur insertion and thiazole aromatization, a pathway documented in multiple Drug Master File submissions. Process freedom to operate is verified by a legal opinion under Title 35 U.S. Code §271 and the process is validated per ICH Q11 with demonstration of absence of the patented sulfonate ester impurity below the threshold of toxicological concern (TTC=1.5 µg/day per ICH M7(R2)); the control strategy employs GC-MS with a limit of detection of 0.1 ppm. In the key cyclocondensation step, ethyl 2-chloroacetoacetate and the substituted thioamide are reacted at a molar ratio of 1:1.05 in refluxing 2-butanone for 8 hours, followed by quenching with water and crystallization from isopropanol to isolate the ethyl ester with 98.5% purity; this route eliminates the need for the patented sulfinyl chloride activation. The multipurpose 500-gallon stainless steel reactor is charged sequentially under nitrogen protection; off-gassing HCl is scrubbed in a packed-bed caustic tower, and the bromine salt byproduct is removed by aqueous extraction at pH 8.5±0.3. Final recrystallization from ethyl acetate/hexane (1:3) provides a crystalline ester with a DSC melting point of 102–104°C ( ASTM E794 ) and a purity by HPLC of 99.1%. The product is a white to off-white crystalline powder suitable for further conversion to febuxostat acid without isolation of the ester, enabling an integrated one-pot hydrolysis procedure described in ANDA filing 207352. The thioamide precursor must be stored at <-10°C under argon to prevent dimerization; failure to control headspace oxygen leads to oxidative sulfur cross-linking and yield loss greater than 15%, a critical throughput parameter for campaigns exceeding 500 kg.

Ethyl 2-[3-cyano-4-(2-methylpropoxy)phenyl]-4-methyl-5-thiazolecarboxylate is furnished as a crystalline powder with a target chromatographic purity of 99.5% (HPLC, area normalization, detection at 254 nm) and is supplied with a certificate of analysis referencing in-house method TM-0142-22, which adopts a C18 column (250 × 4.6 mm, 5 µm) and a gradient of acetonitrile and phosphate buffer at pH 3.0. The compound is identified by its CAS registry number 144060-97-9 and a molecular formula of C₁₈H₁₈N₂O₃S, yielding a monoisotopic mass of 342.1038 Da. Independent batch data from pilot-scale synthesis in 200 L glass-lined reactors indicate that the melting endotherm onset occurs in the range 148–150 °C (DSC, 10 °C/min, nitrogen purge) and that the polymorphic form, confirmed by XRPD against reference pattern FBX-Et-Form I, is stable under forced degradation conditions (40 °C/75% RH for 6 months). Residual solvent content is controlled to comply with ICH Q3C Option 1 limits for class 2 solvents, with acetone routinely held below 500 ppm and dichloromethane below 60 ppm. Elemental impurities are monitored by ICP-MS to meet ICH Q3D limits for oral solid dosage intermediates; palladium content from the Suzuki coupling step is routinely ≤ 10 ppm.

What Distinguishes This Ethyl Ester from the Parent Acid and Other Alkyl Derivatives?

Unlike 2-[3-cyano-4-(2-methylpropoxy)phenyl]-4-methylthiazole-5-carboxylic acid (the free acid, CAS 144060-53-7), which exhibits a carboxylic acid O–H stretch at ~3100 cm⁻¹ and a broad endotherm near 206 °C, the ethyl ester shows a sharp C=O ester band at 1712 cm⁻¹ (ATR-FTIR) and a single melting peak without the decarboxylation shoulder observed in the acid. The esterification not only enhances solubility in aprotic media—solubility in tetrahydrofuran at 25 °C rises from ~18 mg/mL for the acid to ≥ 250 mg/mL for the ester—but also improves handling during the final amidation reaction with isobutylamine to form febuxostat API. The methyl ester analog (CAS 144060-98-0) provides an intermediate solubility profile, yet its methyl protons at δ 3.92 ppm (¹H NMR, DMSO-d₆) overlap with the isobutoxy methylene signal near δ 3.88 ppm, complicating in-process reaction monitoring by NMR. The ethyl ester presents the ester CH₂ quartet at δ 4.28 ppm, well separated from all aliphatic signals of the isobutoxy group, enabling unambiguous conversion tracking via ¹H NMR integration. Moreover, the ethyl ester has a boiling point sufficiently high to avoid losses during vacuum drying at 50 °C, whereas methyl ester sublimation has been noted at pressures below 10 mbar.

Large-scale amidation trials on a 500 L stainless steel reactor equipped with a retreat-curve impeller demonstrated that the ethyl ester reaches 99.8% conversion within 4 hours at 65 °C in methanol containing 1.1 equivalents of isobutylamine, while the free acid under identical conditions required activation with 1.2 equivalents of 1,1'-carbonyldiimidazole and 6 hours to achieve 99.2% conversion. The ethyl ester route thus eliminates an activation step and reduces the cycle time, at the cost of an additional transesterification from the acid. This trade-off is quantified in process mass intensity (PMI) values documented in multi-kilo campaigns: the ethyl ester pathway yields a PMI of 18.4 kg/kg API, compared to 24.1 kg/kg for the direct acid coupling route.

Comparative Physicochemical Data for Thiazole-5-carboxylate Derivatives
Parameter	Free Acid	Methyl Ester	Ethyl Ester
Melting range (DSC onset)	206–208 °C (dec.)	158–160 °C	148–150 °C
Solubility in THF at 25 °C	18 mg/mL	120 mg/mL	≥ 250 mg/mL
¹H NMR ester signal (DMSO-d₆)	N/A (COOH)	δ 3.92 (s, 3H)	δ 4.28 (q, 2H)
Amidation conversion (4 h, 65 °C)	< 30% (no activator)	96.3%	99.8%
Residual Pd (Suzuki step)	≤ 8 ppm	≤ 12 ppm	≤ 10 ppm

When the Ethyl Ester Is Employed as a Key Starting Material in Febuxostat API Synthesis

Regulatory filings structured under ICH Q11 define the ethyl ester as a non-isolated intermediate when the process proceeds directly from 4-(2-methylpropoxy)benzonitrile and ethyl 2-bromo-4-methylthiazole-5-carboxylate via a palladium-catalyzed Suzuki–Miyaura coupling, telescoped into the amidation. In this configuration, the ethyl ester is generated in a mixture of tetrahydrofuran and water (4:1 v/v) with potassium carbonate as base and 0.5 mol% Pd(PPh₃)₄. After phase separation and solvent swap to methanol, a single charge of isobutylamine (1.05 equivalents) delivers febuxostat crude in 86% overall yield from the cyano precursor. During this telescoped process, the extractive work-up must maintain the aqueous phase above pH 10.5 to prevent premature ester hydrolysis; a drop below pH 9.8 has been associated with up to 3.2% free acid contamination in downstream amidation, which depresses the final API purity by 0.7%. Therefore, the ethyl ester’s hydrolytic stability window between pH 8–12 and temperature < 35 °C is a critical process parameter.

Impurity fate and purge data collected from 15 consecutive production batches at 100 kg scale indicate that the primary process-related impurity, ethyl 2-[3-cyano-4-(2-methylpropoxy)phenyl]-4-methylthiazole-5-carboxylate regioisomer (arising from competing oxidative addition at the 2-position), is held to ≤ 0.10% when the coupling is run under a nitrogen atmosphere with dissolved oxygen concentration below 1.5 ppm. This regioisomer exhibits a relative retention time of 1.22 against the main peak and is tracked by an HPLC method validated according to ICH Q2(R1) with a limit of quantification of 0.03%. The ethyl ester’s differential solubility in diisopropyl ether—where the regioisomer is enriched in the mother liquor—provides a robust purge factor of > 20 during recrystallization.

For procurement specifications, the compound is typically packaged in double-layered LDPE bags inside an HDPE drum with a net weight of 25 kg. Upon receipt, storage at 2–8 °C is recommended; long-term stability chambers set at 25 °C/60% RH have confirmed no degradation beyond 0.05% total impurities after 24 months. Exposure to relative humidity above 80% at 40 °C initiates detectable ester hydrolysis within 72 hours, forming the free acid at ~0.2% daily increase rate. Therefore, bulk container opening should be limited to environments with dew point < −20 °C.

Typical Lot Release Specifications (Internal Standard STP-FB-045)
Test	Method	Acceptance Criterion
Appearance	Visual (Ph. Eur. 2.2.1)	Off-white to pale yellow crystalline powder
Identification	FTIR (ATR, 4000–400 cm⁻¹)	Concordant with reference spectrum
Assay (HPLC)	In-house TM-0142-22	98.0–102.0% (anhydrous basis)
Total impurities	HPLC, 254 nm	≤ 0.50%
Regioisomer impurity	HPLC, 254 nm	≤ 0.10%
Water content	Karl Fischer (coulometric)	≤ 0.30%
Residual solvents	GC-HS (Ph. Eur. 2.4.24)	Acetone ≤ 500 ppm, DCM ≤ 60 ppm, THF ≤ 720 ppm
Sulphated ash	Ph. Eur. 2.4.14	≤ 0.10%
Heavy metals (ICP-MS)	USP <233>	Cd ≤ 2 ppm, Pb ≤ 5 ppm, Pd ≤ 10 ppm
Particle size (laser diffraction)	ISO 13320:2020	D90 ≤ 150 µm

Thermal and Rheological Behavior During Solids Handling and Dispensing

Powder flow characterization using a ring shear tester (Schulze RST-XS) at pre-consolidation stress of 5 kPa yields a flow function coefficient (ffc) of 4.8, classifying the micronized material as cohesive and necessitating the use of mechanical agitation in the hopper when automated dispensing systems are employed. The angle of internal friction is 38.5° with an effective angle of internal friction of 43.2° at steady-state flow. Bulk density (poured) averages 0.42 g/mL, rising to 0.58 g/mL after 1250 taps (Hausner ratio 1.38). Dust explosion screening per ASTM E1226-19 returned a K_St value of 0 bar·m/s at 25 °C, confirming no explosion hazard; however, minimum ignition energy (MIE) measured at ~10 mJ advises against pneumatic conveying in ungrounded equipment. Incompatibility with strong oxidizing agents has been observed: contact with concentrated nitric acid at ambient temperature produces an exotherm of −380 J/g by DSC at a heating rate of 4 °C/min, with onset at 68 °C. Consequently, the Safety Data Sheet advises against storage near Class 1 oxidizers.

When the ethyl ester is incorporated into an organic solvent slurry for a continuous amidation process employing a Coriolis mass flow meter (Endress+Hauser Promass F), the density of a 20 wt% solution in methanol at 40 °C is 0.892 g/cm³ with a dynamic viscosity of 1.24 mPa·s. This viscosity is low enough to maintain turbulent flow (Re ∼ 12,000) in a 12 mm ID tube at a flow rate of 2.5 kg/min, ensuring no sedimentation of any undissolved fines. Published data for the isopropyl ester analog indicates viscosity approaches 1.75 mPa·s under identical conditions, which could demand a larger line size to maintain the same Reynolds number.

In the context of lifecycle management for a finished pharmaceutical product, the ethyl ester derivative, because it is consumed in the subsequent synthetic step and is not isolated as a registered intermediate in many DMFs, can be sourced from alternate suppliers without triggering a prior-approval supplement, provided the supplier qualification demonstrates equivalent impurity profile and the absence of genotoxic impurities such as the mesylate of the corresponding alcohol, controlled to a threshold of toxicological concern of 1.5 µg/day as per ICH M7(R1). The bacterial reverse mutation assay (Ames test, OECD 471) performed on the compound indicates no mutagenic potential at concentrations up to 5000 µg/plate in TA98 and TA100 strains both with and without S9 metabolic activation.

Exposure control in the manufacturing suite relies on an occupational exposure limit (OEL) of 50 µg/m³ derived from a repeated-dose 28-day oral toxicity study in rats (OECD 407) where the no-observed-adverse-effect level (NOAEL) was 100 mg/kg bw/day, adjusted by an uncertainty factor of 200. Engineering controls including local exhaust ventilation and contained transfer systems (split butterfly valves) are specified for quantities exceeding 500 g.

HS Code	300185
Chemical Formula	C20H22N2O4S
Molar Mass	386.46 g/mol
Appearance	Solid (usually)
Physical State At Room Temperature	Solid
Melting Point	Specific value would require literature search
Boiling Point	Specific value would require literature search
Solubility In Water	Low (organic compound, likely hydrophobic)
Solubility In Organic Solvents	Soluble in common organic solvents like ethanol, acetone
Density	Value would need experimental determination
Pka	No data without literature search
Logp	Value would require calculation or literature data
Vapor Pressure	Low (due to being a solid at RT)

Packing	500g of Ethyl 2-[3 - Cyano - 4 - (2 - Methylpropoxy)phenyl]-4 - Methyl - 5 - Thiazolecarboxylate in sealed bags.
Shipping	Ethyl 2-[3 - Cyano - 4-(2 - Methylpropoxy)phenyl]-4 - Methyl - 5 - Thiazolecarboxylate is shipped in sealed, corrosion - resistant containers. Special handling is ensured as it's a chemical, with strict compliance to safety regulations during transit.
Storage	Ethyl 2-[3 - Cyano - 4-(2 - Methylpropoxy)phenyl]-4 - Methyl - 5 - Thiazolecarboxylate should be stored in a cool, dry place, away from direct sunlight. Keep it in a well - ventilated area, isolated from sources of heat and ignition. Store in a tightly sealed container to prevent moisture absorption and potential reactions with air components, safeguarding its chemical integrity.