2-N-Boc-Amino-Thiazole-5-Carboxylic Acid

In the commercial synthesis of dasatinib monohydrate and its structurally related 2‑aminothiazole‑5‑carboxamide clinical candidates, the N‑Boc‑protected building block serves as the cornerstone for constructing the hinge‑binding pharmacophore. The Boc group suppresses premature metalation or nucleophilic addition at the exocyclic amine during lithiation or palladium‑catalysed cross‑coupling steps that are required when the thiazole 5‑position is further elaborated before final deprotection. Anchoring the entire downstream workflow to the standards of ICH Q11 (Section 5.3, designation of starting materials) and supporting a Type II Active Pharmaceutical Ingredient Master File, this intermediate is routinely controlled to a chromatographic purity of ≥ 99.5% (HPLC, 220 nm) with a single unknown impurity ceiling of ≤ 0.10%, a residual palladium limit of < 10 ppm (USP <232>), and volatile organic impurities validated against USP <467> Option 1 for Class 2 solvents. In the pivotal amide bond‑forming step, 2‑N‑Boc‑amino‑thiazole‑5‑carboxylic acid is charged at a molar ratio of 1.0 eq relative to the coupling partner 2‑chloro‑6‑methylaniline hydrochloride, which is itself employed at a 5 mol% excess (1.05 eq) to drive consumption of the activated acid. Activation is performed in a 500 L glass‑lined reactor (Pfaudler AE‑type, jacket service fluid temperature −5 to 0 °C) by slow addition of 1.2 eq 1‑ethyl‑3‑(3‑dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) and 0.1 eq 1‑hydroxybenzotriazole monohydrate (HOBt·H₂O) into an anhydrous tetrahydrofuran solution, maintaining the internal temperature below +5 °C with a calibrated Pt100 probe in a dead‑leg‑free thermowell. The mixture is agitated at 75 rpm with a retreat‑curve impeller for 30 minutes, after which a pre‑cooled (0–5 °C) solution of 2‑chloro‑6‑methylaniline and N‑methylmorpholine (2.2 eq) in THF is metered via a mass flow controller at a rate of 3.2 kg·min⁻¹. Post‑reaction work‑up includes gravity settling of the urea by‑product, brine washing at 15 °C to minimise emulsion formation, and solvent displacement into isopropyl acetate before Boc cleavage. Deprotection is conducted in a Hastelloy C‑22 pressure‑rated vessel by treating the dried intermediate with 2.5 M HCl in 1,4‑dioxane (6.0 L/kg of substrate), venting the copious CO₂ and isobutylene through a chilled (−15 °C) trap to reduce volatile organic emission load on the abatement system. The resulting dasatinib precursor dihydrochloride is isolated in 87–91% corrected yield after reslurrying in tert‑butyl methyl ether and vacuum drying at 45 °C and 10 mbar for 18 hours, targeting a residual dioxane level below 380 ppm to comply with ICH Q3C concentration limits for the final API.

In the assembly of the HIV‑1 protease inhibitor pharmacophore that characterises ritonavir and the clinical backup lopinavir, the thiazole‑5‑carboxylate moiety is initially preserved as a latent hydroxymethyl or chloromethyl handle for the construction of the (5‑thiazolyl)methyl carbamate motif. A mixed anhydride reduction sequence begins with the dissolution of 2‑N‑Boc‑amino‑thiazole‑5‑carboxylic acid in anhydrous tetrahydrofuran (10 L/kg) in a cryogenic vessel set to −25 °C. Isobutyl chloroformate (1.10 eq) is added in a single portion, followed by a controlled drip of N‑methylmorpholine (1.20 eq) over 45 minutes while maintaining the jacket outlet temperature no warmer than −18 °C. The resulting mixed anhydride suspension is immediately metered into a vigorously stirred solution of sodium borohydride (1.50 eq) in a 4:1 v/v THF/methanol mixture held at −30 °C; the borohydride quenching protocol employs a 1.0 M potassium dihydrogen phosphate buffer (pH 4.2) to avoid the violent hydrogen evolution that accompanies direct acetic acid quench at this scale. After phase separation and solvent swap into dichloromethane, the primary alcohol intermediate is isolated with a typical in‑process yield of 82%. Conversion to the carbamate fragment proceeds via activation with 1,1’‑carbonyldiimidazole (CDI, 1.05 eq) in dichloromethane at 20 °C for 3 hours, followed by coupling with the requisite amine nucleophile. This step is executed under nitrogen in a dedicated 200 L 316L stainless steel reactor with a double mechanical seal and a rupture disc rated for 1.5 barg, because residual water ingress above a concentration of 300 ppm (determined by online NIR spectroscopy) leads to CDI hydrolysis and an uncontrolled drop in coupling efficiency below 70%. The terminal API family includes ritonavir (AbbVie, CAS 155213‑67‑5) and the co‑formulated lopinavir (CAS 192725‑17‑0), where the regulatory starting material definition requires the Boc‑protected acid to meet cGMP 21 CFR Part 211 with a dedicated annual stability programme per ICH Q1A(R2). A documented processing conflict emerges when Boc deprotection is deliberately postponed to the final synthetic stage: traces of tert‑butyl carbocation, generated under the strongly acidic conditions of 33% HBr in acetic acid, are capable of alkylating the electron‑rich C‑5 position of the thiazole ring, yielding a purple‑coloured oligomeric impurity that precipitates during antisolvent crystallisation and can elevate total related substances above 0.6% if the reaction time exceeds 4 hours. Mitigation involves in‑line FT‑IR monitoring of the Boc carbonyl stretch at 1704 cm⁻¹ and termination of the cleavage step at ≤ 95% conversion to suppress the competing pathway.

琥珀酸脱氢酶抑制剂噻唑酰胺家族：从中间体到制剂的原药合成

Thifluzamide and the wider benzanilide‑type succinate dehydrogenase inhibitor (SDHI) fungicide class rely on 2‑aminothiazole‑5‑carboxylic acid as the heterocyclic linchpin, with the Boc‑protected variant providing better solubility in the toluene‑based acylation medium and significantly reduced tar formation during acid chloride generation. The technical concentrate manufacturing process is aligned with the FAO/WHO Joint Meeting on Pesticide Specifications (JMPS) manual and the OECD GLP Principles for physico‑chemical property testing, necessitating a five‑batch toxicology bridging package where the Boc intermediate is characterised for content, impurities, and stability. The acid chloride route is favoured in campaigns exceeding 500 kg of final active ingredient because the alternative mixed‑anhydride route introduces isobutanol, which can undergo transesterification with the trifluoromethoxy substituent under the thermal conditions of the coupling step. In a dedicated 1000 L glass‑lined reactor equipped with a chlorine‑resistant graphite condenser, the carboxylic acid (1.0 eq) is suspended in toluene and treated with thionyl chloride (1.55 eq) and a catalytic aliquot of N,N‑dimethylformamide (0.02 eq) at 65 °C until gas evolution ceases. The excess thionyl chloride is stripped under reduced pressure (50 mbar, jacket 80 °C) to a final chloride concentration of < 50 ppm as measured by argentometric titration of a hydrolysed sample. The resulting acid chloride solution is cooled to 5 °C and added slowly to a dichloromethane slurry of 2’,6’‑dibromo‑4‑(trifluoromethoxy)aniline (0.98 eq, deliberately substoichiometric to avoid free amine carry‑over into the crystallisation) and triethylamine (1.10 eq). The condensation exotherm is managed by a split‑range cascade controlling both the jacket brine valve and the reagent dosing pump, and the internal temperature is never allowed to exceed 12 °C to prevent de‑Boc side reactions initiated by liberated HCl. After aqueous work‑up, the N‑Boc intermediate is deprotected neat using trifluoroacetic acid (3.0 L/kg) containing 2% v/v triisopropylsilane as a carbocation scavenger; failure to include the silane scavenger results in a 4–7% yield reduction due to oligomeric impurities identical in nature to those described for the antiviral pathway. The technical‑grade thifluzamide (CAS 130000‑40‑7) is subsequently crystallised from 1‑propanol/water to achieve a purity of ≥ 98.0%, conforming to EPA 40 CFR §158.1100 data requirements and forming the basis for commercial suspension concentrate formulations at 240 g·L⁻¹ active ingredient loading. A documented operational boundary arises from the moisture sensitivity of the thionyl chloride activation step: the toluene charge must be dried to a Karl Fischer titre below 150 ppm, and the blanketed reactor headspace is maintained at a dew point of < −40 °C via a pressure‑swing adsorption nitrogen generator, otherwise corrosion of the vessel’s tantalum thermowell becomes detectable after only 12–15 production batches.

Custom synthesis service providers engaged in the assembly of targeted protein kinase probe libraries routinely adopt a diversity‑oriented synthetic platform in which 2‑N‑Boc‑amino‑thiazole‑5‑carboxylic acid functions as the invariant “core monomer” reacted in parallel with a set of structurally diverse aryl and heteroaryl amines. Operating outside the scope of current good manufacturing practice but under a quality management system certified to ISO 9001:2015 and with occupational hygiene controls following ISO 45001:2018, the workflow utilises a Chemspeed SWING XL automated solid‑phase extraction platform coupled to a Tecan Freedom EVO liquid handler. Each reaction well is charged with 0.05 mmol of the Boc‑acid, 0.05 mmol of the amine partner, and a coupling cocktail consisting of 1.10 eq (2‑(7‑aza‑1H‑benzotriazole‑1‑yl)‑1,1,3,3‑tetramethyluronium hexafluorophosphate) (HATU) and 3.0 eq N,N‑diisopropylethylamine in anhydrous N,N‑dimethylacetamide, with a total well volume of 0.4 mL. After 16 hours of orbital shaking at 22 °C in a nitrogen‑purged glovebox with O₂ < 5 ppm, the crude mixtures are purified by strong cation exchange solid‑phase extraction, followed by a dual‑filter layer of SiliaCat DPP palladium scavenger and QuadraSil MP thiourea to sequester metal and electrophilic impurities that would otherwise attenuate kinase inhibition readouts in the subsequent ADP‑Glo™ assay. The final products are a series of N‑(substituted)‑2‑amino‑thiazole‑5‑carboxamides with calculated logD 1.8–3.4, each delivered as a 10 mM DMSO stock solution with a minimum purity of 90% (ELSD‑LCMS). The downstream application of these compounds spans primary screening hits against wild‑type and mutant BCR‑ABL, SRC family kinases, and IRAK‑4, and the Boc‑protected acid’s utility lies in the fact that the carboxamide coupling can be conducted without protection/deprotection cycles at the 2‑amino position, thereby reducing by two synthetic steps the average linear sequence length relative to routes that start from unprotected 2‑amino‑thiazole‑5‑carboxylic acid. A documented processing bottleneck concerns the removal of HATU‑derived tetramethylguanidine by‑product: when the subsequent de‑Boc step is performed with 25% v/v trifluoroacetic acid in dichloromethane followed by simple evaporation, the residual guanidinium salt co‑concentrates and, if not scavenged, can interfere with isothermal titration calorimetry measurements on the target protein, leading to false negatives that obscure genuine low‑micromolar binders. The scavenging cartridge is therefore treated as a mandatory unit operation, and the final well plate undergoes an additional methanol rinse and a 4‑hour vacuum centrifugation step at 30 °C to meet the residual solvent specification for biological assay submission.