Ethyl 2-Aminothiazole-5-Carboxylate Hydrochloride

The hydrochloride salt is introduced into the acylation reactor at 1.00–1.05 molar equivalents relative to 2,4-dichloro-6-methylbenzoic acid in the commercial manufacturing route to the BCR-ABL tyrosine kinase inhibitor dasatinib. The process is executed in a glass-lined vessel charged with anhydrous N,N-dimethylformamide and N,N-diisopropylethylamine at 2.2–2.5 equivalents under a dry nitrogen sweep to prevent premature hydrolysis of the acid chloride intermediate. Cooling jacket fluid is maintained at 0–5 °C during the addition of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 1-hydroxybenzotriazole as the coupling system; adiabatic temperature rise beyond 8 °C causes a detectable increase in the N-acyl urea by-product and must be avoided by staged solid dosing. Agitation is sustained at 120–150 rpm with a retreat-curve impeller for 16–20 hours post-addition, after which in-process HPLC (C18 column, UV 254 nm) confirms residual free amine below 0.5 area-%. The batch is quenched into purified water at 2–5 °C, the crude ethyl 2-(2,4-dichloro-6-methylbenzamido)thiazole-5-carboxylate is isolated by centrifuge filtration and washed with chilled ethanol/water (1:1 v/v) until conductivity of the filtrate drops below 50 µS/cm. The wet cake is dried in a double-cone vacuum dryer at 45±5 °C and ≤10 mbar to a loss-on-drying specification of <0.5%. This intermediate then undergoes chlorination with phosphorus oxychloride, substitution with N-(2-hydroxyethyl)piperazine, and final deprotection to yield dasatinib monohydrate. Compliance with ICH Q7 requires dedicated equipment or validated cleaning protocols because of the cytostatic potency; residual solvent limits conform to ICH Q3C Option 1 concentrations—DMF <880 ppm, methylene chloride <600 ppm, and ethanol <5000 ppm. A nitrosamine risk assessment conducted per EMA/CHMP/592412/2021 evaluates secondary amine sources and nitrite traces in excipients; the synthetic sequence is designed to avoid nitrosating conditions by eliminating nitrite quenching steps in acidic media. The terminal dosage form is dasatinib monohydrate film-coated tablets presenting at strengths of 20 mg, 50 mg, 70 mg, 80 mg, 100 mg, and 140 mg for chronic myeloid leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia indications.

To What Extent Does Ring Planarity Alter Peptide Backbone Dihedral Angles When the Thiazole is Incorporated as a Proline Bioisostere?

In solid-phase peptide synthesis, 2-aminothiazole-5-carboxylic acid—obtained via quantitative saponification of the ethyl ester hydrochloride with 2 N lithium hydroxide in THF/water (3:1) at 0–5 °C followed by acidification to pH 3–4 and ethyl acetate extraction—is converted to its N-Boc or N-Fmoc derivative for use as a conformationally restricted proline surrogate. The ethyl ester hydrochloride serves as the long-term storage form because the free acid dimerizes slowly via head-to-tail zwitterion interactions at ambient humidity; the hydrochloride salt shows less than 0.3% dimer after 12 months at 25 °C/60% RH in sealed foil laminate packaging. For manual Fmoc-solid-phase protocols on Wang resin preloaded with the first amino acid, Boc-Atc-OH is coupled at 3.0 equivalents relative to the resin loading using PyBOP (3.0 equiv.) and DIPEA (6.0 equiv.) in DMF for 2 hours with vortex agitation. Double-coupling is mandatory when the preceding residue is sterically hindered (e.g. valine or α,α-disubstituted amino acids); a Kaiser test after the first coupling must read faint blue before proceeding. The ring nitrogen of the thiazole does not require side-chain protection under standard Fmoc deprotection conditions with 20% piperidine/DMF, but prolonged exposure beyond 2 × 5 minutes can cause slight ring-opening to a thiourea detected by a low-intensity peak at m/z +18 in ESI-MS. Cleavage from the resin with TFA/TIS/H₂O (95:2.5:2.5 v/v) for 2.5 hours releases the crude peptide-thiazole conjugate, which is precipitated in cold diethyl ether and purified by reverse-phase flash chromatography (C18, 0.1% TFA in acetonitrile/water gradient). The thiazole unit reduces the trans/cis amide isomerization of the Xaa-Atc bond to a ratio exceeding 9:1 compared to approximately 4:1 for the corresponding proline peptide, as measured by 1H-1H NOESY integration of Hα sequential cross-peaks. There is no formal regulatory monograph governing these building blocks for research use; contract laboratories typically operate under ISO 9001:2015 and supply a certificate of analysis reporting HPLC purity >98 area-% at 220 nm, water content by Karl Fischer coulometry <0.5%, and residual lithium by ICP-OES <50 ppm. The terminal products are linear or cyclic modified peptides evaluated as opioid receptor ligands, thrombin inhibitors, or integrin antagonists in early discovery; published data for in vivo pharmacokinetics of 2-aminothiazole-containing peptide leads beyond rodent metabolic stability assays remain sparse.

When the hydrochloride is dissolved directly in 15% w/w hydrochloric acid at ambient temperature and combined with 0.05–0.10% w/w potassium iodide as a synergist, the resulting inhibitor cocktail exhibits a corrosion rate reduction exceeding 95% on AISI 1020 carbon steel coupons under static immersion conditions per ASTM G31-72(2004) at 60 °C for 6 hours. The addition ratio is maintained at 0.2–0.5% w/w of the hydrochloride relative to the total acidizing fluid weight; excursions above 0.6% provide no incremental protection and instead raise the total organic carbon load in the spent acid return, complicating downstream wastewater treatment in the field. Blending is performed in a polyethylene tote fitted with an air-driven mixer at 200–300 rpm until the salt is fully solvated; a slight exotherm of ΔT ≈ 3–5 °C is dissipated by natural convection if the mixing vessel is kept at ambient outdoor temperatures below 35 °C. Electrochemical impedance spectroscopy conducted with a three-electrode flat cell (Ag/AgCl reference, platinum counter electrode) at open-circuit potential over 24 hours shows the charge transfer resistance increasing from approximately 80 Ω·cm² for uninhibited acid to >2000 Ω·cm² with the full inhibitor package. The primary mechanism is anodic site blocking through chemisorption of the thiazole ring onto the steel surface, confirmed by X-ray photoelectron spectroscopy detection of N 1s and S 2p peaks after rinsing. Compliance with NACE TM0169 test protocol is standard for supplier qualification, and the product is shipped as a liquid inhibitor blend under UN 3265 (Corrosive liquid, acidic, organic, n.o.s.) classification. End-use formulations are deployed in matrix acidizing of sandstone formations and in pickling baths preceding hot-dip galvanizing lines; the final articles are inhibited acid packages sold by oilfield service companies as stimulation fluid additives.

Coupling with Pyridone Derivatives for Polyester Shade Range Extension

The ethyl ester undergoes diazotization in concentrated sulfuric acid (96–98%) at −5 to 0 °C using a slight molar excess of nitrosylsulfuric acid (1.02–1.05 equivalents), generated in situ from sodium nitrite. The resulting diazonium solution is added dropwise over 45–60 minutes to a stirred suspension of N-ethyl-3-cyano-4-methyl-6-hydroxypyridone in ice/water/methanol buffered with sodium acetate to maintain pH 3.5–4.5, and the coupling is continued for an additional 3 hours at 0–5 °C. The molar ratio of coupler to diazo component is held at 1:1.02 to ensure complete conversion of the high-value thiazole intermediate. The precipitated crude dye is isolated by vacuum filtration, washed with deionized water until the rinse is sulfate-free (tested with 10% BaCl₂ solution), and dried in a fluid-bed dryer at 80 °C for 8 hours to a moisture content below 1.0%. Adsorbable organic halogens (AOX) in the combined wastewater are controlled to a maximum of 0.5 kg per ton of dye through alkaline hydrolysis of residual diazonium before discharge, in line with OECD emission scenario document values for textile dyes. The resultant monoazo disperse dye, characterized by λₘₐₓ at 438–442 nm in acetone, is formulated as a 40% strength presscake or as a 200% granular product by standardized milling with lignosulfonate dispersants in a bead mill to a particle size D₉₀ < 2 µm. High-temperature exhaust dyeing of polyethylene terephthalate woven fabric at 130 °C for 45 minutes at a 3.0% o.w.f. shade depth yields a bright greenish-yellow hue with dry heat fixation fastness rated 4–5 under ISO 105-P01:1993. Compliance testing screens for the 24 carcinogenic amines listed in EU Regulation 2020/2096 (amending Annex XVII of REACH) with a detection limit of 30 mg/kg in the dyed textile extract; the azo linkage of this diarylide structure is proven stable under the reductive cleavage conditions of EN ISO 14362-1:2017. The terminal commercial article is a C.I. disperse yellow or orange shade used in automotive interior upholstery, where light fastness requirements exceed grade 6 under ISO 105-B02:2014 xenon-arc testing at 120 hours.

Lithiation at the 5-position of the thiazole ring is suppressed by the ester carbonyl; instead, the free amine is exploited to build a pincer-type P,N,N-ligand after condensation with 2-diphenylphosphinobenzaldehyde in refluxing toluene with azeotropic removal of water. The hydrochloride is free-based using saturated sodium bicarbonate and extracted into dichloromethane, then reacted with 1.0 equivalent of the aldehyde and 0.95 equivalents of the aminoethylpyridine fragment under Dean-Stark conditions for 6 hours. The resulting imine-thiazole intermediate is reduced with sodium triacetoxyborohydride (1.5 equiv.) in 1,2-dichloroethane at ambient temperature for 12 hours to give the ligand precursor, which is then metallated with [PdCl₂(cod)] in dichloromethane to yield the palladium(II) pre-catalyst. When employed in the asymmetric allylic alkylation of rac-1,3-diphenylprop-2-enyl acetate with dimethyl malonate, this catalyst—at a loading of 2 mol% and using N,O-bis(trimethylsilyl)acetamide as base in dichloromethane at −20 °C—delivers the alkylated product in 92% isolated yield and 88% enantiomeric excess as measured by chiral stationary phase HPLC (Chiralpak AD-H, hexane/2-propanol 90:10, 1.0 mL/min). There is no vertical standard governing the supply of such custom-synthesized ligand intermediates; the buyer’s specification typically references residual palladium by ICP-MS <10 ppm, ³¹P NMR purity >95%, and absence of free aldehyde above 0.2 area-% by GC. The tangible product at the terminal downstream stage is not a finished pharmaceutical article but an experimental chiral catalyst kit shipped under refrigeration to medicinal chemistry laboratories pursuing enantioselective C–C bond formations against prostaglandin or leukotriene receptor targets; batch-to-batch consistency in enantiomeric bias is validated by the customer upon receipt by standard test reactions with pre-agreed acceptance windows of ±3% ee.

From Laboratory Bench to Multi-Kilogram Pilot Batches Under a Quality Management System

When supplied as a non-GMP advanced intermediate for discovery screening libraries and hit-to-lead optimization, this building block is manufactured by a Hantzsch-type cyclocondensation of ethyl 2-chloro-3-oxobutanoate with thiourea in refluxing ethanol, followed by hydrochloride salt formation using HCl gas in ethyl acetate at 0–10 °C and recrystallization from ethanol/methyl tert-butyl ether. The addition ratio in the Hantzsch step is kept at thiourea 1.08 equiv. and sodium acetate 1.15 equiv. as acid scavenger; the crude ethyl 2-aminothiazole-5-carboxylate base is isolated by drowning in water, filtered, and washed until the filtrate tests neutral to litmus. The entire process is documented according to ISO 9001:2015 with a change-control log maintained for raw material source substitutions, and each lot is assigned a batch record number cross-referenced to the equipment cleaning log. A master specification sheet requires HPLC purity (220 nm) ≥ 98.0 area-%, residual chloride content 13.8–14.5% w/w by argentometric titration, water content ≤ 0.3% by Karl Fischer, heavy metals ≤ 20 ppm by USP ⟨231⟩ method II, and sulfated ash ≤ 0.1%. No material is released without a negative test for genotoxic impurities derived from the chloroester starting material, quantified by a validated UPLC-MS/MS procedure with a reporting threshold of 1 ppm. Packaging is in 1 kg, 5 kg, and 25 kg HDPE drums with double-liner LDPE bags under nitrogen purge, labeled with the Globally Harmonized System pictograms for Skin Irritant (Category 2) and Eye Irritant (Category 2A). The terminal articles that emerge from the customer’s synthesis routes include milligram-scale focused kinase libraries, gram quantities of pharmacological tool compounds dosed in rodent efficacy models, and millimoles of photoaffinity probes for target identification pull-down experiments. Published data on the long-term thermal stability of the neat hydrochloride under stressed conditions state a decomposition onset at 162±3 °C by differential scanning calorimetry at 10 K/min under nitrogen, with no exothermic event detectable below 150 °C, confirming safe headspace for kilogram-scale vacuum drying.