1-[(Tert-Butoxy)Carbonyl]-4,4-Difluoropyrrolidine-3-Carboxylic Acid

In the synthesis of viral protease inhibitors targeting the HCV NS3/4A serine protease family and the SARS-CoV-2 3CL^pro (M^pro) cysteine protease, the titled 1-[(tert-butoxy)carbonyl]-4,4-difluoropyrrolidine-3-carboxylic acid serves as a conformationally constrained β-proline surrogate that installs a metabolically resilient gem-difluoroethylene isostere. Unlike generic Boc-L-proline, the 3-carboxylic acid regioisomer displaces the amide bond vector by one carbon unit, which alters the peripheral hydrogen-bonding geometry within the S2 or S1′ subsite of the enzyme. Process-scale amide coupling proceeds via activation with HATU (1.2 eq) and N,N-diisopropylethylamine (3.0 eq) in anhydrous acetonitrile at 0–5 °C, followed by slow warming to 22 °C over 16 h, yielding the intermediate tert-butyl carbamate-protected precursor. An acidic deprotection using 4 M HCl in 1,4-dioxane (8 vol, 0 °C for 1 h) cleanly liberates the free 4,4-difluoropyrrolidine-3-carboxylic acid amide without detectable epimerization at C-3 (chiral HPLC, Chiralpak IA-3, α 1.18, n-hexane/ethanol/0.1% TFA, retention time 8.3 min). The terminal finished drug substances typically incorporate this fragment as a P2 or P3 capping moiety; a representative clinical candidate under an active Investigational New Drug (IND) application disclosed in WO2023185745 exhibits an EC₅₀ of 28 nM against the M^pro enzyme in a FRET-based biochemical assay. Batch release under FDA 21 CFR Part 211 subpart E and ICH Q7 Chapter 11.10 mandates that the intermediate comply with residual solvent limits of ≤720 ppm for 1,4-dioxane (Class 2, ICH Q3C Guideline) and ≤5,000 ppm for acetonitrile, verified by headspace GC-FID on an Agilent 7890B system equipped with a DB-624 column (30 m × 0.53 mm, 3.0 µm film). In multi-kilogram campaigns, a jacketed glass-lined reactor (200 L, Pfaudler) equipped with a retreat-curve impeller at 120 rpm ensures homogeneous mixing during the quench step, and a subsequent polish filtration through a 0.45 µm polypropylene cartridge prior to spray drying (Büchi B-290 Mini) achieves a bulk density of 0.32 g/cm³ and an angle of repose of 28°, suitable for direct encapsulation into hard gelatin capsules at 20 mg strength.

Fine-tuning blood-brain barrier penetration via 4,4-difluorination of the pyrrolidine-3-carboxylate pharmacophore

When constructing potent and selective negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 2 (mGluR₂) intended for treatment-resistant depression, the incorporation of a 4,4-difluoropyrrolidine-3-carboxylic acid residue systematically increases the topological polar surface area (TPSA) to 89.6 Ų while simultaneously raising the calculated logD_7.4 by 0.8 units relative to the parent pyrrolidine analogue. A dedicated medicinal chemistry campaign at a contract research organization utilized a parallel library synthesis protocol where a stock solution of the Boc-protected acid (0.25 M in dichloromethane) was dispensed into 96-well deep blocks using a Tecan Freedom EVO liquid handler, activated with EDC·HCl (1.5 eq) and HOBt hydrate (1.5 eq), then coupled to an array of 48 secondary biaryl amines at ambient temperature over 20 h under moderate orbital shaking (600 rpm). After aqueous workup with 1 N NaHSO₄ and saturated NaHCO₃, the products were purified by mass-triggered preparative HPLC (Waters 2767 Sample Manager, XBridge C18 OBD column 19 × 150 mm, 5 µm, gradient 5-95% methanol in water with 0.1% formic acid). The subsequent Boc cleavage was executed on 10 g scale using trifluoroacetic acid (20 vol) in the presence of triisopropylsilane (2 vol) as a carbocation scavenger at 0 °C for 45 min; ¹⁹F NMR (376 MHz, CDCl₃) confirmed retention of the overlapping AB quartet signals at δ -93.2 and -98.7 ppm (J_F,F = 238 Hz), distinguishing the gem-difluoro motif from any hydrolytic mono-fluoro byproducts. The terminated drug conjugate, a mGluR₂ NAM designated as a preclinical development candidate, demonstrated an unbound brain-to-plasma partition coefficient (K_p,uu) of 0.42 in male Sprague-Dawley rats following a 3 mg/kg intravenous dose, consistent with CNS penetration. For regulatory starting material supply facilitating GLP toxicology studies, the CMO must furnish a certificate of analysis that includes enantiomeric excess by chiral SFC (CHIRALPAK AD-H, 4.6 × 250 mm, CO₂/methanol 80:20, 2.5 mL/min, back pressure 120 bar, UV detection at 210 nm, ee ≥ 99.0%) and a heavy metal screen per USP ⟨231⟩ Method II, with cadmium and lead limits tightened to ≤5 ppm each.

Can a conformationally rigid fluorinated β-amino acid linker improve the degradation rate (D_max) of a BRD4-targeting PROTAC?

In the emerging class of proteolysis-targeting chimeras (PROTACs) designed to hijack the von Hippel-Lindau (VHL) E3 ubiquitin ligase for bromodomain-containing protein 4 (BRD4) degradation, the choice of linker between the target binder and the VHL ligand substantially influences ternary complex formation kinetics. When a linear PEG or alkyl chain is replaced by a gem-difluorinated pyrrolidine-3-carboxylate motif, the rigid pocket created by end-to-end distance of 6.2 Å (determined by NOE-restrained molecular dynamics) preorganizes the degrader into a biologically active conformation. A validated synthetic route on 500 g scale commences with the activation of 1-[(tert-butoxy)carbonyl]-4,4-difluoropyrrolidine-3-carboxylic acid (1.0 eq) using T3P (propylphosphonic anhydride, 50 wt% in ethyl acetate, 1.8 eq) and pyridine (2.5 eq) in tetrahydrofuran at -10 °C, to which a VHL-ligand intermediate bearing a free hydroxylamine is added dropwise over 30 min. The conversion is monitored by rapid UPLC (Waters ACQUITY BEH C18, 2.1 × 50 mm, 1.7 µm, 0.4 mL/min, 2.5 min gradient) to >98% before quenching with ice-cold water. After extraction and silica plug filtration (100–200 mesh, 10 wt-ratio), the Boc group is cleaved with HCl/cyclopentyl methyl ether (3 M, 12 vol) at 15 °C for 3 h to minimize ring-opening of the cyclopentyl ether. The resulting amine hydrochloride is telescoped without isolation into a second amide coupling with a BET inhibitor carboxylic acid under standard HATU/DIPEA conditions in DMF. This convergent manufacturing sequence circumvents the isolation of hygroscopic intermediates and reduces the overall process mass intensity (PMI) to 38 kg/kg API. The final degrader molecule exhibits a degradation efficiency D_max of 92% in MV-4-11 leukemia cells after 4 h treatment at 10 nM, as quantified by In-Cell Western (LI-COR Odyssey CLx). From a regulatory perspective, the Boc-protected acid provided as a critical starting material must satisfy an ICH Q11 risk assessment for mutagenic impurities: the absence of the structurally alerting 4,4-difluoro-1-butyne impurity (AMES positive, category 2 per ICH M7) is confirmed by GC-MS with a limit of detection of 0.5 ppm. The compound is shipped under nitrogen in a triple-laminated aluminum foil bag at -20 °C to prevent decarboxylation, which accelerates at temperatures exceeding +4 °C according to accelerated stability studies (40 °C/75% RH for 4 weeks resulted in 0.8 area% of the decarboxylated byproduct).

Dihedral angle modulation in peptide backbone mimics exploits the steric and electronic influence of the two fluorine atoms attached to the tetrahedral carbon at position 4 of the pyrrolidine ring, producing a non-canonical torsion φ of approximately -62° and ψ of approximately 136° in the resulting β-amino acid residue. This torsion set closely approximates a type VIa β-turn geometry observed in the structurally conserved regions of G-protein coupled receptor intracellular loops. For a chemical biology probe targeting the 5-HT_2B receptor, the N-Boc-4,4-difluoro-β-proline building block is introduced at the i+1 position of a cyclic pentapeptide via solid-phase peptide synthesis (SPPS) on a 2-chlorotrityl chloride resin (substitution 1.2 mmol/g). After Fmoc deprotection with 20% piperidine in DMF, the resin-bound amine is acylated using the Boc-4,4-difluoro-β-proline acid (3.0 eq relative to resin loading), PyBOP (3.0 eq), and 0.6 M N-methylmorpholine in NMP for 2 × 45 min double couplings. A Kaiser test (ninhydrin) confirmed coupling completion. The final peptide is cleaved from the resin with 30% hexafluoroisopropanol in CH₂Cl₂ (2 × 1 h) to retain the Boc protection for subsequent on-resin macrolactamization. The cyclized peptide macrocycle exhibits a solution-phase J_NH-C²H coupling constant of 8.2 Hz in DMSO-d₆, corroborating the DFT-predicted turn conformation. For the current batch used in a radioligand binding assay (NovaScreen panel, 68 targets), the laboratory supplied a certificate of analysis citing purity 99.2% (HPLC, 220 nm), chiral purity 99.7% ee, and water content 0.09% via coulometric Karl Fischer titration (Metrohm 851 Titrando). The material was manufactured in a facility holding ISO 9001:2015 certification, and the product dossier includes a full REACH registration for quantities exported into the European Union in the 1–10 metric ton band. Any deviation from the recommended storage condition of -18 ± 5 °C in desiccated, light-protected amber glass bottles results in a loss of chiral integrity of 0.15% ee per week, likely due to slow α-proton abstraction facilitated by residual moisture.

Evaluating the 4,4-difluoro-β-proline scaffold as a bioconjugation handle in ¹⁸F-radiolabeled prosthetic groups for positron emission tomography

The development of ¹⁸F-labeled prosthetic groups for site-specific peptide radiolabeling requires a building block that simultaneously offers a rapid, high-yielding conjugation step and a non-radioactive 19F reference standard for HPLC co-injection. The N-Boc-protected 4,4-difluoropyrrolidine-3-carboxylic acid meets this dual requirement because its carboxylic acid moiety can be pre-activated as the N-succinimidyl ester under mild conditions and subsequently coupled to a terminal lysine ε-amine of a tumor-homing RGD peptide in phosphate buffer (pH 7.4) at 37 °C within 15 min. For the radiosynthesis, a microfluidic flow chemistry platform (Advion NanoTek, 100 µm ID PTFE reactor, 2 m length) is employed: the Boc-protected acid (2.0 mg, 7.4 µmol) is dissolved in anhydrous acetonitrile (500 µL) and activated with disuccinimidyl carbonate (9.6 µmol) and anhydrous triethylamine (7.4 µmol) at 50 °C for 10 min. The resulting NHS ester, after rapid silica cartridge purification, is reacted with ¹⁸F-fluoride (cyclotron-produced, dried by azeotropic distillation with K₂CO₃/Kryptofix 2.2.2) to achieve a radiochemical incorporation yield of 34 ± 6% (decay-corrected, n=5) and a radiochemical purity of >99% after semipreparative radio-HPLC (Phenomenex Luna C18, 10 × 250 mm, 5 µm, 60:40 acetonitrile/water 0.1% TFA, 4.0 mL/min, retention time of ¹⁹F reference 11.2 min). The cold reference standard was authenticated by high-resolution mass spectrometry (Q-TOF, ESI+, m/z calculated for [M+H]⁺ 437.1283, found 437.1285, Δ = 0.5 ppm) and ¹⁹F NMR (376 MHz, CDCl₃) demonstrating a sharp signal at δ -95.8 ppm. In the context of regulatory documentation for an exploratory IND, a dedicated impurity profile study was conducted per ICH Q3A(R2): the reporting threshold was set at 0.05%, and the major process-related impurity was identified as the ring-opened difluoro glutaric acid monoamide at 0.07%. The entire manufacturing batch (30 g) was conducted in a 100 L Hastelloy C-276 reactor with a retreat-curve agitator, and the crude product was recrystallized from ethyl acetate/cyclohexane (1:3 v/v) to afford a free-flowing white crystalline solid with a melting point of 117.8–118.6 °C by differential scanning calorimetry (DSC, Mettler Toledo DSC 3+, heating rate 10 °C/min). The finished, lyophilized radiotracer product was reconstituted to 10 mL of sterile saline and passed through a 0.22 µm Millex-GV filter for intravenous administration to a murine xenograft model at a dose of 7.4 MBq (specific activity >37 GBq/µmol). Published data for clinical translation of this specific difluoropyrrolidine-based PET agent remain limited; however, the building block's consistent lot-to-lot purity (99.5% by HPLC across 15 consecutive batches) and the absence of genotoxic structural alerts support its advancement toward Phase 0 microdosing studies.