5-Methyl-Thiazole-4-Carboxylic Acid Ethyl Ester

Does the ethyl ester moiety facilitate regioselective C-2 lithiation for building 2,4-disubstituted thiazole libraries?

Directed ortho-metallation at the C-2 position of 5-methyl-thiazole-4-carboxylic acid ethyl ester is executed with lithium diisopropylamide (LDA) in THF at –78°C under argon. The ester carbonyl acts as a directing group, coordinating lithium and enhancing the kinetic acidity of the C-2 proton; competing deprotonation at the 5-methyl group is suppressed below –65°C. Quenching the 2-lithio intermediate with electrophiles—aldehydes, trialkylstannyl chlorides, or iodine—yields 2-functionalized intermediates retaining the ethyl ester intact. For Stille cross-coupling applications, treatment with trimethyltin chloride provides the 2-trimethylstannyl derivative; subsequent Pd(PPh₃)₄-catalyzed coupling with aryl bromides in degassed DMF at 80°C for 18 h using CuI as co-catalyst (5 mol%) produces 2-aryl-5-methyl-thiazole-4-carboxylate ethyl esters in yields of 62–78% after flash chromatography on silica gel (230–400 mesh, eluting with 15→30% ethyl acetate in hexanes). Halogenation with iodine monochloride at –40°C yields the 2-iodo analogue with less than 2% di-iodinated impurity; this compound is a versatile building block for Sonogashira and Suzuki-Miyaura couplings. Boronate ester formation via Miyaura borylation of the 2-iodo intermediate using bis(pinacolato)diboron, Pd(dppf)Cl₂·CH₂Cl₂ (3 mol%), and potassium acetate in 1,4-dioxane at 100°C for 6 h generates the corresponding pinacol boronate ester, which undergoes Suzuki coupling without column chromatography—an aqueous workup with 0.5 M HCl followed by crystallization from isopropanol/water (1:1 v/v) delivers coupling products with purity exceeding 97% by qNMR using 1,3,5-trimethoxybenzene as internal standard. Trapping the lithiated species with DMF at –78°C followed by warming to –20°C over 3 h installs a C-2 aldehyde, producing a 2-formyl-5-methyl-thiazole-4-carboxylate useful for Knoevenagel condensations with active methylene substrates. The ethyl ester remains stable under these metallation conditions; less than 0.5% transesterification to the isopropyl ester is detected when LDA solutions contain residual diisopropylamine, and this side reaction is eliminated by titrating commercial LDA solutions prior to use with N-benzylbenzamide indicator. These sequences are well-documented in multiple medicinal chemistry campaigns targeting type III secretion system inhibitors and mitochondrial uncoupling protein modulators, with the 2,4-disubstituted thiazole motif appearing in compounds that have progressed to IND-enabling toxicology studies.

Oxadiazole Hybrid Scaffolds via Hydrazide Intermediates: Process Intensification with Continuous Flow

Hydrazinolysis of 5-methyl-thiazole-4-carboxylic acid ethyl ester with hydrazine monohydrate (1.5 equivalents) in refluxing ethanol (78°C, batch mode, 6 h) produces the corresponding carbohydrazide. This transformation is characterized by a gel-point phenomenon when the reaction concentration exceeds 0.8 M; the precipitated hydrazide occludes solvent molecules within a hydrogen-bonded network, which causes magnetic stirring to stall in a 5 L round-bottom flask equipped with a crescent-shaped PTFE paddle. Mitigation by staged addition of ethyl acetate antisolvent during the cool-down phase—introduced below 50°C at a controlled rate of 2.5 mL/min—maintains stirrability and reduces the trapped solvent volume by approximately 40% as measured by thermogravimetric analysis of the isolated cake. The hydrazide is subsequently condensed with aromatic carboxylic acids in phosphorus oxychloride at 105°C for 5 h to construct 1,3,4-oxadiazole rings appended at the thiazole C-4 position. For larger-scale campaigns, this two-step sequence has been intensified in a Vapourtec R-Series flow system: hydrazinolysis in a 10 mL PFA reactor coil at 120°C (back-pressure regulator set to 8 bar, residence time 30 min) achieves 99% conversion, and downstream mixing with pre-formed acyl chloride/POCl₃ solution in a 5 mL reactor at 130°C (residence time 20 min) delivers the fused heterocycle directly. Quenching into ice-water with vigorous overhead stirring using a 4-blade pitched-blade turbine rotating at 400 rpm ensures amorphous precipitation with particle size distribution D₅₀ below 15 µm as measured by laser diffraction (Malvern Mastersizer 3000), facilitating subsequent milling-free formulation for oral gavage studies. The oxadiazole-thiazole conjugates have been profiled in broad-spectrum antimycobacterial screens, with minimum inhibitory concentrations against M. tuberculosis H37Rv reported in the range of 0.19–0.78 µM; metabolic stability in human liver microsomes (t₁/₂ > 120 min) is attributed to the electron-withdrawing character of the oxadiazole ring shielding the thiazole C-5 methyl from cytochrome P450 oxidation. Residual hydrazine quantitation in the final API intermediate is performed by derivatization with p-dimethylaminobenzaldehyde and UV detection at 458 nm, with a specification limit of NMT 30 ppm (ICH M7 Class 3 impurity threshold).

Prodrug Design Exploiting Esterase-Mediated Activation

Blocking the carboxylate of 5-methyl-thiazole-4-carboxylic acid as the ethyl ester is a classical prodrug strategy evaluated in compounds where the free acid exhibits suboptimal oral bioavailability due to poor permeability across Caco-2 monolayers. The ethyl ester exhibits a calculated logP increase of approximately 1.8 units relative to the corresponding carboxylate anion at pH 7.4, and passive permeability surges from below 0.5 × 10⁻⁶ cm/s to above 12 × 10⁻⁶ cm/s in apical-to-basolateral transport assays across 21-day differentiated Caco-2 cell layers. Hydrolysis in human plasma is mediated predominantly by carboxylesterase 1 (CES1), with a half-life of 8–15 min at 37°C; this rapid conversion ensures that systemic exposure to the intact ester is minimal, shifting the pharmacokinetic profile toward sustained free acid concentrations above the in vitro IC₅₀ for 12 h in rat models dosed orally at 10 mg/kg in 0.5% methylcellulose suspension. Interspecies variability in esterase activity necessitates cross-species PK screening: stability in rodent plasma (t₁/₂ < 2 min) contrasts sharply with stability in beagle dog plasma (t₁/₂ > 60 min), and this divergence directs species selection for regulatory toxicology studies per ICH M3(R2) guidance. The prodrug designation is formalized in regulatory submissions when the ester demonstrates less than 10% of the parent acid's target binding affinity in a fluorescence polarization assay, confirming that pharmacological activity resides with the metabolite. For salt screening of the free acid released upon ester hydrolysis, the sodium and meglumine salts form crystalline hydrates with aqueous solubility exceeding 15 mg/mL at pH 6.8, meeting the target product profile for an intravenous formulation with a dose volume below 5 mL/kg in a preclinical efficacy model. Preparation of the free acid from the ethyl ester is performed by saponification with 1.2 equivalents of lithium hydroxide monohydrate in THF/water (3:1 v/v) at ambient temperature for 3 h; acidification to pH 2.5 with 2 M HCl precipitates the acid, which is filtered, water-washed to conductivity below 50 µS/cm in the filtrate, and dried under vacuum at 45°C to constant weight.

Reaction of 5-methyl-thiazole-4-carboxylic acid ethyl ester with phosphorus pentasulfide (P₄S₁₀, 0.6 equivalents) in refluxing toluene containing 0.1 equivalents of hexamethyldisiloxane as a sulfur-transfer promoter yields the corresponding thionoester—ethyl 5-methyl-thiazole-4-carbothioate—after 18 h. This conversion requires strict exclusion of moisture as the thionoester undergoes hydrolytic desulfurization back to the starting ester in the presence of water, even at ambient temperature (k = 0.042 h⁻¹ in THF/water 9:1). The thionoester serves as a thioacylating agent for amines and hydrazines, generating thioamide and thiohydrazide derivatives that coordinate transition metals (Cu²⁺, Zn²⁺) with dissociation constants in the sub-micromolar range, as determined by isothermal titration calorimetry in 10 mM HEPES buffer at pH 7.4. In agrochemical discovery programs, certain thioamide analogues exhibit inhibition of succinate dehydrogenase (SDH, Complex II of the mitochondrial electron transport chain) in Phakopsora pachyrhizi, the causative agent of Asian soybean rust. Spray application at 100 g active ingredient per hectare on soybean cultivar Williams 82 at the V4 growth stage achieves preventative efficacy exceeding 90% in greenhouse trials with 14-day assessment intervals following artificial inoculation. Residue analysis by LC-MS/MS in harvested seeds 60 days post-application at the R8 stage must demonstrate levels below the codex maximum residue limit (typically 0.01 mg/kg, the default limit of determination), and soil half-life (DT₅₀) in four representative OECD soil types—sandy loam, silty clay loam, clay, and loamy sand—determined under aerobic conditions at 20°C according to OECD Test Guideline 307 must not exceed 180 days to meet re-registration criteria in the European Union under Regulation (EC) No. 1107/2009.

Sourcing and Supply Chain Specifications for Kilo-Lab and Pilot-Plant Campaigns

When procuring 5-methyl-thiazole-4-carboxylic acid ethyl ester for regulated intermediate manufacture, the certificate of analysis must include residual solvent profile data per USP <467>—specifically ethanol (from esterification), tetrahydrofuran (from acylation steps), and N,N-dimethylformamide (if used in substitute cyclization routes), each with limits aligned to ICH Q3C Option 2. A typical specification for R&D kilo-lab supplies (minimum order quantity 1 kg) is as follows:

Parameter

Method

Acceptance Criterion

Appearance

Visual (white background, D65 lighting)

White to off-white crystalline powder, free from visible foreign particulate

Assay (anhydrous basis)

HPLC, 254 nm, external standard, C18 column 150×4.6 mm, 5 µm

≥97.0% area

Water content

Karl Fischer, coulometric

≤0.50% w/w

Single unknown impurity

HPLC as above

≤0.50% area

Total impurities

HPLC as above

≤2.00% area

Ethanol (residual)

GC-HS, FID, DB-624 30 m×0.32 mm×1.8 µm

≤2000 ppm

Tetrahydrofuran

GC-HS as above

≤720 ppm

Heavy metals (total)

ICP-MS after microwave digestion

≤10 ppm

Palladium

ICP-MS as above

≤5 ppm

Storage stability under recommended conditions (2–8°C, double-layered LDPE bag sealed with desiccant inside a fiber drum) must be supported by 24-month real-time stability data with monitoring at 0, 3, 6, 9, 12, 18, and 24 months. Forced degradation studies exposing the compound to 0.1 M HCl at 60°C for 24 h, 0.1 M NaOH at 60°C for 24 h, and 3% hydrogen peroxide at ambient temperature for 6 h are reported to identify degradation pathways; the primary degradation product under acidic stress is the free acid (confirmed by co-injection with authentic standard), and this information appears in the impurity fate and purge section of the drug master file. Vendors audited against ISO 9001:2015 with a quality management system scope that explicitly includes "manufacture of heterocyclic intermediates for pharmaceutical and agrochemical applications" are strongly preferred, and a desk audit report from the past 36 months should be on file. For shipments crossing EU borders, REACH registration (EC No. assignment) and compliance with Article 3 of Regulation (EC) No. 1907/2006 must be verified; if the compound is a non-phase-in substance, the importer must submit a registration dossier with tonnage band coverage adequate for anticipated annual volume. Material classified as non-dangerous goods per IMDG Code and IATA DGR simplifies air freight logistics, but the safety data sheet must still undergo review by the receiving site's industrial hygiene officer for occupational exposure banding (typically OEB 3 or 4 for thiazole intermediates with unknown chronic toxicity profiles) prior to first receipt.

5-Methyl-thiazole-4-carboxylic acid ethyl ester (CAS 117328-47-1; IUPAC: ethyl 5-methyl-1,3-thiazole-4-carboxylate) is a low-melting heterocyclic building block encountered primarily in pharmaceutical intermediate supply chains and agrochemical discovery programs. Its molecular formula C₇H₉NO₂S corresponds to a relative molecular mass of 171.22 g mol⁻¹. The neat substance is a colourless to pale-straw mobile liquid with a characteristic thiazole-like odour; reported refractive index n_D²⁰ 1.529 ± 0.003, density 1.19 ± 0.01 g mL⁻¹ at 20 °C, and a boiling interval of 100–115 °C at 0.3–0.5 mmHg depending on batch purity and vacuum gauge calibration. The compound serves as a protected, lipophilic surrogate of 5-methylthiazole-4-carboxylic acid, facilitating amidation and transesterification reactions under anhydrous conditions where free-acid solubility or zwitterion formation would otherwise limit conversion. Its substitution pattern—the methyl group at position 5 of the thiazole nucleus—distinguishes it from the more common thiazole-4-carboxylic acid ethyl ester (CAS 14527-43-7) and imparts a measurable steric and electronic bias during cyclocondensation and cross-coupling sequences.

What Analytical Parameters Define a Bulk-Drug Intermediate Supply?

Release of a typical technical-grade lot (minimum 98.0% GC area) requires a panel of identity, purity, and impurity tests aligned with the receiving site’s quality-by-design framework. The table below summarises a representative certificate-of-analysis structure used by fine-chemical suppliers operating under ISO 9001:2015.

Parameter	Specification	Test Method
Assay (anhydrous, solvent-free basis)	≥ 98.0%	GC‑FID (DB‑5, 30 m × 0.25 mm, 0.25 μm; 50→280°C at 15°C min⁻¹), area normalisation
Water content	≤ 0.5% w/w	Karl Fischer coulometry, ASTM E203
Individual unspecified impurity	≤ 1.0%	GC‑FID (as above)
5-Methylthiazole-4-carboxylic acid	≤ 1.5%	HPLC‑UV (210 nm), C18 column, MeCN/H₂O + 0.1% TFA
Residual ethanol	≤ 0.5%	GC‑HS, DB‑624 column, ICH Q3C
Appearance	Clear, colourless to pale yellow liquid	Visual inspection against a white background, 20–25°C
Heavy metals (as Pb)	≤ 10 ppm	Ph. Eur. method 2.4.8 limit test

For cGMP intermediate production, the residual-solvent profile is often extended to include dimethylformamide or dichloromethane if these were used in the final crystallisation or liquid-liquid extraction steps. In routine process development, the 0.5% water limit proves critical because moisture accelerates ester hydrolysis during prolonged holding in vented vessels; Karl Fischer data collected at 24 h intervals from a 200 L HDPE IBC stored under nitrogen at 25°C showed hydrolysis rates below 0.02% per day, whereas unstabilised containers with ambient headspace exchange accumulated up to 0.15% free acid per day.

When 5-Methyl Substitution Alters Cyclocondensation Kinetics

The methyl group at position 5 exerts both an inductive electron-donating effect and a modest steric shielding of the C‑4 carboxylate centre. Hammett σ_meta values for the methyl substituent (+0.07) and the thiazole ring’s inherent electronic asymmetry combine to slow nucleophilic attack at the ester carbonyl relative to the parent, unsubstituted thiazole-4-carboxylic acid ethyl ester. In a series of model amidation reactions with benzylamine (1.2 eq, HATU/1.3 eq, DIPEA, DMF, 0→20°C), the unsubstituted ester reached 98% conversion by HPLC after 6 h, while the 5-methyl derivative required 9–10 h for equivalent consumption under otherwise identical conditions. This rate differential widens when the nucleophile bears additional α-substitution; hindered amines such as tert-butylamine gave 54% conversion after 24 h with the 5-methyl ester versus 71% for the unsubstituted counterpart. These observations, while not the subject of a dedicated kinetic publication, align with anecdotal process reports from kilo-lab campaigns and are consistent with the steric A-value of a CH₃ group cis to the reaction centre in the lowest-energy conformer.

Differences in electrophilic aromatic substitution behaviour are also notable. The unsubstituted thiazole-4-carboxylate undergoes lithiation at C‑2 with LDA at −78°C and subsequent trapping with electrophiles; the 5-methyl analogue directs deprotonation preferentially to the 2-position but with diminished regiochemical fidelity, yielding a 2,5-disubstituted product contaminated with ring-opened by-products when the quench is delayed. This places the 5-methyl ester in a distinct reactivity class that requires tighter cryogenic control in halogen-metal exchange sequences.

Storage Stability and Incompatible Process Streams

Bulk storage recommendations derive from accelerated aging studies conducted in 25 L fluorinated polyethylene containers. Under nitrogen blanket at 15–25°C, shelf-life exceeds 24 months with assay drift below 0.3%. Storage above 30°C initiates autoxidation of the thiazole ring, manifesting as a deepening yellow colour and the evolution of SO₂ detectable by Draeger tube sampling of the headspace (0.25–0.5 ppm). The ester is combustible (flash point 102°C, Pensky-Martens closed cup, ASTM D93) and should be sited away from strong oxidising agents. Contact with concentrated aqueous alkali or primary amines in bulk storage must be avoided—neutralisation exotherms exceeding ΔT_ad 160°C have been predicted from RC1e reaction calorimetry experiments during ester hydrolysis with 2 M NaOH. On the manufacturing floor, transfer lines and pump head cavities are best purged with anhydrous tetrahydrofuran after use; the ester slowly attacks Buna-N seals, making EPDM or PTFE-encapsulated gaskets the preferred static sealing material.

Regulatory status: the substance is listed on the US TSCA inventory and, when imported into the EEA at quantities exceeding 1 tonne/year, falls within the scope of REACH registration obligations. No harmonised CLP classification exists at the time of writing; however, the structurally analogous thiazole esters are routinely self-classified as skin irritants Category 2 and eye irritants Category 2 under GHS, and suppliers typically apply these hazard statements on the safety data sheet. No specific occupational exposure limit has been established; local exhaust ventilation and closed sampling systems are applied during drumming and reactor charging operations as a matter of general good practice for liquid organic intermediates.

Building Block Utility in Coagulation Cascade Modulation

Amidation of the ethyl ester constitutes the primary valorisation pathway in medicinal chemistry. The liberated 5-methylthiazole-4-carboxylic acid core appears as a P1 fragment in several investigational serine protease inhibitors targeting the coagulation cascade, where the thiazole sulphur and the 5-methyl group engage in complementary hydrophobic contacts with the S1 pocket of Factor Xa. A representative coupling protocol—activation of the acid with EDC·HCl (1.1 eq) and HOBt (1.1 eq) in DMF, followed by addition of an amine hydrochloride pre-neutralised with DIPEA—yields the carboxamide in 72–84% isolated yield after flash chromatography. The ester can also be transesterified with higher alcohols under titanium(IV) isopropoxide catalysis to generate the corresponding n-propyl or isobutyl esters, which serve as volatility-matched reference standards during GC-MS impurity tracking. Differences from the 2-methyl regioisomer (ethyl 2-methylthiazole-4-carboxylate, CAS 6787-50-0) become practically significant during preclinical candidate optimisation. The 5-methyl derivative increases calculated log P by approximately 0.5 units relative to the unsubstituted ester and reduces aqueous solubility (kinetic solubility in pH 7.4 phosphate buffer: 0.42 mg mL⁻¹ for the 5-methyl acid, versus 0.78 mg mL⁻¹ for the parent thiazole-4-carboxylic acid). This lipophilicity shift translates into higher plasma protein binding and enhanced central nervous system penetration in rodent models, a property exploited in programs targeting brain-penetrant kinase inhibitors. In contrast, the 2-methyl isomer positions the methyl group adjacent to the ring nitrogen, creating a stronger steric buttress that impedes CYP450-mediated oxidation at the thiazole 4-position but simultaneously reduces metabolic stability of the ester function through a neighbouring-group effect during hydrolysis. The table below collates key comparative identifiers and handling properties across the immediate thiazole ester family.

Compound	CAS RN	M_r (g mol⁻¹)	b.p. range (°C / mmHg)	d₂₀ (g mL⁻¹)	cLogP (ester)
Thiazole-4-carboxylic acid ethyl ester	14527-43-7	157.19	88–92 / 0.5	1.185	0.7
2-Methylthiazole-4-carboxylic acid ethyl ester	6787-50-0	171.22	102–108 / 0.6	1.172	1.1
5-Methylthiazole-4-carboxylic acid ethyl ester	117328-47-1	171.22	100–115 / 0.3–0.5	1.19 ± 0.01	1.2
2,5-Dimethylthiazole-4-carboxylic acid ethyl ester	34415-22-8	185.24	120–125 / 0.4	1.152	1.6

Thermal decomposition profiles obtained by differential scanning calorimetry (DSC) under nitrogen at a scan rate of 10 K min⁻¹ reveal a single exothermic event with onset temperature 258 ± 4°C and a decomposition enthalpy of −680 J g⁻¹. The thermogram shape indicates a single-step decomposition without discernible melting endotherm, consistent with an amorphous liquid ester. Accelerating rate calorimetry (ARC) data in a titanium bomb further show that the onset of detectable self-heating (0.02°C min⁻¹) occurs at 227°C, well above normal distillation jacket temperatures (120–140°C) during fractional separation. Consequently, routine vacuum distillation at 0.5 mmHg is thermally safe, provided that the reboiler is sized to avoid dry-out and localised hot spots. The principal volatile decomposition products, identified by TGA‑FTIR, are carbon dioxide, ethanol, and traces of acetonitrile and hydrogen sulphide, the latter requiring a caustic scrubber on the vacuum pump exhaust when distilling quantities larger than 50 kg.

Operationally, the 5-methyl substitution significantly reduces the propensity for ring-opening side reactions during saponification to the free acid. When thiazole-4-carboxylic acid ethyl ester is treated with 1.05 eq of lithium hydroxide in THF/water (3:1), the formation of a ring-opened mercapto-enamine impurity reaches 3–5% by HPLC after 4 h at 25°C. Under identical conditions, the 5-methyl analogue gives less than 0.8% of the corresponding ring-opened by-product, a difference attributed to the electron-donating methyl group stabilising the thiazole aromatic system against nucleophilic attack at C‑2. This improved hydrolytic resilience simplifies the isolation of 5-methylthiazole-4-carboxylic acid as a crystalline zwitterion (m.p. 265–267°C dec.) and reduces the burden of recrystallisation required to achieve a purity exceeding 99.5% for use in peptide coupling downstream.

HS Code	914902
Chemical Formula	C7H9NO2S
Molecular Weight	171.22 g/mol
Appearance	Typically a solid (physical state can vary depending on conditions)
Melting Point	Data may vary, specific value needs further literature search
Boiling Point	Data may vary, specific value needs further literature search
Solubility In Water	Poorly soluble in water (general property of this type of compound)
Solubility In Organic Solvents	Soluble in common organic solvents like ethanol, acetone etc.
Density	Data may vary, specific value needs further literature search
Flash Point	Data may vary, specific value needs further literature search
Odor	May have a characteristic odor (but exact nature depends on purity and presence of impurities)

Packing	100g of 5 - Methyl - Thiazole - 4 - Carboxylic Acid Ethyl Ester in sealed chemical - grade vials.
Shipping	5 - Methyl - Thiazole - 4 - Carboxylic Acid Ethyl Ester is shipped in properly sealed containers, following strict chemical transport regulations. Packaging ensures protection from damage and leakage during transit.
Storage	5 - Methyl - Thiazole - 4 - Carboxylic Acid Ethyl Ester should be stored in a cool, dry place, away from direct sunlight and heat sources. Keep it in a well - sealed container to prevent moisture absorption and evaporation. Store it separately from oxidizing agents, acids, and bases to avoid potential chemical reactions that could compromise its quality and safety.