(3S,4R)-4-(2,3-Dichlorophenyl)Pyrrolidine-3-Carboxylic Acid

In the production of a clinical-stage neurokinin-1 (NK1) receptor antagonist intended for chemotherapy-induced nausea and vomiting (CINV) prophylaxis, the chiral intermediate (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid is introduced as the stereochemical anchor in the final acylation step. The downstream process involves manual charging of the intermediate into a 500 L glass-lined reactor at 20 ± 2 °C under nitrogen, followed by dissolution in anhydrous tetrahydrofuran (THF, water content ≤ 0.01% as determined by Karl Fischer titration per USP <921> Method Ic). Activation of the carboxyl group is achieved using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) at a molar ratio of 1.05 equivalents relative to the API penultimate amine fragment, combined with 1-hydroxybenzotriazole hydrate (HOBt·H₂O) at 1.10 equivalents. Because the pKₐ of the pyrrolidine nitrogen is 8.2, proton scavenging with N-methylmorpholine (NMM, 2.5 equivalents) is critical to avoid premature salt formation that reduces the effective nucleophilicity of the coupling partner. The reaction mass is held at -5 to 0 °C during dropwise addition of the amine component over 90 minutes, a processing window that must be maintained to suppress base-catalyzed epimerization at the C3 center; excursions above +2 °C have been observed on a 300 mm Hastelloy C-22 batch reactor to cause a 0.4–0.6% increase in the (3R,4S) enantiomer as measured by chiral HPLC (Chiralpak IA-3, 4.6 × 150 mm, mobile phase hexane/ethanol/TFA 80:20:0.1 v/v/v, flow rate 0.8 mL/min, UV detection at 215 nm). The crude product is extracted into ethyl acetate, washed with 5% w/w aqueous citric acid and 7% w/w sodium bicarbonate, and concentrated under reduced pressure at a jacket temperature not exceeding 35 °C. Final purification by flash chromatography on silica gel 60 (particle size 40–63 μm) using a step gradient of ethyl acetate/heptane yields the coupled material with a typical recovery of 87–92% and an enantiomeric excess validated at ≥ 99.5% by the same chiral method. The coupling product is then telescoped directly into a Boc-deprotection sequence using 4 M HCl in dioxane to furnish the stage-1 API, which is tested against ICH Q3A guidelines for residual solvents (headspace GC per USP <467>) and elemental impurities (ICP-MS per USP <233>) before advancing to salt formation and micronization. Equipment train validation includes quantification of carryover of the free acid into the next batch; carryover ≤ 0.15% of the total batch weight is considered acceptable based on a toxicological qualification study conforming to EMA/CHMP/CVMP/QWP/79816/2011.

What Impact Does the 2,3-Dichlorophenyl Substituent Have on Dopamine D3 Receptor Selectivity Over D2 in Agonist-Lead Optimization?

Medicinal chemistry programs targeting dopamine D3 receptor partial agonists for opioid and stimulant use disorders frequently employ (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid as the core scaffolding element, capitalizing on the orthogonal substitution pattern that creates a steric clash in the D2 orthosteric pocket while preserving key hydrogen-bonding interactions at D3. In a representative synthesis, the acid is converted to the methyl ester using thionyl chloride in methanol at 0 °C → reflux, a step that must simultaneously control for formation of the hydrochloride salt of the pyrrolidine nitrogen. The methyl ester is then reduced with lithium aluminium hydride (LiAlH₄, 2.2 equivalents) in THF at -10 °C over 2 h, producing the corresponding primary alcohol, which serves as the handle for subsequent mesylation and nucleophilic displacement with a substituted piperazine. Throughout this sequence, the dichlorophenyl ring orientation forces the piperazine side chain into a pseudo-equatorial disposition, a conformational feature confirmed by NOESY experiments that correlates with a 16-fold D3/D2 selectivity ratio in radioligand binding assays performed according to a standardized protocol using CHO cell membranes expressing human D3s and D2L receptors (assay buffer 50 mM Tris-HCl, pH 7.4, 10 mM MgCl₂, 0.1% BSA). Formulation of the lead compound for in vivo efficacy studies in a rat reinstatement model requires preparation as a 0.5% w/v hydroxypropyl-β-cyclodextrin solution with the drug substance milled to a particle size D₉₀ ≤ 5 μm (Laser diffraction, Mastersizer 3000) to ensure adequate oral bioavailability. Regulatory starting material designation for the chiral acid under ICH Q11 is recommended, and its certificate of analysis must document residual palladium below 10 ppm (JP 2.68 Method 1) if any preceding steps involved Pd-catalyzed cross-coupling to introduce the aryl group.

Serotonin 5-HT2C Agonist Intermediates Targeting Weight Management: Nonlinear Manufacturing Cost Drivers

Academic and biotech consortia investigating pyrrolidine-based 5-HT2C receptor agonists for obesity and binge-eating disorder utilize the carboxylic acid moiety of (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid to construct spirocyclic or fused heterocyclic replacements for the labile N-benzyl motif commonly encountered in early leads. The initial activation as the mixed anhydride with isobutyl chloroformate (IBCF, 1.0 equiv) in dichloromethane at -20 °C, followed by trapping with a 3-amino-1,2,4‑oxadiazole entity, proceeds with a yield sensitivity of ±12% for every 0.1 equivalent deviation in IBCF stoichiometry, a response surface identified during a three-factor Box–Behnken design of experiments. The FDA Animal Efficacy Rule considerations for obesity agents demand that the Active Pharmaceutical Ingredient (API) derived from this intermediate comply with 21 CFR 58 Good Laboratory Practice for non-clinical laboratory studies during the chronic dog toxicology phase; hence the chiral acid used for GLP tox batches is released with an additional test for mutagenic azide impurities (HPLC–CAD detection, LOQ 0.03 ppm) if a diphenylphosphoryl azide (DPPA) coupling was employed earlier in the route. Crystallization of the final API free base is performed from 2‑propanol/water 3:1 v/v, with seeding from a polymorphic form designated “Form A” having a characteristic DSC endotherm at 168.2 °C (onset). A typical batch size of 12–15 kg for Phase IIa clinical supplies uses a commercial-scale filter dryer with a 0.5 m² sintered plate, and the drying curve is ended at a residual moisture of ≤ 0.2% w/w (Mettler Toledo HX204 halogen moisture analyzer, 105 °C, switch-off criterion 1 mg/50 s). Process analytical technology (PAT) at this stage includes in-line Raman monitoring for the appearance of the hemi‑hydrate, which must remain below 1.5% of total solids because its conversion to Form A upon heating can generate internal stress fractures in the compressed tablet cores.

When sigma-1 receptor antagonist programs for chemotherapy‑induced peripheral neuropathy evaluate the scaffold, the pendent carboxylic acid of the (3S,4R) chiral pyrrolidine is frequently elaborated into a primary carboxamide via ammonolysis of the corresponding methyl ester under microwave irradiation (100 W, 80 °C, 30 min, solvent: 7 N ammonia in methanol). This transformation proceeds with retention of configuration as confirmed by VCD spectroscopy, and the crude amide is directly subjected to borane‑tetrahydrofuran complex reduction to the aminomethyl derivative, which is then functionalized with a guanidyl group for receptor anchoring. The guanidinylation protocol using N,N'-bis-Boc-1-guanylpyrazole requires that the free amine be maintained at strictly less than 0.3% water content (coulometric KF) to prevent premature pyrazole hydrolysis, a control parameter validated through stability studies conducted under ICH Q1A(R2) accelerated conditions at 40 °C/75% RH. Registration batches of the API manufactured in an EU-based facility must comply with the EU GMP Part II (ICH Q7) and REACH Regulation (EC) No 1907/2006 for the imported chiral intermediate; a representative specification requires assay by HPLC (area %) of the acid between 98.0% and 102.0% on anhydrous basis, with the (3R,4S) enantiomer limited to ≤ 0.5% and total related substances ≤ 1.0%. Equipment qualification for the hydrogenation step occasionally used to remove a benzylic protecting group relies on a HEL AutoMATE parallel reactor system to establish a design space where hydrogen uptake at 3.0–3.5 bar(g) and 25 ± 3 °C avoids over-reduction of the dichlorophenyl ring, an event that would generate a 4‑(3‑chlorophenyl) impurity that is difficult to purge and exhibits 15% of the target pharmacological activity.

During the development of a dual NK1/μ-opioid hybrid agonist intended for pain management, the coupling of (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid with a tertiary amine-bearing opioid pharmacophore is effected using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate (HATU) and N,N-diisopropylethylamine (DIPEA, 3.0 equivalents) in N,N-dimethylformamide at 0–5 °C for 45 min. At scale, the reaction exotherm can raise the internal temperature by 2–8 °C depending on agitator tip speed (optimum 1.8 m/s for an Ekato Paravisc impeller in a 400 L vessel), and poorly controlled heat dissipation leads to racemization at the C3 carbon via an oxazolone intermediate, with an apparent activation energy of 63 kJ/mol. The subsequent separation employs a biphasic quench into 10% w/w sodium chloride solution and ethyl acetate, followed by polishing filtration through a Zeta Plus 10SP depth filter to remove residual palladium originating from the earlier halogen-exchange step. The final API isolate must not exceed 20 mg/m² surface area palladium contamination by X-ray photoelectron spectroscopy. Tablet cores incorporating the compound are manufactured on a Korsch XL 100 rotary press with 14 station tooling, where the compression force is limited to 8–12 kN to preserve the integrity of a thermosensitive salt form whose glass transition temperature is 47 °C; out-of-specification hardness (target 60–80 N) has been traced to electrostatic charging of the micronized drug substance when processed at relative humidity below 35%, which is mitigated by pre-conditioning the drug substance in a climate chamber at 50% RH for 12 h.

Method Transfer Artifacts When Scaling Chiral Resolution from CO₂ SFC to Preparative HPLC

For a development program relying on late‑stage enantiomeric enrichment of the final intermediate derived from (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid, a switch from supercritical fluid chromatography (SFC, Chiralpak AD-H 20 μm, 30 x 250 mm column, co‑solvent methanol with 0.2% isopropylamine, 50 g/min total flow, back‑pressure 120 bar, column temperature 35 °C) to a traditional preparative HPLC system (Chiralpak IA 20 μm, 50 x 500 mm, mobile phase n-hexane/ethanol/diethylamine 75:25:0.1, flow rate 80 mL/min) introduced a previously unobserved self‑association of the solute in the mobile phase at concentrations above 35 mg/mL. This association manifested as a shoulder on the trailing edge of the desired peak that, upon fraction collection, led to 1.2–1.8% contamination with the opposite enantiomer, exceeding the acceptance criterion of ≤ 0.2%. The phenomenon was eliminated by incorporating 5% v/v tetrahydrofuran into the diluent and applying a sample loop filling of ≤ 80% of the calibrated volume, with the column thermostatted at 25 °C to reduce viscosity effects. System suitability test criteria transferred between sites per a protocol aligned with ICH Q2(R1) required that the resolution factor (Rs) between the (3S,4R) and (3R,4S) peaks remain above 2.5 across three consecutive injections; deviations triggered an investigation under a corrective and preventive action plan that included sonic cleaning of the column with 100% ethanol at 50 °C for 2 h. Materials certified under ISO 9001:2015 for R&D‑phase delivery are shipped in amber glass containers with aluminum seals with an inner liner of PTFE/FEP, with a retest interval of 12 months when stored at -20 °C in an argon atmosphere.

When the scaffold is incorporated into a C–H functionalization manifold for a late‑stage diversification library, the free carboxylic acid of (3S,4R)-4-(2,3-dichlorophenyl)pyrrolidine-3-carboxylic acid is protected in situ as a trimethylsilylethyl ester (TMSE, 1.2 equiv of trimethylsilylethanol, DCC/DMAP in CH₂Cl₂, 0 °C → rt, 16 h) to enable a palladium‑catalyzed directed C(sp³)–H arylation of the pyrrolidine ring at the 2‑position. A ligand screening performed on a Chemspeed SWING platform identified a combination of Pd(OAc)₂ (10 mol%), 1,1'-bis(diphenylphosphino)ferrocene (dppf, 12 mol%), and cesium pivalate (1.5 equiv) in toluene at 110 °C as the sole condition that suppressed a competing decarboxylation side reaction to below 3% conversion. The crude reaction stream is quenched with aqueous EDTA disodium salt (10% w/w) to chelate palladium, and the TMSE ester is cleaved with TBAF on silica gel to regenerate the acid, yielding a 2‑aryl substituted analogue in an overall isolated yield of 47–53% over two steps. The resulting compounds are tested as PET tracer precursors in accordance with 21 CFR 361.1, requiring that residual tin or palladium in the final injectable formulation be verified by a validated ICP‑MS method with a detection limit of 0.1 ppb. The use of the single enantiomer starting material ensures that the absolute configuration at C3 and C4 is retained, a point confirmed by single‑crystal X‑ray diffraction of a heavy‑atom derivative (Mo Kα radiation, λ = 0.71073 Å, Flack parameter 0.02(3)).