Ethyl 2-Bromo-4-Methyl-1,3-Thiazole-5-Carboxylate

Starting Point for Type I MET Kinase Inhibitors — Exhaustive Process-Scale Protocol

In the synthesis of small-molecule ATP-competitive MET kinase inhibitors, the thiazole-5-carboxylate scaffold serves as a urea or amide bioisostere, with the 2-bromo substituent enabling late-stage diversification. Fabrication of the core intermediate 2-bromo-4-methylthiazole-5-carboxylic acid ethyl ester at scale proceeds via Hantzsch cyclocondensation of ethyl 2-chloroacetoacetate with thiourea, followed by diazotization-bromination in aqueous HBr (48% w/w) using NaNO₂ at –5 °C to 0 °C; the exothermic N₂ evolution requires a jacketed glass-lined reactor (Pfaudler, DN600–1000) equipped with a back-pressure regulator and scrubbed vent. Isolation of the product via toluene extraction (2 × 1.5 L per mole of substrate), drying over anhydrous MgSO₄ (10% w/w relative to theoretical yield), and fractional vacuum distillation (2.5–3.0 mbar, boiling range 108–114 °C) yields a pale-yellow oil with purity ≥98.5% by GC-FID (Agilent DB-5 column, 30 m × 0.25 mm, 0.25 µm film). Industry-wide compliance for a penultimate intermediate destined for an oral oncolytic (e.g., capmatinib dihydrochloride monohydrate) mandates adherence to ICH Q7 GMP for active pharmaceutical ingredients, ICH Q3C(R8) for residual solvents (toluene ≤890 ppm, 1,2-dichloroethane ≤5 ppm), and ICH Q3D(R2) elemental impurity limits — palladium ≤10 µg/g, copper ≤300 µg/g — unless pre-validated salt removal protocols are applied.

Typical Pd-mediated cross-coupling of this bromide with an arylboronic acid pinacol ester to install the pendant benzene ring employs 1.05–1.15 equiv of the thiazole intermediate in a degassed toluene/ethanol/water (4:1:1 v/v/v) ternary system under nitrogen blanket, with Pd(PPh₃)₄ loading at 1.5–2.0 mol% and K₂CO₃ at 2.0 equiv acting as base. Process development studies in a HEL AutoMATE parallel reactor have identified that a temperature ramp from 68 °C to 78 °C over 6–8 h suppresses protodebromination while maintaining conversion > 97% (IPC by HPLC, Waters XBridge C18, 0.1% TFA in water/acetonitrile gradient). Upon completion, the reaction mass is filtered through a Celite pad (Sartorius depth filter, 0.45 µm) to remove Pd black, washed with 10% w/v aqueous Na₂EDTA (pH 7.2) to chelate residual Pd²⁺, and concentrated on a Buchi Rotavapor R-300 at 40 °C/20 mbar. The crude coupling product is purified by flash chromatography on a Teledyne Isco Combiflash system (330 g RediSep Gold silica column, gradient ethyl acetate in heptane from 5% to 40%) delivering the isolated 2-aryl-4-methylthiazole-5-carboxylate ethyl ester in 82–88% yield with HPLC area% > 99.0% at 254 nm. The terminal finished drug substance — a potent MET inhibitor — is formulated as immediate-release tablets containing 150–400 mg of the free base equivalent, manufactured according to FDA 21 CFR Part 211 under full cGMP and released per USP monograph specifications for content uniformity (USP <905>) and dissolution (USP <711> Apparatus 2, 900 mL 0.01 M HCl, 50 rpm).

When the 2-bromo substituent is deliberately retained for a subsequent ortho-directed C–H activation, the process profile shifts significantly because the thiazole ring’s electron-deficient character retards oxidative addition unless a ferrocenylphosphine ligand is employed. Under those conditions, the reaction mixture is maintained anhydrous (KF titration ≤50 ppm H₂O) and the bromide is used in slight deficit (0.95 equiv) relative to the arene coupling partner to minimise homocoupling byproducts. Incompatibility arises with amine bases stronger than pKₐ 10.5; triethylamine causes premature quaternisation of the palladium intermediate, reducing turnover number below 200. In all protocols, storage of the neat bromide at –20 °C under argon in Type III amber glass bottles (Schott Duran) with PTFE-lined caps is required; exposure to ambient light and relative humidity above 60% leads to discernible ester hydrolysis within 72 h, forming the free carboxylic acid as an inactive coupling partner.

Where does the ester moiety offer an advantage over the acid in solid-phase peptide synthesis? In an emerging drug-linker conjugate platform, Ethyl 2-bromo-4-methyl-1,3-thiazole-5-carboxylate is directly converted into a hydroxamic acid via treatment with hydroxylamine hydrochloride and potassium hydroxide in methanol at 0–25 °C, bypassing acid-labile protection schemes. The resulting zinc-binding warhead is incorporated into a peptide backbone on 2-chlorotrityl chloride resin (loading 0.8–1.2 mmol/g) using Fmoc-solid-phase chemistry (Liberty Blue automated microwave peptide synthesizer, CEM Corporation, coupling temperature 50 °C, HBTU/DIEA activation). Cleavage with trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5 v/v/v) and preparative HPLC purification (Phenomenex Luna C18, 10 µm, acetonitrile/0.1% TFA gradient) yields the > 95% pure peptide hydroxamic acid that acts as a selective HDAC6 inhibitor candidate. The material is lyophilized in an SP VirTis Advantage Plus freeze dryer and formulated as a lyophilised powder for reconstitution in sterile phosphate-buffered saline, tested per USP <61> and <62> for bioburden and objectionable microorganisms.

Elaboration of 2-Thiazoleacrylonitrile Building Blocks for Succinate Dehydrogenase Inhibitor (SDHI) Fungicides

Agrochemical process chemistry relies on this bromothiazole ester as a precursor to 2-cyano-3-(4-methylthiazol-5-yl)acrylate intermediates that form the pharmacophore of modern SDHI fungicides (IRAC Code FRAC 7). The cyanoacrylate is generated via Knoevenagel condensation of the aldehyde obtained by sequential DIBAL-H reduction of the ester at –78 °C (THF, 1.1 equiv DIBAL-H, quench with sat. NH₄Cl) and Dess-Martin periodinane oxidation (1.2 equiv, CH₂Cl₂, 0 °C to room temperature) to give 2-bromo-4-methylthiazole-5-carboxaldehyde in 74% yield over two steps. The aldehyde is condensed with ethyl cyanoacetate in toluene in the presence of ammonium acetate and glacial acetic acid (5 mol%) under Dean-Stark reflux (110 °C, 6 h), removing water azeotropically; the product, (E)-ethyl 2-cyano-3-(2-bromo-4-methylthiazol-5-yl)acrylate, is crystallised from isopropanol/water (3:1) to provide a single geometric isomer (GC purity 99.2%).

The final coupling with a substituted aniline in a Buchwald-Hartwig amination installs the amide tail necessary for binding to ubiquinone site Qp of complex II. Palladium acetate (2 mol%) and BINAP (2.4 mol%) catalyze the C–N bond formation in refluxing 1,4-dioxane with Cs₂CO₃ (1.4 equiv); the crude active ingredient is recrystallised from acetonitrile to meet FAO specifications for technical material (purity ≥95% m/m). Formulated as a suspension concentrate (SC) containing 200 g/L active ingredient, ethylene oxide-propylene oxide block copolymer emulsifier (40 g/L), xanthan gum thickener (2 g/L), and 1,2-benzisothiazolin-3-one preservative (0.1 g/L), the terminal product is registered under the provisions of Regulation (EC) No. 1107/2009, with MRLs established by Codex Alimentarius per CAC/GL 40-1993 for cereals (0.05 mg/kg) and pome fruit (0.1 mg/kg). Incompatibility with calcium-containing tank mixes is noted; precipitation occurs when SC formulation is co-applied with calcium nitrate foliar fertilisers at concentrations exceeding 0.5% w/v.

Controlled incorporation of Atom Transfer Radical Polymerization (ATRP) initiator sites into poly(methyl methacrylate) scaffolds begins with the transesterification of Ethyl 2-bromo-4-methyl-1,3-thiazole-5-carboxylate with poly(ethylene glycol) monomethyl ether (mPEG, Mₙ 5000 g/mol) using titanium(IV) isopropoxide (0.1 equiv) in refluxing toluene under nitrogen for 48 h. The resultant mPEG-thiazole ester is converted to the α-bromoisobutyrate macroinitiator by sequentially hydrolysing the ethyl ester (LiOH, THF/H₂O, 0 °C, 2 h), activating the carboxylic acid with N,N'-dicyclohexylcarbodiimide (DCC, 1.2 equiv) in dichloromethane, and esterifying with 2-hydroxyethyl 2-bromoisobutyrate (1.1 equiv) in the presence of 4-dimethylaminopyridine (DMAP, 0.15 equiv) at 25 °C for 18 h. After filtration of DCU and column chromatography (silica gel, gradient methanol in chloroform), the isolated macroinitiator is stored under argon at –20 °C. ATRP of styrene is conducted in a glovebox (MBraun LABmaster SP, O₂ <0.1 ppm) using CuBr (1.0 equiv relative to initiator) and N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA, 1.2 equiv) in anisole at 90 °C for times varying from 10 min to 4 h to achieve target number-average molar masses between 8000 and 45 000 g/mol with dispersity Đ below 1.15 (THF-SEC, RI detector, PS standards). The terminal bromine at the polymer chain end remains available for further chain extension or conversion to an azide for click functionalisation. The resulting block copolymer amphiphile self-assembles into micelles of hydrodynamic diameter 85±12 nm (DLS, Malvern Zetasizer Nano ZS) and is evaluated for controlled release of a hydrophobic thiazole carboxamide fungicide, with dialysable release profiles measured in PBS (pH 7.4, 37 °C) using a Float-A-Lyzer device (MWCO 3.5 kDa, Spectrum Labs). Compliance with OECD 301F biodegradability (readily biodegradable, >60% CO₂ evolution within 28 days) and absence of tin catalyst residues (<5 ppm by ICP-MS) is required for consumer contact materials; where Sn-free initiation is essential, an organocatalysed system based on 10-phenylphenothiazine (0.1 mol%) under UV irradiation at 365 nm is employed instead.

An electron-deficient thiazole carboxylate exhibits a lowest unoccupied molecular orbital (LUMO) energy of approximately –2.8 eV (DFT B3LYP/6-31G*), rendering it suitable as an acceptor monomer in donor-acceptor conjugated copolymers for solution-processed organic light-emitting diodes (OLEDs). A typical fabrication route homopolymerizes Ethyl 2-bromo-4-methyl-1,3-thiazole-5-carboxylate after Stille polycondensation with 2,5-bis(trimethylstannyl)thiophene in chlorobenzene at 120 °C using Pd₂(dba)₃ (2 mol%) and P(o-tol)₃ (8 mol%) devoid of CuI to prevent Glaser-type alkyne dimerisation. The crude polymer is precipitated into methanol, then purified by sequential Soxhlet extraction with acetone, hexane, and finally dichloromethane; the dichloromethane fraction exhibits weight-average molar mass Mw 24 000 g/mol (GPC, 1,2,4-trichlorobenzene, 150 °C). For device fabrication, a 15 wt% solution in o-xylene is spin-coated onto an ITO-coated glass substrate (sheet resistance 10 Ω/sq) pre-cleaned by UV-ozone treatment for 20 min, dried on a hotplate at 120 °C for 30 min under N₂, leading to an emissive layer thickness of 80±5 nm (Dektak profilometer). Cathode evaporation of LiF (1 nm) / Al (100 nm) at 5×10⁻⁷ mbar completes the device stack. The unoptimised device yields an electroluminescence onset at 3.1 V and a maximum external quantum efficiency of 2.8% (Keithley 2400 sourcemeter, Labsphere integrating sphere). RoHS Directive 2011/65/EU compliance (Pb, Hg, Cd, Cr6+, PBBs, PBDEs below threshold) and compliance with IEC 62321-2:2013 for halogen content are verified for the final OLED panel destined for indoor signage. A critical processing boundary is that the brominated monomer must be stored under dark conditions at –20 °C and processed within 48 h after preparation of its tin functionalised coupler to avoid proto-destannylation side reactions.

Consistency of a Primary Impurity Reference Standard Against ICH Q3A(R2) Thresholds

Manufacture of a validated batch of Ethyl 2-bromo-4-methyl-1,3-thiazole-5-carboxylate for use as a primary reference standard in impurity profiling follows a documented multi-column recrystallisation screening protocol. Starting from technical grade (94% purity), the ester is dissolved in hot heptane/isopropanol (8:2 v/v, 65 °C), treated with activated charcoal (Norit CN1, 5% w/w), filtered through a Büchner funnel fitted with Whatman Grade 1 paper, and cooled stepwise to –5 °C at 0.1 °C/min. The resulting crystals are isolated, washed with chilled heptane, and dried in vacuo (25 °C, 0.5 mbar, 48 h) to a constant weight. The final candidate standard is subjected to: quantitative ¹H-NMR (internal standard dimethyl sulfone, UKAS-accredited laboratory), differential scanning calorimetry (DSC, TA Instruments DSC2500, melting endotherm onset 58.2 °C, enthalpy 87.3 J/g), and UHPLC-DAD/MS purity assessment using a Waters ACQUITY UPLC H-Class with QDa detector (ACQUITY BEH C18, 1.7 µm particles, detection 210–400 nm). Identified impurities — the debrominated analogue (4-methylthiazole-5-carboxylate ethyl ester) and the hydrolysed free acid — are quantified and must not exceed 0.10% area each. The standard is packaged in 100 mg amber vials flame-sealed under argon and assigned a shelf life of 24 months at storage temperature –20±5 °C. In a cGMP pharmaceutical quality control setting per 21 CFR 211.194, this reference standard is used to spike active pharmaceutical ingredient batches at the reporting threshold (0.05%), identification threshold (0.10%), and qualification threshold (0.15%) as per ICH Q3A(R2) to ensure system suitability and method linearity (r² ≥ 0.999) during HPLC batch release testing of oncolytic tablet formulations. Each lot is accompanied by a Certificate of Analysis referencing the standard's internal control number, retest date, and the thermogravimetric analysis (TGA, TA Discovery TGA5500) showing residual solvent weight loss of <0.1%.

The utility of a late-stage C–H arylation manifold transforms the thiazole 5-carboxylate into a structural isostere of the epothilone macrolide side chain when 2-bromo is replaced by a styryl motif. A 1,3-dipolar cycloaddition sequence, often overlooked in medicinal chemistry, exploits the electron-poor thiazole ring as a dipolarophile: the compound reacts with nitrile oxides generated in situ from hydroxamoyl chlorides and Et₃N in CH₂Cl₂ at 0 °C to form isoxazoline-fused thiazole bicycles with complete regioselectivity (single isomer by X-ray crystallography). The ester group withstands these conditions provided the reaction pH remains below 8.0; above that threshold, saponification competes, lowering the yield of fused bicycle to <30%. Subsequent global deprotection and macrolactonisation with a BOP reagent (benzotriazol-1-yl-oxy-tris-(dimethylamino)phosphonium hexafluorophosphate) delivers a 16-membered ring epothilone B hybrid. Purification of the macrocycle by reversed-phase medium-pressure liquid chromatography (Biotage Isolera Dalton 2000, KP-C18-HS column, acetonitrile/water gradient) and lyophilisation afford a white amorphous powder with IC₅₀ 42 nM against A549 non-small-cell lung cancer cells (MTT assay, 72 h exposure). All synthetic steps adhere to European Pharmacopoeia monograph 5.1.10 for the control of contamination during manufacture. The stability of the intermediate bromothiazole ester in cell culture-grade DMSO stock solutions is limited to 72 h at room temperature before detectable solvolysis (HPLC evidence of free acid).