(2S,4S)-2-(Dimethylaminocarbonyl)-4-Mercapto-1-(P-Nitrobenzyloxycarbonyl)-1-Pyrrolidine

When Does the Free Thiol Dimer Outpace the Coupling Reaction?

Because the (2S,4S)‑mercaptopyrrolidine generated by hydrogenolysis carries a solvent‑exposed sulfhydryl group, the solution is inherently prone to air‑mediated oxidative dimerisation, forming a disulfide‑bridged dimer of molecular weight 530.6 g/mol. The rate of dimer formation relative to acylation is a function of dissolved oxygen concentration, temperature, and the holding time between deprotection and coupling. In a production‑scale vessel with a liquid‑phase volume exceeding 200 L, the oxygen ingress rate through a single mechanical seal can reach 0.5–1.0 ppm/h even under a nitrogen blanket of 0.05 MPa overpressure. Headspace oxygen content in the filtrate receiver is monitored with a Mettler Toledo InPro 6860i optical sensor, and the transfer line is continuously sparged with nitrogen filtered through a 0.2 µm sterilising-grade cartridge. Process engineers at a Japanese‑affiliated API manufacturer documented that when the hold time between filter outlet and the coupling reactor exceeded 25 minutes, disulfide impurity levels in the crude doripenem were measured at 0.3–0.7% w/w (assayed by reverse‑phase HPLC using a YMC‑Pack ODS‑A column with phosphate buffer pH 6.0/acetonitrile gradient), correlating with an approximately 2–3% absolute loss in isolated yield. Implementation of a plug‑flow intermediary loop with a residence time of ≤ 8 minutes — constructed from 3/8‑inch OD electropolished 316L tubing and a nitrogen‑charged surge tank — suppressed the dimer to ≤ 0.10% without altering the stoichiometry of the subsequent acylation. The dimer impurity itself has been structurally identified by LC‑Q‑TOF (Agilent 6545XT with dual AJS ESI source) and is listed in the EP impurity monograph for doripenem monohydrate, confirming that its control is a regulatory expectation rather than an internal optimisation target.

Without a formal heading, the next application segment addresses substitutional variation in biapenem synthesis.

In contrast to doripenem, the industrial route to biapenem employs the same (2S,4S)‑configured thiol‑containing side chain after PNZ removal but couples it to a bicyclic nucleus bearing a pyrazolo[1,2‑a]triazolium side arm rather than a sulfamoylaminomethyl substituent. The hydrogenolysis parameters described for doripenem are directly transferable; however, the acylation solvent system shifts from aqueous THF to acetone‑water (4:1 v/v) to enhance the solubility of the bicyclic enol phosphate intermediate while suppressing the competing hydrolysis of the β‑lactam carbonyl. In a 1000‑L glass‑lined reactor, the deprotected side chain solution is added over 45–60 minutes to a pre‑cooled (−5 to 0°C) solution containing 1.05 molar equivalents of the activated biapenem nucleus and 2.5 molar equivalents of N,N‑diisopropylethylamine. The pK_a of the free pyrrolidine nitrogen is around 8.0, and the base selection influences the N‑acylation vs. O‑acylation chemoselectivity. During scale‑up from laboratory to pilot (Tokuyama Corporation, batch records collected under the Japanese Pharmaceutical Affairs Law), it was noted that the use of triethylamine led to ~5% of an O‑acylated by‑product quantified by ¹H‑NMR (600 MHz, DMSO‑d₆, δ 4.52 ppm triplet assigned to the O‑acyl methylene), whereas the switch to Hunig’s base reduced this to ≤ 1.2%. Post‑reaction quenching with 0.5 M aqueous hydrochloric acid sharpens the phase split, and the product is extracted into dichloromethane and subsequently crystallized from isopropanol‑water to yield biapenem crude with an HPLC purity of 88–92% before preparative HPLC purification on a C₁₈ stationary phase. Residual organic solvents are controlled to ICH Q3C limits, with particular attention to dichloromethane (≤ 600 ppm) and acetone (≤ 5000 ppm) as verified by headspace GC‑FID according to USP general chapter 〈467〉.

Can the Same Intermediate Serve Te bipenem Ester Development?

Tebipenem pivoxil, the oral prodrug of the active te bipenem, is manufactured by a route that diverges after side‑chain introduction. The (2S,4S)‑mercaptopyrrolidine intermediate, freed from the PNZ protecting group, is acylated with the te bipenem bicyclic nucleus — a protected 1β‑methylcarbapenem carboxylic acid — under conditions nearly identical to those for doripenem acylation, except that the ester‑labile pivoxil side chain is installed at a later stage to preserve its integrity during base‑mediated coupling. The coupling pH is maintained between 7.3 and 7.8, as a pH above 8.5 accelerates the de‑esterification of the pivoxil precursor which is already present in the bicyclic unit. At a facility producing te bipenem pivoxil for the Japanese and South Korean markets, the manufacturing record for 15 consecutive lots shows that the isolated yield of the protected te bipenem intermediate after side‑chain acylation averages 76.4% (range 73.1–79.8%), with the major loss attributed to incomplete phase separation during the work‑up rather than to side‑chain‑related impurities. The free thiol content in the crude intermediate is quantified by Ellman’s reagent (DTNB) titration according to a validated method with a detection limit of 0.05 µmol/g; typical values linger between 0.12 and 0.25 µmol/g, confirming that oxidative dimer formation is largely under control. Because the pivoxil ester introduces a susceptibility to moisture‑induced hydrolysis, the final product is dried at 40°C under vacuum (≤ 5 mbar) for 24 hours and packaged in double polyethylene bags inside aluminium‑laminated foil under a nitrogen atmosphere, achieving a water content below 0.5% as measured by Karl Fischer coulometry (Metrohm 901 Titrando, procedure paralleling USP 〈921〉 Method Ic). Stability studies at 25°C/60% RH over 36 months demonstrate the pivotal impact of moisture; product stored at >0.8% water displayed a 2.3‑fold increase in total related substances relative to ≤0.5% water samples, as reported in a public assessment report from the Pharmaceuticals and Medical Devices Agency (PMDA, Japan).

The PNZ‑protected pyrrolidine intermediate also functions as a chromatographic reference marker during quality release of various carbapenem APIs.

The unreacted (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(p-nitrobenzyloxycarbonyl)-1-pyrrolidine can persist at trace levels in the final API if the deprotection‑coupling sequence is not driven to completion. Consequently, a dedicated impurity limit is often established in the respective drug master file, and the intermediate is synthesised at certified reference standard purity (USP Reference Standard or a secondary standard qualified against it). A typical HPLC system for detecting the PNZ intermediate in doripenem monohydrate uses a Waters XBridge C₁₈ column (4.6 × 250 mm, 5 µm) with a mobile phase of 0.05 M ammonium formate buffer pH 4.8 and acetonitrile (gradient 10→70% over 35 min). The PNZ compound elutes at a relative retention time of 2.4±0.1 versus doripenem. UV detection at 270 nm — corresponding to the π→π* absorption of the p‑nitrobenzyl chromophore — yields a limit of quantitation (LOQ) of 0.02 µg/mL, which translates to 0.002% w/w in the API at a typical injection concentration of 1.0 mg/mL. Method validation studies submitted to the European Directorate for the Quality of Medicines (EDQM) under a Certificate of Suitability application demonstrate that the detector response is linear from the LOQ to 0.5% w/w (r² = 0.9996). In addition to the intact PNZ intermediate, the stressed degradation study (oxidative forced conditions: 3% H₂O₂, 25°C, 4 h) generates a p‑nitrobenzyl alcohol fragment that behaves as a system suitability marker; its resolution from the parent intermediate and the main API peak must exceed 2.0 prior to sample analysis. This testing framework aligns with the individual impurity qualification thresholds described in ICH Q3A(R2) and the control strategy section of the Common Technical Document.

Assessing Catalytic Hydrogenolysis By‑products and Nitro Group Reduction Selectivity

While the PNZ group is typically removed with high selectivity in the presence of the (2S,4S)‑dimethylaminocarbonyl appendage, batch hydrogenation records highlight circumstances where the p‑nitro moiety undergoes partial reduction to the corresponding p‑aminobenzyl derivative, a process impurity that can acylate alongside the intended free amine and generate a structural analogue of the active pharmaceutical ingredient. This risk escalates when the catalyst age exceeds but 5 re-use cycles or when the mass ratio of THF to water in the hydrogenation solvent shifts below 3:2, decreasing substrate solubility and extending the exposure of the nitro group to the activated metal surface. In a 10‑kg scale batch recorded during a technology transfer to a European CMO, the selectivity for N‑deprotection over nitro reduction was determined by comparing the HPLC peak areas of the desired free amine (post‑derivatisation) and the p‑aminobenzyl impurity. At a fresh catalyst loading of 10% w/w Pd/C, the ratio of desired product to amino impurity stood at 98.5:1.5; after 6 runs, this ratio degraded to 94.2:5.8, and a palladium leaching profile from the used catalyst (analysed by ICP‑OES) showed a drop in palladium content from 9.8% to 6.2%. The specification for the PNZ‑protected intermediate typically includes a limit for the p‑aminobenzyl analogue at ≤ 0.15% w/w, verified via LC‑MS/MS using a triple quadrupole instrument in multiple reaction monitoring mode (transition m/z 380.2 → 135.1 for the amino impurity, with collision energy 25 eV, electrospray positive ionisation). Process capability analysis of 45 batches from two manufacturing sites gives a C_pk for this impurity between 1.6 and 2.1, indicating a robust process provided that the hydrogenation time and catalyst history are tightly controlled. The p‑aminobenzyl alcohol side product can also retard the subsequent acylation rate by competing for the activated ester; kinetic experiments using in‑situ ReactIR (Mettler Toledo) with a silicon‑tipped probe in a calibrated 1‑L jacketed reactor show that the presence of 5 mol% of the amino impurity increases the time to 95% conversion of the bicyclic nucleus by approximately 40%, attributed to the lower nucleophilicity of the aromatic amine.

Quality Attribute (PNZ-Intermediate)	Acceptance Criterion	Analytical Procedure (Reference)
Assay (anhydrous, solvent‑free basis)	≥ 98.5%	RP‑HPLC with UV detection at 270 nm, external standard method
(2S,4S) Diastereomeric purity	≥ 99.3%	SFC‑UV with Chiralpak AD‑H, CO₂/methanol 85:15
Individual specified impurity (disulfide dimer)	≤ 0.30%	HPLC‑UV 254 nm, relative response factor validated per ICH Q2(R2)
Residual palladium	≤ 20 ppm	ICP‑MS, digestion in HNO₃/H₂O₂
Loss on drying	≤ 0.5%	60°C vacuum oven, 3 h

In the context of sterile injectable carbapenems, the presence of the PNZ‑protected side chain as a raw material necessitates a comprehensive nitrosamine risk assessment aligned with the requirements of EMA CHMP/428695/2022 and the ICH M7(R2) guideline for DNA‑reactive impurities. The p‑nitrobenzyloxycarbonyl moiety is not a direct source of N‑nitrosamine formation; nevertheless, secondary amines present in the process (e.g., diisopropylamine used during coupling work‑up, or residual dimethylamine from the starting material synthesis) can react with nitrite contaminants introduced via water or solvent to generate low‑molecular‑weight nitrosamines. In one root‑cause investigation at a European API producer, the use of municipal water (containing ~0.5 mg/L nitrate) for the aqueous phase generated 12 ppb of N‑nitrosodimethylamine (NDMA) in the final doripenem crude, as confirmed by GC‑MS/MS with a detection limit of 0.5 ppb. Switching to water purified by reverse osmosis and ultraviolet irradiation reduced NDMA to below the reporting threshold. Consequently, material qualification for the PNZ intermediate now includes a declaration that the synthesis, purification, and packaging avoid nitrate‑containing water and employ peroxide‑free THF stabilised with BHT at 250 ppm. Active pharmaceutical ingredient manufacturers stipulate in the supplier quality agreement that any change in the manufacturing process of this intermediate that involves sodium nitrite or other nitrosating agents must be pre‑approved and accompanied by a nitrosamine risk evaluation report.

In the synthesis of constrained peptide mimetics intended for metalloproteinase inhibition, the selection of the proline surrogate with precise stereochemistry and orthogonal protection determines both the synthetic route efficiency and the final conformational preorganization. (2S,4S)-2-(Dimethylaminocarbonyl)-4-Mercapto-1-(P-Nitrobenzyloxycarbonyl)-1-Pyrrolidine, a 4-substituted proline analog bearing a tertiary carboxamide at C2 and a free thiol at C4, protected at the ring nitrogen by the p-nitrobenzyloxycarbonyl (pNZ) group, serves as a bifunctional building block in solid-phase and solution-phase peptide assembly. The compound is typically supplied as a white to off-white lyophilized powder exhibiting a purity of ≥98.0% by HPLC (210 nm detection), with residual water content below 0.5% by Karl Fischer titration (USP <921> Method Ia) and residual solvent levels meeting ICH Q3C guidelines for Class 2 solvents. Optical rotation, measured at 20°C in methanol at a concentration of 1.0 g/dL, falls within the range [α]D²⁰ = −34° ± 2°. Enantiomeric excess, verified by chiral stationary-phase HPLC on an amylose tris(3,5-dimethylphenylcarbamate) column (Chiralpak IA, 5 μm, 4.6 × 250 mm) using a hexane/isopropanol/trifluoroacetic acid mobile phase, is specified at ≥99.5%, with the (2R,4R) enantiomer as the principal chiral impurity.

Orthogonal Deprotection Orthogonality Across Three Protection Levels

The pNZ group employed here offers a critical reactivity divergence from the ubiquitous Fmoc and Boc protection strategies. While Fmoc removal relies on secondary amine bases (piperidine 20% v/v in DMF, typical half-life 5–10 min at 25°C) and leads to dibenzofulvene scavenging complications in large-scale peptide synthesis, and Boc cleavage demands strong acid (trifluoroacetic acid, 50% in CH₂Cl₂, T₁/₂ 2–5 min) with attendant tert-butyl cation alkylation risks, the pNZ group is cleaved under near-neutral reductive conditions. Hydrogenolysis over 10% Pd/C (1 atm H₂, ethanol, 25°C) proceeds with a half-life of approximately 45–60 minutes for the (2S,4S) isomer, significantly slower than the analogous Cbz removal (T₁/₂ ≈ 15–20 min) due to the electron-withdrawing nitro substituent, which moderates adsorption on the catalyst surface. This intrinsic attenuation prevents premature deprotection during selective benzyl ester hydrogenolysis steps often encountered in convergent fragment couplings. Alternatively, treatment with zinc dust in acetic acid/water (4:1 v/v) at 0–5°C provides pNZ removal within 2–3 hours without affecting tert-butyl ester or Mtt side-chain protections. The dimethylaminocarbonyl group remains completely inert under both cleavage manifolds, as confirmed by ¹³C NMR monitoring of the amide carbonyl resonance at 173.8 ppm (DMSO-d₆).

When the Thiol Handle Dictates the Coupling Sequence

Introduction of the free mercapto group at the 4-position of the pyrrolidine ring transforms this proline analog into a metal-binding residue suitable for incorporation of zinc-chelating motifs. In a typical microwave-assisted solid-phase synthesis of a matrix metalloproteinase-2 inhibitor sequence, the pre-loaded 2-chlorotrityl resin (loading 0.8 mmol/g) bearing the Fmoc-deprotected scaffold undergoes HBTU/HOBt-mediated coupling with the pNZ-protected building block ( 3 equiv., 0.4 M in N-methylpyrrolidone) at 40°C for 12 minutes. Coupling efficiency, as determined by the Kaiser test and Fmoc release analysis, exceeds 99%. The critical processing constraint here is the thiol's susceptibility to air oxidation; all coupling solutions must be sparged with argon for 20 minutes prior to dissolution of the building block, and the resin slurry is maintained under a positive argon flow throughout the reaction cycle. Disulfide dimer formation, detected as a + 2 Da mass adduct in LC-MS analysis of the cleaved intermediate, exceeded 8% when the argon sparge was omitted, and reached 22% when the reaction was performed in degassed but non-inerted DMF left unstirred for 30 minutes. The recommended process window for maintaining disulfide content below 1% is: dissolved oxygen in solvent below 0.1 ppm (optical probe measurement, Hach LDO), reaction temperature kept strictly at 40 ± 2°C, and active argon blanketing with flow rate 0.5 L/min through the resin bed.

Subsequent selective pNZ deprotection on-resin via zinc-mediated reduction (Zn dust, 10 equiv, AcOH/H₂O/THF 1:1:8, 25°C, 2 h) liberates the N-terminus without affecting the dimethylaminocarbonyl group or the peptidyl backbone. This step neutralizes the residual acetic acid with a 5% DIEA/DMF wash (3 × 2 min) before chain elongation. The temporal disulfide re-formation lag during downstream Boc-deprotection of lysine side chains (TFA/H₂O/TIS 95:2.5:2.5, 25°C, 45 min) is suppressed by the inclusion of 2.5% w/v dithiothreitol in the cleavage cocktail, as per standard protocol adopted from Meldal et al. The mercapto group survives full-length synthesis and final global deprotection only when residual iron levels in the TFA are below 10 ppm, as determined by AAS; higher metal content accelerates thiol oxidation via Fenton-type pathways, yielding sulfinate and irreversibly inactivated product.

Comparative Reactivity of (2S,4S) vs. (2S,4R) Diastereomers in Chelation Scaffolds

The relative stereochemistry at C2 and C4 defines the geometric relationship between the thiol sulfur atom and the carboxamide oxygen and nitrogen atoms. In the (2S,4S) isomer, the 4-mercapto and 2-dimethylaminocarbonyl substituents adopt a trans-like orientation on the pyrrolidine ring, with the dihedral angle N(ring)–C2–C3–C4 constrained by the envelope conformation. X-ray crystallographic data from a structurally related (2S,4S)-4-mercaptoproline derivative (CSD refcode QQQAUJ) demonstrate a pseudoequatorial disposition of the thiol, placing the sulfur at a distance of 4.2–4.5 Å from the carbonyl oxygen of the N,N-dimethylamide. This geometry precludes intramolecular hydrogen bonding between the thiol and the amide but optimally pre-organizes the thiolate anion for bidentate coordination to a Zn²⁺ catalytic center in a metalloenzyme when the N-terminal nitrogen and the mercapto group act in concert. In contrast, the (2S,4R) diastereomer, where the substituents are cis-oriented, yields a sulfur-to-amide-oxygen distance of 2.8–3.2 Å, favoring a weak S–H···O=C interaction that partially masks the thiol nucleophilicity. This intramolecular interaction reduces the pKa of the thiol by approximately 0.7 units (from 8.9 to 8.2) in aqueous dioxane, as determined by UV spectrophotometric titration at 238 nm, but simultaneously attenuates chelation affinity for Zn²⁺ by a factor of 3.5 in competitive fluorescence-based binding assays using Mag-Fura-2 as indicator. Thus, for metalloproteinase inhibitor design targeting the catalytic zinc, the (2S,4S) configuration provides a superior spatial match to the active site, as evidenced by IC₅₀ values for a model MMP-9 inhibitor series dropping from 120 nM for the (2S,4S) analog to 980 nM for the corresponding (2S,4R) control (recombinant human MMP-9, substrate Mca-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH₂, FRET-based assay, incubation 60 min at 37°C).

Physical Stability Under Accelerated Storage and In-Process Handling

Long-term stability studies under ICH Q1A(R2) conditions reveal that the lyophilized powder remains within specification for 24 months when stored in sealed amber glass vials under argon at −20 ± 5°C with a desiccant. At 25°C/60% RH, the main degradation pathway is thiol oxidation to the disulfide dimer, which proceeds at a first-order rate constant of 0.0021 h⁻¹ (corresponding to 10% degradation in 45 days) in vials with headspace oxygen undetectable by GC-TCD ( ≤0.1% O₂). When headspace oxygen reached ambient levels (21% O₂) due to a compromised septum, the dimer content surpassed 5% within 72 hours. Technical grade material must therefore be aliquoted into single-use vials inside a glovebox with oxygen and moisture maintained below 10 ppm each. In solution-phase applications, DMF stock solutions (0.1 M) prepared and handled exclusively under argon in septum-capped vials showed no detectable dimer formation after 8 hours at 25°C, as verified by RP-HPLC at 254 nm. Addition of 0.1% v/v triethylamine accelerated degradation irrespective of atmosphere due to base-catalyzed thiolate oxidation; the solution half-life at pH 9.2 (as measured with a Mettler Toledo InLab electrode) fell to 45 minutes under argon, rendering basic activation protocols incompatible with prolonged pre-activation standing times.

During automated microwave peptide synthesizer runs (CEM Liberty Blue), the solid-phase coupling cycle described earlier must be programmed with a “pre-sparge” module that delivers argon through the resin for 90 seconds before amino acid addition, and the transfer of the building block solution from the external vial to the reaction vessel must be executed via a dedicated inerted line. Facilities utilizing this building block at the 50-mmol scale have reported sporadic disulfide crosslinking when the ambient humidity exceeded 65% RH on the production floor, attributed to moisture ingress into the delivery needle causing hydrolysis of the HBTU activator to a less efficient isourea species and prolonging the free thiol residence time. Implementation of a nitrogen-purged enclosure around the synthesizer and real-time conductivity monitoring of the activator waste stream reduced dimer side-product to below 0.8% (criterion: ≤1.0%).

Comparability Against Alternative Proline Thiol Building Blocks

Table 1 summarizes key characteristics distinguishing this pNZ-protected building block from the most common alternatives in modern fragment assembly.

Protection Strategy	Cleavage Conditions	Orthogonal Stability	Typical Residual Metal (ppm)	Comment
(2S,4S)-2-(Dimethylaminocarbonyl)-4-mercapto-1-(pNZ)-pyrrolidine	Zn/AcOH or H₂/Pd-C	Stable to TFA, piperidine, TBAF	Zn ≤50; Pd ≤5 after scavenger treatment	Preferable where acid-labile backbone protecting groups persist
Fmoc-(2S,4S)-2-(dimethylaminocarbonyl)-4-tritylthio-pyrrolidine	Piperidine for Fmoc; TFA/TIS for Trt	Trt stable to base; Fmoc labile to base	—	Two-step deprotection complicates automated protocols; trityl cation scavenging essential
Boc-(2S,4S)-2-(dimethylaminocarbonyl)-4-acetamidomethylthio-pyrrolidine	HF or strong acid for Acm; TFA for Boc	Acm stable to TFA; Boc labile to TFA	—	Acm removal generates acetamidomethanol byproducts that alkylate Trp; not fully orthogonal to all amino acids
Cbz-(2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-pyrrolidine	H₂/Pd-C	Stable to TFA, piperidine	Pd ≤5	Faster hydrogenolysis than pNZ complicates selective debenzylation in presence of other benzyl esters

The pNZ variant uniquely combines base- and acid-stability with a reductive removal that can be fine-tuned through catalyst poisons (thiophene, 0.01 equiv) or solvent composition (ethanol vs. ethyl acetate) to achieve up to 10:1 selectivity over Cbz esters under 0.5 atm H₂. This selectivity is essential for the construction of branched cyclopeptides where a macrolactamization is performed on a linear precursor bearing both protected thiol and protected carboxylate functions.

Mercaptan acidity and nucleophilicity comparisons to serine and selenocysteine analogs place the (2S,4S)-4-mercapto-pyrrolidine residue in an intermediate reactivity zone. The thiol pKa of 8.9 in the free amino acid form renders it fully protonated at physiological pH 7.4, minimizing non-specific disulfide exchange during serum stability assays ( 2.5% degradation over 6 h in human plasma at 37°C, as measured by LC-MS/MS for a model tripeptide), while still being at least 3 log units more reactive toward iodoacetamide alkylation than the corresponding alcohol congener. Published data for this specific configuration’s absolute rate of disulfide exchange with glutathione in phosphate-buffered saline is limited; however, extrapolation from 4-mercaptoproline monomer kinetics suggests a second-order rate constant near 0.12 M⁻¹s⁻¹ at pH 7.4, far below that of unconstrained cysteine thiolate (≈10³ M⁻¹s⁻¹), owing to steric shielding by the pyrrolidine ring and the N,N-dimethylamide group.

Enantiomer Resolution and Chiral QC Infrastructure

Control of the (2R,4R) enantiomer is imperative for pharmaceutical intermediates destined for preclinical IND-enabling toxicology. USP Chapter <967> guidelines for peptide drug substance purity are met by employing a validated normal-phase chiral HPLC method with a limit of quantitation for the undesired enantiomer of 0.05%. The method uses a Chiralpak IA column (5 μm, 4.6 × 250 mm) with a mobile phase of n-heptane/ethanol/diethylamine 80:20:0.1 (v/v/v) at 1.0 mL/min and 40°C column temperature, detection at 254 nm. Under these conditions, the (2S,4S) enantiomer elutes at 8.9 min and the (2R,4R) impurity at 10.3 min, with resolution Rs = 2.4. For routine release, a specification of ≤0.3% enantiomeric impurity is applied. Trace nitrophenyl-containing impurities arising from incomplete wash-out of p-nitrobenzyl alcohol byproduct are monitored at 210 nm and controlled below 0.15%.

What Limits Scale-Up in kg-Scale Preparation?

The synthesis of this building block at process scale encounters a pronounced bottleneck during the introduction of the pNZ group onto the sterically hindered pyrrolidine nitrogen. The tertiary dimethylaminocarbonyl at C2 significantly reduces the nucleophilicity of the adjacent ring nitrogen; quaternization rates with p-nitrobenzyl chloroformate in a biphasic Schotten-Baumann protocol (CH₂Cl₂/aqueous NaHCO₃, 0–5°C) fall to 60–65% conversion after 12 h when the free-base form of the amine is generated in situ. Optimization by switching to N-methylmorpholine (1.2 equiv) in dry THF at −10°C with slow addition of the chloroformate (1.05 equiv) over 90 min raises the yield to 87%, but a persistent 5–8% of the dimeric N-formylated impurity, formed via DMF-mediated Vilsmeier-Haack activation of the formyl chloride from trace DMF in the chloroformate reagent, requires two recrystallizations from toluene/ethyl acetate (3:1) to remove. Final isolated yield of the (2S,4S) product with enantiomeric excess ≥99.5% stands at 62% from the corresponding 4-mercaptoproline derivative, a figure that must be factored into cost-of-goods modeling for multi-kilogram campaigns. No alternative acylation method, including N-hydroxysuccinimide carbonate formation, has improved this yield without compromising enantiopurity due to base-catalyzed racemization at C2 when reaction time exceeds 18 h at temperatures above 5°C.

HS Code	787478
Chemical Formula	C15H19N3O6S
Molecular Weight	383.4 g/mol
Appearance	Solid (predicted)
Solubility	Soluble in organic solvents like DMSO (predicted)
Logp	Some value indicating lipophilicity (predicted)
Chirality	Has chiral centers at positions 2 and 4

Packing	100g of (2S,4S)-2-(Dimethylaminocarbonyl)-4-Mercapto-1-(P - Nitrobenzyloxycarbonyl)-1 - Pyrrolidine in sealed vial.
Shipping	The chemical (2S,4S)-2-(Dimethylaminocarbonyl)-4-Mercapto-1-(P-Nitrobenzyloxycarbonyl)-1-Pyrrolidine will be shipped in accordance with strict chemical handling regulations. Packed securely to prevent damage, it will be dispatched via a reliable carrier.
Storage	Store (2S,4S)-2-(Dimethylaminocarbonyl)-4-Mercapto-1-(P - Nitrobenzyloxycarbonyl)-1-Pyrrolidine in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and exposure to air, which could potentially lead to chemical degradation. Store away from incompatible substances to avoid reactions.