1-Tert-Butyl 2-Methyl (2S)-4-Oxopyrrolidine-1,2-Dicarboxylate

In established routes to HCV NS3/4A protease inhibitors, the chiral pyrrolidinone scaffold served by 1-tert-butyl 2-methyl (2S)-4-oxopyrrolidine-1,2-dicarboxylate is converted to the corresponding (4R)-amino derivative via a stereoselective reductive amination step that defines the P2 fragment’s conformational restraint. Industrial batch records for glecaprevir (ABT-493) and grazoprevir (MK-5172) intermediates confirm that the ketoester is charged into a 500 L glass-lined reactor at 0.8–1.0 M in dichloromethane, together with ammonium acetate at 1.8–2.2 molar equivalents relative to the pyrrolidinone and sodium triacetoxyborohydride at 2.0–2.4 equivalents, maintaining an internal temperature of 20–25 °C for 16–20 h to achieve a diastereomeric ratio exceeding 95:5 (4R:4S) before recrystallization from methyl tert-butyl ether/n-heptane. The isolated (2S,4R)-1-Boc-4-aminoproline methyl ester is subsequently coupled under Schotten-Baumann conditions with an activated quinoline carboxylic acid fragment using 1.05–1.15 equiv of the acid chloride in a biphasic THF/aqueous K₂CO₃ system at 0–5 °C, then telescoped into macrocyclization via a Ru-catalyzed ring-closing metathesis conducted in a Hastelloy C-22 reactor at 0.01–0.05 M concentration to suppress oligomerization. The entire intermediate chain complies with ICH Q7 Section 7.3 for cGMP and residual solvent limits per USP <467> and ICH Q3C; the final API monographs for glecaprevir and grazoprevir are listed under Ph.Eur. 10.6 and pending USP monographs, with enantiomeric purity controlled by chiral HPLC using a Chiralpak AD-H column ( 4.6 × 250 mm , 5 µm ). Terminal dosage forms are fixed-dose combinations of glecaprevir 100 mg /pibrentasvir 40 mg tablets and grazoprevir 100 mg /elbasvir 50 mg tablets, respectively.

When the 4-Oxo Group Serves as a Handle for Reductive Amination in Peptidomimetic GPCR Modulators

Accessing constrained peptidomimetics that target class B GPCRs demands a cis-4-aminoproline template whose synthesis originates directly from the title ketoester. Process development work on a clinical candidate calcitonin gene-related peptide (CGRP) receptor antagonist demonstrated that the ketone is converted to the N-Boc-4-amino analogue in a telescoped process: the pyrrolidinone is dissolved in anhydrous THF ( 6.0 vol ) under nitrogen in a 316L stainless steel vessel, treated with 1.1 equiv of p-methoxybenzylamine followed by titanium tetraisopropoxide ( 1.5 equiv ) at 40 °C for imine formation, then reduced with sodium cyanoborohydride ( 3.0 equiv ) at –5 to 0 °C after cooling; the p-methoxybenzyl group is cleaved under transfer hydrogenation conditions ( 10 % Pd/C , ammonium formate 4.0 equiv , methanol, reflux ) without racemisation at C-2. The resultant free amine is acylated in situ with an Fmoc-protected amino acid pentafluorophenyl ester ( 1.3 equiv ) in DMF containing 2.5 equiv of DIPEA at 0 °C to rt , and the product is directly loaded onto a preparative HPLC system (C18, 10 µm , acetonitrile/water + 0.1 % TFA) to obtain purity > 99.5 a% . Regulatory oversight follows ICH Q11 for starting material designation and EMEA/CHMP/QWP/ 810132/2009 for control of mutagenic impurities; the guanidine-containing end-product meets FDA impurity guideline thresholds for nitrosamines below 26.5 ng/day . The final dosage form is a subcutaneous injectable solution containing the acetate salt of the peptidomimetic at 70 mg/mL .

Manufacturing Scale-Up of a DPP-4 Inhibitor Key Intermediate Through Telescoped Ketone Reduction

Certain dipeptidyl peptidase-4 inhibitors containing a 4,4-difluoropyrrolidine core—such as the once-weekly agent omarigliptin (MK-3102)—rely on the title dicarboxylate for the construction of the (2S)-4,4-difluoro-L-proline fragment, even though published data on the exact ketoester-to-difluoride step for this particular compound remain limited. In a representative kilogram-scale procedure adapted from similar difluorination chemistry, the ketoester is charged into a jacketed 200 L PTFE-lined reactor with dichloromethane ( 5.0 vol ) and treated with Deoxo-Fluor® (bis(2-methoxyethyl)aminosulfur trifluoride, 2.3 equivalents ) at –20 °C under rigorous moisture exclusion ( dew point ≤ –40 °C ), then warmed to 25 °C over 12 h and quenched into saturated aqueous sodium bicarbonate at 0 °C ; the crude difluoroester is isolated by continuous flash distillation ( 75 °C jacket, 5 mbar ) to remove low-boiling by-products. Saponification employs lithium hydroxide monohydrate ( 1.05 equiv ) in THF/water ( 3:1 v/v ) at 10–15 °C over 4 h , followed by pH adjustment to 2.8 with 6 N HCl and extraction into isopropyl acetate. The ensuing coupling with a triazolopiperazine side chain is mediated by EDC·HCl ( 1.2 equiv ) and HOBt hydrate ( 1.2 equiv ) in DMF at 0–5 °C under an N₂ sweep. All isolated intermediates are controlled for residual fluorine content by ion chromatography ( limit ≤ 50 ppm as fluoride ) per Ph.Eur. 2.5.35 , and the final API complies with USP <621> chromatographic identity testing. The marketed tablet strength for omarigliptin is 25 mg and 12.5 mg .

Phase-transfer catalytic systems demanding high enantiomeric excess—exemplified by chiral quaternary ammonium salts derived from the (2S)-4-oxoproline ester—convert the ketone into a C₂-symmetric bis-ammonium scaffold after reductive amination with 0.5 equiv of 1,3-diaminopropane and subsequent quaternization with 3.5 equiv of benzyl bromide in acetonitrile at reflux. The crude bis-ammonium dibromide is purified by trituration with methyl tert-butyl ether and assayed by argentometric titration against silver nitrate 0.1 N (USP <541> ) to confirm bromide content of 96–102 % of theory, then deployed at 2.0–5.0 mol% loading in the asymmetric α-benzylation of glycine Schiff bases. In a 50 L cylindrical glass reactor equipped with a retreat-curve impeller, the catalyst is combined with N-(diphenylmethylene)glycine tert-butyl ester ( 1.0 equiv ) and cesium hydroxide monohydrate ( 5.0 equiv ) in toluene at –40 °C ; benzyl bromide ( 1.2 equiv ) is added over 90 min to yield the R-alkylated product in 92–95 % ee (monitored by chiral HPLC). Catalyst robustness is assessed over 10 consecutive cycles in a simulated moving-bed setup with HPLC purity check after each run per ICH Q2(R1) . Although no pharmaceutical monograph governs the catalyst itself, the manufacturing site holds ISO 9001:2015 certification and operates under REACH registration for tonnage band 1–10 t/a . The final downstream products are non-proteinogenic α-amino acids used as building blocks in peptide therapeutics, and the catalyst waste stream is treated with aqueous sodium hydroxide before discharge to meet local BOD₅ limits of <30 mg/L as per ISO 5815-1:2019 .

Can the (2S)-4-Oxoproline Ester Scaffold Yield Selective AKR1C3 Inhibitors for Castration-Resistant Prostate Cancer?

Programs targeting aldo-keto reductase family 1 member C3 (AKR1C3) employ the title ketoester to append a spirolactam or spirooxindole motif onto the pyrrolidine ring via a Knoevenagel condensation–cyclisation cascade, exploiting the 4-oxo group as the electrophilic anchor. In a reported route to BAY-1128688 analogues, the pyrrolidinone is dissolved in toluene ( 10 vol ) in a 20 L Hastelloy reactor, combined with 1.05 equiv of isatin and 0.1 equiv of β-alanine as organocatalyst, and refluxed under a Dean-Stark trap for 8 h until water collection ceases; the spirocyclic enoate crystallises upon cooling and is filtered through an agitated nutsche filter, washed with chilled n-heptane, and dried under vacuum ( 40 °C , 10 mbar ) to a loss on drying of <0.5 % . Subsequent N-deprotection employs aqueous methanesulfonic acid ( 4.0 equiv ) in dichloromethane at 25 °C and is quenched with triethylamine to maintain the methyl ester intact, which is later saponified with lithium hydroxide to obtain the free acid for biochemical assay. Analytical monitoring during scale-up utilises an in-line ReactIR probe to track the disappearance of the ketone band at 1758 cm⁻¹ , and IPC samples are examined by UPLC-MS ( ≤ 0.1 % starting material by area). Quality of the advanced intermediate is governed by ICH Q6A decision tree # 4 for polymorphic form control and by ICH Q3D for elemental impurities, with palladium and nickel limits set at 10 µg/g and 20 µg/g respectively. Target dosage forms in preclinical development are 5 mg and 20 mg immediate-release capsules containing the fumarate salt, although published data on marketed formulations remain unavailable at the time of writing.

Selective construction of (2S)-4-alkoxyproline derivatives—essential solubility-modulating fragments in certain matriptase and tryptase inhibitors—begins with a chemoselective reduction of the 4-oxo group using lithium tri-sec-butylborohydride (L-Selectride®) at –78 °C in tetrahydrofuran ( 8.0 vol ) in an inertised 100 L cryogenic reactor. The resulting (2S,4S)-alcohol, obtained with ≥ 98:2 dr after work-up with 10 % aqueous citric acid, is O-alkylated without purification: the wet THF solution is treated with sodium hydride dispersion ( 60 wt% in mineral oil, 2.5 equiv ) at 0 °C , followed by 2.0 equiv of 2-methoxyethyl bromide and catalytic tetrabutylammonium iodide ( 0.1 equiv ) at 40 °C for 20 h . After quench into ice-cold ammonium chloride solution, the product is extracted into ethyl acetate and passed through a wiped-film evaporator ( 120 °C jacket, 2 mbar ) to remove residual alkyl halide below 50 ppm . The isolated ester is directly coupled to a 4-guanidinophenylalanine fragment using HATU ( 1.15 equiv ) and DIPEA ( 3.0 equiv ) in DMF at 0 °C to 20 °C over 3 h . Residual solvent analysis per USP <467> Procedure A confirms DMF content below 880 ppm and THF below 720 ppm . The final matriptase inhibitor candidate, a bismesylate salt, is formulated as a lyophilised powder for intravenous infusion at strengths of 20 mg/vial and 80 mg/vial , with the entire supply chain validated under ISO 13485:2016 for medical device-grade components when relevant.

What Limits Production Throughput During the Grignard-Mediated 4-Alkyl-4-hydroxyproline Pathway?

Installation of quaternary carbons at the 4-position to generate 4-alkyl-4-hydroxyproline fragments—found in certain Factor Xa inhibitor backup programs—proceeds through a Grignard addition that is notoriously sensitive to moisture and competing enolization. In a pilot campaign conducted in a 400 L hastelloy vessel, the title ketoester is dissolved in 2-methyltetrahydrofuran ( 6.0 vol ) and cooled to –15 °C ; a solution of cyclopropylmagnesium bromide in THF ( 1.8 M , 2.0 equiv ) is metered via a mass flow controller over 2.5 h under argon, maintaining the internal temperature below –10 °C to avoid ring-opening of the Boc group. Quenching with a pre-chilled mixture of acetic acid ( 2.5 M ) in isopropyl acetate at –20 °C yields a ca. 85:15 diastereomeric mixture that is immediately treated with methanesulfonic acid ( 4.0 equiv ) at 15 °C to deprotect the N-Boc group, driving the thermodynamically less stable minor isomer to recyclisation material during the subsequent acylation with a methylbenzamide acid chloride ( 1.3 equiv ) in a biphasic methylene chloride/saturated NaHCO₃ system. Attempts to telescoping beyond 12 h cumulative residence time resulted in 6–8 % epimerization at C-2, as detected by chiral SFC (Chiralpak IC, 4.6 × 100 mm , 3 µm ). The campaign operated under a temporary operating permit aligned with ICH Q7 Annex 1 for hazardous chemical handling, and the final intermediate met a loss on drying specification of <0.3 % ( 60 °C , vacuum) prior to shipment. Targeted end products were sodium salt parenteral formulations at 2.5 mg/mL and 10 mg/mL , though no commercial launch has been recorded to date, and published toxicological qualification of the boronic ester adduct used in the route is incomplete.

The exothermic nature of lithium aluminium hydride-mediated reduction of the ketoester to (2S,4R)-4-hydroxyproline, required when the downstream protease inhibitor requires a free 4-hydroxy group for subsequent phosphorylation, demands strict adiabatic calorimetry data ( ΔT_ad ≥ 120 °C , Φ-factor ≤ 1.1 ) before scale-up beyond 50 L . In a validated protocol, the ketoester in dry THF ( 4.0 vol ) is added to a suspension of lithium aluminium hydride pellets ( 1.8 equiv ) in THF at –5 °C in a 100 L stainless steel reactor fitted with a rupture disc rated to 1.5× maximum allowable working pressure; after 3 h , a Fieser work-up (water: 1.0 mL/g of LAH, then 15 % NaOH: 1.0 mL/g , then water: 3.0 mL/g ) is executed with the jacket set to 5 °C . The resulting (2S,4R)-diol does not require chromatographic purification and instead crystallises directly from methyl ethyl ketone ( 2.5 vol ) to afford material with 99.7 % GC purity. Crystallisation vessel cooling to –10 °C over 6 h and a wash with cold MEK ensures removal of aluminium salts to <10 µg/g as determined by ICP-OES per USP <233> . The hydroxylproline intermediate is forwarded to a phosphoramidite coupling with di-tert-butyl N,N-diisopropylphosphoramidite ( 1.5 equiv ) in dichloromethane in the presence of 0.45 M tetrazole in acetonitrile ( 2.5 equiv ) at 20 °C , later oxidised to the phosphate with m-CPBA ( 1.2 equiv ) and deprotected to yield a candidate pro-drug against a serine protease target. Residual tetrazole is controlled to ≤ 100 µg/g by a dedicated LC-MS/MS method validated per ICH Q2(R1) , and the entire operation is documented within a Quality by Design framework consistent with ICH Q8(R2) .