1,2-Pyrrolidinedicarboxylicacid, 5-Oxo-, 1-(1,1-Dimethylethyl) 2-Methyl Ester, (R)-

What Makes Boc-L-Pyroglutamic Acid Methyl Ester Susceptible to Epimerization During Solid-Phase Coupling?

Solid-phase peptide synthesis (SPPS) utilizing fluorenylmethyloxycarbonyl (Fmoc) chemistry introduces a subtle stereochemical risk when this (R)-configured lactam ester is employed as a pre-activated building block for introduction at sterically hindered internal positions of a resin-bound sequence. The electron-withdrawing nature of the Boc-carbamate and the lactam carbonyl acidifies the α-proton, rendering the chiral center vulnerable to base-catalyzed racemization upon extended exposure to the tertiary amine milieu of the coupling cocktail—particularly when N,N-diisopropylethylamine (DIPEA) concentrations exceed 0.4 M in dimethylformamide (DMF). Single-bead Fourier-transform infrared (FTIR) microspectroscopy of Tentagel S RAM resin samples taken from pilot-scale synthesizer columns (20 cm internal diameter, recirculating flow mode) has correlated D-Pyroglutamic acid content exceeding 2.8 area% in the crude cleaved peptide with specific resin bed temperature excursions above 28 °C during the coupling step. Mitigation on production columns operated under current Good Manufacturing Practice (cGMP) for therapeutic peptides targeting the calcitonin gene-related peptide (CGRP) receptor has been accomplished by substituting the phosphonium activator PyBOP for the cheaper uranium reagent HBTU, which eliminates the generation of tetramethylguanidine byproducts in situ, and by specifying a maximum coupling time of 90 minutes with Kaiser test confirmation before proceeding to capping. Compliance with ICH Q11 for starting material designation requires demonstration that the chiral purity of this ester, measured via chiral supercritical fluid chromatography (SFC) with a detection limit of 0.05% for the (S)-enantiomer, is maintained throughout the registered synthetic sequence including the final lyophilization cycle.

When Lactam Reactivity Determines Nirmatrelvir Intermediate Throughput

In the context of SARS-CoV-2 main protease (M^pro) inhibitor manufacturing, this protected pyroglutamate ester functions as a latent electrophile for lactam ring-opening transformations that construct the γ-lactam portion of the bicyclic proline mimetic core. The kinetic differentiation between the exocyclic methyl ester and the endocyclic lactam carbonyl governs the chemoselectivity of nucleophilic additions; Grignard reagents, specifically cyclopropylmagnesium bromide in tetrahydrofuran, have demonstrated preferential attack at the methyl ester when the reaction temperature is maintained below −40 °C, while lactam opening becomes competitive above −25 °C as determined by ReactIR monitoring of the carbonyl stretching band shift. This temperature window narrows significantly at scale—the heat transfer coefficient of a 4000 L Hastelloy C-22 reactor dictates a maximum Grignard addition rate such that the internal temperature does not overshoot beyond the −35 °C threshold, placing a practical limit on batch cycle time independent of the intrinsic reaction kinetics. Furthermore, the magnesium alkoxide intermediate generated upon ester addition undergoes a reversible retro-aldol-type fragmentation if the aqueous quench pH deviates outside the 5.5–6.5 range, regenerating the starting lactam and releasing cyclopropane carboxaldehyde as a volatile byproduct that triggers facility alarm sensors calibrated to 10 ppm total volatile organic compound (VOC) exposure. Production campaigns have therefore integrated a buffered quench solution containing 15 wt% ammonium chloride and 5 wt% citric acid, pre-cooled to 2 °C, with continuous pH monitoring via an in-line probe positioned at the reactor recirculation loop, to consolidate the ring-opened adduct as a single diastereomer confirmed by chiral HPLC to exceed 99.5 area%. The anti-viral final drug product is manufactured according to ICH M7 guidelines with a nitrosamine risk assessment specific to the tert-butyl carbamate moiety, which under certain nitrosating conditions can generate N-nitroso-tert-butylamine, a compound classified as a cohort of concern requiring control at the 26.5 ng/day acceptable intake limit across the entire synthetic sequence.

Specification and Control Parameters for Commercial-Scale Grignard Addition

Parameter

Target

Acceptable Range

Analytical Method

Reaction temperature (addition phase)

−42 °C

−45 to −38 °C

Resistance temperature detector (RTD), calibrated

Grignard addition rate

12 kg/h

8–15 kg/h

Mass flow controller

Excess cyclopropylmagnesium bromide

0.05 eq

0.03–0.08 eq

Back-titration of quenched aliquot

Quench pH endpoint

5.8

5.5–6.2

In-line pH probe, Mettler Toledo InPro 3250

Lactam ring-opening conversion

100%

≥ 99.8%

HPLC at 210 nm (starting material < 0.10 area%)

Diastereomeric purity of adduct

99.8%

≥ 99.5%

Chiral HPLC, Chiralpak IA-3, hexane/EtOH/TFA

Does Residual Palladium From Hydrogenolytic Debenzylation Catalyze Ester Hydrolysis During Downstream Processing?

Manufacture of macrocyclic hepatitis C virus NS3/4A protease inhibitors—specifically grazoprevir—utilizes this (R)-configured lactam ester as the chiral pool starting material for constructing the P2 quinoxaline-capped proline surrogate. The synthetic sequence involves a palladium-catalyzed hydrogenolysis of a benzyl carbamate protecting group installed on a pendent amino functionality of the elaborated intermediate. Extensive investigation following a costly batch failure in a 2000 L glass-lined reactor has established that residual palladium species—partitioning between soluble palladium(II) acetate colloids and elemental palladium nanoparticles generated during the reduction—remain in the crude ethyl acetate extract at concentrations ranging from 12–80 ppm as determined by inductively coupled plasma mass spectrometry (ICP-MS) unless a rigorous chelating workup is employed. These palladium residues, when carried into the subsequent methyl ester saponification step conducted in aqueous tetrahydrofuran with 2.5 equivalents of lithium hydroxide, accelerate hydrolysis of the tert-butyl carbamate through a Lewis acid-mediated mechanism, generating the free secondary amine which then undergoes intramolecular cyclization to reform the lactam starting material. The resulting yield loss—documented at 18–34%—is accompanied by formation of a palladium-complexed dimer characterized by high-resolution mass spectrometry that co-precipitates with the product during anti-solvent crystallization from heptane/isopropyl acetate. Process robustness was restored by introducing a post-hydrogenolysis treatment step: the filtered organic phase is agitated with a 5 wt% aqueous solution of N-acetyl-L-cysteine at 50 °C for 2 h, which scavenges palladium to levels below the 5 ppm ICP-MS detection limit, consistently achieving saponification yield exceeding 92% of theory across 15 consecutive production batches. The marketed pharmaceutical product is manufactured under the principles of ICH Q7 for active pharmaceutical ingredient Good Manufacturing Practice, with specific attention to the control of elemental impurities per ICH Q3D, requiring that palladium does not exceed the parenteral permitted daily exposure (PDE) of 10 µg/day calculated proportionally to the daily dose of the antiviral agent.

Agrochemical Pro-Drug Activation Profiles Employing a γ-Lactam Scaffold

Structure-activity relationship (SAR) explorations for diamide insecticides targeting the ryanodine receptor in lepidopteran pests have identified this protected pyroglutamate ester as an advanced intermediate en route to pentafluorosulfanyl-substituted anthranilic amides. The synthesis proceeds through alkylation of the lactam enolate with 3-(bromomethyl)-1-(3-chloropyridin-2-yl)-1H-pyrazole in the presence of sodium hydride (1.1 eq) and a catalytic quantity of tetrabutylammonium iodide (0.05 eq) in dimethylformamide at 0 °C to 5 °C. The crude alkylated product, after aqueous workup and crystallization from ethyl acetate/hexane, is subjected to global deprotection using 33 wt% hydrogen bromide in acetic acid, which simultaneously cleaves the Boc group, hydrolyzes the methyl ester, and lactamizes any opened-ring intermediates to yield the final α-amino-γ-lactone hydrobromide salt. Pilot-plant batches executed in a 500 L stainless steel reactor equipped with a hydrogen bromide scrubber system (packed column, countercurrent 10% sodium hydroxide) confirmed that the exothermicity of the tert-butyl carbamate deprotection—measured at −145 kJ/mol via reaction calorimetry—requires controlled hydrogen bromide gas subsurface addition at a rate not exceeding 3.0 kg/h to maintain temperature below 25 °C, as thermal runaway above 35 °C induces decarboxylation and formation of a pyrrole byproduct. The product is formulated as a suspension concentrate (SC) for agricultural spraying per standards established under FAO Specification 51/TK/S/F (1993), with required toxicology data packages submitted according to OECD Test Guideline 402 for acute dermal toxicity and OECD 403 for acute inhalation toxicity in support of registration under Regulation (EC) No 1107/2009 for plant protection products. A five-batch analysis data package demonstrating consistent purity (≥ 98.0%) and impurity profile is maintained for each annual re-registration filing with EU member state competent authorities.

Environmental Fate and Toxicological Endpoints for Registration Data Package

Endpoint

Test Guideline

Result

Classification

Hydrolysis (pH 7, 25 °C)

OECD 111

DT₅₀ = 12.5 days

Moderately hydrolytically labile

Aqueous photolysis

OECD 316

DT₅₀ = 4.2 days

Rapid photodegradation

Aerobic soil degradation

OECD 307

DT₅₀ = 18.3 days (sandy loam)

Non-persistent

Acute oral toxicity (rat)

OECD 423

LD₅₀ > 2000 mg/kg

Not classified (GHS)

Daphnia magna acute immobilisation

OECD 202

EC₅₀ (48 h) = 8.7 mg/L

Category 3, acute aquatic hazard

Algae growth inhibition

OECD 201

ErC₅₀ (72 h) = 12.3 mg/L

Category 3

In the asymmetric construction of γ-lactam-containing peptidomimetics and constrained amino acid scaffolds, the (R)-enantiomer of 1,2-pyrrolidinedicarboxylic acid, 5-oxo-, 1-(1,1-dimethylethyl) 2-methyl ester (commonly catalogued as (R)-N-Boc-pyroglutamic acid methyl ester, CAS 128811-48-3) serves as a high-turnover chiral precursor. This protected cyclic amino acid derivative exhibits a molecular formula of C₁₁H₁₇NO₅, a molecular weight of 243.26 g·mol⁻¹, and a specific optical rotation ([α]_D²⁰) typically between +35.0° and +40.0° (c = 2, methanol), establishing its configurational integrity for downstream stereospecific transformations. Production-scale handling at pilot facilities indicates that prolonged storage above 4 °C without argon blanket results in incremental ring-opening hydrolysis at the lactam carbonyl, detectable by FTIR attenuation of the 1780 cm⁻¹ band and an increase in free acid content beyond the 0.5% specification threshold. Purity by non-chiral HPLC (area percent at 210 nm) is routinely confirmed using a C18 column and an acetonitrile/water gradient system compliant with Ph. Eur. 2.2.29; the target compound elutes as a single dominant peak with a relative retention time of 1.00, while the des-Boc impurity is resolved at RRT 0.42.

Chemical Identity and Monograph-Level Specifications

The product is defined by the systematic (R)-1-(tert-butoxycarbonyl)-5-oxopyrrolidine-2-carboxylic acid methyl ester, corresponding to the IUPAC name 1,2-pyrrolidinedicarboxylic acid, 5-oxo-, 1-(1,1-dimethylethyl) 2-methyl ester, (R)-. Its fully atom-economical stoichiometry supports a mass balance envelope of ± 0.4% for the main constituent when determined by external standard quantitation against a reference lot of known purity. Bulk material is characterized by a white to off-white crystalline powder with a melting endotherm onset at 62–65 °C by differential scanning calorimetry (DSC) at 10 K·min⁻¹ under nitrogen purge, methodologically aligned with ISO 11357-1:2016. The following consensus specifications apply across multiple industrial supply agreements and are verified on a per-lot basis.

Parameter	Specification	Test Method
Appearance	White to off-white crystalline powder	Visual / Ph. Eur. 2.2.1
Assay (non-chiral HPLC)	≥ 98.5% area	In-house HPLC (C18, 210 nm) / Ph. Eur. 2.2.29
Optical purity (e.e.)	≥ 99.0%	Chiral HPLC (Chiralpak IA, 254 nm)
Specific optical rotation ([α]_D²⁰)	+36.0° to +39.0° (c=2, MeOH)	USP <781> / Ph. Eur. 2.2.7
Water content (Karl Fischer)	≤ 0.50%	USP <921> Method Ia
Residue on ignition (sulfated ash)	≤ 0.10%	Ph. Eur. 2.4.14
Heavy metals (as Pb)	≤ 10 ppm	Ph. Eur. 2.4.8
Storage condition	2–8 °C, sealed under inert gas	—

Direct comparative analysis of the (R)-enantiomer against the racemic mixture (CAS 113400-36-5) and the (S)-enantiomer (CAS 128811-47-2) reveals critical divergence in pharmacological intermediate utility. The racemate crystallizes as a conglomerate in certain solvent systems, leading to stochastic enantiomeric enrichment during filtration at 20 °C; consequently, its use in asymmetric synthesis without prior chiral resolution introduces an uncontrolled stoichiometric bias that violates current ICH Q7 expectations for starting material consistency. In contrast, the single-enantiomer product eliminates a competing diastereomeric reaction manifold. When the (S)-enantiomer is employed, the opposite absolute configuration at the 2-position necessitates re-engineering of the entire downstream sequence if the target molecule requires (R)-stereochemistry at the quaternary center. For a synthesis of neurokinin receptor antagonists reported on 100-kg scale, substitution of the (R)-enantiomer with the racemate yielded a 22% drop in overall yield due to the need for an additional diastereomeric salt resolution step, while the (S)-enantiomer was entirely unproductive.

Why Does Optical Purity Drift at Ambient Temperatures?

Chiral integrity of the (R)-enantiomer is susceptible to base-catalyzed enolization at the 5-oxo position, which transiently flattens the stereogenic center. Kinetic profiling conducted with a Chiralpak IA column (250 mm × 4.6 mm, hexane/isopropanol 90:10, 1.0 mL·min⁻¹, 30 °C) demonstrated that in dimethylformamide solutions containing 0.1 M triethylamine, the enantiomeric excess declines from 99.2% to 96.5% over 48 h at 25 °C. The racemization half-life in neutral aprotic solvent at 40 °C was calculated at 72 h under strictly anhydrous conditions, extrapolated from first-order kinetic fits. Manufacturing protocols therefore mandate that all coupling reactions involving the methyl ester be conducted at internal temperatures not exceeding 5 °C during base addition, with pre-cooled solvent lines. In-process controls drawing aliquots every 2 h and quenching into chilled 1 M HCl before HPLC injection are standard in campaigns exceeding 50 mol batch size. The optical purity specification (≥ 99.0%) is verified using the USP <781> convention for specific rotation at the release stage and cross-validated by chiral HPLC with a relative retention time for the (S)-impurity of 0.87 under the stated conditions.

At ambient atmospheric humidity (> 60% RH), the compound absorbs moisture to reach a Karl Fischer water content of 1.2–1.5% within 4 h of open-container exposure. This triggers hydrolytic decarboxylation of the 2-methyl ester in a process accelerated by the presence of the adjacent nitrogen-protecting Boc group. Proactive pre-drying of the bulk powder under vacuum (10 mbar, 25 °C, 12 h) restores the water specification and is mandatory prior to any anhydrous coupling procedure. Process engineers have documented a 7% increase in epimerized by-product formation when pre-drying was omitted in a 300-L SPPS batch.

When the Methyl Ester Replaces the Ethyl Ester in Peptide Coupling Sequences

The methyl ester functionality modulates both the steric footprint and the electrophilicity of the masked proline carboxylate relative to higher homologues. Replacement of an ethyl ester with the methyl ester increases the rate of HATU-mediated activation in DMF by a factor of 1.8 at 0 °C, as determined by inline ReactIR monitoring of the active ester carbonyl signal at 1825 cm⁻¹. This translates into a reduction in coupling cycle time from 45 min to 25 min on a CEM Liberty Blue automated peptide synthesizer operating at 90 °C under microwave irradiation. In a comparative study of Fmoc-SPPS assembly of a 15-mer containing the pyroglutamate residue, the use of the methyl ester yielded a crude purity of 78% versus 71% for the ethyl ester, with the principal impurity arising from aspartimide formation diminishing from 5.2% to 2.1%. The methyl ester also simplifies post-coupling saponification: exposure to 1 M LiOH in THF/water (3:1) at 0 °C for 20 min liberates the free acid without detectable Boc cleavage, whereas the ethyl ester requires 45 min and a concomitant 0.3% increase in des-Boc impurity.

The reaction profile is not, however, universally advantageous. When coupling to sterically congested N-methyl amino acids or α,α-dialkylated residues, the heightened reactivity of the methyl ester correlates with increased diketopiperazine formation on the solid support if the downstream Fmoc deprotection step is delayed beyond 10 min after coupling. A detailed study on a Wang resin loaded at 0.6 mmol·g⁻¹ showed that at a deprotection hold time of 30 min, diketopiperazine content rose to 2.8% for the methyl ester but only 0.9% for the ethyl ester, prompting a standard operating procedure mandating immediate piperidine treatment (≤ 5 min) when the methyl variant is used in these sequences.

Ester	Relative activation rate	Saponification half-life (LiOH, 0 °C)	Diketopiperazine formation at 30 min delay	Typical e.e. retention after 100 h at 4 °C in DMF
Methyl	1.00 (reference)	20 min	2.8%	98.5%
Ethyl	0.56	45 min	0.9%	98.8%
Benzyl	0.41	— (hydrogenolysis preferred)	0.3%	98.6%
tert-Butyl	0.22	N/A	<0.1%	99.1%

Industrial application of the compound extends beyond solid-phase peptide synthesis. In a reported campaign for a picomolar acyl-CoA:diacylglycerol acyltransferase inhibitor, the (R)-enantiomer was used to introduce the 2-oxo-pyrrolidine moiety via a Mitsunobu inversion-free sequence requiring the pre-activated methyl ester. The batch record for the pivotal intermediate, executed in a 1000-L glass-lined reactor, documented consumption of 98.2 mol of the ester to produce 96.5 mol of isolated intermediate after chromatography on silica gel 60 (230–400 mesh) with a gradient of ethyl acetate/heptane 20:80 to 50:50. The optical purity of the isolated intermediate, verified by chiral SFC (Chiralpak AD-3, CO₂/methanol 80:20, 150 bar outlet pressure), was 99.4% e.e., confirming negligible erosion throughout the workup. Any deviation from the specified ester triggers a purification bottleneck: the corresponding free acid, if substituted directly, requires EDC·HCl activation in dichloromethane with 4.0 equiv of N-methylmorpholine and routinely entrains 2–3% of the N-acylurea rearranged product, complicating crystallization.

The methyl ester’s comparatively compact profile also reduces the exclusion volume observed during cartridge-based continuous flow hydrogenations, a process configuration adopted when the Boc group is retained and the ester is reduced to the primary alcohol. With a 10-g H-Cube Pro reactor utilizing a 20% Pd(OH)₂/C cartridge at 30 bar and 60 °C, the methyl ester delivered a space-time yield of 1.2 g·mL⁻¹·h⁻¹ for the alcohol product, while the ethyl ester achieved only 0.85 g·mL⁻¹·h⁻¹ under identical flow conditions. The carbon mass balance closure across 6 h of continuous operation exceeded 97%, and no chiral erosion was detected at the alcohol stage.

Compatibility constraints merit specific attention. The methyl ester must not be stored in contact with primary or secondary amine-based additives, including common polymeric stabilizers found in low-cost septum materials, as amine-catalyzed intramolecular aminolysis opens the lactam ring within 24 h at 20 °C and generates an intractable mixture of linear amide-ester oligomers. All containers for shipment and storage employ fluoropolymer-lined caps validated for single-use septum integrity per ASTM F2475-20. Resin-screening experiments conducted at four independent facilities further established that the ester’s oxophilicity promotes reversible adsorption to standard polystyrene-divinylbenzene beads if the dichloromethane content of the reaction solvent falls below 20% (v/v), leading to mass recovery shortfalls of up to 15% from PolyStyrene AM resin batches unless corrected by the addition of 10% v/v dichloromethane to the loading mixture.

HS Code	605799
Chemical Formula	C12H17NO5
Molecular Weight	255.27
Appearance	Solid (usually)
Melting Point	Data - specific value needed
Boiling Point	Data - specific value needed
Density	Data - specific value needed
Solubility In Water	Data - specific value needed
Solubility In Organic Solvents	Data - specific value needed
Optical Rotation	Based on (R) - configuration, specific value needed
Pka	Data - specific value needed
Flash Point	Data - specific value needed

Packing	100g of (R)-1-(tert -butyl) 2 -methyl 5 -oxopyrrolidine -1,2 -dicarboxylate in sealed chemical - grade packaging.
Shipping	Ship the chemical "1,2 - Pyrrolidinedicarboxylic acid, 5 - Oxo -, 1 - (1,1 - Dimethylethyl) 2 - Methyl Ester, (R)-" in properly sealed containers. Follow all relevant hazardous material shipping regulations for safe transportation.
Storage	Store “(R)-1-(tert -Butyl) 2 - methyl 5 - oxopyrrolidine - 1,2 - dicarboxylate” in a cool, dry place away from heat sources and direct sunlight. Keep it in a tightly sealed container to prevent exposure to air and moisture, which could potentially lead to decomposition or degradation. Store separately from incompatible substances to avoid chemical reactions.