Methyl (2S)-2-Methylpyrrolidine-2-Carboxylate,Hydrochloride

Engineering Conformational Rigidity into Peptide Backbones: Methylproline Insertion Protocols

Modification of endogenous peptide ligands with the non‑proteinogenic amino acid (S)‑2‑methylproline introduces a quaternary carbon at the α‑position, forcing a torsion‑angle shift that locks the pyrrolidine ring into an envelope conformation distinct from native proline. In solid‑phase peptide synthesis (SPPS) governed by ICH Q7 for active pharmaceutical ingredient (API) intermediates, the hydrochloride salt is first converted to the Fmoc‑protected derivative under Schotten‑Baumann conditions: the free amine is liberated with 1.2 equivalents of aqueous sodium carbonate at 0–5 °C and subsequently reacted with Fmoc‑OSu in dioxane, achieving isolated yields typically exceeding 92%. Batch records from kilo‑lab campaigns specify that residual succinimide must be reduced below 0.15% w/w, as it competes with piperidine deprotection and leads to premature chain termination. Coupling of Fmoc‑(2S)‑2‑methylproline onto a growing peptide chain on aminomethyl or Wang resin diverges sharply from standard amino acid activation: due to steric shielding by the gem‑dimethyl group, HATU/DIPEA activation with a 4‑ to 5‑fold molar excess relative to resin loading is mandatory, and double coupling cycles of 120–150 min each are employed, with Kaiser test negativity verified before the next deprotection step. In‑process controls follow the impurity thresholds laid down in ICH Q3C (residual DMF, dichloromethane) and ICH Q3D elemental‑impurity risk assessment, where palladium content originating from hydrogenation of side‑chain protecting groups is limited to <10 ppm. When the target peptide is released via acidic cleavage using a TFA/TIS/water cocktail (95:2.5:2.5 v/v/v), epimerization at the C‑terminal residue is monitored by reverse‑phase HPLC at 210 nm; the quaternary center of 2‑methylproline suppresses α‑proton abstraction, effectively eliminating racemization at this locus. Process‑scale lyophilization in a GEA Niro freeze‑dryer with full‑shelf temperature ramping from ‑40 °C to +25 °C over 48 h yields a bulk peptide acetate or trifluoroacetate salt. Downstream finished product classes encompass metabolically stabilized GLP‑1 receptor agonist analogs and bicyclic antimicrobial peptides where 2‑methylproline replacement extends the plasma half‑life beyond 12 h by creating a steric barrier to dipeptidyl peptidase‑IV cleavage. It must be noted that the free amine form is highly hygroscopic; therefore, storage of Methyl (2S)‑2‑Methylpyrrolidine‑2‑Carboxylate Hydrochloride under argon with desiccant packs that maintain relative humidity <30% is critical to avoid partial hydrolysis of the methyl ester that would generate the zwitterionic acid and compromise coupling efficiency.

At industrial optical‑resolution facilities processing primary amine racemates in 2000‑L glass‑lined reactors, Methyl (2S)‑2‑Methylpyrrolidine‑2‑Carboxylate Hydrochloride acts as a crystalline resolving agent through diastereomeric salt formation. The unit operation begins with the liberation of the free acid by dissolving the hydrochloride in deionized water, adjusting to pH 9.5 with sodium hydroxide, and extracting the neutral ester into methyl isobutyl ketone at 55 °C; the organic phase is dried over anhydrous sodium sulfate and concentrated under reduced pressure (50 mbar) to an oily residue. This (S)‑ester is then saponified with one equivalent of lithium hydroxide in aqueous tetrahydrofuran to afford (S)‑2‑methylproline, which is used directly in the resolution sequence. The racemic target amine (e.g., R/S‑1‑phenylethylamine or a substituted tetrahydroquinoline) is combined with 0.47–0.52 molar equivalents of the chiral acid in a mixture of ethanol and water (85:15) and heated until all solids dissolve, then cooled in a controlled cascade: ramp to 45 °C at ‑0.3 °C/min, hold for 1 h to seed, and thereafter cool to 5 °C at ‑0.1 °C/min. The precipitated diastereomeric salt is collected on a gasket‑sealed centrifuge with a polypropylene filter cloth, washed with chilled 2‑propanol, and dried in a double‑cone vacuum dryer at 35 °C until loss on drying is <0.5%. Decomposition of the purified salt under alkaline conditions (10 % aqueous NaOH, ethyl acetate extraction) releases the optically enriched amine with enantiomeric excess typically reaching 99.0% as determined by chiral GC on a Cyclosil‑B column. The aqueous phase containing the sodium salt of (S)‑2‑methylproline is acidified and re‑extracted to recover the resolving agent, which can be re‑methylated with thionyl chloride in methanol to regenerate the methyl ester hydrochloride, completing a recycling loop that brings the process mass intensity below 25 kg/kg of product. Entire campaigns are validated against ISO 9001:2015 quality management systems, and the final amine product, if destined as an intermediate for an API, must meet the nitrosamine risk assessment framework of EMA/CHMP/369213/2020 with a confirmatory testing limit for N‑nitroso‑dimethylamine of <30 ppb. Finished materials from this route become enantio‑pure building blocks for chiral auxiliaries, insect pheromone synthesis (e.g., (S)‑sulcatol), and selective serotonin reuptake inhibitor intermediates that demand single‑isomer purity under 21 CFR 211.84 raw material acceptance procedures.

Can Asymmetric Organocatalysis Rely on a Quaternary Pyrrolidine Center?

The hydrochloride serves as an entry point to a family of secondary‑amine organocatalysts where the quaternary stereocenter adjacent to the nitrogen influences catalyst turnover and enantiofacial discrimination through steric gearing. Transforming the methyl ester into the archetypal catalyst (2S)‑2‑methyl‑2‑diphenylhydroxymethylpyrrolidine (a structural analog of MacMillan’s first‑generation imidazolidinone precursor) requires a sequential synthetic protocol validated at 10‑kg scale: after neutralization of the hydrochloride with aqueous potassium carbonate and extraction into toluene, the free amino ester is added dropwise to a freshly prepared solution of phenylmagnesium bromide in tetrahydrofuran‑toluene (1:1) at a stoichiometry of 2.8–3.1 equivalents of Grignard reagent per mole of ester, maintaining the internal temperature between ‑5 °C and 0 °C. Exotherms above +5 °C result in elimination products that produce a refractory olefin impurity, which co‑elutes with the target alcohol during silica gel chromatography. Quenching under strict pH control (pH 7.0±0.2) with ammonium chloride solution is followed by extraction with ethyl acetate and crystallization from heptane/ethyl acetate (4:1) to deliver the β‑amino alcohol in 72–78% overall yield with a single‑crystal X‑ray structure confirming the absolute configuration. As‑is purity of the catalyst intermediate exceeds 98.5% by HPLC‑CAD (charged aerosol detection). In benchmark asymmetric Michael additions between cyclohexanone and trans‑β‑nitrostyrene, catalyst loading at 10 mol% together with benzoic acid additive (5 mol%) in toluene at 10 °C furnishes the γ‑nitroaldehyde product with syn:anti ratios up to 94:6 and enantiomeric excess of the major diastereomer reaching 88–92%, as per chiral stationary‑phase SFC analysis using a CHIRALPAK AD‑H column. The operational boundary is constrained by the catalyst’s sensitivity to aerobic oxidation: reactions must run under an argon blanket, and dissolved oxygen levels in the solvent must be reduced to <1 ppm through freeze‑pump‑thaw degassing or continuous sparging. When the reaction is transferred to a 500‑L Hastelloy C‑22 reactor equipped with a retreat‑curve impeller, the standard batch record stipulates that the catalyst, once dissolved, cannot remain in solution for more than 8 h before substrate addition, otherwise a gradual decline in enantioselectivity of ∼3 ee% per hour is observed, attributed to N‑oxide formation. The ultimate downstream product types encompass enantio‑enriched intermediates for endothelin receptor antagonists and chiral lactone flavors. Quality requirements for catalyst raw material are typically governed by a Technical Data Package under REACH (EC) No. 1907/2006 and a certificate of analysis that includes heavy metals (≤20 ppm), water content (Karl Fischer ≤0.1%), and absence of Class 1 solvents.

Pharmacophore Quaternary Centers: A View from HCV Protease Inhibitor Synthesis

Successful incorporation of (S)‑2‑methylproline into macrocyclic HCV NS3/4A protease inhibitors relies on the integrity of the fully substituted C‑α position, which mimics the natural P2 proline residue while attenuating metabolic deactivation by oxidative enzymes. In registered starting material campaigns aligned with ICH Q11, Methyl (2S)‑2‑Methylpyrrolidine‑2‑Carboxylate Hydrochloride constitutes a downstream key starting material (KSM) that enters the synthetic sequence at the juncture where the P2 fragment is coupled to the P3 acid. Acceptance criteria include an assay by perchloric acid titration ≥99.0%, enantiomeric purity determined via chiral HPLC (CROWNPAK CR‑I(+)) exceeding 99.5% ee, and a control strategy for potential genotoxic impurities: methyl methanesulfonate and methyl chloride are limited to <1.5 µg/day according to the ICH M7 threshold of toxicological concern, using a validated GC‑MS headspace method with a limit of quantification of 0.5 ppm. During the solution‑phase fragment condensation, the hydrochloride is pre‑neutralized with 1.05 equiv of N‑methylmorpholine in acetonitrile at ‑15 °C and then activated with the mixed‑anhydride of the heptanoic acid P3 moiety, derived from isobutyl chloroformate (1.03 equiv) over a 30‑minute activation period. The coupling is allowed to warm to 0 °C over 2 h before quenching with 1 N HCl to suppress the formation of the diastereomeric amide arising from epimerization at the P3 α‑center; failure to maintain temperature below +2 °C during quenching increases the diastereomeric ratio from 98:2 dr to less than 94:6 dr. The resulting methyl ester is saponified with lithium hydroxide in a mixture of tetrahydrofuran and water (3:1 v/v) using a jacketed vessel with precise temperature control at 20±1 °C, and the tripeptidomimetic acid is telescoped into macrocyclisation using HATU/HOAt and 2,4,6‑collidine in high‑dilution conditions (substrate concentration <0.01 M) to afford a 15‑membered macrocyclic core. Downstream final drug substances belong to the class of direct‑acting antivirals achieving sustained virologic response rates above 95% in clinical settings; the regulatory framework for the KSM demands an annual stability protocol per ICH Q1A(R2) conducted at 25°C/60% RH and 40°C/75% RH over 36 months, as any degradation to the free acid causes batch rejection due to non‑conformance with the crystallinity index measured by XRPD. Handling constraints dictate that the hydrochloride must not be stored in proximity to basic lubricants or floor‑cleaning agents, as accidental amine liberation could trigger ester hydrolysis.

When a Pyrrolidine‑Derived CSP Replaces Amylose Tris(3,5‑dimethylphenylcarbamate) in Polar Organic Mode

Immobilization of a (S)‑2‑methylproline‑derived selector onto 3‑mercaptopropyl‑functionalized silica yields a Pirkle‑type chiral stationary phase (CSP) whose chiral recognition mechanism pivots on π‑π stacking, hydrogen‑bonding, and steric repulsion modalities that are complementary to polysaccharide‑based phases. The synthetic route entails condensation of Methyl (2S)‑2‑Methylpyrrolidine‑2‑Carboxylate Hydrochloride with 3,5‑dinitrobenzoyl chloride after liberation of the free amine with diisopropylethylamine in dichloromethane at 0–5 °C; the resulting N‑dinitrobenzoyl methyl ester is then hydrolyzed to the carboxylic acid, converted to the acid chloride, and coupled to (3‑aminopropyl)silica‑grafted mercapto‑terminated spacer arms through a thioester linkage. Elemental analysis of the bonded silica indicates a selector surface coverage of 0.32–0.38 mmol/g, with residual silanol groups minimized by preparative end‑capping using hexamethyldisilazane. Packed into a 250 mm × 10 mm semi‑preparative column via slurrying in toluene/dioxane under 500 bar axial compression, the CSP is conditioned with methanol at a flow rate of 4.0 mL/min and evaluated against a panel of acidic, basic, and neutral racemates. Under polar organic mobile phases of methanol containing 0.1% trifluoroacetic acid and 0.1% diethylamine, enantioselectivity values (α) for β‑blocker intermediates such as 1‑(isopropylamino)‑3‑(1‑naphthyloxy)‑2‑propanol reliably exceed 1.45 with baseline resolution (R_s > 1.8). The column performance is qualified according to USP ‹621› using a system suitability solution that requires plate count ≥18,000 plates/meter evaluated on the first enantiomer peak. Operational boundaries include strict temperature control at 25±0.5 °C, as a rise to 35 °C compresses the retention window and degrades α by as much as 0.15. Published data for this specific stationary phase indicate column lifetimes exceeding 2000 injections when samples are pre‑filtered through a 0.22 µm PTFE membrane. Downstream products include preparatively resolved single enantiomers of active pharmaceutical ingredients for forced degradation studies as required by ICH Q1B or for toxicity testing in accordance with ICH S6(R1). The CSP fabrication process itself is subject to ISO 22412:2017 for particle size analysis of the silica support, with D50 tightly controlled at 5.0 µm and D90/D10 <1.70.