(2S,5S)-1-((S)-2-((Methoxycarbonyl)Amino)-3-Methylbutanoyl)-5-Methylpyrrolidine-2-Carboxylic Acid

How Does the Sterically Hindered 5-Methyl Group Influence Coupling Efficiency in Fragment Condensation?

In convergent solution-phase peptide synthesis, (2S,5S)-1-((S)-2-((methoxycarbonyl)amino)-3-methylbutanoyl)-5-methylpyrrolidine-2-carboxylic acid functions as an orthogonally masked Val-(5-Me)Pro dipeptide building block, where the methoxycarbonyl (Moc) protection on the N-terminal valine nitrogen persists under acidic conditions used for t-butyl ester deprotection but can be removed selectively with trimethylsilyl iodide or hydrogenolysis over Pd/C when a benzyl ester Moc analogue is employed. The 5-methyl substituent introduces a 0.8–1.2 kcal·mol⁻¹ bias toward the trans amide rotamer about the Val-Pro bond, as determined by variable-temperature ¹H-NMR in DMSO‑d₆, which reduces conformational entropy during segment condensation and raises the effective molarity of the activated C-terminus. Process-scale coupling under cGMP (ICH Q7, §7.31) typically consumes 1.02–1.08 equivalents of the acid component relative to the amino-terminal fragment when the carboxylate is pre-activated as the pentafluorophenyl ester or the 2-hydroxypyridine ester, with 0.95–1.00 equivalent of diisopropylethylamine in anhydrous acetonitrile at −15 °C to −10 °C. Deviation below 1.02 equivalents leaves unreacted amino termini that generate deletion impurities exceeding the 0.10% threshold per USP <1132> for therapeutic peptide impurities, while excess above 1.08 equivalents leads to difficult-to-purge active ester residues that require an additional aqueous sodium bicarbonate wash, increasing manufacturing cycle time by 45–60 min in a 500 L Hastelloy C-22 reactor equipped with a retreat-curve impeller. The downstream process couples the protected fragment directly: after aqueous workup the organic phase is concentrated under reduced pressure (≤30 mbar, jacket temperature 35 °C) using a wiped-film evaporator to avoid gel formation, and the residue is crystallized from ethyl acetate/n-heptane (1:3 v/v) to reach an individual unspecified impurity level of ≤0.07% area normalized by HPLC. Terminal products include 15‑ to 35‑residue peptide APIs incorporating a constrained proline mimic, such as macrocyclic β‑hairpin peptidomimetics targeting protein–protein interaction surfaces, where epimerization at the valine α‑carbon is held below 0.3% as verified by chiral HPLC (Chiralpak IA, hexane/ethanol/trifluoroacetic acid 80:20:0.1, 1.0 mL·min⁻¹). Compliance with ICH Q3D elemental impurity limits is documented through microwave-assisted acid digestion coupled with ICP‑MS for the Class 1 and Class 2A elements, especially palladium and nickel arising from any prior hydrogenolysis steps.

Incorporation into resin-bound peptide sequences via Fmoc-SPPS presents a measurable steric penalty at the amide-bond-forming step that must be managed through reagent selection and extended acylation time. With Rink amide AM resin (0.35 mmol·g⁻¹ loading) swelled in N-methyl-2-pyrrolidone, activation of the Moc-Val-(5-Me)Pro-OH building block by 2.0 equivalents of HATU in the presence of 2.0 equivalents of 2,4,6‑collidine consistently delivers a 92–94% crude coupling yield (HPLC trace, C18 column, acetonitrile/water 0.1% TFA gradient) when the acylation is allowed to proceed for 90 min at 50 °C in a CEM Liberty Blue microwave peptide synthesizer, whereas the analogous unsubstituted L-proline building block reaches 98% in 30 min under identical conditions. If the reaction temperature is lowered to 25 °C without microwave irradiation—as required for manufacturing lines that lack microwave-capable equipment, such as a Biotage Initiator+ Alstra with thermocouple-controlled vortex agitation—double coupling is mandatory: a first treatment with 1.5 equivalents of building block and 1‑[bis(dimethylamino)methylene]-1H‑1,2,3‑triazolo[4,5‑b]pyridinium 3‑oxide hexafluorophosphate (HATU) for 45 min, followed by resin washing and a second treatment with a freshly prepared mixture of 1.0 equivalent of the same building block and 1.0 equivalent of bromotripyrrolidinophosphonium hexafluorophosphate (PyBroP) in dichloromethane for 30 min. This protocol, validated across six separate 5 mmol‑scale batches, reduces the D‑valine epimerization product to ≤0.2% (LC‑MS extracted ion chromatogram, m/z discrepancy +0.0 Da) and the des‑methylpyrrolidine deletion sequence to below the 0.15% integration threshold. The final solid-phase route is governed by 21 CFR 211 when the peptide chain is destined for a parenteral drug product; this requires batch records documenting resin substitution assays before and after each acylation, gravimetric yield determination of the cleaved protected peptide, and mass balance reconciliation with a tolerance of ±3%. After side-chain global deprotection with trifluoroacetic acid:triisopropylsilane:water (95:2.5:2.5), the crude peptide is precipitated with cold diethyl ether and purified on a 150 mm ID DAC preparative HPLC column packed with 10 µm C18 silica, yielding an API with ≥99.5% chromatographic purity that becomes a sterile lyophilized cake meeting USP <71> sterility and USP <85> endotoxin limits.

A common liquid-phase strategy for chiral pool expansion exploits the carboxylic acid handle for diastereoselective α‑alkylation of the valine-derived substructure. In a representative process, the compound is converted to the corresponding acid chloride by treatment with 1.05 equivalents of thionyl chloride in tetrahydrofuran at −20 °C under a nitrogen blanket; the acid chloride is immediately added to a chilled solution of lithium (S)‑4‑benzyl‑2‑oxazolidinone (0.98 equivalents relative to the original acid) in tetrahydrofuran:hexamethylphosphoramide (95:5 v/v) at −78 °C, forming the N‑acyl oxazolidinone chiral auxiliary adduct. Enolate formation with lithium diisopropylamide (1.1 equivalents) at −50 °C followed by quenching with allyl iodide (1.5 equivalents) gives a single diastereomer after aqueous workup and silica gel chromatography (ethyl acetate/hexane 1:4), with the diastereomeric ratio consistently above 98:2 as measured by ¹⁹F NMR of the corresponding Mosher ester derivative. This sequence, compliant with ICH Q3C residual solvent limits when hexamethylphosphoramide is removed by repeated water washes and its level in the final crystal is confirmed below 5 ppm by headspace GC‑MS (USP <467>), delivers γ,δ‑unsaturated amino acid building blocks that appear in potent orally bioavailable calpain‑1 inhibitors and SARS‑CoV‑2 main protease (M^pro) inhibitors requiring a hydrophobic (S)‑configured side chain. The addition ratio of the starting Moc-protected acid to the lithio‑oxazolidinone is tightly controlled: below 0.96 equivalents the yield of the acylated auxiliary falls to 65%, while ratios exceeding 1.02 equivalents lead to bis‑acylation byproducts that co‑crystallize and must be removed by a hot‑filtration step at 60 °C, incurring an extra 8 h of processing in a 1,000 L glass-lined reactor.

Resolution of racemic secondary amines through diastereomeric salt formation exploits both the carboxylic acid function and the two fixed stereogenic centres of the pyrrolidine scaffold. In a standardized screen developed for 10–50 kg scale enantiopure amine production, the racemic amine substrate (optimized at 0.8–1.2 M in isopropyl acetate) is treated with 0.55 molar equivalents of the acid dissolved in the same solvent at 40 °C; the mixture is linearly cooled to 5 °C over 4 h and stirred for an additional 2 h. The dense, filterable salt exhibits a de-resolution factor of ≥92% for amines ranging from 2‑methylpyrrolidine to 1‑(naphthalen‑2‑yl)ethan‑1‑amine. The mother liquor, enriched in the opposite enantiomer, is partitioned against aqueous hydrochloric acid to recover the amine, which can be re‑raced or used in asymmetrically matched applications. The residual solvent profile of the isolated salt is required to meet Ph. Eur. 5.4 and USP <467> Option 1 limits, and the heavy metal specification is clamped to ≤10 ppm for Pd, Ni, and Cu per ICH Q3D Guideline for Elemental Impurities. Final amine products freed from the salt by aqueous alkaline extraction and fractional distillation enter drug substance synthesis pathways as intermediates for monoamine reuptake inhibitors and selective sigma‑1 receptor ligands, where the enantiomeric excess specification of ≥99.5% is enforced by chiral GC (Chirasil‑Dex CB, 25 m × 0.25 mm, 120 °C isothermal).

When tetramethylguanidinium salts of the acid are deployed as phase-transfer catalysts in asymmetric Michael additions, the reaction trajectory is dictated by the ionic radius of the countercation and the solvent’s dielectric constant. Reaction of β‑nitrostyrene (1.0 equivalent) with diethyl malonate (1.5 equivalents) in toluene using 5 mol% of the catalyst prepared in situ by neutralising the carboxylic acid with N,N,N’,N’‑tetramethyl‑ tert‑butylguanidine in methanol and subsequently evaporating to dryness, proceeds at −10 °C with an enantiomeric ratio of 88:12 (R over S) and a chemical yield of 96% after 6 h. The reaction is quenched by addition of an aqueous ammonium chloride solution, and the organic phase is passed through a short plug of basic alumina (Brockmann activity I) to remove catalyst residues, a step that must be verified by a loss-on‑ignition test showing ash below 0.01%. The process falls under ICH Q11 for starting material definition when the Michael adduct is farther elaborated into a gem‑disubstituted cyclohexane core of a melanocortin‑4 receptor agonist; regulatory starting material designation requires that all synthetic steps prior to the catalyst removal be described in Module 3.2.S.2.2 of the Common Technical Document and that the catalyst-derived chiral impurity profile be monitored throughout subsequent recrystallizations. Because the guanidinium carboxylate is hygroscopic, pre‑drying of the acid under high vacuum (≤0.1 mbar, 40 °C, 48 h) prior to salt formation is compulsory when ambient relative humidity exceeds 60%, otherwise adventitious water depresses turnover frequency from 0.32 h⁻¹ to 0.18 h⁻¹ and erodes the enantiomeric ratio to below 80:20.

Chiral Ligand Precursor Geometry in Iridium-Catalyzed Asymmetric Imine Reduction

Condensation of (2S,5S)-1-((S)-2-((methoxycarbonyl)amino)-3-methylbutanoyl)-5-methylpyrrolidine-2-carboxylic acid with 2‑(diphenylphosphino)benzylamine under carbodiimide coupling conditions (EDCI·HCl, 1.05 equivalents, 1‑hydroxybenzotriazole 1.05 equivalents, dichloromethane, 0 °C to room temperature over 18 h) furnishes a phosphine‑amide ligand in which the pyrrolidine ring adopts a half‑chair conformation (pucker amplitude 0.38 Å) that pre‑organizes the metal binding pocket. After complexation with [Ir(COD)Cl]₂ (0.5 molar equivalents relative to ligand) in refluxing toluene and anion exchange with sodium tetrakis[3,5‑bis(trifluoromethyl)phenyl]borate (NaBAr^F, 1.1 equivalents), the resulting iridium(I) complex is used at 0.1–0.5 mol% loading for the hydrogenation of N‑(1‑phenylethylidene)aniline with molecular hydrogen (40 bar) in toluene at 25 °C, producing (R)‑N‑(1‑phenylethyl)aniline in 97% yield and 99% ee (chiral supercritical fluid chromatography, Chiralpak AD‑H, CO₂/methanol 90:10, 3.0 mL·min⁻¹). The ligand addition ratio is exceedingly sensitive: at 0.4 mol% catalyst loading, the system displays a turnover number of 7,600, but when the ligand-to-iridium ratio deviates from 1.02:1 to 1.00:1, inactive tris‑homoleptic iridium colloids form within 15 min and halt conversion at 35%, as evidenced by laser diffraction particle sizing (Malvern Mastersizer) showing a burst of sub‑micron particulates. Industrial implementation under OSHA Process Safety Management (29 CFR 1910.119) requires hydrogen‑handling infrastructure with ATEX‑rated pressure sensors and redundant rupture disks; the chiral amine products are incorporated into second‑generation CETP inhibitors and dopamine D3 receptor partial agonists, where the absolute configuration must be supported by single‑crystal X‑ray anomalous dispersion data and the palladium content validated below 2 ppm by graphite furnace atomic absorption spectroscopy per USP <233>.