(2S,4R)-4-Fluoropyrrolidine-2-Carboxylic Acid Hydrochloride

Why does residual 4‑fluoropyrrolidine impurity above 0.15% trigger batch rejection in grazoprevir synthesis?

The NS3/4A serine protease inhibitor grazoprevir (MK‑5172) incorporates the (2S,4R)‑4‑fluoropyrrolidine‑2‑carboxamide fragment at the P2 moiety of its 15‑membered macrocyclic core. During the convergent assembly performed on a multi‑100‑kg scale, (2S,4R)‑4‑fluoropyrrolidine‑2‑carboxylic acid hydrochloride is first converted to its N‑Boc‑protected derivative using di‑tert‑butyl dicarbonate (1.20 eq.) in 2‑methyltetrahydrofuran at 20–25 °C under pH‑stat control maintaining pH 8.5–9.0 with 2.0 M Na₂CO₃. The coupling to the macrocyclic amino ester core employs the mixed anhydride method with pivaloyl chloride and 3.0 eq. of N‑methylmorpholine in dichloromethane at ‑20 °C; the stoichiometry is set at 1.00 ± 0.02 eq. relative to the amino component because even an 0.25% molar excess of the fluorinated fragment introduces a persistent 4‑fluoropyrrolidine‑amide dimer impurity (m/z 365.17) that co‑crystallizes with grazoprevir in the final acetone/water recrystallization. The acceptance criterion of ≤ 0.15% for residual free 4‑fluoropyrrolidine‑2‑carboxylic acid, measured by ion‑exchange chromatography with suppressed conductivity detection (Dionex ICS‑6000, IonPac AS20, 35 mM KOH gradient), is driven by a critical process anomaly observed during pilot‑scale campaigns at a contract manufacturing site in Lonza, Visp: when free acid levels exceeded 0.18%, the subsequent Boc‑deprotection step with methanesulfonic acid in isopropyl acetate generated covalent piperidine‑fluoro adducts that increased the final genotoxic impurity profile above the TTC‑derived limit of 1.5 µg/day. Downstream processing is executed in a GMP‑compliant Supraton S‑PEEK inline high‑shear mixer to ensure rapid mass transfer during the carbonate‑base neutralization, and the final product is milled through a jet‑mill to achieve an aerodynamic particle size D₅₀ of 2.5–4.0 µm — appropriate for the spray‑dried amorphous solid dispersion loaded into Zepatier® (100 mg grazoprevir / 50 mg elbasvir) tablets. Relevant compendial references include ICH M7(R1) Addendum for mutagenic impurity risk assessment, Ph. Eur. 2.2.46 for chromatographic separation of diastereomers, and ASTM E2817‑21 for the determination of specific surface area of the micronized drug substance.

Facilities producing 4‑[¹⁸F]fluoro‑L‑proline for oncologic PET imaging — typically as the trans‑4‑[¹⁸F]fluoroproline diastereomer derived from the same (2S,4R)‑configured tosylate precursor — operate under the simultaneous jurisdiction of GMP for PET pharmaceuticals (EudraLex Volume 4 Part II, Annex 3) and the monographs of the European Pharmacopoeia chapter 0126 on radiopharmaceutical preparations. The precursor, derived from (2S,4R)‑4‑hydroxy‑L‑proline through a fluorination‑salt exchange sequence, is formulated as a sterile, dry residue in septum‑sealed GE F‑300 reaction vials; the ratio of precursor to Kryptofix 2.2.2 / K₂CO₃ complex is critical and must be maintained at 1:0.85 (w/w) to suppress competing [¹⁸F]‑fluoride protonation while avoiding excessive kryptofix carryover (> 50 µg/mL causes positive Ames test in final product release). Nucleophilic ¹⁸F‑fluorination is carried out on an Eckert & Ziegler Modular‑Lab system by heating the precursor (5.0 ± 0.2 mg) in anhydrous CH₃CN at 85 °C for 12 minutes, followed by hydrolysis with 1.0 M HCl at 110 °C for 8 minutes to liberate the free acid and cleave the protecting groups. The crude radiotracer is purified on a Phenomenex Synergi Hydro‑RP C₁₈ column (250 × 10 mm, 4 µm, ethanol:water 5:95 containing 10 mM NH₄H₂PO₄) and sterilized by 0.22 µm membrane filtration into a final vial suitable for multi‑dose dispensing. The radiochemical purity specification is ≥ 98.0% by radio‑HPLC and the enantiomeric ratio is verified to exceed 97% trans‑4‑[¹⁸F]fluoroproline by chiral radio‑TLC on silica gel impregnated with copper‑proline complex. Terminal products are prepared as injectable solutions with an activity concentration of 400–600 MBq/mL and administered at doses of 300–400 MBq for PET‑CT scanning of L‑type amino acid transporter overexpression in gliomas and prostate carcinomas. The entire batch record is aligned with USP <823> and the sterile preparation must satisfy film‑on‑glass endotoxin testing per Ph. Eur. 2.6.14 (endotoxin limit ≤ 2.0 EU/mL before decay correction).

When a sarcosine‑to‑fluoroproline substitution is engineered at the P2′ position of a peptide macrocycle

Stapled and backbone‑cyclized therapeutic peptides — particularly those targeting intracellular protein‑protein interactions — exploit the conformational pre‑organization imparted by (2S,4R)‑4‑fluoroproline. The fluorine atom enforces a C^γ‑exo pucker with a measured pseudo‑rotational phase angle P of 18 ± 4° and a ϕ torsion of ‑55 ± 3° in an Ala‑FP‑Ala tripeptide crystal lattice (CCDC deposition 1845722), producing a local polyproline II‑type twist that persists through 12‑residue helices. In automated solid‑phase synthesis on a Protein Technologies Prelude X instrument, the hydrochloride salt is coupled as its Fmoc‑(2S,4R)‑4‑fluoroproline‑OH derivative using 3.0 eq. of HATU/6.0 eq. of DIEA in NMP, with a double‑coupling protocol of 2 × 30 minutes to overcome the reduced acylation rate (about 0.4× that of proline) caused by the electron‑withdrawing fluoro substituent lowering the amine nucleophilicity. The incorporation ratio is precisely 1:1 stoichiometric substitution at the targeted position; at positions where the peptide sequence carries an aspartimide‑prone Asp‑Gly junction, the fluoroproline must not be placed within two residues of the aspartate residue because the fluoride‑catalyzed succinimide ring opening leads to 3‑(4‑fluoropiperidine)‑2,6‑diketopiperazine adducts with a mass shift of +18 Da, a phenomenon documented during the scale‑up of a 32‑residue MDM2‑binding peptide at a kilo‑lab facility (Thermo Fisher Alfa Aesar bench‑scale data). Cleavage from the resin uses a Reagent K cocktail (TFA/thioanisole/water/phenol/EDT 82.5:5:5:5:2.5 v/v) at 25 °C for 3 hours, and crude purity routinely exceeds 78% by RP‑HPLC before preparative purification on a Waters Prep 150 LC system with a XBridge BEH130 C₁₈ column. The terminal products are lyophilized peptide acetates intended for subcutaneous injection; when the sequence contains a fluoroproline‑enforced turn, the peptide exhibits a thermal stability improvement with a T_m increase of 8–12 °C relative to the unfluorinated proline analogue as measured by variable‑temperature CD spectroscopy in phosphate buffer per ASTM E3066‑16a. Quality conformance encompasses ICH Q6B test procedures for peptide biotechnology products, specific optical rotation (Ph. Eur. 2.2.28), and mass accuracy within 3 ppm on a Bruker maXis II ESI‑QTOF mass spectrometer.

Organocatalytic aldol condensations employing (2S,4R)‑4‑fluoroproline as a catalyst exploit the fluorine‑induced increase in carboxylic‑acid acidity (pK_a 2.9 ± 0.1 vs. 4.1 for L‑proline in dimethyl sulfoxide‑water mixtures) to enhance the rate of enamine formation in aqueous‑organic biphasic media. In a jacketed 20 L QVF glass reactor operating at ‑5 °C, 5 mol% of (2S,4R)‑4‑fluoropyrrolidine‑2‑carboxylic acid hydrochloride — pre‑neutralized in situ with 1.00 eq. of LiOH·H₂O to generate the active zwitterionic form — is combined with 1.20 eq. of 4‑nitrobenzaldehyde and 3.0 eq. of acetone in a 3:1 (v/v) mixture of DMSO and brine. The reaction achieves ≥ 92% conversion within 48 hours and returns the isolated β‑hydroxyketone product with an anti:syn ratio of 96:4 and 94% ee as determined by chiral‑phase GC on a Restek Rt‑βDEXse column (30 m × 0.32 mm, 0.25 µm film thickness). Addition levels outside the 4–6 mol% window generate complexity: below 4 mol% the turnover frequency drops below economically viable 0.45 h⁻¹, while above 6 mol% the catalyst self‑condenses through intermolecular amide bond formation, forming a diketopiperazine‑type dimer that precipitates and blocks the bottom drain valve. This specific deactivation pathway necessitates the installation of a 100 µm inline PTFE mesh filter upstream of the product‑outlet diaphragm pump. The downstream workup includes adjustment to pH 3.0 with 3 M HCl, extraction into MTBE, and fractional distillation under reduced pressure (bath temperature ≤ 70 °C) to recover the chiral product without retro‑aldol degradation. The entire process is subject to environmental release constraints under REACH Annex XVII entry 72 for fluorinated by‑product waste incineration with HF scrubbing efficiency > 99.9%, and occupational exposure limits are enforced for airborne 4‑fluoropyrrolidine hydrochloride at a TWA of 0.05 mg/m³ (internal OEL derived from ICH Q3C‑based PDE calculation). The target molecule isolated via this route serves as a key intermediate in the preparation of fluoro‑oxazolidinone chirons for downstream non‑β‑lactam β‑lactamase inhibitors currently under Phase II evaluation.

DPP‑4 inhibitor omarigliptin exploits the trans‑4‑fluoropyrrolidine‑2‑carboxamide pharmacophore to achieve a trough‑to‑peak ratio exceeding 12

Omarigliptin (MK‑3102), a once‑weekly dipeptidyl peptidase‑4 inhibitor commercialized as Marizev®, requires the (2S,4R)‑4‑fluoropyrrolidine‑2‑carboxamide fragment to be coupled to the pyrazolo‑pyrimidine acetic acid core through a tandem acylation‑dehydration sequence in which the hydrochloride salt is converted to the free amine in 2‑MeTHF using 2.5 eq. of aqueous K₃PO₄ prior to reaction with 1.03 eq. of the mixed anhydride formed from the acetic acid core and trimethylacetyl chloride. This coupling is performed in a Corning Advanced‑Flow G1 silicon‑carbide microreactor with a residence‑time distribution volume of 8.2 mL at ‑15 °C and a total flow rate of 12 mL/min to suppress the formation of the thermodynamically favored but pharmacologically inactive cis‑amide rotamer; the microreactor plates are configured with a heat‑transfer oil circuit controlled to ±0.5 °C and the process generates the desired trans‑amide with > 98.5% diastereomeric purity by ¹⁹F NMR (δ −181.2 ppm vs. CFCl₃, Bruker 400 MHz, CD₃OD). The formulation requirement imposes an additional constraint: the free‑base amine must be generated in a strictly chloride‑free environment because residual chloride as low as 120 ppm in the final drug substance promotes salt‑bridge re‑formation with the carboxylic acid moiety of the acetic acid core, reducing the dissolution rate of the spray‑dried amorphous dispersion by approximately 40% in FaSSIF biorelevant media at pH 6.5 as demonstrated in USP <711> apparatus 2 dissolution profiling (paddle, 75 rpm, 900 mL). The terminal dosage form is a 25 mg and 12.5 mg round film‑coated tablet meeting USP <905> content uniformity and Ph. Eur. 2.9.40 assay specifications, with a shelf‑life stability indicating method that monitors for the toxic degradation product hydrazine generated from pyrazole ring fragmentation, controlled at ≤ 0.033 ppm (LOQ, LC‑MS/MS). Approval‑critical documentation references FDA guidance “ANDAs: Stability Testing of Drug Substances and Products Q&A” (Revision 1, 2024) and the Japanese PMDA PFSB/ELD Notification No. 0426‑1 on mutagenic impurities.

The deployment of (2S,4R)-4-fluoropyrrolidine-2-carboxylic acid hydrochloride in the synthesis of elbasvir — the second NS5A inhibitor paired with grazoprevir — diverges from the velpatasvir route in one critical parameter: the heterodimeric symmetrical architecture of elbasvir demands that the fluoroprolinamide arm be installed twice, once at each terminus of the central biphenyl‑imidazole scaffold. During the double amidation executed in a Büchi 160‑L glass‑lined vessel, the hydrochloride is pre‑dissolved in a 4:1 (v/v) mixture of dichloromethane and hexafluoroisopropanol and added as a solution via a peristaltic pump over 45 ± 5 min to a pre‑cooled (‑30 °C) suspension of the bis‑activated pentafluorophenyl ester. The addition ratio is strictly 2.05–2.10 eq. total, with an interim in‑process TLC check (silica gel 60 F₂₅₄, ethyl acetate/hexane 7:3) after the first 1.0 eq. is consumed to verify that the mono‑amidated intermediate does not persist longer than 15 minutes; prolonged holding produces an N‑carbonyl‑bridged macrocyclic impurity that reduces elbasvir yield by 6–8%. The terminal dosage form is a fixed‑dose combination tablet (Zepatier®) containing 50 mg elbasvir and 100 mg grazoprevir, manufactured under a process analytical technology (PAT) framework with online Raman monitoring of the final spray‑dried dispersion per ASTM E2891‑20. The impurity profile of the fluoro‑proline diastereoisomer must also meet the Ph. Eur. monograph 3096 standards for any epimerised (2R,4R)‑isomer, controlled at ≤ 0.30% by UPLC (ACQUITY UPLC H‑Class, BEH C₁₈ 1.7 µm, 50 mM NH₄HCO₃ pH 9.5/CH₃CN).