4-Methyl-2-Pyridin-3-Yl-1,3-Thiazole-5-Carboxylate

How Does Acid Cleavage of the Methyl Ester Proceed Without Decarboxylation in GMP Manufacturing?

The selective conversion of the methyl ester to the corresponding 4-methyl-2-pyridin-3-yl-1,3-thiazole-5-carboxylic acid demands rigorous control over temperature and acid concentration to suppress the thermally sensitive thiazole ring decarboxylation pathway. In a 500-L glass-lined reactor (Pfaudler AE series with dished top and half-coil jacket) under ICH Q7 active pharmaceutical ingredient GMP, a 285 kg charge of the ester is suspended in 1,250 L of purified water and 180 L of concentrated hydrochloric acid (37 wt%). The batch is heated via jacket circulation at a ramp rate of 0.5 °C/min to an internal temperature of 68 ± 2 °C — exceeding 70 °C triggers decarboxylation yielding 4-methyl-2-pyridin-3-ylthiazole as the main impurity, detected by inline ReactIR (Mettler Toledo 4000) at 1,714 cm⁻¹ (carbonyl loss) and 1,598 cm⁻¹ (ring vibration). Hydrolysis is maintained for 14–16 h with overhead agitation at 120 rpm using a retreat-curve impeller. After confirmatory HPLC (Hypersil BDS C8, 250 × 4.6 mm, 5 µm, mobile phase 0.1 % trifluoroacetic acid in water/acetonitrile gradient, UV 254 nm) shows residual ester < 0.5 %, the mixture is cooled to 5 °C and neutralized to pH 3.5 with 50 % aqueous sodium hydroxide. The precipitated carboxylic acid is isolated in a 0.72-m² Hastelloy C22 pressure nutsche filter, rinsed with cold water (2 × 200 L), and dried under vacuum (10 mbar, 45 °C) for 18 h to a water content by Karl Fischer (Metrohm 831) < 0.5 %. Molar yield: 92–94 %. A process impurity control strategy per ICH Q3A(R2) reports the decarboxylated by-product at < 0.10 % and the unreacted ester at < 0.15 %. The final acid is handled under a nitrogen blanket and stored in double PE liners inside fiber drums at ≤ 25 °C to prevent moisture uptake, which would retard the subsequent amide coupling step used in kinase inhibitor synthesis (e.g., coupling with 3-bromo-4-fluoroaniline via EDC·HCl and HOBt in DMF to form the pharmacologically critical amide intermediate).

Comparison of Hydrolysis Acids on Purity and Decarboxylation By-Product Formation

Parameter

6 N HCl

48% HBr

Test Method

Reaction completion time (h)

HPLC (in-process)

Carboxylic acid purity (%)

99.4

99.1

USP <621>

Decarboxylated impurity (%)

0.08

0.22

HPLC-UV/MS

Residual bromide/chloride (ppm)

< 100

340

Ion chromatography (USP <1065>)

Post-hydrolysis, the free acid is dissolved in anhydrous tetrahydrofuran (15 L/kg) and treated with 1.1 eq. of carbonyldiimidazole to generate the acyl imidazolide; subsequent addition of 1.05 eq. of aniline derivative yields the corresponding amide, forming the backbone of a clinical-stage colony-stimulating factor-1 receptor inhibitor. All raw materials and intermediates conform to REACH registration dossiers and to monograph EN 71-3 for extractable heavy metals.

Neonicotinoid Analog Lead Optimization via C5-Carboxylate Derivatization

Derivatization of the C5-carboxylate center with N-cyano-S-ethyl isothiourea in dimethylacetamide enables the construction of a imidacloprid-analog scaffold where the pyridine-thiazole core mimics the chloronicotinyl pharmacophore. In a 20-L stirred jacketed glass reactor (Radleys Reactor-Ready), 1.8 kg of 4-methyl-2-pyridin-3-yl-1,3-thiazole-5-carboxylic acid, prepared as described, is activated with 1.15 eq. of thionyl chloride in 4.5 L of dichloromethane containing 0.5 mol% dimethylformamide at 0–5 °C. After 3 h under anhydrous conditions, volatiles are stripped in vacuo (Büchi R-220 rotary evaporator, 60 °C bath) and the residual acid chloride is taken up in 3 L of dry acetonitrile. To this is slowly added a suspension of 1.0 eq. N-cyano-S-ethyl isothiourea and 1.3 eq. triethylamine in 5 L acetonitrile at −10 °C to control the exotherm; the mixture is then warmed to 25 °C and stirred for 12 h. Following filtration of triethylamine hydrochloride and concentration, the crude N-cyanoamidine is purified by trituration with isopropanol (3 × 1.5 L) to reach a purity of 97.2 % by HPLC (Agilent 1260, ZORBAX SB-Aq, 4.6 × 250 mm, 5 µm, acetonitrile/0.05 M KH₂PO₄ pH 3.0 gradient). Isolated yield: 74 %. For technical-grade agrochemical synthetic campaigns, the acetamiprid-type product is spray-dried (Büchi Mini Spray Dryer B-290, inlet temperature 180 °C) to yield a free-flowing powder with d₅₀ particle size 12 µm. The lot is tested per CIPAC MT 46 for wettability and MT 53 for suspensibility; a 5 % SC formulation exhibits suspensibility 92 % after 30 min. Residual solvents comply with FAO specification 472/TC (thiacloprid technical concentrate) benchmark limits: DMF < 0.1 %, acetonitrile < 0.04 %. Ecotoxicological classification under EC 1107/2009 requires a honeybee acute oral LD₅₀ > 100 µg/bee to meet active substance listing criteria for foliar application on Brassica crops; the pyridine-thiazole analog demonstrates a value of 118 µg/bee in limit tests according to OECD 213.

When Trace Pd Residuals from Ligand Synthesis Poison Cross-Coupling Catalyst Activity

When synthesizing a palladium(II) catalyst by anchoring the pyridine-thiazole carboxylate onto aminopropyl-functionalized mesoporous silica (SBA-15, surface area 672 m²/g per ASTM D4222-03, pore size 6.8 nm), the residual palladium from a prior coupling step used to attach a biphenyl spacer must be quantified. The immobilization starts with refluxing 12 g of SBA-15-NH₂ (loading 1.9 mmol NH₂/g) with 1.5 eq. of the hydrolyzed acid chloride (generated in situ from the ester) in toluene for 24 h under a Soxhlet purge to drive off HCl. After filtration and washing with toluene, dichloromethane, and diethyl ether, the ligand-grafted silica is dried, and elemental analysis yields 1.32 % N and 2.46 % C, corresponding to a ligand loading of 0.68 mmol/g. The material is then stirred with 0.5 eq. Pd(OAc)₂ in acetone at 40 °C for 6 h to form the immobilized catalyst. A critical failure mode emerges: the earlier Pd-catalyzed Suzuki coupling used to introduce a 4-biphenyl group left 1,840 ppm Pd in the intermediate ligand precursor. This excess palladium, when introduced into the final catalyst bed, leads to metal agglomeration during the subsequent Suzuki–Miyaura cross-coupling of 4-bromotoluene and phenylboronic acid, reducing turnover frequency from a design target of 1,250 h⁻¹ to 390 h⁻¹ as measured by hot filtration test and inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer Avio 500). To meet the catalyst specification of < 50 ppm residual Pd in the ligand, an additional scavenging step using trimercaptotriazine-functionalized silica (2 wt% relative to substrate) in refluxing tetrahydrofuran for 4 h reduces Pd to 32 ppm. The decontaminated ligand then yields a final Pd catalyst with 0.47 wt% Pd and dispersion of 34 % by CO chemisorption (Micromeritics AutoChem II 2920). Hot filtration test confirms that leaching of catalytically active species is below 0.8 % of total Pd. The immobilized catalyst maintains > 95 % conversion over 6 consecutive cycles in bath reactor setup at 80 °C, with no detectable Pd in the product stream by ICP-MS (< 10 ppb), meeting EMA guideline CHMP/SWP/4446/2000 for heavy metal residues in pharmaceutical intermediates.

Aminolysis of the methyl ester with 2-(2,6-dichlorophenylamino)ethylamine in isopropanol at reflux under a nitrogen atmosphere proceeds with a half-life of 47 min at 82 °C, forming a key intermediate for a novel veterinary anthelmintic in the paraherquamide structural class. To a 200-L glass-lined steel reactor (THALETEC Type BE) equipped with a double-acting mechanical seal and six-blade pitched-turbine agitator, 18.3 kg of the ester and 11.9 kg of the amine are charged with 130 L of isopropanol. The jacket is set to 95 °C with a temperature ramp of 1 °C/min; at an internal temperature of 70 °C, an exotherm initiates and must be managed by partial jacket cooling to keep the batch at 78–82 °C—adiabatic temperature rise calculations (ARSSST, Fauske & Associates) predict a ΔT_ad of 56 °C if cooling is lost, which would generate off-spec dark oligomers. After 5 h, HPLC monitoring (Phenomenex Luna C18, 150 × 3.0 mm, 5 µm, water/acetonitrile 0.1 % TFA gradient) confirms < 0.3 % residual ester. The mixture is cooled to −5 °C and held for 4 h to crystallize the amide product. Filtration through a 0.45-m² polypropylene filter cloth in a Nutsche filter, followed by a wash with cold isopropanol (2 × 15 L) and vacuum drying at 40 °C (5 mbar) affords the off-white crystalline solid in 86 % yield with a melting point of 194–196 °C by DSC (Mettler Toledo DSC1, 10 °C/min under nitrogen). Purity by area normalization HPLC is 99.6 %; the primary impurity, the corresponding acid from ester hydrolysis by adventitious moisture, is controlled at < 0.15 %. For veterinary drug master file support, the batch is tested per VICH GL10 for impurities and per USP <231> for heavy metals (< 20 ppm). The isolated amide is then used in a high-pressure hydrogenation step (Büchi Limbo reactor, 50 bar H₂, 10 % Pd/C) to complete the anthelmintic core.

Corrosion Inhibitor Efficiency Index in 1 M HCl: Potentiodynamic Polarization Parameters for Copper

Immersion of C11000 copper coupons in uninhibited and inhibited 1 M hydrochloric acid at 25 °C with the thiazole ester as an organic inhibitor reveals an inhibition efficiency of 87.4 % at a concentration of 2.0 mM according to ASTM G5-14 potentiodynamic anodic polarization scan at 0.166 mV/s from −250 mV to +250 mV versus open circuit potential. The ester is dissolved in a minimal volume of ethanol (2 % v/v in the test solution) to avoid surfactant effects. Tafel extrapolation yields a corrosion current density (i_corr) of 12.7 µA/cm² in the uninhibited electrolyte, decreasing to 1.6 µA/cm² with inhibitor. Langmuir adsorption isotherm fitting of polarization resistance data (Gamry Reference 600+ potentiostat, EIS at 10 mV rms from 100 kHz to 0.01 Hz) gives an equilibrium constant of 2.8 × 10⁴ L/mol, indicative of strong chemisorption through pyridine nitrogen lone pair and thiazole sulfur π-donation to Cu(111) sites. The inhibitor film persists for 48 h in stagnant acid; coupon weight loss measurement per ASTM G31-72 provides a corrosion rate of 0.032 mm/year versus 0.258 mm/year without inhibitor. Compatibility with subsequent electroplating processes is assessed by Hull cell test (267 mL, 2 A for 10 min): the additive does not cause streaking or pitting in the 1–3 A/dm² current density range, making it suitable for use in through-hole copper plating baths where acid copper sulfate solutions (200 g/L CuSO₄·5H₂O, 60 g/L H₂SO₄) require 5–10 mg/L inhibitor for uniform deposition. Toxicological profiling for industrial use includes an acute fish toxicity test (OECD 203) with an LC₅₀ (96 h) > 100 mg/L for Danio rerio, classifying it as category 3 under GHS. Wastewater treatability is verified by ISO 7827 28-day ready biodegradability test: 62 % degradation, surpassing the 60 % threshold for “readily biodegradable.”

Electrochemical Parameters for Copper Corrosion Inhibition by 0.5–2.0 mM Thiazole Ester in 1 M HCl

Concentration (mM)

E_corr (mV vs. SCE)

i_corr (µA/cm²)

Inhibition Efficiency (%)

Polarization Resistance (Ω·cm²)

0 (blank)

−212

12.7

—

342

0.5

−225

5.8

54.3

748

1.0

−218

2.9

77.2

1,493

2.0

−234

1.6

87.4

2,633

As a 1,3-thiazole-5-carboxylate derivative bearing a pyridin-3-yl substituent at the 2-position and a methyl group at the 4-position, 4-Methyl-2-(pyridin-3-yl)-1,3-thiazole-5-carboxylate (CAS 1005197-15-6, molecular formula C₁₁H₁₀N₂O₂S, molecular weight 234.27 g·mol⁻¹) serves as a heterocyclic building block for medicinal chemistry and materials science research. The ester moiety is typically presented as the methyl carboxylate, enabling selective functionalization at the 5-position via hydrolysis or transesterification while the pyridine nitrogen offers a scaffold for coordination chemistry or late-stage N-oxidation. This compound is supplied as a crystalline solid with a melting range of 112–115 °C (determined by ASTM E794-06 differential scanning calorimetry) and an HPLC purity specification of ≥98.0 area% at 254 nm. Distinct from 2-phenyl- or 2-thienyl-thiazole carboxylates, the electron-deficient pyridin-3-yl group modulates the electron density on the thiazole ring, shifting reactivity in electrophilic substitutions and altering the acidity of the 5-carboxyl proton in the free acid form.

What Role Does the Pyridine Nitrogen Play in Direct Arylation Protocols?

In palladium-catalyzed C–H activation at the thiazole 5-position, the pyridin-3-yl substituent acts as an intramolecular directing group, coordinating Pd(II) intermediates through the pyridine nitrogen lone pair. This chelation-controlled pathway, confirmed by stoichiometric NMR titration experiments using Pd(OAc)₂ in DMSO-d₆, lowers the activation energy for C–H bond cleavage relative to non-directing aryl groups. Under standard conditions—5 mol% Pd(OAc)₂, 2 equiv K₂CO₃, DMAc at 110 °C for 16 h—the regioselectivity for 5-arylation exceeds 20:1 over competitive 3-position functionalization, whereas the 2-phenyl analogue gives a 6:1 ratio under identical conditions. The presence of the pyridine also enables post-functionalization: quaternization with methyl iodide at the pyridine nitrogen converts the directing group into an electron-withdrawing pyridinium, deactivating the thiazole toward further electrophilic attack and providing a synthetic branching point that is unavailable with carbocyclic aryl-substituted thiazole esters.

This reactivity profile places 4-methyl-2-(pyridin-3-yl)-1,3-thiazole-5-carboxylate in a distinct category relative to thiazoles bearing 4-pyridyl or 4-fluorophenyl substituents. The meta-relationship of the pyridine nitrogen to the thiazole ring in the 3-pyridyl isomer minimizes steric congestion at the Pd center during cis-coordination, preserving catalytic turnover frequency. In a head-to-head comparison of initial rates under identical catalyst loading, the 3-pyridyl substrate exhibited a turnover frequency of 1.8 h⁻¹, versus 0.6 h⁻¹ for the 4-pyridyl isomer and 0.3 h⁻¹ for the 2-pyridyl analogue (rate data derived from ¹H NMR monitoring of substrate consumption at 15% conversion). This difference is exploited in convergent syntheses of kinase inhibitor libraries where precise control over arylation sequence is required.

Analytical Specifications and Release Methodology

Each manufactured batch of the methyl ester is released against a specification that combines chromatographic purity, residual solvents, water content, and identity confirmation. The primary assay uses reversed-phase HPLC on a C₁₈ column (150 × 4.6 mm, 5 μm) with a mobile phase gradient of 0.1% trifluoroacetic acid in water/acetonitrile. Detection at 254 nm is supplemented by diode-array purity assessment at 210–400 nm. Identity is confirmed by ¹H (DMSO-d₆, 400 MHz) and ¹³C NMR against a certified reference spectrum; the characteristic singlet for the 5-methyl ester appears at δ 3.88 ± 0.03 ppm, while the pyridine C₂-H resonance is observed as a doublet at δ 9.12 (J = 2.1 Hz). LC-MS (ESI+) yields the [M+H]⁺ ion at m/z 235.1.

Release specifications for 4-methyl-2-(pyridin-3-yl)-1,3-thiazole-5-carboxylate
Test	Method/Standard Reference	Acceptance Criterion
Assay (HPLC, area%)	In-house SOP-QC-2041 (USP <621>)	≥98.0%
Water content	Karl Fischer coulometry, USP <921> Method Ic	≤0.5% w/w
Residual solvents (GC-HS)	USP <467> Procedure A	MeOH ≤3000 ppm, EtOAc ≤5000 ppm, DMF ≤880 ppm
Melting point (DSC onset)	ASTM E794-06	112–115 °C
Heavy metals (ICP-MS)	USP <232>/<233>	Pb ≤10 ppm, Cd ≤2 ppm, As ≤1.5 ppm
Palladium content	Acid digestion followed by ICP-OES	≤20 ppm

Storage stability studies under ICH Q1A(R2) conditions demonstrate no significant degradation at 25 °C/60% RH in sealed LDPE double-bagged packaging over 24 months. Exposing the powder to 75% RH at 40 °C without primary desiccant results in a 1.2% increase in the free acid impurity after 4 weeks, attributed to ester hydrolysis by absorbed moisture. Consequently, handling at ambient relative humidity above 60% should be limited to ≤30 min, or the compound should be pre-dried (40 °C under vacuum for 4 h) before use in water-sensitive chemistries.

In continuous flow hydrogenation processes utilizing a ThalesNano H-Cube Pro reactor with a 10 mol% Pd/C cartridge, the pyridine ring of 4-methyl-2-(pyridin-3-yl)-1,3-thiazole-5-carboxylate undergoes partial hydrogenation to piperidine at hydrogen pressures exceeding 50 bar and temperatures above 80 °C. Carefully controlling the H₂ flow to 30 mL·min⁻¹ and substrate concentration to 0.05 M in ethanol limits over-reduction to <3 area% while still permitting clean dehalogenation of companion aryl bromide intermediates in stepwise library synthesis. This sensitivity to over-reduction differs markedly from the 2-phenyl-thiazole analogue, which tolerates 90 bar without ring saturation, making the pyridyl derivative the preferred substrate only when downstream functionalization of the saturated heterocycle is desired.

When 4-Methyl-2-Pyridin-3-Yl-1,3-Thiazole-5-Carboxylate Replaces 2-Phenyl Analogues in Fragment-Based Screening

In fragment-based drug discovery campaigns targeting bromodomain or kinase ATP-binding pockets, the 3-pyridyl thiazole carboxylate provides a bidirectional hydrogen-bonding motif that is absent in the purely hydrophobic 2-phenyl scaffold. Isothermal titration calorimetry measurements (performed on a Malvern MicroCal PEAQ-ITC at 298 K) against the BRD4(1) bromodomain revealed a K_d of 85 ± 12 μM for the methyl ester, whereas the 2-phenyl analogue showed no detectable binding at concentrations up to 500 μM. The pyridine nitrogen accepts a hydrogen bond from the conserved Asn140 side chain, while the ester carbonyl engages a structural water molecule coordinated to Tyr97. X-ray co-crystal structures deposited under PDB 6Y3M confirm the binding pose, with the 4-methyl group occupying a small hydrophobic subpocket formed by Trp81 and Pro82. This validated fragment hit is used as a starting point for structure-guided elaboration, where the methyl ester is hydrolyzed to the carboxylic acid (using LiOH in THF/water, 0 °C to rt, 2 h) to improve solubility and engage additional salt-bridge interactions.

Unlike the corresponding ethyl or tert-butyl esters, the methyl carboxylate hydrolyzes cleanly without competing decarboxylation at the 5-position. Thermogravimetric analysis coupled with mass spectrometry (TGA-MS) of the free acid derivative shows decarboxylation onset at 190 °C, well above typical reaction workup temperatures, whereas the ethyl ester eliminates ethylene at 170 °C in the presence of trace base. This thermal stability margin makes the methyl ester the default choice for library production where final compounds must survive high-temperature amidation or microwave-assisted coupling. In an amidation screen with 48 aliphatic and aromatic amines using HATU in DMF at 25 °C—with the substrate pre-dried at 40 °C under vacuum to a water content ≤0.1% w/w—the average isolated yield was 84 ± 6%, with the main side product being the N-acylurea adduct (≤5% when 2.5 equiv of DIPEA is employed).

While the ester functionality provides a convenient handle for subsequent hydrolysis or aminolysis, direct handling of the neat compound on a kilogram scale in a pilot plant setting requires attention to dust generation. Particle size distribution analysis (Malvern Mastersizer 3000, dry dispersion at 2 bar) on three representative lots showed a D_v90 of 180–220 μm and a D_v10 below 30 μm. The fines fraction (≤10 μm) exhibited adhesive cohesion on stainless steel surfaces at relative humidity above 40%, necessitating the use of conductive FIBC (Type C) with grounding during transfer. Nitrogen-inerted glovebox conditions (<1 ppm O₂, <1 ppm H₂O) extend open handling windows by preventing static-induced agglomeration and hydrolytic degradation of the methyl ester.

A direct comparison of physical properties and reactivity profiles between 4-methyl-2-(pyridin-3-yl)-1,3-thiazole-5-carboxylate and two close structural analogues is summarized below. These data were generated on a single HPLC system and DSC instrument to minimize inter-laboratory variance.

Comparative analysis of 2-aryl-4-methyl-1,3-thiazole-5-carboxylic acid methyl esters
Property	2-(Pyridin-3-yl)	2-Phenyl	2-(Thien-2-yl)
HPLC retention time (min)	4.82	6.31	6.02
Melt onset (DSC, °C)	112.4	94.8	107.2
Solubility in DMF at 25 °C (mg·mL⁻¹)	>200	>200	>200
Hydrolysis half-life at pH 10, 25 °C (h)	2.1	8.7	5.3
Pd catalyst compatibility (initial TOF, h⁻¹)	1.8	0.4	1.2
N-Oxidation selectivity (mCPBA, CH₂Cl₂)	Pyridine N-oxide exclusive	N/A	S-Oxide 8:1 favouring thiazole

The enhanced alkaline lability of the pyridyl derivative, driven by the electron-withdrawing effect of the pyridine ring on the thiazole π-system, means that saponification protocols must use exactly 1.05 equiv of LiOH at 0–5 °C to avoid decarboxylation. When the same protocol is applied to the 2-phenyl analogue, 2.0 equiv of base and 25 °C are routinely tolerated, underlining the need for tailored workups rather than a uniform procedure across 2-arylthiazole esters. Regulatory classification: the compound is listed in the EINECS inventory under generic 1,3-thiazole derivatives and does not fall under REACH Annex XVII restrictions. For shipment, it is classified as non-hazardous under DOT/ADR 49 CFR 172.101, though local regulations concerning nitrile-containing dusts should be consulted when quantities exceed 25 kg per container.

HS Code	217948
Chemical Formula	C11H9N3O2S
Molecular Weight	247.27
Appearance	Solid (usually)
Physical State At Room Temp	Solid
Melting Point	Varies, needs experimental determination
Boiling Point	Varies, needs experimental determination
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in some organic solvents like DMSO
Odor	Typically odorless or with a faint, characteristic odor
Stability	Stable under normal conditions, but may react with strong oxidizing agents
Pka	Data may be needed from specific studies
Logp	Calculated value would depend on specific methods, indicates lipophilicity

Packing	100g of 4 - Methyl - 2 - Pyridin - 3 - Yl - 1,3 - Thiazole - 5 - Carboxylate in sealed chemical - grade bag.
Shipping	4 - Methyl - 2 - pyridin - 3 - yl - 1,3 - thiazole - 5 - carboxylate is shipped in properly labeled, sealed containers. Shipment follows strict chemical transport regulations to ensure safety during transit.
Storage	4 - Methyl - 2 - pyridin - 3 - yl - 1,3 - thiazole - 5 - carboxylate should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and exposure to air, which could potentially cause degradation. Store separately from incompatible substances, like strong oxidizing agents or acids, to avoid chemical reactions.