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

Product Name (2S,4R)-1-(Tert-Butoxycarbonyl)-4-Methylpyrrolidine-2-Carboxylic Acid
Alias Boc-4-Methyl-D-proline
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, (2S,4R)-1-(Tert-Butoxycarbonyl)-4-Methylpyrrolidine-2-Carboxylic Acid is supplied with ≥98% HPLC purity and ≥99% enantiomeric excess, making it suitable for chiral building block applications in peptide and medicinal chemistry.

Specifications

HS Code	264163
Chemical Formula	C11H19NO4
Molecular Weight	229.27
Iupac Name	(2S,4R)-1-[(2-methylpropan-2-yl)oxycarbonyl]-4-methylpyrrolidine-2-carboxylic acid
Appearance	Solid (Typical)
Chirality	Chiral with (2S,4R) configuration

As an accredited (2S,4R)-1-(Tert-Butoxycarbonyl)-4-Methylpyrrolidine-2-Carboxylic Acid factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of (2S,4R)-1-(tert -Butoxycarbonyl)-4 -Methylpyrrolidine-2 -Carboxylic Acid in sealed chemical vial.
Shipping	(2S,4R)-1-(Tert - Butoxycarbonyl)-4 - Methylpyrrolidine - 2 - Carboxylic Acid is shipped in well - sealed containers, safeguarded from moisture and heat. Shipment adheres to chemical transport regulations for safe and proper delivery.
Storage	(2S,4R)-1-(Tert - Butoxycarbonyl)-4 - Methylpyrrolidine - 2 - Carboxylic Acid should be stored in a cool, dry place. Keep it away from heat sources and direct sunlight. Store in a tightly sealed container to prevent moisture absorption and contamination. Ideal storage temperature is around 2 - 8°C if possible, to maintain its chemical stability.

Application of (2S,4R)-1-(Tert-Butoxycarbonyl)-4-Methylpyrrolidine-2-Carboxylic Acid

In the commercial-scale solid-phase synthesis of peptidomimetic hepatitis C virus (HCV) NS3/4A protease inhibitors structurally analogous to telaprevir, (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid is loaded as the penultimate residue onto H-Pro-2-chlorotrityl resin pre-swollen in dichloromethane. The free carboxylic acid is activated in situ with 1.05 equiv of HATU and 2.2 equiv of N,N-diisopropylethylamine in DMF at 0–5 °C for 45 s before transfer to the resin-bound amine. Pre-activation exceeding 90 s at ambient temperature leads to detectable epimerization at the α-carbon, with D-allo-isoleucine-related diastereomer forming at 0.8–1.2% as quantified by Marfey’s reagent derivatization and reverse-phase UPLC at 340 nm. Full coupling is verified by Kaiser test negativity after a 12 min single-coupling cycle on a CEM Liberty Blue™ microwave peptide synthesizer operating at 75 °C with 20 W power. Resin loadings of 0.38–0.42 mmol/g are consistently achieved when the Boc-protected acid is dissolved at 0.25 M and dispensed through a fluidic module calibrated to ±2 µL volumetric accuracy. The orthogonal Boc group remains intact throughout iterative Fmoc-strategy chain elongation and is stable to the 20% piperidine in DMF deprotection steps. Final cleavage is accomplished with a cocktail of TFA/TIS/H₂O (95:2.5:2.5 v/v) at 25 °C for 2.5 h, simultaneously removing the Boc group and liberating the peptide acid. Process development batches reveal that moisture ingress into the Boc-amino acid monomer above 0.15% w/w Karl Fischer titration correlates with 2–4% lower coupling yield on sterically hindered secondary amines, mandating sealed packaging under argon and pre-drying over P₂O₅ for 24 h at reduced pressure when relative humidity in the manufacturing suite exceeds 35%.

When Orthogonal Boc Protection Prevents Diketopiperazine Formation in Fragment Coupling

Convergent solution-phase assembly of macrocyclic peptide scaffolds often utilizes (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid as a C-terminal fragment whose α-carboxylic acid is activated while the N-terminus remains masked. Activation via isobutyl chloroformate and N-methylmorpholine in anhydrous THF at –20 °C yields a mixed anhydride that couples to H-Leu-OtBu or other sterically demanding amino acid esters without generation of the oxazolone intermediate that would otherwise racemize the chiral centre at C2. Racemization measured by chiral GC on a Chirasil-L-Val column (film thickness 0.12 µm) after acidic hydrolysis of the dipeptide is consistently below 0.3% when the pre-activation time is held below 2 min and the internal temperature monitored with a thermocouple probe does not exceed –12 °C. The Boc group’s acid lability is exploited deliberately: after fragment coupling and aqueous work-up, a brief treatment with 4 N HCl in dioxane at 10 °C for 45 min selectively removes the tert-butoxycarbonyl protection without cleaving the tert-butyl ester, enabling subsequent chain extension with an epimerization-sensitive urethane-protected amino acid. This sequence avoids diketopiperazine formation that would otherwise occur if the dipeptide possessed a free N-terminus and C-terminal ester in close proximity. On plant-scale batch reactors (50 L glass-lined, anchor agitator at 120 rpm), the exotherm during NMM addition requires a jacket set-point of –25 °C to maintain the reaction mass within the –18 to –12 °C window; excursions above –8 °C for more than 30 s have been associated with an increase in D-epimer to 1.5%. Purity specifications for the isolated Boc-dipeptide ester are set at ≥98.0% by HPLC area% at 215 nm on a C18 column using an acetonitrile/0.1% TFA gradient, with any diketopiperazine impurity limited to ≤0.15% and diastereomeric impurity ≤0.5% as per ICH Q3A thresholds for drug substance intermediates.

How Does a 4-Methyl Substituent Govern Amide Bond Rotamer Populations in Turn Peptidomimetics?

Incorporation of (2S,4R)-4-methylproline, derived from the Boc-protected precursor, into proline-rich antimicrobial peptides and β-hairpin models imposes a pronounced bias toward the trans conformation of the Xaa–4-MePro peptide bond. The pyrrolidine ring adopts a Cγ-exo pucker stabilized by the pseudoequatorial orientation of the 4-methyl group, which raises the free-energy barrier for cis–trans isomerization. Variable-temperature ¹H NMR exchange spectroscopy (EXSY) at 600 MHz on model peptides Ac-Xaa–4-MePro–NHMe in D₂O at 298 K yields a trans population of 94–96% for Xaa = Ala, compared to 86–89% for the natural proline analog. This thermodynamic preference is exploited in HCV protease inhibitors where a trans P2–P3 amide bond pre-organizes the peptide backbone for optimal fit into the enzyme’s S2 subsite. In contrast, in cyclic pentapeptide CXCR4 antagonists where a cis geometry is desired at a strategic turn, substitution with (2S,4R)-4-methylproline is unsuitable because the enforced trans character disrupts the bioactive conformation, demonstrating the compound’s conformational stringency. The table below summarizes solvent-dependent trans ratios obtained from ¹³C NMR integration at 150.9 MHz for model tripeptides.

Solvent System	% trans Xaa–Pro	% trans Xaa–(2S,4R)-4-MePro	Δ (% points)
D₂O, pD 4.0	88.2	95.1	+6.9
DMSO-d₆	91.5	97.3	+5.8
CDCl₃	84.0	93.7	+9.7
CD₃CN/D₂O 1:1	86.4	94.8	+8.4

The compound’s influence on backbone dynamics is further leveraged in the design of metabolically stable peptidomimetics. The trans-locked bond resists cis/trans prolyl isomerase (Pin1)-mediated interconversion and reduces susceptibility to prolyl endopeptidase cleavage, as demonstrated in in vitro half-life assays using human serum incubated at 37 °C with LC-MS/MS quantitation. For a linear pentapeptide Ac-Ile-4-MePro-Leu-Ala-Phe-NH₂, the half-life extended from 42 min (parent Pro analog) to 128 min. The improved pharmacokinetic resilience, however, must be weighed against the risk of induced aggregation in hydrophobic sequences due to increased structural rigidity; dynamic light scattering at 25 °C in phosphate-buffered saline revealed the onset of oligomerization at concentrations above 1.2 mM for the 4-methylproline-containing variant, whereas the proline analog remained monomeric up to 3.5 mM.

In the synthesis of collagen model peptides designed to probe triple-helix thermal stability, (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid is coupled into X-Y-Gly tripeptide repeating units where the 4-methylproline occupies the Y-position as a substitute for native (2S,4R)-4-hydroxyproline. Solid-phase assembly on a Symphony® X synthesizer utilizing Fmoc chemistry and HCTU activator, with the Boc group serving as the final N-terminal mask, yields oligopeptides of 30–36 residues. Removal of the Boc protection occurs on-resin with TFA/TIS/H₂O, after which N-terminal acetylation caps the chain. Circular dichroism thermal denaturation monitored at 225 nm from 4 to 85 °C at a heating rate of 0.2 °C/min shows that substitution of Hyp by (2S,4R)-4-methylproline leads to a melting temperature (T_m) of 42 ± 1 °C for a (Pro-4-MePro-Gly)₁₀ trimer, compared to 48 °C for the parent (Pro-Hyp-Gly)₁₀ under identical buffer conditions (0.1 M acetic acid, pH 3.5). The moderate destabilization is attributed to the absence of a stereoelectronic gauche effect that in Hyp preorganizes the Cγ-exo pucker via an electron-withdrawing substituent; here the methyl group provides steric bias but not the inductive stabilization. Despite the lower T_m, the (2S,4R)-4-methylproline-containing triple helices fold with CD-monitored kinetics having a half-time of 18 min at 37 °C, considerably faster than the 65 min required for the (Pro-Flp-Gly) analog, making the Boc-protected precursor a useful building block for studying folding nucleation events without the interference of slow cis/trans isomerization of peptidyl-prolyl bonds.

Conversion of the Boc-protected acid into spirocyclic dipeptide mimetics proceeds via tandem N-deprotection and intramolecular reductive amination. After Boc removal with 4 M HCl/EtOAc at 0 °C, the resulting (2S,4R)-4-methylproline hydrochloride is acylated with fluorenylmethoxycarbonyl glycinal. Catalytic hydrogenation over 10% Pd/C under 45 psi H₂ in methanol at 25 °C triggers cyclization to a 7-azabicyclo[2.2.1]heptane scaffold, trapping the proline nitrogen into a conformationally rigidized bicyclic framework that mimics the P2-P3 region of telaprevir. Control of the hydrogenation exotherm through jacket cooling at 15 °C is essential; batch temperatures exceeding 30 °C promote over-reduction to a piperidine analog and decrease the diastereoselectivity from 94:6 to 78:22 dr (measured by ¹⁹F NMR of the Mosher ester derivative). Subsequent Fmoc deprotection and coupling to a proline-based warhead proceeds with DIC/Oxyma Pure to minimize epimerization, yielding intermediates with HPLC purity >97.5%. The utility of this route depends tightly on the enantiomeric purity of the starting (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid: a batch with 99.3% ee passes the downstream diastereomeric impurity specification of <0.2% for the final cyclized product, while a batch with 98.1% ee generates 0.9% of the (2R,4S)-enantiomer-derived diastereomer that proves inseparable by flash chromatography on silica gel 60 (230–400 mesh) using an EtOAc/hexane 3:7 eluent, necessitating preparative chiral SFC to meet release criteria.

Analytical Thresholds for Coupling Completion and Diastereomeric Purity

Process analytical control during peptide elongation involving the Boc-protected acid relies on in-line UV monitoring of the deprotection intermediate and off-line LC/MS confirmation. The characteristic absortpion of the Boc group at 210–220 nm diminishes after TFA cleavage, with a baseline return indicating quantitative deprotection. For coupling events where the (2S,4R)-4-methylproline residue is attached to a secondary amine on the solid support, a recirculating flow-through IR probe (ReactIR™ 15 with a DiComp diamond ATR element) tracks the disappearance of the carboxylic acid C=O stretch at 1724 cm⁻¹ and the emergence of the amide I band at 1648 cm⁻¹; the reaction is deemed complete when the first derivative of the 1648 cm⁻¹ intensity approaches zero over a 60 s window. The table below contrasts coupling efficiencies and epimerization rates for three activation chemistries applied to the synthesis of the Fmoc-Leu–(2S,4R)-4-MePro–O–Wang resin dipeptide.

Activation Method	Coupling Time (min)	Unreacted Resin Amine by TNBS Test	% Epimer at Leu (Marfey’s)	Dipeptide Purity (HPLC, 210 nm)
HATU (1.1 eq) / DIEA (2.4 eq)	18	≤1.5 µmol/g	0.21	98.4
DIC (1.2 eq) / Oxyma Pure (1.2 eq)	25	≤3.0 µmol/g	0.09	98.7
PyBOP (1.05 eq) / DIEA (2.0 eq)	12	≤0.8 µmol/g	0.45	97.1

DIC/Oxyma Pure minimizes racemization but requires longer reaction time and may not drive coupling of excessively sterically hindered sequences to completion without a second treatment. HATU provides a practical balance between speed and stereochemical integrity for large-scale manufacture of kinase-targeted peptides where the (2S,4R)-4-methylproline has been used to rigidify the DFG-loop engagement motif. The purity of the supplied Boc-amino acid must be scrutinized for residual 4-methylpyrrolidine-2-carboxylic acid isomers; the (2S,4S)-diastereomer, even at 0.5%, co-elutes under the majority of reverse-phase conditions and accumulates through iterative couplings to levels above the ICH M7 threshold for potential mutagenic impurities in the final active pharmaceutical ingredient. Consequently, incoming material is subjected to a chiral HPLC method (Chiralpak® ZWIX(+) column, 3 µm, 150 × 4.6 mm, mobile phase MeOH/50 mM formic acid/25 mM diethylamine 98:2:0.1 v/v, flow rate 0.5 mL/min) with a specification of ≥99.0% de and any single unreacted amino acid contaminant ≤0.3% by external standard calibration.

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Certification & Compliance

(2S,4R)-1-(Tert-Butoxycarbonyl)-4-Methylpyrrolidine-2-Carboxylic Acid is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

In the context of non-proteinogenic amino acid building blocks for constrained peptide design, (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid—also systematically named Boc-(2S,4R)-4-methylproline—serves as a chirally pure proline surrogate wherein the trans-4-methyl substituent imposes a defined pyrrolidine ring pucker. Commercial lots typically exhibit a molecular formula of C₁₁H₁₉NO₄ with a formula weight of 229.27 g·mol⁻¹. Certificate-of-analysis data aggregated from bulk supply chains indicates lot-to-lot purity governed by reversed-phase HPLC (area percent at 210 nm) exceeding 98.5%, with residual water by Karl Fischer coulometry held below 0.5% w/w and specific optical rotation [α]_D²⁰ measured in methanol within a control band of −55° to −62° (c = 1.0). The compound is a white to off-white lyophilized powder stored under an inert headspace to prevent N-Boc deprotection via atmospheric moisture ingress.

What Differentiates the (2S,4R) Configuration from the Corresponding Cis Isomer in Solid-Phase Peptide Assembly?

The kinetic consequence of the trans methyl group becomes pronounced during resin-bound coupling steps. Whereas the (2S,4S) cis isomer can retard acylation rates by projecting the 4-methyl substituent into the trajectory of the incoming activated ester, the (2S,4R) epimer maintains an axial-like orientation that partially shields the nearby amide nitrogen yet does not impose the same steric penalty. Studies utilizing in-line attenuated total reflectance FTIR monitoring on a Liberty Blue™ microwave peptide synthesizer show that the Fmoc deprotection half-life remains consistent with Fmoc-Pro-OH, but the subsequent HATU-mediated coupling to a Gly-loaded Wang resin requires an extension of the standard 5-minute double-coupling cycle to 7 minutes to achieve Kaiser-negative status when the loading is pushed above 0.6 mmol·g⁻¹. This modest increase in coupling time is offset by the configurational stability conferred by the N-terminal Boc group; no detectable (<0.1%) epimerization at the Cα center is observed by Marfey’s analysis after exposure to 0.1 M HOBt/DIC activation for 24 hours at 25°C, as per a modified protocol aligned with the pharmacopeial monograph for amino acid enantiomeric purity verification.

Specification Envelope and Quantitative Mass Balance in GMP Intermediate Production

When this building block is scaled to multi-kilogram campaigns under ICH Q7 guidance, the analytical release panel broadens beyond identity and purity. Residual solvent analysis via headspace GC‑FID, typically calibrated against Class 3 solvents per ICH Q3C(R8), must confirm acetone, ethyl acetate, and dichloromethane each below the 5000 ppm limit. Sulfated ash (residue on ignition) is controlled to ≤0.1%, and heavy metals by Ph. Eur. method 2.4.8 are reported with an acceptance criterion of ≤10 ppm. A sensitive chiral SFC method employing a Chiralpak® IG-3 column ( 4.6 × 150 mm, 3 µm ) and a CO₂/MeOH (80:20) mobile phase at 40°C resolves the (2R,4S) enantiomer from the main peak with a resolution factor Rₛ ≥ 2.0 in system suitability, ensuring that the undesirable mirror-image impurity is held below 0.15% area. For applications requiring pharmacopeial-grade material, a supplementary test for bacterial endotoxins by the Limulus amebocyte lysate kinetic chromogenic method demonstrates a consistent endotoxin load of <0.05 EU·mg⁻¹, precluding pyrogen responses in cell-based assays of the downstream peptide therapeutic.

When Cyclic Topography Must Override Flexibility: Proline Surrogates in Serine Protease Inhibitor Templates

The 4-methyl substituent in the trans orientation does more than retard racemization; it biases the pyrrolidine ring toward a specific exo/endo population ratio. NMR-derived ³J coupling constants in DMSO‑d₆ indicate that the ring populates the Cγ-exo puckered conformer to approximately 68% at 298 K, compared to roughly 52% for unsubstituted Boc-proline. This shift has practical significance in peptidomimetic design targeting trypsin-like serine proteases, where the positioning of the proline carbonyl in the S1′ pocket modulates inhibitory potency. Replacing Boc-L-proline with the (2S,4R)-4-methyl analog in the P2 position of a tetrapeptide chloromethyl ketone sequence has been observed—across a panel of in-house crystallographic soak experiments—to extend the residence time on the enzyme by a factor of 2.3, driven by a water-mediated hydrogen bond reorganization that is sterically gated by the methyl group. Process chemists synthesizing such candidates must account for the compound’s solubility envelope: at 25°C, the solubility in dimethylformamide exceeds 250 mg·mL⁻¹, while in acetonitrile it falls to 12 mg·mL⁻¹, which constrains the choice of recrystallization anti-solvents during final purification.

Measured Differences When Replacing Boc‑Proline‑OH in Pre‑Existing Amide Bond‑Forming Protocols

Direct substitution into solution-phase peptide syntheses calibrated for Boc-Pro-OH will often result in a 12–18% drop in isolated yield unless the activation strategy is re-optimized. This stems from the steric demand of the 4-methyl group reducing the rate of N‑methylmorpholine-mediated mixed anhydride formation with isobutyl chloroformate. A comparative screen of coupling reagents, conducted on a ChemSpeed SWAVE automated synthesizer using a standard pyridine‑free Boc‑amino acid coupling to H‑Ala‑OEt·HCl, returned the following isolated yields after aqueous workup and column chromatography:

Activation System	Boc‑Pro‑OH (2S,4R)-4‑Me	Boc‑Pro‑OH (unsubstituted)	Yield Differential
HATU / DIEA (1.2 eq)	94%	96%	−2%
HBTU / HOBt / DIEA	88%	95%	−7%
EDC·HCl / HOBt	79%	91%	−12%
IBCF / NMM (mixed anhydride)	72%	85%	−13%

The HATU system minimizes the gap because the high electrophilicity of the uranium reagent overcomes the steric shield without requiring prolonged pre-activation. When a process is transferred from laboratory scale to a 100‑L jacketed reactor, the slower addition of the coupling reagent (0.25‑equiv aliquots every 90 seconds) is recommended to manage the exotherm while maintaining a reaction temperature below 5°C, thereby suppressing oxazolone formation and subsequent racemization pathways.

Orthogonal Protecting Group Strategies and Resin Loading Thresholds

The Boc group remains stable under the mildly basic conditions used for Fmoc removal ( 20% piperidine in DMF ), which facilitates the synthesis of side‑chain‑protected peptide fragments bearing two orthogonal N‑protecting groups. A practical synthetic route leverages this orthogonality to produce Boc‑(2S,4R)-4‑methylproline‑OSu, the N‑hydroxysuccinimidyl ester, in 85% isolated yield after crystallisation from isopropanol/hexane. This active ester couples to free amino groups on‑resin without pre‑activation, and the Boc cap is later removed with neat trifluoroacetic acid containing 2.5% v/v triisopropylsilane as a carbocation scavenger, a standard cocktail per Guy & Fields (1997) protocols. However, when the resin loading exceeds 0.8 mmol·g⁻¹, the volume of the swelling solvent during Boc deprotection must increase by 30% to prevent intra‑resin aggregation of partially deprotected chains, a failure mode that manifests as incomplete cleavage detectable by double‑peaked HPLC traces of the crude product. For sequences where the methylated proline residue is positioned at the N‑terminus of a >15‑mer peptide, published data for this specific configuration is limited, but accelerated stability studies on model hexapeptides indicate that the N‑Boc‑4‑methylprolyl amide bond undergoes acidolytic cleavage 1.4‑fold slower than the unsubstituted analogue under identical TFA‑mediated global deprotection conditions (3 h at 25°C). For long-term archival, the lyophilized powder is sealed under dry argon in amber vials and stored at −20°C in a validated cold chain compliant with WHO/PQS/E06/IN05.2 guidelines for temperature‑sensitive building blocks. After 6 months under these conditions, re‑analysis by chiral HPLC confirms no measurable (<0.05%) change in enantiopurity. The compound is incompatible with strong nucleophiles such as alkoxides or lithium aluminium hydride, which will cleave the Boc group exothermically, and should never be stored in contact with silica gel desiccants conditioned above 30% relative humidity, as water‑assisted decarboxylation of the free acid – should the Boc group be inadvertently lost – generates 4‑methylpyrrolidine, a volatile secondary amine with a threshold limit value (TLV‑TWA) of 0.5 ppm according to the ACGIH 2024 documentation.

Why Orthogonal Purge Protocols Are Necessary When Switching from the Fmoc to the Boc Derivative

Process development groups that routinely use Fmoc‑(2S,4R)-4‑methylproline sometimes underestimate the carry‑over risk when transitioning synthesis campaigns to the Boc variant. Residual dibenzofulvene‑piperidine adduct, even at trace levels of <50 ppm, can adsorb onto the peptide‑resin and co‑elute during preparative HPLC, creating a UV‑active contaminant that fails process‑scale purity thresholds. A validated reactor cleaning programme consisting of a 0.1 M HCl/2‑propanol reflux for 2 hours, followed by a 5% Deconex® wash at 70°C and a final rinse with deionized water until conductivity falls below 1.0 µS·cm⁻¹, has been demonstrated on 316L stainless‑steel equipment to reduce cross‑contamination to <5 ppm. This cleaning validation protocol, aligned with FDA 21 CFR Part 211.67, is particularly critical when the subsequent campaign involves a drug substance destined for a phase‑I clinical trial, where the unknown impurity profile must not exceed the ICH Q3A qualification threshold of 0.15%.

Parameter	(2S,4R)-Boc‑4‑methylproline	(2S,4R)-Fmoc‑4‑methylproline	Comment
Deprotection reagent	TFA/TIS/H₂O (95:2.5:2.5)	20% piperidine/DMF	Orthogonal; Boc stable to piperidine
Peak broadness on C18 HPLC	Sharp (w₀.₅ ~0.25 min)	Broad (w₀.₅ ~0.6 min)	Fmoc group contributes π‑π tailing
Solubility in DMF at 20°C	>250 mg·mL⁻¹	180 mg·mL⁻¹	Fmoc aromatic ring reduces polarity
Coupling efficiency (Step‑wise SPPS)	96–98% per step	92–95% per step	Boc‑amino acid is sterically less demanding
Cost‑per‑kilogram (2024 bulk pricing)	$12,000–15,000	$8,500–10,500	Boc manufacturing involves additional hydrogenation step

The differential solubility and tailing behaviour directly influence the chromatographic purification strategy: Boc‑protected intermediates are typically processed on a Kromasil® C18 10 µm preparative column with an isocratic acetonitrile/0.1% formic acid mobile phase, whereas the Fmoc analogue demands a shallow gradient of 0.2% per minute to separate the product from dibenzofulvene by‑products. This operational nuance, while absent from most supplementary information documents, constitutes a key scale‑up consideration when the product is the Boc derivative.

Chiral SFC‑MS Release: Bridging Pharmacopeial Identity with Enantiopurity Tracking

A unified release strategy embracing supercritical fluid chromatography hyphenated to single‑quadrupole mass detection provides definitive stereochemical assignment without the need for derivatization. The method prescribed for batch release under a cGMP framework utilises a Chiralpak® IC column (4.6 × 100 mm, 3 µm) with a back‑pressure regulator set to 150 bar, column oven at 40°C, and a co‑solvent gradient of MeOH containing 20 mM ammonium acetate from 5% to 40% over 8 minutes. Under these conditions, the (2S,4R) target elutes at 3.7 minutes, while the (2R,4S) enantiomer separates with an α = 1.12 eluting at 4.1 minutes. The molecular ion [M+H]⁺ at m/z 230.1 confirms integrity of the Boc group, and the absence of a fragment peak at m/z 130.1 (corresponding to decarboxylated pyrrolidine) serves as an in‑process control against thermal degradation during analysis. This method has been validated for repeatability with an RSD of 0.4% for the main peak area across six injections, meeting the system precision requirements of ICH Q2(R2). The tendency of the methyl substituent to suppress non‑specific binding to stainless‑steel autosampler components is notable. Unlike unprotected 4‑methylproline, which can chelate metal ions and cause carry‑over peaks in the subsequent blank injection, the Boc‑protected carbamate eliminates this interference; blank gradient runs consistently return a baseline devoid of ghost peaks at retention times corresponding to either enantiomer. When the analytical method is transferred to a quality‑control laboratory operating under ISO 17025:2017, the intermediate precision study involving a second instrument at site B yields a maximum allowable difference of ±0.03 for the enantiomeric purity determination, a figure that aligns with the pharmacopeial concept of a chromatographic procedure’s reproducibility index.

Microwave‑Assisted Boc‑Deprotection Kinetics in Contrast to Conventional Oil‑Bath Heating

Where standard Fmoc‑SPPS dominates, Boc‑deprotection with TFA is conventionally performed at ambient temperature for 30 minutes. When the synthesis is accelerated using microwave energy, the (2S,4R)-4‑methyl congener requires recalibration of the standard protocol. In instrumented CEM Liberty Blue™ reactors, applying 100 W microwave power to achieve an internal temperature of 38°C reduces the deprotection time to 3 minutes for Boc‑Pro‑OH, but the steric hindrance of the methyl group extends the necessary time for complete removal to 4 minutes 30 seconds at the same power setting. Quantitative ¹⁹F NMR monitoring of the liberated trifluoroacetamide by‑product at −76 ppm (referenced to α,α,α‑trifluorotoluene) confirms that 97% of the Boc groups are cleaved within the first 2 minutes for the unsubstituted proline, while the methylated variant reaches the same conversion only after 3 minutes 45 seconds. This lag is attributed to a hydrophobic pocket formed around the carbamate by the trans methyl group, hindering the approach of charged TFA molecules. Process descriptions for automated peptide synthesizers must distinguish between the deprotection cycle recipes stored for Boc‑Pro and Boc‑(2S,4R)-4‑methylproline, or else a pre‑mature termination of the step will accumulate deletion sequences detectable by mass spectrometry after cleavage.