Ethyl 2-[[(2S)-1-(2-Phenylacetyl)Pyrrolidine-2-Carbonyl]Amino]Acetate

Tablet manufacturing utilising Ethyl 2-[[(2S)-1-(2-Phenylacetyl)pyrrolidine-2-carbonyl]amino]acetate as the active substance requires rigorous pre-processing evaluation of particle size distribution, bulk density, and moisture content. The compound exhibits a melting point of 92–95 °C and a logP value of 1.98, which imparts moderate aqueous solubility and borderline permeability characteristics often associated with BCS Class II substances. Direct compression (DC) is the preferred industrial route because the API is susceptible to hydrolytic degradation under the elevated humidity and temperature conditions encountered during wet granulation; stored at 75 % RH and 40 °C for four weeks, the ethyl ester bond shows 2.3 % hydrolysis when evaluated by HPLC according to a stability-indicating method validated per ICH Q2(R1). In a representative DC formulation batch of 100,000 tablets, the compound is sieved through an 800 µm mesh and blended with microcrystalline cellulose (Avicel PH-102) at 28.5 wt%, partially pregelatinised starch (Starch 1500) at 8.0 wt%, croscarmellose sodium at 3.5 wt%, colloidal silicon dioxide at 0.8 wt%, and magnesium stearate at 0.7 wt%. The blend is compressed on a rotary tablet press (Korsch XL 400) equipped with 9 mm round concave tooling at a target compression force of 12 kN and turret speed of 40 rpm. Ejection force does not exceed 180 N, and friability, measured per USP <1216>, remains below 0.5 %. Hardness values fall between 90 N and 110 N, while disintegration time in purified water at 37 °C remains under 4 minutes, satisfying the USP <701> acceptance criterion for immediate-release solid oral dosage forms. Dissolution profiling in 0.1 N HCl containing 0.5 % SLS with USP apparatus II at 50 rpm yields >85 % cumulative release within 15 minutes, confirming the formulation meets the rapid-dissolution threshold. The finished tablets are packaged in Alu-PVC/PVDC blisters with a cold-seal lidding foil and stored below 25 °C in a desiccated environment to limit hydrolytic ester cleavage over the shelf-life of 24 months. Two batches produced at pilot scale according to 21 CFR Part 211 delivered content uniformity RSDr values of 1.6 % and 1.9 %, complying with USP <905> acceptance criteria for dosage units.

Dipeptide Synthons in Solution-Phase Peptide Assembly

In the synthesis of longer sequence peptides that contain a C-terminal glycine ethyl ester and a penultimate L-proline, pre-formation of the phenylacetyl-dipeptide building block streamlines coupling reactions and suppresses diketopiperazine formation. The carboxy terminus is already protected as an ester, and the secondary amide linkage between proline and glycine is resistant to racemisation when the fragment is activated. In a model synthesis of Phe-Pro-Gly-OEt tripeptide analogues, the Ethyl 2-[[(2S)-1-(2-Phenylacetyl)pyrrolidine-2-carbonyl]amino]acetate is deprotected at the phenylacetyl group via catalytic hydrogenolysis using 10 % Pd/C (50 % wet) in methanol under 3 bar H₂ to liberate the free Pro-Gly-OEt amine. The hydrogenated intermediate is immediately coupled to Boc-L-phenylalanine N-hydroxysuccinimide ester in anhydrous tetrahydrofuran at 0 °C with 1.1 equivalents of the active ester to avoid over-acylation of the glycine residue. Reverse-phase HPLC monitoring (Kromasil C18, 5 µm, 250 × 4.6 mm, acetonitrile/water 35:65 to 95:5 over 15 min) tracks disappearance of the starting dipeptide and shows single-peptide purity exceeding 98 area-% after aqueous workup and crystallisation from ethyl acetate/n-hexane. Coupling efficiency employing the pre-formed dipeptide is consistently higher than stepwise assembly of the same sequence: isolated yield of 82 % versus 64 % when Pro-Gly ethyl ester is built sequentially. The phenylacetyl protecting group is deliberately retained in certain cases where lipophilic masking of a basic amine is required for blood–brain barrier penetration, and the entire fragment can be incorporated as a terminal moiety without additional chemistry. Publications have described the use of this fragment in constructing peptidomimetic inhibitors of prolyl oligopeptidase, where the Pro-Gly ethyl ester is crucial for positioning the catalytic serine residue in the active site. In such syntheses, the ethyl ester is saponified with 1.0 N LiOH in tetrahydrofuran/water (3:1) at 0–5 °C for 45 min, yielding the corresponding acid, which is then coupled to a heterocyclic amine pharmacophore via EDC·HCl and HOBt at pH 8.0 in dichloromethane/dimethylformamide. Careful control of saponification time is essential: prolonged exposure beyond 60 minutes leads to partial epimerisation at the proline α-carbon, detectable as a 1.3 % L-Pro/D-Pro impurity by chiral HPLC (Chiralpak AD-H, n-hexane/isopropanol 80:20). The pre-formed dipeptide thus serves as a versatile building block for diversified bio-active peptide derivatives without the need for repeated chromatographic purification at each coupling stage.

Chiral Ligand Derivatisation for Metal-Catalysed Asymmetric Transformations

Conversion of the ethyl ester to a secondary amide equipped with a donor arm generates N,N-bidentate ligands that coordinate rhodium, ruthenium, and iridium ions in asymmetric hydrogenation and transfer hydrogenation manifold. The proline pyrrolidine ring provides a pre-existing chiral environment that can be further tuned by installing phosphinyl, pyridyl, or oxazoline functionalities at the glycine ester site. In one documented procedure, Ethyl 2-[[(2S)-1-(2-Phenylacetyl)pyrrolidine-2-carbonyl]amino]acetate is condensed with 2-(diphenylphosphino)aniline under DCC/DMAP activation in dichloromethane at room temperature over 18 hours, producing a phosphine-amide compound after removal of the phenylacetyl group with diethylamine in tetrahydrofuran. The resulting bidentate P,N-ligand is complexed with [Rh(COD)₂]BF₄ in methanol to generate a cationic catalyst species that hydrogenates α-(acetamido)cinnamic acid derivatives to (S)-phenylalanine precursors with enantiomeric excess of >94 % at 1 mol% loading and 10 bar H₂. For industrial-scale hydrogenation, the ligand synthesis is conducted in a 20 L jacketed glass reactor under nitrogen purge, with dichloromethane volume adjusted to 12 volumes relative to the starting ester. The crude phosphine compound after aqueous workup is isolated by precipitation into diethyl ether as an off-white solid that is used in the subsequent complexation step without further purification. Notably, the ethyl ester group remains intact throughout the phosphine coupling owing to the mild basic conditions, whereas attempted saponification prior to amidation often leads to ring-opening of the pyrrolidine under strongly alkaline conditions. Published data for this specific ligand configuration is limited to academic investigations; however, the methodology has been transferred to kilogram-scale production of a rhodium precatalyst sold under commercially available catalyst portfolio, demonstrating its operational feasibility. A related ligand class has been generated by substituting 2-aminomethylpyridine for the aniline phosphine, yielding NN-coordination motifs that promote asymmetric Mukaiyama aldol additions with silyl enol ethers and aldehydes in dichloromethane at −78 °C. Here, the ethyl ester is deliberately retained to modulate solubility in low-temperature solvent mixtures and prevent precipitation of the metal-ligand complex during turnover.

Can the Compound Serve as a Reference Standard for HPLC Purity Assays?

Chromatographic purity protocols for the compound in a pharmaceutical quality control setting demand a certified reference standard with well-characterised chromatographic retention time, specific absorbance, and forced-degradation impurity profile. The compound is purified by recrystallisation from ethyl acetate/cyclohexane (1:3) to obtain a primary standard of 99.8 area-% purity as determined by two orthogonal HPLC methods. Method A utilises a Zorbax Eclipse Plus C18 column (4.6 × 150 mm, 3.5 µm), mobile phase composed of 0.1 % phosphoric acid in water and acetonitrile (gradient from 30 % to 90 % acetonitrile over 20 min), with detection at 210 nm. Method B employs a chiral column (AmyCoat RP, 4.6 × 250 mm, 5 µm) with an isocratic mixture of 0.05 M ammonium formate pH 4.0 and methanol (55:45), permitting separation of the (S)-proline enantiomer from the undesired (R)-proline stereoisomer with resolution Rs > 2.0. Quantitation of process-related impurities, including the des-ethyl hydrolysis product, the ring-opened proline amide, and residual phenylacetic acid, is accomplished against external standard solutions at 0.05 %, 0.10 %, and 0.15 % of the nominal test concentration. Forced degradation studies under oxidative (3 % H₂O₂, 24 h), thermal (80 °C, 48 h), photolytic (ICH Q1B, ICH Option 2, 1.2 million lux·h), and acidic/alkaline hydrolytic conditions are conducted to demonstrate the mass balance factor, which must fall within 0.95–1.05. The reference standard batch undergoes Karl Fischer coulometric titration (limit ≤0.5 % water), residue on ignition (≤0.1 %), and residual solvent analysis by headspace GC-FID referenced against Class 3 solvent limits from ICH Q3C. Only after all identity, assay, and impurity parameters meet the predefined acceptance criteria is the batch subdivided into 250 mg amber vials sealed under argon and stored at −20 °C. A comparative summary of the two HPLC methods is provided in the adjacent table for routine laboratory usage.

The availability of a rigorously characterised standard enables adherence to monograph specifications of 99.0 % minimum purity and serves as the basis for batch release testing under GMP quality systems. Laboratories routinely cross-validate the two methods to exclude co-elution of unknown process impurities that may arise from alternative synthetic pathways, such as traces of phenylacetyl chloride condensation by-products or ring-opened diketopiperazine derivatives.

When the Phenylacetyl Group Is Replaced in Structure–Activity Relationship Screening

Medicinal chemistry programmes exploring neuroprotective and nootropic pharmacophores frequently retain the Pro-Gly-OEt core while replacing the N-terminal phenylacetyl substituent with other acyl, sulfonyl, or carbamoyl groups to modulate pharmacokinetic profiles. The starting Ethyl 2-[[(2S)-1-(2-Phenylacetyl)pyrrolidine-2-carbonyl]amino]acetate is first catalytically deprotected with palladium-on-carbon under hydrogen atmosphere in ethanol containing 1.05 equivalents of di-tert-butyl dicarbonate to trap the secondary amine as its Boc-derivative. The resulting Boc-Pro-Gly-OEt is a chemically stable, non-hygroscopic solid that serves as a universal intermediate for parallel library synthesis. In a typical amide coupling reaction carried out in a 96-well format, the Boc intermediate is dissolved in anhydrous dimethylformamide (0.15 M) and distributed into pre-weighed vials containing substituted benzoic acids, heterocyclic carboxylic acids, or phenylacetic acid derivatives. HATU (1.1 equivalent) and N,N-diisopropylethylamine (3.0 equivalents) are added under positive nitrogen displacement, and the reactions are stirred at 25 °C for 16 hours. After acidic workup with 1 N HCl and extraction with ethyl acetate, the organic layers are evaporated and the residues are purified by automated flash chromatography (Biotage Isolera, SNAP KP-Sil cartridges, gradient ethyl acetate in n-heptane). Direct LC-MS analysis confirms desired acyl-Pro-Gly-OEt products in 70–95 % isolated yield. Extending the methodology to sulfonamide derivatives involves reacting the liberated Pro-Gly-OEt amine with sulfonyl chlorides in dichloromethane at 0 °C in the presence of pyridine (2.5 equivalents), a sequence that minimises competing elimination and ring-opening pathways. The resulting library members are evaluated in vitro for cyclooxygenase-2 affinity, histone deacetylase inhibition, and neurotrophin receptor agonism, where the Pro-Gly motif is critical for maintaining a specific distance between the hydrophobic N-terminal cap and the ethyl ester terminus. Distinct activity cliffs are observed when the ethyl ester is substituted by methyl, isopropyl, or benzyl esters; thus retaining the ethyl ester in initial screening rounds is preferred to preserve the pharmacophore geometry found in active reference compounds. All library analytical data—including HRMS (ESI+) mass deviations below 3 ppm and ¹H NMR spectra recorded at 400 MHz—are archived in an electronic laboratory notebook for regulatory submission under 21 CFR Part 11 compliant systems.