L-Isoleucine ethyl ester hydrochloride, model designation H-Ile-OEt·HCl peptide synthesis grade, is the crystalline hydrochloride of the ethyl ester of L-isoleucine. Its molecular formula is C₈H₁₇NO₂·HCl, with a molecular weight of 195.69 g/mol and CAS registry number 23356-96-9. The product is supplied as a white to off-white crystalline powder, typically assaying ≥98.0% by HPLC area normalization at 210 nm, with a specific rotation [α]_D²⁰ of +16.0° to +18.0° (c=2.0, ethanol), loss on drying ≤0.5%, and residue on ignition ≤0.1%. The hydrochloride form offers defined stoichiometry and crystalline handling properties that differ from the free amino acid L-isoleucine, which is amphoteric and typically requires aqueous or strongly hydrogen-bonding solvent systems. As an ethyl ester salt, the compound is used primarily as a chiral building block for N-functionalization, for solution-phase peptide fragment ligation, and for the preparation of N-protected amino acid esters such as Boc-Ile-OEt.

Which Release Specifications and Pharmacopoeial Methods Define Peptide Synthesis Grade H-L-Ile-OEt·HCl?

Release specifications for H-L-Ile-OEt·HCl are generated under a quality system aligned with USP general chapters and ICH Q3D. Because the ethyl ester can retain moisture in the hydrochloride lattice, loss on drying is controlled by vacuum drying at 60 °C for 4 h. Optical rotation is measured in ethanol at 20 °C at the sodium D line with a 1 dm cell. Chloride content is monitored argentometrically as a stoichiometric verification of salt formation. The following table summarizes a representative release specification that would accompany a peptide synthesis grade lot.

For N-Boc protection of H-L-Ile-OEt·HCl, the hydrochloride is suspended in tetrahydrofuran and neutralized with aqueous sodium bicarbonate or N,N-diisopropylethylamine at 0–5 °C. Di-tert-butyl dicarbonate is added in stoichiometric excess, maintaining pH between 8.0 and 8.5. Pilot-scale batches in 50 L glass-lined reactors with retreat-blade impellers have shown that uncontrolled exothermic neutralization can raise internal temperature above 15 °C, increasing ethyl ester hydrolysis and reducing isolated yield. Conversion is typically monitored by thin-layer chromatography and then by HPLC using USP <621>; complete disappearance of the free amine is observed within 2–4 h when the temperature is held below 5 °C. The resulting Boc-Ile-OEt is extracted into ethyl acetate and washed with 1 M citric acid, 5% sodium bicarbonate, and brine. Residual DIPEA salts must be removed before saponification because amine residues accelerate ester cleavage and can promote epimerization at the α-carbon.

When Ethyl Ester Hydrochloride Replaces Free L-Isoleucine in Solution-Phase Fragment Coupling

In solution-phase fragment coupling, H-Ile-OEt·HCl is used as the nucleophilic amino component rather than as the carboxylic acid donor. The hydrochloride is pre-dried at 40 °C under vacuum for 12 h when ambient relative humidity exceeds 60%, because lattice moisture quenches uronium-type coupling reagents. The salt is suspended in anhydrous N,N-dimethylformamide or dichloromethane, and 2.0–2.2 equivalents of a tertiary amine such as DIPEA or N-methylmorpholine are added to liberate the free amine. Immediate addition of the pre-activated carboxylic acid fragment is required; delayed addition after neutralization increases competing lactam formation and ketene loss in the activation partner. For HATU-mediated couplings, the carboxylic acid component, HATU, and DIPEA are pre-mixed for 30–60 seconds at 0 °C before addition to the neutralized isoleucine ester. In 100 L jacket-cooled stirred reactors, the resulting coupling exotherm is controlled by jacket setpoint −5 °C to keep internal temperature below 5 °C; excursions above 8 °C accelerate epimerization via oxazolone intermediates. Coupling conversion is followed by HPLC at 210 nm and typically exceeds 95% within 3 h when the activation stoichiometry is maintained at 1.05 equivalents relative to the carboxyl donor.

The process conflict in this system is the competing consumption of the tertiary amine by the hydrochloride counterion. Insufficient base leaves the isoleucine nitrogen protonated and non-nucleophilic; excessive base raises pH above 8.5 and promotes base-catalyzed ethyl ester hydrolysis. The operational window is therefore controlled by pH measurement in wet DMF, with a target of 7.5–8.0 after neutralization. Published data for this specific configuration is limited, but inline infrared monitoring has documented the disappearance of the activated ester carbonyl near 1805 cm⁻¹ when HATU activation is complete, providing a real-time indication that the hydrochloride has been fully neutralized.

In EDC/HOBt-mediated couplings, the hydrochloride is pre-neutralized with N-methylmorpholine in DMF at 0 °C before the addition of 1-hydroxybenzotriazole and the N-protected carboxyl component. With water-soluble carbodiimide, the reaction medium must remain anhydrous after neutralization because EDC hydrolysis is first-order in water. Process development data from pilot-scale runs indicate that residual water above 0.1% lowers isolated yield by 5–10% due to competing urea by-product formation and acylation of water. The formed dicyclohexylurea precipitates from dichloromethane and is removed by filtration; residual isoleucine hydrochloride can be detected as a polar baseline peak by HPLC at 210 nm. The acylation is typically complete in 4–6 h at 0–5 °C when 1.2 equivalents of carbodiimide are used.

Saponification of H-L-Ile-OEt·HCl-derived intermediates is preferably conducted with lithium hydroxide monohydrate in tetrahydrofuran/water 3:1 at 0–5 °C. The ethyl ester is consumed within 30–60 min; extended reaction time or temperatures above 10 °C can racemize the α-carbon. The reaction is quenched with 1 M potassium hydrogen sulfate to pH 3.0–3.5 and extracted into ethyl acetate. The carboxylic acid product must not be stored in basic aqueous solution, because α-proton abstraction is reversible and leads to enantiomeric erosion. Chiral HPLC analysis of the crude saponification product typically shows ≤0.5% D-isoleucine when the temperature is maintained at 5 °C or below; above 15 °C, the D-isomer content can rise above 1.0% within 2 h. Published data for this specific configuration is limited, but the trend is consistent with known α-proton exchange rates for amino acid derivatives.

Differences between H-L-Ile-OEt·HCl and related isoleucine derivatives are most pronounced in solubility, deprotection orthogonality, and hydrolytic stability. The following table compares properties relevant to route selection in peptide synthesis and pharmaceutical intermediate manufacture.

Replacement of the ethyl ester with the tert-butyl ester alters deprotection orthogonality. The tert-butyl ester is selectively removed with trifluoroacetic acid in dichloromethane, typically 30–50% v/v TFA, while the ethyl ester remains intact under these acidic conditions. Conversely, saponification of the ethyl ester with lithium hydroxide in tetrahydrofuran/water at 0–10 °C leaves a tert-butyl ester unaffected. The choice between H-L-Ile-OEt·HCl and H-L-Ile-OtBu·HCl is therefore determined by the downstream deprotection schedule. If a later fragment requires TFA deprotection of a side-chain protecting group, the ethyl ester is preferred to avoid simultaneous loss of the C-terminal carboxyl protection. If the final global deprotection includes saponification, the tert-butyl ester may be preferred.

Counterion, Moisture, and Thermal Storage Boundaries

H-L-Ile-OEt·HCl is stored in sealed containers under inert gas at 2–8 °C. The hydrochloride is hygroscopic above 60% relative humidity; pre-drying at 40 °C under vacuum for 12 h is specified before use in water-sensitive couplings. Differential scanning calorimetry shows a melt endotherm near 118–121 °C with decomposition onset above 180 °C. Avoid combination with strong aqueous bases, amine-based additives, or anhydrous nucleophilic reagents in storage, because free amine generation and ethyl ester hydrolysis occur under alkaline conditions. Exposure to protic acids in solution can shift the dissociation equilibrium of the hydrochloride salt and reduce coupling efficiency.

In chiral HPLC method development, the enantiomeric purity of H-L-Ile-OEt·HCl is evaluated after derivatization to the free amine or after direct chiral separation on a Chiralpak IA column. The method uses a mobile phase of heptane/ethanol/trifluoroacetic acid 80/20/0.1 with a flow rate of 1.0 mL/min and UV detection at 210 nm. The D-enantiomer elutes before the L-enantiomer under these conditions; quantification is by area normalization against a reference standard of known chiral purity. For lot release, the acceptance limit is ≥99.0% L-enantiomer. This specification is stricter than the routine assay limit because low-level enantiomeric contamination can propagate through peptide coupling and deprotection steps and is not removed by simple crystallization.

Compared with N-Boc-L-isoleucine ethyl ester, H-L-Ile-OEt·HCl contains a free amino group in protonated form, making it directly available for N-acylation or reductive alkylation without a pre-deprotection step. The trade-off is the extra base requirement and salt removal burden. Differentiation from the methyl ester also appears in residual solvent profiles: methyl ester batches may retain methanol and require USP <467> control below 3000 ppm, while ethyl ester batches are controlled for ethanol below 5000 ppm. This difference can affect downstream solvent selection in active pharmaceutical ingredient manufacturing.