3-(1-Piperazizyl)-1,2-Benzisothiazole

In the preparation of ziprasidone hydrochloride monohydrate active pharmaceutical ingredient, the alkylation of 5-(2-chloroethyl)-6-chloro-1,3-dihydro-2H-indol-2-one with 3-(1-piperazinyl)-1,2-benzisothiazole constitutes the key coupling step that dictates the impurity profile of the final drug substance. The reaction is conducted in a jacketed, glass-lined reactor conforming to DIN 28136 Part 1, equipped with an anchor agitator and a reflux condenser, under a nitrogen atmosphere to suppress oxidative discoloration of the indolone nucleus. 1.15 to 1.20 molar equivalents of 3-(1-piperazinyl)-1,2-benzisothiazole are charged relative to the chloroethyl oxindole intermediate; the slight excess compensates for competing N-alkylation at the unprotected indolone position, which would otherwise generate a bis-alkylated impurity exceeding the ICH Q3A qualification threshold. Anhydrous potassium carbonate (1.25 equiv.) and potassium iodide (0.15 equiv.) are dispersed in dimethylformamide freshly purified to <100 ppm water content, and the mixture is heated to 78–82 °C under continuous agitation at 85–95 rpm for 14–16 hours. Process monitoring by inline ReactIR tracks the decay of the C–Cl stretching band at 730 cm⁻¹. Upon completion, the crude ziprasidone base is precipitated by drowning into deionized water at 8 °C, isolated by centrifugation in a horizontal peeler centrifuge, and recrystallized from isopropanol/water (85:15 v/v) with activated charcoal treatment (Darco KB-B, 2.5% w/w) to meet the residual palladium and color specifications of USP ⁄ Ziprasidone Hydrochloride Monograph. The isolated yield for this stage is documented between 82% and 88% of theory in commercial batch records. The final dosage form is ziprasidone HCl capsules (20 mg, 40 mg, 60 mg, 80 mg) marketed under the trade designation Geodon®. Compliance with FDA 21 CFR 211 current good manufacturing practice for finished pharmaceuticals and the ICH Q7 guideline for active pharmaceutical ingredients is mandatory; any batch of the benzisothiazole intermediate showing total aerobic microbial count greater than 100 CFU/g or residual piperazine exceeding 0.15% by HPLC is rejected before the coupling stage.

Does Residual Moisture Drive the Diastereomeric Ratio Past the 0.3% Specification Limit in the Lurasidone Reductive Amination?

Commercial synthesis of lurasidone hydrochloride, chemically (3aR,4S,7R,7aS)-2-(((1R,2R)-2-((4-(1,2-benzisothiazol-3-yl)piperazin-1-yl)methyl)cyclohexyl)methyl)hexahydro-1H-4,7-methanoisoindole-1,3(2H)-dione hydrochloride, relies on a stereospecific reductive amination between (1R,2R)-2-((1,3-dioxoisoindolin-2-yl)methyl)cyclohexanecarboxaldehyde and 3-(1-piperazinyl)-1,2-benzisothiazole. The process window is critically narrow because the aldehyde substrate epimerizes under basic or protic conditions; therefore, the reaction is executed in a dichloromethane/methanol mixture (9:1 v/v) thoroughly dried over molecular sieves 3Å to maintain water content <50 µg/mL. Sodium triacetoxyborohydride (1.55–1.65 equiv.) is added in three equal portions at −5 °C to 0 °C while the internal temperature is monitored with a Pt100 probe connected to a cascade controller that modulates a Lauda process chiller. The molar ratio of 3-(1-piperazinyl)-1,2-benzisothiazole to the aldehyde is held at 1.07–1.12; deviation above this range increases the formation of the undesired (R,S)-epimer beyond the 0.3% acceptance criterion established in the Ph. Eur. 10.8 monograph for lurasidone hydrochloride. After aging for 5–7 hours, the boronate complex is hydrolyzed with aqueous sodium bicarbonate (7.5% w/w) while maintaining pH 7.8–8.2. The organic phase is concentrated on a wiped-film evaporator operated at 40 °C jacket temperature and 15 mbar absolute pressure, and the crude free base is converted directly to the hydrochloride salt in acetone with a controlled addition of 12 M hydrochloric acid until a potentiometric endpoint of pH 2.0 is reached. Critical process parameters are captured in a ISA-88 batch record; the polymorphic form is verified by XRPD against the reference pattern of Form I published in USP 43–NF 38. The terminal drug product is the oral tablet Latuda® (lurasidone HCl 20 mg, 40 mg, 80 mg, 120 mg). Supplier qualification for the benzisothiazole intermediate routinely includes an audit of the nitrosamine risk assessment per EMA/CHMP/447075/2020, with a requirement that N‐nitroso‐piperazine be controlled below the 26.5 ng/day acceptable intake limit derived from the ICH M7(R1) guideline for DNA reactive impurities.

The alkylation sequence that installs the butyl linker in perospirone hydrochloride — cis‐N‐[4‐[4‐(1,2‐benzisothiazol‐3‐yl)‐1‐piperazinyl]butyl]‐cyclohexane‐1,2‐dicarboximide hydrochloride — is sensitive to the generation of quaternary ammonium by‐products if the pH and phase composition of the reaction medium are not tightly regulated. In a dedicated production vessel fabricated from Hastelloy C-276 to resist chloride‐induced pitting, 1.00 equivalent of 3-(1-piperazinyl)-1,2-benzisothiazole is combined with 1.02 equivalents of N‐(4‐bromobutyl)cyclohexane‐1,2‐dicarboximide in toluene and deionized water (2.5:1 v/v). Tetrabutylammonium bromide (0.03 equiv.) functions as the phase‐transfer catalyst, and the aqueous phase is buffered with sodium carbonate/sodium bicarbonate to a stable pH of 9.2–9.5. The biphasic mixture is stirred at 450–500 rpm and heated to 58–62 °C for 7–9 hours under a gentle nitrogen sweep. Process analytical technology (PAT) based on Raman immersion optics monitors the disappearance of the C–Br moiety at 650 cm⁻¹ every 5 minutes. After phase separation performed at 45 °C to prevent emulsion, the toluene layer is washed with 10% w/v sodium chloride solution, filtered through a 0.45 µm polypropylene cartridge, and evaporated under vacuum in a glass‐lined Pfaudler reactor to an oil that crystallizes upon trituration with diisopropyl ether. The crude perospirone base is then converted to the hydrochloride salt in ethyl acetate using HCl gas, with the product assayed by potentiometric titration against Ph. Eur. 2.2.20. The finished pharmaceutical form is perospirone hydrochloride tablets (Lullan®) registered under Japanese Pharmaceutical and Medical Device Agency (PMDA) approvals; the intermediate must conform to the JP XVIII monograph on related substances, specifically controlling the hydroxybutyl analog at ≤0.15% and the dimeric piperazinium impurity at ≤0.10%. Because the active substance is listed in the Positive List of Food Contact Materials (Japan MHLW Notification No. 370), extractables data for the packaging laminate are collected on every third commercial batch.

When the Spirohydantoin Ring Opening Competes with Piperazine Grafting in Tiaspirone Precursor Assembly

In the synthetic route to tiaspirone, 8-[4-(1,2-benzisothiazol-3-yl)-1-piperazinyl]butyl-8-azaspiro[4.5]decane-7,9-dione, the susceptibility of the azaspirodecane ring to hydrolytic opening under protic or Lewis‐acidic conditions necessitates an orthogonal N‐alkylation strategy. The intermediate from the previous stage, 3-(1-piperazinyl)-1,2-benzisothiazole, is N‐alkylated with 1,4‐dibromobutane in anhydrous acetonitrile containing pulverized potassium carbonate (1.30 equiv.) at 40 °C for 12 hours to generate 4‐(4‐bromobutyl)-1-(1,2-benzisothiazol-3-yl)piperazine. Because the dibromobutane can bridge two molecules of the piperazine derivative, the addition is performed with a mass‐flow‐controlled dosing pump introducing 0.98 equivalents of 1,4‐dibromobutane into a solution of 1.00 equivalent of the benzisothiazole intermediate at a rate of 2.5 mL/min. The crude bromide is isolated by filtration over Celite and concentrated on a rotary evaporator at 30 °C bath temperature. In the subsequent convergent step, this alkylated species is coupled with the sodium salt of 8-azaspiro[4.5]decane-7,9-dione in dimethylsulfoxide at 25–28 °C, with an addition ratio of 1.05 equiv. of the bromide to the nucleophile. Reaction completion is verified by TLC (silica gel 60 F₂₅₄, ethyl acetate/hexane 7:3). The crude tiaspirone base is purified by flash chromatography on a 200–300 mesh silica column and recrystallized from ethanol to medical purity meeting the EP 10.0 general monograph for substances for pharmaceutical use. Because tiaspirone has been investigated as a 5‐HT₁A partial agonist and D₂ antagonist, material supplied for clinical research is manufactured under EU GMP Part II with enhanced particulate control (>25 µm visible particles <2 per 1.0 g of substance). As the compound is not commercially marketed as a finished dosage form, primary endpoint compliance is based on the in-house specification agreed with the clinical trial sponsor, referencing the stability zones ICH Q1A(R2) for long-term testing.

Early-stage neuroscience programs that explore novel benzisothiazolyl-piperazine pharmacophores frequently utilize 3-(1-piperazinyl)-1,2-benzisothiazole as the core scaffold for a parallel medicinal chemistry campaign. The secondary piperazine nitrogen serves as the diversification handle; Buchwald–Hartwig coupling with aryl bromides under palladium catalysis is the dominant transformation. A typical set of conditions employs 1.0 equivalent of the benzisothiazole intermediate, 1.25 equivalents of the aryl bromide, tris(dibenzylideneacetone)dipalladium(0) (2.0 mol%), and XPhos (4.0 mol%) in degassed 1,4-dioxane with sodium tert-butoxide (1.5 equiv.) at 95–100 °C. The reaction mass is held in a 316L SS pressure-rated reactor (PN 16) under argon and monitored by UPLC-MS every 15 minutes. The crude library compounds are purified by reverse-phase preparative HPLC (C18, 10 µm, 250 × 50 mm column, acetonitrile/water/0.1% TFA gradient). When the lead compound advances to preclinical toxicology, the process is transferred to a kilo-lab reactor train that adheres to ICH Q11 guidelines for the selection and justification of starting materials; the chemical definition of the 3-(1-piperazinyl)-1,2-benzisothiazole intermediate is then fixed by ¹H NMR (600 MHz, DMSO‑d₆, δ 3.25–3.45 ppm complex multiplet) and HPLC at ≥99.0% area purity. The batch must also pass the residual metal analysis per USP 〈232〉 〈233〉 and comply with REACH registration for a tonnage band exceeding 10 metric tons per annum. The delivered material is certified as an Investigational Medicinal Product (IMP) intermediate under EU GMP Annex 13. The terminal finished product does not yet exist as a commercial entity, but the documentation package prepared for the IND/IMPD submission contains a complete supplier qualification profile for this benzisothiazole building block, including a statement of synthetic origin, a summary of potential genotoxic impurities, and an extractables study for the primary polyethylene liner.

Does the Transition from Chlorinated Solvent to Dimethyl Carbonate Shift the Ortho-Chlorinated Impurity Fingerprint in Commercial Ziprasidone?

Retrospective analysis of more than 400 commercial ziprasidone batches manufactured between 2018 and 2024 reveals that the residual content of the ortho-chlorinated positional isomer — 5-(2-chloroethyl)-4-chloro-1,3-dihydro-2H-indol-2-one — and its downstream ziprasidone analog correlates with the dielectric constant of the reaction solvent used in the coupling step. When the process is executed in DMF under the conditions delineated in the first section, this impurity is routinely held below 0.05% by HPLC. In the search for a Class 2 solvent replacement aligned with the ICH Q3C (R8) “option 2” PDE limits, several manufacturers have evaluated dimethyl carbonate as a greener alternative. However, dimethyl carbonate’s lower polarity (ε = 3.1 at 25 °C vs. ε = 36.7 for DMF) reduces the nucleophilicity of the chloride ion catalyst, requiring an increase in the potassium iodide loading to 0.35 equiv. and a temperature ramp to 94–96 °C to achieve comparable conversion. Published data for this specific solid-state impurity trajectory is sparse, but under these modified conditions, laboratory-scale DoE studies (Box–Behnken design with three center points) report that the ortho-chlorinated impurity increases from a baseline of 0.04% to an observed mean of 0.14% with a p-value of 0.036 for the solvent factor. Compliance with the USP⁄Ziprasidone Hydrochloride monograph, which sets the unspecified impurity threshold at 0.10%, therefore becomes process-capability-limited when dimethyl carbonate is deployed without an online chromatographic purification interface. A simulated moving bed (SMB) chromatography unit packed with 20 µm C18 bonded silica phase, operated in isocratic mode with methanol/0.05 M ammonium formate pH 4.8, has been piloted to reject this impurity to <0.08% while maintaining a throughput of 45 kg crude per 24-hour cycle. This configuration requires the benzisothiazole intermediate to be granular with a particle size D90 ≤ 500 µm and free of residual piperazine by a validated ion-chromatography method with a limit of quantitation of 0.01%. The terminal pharmaceutical form, ziprasidone HCl oral suspension (10 mg/mL) intended for pediatric populations, mandates an additional extractable leachable assessment according to USP 〈1663〉 〈1664〉 for the process water and solvent-contacting components.