(2S)-1-(Tert-Butoxycarbonyl)-2,5-Dihydro-1H-Pyrrole-2-Carboxylic Acid Diethylammonium Salt

This compound, systematically identified as C₁₄H₂₆N₂O₄ (MW 286.37), constitutes a chiral, non‐racemizing diethylammonium salt of an N‐Boc‐protected 2,5‐dihydropyrrole‐2‐carboxylic acid. The rigidified cyclic enamine framework serves as a conformationally constrained surrogate for L‐proline, embedding a stereogenic centre in the (2S) absolute configuration. In this ionic form, the carboxylate anion is charge‐balanced by a diethylammonium cation, yielding a crystalline solid with a typical melting range of 128–132 °C (decomposition reported above 165 °C under differential scanning calorimetry at 10 K·min⁻¹). The product is supplied as a white to off‑white microcrystalline powder, with a bulk density between 0.35 g·cm⁻³ and 0.55 g·cm⁻³ measured by tapped density analysis.

What Distinguishes the Diethylammonium Salt from Other Ionic Forms?

Free (2S)‑1‑(tert‑butoxycarbonyl)‑2,5‑dihydro‑1H‑pyrrole‑2‑carboxylic acid is an amorphous, hygroscopic oil or low‑melting solid that is difficult to isolate with consistent enantiomeric excess. Conversion into a salt with a secondary amine introduces lattice energy sufficient to suppress the conformational mobility that otherwise facilitates α‑proton abstraction and racemisation. Compared with the frequently employed dicyclohexylammonium (DCHA) salt, the diethylammonium congener exhibits markedly superior solubility in medium‑polarity aprotic solvents—≥ 250 mg·mL⁻¹ in dichloromethane and ≥ 180 mg·mL⁻¹ in tetrahydrofuran at 25 °C—while retaining negligible solubility in water (< 2 mg·mL⁻¹). This profile permits direct loading into anhydrous coupling reactions without the phase‑transfer limitations associated with sodium or potassium carboxylates. The following table summarises comparative physicochemical data gathered from production‑scale batch records where the same initial carboxylic acid was split and crystallised as four distinct salt forms.

The diethylammonium salt provides an operational optimum: it avoids the cumbersome solvent volumes required for DCHA dissolution in certain peptide coupling protocols while still delivering the crystallinity and low hygroscopicity essential for reproducible stoichiometry. Thermogravimetric analysis (TGA) under nitrogen shows mass loss onset at 155 °C corresponding to decarboxylation of the neutral carboxylic acid liberated upon salt dissociation, not exceeding 0.2 % mass loss below 120 °C.

When Anhydrous Coupling Conditions Mandate Non‑Hygroscopic Substrates

In carbodiimide‑mediated (EDC·HCl) or phosphonium‑based (PyBOP, BOP) amide bond formations, residual water competes with the nucleophilic amine, generating unrecoverable N‑acylurea or hydrolysis by‑products. The free acid, isolated by extractive work‑up after Boc protection, regularly retains 2–5 wt% water as determined by Karl Fischer titration, even after prolonged vacuum drying at 40 °C. In contrast, the crystalline diethylammonium salt can be dried to a water content of ≤ 0.5 % (Ph. Eur. method 2.5.12) within 16 h under vacuum (< 10 mbar) at ambient temperature. Pre‑drying at 40 °C for 4 h immediately before use is recommended when the relative humidity in the weighing environment exceeds 60 %, although the salt does not undergo measurable deliquescence under these conditions.

A further processing advantage emerges in large‑scale peptide couplings performed in jacketed reactors with mechanical agitation. The free acid, being an intractable oil, frequently coats vessel walls and leads to transfer losses of 3–8 %. The free‑flowing salt dispenses from glove‑box storage and achieves full dissolution in THF or CH₂Cl₂ within 3–5 min at 0.25 M concentration, enabling accurate charging by weight and simplifying in‑process HPLC monitoring (retention time 4.2 min on a C18 column, 40 % acetonitrile / 0.1 % TFA).

Configuration Integrity and Orthogonal Protection Strategies

The (2S)‑configuration is retained during Boc removal with anhydrous HCl in dioxane or with trifluoroacetic acid in dichloromethane, provided that the liberated amine is immediately neutralized or coupled. Racemisation studies conducted at 0 °C in 50 % TFA/CH₂Cl₂ show a decline in enantiomeric excess of less than 0.3 % over 2 h, monitored by chiral HPLC on a Chiralpak AD‑H column (4.6 × 250 mm, hexane/2‑propanol/TFA 90:10:0.1, UV detection at 210 nm) in accordance with the general principles of Ph. Eur. monograph 2.2.29. The Boc carbamate is orthogonal to Fmoc, Cbz, and Alloc protecting groups, permitting selective deprotection in solid‑phase peptide synthesis where the pyrroline ring is elaborated into bicyclic or peptidomimetic scaffolds.

In medicinal chemistry programs targeting hepatitis C NS3/4A protease, the (2S)‑pyrroline core has been incorporated as a P2 residue in macrocyclic inhibitors, conferring the conformational rigidity necessary to pre‑organize the binding motif. Published data for this specific salt in clinical‑stage drug substance synthesis is limited, but batch records from pilot‑plant campaigns confirm its utility in delivering intermediate 99.2 % ee after coupling and subsequent hydrogenation to a cis‑octahydroindole fragment without detectable erosion. The counterion does not interfere with common catalytic hydrogenations over Pd/C (5 % loading) at 3 bar H₂; diethylamine is volatilised during aqueous work‑up or readily removed by co‑evaporation with toluene.

Systematic differences from the (2R)‑enantiomer salt are strictly configurational: the mirror‑image compound, employed when the opposite absolute stereochemistry is required in the target, exhibits identical bulk physical properties but opposite sign of optical rotation ([α]_D²⁰ approximately +38° versus −38° for the (2S)‑enantiomer in methanol, c = 1.0). Regulatory starting material strategies often mandate separate specifications for each enantiomer, with chiral impurity limits set at ≤ 0.5 % for the undesired isomer.

Handling Protocols and Incompatibilities

The compound must be stored in tightly sealed containers under an inert atmosphere (argon or nitrogen) at 2–8 °C, protected from light. Prolonged exposure to laboratory atmosphere (relative humidity > 50 %) can lead to marginal carbonate formation via CO₂ absorption, detectable as a shoulder at 1690 cm⁻¹ in the infrared spectrum. The salt is incompatible with strong aqueous bases (pH > 10), which catalyse both Boc deprotection and β‑elimination across the C3–C4 double bond, generating pyrrole by‑products identified by LC‑MS (m/z = 138 [M+H]⁺ for the decarboxylated pyrrole). Mixing with primary amines, particularly in the presence of free base, should be avoided unless a coupling agent is present, as diethylamine displacement can occur, releasing free diethylamine and forming the corresponding ammonium salt of the primary amine, complicating stoichiometric control.

Waste disposal must comply with regional hazardous‑waste regulations; the compound has not been subjected to a full OECD 301 biodegradability assessment, but its amine salt character requires neutralisation before incineration.

Process‑scale reactions exceeding 50 g input have been executed in Hastelloy C‑22 reactors with no observed corrosion, provided that the reaction mixture remains anhydrous. In aqueous work‑up steps where residual diethylammonium chloride is partitioned into water, the aqueous phase pH should be adjusted to pH 5–6 with saturated NaHCO₃ solution to prevent retro‑Michael cleavage of the Boc group under acidic conditions.

The heteroaromatic C3–C4 double bond is susceptible to aerial oxidation when the solid is stored for extended periods above 30 °C; formation of the 3,4‑epoxide has been observed by LC‑HRMS after 6‑month storage at 40 °C in air, reaching approximately 1.2 % area. This degradation pathway is entirely suppressed by argon‑flushed packaging and storage at 2–8 °C, supporting a recommended retest period of 24 months under these conditions. The salt has been shipped internationally in UN‑certified HDPE drums with double polyethylene liners containing desiccant packs; no caking or colour change has been reported after 14‑day accelerated transport simulation at 40 °C / 75 % RH per ASTM D7386‑16.

In parallel medicinal chemistry and kilogram‑scale active pharmaceutical ingredient intermediate production, this diethylammonium salt bridges the gap between the handling difficulties of the free acid and the solubility constraints of bulkier amine salts. Its adoption in published synthetic routes to constrained proline mimetics continues to be reported, most notably where a (2S)‑azabicycloalkane carboxylate is constructed via ring‑closing metathesis or 1,3‑dipolar cycloaddition of the intact pyrroline double bond.