(2S,4R)-1-[(Tert-Butoxy)Carbonyl]-4-Fluoropyrrolidine-2-Carboxylic Acid

How Incorporating Fluoroproline Residues into Peptidomimetics Alters Coupling Efficiency and Racemisation Risk in Fragment Condensation

In solid‑phase peptide synthesis (SPPS) operated under the Boc‑benzyl protection strategy, (2S,4R)-1-[(tert‑butoxy)carbonyl]-4-fluoropyrrolidine-2-carboxylic acid is introduced as a pre‑activated building block that eliminates the need for in‑situ amine deprotection prior to chain elongation. The inductive electron withdrawal exerted by the C‑4 fluorine reduces the nucleophilicity of the adjacent amine after Boc removal, directly translating into extended coupling intervals when sterically hindered Fmoc‑SPPS or solution‑phase amidation is substituted. On a manufacturing line employing a 300 mmol scale automated peptide synthesizer with 0.4 mmol/g aminomethyl resin loading, acylation with 2.5 equiv. of the fluoroproline derivative activated by HATU (2.45 equiv.) and 4.0 equiv. of N,N‑diisopropylethylamine in anhydrous N‑methyl‑2‑pyrrolidone at 0–5 °C routinely requires a double‑coupling protocol of 2 × 90 min to bring the free‑amine Kaiser test to endpoint. Failure to maintain the jacket temperature below 8 °C during activation leads to measurable epimerisation at the C‑2 stereocentre, with the diastereomeric excess dropping from >99.5 % to 97.0 % when the same mixture is held at 20 °C for 45 min before resin addition, as tracked by C18‑UPLC separation of the 4‑fluoroproline‑containing hexapeptide fragment (column: Acquity UPLC CSH 130 Å, 1.7 µm; gradient 5–50 % MeCN in 0.1 % TFA over 8 min). Residual N,N‑dimethylformamide, which replaces NMP in related protocols run at tonne scale, is controlled to ≤880 ppm per ICH Q3C Guideline for residual solvents, while tin‑ and palladium‑derived metals originating from earlier catalytic deprotection steps are monitored to ≤10 ppm for Pd, ≤20 ppm for Sn as mandated by the elemental impurity profile outlined in USP <232>/<233> and ICH Q3D. The resulting peptide mimetic, frequently a macrocyclic acylsulfonamide or a linear tripeptide aldehyde, exhibits a shifted cis–trans amide equilibrium around the 4‑fluoroproline residue, a property that is exploited in the final drug substance to pre‑organise the bound conformation in serine protease active sites.

There is no header here — the section opens with a direct technical observation. In kilo‑laboratory campaigns preparing late‑stage intermediates for macrocyclic HCV NS3/4A protease inhibitors such as glecaprevir, the (2S,4R)-4-fluoroproline moiety is incorporated via an N‑acylative route that proceeds with 1.08 eq of the Boc‑protected amino acid relative to the quinoline‑carboxylic acid activated partner, using a combination of 1‑ethyl‑3‑(3‑dimethylaminopropyl)carbodiimide hydrochloride (1.15 eq) and 0.10 eq of 1‑hydroxy‑7‑azabenzotriazole in dichloromethane at −10 °C to suppress diketopiperazine formation. Process analytical technology (PAT) utilising ReactIR 15 with a diamond ATR probe tracks the consumption of the carbonyl band at 1812 cm⁻¹ that corresponds to the mixed anhydride intermediate; reaction quenching is triggered automatically when the derivative peak area falls below 2% of its initial value. The crystallisation is induced by antisolvent addition of n‑heptane at 45 °C with a controlled cooling ramp of 0.3 K/min, delivering the amide coupling product with a consistent particle size distribution characterised by d₅₀ 120–180 µm, as measured by laser diffraction according to ISO 22412:2017. Recovered yields exceed 92% after vacuum drying at 40 °C, and the diastereomeric purity of the isolated intermediate is held above 99.8% by chiral normal‑phase HPLC (Chiralpak IA‑3 3 µm, 250 × 4.6 mm; n‑hexane/ethanol/trifluoroacetic acid 80/20/0.1). The terminal API is tested for mutagenic azide impurities stemming from earlier click‑chemistry steps, with an acceptance criterion of ≤8.3 ppm, compliant with the staged TTC approach of ICH M7(R2).

What Process Controls Prevent Epimerisation at the C‑2 Chiral Centre During Amide Bond Formation?

When (2S,4R)-1-[(tert‑butoxy)carbonyl]-4-fluoropyrrolidine-2-carboxylic acid is deployed as an electrophilic partner in direct amination sequences with bulky C‑terminal anilines or cyclopropylamines, the primary process conflict arises between the necessity to drive the reaction to completion and the tendency of the activated carboxylate to undergo oxazolone‑mediated racemisation. Pilot‑scale campaigns have documented that replacing classic carbodiimide‑based activation with bis(2‑oxo‑3‑oxazolidinyl)phosphonic chloride (BOP‑Cl) or propylphosphonic anhydride (T3P) moderates this risk by maintaining a slightly acidic pH profile that retards oxazolone formation. In a specific coupling with (1R,2S)‑1‑amino‑2‑vinylcyclopropanecarboxylic acid ethyl ester run in a 50 L jacketed glass reactor, the preferred charge sequence is: the Boc‑fluoroproline (1.0 eq), the amine hydrochloride (0.93 eq), and a pyridine‑buffered dichloromethane solution of T3P (1.3 eq, 50 w% in EtOAc) added in a single portion at −15 °C. The internal temperature is held at −12 ± 3 °C for the first 30 min, then allowed to rise to 5 °C over 2.5 h. Work‑up involves quench into 2 M aqueous citric acid (10 v/w relative to fluoroproline input), back‑extraction with MTBE, and solvent swap into isopropyl acetate before the Boc group is selectively removed with anhydrous 4 M HCl in dioxane at 10–15 °C. The hydrochloride salt that precipitates is washed with diisopropyl ether to afford material that meets the water content limit of ≤0.5 % by Karl Fischer titration (ASTM E203‑16), necessary to avoid lactamisation during subsequent hot‑melt extrusion of the final pharmaceutical formulation.

Cold Reference Standard Material Utilisation in ¹⁸F Radiopharmaceutical Quality Control

Positron emission tomography (PET) radiopharmacies rely on (2S,4R)-1-[(tert‑butoxy)carbonyl]-4-fluoropyrrolidine-2-carboxylic acid primarily as a synthetic precursor and identity standard for the preparation and validation of trans‑4‑[¹⁸F]fluoro‑L‑proline, a tracer that maps abnormal collagen biosynthesis in fibrotic and neoplastic lesions. The non‑radioactive Boc‑protected acid is dissolved in 25 mM phosphate buffer (pH 7.4) at a concentration of 0.100 mg/mL and co‑injected with the radiochemical product on a Radio‑HPLC system equipped with a dual‑wavelength UV (210 nm) and sodium iodide scintillation detector. The acceptance criterion for radiochemical identity is that the retention time of the ¹⁹F‑reference must differ by no more than ±0.2 min from that of the 511 keV peak on a Luna C18(2) 3 µm, 150 × 4.6 mm column eluted with 99 % aqueous 0.1 % formic acid and 1 % acetonitrile at 0.8 mL/min. The material is also used to spike the formulated [¹⁸F]fluoroproline batch at 5 µg/mL as a system suitability marker to confirm no carrier‑to‑tracer peak distortion. Regulatory starting material governance follows 21 CFR 212.60 and 212.61 with respect to in‑process controls, and a Certificate of Analysis must accompany each 500 mg unit, documenting chiral purity (≥99.0 % by chiral GC‑FID after derivatisation to the methyl ester), residual 4‑dimethylaminopyridine (≤10 ppm if used in any pre‑activation step), and endotoxin level (≤0.25 EU/mg per USP <85>). Published data for the specific molar activity achievable from this precursor in automated modules is limited, and yield‑optimisation studies remain proprietary to individual compounding centres.

The substitution of a 4‑fluorine atom on the pyrrolidine ring of dipeptidyl peptidase‑4 (DPP‑4) inhibitor pharmacophores is a well‑established strategy to modulate metabolic soft spots on the cyanopyrrolidine warhead without sacrificing the critical hydrogen bond between the nitrile and the S1 serine hydroxyl. Lab‑scale analogue synthesis often proceeds through (2S,4R)-1-Boc‑4-fluoroproline‑2‑carboxylic acid as a key acylation reagent onto N‑substituted glycine amides or hexahydro‑2H‑pyrido[3,2‑b][1,4]oxazin‑3(4H)‑one cores. A representative procedure executed on a 25 g substrate charge employed 1.18 eq of the fluoroproline acid, O‑(7‑azabenzotriazol‑1‑yl)‑N,N,N‘,N’‑tetramethyluronium hexafluorophosphate (1.15 eq), and 2.6 eq of N‑methylmorpholine in 8 v/w of acetonitrile at 22 °C for 16 h. The crude extract was subjected to silica gel chromatography (ethyl acetate/heptane 3:7) to remove the residual HATU byproduct, and the Boc group was cleaved with methanolic HCl to precipitate the ammonium salt, which was collected in 89 % isolated yield with 99.3 % area purity by HPLC‑UV. Quantitative ¹⁹F NMR (Bruker AVANCE 400 MHz, internal standard: 0.05 M α,α,α‑trifluorotoluene) confirmed the absence of regioisomeric 4,4‑difluoro contaminants that can form via elimination during prolonged basic exposure. At the drug‑substance level, Phase I microsomal intrinsic clearance in human liver microsomes (HLM) incubated at 37 °C with 1 µM test article for 45 min typically indicates a 2‑ to 4‑fold reduction in Clint compared to the non‑fluorinated parent, with the added advantage that the fluorine serves as a ¹⁹F NMR probe for binding‑mode studies using WaterLOGSY and saturation transfer difference (STD) experiments. Compliance with ICH Q6A is assured by X‑ray powder diffraction monitoring of the final polymorph, with any batch exhibiting additional peaks at 2‑theta 9.8° or 16.4° (Cu Kα) being reprocessed through a maturation slurry in ethyl acetate/cyclohexane.

When the Boc‑Protected Amino Acid Serves as a Direct Building Block in Solution‑Phase HCV Protease Inhibitor Assembly

Linear assembly of the tripeptide mimetic core for certain NS3/4A protease inhibitors exploits the orthogonal protection offered by the Boc group to allow selective C‑terminus coupling while leaving the N‑terminus masked for later global deprotection. In a documented four‑step sequence that delivers the (1R,2S)‑1‑amino‑N‑(cyclopropylsulfonyl)‑2‑ethenylcyclopropane carboxamide‑terminated macrocyclisation precursor, (2S,4R)-1-[(tert‑butoxy)carbonyl]-4-fluoropyrrolidine-2-carboxylic acid (1.05 eq) is condensed with the free amine of an intermediate bearing a hex‑5‑enoic acid side chain using N,N’‑diisopropylcarbodiimide (1.10 eq) and 0.05 eq of pentafluorophenol in a mixed solvent of dichloromethane and DMF (4:1 v/v) over 18 h at 0 °C ➔ 20 °C. The reaction mass is washed sequentially with 5 % sodium bicarbonate and 1 M KHSO₄ to remove unreacted coupling agents; residual carbodiimide‑urea byproduct is monitored by FT‑IR for the characteristic asymmetric N=C=N stretch at 2120 cm⁻¹ and must be ≤0.15 % w/w in the oily intermediate before proceeding to ring‑closing metathesis. That metathesis step, catalysed by Hoveyda‑Grubbs II catalyst (1.5 mol%) under an ethylene atmosphere in toluene at 60 °C, forms the macrocyclic scaffold. The process engineering challenge is the competing β‑hydride elimination that liberates HF from the fluoropyrrolidine ring under the action of the ruthenium alkylidene, poisoning the catalyst and driving the required catalyst loading from a straightforward 0.5 mol% (for non‑fluorine macrocycles) to the aforementioned 1.5 mol%. After n‑heptane precipitation, the macrocyclic ester is saponified to the acid and directly re‑n‑amplified with the cyclopropylsulfonamide arm, where the intact Boc group preserves the N‑terminus. Global deprotection with trifluoroacetic acid/triisopropylsilane/water (95/2.5/2.5 v/v) at 25 °C for 2 h liberates the final inhibitor, which is then crystallised as a mesylate salt from acetone/diethyl ether. The entire campaign operates under an environmental emissions control plan aligned with the EU REACH regulation, specifically targeting an atmospheric release limit of ≤25 mg/Nm³ for fluorinated volatile organic compounds (F‑VOCs) as captured by activated carbon bed monitors.

Asymmetric Organocatalysis with 4‑Fluoroproline: Tuning Enamine Reactivity Through Fluorine Substitution

Outside the pharmaceutical supply chain, (2S,4R)-1-[(tert‑butoxy)carbonyl]-4-fluoropyrrolidine-2-carboxylic acid functions as a stable precursor to the free secondary amine catalyst (2S,4R)-4-fluoropyrrolidine-2-carboxylic acid, which, after quantitative Boc removal via HCl/dioxane and neutralisation, is screened in direct asymmetric aldol additions. The electron‑withdrawing fluorine at C‑4 lowers the electrophilicity of the intermediate enamine, favouring transition states with greater C–C bond‑forming selectivity in cyclohexanone additions to electron‑deficient benzaldehydes. A standard screening protocol loads the freshly prepared catalyst at 15 mol% along with the aldehyde (0.5 mmol) and 5.0 equiv. of cyclohexanone in wet DMSO (3 % v/v water) at 4 °C for 48–96 h. Diastereoselectivity (syn/anti) and enantiomeric excess are determined by chiral stationary‑phase SFC (Chiralpak AD‑H 5 µm, 250 × 4.6 mm; CO₂/MeOH 85:15, 3 mL/min, 40 °C, backpressure 120 bar). Reported anti:syn ratios reach >20:1 with ee values exceeding 90 % in certain p‑nitrobenzaldehyde cases, though published data for this specific configuration with aliphatic acceptor substrates is limited and frequently reveals a sharp drop in turnover when the ketone component bears α‑branching. Because the catalyst is a secondary amine, post‑reaction scavenging with polymer‑bound isocyanate resin (1.5 equiv. of NCO groups relative to theoretical catalyst charge) at 30 °C for 1 h followed by filtration through a 0.2 µm PTFE membrane removes catalyst‑derived impurities below the reporting threshold of the subsequent evaporative light‑scattering detection analysis. All solvents used in the catalytic cycle are tested for peroxides (limit ≤10 ppm as H₂O₂) prior to batch release, in accordance with the safety requirements of ASTM E298‑17 for reactive peroxide‑forming materials.