Ethyl 2-Methylthiazole-4-Carboxylate

Leveraging 2-Methylthiazole-4-Carbonyl Electrophiles in Parallel Medicinal Chemistry

Parallel synthesis libraries targeting the bacterial enoyl-ACP reductase (FabI) often start from ethyl 2-methylthiazole-4-carboxylate. The ester is first saponified to the free carboxylic acid by stirring with 1.05 equivalents of sodium hydroxide in aqueous ethanol at 60°C for 2 hours; after pH adjustment to 2.5–3.0 with dilute hydrochloric acid, the crystalline acid precipitates in yields of 88–94% (W/W). This acid is then activated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC.HCl) and 1-hydroxybenzotriazole (HOBt) in dry N,N-dimethylformamide at 0–5°C, followed by coupling with structurally diverse primary amines. When carried out on a 10–50 mmol scale in a jacketed glass reactor with overhead stirring, typical amidation completes within 8–16 hours. The crude 2-methylthiazole-4-carboxamides are purified by flash chromatography over silica gel (230–400 mesh) using ethyl acetate/hexane gradients, and the purity of each library member is confirmed by LC-MS with UV detection at 254 nm. In a medicinal chemistry campaign directed at methicillin-resistant Staphylococcus aureus (MRSA), preliminary MIC values for the most active congeners ranged between 2 and 16 µg/mL when tested according to CLSI M07-A10 broth microdilution methodology—published data for this specific scaffold remains fragmented, but the consistent activity of the 2-methylthiazole core has prompted multiple contract research organizations to stockpile the ester as a key enabling building block. Critical process parameters include strict exclusion of water during the coupling step, as adventitious moisture hydrolyses the active ester intermediate and reduces coupling efficiency to below 40%. The manufacturing intermediate supplied for GLP toxicology studies must comply with ICH Q7 active pharmaceutical ingredient GMPs, with residual DMF, EDC byproducts, and HOBt quantified by headspace GC-MS against ICH Q3C thresholds. Operators frequently note a moderate exotherm during the acid chloride preparation, making controlled addition rates and a jacket temperature of −5°C essential for kilogram- scale work executed in a 20 L Hastelloy C-22 reactor.

When a fine chemical manufacturer requires a kilogram-scale supply of 2-methylthiazole-4-carboxylic acid for phosphorylation studies targeting Fusarium graminearum in wheat, ethyl 2-methylthiazole-4-carboxylate serves as the preferred hydrolysis precursor. The saponification is executed by charging a 1000 L glass-lined reactor with the ester (1.0 kg mol), 30% (w/w) aqueous sodium hydroxide (1.05 mol equiv), and process water to a total volume of 600 L. The mass is heated to 70–75°C under nitrogen blanket and held under gentle reflux for 3 hours until in-process HPLC analysis confirms residual ester below 0.5 area%. After cooling to 20–25°C, the hydrolysate is acidified with 32% hydrochloric acid to pH 2.0–2.5, precipitating the thiazole carboxylic acid as a light tan solid. The slurry is filtered through a Nutsche filter-dryer, washed with deionized water until the filtrate conductivity drops below 100 µS/cm, and dried under vacuum at 50°C for 12 hours. Final acid purity by non-aqueous titration averages 99.1%, with loss on drying below 0.5%. This intermediate is subsequently converted to a thiophosphoramidate fungicide lead by reaction with O,O-diethyl phosphorochloridothioate in tetrahydrofuran in the presence of triethylamine at 0–10°C. The phosphoramidate crude is crystallized from toluene/heptane to give a 98+% pure active ingredient that is formulated as a 250 g/L EC for field trials. Regulatory documentation for export shipments includes a REACH registration dossier for the intermediate (tonnage band 1–10 t/a), a safety data sheet classifying the acid as Skin Corr. 1B, and an analytical certificate demonstrating compliance with the absence of 2-aminothiazole (a suspected mutagen) at a limit of detection of 50 ppm. Process waste waters, which contain NaCl and traces of ethanol, are neutralized to pH 6.5–8.5 and treated in an on-site biological oxidation plant before release.

During Banbury mixing of a silica-reinforced SBR compound at 135°C, the addition of 0.15 phr of an activatable thiazole precursor derived from ethyl 2-methylthiazole-4-carboxylate suppresses prevulcanization by 42% without impairing the delta torque in an MDR rheometer at 160°C. The precursor itself—prepared by converting the ester directly to the corresponding thioamide using gaseous ammonia and elemental sulfur in a pressure autoclave at 130°C and 5 bar—functions as a latent accelerator that remains inactive during compound mixing and storage but releases the active 2-methylthiazole-4-thiocarboxamide species when the rubber temperature exceeds 140°C. In a typical passenger tire tread formulation composed of solution SBR (70 phr), butadiene rubber (30 phr), precipitated silica (80 phr), silane coupling agent (6.4 phr), zinc oxide (3 phr), stearic acid (2 phr), antioxidant 6PPD (2 phr), and sulfur (1.5 phr), the thiazole derivative is introduced at the same time as the silane during the first mixing pass. A maximum dump temperature of 110°C must be strictly observed, verified by a needle pyrometer inserted into the batch, because premature activation results in a scorched compound with Mooney viscosity exceeding 120 MU and a loss of silanization efficiency. Vulcanization follows at 160°C for t₉₀ + 2 minutes (typically 12–16 minutes), yielding vulcanizates with tensile strength of 18.5–21.0 MPa (ISO 37:2017), elongation at break 380–420%, and DIN abrasion values of 105–115 mm³ (ISO 4649:2017). The final cured components—automotive weatherstrip profiles and engine mount bushings—demonstrate heat aging stability for 1000 hours at 100°C with less than 15% loss in elongation, a performance attribute attributed to the thiazole accelerator’s resistance to reversion. Regulatory compliance for all export markets is documented through an SDS aligned with GHS Rev.8, a certificate of analysis attesting to polycyclic aromatic hydrocarbon content below 1 mg/kg per AfPS GS 2019:01 PAK, and a REACH confirmation that the substance is not listed on the Candidate List of substances of very high concern.

Directed Ortho-Metalation as a Gateway to 2,4-Disubstituted Thiazoles

The C-5 position of the thiazole ring in ethyl 2-methylthiazole-4-carboxylate undergoes regiospecific lithiation when treated with 1.1 equivalents of lithium diisopropylamide (LDA) in tetrahydrofuran at −78°C. The resulting organolithium intermediate is remarkably stable at cryogenic temperatures for up to 2 hours, allowing subsequent trapping with a wide range of electrophiles—aldehydes, ketones, chlorotrimethylsilane, dimethylformamide—to produce 5-substituted derivatives in yields of 65–92%. This one-pot functionalization eliminates the need for protecting group strategies and has been adopted by research labs synthesizing custom thiazole building blocks for peptide isosteres and metal-organic frameworks. On a 5 L laboratory scale, the protocol requires a jacketed flask equipped with a calibrated J-KEM temperature controller and an overhead mechanical stirrer capable of maintaining a steady −78°C via a dry ice–acetone bath. Quenching with saturated ammonium chloride at −60°C and extraction with methyl tert-butyl ether, followed by column chromatography on silica (eluting with 5–15% ethyl acetate in hexanes), delivers the product in >95% purity by HPLC-UV at 270 nm. Laboratories performing this transformation routinely characterize the product by ¹H NMR (400 MHz), ¹³C NMR, and high-resolution mass spectrometry. The method’s tolerance for ketone electrophiles has enabled the synthesis of a 5-benzoyl derivative that serves as a photoinitiator candidate in acrylate-based UV-curable coatings—initial photocure speed measurements under a 365 nm LED array indicate that a blend containing 0.5 wt% of the benzoyl thiazole initiates polymerization more rapidly than benzophenone at identical loading, based on real-time FTIR conversion of the acrylate double bond at 810 cm⁻¹. The intermediate itself is classified as flammable liquid (Flash Point 92°C, closed cup) and must be stored in a certified flammable materials cabinet when quantities exceed 500 mL. An approved standard operating procedure requires a secondary containment tray and a portable carbon dioxide extinguisher within immediate reach. No harmonized classification for chronic aquatic toxicity has been assigned, but manufacturers voluntarily provide a cautionary statement advising against environmental release based on biodegradation simulation tests performed in accordance with OECD 301F.

Key Application Parameters and Regulatory References

Application Segment

Critical Conversion

Typical Catalyst/Reagent

Processing Window

Final Product Certification Framework

High-impact savory flavour ingredient

Ester → 4-hydroxymethyl derivative

NaBH₄/ZnCl₂ (aq. organic biphasic)

0–5°C, 4–6 h

21 CFR 172.515; EC 1334/2008; JECFA; USP<467>

Antimicrobial lead optimization

Ester → carboxamide library

EDC·HCl/HOBt in DMF

0–25°C, 8–16 h

ICH Q3C, Q7; CLSI M07-A10

Phosphoramidate fungicide intermediate

Ester → carboxylic acid → thiophoshoramidate

NaOH aq.; Et₃N, THF

70–75°C; 0–10°C

REACH Annex VII; 98/8/EC

Silica-filled rubber vulcanization

Ester → thioamide latent accelerator

S₈/NH₃ pressure autoclave

130°C, 5 bar; rubber mixing <110°C

ISO 37:2017; AfPS GS 2019:01 PAK

Specialty building block synthesis

C-5 lithiation–electrophile trapping

LDA, THF, –78°C

–78°C, 1–2 h, then quench

OECD 301F; in-house SOP for flammables

Ethyl 2-methylthiazole-4-carboxylate (CAS 7210-73-3; molecular formula C₇H₉NO₂S, molecular weight 171.22 g·mol⁻¹) is a heterocyclic building block manufactured to meet the demands of convergent pharmaceutical syntheses and crop protection discovery programs. Commercial specifications set a minimum assay of 98.0% by GC (area normalization), with a premium grade reaching 99.5% individual purity, residual ethanol below 500 ppm, and moisture content controlled below 0.10% (Karl Fischer). The pale-yellow to colorless liquid exhibits a boiling point of 234–236 °C at atmospheric pressure and a flash point of 107 °C (closed cup, ASTM D93-20). Unlike its methyl or isopropyl homologues, the ethyl ester offers a distinct combination of steric bulk and leaving-group aptitude that balances transesterification kinetics against premature hydrolysis during multi-step sequences involving reactive organometallic intermediates.

When the 2-methyl substitution pattern becomes non-negotiable in heterocycle elaboration

In the synthesis of febuxostat precursor fragments and related 2-arylthiazole-4-carboxylic acid derivatives, the integrity of the 2-methyl group must survive Suzuki-Miyaura coupling conditions without participating in β-hydride elimination side reactions. Batch reactor data from kilo-lab campaigns indicate that the ethyl ester of 2-methylthiazole-4-carboxylic acid withstands Pd(0)-catalyzed cross-coupling at 80 °C in dioxane/water mixtures with a loss of the methyl substituent of less than 0.3% over 18 hours, as monitored by HPLC at 254 nm. The corresponding methyl ester, though slightly more reactive in nucleophilic acyl substitution, generates methanol as a byproduct during saponification, which complicates solvent recovery in continuous processing. Industrial preference for the ethyl ester arises from its azeotropic compatibility with toluene and ethanol, enabling straightforward distillative removal of the liberated alcohol during ester-activation steps.

What governs the hydrolysis window in scaled-up amidation?

Conversion of the ester to the corresponding primary amide via ammonolysis in methanolic ammonia at 0–5 °C proceeds with a reported isolated yield of 92% when the ethyl ester is introduced over 4 hours under strictly anhydrous conditions. In contrast, the methyl ester under identical stoichiometry yields 87% due to competitive amidation of the 4-position and traces of ring-opened byproducts, as confirmed by ¹H NMR kinetic sampling. Process safety evaluations highlight that the exotherm associated with the addition of 7N ammonia in methanol to the neat ethyl ester can elevate the internal temperature by 12–15 °C within 30 seconds if jacket cooling fails to respond within the spec of ΔT 5 °C·min⁻¹. For this reason, semi-batch mode with the ester dissolved in 2 volumes of THF is specified on any scale exceeding 50 L.

Residual 2-methylthiazole-4-carboxylic acid, the hydrolysis product present at 0.05–0.2% even in freshly distilled material, acts as an autocatalytic hot-spot during storage. Exposure to ambient humidity above 60% RH at 25 °C for 48 hours elevates acid content to 0.8%, triggering further degradation. Storage under nitrogen headspace with a desiccant breather vent is mandated; typical container sizes in distribution range from 1 kg HDPE bottles to 200 kg epoxy-lined steel drums compliant with UN 6.1/8 packing group requirements for marine transport.

Comparative impurity fingerprint: ethyl versus n-propyl and benzyl esters

Parameter	Ethyl 2-methylthiazole-4-carboxylate	Methyl 2-methylthiazole-4-carboxylate	2-Methylthiazole-4-carboxylic acid
Boiling point (lit., °C)	234–236	218–220	Decomposes > 260
Typical assay range (% GC)	98.5–99.8	98.0–99.5	97.0–99.0 (HPLC)
Key process impurity	Ethyl 2-methylthiazole-5-carboxylate (< 0.15%)	Dimethyl sulfate residue detectable at < 2 ppm	2-Methylthiazole (< 0.05%)
Solubility in MTBE (g/100 mL, 25 °C)	52	48	6
Preferred activation method	NaOH/EtOH hydrolysis or LiOH/THF-H₂O	LiOH/MeOH-H₂O, faster but foams	Direct coupling via CDI or HATU

The regioisomeric impurity ethyl 2-methylthiazole-5-carboxylate, which can originate from Hantzsch cyclization under inadequate pH control, co-elutes with the desired product on standard DB-5 GC columns but is resolved using a 30 m × 0.25 mm CycloSil-B column with a 110 °C isothermal hold for 20 minutes. Acceptance criteria per USP < 621 > specify resolution ≥ 2.0 between the 4- and 5-isomers. Customers integrating this intermediate into cGMP sequences rely on the ethyl ester because its higher boiling point reduces losses during solvent swap after extractive work-up relative to the methyl analogue, which can suffer 3–5% evaporative loss under vacuum distillation at 40 mbar.

In an application without explicit labeling, the compound serves as a masked carboxyl synthon during organozinc-mediated Negishi couplings. THF solutions of the ethyl ester are amenable to transmetalation with ethylzinc bromide generated in situ, and the resulting zincate does not attack the ester carbonyl at temperatures below −20 °C, a processing window that collapses to −30 °C for the more electrophilic methyl ester. Published data for this specific configuration is limited, but pilot-plant reports document 78% isolated yield of the coupled biaryl after 16 hours at −15 °C in the presence of 2 mol% Pd(PPh₃)₄, with no detectable decarboxylation.

Specification sheet compliance boundaries

Test	Method	Limit
Appearance	Visual (Ph. Eur. 2.2.1)	Clear, colorless to pale yellow liquid
Identification	FTIR; ¹H NMR (400 MHz, CDCl₃)	Matches reference spectrum; δ 2.73 (s, 3H), δ 1.40 (t, J = 7.1 Hz, 3H)
Assay (GC)	ASTM D3465-21 (capillary GC, FID)	≥ 98.0% (area %)
Water (KF)	USP < 921 >	≤ 0.10%
Residual ethanol	GC headspace (EP 2.4.24)	≤ 500 ppm
Heavy metals (Pb, Cd, Hg)	ICP-MS (ICH Q3D)	Class 1 elements ≤ 1 ppm; Class 2A ≤ 10 ppm
Sulfated ash	EP 2.4.14	≤ 0.1%
Peroxide value	EP 2.5.5 (iodometric)	≤ 2.0 meq/kg

The ethyl ester is incompatible with strong oxidizing agents and should not be stored in proximity to peroxidizable solvents such as diethyl ether unless inhibited. In processes where the ester is reduced with LiAlH₄ in THF, the addition rate must be controlled to keep the internal temperature below 10 °C; a deviation above 15 °C leads to ring reduction and the appearance of a thiazolidine byproduct detectable at R_f 0.35 (TLC, silica gel, EtOAc:hexane 1:4). No combination with amine-based additives is permitted during acylation steps when DCC or EDC is used, because the additive accelerates N-acylurea formation to levels exceeding 5% within 1 hour.

For bulk shipments, the product is classified under HS code 2934.10.00. The vapor pressure at 25 °C is below 0.01 mmHg, but good industrial hygiene practice requires local exhaust ventilation when heating above 100 °C to avoid exposure to trace thiazole decomposition vapors. The difference from isopropyl 2-methylthiazole-4-carboxylate—rarely commercialized but available on custom synthesis—lies in the greater steric hindrance of the isopropyl ester; enzymatic hydrolysis screens with pig liver esterase (PLE) reveal a rate differential of 1:0.3 (ethyl:isopropyl) under identical conditions, which is exploited in prodrug strategies but represents a liability in large-scale deprotection sequences where the ethyl ester’s predictability is preferred.

HS Code	247690
Chemical Formula	C7H9NO2S
Molar Mass	171.217 g/mol
Appearance	A colorless to light yellow liquid
Boiling Point	Approximately 253 - 255 °C
Density	Around 1.186 g/cm³
Solubility In Water	Insoluble in water
Solubility In Organic Solvents	Soluble in common organic solvents like ethanol, acetone
Flash Point	Greater than 110 °C
Odor	Characteristic, somewhat pungent

Packing	100g of Ethyl 2 - Methylthiazole - 4 - Carboxylate in a sealed, chemical - resistant bottle.
Shipping	Ethyl 2 - Methylthiazole - 4 - Carboxylate is shipped in properly sealed containers. Packaging adheres to chemical transport regulations. Shipment is via approved carriers, ensuring safe and timely delivery while maintaining product integrity.
Storage	Ethyl 2 - Methylthiazole - 4 - Carboxylate should be stored in a cool, dry, well - ventilated area. Keep it away from heat sources, open flames, and oxidizing agents. Store in a tightly closed container to prevent vapor leakage. Due to its chemical nature, proper storage helps maintain its stability and reduces the risk of potential reactions or degradation.