2R-Thiocarbamoyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester

Automated Fmoc-based solid-phase peptide synthesis (SPPS) incorporating a thioamide isostere at the i+1 residue of a key protease cleavage site routinely employs (R)-1-Boc-2-thiocarbamoyl-pyrrolidine as the masked chiral building block. The tertiary butyl carbamate protection permits orthogonal deblocking under conditions that minimise racemisation at the 2-position. Loading onto 2-chlorotrityl chloride resin pre-swelled in anhydrous NMP is performed with 4.0 equivalents of the amino acid derivative and 8.0 equivalents of DIPEA in DCM at 20–22 °C for 90 minutes; final substitution levels are held at 0.6–0.8 mmol/g to reduce inter-chain aggregation of the thioamide-rich sequence. Subsequent Fmoc removal uses 20% v/v piperidine in DMF, with exposure strictly limited to 3+7 minutes because extended contact with secondary amine nucleophiles attacks the thiocarbonyl moiety. Chain elongation proceeds with standard Fmoc-Xaa-OH monomers activated by 0.95 equivalents of HATU and 2.0 equivalents of NMM in DMF; coupling cycles are sonicated at 37 °C to compensate for the reduced reactivity of the sterically hindered thioamide amine. Crucially, the thioamide bond exhibits marked acid lability. Global cleavage and side-chain deprotection therefore avoid standard Reagent R. Instead, a modified cocktail of TFA/thioanisole/1,2-ethanedithiol/anisole (90:5:3:2 v/v) is applied over 25 minutes at 0 °C, immediately quenched into cold tert-butyl methyl ether, and lyophilised from aqueous acetonitrile containing 0.1% v/v formic acid. Process intermediates are monitored for thioamide integrity on a Waters ACQUITY UPLC H-Class system equipped with a BEH C18 1.7 µm column, using a 10–90% MeCN/water gradient over 6 minutes. Residual piperidine, DMF, and DCM are controlled to limits prescribed by ICH Q3C for class 2 solvents. The final pseudo-peptide, incorporating a (R)-thiazolidine-like isostere, resists hydrolysis by falcipain-2 and cathepsin L with serum half-lives exceeding 48 hours in pH 7.4 PBS at 37 °C—a performance benchmark measured by LC-MS/MS quantitation of the intact parent.

Can the (R)-thiocarbamoyl intermediate circumvent racemisation in thiazoline API construction?

During the kilogram-scale preparation of a lead antitubercular agent containing a chiral thiazoline core, the (R)-Boc-thiocarbamoyl pyrrolidine is condensed with 2-bromo-4′-fluoroacetophenone to exploit Hantzsch-type heterocyclisation. The process is inherently prone to epimerisation because the transient thioimidate intermediate enolises at the α-carbon under basic conditions. Mitigation relies on precise reverse addition: a solution of the chiral thioamide (1.0 eq) in anhydrous MeCN is added over 45 minutes to a slurry of the α-bromoketone (1.08 eq) and finely ground K₂CO₃ (1.5 eq) at −5 to 0 °C in a 50 L jacketed glass-lined reactor. The formation of a hydrogen bond network between the thiocarbonyl and the protonated pyridine nitrogen is exploited by introducing 0.3 equivalents of 2,6-lutidine as a non-nucleophilic buffering additive; this suppresses the local pH spike responsible for stereochemical erosion. Reaction progress is tracked with an in-line ReactIR probe monitoring the disappearance of the characteristic thioamide C=S stretch at 1250 cm⁻¹. After 4 hours, the mixture is quenched with 10% w/v aqueous NH₄Cl, extracted into MTBE, and the organic layer is dried over MgSO₄ and concentrated at ≤30 °C under reduced pressure. Crude thiazoline is purified by flash chromatography on silica gel 60 (230–400 mesh) with a heptane/EtOAc gradient, affording an isolated yield of 82–85% and chiral purity of 99.2% ee as determined by chiral HPLC on a Chiralcel OD-H column (hexane/i-PrOH 90:10, 1.0 mL/min, λ=254 nm). Heavy metal contaminants—particularly residual palladium from an upstream cross-coupling—are restricted to the 10 ppm threshold for oral solid dosage forms under ICH Q3D Elemental Class 1 and 2A guidelines. The isolated Boc-protected heterocycle serves as a direct penultimate intermediate to a nanomolar inhibitor of Mycobacterium tuberculosis InhA enoyl-ACP reductase, with the downstream ethionamide-resistant strain susceptibility fully retained in in vitro microdilution assays.

In asymmetric organocatalysis, transformation of the primary thioamide group into a tailored thiourea array proceeds through sequential deprotection and functionalisation without disturbing the pyrrolidine α-stereocenter. Boc removal is conducted with 30% v/v TFA in DCM containing 5% v/v iPr₃SiH as carbocation scavenger; the resulting (R)-2-thiocarbamoylpyrrolidinium trifluoroacetate salt is isolated by precipitation from tert-butyl methyl ether in quantitative yield. The free amine is liberated with 1M NaOH and immediately reacted with 3,5-bis(trifluoromethyl)phenyl isothiocyanate (1.05 equiv) in DCM at 0–5 °C. The product, (R)-2-[3-(3,5-bis-trifluoromethylphenyl)thioureido]-2-thiocarbamoylpyrrolidine, retains the primary thioamide as a secondary hydrogen-bond donor. This bifunctional catalyst operates in the conjugate addition of dimethyl malonate to trans-β-nitrostyrene: with a catalyst loading of 5 mol% in toluene at −20 °C, the adduct is obtained in 95% conversion and 87% ee after 24 hours. Systematic evaluation of donor scope is summarised below. Each trial employs catalyst recrystallised from i-PrOH to ≥99.5% purity, with water content verified by coulometric Karl Fischer titration (≤0.01% w/w) to exclude moisture-induced retro-thioamide hydrolysis. The unreacted isothiocyanate is scavenged by polymer-bound tris(aminoethyl)amine resin prior to column work-up.

Data reflect enantiomeric excess determined on a Chiralpak AD-H column under isocratic conditions at 25 °C. Reported values are averages of duplicate runs with ±2% reproducibility.

When agrochemical discovery programs require a chiral pyrrolidine-thioamide scaffold

A succinate dehydrogenase inhibitor (SDHI) fungicide series targeting Zymoseptoria tritici incorporates the (R)-thiocarbamoyl-pyrrolidine motif to enhance binding within the ubiquinone pocket of complex II. Scale-up synthesis begins with (R)-Boc-thiocarbamoylpyrrolidine, which is coupled to 4-(trifluoromethyl)benzoic acid using EDC·HCl (1.2 eq) and HOBt (1.2 eq) in DMF at 0–5 °C to generate the corresponding N-Boc protected thioamide-ester intermediate. After aqueous work-up with 5% NaHCO₃, the Boc group is cleaved with anhydrous HCl in dioxane (4M, 2 hours), and the liberated amine is acylated with 3-difluoromethyl-1-methylpyrazole-4-carbonyl chloride in the presence of Et₃N. Residual chloride reagents are hydrolysed with a controlled pH 8.5 phosphate buffer quench to avoid thiocarbonyl desulfurisation. Final product is crystallised from hot heptane/EtOAc (4:1) to 99.0% area purity by GC-FID and conforms to the ≤0.15% w/w individual impurity threshold defined by FAO Specification 581/TC for technical-grade active ingredients. The metabolite profiling study under aerobic soil metabolism (OECD 307, 20 °C, 45% WHC) showed DT₅₀ of 34 days for the parent, with the thioamide moiety undergoing gradual oxidative desulfuration to the corresponding amide, a transformation confirmed by ¹⁸O-isotope labeling mass spectrometry. Field trials conducted on Puccinia recondita in winter wheat at application rates of 75–100 g a.i./ha demonstrated curative activity equivalent to fluxapyroxad standards, while the (R)-enantiomer proved at least 20-fold more potent than the (S)-counterpart in detached leaf assays.

Coordination polymer nodes for soft metal sequestration

(R)-Boc-thiocarbamoylpyrrolidine functions as a monotopic ligand building block in the solvothermal synthesis of a highly crystalline Ag(I) coordination polymer exhibiting selective Hg²⁺ uptake from simulated industrial wastewater. A Pyrex pressure tube is charged with the thioamide ligand (0.20 mmol), AgBF₄ (0.22 mmol), and 4.0 mL of a mixed solvent system composed of MeCN/EtOH/H₂O (2:1:1 v/v). The tube is sealed under argon and heated in a programmable oven at 85 °C for 48 hours, then cooled to ambient temperature at 5 °C/h. Colourless block-shaped crystals are isolated by filtration, washed with cold MeCN, and dried under vacuum at 40 °C for 12 hours. Single-crystal X-ray diffraction reveals a 1D zigzag chain where the silver centre adopts a near-linear S–Ag–N coordination geometry (Ag(1)–S(1) = 2.412 Å, Ag(1)–N(pyrrolidine) = 2.196 Å). The tert-butyl carbamate groups remain intact and fill the inter-chain void spaces, conferring a BET surface area of 420 m²/g measured by N₂ adsorption at 77 K according to ISO 9277:2010. For mercury capture, a batch experiment using 10 mg of activated polymer in 50 mL of aqueous Hg(NO₃)₂ solution (50 mg/L, pH 5.5) reaches equilibrium within 30 minutes, achieving 98.3% removal efficiency and a distribution coefficient (K_d) of 8.2 × 10⁵ mL/g. Regeneration with 0.1M thiourea solution over 5 cycles causes less than 5% loss in capacity, and leached silver is quantified by ICP-OES to remain below the 0.1 mg/L EU drinking water limit.

A mechanistic probe of prolyl oligopeptidase (POP, EC 3.4.21.26) inhibition deploys the free amine form of (R)-2-thiocarbamoylpyrrolidine as an active-site titrant that exploits the catalytic serine nucleophile. The Boc-protected precursor is first deprotected using HCl/dioxane and the hydrochloride salt is recrystallised from MeOH/Et₂O. The resulting (R)-2-thiocarbamoylpyrrolidine·HCl is dissolved in DMSO-d₆ and the stock concentration is validated by ¹H NMR integration against an internal standard of 1,3,5-trimethoxybenzene. POP activity is assayed fluorimetrically (λ_ex=380 nm, λ_em=460 nm) in 50 mM HEPES, 1 mM EDTA, pH 7.5 at 30 °C using Z-Gly-Pro-7-amino-4-methylcoumarin (100 µM). Upon addition of the thioamide probe at 50 µM, time-dependent inactivation is observed, with a measured pseudo-first-order rate constant k_obs of 0.023 s⁻¹ and no recovery of activity after 1000-fold dilution—a hallmark of covalent acylation of the active-site serine. Dialysis against assay buffer followed by ESI-TOF MS indicates a mass shift of +113 Da on the intact enzyme, consistent with thioacylation. Control experiments with the (S)-enantiomer show negligible inhibition at 200 µM, confirming chiral recognition. The probe is further utilised in competitive ABPP with a TAMRA-fluorophosphonate activity-based probe to quantify occupied POP in mouse brain homogenates; densitometry of SDS-PAGE bands indicates 85% blockade at 10 µM probe concentration, meeting the sensitivity requirement for CNS target-engagement studies mandated by EMA/CAT/CPWP in early phenotypic screening.