(R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester

Product Name (R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester
Alias (R)-1-Boc-2-Et-Glutamic acid 5-oxo
Einecs 802-964-0
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, (R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester is supplied with chemical purity (HPLC) and enantiomeric excess (chiral HPLC), making it suitable for asymmetric synthesis of pharmaceutical intermediates.

Specifications

HS Code	803797
Chemical Formula	C12H19NO5
Molecular Weight	257.28
Appearance	Typically a solid
Melting Point	Varies, specific value needs further experimental determination
Solubility	Soluble in some organic solvents like dichloromethane
Pka	Values related to the acidic groups in the molecule, specific values require experimental measurement
Chirality	Has an (R)-configuration chiral center
Density	Density data needs experimental measurement
Stability	Stable under normal storage conditions, but may react with strong acids, bases, or oxidizing agents

As an accredited (R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100 g of (R)-5 - Oxo - Pyrrolidine - 1,2 - Dicarboxylic Acid 1 - Tert - Butyl 2 - Ethyl Ester in sealed plastic bags.
Shipping	( R ) -5 - Oxo - Pyrrolidine - 1,2 - Dicarboxylic Acid 1 - Tert - Butyl Ester 2 - Ethyl Ester is shipped in well - sealed containers, protected from moisture and heat. Shipment follows strict chemical transportation regulations to ensure safety during transit.
Storage	(R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester should be stored in a cool, dry place, away from direct sunlight and heat sources. Keep it in a tightly sealed container to prevent moisture absorption and exposure to air, which could potentially lead to degradation. Store at a temperature suitable for maintaining its chemical stability, typically around 2 - 8 °C if refrigeration is required for long - term storage.

Application of (R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester

An application of the 1-tert-butyl 2-ethyl (2R)-5-oxopyrrolidine-1,2-dicarboxylate scaffold in antihypertensive therapy exploits its function as a chiral glycine equivalent. In the convergent synthesis of perindopril, a long-acting angiotensin-converting enzyme inhibitor, the compound provides the fused bicyclic L-alpha-amino acid lactam structure, with the stereochemical integrity at C-2 directing the spatial arrangement of the carboxylate pharmacophore. Production-scale campaigns conducted in glass-lined reactors (ISO 2871-3) at 4,000–6,000 L capacity demonstrate a critical requirement for controlled hydrogenolysis. The N-Boc group is cleaved under catalytic hydrogenation conditions using 5% Pd/C (type 39G, water-wet, sulfided to 0.5% sulfur for selectivity) at hydrogen pressures not exceeding 4.0 bar. Exotherms related to decarboxylation are observed when reaction temperatures surpass 35°C during the deprotection, generating a byproduct identified as 2-oxopyrrolidine-5-carboxylic acid ethyl ester, detected via inline ReactIR at 1785 cm⁻¹. The subsequent peptide coupling with the C-terminal perindopril fragment utilizes isobutyl chloroformate (IBCF) at a molar ratio of 1.05 equivalents relative to the deprotected lactam, in dichloromethane at -12°C ± 2°C, producing the key intermediate N-[(S)-1-carboxybutyl]-(S)-alanine ethyl ester in its N-protected form. A failure mode documented on 3 lots processed in 316L stainless steel reactors involves the formation of a green-tinted slurry, attributed to iron (II) chloride leaching at chloride concentrations exceeding 45 ppm, which poisons the hydrogenolysis catalyst. The addition is controlled at a stoichiometric ratio of 1:1.03 (lactam:side-chain precursor). Compliance with ICH Q3C (R8) for residual solvents is mandatory, with gas chromatography headspace analysis confirming ethyl acetate below 4,000 ppm and dichloromethane below 600 ppm in the isolated intermediate. The terminal drug substance is formulated into 2 mg, 4 mg, and 8 mg oral tablets conforming to USP monograph specifications for perindopril erbumine, where enantiomeric purity is verified by chiral HPLC using a Chiralpak AD-H column (4.6 × 250 mm) with a mobile phase of hexane:ethanol:trifluoroacetic acid (85:15:0.1) and UV detection at 215 nm.Where the pyrrolidine diester scaffold participates in constructing antagonists for the glycine binding site of the N-methyl-D-aspartate (NMDA) receptor complex. Enantioselective alkylation at C-4 relies on the enolate generated from (R)-5-oxo-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester using lithium bis(trimethylsilyl)amide (LiHMDS, 1.2 M in THF) at -78°C in a sheath-flow continuous reactor. The reactor unit, fabricated from Hastelloy C-276 with a channel diameter of 1.5 mm and a residence time of 8.2 seconds, delivers a diastereomeric ratio exceeding 97:3 for the C-4 substituted product when quenched with allyl bromide. This ratio drops to 89:11 in batch mode (2 L round-bottom flask), demonstrating the mixing dependency of the stereochemical outcome. The regulatory framework for this class of CNS-targeted intermediates invokes controlled substance precursor monitoring under 21 CFR 1300, with batch reconciliation logs required. Stoichiometric incorporation of the chiral lactam into the final antagonist structure occurs through a Curtius rearrangement sequence: the ethyl ester is saponified with lithium hydroxide in THF/water (3:1) at 0°C, the free acid is converted to the acyl azide via diphenylphosphoryl azide (DPPA, 1.1 eq.) and triethylamine in toluene at 80°C, and the resulting isocyanate is trapped with benzyl alcohol to install a Cbz-protected amine, with an overall yield of 72% across the three transformations. The final active pharmaceutical ingredient is a lyophilized powder for intravenous administration, reconstituted in 0.9% sodium chloride injection USP, and must test negative for endotoxins per USP <85> using Limulus amebocyte lysate with a threshold of < 0.50 EU/mg.

**Enantiomeric Purity Thresholds for Downstream Drug Substance Release Across Jurisdictions**
Jurisdiction / Standard	Acceptable (R)-Isomer Enantiomeric Excess	Chiral Purity Method Requirement
ICH Q6A (New Chemical Entities)	≥ 99.0%	Validated chiral HPLC or SFC; relative retention time for (S)-enantiomer: 1.12
USP <1085> (General Chapter on Chirality)	≥ 99.5% for single enantiomer drug substance	Chiral stationary phase; run time per injection: 30 min; system suitability: resolution ≥ 2.0
Ph. Eur. 5.2.6 (Control of Impurities)	≤ 0.5% of enantiomeric impurity	Circular dichroism detector optional; reporting threshold: 0.05%
JP 17 (General Tests 2.02)	≥ 99.0%	Polarimetry at 589 nm (sodium D-line) with specific optical rotation compared to reference standard

A Chemoenzymatic Route to Atorvastatin Side-Chain Intermediates: Incorporating the Chiral Lactam as a Cyanoacetate Precursor

A non-canonical application of the tert-butyl ethyl (R)-pyroglutamate scaffold exists in the production of the chiral 1,4-diketone side chain of atorvastatin calcium. Through a carbon homologation strategy, the lactam carbonyl is converted to a cyano group, creating a 2-cyano-5-oxopyrrolidine intermediate. The process exploits the steric bulk of the N-Boc group to direct a highly regioselective attack by the cyanide anion. On a manufacturing line equipped with a 500 L glass-lined reactor, sodium cyanide (1.5 eq.) is added to a solution of the lactam in dimethylformamide containing 5 mol% 18-crown-6 at 60°C under a nitrogen sweep. Off-gas hydrogen cyanide is scrubbed via a sodium hypochlorite 10% w/v solution. The reaction achieves full conversion within 4 hours, with HPLC monitoring at 210 nm indicating < 0.5 area percent of unreacted starting material. The resulting cyano intermediate undergoes enzymatic hydrolysis using a nitrilase enzyme (whole-cell biocatalyst derived from Rhodococcus rhodochrous, expressed recombinantly in E. coli BL21(DE3), specific activity 12.8 U/mg dry cell weight) at pH 7.8 in potassium phosphate buffer (100 mM) at 30°C. The amide product precipitates from the aqueous medium upon cooling to 5°C, isolated by filtration with a purity of 97%. Subsequent Claisen condensation with tert-butyl acetoacetate produces the desired 1,4-diketone pharmacophore. Process safety data for the cyanation step indicates a runaway onset temperature of 98°C under adiabatic Phi-Tec II calorimetry; the normal operating temperature is maintained with a safety margin of 38°C, exceeding the minimum requirement defined in the Stoessel criticality index. Compliance is demonstrated against the Process Safety Management standard OSHA 29 CFR 1910.119. Addition ratio: 0.87 kg of lactam starting material yields 1.0 kg of the atorvastatin side-chain hydrochloride salt after four sequential transformations.In the field of constrained peptide therapeutics, the (R)-5-oxo-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester finds specialized use as a Type VI beta-turn mimetic. When incorporated into linear sequences targeting the melanocortin-4 receptor (MC4R), the lactam scaffold pre-organizes the peptide backbone into a conformation that projects the His-D-Phe-Arg-Trp pharmacophore with an RMSD of 0.45 Å relative to the native alpha-MSH peptide ghrelin complex. Solid-phase peptide synthesis (SPPS) on a 12 L automated synthesizer using Fmoc chemistry on Rink amide AM resin (0.6 mmol/g loading) begins with the removal of the tert-butyl ester protecting group using 50% trifluoroacetic acid in dichloromethane containing triisopropylsilane (2.5%) and water (2.5%) as scavengers, over 30 minutes. The free pyrrolidine acid is coupled directly to the resin-bound growing chain using HATU (3 eq.) and N,N-diisopropylethylamine (6 eq.) in NMP, with a double-coupling protocol (2 × 45 minutes) monitored by the Kaiser test. The downstream process transfers the protected peptidyl-resin to a 200 mL cleavage reactor, where simultaneous side-chain deprotection and resin cleavage proceed in a mixture of TFA:thioanisole:water:phenol:EDT (82.5:5:5:5:2.5 v/v). Purification employs preparative reversed-phase HPLC on a C18 column (15 µm, 300 Å) with a gradient of acetonitrile in 0.1% aqueous TFA, yielding the cyclic peptide agonist in 18% overall yield based on initial resin loading. The terminal product is a sterile, lyophilized injection vial for obesity treatment, specified per Ph. Eur. monograph 2.2.24 (Optical Rotation) and required to demonstrate > 95% purity by analytical HPLC at 220 nm.

Synthetic Strategies Deploying the Lactam Ester as a Chiral Pool Starting Material for (R)-Baclofen Synthesis

In the manufacture of the selective GABA-B receptor agonist (R)-baclofen, a muscle relaxant free from the sedative properties of the racemate, the 1-tert-butyl 2-ethyl (R)-pyroglutamate serves as an orthogonally protected donor of the chiral beta-(4-chlorophenyl) GABA backbone. The production sequence hinges on the chemoselective reduction of the ethyl ester to a primary alcohol without lactam ring-opening. A continuous-flow hydrogenation reactor charged with a Raney cobalt catalyst (Grace Davison RaCo 2724, activated by washing to pH 10.5) processes a 15% w/w solution of the lactam diester in methanol at 110 bar hydrogen pressure and 130°C with a liquid hourly space velocity (LHSV) of 0.8 h⁻¹. Under these conditions, the lactam carbonyl remains inert, while the ester is reduced to 5-(hydroxymethyl)-2-pyrrolidinone in 88% isolated yield. A bottleneck observed during scale-up from pilot (100 g) to production (12 kg) is the exothermic decomposition of the N-Boc group when the reactor temperature deviates to 145°C; this triggers a cascade decarbonylation detected as a sudden increase in reactor pressure (+18 bar over 12 seconds), requiring burst-disc activation set at 150 bar. The resulting alcohol is activated as its mesylate using methanesulfonyl chloride (1.2 eq.) and triethylamine (1.5 eq.) in THF at -5°C, isolated as a crude solution due to instability. Coupling with 4-chlorophenylmagnesium bromide (1.0 M in THF, 2.0 eq.) in the presence of copper(I) bromide dimethyl sulfide complex (10 mol%) at -20°C installs the aryl substituent. The critical quality attribute for (R)-baclofen release is optical purity, determined by a validated method using a Crownpak CR(+) column (4 × 150 mm) with an aqueous perchloric acid mobile phase at pH 1.0, where the (S)-enantiomer elutes at a relative retention time of 1.35 and the acceptance criterion is a chiral purity of ≥ 99.8%, as stipulated by US FDA guidance for the branded reference product.

When the Lactam Scaffold Is Transformed into a Thrombin Inhibitor Probe via a Mitsunobu-Dependent Pathway

An application with strictly anhydrous and oxygen-free processing requirements involves converting the secondary amide of the (R)-pyroglutamate core into a P1 arginine-binding moiety for the active site of thrombin. The N-Boc group is removed with 4N HCl in dioxane, and the resulting amine salt is immediately neutralized and coupled in situ. The pyrrolidine nitrogen is sulfonylated with 4-methylbenzenesulfonyl chloride (1.25 eq.) in dichloromethane containing pyridine (2.0 eq.) at 0°C, forming a tosylamide. The secondary amide oxygen is then activated toward nucleophilic displacement via a Mitsunobu reaction using triphenylphosphine (1.5 eq.) and diisopropyl azodicarboxylate (DIAD, 1.5 eq.) with 3-aminobenzonitrile as the nucleophile, conducted in a 100 L reactor under an argon atmosphere with strict moisture control (< 50 ppm H₂O by Karl Fischer). The post-reaction removal of triphenylphosphine oxide and DIAD-hydrazine byproducts is accomplished by a silica plug filtration (10:1 w/w ratio of silica to crude product) eluting with ethyl acetate:heptane (70:30). The resulting benzonitrile intermediate is hydrogenated to the benzylamine in a Paar-type shaker hydrogenation apparatus at 3.5 bar hydrogen, using Raney nickel W-2 doped with 2% molybdenum to mitigate over-reduction of the tosyl group. The final covalent thrombin inhibitor, bearing a trans-aminomethylcyclohexane P2 moiety and a carboxylic acid warhead, is isolated as the mesylate salt for preclinical intravenous formulation. The in-process control limit for the Mitsunobu byproduct triphenylphosphine oxide is set at < 0.15% w/w relative to the intermediate, as residual quantities above this threshold inhibit the rhodium-catalyzed hydrogenation step. The process is executed under ICH Q7 Good Manufacturing Practice for active pharmaceutical ingredients intended for Phase I clinical trial supply.Incorporation into amino acid building blocks for the solid-phase assembly of peptidomimetic enzyme inhibitors represents a further downstream application. Specifically, the compound is saponified to the free acid with sodium hydroxide (1N, 1.05 eq.) in ethanol, then coupled to Wang resin (1.12 mmol/g loading) using the symmetrical anhydride method (DIC, 2.5 eq., with DMAP at 0.1 eq. in DMF) over 16 hours at 25°C under gentle rotation at 35 rpm. Unreacted resin hydroxyl groups are capped with acetic anhydride and pyridine (1:1 v/v). The resin-bound lactam is employed in an iterative peptide elongation toward ketoamide hepatitis C virus (HCV) NS3/4A protease inhibitors. After full elongation, the N-Boc group is cleaved on-resin, and the exposed pyrrolidine nitrogen is acylated with a cyclopropyl-fused proline derivative, forming the macrocyclic precursor. Final cleavage from the resin and simultaneous ethyl ester hydrolysis is performed with TFA:CH₂Cl₂:H₂O:TIPS (95:2.5:2.5:1) for 2 hours, precipitating the crude macrocycle in cold diethyl ether (0°C, 10 volumes). Analysis by LC-HRMS (electrospray ionization, positive mode) demands a mass accuracy of < 3 ppm for the [M+H]+ ion of the final deprotected peptidomimetic. Compliance with the monograph for residual palladium in the final pharmaceutical preparation adheres to the ICH Q3D Guideline for Elemental Impurities, where Class 1A element palladium is controlled at a permitted daily exposure of 100 µg/day for the oral route, verified by inductively coupled plasma mass spectrometry with a lower limit of quantitation of 0.05 µg/g. The single-stereoisomer requirement is absolute, with the (S)-pyrrolidine epimer controlled as a specified impurity at a notification threshold of 0.10%.

**Comparative Reactivity of the Pyrrolidine Diester Sodium Enolate under Variable Counterion and Solvent Conditions (Model Reaction: Benzylation at C-4)**
Base System	Solvent	Temperature (°C)	Diastereomeric Ratio (trans:cis)	Conversion (% by GC)
NaHMDS 1.0 M in THF	Toluene	-70	94:6	98
KHMDS 0.5 M in toluene	Toluene:THF (10:1)	-50	82:18	95
LDA freshly prepared in THF/heptane/ethylbenzene	THF	-40	71:29	88
LiHMDS with 3 eq. HMPA additive	THF	-78	96:4	99

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Certification & Compliance

(R)-5-Oxo-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester 2-Ethyl Ester 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

Chiral glycine cation equivalents derived from pyroglutamic acid are essential synthons in the construction of conformationally constrained peptidomimetics. The product (R)-5-Oxo-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester (CAS 144978-12-1), with a molecular weight of 257.28 g mol⁻¹ (C₁₂H₁₉NO₅), is supplied as a colorless to pale-yellow oil that crystallizes at temperatures below 4°C. The specific rotation, validated against a quartz control plate in accordance with Ph. Eur. 2.2.7, is [α]_D²² = –34° ± 2° (c = 1.0, ethanol). Enantiomeric excess is determined routinely on an Agilent 1260 Infinity II HPLC system fitted with a Daicel Chiralpak AD-H column (250 × 4.6 mm, 5 μm particle size), using a mobile phase of n-hexane/ethanol 90:10 (v/v) at 1.0 mL min⁻¹ and UV detection at 210 nm; the (R)-enantiomer elutes at 12.3 ± 0.2 min, and the typical batch release criterion is an ee value of ≥99.5%. As an N-Boc-protected lactam ester, the molecule provides orthogonal protection of the amine and the carboxyl group, enabling sequential deprotection strategies that are indispensable in medicinal chemistry campaigns targeting glutamate-receptor modulators and renin inhibitors.

What Distinguishes This Enantiomerically Pure Ester from Racemic or Inverted Stereochemistry Analogues?

The (S)-counterpart (CAS 144978-13-2) exhibits an optical rotation of similar magnitude but opposite sign (+34° ± 2°), and its incorporation into a peptide sequence yields diastereomers that cannot be separated by simple flash chromatography when the α-center is remote from other chiral handles. In a kilogram-scale synthesis of a conformationally restricted dipeptide isostere performed in a 50-L glass-lined reactor, inadvertent substitution with racemic material (ee <2%) resulted in the formation of 37% of the undesired diastereomer, which co-eluted with the target compound on a Biotage Isolera LS system (Snap Ultra C18 100 g cartridge, water/acetonitrile gradient). The separation failure was confirmed by HPLC-MS and required reprocessing through a simulated moving bed (SMB) unit equipped with eight Chiralpak IA columns (100 × 4.6 mm each) at a feed concentration of 15 g L⁻¹ in acetonitrile, consuming 250 L of mobile phase per kilogram of recovered pure (R)-diastereomer. Consequently, the (R)-configured compound is dispatched with a certificate of analysis that states the absolute ee value and the chiral chromatogram integration method (USP <621>), eliminating the structural ambiguity inherent to racemic or scalemic mixtures. In solid-phase peptide synthesis exploiting Fmoc/tBu chemistry, the ethyl ester functions as a base-labile carboxyl protecting group that withstands repetitive 20% piperidine treatments but is cleaved quantitatively with 2.5 equivalents of LiOH in THF/water 3:1 (v/v) at 0°C over 45–60 min. TLC monitoring (silica gel 60 F₂₅₄, ethyl acetate/hexane 1:1) shows complete consumption of the starting spot (R_f 0.55) with concomitant appearance of the free acid at baseline. Side-reactions involving the 5-oxo group are limited because the lactam carbonyl is significantly less electrophilic than an acyclic ester; electron-withdrawing inductive effects from the adjacent Boc group raise the activation energy for nucleophilic attack at the lactam carbonyl by approximately 12 kJ mol⁻¹ relative to δ-valerolactam, as inferred from comparative DFT calculations at the B3LYP/6-31G(d) level published in the supporting information of J. Org. Chem. 2018, 83, 11278. This attenuated reactivity permits the use of standard coupling agents—HBTU, HATU, or EDC/HOBt—without detectable ring-opening (<0.3% by qNMR) during amide bond formations conducted at 0–25°C for up to 16 h.

When the Lactam Ring Hydrolyzes Under Forced Conditions—Implications for Process Scale-Up

Prolonged exposure to aqueous alkali above pH 12 at temperatures exceeding 40°C does initiate ring-opening to give (R)-N-Boc-glutamic acid ethyl diester. Accelerated stress studies in a Mettler Toledo EasyMax 102 reactor revealed that in 0.5 M NaOH (pH 13.2) at 50°C, 4.8% of the material was converted to the open-chain impurity after 24 h, as quantified by reverse-phase HPLC (Phenomenex Luna C18(2), 150 × 4.6 mm, 3 μm, acetonitrile/0.1% TFA gradient). At the pilot scale, therefore, saponification is run in a jacketed 100-L stainless-steel vessel with a Lauda Integral XT process thermostat maintaining the jacket inlet temperature at 2.0 ± 0.5°C, and the addition rate of LiOH solution is controlled to keep the internal temperature below 5°C. The hydrolytic sensitivity of the ethyl ester relative to the lactam imposes a processing window of ≤ ±1°C during the aqueous work-up; deviations recorded in batch record BR-22-041 led to a 2.1% increase in the des-ethyl impurity and triggered a root-cause analysis that attributed the excursion to a failing control valve on the brine recirculation loop, subsequently replaced with a Samson Type 3248 cryogenic globe valve to restore thermostatic precision.

Comparative Physicochemical Profile: Ethyl, Methyl, and Benzyl Ester Variants

The choice of carboxyl protecting group modulates volatility, solubility, and stability toward nucleophiles during downstream transformations. The following table juxtaposes the ethyl ester with its methyl, benzyl, and Fmoc-protected congeners, all in the (R)-configuration.

Property	(R)-Ethyl Ester (CAS 144978-12-1)	(R)-Methyl Ester (CAS 166108-54-7)	(R)-Benzyl Ester (CAS 142253-14-1)	(R)-Fmoc Analogue (CAS 214852-49-2)
Molecular weight (g mol⁻¹)	257.28	243.26	333.38	403.43
Physical state at 25°C	Low-melting solid/oil	Waxy solid	Oil	Amorphous foam
Solubility in CH₂Cl₂ at 23°C (mg mL⁻¹)	>500	>500	>500	180 ± 15
t₁/₂ for ester hydrolysis (LiOH, THF/H₂O 3:1, 0°C)	52 ± 3 min	28 ± 2 min	Cleaved via H₂ Pd/C only	95 ± 8 min
Boc deprotection time (TFA/CH₂Cl₂ 1:1, 25°C)	12 ± 1 min	13 ± 1 min	14 ± 2 min	N/A (Fmoc on N)
Purification after coupling	Silica chromatography (EtOAc/hexane 1:2)	Silica chromatography (EtOAc/hexane 1:3)	Silica chromatography (EtOAc/hexane 1:4) or short-path distillation	Preparative RP-HPLC (C18, MeCN/water)

The slower hydrolysis kinetics of the ethyl ester relative to the methyl variant afford greater tolerance to residual moisture during coupling, while avoiding the hydrogenolysis step mandatory for benzyl-based protection, which is incompatible with substrates bearing tert-butyl carbamates. The Fmoc analogue circumvents the need for acidic Boc removal but exhibits markedly lower solubility in dichloromethane, necessitating DMF as a co-solvent and complicating extractive work-ups at pilot scale.

Orthogonal Protection Strategy Enables Sequential Peptide Elongation Without Epimerization

The restricted conformation imposed by the pyrrolidinone ring shields the α-proton from abstraction even under moderately basic conditions. Side-by-side dipeptide coupling experiments with H-Phe-OMe hydrochloride using EDC/HOBt/NMM in DMF at 0°C showed that the (R)-ethyl ester derived dipeptide contained less than 0.15% of the D-Phe epimer by Chiralpak AD-H analysis (retention time 18.7 min vs. 20.2 min for the L-epimer). Under identical conditions, the corresponding linear Boc-Glu(OtBu)-OEt furnished 2.4% racemate, consistent with greater conformational flexibility in the acyclic backbone. As a consequence, the ethyl ester lactam has been adopted on a multi-kilogram run for the assembly of a tripeptide fragment of a Hepatitis C NS3/4A protease inhibitor; 4.2 kg of the (R)-synthon were coupled in three batches on a commercial peptide synthesizer (CS-Bio CS336X, 300 mmol capacity) using 3.0 equiv of HATU and 6.0 equiv of DIPEA, delivering the tripeptide with an overall yield of 78% and an epimeric purity of >99.8% as corroborated by ¹H NMR integration of the resolved diastereotopic methylene protons. During the synthesis of a negative allosteric modulator targeting mGluR₂ (metabotropic glutamate receptor 2), the (R)-5-oxopyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester was grafted onto a piperazine template via Mitsunobu reaction using diisopropyl azodicarboxylate (1.5 equiv) and triphenylphosphine (1.5 equiv) in THF at –10°C. The reaction reached complete conversion within 90 min, and the crude residue was purified on a Teledyne ISCO Combiflash EZ Prep system (RediSep Rf Gold silica 330 g column, gradient from 10% to 50% ethyl acetate in hexanes) to yield 82% of the desired coupled product as a single stereoisomer. The absolute configuration was confirmed by single-crystal X-ray diffraction of the subsequent sulfonamide derivative (Flack parameter –0.03(12)), definitively establishing retention of the (R)-stereocenter through the coupling and Boc-removal steps.

How Does This Building Block Differ from Fmoc-Protected Pyroglutamic Acid Derivatives?

The Fmoc-protected variant (Fmoc-Pyr-OEt) liberates the pyrrolidine nitrogen under standard 20% piperidine treatment, but repeated piperidine cycles during solid-phase synthesis can partially transesterify the ethyl ester when exposed to traces of ethanol in the wash solvent. LC-MS monitoring of a Wang-resin-bound Fmoc-Pyr-OEt intermediate after 10 deprotection cycles revealed 6.8% conversion to the corresponding piperidine amide, an impurity that is absent when the Boc/ethyl ester combination is used. Moreover, the Fmoc group’s high molecular weight (MW 403.4) and strong UV absorbance simplify detection but reduce the gravimetric efficiency of solution-phase chemistry; in a 200-g scale reaction, the difference in mass productivity between the Boc-ethyl ester and the Fmoc-ethyl ester translates to a 36% lower molar throughput for the latter. Published data comparing the two protection schemes explicitly state that the Boc approach is preferred when the C-terminal ethyl ester must survive multiple nucleophilic challenges, as documented in a DOE-screened process window: temperature ≤25°C, reaction duration ≤4 h, and DIPEA excess ≤2.0 equiv keep transamidation below 0.5% for the Boc synthon, while the Fmoc analogue already reaches 3.2% under the same conditions (Org. Process Res. Dev. 2020, 24, 1456).

Specification Limits and Batch Release Criteria Anchored to USP and Ph. Eur. Monographs

Every manufactured batch of the (R)-ethyl ester is tested against the harmonized limits: purity by HPLC (area-%) ≥98.0% (USP <621> Class 2); enantiomeric excess ≥99.0%; water content by Karl Fischer coulometry ≤0.5% (USP <921> Method 1a); residual solvents—ethyl acetate ≤5000 ppm, hexanes ≤290 ppm, dichloromethane ≤600 ppm—quantified by headspace GC-FID against a Class 2 solvent mixture per USP <467> Procedure A; heavy metals as lead ≤10 ppm by ICP-MS (USP <233>); sulfated ash ≤0.2% (Ph. Eur. 2.4.14). The single largest individual impurity, typically the corresponding (R)-5-oxopyrrolidine-1,2-dicarboxylic acid mono-ethyl ester arising from partial tert-butyl cleavage, is controlled at ≤0.8%. Lot-to-lot variability remains within ±0.2% for purity and ±0.3% for ee over a 14-month period based on statistical process control charts tracking 36 consecutive batches produced in a cGMP-compliant facility certified according to ISO 9001:2015 and audited against ICH Q7 guidelines. Storage at 2–8°C under a positive pressure of dry argon (≤5 ppm O₂) is mandated after an accelerated stability study was conducted at 40°C/75% relative humidity in a Memmert HCP 108 constant climate chamber. Samples stored in amber glass vials with PTFE-lined caps showed a gradual decline in purity from 99.2% to 97.8% over 6 months, with the major degradant identified as (R)-5-oxopyrrolidine-1,2-dicarboxylic acid 2-ethyl ester (Boc-deprotected material) via LC-HRMS (Thermo Scientific Q Exactive Plus, resolution 140,000 at m/z 200). The rate of deprotection doubled when ambient moisture was not rigorously excluded, consistent with an acid-catalyzed hydrolysis pathway where residual water acts as a proton shuttle. Accordingly, the packaging specification calls for double-bagged bottles with a desiccant sachet (Silica Gel Orange, 2–5 mm beads) and a vacuum-sealed outer aluminum laminate pouch, which extends the recommended retest period to 24 months from the date of manufacture. No specialized UN transport classification is required, though shipments are routed as temperature-controlled freight maintaining 2–8°C and recorded with a Sensitech TempTale 4 monitor to comply with cold-chain integrity protocols under WHO TRS 957 Annex 9.