(R)-2-Carboxymethyl-Pyrrolidine-1-Carboxylic Acid Tert-Butyl Ester

Direct incorporation of (R)-2-carboxymethyl-pyrrolidine-1-carboxylic acid tert-butyl ester into peptide backbones via Boc/Benzyl solid-phase synthesis enables the reliable introduction of a β‑homoproline residue—often intended as a turn‑inducing motif in enzyme inhibitor sequences. The monomer is pre‑activated as a mixed carbonic anhydride or uronium salt and coupled to 4‑methylbenzhydrylamine (MBHA) resin pre‑swollen in dichloromethane at 25 °C. A standard coupling cocktail employs 4.0 equivalents of the protected amino acid, 3.9 equivalents of hexafluorophosphate azabenzotriazole tetramethyl uronium (HATU), and 8.0 equivalents of N,N‑diisopropylethylamine (DIEA) in anhydrous dimethylformamide, with reaction times of 45–90 min. Because the free 2‑carboxymethyl side chain presents an acidity risk for oxazolone‑mediated epimerisation, the activation and coupling stages are executed at a chilled jacket temperature of 0–5 °C; epimerisation levels measured by Marfey’s analysis must remain below 2.5 % to conform to ICH Q6B decision thresholds for peptide‑related impurities. On‑resin monitoring follows the quantitative ninhydrin (Kaiser) protocol, with double coupling performed when the residual free amine exceeds 3 µmol/g. Following complete chain assembly, the N‑terminal Boc group is removed with 50 % trifluoroacetic acid in dichloromethane, and final cleavage from the resin together with side‑chain deprotection is accomplished with anhydrous hydrogen fluoride/anisole (9:1 v/v) at −5 °C for 60 min. The crude peptide is precipitated in cold diethyl ether, recovered by centrifugation, and purified by preparative reversed‑phase HPLC on a C18 column with a mobile phase gradient of acetonitrile in 0.1 % aqueous trifluoroacetic acid. The entire downstream sequence operates under ICH Q7 active pharmaceutical ingredient GMP, with critical process parameters logged in batch manufacturing records on automated peptide synthesizers such as the CEM Liberty Blue or the C S Bio CS336X. The terminal products are therapeutic peptides—most frequently engineered protease inhibitors or receptor antagonists—where the β‑homoproline carboxylate can be further functionalised with polyethylene glycol chains through amide coupling to modulate half‑life.

What Residual Solvent Profile Governs the Use of (R)‑Boc‑β‑Homoproline in Late‑Stage API Intermediates?

In the kilogram‑scale synthesis of dipeptidyl peptidase IV (DPP‑IV) inhibitor scaffolds and integrin‑antagonist building blocks, the (R)‑2‑carboxymethyl‑pyrrolidine‑1‑carboxylic acid tert‑butyl ester serves as a chirality‑conserving intermediate that delivers the pyrrolidine‑2‑ethanoic acid framework. The free carboxymethyl appendage is activated as a mixed pivaloyl anhydride in tetrahydrofuran at −15 to −10 °C and subsequently condensed with a primary or secondary amine nucleophile under strictly anhydrous conditions. To minimise racemisation, the feeding ratio is tightly controlled at 1.00–1.30 molar equivalents of the activated Boc‑amino acid relative to the amine substrate, with a dosing rate of 0.5–1.0 mL/min via syringe pump over 30–120 min. After aqueous work‑up, the N‑Boc group is cleaved with 4 M hydrogen chloride in 1,4‑dioxane at 20–25 °C for 2–4 h, precipitating the hydrochloride salt directly. Residual solvent compliance is critical: the isolated intermediate must meet ICH Q3C (R8) limits for Class 2 solvents, specifically ≤600 ppm for dichloromethane and ≤720 ppm for tetrahydrofuran. Analytical quantification employs headspace GC‑FID per USP <467> with a DB‑624 capillary column, while enantiomeric purity is determined on a Chiralpak AD‑H column (250 × 4.6 mm) using hexane/ethanol/trifluoroacetic acid (90:10:0.1) as mobile phase, with an acceptance criterion of ≥99.5 % enantiomeric excess. Polishing via multi‑stage reslurry in n‑heptane/ethyl acetate (4:1) at 40 °C reduces single impurities below 0.10 %. The final finished product is a crystalline hydrochloride salt of a pyrrolidine‑based active pharmaceutical ingredient, typically subjected to additional salt screening and particle size reduction via jet milling to target a D₉₀ of <10 µm prior to formulation.

As a chiral pool entry to non‑proteinogenic phosphine and N‑heterocyclic carbene ligand architectures required for asymmetric hydrogenation and cross‑coupling, the (R)‑Boc‑protected scaffold is first reduced with borane‑dimethyl sulfide in tetrahydrofuran at reflux to convert the 2‑carboxymethyl side chain into the corresponding primary alcohol. Mesylation with methanesulfonyl chloride and triethylamine at 0 °C is followed by nucleophilic displacement with potassium diphenylphosphide at −78 °C, yielding the phosphine intermediate. An overall stoichiometric efficiency of 85 % is obtained over three steps (1.0 mmol of Boc‑amino acid furnishes 0.85 mmol of isolated phosphine product). The pyrrolidine nitrogen is deprotected with trimethylsilyl bromide and 2,6‑lutidine in dichloromethane at 0 °C before complexation with [Rh(COD)₂]BF₄ or [RuCl₂(p‑cymene)]₂. Ligand manufacturing for preclinical catalyst supply is conducted under ISO 9001:2015, with in‑process control by ³¹P NMR (purity >98 %) and chiral HPLC. The isolated bidentate P,N‑ligands demonstrate catalyst loadings as low as 0.1 mol % in the asymmetric hydrogenation of dehydroamino acid derivatives, and the commercial product is distributed as a sealed ampoule under argon atmosphere.

Ligand‑Exchange Chromatographic Chiral Selector Derivatisation

Immobilisation of (R)‑2‑carboxymethyl‑pyrrolidine‑1‑carboxylic acid tert‑butyl ester onto aminopropyl‑functionalised silica gel generates a brush‑type chiral stationary phase suited for the ligand‑exchange enantioseparation of underivatised α‑amino acids. The bonding protocol treats 3‑aminopropyl silica (fully porous, particle size 5 µm, pore diameter 120 Å, specific surface area 300 m²/g) with 2.5 equivalents of the chiral selector in the presence of 1‑ethyl‑3‑(3‑dimethylaminopropyl)carbodiimide (EDC, 3.0 eq.) and N‑hydroxysuccinimide (2.0 eq.) in anhydrous DMF at room temperature for 24 h. Elemental analysis of the dried, capped (acetic anhydride/pyridine) silica typically shows a ligand loading of 0.25–0.35 mmol/g, which corresponds to a surface coverage that balances recognition kinetics and mass transfer. The bonded silica is slurry‑packed into 250 × 4.6 mm stainless‑steel columns under 400 bar packing pressure and conditioned with 5 mM copper(II) acetate. Enantioseparation of phenylalanine and tyrosine derivatives is achieved with selectivity factors (α) ranging from 1.12 to 1.45 and resolution values (R_s) >1.5 under purely aqueous mobile phase conditions containing 0.5 mM CuSO₄. Column manufacturing and quality release are governed by ISO 13485:2016, while the analytical method validation—covering linearity, accuracy, and precision—follows ICH Q2(R1). The end product is a reusable analytical HPLC column supplied with a certificate of analysis stating plate count and asymmetry factor, used in pharmaceutical quality control for the enantiomeric purity determination of amino acid drug substances.

If the tert‑butyloxycarbonyl protective group of (R)‑2‑carboxymethyl‑pyrrolidine‑1‑carboxylic acid tert‑butyl ester is removed with a 95:5 v/v mixture of trifluoroacetic acid and triisopropylsilane at 0 °C over 1 h, the liberated secondary amine—(R)‑2‑carboxymethyl‑pyrrolidine—functions as a bifunctional organocatalyst for direct asymmetric aldol additions. The catalyst loading is typically 10 mol % in a biphasic brine/dichloromethane system at 4 °C, where the carboxylic acid group acts as a Brønsted acid co‑catalyst within the same pyrrolidine framework. In a validated substrate scope, reaction of 4‑nitrobenzaldehyde with cyclohexanone yields the anti‑aldol product with 92 % ee and a diastereomeric ratio >95:5. The preparative procedure is scalable: after deprotection and removal of volatiles under reduced pressure, the crude trifluoroacetate salt is neutralised with triethylamine and the free amino acid crystallised from ethanol/diethyl ether into a white crystalline powder with purity >99 % by HPLC. Production of research‑scale batches complies with ISO 9001:2015 and the catalyst is characterised by specific optical rotation ([α]_D²⁰ = −31.0° (c 1.0, H₂O)) and ¹H NMR (500 MHz, D₂O). The commercial product is an off‑the‑shelf vial containing the organocatalyst stored under inert atmosphere for direct use in medicinal chemistry asymmetric reaction screening.

When Monomer Architecture Dictates the Helical Sense of Polyamides

Interfacial polycondensation of the (R)‑Boc‑protected chiral diacid monomer—after transient protection of the carboxymethyl group with N,O‑bis(trimethylsilyl)trifluoroacetamide—with terephthaloyl chloride in a water/dichloromethane biphasic system introduces predetermined chirality into the main chain of an aromatic–aliphatic polyamide. The monomer feed ratio is maintained at 10–30 mol % relative to a flexible comonomer such as 1,6‑hexanediamine, which controls chain stiffness and optical activity while avoiding excessive crystallinity. The reaction is carried out at 5–10 °C with sodium carbonate as acid scavenger, and the polymer precipitated directly into methanol. Gel permeation chromatography in hexafluoroisopropanol against polymethyl methacrylate standards indicates number‑average molecular weights between 15 000 and 35 000 Da, sufficient to meet the polymer exemption criteria under REACH Regulation (EC) No 1907/2006 for substances with molecular weight above 10 000 Da. As the polymer may find application in food‑contact or biomedical device components, extractables evaluation per ISO 10993‑12:2021 and residual monomer limits are assessed. Following casting or electrospinning, the Boc groups are removed in the solid state by exposure to trifluoroacetic acid vapour, generating secondary amine functionalities that can be post‑modified with fluorescent probes or cross‑linking agents to stabilise fibre morphology. The terminal product is an enantiopure polyamide film or non‑woven mesh, employed as a chiral‑selective membrane material in static or flow‑through resolution of racemic mixtures.