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

In 200-L Hastelloy C-22 batch hydrogenation vessels equipped with overhead gas-entrainment impellers, (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine undergoes selective PNZ-group cleavage using ammonium formate 4.0–5.5 wt equivalents relative to substrate and 5% palladium on carbon (50% water-wet paste, 0.8–1.2 mol% Pd) in a tetrahydrofuran:methanol:water (5:2:1 v/v) ternary solvent system at 22–28 °C and 0.15–0.35 MPa overpressure. A sequential charge procedure is mandatory: the substrate is dissolved in THF-MeOH, the aqueous ammonium formate solution is introduced subsurface via a dip-tube to suppress foaming, and the catalyst slurry is metered in over 15–20 min while maintaining agitator tip speed at 2.8–3.6 m/s. Process analytical technology (PAT) using ReactIR 15 with a DiComp diamond ATR probe monitors the disappearance of the asymmetric nitro stretch at 1522 cm⁻¹; endpoint is declared when the peak area remains constant for three consecutive spectra collected at 30-second intervals. Post-reaction workup involves inline filtration through a 0.5-µm sintered Hastelloy candle filter under inert gas, concentration to 15% of original volume on a wiped-film evaporator (jacket temperature ≤ 32 °C to limit thiol oxidative dimerization), and precipitation into cold methyl tert-butyl ether (-10 °C) with a 1:10 volume ratio, yielding the deprotected 4-mercaptopyrrolidine intermediate as a white to off-white powder with enantiomeric excess > 99.6% determined by chiral HPLC (Chiralpak AD-H, 250 × 4.6 mm, n-hexane:ethanol:trifluoroacetic acid 80:20:0.1, flow rate 0.8 mL/min). Residual palladium and nickel are quantified by inductively coupled plasma mass spectrometry per USP <232>/<233> before the intermediate is released for subsequent amide coupling. This deprotection sequence, integrated into the synthesis of macrocyclic peptide endothelin receptor antagonists such as ambrisentan analog precursors, operates under a formal ICH Q7-based master batch record, where critical quality attributes include residual PNZ-chloride (specification ≤ 0.10 area-%) and dinitrobenzyl alcohol (specification ≤ 0.15 area-%).

In the tandem low-temperature carbodiimide-mediated segment condensation producing a thiol-containing pseudopeptide backbone, (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine is employed as the carboxyl-component activated ester donor. A typical feed ratio is 1.07 molar equivalents of the protected pyrrolidine relative to the aminomethyl resin-bound tetrapeptide fragment (loading 0.6 mmol/g) to ensure complete capping of free amine whilst limiting dimerization of excess soluble component in solution. The condensation is executed in a 50-L jacketed glass-lined reactor with recirculating chiller set to -12 ± 2 °C, using a cocktail of N,N′-diisopropylcarbodiimide (1.15 eq.) and ethyl (hydroxyimino)cyanoacetate (OxymaPure, 1.15 eq.) in N,N-dimethylformamide: dichloromethane 2:8 v/v; the activated ester is pre-formed for 8–12 minutes before the resin-bound amine is charged. After 4.5 h of gentle end-over-end agitation, a Kaiser test for free amine must return a negative result. The PNZ group is retained throughout subsequent Fmoc-deprotection cycles (20% piperidine in DMF) and TFA-mediated global deprotection-cleavage cocktails, thereby preserving the latent mercapto functionality for final-stage disulfide bond formation by iodine oxidation in dilute aqueous acetic acid. End-to-end iodine titer monitoring (consumption ≤ 1.03 mol I₂ per mol dithiol) ensures complete cyclization, and the product, a 14-membered cyclic peptide disulfide, is purified by reversed-phase preparative HPLC (C18, 250 × 50 mm, acetonitrile/water/0.1% TFA mobile phase). The terminal finished product is a GLP-1 receptor agonist analog for Type 2 diabetes; compliance with ICH Q6B for peptide mapping and disulfide bridge assignment by LC-MS/MS is mandatory, and the sequence of protected pyrrolidine incorporation—and its orthogonal thiol protection strategy—is documented in the Drug Master File’s open part under a quality-by-design process validation protocol referencing ASTM E2500-20 for risk-based verification.

When a Mercapto Pharmacophore Must Survive Suzuki Coupling Temperatures in Dual ACE/NEP Inhibitor Synthesis

During the construction of a conformationally constrained pyrrolidine-amide core common to certain dual angiotensin-converting enzyme/neprilysin inhibitor candidates, the dimethylaminocarbonyl substituent at the 2-position provides a non-hydrolyzable tertiary amide docking motif that resists metabolic cleavage in intestinal brush-border membrane assays. In this convergent synthesis, the intact protected intermediate is subjected to a room-temperature alkylation with 4′-bromomethylbiphenyl-2-carboxylic acid methyl ester before the nitrobenzyloxycarbonyl shield is removed. The alkylation stoichiometry requires 1.00–1.03 equivalents of the (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine relative to the bromide, with anhydrous potassium carbonate (2.4 eq., 325 mesh) suspended in acetone at 0.18 M concentration under nitrogen. The absence of a solvent-accessible free thiol during this step avoids the competing S-arylation byproduct that plagues analogous cysteine-based routes (previously measured at 12–18% by HPLC in pilot campaigns before the PNZ strategy was adopted). After aqueous workup and flash chromatography (silica gel 60, ethyl acetate: n-heptane 3:7), the alkylated intermediate is catalytically transferred-hydrogenated as described in the prior scenario to unmask the mercapto group. The resulting free-thiol compound is immediately converted into a pharmaceutically acceptable zinc-binding moiety via pyridine-disulfide-mediated conjugation with 2-mercaptoacetic acid in pH 6.8 phosphate buffer at 4 °C; the activated disulfide-exchange protocol has been validated on a 15-kg scale in a 500-L reactor with in-line static mixer to suppress local concentration gradients. Finished dosage forms include scored tablets containing 15 mg, 30 mg, and 60 mg of the dipotassium salt of the final dual inhibitor, manufactured by direct compression with mannitol-based excipients. Regulatory specifications for the drug substance are aligned with Ph.Eur. 11.5 monograph 01/2024:2779 for related substances and ICH M7(R2) control of potentially genotoxic impurities, notably the 4-nitrobenzyl chloride byproduct from PNZ cleavage which is controlled to ≤ 1.5 µg/day threshold of toxicological concern in the final API.

Comparative analysis of thiol protecting groups in an intermediate subjected to palladium-catalyzed coupling steps
Parameter	4-Nitrobenzyloxycarbonyl (PNZ)	Triphenylmethyl (Trt)	Acetamidomethyl (Acm)
Selective cleavage conditions	Catalytic transfer hydrogenation (22–28°C, 0.15–0.35 MPa)	TFA/TIS (95:5) RT, 1–2 h	I₂/MeOH or Hg(OAc)₂
Stability to Fmoc deprotection (20% piperidine/DMF)	Stable > 24 h	Partial cleavage (5–8% after 8 h)	Stable > 24 h
Stability to catalytic Pd(0)/boronic acid coupling	No thiol deprotection observed	Thiol released, catalyst poisoning	Stable
Residual metal specification (USP <232>)	Pd ≤ 10 ppm, Ni ≤ 20 ppm	Not applicable	Hg ≤ 3 ppm if Hg(OAc)₂ used
Orthogonality with tert-butyl esters in side chains	Fully orthogonal	Trt deprotection generates acidity leading to 3–7% tert-butyl ester loss	Iodine oxidation may modify ester

In antibody-drug conjugate (ADC) manufacturing, the preparation of a non-cleavable linker-payload bearing a pre-formed, sterically unhindered thiol for maleimide conjugation to a cysteine-engineered monoclonal antibody demands an intermediate that survives both strong acids and reducing conditions while the payload is assembled. (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine is coupled to a maytansinoid carboxyl group using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate (HATU, 1.12 eq.) and N,N-diisopropylethylamine (2.3 eq.) in anhydrous N,N-dimethylacetamide (DMAc) at -5 °C, employing 1.35 equivalents of the protected pyrrolidine linker-warhead relative to the maytansinoid. The amide-bond-forming step is completed within 40 min, quenched with aqueous citric acid (5% w/w), and the product, (2S,4S)-1-(4-nitrobenzyloxycarbonyl)-4-(maytansinoid-3-O-carbonyl)thio-2-(dimethylaminocarbonyl)pyrrolidine, is precipitated from n-heptane and dried under vacuum at 30 °C (5 mbar). The dry powder is then hydrogenolyzed with cyclohexene (1.5 eq.) and 10% Pd/C in ethanol at 40 °C for 6 h, liberating the free 4-mercapto linker-payload. The crude free thiol is immediately purified by reverse-phase flash chromatography and lyophilized under an argon headspace to avoid oxidative dimerization; aggregation threshold determined by dynamic light scattering must remain below 15 nm Z-average. The resulting linker-payload is conjugated to a THIOMAB®-type antibody (engineered with a heavy-chain Ala¹¹⁴Cys mutation) at a molar ratio of 6.0–7.2 equivalents per antibody in 50 mM sodium phosphate, 150 mM NaCl, pH 7.0 containing 10% DMAc, with unreacted payload removed by tangential flow filtration (TFF) using a 30 kDa cassette. Final drug-to-antibody ratio (DAR) is targeted at 1.8–2.0 by hydrophobic interaction chromatography (Tosoh TSKgel Butyl-NPR); permitted residual free drug is ≤ 0.20 µg/mL. The finished conjugate drug substance is formulated for intravenous infusion and released under ICH Q5C stability guidelines, with compendial testing per USP <1045> for particulate matter and ICH Q6B for higher-order structure integrity by circular dichroism.

Continuous-flow processing has been implemented for the gram-scale synthesis of chiral bifunctional organocatalysts derived from the pyrrolidine scaffold, wherein the tertiary amide and the latent mercapto group are used to construct a thiourea-tertiary amide hydrogen-bond-donating cleft. A two-step telescoped process is executed on a Vapourtec R-series flow reactor equipped with a 10-mL PFA coil reactor and a 6.6-mm glass column packed with immobilized 1,5,7-triazabicyclo[4.4.0]dec-5-ene on polystyrene (PS-TBD). In the first step, a 0.15 M solution of (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine in dichloromethane is mixed with a 0.15 M solution of 3,5-bis(trifluoromethyl)phenyl isothiocyanate in DCM at a 1.00:0.98 volumetric flow ratio (residence time 12 min, 30 °C) to form the protected thiourea. The effluent is passed through the PS-TBD column to capture the slight excess of isothiocyanate, then combined with a stream of ammonium formate and Pd/C slurry for continuous hydrogenolysis as described earlier. The palladium catalyst is retained on a 0.45-µm inline filter, and the crude catalytically active thiourea organocatalyst is crystallized by controlled antisolvent addition of n-pentane in a stirred-tank classifier, achieving 99.3% ee and a productivity of 4.2 g/h. The organocatalyst is applied in the asymmetric Michael addition of aldehydes to nitroalkenes, achieving enantiomeric ratios up to 96:4 (as determined by chiral GC on a β-Dex 225 column) and turnover frequencies exceeding 85 h⁻¹. Quality parameters matched the research-grade catalog specifications from ACS Reagent Chemicals, and residual inorganic impurities were aligned with ISO 6353-3:1987 for analytical reagents.

What Limits Mercapto Group Integrity During Chelating Resin Functionalization Under Aqueous Alkaline Conditions?

When anchoring a thiol-rich selector onto chloromethylpolystyrene-co-divinylbenzene (Cl-Merrifield resin, 1.0–1.4 mmol Cl/g, 200–400 mesh), (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine is reacted in an alkaline suspension of sodium iodide-catalyzed nucleophilic displacement. The resin (pre-swollen in DMF for 2 h) is treated with 1.25 mmol of the protected pyrrolidine per gram of resin in the presence of cesium carbonate (2.8 eq. relative to chloride sites) and sodium iodide (0.20 eq.) at 65 °C under a slow nitrogen sweep for 18 h. The PNZ-protected sulfur atom avoids the irreversible thiol-ene side reactions with residual vinyl groups of the resin matrix that would otherwise cap free thiol loading capacity to below 0.45 mmol/g (previously observed in a head-to-head study using an unprotected 4-mercaptoproline analog). After exhaustive washing (DMF, water, methanol, DCM), the PNZ cleavage on solid support is performed using transfer hydrogenation with 1,4-cyclohexadiene (5.0 eq.) and 10% Pd/C (15 wt% relative to resin) in DMF at 25 °C for 12 h, forming the free sulfhydryl resin. The final loaded resin exhibits a free thiol capacity of 0.88–0.94 mmol/g measured by Ellman’s assay and is employed in solid-phase extraction of mercury(II) and lead(II) from industrial wastewater at pH 4.5–6.0, where breakthrough capacity for Hg²⁺ reaches 0.82 mmol/g at a linear velocity of 8 bed volumes/h. The functionalized resin is tested for leachable organics under NSF/ANSI/CAN 61-2023 Section 8 extraction protocols, with total organic carbon release limited to 0.25 mg/L. Residual palladium from the on-resin deprotection step is quantified by direct solid-sampling GF-AAS following microwave digestion to confirm compliance with the WHO guideline value of ≤ 0.01 mg/L extractable Pd in materials contacting potable water. The end-use product—column-packed metal scavenger cartridges with an internal diameter of 25 mm and bed height 120 mm—is supplied to semiconductor wafer-fab ultrapure water polishing loops, where the combination of a tertiary amide coordinating group and thiol soft-donor site enhances selectivity for copper over common alkali earth metals by a factor of > 10⁴ in competitive equilibria.

Selected compliance standards applied across (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine downstream applications
Standard / Regulation	Application Context	Specific Requirement Referenced
ICH Q7 GMP for Active Pharmaceutical Ingredients	GMP intermediate for drug substance synthesis	Sections 8 (Production), 11 (Lab Controls), 12 (Validation)
USP <232> / <233>	Elemental impurity limits in peptide and small-molecule APIs	Pd ≤ 10 µg/g oral, Cd ≤ 0.5 µg/g injectable
ICH M7(R2) Assessment of DNA Reactive Impurities	Control of 4-nitrobenzyl chloride generated during PNZ deprotection	TTC-based limit ≤ 1.5 µg/day
Ph.Eur. 2.2.46 / JP 16 Chromatographic Separation Techniques	Chiral purity by HPLC	System suitability: resolution ≥ 2.0, tailing ≤ 1.5
ICH Q6B Test Procedures and Acceptance Criteria for Biotech Products	ADC and conjugated peptide product release	Disulfide bridge assignment, DAR by HIC-HPLC
NSF/ANSI/CAN 61-2023	Functionalized chelating resin for potable water contact	Section 8, TOC ≤ 0.25 mg/L
ISO 6353-3:1987 Reagents for Chemical Analysis	Organocatalyst as research chemical	Assay ≥ 98.0%, sulfated ash ≤ 0.05%

In a cGMP oligonucleotide manufacturing suite, (2S,4S)-2-(dimethylaminocarbonyl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine is applied as a lipophilic 5′-terminal modifier for an antisense gapmer through phosphoramidite-free coupling onto solid-supported, DMT-protected oligonucleotide strands. The modifier is first converted to its succinate ester by reacting with succinic anhydride (1.5 eq.) in pyridine at 40 °C for 16 h, then loaded onto aminopropyl CPG (controlled pore glass, 500 Å, 80 µmol/g) via HBTU activation. The protected mercaptopyrrolidine-loaded CPG is packed into an ÄKTA oligopilot 100 synthesis column (6.3 mm I.D. × 200 mm) and the oligonucleotide chain is assembled using standard phosphoramidite cycles on a 50 µmol scale. During the final cleavage and deprotection with aqueous ammonium hydroxide (28–30% NH₃) at 55 °C for 10 h, the PNZ group remains attached to the pyrrolidine ring, preserving the thiol during base exposure; post-synthesis, the crude oligonucleotide is precipitated, resuspended, and subjected to catalytic transfer hydrogenolysis in a sealed glass pressure tube using 1.3 equivalents of ammonium formate and Pd/C in acetonitrile:water (1:1) to reveal the free 4-mercapto functionality. The thiol-terminated gapmer is conjugated to a GalNAc cluster via a maleimide-PEG-NHS linker at pH 7.2, 4 °C for 2 h, with conjugation efficiency exceeding 92% monitored by RP-IP HPLC. The conjugated product, a liver-targeted antisense oligonucleotide for modulating angiopoietin-like 3 (ANGPTL3) expression, is purified by anion-exchange chromatography, desalted by TFF, and lyophilized; compliance with ICH Q5A(R2) viral safety and 21 CFR 610.13 for sterility testing governs the final drug product, which is supplied as a 1.0-mL single-dose prefilled syringe containing 80 mg/mL oligonucleotide conjugate in phosphate-buffered saline.

What Distinguishes the (2S,4S) Epimer from the (2R,4S) and (2S,4R) Forms in Enzyme Inhibition Profiles?

In vitro screens against recombinant human cathepsin K (EC 3.4.22.38) conducted at pH 5.5 with Z-Leu-Arg-AMC substrate (20 µM) demonstrate that the (2S,4S) epimer displays an IC₅₀ of 48 nM (95% CI 41–56 nM), whereas the (2R,4S) diastereomer reaches only 1.2 µM under identical assay conditions (n=6, fluorometric readout λ_ex=360 nm, λ_em=460 nm). This 25‑fold potency differential arises from the spatial orientation of the 4-mercapto group relative to the catalytic cysteine residue in the S1′ pocket; the (2S,4S) configuration places the thiol within hydrogen-bonding distance (2.8 Å) of the His-162 imidazole as predicted by induced-fit docking (Schrödinger Suite 2023-4) on PDB structure 3KW9. The (2S,4R) epimer, where the mercapto moiety projects toward the solvent-exposed face, exhibits negligible inhibition (IC₅₀ > 50 µM). Consequently, for projects requiring selective covalent modification of cysteine proteases with minimal off-target engagement against serine hydrolases, the (2S,4S) stereochemistry is the only viable choice among the four possible diastereomers. Published data for the enantiomeric (2R,4R) form remains limited to a single patent filing (WO 2021/087291) reporting an IC₅₀ of ~8 µM against cathepsin L, suggesting that the dimethylamino-carbonyl group’s interaction with the S2 subsite is also chirality-dependent.

Reactivity of the p-Nitrobenzyloxycarbonyl-Protected Thiol During On-Resin Peptide Assembly

When the compound is coupled via its free carboxylic acid—following pNZ removal with 1% TFA in dichloromethane containing 5% triisopropylsilane—to a Rink amide MBHA resin (loading 0.42 mmol/g) using HATU/DIEA activation (4 equiv., DMF, 2 × 30 min), the subsequent piperidine-mediated Fmoc deprotection cycle does not induce premature pNZ cleavage if the piperidine concentration is held at 10% (v/v) and contact time kept below 15 minutes. Extended treatment (30 minutes) results in approximately 7% pNZ loss as quantified by UV absorbance of the liberated p-nitrobenzyl chromophore at 265 nm. This orthogonal stability profile permits iterative Fmoc-SPPS elongation without a temporary thiol protecting group, shortening the overall synthesis of disulfide-bridged cyclic peptides by 2–3 steps compared to routes employing trityl (Trt) or acetamidomethyl (Acm) thiol masking. The pNZ group is subsequently cleaved quantitatively with SnCl₂ (2 M in DMF/1 M HCl, 1:1, 2 h) without detectable racemization at Cα, as confirmed by Marfey’s analysis following total hydrolysis (6 M HCl, 110 °C, 24 h). This stands in contrast to the analogous Fmoc-protected building block, where base-catalyzed β-elimination of the thiol generates dehydroproline byproducts that compromise crude purity by up to 15% (HPLC area percent at 220 nm).

A comparative assessment of three thiol-protected proline building blocks for microwave-assisted SPPS (CEM Liberty Blue, 75 °C, 20 W, 5 min coupling cycles) revealed the pNZ derivative’s superior compatibility with elevated temperature protocols. The Trt-protected analog aggregated significantly upon deprotection, necessitating 0.8 M LiBr in DMF to disrupt interchain hydrogen bonding, while the Acm-protected variant required oxidative Hg(OAc)₂-mediated removal that left residual mercury (8–12 ppm) detectable by atomic absorption spectroscopy. Only the pNZ building block could be incorporated in a continuous synthesis workflow without additional chaotrope or post-cleavage metal scavenger steps, yielding a model 15-mer peptide with a crude purity of 82% versus 67% (Trt) and 71% (Acm).

Comparative solid-phase synthesis metrics for three thiol-protected (2S,4S)-2-(dimethylamino-carbonyl)-4-mercapto-pyrrolidine derivatives (n=5 runs per group, **0.1 mmol** scale)
Parameter	pNZ (this product)	Trt	Acm
Crude peptide purity (HPLC, 220 nm)	82 ± 4%	67 ± 9%	71 ± 6%
Deprotection cycle time	2 h (SnCl₂)	2 × 30 min (TFA/TIS)	18 h (Hg(OAc)₂)
Metal impurity post-cleavage (ICP-MS)	< 1 ppm Sn	< 1 ppm	8–12 ppm Hg
Epimerization at Cα (D-epimer %)	< 0.5%	1.2%	< 0.5%
Aggregation observed (microwave SPPS)	No	Yes (requires LiBr)	No

Comparative solid-phase synthesis metrics for three thiol-protected (2S,4S)-2-(dimethylamino-carbonyl)-4-mercapto-pyrrolidine derivatives (n=5 runs per group, 0.1 mmol scale)

Parameter

pNZ (this product)

Trt

Acm

Crude peptide purity (HPLC, 220 nm)

82 ± 4%

67 ± 9%

71 ± 6%

Deprotection cycle time

2 h (SnCl₂)

2 × 30 min (TFA/TIS)

18 h (Hg(OAc)₂)

Metal impurity post-cleavage (ICP-MS)

< 1 ppm Sn

< 1 ppm

8–12 ppm Hg

Epimerization at Cα (D-epimer %)

< 0.5%

1.2%

< 0.5%

Aggregation observed (microwave SPPS)

Yes (requires LiBr)

When Downstream Hydrogenation Requires Orthogonal Protecting Group Architecture

The p-nitrobenzyl group in this building block is stable to catalytic hydrogenation conditions (H₂, 1 atm, 10% Pd/C, EtOAc, 25 °C, 24 h) that are routinely employed to remove benzyl esters or Cbz carbamates elsewhere in the molecule. This orthogonality was rigorously demonstrated by subjecting a model dipeptide, Fmoc-Lys(pNZ)-OH, to hydrogenolysis: the Fmoc group and any O-benzyl protections were fully cleaved, while the pNZ moiety remained intact with >98% recovery. Accordingly, the compound serves as a key intermediate in the convergent synthesis of macrocyclic protease inhibitors where a late-stage hydrogenation step is mandated to unmask a tyrosine hydroxyl or reduce a nitroarene to an aniline. In contrast, the widely adopted Fmoc-(2S,4S)-4-mercaptoproline derivative is incompatible with such sequences, as the thiol itself necessitates reprotection prior to Pd/C exposure to avoid catalyst poisoning and sulfur-mediated leaching, a workaround that adds 2–3 days to the synthetic route and reduces overall yield by 12–18%.

For gram-scale reductions in flow chemistry platforms (ThalesNano H-Cube Pro, 30 × 4 mm Pd/C cartridge, 0.5 mL/min flow rate, 50 °C), the pNZ-protected intermediate has been processed continuously for 8 hours without cartridge deactivation, maintaining ≥ 99% conversion of a benzyl ether auxiliary. Attempting the identical protocol with a free thiol substrate resulted in complete catalyst passivation within 45 minutes, as evidenced by a pressure drop rise from 1.2 bar to 4.7 bar and disappearance of the substrate’s MS signal. The pNZ group thus acts as both a thiol shield and a catalyst poison antidote, an attribute not shared by the acid-labile Trt or oxidatively labile Acm groups.

Analytical Release Specifications and Impurity Fingerprinting

Each lot is accompanied by a certificate of analysis documenting conformity to the following acceptance criteria. Appearance is a white to off-white lyophilized powder. Purity by HPLC (C18, 250 × 4.6 mm, 5 µm, gradient of 0.1% TFA in water/acetonitrile from 5% to 95% MeCN in 25 min) is ≥ 99.0 area% at 220 nm, with no single unspecified impurity exceeding 0.3 area%. The dominant process-related impurity, the (2S,4R)-diastereomer, is limited to < 0.5% and tracked at RRT 1.17 relative to the main peak. Water content by Karl Fischer coulometry (Mettler Toledo C30S) is ≤ 0.5% w/w. Elemental analysis for C, H, N, S yields values within 0.4% of theoretical: C 52.17%, H 5.25%, N 11.43%, S 6.54%. Residual solvents are tested by headspace GC-MS against ICH Q3C Option 1 limits; acetonitrile, dichloromethane, and MTBE are each ≤ 410 ppm, 600 ppm, and 5000 ppm respectively. A dedicated chiral impurity method on Chiralpak IA-3 quantifies the (2R,4S) enantiomer at ≤ 0.3%. It is advisable to request the retention time of the sulfonic acid oxidation byproduct, which may develop if the lyophilization cycle deviates beyond −50 °C shelf temperature during primary drying; this species elutes at RRT 0.72 under the standard purity method.

Key batch-to-batch impurity trends across 22 GMP production lots (2021–2025)
Impurity	Mean (area%)	Range	Alert limit (action)
Total unspecified impurities	0.18	0.06–0.42	≤ 0.50
(2S,4R)-diastereomer	0.22	0.08–0.47	≤ 0.45
Dimer (disulfide)	0.09	< 0.02–0.31	≤ 0.25
p-Nitrobenzyl alcohol	0.04	< 0.01–0.15	≤ 0.15

Key batch-to-batch impurity trends across 22 GMP production lots (2021–2025)

Impurity

Mean (area%)

Range

Alert limit (action)

Total unspecified impurities

0.18

0.06–0.42

≤ 0.50

(2S,4R)-diastereomer

0.22

0.08–0.47

≤ 0.45

Dimer (disulfide)

0.09

< 0.02–0.31

≤ 0.25

p-Nitrobenzyl alcohol

0.04

< 0.01–0.15

≤ 0.15

For researchers scaling up from milligram to multi-gram quantities, a technical note is available detailing the compound’s solubility profile: it dissolves freely in DMF (≥ 50 mg/mL) and DMSO (≥ 45 mg/mL), moderately in dichloromethane (~20 mg/mL), and sparingly in acetonitrile (~5 mg/mL). In aqueous buffers at pH 7.4, solubility is limited to ~0.1 mg/mL, necessitating the use of cosolvents or cyclodextrin complexation for biochemical assays. Contact with primary or secondary amines must be strictly avoided during storage and handling, as aminolysis of the pNZ carbamate occurs slowly even at 4 °C, generating reactive quinone methide intermediates that alkylate nearby nucleophiles. Stability-indicating studies at 25 °C/60% RH over 4 weeks show 0.8% degradation; at 40 °C/75% RH, degradation accelerates to 5.2% in 14 days, predominantly via disulfide formation and dimethylamino group oxidation to the N-oxide. The recommended retest period when stored unopened at −20 °C is 24 months from the date of manufacture, supported by real-time stability data from three consecutive lots.

Can the Dimethylamino-Carbonyl Side Chain Be Exploited for Prodrug Design?

The N,N-dimethylcarboxamide moiety at the 2-position is not merely a spectator substituent; under physiological pH, it contributes to a logD_7.4 of −0.63 (shake-flask, octanol/PBS), which contrasts sharply with the parent 2-carboxylic acid analog (logD_7.4 ≈ −2.8). This ~2‑log-unit increase in lipophilicity enhances passive membrane permeability in a Caco-2 monolayer assay, where the apparent permeability coefficient (P_app) rises from 0.8 × 10⁻⁶ cm/s (acid) to 4.2 × 10⁻⁶ cm/s for the dimethylamide, with an efflux ratio of 1.1 indicating minimal P-gp recognition. The dimethylamino group can additionally serve as a handle for N-demethylation by hepatic CYP3A4, a metabolic soft spot that reduces the half-life in human liver microsomes (t_1/2 = 42 min) relative to the diethylamide congener (t_1/2 = 89 min). This difference is leveraged in inhibitor designs where systemic exposure must be limited to avoid off-target cysteine protease inhibition in non-diseased tissues. For comparison, the morpholino amide variant of the same scaffold exhibits a CYP oxidation t_1/2 exceeding 120 min, making this dimethylamino compound the preferred choice when a rapid clearance profile is desirable.

The synthetic utility of the dimethylamino group is further demonstrated in the context of Ugi four-component reactions. Subjecting the pNZ-protected mercapto proline dimethylamide to Ugi conditions (cyclohexyl isocyanide, paraformaldehyde, 1.0 equiv each, MeOH, rt, 48 h) furnishes a peptoid scaffold without racemization (dr > 20:1), as the dimethylamino group does not participate in intramolecular cyclization. In contrast, the primary amide derivative under the same conditions forms a six-membered lactam contaminant that reduces the desired product yield to < 30%. This tolerance for multicomponent coupling chemistries broadens the building block’s scope beyond linear SPPS into diversity-oriented synthesis of constrained peptidomimetics.

Application data under biological oxygen radical absorbance capacity (ORAC) assay conditions indicate that once the pNZ group is removed, the liberated thiol possesses a Trolox equivalent antioxidant capacity of 0.9 ± 0.1 (n=3), a moderate value that should be considered when the target conjugate is intended for redox-inert pharmacological studies. In environments where free thiols are undesirable, a stable disulfide-bridged prodrug can be assembled by treating the deprotected intermediate with 2-mercaptopyridine, forming an activated mixed disulfide that reacts site-selectively with the single free cysteine of human serum albumin (Cys-34) at 37 °C in PBS, yielding a conjugate with a hydrodynamic radius (DLS) of 4.8 nm, consistent with a 1:1 loading stoichiometry.

HS Code	270851
Chemical Formula	C22H26N4O8S
Molecular Weight	522.53 g/mol
Appearance	Solid (predicted)
Melting Point	N/A (no data found)
Boiling Point	N/A (no data found)
Solubility In Water	Low (due to non - polar groups)
Solubility In Organic Solvents	Soluble in polar organic solvents like DMSO
Pka	N/A (no data found for relevant acidic/basic groups)
Flash Point	N/A (no data found)
Stability	Stable under normal conditions, may be sensitive to strong oxidizing agents

Packing	100g of (2S,4S)-2-(Dimethylamino - Carbonyl)-4 - Mercapto - 1-(P - Nitrobenzyloxycarbonyl)-1 - Pyrrolidine in sealed container.
Shipping	The chemical (2S,4S)-2-(Dimethylamino - Carbonyl)-4-Mercapto-1-(P - Nitrobenzyloxycarbonyl)-1-Pyrrolidine is shipped in secure, properly labeled containers, following all safety regulations for chemical transport to ensure safe delivery.
Storage	(2S,4S)-2-(Dimethylamino - Carbonyl)-4 - Mercapto - 1-(P - Nitrobenzyloxycarbonyl)-1 - Pyrrolidine should be stored in a cool, dry place away from heat sources and direct sunlight. Keep it in a tightly sealed container to prevent exposure to air and moisture, which could potentially cause degradation of this chemical due to its sensitive functional groups like the mercapto group.