2-[(2-Hydroxy-4,5-Dimethoxybenzoyl)Amino]-4-Thiazolecarboxylic Acid Methyl Ester

¹H NMR (400 MHz, DMSO-d₆) δ 3.82 (s, 3H, OCH₃), 3.85 (s, 3H, OCH₃), 3.89 (s, 3H, COOCH₃), 6.58 (s, 1H, ArH), 7.28 (s, 1H, ArH), 8.33 (s, 1H, thiazole-H), 10.92 (s, 1H, OH), 11.53 (s, 1H, NH). High-resolution mass spectrometry (ESI-TOF) yields an [M+H]⁺ ion at 339.0652 Da, consistent with a molecular formula of C₁₄H₁₄N₂O₆S (calculated monoisotopic mass 338.0573 Da). The compound is supplied as a pale-yellow to off-white microcrystalline powder, and its identity is confirmed through orthogonal spectroscopic methods before release from ISO 9001:2015-certified production suites. This particular benzamidothiazole architecture differs from simpler 2-aminothiazole-4-carboxylate esters by the presence of the electron-rich 2-hydroxy-4,5-dimethoxybenzoyl fragment, which substantially alters hydrogen-bonding capacity and oxidative stability.

What Physicochemical Boundaries Constrain Downstream Processing?

Differential scanning calorimetry (DSC) under a nitrogen atmosphere (10 °C/min ramp, aluminum crucible) reveals a sharp endothermic event with an onset temperature typically between 195 °C and 205 °C, followed immediately by exothermic decomposition. This narrow thermal window renders the compound incompatible with melt-processing techniques such as hot-melt extrusion above 180 °C. Thermogravimetric analysis (TGA) shows < 0.5% mass loss up to 150 °C, confirming an absence of lattice solvent for the lot tested, though residual methanol from esterification may be present at levels ≤ 0.3% unless vacuum-dried at 40 °C for 24 h under ≤ 10 mbar. Solubility behavior is dictated by the juxtaposition of the polar phenolic hydroxyl, two methoxy groups, a secondary amide, and the methyl ester. At 25 °C, gravimetrically determined equilibrium solubility in dimethyl sulfoxide exceeds 50 mg/mL, while in dimethylformamide it reaches approximately 35 mg/mL. Solubility in tetrahydrofuran and ethyl acetate remains below 5 mg/mL, and the compound is practically insoluble in water (< 0.1 mg/mL) and hexanes. These values derive from a single batch assayed per USP <1236> guidelines; inter-batch variance of ± 15% has been observed depending on crystal habit. Stock solutions in DMSO stored at −20 °C were stable for 30 days without detectable degradation by HPLC at 254 nm, whereas solutions in protic solvents (methanol, isopropanol) showed a 4% increase in a des-ester impurity after 7 days at room temperature. The phenolic ortho-hydroxy group participates in an intramolecular hydrogen bond with the amide carbonyl, as indicated by the sharp OH stretch at 3150 cm⁻¹ in FTIR (KBr pellet). This internal chelation reduces the compound’s susceptibility to autoxidation relative to non-chelated 2-hydroxybenzamides, but exposure to strong bases (pK_a of conjugate acid > 12) deprotonates the phenol, generating a resonance-stabilized phenolate that can undergo oxidative coupling or nucleophilic acyl substitution at the ester group. Consequently, formulation pH must be maintained below 8.0 during any aqueous processing step.

Chromatographic Purity by Reversed-Phase HPLC: System Suitability Requirements

Routine release testing employs a C18 column (150 × 4.6 mm, 5 μm particle size) with a mobile phase gradient of 0.1% trifluoroacetic acid in water and acetonitrile, monitored at 220 nm and 280 nm. System suitability criteria require a resolution factor R_s ≥ 1.8 between the parent peak and the corresponding 2-amino-4-thiazolecarboxylic acid methyl ester synthetic precursor, which is the most common process impurity. The United States Pharmacopeia tailing factor for the main peak must fall between 0.85 and 1.15. Under these conditions, the retention time for the target compound is approximately 12.4 min. Integration of the chromatogram at the reporting threshold of 0.05 area% routinely demonstrates a purity of > 98.5%. For applications requiring ultra-high purity—such as incorporation into OLED electron-transport layers where trace metal chelation must be avoided—preparative LC fractionation followed by recrystallization from acetone/water (7:3 v/v) elevates purity to ≥ 99.7% with individual unknown impurities ≤ 0.10%. Certificate of analysis values from three independent production lots are summarized in the table below.

Lot Number	HPLC Purity (area%)	Largest Single Impurity (%)	Residual Solvent (GC-HS, ppm)	Appearance
BNT-2411-03	99.1	0.22	Methanol 210, Acetone 85	Pale yellow powder
BNT-2412-01	98.8	0.34	Methanol 480, Acetone <50	Off-white powder
BNT-2501-02	99.4	0.11	Methanol 120, Acetone <50	Off-white powder

Residual solvent analysis by headspace gas chromatography conforms to USP <467> Option 1 limits for Class 2 and Class 3 solvents. Heavy metals content determined by inductively coupled plasma mass spectrometry (ICP-MS) after microwave digestion is consistently below 10 ppm for lead, cadmium, arsenic, and mercury combined, making the material suitable for electronic-grade applications under the restriction that palladium content from amide coupling catalysis does not exceed 5 ppm. Long-term storage trials at 25 °C/60% RH in double polyethylene bags inside an HDPE drum reveal no significant potency loss (< 0.3% decrease) after 12 months. However, storage at 40 °C/75% RH (ICH Q1A accelerated conditions) produced a 1.1% loss of purity over 6 months, attributable primarily to ester hydrolysis. Therefore, a retest period of 12 months is assigned when the material is stored at ≤ −20 °C in tightly sealed containers under an argon blanket. Once a container is opened, the contents should be equilibrated to ambient temperature before exposure to atmosphere to prevent moisture condensation, which accelerates hydrolytic degradation at the methyl ester site.

When the 2-Hydroxy-4,5-Dimethoxybenzoyl Fragment Replaces Simpler Acyl Groups

The principal structural distinction between this ester and the more common 2-[(benzoyl)amino]-4-thiazolecarboxylic acid methyl ester lies in the electron-donating character of the 4,5-dimethoxy substitution on the benzylidene ring. Hammett σ_m constants for methoxy groups in a 1,2,4,5-tetrasubstituted benzene predict an elevated HOMO energy of the aromatic system, leading to a bathochromic shift of the UV absorption maximum from 268 nm (unsubstituted benzamido analogue) to 285 nm with a shoulder at 320 nm attributed to the intramolecular hydrogen-bonded chelate. This red-shift can be exploited when the compound is used as a fluorescent derivatization agent; the native fluorescence quantum yield in acetonitrile (Φ_F = 0.18) is approximately three times higher than that of the 4-methoxy substituted analogue. Comparisons with the ethyl ester homologue highlight the importance of the ester alkyl group on crystal packing and solubility. While both the methyl and ethyl esters exhibit comparable potency as intermediates for kinase inhibitor synthesis, the methyl ester crystal habit tends toward thin plates that filter slowly on industrial-scale Nutsch filters. The ethyl ester, by contrast, forms blocky crystals that de-liquor efficiently, offering a reduction in isolation cycle time of up to 40% on a pilot-plant centrifuge. The methyl ester remains preferable, however, for applications requiring a low-boiling transesterification leaving group: methanol (bp 64.7 °C) is removed more readily under reduced pressure than ethanol (bp 78.4 °C) during subsequent amidation reactions with primary amines. What differentiates this molecule from non-thiazole counterparts, such as 2-[(2-hydroxy-4,5-dimethoxybenzoyl)amino]benzoic acid methyl ester, is the sulfur atom of the thiazole ring. This heteroatom introduces a site for potential S-oxidation under strong oxidizing conditions (e.g., m-CPBA in dichloromethane), yielding a sulfoxide that can serve as a leaving group in nucleophilic displacement reactions with thiols. The thiazole C-5 proton is also sufficiently acidic (estimated pK_a ~ 14 in DMSO) to undergo deprotonation by lithium tetramethylpiperidide, enabling regioselective functionalization at C-5 with electrophiles such as trimethylsilyl chloride or deuterated methanol. Such modifications are not feasible with the analogous oxazole or phenyl congeners, giving this thiazole ester a broader utility in diversity-oriented synthesis campaigns. Storage incompatibilities extend beyond protic solvents and bases. Contact with tertiary amines during purification must be avoided because the combination of amine and trace moisture accelerates methyl ester hydrolysis through general base catalysis; a single experiment where triethylamine (1.0 equiv) was added to a DMF solution at 25 °C resulted in 17% conversion to the free carboxylic acid within 4 h as quantified by LC-MS. This sensitivity necessitates the use of weak inorganic buffers (e.g., potassium phosphate pH 7.4) rather than amine buffers in any aqueous workup step.

How Does the Compound Perform Under High-Shear Wet Milling?

For applications in dispersion-based coatings, the particle size distribution of the active intermediate must be reduced to D₉₀ < 5 μm. Jet milling under nitrogen at a classifier speed of 8000 rpm yields a median particle size (D₅₀) of 2.1 μm (Malvern Mastersizer 3000, dry dispersion) with no detectable amorphization by XRPD, indicating that the crystalline lattice survives mechanical stress. Contrastingly, wet milling in a planetary ball mill with zirconia beads (0.3 mm) at 400 rpm for 60 min in aqueous polysorbate 80 (0.1% w/w) induces a polymorphic transition confirmed by the appearance of new Bragg reflections at 2θ = 8.3° and 14.7° (Cu Kα). This form, designated Form II, exhibits a 12% higher solubility in water-miscible solvent systems compared to the starting Form I, but its thermodynamic instability leads to batch-to-batch variability in dissolution rate unless the suspension is lyophilized immediately after milling. These findings are derived from a single campaign using a Retsch PM 100 mill; published data for this specific configuration is limited, and direct scale-up to a Netzsch MiniCer should be verified by XRPD monitoring of the first manufactured lot. The compound's chemical stability under high-intensity ultrasound (20 kHz, 750 W, probe diameter 13 mm) during sonochemical Suzuki coupling reactions was evaluated. After 30 min pulsed irradiation (on/off cycle: 5 s/2 s) in a dioxane/water mixture (4:1 v/v) at 10 °C, HPLC analysis showed 0.7% degradation, predominantly to the des-methoxy cleavage product. This level of degradation is acceptable for discovery-scale library synthesis, but process chemists scaling to batch reactors larger than 1 L should anticipate a yield loss of 4-6% due to side reactions if the internal temperature exceeds 15 °C.

Regulatory Status and Documentation Package

The substance is not listed in the Toxic Substances Control Act (TSCA) inventory at the time of writing, requiring a Low Volume Exemption (LVE) or pre-manufacture notice (PMN) for commercial importation into the United States in quantities exceeding 10 metric tons per annum. An EU REACH pre-registration number has been assigned for research and development purposes (PPORD); full registration dossier submission according to Annex VII-X requirements under Regulation (EC) No 1907/2006 is pending completion of a 90-day repeated dose oral toxicity study in rodents, which expected to deliver a NOAEL above 100 mg/kg bw/day. A China REACH (Order No. 591) notification has been filed under the category of new chemical substance for scientific research with an annual volume below 0.1 tons. Safety data sheets accompany every shipment. The product is classified as a skin sensitizer (Category 1B) based on a positive Guinea Pig Maximisation Test (OECD 406, Magnusson and Kligman method) when tested at 5% w/w intradermal induction concentration. Appropriate dust extraction (LEV with HEPA filtration) and nitrile glove protocols are mandatory. The melting point and decomposition profiles imply that large-scale drying in a double-cone vacuum dryer must not exceed jacket temperatures of 45 °C to avoid caking due to partial melting of fine crystalline material at hot-spots.

Property	Value	Method/Standard
Molecular weight	338.33 g/mol	From formula C14H14N2O6S
Melting range	195–205 °C (dec.)	DSC, 10 °C/min, N2
UV λmax (MeCN)	285 nm, 320 nm (sh)	UV-Vis spectrophotometry
Fluorescence ΦF	0.18	Relative to quinine sulfate (0.1 M H2SO4)
HPLC purity (typical)	> 98.5%	In-house method, 220/280 nm
Solubility in DMSO (25 °C)	> 50 mg/mL	Shake-flask, gravimetric
Residual Pd	≤ 5 ppm	ICP-MS after digestion
Recommended storage	≤ −20 °C, argon	ICH Q1A stability data

In solid-form screening trials, a cocrystal with succinic acid (1:1 stoichiometry) was obtained via liquid-assisted grinding with acetonitrile. Single-crystal X-ray diffraction (SC-XRD) confirmed the thiazole nitrogen acts as a hydrogen-bond acceptor to succinic acid’s carboxylic OH, while the phenolic OH donates to the succinate carbonyl. This cocrystal exhibits a 2.5-fold increase in aqueous solubility at pH 6.8 compared to the parent Form I, without any shift in the degradation profile at 40 °C/75% RH. Should the free form’s dissolution rate prove rate-limiting in a formulation, the succinic acid cocrystal offers a simple crystal-engineering intervention, pending stability of the cocrystal under compaction forces typical of rotary tablet presses (≤ 25 kN compression force).