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

Enzymatic Inhibition Profiles Reshaped via the 4-Methylthiazole-5-Carboxylate Core

In the campaign-scale manufacture of non-purine xanthine oxidase inhibitors destined for oral solid dosage forms at 40 mg and 80 mg label strengths, the ethyl 2-(4-hydroxyphenyl)-4-methylthiazole-5-carboxylate scaffold serves as the direct precursor to the free carboxylic acid intermediate upon controlled alkaline hydrolysis. A 2.0–2.5 M sodium hydroxide solution in ethanol/water (85:15 v/v) is charged into a glass-lined, half-pipe coil reactor with a jacket temperature fixed at 48±2 °C, and the ester is introduced under nitrogen sweep at a controlled rate of 1.2 kg/min per 3500 L batch to maintain a reaction mass temperature that does not overshoot 52 °C. The molar ratio of hydroxide to ester is maintained at 2.18:1; deviations beyond 2.30:1 trigger measurable ring-opening of the thiazole, identified as an undesired mercaptoacrylonitrile-related impurity detectable by UPLC–MS at retention time 1.37 min relative to the main peak. After 4.5 h, in-process HPLC (C18 column, acetonitrile/0.1% phosphoric acid) confirms residual ester ≤0.8 area%, and the batch is cooled to 8 °C before pH adjustment to 2.0 with 6 N HCl. The precipitated 2-(4-hydroxyphenyl)-4-methylthiazole-5-carboxylic acid is isolated on a 0.5 m² polypropylene filter press, washed with chilled deionized water until washings show conductivity ≤28 μS/cm, and dried in an agitated vacuum pan drier at 65 °C, −0.095 MPa for 10 h. The isolated acid—typically obtained at 94–96% yield with loss-on-drying 0.22%—is immediately reacted in a subsequent step with hexamethylenetetramine in trifluoroacetic acid to install the formyl group ortho to the phenolic hydroxyl, a transformation that demands the ethyl ester be removed beforehand because residual ester functions as a competing electrophile and reduces formylation regioselectivity below 80%. All vessels downstream of the hydrolysis stage are constructed of Hastelloy C-276 to resist chloride-induced pitting, and process analytical technology including ReactIR 15 probe monitoring tracks the disappearance of the ester carbonyl stretch at 1714 cm⁻¹. Quality specifications for the ethyl ester entering this route are aligned with ICH Q7 and WHO GMP guidelines for active pharmaceutical ingredient starting materials: assay by HPLC (230 nm) ≥99.5 area%, single unspecified impurity ≤0.10%, total impurities ≤0.5%, residual ethanol ≤420 ppm (ICH Q3C limit), palladium ≤3 ppm by ICP-MS, and water content ≤0.25% by Karl Fischer titration. Failure to pre-dry the feedstock at RH > 55% storage conditions leads to partial ester cleavage in the drum and raises the free acid content above 0.7%, a drift that shifts the hydrolysis stoichiometry and yields an acidic batch that requires re-crystallization of the downstream acid from 2:1 ethanol/water with a 12% mass loss. The ultimate therapeutic compound—a regulated product prescribed under the ATC code M04AA—is formulated with lactose monohydrate, microcrystalline cellulose, and croscarmellose sodium, and its dissolution profile is governed by USP monograph test 711 using Apparatus II at 75 rpm in 900 mL of pH 6.8 phosphate buffer. Thermal cycling stability studies per ICH Q1A(R2) at 40°C/75% RH for 6 months show the ethyl ester-attributed process impurity at RRT 0.91 does not exceed 0.15% when the intermediate complies with the above purity boundaries.

In high-solubility disperse dye formulations designed for polyester resource-efficient exhaust dyeing at 130 °C without carriers, the coupling of the title compound—functioning as a pyrazolone-free yellow coupler—with diazotised 2,4-dinitroaniline proceeds at 0–5 °C under strictly maintained pH 4.5–5.0 using a sodium acetate buffer prepared with 4.0 kg of sodium acetate trihydrate per 100 L of reaction liquor. The coupling vessel is an AISI 316L jacketed reactor fitted with a retreat-curve impeller turning at 95 rpm; brine at −8 °C is circulated through the jacket to absorb the exotherm generated when the diazonium salt solution—pumped from a pre-chilled, PTFE-lined header tank—contacts the coupler suspension. At a molar ratio of diazonium salt to coupler of 1.03:1, the reaction achieves 98.2% conversion within 90 min when the ionic strength is held below 0.15 M using a conductivity probe that triggers an automated DI water addition loop. A critical processing boundary exists at pH 3.8: below this value the phenolic hydroxyl remains substantially protonated, the coupling rate drops below 40% of the design half-life, and the competitive formation of bis-coupled byproducts increases above 1.8 area% detectable by HPLC at 420 nm. The filtered cake, washed with 0.01 N acetic acid until chloride negative, is dried in a double-cone rotary vacuum dryer at 78 °C, −0.092 MPa, after which the crude dye powder is subjected to particle size reduction in a horizontal bead mill charged with 0.6–0.8 mm yttria-stabilized zirconia beads at 80% loading. Milling residence time is controlled to achieve a D90 particle size of 1.0–1.5 µm as determined by laser diffraction (Malvern Mastersizer 3000 with wet dispersion in deionized water containing 0.5 g/L sodium hexametaphosphate). The dispersing agent is a high-purity sodium lignosulfonate dosed at 35% w/w relative to the dry dye weight; batch-to-batch variability in lignosulfonate molecular weight distribution has been shown to shift the dispersion stability index (assessed by Turbiscan LAb Expert) by up to 15%, requiring tight supplier qualification per ISO 21305-2. The finished yellow liquid or spray-dried powder gains a commercial strength of 200% or 400% and a shade deviation ΔE ≤0.55 against the master standard when measured on polyester woven twill under D65/10° conditions. Fastness properties determined under ISO 105-C06:2010 domestic/commercial laundering conditions (procedure A2S, 40 °C) deliver a shade change rating of 4–5 and staining of undyed polyester of 4, while light fastness assessed per ISO 105-B02:2014 with a Xenotest 150 S+ at a black-panel temperature of 45 °C achieves a Blue Wool rating of 6–7 at the 1/1 standard depth. Sublimation fastness per ISO 105-P01 at 180 °C for 30 s remains above 4 because the ester substituent does not contribute to thermomigration under polyamide-free exposure conditions. The dye intermediate complies with the chemical restrictions of the OEKO-TEX Standard 100 Annex 4 limit value catalogue, with free o-anisidine and p-chloroaniline levels each below 18 mg/kg when analysed by HPLC–DAD after reductive cleavage per EN ISO 14362-1:2017.

When the 4-Hydroxyphenyl Moieties Must Remain Unprotected During Glycidyl Methacrylate Addition

High-refractive-index UV-curable monomers for optical fibre primary and secondary coatings require a balance of aromatic content sufficient to achieve a refractive index of 1.540–1.560 at 589 nm and a gel-phase mobility that prevents microbending loss during −40 °C thermal cycling. The ethyl ester is subjected to a one-pot glycidyl methacrylate (GMA) opening without prior silane protection of the phenol: a stoichiometric run of 1.0 mol of the thiazole ethyl ester and 1.03 mol of GMA (purity ≥97.5%, epoxide equivalent 142–143 g/eq) is heated to 88 °C in the presence of triphenylphosphine catalyst at 0.32 mol% and 4-methoxyphenol as an in-situ inhibitor at 210 ppm relative to the combined monomer mass. The reaction exotherm—monitored by a cascade controller that modulates jacket temperature to keep the bulk at 92±1.5 °C—completes within 7.5–8.5 h, signalled by the epoxide IR band at 912 cm⁻¹ diminishing to < 0.2% of the initial absorbance. A critical failure scenario arises when the jacket response lags beyond +3 °C during the first 40 min; thermal overshoot accelerates the etherification of the secondary hydroxyl generated at the epoxy ring by unreacted phenol, producing oligomeric dimers and trimers that elevate the dynamic viscosity at 25 °C from a specification window of 1.3–1.9 Pa·s to above 3.0 Pa·s. Continuous nitrogen sparging at 0.15 vvm strips dissolved oxygen and stabilises colour to an APHA value ≤80. The product—ethyl 2-(4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)-4-methylthiazole-5-carboxylate—is isolated as an amber liquid after filtration through a 1 µm absolute polypropylene depth filter and vacuum devolatilisation at 85 °C, 0.5 kPa for 45 min to remove residual GMA below 25 ppm. In primary coating formulations for bend-insensitive single-mode fibre compliant with ITU-T G.657, the monomer is combined with aliphatic urethane acrylate oligomers at a weight ratio of 25–35 phr, a photoinitiator package of 2,4,6-trimethylbenzoyldiphenylphosphine oxide at 2.2 phr, and a γ-mercaptopropyltrimethoxysilane adhesion promoter at 0.8 phr. Draw-tower curing under a 395 nm LED array delivering 1.8 J/cm² yields a cured film with a storage modulus at 25 °C (DMTA, 1 Hz, ASTM D 4065) of 8–12 MPa, elongation-at-break of 48–52% (ASTM D 638, dry, gauge length 25.4 mm), and water absorption of 1.4% after 24 h immersion at 23 °C. Outgassing measured per ASTM E 595 yields a total mass loss of 0.78% and collected volatile condensable material of 0.09%, a boundary value that restricts the monomer content above 30 phr when the coating is specified for ultra-low-attenuation submarine fibres. Registration for export under REACH mandates a robust skin sensitisation hazard assessment; the intermediate triggered a stimulation index of 1.7 in the local lymph node assay (OECD TG 442B), placing it outside the CLP classification for skin sensitiser but requiring nitrile glove protection with a permeation breakthrough time exceeding 240 min measured per EN 16523-1. The downstream finished fibre coating product must also comply with the heavy metal extraction limits of EN 71-3 at Category III, where barium, antimony, and arsenic each remain below 60 mg/kg, 45 mg/kg, and 25 mg/kg respectively when tested on a coating specimen cured to 97% conversion verified by FTIR acrylate peak area ratio.

Why Does the Free Phenol Compete with Melt-Phase Transesterification When Producing High-Molecular-Weight Stabilisers?

Industrial synthesis of high-spin-pin tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]pentaerythritol type macromolecular antioxidants frequently employs the thiazole ethyl ester as a template to introduce an auxiliary heterocyclic ring that retards gas-fade yellowing in polypropylene fibre-grade resins. The ester is first hydrolysed to the corresponding acid as detailed in the pharmaceutical pathway; thereafter, the phenolic hydroxyl is treated with isobutylene in a pressurised alkylation reactor at 90–95 °C and 0.45–0.55 MPa using a fixed-bed Amberlyst 15 dry ion-exchange resin catalyst with an acid capacity of 4.6 meq/g. The tert-butylation proceeds selectively at the ortho positions to the –OH group to yield 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylic acid, provided the feed moisture content is held below 320 ppm; water ingress above this level strips the acid site and favours oligomerisation of isobutylene, depositing polyisobutylene gums on the catalyst bed that cause backpressure to rise beyond 150 kPa within 12 h of continuous operation. After catalyst separation and distillation of unreacted olefin, the hindered phenolic acid is esterified with pentaerythritol using a 1.15:1 molar stoichiometry of acid-to-hydroxyl in a high-temperature screw reactor with L/D ratio 48 and segmented barrel zones set at 200 °C, 215 °C, and 190 °C from feed to die, operating at a specific mechanical energy input of 0.22 kWh/kg. In this configuration, any residual unalkylated mono-phenolic acid stemming from incomplete conversion of the ethyl ester is esterified onto the pentaerythritol backbone, creating asymmetrically substituted analogues that reduce crystallinity of the antioxidant and promote migration to the polymer surface. Migration testing executed per EU Regulation 10/2011 on a 50 µm cast polypropylene film containing 0.12 wt% of the fully formulated stabiliser yields a specific migration limit (SML) for the sum of phenolic methylester species of 0.047 mg/kg when using food simulant D1 (ethanol 50%) for 10 days at 40 °C, well within the generic SML of 0.05 mg/kg. The overall migration in simulant D2 (vegetable oil) remains under 2.8 mg/dm², as determined gravimetrically per EN 1186-2. The ethyl ester therefore must be delivered with a free phenol titrimetric value (expressed as p-cresol equivalents) not exceeding 0.8 meq/kg so that the downstream dialkylation step is not perturbed by unproductive acid-base consumption. A processing window of ±3 °C around the glass-transition melt-blend zone of the twin-screw—established through DSC isothermal osidometric scans at 190 °C under oxygen—prevents premature radical generation from thiazole ring cleavage, which otherwise initiates chain scission in the host polypropylene, evidenced by a melt flow index (ISO 1133-1:2022, 2.16 kg at 230 °C) increase from 2.5 g/10 min to 4.1 g/10 min within the first compound pass.

Transforming the ethyl ester into the corresponding hydrazide and subsequent condensation with 3,5-dichlorobenzaldehyde under acidic ethanol reflux generates a crystalline thiazole hydrazone with curative activity against Botrytis cinerea in greenhouse tomatoes and Uncinula necator in vineyards. The hydrazide formation is carried out by charging 1.0 kg of the ester and 0.98 kg of hydrazine hydrate (80% technical grade) into 4.5 L of n-propanol held at 80 °C for 6.5 h in a Hastelloy C-22 reactor equipped with a variable-frequency magnetic drive agitator. The target hydrazide precipitates on cooling to 3 °C, is filtered, and is washed with ice-cold isopropanol, then dried to a melting range of 182–185 °C. The critical quality attribute at this stage is the residual ester content that does not participate in hydrazinolysis: levels exceeding 2.6 area% result in competing side-products during the aldehyde coupling that are difficult to purge because their solubility profile mimics the product. The condensation with 3,5-dichlorobenzaldehyde proceeds at 1:1.03 aldehyde-to-hydrazide molar proportion in ethanol with 0.5 vol% glacial acetic acid catalyst, stirred at 72 °C for 9 h, after which the thick slurry is cooled to 18 °C and filtered. The wet cake—washed with denatured ethanol and dried at 62 °C, −0.090 MPa—yields a technical-grade fungicide of 95.8% purity (HPLC area, 254 nm) with a loss-on-drying below 1.0%. Milling together with kaolin carrier at a 1:4 weight ratio and sodium alkylnaphthalenesulfonate wetting agent at 2.5 wt% in an air jet mill operated at 0.7 MPa grinding pressure produces a water-dispersible granule formulation with a suspensibility of 92% when tested by CIPAC method MT 15.1 after 30 min in CIPAC standard hard water. A thin-layer chromatography fingerprint per CIPAC method 1A distinguishes the active from synthetic by-products, and storage stability testing at 54 °C for 14 days shows degradation below 1.7%. The plant-scale bottleneck experienced during campaign changeovers is the formation of a crust of hydrazone on the reactor wall above the liquid line when the ethanol vapour does not adequately wet the surface; a nitrogen line retrofitted at 0.05 MPa to induce a gentle reflux drip has been shown to reduce wall fouling to < 0.2 kg per batch. The technical material conforms to the FAO specification on pest control agent identity and impurity of carcinogenic concerns—hydrazine content is verified to ≤12 ppm by derivatisation–LC/MS against the limit prescribed under CLP Regulation Annex VI. In end-use trials on Vitis vinifera at a spray concentration of 250 g a.i./ha applied with a backpack mist blower, a disease control efficacy of 78–83% was recorded against powdery mildew, although published data for this specific configuration is limited to two growing cycles and should not be extrapolated without regional pathogen pressure profiling.