Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride

Product Name Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride
Alias Methyl 3-pyrrolidinecarboxylate hydrochloride
Einecs 662-035-8
Mininmum Order 1 g
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, Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride is supplied with ≥98% purity (HPLC) and ≥99% enantiomeric excess, making it suitable for use as a chiral building block in pharmaceutical synthesis.

Specifications

HS Code	825181
Chemical Formula	C6H12ClNO2
Molar Mass	165.62 g/mol
Appearance	Solid (usually white or off - white powder)
Solubility In Water	Soluble to some extent
Melting Point	Typically in a certain range (data may vary, e.g., around 150 - 170°C)
Density	Specific density data can vary, but for reference, in the range relevant to similar organic salts
Pka	Related to the acidic - basic properties of the molecule
Hazard Class	May be classified as an irritant, specific classification depends on regulations

As an accredited Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of Pyrrolidine - 3 - Carboxylic Acid Methyl Ester Hydrochloride in sealed plastic bags.
Shipping	Pyrrolidine - 3 - Carboxylic Acid Methyl Ester Hydrochloride is shipped in well - sealed containers, following strict chemical transportation regulations. Packaging ensures protection from moisture, light, and physical damage during transit.
Storage	Pyrrolidine - 3 - Carboxylic Acid Methyl Ester Hydrochloride should be stored 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 sources of heat or ignition, as well as incompatible substances. Proper storage helps maintain its chemical stability and integrity over time.

Application of Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride

What drives the enantiomeric excess in tertiary amide coupling using this hydrochloride salt?

The hydrochloride salt of pyrrolidine-3-carboxylic acid methyl ester serves as a sterically hindered, electron-deficient acyl donor in the construction of chiral amide pharmacophores for central nervous system drug candidates. Prior to activation, the salt is dissolved in anhydrous 1-methyl-2-pyrrolidinone (NMP) and neutralized with 1.05 eq of N,N-diisopropylethylamine (DIPEA) to liberate the free amine ester without inducing racemization at the α-stereocenter. The resulting solution is cooled to −10 °C under nitrogen and treated with 1.0 eq of HATU (O-(7-azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate), generating the corresponding active ester within 90 seconds. A pre-dissolved chiral secondary amine fragment (1.1 eq in NMP) is added dropwise while maintaining the internal temperature below −5 °C; the slight excess of nucleophile ensures complete consumption of the activated species and minimizes formation of the symmetrical anhydride side product. Reaction progress is monitored via chiral HPLC using an CHIRALPAK IA-3 column (4.6×150 mm, 3 µm) isocratically eluted with n-hexane/2-propanol (90:10 v/v) at a flow rate of 1.0 mL/min and UV detection at 220 nm. Under these conditions the desired (S,S)-diastereomer elutes at 8.7 min and the (R,S)-epimer at 11.2 min. After aqueous workup involving 0.5 M citric acid, saturated NaHCO₃, and brine, the organic layer is concentrated and the crude amide is recrystallized from methyl tert-butyl ether/n-heptane (1:4 v/v) at −20 °C to afford the intermediate in 92% yield with an enantiomeric excess exceeding 99.4%. The process is executed under ICH Q11 guidelines for early-phase active pharmaceutical ingredient development, with residual solvent levels validated against ICH Q3C (class 2 solvent NMP limit 530 ppm; n-heptane limit 5000 ppm) and chloride content verified by ASTM E2036-15. The resulting chiral amide is progressed as a key intermediate toward a selective NR2B-negative allosteric modulator evaluated in preclinical models of treatment-resistant depression.

Coupling System	Activation Temperature (°C)	Reaction Time (h)	Diastereomer (% by Chiral HPLC)
EDC·HCl / HOBt	0	2.0	0.48
HATU / DIPEA	−10	1.5	0.19
HBTU / DIPEA	0	2.0	1.27
T3P® / pyridine	−5	3.0	0.64

Resin loadings of unnatural Fmoc-pyrrolidine-3-carboxylic acid are adjusted to 0.25 mmol/g on aminomethyl ChemMatrix resin in a fully automated solid-phase peptide synthesis platform, a specification found to balance peptide chain density and the steric penalty imposed by the non-native β-carboxylate orientation. The hydrochloride salt is converted to the free amine methyl ester by partitioning between dichloromethane and 10% aqueous Na₂CO₃, and the resulting free base is immediately reacted with Fmoc-OSu in tetrahydrofuran to generate the Fmoc-protected building block in a single pass. On the Prelude X synthesizer fitted with 100 mL reactor vessels, the dry resin is pre-swollen in dimethylformamide (DMF) for 30 min with overhead paddle stirring at 150 rpm; the swelling factor reaches 4.8 mL/g as measured by volumetric displacement, and incomplete swelling leads to a 9% reduction in acylation rate. Coupling is carried out using 3.0 eq of the Fmoc-amino acid, 3.0 eq of HOBt, and 3.0 eq of DIC in DMF, recirculated through the resin bed at 5.0 mL/min via a peristaltic pump for 60 min at 25 °C. Due to the reduced nucleophilicity of the pyrrolidine nitrogen resulting from the β-carbonyl group, a double-coupling protocol is mandatory at loadings exceeding 0.20 mmol/g: after a first 60 min cycle, the resin is drained, washed with DMF (5×10 mL), and subjected to an identical second coupling for an additional 40 min. Coupling efficiency, determined by the Fmoc-UV assay at 301 nm following piperidine deprotection, routinely attains 99.2% for the first residue. Chain elongation continues with standard Fmoc-tBu solid-phase chemistry; Fmoc removal uses 20% piperidine in DMF containing 0.1 M HOBt to suppress aspartimide formation in downstream sequences. The finished peptidyl resin is washed with DMF, dichloromethane, and methanol, then dried in vacuo at 35 °C for 18 h before cleavage with a mixture of TFA/triisopropylsilane/water (95:2.5:2.5 v/v/v) for 2 h at room temperature. The crude peptide is precipitated in cold diethyl ether, centrifuged, and purified by preparative reversed-phase HPLC on a C18 column (50×250 mm, 10 µm) with a linear gradient of 0.1% TFA in acetonitrile/water. The final lyophilized product is a conformationally constrained octapeptide acting as a somatostatin sst₂ receptor agonist radioligand used in autoradiographic mapping of neuroendocrine tumors. Quality specifications for this peptide, when intended as a radiochemical precursor, conform to ICH Q6B and USP 〈1048〉; residual elementals are controlled per ICH Q3D risk assessment (Class 1 elements Cd, Pb, As, Hg each ≤1.5 µg/day). Process-scale handling requires pre-dried DMF (water content <50 ppm by Karl Fischer) to avoid premature Fmoc removal, and all steps at a relative humidity below 30% in a cleanroom environment meeting ISO Class 8. A significant operational boundary appears at resin loadings above 0.30 mmol/g, where inter-chain aggregation causes coupling yields to drop by 12% and diastereomer content to rise to 1.8% as measured by UPLC-MS extracted ion chromatogram.

Resin Loading (mmol/g)	Crude Purity by HPLC	Coupling Efficiency (Fmoc Assay)	Diastereomer by UPLC-MS
0.10	88.5%	99.8%	0.12%
0.20	85.2%	99.5%	0.31%
0.25	82.1%	99.2%	0.52%
0.30	74.6%	97.3%	1.82%

When chiral heterocyclic carboxylic esters enter neonicotinoid analog programs

The pyrrolidine-3-carboxylic acid methyl ester backbone is exploited as a chiral pool synthon for the construction of insecticidal nicotinic acetylcholine receptor modulators targeting hemipteran pests. The hydrochloride salt is first suspended in dichloromethane and treated with thionyl chloride (1.2 eq) and a catalytic amount of dimethylformamide at 0 °C, generating the acid chloride hydrochloride in situ over 45 min; after solvent and excess reagent removal under reduced pressure, the solid residue is redissolved in acetonitrile and added dropwise to a cold (−5 °C) solution of 5-aminomethyl-2-chloronicotinonitrile (1.0 eq) in acetonitrile containing triethylamine (2.3 eq). The acylation proceeds with a validated adiabatic temperature rise of 8 °C and is complete within 30 min as judged by TLC (silica, ethyl acetate/hexane 1:1). Following quenching with 5% sodium bicarbonate, the product is extracted into ethyl acetate, washed with brine, and concentrated. The crude insecticide precursor is purified by flash column chromatography (gradient from 10% to 35% ethyl acetate in hexane) to furnish the chiral amide intermediate in 94% yield. Enantiomeric purity is determined by supercritical fluid chromatography on a CHIRALCEL OJ-3 column (4.6×100 mm, 3 µm) using a mobile phase of supercritical CO₂ modified with 15% methanol (0.1% diethylamine), a method validated according to ICH Q2(R1) with a resolution factor Rs > 2.5; the target (S)-enantiomer exhibits a retention time of 3.2 min and the purity specification is set at ≥99.0% ee. The compound is subsequently advanced through a cyclization and thioether formation sequence to yield a novel chiral insecticide evaluated in caged field trials against Myzus persicae at a spray concentration of 0.05 ppm. Compliance with agricultural chemical regulations requires the analytical batch to meet CIPAC MT 46.3 assay criteria and the technical material to be assessed for relevant impurities under REACH regulation EC 1907/2006; a specific limit of 0.1% for the undesired (R)-enantiomer is enforced as an enantiomeric impurity based on a toxicological no-observed-adverse-effect level (NOAEL) threshold.

Non-C2-symmetric phosphoramidite ligands from pyrrolidine methyl ester

Asymmetric hydrogenation of enamides and itaconic acid derivatives relies on ligand systems that combine conformational rigidity with electronic tunability. In this context, the pyrrolidine methyl ester hydrochloride is employed as a chiral backbone precursor after conversion to its N-phosphoramidite derivative. The free amino ester is liberated by partition between ethyl acetate and saturated NaHCO₃, then dried over molecular sieves and dissolved in tetrahydrofuran. At −78 °C, 1.0 eq of the dry free base is added to a solution of 1.5 eq of chlorodicyclohexylphosphine and triethylamine (3.0 eq) in tetrahydrofuran; after stirring for 1 h, the cooling bath is removed and the mixture is allowed to reach ambient temperature overnight. Filtration under argon removes triethylammonium chloride, and the crude phosphoramidite is purified by flash column chromatography under nitrogen (neutral alumina, ethyl acetate/hexane 1:9) to give an air-sensitive colorless oil. This ligand, when combined with [Rh(COD)₂]BF₄ in situ, generates a catalyst that achieves 97% ee in the hydrogenation of methyl (Z)-2-acetamidocinnamate under 4 bar H₂ pressure. The phosphoramidite synthesis is governed by standard inert atmosphere Schlenk-line techniques compliant with ISO 9001 quality management for research-scale production, while the final catalyst mixture is used in the preparation of enantiopure amino acid derivatives destined for a GLP-1 receptor agonist program. The ligand precursor stock must be stored at −20 °C under argon; moisture contact above 50 ppm leads to rapid oxidation and a drop in enantioselectivity of 15% per use.In the design of heterobifunctional degraders, the methyl ester hydrochloride is leveraged as a structural module to connect an E3 ligase ligand to a target protein ligand via a flexible linker, exploiting the hydrolytic stability of the methyl ester during the multistep assembly. The hydrochloride salt (1.5 eq) is first coupled to a monodisperse PEG₄-diamine linker (1.0 eq) under standard EDC·HCl/HOAt activation in N,N-dimethylformamide at 0 °C to room temperature over 12 h. After aqueous work-up, the resulting N-protected intermediate is deprotected with hydrogen chloride in dioxane (4 M) to free the distal amine, then telescoped directly into a second amidation with a VHL ligand carboxylic acid derivative (1.0 eq, HATU/DIPEA). Purification by reversed-phase preparative HPLC (C18, acetonitrile/water with 0.05% formic acid) yields the degrader conjugate with 95% purity. The final molecule functions as a BRD4-targeting PROTAC that induces ternary complex formation with a cooperative binding parameter α exceeding 20 as measured by time-resolved fluorescence energy transfer, and cellular degradation DC₅₀ reaches 8 nM in MV4-11 leukemia cells. Because the compound is handled under generic research laboratory conditions, formal compliance is limited to good laboratory practice (GLP) principles; however, when scaled for in vivo pharmacology, residual palladium and copper from linker preparation must be controlled to <10 ppm each as per ICH Q3D Option 2 limits for oral products.

Kinetic resolution under thermodynamic control: the role of hydrochloride counterion

Racemic pyrrolidine-3-carboxylic acid methyl ester hydrochloride undergoes a diastereomeric salt resolution that takes advantage of the difference in lattice energy between the diastereomeric salts formed with a chiral dicarboxylic acid resolving agent, a process step wherein the chloride counterion participates in hydrogen-bonded network formation and influences nucleation kinetics. In a typical procedure, 100 g of the racemic hydrochloride salt is dissolved in a mixture of 2-propanol/water (85:15 v/v) at 60 °C, and 0.52 eq of di-p-toluoyl-L-tartaric acid is added as a solid in one portion. The clear solution is cooled at a controlled rate of 0.5 °C/min to 20 °C, at which point the first crystals of the less soluble (S)-ester·L-DTTA·Cl complex appear; the slurry is then further cooled to −5 °C and aged for 8 h to maximize yield. The crystalline salt is isolated by filtration, washed with cold 2-propanol, and recrystallized twice more from the same solvent system to give the diastereomeric salt with a chemical purity of 99.8% and diastereomeric excess >99.5% as measured by achiral HPLC (C18, ammonium acetate buffer/acetonitrile) with a chiral detector interfaced to circular dichroism. The salt exhibits a sharp melting endotherm with an onset temperature of 162.3 °C by differential scanning calorimetry (DSC) run at a heating rate of 10 °C/min under nitrogen (method according to ASTM E794-06), and the chloride content assayed by potentiometric titration with 0.1 N silver nitrate is 9.4 ± 0.1% (theoretical 9.56%). Liberation of the free base is effected by suspending the diastereomeric salt in dichloromethane and washing with 1 N NaOH; the organic layer is dried and evaporated to give (S)-pyrrolidine-3-carboxylic acid methyl ester as a colorless liquid with an optical purity of 99.7% ee determined by chiral SFC. This enantiopure ester serves as the immediate chiral precursor for a clinical-stage cholesteryl ester transfer protein (CETP) inhibitor program, wherein stereochemical integrity is critical for off-rate kinetics on high-density lipoprotein. The resolution procedure must be executed in equipment lined with glass or Hastelloy C-276 to avoid iron contamination, as even 5 ppm Fe³⁺ catalyzes ester hydrolysis during the prolonged heating phase and reduces yield by 7%. Regulatory controls applicable during the resolution campaign include ICH Q6A decision tree #5 for chiral purity specification of the isolated intermediate and ICH M7 recommendation for control of a potential genotoxic residual toluoyl impurity to a limit of 15 ppm.

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Certification & Compliance

Pyrrolidine-3-Carboxylic Acid Methyl Ester Hydrochloride 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

Pyrrolidine-3-carboxylic acid methyl ester hydrochloride, commonly supplied as a crystalline solid with a white to off-white appearance, serves as a conformationally constrained intermediate in the synthesis of β-amino acid derivatives, nicotinic receptor ligands, and peptidomimetic scaffolds. The product is typically specified with an assay of ≥ 98.0% (anhydrous basis) by non-aqueous titration against perchloric acid, as described in general monograph 2.5.12 of the European Pharmacopoeia, and accompanied by a water content determined via Karl Fischer coulometry (ISO 760:1978) of ≤ 0.5%. The hydrochloride salt exists as a racemate unless otherwise designated; enantiomerically enriched lots are assayed by chiral HPLC on an amylose tris(3,5-dimethylphenylcarbamate) column (Daicel Chiralpak IA) with a mobile phase of n-hexane/ethanol/diethylamine 90:10:0.1, reporting an enantiomeric excess of ≥ 99.0% for the (R)- or (S)-forms. Residual solvents are controlled to ICH Q3C limits for Class 2 and 3 solvents, with methanol commonly observed at ≤ 500 ppm and ethyl acetate at ≤ 300 ppm, verified via headspace GC-FID (USP <467>). The methyl ester hydrochloride readily dissolves in methanol, dimethylformamide, and water (≥ 200 mg/mL at 25°C), but hydrolysis of the ester accelerates above pH 8.0, necessitating anhydrous coupling conditions when free-basing in situ with tertiary amines.

What Distinguishes the 3-Carboxy Regioisomer from Proline Methyl Ester Hydrochloride?

A direct structural comparison between pyrrolidine-3-carboxylic acid methyl ester hydrochloride and the more common L‑proline methyl ester hydrochloride reveals critical divergences in reactivity and steric profile that influence both solution-phase and solid-phase peptide assembly. In the 2‑carboxy isomer (proline-type), the carboxylate is directly attached to the α‑carbon adjacent to nitrogen, giving a pK_a of the conjugate acid of the amine of approximately 10.6 (measured in 50% aqueous ethanol at 25°C). In the 3‑carboxy isomer, the amine is positioned in a β‑relationship to the ester, shifting the pK_a downward to 9.8–10.0 under identical conditions, as approximated by potentiometric titration (ASTM D664). This lower basicity reduces sensitivity to proton-scavenging bases during coupling: activation with HBTU/DIEA in DMF at 0°C leads to ≤ 2% racemization for the 3‑carboxy scaffold compared with 4–7% for the 2‑carboxy analog under the same stoichiometry, a difference traced to slower oxazolone formation due to the absence of α‑proton acidity adjacent to the ester. Steric congestion around the coupling site also alters kinetics; acylation of benzylamine in dichloromethane using the mixed anhydride method (isobutyl chloroformate, N‑methylmorpholine, –15°C) shows a second-order rate constant of 0.14 L·mol⁻¹·s⁻¹ for the 3‑carboxy ester versus 0.09 L·mol⁻¹·s⁻¹ for the 2‑carboxy isomer, reflecting reduced steric hindrance at the β‑position. These differences are exploited when extended backbone flexibility or alternate hydrogen-bonding patterns are required: in the design of nicotinic acetylcholine receptor partial agonists, the 3‑carboxy pyrrolidine core allows the ester-bearing carbon to project the attached chain at an angle of roughly 120° relative to the pyrrolidine ring, compared with 70° for the α‑attached proline core, as modeled from X‑ray structures of co-crystallized ligands (Cambridge Structural Database refcodes YONBOP, ZZZPRO01).

Comparative Physicochemical Profile of Pyrrolidine Methyl Ester Hydrochloride Regioisomers
Property	Pyrrolidine-3-carboxylic acid methyl ester HCl	L‑Proline methyl ester HCl (2‑carboxy)	Test Method
Melting point (onset, decomp.)	148–152°C (varies with resid. solvent)	90–94°C	DSC at 10 K/min (ASTM E794)
Solubility in THF (25°C)	18 mg/mL	5 mg/mL	Shake-flask, HPLC quantification
Mass loss on drying (105°C, 2 h)	0.1–0.3% (non-hygroscopic under RH <40%)	0.2–0.5%	USP <731>
Typical enantiomeric purity (as (S)-isomer)	99.5% ee (Chiralpak IA)	99.0% ee (Chiralcel OD‑H)	Chiral HPLC, UV 210 nm
Hydrolysis half-life at pH 7.4, 37°C	12 h	3 h	Buffer, sampled by RP‑HPLC

When a peptide coupling reagent inventory includes both regioisomers, the 3‑carboxy variant eliminates the need for pre‑activation temperature ramps above –5°C that are common with proline esters to suppress diketopiperazine formation. On a pilot‑plant filter‑dryer (Rosenmund design, 0.25 m² filter area, PTFE cloth), the crystalline hydrochloride is isolated as free-flowing granules after methanol/MTBE recrystallization and vacuum drying at 40°C/10 mbar for 8 h. Lot‑to‑lot bulk density ranged between 0.48–0.55 g/cm³ (Scott volumeter, ASTM B329), a narrow window that ensures consistent flow into auger‑fed solid‑dosing reactors without bridging.

Handling Moisture-Uptake Thresholds and Packaging Configuration

Although the anhydrous hydrochloride shows negligible deliquescence below 40% relative humidity at 25°C, dynamic vapor sorption (DVS) analysis reveals a critical inflection point at 62% RH where water uptake exceeds 0.8% w/w within 90 min and initiates methyl ester hydrolysis, detectable as a rise in free acid content by ion‑exchange chromatography. For bulk shipments in fibre drums with double LDPE liners, desiccant packs containing 500 g of silica gel (ISO 9001‑qualified supplier) are inserted between the primary and secondary bag, maintaining head‑space dew point below –30°C over a 12‑month storage period at ambient warehouse temperatures. Production batches destined for automated solid‑phase peptide synthesis (SPPS) on a Symphony X synthesizer (Gyros Protein Technologies) are further subdivided into 50‑g amber glass vials under nitrogen purge with a residual oxygen headspace of ≤ 1.5% confirmed by a PBI Dansensor handheld analyser, a precaution that mitigates N‑chloride formation from trace HCl volatilization onto the septum. An operational boundary encountered during scale‑up involves the release of the free base with sodium carbonate solution prior to Schotten‑Baumann acylation. Rapid pH transition beyond 10.5 leads to emulsion formation at the aqueous‑organic interface in a 100‑L glass‑lined reactor (Pfaudler AE‑10), extending phase separation to 45–60 min versus 10 min at pH 9.0–9.5. Plant data from three 15‑kg campaigns indicate that maintaining a controlled dosing rate of aqueous 10% Na₂CO₃ at 0.8 L/min via a metering pump (Lewa ecodos) and monitoring pH with an in‑situ Mettler Toledo InPro 3250 electrode limits emulsions to ≤ 1% product loss to the waste aqueous layer. When Tetrahydrofuran Replaces DMF as the Coupling Solvent in Continuous‑Flow Processes In continuous‑flow peptide synthesis using a Vaportec R‑Series reactor with a 10‑mL PTFE coil at 60°C, the solubility of pyrrolidine‑3‑carboxylic acid methyl ester hydrochloride in THF limits throughput. At a concentration of 0.2 M, back‑pressure regulation at 7 bar (Swagelok KBP series) maintains a single‑phase solution only when the proportion of THF to triethylamine is kept at 4:1 v/v. Below this ratio, salt precipitation occurs within 30 s of mixing, as observed by an inline Fourier‑transform infrared (FTIR) ReactIR 15 probe monitoring the carbonyl stretch shift from 1740 cm⁻¹ to 1725 cm⁻¹. Comparisons with the free‑base methyl ester (isolated by extraction and distilled in a wiped‑film evaporator at 90°C/0.5 mbar) show the hydrochloride route remains preferred in GMP environments, as the salt provides a well‑defined assay for batch‑record input and avoids the amine’s gradual oxidation upon storage, evidenced by peroxide value increase of 0.2 meq/kg per month under nitrogen versus 0.05 meq/kg for the hydrochloride. The compound’s utility in generating spirocyclic β‑lactam precursors highlights another contrast with the 2‑carboxy ester. Treatment with methanesulfonyl chloride in pyridine at –20°C yields a crystalline mesylate‑activated intermediate that cyclizes upon mild heating to 50°C in toluene, giving a spiro‑β‑lactam with 67% isolated yield after silica gel chromatography (ASTM D92‑equivalent flash point monitoring for flammable solvent). Under identical conditions, the proline ester mesylate undergoes elimination to the α,β‑unsaturated ester in >30% yield, a side reaction attributed to anti‑periplanar alignment of the β‑hydrogen in the 2‑carboxy isomer. The 3‑carboxy regioisomer’s β‑methylene group lacks equivalent geometry, suppressing this route to impurities.

Typical Batch Release Specifications and Associated Methods
Parameter	Specification	Method Reference
Appearance	White crystalline powder	Visual, Ph.Eur. 2.2.1
Assay (anhydrous, non‑aqueous titration)	98.0–102.0%	USP <541> (perchloric acid)
Water (Karl Fischer)	≤ 0.5%	ISO 760:1978, coulometric
Residue on ignition (sulfated ash)	≤ 0.1%	Ph.Eur. 2.4.14
Chloride content (argentometric)	16.0–17.5% (theory 17.0%)	USP <221> (Mohr)
Enantiomeric purity (if chiral)	≥ 99.0% ee	In‑house chiral HPLC, Daicel IA
Residual methanol	≤ 500 ppm	USP <467> Procedure A
Lead, cadmium, mercury, arsenic (total)	≤ 10 ppm each	USP <232>/<233> (ICP‑MS)

The absence of a coplanar arrangement between the ester and the amino group eliminates the possibility of intramolecular aminolysis to a bicyclic lactam under high‑dilution coupling; such cyclization is a documented failure mode for proline methyl ester in DMF at concentrations below 5 mM at 80°C. Long‑term stability tests (ICH Q1A) on three production lots stored at 25°C/60% RH confirmed ≤ 0.2% growth in total related substances over 36 months, with the main degradant identified as the free acid by LC‑MS, providing a shelf‑life assignment of 36 months under the recommended refrigerated condition (2–8°C). The compound is listed in the EINECS inventory with REACH pre‑registration and has been examined for residual ethylene oxide from esterification; limits are set at ≤ 1 ppm per ISO 10993‑7 when the ester is intended for medical‑device intermediates. From the perspective of a kilo‑laboratory operator running 5‑L jacketed reactors with retreat‑curve impellers, the hydrochloride’s dissolution exotherm (ΔH ≈ +12 kJ/mol in water) requires cooling capacity to maintain 20 ± 2°C during make‑up of a 1 M stock solution, a parameter incorporated into automated recipe management on a Siemens SIMATIC BATCH system. Differences from the hydrobromide salt, occasionally encountered from brominating cyclizations, are marked: the hydrobromide displays a melting endotherm at 168–172°C but discolours rapidly above 120°C in air due to bromide‑mediated oxidation, ruling it out for high‑temperature amidations without inerting. These distinctions underpin the selection of the hydrochloride as the reference standard form for this building block.