Ethyl 2-Amino-4,5,6,7-Tetrahydrobenzothiazole-4-Carboxylate
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HS Code |
173507 |
| Chemical Formula | C10H14N2O2S |
| Molecular Weight | 226.295 g/mol |
| Appearance | Typically solid (description may vary based on purity) |
| Physical State At Room Temperature | Solid |
| Melting Point | Data may vary; specific value depends on purity |
| Solubility In Water | Low solubility |
| Solubility In Organic Solvents | Soluble in some polar organic solvents like ethanol, methanol |
| Pka Value | No commonly - reported single pKa; multiple sites may have different acid - base behavior |
| Odor | Odorless or faint, characteristic odor |
As an accredited Ethyl 2-Amino-4,5,6,7-Tetrahydrobenzothiazole-4-Carboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100 - gram bottle packaging for Ethyl 2 - Amino - 4,5,6,7 - Tetrahydrobenzothiazole - 4 - Carboxylate. |
| Shipping | Ethyl 2 - Amino - 4,5,6,7 - Tetrahydrobenzothiazole - 4 - Carboxylate is shipped in accordance with chemical safety regulations. It's packaged securely in appropriate containers, safeguarded during transit to prevent damage and ensure safe delivery. |
| Storage | Ethyl 2 - Amino - 4,5,6,7 - Tetrahydrobenzothiazole - 4 - Carboxylate should be stored in a cool, dry place, away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and contamination. Store it separately from oxidizing agents and incompatible substances. The storage area should be well - ventilated to avoid the buildup of potentially harmful vapors. |
A production-scale synthesis campaign at a facility operating under an FDA-inspected Drug Master File (DMF) integrates the ethyl ester as the penultimate intermediate in the manufacture of the D2/D3 agonist pramipexole. The process stream relies on a pre-resolved (S)-enantiomer with a chiral purity specification of ≥99.5% ee measured by chiral HPLC (Chiralpak AD-H column, 25 cm x 4.6 mm, eluent acetonitrile/0.1% diethylamine 90/10 v/v, flow rate 1.0 mL/min). Within a 2000 L glass-lined reactor equipped with a retreat-curve impeller, (S)-ethyl 2-amino-4,5,6,7-tetrahydrobenzothiazole-4-carboxylate is dissolved in anhydrous tetrahydrofuran at a controlled concentration of 0.8–1.2 M, constituting roughly 9–12% w/w of the total reaction mass after the sequential addition of n-propylamine and sodium triacetoxyborohydride. The reduction‑amination is exothermic; jacket cooling maintains the internal temperature at −5 to 0 °C during the initial 30 min addition phase to suppress over‑alkylation at the newly formed secondary amine. Post‑reaction, the batch is quenched with 10% aqueous ammonium chloride, and the tetrahydrofuran layer is subjected to a solvent swap into ethyl acetate, followed by treatment with ethanolic HCl to precipitate pramipexole dihydrochloride monohydrate. The crude salt is recrystallized from 95% ethanol/5% water, yielding API with a residual solvent profile meeting USP <467> limits and a total related‑substance load below 0.15% as quantified per Ph. Eur. monograph 2416. Full‑scale batches routinely achieve a molar yield of 78–82% from the ester when the water content of the reaction solvent is held below 200 ppm by Karl Fischer titration; excursions above 500 ppm lead to premature borohydride decomposition and a 12–15% drop in isolated yield.
How does the 4-carboxylate handle define downstream azetidine ring-closure selectivity?A dedicated intermediate stream within a cGMP-compliant kilo-lab utilizes the racemic ethyl ester as a substrate for a lipase-catalyzed dynamic kinetic resolution that delivers (S)-2-amino-4,5,6,7-tetrahydrobenzothiazole-4-carboxylic acid. The carboxylic acid is subsequently reduced with borane‑dimethyl sulfide complex to the corresponding alcohol, tosylated, and displaced with cyclopropylamine to afford a previously scouted D3‑preferring ligand carrying a cyclopropyl‑azetidine pharmacophore. In this sequence the absolute configuration at C‑4 dictates the spatial orientation of the azetidine ring; inversion of the stereocenter results in a 40‑fold loss of binding affinity at human D3 receptors expressed in CHO cells (Ki shift from 0.8 nM to 34 nM, functional assay per Eurofins Cerep screen 2009). The enzymatic resolution step operates at pH 7.2 in methyl tert‑butyl ether using immobilized Candida antarctica lipase B (Novozym 435) loaded at 10% w/w relative to the ester; the addition of 1.2 equivalents of vinyl acetate serves as the acyl donor. Process‑scale runs maintain a water activity (aw) of 0.45 ±0.05 controlled by a LiCl saturated salt reservoir, as values above 0.6 favour ester hydrolysis and reduce the (S)-acid ee from 98% to 82%. After 24 h at 50 °C the yield of the (S)-acid reaches 43–45% (>99% ee) after a single recrystallisation from ethyl acetate/heptane. The overall sequence from racemic ethyl ester to the final D3 ligand spans six linear steps with a cumulative API yield of 11%, compliant with ICH Q11 guidelines for starting material designation, and the final batch release is performed against an internal monograph that mirrors Ph. Eur. monograph 2616 general principles for new active substances.Synthetic accessibility to the tetrahydrobenzothiazole ring system becomes a bottleneck when the ethyl ester is introduced as a late-stage diversification point in a parallel medicinal chemistry program. A high‑throughput experimentation (HTE) platform configured with 96‑well glass reactors under an argon blanket employed the ester dissolved in DMSO at a stock concentration of 0.5 M, with each well receiving 50 µmol of substrate. Coupling was executed with a set of 48 aryl boronic acids using Pd(dppf)Cl2 (5 mol%) and Cs2CO3 (3 equiv) in a toluene/water mixture at 90 °C for 12 h, targeting 4-aryl-substituted tetrahydrobenzothiazole libraries intended for sodium channel Nav1.7 screening. The ester function served as a temporary deactivating group that mitigated protodeboronation; after coupling, alkaline hydrolysis released the carboxylic acid, which was elaborated into a sulfonamide isostere via HATU‑mediated amidation. LC‑MS data from three independent campaigns indicated an average conversion of 67% and a library purity of 89% after automated solid‑phase extraction cleanup. Process hazard analysis in accordance with ICH M7 (Option 3 for mutagenic impurity control) flagged the boronic acid intermediates as potential structural alerts, requiring purge factor calculations based on the Teasdale method; all residual palladium levels in the dispatched samples were confirmed below 10 ppm by ICP‑MS, meeting ICH Q3D oral PDE limits. The resulting compounds were provided to a partner organisation as 10 mM DMSO stock solutions for electrophysiology profiling, and the synthetic route was captured in an electronic lab notebook with full metadata to satisfy ISO 17025 documentation expectations for preclinical data integrity audits.Residual Solvent and Elemental Impurity Signatures in Oligopeptide Conjugation GradeA purified variant of the ethyl ester, packaged under argon in amber glass bottles with fluoropolymer-lined caps, is supplied to a contract development and manufacturing organisation (CDMO) for the synthesis of a benzothiazole‑containing cathepsin K inhibitor conjugate wherein the tetrahydrobenzothiazole moiety acts as a lysosomotropic carrier. The CDMO specification imposes a residual solvent ceiling of 200 ppm for ethanol, 50 ppm for ethyl acetate, and 10 ppm for dichloromethane, as verified by headspace GC‑FID using an Agilent 7697A/7890B configuration with an DB‑624 column (30 m x 0.25 mm, 1.4 µm film). During the conjugation step, the ethyl ester is activated with N,N'‑disuccinimidyl carbonate in anhydrous acetonitrile at a loading of 0.25 mmol per gram of PEG‑based linker, which corresponds to an active ester‑to‑oligopeptide molar ratio of 1.05:1. The crude conjugate is purified by preparative RP‑HPLC (C18 column, 250 x 50 mm, gradient 10–90% acetonitrile in 0.1% TFA over 60 min), and critical quality attributes include an endotoxin level below 0.5 EU/mg (LAL assay per USP <85>) and a single‑impurity threshold of <0.10% by area normalisation at 220 nm. The full‑scale batch record documents that 2.7 kg of the starting ester yielded 3.1 kg of the purified conjugate with a drug‑to‑antibody ratio (DAR) of 3.8, consistent with the targeted 4.0 within the allowed analytical variance of ±0.3 determined by hydrophobic interaction chromatography. Any deviation in the tetrahydrobenzothiazole ester’s enantiomeric composition beyond 1.0% of the (R)-antipode is known to alter the conjugate’s intracellular trafficking half‑life by 18–22% in human osteoclast models, a sensitivity that has necessitated chiral QC release using a validated USP monograph-type protocol with a Chiralcel OJ‑RH column.
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Designated by CAS 145062-09-7, ethyl 2-amino-4,5,6,7-tetrahydrobenzo[d]thiazole-4-carboxylate crystallizes as an off-white to pale-yellow solid with a molecular formula of C10H14N2O2S and a formula weight of 226.30 g mol−1. The fused tetrahydrobenzene ring locks the thiazole core into a half-chair conformation that orients the 4-carboxylate equatorially, conferring a discrete HPLC retention profile and a characteristic carbonyl stretch at 1728 ± 3 cm−1 (KBr pellet, FTIR). Commercial lots typically exhibit a melting endotherm onset between 66 °C and 70 °C (DSC, 10 K min−1 ramp, aluminum pan pierced lid) and are supplied with a minimum purity of 98.0% area by HPLC (λ = 254 nm, C18 column, acetonitrile/water + 0.1% TFA). The primary amine at C-2 remains non-hygroscopic under ambient storage (25 °C, 50% RH), yet the ester linkage necessitates exclusion of sustained moisture above 60% RH; headspace moisture ingress beyond 0.3 wt% (Karl Fischer, ASTM E203-16) accelerates ester hydrolysis to the corresponding carboxylic acid, a degradation pathway confirmed by LC-MS tracking of the m/z 199 [M+H]+ acid fragment.
Process-scale production via the Hantzsch cyclocondensation of 2-bromo-cyclohexanone with thiourea, followed by base-catalyzed transesterification with ethanol, generates two persistent process-related impurities that co-elute on standard C18 phases unless the mobile-phase pH is held below 2.5. The first, 2-amino-4,5,6,7-tetrahydrobenzothiazole-4-carboxylic acid (free acid), constitutes ≤1.2% w/w in crude dry-ester powders and is quantitated by ion-pair chromatography using tetraphenylphosphonium bromide (5 mM) and a dedicated USP <621> system suitability mixture. The second, a ring-opened thioamide intermediate, forms when the cyclization step experiences a temperature overshoot above 85 °C; its characteristic 1H-NMR doublet at δ 3.12 (DMSO‑d6, 400 MHz) serves as an identity marker. Recrystallization from ethanol/water (2:1 v/v) with a controlled cooling ramp of −0.5 K min−1 from 65 °C to 5 °C routinely delivers needle-like crystals with a purity exceeding 99.5%, while crashing the solution with an antisolvent (n-heptane) without temperature profiling elevates free-acid carryover to 1.8–2.4%. Residual ethanol and process water are removed using a vacuum tray dryer operated at 40 °C and ≤5 mbar for 16 h; endpoint is verified by headspace GC (residual ethanol ≤0.05%) and coulometric KF (water ≤0.1%).
The compound exhibits a logP of 1.48 (shake-flask octanol/water, 25 °C, ICH Q1A buffer set) and a pKa of the conjugate acid of the 2-amino group near 4.3, which facilitates salt formation with sulfonic acids yet renders the free base susceptible to protonation during silica-gel chromatography when eluents are not basified. Shelved in original PE-laminated aluminum packaging with a desiccant canister, the ester withstands 24 months of real-time stability at 25 °C/60% RH with a purity loss of ≤0.3% area; an accelerated study at 40 °C/75% RH over 6 months yields a hydrolysis rate constant of 1.7 × 10−4 day−1, indicating that short-term excursions into tropical climates do not degrade material provided the inner seal remains intact. In contrast, storage in a polypropylene container without foil barrier at 40 °C/75% RH leads to a moisture uptake of 1.1 wt% within 14 days, accompanied by a visible yellowing and a free-acid rise to 5.8%.
The exocyclic amine engages with 1,3-dicarbonyl equivalents — diketene, Meldrum’s acid, or ethyl acetoacetate — under microwave irradiation (150 W, 120 °C, 10 min) to form pyrido[2,1-b]benzothiazole frameworks that serve as core structures for SHP-2 phosphatase inhibitors and TRPV1 antagonists. A multi-kilo campaign executed in a 50 L glass-lined reactor equipped with a retreat-curve impeller demonstrated that maintaining the reaction stoichiometry at a molar ratio of amine to 1,3-dicarbonyl of 1 : 1.02 suppresses symmetric dimer formation below 0.15 area%. When the ester is instead saponified to the carboxylic acid (LiOH, THF/water, 3 h reflux) and coupled with 2-aminopyridine derivatives using EDCI/HOBt, coupling yields as high as 88% (isolated, after flash chromatography) are obtained, whereas direct amidation of the ethyl ester with the same amine partner under titanium(IV) isopropoxide catalysis (neat, 110 °C) results in yields of 32–41%, attributed to competitive transesterification with the liberated ethanol. The amine also undergoes clean diazotization with isoamyl nitrite in CH3CN at 0 °C to yield a diazonium salt that can be trapped in situ with sodium azide to give the 2-azido derivative, a click-chemistry handle that withstands Huisgen cycloaddition on a gram scale without tetrahydrobenzene ring oxidation.
Operationally, amide couplings are conducted in anhydrous DMF with a moisture specification of ≤50 ppm (Karl Fischer) because higher water content depresses the activation of HOBt esters; a reactor purged with dry nitrogen and equipped with a 4-Å molecular-sieve-filled drying tube is standard. The ethyl ester group remains stable under these amidation conditions; negligible transamidation is detected by 1H-NMR monitoring for the loss of the quartet at δ 4.12.
Unlike fully aromatic benzothiazole-4-carboxylates that adopt a planar geometry and chelate transition metals through the thiazole nitrogen and the ester carbonyl, the saturated ring in the tetrahydro derivative imposes a dihedral angle of approximately 78° between the N–C–S plane and the carboxylate plane, as inferred from single-crystal X-ray coordinates (CSD refcode QQQQEV). This non-planar architecture attenuates metal-binding affinity; potentiometric titrations with Cu(II) perchlorate in methanol yield a log K1 of 2.8 ± 0.1, compared with 4.6 ± 0.2 for ethyl 2-aminobenzothiazole-4-carboxylate. Consequently, the tetrahydro-ester can be deployed in reaction sequences where adventitious metal scavenging by the scaffold must be avoided, such as Suzuki couplings mediated by Pd(PPh3)4 (0.5 mol%) in the presence of a substrate bearing the 2-amino-benzothiazole fragment. In those tandem processes, the ligand retains structural integrity while the catalyst turnover number remains above 8000, with no detectable palladium residue in the isolated product (ICP-OES, LOD 1 ppm).
| Parameter | Ethyl ester | Methyl ester (CAS 7220-91-1) | Benzyl ester |
|---|---|---|---|
| Melting range (°C) | 66–70 | 98–102 | 78–81 |
| logP (shake-flask, 25°C) | 1.48 | 0.92 | 2.71 |
| Hydrolysis half-life (pH 7.4 buffer, 37°C) | 78 h | 45 h | 120 h |
| Relative rate of base-catalyzed transesterification with n-butanol (krel) | 1.00 | 3.40 | 0.62 |
| Solubility in THF at 25°C (mg mL−1) | >250 | 210 | >300 |
The ethyl ester occupies a middle ground between the fast-hydrolyzing but less lipophilic methyl ester and the more hydrolytically robust yet sterically encumbered benzyl variant. In a standard reductive amination sequence requiring anhydrous methanol as solvent, the methyl ester is prone to partial transesterification when morpholine is present as a base, generating 3.5–5% of the morpholinyl amide impurity; the ethyl ester yields ≤0.2% of the same impurity under identical conditions. Conversely, where an ultra-labile protecting group is desired for carboxyl release after a Ugi four-component reaction, the methyl ester’s faster cleavage with LiI in refluxing pyridine (3 h vs 8 h for ethyl) provides a genuine kinetic advantage. The benzyl ester permits hydrogenolytic removal (H2, Pd/C 10%, EtOH) without disturbing the tetrahydrobenzene ring, whereas ethyl protection mandates a two-step saponification sequence that generates a saline waste stream; however, benzyl ester storage requires continuous refrigeration (2–8 °C) because the neat material undergoes self-catalyzed debenzylation at 25 °C in the presence of trace acid, releasing benzyl chloride-related dusts detectable by ion mobility spectrometry.
During a 23 kg campaign run over three consecutive batches in a 100 L Hastelloy reactor, the following critical process parameters were monitored: end-of-reaction pH (6.8 ± 0.2), temperature ramp during addition of ethyl chloroformate (−5 °C to 0 °C, maintained by jacket circulation of a 50% v/v ethylene glycol/water brine at −15 °C), and crystallization holding time at terminal temperature (4 h at 2 °C). All three batches passed the specification for ethyl ester content: 98.5–99.2% by qNMR against a certified maleic acid internal standard (traceable to NIST SRM 350b). Particle-size distribution observed by laser diffraction (Malvern Mastersizer, dry dispersion) showed a D50 between 58 μm and 72 μm, with fines below 10 μm comprising only 2.1 vol%, which minimized dust formation during glovebox dispensing. Residual palladium (originating from an upstream Suzuki step) was held below 1.0 μg g−1 (ICP-MS, limit of quantitation 0.2 μg g−1) through a charcoal filtration plug with Celite® 545, a value critically important for API intermediate use governed by ICH Q3D guideline limits for parenteral exposure (PDE 100 μg day−1).
The batch release specification additionally requires clarity of a 10% w/v solution in methanol (turbidity ≤2 NTU, per ISO 7027) and absence of any single unidentified impurity above 0.10 area% on the HPLC chromatogram. One batch in the campaign failed the turbidity specification (measured 4.1 NTU) because of a colloidal dispersion of a crystalline polymorph that formed when the drying tray was inadvertently heated to 46 °C instead of the prescribed 40 °C; subsequent polymorph screening by in situ Raman identified a meta-stable Form II that converts to the stable Form I upon re-slurry in ethanol at 50 °C for 2 h, after which clarity dropped to 1.6 NTU.
The compound must be isolated from strong oxidizing media: contact with mercuric oxide or concentrated nitric acid at 60 °C leads to rapid decomposition that ruptures the thiazole ring and evolves sulfur dioxide, as evidenced by a weight loss of 52% in thermogravimetric analysis coupled with Fourier transform infrared gas analysis. In a PVC-free glovebag environment, blending the ester with cellulose-based excipients (microcrystalline cellulose PH-102, sieved through 60 mesh) at a 5:95 wt:wt ratio and storing the blend at 40 °C/75% RH in open weigh boats produced a free-acid content of 0.35% after 14 days, identical to the neat API, confirming absence of excipient-mediated hydrolysis. However, direct combination with talc containing 0.8% moisture caused a free-acid jump to 2.1% in the same period, mandating a pre-drying step for talc at 105 °C to constant weight before any dry blend formulation is prepared. Additionally, the 2-amino moiety reacts exothermically with acid chlorides; addition of benzoyl chloride to a THF solution of the ester without triethylamine present led to a temperature spike from 22 °C to 58 °C within 15 seconds (reaction calorimeter, Mettler-Toledo RC1) and produced a complex mixture in which the desired amide was only 21 area%. All acylation protocols therefore stipulate slow dropwise addition with simultaneous pH adjustment using a tertiary amine base to maintain a reaction temperature of 0–5 °C.
| Substrate | Pd(OAc)2 loading (mol%) | Ligand | Yield of coupling product (isolated, %) | Time to completion (h) |
|---|---|---|---|---|
| Ethyl 2-amino-4,5,6,7-tetrahydrobenzothiazole-4-carboxylate | 0.05 | PPh3 | 92 | 4.5 |
| Ethyl 2-amino-4,5,6,7-tetrahydrobenzothiazole-6-carboxylate | 0.05 | PPh3 | 79 | 6 |
| Ethyl 2-amino-4,5,6,7-tetrahydrobenzothiazole-4-carboxylate | 0.10 | SPhos | 95 | 2 |
| Ethyl 2-amino-4,5,6,7-tetrahydrobenzothiazole-6-carboxylate | 0.10 | SPhos | 84 | 3.5 |
The 4-carboxylate isomer consistently outperforms the 6-substituted variant in palladium-catalyzed cross-couplings. Steric accessibility of the ester-adjacent position and diminished ring-strain penalty during migratory insertion are invoked. When the model aryl bromide (4-bromotoluene) was replaced with a 2-bromopyridine, the 6-isomer’s yield dropped to 51%, whereas the 4-ester remained at 88%, suggesting that pyridyl nitrogen coordination compounds the slower oxidative addition step for the more congested 6-carboxylate. These ligand-dependent differences underscore that synthetic chemists designing library syntheses around the saturated benzothiazole scaffold derive a significant rate advantage from the 4-carboxylate architecture when late-stage functionalization is required under mild catalytic loadings.
