2-Methyl-4-(Trifluoromethyl)Thiazole-5-Carboxylic Acid Ethyl Ester

What Are the Critical Process Parameters for Converting the Ester into a Class of SDHI Fungicide Amide Scaffolds?

Commercial manufacture of SDHI (succinate dehydrogenase inhibitor) fungicides targeting the Basidiomycete and Ascomycete spectra has increasingly adopted 2-methyl-4-(trifluoromethyl)thiazole-5-carboxylic acid as a core pharmacophore, derived from the ethyl ester via alkaline hydrolysis. The ester is saponified in a 30% (w/w) aqueous NaOH regime within a glass-lined (DIN 28120) or Hastelloy C-22 reactor at 35–40 °C, with the dosing rate of the ester strictly limited to ≤5 mL/min per kilogram batch mass to contain the exotherm within a ±2 °C band; exceeding this window elevates the yield of the decarboxylated by-product 2-methyl-4-(trifluoromethyl)thiazole beyond 4.2% area-by-HPLC, rendering the batch non-compliant with downstream acylation purity thresholds. Following acidification to pH 2.8–3.1 with 6 N HCl at ≤10 °C, the free acid is isolated by centrifugation (RCF ≥ 400 × g) and dried under vacuum (50 °C, 10 mbar) to reduce residual moisture below 0.5% (Karl Fischer). Subsequent conversion to the acid chloride employs thionyl chloride at a molar ratio of 1.8:1 (SOCl₂:acid) in toluene containing 0.5 mol% DMF catalyst at 70–75 °C for 4 hours; off-gas scrubbing through 15% aqueous NaOH traps ensures workplace exposure limits (ACGIH TLV-TWA 1 ppm for SO₂/HCl) are not breached. Coupling with substituted anilines—typically 2-(1,3-dimethylbutyl)aniline or its fluoroalkyl variants—is executed at −5 to 0 °C in dichloromethane with 1.05 molar equivalents of the acid chloride relative to the amine, followed by a triethylamine proton scavenger equivalent to 1.2 eq. Process-scale observation: in campaigns exceeding 500 kg final amide output, nitrogen blanketing (dew point ≤ −40 °C) during acid chloride storage becomes indispensable; moisture ingress above 120 ppm leads to dimeric anhydride impurities that co-crystallise with the target amide, lowering isolated yield to 78–82% from a benchmark of 90–93%.

Formulation addition rate: The resulting active ingredient (AI) derived from the ester typically constitutes 200–250 g/L in suspension concentrate (SC) or 50–70% w/w in water-dispersible granule (WG) formulations. The molar utilisation factor of the original ethyl ester building block to the final AI, accounting for synthesis losses, is 0.82–0.88.

Downstream manufacturing process: Alkaline hydrolysis → acidification/crystallisation → vacuum drying → acid chloride formation → Schotten-Baumann-type amidation → solvent exchange into xylene → crystallisation → milling/air-jet micronisation to D₉₀ ≤ 5 µm → formulation blending with naphthalene sulfonate formaldehyde condensate dispersants (e.g., Morwet D-425) and defoamers.

Terminal product types: SDHI fungicide SC, WG, and emulsifiable concentrate (EC) co-formulated with triazole or strobilurin partners, registered for use on cereals, soybean, and specialty cucurbits.

Regulatory compliance standards: Residue tolerances are evaluated under EPA 40 CFR Part 180 (including subpart C agronomic field trial protocols) and Regulation (EC) No 396/2005 MRL frameworks. Environmental fate complies with OECD Test Guideline 307 (aerobic soil degradation) and 308 (aquatic sediment). The five-batch analysis for pre-registration must demonstrate purity ≥ 98.5% with individual unspecified impurities ≤ 0.15%, validated per SANCO/3030/99 rev.5.

Precursor for a 2-methyl-4-trifluoromethylthiazole-containing NS5A replication complex inhibitor necessitates strict control of genotoxic impurity carryover from the ethyl ester synthesis, particularly sulfonate esters arising during crystallisation. The ethyl ester is first reduced to the primary alcohol with lithium aluminium hydride (1.5 eq in THF, 0–5 °C addition, then reflux 4 h) in a multipurpose stainless steel (SS316L) reactor equipped with a condenser rated for hydrogen off-gas below 25% LEL. After Fieser work-up and distillation (b.p. 92–94 °C at 2 mbar), mesylation with methanesulfonyl chloride (1.05 eq, Et₃N 1.2 eq, DCM, −10 °C) generates the alkyl mesylate that undergoes nucleophilic displacement with a biphenylimidazole-pyrrolidine derivative in DMF containing K₂CO₃ (2.0 eq) at 50 °C. The resulting free base is treated with 1.0 eq of the ethyl ester-derived sulfonate ester in a Mitsunobu-like sequence where the addition of 0.03 eq tetrabutylammonium iodide suppresses dialkylation side-products below 0.8% by UPLC. Purification via preparative reverse-phase HPLC (C18, acetonitrile/0.1% TFA) yields the inhibitor as a di-trifluoroacetate salt. The isolated yield of this GMP step is 71–76%. Acceptance limits for residual Pd (from an earlier Suzuki step), Co, and Ni in the final API are ≤ 10 ppm, ≤ 5 ppm, and ≤ 20 ppm respectively, quantified by ICP-MS per USP ⟨232⟩. Purge factor calculations according to ICH M7 (R2) require demonstration that the ethyl ester-derived mesylate intermediate is cleared by a factor of at least 3.2 × 10⁴ relative to the API.

Formulation addition rate: The API, stoichiometrically traced back to the ethyl ester, is formulated in an immediate-release tablet core with a unit dose strength of 60 mg (free base equivalent). The ester-derived fragment accounts for approximately 18% w/w of the API molecular mass.

Downstream manufacturing process: Ester reduction → mesylation → N-alkylation → acid-base extraction → preparative HPLC → salt formation/lyophilisation → dry blending with mannitol 200 SD, croscarmellose sodium, and colloidal SiO₂ → direct compression using a 19-station Fette 2090i rotary press with compaction force of 8–12 kN → aqueous film-coating (Opadry II).

Terminal product types: Fil-coated immediate-release tablets containing 60 mg or 90 mg of the active moiety, prescribed in combination with sofosbuvir for genotype 1 HCV infection.

Regulatory compliance standards: Manufacturing environment classified as ISO 14644-1 Class 8 (Grade C) with Class 7 (Grade B) surrounding open processing steps. Cleaning validation acceptance per PIC/S PI 006-3 with MACO-based limits. Intermediate and API specifications aligned with ICH Q3A, Q3C, and the guideline on setting specifications for related substances (EMA/CHMP/CVMP/QWP/199250/2006). Stability testing per ICH Q1A(R2) in ICH climatic zones II and IVb.

Negative Dielectric Anisotropy Liquid-Crystal Monomers Incorporating the 2-Methyl-4-trifluoromethylthiazole Core

Monomer design for negative Δε liquid-crystal mixtures increasingly exploits the polarisable 2-methyl-4-(trifluoromethyl)thiazole ring as a terminal head group to enhance vertical dipole moments while preserving low rotational viscosity (γ₁). The ethyl ester serves as a building block for a lateral 2,3-difluorobenzene-tolane-thiazole architecture: the acid (liberated from the ester by the enzymatic process outlined earlier) is converted to the Weinreb amide using N,O-dimethylhydroxylamine hydrochloride (1.3 eq) and EDC·HCl (1.2 eq) in dichloromethane at 25 °C. Reaction with 3,4,5-trifluorophenylmagnesium bromide (1.1 eq, THF, −20 °C) and subsequent desoxo-fluorination with DAST (1.05 eq, DCM, −78 °C to RT) installs the difluoro olefin linker. Final Sonogashira coupling to 4-(trans-4-propylcyclohexyl)ethynylbenzene under Pd(PPh₃)₂Cl₂ (2 mol%), CuI (4 mol%), and triethylamine at 60 °C delivers the target monomer after flash chromatography and recrystallisation from ethanol/ethyl acetate (85:15) to 99.8% GC purity. Production-scale batch records for the desoxo-fluorination step indicate that water content in the DAST reagent must be maintained below 50 µg/g, otherwise exothermic decomposition leading to SF₄ off-gassing occurs, requiring a Halar®-lined reactor and scrubber.

Formulation addition rate: In final negative Δε VA-mode mixtures, the thiazole-based monomer is present at 4–8% w/w, typically in combination with a bicyclohexyl-fluorobenzene nematic host. Adding the monomer at 6% w/w increases the absolute value of Δε from −3.5 to −4.2 at 1 kHz, measured per IEC 61747-2-2.

Downstream manufacturing process: Ester enzymatic hydrolysis (immobilised Candida antarctica lipase B, Novozym 435®, pH 7.0 phosphate buffer) → Weinreb amide formation → Grignard ketone synthesis → DAST fluorination → Sonogashira coupling → hydrogenation of protecting groups over 5% Pd/C, 3 bar H₂ → triple-zone vacuum sublimation (10⁻⁶ mbar, gradient 180–220 °C) → filling under nitrogen into aluminium twin-bottle packs.

Terminal product types: Negative Δε liquid-crystal mixtures for vertical alignment (VA) and fringe-field switching (FFS) panels in ultra-high-definition television (UHD-TV) and medical-grade monitors with response time below 5 ms.

Regulatory compliance standards: Monomer purity tests follow VDI 2083 cleanroom protocols; halogen-free declaration per IEC 61249-2-21 for the final laminate. RoHS Substance Restrictions set under 2011/65/EU with Annex III exemptions monitored via IEC 62321-7-2:2017. Monomer-specific occupational exposure bands are aligned with EN 689:2018 for chronic inhalation toxicity.

For TRPV1 antagonist clinical candidates targeting chronic neuropathic pain, the ethyl ester provides the 2-methyl-4-(trifluoromethyl)thiazole head that mimics the capsaicinoid A-region motif. Process chemistry at the pilot scale (50–100 L reactor) generates the acid chloride as described, then couples it to a 3-chloro-4-aminophenyl piperazine derivative in isopropyl acetate containing aqueous K₂CO₃ (10% w/v) under Schotten-Baumann biphasic conditions at −5 to 0 °C. The molar ratio of acid chloride to aminopiperazine is fixed at 0.98:1.00 to prevent excess acylation at the piperazine N1 position; even 2% molar excess of the acid chloride results in a bis-acylated impurity that is extremely difficult to purge to the <0.10% acceptance threshold in the API. Post-acylation, the isopropyl acetate layer is washed with 0.5 N HCl to remove unreacted amine, followed by brine and water. Crystallisation from isopropanol/water (70:30 v/v) with controlled cooling ramp from 75 °C to 20 °C at 0.2 °C/min delivers Form A polymorph exclusively (DSC onset 178.5 °C, ASTM E1356-08) with a PSD D₅₀ of 35 µm. Micronisation via a jet mill (Roto-Jet 5) using nitrogen at 6 bar reduces D₉₀ to 12 µm for transdermal patch compatibility.

Formulation addition rate: In the final transdermal reservoir patch, the API—derived in a three-step sequence from the ethyl ester—is dissolved in a silicone adhesive matrix at 3.5–5.0% w/w (corresponding to 15–22 mg API per 20 cm² patch). For oral capsules under development, the drug load is 4 mg free base per size 4 hypromellose capsule, utilising a wet granulation process.

Downstream manufacturing process: Acid chloride formation → interfacial acylation → extractive work-up → crystallisation → polymorph control → delumping → micronisation → adhesive blending (for patch) or ribbon blending with lactose monohydrate 310/MCC 101 (for capsule granulation).

Terminal product types: Transdermal therapeutic system (TTS) with a 72-hour wear period and hard capsule presentations for Phase II clinical evaluation.

Regulatory compliance standards: Residual solvent Class 2 limits enforced per ICH Q3C(R8), with special attention to isopropyl acetate (≤ 5000 ppm) and dichloromethane (≤ 600 ppm). Mutagenic impurity risk assessment conducted per ICH M7(R2), using QSAR (DEREK Nexus 6.2) predictions. GMP adherence for investigational medicinal products per EU GMP Annex 13 and 21 CFR 312. Percutaneous absorption validated using human cadaver skin in Franz diffusion cells per OECD Test Guideline 428.

When Disperse Dyes for Supercritical CO₂ Textile Processing Require a Thiazole-Based Heterocyclic Diazo Component

Elimination of aqueous effluent in polyester dyeing drives adoption of supercritical carbon dioxide (scCO₂) as a transfer medium at 250–280 bar and 120–130 °C. The thiazole heterocycle, carrying the electron-withdrawing trifluoromethyl group, functions as the diazo component after ester hydrolysis and Curtius rearrangement to the 5-amino-2-methyl-4-(trifluoromethyl)thiazole intermediate. Diazotisation proceeds smoothly in 85% phosphoric acid with nitrosylsulfuric acid (1.05 eq, prepared at −5 °C), giving a diazonium solution that couples with N-ethyl-N-(2-hydroxyethyl)-m-toluidine in a coground acetic acid/ice mixture at 0–5 °C while maintaining free nitrite below 10 mg/L (detected by starch-iodide paper) to avoid nitrosoamine by-product formation exceeding the 0.5% area limit. The crude press cake is washed to neutral conductivity (<50 µS/cm) and dried in a paddle dryer (65 °C, 40 mbar) to yield a high-molar-extinction-coefficient red dye (λmax 518 nm in DMF, ε = 54,000 L·mol⁻¹·cm⁻¹). The ethyl ester contributes approximately 38% by mass of the final dye chromophore.

Formulation addition rate: The neat dye is standardised with a lignosulfonate dispersant (e.g., Reax® 85A) and dedusting oil to a strength of 100% Type I (CI method); the actual thiazole-based dye content in the formulated granulate is 42–48% w/w. The injection ratio into the scCO₂ dyeing vessel is 0.8–1.2% on weight of polyester fabric.

Downstream manufacturing process: Alkaline ester hydrolysis → acidification → Curtius rearrangement with diphenylphosphoryl azide (1.1 eq, toluene, 90 °C) to the amine → diazotisation in mixed acid → coupling → filtration → counter-current washing → drying → standardisation → spray-drying into dust-free granules (particle size 200–400 µm).

Terminal product types: High-strength azo disperse dyes dedicated to supercritical CO₂ polyester coloration, resulting in tricot, warp-knit, and woven goods intended for sportswear and automotive interior upholstery.

Regulatory compliance standards: Dye chemistry adheres to the ZDHC Manufacturing Restricted Substances List (MRSL) v3.0, particularly regarding prohibited arylamines (detection limit 20 mg/kg per EN 14362-1:2017). Extractable heavy-metal content must satisfy OEKO-TEX® Standard 100 Annex 4 limits for infants (chromium VI ≤ 0.5 mg/kg, cadmium ≤ 0.1 mg/kg). Biodegradation screening per OECD 302B (Zahn-Wellens test) ensures > 70% DOC removal over 28 days.

In veterinary coccidiostat repositioning programs, the 2-methyl-4-(trifluoromethyl)thiazole nucleus is employed as a bioisostere for the benzamide moiety in earlier-generation triazine anticoccidials. The ethyl ester is transformed into the corresponding hydrazide by treatment with hydrazine monohydrate (1.5 eq) in ethanol at reflux (12 h), giving a hydrazide intermediate with a melting point of 161–163 °C. Condensation with cyanamide (50% aq., 1.2 eq) in 2 N HCl at 95 °C followed by cyclisation into the triazole ring generates the 2-methyl-4-(trifluoromethyl)thiazole-5-triazole hybrid. Pilot batch notes require that the hydrazide formation step be executed in nitrogen-inerted vessels to suppress oxidative dimerisation to the diacylhydrazide, which otherwise precipitates as a yellow solid and reduces the yield to ≤34%. The crude triazole product is purified via hot filtration through 0.5 µm PTFE membrane, then recrystallised from acetonitrile/water to 99.3% w/w purity.

Formulation addition rate: The resulting thiazole-triazole active, traceable to the ethyl ester via the hydrazide, is incorporated into pelleted feed premixes at 0.5–1.0% w/w (equivalent to 50–100 g/tonne finished feed) or into a 5% oral solution for drench application.

Downstream manufacturing process: Ester hydrazinolysis → azine formation → triazole ring closure → charcoal decolourisation → crystallisation → fluid-bed drying (inlet air 70 °C, residual moisture <0.3%) → micro-dosing into a mineral-carrier premix.

Terminal product types: Medicated feed premix, oral drench solution, and sustained-release intraruminal bolus devices delivering a continuous 6-month prophylactic dose against Eimeria spp. in lambs and calves.

Regulatory compliance standards: Premix homogeneity testing per USP General Chapter ⟨905⟩ (acceptance value ≤ 15.0). Maximum residue limits for edible tissues are established under VICH GL 49 (EMA/CVMP/VICH/463202/2009) with the marker residue defined as the parent triazole-thiazole quantified by LC-MS/MS. Premix stability evaluated in accelerated conditions at 40 °C/75% RH for 6 months per VICH GL 18R. Feed additive registration dossier complies with Regulation (EC) No 1831/2003, Article 7 data requirements, and the technical additive category.