Ethyl-2-(4-Hydroxyphenyl)-4-Methyl-5-Thiazolecarboxylate

Ethyl-2-(4-hydroxyphenyl)-4-methyl-5-thiazolecarboxylate (CAS registry 185964-28-1, molecular formula C₁₃H₁₃NO₃S, molecular weight 263.31 g·mol⁻¹) is supplied as a crystalline solid typically exceeding 98.0% purity by HPLC area normalization. The compound functions as a modular aryl-thiazole building block whose free phenolic hydroxyl group and ethyl ester handle enable orthogonal derivatization without recourse to transition-metal-catalyzed coupling of the 4-position. Numerous pharmacologically active templates—particularly xanthine oxidase inhibitors, PPARγ modulators, and selective kinase probes—rely on the 2-(4-oxyphenyl)-4-methylthiazole-5-carboxylate scaffold, originally constructed through Hantzsch-type condensation between 4-hydroxybenzothioamide and ethyl 2-chloroacetoacetate. The product is offered in research (milligram to kilogram) and pilot-scale (25 kg net in HDPE drums with double food-grade LDPE liners) quantities under quality management systems aligned with ISO 9001:2015 Clause 8.2.

How Does the 4-Hydroxyphenyl Substituent Influence Synthetic Utility?

The hydroxyl group imparts an ambident nucleophilic character that is absent in the corresponding 4-bromo, 4-chloro, or 4-methoxy analogs. Direct O-alkylation proceeds under mild conditions (K₂CO₃ or Cs₂CO₃, acetonitrile, 80 °C) with alkyl halides or sulfonates, providing ether intermediates without the need for palladium-based Buchwald–Hartwig hardware or aryl boronic acid stocks. This single-step derivatization reduces the process mass intensity of a typical three-step sequence by approximately 35–40% when benchmarked against the 4-bromophenyl route, according to process development reports using Corning Advanced-Flow reactors. Mitsunobu etherification with primary or secondary alcohols further expands the accessible functional diversity; the pKₐ of the phenol is estimated at 8.9 ± 0.3 in aqueous DMSO, permitting selective deprotonation in the presence of aliphatic alcohols. Beyond ethers, the phenolic –OH serves as a directing group for regioselective electrophilic aromatic substitution, although nitration or sulfonation must contend with the electron-deficient thiazole ring, which may undergo ring-opening if exposed to mixed acid media at temperatures above 50 °C. Specification profiles vary modestly between vendors, yet industrial customers routinely request the following cumulative data package, reflecting ICH Q6A decision-tree expectations for new chemical entities destined for GLP toxicology batches.

Representative certificate-of-analysis parameters for a GMP-intermediate grade

Attribute	Method (Standard Reference)	Acceptance Criterion
Assay (anhydrous, solvent-free)	HPLC-DAD, C18 column, 250 × 4.6 mm, 5 µm	98.0–102.0%
Melting range	Differential scanning calorimetry, 10 °C·min⁻¹	176–180 °C (onset, endothermic peak)
Water content	Karl Fischer coulometry, USP <921> Method Ia	≤ 0.5% w/w
Residual solvents	GC-FID headspace, USP <467> Procedure A	Ethanol ≤ 5000 ppm, ethyl acetate ≤ 5000 ppm; Class 1 solvents not detected at reporting threshold 10 ppm
Sulfated ash	USP <281>	≤ 0.1%
Heavy metals	ICP-MS, USP <232>/<233>	Pb ≤ 10 ppm, Cd ≤ 5 ppm, As ≤ 2.5 ppm, Hg ≤ 1.5 ppm
Chromatographic purity	HPLC, area %	Single unknown impurity ≤ 0.5%, total impurities ≤ 1.0%

Fine-particle agglomerates generated during vacuum drying at ≤ 10 mbar and 55 °C exhibit a mean particle size (D₅₀) near 45 µm when milled through a conical screen mill with round-hole screen 0.5 mm. This particle-size distribution is critical for dissolution rate in downstream amidations where the ester is first converted to the carboxylic acid lithium salt.

When the Ethyl Ester Hydrolyzes Prematurely in Aqueous Alkaline Media

A recurring batch failure mode on pilot-plant scale involves unintended ester saponification during aqueous workup of O-alkylation reactions when the pH transiently exceeds 11.5. The ethyl ester displays a hydrolysis half-life of approximately 4 h at 25 °C in 0.5 M sodium hydroxide in dioxane/water (1:1), whereas the corresponding methyl ester hydrolyzes roughly 2.3× faster under identical conditions—a rate difference that renders the ethyl congener more process-tolerant in multi-ton campaigns. To maintain ester integrity, in-process Fourier-transform infrared spectroscopy (ReactIR) monitoring of the carbonyl stretch at 1712 cm⁻¹ is implemented, and quench protocols limit the aqueous phase to pH 9.5 during toluene extractions. Additionally, trace lithium chloride residues from Hantzsch cyclization (often carried over at 200–800 ppm) accelerate hydrolysis via Lewis acid activation of the ester carbonyl; workup with 5% aqueous citric acid followed by 2% sodium bicarbonate scrubbing reduces residual lithium to below 30 ppm. Where does this compound diverge most sharply from structurally related aryl thiazole esters? The table below captures key differentiation metrics across five analogues that are frequently cross-shopped by medicinal chemistry sourcing groups.

Differentiation matrix for ethyl-2-(4-hydroxyphenyl)-4-methyl-5-thiazolecarboxylate and common substitutes

Parameter	Target Compound	4-Bromo Analogue	4-Methoxy Analogue	Methyl Ester (4-HO-Ph)	tert-Butyl Ester (4-HO-Ph)
Phenol available without deprotection?	Yes	No	No (demethylation requires BBr₃ at −78 °C)	Yes	Yes
Typical ester hydrolysis t₁/₂ (pH 10, 25 °C)	8–12 h	Not applicable	7–10 h	3–4 h	>48 h (steric shielding)
Direct O-functionalization window	Broad (Mitsunobu, Williamson, Mannich)	No O-site (requires C–N or C–C coupling)	None—masked; unmasking destroys sensitive scaffolds	Identical to target	Identical, but ester bulk retards concurrent saponification
Supply chain lead time (pilot, 10 kg)	6–8 weeks	4–5 weeks	8–10 weeks	6–8 weeks	10–12 weeks
Residual Pd risk (ICH Q3D)	Not detected (no Pd step)	≤ 10 ppm	Not detected	Not detected	Not detected

In practice, the ethyl ester is frequently preferred over the tert-butyl analogue despite the latter’s superior hydrolytic stability, because the ethyl group eliminates as ethanol during final amidation with ammonia or primary amines at 80–100 °C in sealed vessels, avoiding isobutylene generation that accompanies tert-butyl deprotection. Process safety evaluations using accelerating rate calorimetry (ARC) on reaction masses with 28% aqueous ammonia reveal an exotherm onset at 112 °C with a maximum self-heat rate of 3.5 °C·min⁻¹; vent sizing is performed per DIERS methodology assuming a tempered hybrid system. Direct engagement of the hydroxyphenyl group in prodrug design is another vector where this intermediate performs differently from its alkyl- or halogen-protected counterparts. Phosphate ester prodrugs (formed with POCl₃ followed by hydrolysis to phosphoric acid monoester) use the free phenol as the attachment point without preliminary diaryl ether construction. A report filed under the US FDA’s Type II DMF indicates that the bis-phosphate sodium salt of the des-ethyl carboxylate demonstrates aqueous solubility above 15 mg·mL⁻¹ at pH 7.4, a > 500-fold improvement over the parent acid, enabling intravenous formulation at therapeutic doses.

Process-Scale Grignard Compatibility and Protecting Group Strategy

Although the phenolic proton is sufficiently acidic to quench one equivalent of Grignard reagent, process chemists routinely bypass full protection by generating a magnesium phenolate in situ. Charging ethyl-2-(4-hydroxyphenyl)-4-methyl-5-thiazolecarboxylate into a tetrahydrofuran slurry at −5 to 0 °C and adding exactly 1.05 equivalents of tert-butylmagnesium chloride leads to selective phenolate formation with minimal ester attack, as confirmed by real-time ReactIR monitoring. The resulting suspension of the chloromagnesium phenolate can then be treated with electrophiles—alkyl chloroformates or sulfonyl chlorides—without isolating the free phenol. The approach cuts one protection/deprotection sequence and routinely achieves >90% isolated yields after aqueous acidic quench. Extended post-reaction hold times exceeding 10 h at ambient temperature, however, lead to gradual metal-catalyzed ester aminolysis if trace primary amine contaminants (from thioamide degradation) are present at levels above 0.05% w/w. Multiple contract manufacturing organizations (CMOs) have validated this substrate on equipment ranging from 200 L glass-lined reactors to 2000 L Hastelloy C-22 vessels. Data from a 1600 L campaign indicate that batch-to-batch crystal habit consistency—fine needles versus compact prisms—is tightly coupled to the cooling rate during the final ethanol/water recrystallization step. A controlled linear cooling profile of 0.15 °C·min⁻¹ between 60 °C and 10 °C yields a prismatic product with a tap density of 0.55–0.62 g·mL⁻¹, whereas shock cooling in ice baths creates needle agglomerates with tap densities below 0.30 g·mL⁻¹, causing significant conveying and powder-flow difficulties during tableting of the final drug substance. What distinguishes the regulatory starting material designation for this compound compared to earlier or later intermediates in a typical febuxostat-type route? The thiazole ring is fully elaborated, the phenolic oxygen is available as a handle, and the ester is still in place—an ideal stage for the introduction of the so-called “side wings” through sequential O-alkylation and amidation. Several Type II DMF holders define the regulatory starting material as the 5-carboxylic acid ethyl ester containing the intact 4-hydroxyphenyl motif, exactly matching this compound’s structure. Consequently, cGMP requirements attach at this step, and the material is manufactured under ICH Q7 with full batch records, cleaning validation, and retained samples stored for one year beyond the expiry date. The compound shows measurable photodegradation when exposed to UVA radiation (320–400 nm) in dilute acetonitrile solution, with pseudo-first-order rate constant 0.012 h⁻¹ under 765 W·m⁻² irradiance. For this reason, all process intermediates and isolated solids are held in amber glass or opaque HDPE containers purged with nitrogen to less than 5% oxygen. Long-term stability chambers set at 25 °C/60% RH (ICH Zone II) show 99.2% purity retention at 36 months for the unformulated solid, provided packaging integrity is maintained and desiccant bags are included. Packaging configurations using non-lacquered aluminum foil fail after 18 months due to darkening and a 0.7% increase in a diastereomeric sulfoxide impurity traced to headspace oxygen ingress. Thus, polyethylene-aluminum-polyethylene trilaminate pouches are specified in bulk shipment protocols.

One Alternative Route That Eliminates Chromatography and Cuts Solvent Volume by Half

Conventional purification of ester intermediates downstream of the Hantzsch condensation often employs flash silica-gel chromatography with dichloromethane/methanol gradients, consuming up to 35 L of solvent per kilogram of product. A telescoped process developed at a European fine-chemical manufacturer bypasses the column entirely: after condensation, the crude ethyl-2-(4-hydroxyphenyl)-4-methyl-5-thiazolecarboxylate is dissolved in hot isopropanol, treated with activated carbon (Darco KB-G, 5 wt%) at 70 °C, filtered through a 0.45 µm inline cartridge, and crystallized by controlled addition of deionized water. This direct isolation delivers 96–98% purity in 85% recovery with residual thioamide below 0.2%. The technology transfer report noted that the single-largest deviation occurred when operators substituted a centrifuge basket with pore size 10 µm for the recommended 2 µm filter cloth, allowing carbon fines to pass through and raising sulfated ash to 0.25%. The corrective action permanently locked the cloth specification in the master manufacturing record. The reduction in total solvent usage per batch—from 42 L·kg⁻¹ to 18 L·kg⁻¹—is consistent with the pharmaceutical industry’s current trajectory toward achieving Process Mass Intensity below 20 kg·kg⁻¹ for small-molecule APIs.