(-)-N-Ethyl-2-(3-Bromo-2,6-Dimethoxybenzamidomethyl)Pyrrolidine

In automated synthesis modules operated under cGMP 21 CFR 212.71 for positron emission tomography (PET) radiopharmaceuticals, the title bromobenzamide intermediate functions as a precursor for aromatic nucleophilic radiofluorination. Azeotropically dried no-carrier-added [¹⁸F]fluoride, eluted with 0.75 mL of a 10:1 (v/v) acetonitrile–water mixture containing K₂CO₃ (1.5 mg) and Kryptofix 2.2.2 (5.0 mg), is combined with the intermediate (2.0 ± 0.1 mg, 4.6 µmol) in anhydrous DMSO (0.3 mL). The substitution of the 3-bromo substituent proceeds in a capped vial at 140 °C for 8 minutes under controlled microwave irradiation (Biotage Initiator+, 100 W maximum power), yielding the 3-[¹⁸F]fluoro-2,6-dimethoxy analogue. During process qualification per USP <823>, radiochemical purity is assessed by radio-TLC (silica gel 60 F₂₅₄, ethyl acetate–hexane 70:30 v/v) and radio-HPLC (Phenomenex Luna C18(2), 5 µm, 250 × 4.6 mm, 45:55 acetonitrile–0.1% trifluoroacetic acid aqueous, 1.0 mL/min, retention time of the labelled product at 11.3 minutes). Residual precursor removal is executed via semi-preparative HPLC on a reversed-phase column with UV detection at 254 nm, and the collected fraction is formulated in 10% ethanolic saline after solid-phase extraction and sterile filtration through a 0.22 µm membrane. The final injectable solution (≤ 15 mL, pH 6.5–7.5) exhibits a specific activity not less than 37 GBq/µmol at end of synthesis, serving as a diagnostic radiopharmaceutical for D2/D3 receptor imaging in extrapyramidal disorders. Process validation batches confirm that residual palladium, when introduced via trace Pd-catalysed side-pathways during preceding amide coupling, remains below the 10 ppm threshold mandated by ICH Q3D Guideline for Elemental Impurities, as determined by ICP-MS after microwave-assisted acid digestion.

What Restricts Scale-up Enantiomeric Excess When (S)-Aminomethylpyrrolidine Is Coupled to the Benzoyl Chloride?

The condensation between (S)-(–)-N-ethyl-2-aminomethylpyrrolidine and 3-bromo-2,6-dimethoxybenzoyl chloride, carried out in the presence of a hindered tertiary amine base such as N,N-diisopropylethylamine (1.05 eq) in dichloromethane at −5 °C to 0 °C, delivers the target amide with an enantiomeric excess as high as 99.5% only when strict moisture control (< 0.005% water by Karl Fischer titration) and slow addition of the acid chloride (0.55 mol/h) are maintained. In production-scale agitated reactors (jacketed glass-lined, 100 L nominal capacity, retreat-blade impeller at 95 rpm), deviations beyond +2 °C from the setpoint lead to partial racemisation via oxazolone formation, reducing the ee to 96.2–97.8%, as quantified by chiral HPLC (Chiralpak IA column, 5 µm, 250 × 4.6 mm, hexane–isopropanol–diethylamine 85:15:0.1 v/v/v, 0.8 mL/min, UV 280 nm) in accordance with Ph. Eur. 2.2.29. The reaction mass is subsequently washed with 1 N HCl (2 × 20 L) and 5% NaHCO₃ solution (2 × 20 L), dried over Na₂SO₄, and concentrated under reduced pressure (≤ 45 °C bath). The crude residue is crystallised from isopropanol–diisopropyl ether (1:4 v/v) to afford a white crystalline solid with a melting point of 108–110 °C (DSC onset, 5 °C/min ramp). Specification limits for the isolated intermediate include chiral purity not less than 99.0%, HPLC purity not less than 99.5 area%, and residual diisopropylethylamine below 20 ppm. For integration into active pharmaceutical ingredient (API) synthesis, this intermediate serves as a gateway to enantiopure benzamide-based dopamine D2 receptor antagonists; the 3-bromo atom provides a synthetic handle for downstream diversification via palladium-catalysed reactions without disturbing the stereogenic centre, a design feature that permits late-stage functionalisation in multi-kilogram campaigns under ICH Q7 active pharmaceutical ingredient guidelines.

When parallel structure–activity relationship (SAR) batches demand rapid analogue generation, the intermediate is loaded into a glovebox-compatible 24-position carousel reactor (Radleys GreenHouse, 40 mL tubes) with 0.18–0.25 mmol of substrate per tube, along with the desired boronic acid or pinacol ester (1.2 equivalent), bis(di-tert-butylphosphino)ferrocene palladium dichloride (2 mol% relative to boronate, CAS 95408-45-0), and potassium phosphate tribasic (3.0 equivalents) in degassed dioxane–water (4:1 v/v, 1.0 mL). The sealed vessels are heated to 85 °C with orbital shaking at 650 rpm, and conversion surpasses 95% (HPLC, 220 nm) within 4 hours for aryl and heteroaryl boronates, as referenced against the general Suzuki coupling monograph of Org. Process Res. Dev. 2018, 22, 758–768. After cooling, the biphasic mixture is filtered through a short pad of Celite, diluted with ethyl acetate, and the organic phase is passed through a Biotage Isolera One unit equipped with a SNAP Ultra C18 cartridge (acetonitrile–water gradient, 0.1% formic acid) to isolate the coupled tertiary amide in typical yields of 68–84% with retained enantiopurity (> 99% ee). The resulting compounds constitute a library of investigational D2/D3 receptor ligands intended for in vitro binding assay panels (CHO-K1 cells expressing human D2_Long receptors, displacement of [³H]raclopride, per ICH S7B safety pharmacology profiles). Here the compliance focus shifts to Research Use Only (RUO) specifications: the supplier-provided certificate of analysis for the bromo intermediate must detail heavy metal content (< 20 ppm Pb, Cd, Hg, As by USP <232>) and residual solvents (dichloromethane < 600 ppm, isopropanol < 5000 ppm) per ICH Q3C, because even non-GMP libraries can introduce toxicological noise if metal contamination exceeds threshold limits.

Genotoxic Impurity Fate Mapping When the Bromo Intermediate Becomes the API Starting Material

In regulatory filings under ICH M7(R1), the intermediate is assessed for its potential to carry through structurally alerting impurities into the final drug substance. The 3-bromo-2,6-dimethoxybenzamide substructure, while not inherently mutagenic in standard Ames OECD 471 tests, can generate trace levels of 3-bromo-2,6-dimethoxyaniline upon prolonged storage under acidic conditions (pH < 3.0, 40 °C, 75% RH), a degradant that exhibits a positive alert in (Q)SAR analysis (DEREK Nexus 6.1.0). Consequently, the acceptance criterion for this specified impurity is set at ≤ 15 ppm relative to the API, corresponding to a theoretical cancer risk below 1 in 10⁵ under the staged TTC of 1.5 µg/day for a 150 mg/day maximum daily dose. Quantification utilises a validated UPLC-MS/MS method (Acquity BEH C18, 1.7 µm, 50 × 2.1 mm; mobile phase: 10 mM ammonium formate pH 3.5–acetonitrile gradient; retention time 4.8 min; MRM transition m/z 231.9 → 121.0; LOD 0.5 ppm). Downstream process teams exploit this stability data to design a synthetic route wherein the bromo intermediate is subjected to a Buchwald–Hartwig amination or a copper(I)-catalysed azide–alkyne cycloaddition within 48 hours of release, before amine hydrolysis can accumulate. For a commercial-scale batch intended for a D2 antagonist API manufactured in an EU GMP Part II-compliant facility, the typical charging ratio is 1.00 kg of intermediate generating 1.55–1.62 kg of the subsequent penultimate after workup and column chromatography (silica gel 60–120 mesh, gradient elution 0–5% methanol in dichloromethane with 0.1% triethylamine), reflecting a molar yield of 88–92%. The final API, formulated as an oral tablet with a 50 mg label claim, is blister-packed under nitrogen to limit bromine-related photodegradation identified during ICH Q1B photostability testing.

Suzuki Coupling vs. Copper-Mediated Fluorination: Thermochemical Boundary Conditions at the Bromine Site

Divergent process sequences exploit the 3-bromo handle through catalyst-controlled chemoselectivity. In one workflow, a Pd-XPhos-G2 precatalyst (1.5 mol%) and potassium tert-butoxide (1.8 eq) in 2-methyltetrahydrofuran at 50 °C promote C–O bond formation with alkanols, while in a parallel copper(I)-mediated system (CuI, 10 mol%, N,N′-dimethylethylenediamine, 20 mol%, KF 3.0 eq, DMF, 120 °C, 24 h) the bromide is directly displaced by fluoride to produce the cold 3-fluoro reference standard required for specific activity calculations of [¹⁸F] PET tracers. This latter transformation exhibits a pronounced kinetic cliff: at temperatures below 115 °C conversion stalls at < 15% after 24 h, whereas at 125 °C competing amide hydrolysis generates 8–12% of the free carboxylic acid by-product, as identified by LC-MS. The narrow processing window necessitates jacketed reactor control with thermocouple redundancy and cascade failure interlocks, characteristics documented in the production of GMP-grade reference standards for clinical imaging sites. For the ultimate radiopharmaceutical batch record, the [¹⁹F]cold standard is added to the formulation at 0.5–1.0 µg/mL to minimise non-specific binding to container surfaces during dose transport, a procedure aligned with the guidelines of the European Pharmacopoeia monograph 01/2023:2485 for fluorodeoxyglucose analogues and applied here by amendment to investigational D2-ligand injections.

In hollow-fibre continuous flow reactors (Corning Advanced-Flow G1, 2.0 mL internal volume, PFA construction), the bromo intermediate dissolved in propylene carbonate (0.12 M) is combined with tetrakis(triphenylphosphine)palladium(0) (0.5 mol%) and an arylzinc bromide generated in situ (1.15 eq) at a combined flow rate of 2.5 mL/min, achieving a residence time of 48 seconds at 92 °C with back-pressure regulation at 5 bar. Negishi coupling under these conditions delivers conversion of 97.6% (in-line FTIR monitoring at 1640 cm⁻¹) with an enantiomeric excess retention of 99.7%, outperforming batch protocols where prolonged thermal exposure at 70 °C for 4 hours results in ee erosion of 0.6–1.2%. This process is particularly relevant for manufacturing kilogram quantities of late-stage intermediates destined for oral solid dosage forms of amisulpride-structure D2/D3 antagonists, where regulatory starting materials must be designated with justified specifications after a minimum of three consecutive validation campaigns under EMA/CHMP/CVMP/QWP/59840/2017 reflection paper conditions. The terminal API powder is sieved through 250 µm mesh, blended with lactose monohydrate (60% w/w), microcrystalline cellulose (20% w/w), crospovidone (5% w/w), and magnesium stearate (0.5% w/w), compressed into oval tablets (12 × 5.5 mm, 150 mg tablet mass) at a target hardness of 70–85 N (Pharma Test PTB 311E), with disintegration time < 4 min in 0.1 N HCl (37 °C) measured per Ph. Eur. 2.9.1. At each of these downstream formulation stages, the bromo intermediate’s fingerprint is traced via the amide carbonyl stretch (1625 cm⁻¹) in process analytical technology (PAT) models to verify that no unreacted remnant migrates into the finished drug product.