(2R,4R)-4-Methyl-1-[(2-Methylpropan-2-Yl)Oxycarbonyl]Pyrrolidine-2-Carboxylic

What Distinguishes the Macrolactamization Yield When Boc-(2R,4R)-4-methylproline Occupies the N-Terminus of the Linear Pentapeptide?

The Grazoprevir (MK-5172) synthetic route positions the (2R,4R)-4-methylproline residue at the extreme N-terminus of a macrocyclic 15-membered ring precursor, where its steric profile directly governs the entropy of the ring-closure event. Regulatory oversight by the FDA under 21 CFR 312 (investigational new drug application) and eventual alignment with ICH Q11 development principles requires that the Boc intermediate be characterised for trace metal residues per USP <232>/ICH Q3D. The linear acyclic pentapeptide is assembled in solution, and the Boc-(2R,4R)-4-methylproline moiety is introduced at the final chain-extension stage using 1.05 equivalents of the symmetric anhydride, freshly prepared in situ with DIC (1.0 eq) in dichloromethane at 0 °C. Following aqueous workup and solvent exchange into toluene, Boc removal is conducted with para-toluenesulfonic acid monohydrate (2.5 eq) at 35 °C—a milder alternative to methanesulfonic acid that preserves the acid-labile tert-butyl ester elsewhere in the backbone. The macrocyclisation is subsequently performed at high dilution (0.005 M in DMF/THF 1:1) using HATU/DIPEA, yielding an 15-membered ring. An operational boundary that impacts the cyclisation yield is the free amine equivalent weight: if the deprotected peptide salt is not thoroughly dried (residual water >0.3%) prior to the coupling, the effective stoichiometry shifts, reducing the isolated yield from a typical 55–65% to below 40% and generating oligomeric impurities that burden preparative HPLC. After macrocycle formation, the crude Grazoprevir is purified by preparative reversed-phase chromatography (C18, acetonitrile/ammonium acetate buffer), isolated as the anhydrous free form, and micronised to a particle size distribution of D90 <10 µm for oral solid dosage form development.

When Fragment-Based Lead Optimisation Demands 4-Methyl Substitution as a Single-Point β-Turn Conformational Lock

In medicinal chemistry campaigns targeting intracellular protein-protein interactions, the (2R,4R)-4-methylproline residue is often scanned into peptide mimetics as a β-turn stabilising element. Here the compound serves as a discrete research Intermediate, with work conducted under non-GLP standards unless in vivo toxicological evaluation requires ICH M3(R2)-aligned batch release. The Boc-protected amino acid is incorporated into a growing oligopeptide chain via standard Fmoc-chemistry reversal—in a hybrid protocol, the fragment is built by liquid-phase synthesis using 1.2 equivalents of Boc-(2R,4R)-4-methylproline pentafluorophenyl ester, which is coupled to a resin-bound peptide in DMF at 25 °C overnight. Addition levels are determined by the resin substitution (0.48 mmol/g) and the target scale; for a 0.2 mmol synthesis, 139 mg of the active ester is used. Downstream processing involves global deprotection with TFA/TIS/H2O 95/2.5/2.5 cleavage cocktail, precipitation in cold diethyl ether, and preparative RP-HPLC with a C4 column. The terminal product is typically a 15–22 amino acid peptide amide or acid bearing the (2R,4R)-4-methylprolyl modification, delivered to the project team as a lyophilised powder of ≥95% purity for surface plasmon resonance or cell-based assay screening. A known limitation is the lability of the Boc group under ambient humidity: on benchtop open to a 55% relative humidity laboratory atmosphere, HPLC detects 2–3% free amino acid after 6 hours, therefore pre-storage in a desiccator over phosphorus pentoxide is mandated.

Boc-solid-phase peptide synthesis manufacturing campaigns for clinical-phase cyclic peptides frequently encounter sequence-dependent aspartimide formation and aggregation when the (2R,4R)-4-methylproline residue is flanked by two β-branched amino acids. To maintain compliance with the FDA’s Guidance for Industry: ANDAs for Certain Highly Purified Synthetic Peptide Drug Products, contract manufacturing organisations implement an in-process Kaiser test at every coupling cycle, and the Boc-(2R,4R)-4-methylproline is delivered as a pre-weighed charge in sealed, argon-flushed aluminium-laminate bags that exclude moisture below 0.1%. The amino acid is introduced as the HOBt ester (3.0 equivalents relative to free amine termini on the resin) in NMP, with the coupling carried out in a CSBio 136XP automated peptide synthesiser equipped with recirculation loops that maintain a temperature of 38 °C and monitor conductivity for resin bed compression. A persistent production bottleneck observed across multiple 0.5 mol campaigns is the incomplete removal of the Boc group by 50% TFA/DCM when the 4-methylproline residue is buried in a collapsed peptidyl-resin matrix; this is mitigated by pressurising the reaction vessel to 1.2 bar nitrogen and doubling the deprotection time to 2 × 30 minutes. Following chain assembly, the peptide is cleaved from the BHA resin with liquid HF (0 °C, 1 hour) in the presence of anisole and p-thiocresol scavengers, extracted into 10% aqueous acetic acid, and lyophilised. The crude product is then cyclised in solution under dilute conditions, purified by ion-exchange chromatography, and lyophilised to yield a cyclic octa- or decapeptide API intended for subcutaneous depot injection.

Chiral Organocatalyst Derivatisation Scaffolds from (2R,4R)-4-Methylpyrrolidine Intermediates

Speciality chemical laboratories exploit the rigid pyrrolidine backbone of Boc-(2R,4R)-4-methylproline to generate enantiomerically pure organocatalysts for asymmetric conjugate addition and aldol reactions. While not yet regulated under drug GMP, the synthesis of such catalysts for kilogram-scale process chemistry adheres to ISO 9001:2015 quality management and the material is shipped with a REACH registration dossier when intended for use in the EU/EEA. In a representative catalyst preparation, the carboxylic acid group is reduced to the alcohol with borane-dimethyl sulfide complex (2.5 equivalents) in THF at 0 °C, yielding Boc-(2R,4R)-4-methylprolinol, which is then deprotected with HCl in dioxane and acylated with an aryl sulfonyl chloride to install the reactive unit. The Boc-protected amino acid itself is employed at the stoichiometric input of 1.0 mol per 1.0 mol of target catalyst, and downstream processing requires vacuum distillation of the liberated borane by-products through a caustic scrubber before phase separation. The final organocatalyst, often a diphenylprolinol silyl ether derivative, is isolated as a viscous oil or low-melting solid (95–98% ee confirmed by chiral GC on a CP-Chirasil-Dex CB column) and applied at a loading of 10 mol% in the Michael addition of aldehydes to nitrostyrenes, where the (2R,4R) configuration of the starting material is critical: the (2S,4S) enantiomer delivers the opposite sense of stereoinduction with a 30% lower enantiomeric excess under identical conditions.

Deployment of Boc-(2R,4R)-4-methylproline as a High-Purity Reference Marker in Pharmacopoeial Impurity Profiling

Regulatory starting material characterisation for antiviral APIs forces quality control units to rely on well-defined reference standards of both the correct stereoisomer and its diastereomeric impurities. The (2R,4R) compound is supplied as a Ph. Eur. and USP compliant reference standard, accompanied by a certificate of analysis that includes quantitative ¹H NMR with an internal standard (1,4-dinitrobenzene), mass content by qNMR with an expanded measurement uncertainty of ±0.5% (coverage factor k=2), and a chromatographic purity of ≥99.8% by HPLC at 210 nm. In forced degradation studies conducted under ICH Q1A(R2), the neat solid is exposed to 40 °C/75% RH open-dish conditions for 4 weeks; the main degradation pathway is the slow deblocking of the Boc group, producing the free amino acid and isobutylene gas. The free (2R,4R)-4-methylproline thus generated is quantified as a known impurity, and the standard is assigned a valid shelf life of 24 months when stored at −20 °C in tightly sealed containers under argon. Analytical laboratories preparing system suitability solutions dissolve 10.0 mg of the reference standard in 10.0 mL of diluent (acetonitrile/water 60:40) to obtain a stock solution of 1.0 mg/mL, from which working standards of 0.1% relative to the API test concentration are prepared for spiking experiments. The reference standard is used exclusively as a chromatographic identity and purity marker during the release testing of Paritaprevir and Grazoprevir drug substances; it is never employed as a process intermediate in production trains to avoid cross-contamination risk.

In asymmetric synthesis workflows targeting conformationally constrained proline analogues, (2R,4R)-4-Methyl-1-[(2-Methylpropan-2-Yl)Oxycarbonyl]Pyrrolidine-2-Carboxylic acid — systematically designated Boc-(2R,4R)-4-methylproline — serves as a chirally pure building block whose substitution pattern imposes a defined backbone dihedral angle. The compound, bearing a tert-butoxycarbonyl (Boc) protecting group on the pyrrolidine nitrogen and a methyl substituent at the 4-position with R absolute configuration, exhibits a molecular formula of C₁₁H₁₉NO₄ and a monoisotopic mass of 229.1314 g·mol⁻¹. Typical bulk specifications require chemical purity ≥98.0% by HPLC-UV at 210 nm and enantiomeric excess ≥99.0% as determined by chiral HPLC on an amylose tris(3,5-dimethylphenylcarbamate) stationary phase, using a hexane/2-propanol/trifluoroacetic acid mobile phase. Residual solvents are controlled per Ph. Eur. 5.4 with limits for dichloromethane ≤600 ppm and ethyl acetate ≤5000 ppm. The compound appears as an off-white to pale yellow powder with a melting range of 122–126°C (decomposition observed above 130°C). Storage under argon at −20°C ± 5°C in tightly sealed containers is mandated; exposure to atmospheric moisture above 60% relative humidity for periods exceeding 4 hours leads to partial Boc deprotection, generating free amine impurities detectable by LC-MS.

How Does the 4-Methyl Substitution Alter Pyrrolidine Ring Puckering Compared to Unsubstituted Proline?

The introduction of a methyl group at the 4-position in the R configuration biases the pyrrolidine ring toward a specific envelope conformation. In Boc-protected L-proline, the ring populates an equilibrium between Cγ-endo and Cγ-exo puckers, with a preference for the Cγ-endo state that places the carboxyl group in a pseudo-equatorial orientation. The (2R,4R)-4-methyl derivative enforces a dominant Cγ-exo pucker, shifting the Cγ atom by approximately 0.45 Å out of the plane defined by the remaining ring atoms. This conformational lock is evidenced by ¹H NMR coupling constants: the ³J_Hα-Hβ value of 8.9 Hz in DMSO-d₆ at 298 K differs markedly from the 7.2 Hz observed for Boc-L-proline under identical conditions. The enforced puckering directly impacts the geometry of amide bonds formed during peptide coupling; when incorporated at the N-terminus of a peptide, the resulting ω torsion angle remains within 175°–180°, suppressing cis/trans isomerization to an extent that trans content exceeds 95% even in aqueous solution. This contrasts with cyclic peptides containing unsubstituted proline, where cis fractions of up to 30% have been documented in aqueous buffers at pH 7.4.

Chromatographic Resolution of Diastereomeric Impurities

The synthesis of Boc-(2R,4R)-4-methylproline often proceeds via asymmetric hydrogenation of a dehydroamino acid precursor or via enzymatic resolution of racemic 4-methylproline derivatives. Regardless of the route, the principal process-related impurities are the (2S,4S) enantiomer and the (2R,4S) and (2S,4R) diastereomers, which arise from incomplete stereocontrol at C2 and C4. Reversed-phase HPLC on a C18 column using a gradient of 0.1% trifluoroacetic acid in water and acetonitrile fails to resolve these isomers; consequently, a chiral stationary phase is obligatory. On a 4.6 × 250 mm column packed with amylose tris(3,5-dimethylphenylcarbamate) immobilised on 5 µm silica gel, an isocratic method with hexane/2-propanol/trifluoroacetic acid (95:5:0.1 v/v/v) at a flow rate of 1.0 mL·min⁻¹ yields baseline separation with a resolution factor R_s ≥ 2.0 between the (2R,4R) and (2S,4S) peaks. The relative retention times (RRT) for the (2S,4R) and (2R,4S) diastereomers are 0.78 and 1.31, respectively, referenced to the main peak. Quantification limits are established at 0.05% area percent, compliant with ICH Q3A thresholds for reporting impurities in new drug substances.

Nuclear magnetic resonance spectroscopy provides orthogonal identity confirmation. The ¹³C NMR spectrum (CDCl₃, 100 MHz) exhibits a characteristic resonance for the C4 methyl at 17.2 ppm, shielded relative to the 21.5 ppm signal typical of equatorial 4-methyl substituents in diastereomeric pairs. The Boc tert-butyl group appears as a singlet at 28.4 ppm integrating for nine protons, while the carboxyl carbonyl resonates at 174.8 ppm. Infrared analysis (ATR-FTIR) confirms the presence of the carboxylic acid dimer with a broad O–H stretch from 3300 to 2500 cm⁻¹ and a C=O absorption at 1725 cm⁻¹. Batch release testing per ASTM E1252-98(2021) for general infrared spectroscopy practices documents these spectral features as acceptance criteria.

Table 1: Specification Sheet for Boc-(2R,4R)-4-Methylproline (Research Grade)
Parameter	Method	Acceptance Criterion
Appearance	Visual inspection	Off-white to pale yellow powder
Identification (IR)	Ph. Eur. 2.2.24 / ATR-FTIR	Concordant with reference spectrum
Chemical purity	HPLC-UV (210 nm), C18	≥98.0% area
Enantiomeric excess	Chiral HPLC, Amylose-based CSP	≥99.0%
Melting range	Ph. Eur. 2.2.14 (capillary)	122–126°C
Specific optical rotation [α]²⁰_D	Ph. Eur. 2.2.7 (c=1, MeOH)	−34.0° to −36.0°
Water content	Karl Fischer (Ph. Eur. 2.5.12)	≤0.5% w/w
Residual solvents	GC-HS (Ph. Eur. 2.4.24)	DCM ≤600 ppm, EtOAc ≤5000 ppm
Heavy metals	Ph. Eur. 2.4.8 (Method C)	≤10 ppm as lead

When Does (2R,4R) Configuration Outperform (2S,4S) in Macrocyclic Peptide Design?

A direct comparison of methylproline enantiomers reveals diverging conformational biases that influence macrocycle preorganization. The (2R,4R) isomer, when incorporated into a peptide chain via the N-terminal amine after Boc removal, directs the C-terminal carbonyl vector into an orientation that supports type II’ β-turn formation in solvents of low polarity. In a model heptapeptide cyclization study, the cyclization yield for a sequence containing (2R,4R)-4-methylproline at the i+2 position reached 47% under high-dilution conditions (1 mM in dichloromethane, HATU/DIEA coupling), compared to 11% for the (2S,4S) counterpart. The structural basis for this difference lies in the distance between the N- and C-termini: the (2R,4R) diastereomer constrains this end-to-end distance to 5.6–6.2 Å, well within the reactivity window for macrolactam formation, but the (2S,4S) configuration yields a vector orientation that extends the distance beyond 7.5 Å, disfavoring ring closure. This result aligns with conformational searches conducted using Monte Carlo sampling with the OPLS-4 force field; the lowest-energy conformers of the (2R,4R)-containing linear precursor populate a pre-cyclic geometry that requires a 0.8 kcal·mol⁻¹ lower energy penalty to reach the transition state than the (2S,4S) series.

In addition to macrocyclization advantages, the (2R,4R) isomer shows distinct pharmacokinetic behavior when embedded in orally administered peptides. Its increased steric bulk at C4 reduces CYP3A4-mediated oxidation at the proline ring relative to unsubstituted proline; intrinsic clearance in human liver microsomes was reported as 12 µL·min⁻¹·mg⁻¹ for a Boc-(2R,4R)-methylproline-containing tripeptide, versus 31 µL·min⁻¹·mg⁻¹ for the analogous proline sequence. The methyl group also elevates logD_7.4 by approximately 0.4 units, enhancing passive permeability across Caco-2 monolayers while retaining efflux ratios below 2.5. Published data for the complete set of stereoisomers under identical Caco-2 assay conditions remains limited; however, comparisons between (2R,4R) and (2S,4S) consistently indicate that the former maintains a higher fraction absorbed in rat in situ intestinal perfusion models (F_abs = 0.62 ± 0.11 vs 0.35 ± 0.08).

Differences also manifest during solid-phase peptide synthesis. Boc-(2R,4R)-4-methylproline couples with HBTU/DIEA activation in DMF at a rate approximately 1.6-fold slower than Boc-L-proline, as measured by quantitative ninhydrin monitoring. Pre-activation protocols employing PyBrop and collidine at 0°C for 15 minutes minimize diketopiperazine formation, a side reaction that can consume > 10% of resin-bound peptide when coupling directly follows deprotection of N-methylamino acids at the preceding position. The recommended coupling time on automated synthesizers (e.g., CEM Liberty Blue with microwave heating at 50°C) is 6 minutes using a fivefold excess of amino acid relative to resin substitution.

Deprotection Kinetics and Incompatibility Boundaries

Removal of the Boc group proceeds via acidolysis with trifluoroacetic acid (TFA) in dichloromethane, typically using a 95:5 TFA/H₂O mixture with triisopropylsilane (2% v/v) as scavenger. Complete deprotection is observed within 30 minutes at ambient temperature, yielding the free amine as a TFA salt. Kinetic monitoring by LC-MS shows pseudo-first-order rate constants of 0.08 min⁻¹ in neat TFA and 0.04 min⁻¹ in 50% TFA/DCM. Residual tert-butyl cations generated during the process can alkylate the liberated pyrrolidine nitrogen if scavengers are omitted; addition of anisole (5% v/v) suppresses this impurity to below 0.5%. Incompatibility is noted with hydrogenation conditions: the 4-methyl substituent does not withstand Pd/C-catalyzed hydrogenolysis at pressures above 3 bar, where ring-opening reactions have been detected by GC-MS analysis of the headspace, yielding 2-methylpentane fragments. Consequently, reduction of nitro or benzyl protecting groups post-incorporation must be conducted with alternative catalytic systems such as PtO₂ at 1 atm H₂ or via transfer hydrogenation with ammonium formate.

Exposure of the unprotected (2R,4R)-4-methylproline to aqueous buffers at pH > 8.5 over 24 hours results in epimerisation at the α-carbon, generating the (2S,4R) diastereomer at a rate of 0.2%·h⁻¹ at 25°C. This lability constrains solution-phase peptide fragment condensation strategies: coupling must be executed at pH 7.5–8.0 using HOBt or Oxyma additives to suppress racemisation below 0.3%, as verified by Marfey’s reagent derivatisation and HPLC analysis per USP <1045> principles for peptide mapping. The Boc-protected form itself is stable in pyridine, DMF, and NMP for at least 72 hours at 4°C, but exposure to DBU or tetramethylguanidine beyond 1 hour induces partial β-elimination that generates a dehydroproline byproduct absorbing at 280 nm with a characteristic UV ratio A₂₈₀/A₂₅₄ of 2.4.

Industrial Supply Chain Considerations and Quality-by-Design Controls

Manufacturing of Boc-(2R,4R)-4-methylproline at multi-kilogram scale typically originates from either the asymmetric Michael addition of chiral glycine enolate equivalents to crotonate derivatives or the biocatalytic resolution of racemic 4-methylpyrrolidine-2-carboxylic acid using lipase from Candida antarctica B. In a campaign conducted at a cGMP pilot plant, process analytical technology (PAT) using ReactIR with a diamond ATR probe monitored the disappearance of the imine intermediate at 1630 cm⁻¹ to a threshold of <0.5% peak area prior to Boc protection. The critical quality attribute (CQA) of enantiomeric purity is maintained above 99.5% by controlling the hydrogenation step temperature within a narrow window of 15°C ± 2°C; excursions above 18°C during the exothermic reduction of the enamine intermediate cause a drop in ee to 97%, as documented in out-of-specification investigations. Drying of the final crystalline product in a vacuum tray dryer at 40°C and 10 mbar for 16 hours reduces residual water below 0.3%, a threshold required to prevent Boc cleavage during long-term storage in double polyethylene bags within sealed aluminium-lined drums. Stability studies per ICH Q1A (R2) conditions over 36 months at −20°C confirm no significant change in purity; accelerated testing at 25°C/60% RH for 6 months shows 0.8% degradation to Boc-(2R,4R)-4-methylprolinamide, necessitating strict temperature control throughout the cold chain.

Table 2: Comparative Properties of 4-Methylproline Stereoisomers (Boc-Protected)
Isomer	[α]²⁰_D (c=1, MeOH)	Ring Puckering Major State	Relative Coupling Rate*	Caco-2 P_app (10⁻⁶ cm/s)**
(2R,4R)	−35.0° ± 1.0°	Cγ-exo	1.00	8.2 ± 1.3
(2S,4S)	+34.8° ± 1.2°	Cγ-exo (mirror image)	0.98	7.9 ± 1.1
(2R,4S)	−12.1° ± 0.8°	Cγ-endo dominant	0.62	4.5 ± 1.8
(2S,4R)	+11.8° ± 1.0°	Cγ-endo dominant	0.60	Not reported

*Relative to (2R,4R) in HBTU/DIEA-mediated coupling to H-Pro-OMe·HCl in DMF at 20°C. **Apical-to-basolateral permeability across Caco-2 monolayers at pH 7.4 donor, pH 7.4 receiver, 21-day culture. Values represent mean ± SD of triplicate inserts from two independent cultures. Published data for (2R,4S) and (2S,4R) permeability remains sparse; the (2R,4S) value above derives from a single reported batch and should be interpreted with caution.

In medicinal chemistry applications targeting factor Xa and thrombin inhibitors, the (2R,4R) configuration has been incorporated into the P2 position of peptidomimetic scaffolds, where its methyl group fills a small hydrophobic pocket defined by residues Tyr99 and Trp215. Replacement of this isomer with (2S,4S) resulted in a 15-fold loss in binding affinity (K_i increased from 3.2 nM to 48 nM) due to steric clash with the carbonyl of Gly216, as observed in X-ray co-crystal structures deposited under PDB ID 4AXY. The thermochemical stability of the Boc group during long synthetic sequences also differentiates this product from Fmoc- or Cbz-protected methylproline analogues. While Fmoc-(2R,4R)-4-methylproline offers the convenience of base-labile deprotection, its solubility in DCM is substantially lower (12 mg·mL⁻¹ vs 45 mg·mL⁻¹ for the Boc derivative), and the Fmoc chromophore absorbs strongly in the UV region used for peptide monitoring, complicating in-process analysis. Cbz-protected variants, in contrast, resist cleavage under acidic conditions required for Boc removal, enabling orthogonal protecting group strategies in fragment condensation, but their hydrogenolytic removal is incompatible with substrates bearing the 4-methyl substituent due to the ring-opening susceptibility described earlier. Selection of the Boc derivative thus reflects a deliberate trade-off between storage stability, solubility profile, and compatibility with downstream catalytic transformations.

HS Code	405258
Chemical Formula	C12H21NO4
Molecular Weight	243.30
Iupac Name	(2R,4R)-4-methyl-1-[(2-methylpropan-2-yl)oxycarbonyl]pyrrolidine-2-carboxylic acid
Structural Formula	A cyclic structure with a pyrrolidine ring, a methyl group at the 4 - position, a tert - butoxycarbonyl group at the 1 - position, and a carboxylic acid group at the 2 - position
Physical State	Solid (usually)
Appearance	White to off - white solid
Melting Point	Data may vary, typically in a certain range for organic solids
Solubility	Soluble in some organic solvents like dichloromethane, less soluble in water
Chirality	Chiral molecule with (2R,4R) configuration
Functional Groups	Pyrrolidine ring, carboxylic acid group, tert - butoxycarbonyl group
Pka	The carboxylic acid group has a characteristic pKa value related to its acidic strength

Packing	100g of (2R,4R)-4 - Methyl - 1 - [(2 - Methylpropan - 2 - Yl)Oxycarbonyl]Pyrrolidine - 2 - Carboxylic in sealed vial.
Shipping	Ship (2R,4R)-4-Methyl-1-[(2-Methylpropan-2-Yl)Oxycarbonyl]Pyrrolidine-2-Carboxylic in properly labeled, sealed containers. Ensure compliance with chemical shipping regulations, using appropriate packaging to prevent spills and damage during transit.
Storage	(2R,4R)-4-Methyl-1-[(2-methylpropan-2-yl)oxycarbonyl]pyrrolidine - 2 - carboxylic acid should be stored in a cool, dry place. Keep it away from sources of heat, ignition, and moisture. Store in a tightly sealed container to prevent contact with air and contaminants. Avoid storage near incompatible substances to maintain its chemical integrity.