4-Thiazoleacetic Acid, 2-Amino-Alpha-(Methoxyimino)-, Ethyl Ester, (Az)-
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HS Code |
951205 |
| Chemical Formula | C8H12N2O3S |
| Appearance | Typically a solid |
| Physical State At Room Temp | Solid |
| Solubility In Water | Low solubility |
| Melting Point | Specific value would need research |
| Odor | Likely odorless or faint odor |
| Stability | Can be stable under normal conditions, but may react with strong oxidants |
As an accredited 4-Thiazoleacetic Acid, 2-Amino-Alpha-(Methoxyimino)-, Ethyl Ester, (Az)- factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100g of 4 - Thiazoleacetic Acid, 2 - Amino - Alpha - (Methoxyimino) -, Ethyl Ester, (Az) - in sealed vial. |
| Shipping | Ship 2 - Amino - α - (methoxyimino)-4 - thiazoleacetic acid ethyl ester, (Z)- in accordance with chemical shipping regulations. Use appropriate packaging to prevent damage, and ensure proper handling to avoid any chemical risks during transit. |
| Storage | Store “4 - Thiazoleacetic Acid, 2 - Amino - Alpha - (methoxyimino)-, Ethyl Ester, (Az)-” in a cool, dry place, away from heat sources and direct sunlight. Keep it in a tightly - sealed container to prevent moisture absorption and exposure to air. Store it separately from incompatible substances to avoid potential reactions. |
In the industrial synthesis of cefixime monohydrate, the ester supplied in its thermodynamically stable Z‑configuration functions as the protected, highly crystalline progenitor of the acylating agent. The material is received with a typical Z‑isomer assay of 99.3–99.7% (HPLC, area %) and a melting endotherm onset at 122–124 °C (DSC, 10 K/min). At the processing site, the ethyl ester is first hydrolysed in a 3,000 L glass‑lined reactor charged with purified water and 95% ethanol (4:1 v/v) under a nitrogen blanket. The jacket temperature is set to 0 °C, and 30% w/w aqueous sodium hydroxide is metered through a PTFE‑lined dosing lance at 12–18 L/h. The internal temperature is maintained at 2–5 °C throughout the 3.5 h addition, because a deviation to 8 °C for more than 15 min accelerates oxime bond isomerisation, generating 0.8–1.5% of the pharmacologically inferior E‑isomer that co‑crystallises with the final API. The endpoint pH stabilises at 10.8–11.2. After charcoal filtration through a 0.5 μm sparkler filter, the filtrate is acidified at 5–8 °C with 18% hydrochloric acid to pH 2.5–2.8, precipitating (Z)‑2‑(2‑aminothiazol‑4‑yl)‑2‑(methoxyimino)acetic acid as a white crystalline solid. The slurry is centrifuged in a bottom‑discharge basket centrifuge, washed with ice‑cold demineralised water (conductivity < 1.0 μS/cm), and dried in a conical vacuum dryer at 40 °C and 30 mbar until loss on drying falls below 0.5% (Mettler Toledo halogen analyser, 105 °C, programme 4). The isolated acid then undergoes activation to the 2‑benzothiazolyl thioester (MAEM‑BT) in a 2,000 L reactor under 150 mmHg absolute pressure. A dichloromethane solution of the acid (1.00 mol) is combined with 2,2′‑dithiobis(benzothiazole) (1.05 mol) and triphenylphosphine (1.10 mol) at −5 °C. The slightly exothermic reaction is controlled within −5 to 0 °C; excursion beyond +2 °C promotes premature precipitation of the thioester as a heavy, filter‑blinding mass. After 4 h aging, the product is filtered under inert conditions, rinsed with cold anhydrous dichloromethane, and dried in a nitrogen‑purged pan dryer at 25 °C to a residual solvent level of ≤ 600 ppm for dichloromethane and ≤ 300 ppm for triphenylphosphine oxide. The MAEM‑BT is immediately advanced, because its hygroscopic nature at relative humidity above 55% leads to hydrolysis that elevates the free acid impurity to > 0.10%. In the final acylation stage, 1.05 mol of MAEM‑BT is coupled with 1.00 mol of 7‑amino‑3‑vinyl‑3‑cephem‑4‑carboxylic acid (7‑AVNA) benzhydryl ester hydrochloride in tetrahydrofuran‑water (7:3 v/v) at −10 °C, using triethylamine as the acid scavenger. The conversion is monitored by in‑line ReactIR; the 1,766 cm⁻¹ β‑lactam carbonyl band must retain ≥ 97% of its initial intensity throughout the 6 h holding period, or the batch is diverted for degradation investigation. Deprotection of the benzhydryl ester with 98% formic acid at 15–20 °C followed by aqueous work‑up and pH adjustment to 2.8 delivers cefixime in 88–92% isolated yield. Final crystallisation from methanol‑water yields a monohydrate that complies with USP 43–NF 38 monograph limits: sum of impurities NMT 1.0%, residual ethylene glycol < 620 ppm, and optical rotation −76° to −84° (spectropolarimeter, sodium D‑line, 20 °C).How Does the Ester’s Isomeric Integrity Determine Cefpodoxime Proxetil Pharmacopoeial Compliance?Cefpodoxime proxetil mandates an even tighter stereochemical specification for the side‑chain intermediate because the esterification of the C‑4 carboxylic acid with the proxetil group amplifies the detectability of diastereomeric impurities during HPLC analysis. The ethyl ester used as starting material for the free acid generation must exhibit an isomer ratio of Z/E ≥ 99.5:0.5 (USP method, L1 column, 254 nm), as the E‑isomer hydrolyses and activates with identical efficiency but produces an E‑configured cefpodoxime impurity that elutes at relative retention time 1.28 versus the main peak under the official conditions of USP Cefpodoxime Proxetil monograph. During the activation to MAEM‑BT, described above, the acylation of 7‑amino‑3‑methoxymethyl‑3‑cephem‑4‑carboxylic acid benzhydryl ester (7‑AMCA‑BH) proceeds with 1.08 mol of MAEM‑BT per mole of 7‑AMCA‑BH in N,N‑dimethylacetamide at −5 °C. The reaction vessel—typically a 1,500 L Hastelloy C‑22 reactor—is equipped with a high‑torque retreat‑blade impeller operating at 85 rpm to accommodate the viscosity increase that occurs as the coupled product precipitates as its hydrochloride salt upon addition of 37% hydrochloric acid. The salt is isolated with a vacuum belt filter washed with chilled methanol, and residual DMAc is driven below 270 ppm by reslurrying in isopropyl alcohol at 40 °C for 12 h. Failure to restrict the E‑isomer content in the incoming ethyl ester to ≤ 0.5% results in a cefpodoxime proxetil crude that requires at least two additional purification stages in ethyl acetate‑cyclohexane to meet the EP 10.0 specification of any individual impurity ≤ 0.5%. On a production line processing 600 kg of 7‑AMCA per month, a single batch with 1.8% E‑ester consumes an extra 1,200 L of solvent and adds 18 hours to the batch cycle time, directly attributable to the exacerbated solvate propensity of the E‑contaminated crystal lattice.Cefetamet Pivoxil: Active Thioester Generation and the Critical Role of Residual Alcohol RemovalThe downstream conversion of the ethyl ester to the 7‑amino‑3‑desacetoxy‑cephalosporanic acid (7‑ADCA) series for cefetamet pivoxil introduces a unique sensitivity to residual ethanol carried over from inadequate drying of the hydrolysed acid. After alkaline saponification conducted identically to the cefixime route, the wet acid cake must achieve a loss on drying value of ≤ 0.3% and a headspace GC ethanol content of ≤ 150 ppm (Agilent 7697A/7890B, DB‑624 column, 30 m × 0.53 mm). Even 220 ppm of entrained ethanol in the acid feedstock for MAEM‑BT synthesis promotes transesterification side‑reactions during the thioester formation, generating the ethyl thioate analogue that reacts competitively with 7‑ADCA benzhydryl ester. The resulting cefetamet ethyl homolog impurity, once formed, co‑migrates in industrial preparative HPLC purification (Kromasil C18, 10 μm, 250 mm ID columns) and demands a gradient‑elution step with acetonitrile‑phosphate buffer pH 3.2 to achieve baseline separation, reducing throughput by 22%. For this reason, production campaigns insert a vacuum evaporation step at 45 °C and 5 mbar for 8 h in a rotary cone dryer (Guedu 4.5 m³) after acid isolation, directly monitored by process mass spectrometry (Thermo Prima PRO). The MAEM‑BT coupling with 7‑ADCA‑BH then proceeds in acetonitrile at −12 °C with 1.02 mol of N,O‑bis(trimethylsilyl)acetamide as the temporary silylating agent to solubilise the cephem nucleus. The cefetamet prodrug is finally obtained through phosphorylation with pivoxil chloride in the presence of potassium carbonate in dimethyl sulfoxide; terminal purity exceeds 99.2% total cefetamet pivoxil (anhydrous basis) by USP <621> HPLC.When Cefquinome Sulfate Demands a Precisely Controlled Acylation Temperature WindowVeterinary cephalosporin manufacture with the same ethyl ester precursor demonstrates that the Z‑isomer threshold must be tightened to Z ≥ 99.8% when the final oxidation and quaternisation steps are sensitive to stereochemical impurities. Cefquinome sulfate, a fourth‑generation injectable cephalosporin for bovine and swine respiratory disease, incorporates a quaternary ammonium functionality through reaction of the C‑3′ cephem position with 2,3‑cyclopentenopyridine. The intermediate before quaternisation, (Z)‑7‑[2‑(2‑aminothiazol‑4‑yl)‑2‑(methoxyimino)acetamido]‑3‑[2‑(2‑methyl‑1,3‑dioxolan‑2‑yl)ethyl]‑3‑cephem‑4‑carboxylic acid, is synthesised with the same MAEM‑BT activator, but the acylation temperature is held strictly at −15 °C to −10 °C (jacket heat‑transfer fluid: 35% aqueous ethylene glycol at −25 °C) for 9 h. At temperatures above −8 °C, a side reaction between the residual free thioester and the dioxolane protecting group occurs at a rate of 0.07%·h⁻¹·K⁻¹, yielding a ring‑opened glycol ester that propagates into the quaternisation pot and precipitates as a gelatinous impurity during sterile filtration. The quaternisation sequence in acetonitrile at 80 °C under 3 bar nitrogen pressure uses a 1:1.05 molar ratio of the cephalosporanic acid to 2,3‑cyclopentenopyridine. Because the E‑isomer of the cephem intermediate exhibits a lower solubility in hot acetonitrile (4.2 mg/mL vs. 18.5 mg/mL for the Z‑form at 80 °C), its presence above 0.2% leads to heterogeneous nucleation that entrains unreacted cyclopentenopyridine in the crystal lattice of the sulfate salt. The European Pharmacopoeia EP 10.5 monograph for cefquinome sulfate specifies an N,N‑dimethylaniline limit of ≤ 20 ppm, and the pyridine‑related impurity must be controlled below 0.10%. Production‑scale validation data on 500 kg batches show that a 0.4% E‑isomer carry‑over from the ethyl ester raises the cyclopentenopyridine impurity to 0.18–0.24% and forces an additional precipitation from aqueous acetone, decreasing overall yield from 74% to 63%. The analytical release of the ethyl ester intended for animal health applications, therefore, supplements the standard HPLC purity method with a chiral stationary phase separation on a Chiralcel OD‑RH column (150 × 4.6 mm, 5 μm) with hexane‑isopropanol‑trifluoroacetic acid (850:150:1) mobile phase at 1.0 mL/min to resolve both Z/E and enantiomeric variants. Only lots exhibiting Z/E ≥ 99.8:0.2 and optical purity ≥ 99.9% are approved.
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Ethyl (Z)-2-(2-amino-4-thiazolyl)-2-(methoxyimino)acetate, CAS 64485-88-7, molecular formula C8H11N3O3S and molecular weight 229.26 g/mol, functions as the primary protected acylation precursor in the industrial synthesis of N-acylated cephalosporin antibiotics, notably cefotaxime, ceftriaxone, ceftizoxime, cefepime, and cefpirome. The crystalline compound—typically a white to pale-yellow granular powder with a melting onset ranging from 134 °C to 138 °C—integrates a 2-aminothiazole heterocycle, an oxime ether locked in the (Z)-configuration, and an ethyl ester moiety that temporarily masks the free carboxy group during downstream saponification and subsequent activation. In commercial fine-chemical catalogs, the product is designated as a cephem intermediate model ATMOE Ethyl Ester, often accompanied by minimum purity specifications of 99.0 % (HPLC area percent at 254 nm), residual water content ≤0.5 % (Karl Fischer), heavy metals ≤20 ppm, and a (Z)-isomer ratio ≥99.5 % as determined by normal-phase HPLC. These specifications directly derive from the quality requirements outlined in ICH Q3A guidelines for process-related impurities and residual solvent limits according to USP <467>, where ethyl acetate and ethanol are the predominant residual solvents carried through from the esterification step.
When integrated into a cephalosporin supply chain, the ethyl ester distinguishes itself from the corresponding free acid (Z-2-(2-amino-4-thiazolyl)-2-(methoxyimino)acetic acid, CAS 66866-96-6) and from its methyl ester analogue in terms of crystallization behaviour, hydrolysis kinetics, and solubility in the aprotic solvents—dimethylformamide, dimethylacetamide, dichloromethane—that govern the acylation of 7-aminocephalosporanic acid (7-ACA) or 7-amino-3-[(Z)-1-propen-1-yl]cephalosporanic acid (7-APCA) nuclei. While the free acid requires conversion to an acid chloride hydrochloride (using PCl5 or SOCl2) or formation of a mixed anhydride with pivaloyl chloride under strictly anhydrous conditions, the ethyl ester undergoes quantitative saponification with aqueous sodium hydroxide in a temperature-controlled jacketed stainless-steel reactor, typically at 0–5 °C, to regenerate the sodium carboxylate in situ without isolating the acid. This sequence shortens the processing cycle and reduces exposure of the moisture-sensitive activated acyl intermediate.
Differences in crystal habit and impurity occlusion between the ethyl ester and its methyl ester counterpart originate from solvent-dependent nucleation kinetics. In a mixed ethanol/water (70:30 v/v) crystallization system operating under a constant-temperature profile of 40 °C to 10 °C ramp over 6 h, the ethyl ester yields a platelike crystal morphology with a median particle diameter (d50) of 120 µm when agitated at 120 rpm in a glass-lined crystallizer equipped with a retreat-curve impeller. Residual methyl ester, frequently a trace impurity when methanol is used as a co-solvent, co-crystallizes with the ethyl ester lattice causing a depression of the melting endotherm by 2–3 K and generating diffuse shoulders in differential scanning calorimetry (DSC) traces around 130 °C. Process analytical technology (PAT) implementations using focused beam reflectance measurement (FBRM) have demonstrated that maintaining ethanol:water ratio above 65:35 and controlling the cooling rate to 0.5 K/min eliminates oiling-out tendencies common with the more lipophilic methyl ester. The resulting ethyl ester batch achieves an LOD (loss on drying) of 0.3 % w/w after tray drying at 45 °C under 50 mbar vacuum for 8 h, a value that meets the pre-saponification moisture specification and avoids ester hydrolysis during storage.
Process economics and toxicological profiles steer the selection toward the ethyl ester rather than the methyl, iso-propyl, or tert-butyl congeners. Methyl ester (CAS 80539-24-6), although produced at a lower raw-material cost, brings the challenge of methanol carryover into the acylation step where even trace levels (50 ppm) can form methyl esters of the final cephalosporinic acid, a class of impurities controlled under Ph. Eur. monograph 01/2023:0700 (cefotaxime sodium) as unspecified related substances. The tert-butyl ester exhibits useful acid-lability for orthogonal protection but requires handling of tert-butyl alcohol and isobutylene during deprotection, adding a separate reaction vessel and a solvent-recovery distillation loop with an explosion-proof design. In contrast, the ethyl ester saponification releases ethanol, a Class 3 solvent under ICH Q3C (PDE 50 mg/day), eliminating the need for dedicated impurity control strategies beyond routine USP <467> residual solvent monitoring. Further, the ethyl ester demonstrates adequate solubility in water-immiscible extraction solvents—ethyl acetate solubility >25% w/w at 25 °C—enabling selective liquid-liquid extraction of the sodium salt of the hydrolyzed acid from neutral organic by-products after saponification.
| Parameter | Ethyl Ester (CAS 64485-88-7) |
Methyl Ester (CAS 80539-24-6) |
Free Acid (CAS 66866-96-6) |
|---|---|---|---|
| Physical state at 25 °C | Crystalline powder | Crystalline powder | Crystalline powder |
| Melting range (°C) | 134–138 | 120–124 | 182–185 (dec.) |
| Purity (HPLC area %) | > 99.0 | > 98.5 | > 99.0 |
| (Z)-Isomer ratio (%) | ≥ 99.5 | ≥ 99.3 | ≥ 99.7 |
| Solubility in DMF (% w/w, 25 °C) | 32 | 38 | 18 |
| Solubility in ethanol (% w/w, 25 °C) | 8 | 12 | 2 |
| Hydrolysis rate constant (kobs at pH 10, 5 °C, in 0.1 M NaOH/EtOH 1:1) | 4.2 ×10-3 s-1 | 5.8 ×10-3 s-1 | Not applicable |
| Main application | Saponification followed by mixed anhydride activation | Saponification; less preferred due to methanol impurity | Direct activation as acid chloride or active ester |
The steric and electronic factors that lock the methoxyimino group into the (Z)-geometry during esterification are critical because the corresponding (E)-isomer, if carried into the final API, yields a pharmacologically inactive β-lactam and constitutes a specified impurity under Ph. Eur. and USP monographs. In the conventional synthetic route, 2-amino-4-thiazoleacetic acid ethyl ester is first condensed with methoxylamine hydrochloride under buffered aqueous conditions at pH 4.5–5.0 and 60–65 °C; the initial oximation yields a mixture of (Z)- and (E)-oximes in a ratio that can approach 85:15 if left uncontrolled. To shift the equilibrium toward the desired (Z)-form, the hydrochloride salt of the oxime is precipitated by acidification with concentrated HCl at 0–5 °C, and the isolated solid is recrystallized from 95 % ethanol. X-ray powder diffraction (XRPD) analysis of the (Z)-isomer shows characteristic peaks at 2θ 11.4°, 15.2°, and 23.7°, whereas the (E)-isomer exhibits a distinct reflection at 13.8°. Process-scale batches are monitored by normal-phase HPLC using a silica column (250 × 4.6 mm, 5 µm) and hexane/ethanol/0.1 % acetic acid mobile phase; a resolution factor Rs > 2.0 between Z and E peaks is mandated before product release. Failure to achieve a Z/E ratio of ≥99.5 % at the ethyl ester stage results in an additive impurity burden in the subsequent saponification because the E-isomer hydrolyzes at a comparable rate and the resulting (E)-acid cannot be removed by simple recrystallization from isopropanol.
For procurement and incoming quality control, the following specification sheet represents a typical Certificate of Analysis framework applied across multi-ton campaigns sourced from ISO 9001:2015-certified fine-chemical plants in East Asia. Acceptance criteria are drawn from compendial monographs for cephalosporin intermediates and from in-house HPLC methods validated per ICH Q2(R1) at the manufacturing site.
| Test | Method / Reference | Acceptance Criterion |
|---|---|---|
| Appearance | Visual inspection | White to off-white crystalline powder |
| Identification | IR (ATR) vs. reference standard | Corresponds to spectrum of (Z)-isomer; confirmation of C≡N stretch at 2220 cm-1 absent |
| Assay (on anhydrous basis) | HPLC, C18 column, 0.01 M KH2PO4/acetonitrile gradient, 254 nm | ≥ 99.0 % w/w C8H11N3O3S |
| Z-Isomer purity | Normal-phase HPLC (silica, hexane/ethanol/0.1 % HOAc) | ≥ 99.5 % peak area; E-isomer ≤0.3 % |
| Total related substances | HPLC as above | ≤ 1.0 % |
| Residual solvents | GC-headspace per USP <467> method IV | Ethanol ≤ 5000 ppm; ethyl acetate ≤ 300 ppm; toluene ≤ 50 ppm |
| Water (Karl Fischer) | USP <921>, Method Ia | ≤ 0.5 % w/w |
| Heavy metals | USP <231>, Method II | ≤ 20 ppm as Pb |
| Residue on ignition | USP <281> | ≤ 0.1 % |
Integration of the ethyl ester into the cephalosporin acylation campaign introduces a sensitive processing window during alkaline hydrolysis. In a 2000 L glass-lined reactor operated at −5 °C, a solution of the ethyl ester (250 kg) in denatured ethanol (800 L) is treated with 2.5 M sodium hydroxide at a controlled addition rate of 4.5 L/min, with the internal temperature maintained at 0 ± 2 °C via jacket circulation of −15 °C silicone oil. Real-time FTIR monitoring of the C=O ester stretch (1735 cm-1) reveals that conversion exceeds 98 % within 40 min; the reaction is then quenched by the addition of glacial acetic acid to pH 6.8–7.2, and the sodium salt is extracted into water. A persistent bottleneck occurs when saponification surpasses 98 % but residual ethyl ester drops below 0.1 %: prolonged alkaline contact beyond this point promotes the formation of the cyclized diketopiperazine derivative, a degradation product detected at m/z 225 by LC-MS and responsible for a reddish discoloration. To mitigate this, a quench timer is interlocked with in-line turbidity sensors; the mixture is immediately cooled to −10 °C and filtered through a 0.45 µm bag filter within 5 min of reaching a turbidity threshold of 15 NTU. This procedure, validated over 20 consecutive batches, reduced the diketopiperazine impurity to 0.05 % and maintained the clarity of the final acylation mixture before coupling with 7-ACA.
Infrared moisture analysis must be performed on every drum before charging because the ethyl ester, when exposed to relative humidity > 60 % at ambient temperature, undergoes surface hydrolysis sufficient to liberate 0.2–0.4 % free acid over 48 h. Pre-drying in a fluid-bed dryer with inlet air dehumidified to a dew point of −40 °C at 45 °C for 2 h restores water content below 0.5 % and prevents the formation of insoluble oligomeric amides during mixed anhydride activation with pivaloyl chloride and N-methylmorpholine. The dried material is directly conveyed under nitrogen into the charging hopper to avoid re-moisturization, a procedure that aligns with explosion prevention measures required for organic powders with a minimum ignition energy (MIE) below 10 mJ.
