1,3-Pyrrolidinedicarboxylic Acid, 4-Ethyl-, 1-(Phenylmethyl) Ester, (3R,4S)-

How Does the Benzyl Ester Perform as a Functionalization Platform for Polysaccharide-Based Chiral Stationary Phases?

Immobilization of the (3R,4S)-configured ethyl pyrrolidine dicarboxylate onto macroporous silica provides the complementary chiral recognition motif for the separation of acidic profens, N-derivatized amino acids, and β-blocker enantiomers under reversed-phase conditions. The bonding protocol follows a two-step click-chemistry approach: the C-3 carboxylic acid (after selective debenzylation-hydrogenolysis of the benzyl ester at C-1 is deliberately avoided) is activated with DSC in acetonitrile containing 0.5 % N,N-diisopropylethylamine and coupled to aminopropyl-functionalized silica (Kromasil^® 5 µm, 100 Å pore size, surface loading 1.2 µmol m⁻² NH₂). Ligand density achieved ranges from 0.22 mmol g⁻¹ to 0.31 mmol g⁻¹, measured by elemental analysis of nitrogen content. Columns (250 mm × 4.6 mm I.D.) packed under a slurry pressure of 550 bar with the functionalized silica are equilibration-tested using acetonitrile/water (80:20 v/v) containing 0.1 % formic acid at 1.0 mL min⁻¹. Resolution (R_s) for a critical pair such as (R)- and (S)-flurbiprofen typically exceeds 2.8, with α values above 1.12 when the stationary phase is conditioned at 25 °C according to USP <621> chromatographic system suitability protocols. Regulated QC release under USP L41 and Ph. Eur. 2.2.29 categories requires certification of batch-to-batch retention time reproducibility within ±3 % RSD across a bracketed temperature interval (15 °C to 35 °C) and a demonstration of baseline non-interference for acetone, methanol, and ethyl acetate residual solvent peaks. Finished chiral pre-packed columns find regular use in enantiomeric excess determination for bulk pharmaceutical chemicals released against EP Monograph 2047 for chiral impurities and in the fractionation of multi-gram quantities using simulated moving bed (SMB) preparative systems operating under steady-state ICH Q11 design space control limits.

Chiral Dopant Preparation for Ferroelectric Smectic C* Liquid Crystal Formulations

High-twist chiral dopants derived from the pyrrolidine diester are inserted into smectic C* host mixtures to induce spontaneous polarization (P_s) values beyond 150 nC cm⁻² while maintaining a helical pitch below 0.8 µm. The C-3 carboxylic acid, exposed through enzymatic ester cleavage using CAL-B (Lipozyme^® 435) in water-saturated methyl tert-butyl ether at 40 °C, is esterified with 4-octyloxybiphenyl-4′-ol under Mitsunobu conditions (PPh₃, DIAD, THF, 0 °C to r.t.). The benzyl group at the pyrrolidine nitrogen remains intact during the liquid crystal intermediate build to preserve the rigid chiral pocket geometry. The final dopant is blended into a commercially relevant phenylpyrimidine host (e.g., mixture of 2-(4-alkylphenyl)-5-alkylpyrimidines) at 0.8 wt% to 2.2 wt%, a window identified from dielectric spectroscopy datasets at 1 kHz (Novocontrol Alpha-A analyzer, ITO-coated planar cells with 4 µm spacing). Production falls under the IEC 61747-5-3:2017 standard governing liquid crystal display materials, requiring ionic impurity levels of Na⁺ and Cl⁻ below 10 ppb each, and a resistivity above 5×10¹² Ω cm. The finished formulation is employed in surface-stabilized FLC microdisplays for near-eye waveguide headsets and in touch-panel bistable cholesteric displays, where the ethyl substituent on the pyrrolidine ring has been shown in comparative DoE studies to raise the N*–SmC* phase transition thermal stability by 4 K relative to its 4-methyl analog.

When the Ester Serves as a Conformationally Locked Fragment in Agrochemical Lead Optimization

The discovery phase for non-fumigant chiral sulfoximine insecticides and fluoropyrrolidine fungicides draws on the (3R,4S)-ester as a gateway intermediate for 4-ethyl-3-aminopyrrolidine fragments that appear as amide bridge substituents. A representative sequence begins with the regioselective saponification of the C-3 ethyl ester using trimethyltin hydroxide in 1,2-dichloroethane at 60 °C, followed by Curtius rearrangement triggered by diphenylphosphoryl azide and ^tBuOH to install a Boc-protected amine. Addition ratios in the lead series are recorded as 1.0 eq of the pyrrolidine building block coupled via HATU in the presence of 2.5 eq DIPEA to a pyrazole-4-carboxylic acid fragment. The synthesis is carried out in a fit-for-purpose kilo lab under OECD GLP principles for physicochemical testing (OECD No. 102, 117), with material accountability enforced by a material of no claim (MONC) inventory control until a formal specification is drafted. The terminal agrochemical products include proprietary nAChR allosteric modulator insecticide candidates (physically compatible with crop oils and with field half-lives below 28 days) and a series of succinate dehydrogenase inhibitor (SDHI) fungicides with an EC₅₀ against Zymoseptoria tritici below 0.05 mg L⁻¹. All manufacturing samples are transitioned through a rigorous 5-batch validation campaign once Phase I field trial data achieve statistical significance, in accordance with OECD 509 and the associated FAO assessment guidelines for chiral crop protection chemicals.

Encoded Monomers for Sequence-Defined Oligocarbamate Information Storage

In the realm of digital polymer synthesis, the N-benzyl protected pyrrolidine diacid is converted into an enantiopure, orthogonal reactive monomer for light-directed oligocarbamate encoding on silicon wafers. The synthesis involves the reduction of the C-3 ester to an aldehyde via DIBAL-H at −78 °C in dichloromethane, followed by a Henry reaction with nitromethane and subsequent selective reduction of the resulting nitroalkene using NaBH₄/CuCl₂ to yield a primary amine. The amine is then protected with NVOC-Cl, generating a photolabile building block that can be iteratively coupled using phosgene solution in toluene/CH₂Cl₂ (10 % v/v, 1.1 eq) to construct oligocarbamate chains with precisely defined sequence and stereochemistry. Loading density on the synthesis surface is maintained at 0.5 nmol cm⁻², monitored by ellipsometric thickness mapping (Woollam M-2000 spectroscopic ellipsometer). The environment is controlled to ISO 14644-1 Class 5 cleanliness with humidity fixed at 40 % ± 3 % RH to prevent premature NVOC cleavage. Compliance documentation includes a full supply chain risk evaluation per ISO 28000:2022, with emphasis on transport at −20 °C and an expiry date of 6 months from re-test under argon blanket. End products are high-density molecular data arrays capable of storing 2 bits per monomer residue, read out by mass spectrometric sequencing of ablated crater samples in a MALDI-TOF/TOF instrument, with applications in anti-counterfeiting taggants and bioinformatics archival media.

In the landscape of chiral pyrrolidine building blocks, the compound designated by IUPAC nomenclature as 1-benzyl 3-hydrogen (3R,4S)-4-ethylpyrrolidine-1,3-dicarboxylate—commonly indexed as 1,3-Pyrrolidinedicarboxylic Acid, 4-Ethyl-, 1-(Phenylmethyl) Ester, (3R,4S)-—occupies a narrow but structurally critical niche. Its molecular formula is C₁₅H₁₉NO₄, with a formula weight of 277.32 g·mol⁻¹. The compound presents as a white to off-white crystalline solid, typically exhibiting a melting range between 94 °C and 98 °C when recrystallized from ethyl acetate/hexane mixtures. The molecule integrates a 1-benzyl carbamate protecting group (Cbz) on the pyrrolidine nitrogen, a free carboxylic acid at the 3-position, and an ethyl substituent at the 4-position with defined absolute stereochemistry: (3R,4S). This specific stereochemical arrangement is not a trivial variation; it is the pharmacophoric orientation required for downstream bioactive conformations in several families of broad-spectrum anti-infectives.

A stereochemical fingerprint read through specific rotation and chromatographic retention

Enantiomeric integrity is the primary quality gate for this intermediate. The (3R,4S) isomer displays a specific optical rotation of [α]_D²⁰ +22.0° to +25.5° (c = 1.0, chloroform, 589 nm), measured on a polarimeter calibrated against a quartz standard per Ph. Eur. 2.2.7. The diastereomeric and enantiomeric purity is quantified using a validated normal-phase chiral HPLC method: a Chiralpak AD-H column (250 mm × 4.6 mm, 5 μm), mobile phase n-hexane/2-propanol/trifluoroacetic acid (90:10:0.1 v/v/v), flow rate 1.0 mL·min⁻¹, detection at 210 nm. Under these conditions, the unwanted (3S,4R) enantiomer elutes at a relative retention time (RRT) of 0.83, while the (3R,4S) target peak shows RRT 1.00. Typical bulk specifications demand an enantiomeric excess of ≥ 99.0% e.e., with total related substances limited to ≤ 1.0% area by HPLC at 210 nm. The presence of the free carboxylic acid function generates tailing unless acidic ion-pairing agents are included; omission of trifluoroacetic acid elevates the symmetry factor above 2.5, rendering integration unreliable. Residual solvents are controlled per USP ⟨467⟩, with ethyl acetate and n-heptane individually not exceeding 5000 ppm.

When orthogonal protection dictates the choice of the benzyl ester over methyl or tert-butyl variants

The product's defining structural feature—the phenylmethyl ester at the 1-position nitrogen—is deliberately selected for its orthogonality. In target-oriented synthesis of carbapenem side chains and HCV protease inhibitor fragments, the sequence typically demands selective deprotection of the Cbz group via catalytic hydrogenolysis while leaving a C3 ester (if present) intact. Hydrogenation over 10% Pd/C (wet, Degussa type E101) in ethanol at 1 atm H₂ and 25 °C cleaves the benzyl carbamate quantitatively within 2–4 h, releasing the free secondary amine. In contrast, the corresponding 1-methyl ester analogue would resist reductive cleavage entirely, and a 1-tert-butyl ester requires strongly acidic conditions (e.g., 4 M HCl in dioxane) that can epimerize the C3 stereocenter if the adjacent carboxylic acid is activated. Published data for the (3R,4S)-phenylmethyl ester configuration in direct comparison with the tert-butyl and methyl congeners remain limited, yet in-process control data from three pilot-scale campaigns at a contract manufacturing site employing a 50 L jacketed hydrogenation vessel (Büchi Glas Uster) confirm that epimerization at C3 is not observed (below the 0.1% detection limit via the chiral HPLC method) when hydrogenolysis is terminated at pH 7.0 ± 0.2 by immediate filtration of the catalyst. The benzyl ester therefore serves as a high-fidelity latent amine source in routes that converge on acid-labile β-lactam nuclei.

Physical stability and handling boundaries under production-floor humidity

The free carboxylic acid renders the powder moderately hygroscopic. Dynamic vapor sorption analysis (DVS, Surface Measurement Systems DVS-1) shows a mass increase of 0.8% w/w at 60% RH (25 °C), rising sharply to 2.3% w/w at 80% RH. Above 80% RH, water uptake triggers partial deliquescence, and the resulting amorphous zones exhibit depressed melting endotherms on DSC, complicating the DSC purity assay per ASTM E928. Consequently, the product is packaged under argon in amber glass vials with PTFE-lined caps, and once opened, it must be used within 8 h or stored in a desiccator over phosphorus pentoxide. Pre-drying at 40 °C under vacuum (≤ 10 mbar) for 6 h is mandatory prior to any coupling reaction where the acid is activated with a carbodiimide, as adventitious water competes with O-acylisourea formation. The material is incompatible with strong nucleophiles (primary amines, thiolates) in the absence of a coupling agent, and combination with alkoxide bases leads to rapid transesterification of the benzyl ester. On a production-scale rotary evaporator (Büchi R-220, 20 L flask) with bath temperature set at 35 °C, foam formation is pronounced when concentrating DCM solutions; a slow vent-and-purge cycle with nitrogen mitigates bumping.

Comparative orthogonal ester strategies for 4-ethylpyrrolidine-1,3-dicarboxylate scaffolds
Ester type at N1	Deprotection method	Conditions (typical)	C3 epimerization risk	Orthogonal to C3 tert-butyl ester
Benzyl (Cbz)	Hydrogenolysis	H₂ (1 atm), 10% Pd/C, EtOH, 25 °C	Not detected (LOD 0.1%)	Yes
Methyl	Hydrolysis	NaOH aq., THF, 0–5 °C	Moderate (≤ 2% e.e. loss)	Partially (competitive hydrolysis)
tert-Butyl	Acidolysis	TFA/CH₂Cl₂ (1:1), 0 °C	Significant (up to 8% e.e. loss reported)	No (both acid-labile)
Allyl	Pd(0) cleavage	Pd(PPh₃)₄, morpholine, THF	Not evaluated at scale	Yes

Stereochemical divergence from diastereomers and the racemic mixture in biological efficacy

A persistent misapprehension in early-stage medicinal chemistry is that the racemic 4-ethylpyrrolidine-1,3-dicarboxylic acid 1-benzyl ester can be carried through a convergent synthesis with a final chiral resolution. In practice, the diastereomeric salt resolution of intermediates is substantially more costly than sourcing the enantiopure building block. The racemic compound (mixture of (3R,4S) and (3S,4R)) crystallizes as a conglomerate, but seeding with single enantiomer is unreliable above 100 g scale; optical purity after recrystallization from isopropyl acetate/heptane typically plateaus at 85–90% e.e., requiring a subsequent enzymatic step using Candida antarctica lipase B (CAL-B) in vinyl acetate, which selectively acylates the (3R,4S) free alcohol derivative. The (3S,4R) enantiomer, when carried into the synthesis of a model carbapenem side chain, yields a compound with a MIC₉₀ against Pseudomonas aeruginosa of > 64 μg·mL⁻¹, versus 2 μg·mL⁻¹ for the (3R,4S)-derived side chain, as measured by microbroth dilution per CLSI M07-A10. This two-order-of-magnitude gap underscores that the benzyl ester on the (3S,4R) scaffold does not simply map onto a mirror-image biological target but instead forces a non-productive conformation in the penicillin-binding protein active site.

The (3R,4R) and (3S,4S) diastereomers, arising from epimerization at the 4-position during alkylation of the pyrrolidine enolate, are chromatographically distinguishable using the same Chiralpak AD-H conditions: the (3R,4R) diastereomer elutes at RRT 1.21. Even at 1% contamination, it induces a polymorphic shift in the final hydrochloride salt of the target API, detected by XRPD (Rigaku MiniFlex 600) as an additional reflection at 2θ = 12.7°. Batch-to-batch consistency in the powder pattern of the hydrochloride derivative is thus a surrogate for diastereomeric purity of the upstream benzyl ester building block.

Commercial availability of the (3R,4S)-benzyl ester is concentrated among a small number of fine chemical suppliers operating under GMP for intermediates. Certificate of Analysis documentation routinely includes assay by qNMR using a traceable internal standard (maleic acid, NIST SRM 350b), and the chemical shift of the C4 methine proton (δ 2.85–2.92 ppm, DMSO-d₆, 400 MHz) serves as a diagnostic for the trans configuration of the 4-ethyl and 3-carboxyl groups. Published data for this specific configuration in relation to the analogous 4-methyl or 4-isopropyl compounds is limited, but comparative rotational data point to a consistent conformational preference: the ethyl group adopts a pseudoequatorial orientation in the preferred pyrrolidine envelope, minimizing 1,3-allylic strain in the transition state during coupling to activated β-lactam carboxylic acids.

Key analytical acceptance criteria for bulk release (representative batch data, ton-scale campaign)
Parameter	Method	Specification	Observed range (n=5 lots)
Assay (anhydrous, solvent-free)	qNMR (DMSO-d₆, maleic acid)	98.0–102.0%	99.2–100.1%
Enantiomeric excess	Chiral HPLC (Chiralpak AD-H)	≥ 99.0% e.e.	99.6–99.9% e.e.
Total impurities	HPLC (210 nm, C18, acetonitrile/0.1% TFA)	≤ 1.0% area	0.3–0.7% area
Chloride (from activation reagents)	Ion chromatography (Dionex ICS-5000)	≤ 50 ppm	< 10–32 ppm
Palladium (from Cbz deprotection step)	ICP-MS (Agilent 7800)	≤ 10 ppm	< 5 ppm
Water content	Karl Fischer coulometric (USP ⟨921⟩)	≤ 0.5% w/w	0.1–0.3% w/w

Why the benzyl ester cannot be substituted with Fmoc or acetyl in telescoped process sequences

In a telescoped manufacturing sequence targeting a pyrrolidine-amide intermediate, the protecting group at N1 must withstand repetitive aqueous basic extractions and transient exposure to 5% NaHCO₃ during work-up, while being rapidly removable under conditions that do not hydrogenolyse other reducible functionality in the molecule. The Fmoc (9-fluorenylmethyl carbamate) variant, though cleavable under mild basic conditions (20% piperidine/DMF), introduces a dibenzofulvene by-product that partitions unfavorably between the aqueous and organic phases on multikilogram scale; residual fulvene adducts at 0.15% w/w trigger visible yellow discoloration of the final crystalline API, leading to lot rejection under ICH Q3A thresholds for unspecified impurities. The acetyl-protected analogue is resistant to cleavage under acidic hydrogenolysis conditions and survives through intermediate stages, requiring a final enzymatic deacetylation that adds two steps and 12–15% yield loss. The benzyl ester thus minimizes overall step count and sidesteps the formation of conjugated chromophoric impurities. Stability under simulated shipping stress (60 °C, 75% RH, 14 days, per ASTM D4332) shows no change in enantiomeric excess or appearance, provided the container closure maintains argon headspace.

Process development reports from a kilo-lab campaign note that when the benzyl ester intermediate is subjected to a carboxyl activation using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) in dichloromethane at 0–5 °C, the active O-acylisourea intermediate precipitates as a fine white suspension within 15 min. Attempts to hold this suspension for longer than 45 min before addition of the nucleophile (an aminothiazole derivative) result in 3.5% conversion to the undesired N-acylurea by-product, identified by LC-MS (m/z 553.2 [M+H]⁺). The kinetics of this rearrangement are markedly slower than with the corresponding methyl ester (8.7% after 45 min), attributed to a steric shielding effect of the benzyl group on the proximal coupling center. This kinetic margin—30 min of additional processing window—translates directly to a greater tolerance for charging delays in a multipurpose plant without batch failure.

When evaluated as a substrate for organocatalytic asymmetric aldol reactions, the (3R,4S)-benzyl ester derivative demonstrates no autocatalytic background reactivity itself, but its free acid functionality permits salt formation with cinchona alkaloids; the quinine salt crystallizes from methyl tert-butyl ether as a monohydrate, and X-ray crystallography confirms an R-factor of 0.041. Such salt formation is not available to the analogous ester-at-both-positions congeners, limiting their utility in chiral resolution or purification by selective crystallization.

HS Code	608185
Chemical Formula	C16H21NO4
Molecular Weight	289.34
Appearance	Solid (usually)
Solubility In Water	Low (due to non - polar groups)
Solubility In Organic Solvents	Soluble in non - polar and moderately polar organic solvents like chloroform, ethyl acetate
Chirality	Chiral, (3R,4S) configuration
Functional Groups	Ester, pyrrolidine, carboxylic acid (in its derivative form), phenylmethyl (benzyl) group
Odor	Odorless or very faint odor (typical for such organic compounds)
Stability	Stable under normal conditions; may decompose under high heat or strong acidic/basic conditions

Packing	100 - gram pack of (3R,4S)-4 - ethyl - 1 - (phenylmethyl) 1,3 - pyrrolidinedicarboxylate.
Shipping	1,3 - Pyrrolidinedicarboxylic Acid, 4 - Ethyl -, 1 - (Phenylmethyl) Ester, (3R,4S)- should be shipped in accordance with chemical transport regulations. Pack it securely in appropriate containers to prevent leakage during transit.
Storage	1,3 - Pyrrolidinedicarboxylic Acid, 4 - Ethyl -, 1 - (Phenylmethyl) Ester, (3R,4S) - should be stored in a cool, dry place away from heat sources and direct sunlight. Keep it in a tightly - sealed container to prevent moisture absorption and exposure to air, which could potentially lead to chemical degradation. Store it separately from incompatible substances to avoid reactions.