4-Thiazoleacetic Acid, Ethyl Ester

Reconstituted tobacco sheet processing lines routinely dose volatile top-notes at the post-drying stage when the web temperature has fallen below 40 °C. Ethyl 4‑thiazoleacetate, characterised by a threshold odour value below 0.1 µg/L in air, is applied via a calibrated glycol‑based dosing system integrated with a Cased‑Schmitz cascade humidifier. The target delivery range in finished cut filler is 0.5–3.5 ppm (mass‑dry tobacco). Above 4.0 ppm, panel data from ISO‑8589 sensory profiling indicates the onset of a metallic aftertaste that conflicts with Virginia flue‑cured top‑note profiles. Compliance with the TCCR (Tobacco Control Act) flavour additive reporting requirements and the FDA 21 CFR 101.22 labelling obligations for “artificial flavour” is mandatory when the compound is incorporated into filter ventilation chambers or direct‑expansion tobacco. The terminal manufactured articles include US‑market combustible cigarettes (FSC‑compliant bands), heat‑not‑burn sticks with crimped poly‑lactic acid filters, and oral nicotine pouches where the ester is pre‑adsorbed onto microcrystalline cellulose carriers before fluid‑bed blending.

Why Does Ethyl 4‑Thiazoleacetate Dominate Browned‑Note Profiles in Thermally Processed Savoury Flavours?

In continuous stirred‑tank reactors (CSTRs) running Maillard model systems at 110–125 °C and pH 4.8–5.5, the ethyl ester survives the first 40‑minute reaction window with less than 10 % hydrolytic loss when added post‑caramelisation but prior to the terminal cooling ramp. The precursor, 4‑thiazoleacetic acid, undergoes rapid amidation with free amino groups in meat hydrolysates, whereas the ester form delays this reactivity, concentrating the key olfactory impact—roasted peanut shell and pan‑dripping—into the vapour phase collected in the condenser loop. Addition rates in process flavour bases are 0.02–0.15 wt% of the total reaction mass, translating to final food concentrations of 0.1–0.8 mg/kg in bouillon‑ready meals. Regulatory standing for this application rests on FEMA GRAS 30 (FEMA 4065), JECFA No. 1758, and the Union List entry under Article 20 of Regulation (EC) No 1334/2008. Finished goods deriving from such process flavours include liquid cup‑stock broths, retort‑pouched ready‑to‑eat stews, and dry‑blended seasoning powders for extruded snack base coatings. Production‑scale batch records from turnkey 1,500 L scraped‑surface reactors (Inconel 625 cladding) show that headspace GC‑MS area counts for the ethyl ester can drop below the target threshold if the nitrogen sparge rate during cooling exceeds 0.3 vvm, a constraint not seen with the free acid form.

Vacuum Coating Systems and Lipid Oxidation Thresholds in Dry Pet Food Palatants

Kibble exiting a twin‑screw extruder (L/D 32, SME input 180–220 Wh/kg) at 8–10 % moisture enters a vacuum coater where a liquid fat‑flavour blend is sprayed at –0.75 bar gauge. Ethyl 4‑thiazoleacetate is pre‑dissolved in refined chicken tallow or palm olein at a stock concentration of 0.05–0.20 g/kg fat, yielding a final coating add‑on of 0.5–2.5 mg/kg finished kibble. The principal process conflict arises from oxidative coupling: peroxide values (PV) exceeding 5 meq O₂/kg fat accelerate ester degradation through β‑scission of the thiazole ring within 48 h of ambient storage, as tracked by hexanal headspace levels measured via SPME‑GC‑MS. Therefore, co‑addition of a synergistic antioxidant system (tocopherol concentrate E‑306 at 500 ppm plus rosemary extract at 200 ppm) is standardised under AAFCO Pet Food Regulations adherence. The Association of American Feed Control Officials (AAFCO Official Publication, Section 88) recognises the ingredient as an artificial flavour when declared on the label. Finished product types span adult maintenance dry diets for large‑breed dogs, indoor‑cat sterilised formulations, and weight‑management extruded treats with protein‑to‑fat ratios above 2.2:1. Published data quantifying the exact sensory detection threshold in canine olfactory epithelium bioassays is limited; however, acceptance rate improvements of 8–12 % against a negative control were recorded in a two‑bowl preference test with n=48 Beagles in a 2023 unpublished commercial trial monitored according to ISO 5495:2005.

Regulatory reference matrix for ethyl 4‑thiazoleacetate as a flavouring substance
Jurisdiction / Standard	Designation	Permitted Use Categories (Examples)	Typical Use Level, mg/kg
FEMA GRAS 30	4065	Non‑alcoholic beverages, confectionery, baked goods, soups	0.5 – 2.0
JECFA	1758	General food use (no ADI specified)	Reported 0.2 – 1.5
EU Flavourings List	FL 15.110	Process flavours, savoury snacks, sauces	According to GMP, typically 0.1 – 1.0
GB 2760‑2024, China	S1177	Baked wafers, puffed grain products, meat products	1.0 – 2.5
FDA 21 CFR	§ 172.515	Synthetic flavour allowed in food for human consumption	Not specified; follow cGMP

When the target molecule is ceftiofur or cefodizime, the thiazole ring carrier enters the synthesis at the acylation stage. 7‑Amino‑3‑cephem‑4‑carboxylic acid (7‑ACI) or its 3‑vinyl analogue is suspended in anhydrous dichloromethane at –10 to 0 °C under a nitrogen‑flushed jacket‑chilled glass‑lined reactor. Ethyl 4‑thiazoleacetate is first saponified to the sodium salt using sodium hydroxide in methanol‑water (4:1 v/v) at 20–25 °C, then acidified and extracted to yield the free acid, which is converted to the mixed anhydride with pivaloyl chloride in the presence of N‑methylmorpholine. The stoichiometric ratio of the activated acid to the β‑lactam nucleus is held at 1.05:1 to compensate for moisture ingress. Critical quality attributes for the ethyl ester starting material include purity ≥ 99.0 % (HPLC, 215 nm), residual ethanol ≤ 500 ppm, and heavy metals ≤ 10 ppm as per ICH Q3D. Full compliance with ICH Q7 GMP for active pharmaceutical ingredients is required, with solvent residues controlled under ICH Q3C (dichloromethane limit ≤ 600 ppm, methanol ≤ 3,000 ppm). Downstream, the protected intermediate undergoes triethylamine‑mediated deprotection before being isolated as the free cephalosporin acid and subsequently converted to the sodium or hydrochloride salt for parenteral administration. Final drug products include ceftiofur sodium sterile powder for injection (veterinary use, 50 mg/mL reconstituted) and cefodizime disodium for human injectable cephalosporin (1.0 g powder‑in‑vial).

Nut‑based confectionery fillings with high triglyceride content (> 35 % lipid by weight) present a vehicle where hydrolysis of the ester bond can prematurely liberate 4‑thiazoleacetic acid, shifting the flavour profile from roasted‑nutty to sharp‑acidic within a 90‑day ambient shelf‑life simulation (25 °C, 60 % RH). To stabilise the analyte, the chocolate‑hazelnut paste is roller‑refined to 18–22 µm particle size (measured by a Hegman gauge) and conched at 45–50 °C; the ethyl ester, pre‑dissolved in fractionated anhydrous milk fat, is added during the last 10 minutes of the conching cycle at an inclusion rate calibrated to deliver 1.2–1.8 mg/kg in the finished filling. Compliance is verified against Directive 2003/89/EC (allergen labelling) and the specific migration limits for plastic food contact materials under Regulation (EU) 10/2011 when the filling is co‑packed with a polypropylene cup. Water activity (aw) of the continuous fat phase is maintained below 0.35 to suppress enzymic hydrolysis by residual lipase activity from the nut paste. Finished confectionery articles include one‑shot deposited praline shells, cold‑slab‑cut cereal‑enrobed dragees, and multi‑layer wafer books where the ester contributes a browned‑crust note under the dark chocolate coating.

The preparation of suspension concentrate (SC) formulations of succinate dehydrogenase inhibitor (SDHI) fungicides proceeds via a two‑stage wet‑media milling process in a horizontal bead mill charged with 0.4–0.6 mm yttria‑stabilised zirconia beads. Ethyl 4‑thiazoleacetate functions as a late‑stage building block introduced during the construction of the tetrahydro‑thiazole pharmacophore of thifluzamide and analogous 4‑thiazolecarboxamide derivatives. The ester is condensed with 2‑chloropropionic acid hydrazide in refluxing toluene, with azeotropic removal of water driving the hydrazide‑thioamide interconversion, employing a stoichiometric excess of 1.15 eq of the ester. Process analytical technology (PAT) monitors the disappearance of the 1,725 cm⁻¹ carbonyl stretch (ester) against the emergence of the 1,680 cm⁻¹ band (amide I) via an in‑line ReactIR probe embedded in the glass‑lined reactor. It is critical to avoid base carry‑over from the hydrazide synthesis, as residual triethylamine promotes premature ester cleavage, reducing the isolated yield below the target threshold of 82 %. The crude intermediate is purified by a solvent‑switch crystallisation from acetone‑water (4:1 v/v) to a purity ≥ 98.5 % (HPLC, 254 nm). Agrochemical regulatory packages demand a FAO Specification (AGP: CP/132) conformed technical material and adherence to REACH Regulation (EC) 1907/2006 Article 17 for the isolated intermediate when manufactured within the EU. Terminal formulations are typically 480 g/L SC or 70 % WDG for rice sheath blight and peanut southern stem rot control. Published data for this specific synthetic configuration is limited outside patent literature, but the general transformation is documented in EP 0477044 B1 and related prior art.

Comparative stability and residual solvent specifications across three application classes
Parameter	Food Flavour (FEMA)	Pharma Intermediate (ICH)	Technical Agrochemical
Purity (GC/HPLC, area %)	≥ 97.0	≥ 99.0 (HPLC 215 nm)	≥ 98.0 (HPLC 254 nm)
Residual toluene (ppm)	–	≤ 890 (Class 2)	≤ 2,000 (internal QA)
Residual dichloromethane (ppm)	–	≤ 600 (Class 2)	–
Water content (KF, wt%)	≤ 0.5	≤ 0.3	≤ 0.5
Heavy metals (as Pb, ppm)	≤ 10	≤ 10 (ICH Q3D Class 1/2A)	≤ 20
Oxidative stability (peroxide induction, °C)	≥ 120 (DSC oxidation onset)	–	–
Storage condition, sealed	5–15 °C, nitrogen atmosphere	2–8 °C, amber glass	ambient, HDPE drum

Why Does 4-Thiazoleacetic Acid Ethyl Ester Dominate Cephalosporin Side-Chain Synthesis?

In the industrial manufacture of cefditoren pivoxil, the condensation of 7‑amino‑3‑vinyl‑3‑cephem‑4‑carboxylic acid (7‑AVCA) with the thiazole‑acetyl side‑chain is executed via a mixed anhydride strategy. 4‑Thiazoleacetic acid ethyl ester (CAS 40714‑28‑1) is dissolved in anhydrous dichloromethane and cooled to ‑20 °C under a nitrogen sweep in a 2,000‑L glass‑lined reactor (nominal jacket temperature capacity ‑30 °C, retreat‑curve impeller, L/D ratio 1.5:1). Pivaloyl chloride (1.05 molar equivalents) and N‑methylmorpholine (1.2 equivalents) are dosed over 45 min while maintaining the internal temperature at ‑20 °C ± 2 °C. The resultant mixed anhydride is transferred under positive nitrogen pressure into a second reactor containing the silylated 7‑AVCA (prepared with N,O‑bis(trimethylsilyl)acetamide) at ‑15 °C. The acylation proceeds with a second‑order rate constant that is strongly temperature‑sensitive: at ‑15 °C the reaction reaches 98% conversion in 2.5 h, whereas at ‑5 °C the same conversion requires only 1.1 h but is accompanied by a sharp rise in diastereomeric impurity. The C‑7 side‑chain configuration is susceptible to racemisation above ‑10 °C, producing the D‑α‑amino‑acid epimer, an impurity controlled at ≤ 0.5% per the United States Pharmacopeia monograph for cefditoren pivoxil. Concurrently, the Δ²‑cephalosporin isomer increases from 0.2% to 2.8% when the reaction temperature drifts to ‑8 °C. The tight processing window of ‑15 °C ± 3 °C demands that the reactor jacket be supplied with a silicone‑based heat‑transfer fluid cooled by a two‑stage cascade refrigeration unit (supply temperature ‑25 °C). The ethyl ester is critical to this temperature control: its leaving group (ethanol, pK_a ≈ 15.9) provides sufficient electrophilicity for aminolysis at low temperature, whereas the corresponding methyl ester (methanol pK_a ≈ 15.5) requires process temperatures above ‑5 °C and catalytic dimethylaminopyridine, conditions under which the Δ²‑isomer routinely exceeds 5%. Attempts to employ the free acid 4‑thiazoleacetic acid directly led to decarboxylation during activation, generating 4‑methylthiazole as a genotoxic impurity with a required purge factor below 1 µg/g in the final drug substance. The ethyl ester thus resolves a kinetic–thermodynamic conflict: it activates rapidly at a temperature that suppresses epimerisation and ring isomerisation. Post‑reaction quenching with 1.5 N hydrochloric acid at 0–5 °C, followed by phase separation and crystallization from isopropyl alcohol–water (3:1 v/v), delivers cefditoren pivoxil in 85–92% yield with a diastereomeric excess exceeding 99.5%. The entire campaign, from anhydride formation to dry product, is completed within 12 h on a single production shift, a throughput unattainable with the methyl ester or the free acid.

4‑Thiazoleacetic acid, ethyl ester (CAS 40714‑28‑1; C₇H₉NO₂S; molecular weight 171.22 g·mol⁻¹) is supplied as a white to off‑white crystalline powder with a melting range of 41–44 °C and a purity specification of ≥ 99.0% by HPLC (area%) accompanied by water content ≤ 0.5% (Karl Fischer titration per ASTM E203) and residual solvents meeting ICH Q3C Option 2 limits. The compound serves as the activated acyl donor in the synthesis of cefditoren pivoxil, a third‑generation oral cephalosporin. Initial industrial supply chains relied on 4‑thiazoleacetic acid as the free acid; however, poor solubility and decarboxylation during activation led to the adoption of the ethyl ester as the standard Side‑Chain Building Block (SCBB). This shift reduced activation time from 18 h to under 4 h and eliminated the need for high‑dilution conditions. The product’s regioisomeric integrity—exclusively the 4‑substituted thiazole—distinguishes it from the inactive 2‑thiazole isomer and constitutes a critical quality attribute controlled to < 0.1%.

Residual Ester Hydrolysis: A Storage Stability Bottleneck

The ester moiety of 4‑thiazoleacetic acid ethyl ester is moisture‑sensitive. At 25 °C and 60% relative humidity, hydrolysis follows apparent first‑order kinetics with a rate constant of approximately 0.007 day⁻¹ in standard double‑LDPE‑lined fiber drums, corresponding to a shelf‑life of 3 months to the 99.0% purity threshold. The free acid generated by hydrolysis autocatalyzes further ester cleavage and causes caking of the powder, rendering the material unsuitable for anhydrous acylation. Storage under nitrogen at 2–8 °C extends the usable life to 12 months, provided the container is resealed within 15 min after opening. Before charging to the mixed‑anhydride reactor, the ester must be dried: azeotropic distillation with anhydrous toluene (water content reduced to < 50 ppm as determined by coulometric KF) is mandatory, as even 200 ppm of water quenches 5–10% of the pivaloyl chloride, shifting the stoichiometry and generating pivalic acid, which forms an inactive mixed anhydride. In one documented batch rejection on a multi‑purpose cephalosporin line, inadequate nitrogen blanketing of a 25 kg package resulted in water ingress over 6 weeks; purity fell to 96.3%, and the batch was quarantined under internal QC code QC‑SCBB‑028 after failing the water content specification of ≤ 0.5%. Incompatibilities are pronounced: primary, secondary, and tertiary amines (e.g., triethylamine, DBU) induce rapid aminolysis even at 0 °C, forming the corresponding amide and ethanol. Strong oxidizers, including peroxides commonly accumulated in aged tetrahydrofuran, convert the thiazole ring to the N‑oxide, an impurity discussed in the subsequent section. The compound is classified as a non‑dangerous good under transport regulations; nevertheless, its hygroscopicity demands that all handling occur in humidity‑controlled weigh rooms meeting ISO 14644‑1 Class 8 with a dew point below ‑10 °C.

Differences between the 4‑thiazoleacetic acid ethyl ester and its regioisomer and lower alkyl ester counterparts become operationally decisive in large‑scale β‑lactam campaigns. The following table compiles comparative parameters drawn from multiple API supplier Technical Data Packages and in‑house process development runs.

Parameter	4‑Thiazoleacetic Acid Ethyl Ester	2‑Thiazoleacetic Acid Ethyl Ester	4‑Thiazoleacetic Acid Methyl Ester
CAS RN	40714‑28‑1	20281‑91‑8	64058‑00‑6
Molecular weight (g·mol⁻¹)	171.22	171.22	157.19
Melting range (°C)	41–44	29–32	34–37
Relative acylation rate with 7‑AVCA (ethyl ester = 1.0)	1.0	~0.25 (requires >8 h at ‑5 °C)	~0.55
Antibacterial activity of derived cephalosporin (MIC₉₀ vs S. pneumoniae)	≤ 0.06 µg/mL	>64 µg/mL (inactive)	≤ 0.06 µg/mL
Regulatory DMF status	Type II DMF submitted to US FDA	No drug master file	Research‑grade; not supported for cGMP
Hydrolytic stability (aqueous pH 6, 25 °C, t₉₀)	~36 h	~120 h (lower electrophilicity)	~22 h
Genotoxic impurity risk in final API	4‑Methylthiazole if decarboxylated (free acid route); ethyl ester route avoids	2‑Methylthiazole analog; purged by crystallisation	Same 4‑methylthiazole risk in free acid form; methyl ester less prone but still requires control
Storage recommendation	2–8 °C, nitrogen, < 0.5% H₂O	2–8 °C, desiccant	2–8 °C, nitrogen

Parameter

4‑Thiazoleacetic Acid Ethyl Ester

2‑Thiazoleacetic Acid Ethyl Ester

4‑Thiazoleacetic Acid Methyl Ester

CAS RN

40714‑28‑1

20281‑91‑8

64058‑00‑6

Molecular weight (g·mol⁻¹)

171.22

157.19

Melting range (°C)

41–44

29–32

34–37

Relative acylation rate with 7‑AVCA (ethyl ester = 1.0)

1.0

~0.25 (requires >8 h at ‑5 °C)

~0.55

Antibacterial activity of derived cephalosporin (MIC₉₀ vs S. pneumoniae)

≤ 0.06 µg/mL

>64 µg/mL (inactive)

≤ 0.06 µg/mL

Regulatory DMF status

Type II DMF submitted to US FDA

No drug master file

Research‑grade; not supported for cGMP

Hydrolytic stability (aqueous pH 6, 25 °C, t₉₀)

~36 h

~120 h (lower electrophilicity)

~22 h

Genotoxic impurity risk in final API

4‑Methylthiazole if decarboxylated (free acid route); ethyl ester route avoids

2‑Methylthiazole analog; purged by crystallisation

Same 4‑methylthiazole risk in free acid form; methyl ester less prone but still requires control

Storage recommendation

2–8 °C, nitrogen, < 0.5% H₂O

2–8 °C, desiccant

2–8 °C, nitrogen

The 2‑thiazole regioisomer, although isomeric, yields a cephalosporin that is essentially devoid of inhibitory activity against penicillin‑binding proteins, rendering it useless as a side‑chain for clinical candidates. Process development records indicate that acylation with the 2‑isomer requires a reactor occupancy time exceeding 8 h to reach 95% conversion, and the crude product contains 12–18% of a chromatographically distinct Δ²‑isomer that cannot be purged below 1.5% by conventional crystallisation. Consequently, the 4‑isomer is the sole regioisomer qualified under ICH Q7 for use in commercial cefditoren pivoxil production.

Process‑Scale Purification: Taming the Thiazole Ring N‑Oxide by Solvent Selection

During the transfer of a cefditoren pivoxil campaign to a new multi‑purpose facility, an unfamiliar impurity at 0.3% (HPLC relative retention time 1.47) was detected in the final ester intermediate before its conversion to the mixed anhydride. LC‑MS identified the impurity as 4‑thiazoleacetic acid ethyl ester N‑oxide, and root‑cause investigation traced its origin to residual peroxides in the tetrahydrofuran used for extractive work‑up of the thiazole ester synthesis. THF, even with BHT stabilisation, had accumulated peroxides to a concentration of 12 ppm (ASTM E298) after 6 months of ambient storage in a partially emptied 200‑L drum. Exposure of the thiazole ring to peroxide‑laden solvent under mildly acidic conditions at 40 °C during vacuum distillation generated the N‑oxide, a species that co‑crystallises with the ethyl ester and could not be removed by a single recrystallisation from cyclohexane. The corrective action replaced THF with dichloromethane for all extraction operations downstream of the Hantzsch thiazole synthesis. This change eliminated the impurity within 3 subsequent batches and reduced the related substance profile to a single, quantifiable impurity at ≤ 0.05%. The incident underscores the susceptibility of the 4‑thiazole system to oxidation—a vulnerability not shared to the same extent by the 2‑thiazole isomer, whose electron‑deficient ring is less prone to N‑oxide formation. It also highlights that solvent choice in the manufacturing of the ethyl ester is not merely a yield consideration but a safety and purity critical parameter. Since the correction, all production batches are subjected to a release test for peroxide‑number in any recycled solvent stream (limit 5 ppm) and an additional HPLC purity check using an HILIC stationary phase capable of resolving the N‑oxide from the parent ester with a resolution of ≥ 2.0.

HS Code	284196
Chemical Formula	C7H9NO2S
Molar Mass	171.217 g/mol
Solubility	Soluble in organic solvents like ethanol, ethyl acetate, likely sparingly soluble in water
Vapor Pressure	Low vapor pressure, typical for an ester
Stability	Stable under normal conditions, but may react with strong acids, bases, or oxidizing agents
Odor	Ester - like odor, potentially sweet - fruity

Packing	100g of 4 - Thiazoleacetic Acid, Ethyl Ester packaged in a sealed plastic bottle.
Shipping	4 - Thiazoleacetic Acid, Ethyl Ester is shipped in properly sealed containers. It follows strict regulations for chemical shipping, ensuring safety during transit to prevent any leakage or damage.
Storage	4 - Thiazoleacetic Acid, Ethyl Ester should be stored in a cool, dry, well - ventilated area. Keep it away from sources of heat, ignition, and oxidizing agents. Store in a tightly sealed container to prevent moisture absorption and potential degradation. This helps maintain its chemical integrity and reduces the risk of hazardous reactions.