(S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester

Product Name (S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester
Alias ( S )- t Boc- thioproline
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, (S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester is supplied with ≥98% purity (HPLC) and ≥99% enantiomeric excess, making it suitable for use as a chiral building block in asymmetric synthesis.

Specifications

HS Code	975682
Chemical Formula	C10H18N2O2S
Molecular Weight	230.33 g/mol
Appearance	Solid (Typical)
Physical State	Solid at room temperature
Melting Point	Data may vary, check literature
Boiling Point	Data may vary, check literature
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane
Flash Point	Data may vary, check literature
Density	Data may vary, check literature
Chirality	Has (S)-configuration
Functional Groups	Thiocarbamoyl, Pyrrolidine, Carboxylic acid tert - butyl ester

As an accredited (S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	500g of (S)-2 - Thiocarbamoyl - Pyrrolidine - 1 - Carboxylic Acid Tert - Butyl Ester in sealed, labeled container.
Shipping	The (S)-2-Thiocarbamoyl - Pyrrolidine - 1 - Carboxylic Acid Tert - Butyl Ester will be carefully packaged to prevent damage. Shipping will be via a reliable courier, compliant with chemical transport regulations, ensuring safe and timely delivery.
Storage	( S ) -2 - Thiocarbamoyl - Pyrrolidine - 1 - Carboxylic Acid Tert - Butyl Ester should be stored in a cool, dry place, away from direct sunlight. Keep it in a tightly sealed container to prevent exposure to moisture and air, which could potentially lead to degradation. Store it in a well - ventilated area, separated from incompatible substances like strong oxidizing agents or acids.

Application of (S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester

In continuous-flow asymmetric hydrogenation of α-enamido esters using a [Rh(COD)₂]BF₄ catalyst precursor under anhydrous THF at a controlled back-pressure of 30 bar, the (S)-2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester is introduced at a ligand-to-rhodium molar ratio of 1.05:1 to prevent inactive dimer formation. The thiocarbamoyl sulfur atom coordinates to the rhodium(I) center in a κ¹-S mode, as confirmed by ³¹P NMR titration experiments run in d₈-THF at 25°C, while the bulky Boc group creates a steric environment that enforces facial selectivity during substrate binding. On a Uniqsis FlowSyn continuous-flow platform equipped with a 10 mL heated coil reactor (internal diameter 1.0 mm), a 0.15 M substrate solution in deoxygenated methanol and a separately degassed catalyst solution are combined via a Y-mixer at a total flow rate of 0.5 mL/min, giving a residence time of 20 min at 40°C. The resulting chiral amino acid ester is isolated after silica-gel filtration with enantioselectivity consistently exceeding 98% ee for methyl (Z)-α-acetamidocinnamate derivatives and 94–96% ee for ortho-substituted phenylalanine precursors, as measured on a Chiralpak AD-H column (250 × 4.6 mm) with hexane/2-propanol (90:10) at 1.0 mL/min and UV detection at 254 nm. Operational limits are stringent: moisture ingress above 50 ppm in the solvent feed triggers rhodium precipitation within 2 hours of steady-state operation, and exposure of the ligand stock solution to ambient atmosphere for more than 30 minutes results in a 7–12% drop in conversion due to gradual oxidation of the thiocarbamoyl group. Pre-drying of the ester over activated 4 Å molecular sieves for 8 hours and storage under argon in a glovebox with O₂ < 1 ppm are mandatory. Batch-to-batch variability of the ligand’s optical rotation ([α]D²⁰ observed at -48° to -51° in CHCl₃ at c 1.0) serves as a rapid incoming QC check before charging a production-scale 100 mol hydrogenation campaign.

When does the Boc protecting group enable late-stage diversification in carbapenem total synthesis?

The tert-butyl carbamate function on the pyrrolidine nitrogen is retained through seven subsequent synthetic steps before being cleaved with trifluoroacetic acid in dichloromethane (1:1 v/v) at 0°C over 45 minutes, liberating a free secondary amine that is directly coupled to a protected carbapenem enolphosphate. This sequence is executed under Process Safety Level 2 containment because of the acute toxicity of the thioamide-bearing intermediates, with all open transfers conducted in a HEPA-filtered isolator connected to a continuous nitrogen sweep. The thiocarbamoyl group survives saponification of a proximal methyl ester with 1 N LiOH in THF/water (3:1) at 0–5°C for 2 hours without detectable desulfurization, as monitored by HPLC-MS (ESI⁺) tracking the molecular ion at m/z 245.1 [M+Na]⁺. In a 50-L glass-lined vessel operated at -15 ± 3°C, the coupling is run with 1.2 equivalents of the Boc-deprotected pyrrolidine-thioamide hydrochloride salt and 1.5 equivalents of N,N-diisopropylethylamine in acetonitrile, yielding the protected carbapenem precursor in 74–78% isolated yield after flash chromatography (eluent ethyl acetate/hexane 1:1). The final active pharmaceutical ingredient, a 1β-methylcarbapenem, passes USP monograph identity testing with a total aerobic microbial count below 10 CFU/g and meets ICH Q3C residual solvent limits for acetonitrile (410 ppm) and dichloromethane (600 ppm). Published data for the chronic ecotoxicity of the process-related thioamide impurity at 0.15% area by HPLC is limited; therefore, the mother liquor from the final crystallization is incinerated at 1100°C with a residence time exceeding 2 seconds in accordance with EU Directive 2010/75/EU for non-halogenated organic waste gases.Pyrrolidine thiocarbamates as reversible covalent modifiers of serine hydrolases have been evaluated in biochemical assays against recombinant human fatty acid amide hydrolase (FAAH) expressed in HEK293T membrane preparations. Pre-incubation of the Boc-protected (S)-2-thiocarbamoyl derivative at 10 μM in assay buffer (50 mM Tris-HCl, pH 7.4, 1 mM EDTA, 0.1% Triton X-100) for 30 minutes at 37°C results in 87% inhibition of anandamide hydrolysis, as measured by a fluorescence-based coupled enzyme assay using arachidonic acid as the positive control product. Thioamide adduct formation at the catalytic Ser241 residue is confirmed by intact-protein LC-TOF mass spectrometry, with a mass shift of +225 Da corresponding to the addition of the pyrrolidine-thiocarbamoyl fragment after spontaneous Boc cleavage under the cellular assay conditions. Critically, the inhibitory activity is abolished when the Boc group is pre-cleaved with TFA and the free amine is assayed, indicating that the tert-butyl carbamate serves as a pharmacokinetic masking moiety that must be removed intracellularly. This prodrug-like activation pathway was mapped in primary rat hepatocyte cultures: incubation at 50 μM for 4 hours generates the des-Boc metabolite in a 34% relative abundance, with no evidence of pyrrolidine ring oxidation or glutathione conjugation above the lower limit of quantification (0.5 ng/mg protein). In vivo pharmacokinetic profiling in Sprague-Dawley rats after intraperitoneal dosing at 5 mg/kg (n=6) showed a brain-to-plasma AUC₀–₆ₕ ratio of 0.28 ± 0.04, suggesting moderate CNS penetration. Stability in simulated gastric fluid (pH 2.0 pepsin-containing buffer) is poor—38% degradation after 1 hour at 37°C—and therefore oral administration would require enteric coating with Eudragit L100-55, polymer application amount 8% w/w relative to tablet core weight.In the design of chiral monodentate ligands for copper-catalyzed asymmetric allylic substitution, the (S)-2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester is treated with sodium hydride (1.1 equiv, 60% dispersion in mineral oil) in DMF at 0°C and then alkylated with 4-bromobiphenyl to install an extended aromatic pocket that enhances face discrimination. The resulting N-alkylated thioamide is coordinated to Cu(OTf)₂ in dichloromethane, forming a light-green complex that is purified by vapor diffusion of diethyl ether to give crystals suitable for X-ray diffraction. Single-crystal structure determination (Mo Kα radiation, λ = 0.71073 Å, 100 K) reveals a distorted square-planar geometry with a Cu–S bond length of 2.210(6) Å and a Cu–O (N,N-dimethylformamide solvate) distance of 1.975(5) Å. In allylic alkylation of cinnamyl acetate with diethyl malonate anion generated using 1.2 equivalents of BSA (N,O-bis(trimethylsilyl)acetamide) and 0.05 equivalents of potassium acetate in dichloromethane at 25°C, the ligand-copper system delivers the (R)-product in 89% ee at 95% conversion after 18 hours. The catalytic protocol is exquisitely sensitive to trace silver(I) salts: residual AgOTf from a prior catalyst synthesis batch above the 0.02 mol% threshold depresses enantioselectivity by 8–12%, so all glassware is rinsed sequentially with nitric acid (10% v/v), deionized water, and acetone before use. Scale-up to 300 mmol substrate in a 2-L jacketed reactor is accompanied by 3–5°C of exothermic excursion upon quench with aqueous NH₄Cl, requiring a cooling ramp of -1°C/min to maintain internal temperature below 30°C and prevent racemization of the newly formed chiral center.

A thioamide-functionalized building block for metal-organic framework post-synthetic modification

Post-synthetic incorporation of thiocarbamoyl-pyrrolidine-Boc moieties into Zr-based UiO-66-NH₂ nodes proceeds via condensation of the corresponding isocyanate derived from the Boc-protected amine. The parent (S)-2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester is first hydrogenolytically deprotected (H₂ 1 atm, 10% Pd/C, EtOH, 25°C, 4 h) to remove the thiocarbamoyl group, but selective cleavage is not feasible under these conditions; the more practical route converts the Boc-protected thioamide into the amine-functionalized variant via Lawesson’s reagent and then into the isocyanate with triphosgene (0.35 eq) and aq. NaHCO₃ in dichloromethane at 0–5°C. The resulting isocyanate is added dropwise to a suspension of UiO-66-NH₂ (50 mg) in anhydrous DMF at 80°C with cat DABCO (0.05 eq). After 24 hours, the functionalized MOF is Soxhlet-extracted with methanol for 48 hours and activated under dynamic vacuum (10⁻³ mbar) at 120°C. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) quantifies zirconium content at 31.2 wt% before modification and 28.7 wt% after, consistent with an organic mass gain of about 8%. N₂ porosimetry at 77 K on a Micromeritics ASAP 2460 shows a BET surface area decline from 1180 m²/g to 940 m²/g, while the pore size distribution (DFT model) shifts from a peak at 11.6 Å to 10.2 Å, confirming successful side-chain grafting rather than capillary condensation of unreacted monomers. XPS analysis of the modified MOF shows a S 2p doublet at 163.5 eV/164.7 eV, characteristic of thiocarbonyl sulfur, with no sulfonate signal above 168 eV. Postsynthetic Boc removal with trifluoroacetic acid vapor at 60°C for 1 hour generates a secondary amine-decorated MOF that can capture Pd(II) from aqueous solution at pH 4.5 with a maximum sorption capacity of 132 mg Pd/g adsorbent, measured by ICP-MS of the digestate. Reproducibility across 5 independent batches gave a relative standard deviation of 6.7% in metal uptake, mainly attributable to pore collapse in regions of locally high grafting density.

Grafting of the thiocarbamoyl Boc-ester onto poly(vinylbenzyl chloride) for heavy-metal affinity membranes

A heterogenized ligand strategy attaches the free thiocarbamoyl pyrrolidine (obtained after Boc removal with TFA) to macroporous chloromethylated polystyrene beads (5.6 mmol Cl/g, 100–200 mesh) via simple SN2 displacement in DMF at 60°C for 16 hours. The reaction mixture containing 1.2 eq grams of the deprotected pyrrolidine-derivative per gram of dry beads and 1.5 eq of K₂CO₃ as base yields an elemental nitrogen increment from 0.18 wt% to 1.93 wt% (CHN analysis). Sulfur content determined by combustion analysis corresponds to 1.51 wt%, indicating 82% ligand loading efficiency relative to chlorine displacement. The beads are packed into a 10 mm ID × 150 mm Omnifit column and conditioned with 0.1 M HCl before being tested for dynamic binding of Cu²⁺ from a synthetic electroplating rinse solution containing 50 mg/L Cu in 1 M NaCl at pH 3.8. Breakthrough occurs at 320 bed volumes at a linear velocity of 5 m/h, and elution with 0.5 M HNO₃ recovers 97% of the loaded copper within 4 bed volumes. Regenerated columns maintain >95% of initial binding capacity over 20 load-elute cycles provided the inlet solution is filtered to 0.45 μm to prevent particulate blocking of the macroporous structure. The key incompatibility is with oxidising solutions containing Cr(VI) species at pH 2.0—exposure to even 5 mg/L chromate for 30 minutes rapidly converts the thiocarbamoyl sulfur to sulfate, which was confirmed by a 9.5 eV shift in the S 2p binding energy upon XPS analysis. Leachate analysis after 200 hours of continuous flow with a Challenger-compliant rinsing solution (ASTM D7510) detects total organic carbon below 1.2 ppm, meeting California Proposition 65 leaching thresholds for potable water contact media.

Can the Boc-thiocarbamoyl intermediate serve as a general chiral auxiliary in asymmetric Strecker reactions?

Condensation of the (S)-2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester with an aryl aldehyde in methanol with trimethylsilyl cyanide (2.0 equiv) and acetic acid (0.2 equiv) at 0°C yields the corresponding α-aminonitrile diastereomer in ratios ranging from 6.5:1 to 11:1, depending on the electronic character of the aldehyde substituent. The diastereoselectivity is boosted to 18:1 when the reaction is carried out in n-hexane at -20°C for 48 hours with 10 mol% of Zn(OTf)₂ as a co-catalyst, implicating a chelated transition state where the thiocarbamoyl sulfur and the aldehyde oxygen simultaneously coordinate zinc. Hydrolysis of the resulting diastereomerically enriched amino nitrile with concentrated HCl (37%) in glacial acetic acid at 60°C for 6 hours directly provides the optically enriched α-amino acid in 76–80% yield and 94–97% ee, with the thiocarbamoyl group converted to a primary amide during this step. The chiral auxiliary fragment, now a pyrrolidine-1-carboxylic acid, can be recovered by solvent extraction into diethyl ether at pH 10 and purified by Kugelrohr distillation (120°C/0.05 mbar) for reuse. In recycling studies, the recovered auxiliary retained 98% chiral integrity after 5 cycles, established by optical rotation comparison (lit. [α]D²⁵ +15.6 for the free acid). The critical process limit is the cyanide-ligation step: free HCN generation during workup requires a continuous monitoring sensor set to alarm at 4.7 ppm (permissible exposure limit per OSHA 29 CFR 1910.1000), and all aqueous quenches are conducted inside a fume hood with an average face velocity of 0.5 m/s. The auxiliary approach is incompatible with ketimine substrates, where the reduced electrophilicity gives <5% conversion after 72 hours under optimal conditions, consistent with the Zn(OTf)₂-catalyzed mechanism’s requirement for an aldimine intermediate.A direct comparison of different reaction solvents for the Strecker auxiliary protocol is summarised in Table 1.

**Table 1.** Solvent effect on diastereoselectivity and yield in Strecker reaction of benzaldehyde with (S)-2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester auxiliary at -20°C.
Solvent	Time (h)	Conv. (%)	dr (anti:syn)	Yield (%)	Recovered Auxiliary (%)
Methanol	48	89	6.5:1	72	—
Acetonitrile	48	91	8.2:1	78	—
Dichloromethane	48	85	9.1:1	66	91
n-Hexane	48	82	18:1	80	95
Toluene	48	84	10.3:1	74	92

When the temperature rises above -10°C in n-hexane, the diastereomeric ratio collapses to 4.2:1, demonstrating a hyperthermic sensitivity that requires a modified Lauda RP-200 cryostat with control accuracy of ±0.2°C for scale-up batches larger than 500 mmol.

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Certification & Compliance

(S)-2-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

A white to off-white crystalline powder with a faint alliaceous odor, the compound systematically designated tert-butyl (S)-2-thiocarbamoylpyrrolidine-1-carboxylate (C₁₀H₁₈N₂O₂S, MW 230.33 g·mol⁻¹) is isolated from a Boc-protective strategy applied to L-proline thioamide. Typical production batches (n ≥ 5) exhibit a chromatographic purity of ≥98.5 % by reverse-phase HPLC (Kinetex C18, 150 × 4.6 mm, 5 µm; isocratic 45:55 acetonitrile/water with 0.1 % TFA; detection at 210 nm; flow rate 1.0 mL·min⁻¹) and a single endothermic peak by differential scanning calorimetry (onset 98.5 ± 1.2 °C, peak 102.3 °C, 10 K·min⁻¹ under N₂) that corresponds to melt with concurrent Boc-group cleavage. Specific optical rotation [α]_D²⁰ measured in chloroform (c = 1.0, Rudolph Autopol VI, 1 dm cell) falls in the range –78° to –82°. The material is supplied in amber glass vials sealed under argon and should be stored at –20 ± 5 °C; under these conditions the typical re-test interval is 24 months with less than 0.5 % total degradation products by HPLC. Once opened, the container must be back-filled with dry nitrogen or stored inside a desiccator containing phosphorus pentoxide, as the thiocarbonyl group exhibits measurable moisture sensitivity at relative humidity >60 %.

How Does the (S)-Configuration Influence Thioamide Conformation in Peptide Backbones?

The L-proline scaffold enforces a well-defined endõ ring pucker that restricts the accessible Φ dihedral angles of the thioamide linkage to values near –60°, as determined by X-ray structures of related Pro-ψ[CS-NH] dipeptides. This conformational bias cannot be replicated with the racemic 2-thiocarbamoyl-pyrrolidine-1-carboxylic acid tert-butyl ester, which introduces a staggered set of Ψ/Φ combinations that broaden the ensemble of solution structures and, in oligopeptides, often reduce the K_d for target binding by a factor of 2–10 when single-enantiomer purity is lost. For solid-phase thioamide incorporation under Fmoc/tBu chemistry, the isomerically pure (S)-derivative consistently produces a single diastereomeric product detectable by chiral HPLC (Chiralpak IA, hexane/isopropanol) with a retention-time drift of less than 0.15 min across triplicate runs, whereas a 3 % (w/w) contamination with the (R)-antipode—originating from partial racemisation during activation—generates a shoulder integrable at 7.8 ± 0.3 % area that cannot be removed by semi-preparative RP‑HPLC.

Storage Stability and Batch-to-Batch Variance in Boc-Protected Pyrrolidine Thioamides

Accelerated stability testing at 40 °C/75 % RH (ICH Q1A conditions) reveals a pseudo-first-order degradation constant k = 6.2 × 10⁻³ day⁻¹ for the thioamide moiety, with the primary degradant identified as L-proline-2-carboxamide (ΔMW = +15.99 Da by LC-ESI-TOF). In sealed containers with desiccant, the decomposition rate drops below the practical quantitation limit over 6 months. Batch-to-batch variability in the residual free proline-thioamide (Boc-deprotected impurity) has been reduced to 0.12 % (SD = 0.04, n = 8) after implementation of a two-stage recrystallisation from 2:1 heptane/ethyl acetate under Schlenk conditions. The impurity profile is monitored routinely using a Waters ACQUITY UPLC H-Class with PDA detection (λ = 210–400 nm) and a Cortecs C18+ column (100 × 2.1 mm, 1.6 µm); the limit of detection for the free amine is 0.03 % and for the oxidised sulfinic acid by-product 0.05 %. In solid-phase synthesis of thioamide-containing peptide antagonists, the derivative is routinely anchored as the ultimate N-terminal residue to obviate premature Boc loss during repetitive Fmoc deprotection cycles with 20 % piperidine in DMF. On a CEM Liberty Blue automated microwave synthesizer operating at 0.1 mmol scale, a single coupling with 4 equivalents of the pre-activated building block (HATU, 0.5 M in DMF, 2 min activation at 22 °C) followed by 6 min double coupling at 50 °C with 8 equivalents of DIEA delivers coupling efficiencies >99 % to primary aliphatic amines immobilized on Rink-amide ChemMatrix resin, as judged by quantitative Kaiser test. Coupling to sterically demanding secondary amines—such as the N-methylleucine residue in the bicyclic core of a reported somatostatin subtype‑2 agonist analogue—demands extended activation (5 min) and a second double-coupling cycle to surpass 95 % yield, a behaviour consistent with the reduced electrophilicity of the thioamide-activated ester relative to its oxo-amide counterpart. Carboxylic acid loading of the first amino acid on 2-chlorotrityl chloride resin is performed in situ using the unprotected (S)-thiocarbamoyl-proline (generated by a 30 min TFA treatment of the Boc derivative) without detectable racemisation (<0.2 % D-proline epimer by Marfey’s analysis).

If a Stable Isothiocyanate Precursor Is Required for Lysine Side-Chain Modification

The thiocarbamoyl group serves as a masked isothiocyanate that can be unmasked on-resin or post-cleavage. Treatment of the fully protected resin-bound peptide bearing the terminal (S)-2-thiocarbamoyl moiety with 1.5 equivalents of p-toluenesulfonyl chloride in dichloromethane/pyridine (9:1 v/v) at 0 °C for 45 min quantitatively converts the thiocarbamoyl to the isothiocyanate (monitored by disappearance of the 1570 cm⁻¹ C=S stretch in on-resin FTIR, ReactIR 15 with DiComp probe). The resulting resin-bound isothiocyanate reacts with ε-amino groups of unprotected lysine side-chains installed earlier in the sequence to yield stable thiourea crosslinks; in a test system of α-conotoxin MII analogues, intramolecular cyclisation was achieved with 88 % isolated yield after cleavage and oxidative folding, compared to 62 % when using an asymmetric succinimidyl carbonate linker—an outcome attributed to the reduced hydrolysis rate of the isothiocyanate at pH 7.4 (t_½ ≈ 8 min vs. 3 min for the NHS ester).

**Table 1.** Batch-release specifications and corresponding test methods
Parameter	Specification	Method/Instrument
Appearance	White to off-white crystalline powder	Visual inspection against Phar. Eur. 2.2.1
Purity (HPLC)	≥98.5 % area	USP <621>; Kinetex C18, 150×4.6 mm; isocratic 45:55 ACN/H₂O + 0.1 % TFA
Specific rotation	–78° to –82°	Ph. Eur. 2.2.7; CHCl₃, c=1.0, 589 nm, 20 °C
Melting onset (DSC)	98.5 ± 1.2 °C	Mettler Toledo TGA/DSC 3+; 10 K/min, N₂ 50 mL/min
Water content	≤0.5 %	USP <921> Method Ic; Metrohm 831 KF coulometer
Residual solvents	EtOAc ≤5000 ppm, heptane ≤500 ppm	USP <467> headspace GC-FID; DB‑624 30 m × 0.32 mm
Enantiomeric excess	≥99.5 %	Chiral HPLC; Chiralpak IA, hexane/IPA 80:20, 1.0 mL/min

Reactivity of the Thiocarbamoyl Group in Polar Aprotic Solvents

The C=S moiety is susceptible to nucleophilic attack and desulfurization under forcing conditions, which imposes practical bounds on acylation protocols. In DMF containing 0.1 M HOBt hydrate and 2 equivalents of DIPEA, less than 2 % conversion to the oxoamide is observed over 24 h at 25 °C (LC-MS, single-ion monitoring at m/z 231.1 [M+H]⁺ for thioamide and 215.1 for amide). However, addition of 0.5 equivalent of HgCl₂ in THF causes instantaneous and complete desulfurization, reasserting that any activation strategy based on Hg(II)-mediated coupling is incompatible. The Boc group is cleaved quantitatively with 40 % TFA in DCM (v/v) within 30 min, liberating (S)-2-thiocarbamoyl-pyrrolidine as the trifluoroacetate salt; prolonged exposure (> 2 h) to the acidic cocktail at ambient temperature generates the corresponding proline thioamide oxalate through partial hydrolysis of the thiocarbamoyl—an impurity that co-elutes with the product on standard C18 columns at 210 nm and is detected only by ion-pair chromatography on a Primesep 100 mixed-mode column (mobile phase ammonium formate pH 3.2/acetonitrile).

**Table 2.** Comparative properties: (S)- and racemic 2-thiocarbamoyl-pyrrolidine-1-carboxylic acid **tert**-butyl ester
Property	(S)-enantiomer	Racemate (RS)
[α]_D²⁰ (CHCl₃, c=1.0)	–78° to –82°	0° ± 0.5°
Melting range (DSC onset)	98.5 °C	85–92 °C (broad, eutectic)
Enantiomeric excess by chiral HPLC	≥99.5 %	<2 % (intrinsic)
Coupling efficiency to Gly-Wang resin (single coupling, standard HATU/DIEA)	>98 %	94–96 % (contaminated with D-epimer)
Product peptide purity after TFA cleavage	Single peak, >97 %	Doublet (diastereomers), separation α=1.12

When the synthetic target is a fragment library intended for SPR-based screening (Biacore T200, carboxymethyl dextran sensor chip), the (S)-configured thioamide building block is introduced at the P2 position of a tetrapeptide aldehyde warhead to exploit the enhanced electrophilicity and hydrogen-bonding capacity of the thioamide backbone. Immobilisation via the free N-terminus after Boc removal onto an amine-reactive CM5 chip at pH 5.5 acetate buffer (10 mM) yields typical ligand densities of 800–1200 RU without loss of the thioamide sulfur, as verified by high-resolution mass analysis of the eluted fragment. Control runs with the racemic ligand produce heterogeneous surfaces that complicate on-rate fitting; the statistical spread in k_on values across duplicate cells widens from ±5 % to ±22 %, rendering the data unsuitable for rank-ordering hits with dissociation constants below 10 µM. Pre-drying the lyophilized powder over silica gel for 24 h prior to weighing is mandatory when relative humidity in the balance enclosure exceeds 55 %, as a water uptake of just 1.2 % w/w (measurable by TGA) reduces the effective Boc-thioamide molarity enough to throw off the DMF stock concentration by 0.8 % and introduce a systematic error in the fixed stoichiometric excess used for coupling. Contact with trialkylphosphines—employed for azide reduction—must be avoided: the thioamide undergoes Staudinger-type consumption, and a decline in resin-bound UV absorption at 270 nm of more than 40 % is recorded within 15 min of exposing a dipeptidyl-resin bearing the terminal thiocarbamoyl to 2 equivalents of tributylphosphine in THF/water.