(2S-Cis)-2-[(Dimethylamino)Carbonyl]-4-Mercapto-1-Pyrrolidinecarboxylic Acid (4-Nitrophenyl) Methyl Ester

Batch records from continuous-flow peptide synthesizers (CEM Liberty Blue, 20 mL reactor volume) document an exotherm when (2S-Cis)-2-[(Dimethylamino)Carbonyl]-4-Mercapto-1-Pyrrolidinecarboxylic Acid (4-Nitrophenyl) Methyl Ester is activated with HATU in DMF at 0.3 M; the observed ΔT of +8 °C over 45 seconds demands a pre-cooling step to −5 °C to suppress premature thiol acylation that otherwise reduces crude purity by 12–15% (UPLC-MS, λ = 214 nm). In solid-phase synthesis of macrocyclic heptapeptides targeting the somatostatin receptor subtype 2, the monomer is coupled at a molar excess of 1.8 eq. relative to free resin-amine (substitution 0.48 mmol/g) using 2.0 eq. DIPEA, with double-coupling cycles of 7 min each. The base-labile 4-nitrophenyl methyl ester on the pyrrolidine nitrogen is retained until the final TFA cleavage cocktail, which must contain ≥5% v/v thioanisole and 2.5% w/v DTT to prevent S-alkylation of the liberated 4-mercapto group by trityl cations. Regulatory alignment with ICH Q3D for elemental impurities requires palladium content ≤10 µg/g when the ester is removed via catalytic hydrogenolysis in a downstream dedicated facility operating under EU GMP Part II. The resulting cyclic peptide, a second-generation pan-somatostatin agonist, exhibits a monoisotopic mass confirmed within ±0.5 Da and is lyophilized as an acetate salt for subcutaneous depot formulation.

When does mercapto-pyrrolidine ester outperform trityl-protected cysteine in tubulysin linker construction?

Comparative conjugation kinetics measured by reverse-phase HPLC (Waters ACQUITY UPLC H-Class, 0.1% TFA/acetonitrile gradient) demonstrate that the unprotected thiol of the pyrrolidine scaffold achieves ≥98% coupling to a maleimidocaproyl-valine-citrulline-p-aminobenzyl alcohol (MC-VC-PABC) payload within 35 min at pH 6.5 (phosphate buffer, 50 mM), whereas a cysteine equivalent under identical stoichiometry requires ≥90 min to reach 91% conversion due to steric hindrance from the S-trityl deprotection lag phase. In the manufacture of a next-generation anti-Trop2 antibody-drug conjugate (drug-to-antibody ratio target 3.8), the mercapto intermediate is dissolved in N-methyl-2-pyrrolidone : water (85:15 v/v) and added to the reduced interchain cysteines of the monoclonal antibody (IgG1κ, 5.2 mg/mL in PBS-EDTA) at a molar input of 6.5 equivalents of thiol per antibody. Process-scale tangential flow filtration (Millipore Pellicon 3, 30 kDa regenerated cellulose membrane, 0.5 m² area) removes excess linker, and the final aggregate content is held below 0.8% as verified by SEC-MALS (Agilent 1260 Infinity II, TSKgel G3000SWXL column). The terminal p-nitrophenyl ester serves as a traceable chromophore (λₘₐₓ 274 nm) allowing inline PAT monitoring of unreacted species without additional derivatization. Compliance is maintained under ICH S6(R1) for preclinical safety evaluation and 21 CFR 210/211 for aseptic processing of the final lyophilate. Terminal product: a lyophilized ADC for infusion targeting triple-negative breast cancer, formulated with histidine-sucrose stabilizer at 20 mg/vial.

Thiol–maleimide step-growth polymerization monitored by oscillatory rheology

When the pyrrolidine ester is incorporated as an asymmetric chain extender into a poly(tetramethylene oxide) diol-based step-growth system, the thiol–maleimide reaction proceeds with a rate constant k = 4.2 × 10⁻³ L·mol⁻¹·s⁻¹ at 25 °C (FTIR kinetic monitoring, disappearance of maleimide band at 696 cm⁻¹). The 4-nitrophenyl methyl ester remains pendant until post-polymerization deprotection with 10% v/v piperidine in DCM, which unmasks the pyrrolidine nitrogen for subsequent quaternization. In the continuous extrusion process (Leistritz ZSE 18 MAXX, L/D 40, screw speed 180 rpm, barrel zones 60–110 °C), the liquid feed of mercapto monomer is injected at barrel zone 4 via an Eldex metering pump at a stoichiometric ratio [SH]₀/[maleimide]₀ = 1.005 to offset thiol oxidation during residence. Oscillatory time sweeps (Anton Paar MCR 302, 25 mm parallel plate, 1 mm gap, 1% strain) reveal that gelation occurs at 42 s, corresponding to a critical conversion of 0.71 predicted by Flory-Stockmayer theory for f = 2.1 effective functionality. The cured polyadduct, after thermal annealing at 120 °C for 4 h, exhibits a Young’s modulus of 18.5 MPa (ASTM D638-14 Type V, crosshead speed 1 mm/min) and 720% elongation at break. Compliance with ISO 10993-5 for in vitro cytotoxicity is mandatory when the material is intended for a blood-contacting catheter coating; extractable leachables are quantified by LC-QTOF, demanding a total non-volatile residue below 0.15 mg/device. Terminal product: a radiopaque polyurethane-urea catheter with a thromboresistant thiol-maleimide network coating, sterilized by ethylene oxide per ISO 11135:2014.

Production-scale vacuum belt dryers (Bucher Unipektin, 12 m² heated surface) used to isolate the compound after silica gel chromatography encounter a persistent batch-end fouling when residual acetic acid from the quench exceeds 0.2% w/w; the thiol acetylates on the belt at ≥55 °C, generating a sticky film that increases clean-in-place downtime by 4 h. The corrective action implemented in multi-kilogram campaigns involves a pre-drying toluene azeotrope step (Karl Fischer endpoint <0.02% water) and injection of 0.01% w/w BHT into the feed liquor. This compound serves as a key molecular probe in the development of ³H-labeled dipeptidyl peptidase IV inhibitors for receptor autoradiography; tritiation occurs at the 4-nitrophenyl ring under carrier-free conditions (RC TRITEC AG, 50 Ci/mmol target specific activity). Post-labeling, the mercapto group must remain unoxidized during HPLC purification on a C18 column eluting with ammonium acetate ( 20 mM, pH 4.8) / methanol; any disulfide detected by radio-TLC (> 2%) triggers reduction with immobilized TCEP gel (Thermo Scientific Pierce, 2 mL cartridge) before final sterile filtration. The 4-nitrophenyl methyl ester is cleaved by catalytic transfer hydrogenation (10% Pd/C, 1 atm H₂, 2 h) in the hot cell to yield the free carboxylic acid for conjugation to a fluorescent tetramethylrhodamine isothiocyanate tag. Terminal product: a high-specific-activity tritiated peptidomimetic used in quantitative whole-body autoradiography for metabolic disease target engagement studies, supplied in ethanol under argon at −80 °C per ARRIVE guidelines 2.0.

EPR dosimetry using mercapto-pyrrolidine as a radical trap in alanine film badges

The crystalline compound co-milled with L-alanine (95:5 w/w, Retsch MM 400 mixer mill, 30 Hz, 10 min) generates a stable thiyl radical upon γ-irradiation (⁶⁰Co source, dose rate 1.2 kGy/h) detectable by X-band EPR spectroscopy (Bruker EMXplus, 9.8 GHz, 2 mW microwave power). The double integral of the signal at g = 2.008 shows linear dose response up to 45 kGy with a deviation of ±2.2% from the reference alanine dosimeter (ISO/ASTM 51607:2013). This application requires the mercaptan to be stored in sealed amber vials at −20 °C and handled in a glovebox with O₂ < 10 ppm to prevent background radical decay. The processed film badges are laminated between polyethylene terephthalate layers (75 μm thick) and qualified under ISO 11137-1 for sterilization dose audit. No further chemical transformation occurs; the terminal product is the EPR dosimeter film itself, used by contract sterilization facilities for routine dose mapping of electron beam accelerators (10 MeV).

In lyophilization cycle development for a diagnostic peptide precursor, differential scanning calorimetry (TA Instruments Q2000) of the frozen solution containing 10 mg/mL of the mercapto monomer in 5% w/v mannitol revealed a collapse temperature of −24 °C. Accordingly, primary drying shelf temperature was ramped from −40 °C to −28 °C over 6 h under 80 mTorr chamber pressure (SP Scientific LyoStar 3). At deviations exceeding −25 °C, macroscopic cake shrinkage was documented, accompanied by a 3.4% increase in moisture (Karl Fischer coulometer, Mettler Toledo C30S), which accelerates thiol oxidation to disulfide upon storage at 25 °C/60% RH beyond 14 days. The cured cake is then dissolved in anhydrous DMF and used directly in oxime ligation with an aminooxyacetyl-functionalized chelator (DOTA-NHS) to form a metal-chelating peptide for ⁶⁸Ga radiolabeling. The conjugation proportion is held at 1.15 mol chelator per mol of deprotected peptide, and the crude product is purified on a Biotage Isolera Dalton 2000 mass-directed system. Terminal product: a freeze-dried kit for gallium-68 labeling used in PET imaging of neuroendocrine tumors, required to meet Ph. Eur. monograph 01/2024:2462 for radiopharmaceutical preparations and ICH Q8(R2) design space verification.