(2R,4S)-1-(Tert-Butoxycarbonyl)-4-Fluoropyrrolidine-2-Carboxylic Acid

For Fmoc/t-Bu hybrid and Boc-strategy solution-phase fragment condensations where a (2R,4S)-4-fluoroprolyl residue must be incorporated adjacent to a sterically congested α,α-disubstituted amine, the free carboxylic acid form is routinely activated without pre‑neutralization to avoid diketopiperazine formation. Compliance with ICH Q7 Chapter 7.3 for GMP starting materials requires vendor-certified residual solvent profiles matching USP <467> Method IV, with acceptance criteria for dichloromethane not exceeding 600 ppm and ethyl acetate below 5000 ppm. In a representative kilo‑scale campaign performed in a 50 L jacketed glass reactor (Buchi Chemreactor CR‑50) equipped with retreat‑curve impeller agitation at 120 rpm, the addition level was locked at 1.07–1.10 molar equivalents relative to the free amine component. The downstream process employed isobutyl chloroformate (1.10 eq) and N-methylmorpholine (2.40 eq) in anhydrous tetrahydrofuran at –15 °C to generate the mixed anhydride; the amine partner was introduced as a single portion after 4‑min activation time. After quenching with 0.5 M citric acid and phase cut, the organic layer was washed with saturated sodium bicarbonate until the aqueous phase reached pH 7.4, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure (45 °C bath, 80 mbar). Purification by flash chromatography on Silica Gel 60 (Merck Grade, 40–63 µm) with n-heptane/ethyl acetate 3:2 (v/v) yielded the elongated intermediate in 88–92% isolated yield with a diastereomeric excess monitored by chiral stationary‑phase HPLC (Chiralpak IA‑3, 4.6×250 mm, mobile phase n-hexane/ethanol/trifluoroacetic acid 80:20:0.1, flow 1.0 mL/min, detection at 210 nm) consistently exceeding 99.5% de. The terminal manufactured goods comprise fully elaborated linear or macrocyclic peptide APIs destined for Phase‑I/II clinical investigation, where the 4‑fluoroproline moiety confers a 0.8–1.2 kcal/mol stabilization of the trans-amide rotamer as determined by variable‑temperature ¹H‑NMR in d₆‑DMSO.

How Does Anhydrous Acidic Cleavage Impact Downstream API Purity Profiles?

When a Boc‑protected fluoropyrrolidine intermediate must be deprotected in the presence of acid‑sensitive glycosidic or tertiary alcohol functionalities on the growing molecule, the choice between TFA/triisopropylsilane and anhydrous HCl in 1,4‑dioxane shifts the impurity landscape decisively. The industry‑recognised specification for this building block as a regulatory starting material (RSM) requires a Certificate of Analysis aligned with ICH Q6A decision tree #2, including identity by ¹⁹F NMR (376 MHz, DMSO‑d₆, δ –168.5 ppm, doublet of triplets) and achiral purity ≥ 99.0% by HPLC at 205 nm. During a pilot‑scale batch intended for a direct‑acting antiviral API, the charging ratio of the Boc‑pyrrolidine acid was set at 1.00 eq relative to the azide‑bearing heptapeptide core; after coupling with HATU (1.03 eq) and DIPEA (2.60 eq) in DMF at 3 °C, the crude reaction mass was subjected to a cleavage cocktail consisting of 4 M HCl in dioxane (4.0 volumes) at 18–22 °C for 45 min. Initial attempts using TFA/TIS/DCM 50:5:45 (v/v/v) generated a persistent N-trifluoroacetyl by‑product at 0.3–0.5% AUC, which co‑eluted with the target intermediate on C18 columns. Switching to anhydrous HCl eliminated this adduct but introduced a risk of fluoride displacement under rigorous moisture ingress; consequently, the headspace of the reactor was maintained with a continuous nitrogen sweep containing <10 ppm H₂O (measured by a Systech EC913 dew‑point transmitter), and the dioxane solution was pre‑dried over 4 Å molecular sieves to ≤50 ppm water by Karl Fischer titration. Post‑cleavage, the hydrochloride salt was precipitated with tert‑butyl methyl ether (8 volumes) and filtered through a 0.45 µm PTFE membrane, then vacuum‑dried at 35 °C for 16 h. The process afforded the amine hydrochloride in 96.3% yield with fluoride‑related degradants below the 0.10% threshold when assayed by ion chromatography (Metrohm 930 Compact IC Flex, Metrosep A Supp 5 column). The downstream processing sequence continues with an N‑acylation to install a quinoline‑4‑carboxamide cap, ultimately delivering a macrocyclic HCV NS3/4A protease inhibitor that is administered orally at a 100 mg once‑daily dose strength. All stages are performed under ISO 14644‑1 Class 8 cleanroom conditions with in‑process controls per ICH Q7 Section 11.10.

Utilization of this (2R,4S)-1‑Boc‑4‑fluoropyrrolidine‑2‑carboxylic acid as a matched C‑terminal coupling partner in convergent liquid‑phase synthesis of a peptidomimetic renin inhibitor typically places stringent demands on the pre‑activation method because the electron‑withdrawing 4‑fluoro substituent increases the acidity of the C‑2 methine proton and elevates the risk of epimerization via oxazolone formation when carbodiimides are employed. The certificate of conformance for a customer‑directed supply is issued against ISO 9001:2015 and contains a dedicated statement on the absence of genetically modified organisms and bovine spongiform encephalopathy risk materials, as required by EDQM guidelines for CEP submissions. In the campaign of interest, the building block was loaded at 1.12 molar equivalents versus the amine fragment bearing a hindered 2,6‑dimethylbenzyl group. A two‑step protocol was adopted: the acid was first converted to the pentafluorophenyl ester using pentafluorophenol (1.20 eq) and DCC (1.15 eq) in ethyl acetate at ‑5 °C for 2 h; after filtration of dicyclohexylurea, the activated ester was isolated as a stable white solid after low‑temperature precipitation from n‑heptane. Coupling proceeded by adding the amine component (1.0 eq) and 1‑hydroxy‑7‑azabenzotriazole (1.0 eq) in dry DMF, stirring at 20 °C for 14 h, then quenching with 1 M NaHSO₄ solution. Workup included back‑extraction with dichloromethane, a brine wash (10% w/v NaCl), and crystallisation from isopropanol/water 7:3 to afford the blocked dipeptide in 85% yield. Chiral purity, verified by supercritical fluid chromatography (SFC) on a Chiralpak AD‑H column (4.6×150 mm, CO₂/methanol 85:15, 2.5 mL/min, 40 °C, 150 bar back‑pressure), showed the unwanted (S,R)-diastereomer below 0.15%. The end product emerging from full deprotection and N‑methylation is a tertiary amide‑containing renin inhibitor that passes through tablet compression with a required compaction pressure of 12–16 kN on a Fette 102i rotary tablet press, yielding 7 mm round tablets with breaking force ≥80 N.

[a] Substrate: (2R,4S)-1‑Boc‑4‑fluoropyrrolidine‑2‑carboxylic acid coupled with (S)-2‑amino‑3,3‑dimethylbutanamide HCl. [b] Determined by UPLC‑QToF (Waters Acquity H‑Class with Xevo G2‑XS, ESI+).

Catalytic Transfer Hydrogenolysis Versus Standard Hydrogenation: Effects on Fluoride Displacement

In the preparation of a central nervous system‑penetrant O‑GlcNAcase inhibitor that embeds the 4‑fluoropyrrolidine ring as the solvent‑exposed motif for reducing P‑glycoprotein recognition, the sequence requires removal of a benzyl ester protecting group while preserving the Boc‑protected fluoropyrrolidine intermediate intact. Regulatory filings for the ultimate API reference the building block’s DMF (Type‑II drug master file) filed with the U.S. FDA under 21 CFR 314.420 and simultaneously the active substance master file with PMDA Japan per MHLW Ordinance No. 135. The quantity of the fluorinated intermediate employed in the ester hydrogenolysis step corresponds to 1.05 kg per 1.00 kg of benzyl ester reactant; this slight excess accounts for mechanical losses during transfer of the viscous DMF solution. Initial small‑scale trials employing 10% Pd/C (Johnson Matthey type 39, 50% water wet) at 25 °C and 1 atm H₂ balloon pressure proceeded smoothly, but upon scaling to a 20 L glass hydrogenation vessel (Büchi buchi pressoclave) with 3 bar hydrogen overpressure, the reaction liberated fluoride ions at a level of 0.8–1.5% after 4 h, as confirmed by a fluoride‑ion‑selective electrode (Mettler Toledo perfectION). The fluoride elimination correlated with a pH drop to 2.3 due to in‑situ hydrogen bromide accumulation; switching to transfer hydrogenation with ammonium formate (4.0 eq) and the same catalyst loading under a nitrogen atmosphere at 40 °C completely eliminated detectable fluoride loss. The downstream process employed a filtration through a 0.2 µm inline PTFE cartridge, followed by aqueous workup with 5% w/w citric acid, and finally crystallization from ethyl acetate/cyclohexane (1:4). The isolated batch passed elemental analysis for C, H, N, and F within ±0.3% of theoretical values (C 53.72, H 6.31, N 5.22, F 7.08). The final medicinal product is a capsule containing 50 mg and 200 mg strengths of the O‑GlcNAcase inhibitor, manufactured under 21 CFR Part 211 conditions with dissolution testing per USP <711> Apparatus II at 50 rpm in 0.1 N HCl.

When the objective is production of a ¹⁹F‑enabled probe for in‑cell NMR metabolomic studies, stringent limits on metal‑ion contaminants become the primary specification differentiators. The B2B supply agreement for this application mandates a Certificate of Analysis that passes ICH Q3D elemental impurity limits for parenteral administration, with Class 1 elements (As, Cd, Hg, Pb) controlled to ≤ 1.5 µg per daily dose equivalent and Class 2A (Co, Ni, V) to ≤ 5.0 µg per day. The Boc‑fluoropyrrolidine acid is deployed at an extremely low scale — 10–50 mg per batch — but its addition ratio inside the synthetic sequence is maintained at precisely 1.00 molar equivalent against the amine‑terminated pentapeptide scaffold assembled on 2‑chlorotrityl chloride resin (loading 0.45 mmol/g). Coupling is performed manually in a fritted syringe equipped with a PTFE stopcock, using a single 120‑minute acylation with HBTU (4.0 eq relative to resin loading) and DIPEA (8.0 eq) in N‑methyl‑2‑pyrrolidone. Following global deprotection with 95% TFA/2.5% water/2.5% triisopropylsilane, the crude peptide is precipitated in cold diethyl ether, centrifuged at 5000 rpm, and purified by preparative HPLC on a C18 column (Phenomenex Luna, 10 µm, 250×21.2 mm) with a gradient of 10–45% acetonitrile in 0.1% TFA over 45 min. Fractions containing the pure ¹⁹F‑labeled reporter peptide are pooled, lyophilized, and analyzed via direct‑infusion Orbitrap MS (Thermo Q Exactive, resolution 140,000 at m/z 200) to confirm monoisotopic mass within 2 ppm. The resulting research tools are not intended for clinical use; nevertheless, the compound itself conforms to REACH Regulation (EC 1907/2006) and is classified under L−CODE L120203 for peptide synthesis reagents.

When the C‑4 Fluorine Substituent Dictates Conformational Locking in a Prolyl Amide Bond

In a development program targeting an orally bioavailable cyclophilin inhibitor, the conformational bias imparted by the 4‑fluoropyrrolidine ring becomes a critical quality attribute, warranting circular dichroism (CD) and ¹⁹F‑NMR batch‑to‑batch correlation. The industry‑adopted acceptance standard for the dihedral angle fidelity is documented by comparison of the measured ³J_HF coupling constant (52.4 Hz) against a reference spectrum archived at the time of DMF submission; any deviation exceeding ± 0.6 Hz triggers a full root‑cause investigation under ICH Q10 corrective action framework. The building block is introduced into the synthetic pathway at a weight‑based addition ratio of 0.94 kg per 1.00 kg of the des‑fluoro precursor, compensating for its higher molecular weight while delivering the same mole count. The downstream process utilises a one‑pot N‑deprotection–reductive amination strategy: the Boc group is removed with 4 M HCl in cyclopentyl methyl ether (5 volumes) at 35 °C for 55 min; the volatiles are stripped to ≤2.5% residual solvent by in vacuo distillation, then the residue is reslurried in tetrahydrofuran and treated with benzaldehyde (1.03 eq) and sodium triacetoxyborohydride (1.40 eq) at 22 °C. After aqueous quenching and extraction, the N‑benzyl‑4‑fluoroproline derivative is crystallised from toluene/heptane to achieve 99.1% chromatographic purity. The ultimate therapeutic entity is a non‑immunosuppressive cyclophilin inhibitor formulated as a lyophilised powder for injection; sterility assurance follows Ph. Eur. 5.1.1 terminal sterilisation validations with a F₀ ≥ 8 min at 121.1 °C.

A less common but commercially sustained deployment of this protected fluoropyrrolidine is as the stoichiometric core in collagen‑related heterotrimeric peptide self‑assembly studies. The lot‑specific release specification for this outlet includes a bioburden limit of ≤ 10 CFU/g and endotoxin ≤ 0.05 EU/mg when tested per USP <85> gel‑clot method, because the assembled triple‑helical probes are incubated with primary fibroblast cultures. The addition ratio diverges markedly from peptide‑linker campaigns: the fluorinated monomer is incorporated at 33 mol% alongside glycine (33 mol%) and (2S,4R)-4‑hydroxyproline (34 mol%) during automated microwave‑assisted solid‑phase synthesis on a CEM Liberty Blue instrument running 0.10 mmol scale cycles at 90 °C. Each coupling cycle of the Boc‑protected fluoroprolin acid uses 5.0 eq of the acid activated with DIC (2.5 eq) and Oxyma Pure (2.5 eq) in DMF, with a deprotection step of neat TFA (2×1‑min pulses) to liberate the N‑terminus. The cleaved peptide is purified at room temperature by semipreparative HPLC and subsequently dialysed against ultrapure water through a 1 kDa membrane. Annealing of the three single chains is performed in phosphate‑buffered saline at pH 7.0, ramping from 80 °C to 4 °C over 18 h. The resulting triple‑helical peptide containing (2R,4S)-4‑fluoroproline exhibits a melting temperature (T_m) increase of 6–9 °C relative to the non‑fluorinated analogue as determined by differential scanning calorimetry (TA Instruments Nano DSC, 1 °C/min scan rate). The final commercial article is a lyophilised peptide supplied as a research‑grade biochemical under ISO 13485:2016 quality management, exempt from therapeutic regulatory filing.