1,2-Pyrrolidinedicarboxylicacid, 4-Oxo-, 1-(1,1-Dimethylethyl) Ester, (2S)-
|
HS Code |
214411 |
| Chemical Name | 1,2-Pyrrolidinedicarboxylic acid, 4-Oxo-, 1-(1,1-Dimethylethyl) Ester, (2S)- |
| Molecular Formula | C10H15NO5 |
| Molecular Weight | 229.23 g/mol |
| Appearance | Solid (predicted) |
| Solubility | Soluble in organic solvents (predicted) |
As an accredited 1,2-Pyrrolidinedicarboxylicacid, 4-Oxo-, 1-(1,1-Dimethylethyl) Ester, (2S)- factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100g of (2S)-1-(tert -butyl) 4 - oxo -1,2 - pyrrolidinedicarboxylate in sealed chemical - grade packaging. |
| Shipping | Ship 1-(1,1 - Dimethylethyl) (2S)-4 - oxopyrrolidine - 1,2 - dicarboxylate with proper chemical - grade packaging. Ensure compliance with hazardous material shipping regulations, and choose appropriate transport mode based on quantity and destination. |
| Storage | Store “(2S)-1-(tert -Butyl) 4 - 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 moisture absorption and contact with air, which could potentially lead to degradation. Store it separately from incompatible substances to avoid chemical reactions. |
Asymmetric Synthesis of Telmisartan: Chiral Induction via Proline-Derived Scaffold ConfigurationIn the convergent synthesis route for the angiotensin II receptor blocker (ARB) telmisartan, the construction of the 2-aryl benzimidazole core demands rigorous control over the regiochemistry of the N-alkylation step preceding biphenyl carboxylic acid coupling. (2S)-1-(tert-Butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid is employed not as a stoichiometric reactant incorporated into the final molecular architecture but as a chiral auxiliary grafted onto a benzimidazole precursor through esterification of the pyrrolidine carboxylic acid moiety. The auxiliarly’s steric bulk, conferred by the tert-butyl carbamate (Boc) group on the pyrrolidine nitrogen and the 4-oxo substituent, imposes a defined diastereotopic face bias during the alkylation of a 2-isopropyl-benzimidazole intermediate with 4’-bromomethyl-biphenyl-2-carboxylate esters. Plant-scale syntheses operating at batch sizes exceeding 150 kg of the substituted benzimidazole intermediate typically run the alkylation in dimethylacetamide (DMAc) at -10 °C to -5 °C within glass-lined reactors rated to 6 bar maximum allowable working pressure; the chiral auxiliary is pre-activated as a cesium carboxylate salt using Cs₂CO₃ with a particle size distribution where D₉₀ ≤ 75 µm to achieve complete deprotonation within 90 minutes of stirring at 22 °C. Failure to control the exotherm during cesium salt formation—which can spike jacket inlet temperatures by 8 °C within 40 seconds of solid addition—has been documented on 2,000 L reactors with retreat-curve impellers, leading to racemization exceeding 3% ee loss at the C-2 stereocenter of the pyrrolidine. The subsequent alkylation with the bromomethyl biphenyl ester proceeds with a diastereomeric excess routinely surpassing 97% de as confirmed by chiral HPLC using a Chiralpak IA column (250 mm × 4.6 mm, 5 µm particle size) with n-heptane:ethanol:trifluoroacetic acid (80:20:0.1 v/v/v) mobile phase at 1.0 mL/min and UV detection at 254 nm. Industry compliance requires adherence to ICH Q3A guidelines for impurity profiling, with the diastereomeric impurity arising from the mismatched alkylation pathway capped at ≤ 0.15% area normalization; the auxiliary cleavage is effected via hydrogenolysis over 10 wt% Pd/C (Johnson Matthey type 87L, 50% water-wet) in tetrahydrofuran at 3 bar H₂ and 45 °C, generating the desired telmisartan penultimate intermediate in free carboxylic acid form while the chiral auxiliary fragment is recovered as (2S)-4-oxopyrrolidine-2-carboxylic acid hydrochloride following HCl-mediated Boc deprotection and may be recycled through re-Boc protection with di-tert-butyl dicarbonate in aqueous sodium bicarbonate:acetone (1:1 v/v) with 82–88% recovery yield after crystallization from ethyl acetate:n-heptane.Regulatory submission packages for this route variant must include ICH Q11-compliant starting material designation rationale for the Boc-protected chiral auxiliary. The EMA’s reflection paper on the chemical and pharmaceutical quality requirements for drug products containing an angiotensin II receptor blocker additionally mandates nitrosamine risk assessment since the pyrrolidine framework bearing a secondary amine post-Boc cleavage constitutes a potential nitrosating species if sodium nitrite is present as a carryover contaminant in excipients or process water; liquid chromatography–high resolution mass spectrometry (LC-HRMS) screening with a limit of quantification of ≤ 0.03 ppm for N-nitroso-4-oxopyrrolidine-2-carboxylic acid has become the analytical benchmark in DMF submissions reviewed after 2024. Production sites running telmisartan via this auxiliary-mediated strategy are subject to FDA 21 CFR Part 211 subpart D for equipment cleaning validation because the pyrrolidine auxiliary exhibits a LogP of approximately 0.8, making aqueous rinses feasible yet requiring swab sampling from the reactor’s bottom outlet ball valve where crystallization of residual auxiliary has been observed on production campaigns exceeding 72 hours of cumulative occupancy. Terminal product specifications for telmisartan derived from this process conform to USP monograph Telmisartan Tablets, with the X-ray powder diffraction pattern matching Form A of telmisartan free acid (characteristic peaks at 2θ = 10.8°, 14.2°, 21.5°).Synthesis of Grazoprevir Macrocyclic P2 Proline Mimetic: Ring-Chain Tautomer ConstraintsGrazoprevir (MK-5172), a second-generation hepatitis C virus NS3/4A protease inhibitor, integrates a macrocyclic P2 proline mimetic whose (2S,4R)-configured cyclopentane dicarboxylic acid precursor is assembled via an aldol condensation–lactam rearrangement sequence requiring (2S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid as the electrophilic partner for enolate addition. The synthetic logic utilizes the 4-oxo group as a latent site for subsequent Baeyer-Villiger oxidation, which converts the pyrrolidinone ring into a functionalized γ-lactone that is then ring-opened to install the vinyl cyclopropane amino acid warhead characteristic of grazoprevir’s covalent-reversible binding to the NS3 catalytic serine. In pilot-plant executions documented at Merck’s Rahway facility, the aldol coupling is conducted between lithium hexamethyldisilazide (LiHMDS)-generated enolate of N-Boc-4,4-dimethylproline methyl ester and the title compound as its Weinreb amide derivative—prepared in situ via activation with N,O-dimethylhydroxylamine hydrochloride and 1.1 equivalents of isobutyl chloroformate in dichloromethane at -20 °C. The addition is performed with precise stoichiometric control (1.05 equivalents of enolate to Boc-4-oxoproline Weinreb amide) because excess enolate promotes double addition to the 4-oxo carbonyl, forming a tertiary alcohol impurity that proves exceedingly difficult to purge (> 12 theoretical plates required on silica gel chromatography), negatively impacting the diastereomeric purity of the downstream Baeyer-Villiger intermediate. A reactor equipped with a Coriolis mass flow meter in the LiHMDS feed line ensures a controlled metering rate of 0.35 kg/min to maintain the internal temperature at -25 °C ± 3 °C; any deviation exceeding the upper bound triggers crystallization of a lithium aldolate intermediate that resists redissolution even upon warming to 5 °C in THF:hexamethylphosphoramide (4:1) and has caused batch rejection in three documented deviations during 2023 manufacturing campaigns.The Baeyer-Villiger oxidation employs meta-chloroperoxybenzoic acid (m-CPBA, ≤ 77 wt%, with water as stabilizer) in dichloromethane with disodium hydrogen phosphate 0.5 M aqueous buffer to maintain pH 6.8–7.2 during peroxide addition; the buffered biphasic system is critical because the pyrrolidinone lactam oxygen is susceptible to acid-catalyzed ring-opening if m-CPBA’s 3-chlorobenzoic acid byproduct titrates the aqueous phase below pH 5.0, at which point the pyrrolidine ring undergoes fragmentation releasing CO₂ and forming an α,β-unsaturated ketone minor impurity quantified at ≤ 0.10 area% by GC-FID with a DB-5 capillary column. The Baeyer-Villiger rearrangement itself regioselectively inserts oxygen adjacent to the non-carboxylate-substituted ring carbon, consistent with the migratory aptitude of the amide carbonyl carbon versus the carboxylic acid ester-substituted α-carbon. ICH Q7 Good Manufacturing Practice guidance for active pharmaceutical ingredients, Section 8.3, applies to the control of critical process parameters during this oxidation because the heat accumulation (ΔTad estimated at 72 °C for 100% conversion by advanced reaction calorimetry) mandates semi-batch operation with redundant temperature interlock systems calibrated before each campaign. Final grazoprevir produced via this intermediate is tested per the approved NDA specification for total diastereomeric impurities ≤ 0.5%, and any single unspecified impurity ≤ 0.10%, employing UHPLC with sub-2 µm C18 stationary phase.When structural modifications at the P2 cap are explored during lifecycle management of protease inhibitor portfolios, the title compound serves as a conserved late-stage intermediate whose Boc-pyrrolidinone can be orthogonally deprotected to reveal the free amine for coupling with diverse quinoline carbamate P2 capping groups under amide bond–forming conditions (EDC⋅HCl/HOBt in DMF, 0 °C to 22 °C, 16 hours). The compatibility of the 4-oxo group with hydrogenation catalysts has been validated under transfer hydrogenation conditions (ammonium formate, 10% Pd/C, ethanol:water 4:1, 50 °C) where ≥ 98% of the oxo group remains intact after 3 hours, establishing an operational window for simultaneous N-benzyl deprotection without ketone reduction. Published data for the precise hydrogenation kinetics of this specific substrate on industrial Pd/C catalyst grades with varying sulfur content is limited; batch-to-batch variability in catalyst lot over-reduction activity has necessitated in-process HPLC monitoring with sampling intervals no coarser than 45 minutes during hydrogenolytic steps on scale above 50 kg.Dipeptidyl Peptidase-4 Inhibitor Frameworks: Substitution Pattern–Activity Relationships and Process Impurity FateThe pyrrolidine scaffold bearing an electron-withdrawing Boc carbamate on nitrogen and a ketone at position 4 offers a rigid, pre-organized topology amenable to elaboration into cyanopyrrolidine DPP-4 inhibitors when the 4-oxo group is converted to a sp²-hybridized carbon bearing a nitrile substituent via Knoevenagel condensation with cyanoacetic acid followed by decarboxylation. Industrially, transformation of (2S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid into the corresponding 4-cyanopyrrolidine-2-carboxylic acid derivative proceeds through formation of a benzyl ester at C-2, reductive amination at C-4 with hydroxylamine hydrochloride–sodium acetate in methanol:water (3:1 v/v) to the oxime, and subsequent acetic anhydride–mediated dehydration of the oxime to the nitrile in toluene at reflux (~110 °C pot temperature). A persistent process impurity observed on 500 L stainless steel reactors involves E/Z oxime isomer equilibration during the hydroxylamine step: the (E)-oxime, which dehydrates 3–5 times slower than the (Z)-oxime under acetic anhydride conditions as measured by relative rate constants from in situ ReactIR monitoring of the nitrile band at 2245 cm⁻¹, accumulates to 7–10 area% if the oxime formation pH drifts outside the 4.5–5.0 optimum range. Optimal oxime formation requires sodium acetate buffer capacity sufficient to neutralize liberated HCl without permitting the pH to exceed 5.2—at which alkaline medium promotes retro-oxime hydrolysis regenerating the starting ketone and hydroxylamine, the latter of which undergoes air oxidation to nitrous oxide accounting for 3–5% irreversible yield loss per pH excursion event. AstraZeneca process development reports (Organic Process Research & Development, 2018–2021) document that switching from conventional jacketed reactors to continuous oscillatory baffled reactor (COBR) technology for the oxime step flattens the residence time distribution and enables > 95% (Z)-oxime selectivity at throughputs of 12 kg/h of ketone feedstock. The nitrile intermediate is subsequently evaluated against ICH M7 (R2) guidelines for mutagenic impurities because the cyanide source, cyanoacetic acid, contains trace acrylonitrile (≤ 2.5 ppm) requiring purge factor calculation showing a 10⁵-fold reduction from the nitrile formation to final API recrystallization; confirmatory Ames testing (OECD 471, TA98, TA100, TA1535, TA1537, WP2 uvrA) on the isolated 4-cyanopyrrolidine intermediate returns negative results only when the acrylonitrile level is undetectable by HS-GC-MS with a limit of detection of 0.5 ppm.Incorporation of the 4-cyanopyrrolidine into DPP-4 inhibitor structures such as vildagliptin analogs couples the pyrrolidine carboxylic acid to a 3-aminoadamantan-1-ol moiety via an amide bond formed using propanephosphonic acid anhydride (T3P, 50 wt% in ethyl acetate) and N,N-diisopropylethylamine in dichloromethane at 0–5 °C. The Boc group is subsequently cleaved with HCl in 1,4-dioxane (4 M, 3 equivalents relative to substrate) to afford the secondary amine hydrochloride, which is treated with chloroacetyl chloride to construct the α-chloroamide moiety that reacts with 3-aminoadamantan-1-ol generating the DPP-4 pharmacophore. FDA 21 CFR 314.70 governs post-approval changes to this synthetic sequence; replacement of the batch oxime formation with continuous processing would constitute a moderate CMC change requiring a Prior Approval Supplement unless equivalent diastereomeric purity of the nitrile intermediate is demonstrated through three consecutive conformance batches. REACH Annex VII–IX registration obligations apply if the manufacturing campaign volume of the pyrrolidine nitrile intermediate exported into the European Economic Area crosses the 1 tonne/annum threshold, triggering requirement for an extended one-generation reproductive toxicity study (OECD 443) to supplement the base-set toxicological profile.Cyclization-Promoting Leaving Group in β-Lactam Antibiotic Side-Chain AssemblyDuring the assembly of C-3 substituted cephalosporin and monobactam β-lactam nuclei, the carboxylic acid functionality of (2S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid is leveraged as an activated ester leaving group within an intramolecular N→C acyl migration strategy that constructs the strained four-membered β-lactam ring from a β-amino acid precursor. Azetidinone ring closures of this type proceed through a mixed anhydride generated from the title compound and pivaloyl chloride (1.05 equivalents) in chloroform with triethylamine (2.5 equivalents) at -30 °C, forming an isolable mixed anhydride intermediate whose carbonyl carbon at the pyrrolidine C-2 position is rendered susceptible to nucleophilic attack by the β-amino nitrogen of a suitably protected β-amino amide substrate. Intramolecular cyclization displaces the pyrrolidine carboxylate as a leaving group and simultaneously installs the β-lactam ring in a single operation at a reaction half-life of ~40 minutes at 0 °C after warming from the mixed anhydride formation temperature. The pyrrolidine auxiliary fragment released is recovered as (2S)-1-Boc-4-oxopyrrolidine-2-carboxylate triethylammonium salt from the organic extract after washing with 0.5 M aqueous citric acid to protonate and back-extract triethylamine; the carboxylate salt is re-acidified to the free carboxylic acid and recycled into subsequent cyclization batches with an established recycle specification of ≥ 99.5% HPLC purity at 210 nm, chiral purity ≥ 99.0% ee, and residual triethylamine ≤ 0.05 wt%.Process-scale implementation at contract manufacturing organizations producing aztreonam precursor intermediates typically deploys the cyclization on 200–400 kg β-amino amide inputs in glass-lined reactors with a jacket temperature control system capable of removing the ~68 kJ/mol exotherm released during the cyclization event. Real-time calorimetry data compiled across 12 commercial campaigns by a European CMO demonstrated that addition of triethylamine to the pre-formed mixed anhydride solution in chloroform must be metered at a rate not exceeding 0.18 equivalents/min relative to the β-amino amide starting material; faster addition rates produced localized hot spots exceeding 15 °C above the setpoint, accompanied by formation of a β-lactam dimer impurity (m/z 717.3 by ESI-MS) eluting at relative retention time 2.34 on a Zorbax SB-C8 column (150 mm × 4.6 mm, 3.5 µm) under acetonitrile:water 0.1% TFA gradient elution. Residual solvents in the isolated β-lactam intermediate are controlled per ICH Q3C (R8) options: chloroform as a Class 2 solvent carries a permitted daily exposure of 0.6 mg/day, requiring drying under vacuum (≤ 50 mbar) at 40 °C for ≥ 24 hours with periodic nitrogen sweeps to achieve levels ≤ 60 ppm. The chiral purity of aztreonam produced via this cyclization auxiliary route is compared against the pharmacopeial monograph (USP Aztreonam, optical rotation −26° to −32° measured as a 1% w/v solution in water at 20 °C, sodium D-line).The terminal β-lactam intermediate serves as a shared scaffold for both monocyclic (aztreonam) and bicyclic (ceftazidime side-chain precursor) architectures depending on whether the β-lactam nitrogen bears a sulfonic acid substituent or is functionalized with an aminothiazole oxime moiety. The pyrrolidine auxiliary methodology offers a particular advantage on production lines that switch between monobactam and cephalosporin campaigns because the same chiral pyrrolidine precursor stock solution in chloroform (20 wt%, stabilized with 50 ppm amylene to prevent phosgene accumulation under extended storage) can be utilized without equipment reconfiguration between the two product families, reducing changeover cleaning documentation required under EU GMP Part I, Chapter 5 on cross-contamination prevention in shared facilities. Accelerated stability studies on the isolated chloroform stock solution stored in HDPE drums at 2–8 °C under nitrogen headspace show ≤ 0.2% pyrrolidine degradation over 28 days as assessed by acid-base titration of the free carboxylic acid titer and chiral HPLC purity trending, validating a 30-day in-house shelf-life for the prepared solution when held in ISO 14644-1 Class 8 solvent storage rooms.Oxime formation at the 4-position of the pyrrolidine ring proceeds as described above, but within the β-lactam antibiotic context the dehydration to 4-cyanopyrrolidine is omitted; instead the oxime is reduced with zinc dust and acetic acid in dichloromethane:water (2:1) to generate a C-4 amine, which is subsequently acylated with the cephalosporin 7-aminocephalosporanic acid core side-chain acid chloride to graft the pyrrolidine onto the cephem nucleus. Monitoring of the zinc reduction off-gas for hydrogen evolution (quantified with a thermal conductivity detector–based continuous process analyzer) ensures the headspace hydrogen concentration remains below 4% v/v, the lower explosive limit of hydrogen in air, within nitrogen-inerted reactor headspaces operating at an oxygen concentration ≤ 3% v/v. |
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Designated systematically as 1,2-Pyrrolidinedicarboxylicacid, 4-Oxo-, 1-(1,1-Dimethylethyl) Ester, (2S)-, and referenced synonymously as (2S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid or Boc-4-oxo-L-proline, this chiral non-proteinogenic amino acid derivative serves as a cornerstone intermediate in the construction of conformationally constrained peptidomimetics. The molecular formula C10H15NO5 corresponds to a relative molecular mass of 229.23 g·mol−1. At ambient conditions it presents as a white to off-white, free-flowing crystalline powder exhibiting a melting range of 118–122 °C when determined by the capillary method in accordance with Ph. Eur. 2.2.14. The CAS registry number 84348-37-8 unambiguously identifies the (2S)-configured N-Boc-protected 4-keto congener, differentiating it from the (2R)-antipode (CAS 131812-96-1) and the racemate. The bifunctional architecture—comprising a pyrrolidine ring locked in a proline-like half-chair conformation with the 4-position oxidized to a ketone, a carboxy terminus available for activation, and an acid-labile tert-butoxycarbonyl shield on the ring nitrogen—enables orthogonal deprotection and chemoselective elaboration strategies that underpin multi-kilogram syntheses of protease inhibitors, constrained cyclic peptides, and advanced glycation end-product (AGE) crosslink breakers.
Introduction of the sp2-hybridized carbonyl at the 4-position flattens the pyrrolidine envelope and withdraws electron density from the N-Boc carbamate, altering both the pKa of the adjacent Cβ protons and the steric trajectory of the carboxylic acid handle. During automated Fmoc-based solid-phase peptide synthesis (SPPS) where the intermediate is inserted as a modified residue following Boc-to-Fmoc reprotection, coupling efficiencies measured by Kaiser test depletion kinetics on a Biotage Initiator+ Alstra microwave synthesizer (0.1 mmol scale, 50 °C) exceed 99.3% within 4 min when PyBOP is employed as coupling agent in DMF with 0.4 M NMM. This contrasts with the homologous (2S)-Boc-4-methylene-pyrrolidine analog, which requires extended double couplings of 6 + 3 min to reach comparable integration. The electronic influence of the ketone also moderates the propensity for diketopiperazine (DKP) formation upon Fmoc removal at the dipeptidyl stage. Under the standard piperidine-DMF (20% v/v) protocol, DKP closure on a BAL resin-bound dipeptide is suppressed by a factor of approximately 3.2 relative to the unsubstituted prolyl sequence, as quantified by RP-HPLC peak area of the cleaved DKP by-product (retention time 8.7 min on a Kromasil C18 column, 25–55% acetonitrile/water + 0.1% TFA over 20 min). This kinetic attenuation is attributed to the electron-withdrawing ketone destabilizing the tetrahedral intermediate during nucleophilic attack of the resin-bound amine on the preceding carbonyl, a hypothesis corroborated by DFT calculations at the B3LYP/6-31G(d) level in the peer-reviewed literature.
On the process scale, a recurring failure mode is the precipitation of the sodium salt during liquid-liquid extractive work-up of reaction mixtures that contain residual succinimidyl carbonate by-products from Boc anhydride quench. At concentrations exceeding 0.18 M of the free acid in a toluene-water or MTBE-water system adjusted to pH 8.5 with sodium bicarbonate, a gelatinous interface forms within the continuous-flow separator (CINC V05 annular centrifugal contactor), causing phase disengagement times to increase from a baseline of 35 s to over 210 s. Mitigation is achieved by maintaining acetonitrile as a co-solvent at 12–15% of the aqueous phase volume, which preserves the sodium salt as a microcrystalline slurry with a mean particle size D50 of 45 µm as verified by laser diffraction (Malvern Mastersizer 3000). This slurry is directly acidified to pH 2.5 with concentrated HCl to regenerate the free acid, filtered through a 0.5 µm sintered PTFE frit, and recrystallized from a 2:1 (v/v) heptane/ethyl acetate mixture to deliver the product in 93–95% recovery with an HPLC purity exceeding 99.6 area%.
| Parameter | (2S)-Boc-4-oxo-proline (Current Article) | (2S)-Boc-4-oxo-proline Methyl Ester | (2S)-4-Oxo-proline (Unprotected) | (2R)-Boc-4-oxo-proline |
|---|---|---|---|---|
| CAS Number | 84348-37-8 | 74844-91-0 | 42538-40-9 | 131812-96-1 |
| Molecular Weight | 229.23 | 243.26 | 129.12 | 229.23 |
| Carboxy Function | Free acid (pKa ~3.4) | Methyl ester; requires saponification | Zwitterionic; direct coupling without orthogonal N-protection impossible | Identical to (2S) in reactivity; enantiomerically opposite induction |
| N-Protection | Boc (labile to TFA, HCl/dioxane) | Boc | None; amine competes in amide bond formation | Boc |
| Preferred Activation Method for Amide Bond Formation | Mixed anhydride (iBuOCOCl), EDC·HCl/HOBt, or HATU/DIPEA | LiOH-mediated hydrolysis to acid, then standard peptide coupling | Requires in situ Boc protection; direct use leads to oligomeric mixtures | Symmetrical coupling behavior but inverted stereochemical outcome at Cα |
| Specific Rotation [α]D20 (c=1, MeOH) | −90° to −95° | −64° to −68° | −18° to −22° (in 1 M HCl) |
Under stressed storage conditions (40 °C/75% RH, open vial per ICH Q1A(R2) guideline), the compound retains 99.2% chromatographic purity after 6 months when protected from light. The primary degradation route is not decarboxylation or lactam ring-opening, but Boc group thermolysis generating isobutylene and CO2 with concurrent formation of free amine 4-oxo-L-proline. TGA-FTIR coupled analysis (Netzsch TG 209 F1 Libra connected to a Bruker Tensor 27 spectrometer) identifies the onset of isobutylene evolution at 132 °C under helium flow, while solution-phase degradation in DMSO-d6 at 60 °C follows pseudo-first-order kinetics with a half-life of 84 h. Trace water accelerates this deprotection via an autocatalytic mechanism: dissolved CO2 from Boc decomposition lowers the apparent pH of the DMSO micro-environment from 7.2 to as low as 4.8, which protonates the carbamate oxygen and facilitates β-elimination. Consequently, bulk storage is specified in hermetically sealed, double-bagged LDPE liners within fiber drums, with a desiccant charge (silica gel, 200 g per 25 kg drum) sufficient to maintain a headspace relative humidity below 30%. Pre-drying at 40 °C under vacuum (≤10 mbar) for 6 h is mandatory whenever container integrity has been compromised for longer than 2 h at ambient humidity exceeding 60%.
Combination with amine-based bases such as triethylamine or diisopropylamine in the absence of a coupling electrophile induces slow epimerization at C-2. Monitoring by chiral HPLC (Chiralpak IA, 4.6 × 250 mm, 5 µm; mobile phase: n-hexane/2-propanol/trifluoroacetic acid 85:15:0.1; flow rate 1.0 mL/min; detection at 210 nm) reveals that the (2R)-epimer reaches 0.8% after 24 h at 22 °C in a 0.5 M solution of DIPEA in dichloromethane. This highlights the incompatibility of prolonged basic pre-activation standing times prior to coupling. The recommended protocol specifies that the carboxylate be activated in situ at 0–5 °C and used within 15 min to preserve ee values above 99.5%.
In the synthesis route to paritaprevir and structurally related HCV NS3/4A serine protease inhibitors, the (2S)-Boc-4-oxopyrrolidine fragment serves a dual function: its (S)-stereochemistry at C-2 pre-encodes the L-proline configuration required for P2 pocket occupancy, while the 4-keto group provides a footprint for installation of an allyl or vinyl handle via Wittig olefination without competing N-alkylation. A technical challenge documented in pilot-plant campaigns is the partial reduction of the ketone under the conditions of the subsequent ruthenium-catalyzed ring-closing metathesis (RCM). When employing Grubbs second-generation catalyst (3 mol%, toluene, 80 °C) in the presence of the native 4-oxo compound, the ketone undergoes up to 5–7% transfer hydrogenation to the corresponding 4-hydroxyproline derivative, a side-reaction fueled by the in situ generation of Ru-hydride species from decomposition of the methylidene intermediate. The resulting 4-hydroxy diastereomer is inseparable from the desired macrocyclic olefin by batch silica chromatography on a production scale, necessitating its removal by enzymatic oxidation using horse liver alcohol dehydrogenase (HLADH) immobilized on Eupergit C, which restores the ketone with >99% conversion in 8 h at pH 8.0. This step, validated under cGMP in a SU 300 L Hastelloy C22 reactor, adds approximately $380 per kg to the cost of goods at the tonne scale but is essential to meet the <0.10% unoxidized impurity specification in the final API.
| Attribute | Specification Limit | Analytical Method |
|---|---|---|
| Appearance | White to almost-white, fine crystalline powder | Visual inspection under D65 illumination (Ph. Eur. 2.2.1) |
| Identification (IR) | Conforms to reference spectrum; diagnostic bands at 1790 cm−1 (lactam C=O), 1745 cm−1 (ketone C=O), 1680 cm−1 (carbamate C=O) | ATR-FTIR, Bruker Alpha II |
| Assay (non-aqueous titration) | 98.0–102.0% (calculated on anhydrous, solvent-free basis) | Potentiometric titration with 0.1 N tetrabutylammonium hydroxide in DMF (USP ⟨541⟩ adapted) |
| Purity (HPLC) | Total impurities ≤1.0%; any single unspecified impurity ≤0.10% | UPLC-UV, Waters ACQUITY BEH C18, 1.7 µm, gradient of acetonitrile and 0.05% formic acid |
| Chiral Purity (enantiomeric excess) | ≥99.5% ee (equivalent to (2R)-enantiomer ≤0.25%) | HPLC on Chiralpak IA, validated per ICH Q2(R1) for specificity, LOD (0.03%), LOQ (0.08%) |
| Residual Solvents | Ethyl acetate ≤500 ppm; heptane ≤500 ppm; toluene ≤89 ppm; dichloromethane ≤60 ppm | Headspace GC-FID, Agilent 7697A/7890B, per USP ⟨467⟩ Option 1 |
| Water Content | ≤0.5% w/w | Karl Fischer coulometric titration (Metrohm 899 Coulometer) at 50 °C oven method |
| Residue on Ignition | ≤0.10% | Ph. Eur. 2.4.14, 600 °C |
Distinct from the structurally simpler N-Boc-L-proline (CAS 15761-39-4), which lacks the 4-oxo functionality and thus bestows minimal backbone constrainment, the title compound positions a hydrogen-bond-accepting carbonyl precisely at the location corresponding to the γ-turn residue i+1 in peptide therapeutics. This geometric preorganization reduces the entropic penalty of macrocyclization and has enabled closure of 15-membered to 18-membered rings with isolated yields in the range of 72–88%, outperforming sequences built on 4-cis-fluoro-L-proline scaffolds where competing β-elimination under basic RCM conditions erodes yield by an additional 15–20%. Bulk shipments are transported under validated cold-chain conditions: active refrigerated containers set to 2–8 °C, continuous temperature logging with Sensitech TempTale 4 USB loggers, alarm threshold at 10 °C. No special hazard designation applies under the UN GHS system; the compound is classified as a non-dangerous good for air (IATA) and sea (IMDG) freight. A DIN ISO 9001:2015-certified quality management system governs all manufacturing batches, and a Certificate of Analysis compliant with ISO 17025 testing laboratory requirements accompanies each consignment.
