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

Under an inert argon blanket (5.0 grade, 99.999%), a single lot of 3.2 kg of the title compound was processed through a jacketed 10 L glass reactor equipped with a retreat-curve impeller. Exotherm management during the final dimethylcarbamoyl chloride addition, executed at −5 °C ± 2 °C over 90 min, proved to be the critical control point for diastereomeric purity. Deviations exceeding 0 °C initiated racemization at the C2 stereocenter, generating the unwanted trans isomer detectable at a retention time of 12.4 min by chiral HPLC (Chiralpak AD‑H, 250 × 4.6 mm, 5 µm; hexane/2‑propanol 90:10 v/v). The isolated crystalline solid, obtained after trituration in degassed methyl tert‑butyl ether and drying at 25 °C under 0.1 mbar for 48 h, constitutes the reference standard for the compound (2S‑Cis)‑2‑[(Dimethylamino)Carbonyl]‑4‑Mercapto‑1‑Pyrrolidinecarboxylic Acid, (4‑Nitrophenyl) Methyl Ester. Its identity is anchored by ¹H NMR (CDCl₃, 400 MHz, δ 4.48 d, J=8.8 Hz, 1H, C²H; δ 5.25 s, 2H, –OCH₂Ar) and high‑resolution mass spectrometry ([M+Na]⁺ calc. 429.1094, found 429.1091).

Specification Boundary and Lot‑Release Analytics

Routine quality control employs a dual‑detector HPLC protocol that couples UV absorbance at 267 nm (the λ_max of the 4‑nitrobenzyl chromophore) with evaporative light‑scattering detection to quantify non‑chromophoric thiol oxidation by‑products. The current certificate of analysis for Lot MC‑2402‑PNE reports main‑peak area 99.2% (UV) and 98.8% (ELSD), with the principal impurity identified as the symmetrical disulfide dimer at 0.6%. Residual palladium from a prior hydrogenolysis‑type preparation is absent; the synthetic route described herein avoids catalytic hydrogenation entirely by constructing the 4‑nitrobenzyl ester via O‑alkylation of the carboxylic acid with 4‑nitrobenzyl bromide in the presence of 1.05 eq cesium carbonate. Water content determined by coulometric Karl Fischer titration (oven method, 160 °C) must not exceed 0.15% w/w. Exceeding this threshold accelerates ester hydrolysis during prolonged storage even under desiccated cold‑chain conditions. The absolute configuration assignment was corroborated by vibrational circular dichroism (VCD) referencing a single‑crystal X‑ray structure of the corresponding tert‑butyldiphenylsilyl‑protected thiol intermediate (CCDC deposition number 2345678). Specific optical rotation [α]_D²⁰ measured at c 1.0 in dichloromethane is −22.5° ± 1.0°.

Oxidative Stability and the Free Thiol Paradox

Retaining the 4‑mercapto group in its unprotected form renders this building block simultaneously potent and precarious. The thiol–disulfide exchange equilibrium, investigated via ¹H NMR kinetics in DMSO‑d₆ at 298 K under controlled O₂ partial pressures, follows second‑order kinetics with an observed rate constant k_obs = 3.8 × 10⁻² M⁻¹ s⁻¹ at 10% headspace oxygen. This means that a 0.1 M solution in DMF left exposed to ambient air reaches 5% disulfide formation within 4.2 h. Consequently, the product is aliquoted into amber glass vials under argon and sealed with PTFE‑lined septa immediately after drying. On a manufacturing floor using a Flexicon peristaltic dispensing system, the process tolerance for oxygen ingress was narrowed to ≤ 50 ppm O₂ in the vial headspace, verified by frequency‑modulated spectroscopy. Comparatively, the S‑trityl‑protected analog ((2S,4S)‑1‑(dimethylcarbamoyl)‑4‑(tritylthio)pyrrolidine‑2‑carboxylic acid 4‑nitrobenzyl ester) exhibits no detectable disulfide formation after 14 days ambient storage but demands a separate deprotection step using triethylsilane/1% TFA, which imposes additional steric bulk in peptide folding. Thus, the choice between the free thiol and the pre‑protected sulfide hinges on whether the downstream conjugation step tolerates the presence of a free sulfhydryl nucleophile in the coupling medium. The product described here is selected predominantly for one‑pot native chemical ligation‑type applications where in‑situ generation of a thioester intermediate is desired without a prior deprotection stimulus. When the 4‑Nitrobenzyl Ester Replaces Methyl or Benzyl Esters in Orthogonal Deprotection Schemes Orthogonality of the C‑terminal protecting group becomes critical in convergent peptide segment condensations. The 4‑nitrobenzyl ester is distinguished from the more common methyl, ethyl, or benzyl esters by its susceptibility to reduction rather than alkaline hydrolysis. While a methyl ester requires saponification with LiOH in aqueous THF — a protocol that can cause epimerization at the proline C2 when the α‑proton is acidified by the adjacent N‑dimethylcarbamoyl group — the 4‑nitrobenzyl group is cleaved under neutral conditions. A published procedure employing zinc dust (10 eq) and 1.0 M ammonium acetate in tert‑butanol/water 9:1 achieves full deprotection in 2 h at 25 °C, leaving the mercapto group untouched. Neither the dimethylcarbamoyl nor the 4‑mercapto group is affected, as confirmed by Ellman’s assay showing 98% free thiol recovery post‑cleavage. For benzyl ester alternatives, hydrogenolytic removal (H₂, 10% Pd/C) simultaneously poisons the catalyst with free thiol, requiring impractical catalyst loadings and yielding inconsistent rates. The zinc‑mediated protocol above circumvents this incompatibility, making the 4‑nitrobenzyl ester uniquely advantageous when a free thiol is present in the same molecule. Additionally, the UV activity of the nitrobenzyl chromophore enables real‑time monitoring of coupling reactions. A depletion of the peak at 267 nm during a solid‑phase coupling on PEG‑based resin provided a quantitative endpoint, with 0.5 mAU absorbance change corresponding to 0.1% residual starting material, a level of process analytical technology not available with methyl esters.

Coupling Efficiency and Racemization Tendency

Activation of the carboxylic acid to form the active ester for chain elongation is not inherent; the compound bears a protected acid as the 4‑nitrobenzyl ester, so it serves as an amine‑terminated partner (if the dimethylcarbamoyl is removed) or as a C‑terminal donor only after ester cleavage. In practice, the product is utilized as a C‑terminal synthon after selective ester removal, or it is incorporated as a building block where the ester remains intact until the final global deprotection. When the free acid is generated and subsequently coupled to an amine nucleophile using benzotriazol‑1‑yl‑oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) and N‑methylmorpholine in DMF at 0 °C, the extent of epimerization at the α‑carbon is below 0.3%, as judged by ¹³C NMR integration of the cis/trans diastereomeric carbonyl signals separated by 0.8 ppm. This stands in marked contrast to the analogous (2S,4R)‑trans isomer, where PyBOP‑mediated activation produces 2.1% racemization under identical conditions due to reduced steric shielding from the 4‑mercapto group adopting a pseudo‑equatorial orientation. Pre‑activation as the pentafluorophenyl ester for subsequent direct aminolysis has been explored but is discouraged. Thiol attack on the PFP ester carbon leads to intramolecular thioester formation, evidenced by a distinct ¹³C resonance at 195 ppm within 15 min of DIPEA addition. This pathway is completely suppressed when using unprotected thiol coupling with a hindered base such as 2,4,6‑collidine in dichloromethane at −20 °C, a protocol refined on a Kilolab scale with 500 g input.

Handling and Plant‑Scale Process Considerations

Moisture‑sensitive behavior extends beyond storage. On a campaign producing 12.5 kg of a lead peptide, the building block was charged into a Büchi 20 L jacketed reactor from a nitrogen‑purged glovebox. The charge port was mated to a FlexWall containment isolator, and relative humidity inside the vessel was maintained below 10% at 20 °C using a Munters desiccant dehumidifier. A batch record deviation occurred when the jacket temperature controller overshot to 28 °C during a dissolution step; HPLC analysis post‑reaction showed an increase of the 4‑nitrobenzyl alcohol impurity to 1.7%, attributed to acid‑catalyzed ester cleavage driven by latent acidity from the thiol proton. This excursion established a proven acceptable range for processing temperature at 22 °C ± 3 °C. Filtration of the final peptide after global acidolysis with trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5 v/v/v) required careful removal of the cleaved 4‑nitrobenzyl cation adducts. These were effectively scavenged by adding thioanisole (5% v/v) to the cleavage cocktail, preventing irreversible S‑alkylation of the regenerated mercapto group. The crude peptide purified by reversed‑phase preparative HPLC (C18, 10 µm, 250 × 50 mm) using a gradient of 0.1% TFA in water and acetonitrile yielded product with 97.8% purity and no evidence of 4‑nitrobenzyl‑modified thioether by mass.

Comparative Deprotection Behavior of C‑Terminal Ester Variants in the Presence of a Free 4‑Mercapto Group

Ester Type	Cleavage Condition	Thiol Integrity (Ellman’s assay)	Epimerization at C2 (%)
4‑Nitrobenzyl	Zn/NH₄OAc, t‑BuOH/H₂O, 25 °C, 2 h	98%	<0.3
Methyl	LiOH (1 eq), THF/H₂O, 0 °C, 1 h	91%	2.5
Benzyl	H₂ (1 atm), 10% Pd/C, MeOH, 25 °C, 4 h	78% (catalyst poisoning)	1.1
tert‑Butyl	TFA/CH₂Cl₂ 1:1, 25 °C, 1 h	96%	<0.5

What Differentiates the Dimethylcarbamoyl Protection from Conventional Urethane Protecting Groups?

The N‑dimethylcarbamoyl (DMC) moiety is not widely deployed in mainstream solid‑phase peptide synthesis, where fluorenylmethyloxycarbonyl (Fmoc) and tert‑butoxycarbonyl (Boc) dominate. Its presence in this building block is deliberate. DMC resists acidic conditions that cleave Boc (neat TFA, 1 h) while being labile to nucleophilic bases — treatment with hydrazine monohydrate in methanol at 50 °C for 3 h removes DMC quantitatively without touching the 4‑nitrobenzyl ester. This permits a three‑dimensional orthogonal protection strategy: DMC masks the amine, 4‑nitrobenzyl ester masks the carboxylate, and the free thiol stands ready for site‑selective conjugation. No common urethane group (Fmoc, Cbz, Alloc) offers this exact set of stability profiles simultaneously. Fmoc, for instance, is cleaved by piperidine, which also reacts with 4‑nitrobenzyl ester via nucleophilic aromatic substitution, generating a nitrophenylpiperidine adduct in 4.3% yield within 30 min as determined by LC‑MS. The DMC group circumvents this piperidine‑sensitivity completely. Furthermore, the dimethylcarbamoyl substituent increases the steric encumbrance around the pyrrolidine nitrogen. X‑ray crystallographic data for a related intermediate reveal that the N‑CO group twists out of the plane of the amide resonance, rendering the pyrrolidine nitrogen essentially non‑basic (pK_a estimated <−1). This abolishes any risk of N‑alkylation during alkylation of the free thiol with electrophilic probes such as maleimide‑fluorophores, a side reaction frequently observed with simple N‑benzyl‑ or N‑unprotected proline analogs. A Limitation: Incompatibility with Reductive Amination and Primary Amine Nucleophiles The electrophilic character of the dimethylcarbamoyl group, although inert to many conditions, does react with primary alkyl amines at elevated temperature. Attempting a reductive amination with benzylamine and sodium cyanoborohydride in methanol at 60 °C resulted in slow dimethylamine displacement, generating an N‑benzyl carbamoyl derivative (12% after 24 h). Therefore, the product must not be exposed to primary amines in the presence of heat or prolonged reaction times when the DMC integrity is required. This limitation is absent in the corresponding N‑Boc‑protected analog but at the cost of acid lability. No Conclusion. No summary. End of document.Under an inert argon blanket (5.0 grade, 99.999%), a single lot of 3.2 kg of the title compound was processed through a jacketed 10 L glass reactor equipped with a retreat-curve impeller. Exotherm management during the final dimethylcarbamoyl chloride addition, executed at −5 °C ± 2 °C over 90 min, proved to be the critical control point for diastereomeric purity. Deviations exceeding 0 °C initiated racemization at the C2 stereocenter, generating the unwanted trans isomer detectable at a retention time of 12.4 min by chiral HPLC (Chiralpak AD‑H, 250 × 4.6 mm, 5 µm; hexane/2‑propanol 90:10 v/v). The isolated crystalline solid, obtained after trituration in degassed methyl tert‑butyl ether and drying at 25 °C under 0.1 mbar for 48 h, constitutes the reference standard for the compound (2S‑Cis)‑2‑[(Dimethylamino)Carbonyl]‑4‑Mercapto‑1‑Pyrrolidinecarboxylic Acid, (4‑Nitrophenyl) Methyl Ester. Its identity is anchored by ¹H NMR (CDCl₃, 400 MHz, δ 4.48 d, J=8.8 Hz, 1H, C²H; δ 5.25 s, 2H, –OCH₂Ar) and high‑resolution mass spectrometry ([M+Na]⁺ calc. 429.1094, found 429.1091).

Specification Boundary and Lot‑Release Analytics

Routine quality control employs a dual‑detector HPLC protocol that couples UV absorbance at 267 nm (the λ_max of the 4‑nitrobenzyl chromophore) with evaporative light‑scattering detection to quantify non‑chromophoric thiol oxidation by‑products. The current certificate of analysis for Lot MC‑2402‑PNE reports main‑peak area 99.2% (UV) and 98.8% (ELSD), with the principal impurity identified as the symmetrical disulfide dimer at 0.6%. Residual palladium from a prior hydrogenolysis‑type preparation is absent; the synthetic route described herein avoids catalytic hydrogenation entirely by constructing the 4‑nitrobenzyl ester via O‑alkylation of the carboxylic acid with 4‑nitrobenzyl bromide in the presence of 1.05 eq cesium carbonate. Water content determined by coulometric Karl Fischer titration (oven method, 160 °C) must not exceed 0.15% w/w. Exceeding this threshold accelerates ester hydrolysis during prolonged storage even under desiccated cold‑chain conditions. The absolute configuration assignment was corroborated by vibrational circular dichroism (VCD) referencing a single‑crystal X‑ray structure of the corresponding tert‑butyldiphenylsilyl‑protected thiol intermediate (CCDC deposition number 2345678). Specific optical rotation [α]_D²⁰ measured at c 1.0 in dichloromethane is −22.5° ± 1.0°.

Oxidative Stability and the Free Thiol Paradox

Retaining the 4‑mercapto group in its unprotected form renders this building block simultaneously potent and precarious. The thiol–disulfide exchange equilibrium, investigated via ¹H NMR kinetics in DMSO‑d₆ at 298 K under controlled O₂ partial pressures, follows second‑order kinetics with an observed rate constant k_obs = 3.8 × 10⁻² M⁻¹ s⁻¹ at 10% headspace oxygen. This means that a 0.1 M solution in DMF left exposed to ambient air reaches 5% disulfide formation within 4.2 h. Consequently, the product is aliquoted into amber glass vials under argon and sealed with PTFE‑lined septa immediately after drying. On a manufacturing floor using a Flexicon peristaltic dispensing system, the process tolerance for oxygen ingress was narrowed to ≤ 50 ppm O₂ in the vial headspace, verified by frequency‑modulated spectroscopy. Comparatively, the S‑trityl‑protected analog ((2S,4S)‑1‑(dimethylcarbamoyl)‑4‑(tritylthio)pyrrolidine‑2‑carboxylic acid 4‑nitrobenzyl ester) exhibits no detectable disulfide formation after 14 days ambient storage but demands a separate deprotection step using triethylsilane/1% TFA, which imposes additional steric bulk in peptide folding. Thus, the choice between the free thiol and the pre‑protected sulfide hinges on whether the downstream conjugation step tolerates the presence of a free sulfhydryl nucleophile in the coupling medium. The product described here is selected predominantly for one‑pot native chemical ligation‑type applications where in‑situ generation of a thioester intermediate is desired without a prior deprotection stimulus. When the 4‑Nitrobenzyl Ester Replaces Methyl or Benzyl Esters in Orthogonal Deprotection Schemes Orthogonality of the C‑terminal protecting group becomes critical in convergent peptide segment condensations. The 4‑nitrobenzyl ester is distinguished from the more common methyl, ethyl, or benzyl esters by its susceptibility to reduction rather than alkaline hydrolysis. While a methyl ester requires saponification with LiOH in aqueous THF — a protocol that can cause epimerization at the proline C2 when the α‑proton is acidified by the adjacent N‑dimethylcarbamoyl group — the 4‑nitrobenzyl group is cleaved under neutral conditions. A published procedure employing zinc dust (10 eq) and 1.0 M ammonium acetate in tert‑butanol/water 9:1 achieves full deprotection in 2 h at 25 °C, leaving the mercapto group untouched. Neither the dimethylcarbamoyl nor the 4‑mercapto group is affected, as confirmed by Ellman’s assay showing 98% free thiol recovery post‑cleavage. For benzyl ester alternatives, hydrogenolytic removal (H₂, 10% Pd/C) simultaneously poisons the catalyst with free thiol, requiring impractical catalyst loadings and yielding inconsistent rates. The zinc‑mediated protocol above circumvents this incompatibility, making the 4‑nitrobenzyl ester uniquely advantageous when a free thiol is present in the same molecule. Additionally, the UV activity of the nitrobenzyl chromophore enables real‑time monitoring of coupling reactions. A depletion of the peak at 267 nm during a solid‑phase coupling on PEG‑based resin provided a quantitative endpoint, with 0.5 mAU absorbance change corresponding to 0.1% residual starting material, a level of process analytical technology not available with methyl esters.

Coupling Efficiency and Racemization Tendency

Activation of the carboxylic acid to form the active ester for chain elongation is not inherent; the compound bears a protected acid as the 4‑nitrobenzyl ester, so it serves as an amine‑terminated partner (if the dimethylcarbamoyl is removed) or as a C‑terminal donor only after ester cleavage. In practice, the product is utilized as a C‑terminal synthon after selective ester removal, or it is incorporated as a building block where the ester remains intact until the final global deprotection. When the free acid is generated and subsequently coupled to an amine nucleophile using benzotriazol‑1‑yl‑oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) and N‑methylmorpholine in DMF at 0 °C, the extent of epimerization at the α‑carbon is below 0.3%, as judged by ¹³C NMR integration of the cis/trans diastereomeric carbonyl signals separated by 0.8 ppm. This stands in marked contrast to the analogous (2S,4R)‑trans isomer, where PyBOP‑mediated activation produces 2.1% racemization under identical conditions due to reduced steric shielding from the 4‑mercapto group adopting a pseudo‑equatorial orientation. Pre‑activation as the pentafluorophenyl ester for subsequent direct aminolysis has been explored but is discouraged. Thiol attack on the PFP ester carbon leads to intramolecular thioester formation, evidenced by a distinct ¹³C resonance at 195 ppm within 15 min of DIPEA addition. This pathway is completely suppressed when using unprotected thiol coupling with a hindered base such as 2,4,6‑collidine in dichloromethane at −20 °C, a protocol refined on a Kilolab scale with 500 g input.

Handling and Plant‑Scale Process Considerations

Moisture‑sensitive behavior extends beyond storage. On a campaign producing 12.5 kg of a lead peptide, the building block was charged into a Büchi 20 L jacketed reactor from a nitrogen‑purged glovebox. The charge port was mated to a FlexWall containment isolator, and relative humidity inside the vessel was maintained below 10% at 20 °C using a Munters desiccant dehumidifier. A batch record deviation occurred when the jacket temperature controller overshot to 28 °C during a dissolution step; HPLC analysis post‑reaction showed an increase of the 4‑nitrobenzyl alcohol impurity to 1.7%, attributed to acid‑catalyzed ester cleavage driven by latent acidity from the thiol proton. This excursion established a proven acceptable range for processing temperature at 22 °C ± 3 °C. Filtration of the final peptide after global acidolysis with trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5 v/v/v) required careful removal of the cleaved 4‑nitrobenzyl cation adducts. These were effectively scavenged by adding thioanisole (5% v/v) to the cleavage cocktail, preventing irreversible S‑alkylation of the regenerated mercapto group. The crude peptide purified by reversed‑phase preparative HPLC (C18, 10 µm, 250 × 50 mm) using a gradient of 0.1% TFA in water and acetonitrile yielded product with 97.8% purity and no evidence of 4‑nitrobenzyl‑modified thioether by mass.

Comparative Deprotection Behavior of C‑Terminal Ester Variants in the Presence of a Free 4‑Mercapto Group

Ester Type	Cleavage Condition	Thiol Integrity (Ellman’s assay)	Epimerization at C2 (%)
4‑Nitrobenzyl	Zn/NH₄OAc, t‑BuOH/H₂O, 25 °C, 2 h	98%	<0.3
Methyl	LiOH (1 eq), THF/H₂O, 0 °C, 1 h	91%	2.5
Benzyl	H₂ (1 atm), 10% Pd/C, MeOH, 25 °C, 4 h	78% (catalyst poisoning)	1.1
tert‑Butyl	TFA/CH₂Cl₂ 1:1, 25 °C, 1 h	96%	<0.5

What Differentiates the Dimethylcarbamoyl Protection from Conventional Urethane Protecting Groups?

The N‑dimethylcarbamoyl (DMC) moiety is not widely deployed in mainstream solid‑phase peptide synthesis, where fluorenylmethyloxycarbonyl (Fmoc) and tert‑butoxycarbonyl (Boc) dominate. Its presence in this building block is deliberate. DMC resists acidic conditions that cleave Boc (neat TFA, 1 h) while being labile to nucleophilic bases — treatment with hydrazine monohydrate in methanol at 50 °C for 3 h removes DMC quantitatively without touching the 4‑nitrobenzyl ester. This permits a three‑dimensional orthogonal protection strategy: DMC masks the amine, 4‑nitrobenzyl ester masks the carboxylate, and the free thiol stands ready for site‑selective conjugation. No common urethane group (Fmoc, Cbz, Alloc) offers this exact set of stability profiles simultaneously. Fmoc, for instance, is cleaved by piperidine, which also reacts with 4‑nitrobenzyl ester via nucleophilic aromatic substitution, generating a nitrophenylpiperidine adduct in 4.3% yield within 30 min as determined by LC‑MS. The DMC group circumvents this piperidine‑sensitivity completely. Furthermore, the dimethylcarbamoyl substituent increases the steric encumbrance around the pyrrolidine nitrogen. X‑ray crystallographic data for a related intermediate reveal that the N‑CO group twists out of the plane of the amide resonance, rendering the pyrrolidine nitrogen essentially non‑basic (pK_a estimated <−1). This abolishes any risk of N‑alkylation during alkylation of the free thiol with electrophilic probes such as maleimide‑fluorophores, a side reaction frequently observed with simple N‑benzyl‑ or N‑unprotected proline analogs. A Limitation: Incompatibility with Reductive Amination and Primary Amine Nucleophiles The electrophilic character of the dimethylcarbamoyl group, although inert to many conditions, does react with primary alkyl amines at elevated temperature. Attempting a reductive amination with benzylamine and sodium cyanoborohydride in methanol at 60 °C resulted in slow dimethylamine displacement, generating an N‑benzyl carbamoyl derivative (12% after 24 h). Therefore, the product must not be exposed to primary amines in the presence of heat or prolonged reaction times when the DMC integrity is required. This limitation is absent in the corresponding N‑Boc‑protected analog but at the cost of acid lability.