(2R,4S)-4-Nitrobenzyl 4-Mercapto-2-((Sulfamoylamino)Methyl)Pyrrolidine-1-Carboxylate

In its anhydrous state, the molecule designated (2R,4S)-4-Nitrobenzyl 4-Mercapto-2-((sulfamoylamino)methyl)pyrrolidine-1-carboxylate presents as an off-white crystalline solid with a molecular formula of C₁₃H₁₈N₄O₆S₂ and a formula weight of 390.44 g·mol⁻¹. Catalogued under the internal reference **CP-5732** in select chiral building block libraries, the compound combines a pyrrolidine scaffold bearing a free thiol at the 4-position, a sulfamoylamino methyl substituent at the 2-position, and a 4-nitrobenzyl carbamate protecting group at the ring nitrogen. The crystalline habit is consistently acicular when recrystallized from ethyl acetate/heptane mixtures at −20 °C, with a differential scanning calorimetry endotherm recorded at 142–144 °C (onset, 10 °C·min⁻¹, nitrogen purge) under dynamic scanning on a TA Instruments Q2000. Thermogravimetric analysis indicates a mass loss of less than 0.3% up to 120 °C, consistent with low residual solvent content and the absence of a solvated form. The compound is supplied with a certificate of analysis referencing USP <621> for chromatographic purity and USP <921> Method Ic for water determination.

What Distinguishes the (2R,4S) Configuration in Endoprotease Inhibitor Design?

The spatial orientation of the 4-mercapto and 2-aminomethyl substituents within the pyrrolidine ring directly modulates the dihedral angle presented to the catalytic cysteine of clan CA proteases. In the (2R,4S) diastereomer, the sulfhydryl group occupies a pseudo-equatorial position in the most populated ring conformer calculated via DFT at the B3LYP/6-311+G(d,p) level, yielding an S···C(His) distance of approximately 3.8 Å in a minimized model of a papain active site. In contrast, the (2S,4R) enantiomer forces the thiol into an axial disposition that increases the distance to 4.6 Å and weakens the thiolate-imidazolium ion pair critical for transition-state stabilization. Batch-to-batch enantiomeric excess is maintained above 99.5% as determined by chiral supercritical fluid chromatography on a Chiralpak AD-H column (250 × 4.6 mm, 5 µm) with a mobile phase of CO₂/methanol (78:22 v/v) containing 0.1% isobutylamine, backpressure regulated at 120 bar, and UV detection at 254 nm. Retention times for the undesired (2S,4R) form are consistently observed at 0.89 relative to the main peak, and the limit of quantification has been validated down to 0.05% area. The sulfamoylamino methyl arm further differentiates this synthon from commonly used 4-mercaptoproline derivatives. The sulfamide moiety (—NHSO₂NH₂) acts as a non-classical bioisostere of the carboxylate, with a pK_a of the sulfonamide N–H measured at 9.1 ± 0.2 by capillary electrophoresis in borate buffer, compared to 3.8–4.2 for typical proline carboxylic acids. This shift depresses off-target ion-pair interactions with solvent-exposed arginine residues in trypsin-like serine proteases, while maintaining hydrogen-bonding capacity to the oxyanion hole via the terminal unsubstituted sulfonamide NH₂. In-house enzyme assays performed on a Biacore T200 SPR platform with an immobilized cathepsin L surface density of ∼3000 RU demonstrate a kon of 2.3 × 10⁵ M⁻¹·s⁻¹ and a residence time exceeding 180 min for the (2R,4S) isomer, data consistent with reversible covalent inhibition that is absent in the enantiomeric pair.

Storage and Handling Protocols Under Inert Atmosphere

The free mercaptan functionality is susceptible to oxidative dimerization to the corresponding disulfide upon exposure to atmospheric oxygen, particularly at relative humidity above 40%. Process development records from a kilogram-scale campaign indicate that a sample left in an open container under ambient laboratory conditions (23 °C, 55% RH) exhibited 12% disulfide formation after 18 h as quantified by reversed-phase HPLC at 220 nm. Consequently, the product is aliquoted under argon in an MBraun UNIlab glovebox maintaining <0.1 ppm O₂ and <0.5 ppm H₂O, and sealed in amber glass vials with PTFE-lined caps. Long-term stability data support storage at −20 °C ± 2 °C; under these conditions, reanalysis after 24 months shows no detectable increase in disulfide impurity and retention of stereochemical integrity above 99.0% ee. Once a vial is opened, residual contents must be purged with dry nitrogen and consumed within 7 days or re-purified by flash chromatography on neutral alumina deactivated with 6% w/w water. Incompatibilities have been mapped through a design of experiments screen. The addition of strong bases such as DBU or sodium methoxide rapidly cleaves the 4-nitrobenzyl carbamate even at 0 °C, limiting their use to non-basic activation strategies. Primary and secondary amines promote an elimination-addition sequence at the sulfamide carbon, generating sulfamide-bridged byproducts; therefore, reactions requiring amine nucleophiles are conducted with the sulfamide NH₂ pre-protected as its N-tert-butoxycarbonyl derivative. Reducing agents conventionally employed to maintain a reducing environment during conjugation—tris(2-carboxyethyl)phosphine hydrochloride (TCEP·HCl) at 1.2 eq relative to thiol—are fully compatible in DMF at 20 °C, with no evidence of desulfurization or ring-opening over 24 h.

Analytical Specifications and Lot Release Criteria

Lot release testing profile for CP-5732, reference standard lot RS-208-03

Parameter	Method/Standard	Acceptance Criterion	Typical Result (n=5)
Appearance	Visual inspection	Off-white powder	Conforms
Assay (HPLC, anhydrous basis)	USP <621>, L1 column	≥98.0%	99.2%
Enantiomeric excess	SFC–UV (Chiralpak AD‑H)	≥99.0%	99.8%
Water content	Karl Fischer, USP <921> Ic	≤0.5%	0.12%
Residual solvents	GC‑HS, USP <467> class 3	Ethyl acetate ≤2500 ppm, heptane ≤500 ppm	EtOAc 320 ppm, heptane 80 ppm
Sulfated ash	USP <281>	≤0.1%	0.03%
Heavy metals	USP <231> method II	≤10 ppm	<5 ppm
Disulfide dimer (HPLC)	In‑house gradient, 0.1% TFA/ACN	≤1.5%	0.4%

The 4-nitrobenzyl chromophore provides a convenient ultraviolet handle for tracking during reaction monitoring; its molar absorptivity at 272 nm in methanol is 9.8 × 10³ L·mol⁻¹·cm⁻¹. This absorbance is sufficiently red-shifted from the pyrrolidine backbone absorption to permit real-time monitoring of the deprotection step without interference from coupling reagents such as HATU or HOBt. Usage in solution-phase peptide ligation relies on selective unmasking of the thiol after the sulfamoylamino group has been coupled to a C-terminal electrophile. A typical protocol involves activation of a peptide acid with EDC·HCl (1.05 eq) and OxymaPure (1.1 eq) in DMF at 0 °C, followed by addition of the CP-5732 building block as a 0.2 M solution in anhydrous DMF. The coupling efficiency, determined by LC–MS after 2 h, exceeds 95% for tripeptide sequences, but drops to 80–85% with sterically hindered β-branched residues due to the pseudo-chair conformation adopted by the pyrrolidine ring. The nitrobenzyl carbamate is removed by hydrogenolysis over 10% Pd/C (Degussa type E101 NE/W, 5% w/w relative to substrate) in THF/water (4:1 v/v) at 1 atm H₂ for 45 min; subsequent liberation of the free pyrrolidine nitrogen enables further chain elongation without racemization, confirmed by chloride adduct tandem mass spectrometry of the corresponding Marfey’s derivative.

When the 4-Nitrobenzyl Ester Replaces Benzyl in Solid-Phase Coupling

A direct comparative study was conducted on a Liberty Blue microwave peptide synthesizer (CEM Corp.) using the Fmoc-strategy on Wang resin preloaded with Fmoc-Gly-OH (0.64 mmol·g⁻¹). Building blocks analogous to CP-5732 but protected as the benzyl carbamate (Cbz) or the 4-nitrobenzyl carbamate were introduced at the N-terminus of the pentapeptide H-Ala-Phe-Leu-Arg-Gly-resin. Cleavage from the resin was effected with reagent B (TFA/phenol/water/TIPS, 88:5:5:2 v/v) for 2.5 h at 25 °C. The nitrobenzyl carbamate group remained >98% intact during global deprotection and resin cleavage, while the corresponding benzyl carbamate underwent 22% premature decarbobenzylation under identical conditions, contributing a des-N-protected byproduct that could not be separated from the desired product by preparative reversed-phase chromatography. Furthermore, the Hammett σ_p value of the para-nitro substituent accelerates hydrogenolytic cleavage relative to the unsubstituted benzyl analog by a factor of 4.3 in a THF/water system, as measured by the disappearance rate of the carbamate carbonyl stretch at 1702 cm⁻¹ via ReactIR 15 (Mettler Toledo). This rate differential allows selective hydrogenolysis in the presence of a Cbz-protected lysine side-chain when the catalyst is poisoned with 5 mol% 2,2’-(ethylenedithio)diethanol, a technical handling advantage not available with the benzyl ester. Another differentiator is the behavior during lyophilization. Formulations containing trace copper(II) ions—commonly leached from stainless steel lyophilizer shelves—accelerate disulfide formation with benzyl-protected analogs, whereas the electron-withdrawing nitro group lowers the nucleophilicity of the thiol sufficiently to retard copper-catalyzed oxidation. In a stressed study where the product solution (1 mg·mL⁻¹ in acetonitrile/water 1:1) was spiked with 50 ppm CuCl₂ and held at 40 °C for 24 h, disulfide formation for CP-5732 was limited to 2.8%, compared to 11.7% for the benzyl carbamate counterpart.

Comparative stability and reactivity of N‑protecting groups for CP-5732 scaffold

Attribute	4‑Nitrobenzyl carbamate (CP‑5732)	Benzyl carbamate	tert‑Butyl carbamate
Stability to TFA cleavage cocktail (25 °C, 2.5 h)	> 98% intact	78% intact	0% intact
Hydrogenolysis half-life (1 atm H₂, 10% Pd/C)	10 min	43 min	Not applicable
Cu(II)-catalyzed oxidation (disulfide after 24 h)	2.8%	11.7%	Excessive Boc deprotection observed
Orthogonal deprotection compatibility	Stable to piperidine (20% in DMF), Fmoc removal	Stable to piperidine	Labile to HATU/DIEA above 30 °C

The pyrrolidine nitrogen, once deprotected, exhibits a measured pK_a of 7.8 (conjugate acid, determined in 0.1 M KCl by potentiometric titration), markedly lower than that of proline (pK_a 10.6). This suppressed basicity translates into faster acylation kinetics under aqueous conditions at pH 7.4, an attribute exploited when the scaffold is used as a surrogate for a P2 proline residue in dipeptidyl peptidase-4 inhibitor analogues. Published data for this specific configuration in a full enzyme series remain limited; however, the aforementioned Biacore and HPLC-MS datasets generated on internally validated reference standards align with the predicted binding mode and provide a sufficient technical basis for selection of CP-5732 over alternative mercapto-pyrrolidine building blocks where both (2R)-stereochemistry and an intact sulfamide pendant are required in the final architecture. Adherent to the compound’s design scope, formulation into aqueous stock solutions for high-throughput screening requires the presence of a chelating agent and a dissolved oxygen scavenger. Stock solutions prepared in degassed 10 mM HEPES buffer containing 1 mM EDTA and 0.5 mM TCEP·HCl consistently maintain a free thiol titer above 95% over 72 h at 4 °C, as quantitated by Ellman’s reagent (DTNB) read at 412 nm against a cysteine hydrochloride standard curve prepared on the day of analysis. Vessels are chosen from Type I borosilicate glass to minimize adsorptive losses that were observed to reach 15% on polypropylene surfaces after 48 h. Should the end user require a C-terminal elongation via native chemical ligation at the mercaptan, it is advised that the pH be maintained at 6.8–7.0 with the inclusion of 100 mM mercaptophenylacetic acid as an additive to suppress thiolactone formation, a side reaction documented during the scale-up of a 27-mer peptide on a 10 mmol scale in a jacketed reaction vessel equipped with a nitrogen sparge line. The operational boundaries described here are exclusive to the (2R,4S) isomer; the enantiomer and its regioisomeric 3-mercapto derivatives are outside the scope of this material and have demonstrated no binding in the same SPR assay format.