1,2-Pyrrolidinedicarboxylic Acid, 4-Fluoro-, 1-(1,1-Dimethylethyl) Ester, (2R,4S)-

The compound (2R,4S)-1-(tert-butoxycarbonyl)-4-fluoropyrrolidine-2-carboxylic acid—systematically designated 1,2-Pyrrolidinedicarboxylic Acid, 4-Fluoro-, 1-(1,1-Dimethylethyl) Ester, (2R,4S)—is supplied as a white to off-white crystalline powder with a molecular weight of 249.24 g/mol and a molecular formula of C₁₀H₁₆FNO₄. The CAS registry number for this specific diastereomer is not universally harmonized across all supplier databases, and the material is typically referenced by its IUPAC or systematic name to avoid stereochemical ambiguity. Routine lot-release specifications mandate a chromatographic purity of ≥98.0% via HPLC (UV detection at 210 nm) and an enantiomeric excess of ≥99.0% as determined by chiral supercritical fluid chromatography (SFC) on an amylose-based polysaccharide column with CO₂/methanol mobile phase. Water content by Karl Fischer titration is controlled to ≤0.5%, with residual ignition on sulfated ash measured at ≤0.1%.

What Drives the Selection of (2R,4S) Over Other 4-Fluoroproline Isomers in Peptidomimetic Design?

Incorporation of trans-4-fluoroproline into a peptide chain modifies the trans/cis amide bond equilibrium through a stereoelectronic effect governed by the fluorine substituent’s gauche interaction with the acyl moiety. The (2R,4S) configuration yields a Cγ-exo ring pucker preference, which contrasts with the Cγ-endo bias of the (2S,4R) L-proline analogue. This ring pucker reversal shifts the backbone φ and ψ dihedral angles, directly impacting the thermodynamic stability of polyproline II helix versus type I β-turn motifs. Solid-state X-ray diffraction data on model tripeptides incorporating the (2R,4S) residue indicate a mean φ value of approximately −60° and ψ near +140°, placing it in the α-helical quadrant of the Ramachandran plot but with reduced conformational entropy relative to the non-fluorinated parent. When used as a proline surrogate in matrix metalloproteinase inhibitor scaffolds, the (2R,4S) configuration has been observed in published crystallographic complexes (PDB deposition data) to realign the P2’ substituent binding trajectory by 1.2–1.8 Å compared to the (2S,4R) counterpart, altering S1’ pocket occupancy. This property is exploited in the design of selectivity-optimized cathepsin inhibitors where off-target binding to cathepsin B versus cathepsin L must be minimized. It is critical to note that the (2R,4S) isomer is distinct from the cis-4-fluoro variants—(2R,4R) and (2S,4S)—which enforce a markedly different ring geometry and exhibit 3- to 5-fold slower amide bond cis-trans isomerization rates in model peptides at 298 K as measured by stopped-flow ¹⁹F NMR.

Storage and Handling: Stability Boundaries and Incompatibilities

Long-term storage at −20 °C ± 5 °C under argon atmosphere in a sealed amber glass vial is recommended to preserve enantiopurity and prevent decarboxylation or N-Boc cleavage. Thermogravimetric analysis reveals that significant mass loss (> 2%) initiates at 148 °C under nitrogen flow at 10 K/min, which is attributed to tert-butyl cation loss followed by CO₂ evolution. The material is hygroscopic; exposure to ambient humidity (relative humidity ≥ 60% for periods exceeding 6 hours) results in agglomeration and localized hydrolysis of the Boc protecting group to yield the free amino acid, which then undergoes diketopiperazine formation in solution. Compatibility testing demonstrates that the compound is stable in anhydrous acetonitrile and dichloromethane over 72 hours at 4 °C, but prolonged contact with dimethyl sulfoxide (DMSO) at room temperature leads to oxidation of the pyrrolidine nitrogen detectable by LC-MS. Avoid combining the protected amino acid with amine-based additives or coupling reagents such as HATU and DIEA in the absence of the intended reaction partner; uncontrolled oligomerization has been documented in process development batches when reagent addition sequence was altered from the optimized protocol.

Batch-to-Batch Variability in cGMP Production Runs

Three consecutive pilot campaigns executed at 15 kg input scale in a 100 L jacketed glass reactor identified critical process parameters that directly influence the diastereomeric purity of the final Boc-protected product. The key intermediate, (2R,4S)-4-fluoropyrrolidine-2-carboxylic acid, is generated via enzymatic resolution of the racemic N-acetyl ester derivative using Aspergillus melleus aminoacylase immobilized on Eupergit^® C 250 L beads. Batch records show that the specific activity of the biocatalyst, measured as initial rate of L-enantiomer hydrolysis at pH 7.8 and 37 °C, decayed from 218 U/g (batch #F-2024-014) to 167 U/g (batch #F-2024-016) after 14 re-cycles, requiring an increase in residence time from 18 hours to 26 hours to maintain enantiomeric excess above the 99.0% release threshold. The subsequent N-Boc protection using di-tert-butyl dicarbonate (Boc₂O) in a tert-butanol/water biphasic system at controlled pH 10.5 ± 0.3 was sensitive to exotherms: a deviation to pH 11.2 during Boc₂O addition led to 0.4% epimerization at the C-2 position, as quantified by SFC analysis employing a Chiralpak^® IC-3 column with a 3% (v/v) methanol modifier.

When Liquid Chromatography-Mass Spectrometry Confounds Enantiomeric Integrity Assessment

Routine HPLC area percent purity measurements using a C₁₈ reversed-phase column (e.g., 150 mm × 4.6 mm, 3 µm particle size) with a water/acetonitrile gradient containing 0.1% formic acid fail to discriminate the (2R,4S) target isomer from its (2R,4R) diastereomer. Co-elution of the cis and trans diastereomers occurs under virtually all standard reversed-phase conditions. Chiral chromatographic separation therefore constitutes the quality control method with direct regulatory significance. A validated SFC-UV method uses a mobile phase of supercritical CO₂/isopropanol with 0.2% isopropylamine additive, delivering a resolution R_s of ≥2.8 between the (2R,4S) and (2R,4R) peaks within a run time of 8 minutes. The limit of quantitation for the undesired diastereomer is established at 0.05% area ratio relative to the main component. Coupling the SFC effluent to a single-quadrupole mass spectrometer operating in ESI positive mode (cone voltage 25 V, capillary 3.0 kV) provides simultaneous confirmation via the protonated molecular ion at m/z 250.1 [M+H]⁺ and fragment ions corresponding to loss of the Boc group (m/z 150.1) and fluoro-pyrrolidine ring (m/z 104.1). Mass spectral libraries built from in-house reference standards of all four stereoisomers enable unambiguous peak assignment in materials destined for investigational new drug (IND) filings.

A direct comparison of the (2R,4S) configuration with the common (2S,4R) enantiomer underscores the subtle but mechanistically critical differences that dictate utility. Both materials share identical molecular weight and elemental composition, yet the spatial orientation of the fluoro substituent and the carboxyl group generates opposing Cotton effects in circular dichroism spectra in the 190–220 nm region. The (2R,4S) isomer is preferentially utilized when a D-amino acid‑like conformation is required within a peptide sequence to engender resistance to endogenous proteolytic cleavage, because the inverted α-carbon stereochemistry prevents recognition by mammalian aminopeptidases and carboxypeptidases. In fragment-based screening campaigns targeting protein-protein interactions, hits incorporating (2R,4S)-4-fluoroproline have been shown in surface plasmon resonance (SPR) binding assays to yield dissociation constants (K_D) that are 4- to 7-fold lower than those containing the L-fluoroproline counterpart, attributable to a better-fit hydrophobic packing with a conserved leucine zipper region.

Residual Solvent and Elemental Impurity Control Per ICH Q3C and Q3D

Manufacturing of the (2R,4S) N-Boc fluoroproline involves the use of Class 2 solvents: dichloromethane and tert-butanol. Quantitation by headspace GC-FID according to USP <467> Procedure A confirms residual dichloromethane at or below 60 ppm (ICH limit: 600 ppm) and tert-butanol not exceeding 5000 ppm (limit: 5000 ppm). Elemental impurity screening per ICH Q3D guideline (USP <232>/<233>) by closed-vessel microwave digestion followed by ICP-MS measures palladium (deriving from a hydrogenation catalyst used upstream in the fluoropyrrolidine synthesis) at concentrations consistently below 1.0 ppm in the isolated product. Nickel and chromium, potential leachables from the reactor vessel, are monitored at a routine reporting threshold of 0.5 ppm. No Class 1 impurity has ever been detected in archival lot analyses dating to 2019, and the supplier’s Quality Technical Agreement (QTA) mandates that any future process change must be communicated 90 days in advance along with a full risk assessment per ICH Q9.

The N-Boc protection strategy distinguishes this product from its Fmoc- or Cbz-protected counterparts. While the Fmoc version is directly compatible with standard solid-phase Fmoc chemistry, the Boc variant offers broad acid lability that can be exploited in convergent solution-phase syntheses where orthogonal protection schemes are required. A notable application is the staged elaboration of macrocyclic peptide inhibitors where the Boc group is retained through Fmoc-SPPS and removed post cyclization. This orthogonal lability also means that the Boc-protected (2R,4S) fluoroproline must not be exposed to TFA vapors during storage or to strongly acidic scrubbing solutions in ventilation systems, as even gaseous contact will initiate surface deprotection, leading to discoloration and degradation within 72 hours at 25 °C. For applications demanding a free N-terminus, the corresponding free base, (2R,4S)-4-fluoropyrrolidine-2-carboxylic acid, is also cataloged, but its hygroscopic nature and propensity to form stable hydrates reduce the accuracy of gravimetric dispensing in parallel synthesis; consequently, the Boc-protected form is preferred in automated high-throughput chemistry platforms where the Boc group is removed immediately prior to the coupling step.