Ethyl 2-Amino-4,5,6,7-Tetrahydro-1,3-Benzothiazole-4-Carboxylate

Ethyl 2-amino-4,5,6,7-tetrahydro-1,3-benzothiazole-4-carboxylate functions as an advanced, ring-constrained heterocyclic building block in the convergent assembly of non-ergoline dopamine D2/D3 receptor agonists, where the 4‑carboxylate ester serves as a pivotal chiral handle for diastereomeric salt resolution prior to reductive amination sequences. In a representative kilogram‑scale campaign, the racemic ester is dissolved in ethyl acetate at 45°C and treated with 0.52 equivalents of L‑(+)-tartaric acid, precipitating the (R)-enantiomer salt with an optical purity exceeding 98% ee after a single reslurry step in 2‑propanol. The resolved ester is subsequently converted to the corresponding primary amine via a three‑stage telescoped process: ester saponification with 2 M NaOH in methanol under reflux for 4 hours, acidification to the carboxylic acid hydrochloride, and Curtius rearrangement triggered by diphenylphosphoryl azide (1.15 eq) in anhydrous toluene at 80°C. The resulting isocyanate is trapped with tert‑butanol to yield the Boc‑protected intermediate, which is then deprotected with anhydrous HCl in dioxane in a jacketed glass‑lined reactor equipped with an H2S‑scrubbing caustic scrubber, owing to trace thiol elimination at temperatures above 25°C. Compliance with ICH Q7 paragraph 19.1 mandates full traceability of this advanced intermediate through validated HPLC methods using a C18 column ( 250 × 4.6 mm, 5 μm) with detection at 254 nm, where the acceptance criterion for any single unknown impurity is ≤ 0.10%. The downstream incorporation ratio into the final API synthetic step is typically 1.00:1.15 (ester intermediate to N‑propylamine nucleophile), reflecting the excess required to compensate for vapor‑phase loss of propylamine during sealed‑vessel heating to 120°C in N‑methyl‑2‑pyrrolidone. The end-product of this route is an orally bioavailable non‑ergot dopamine agonist hydrochloride salt, dispensed as an immediate‑release tablet in strengths of 0.125 mg, 0.25 mg, 0.5 mg, 1.0 mg, and 1.5 mg for the management of early‑stage Parkinson’s disease.

What Concentration Delivers a Charge‑Transfer Barrier Comparable to Propargyl Alcohol in 15% HCl at 60°C?

When formulated into an acid‑pickling inhibitor package intended for J‑55 and N‑80 oil‑country tubular goods, ethyl 2‑amino‑4,5,6,7‑tetrahydro‑1,3‑benzothiazole‑4‑carboxylate forms a protective chemisorbed film on carbon steel surfaces through electron‑donating interactions of the endocyclic sulfur and the exocyclic amine nitrogen with vacant d‑orbitals of iron. Electrochemical impedance spectroscopy conducted in a three‑electrode flat cell (Ag/AgCl reference, platinum counter, C1018 working electrode) reveals that at a loading of 0.25 wt% in uninhibited industrial grade 15% HCl, the charge‑transfer resistance (Rct) increases from 12 Ω·cm² to 840 Ω·cm², correlating to an inhibitor efficiency of 98.6% as calculated by Tafel extrapolation per ASTM G102‑23. This performance eclipses that of propargyl alcohol at identical concentration when the acid bath temperature exceeds 65°C, because the thiazole‑derived film does not undergo exothermic runaway polymerization at hot metal surfaces, a known hazard with acetylenic inhibitors. The recommended working dosage window for immersion pickling of coiled tubing is 0.15–0.40 wt% based on the mass of the acid solution, with the lower boundary applicable to short contact times under 30 minutes and the upper boundary required when the free acid concentration drops below 7% and ferrous chloride accumulation surpasses 120 g/L. A single‑point inductively coupled plasma (ICP) measurement of dissolved iron after 6‑hour exposure must keep the corrosion rate below 0.6 mm/year to satisfy NACE TM0169‑2012 for quenched‑and‑tempered tubulars with a hardness exceeding 22 HRC. In a continuous pickling line for hot‑rolled coil, the neat ester is injected neat via a positive‑displacement dosing pump into the recirculating acid line ahead of the heat exchanger, with the blending completed by an in‑line static mixer of 12 elements. The inhibitor’s solubility limit is 0.6 wt% in 15% HCl at 20°C; exceeding this threshold results in phase separation of a tacky red‑brown semi‑solid that fouls the acid regeneration unit. The final formulated product is a non‑surfactant‑based corrosion inhibitor concentrate, often co‑formulated with 0.05 wt% potassium iodide as a synergist and 0.02 wt% of an ethoxylated nonylphenol dispersant to prevent localized pitting under deposits of mill scale.

High‑Washfastness Heterocyclic Azo Disperse Dyes via Tetrahydrobenzothiazole Diazo Components

When employed as a diazo component for high‑performance disperse dyes, ethyl 2‑amino‑4,5,6,7‑tetrahydro‑1,3‑benzothiazole‑4‑carboxylate generates a series of bathochromically shifted azo chromophores whose photostability on polyester microfiber meets the staining fastness prerequisites of OEKO‑TEX 100 annex 4 and the ZDHC Manufacturing Restricted Substances List Level 3. The diazotization reaction is executed in a 3,000 L glass‑lined reactor equipped with a brine‑cooled jacket and a bottom‑discharge valve, where 1.00 mol of the finely milled aminothiazole ester is suspended in 1,200 L of 32% hydrochloric acid and treated dropwise with 1.02 mol of sodium nitrite dissolved in 80 L of deionized water, while the internal temperature is maintained rigidly at 0°C to +3°C by circulating a ‑15°C calcium chloride brine. Any excursion above +5°C induces exothermic decomposition of the diazonium salt, evidenced by the evolution of nitrogen gas and a rapid darkening of the reaction mass that correlates with a tripling of the insoluble residue in the final pressed filter cake. The clarified diazonium solution is transferred within 20 minutes via a jacketed PTFE‑lined pipe into a second reactor containing the alkaline coupling component—typically N‑ethyl‑N‑(2‑cyanoethyl)‑m‑toluidine—dissolved in a mixture of ice‑water and 0.5% sulfamic acid to destroy excess nitrite. The coupling pH is maintained at 4.5–5.0 through automated dosing of 20% sodium acetate solution, and the end point is verified by a negative response to H‑acid spot test. The precipitated dye is isolated via a filter press, washed until the filtrate conductivity falls below 200 μS/cm, and dried in a Guedu vacuum paddle dryer at 80°C and 50 mbar for 12 hours. Milling to a final particle size distribution d₉₀ ≤ 1 μm is performed on a horizontal bead mill charged with 0.3 mm yttria‑stabilized zirconia beads and an anionic lignosulfonate dispersant at 30% weight on pigment. The finished product is a blue‑to‑red disperse dye powder or 40% liquid dispersion that provides a build‑up of 2.5% on texturized polyester at 130°C under pressure, delivering a wash fastness rating of 4‑5 grey scale according to ISO 105‑C06 and a light fastness of 6‑7 by ISO 105‑B02.

When Tetrahydrobenzothiazole Carboxylate Replaces 2‑Amino‑5‑mercapto‑1,3,4‑thiadiazole in Pyrazole‑4‑carboxamide Fungicide Backbones

In the discovery route to second‑generation pyrazole‑4‑carboxamide fungicides targeting succinate dehydrogenase (SDH) inhibition, the replacement of a mercaptothiadiazole leaving group with the tetrahydrobenzothiazole carboxylate scaffold attenuates the acute toxicity while retaining the hinge‑binding geometry to the ubiquinone‑binding site as verified by X‑ray crystallography of the protein‑ligand complex at 2.1 Å resolution. The coupling step utilized in pilot‑scale synthesis is an N‑acylation performed in a 1,000 L Hastelloy C‑22 reactor purged to an oxygen level below 1,000 ppm by volume: 1.00 kmol of ethyl 2‑amino‑4,5,6,7‑tetrahydro‑1,3‑benzothiazole‑4‑carboxylate is pre‑dried in a fluidized bed at 55°C for 6 hours under a nitrogen sweep to achieve a water content ≤ 0.05%, then slurried in 800 L of anhydrous tetrahydrofuran with 1.05 kmol of triethylamine. To this suspension, a solution of 1.02 kmol of difluoromethyl‑pyrazole‑4‑carbonyl chloride in 200 L of THF is fed at ‑5°C over a 4‑hour period while the jacket temperature is held at ‑15°C to manage the exotherm. The batch is agitated for an additional 2 hours at 0°C, then quenched with 500 L of deionized water while maintaining the temperature below 10°C. The organic layer is separated, washed with 10% sodium bicarbonate to remove unreacted acid chloride, and crystallized from a 3:1 heptane/ethyl acetate mixture. The isolated pyrazole‑carboxamide intermediate exhibits a purity of 99.2% by GC‑FID, with a residual triethylamine content below 50 ppm. For formulation, the technical active is micronized in an air‑jet mill to a d₅₀ of 2–4 μm and then blended with naphthalene sulfonate condensate, precipitated silica, and a vinyl acetate‑ethylene copolymer to produce a 480 g/L suspension concentrate. The SC formulation must pass CIPAC MT 184 for wet sieve retention ( ≤ 0.3% on a 75 μm screen) and CIPAC MT 75.3 for pourability. The final fungicide product is approved under FAO specification 569/SC and is registered for the control of Ramularia leaf spot on barley at an application rate of 1.0 L/ha.

Incorporation of a 2‑hydroxyphenyl‑benzothiazole‑derived UV absorber carrying the tetrahydrobenzothiazole ester moiety into a linear low‑density polyethylene greenhouse film at a net active concentration of 0.30 wt% provides an extended UV‑B cut‑off at 330 nm while keeping the migration into simulant media below 1.5 mg/kg after 10 d at 40°C as evaluated per EU Regulation 10/2011 migration cell method. The masterbatch containing 12 wt% of the active is produced on a co‑rotating twin‑screw extruder with a L/D ratio of 44:1 and a screw diameter of 75 mm, operating with a temperature profile ascending from 180°C at the feeding zone to 230°C at the die, and with a devolatilization vacuum of ‑0.08 MPa applied at barrel 9 to strip off residual ethyl acetate from the active’s synthesis. Torque spikes exceeding 85% of the machine limit are observed when the melt‑flow index of the carrier resin drops below 2 g/10 min ( 190°C/2.16 kg, ISO 1133‑1:2022), necessitating the use of a butene‑branched LLDPE grade with a MI of 4 g/10 min to prevent localized over‑shear and gel formation. The let‑down ratio during blown film extrusion is 2.4%, and the die gap is set to 1.8 mm with a blow‑up ratio of 2.5:1 to produce a 150 μm asymmetric three‑layer film. Accelerated weathering for 2,000 h in a xenon‑arc apparatus according to ISO 4892‑2 cycle 1 (irradiance 60 W/m², black panel temperature 65°C, relative humidity 50%) must demonstrate a retention of tensile elongation at break of ≥ 70% (ISO 527‑3:2018) for the film to be serviceable in a multi‑season Mediterranean tunnel house. The absorber is cleared as an indirect food additive for polyolefin films under FDA 21 CFR § 178.3297, and its REACH registration dossier documents an aquatic chronic NOEC of 0.12 mg/L for Daphnia magna (OECD 211). The terminal article is a commercially installed 180‑200 μm thick UV‑stabilized LDPE/EVA/LLDPE greenhouse cover film with a warranted service life of four continuous cropping cycles.

Sterically Constrained Proline Mimetic for Fmoc‑SPPS Integration into Macrocyclic Peptide Leads

When the tetrahydrobenzothiazole ring system is employed as a conformational constraint in solid‑phase peptide synthesis, the carboxylic acid derived from saponification of the ethyl ester is protected with Fmoc‑OSu in a two‑phase water/dioxane system containing 10% Na₂CO₃, yielding Fmoc‑amino‑tetrahydrobenzothiazole‑carboxylic acid in 91% isolated yield after crystallization from methylcyclohexane. The protected monomer is dissolved in DMF to a concentration of 0.3 M and coupled onto a Rink amide AM resin (loading 0.6 mmol/g) using 5 equivalents relative to the free amine sites, activated in situ with HCTU (4.9 eq) and 2,4,6‑collidine (10 eq). The coupling cycle proceeds at 40°C for 30 minutes under microwave irradiation in a CEM Liberty Blue automated synthesizer, with a double‑coupling protocol invoked when a Kaiser test after the first 15 minutes indicates incomplete acylation. N‑terminal Fmoc removal is carried out with 20% piperidine in DMF containing 0.1 M Oxyma Pure as a sequence‑rearrangement suppressant, a critical additive because the thiazole sulfur undergoes 0.3% cross‑linking with the aspartimide impurity generated from the neighboring aspartic acid residue if the deprotection temperature strays above 50°C. Cleavage of the linear peptide from the resin is performed with a cocktail of TFA/TIS/H₂O (95:2.5:2.5 v/v) for 2.5 h, followed by precipitation in chilled methyl tert‑butyl ether. Cyclization of the crude linear peptide containing the tetrahydrobenzothiazole residue is accomplished at 1 mM concentration in DMF using 3 equivalents of PyAOP and 6 equivalents of DIPEA over 18 h, resulting in a 22‑membered macrocycle exhibiting an EC₅₀ of 48 nM in a PD‑1/PD‑L1 homogeneous time‑resolved fluorescence binding assay. The entire process from Fmoc‑amino acid to lyophilized final peptide is managed under an ICH M7(R2) purge factor analysis, with periodic control of potential N‑nitrosamine formation in the presence of residual nitrite from the Kaiser reagent through a limit test using UPLC‑MS/MS with a detection threshold of 0.03 ppm. The end‑product is a sterile, lyophilized peptidomimetic lead candidate dispensed in 10‑mg vials for intravenous infusion in oncology indications, manufactured under EU GMP Annex 1 cleanroom classification.