The p-chlorophenylamide of 3-methyl-5-benzoylaminoisothiazole-4-carboxylic acid, systematically designated as N-(4-chlorophenyl)-3-methyl-5-(benzoylamino)-1,2-thiazole-4-carboxamide (C₁₇H₁₂ClN₃O₂S, M
r 357.81 g mol⁻¹), is a synthetic, low-molecular-weight heterocyclic amide supplied as a research-grade biochemical probe. No CAS registry number had been assigned to this specific derivative at the time of initial batch release; identification is instead anchored to retention time, exact mass, and spectral fingerprint. The compound is employed principally as a type-II kinase inhibitor scaffold in early-stage oncology target validation, where the 4-chloro substituent on the terminal phenylamide ring contributes to a residence time advantage not observed with the unsubstituted phenylamide or 4-fluoro analog. Characterization data for the pilot lot are reported alongside handling constraints that differentiate this product from structurally related tool compounds.
Chromatographic purity and identity verification protocols
Batch release specifications mandate a chromatographic purity of
≥98.0 area% by high-performance liquid chromatography on a C₁₈ column (
150 ×
4.6 mm,
5 µm) with a mobile phase gradient of
0.1% trifluoroacetic acid in water and acetonitrile, UV detection at
254 nm. System suitability is evaluated per the signal-to-noise requirements of Ph. Eur.
2.2.46. The certified reference standard for external calibration possesses a mass balance of
99.2% determined by quantitative ¹H‑NMR (qNMR) using dimethyl sulfone as an internal standard. Identity confirmation relies on high-resolution mass spectrometry; the protonated molecular ion [M+H]⁺ at
m/z 358.0614 (±
3 ppm) is acquired on a Q‑TOF instrument with electrospray ionization in positive mode. Residual solvent analysis conforms to USP
<467> Procedure A, with acceptance criteria for DMF (
≤880 ppm) and ethyl acetate (
≤5000 ppm).
Table 1 – Release specifications for the p‑chlorophenylamide research compound
| Parameter | Specification | Test method |
|---|
| Appearance | White to off-white crystalline solid | Visual inspection |
| Purity (HPLC) | ≥98.0 area% | In-house SOP LC‑023 (C₁₈, 254 nm) |
| Melting point (decomp.) | 215–218°C | USP <741>, Class I capillary |
| Water content | ≤0.5% w/w | ASTM E203‑16 (coulometric KF) |
| Solubility (DMSO) | ≥50 mg mL⁻¹ | Gravimetric determination |
| Identity (HRMS) | Mass error ±3 ppm | ESI‑Q‑TOF, positive ion mode |
On a multi‑kilogram scale, the penultimate intermediate 3‑methyl‑5‑benzoylaminoisothiazole‑4‑carboxylic acid is activated with 1‑ethyl‑3‑(3‑dimethylaminopropyl)carbodiimide hydrochloride and 1‑hydroxybenzotriazole in
N,N-dimethylformamide at
0–
5°C before addition of 4‑chloroaniline. Residual aniline is scavenged with polymer‑bound isocyanate resin until inline Fourier‑transform infrared spectroscopy confirms disappearance of the isocyanate stretching absorption at
2270 cm⁻¹. The crude material is crystallized from ethyl acetate/
n-heptane to deliver the monohydrate polymorph; powder X‑ray diffraction against the library pattern of the anhydrous form prevents lot acceptance of the kinetically favored anhydrate. Process analytical technology (PAT) trending of the carbonyl region of the Raman spectrum during cooling crystallization ensures consistent nucleation of the desired crystal phase. These scale‑up constraints differentiate the manufacturing route from that of the corresponding 4‑fluoro analog, where toluene/THF solvent systems are employed and scavenging with activated charcoal suffices.
Why does the p‑chlorophenylamide moiety confer target residence time advantages over meta‑ or ortho‑substituted analogs?
Surface plasmon resonance (SPR) studies conducted on a closely related benzoylaminoisothiazole‑4‑carboxamide series indicate that the 4‑chloro substitution facilitates a halogen‑bonding interaction with a backbone carbonyl oxygen of the kinase hinge region (cyanamide residue Met‑
793 in the insulin‑like growth factor‑1 receptor pocket), whereas the 3‑chloro isomer disrupts the coplanarity of the terminal phenylamide ring and the 2‑chloro isomer sterically clashes with the glycine‑rich loop. The resulting off‑rate (
koff) for the
para‑substituted derivative was slowed by a factor of
3–
7 relative to the unsubstituted phenylamide in a contract research organization’s surface‑based binding assay; complete primary data remain proprietary. By contrast, the 4‑bromo analog exhibits a melting point depression of approximately
12°C and a ten‑fold increase in dimethyl sulfoxide‑mediated aggregation above
10 µM, rendering it unsuitable for isothermal titration calorimetry workflows that the p‑chlorophenylamide tolerates. Literature describing similar scaffolds suggests that the Hammett σ
p value of chlorine (
+0.23) modulates the electron density of the thiazole ring sufficiently to influence hydrogen‑bonding capacity of the 5‑benzoylamino carbonyl without triggering glutathione‑trapping oxidative defluorination seen with the 4‑fluoro congener.
Employing this reagent in cell‑free biochemical assays at concentrations exceeding
10 µM necessitates inclusion of
0.01% (v/v) Triton X‑
100 or
0.1 mg mL⁻¹ bovine serum albumin to mitigate nonspecific colloidal aggregation; dynamic light scattering at a scattering angle of
173° confirms particle formation above that threshold in phosphate‑buffered saline without detergent. For steady‑state kinetic analyses, a pre‑incubation period of
30 minutes at
25°C is prescribed to reach binding equilibrium, a protocol step that is notably shorter than the
60‑minute equilibration required for the morpholino‑substituted isothiazole carboxamide comparator. The difference arises from slower association kinetics attributable to desolvation penalties of the polar morpholine moiety.
If the compound is employed in cellular assays, rigorous controls for off‑target adrenergic receptor binding are advised
Radioligand displacement profiling at
1 µM against a panel of
44 G‑protein‑coupled receptors, performed under Eurofins Pharma Discovery Services catalogue item
GPCRProfiler™, has occasionally identified weak α
2C‑adrenergic activity (inhibition
35–
50% at the screening concentration) that is absent in the structurally orthogonal stilbene‑based comparator. Co‑administration of the selective α
2C antagonist JP‑
1302 is therefore recommended during functional cellular readouts involving cAMP response element activation. This polypharmacology signature distinguishes the p‑chlorophenylamide from the di‑methylpyrazole‑based inhibitor series, where adrenergic counter‑screening is typically negative. Published head‑to‑head data under identical cell‑culture conditions are limited; however, internal batch‑to‑batch surveillance across
five independent syntheses has reproduced this ancillary pharmacology, confirming it as a scaffold‑intrinsic liability rather than a trace impurity artifact.
Table 2 – Comparative physicochemical properties across substituted phenylamide derivatives of 3‑methyl‑5‑benzoylaminoisothiazole‑4‑carboxylic acid
| Substituent | M.p. (decomp.) °C | Calculated logP (cLogP) | KH₂O solubility (µM, pH 7.4) | Aggregation threshold (µM, DLS) |
|---|
| –H | 221–224 | 2.8 | 12 | >100 |
| –Cl (para) | 215–218 | 3.4 | 8 | 10 |
| –F (para) | 207–210 | 2.9 | 18 | >100 |
| –Br (para) | 203–206 | 3.6 | 5 | 3 |
Storage outside an inert, anhydrous atmosphere results in hydrolytic ring‑opening of the isothiazole core within
48 hours at ambient relative humidity exceeding
60%, a failure mode first documented during shipment validation using temperature/RH data loggers compliant with EN
12830:2018. The primary degradation product, identified by LC‑MS/MS as 3‑methyl‑5‑benzoylaminoisothiazole‑4‑carboxylic acid, reverts the amide to the free acid. Aliquoting under argon into
2‑mL amber borosilicate vials with PTFE‑lined silicone septa and storage at
−20°C ±
2°C in a desiccated cabinet maintains chemical integrity for
24 months, as verified by accelerated stability testing at
40°C/
75% RH for
6 months according to ICH
Q1A(R2). Repeated freeze‑thaw cycles induce a polymorphic shift from the monohydrate to the anhydrous Form II; dissolution kinetics in dimethyl sulfoxide slow measurably (time to complete dissolution increases from
<2 minutes to
>8 minutes after
3 cycles), justifying the preparation of single‑use aliquots upon initial solvation.
The operational boundary for dimethyl sulfoxide stock solutions is
100 mM; beyond this concentration, viscosity excursions affect acoustic droplet ejection (Labcyte Echo
550‑series) transfer volumes by more than
5%. Dilution into aqueous assay buffers tolerates a final dimethyl sulfoxide concentration of up to
1% (v/v) without phase separation, a range identical to the morpholino analog but notably narrower than the
2% ceiling reported for the piperazine‑linked comparator. For in vivo pharmacokinetic studies in rodent models, the compound is formulated as a micro‑suspension in
0.5% w/v methylcellulose/
0.1% v/v Tween‑
80 in deionized water; mean particle size (D₉₀) measured by laser diffraction (Malvern Mastersizer
3000 with Hydro MV dispersion unit) is maintained below
10 µm via wet bead milling with
0.3‑mm yttria‑stabilized zirconia beads at
2000 rpm for
20 minutes. Co‑administration with ketoconazole is contraindicated because CYP3A4‑mediated oxidative dechlorination generates a reactive quinone‑imine intermediate that covalently modifies hepatic microsomal protein, a bioactivation pathway absent in the methyl‑substituted phenylamide congener lacking the halogen.
