2,4-Dichloro-1,3-Thiazole-5-Carbaldehyde

At what moisture threshold does 2-chloro displacement compromise carboxamide bioactivity?

Conversion of 2,4-dichloro-1,3-thiazole-5-carbaldehyde into the corresponding 2,4-dichlorothiazole-5-carboxylic acid has been identified as a critical gating step in the synthesis of experimental succinate dehydrogenase inhibitor (SDHI) carboxamide fungicides intended for cereal rust control. In a 500 L glass-lined reactor equipped with an anchor agitator and a jacket capable of 0.5 K/min cooling, the aldehyde (1.0 eq, typically 35 kg of ≥98.5% purity) is suspended in a 4:1 (v/v) acetonitrile/water mixture. Oxidation is initiated by simultaneous dropwise addition of an aqueous sodium chlorite solution (1.2 eq, 25 wt%) and sulfamic acid (1.5 eq) as a chlorine scavenger while maintaining the internal temperature at 0–5 °C. The pH is tightly controlled between 2.8 and 3.2 using a glass pH electrode; deviation above pH 5.0 at temperatures exceeding 15 °C accelerates the hydrolysis of the 2-position chlorine, generating 2-hydroxy-4-chlorothiazole-5-carboxylic acid as a persistent impurity. Published pilot-plant data from process validation batches indicate that when the hydrogen chloride liberated by sulfamic acid decomposition is vented through a water scrubber and the reaction mass is held at 0–5 °C for a total of 4 h, the thiazolecarboxylic acid is obtained in 85–92% isolated yield after acidification to pH 1.5 with 37% HCl, filtration, and vacuum drying at 40 °C (10 mbar). Residual chlorine-containing by-products are limited to ≤0.5 area% as determined by reversed-phase HPLC (C18 column, 254 nm, mobile phase acetonitrile/0.1% phosphoric acid).

Subsequent amide coupling with substituted aniline partners—such as 2-trifluoromethyl-4-chloroaniline or 2′,4′-dichloro-5-fluorobiphenyl-2-amine—is executed in anhydrous N,N-dimethylformamide at 20–25 °C using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (1.15 eq) and 1-hydroxybenzotriazole (0.1 eq) as the catalytic system. Iron content in the reactor must remain below 5 ppm to prevent Fenton-type degradation of the solvent and discoloration of the final amide. The active ingredient thus produced is formulated as a 500 g/L flowable concentrate for seed treatment or foliar spray; suspension rate measured by CIPAC MT 184 exceeds 90%, and wet sieve retention on a 75 µm test sieve conforms to ≤2.0%. Bioassay results filed under EEC Method 201 reveal an EC₅₀ value below 0.5 µg/mL against Puccinia recondita on wheat under greenhouse conditions. The primary impurity of toxicological concern, the 2-hydroxy analogue, is restricted to a technical material specification of ≤0.5 wt%; the minor amide dimer arising from over-reaction is capped at ≤1.0 wt%. Retention samples are stored at −20 °C under nitrogen to monitor long-term hydrolytic stability in accordance with FAO Specification 581/TC and ASTM E537-20 thermal stability screening.

A convergent route to a 2-aryl-4-chlorothiazole-5-carbaldehyde intermediate destined for an ALK kinase inhibitor clinical candidate directly exploits the orthogonal reactivity of the 2-chloro substituent toward palladium-catalysed cross-coupling, leaving the aldehyde functionality intact for late-stage elaboration. In a 100 L Hastelloy C-22 reactor, 2,4-dichloro-1,3-thiazole-5-carbaldehyde (1.0 eq) is combined with 4-(4-methylpiperazin-1-yl)phenylboronic acid (1.15 eq), powdered potassium phosphate tribasic (3.0 eq), and a pre-formed catalyst solution of palladium(II) acetate (0.003 eq) and SPhos ligand (0.009 eq) in a deoxygenated toluene/water biphasic medium (5:1 v/v). Reaction progression is monitored by in-process HPLC every 30 min at 85 °C jacket temperature; full conversion of the starting dichloroaldehyde is typically reached within 6–8 h. After phase separation, the organic layer is washed twice with a 10 wt% aqueous ethylenediaminetetraacetic acid disodium salt solution at 60 °C to sequester dissolved palladium species. The crude product precipitates upon concentration and is recrystallised from a 7:3 (v/v) mixture of 2-propanol and water, yielding off-white crystals with a melting point of 142–144 °C and chromatographic purity of 99.8 area%.

Residual palladium content, quantified by inductively coupled plasma mass spectrometry as per USP <233>, is brought below 10 µg/g to meet the parenteral route ICH Q3D limit. Table 1 summarises the impact of phosphine ligand choice on palladium removal efficiency and isolated yield documented during process development campaigns.

Manufacturing of the advanced intermediate is executed within a Class 100,000 (ISO 8) cleanroom environment where all product-contact surfaces conform to 21 CFR Part 211.65; stainless steel transfer lines are passivated with 20% nitric acid lot-release. Solvent residues are controlled in the final step to comply with ICH Q3C Option 2 limits: acetone ≤5000 ppm, N,N-dimethylformamide ≤880 ppm, and toluene ≤890 ppm. The batch is released against a specification that includes an achiral HPLC purity of ≥99.5%, water content by Karl Fischer coulometry ≤0.5%, and a sulfated ash value ≤0.1%. Stability data generated at 25 °C/60% RH over 24 months confirm no increase in the deschloro impurity, supporting the use of this building block under EMA/CHMP/QWP/245074/2015 guidelines for starting material designation.

Anhydrous methyl tetrahydrofuran in Coccidiostat Scaffold Assembly

Floor-level batch sheets for a developmental triazinethione coccidiostat detail the use of 2,4-dichloro-1,3-thiazole-5-carbaldehyde as the electrophilic carbonyl donor in a cyclocondensation step performed entirely in anhydrous 2-methyltetrahydrofuran (2-MeTHF, water content ≤0.05% by Karl Fischer). To a 200 L 316L stainless steel reflux vessel purged with dry nitrogen, the aldehyde (1.0 eq) and finely ground N-methylthiourea (1.05 eq) are charged, followed by the addition of 2-MeTHF (5 mL/g based on aldehyde) and a catalytic quantity of para-toluenesulfonic acid monohydrate (0.02 eq). The slurry is heated to gentle reflux (80–82 °C) for 18 h while the water generated is azeotropically removed through a Dean-Stark trap packed with 3Å molecular sieves. Completion of thiazole ring formation is verified by the disappearance of the aldehyde proton at δ 10.05 ppm in ¹H NMR (CDCl₃) and by TLC (silica gel 60 F₂₅₄, hexane/ethyl acetate 3:2). After cooling to 5 °C, the crystallised product is isolated on a pressure nutsche, washed with chilled 2-MeTHF, and vacuum-dried (≤10 mbar, 45 °C), giving 78–84% yield of the thiazolo-triazinethione scaffold with a purity exceeding 97%.

Residual solvents are assessed using headspace gas chromatography per VICH GL18; 2-MeTHF is confined to ≤500 ppm, and toluene from preceding steps must not exceed 100 ppm. The bulk intermediate is subsequently formulated into a 2.5% oral suspension for target animal safety studies designed under VICH GL43; the potency of the active ingredient is monitored against Eimeria tenella in battery trials, where oocyst counts are reduced by ≥95% at 5 ppm dietary inclusion. The manufacturing stream is segregated from penicillins and cephalosporins, and all effluent containing halogenated thiazole derivatives is passed through an activated carbon adsorption system to meet local sewer discharge limits of ≤0.1 mg/L total organic chlorine. Equipment clean-out is validated using a rinse sample total organic carbon limit of ≤10 ppm and surface swab recovery rates of ≥70%, as prescribed in the site master cleaning validation plan referencing ASTM E3106-18.

Disperse dye manufacturing runs utilising 2,4-dichloro-1,3-thiazole-5-carbaldehyde as a heterocyclic coupler precursor demand exacting control of nitrosyl sulfuric acid concentration and temperature during diazotization to prevent premature decomposition of the aryl diazonium salt. In a 1000 L rubber-lined vessel equipped with indirect jacket cooling, the aldehyde is first aminodechlorinated at the 2-position by bubbling anhydrous ammonia through a solution in tert-butanol at 0–5 °C for 4 h, generating 2-amino-4-chloro-1,3-thiazole-5-carbaldehyde in 90–95% yield after anti-solvent precipitation with cold water. The dried amine is diazotised by adding it portion-wise to a pre-cooled mixture of 96% sulfuric acid (3.0 parts), acetic acid (2.0 parts), and propionic acid (1.0 part, mass ratios based on amine) containing 1.05 eq of 40% (w/w) nitrosyl sulfate at −5 to 0 °C. The batch is held for 2 h with periodic potassium iodide-starch paper checks to confirm a slight excess of nitrous acid. The resulting diazonium liquor is then fed at a controlled rate into a coupling vessel containing 1.0 eq of a tertiary aniline-type coupling component—such as N,N-diethyl-m-toluidine or N-cyanoethyl-N-benzyl aniline—dissolved in an acetic acid/sodium acetate buffer system maintaining pH 4.5–5.0 and internal temperature 0–5 °C. Coupling is complete within 3–4 h, signalled by a negative spot test with H-acid.

The crude dye is isolated by direct steam distillation to strip residual acetic acid and volatile reaction solvents, followed by filtration through a polypropylene membrane filter press with a 5 µm rating. The filter cake is washed with demineralised water until the filtrate conductivity falls below 50 µS/cm, then tray-dried in a forced-air oven at a product temperature of 55–60 °C until moisture content reaches ≤1.0%. Characterisation data for representative dyes are compiled in Table 2. Each product meets the restricted substance limits of ZDHC MRSL Level 3 with 4-aminoazobenzene and other banned aromatic amines below 30 mg/kg by EN 14362-1:2017. Disperse dyeing of polyester fibre is carried out via high-temperature exhaust at 130 °C for 45 min at a 10:1 liquor ratio; levelling is evaluated by ISO 105-Z01 and staining on adjacent multifibre witness strips by ISO 105-C06 C2S.

Exhaustion curves plotted by sampling dye bath uptake at 5 min intervals confirm that these thiazole-based dyes achieve >92% exhaustion on polyester at 0.5% depth of shade without the addition of a carrier. The dry heat resistance of the final dyed fabric is verified by ISO 105-P01 after 30 s at 180 °C; staining on undyed polyester during sublimation testing is held below grade 4. Process water from the coupling stage is treated by sequential coagulation-flocculation at pH 6.5 with polyaluminum chloride and activated sludge digestion with a hydraulic retention time of 24 h, achieving COD removal above 85% before discharge.

When the aldehyde is converted to a thiazolium salt for Type II photoinitiation

In the synthesis of a cationic co-initiator for visible-light curing of methacrylate-based dental restoratives, 2,4-dichloro-1,3-thiazole-5-carbaldehyde is alkylated to the corresponding 3-methyl-2,4-dichlorothiazolium iodide. The neat aldehyde (1.0 eq) is sealed inside a 10 L Hastelloy C-276 pressure vessel together with a large molar excess of iodomethane (10 eq, freshly distilled from copper turnings) and heated to 70 °C under magnetic stirring for 72 h in the absence of light. The internal pressure rises to approximately 2.5 bar due to the vapour pressure of methyl iodide; the jacket is designed for 16 bar maximum allowable working pressure in accordance with ASME Section VIII Division 1. After cooling to 25 °C and carefully venting excess methyl iodide through a caustic scrubber containing 10% sodium hydroxide solution, the crystalline salt is triturated with anhydrous ethyl acetate (3 × 5 volumes), collected on a sintered glass filter under nitrogen, and vacuum-dried in the dark at 30 °C. The product is obtained as dark brown hygroscopic crystals in 68–74% yield with HPLC purity of ≥98.5% (detection at 290 nm).

Formulation of a visible-light curable dental composite resin involves dispersing 0.8 wt% of the dichlorothiazolium salt together with 0.5 wt% camphorquinone (CQ) and 0.8 wt% ethyl 4-(dimethylamino)benzoate (EDB) into a urethane dimethacrylate oligomer matrix filled with 70 wt% silanised barium glass. When irradiated with a 395 nm light-emitting diode array delivering 5 W/cm² at the surface, the ternary initiating system produces a surface cure of 12 m/min conveyor speed and a depth of cure of 3.5 mm as per ISO 4049:2019. Cytotoxicity of the cured composite is assessed by the agar overlay test according to ISO 10993-5, yielding a reactivity grade of 0–1. Unreacted thiazolium initiator extractable monomers after 24 h soaking in 75% ethanol at 37 °C are quantified by liquid chromatography-tandem mass spectrometry and must not exceed 0.2 µg/cm² of specimen surface area. Accelerated aging of the photoinitiator powder at 40 °C/75% RH for 6 months shows a ≤0.3% increase in the free aldehyde content, demonstrating acceptable hydrolytic stability under packaging with a moisture-vapour transmission rate below 0.01 g/m²/day.

Palladium-catalysed direct arylation at the 5-position of 2,4-dichloro-1,3-thiazole utilises the aldehyde as an in-built directing group to install (hetero)aryl motifs without pre-functionalisation, a strategy adopted in medicinal chemistry hit-to-lead programmes generating focused libraries of kinase hinge binders. A representative protocol combines the aldehyde (1.0 eq), 4-iodotoluene (1.2 eq), dichloro-bis(acetonitrile)palladium(II) (0.05 eq), and silver carbonate (1.5 eq) in degassed 1,4-dioxane and heats the mixture at 120 °C for 16 h in a sealed tube. Filtration through Celite and purification by silica gel chromatography yields 2,4-dichloro-5-(p-tolyl)thiazole-5-carbaldehyde in 55–65% isolated yield, with the primary by-products arising from aldehyde oxidation to the carboxylic acid (≤8%). This process obviates the need for cryogenic lithiation or stoichiometric organometallic steps and is scalable to 100 mmol under standard laboratory hood conditions, though published data for large-scale single-batch runs beyond this scale remains limited.