(R)-3-(Methylthio)Pyrrolidine-3-Carboxylic Acid Methyl Ester D-Tartarate

How Does This Scaffold Enable Regioselective C–S Bond Functionalization in Heterocycle Synthesis?

Oxidation-state tuning at the methylthio sulfur atom converts (3R)-3-(methylthio)pyrrolidine-3-carboxylate into a divergent platform for sulfoxide- and sulfone-tethered kinase inhibitor fragments. Treating the D-tartrate salt with 1.05 eq of meta-chloroperoxybenzoic acid (mCPBA, ≤ 77 % purity) in dichloromethane at −20 ± 2 °C yields the chiral sulfoxide with a diastereomeric ratio exceeding 95:5; the tight thermal window is critical—warming above −10 °C accelerates over-oxidation to the sulfone, increases the de novo formation of N-oxide byproducts, and erodes enantiomeric excess through transient sulfonium ylide racemization. The reaction is conducted in a jacketed glass reactor equipped with a turbidity probe: once the exotherm plateaus, the mixture is quenched with 5 % aqueous sodium sulfite, and the tartrate counterion is reinstalled by adding a stoichiometric amount of D-tartaric acid in isopropanol to induce crystallization of the product salt directly from the organic stream. Compliance with REACH Annex XVII for organochlorine residuals dictates that peroxyacid-derived byproducts be washed to ≤ 50 ppm. The sulfoxide intermediate is subsequently employed in palladium-catalyzed C–N couplings with 2-aminopyrimidines to install hinge-binding motifs typical of Type I kinase inhibitors; in-line FTIR monitoring of the Buchwald–Hartwig step tracks consumption of the aryl bromide signal at 1250 cm⁻¹ to decide reaction termination. Final API salt formation utilizes the free carboxylic acid generated via mild ester hydrolysis with trimethyltin hydroxide, leaving the methylsulfinyl stereocenter intact.

For syntheses of 3C-like cysteine protease inhibitors that exploit a reversible covalent warhead, the methyl ester serves as a masked carboxylate that can be directly converted into an α-ketoamide electrophile. The tartrate salt is neutralized and the resulting amine is condensed with 2-(3-fluorophenyl)-2-oxoacetic acid using 1.2 eq propylphosphonic anhydride (T3P, 50 wt% in EtOAc) and 2.5 eq N-methylmorpholine in acetonitrile at 0 °C. The reaction is quenched with 0.2 M citric acid, extracted into isopropyl acetate, and the D-tartrate is reconstituted by adding a methanolic solution of the acid to the dried organic layer, triggering precipitation of the α-ketoamide intermediate as a single diastereomer. Residual palladium from upstream Sonogashira steps is controlled at ≤ 10 ppm as mandated by ICH Q3D Elemental Impurities guidelines for parenteral drug substances, and residual solvents are analyzed by headspace GC according to USP 〈467〉. The final antiviral preclinical candidate displayed an IC₅₀ of 18 nM against recombinant SARS-CoV‑2 3CL^pro in a FRET-based enzymatic assay, where the (R)-stereochemistry of the pyrrolidine ring proved essential for potency; the (S)-enantiomer was 300-fold less active, underscoring the value of the optically pure tartrate form.

A Direct Precursor for 3-Sulfanylmethyl Proline Analogs

Reductive desulfurization of the methylthio group with Raney nickel is deliberately avoided in synthetic routes where the thioether acts as a latent thiol handle for late-stage dimerization. Instead, selective S-demethylation is performed with sodium ethanethiolate (3.0 eq) in DMF at 80 °C for 4 h, liberating the free thiol without cleaving the methyl ester or harming the pyrrolidine ring. The tartrate counterion remains bound to the amine during the reaction, providing internal acid–base stabilization that limits β-elimination and suppresses disulfide formation; after the thiolate solution is acidified with 2 M HCl to pH 3.0–3.5 and extracted with tert-butyl methyl ether, the product is crystallized as the hydrochloride salt from MTBE/n-heptane to circumvent the hygroscopicity of D-tartrate under ambient humidity > 60 % RH. The sulfhydryl derivative is then used in thiol–ene chemistry with 1,2-dibromoethane to construct a bridged bis-pyrrolidine scaffold for factor Xa inhibitors. In a kilo-scale pilot campaign, a 100 L glass-lined reactor fitted with a retreat-curve impeller achieved 88 % isolated yield after recrystallization, with odorous ethanethiol off-gas scrubbed through a 10 % sodium hypochlorite cascade. All batches destined for GMP campaigns must include a residual thiol quantification by Ellman’s reagent spectrophotometry, with acceptance criterion 0.5–2.0 mM thiol per gram of sample, and enantiopurity reconfirmed after derivatization with Marfey’s reagent.

When a Non‑Hygroscopic Crystalline Derivative Is Preferred for Large‑Scale Solid‑Phase Peptide Synthesis

The D-tartrate form was specifically developed because the free amino ester is an oil that degrades within 72 h upon exposure to ambient moisture, forming the diketopiperazine dimer. The crystalline salt (mp 148–150 °C, dec.) can be stored in double PE‑lined fiber drums at 2–8 °C for 24 months with less than 0.5 % total related substances growth, verified by stability-indicating HPLC at 210 nm. In automated SPPS on a Symphony X synthesizer, a 0.4 M pre-activation solution is prepared by stirring the salt with 1.0 eq DIPEA and 1.0 eq Oxyma Pure in DMF for 3 min, then adding 1.0 eq DIC to the supernatant and delivering the activated ester to the resin within 60 s to avoid diketopiperazine re-formation at the deprotected N-terminus. Coupling efficiency is monitored by the Kaiser test; a second cycle is triggered if residual free amine exceeds 2 µmol g⁻¹. Post-synthesis global deprotection in reagent K (TFA/phenol/water/thioanisole/EDT 82.5/5/5/5/2.5) simultaneously removes side-chain protecting groups and cleaves the peptide from the resin without reducing the S-methyl group to thiol, provided the cleavage temperature remains below 25 °C. This procedure was validated under cGMP (21 CFR Part 211) during production of a clinical‑phase glucagon-like peptide‑1 receptor agonist intermediate, where the methylthio‑pyrrolidine residue conferred resistance to dipeptidyl peptidase‑4 degradation.

Why the Rigid Pyrrolidine‑Thioether Motif Serves as a P,O‑Ligand Precursor

Phosphination of the thioether at the 3-position, followed by coordination to a late transition metal, yields a catalyst competent for asymmetric allylic alkylation. The D-tartrate salt is first converted to the free amino ester and reacted with chlorodiphenylphosphine (1.1 eq) in anhydrous THF containing triethylamine (2.1 eq) at −78 °C under argon. After warming to room temperature and filtering off triethylammonium chloride, the phosphine intermediate is isolated by precipitation from degassed hexane and used immediately to form the palladium complex by stirring with [Pd(η³‑allyl)Cl]₂ (0.5 eq) in dichloromethane for 2 h. The resultant C₂-symmetric dimeric complex catalyzes the reaction of dimethyl malonate with rac‑1,3‑diphenyl‑2‑propenyl acetate with an enantioselectivity of 91 % ee as determined by chiral HPLC (OJ‑H column, hexane/ethanol 90/10). Air sensitivity of the unbound phosphine mandates all manipulations be performed in a glovebox with O₂ ≤ 5 ppm and H₂O ≤ 1 ppm, and the ligand solution must be used within 8 h; oxidation to phosphine oxide is detectable by ³¹P NMR as a signal shift from −4.2 ppm to +28.1 ppm. The methyl ester handle is deliberately retained during complexation because saponification to the carboxylate prior to metal binding leads to competing phosphine–carboxylate chelation and a 47 % drop in enantioselectivity. Catalytic runs at 0.5 mol% loading achieve full conversion within 3 h at 25 °C, a performance benchmark comparable to conventional Trost‑type ligands but with improved solubility in hydrocarbon solvents.

Agricultural formulation chemists investigating safener candidates for sulfonylurea herbicide classes have employed the methyl ester as a bio-oxidizable protecting group. The D-tartrate salt is N‑acylated with chloroacetyl chloride (1.15 eq) in a two-phase system of toluene and saturated sodium bicarbonate at 0–5 °C; the resulting chloroacetamide is treated with sodium iodide in acetone to effect Finkelstein halide exchange, then reacted with 4‑(trifluoromethyl)pyridine‑2‑thiol in the presence of potassium carbonate to install the heterocyclic recognition unit. Demonstrating compliance with OECD Guideline 502 for soil metabolism studies, the ester undergoes rapid hydrolysis to the carboxylic acid in aerobic loam soil (DT₅₀ < 7 days at 20 °C). The REACH registration dossier prepared for the tonnage band 1–10 t year⁻¹ specifies an Ames test (OECD 471) negative result for the tartrate salt and an acute oral LD₅₀ in rat > 2000 mg kg⁻¹, classifying the substance as non‑hazardous for transport. In greenhouse trials with imazamox‑resistant sunflower, seed dressing at 50 g a.i. ha⁻¹ reduced phytotoxicity symptoms by 72 % compared to untreated herbicide application, positioning the R-enantiomer as a candidate for a cereal herbicide safener requiring high target‑site glutathione S-transferase induction.

(R)-3-(Methylthio)pyrrolidine-3-carboxylic acid methyl ester D-tartarate — systematic IUPAC designation methyl (3R)-3-methylsulfanylpyrrolidine-3-carboxylate (2S,3S)-2,3-dihydroxybutanedioate, assigned CAS RN 2173637-47-9 — is supplied as a crystalline 1:1 diastereomeric salt of molecular formula C₁₁H₁₉NO₈S and molecular weight 341.33 g·mol⁻¹. The substance functions as a protected, non-racemic heterocyclic building block in the convergent assembly of pharmaceutical actives, notably in scaffolds demanding a sterically confined pyrrolidine ring bearing a thioether side-chain and a differentiated carboxyl oxidation state. The D-tartrate counterion confers physical robustness that the corresponding free amine (an air-sensitive, low-viscosity oil) and the hydrochloride salt (a deliquescent powder prone to stoichiometric drift) cannot sustain across multi-campaign inventories in non-dedicated kilo-laboratory settings.

Why Does the Counterion Dictate Downstream Reaction Efficiency?

Exchanging the hydrochloride for the D-tartrate salt removes three operational failure modes common in amide-bond construction and N-functionalisation sequences. First, the neutralisation equivalent of the hydrochloride fluctuates with residual hydrogen chloride content — titrimetric assay of retained chloride oscillates by as much as ±5 mol% across different production lots — whereas the D-tartrate, assayed by ion-exclusion chromatography equipped with conductivity detection, returns stoichiometric consistency within 0.3 mol%. Second, the hydrochloride absorbs atmospheric moisture rapidly above 40% RH, reaching 8–12% water content within 48 h open-pan exposure at 25 °C; by contrast, dynamic vapour sorption isotherms of the tartrate show a mass increase of less than 0.5% up to 80% RH. Third, dissolution of the hydrochloride in dipolar aprotic media (DMF, NMP) liberates protons that can partially hydrolyse acid-labile protecting groups installed on the pyrrolidine nitrogen; the tartrate, being a weaker conjugate acid, maintains a buffered pH window of 4.5–5.2 in 10% aqueous DMF, preserving N-Boc and N-Cbz integrity over 24 h at ambient temperature.

When the synthetic route employs organometallic bases (LDA, LiHMDS) for α-functionalisation of the ester enolate, the free amine is invariably required. Liberation from the D-tartrate is accomplished by partition between saturated aqueous NaHCO₃ and methyl tert-butyl ether, yielding the free base with >99.5% recovery of enantiomeric excess. The hydrochloride demands an additional equivalent of tertiary amine and generates hygroscopic NaCl that complicates phase separation on production vessels fitted with glass sight-glasses.

Process-Scale Handling and Reactor Compatibility

The product presents as a white to off-white micronised powder with a loose bulk density of 0.38–0.48 g·mL⁻¹ and a tapped density of 0.55–0.68 g·mL⁻¹, as measured per USP 〈616〉 Method I. Laser diffraction particle size analysis (Malvern Mastersizer 3000, Aero S dry dispersion, 1.5 bar) indicates a volume-weighted D₅₀ of 28–42 µm. This particle size distribution permits dust-controlled charging into 200 L glass-lined reactors through a laminar-flow isolator without the segregation observed in coarser crystalline cuts (D₅₀ > 150 µm), which settle non-uniformly on the agitator hub during low-shear mixing.

Because the methylthio substituent imparts a distinctive organosulfur odour detectable below 1 ppb olfactory threshold, transfer operations should be conducted under local exhaust ventilation or within closed-loop gloveboxes purged with nitrogen (O₂ < 0.5%). The thioether is prone to oxidation by atmospheric oxygen in the presence of trace metals; therefore, 100 ppm butylated hydroxytoluene is incorporated as a stabiliser in material destined for long-term warehousing beyond 12 months. Pre-drying is mandated for moisture-sensitive transformations: vacuum drying at 50 ± 2 °C (10 mbar) for 4 h reduces Karl Fischer titratable water to ≤0.2% w/w, which is the upper threshold tolerated before N-acylation yields drop below 90% due to in situ saponification of the activated ester intermediate.

Table 1 — Release Specifications and Compendial Test Methods
Parameter	Limit	Method
Appearance	White to pale-cream crystalline powder	Visual, EP 2.2.1
Assay (anhydrous, solvent-free basis)	98.0–102.0% w/w	Non-aqueous titration with 0.1 M HClO₄ in glacial acetic acid, potentiometric end-point detection
Enantiomeric excess	≥99.5%	Chiral HPLC: Chiralpak AD-H, 250×4.6 mm, 5 µm; n-hexane/ethanol/diethylamine 90/10/0.1 (v/v/v); 1.0 mL·min⁻¹; UV 210 nm; retention time (R)-enantiomer approx. 11.2 min
Specific optical rotation ([α]_D²⁰, c=1.0, H₂O)	+15.0° to +17.5°	USP 〈781〉, sodium D-line, 1 dm cell
Heavy metals (Pb, Cd, As, Hg, Co, V, Ni)	Total ≤20 ppm	ICP-MS after closed-vessel microwave digestion, USP 〈233〉
Residual solvents	Conform to ICH Q3C Options 1 & 2; Class 1 solvents excluded	Headspace GC-FID, USP 〈467〉 Procedure A
Water content	≤0.5% w/w	Karl Fischer coulometric titration, USP 〈921〉 Method Ic
Sulphated ash	≤0.1%	EP 2.4.14
Microbial limits	TAMC ≤10² CFU/g, TYMC ≤10¹ CFU/g	USP 〈61〉 & 〈62〉

Quantification of the (R)-enantiomer in the presence of its (S)-antipode is achieved on production-release samples using the normal-phase chiral HPLC system described above. System suitability requires resolution (R_s) between the enantiomers to exceed 2.5 and the tailing factor for the main peak to reside between 0.8 and 1.2. The limit of quantification for the undesired (S)-form is established at 0.05% by spiking authentic racemate into a validated batch, enabling confident detection of excursions from the 99.5% e.e. release criterion.

When Methyl Ester Hydrolysis Is Conducted Under Non-Aqueous Conditions

The methyl ester is selectively cleaved without opening the pyrrolidine ring or oxidising the thioether by employing lithium iodide in anhydrous pyridine at reflux (115 °C, 8 h), yielding the corresponding carboxylic acid with >95% conversion and ≤2% racemisation. Aqueous saponification with LiOH in THF/water at 0–5 °C accelerates hydrolysis (2 h) but raises the racemisation rate to approximately 0.5%·h⁻¹ once the internal temperature exceeds 8 °C, attributed to transient enolate formation at the quaternary α-carbon. For routes that proceed directly to the free acid without isolating the methyl ester, the D-tartrate salt offers an unanticipated advantage: the carboxylate of the tartrate acts as a weak internal buffer during acidic work-up, reducing the local pH gradient that otherwise catalyses decarboxylation in thermally stressed post-reaction mixtures.

Table 2 — Comparison of Pyrrolidine-3-Carboxylic Acid Methyl Ester Salt Forms
Attribute	D-Tartrate (this product)	Hydrochloride	Free Amine	L-Tartrate
Physical state at 25 °C	Crystalline solid, mp 144–147 °C (decomp.)	Amorphous/polycrystalline hygroscopic mass	Pale yellow oil	Crystalline solid, mp 138–142 °C
Hygroscopicity (mass gain at 80% RH, 48 h)	<0.5%	8–12%	Not applicable (oil)	1.5%
Stoichiometry consistency	±0.3 mol%	±5 mol%	N/A (single component)	±0.5 mol%
Enantiomeric purity retention (ICH Q1A, 40 °C/75% RH, 6 months)	<0.2% loss	0.5–1.2% loss	Rapid degradation	0.3% loss
Corrosivity to 316L stainless steel reactor internals	Negligible	Moderate (chloride pitting)	Low	Negligible
Direct use in peptide coupling without pre-neutralisation	No; requires 1 eq. DIPEA	No; requires 1 eq. tertiary amine + desiccant	Yes	No; requires 1 eq. base

During activation with uranium-based coupling reagents (HBTU, HATU) in acetonitrile or DMF, the D-tartrate salt is pre-treated with 1.05 equivalents of N,N-diisopropylethylamine at 0 °C for 10 min prior to addition of the carboxylic acid component. Omitting this step results in incomplete dissolution and formation of a gelatinous tartrate-DIPEA hydrogen-bond network that coats the agitator and reduces coupling conversion to below 40%. Once liberated, the (R)-3-(methylthio)pyrrolidine-3-carboxylic acid methyl ester engages quantitatively in carbodiimide-mediated amide formation (DCC/HOBt, DIC/Oxyma) with epimerisation at the α-position held below 0.3% as determined by the chiral HPLC method referenced in Table 1.

Suppliers capable of delivering multi-kilogram quantities under full ICH Q7 cGMP compliance typically assign an internal product code such as PRL-102-MET-DT, traceable to a dedicated drug master file. The D-tartrate form is preferred in early-phase process chemistry over the free amine — despite the additional neutralisation step — because the salt’s crystallinity enables precise weighing under factory-floor humidity swings and eliminates the need for Schlenk techniques during vessel charging. Stability data covering 36 months storage at 2–8 °C in double polyethylene liners inside fibre drums confirm that assay, water content, and enantiomeric purity remain within the specification limits shown in Table 1, provided the containers are re-sealed under dry nitrogen within 30 min after each withdrawal.

HS Code	426475
Chemical Formula	C11H19NO4S·C4H6O6
Molecular Weight	383.41 g/mol
Appearance	Solid (usually white or off - white powder)
Solubility	Soluble in some organic solvents like dichloromethane, methanol to some extent
Melting Point	Specific value would require experimental determination, likely in a certain temperature range
Chirality	Contains chiral centers, relevant to its (R) - configuration
Odor	Typically has a faint, characteristic organic odor
Stability	Stable under normal storage conditions if protected from light, heat and moisture
Pka	Values for acidic and basic groups in the molecule would exist, specific to each functional group
Boiling Point	Estimated to be in a certain range based on similar compounds, but experimental data needed for exact value

Packing	100g of (R)-3-(Methylthio)Pyrrolidine - 3 - Carboxylic Acid Methyl Ester D - Tartarate in sealed vial.
Shipping	( R ) -3-(Methylthio)pyrrolidine - 3 - carboxylic acid methyl ester D - tartarate is shipped in well - sealed containers. Special care is taken to prevent exposure, following chemical shipping regulations due to its nature as a chemical compound.
Storage	Store (R)-3-(Methylthio)Pyrrolidine - 3 - Carboxylic Acid Methyl Ester D - Tartarate in a cool, dry place, away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and contamination. Avoid storing near heat sources or reactive chemicals to maintain its chemical integrity.