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

Performance of 19F-edited NMR experiments on intrinsically disordered proteins (IDPs) demands a fluorine label that introduces minimal structural perturbation while exhibiting a sharp singlet resonance at a chemical shift distinct from buffer components and intracellular metabolites. (S)-1-(tert-butoxycarbonyl)-4,4-difluoropyrrolidine-2-carboxylic acid, after Boc removal and incorporation into a peptide backbone via solid-phase synthesis, yields a 4,4-difluoroproline residue. The geminal fluorines eliminate the cis/trans heterogeneity inherent to native proline, collapsing the 19F signal into a single narrow peak typically observed between −95 and −110 ppm relative to CFCl₃, a region largely free of biological background. In a typical labeling strategy for studying proline isomerization in the FKBP12·calcineurin complex, between 1 and 3 residues of the target mini-protein are substituted with 4,4-difluoroproline at a molar substitution ratio of 4,4-difluoroproline to wild-type proline of 1:1 per labeled position. The required incorporation ratio on-resin is controlled by coupling 1.05 equivalents of the Boc-protected amino acid relative to the free amine loading of the peptidyl-resin, which is determined via quantitative Fmoc cleavage at 301 nm. ISO/IEC 17025:2017 governs the competence of the NMR facility generating cGMP-like research data packages. Solid-phase peptide synthesis proceeds via Boc chemistry on a 4-methylbenzhydrylamine (MBHA) resin with a loading of 0.45–0.60 mmol/g, using 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU)/0.4 M N-methylmorpholine in dimethylformamide for each coupling. The end-of-synthesis cleavage employs liquid hydrogen fluoride containing 10% anisole and 2% dimethyl sulfide at −5°C for 60 minutes, followed by precipitation in cold diethyl ether. The crude peptide is purified by semi-preparative reversed-phase HPLC on a C18 column using a linear gradient of 0.1% trifluoroacetic acid in water/acetonitrile to a purity exceeding 95% by area at 220 nm. The terminal product is a lyophilized 19F-labeled peptide probe, often between 20 and 50 residues, for protein folding and ligand-binding studies monitored on a 600 MHz spectrometer equipped with a 19F/H dual-resonance cryoprobe. Operational boundaries include the absolute requirement for pre-drying the Boc-4,4-difluoroproline under vacuum over phosphorus pentoxide for 12 hours when relative humidity exceeds 60%, as carbamate hydrolysis during storage generates free 4,4-difluoroproline that will cap growing chains upon activation.

What Limits the Double-Coupling Requirement for (S)-Boc-4,4-difluoroproline in Solid-Phase Assembly of DPP-4–Resistant Incretin Mimetics?

Addition of (S)-Boc-4,4-difluoroproline into glucagon-like peptide-1 (GLP-1) receptor agonist sequences at the N-terminal cap or at the penultimate proline position has been explored to block dipeptidyl peptidase-4 (DPP-4) cleavage while preserving receptor activation. The steric demand of the gem-difluoro group combined with the electron-withdrawing effect that lowers the pyrrolidine nitrogen nucleophilicity significantly retards the acylation rate during chain elongation. When employing Fmoc-based solid-phase peptide synthesis on a Rink amide resin at a 0.1 mmol scale in a CS Bio 336X automated synthesizer, the coupling step for the N-terminal (S)-Boc-4,4-difluoroproline is implemented using 4 equivalents of the amino acid, 3.9 equivalents of O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU), and 8 equivalents of N,N-diisopropylethylamine (DIEA) relative to the deprotected resin-bound peptide. Despite these molar excesses, a Kaiser test conducted after 60 minutes at 45°C with vortex agitation frequently remains positive, indicating unreacted amine. A recoupling cycle of equal duration is therefore mandated, raising the total coupling time to 120 minutes and achieving a final coupling efficiency of >99.2% as assessed by UV monitoring of the Fmoc-deprotection solution at 304 nm. The formulation addition ratio corresponds to one (S)-4,4-difluoroproline unit per peptide chain, equivalent to a mass ratio of approximately 0.08–0.12 grams of the Boc-amino acid per gram of final crude peptide for a 30-mer. Following chain assembly, the N-terminal Boc group is removed with trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5) over 30 minutes, and the peptide is cleaved from the resin simultaneously with side-chain deprotection using the same cocktail for 3 hours. The crude incretin analog is purified by ion-exchange chromatography followed by high-resolution reversed-phase HPLC with a purity specification of >97% per USP <621>. Terminal dosage forms under evaluation are subcutaneously administered lyophilized powders comprising a mono-4,4-difluoroproline-substituted GLP-1 analog that exhibits a plasma half-life in Sprague-Dawley rats of >18 hours compared to <2 hours for the native sequence. Regulatory compliance for material destined for IND-enabling chronic toxicology requires adherence to ICH Q7 Good Manufacturing Practice for active pharmaceutical ingredients, with residual solvent limits conforming to ICH Q3C, and endotoxin thresholds below 0.5 EU/mg measured by the limulus amebocyte lysate test per USP <85>. Pre-drying of the Boc-amino acid at 40°C under vacuum until Karl Fischer titration indicates water content below 0.1% w/w prevents incomplete couplings caused by hydrolysis of the activated ester.

Conformational Rigidity and Metabolic Shielding in Macrocyclic NS3/4A Protease Inhibitor Scaffolds

Synthesis of pangenotypic hepatitis C virus NS3/4A protease inhibitors has utilized (S)-1-(tert-butoxycarbonyl)-4,4-difluoropyrrolidine-2-carboxylic acid as a proline surrogate within the P2 unit of macrocyclic peptidomimetics to minimize the entropic penalty of binding while impeding oxidative metabolism at the pyrrolidine ring. The compound is incorporated via solution-phase amide bond formation rather than incremental solid-phase methods due to the macrocyclization step that requires a linear precursor in high purity. In a representative sequence, the Boc-4,4-difluoroproline (1.0 equivalent) is reacted with the free amine of an elaborated quinoline-bearing tripeptide fragment (0.95 equivalents) in dichloromethane at 0–5°C using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl, 1.2 equivalents) and hydroxybenzotriazole (HOBt, 1.2 equivalents) for 16 hours. The batch is then washed with 0.5 M aqueous citric acid and 5% sodium bicarbonate. This stoichiometric addition ratio translates to a loading of 0.45–0.60 kg of the Boc-amino acid per 1.0 kg of the final macrocyclic target compound after cyclization and purification. The downstream process strips the Boc group with 4 M hydrogen chloride in 1,4-dioxane at ambient temperature for 90 minutes, liberating the pyrrolidine nitrogen for subsequent peptide coupling that closes the macrocycle. Purification employs normal-phase silica gel chromatography with a step gradient from hexane/ethyl acetate 4:1 to 1:1, isolating the macrocyclic product in 68–72% yield with a diastereomeric excess exceeding 99.5% as verified by chiral HPLC with a Chiralpak AD-H column. The terminal output is a pre-development candidate drug substance at the 100–500 g scale for pharmacokinetic profiling. Regulatory oversight at this stage applies ICH Q11 for starting material definition, ICH Q3A for impurities in new drug substances, and FDA 21 CFR 312.23 for the chemistry, manufacturing, and control section of an Investigational New Drug application. A process-specific limitation arises during the Boc cleavage: prolonged exposure to the ethereal HCl solution beyond 2 hours induces partial defluorination at the 4-position, detected as a +18 Da mass adduct by LC-MS, which is suppressed by quenching the reaction immediately upon complete consumption of starting material as monitored by TLC (ethyl acetate/hexane 3:7, Rf 0.35 for protected intermediate).

Incorporation of (S)-1-Boc-4,4-difluoroproline into the repeating Pro-Arg-Pro and Pro-His-Pro motifs of proline-rich antimicrobial peptides (PrAMPs), including oncocin VDKPPYLPRPRPPRRIYNR-NH₂ and its truncated analogs, has been investigated to overcome rapid exoprotease-mediated inactivation in serum-containing Mueller-Hinton broth. The gem-difluoro substitution renders the Xaa-Pro bond resistant to cleavage by prolidase and aminopeptidase P, which are prevalent in the bloodstream of mammalian models of systemic infection. The synthetic route employs batch-wise Boc chemistry on a phenylacetamidomethyl (PAM) resin using an in-situ neutralization protocol. For each difluoroproline residue introduced, the formulation calls for 2.0 mmol of (S)-Boc-4,4-difluoroproline per gram of resin with a substitution of 0.65 mmol/g, representing an approximately 3.1-fold molar excess. Coupling proceeds via HBTU/DIEA activation in N-methyl-2-pyrrolidone for 20 minutes per cycle, with a double-coupling routine applied whenever the residual amine is detectable by the bromophenol blue indicator method beyond 99.5% completion. The global deprotection and cleavage from the PAM resin is accomplished with anhydrous hydrogen fluoride containing p-cresol (10% v/v) and p-thiocresol (5% v/v) at 0°C for 1 hour, followed by evaporation of HF under nitrogen flow. The crude peptide is triturated with cold diethyl ether, dissolved in 5% aqueous acetic acid, and lyophilized. Final purification to >98% homogeneity is achieved by preparative reversed-phase HPLC on a C4 column with a gradient of acetonitrile in 0.1% aqueous trifluoroacetic acid. The terminal products are fluorinated oncocin derivatives in which 2 of the 5 native proline residues are replaced by (2S)-4,4-difluoroproline, yielding a typical net mass increase of 36 Da per substitution. Quality control specifications reference CLSI M07-A11 for broth microdilution antimicrobial susceptibility testing and USP <1026> for bacterial endotoxins. In susceptibility assays, the minimal inhibitory concentration against Escherichia coli ATCC 25922 remains within one doubling dilution of the parent peptide, while the half-life in 25% fresh human serum at 37°C increases from approximately 45 minutes to over 300 minutes. The operational boundary for HF cleavage mandates rigorous exclusion of moisture from the reaction vessel and thorough pre-cooling of the entire apparatus to −70°C before HF distillation, as ice contamination during cleavage leads to pyroglutamate formation at N-terminal glutamine residues and a concomitant 18 Da mass shift that complicates purification.

When Pin1-Catalyzed Isomerization Rates Are Extracted from 19F Line-Shape Analysis of a (2S)-4,4-Difluoroproline-Containing Substrate

The serine/threonine-proline specific peptidyl-prolyl cis-trans isomerase Pin1 regulates cellular signaling by accelerating the interconversion of phosphorylated Ser/Thr-Pro motifs. A 19F NMR-based activity assay that replaces the scissile proline with 4,4-difluoroproline converts the conformational exchange into an observable two-site exchange line-broadening phenomenon. The substrate peptide, acetyl-Phe-Phe-pThr-(4,4-difluoroPro)-Phe-NH₂, is synthesized on a 0.1 mmol scale using Fmoc-solid-phase chemistry with H-Rink Amide ChemMatrix resin. The N-terminal (S)-Boc-4,4-difluoroproline residue is coupled in the final position using 4 equivalents of the protected amino acid and PyBOP/DIEA in DMF for a single extended cycle of 4 hours at 35°C with nitrogen bubbling. After Boc removal with trifluoroacetic acid/triisopropylsilane (98:2, 30 minutes) and cleavage from the resin with trifluoroacetic acid/thioanisole/water (90:5:5, 3 hours), the peptide is obtained at a crude purity of approximately 80% and purified by preparative HPLC to >99% with a final yield of 35–40 mg. The addition ratio corresponds to a single 4,4-difluoroproline per substrate molecule. For the kinetic assay, the purified substrate is dissolved at 250 µM in an NMR buffer containing 50 mM HEPES-d₁₈, 100 mM NaCl, 2 mM DTT, pH 7.0, and 10% D₂O (v/v). Nanomolar concentrations of full-length Pin1 are added, and a series of one-dimensional 19F spectra are acquired on a spectrometer operating at 564 MHz for 19F with an acquisition time of 0.5 seconds and a relaxation delay of 2.0 seconds. The line-width at half-height of the 19F singlet broadens from 12 Hz (no enzyme) to 35–90 Hz depending on enzyme concentration, and the isomerization rate constant k_cat/K_m is extracted by fitting the spectra to a two-site McConnell equation implemented in MATLAB. The resulting terminal product is a high-purity NMR-detectable substrate for quantifying Pin1 activity in inhibitor screening cascades. The assay protocol adheres to the OECD Principles of Good Laboratory Practice for in vitro enzymology, and the peptide characterization package includes amino acid analysis, electrospray mass spectrometry (observed [M+H]⁺ within 0.5 Da of calculated), and analytical HPLC with a C18 column showing a single peak integrating to >99% area. This configuration is incompatible with any amine-containing buffer additive such as Tris, which forms an aminal adduct with the liberated N-terminal 4,4-difluoroproline residue upon Boc deprotection, shifting the 19F resonance upfield by 4–6 ppm and invalidating the kinetic model.