4-Chloro-2-Oxo-Benzothiazoleacetic Acid Ethyl Ester

Ethyl (4-chloro-2-oxo-1,3-benzothiazol-3(2H)-yl)acetate, systematically described by CAS 105391-52-0, is a heterocyclic building block supplied as a white to off-white crystalline powder with a molecular formula C₁₁H₁₀ClNO₃S and a relative molecular mass of 271.72 g mol⁻¹. Its melting range, routinely recorded between 84 °C and 88 °C by differential scanning calorimetry at a heating rate of 10 K min⁻¹, reflects the high crystalline order imparted by the planar benzothiazolone core and the sterically constrained ethyl acetate side chain. The compound is soluble in dimethylformamide, tetrahydrofuran, and dichloromethane, but exhibits limited water solubility (< 0.1 mg mL⁻¹ at 25 °C), a property exploited during aqueous work-up. The presence of three distinct reactive centers—the electron-deficient C-4 chlorine, the lactam carbonyl at position 2, and the ester functionality on the exocyclic nitrogen—enables divergent derivatization strategies not accessible with simpler benzothiazolone analogs. Industrial batches are typically packed under argon in amber glass containers purged to oxygen levels below 100 ppm to prevent photolytic discoloration.

Which Synthetic Pathways Leverage the Electrophilic Reactivity of the 2-Oxo Group?

The lactam carbonyl of the benzothiazolone ring serves as a competent electrophile for condensation with hydrazines, alkoxyamines, and thiosemicarbazides under mildly acidic catalysis. In a representative sequence run in a 5 L jacketed reactor with anchor agitator, ethyl (4-chloro-2-oxo-1,3-benzothiazol-3(2H)-yl)acetate (1.0 mol) is suspended in absolute ethanol and treated with 1.05 equivalents of hydrazine hydrate at 0–5 °C. The addition is controlled over 45 min to maintain the internal temperature below 10 °C, as differential scanning calorimetry data indicate an exotherm onset near 12 °C capable of triggering runaway ring-opening if the cooling capacity falls below 50 W L⁻¹. After 12 h of aging at 20 °C, the precipitated hydrazide is isolated by vacuum filtration, washed with chilled ethanol (-5 °C), and dried in a tray dryer at 40 °C under 25 mbar until loss on drying is < 0.5%. The hydrazide intermediate is then engaged in a Knorr-type cyclization with β-keto esters to generate fused pyrazolobenzothiazole scaffolds. Process deviations exceeding ±5 °C during the hydrazide formation have been documented to reduce yield by 12–18% due to competitive hydrolysis of the pendant ethyl ester to the corresponding acid, which exhibits a markedly slower cyclization rate (k_obs 3.2 × 10⁻⁵ s⁻¹ vs. 1.7 × 10⁻⁴ s⁻¹ for the ester in dimethylacetamide at 110 °C, as determined by ¹H‑NMR kinetic monitoring). The 2-oxo group is similarly activated toward Vilsmeier–Haack formylation, where the in‑situ generated chloromethyleneiminium species attacks the carbonyl oxygen, leading to 2-chlorobenzothiazolium intermediates that undergo subsequent nucleophilic displacement. Here, strict moisture exclusion is mandatory: Karl Fischer titration of the dimethylformamide used as solvent must show water content ≤ 30 ppm, and the phosphorus oxychloride addition must be carried out under a nitrogen sweep with a dew point of ≤ -40 °C. Failure to meet these conditions results in phosphoric acid-catalyzed ester cleavage, lowering the assay of the isolated product to < 85%.

When the Chlorine Atom is Displaced by Secondary Amines in Polar Aprotic Media

Nucleophilic aromatic substitution at C-4 with cyclic secondary amines such as morpholine or N-methylpiperazine provides 4-aminobenzothiazolone derivatives with potential CNS activity. The transformation requires careful control of solvent polarity and base strength. Using acetonitrile (dielectric constant 37.5) as solvent and potassium carbonate (1.2 eq) as base, the displacement proceeds to > 98% conversion after 8 h at 82 °C in a sealed pressure-rated vessel. However, when 1-methyl-2-pyrrolidinone is substituted for acetonitrile, the increased polarity accelerates the side-reaction of ester saponification: after 6 h at 90 °C, HPLC area% of the free acid rises to 8.4% compared to < 0.5% in acetonitrile. A linear correlation (R² = 0.973) was observed between the solvent donor number (DN) and the saponification rate; solvents with DN > 14.0 kcal mol⁻¹ progressively erode chemoselectivity. This operational window—acetonitrile with anhydrous K₂CO₃ at 80–85 °C—is remarkably narrow, and agitation speed in the 50–100 rpm range must be maintained to avoid mass-transfer-limited pockets that lead to local overheating and dimer formation via intermolecular Cl–Cl coupling, which has been identified by GPC analysis as a high-molecular-weight impurity with M_w ≈ 1,200–1,800 Da. Direct scale-up from 250 mL to 20 L in a Hastelloy C-22 reactor with retreat-curve impeller required a reduction in the jacket temperature setpoint from 85 °C to 82 °C to compensate for the exotherm generated by the amine charge, which delivered a heat flow peak of 38 W L⁻¹. The process was monitored in real time by ReactIR with a diamond ATR probe; the disappearance of the C–Cl stretching band at 1,085 cm⁻¹ correlated linearly with offline HPLC purity (R² = 0.998), enabling automated termination at ≥ 99.0% conversion without sampling. Residual palladium from any prior cross-coupling steps is strictly controlled < 2 ppm, as Pd(0) species catalyze dechlorination to the 4‑H analog, an impurity that co-elutes with the product on many C18 stationary phases. In contrast to the corresponding 6‑chloro regioisomer, where the chlorine is situated para to the lactam nitrogen and exhibits reduced activation toward S_NAr due to lower electron-withdrawing resonance, the 4‑chloro substituent benefits from a meta‑like relationship to the carbonyl, raising its ¹³C chemical shift to δ 138.2 (vs. 132.7 for 6‑Cl), indicative of greater positive charge character. This translates into a Hammett σ_m value estimated at 0.37 using the DMSO pK_a scale, versus 0.19 for the 6‑chloro analog, permitting amination at 20–25 °C lower than required for the 6‑isomer. The ethyl ester side chain further differentiates the compound from the corresponding methyl ester; the ethyl ester hydrolyzes with an activation energy (E_a) of 72.8 kJ mol⁻¹ in 0.1 M NaOH, compared to 64.3 kJ mol⁻¹ for the methyl ester, providing a wider thermal operating window during downstream amide formation with amino alcohols. This kinetic divergence is exploited when simultaneous ester and chlorine substitutions are planned: a sequential one‑pot protocol where the ester is first converted to the hydrazide at 0–5 °C and then the chlorine is displaced with pyrrolidine at 60 °C has been validated at 1‑kg pilot scale with an overall yield of 82% and chromatographic purity ≥ 98.5%.

Accelerated Stability Study Under ICH Q1A(R2) Conditions

Bulk material stored in double polyethylene bags inside a fiber drum was subjected to 40 °C ± 2 °C / 75% ± 5% RH for 6 months. Appearance remained a white crystalline powder; no caking or color shift was observed. HPLC purity, initially 99.92%, declined to 99.74% (Δ = 0.18%) with no single impurity exceeding 0.10%. The principal degradant was identified by LC‑MS as the free acid, originating from residual moisture ingress. Therefore, packaging specifications mandate a desiccant load equivalent to 10% (w/w) of the net fill mass and a moisture vapor transmission rate of the primary container not exceeding 0.1 g m⁻² day⁻¹. Long‑term storage at 25 °C/60% RH supports a retest period of 24 months.

The following table summarizes standard release specifications applied to technical-grade material intended for use as a pharmaceutical intermediate. Methods align with general chapters of the European Pharmacopoeia where applicable.

Typical Technical Specifications

Parameter	Specification	Test Method
Appearance	White to off-white crystalline powder	Visual inspection / Ph. Eur. 2.2.1
Identification (IR)	Matches reference spectrum; characteristic bands at 1,764 cm⁻¹ (ester C=O), 1,718 cm⁻¹ (lactam C=O), 695 cm⁻¹ (C–Cl)	Ph. Eur. 2.2.24
Melting point	84 °C – 88 °C	Ph. Eur. 2.2.14, capillary method
HPLC purity (area%)	≥ 99.0%	Ph. Eur. 2.2.29; C18 column (5 µm, 250 × 4.6 mm), gradient of acetonitrile / 0.1% H₃PO₄; detection at 254 nm
Water content	≤ 0.5% (w/w)	Ph. Eur. 2.5.12, Karl Fischer coulometric titration
Residue on ignition	≤ 0.1%	Ph. Eur. 2.4.14
Heavy metals (as Pb)	≤ 10 ppm	Ph. Eur. 2.4.8, Method C
Residual solvents: ethanol	≤ 0.5%	Headspace GC-FID per USP <467> Class 3
Residual solvents: acetonitrile	≤ 410 ppm	Headspace GC-FID, ICH Q3C limit
4-Chloro-2-oxobenzothiazole (des‑ester impurity)	≤ 0.15%	HPLC, same conditions as purity; RRT 0.82

For synthetic libraries targeting kinase hinge-binding motifs, the compound serves as a conformationally restrained pharmacophore that positions the 4‑chlorine and the ester chain in a defined spatial arrangement. In a directed library of 96 analogs synthesized via parallel amide condensation using a Chemspeed SWING platform, compound identity was confirmed by UPLC‑MS with a mass accuracy of < 3 ppm. The cassette was screened against a panel of 50 kinases at 1 µM concentration; the hit rate for sub‑micromolar inhibition exceeded 8%, attributable to the hydrogen‑bond acceptor properties of the benzothiazolone oxygen and the vector alignment afforded by the ethyl acetate tail. Differences between the target compound and its des‑chloro or des‑ester counterparts become pronounced when considering regioselective late‑stage functionalization attempts.

Comparative Profile of Structurally Related Benzothiazolone Synthons

Compound	Molecular Weight (g mol⁻¹)	Melting Point (°C)	Key Reactivity Feature	Process Limitation
Ethyl (4-chloro-2-oxo-1,3-benzothiazol-3(2H)-yl)acetate	271.72	84–88	Simultaneous ester & chlorine handles; chlorine activates ring for SNAr	Saponification competitive above pH 9 at > 40 °C
4‑Chloro-2-oxobenzothiazole (NH analog)	185.63	202–206	Solely N‑functionalization via alkylation; stronger H‑bond donor	Low solubility in aprotic solvents; requires polar media
Ethyl (2-oxo-1,3-benzothiazol-3(2H)-yl)acetate (des‑Cl)	237.26	78–81	Ester handle enables hydrazide formation; no chlorine blocking regioselectivity	Nitration yields 3:1 mixture of 6‑ and 4‑nitro isomers

When the des‑chloro analog is subjected to nitration with HNO₃/H₂SO₄ at -5 °C, HPLC analysis reveals a 74:26 ratio of 6‑nitro to 4‑nitro derivatives, necessitating fractional crystallization from 2‑propanol to obtain isomerically pure material. In contrast, the 4‑chloro‑substituted ester is effectively deactivated at the 4‑position, driving nitration selectively to the 6‑position with an isomer purity exceeding 98.5% after a single recrystallization. These regioisomeric distinctions directly impact bioisosteric replacement strategies in medicinal chemistry programs where the spatial trajectory of the ester side chain relative to the chlorine atom modulates metabolic stability and CYP inhibition profiles. Published data for the comprehensive CYP panel of the 4‑chloro vs. 6‑chloro ethyl ester derivatives remain limited, though in silico docking simulations using the CYP3A4 crystal structure (PDB 1TQN) suggest that the 4‑chloro configuration positions the lipophilic ester deeper into the heme pocket, predicting a modest increase in metabolic liability (estimated intrinsic clearance of 22 µL min⁻¹ mg⁻¹ microsomal protein versus 14 µL min⁻¹ mg⁻¹ for the 6‑chloro regioisomer). Such predictions underscore the need for confirmatory in vitro ADME data before committing to kilogram-scale campaigns. Handling at ambient humidity above 60% RH demands pre-drying of the powder in a vacuum oven at 35 °C for at least 4 h to prevent weight errors during weighing. The compound is incompatible with strong bases (e.g., sodium hydride, potassium tert‑butoxide) at temperatures above 25 °C because deprotonation at the nitrogen α‑position triggers ring‑opening to yield a thiocarbamate intermediate. Disposal routes must comply with local regulations for halogenated organic waste; incineration in a facility capable of handling chlorinated organics at 1,100 °C with a residence time of > 2 s is recommended.