L-Isoleucine methyl ester hydrochloride is supplied under CAS 18598-74-8, molecular formula C₇H₁₆ClNO₂, and molar mass 181.66 g·mol⁻¹. The compound is the hydrochloride salt of the C-terminal methyl ester of L-isoleucine. The free base methyl ester is a liquid at ambient temperature, whereas the hydrochloride is a white to off-white crystalline powder with a routine melting range of 98–101 °C. This crystalline form permits gravimetric dispensing in automated solid-dosing equipment and reduces evaporative loss during storage. The methyl ester function masks the carboxylic acid terminus, leaving the α-amino group available for selective acylation, sulfonylation, or ureido formation after neutralization. In preparative work, the salt is dissolved in N,N-dimethylformamide or 1-methyl-2-pyrrolidinone and treated with a tertiary amine such as N-methylmorpholine or N,N-diisopropylethylamine. The liberated free amine then participates in solution-phase or solid-phase peptide coupling. Principal uses include the synthesis of peptide fragments, chiral building blocks, and pharmaceutical intermediates. Solubility is high in water, methanol, and ethanol, lower in dichloromethane and ethyl acetate, and practically negligible in aliphatic hydrocarbons. No universal industry model number applies to this product; manufacturers assign internal catalog codes according to package size, analytical certificate, and grade. Procurement documentation should therefore specify the CAS registry number, assay, water content, residual solvent class, and chiral purity level.

What Limits Direct Ester Hydrochloride Utility in Anhydrous Peptide Coupling?

The protonated amine is the primary limitation in anhydrous coupling. Because the α-amine is present as the hydrochloride, one equivalent of base is required to liberate the free amine before reaction with an activated carboxylate can occur. With uronium salt activators such as 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate, additional base is consumed to neutralize the weakly acidic coupling additives. Incomplete neutralization produces unreacted starting material and N-terminal deletion sequences detectable by mass spectrometry or Edman sequencing. A robust protocol for 2-chlorotrityl chloride resin at 0.8–1.2 mmol·g⁻¹ substitution dissolves the ester hydrochloride and coupling reagent in anhydrous N,N-dimethylformamide at 0.2–0.4 M, cools the batch to 0–5 °C, adds N-methylmorpholine or N,N-diisopropylethylamine in 4–6 equivalents relative to free amine, and holds the mixture for 5–15 min before resin transfer. The exotherm is mild at laboratory scale, but at 0.5 mol scale jacket cooling is normally required to maintain 20 ± 3 °C. Water ingress is a critical process conflict: the hydrochloride powder is hygroscopic, and absorbed water shifts the acid-base equilibrium toward the protonated amine while also decomposing moisture-sensitive activators. For demanding sequences, the powder should be dried under vacuum at 40 °C for 12 h when Karl Fischer water exceeds 0.1%. Coupling protocols commonly use 2.0–2.5 equivalents of HBTU or HATU with 1-hydroxybenzotriazole or 1-hydroxy-7-azabenzotriazole. Epimerization at the isoleucine α-carbon is typically reported below 1% by Marfey’s derivatization or chiral HPLC. Carbodiimide-only methods without a racemization-suppressing additive can exceed this threshold when pre-activation exceeds 30 min or when the batch temperature rises above 25 °C. Published data for this specific configuration across all activator combinations is limited, so process-specific validation is required.

Across commercial lots, analytical characterization should include counterion stoichiometry, residual solvent profile, related substances, and chiral identity. Non-aqueous titration with perchloric acid in glacial acetic acid determines free amine content; chloride content is measured by argentometric titration or ion chromatography. Residual solvent reporting follows the manufacturer’s validated method and should be reviewed against ICH Q3C limits. The following representative release matrix is drawn from common commercial certificates of analysis; actual acceptance criteria vary by supplier and grade.

Because no universal model number exists for L-isoleucine methyl ester hydrochloride, material selection is controlled by the chemical identifier and the grade-specific certificate. Laboratory-grade material is often supplied in 5 g, 25 g, and 100 g bottles. High-purity or GMP starting material may include additional release controls such as assay ≥99.0%, water ≤0.2%, and restricted residual solvent classes. Bulk packaging typically uses double polyethylene liners inside fiber drums under nitrogen. The manufacturer’s certificate of analysis and safety data sheet define the exact model-specific release limits.

When the sec-Butyl Side Chain Slows Coupling Relative to Leucine-Based Esters

When L-isoleucine methyl ester hydrochloride is compared with L-leucine methyl ester hydrochloride, the difference in side-chain branching controls coupling rate and racemization risk. Both compounds share the molecular formula C₇H₁₆ClNO₂ and molar mass 181.66 g·mol⁻¹. Isoleucine is β-branched at the side chain, whereas leucine is branched at the γ-carbon. In hindered acylations the β-methyl group of isoleucine reduces access of the activated carboxylate to the α-nitrogen. The practical consequence is that isoleucine residues often require longer coupling times, a second coupling, or a switch from 1,3-dicyclohexylcarbodiimide to a phosphonium or uronium reagent. A synthesis campaign on a polyethylene glycol-derivatized resin with 0.25 mmol·g⁻¹ loading may require double coupling for isoleucine residues when using HBTU/HOBt at 25 °C, whereas leucine residues can frequently be single-coupled with the same stoichiometry. No universal kinetic constant applies, and the extent of retardation is peptide-sequence dependent. The enantiomeric form, D-isoleucine methyl ester hydrochloride, has the same molecular weight and physical state but opposite optical rotation. D-isoleucine methyl ester hydrochloride is used in chiral method qualification, forced degradation studies, or mirror-image peptide synthesis. It cannot be substituted for the L-form in native peptide sequences without reversing α-carbon stereochemistry.

Storage stability is governed by moisture uptake, ester hydrolysis, and thermal exposure. The hydrochloride salt should be stored at 2–8 °C in sealed, desiccated containers under inert gas. Exposure to relative humidity above 60% without desiccant can produce visible surface wetting and increase water content; transfers should be performed under nitrogen or in a dry room. The methyl ester is stable in neutral dry aprotic solvents, but aqueous or alcoholic solutions above neutral pH slowly hydrolyze to L-isoleucine hydrochloride and methanol. No published Arrhenius data were identified for this specific hydrolysis rate; stress testing under the intended process conditions is required before prolonged solution storage. In pharmaceutical intermediate campaigns, the product is often used as the isolated salt and neutralized directly before coupling. Residual tertiary amine hydrochloride byproduct remains in the process stream. If this is undesirable for downstream purification, the free base can be generated in situ and extracted into an organic phase, but extraction is limited by the lower solubility of the free base in aliphatic hydrocarbons and by its instability under basic aqueous conditions.

In process-scale campaigns, the most common bottleneck is precipitation of the neutralized free base at low temperature or upon addition of chlorinated solvents. The free base methyl ester has limited solubility in dichloromethane under some conditions, and transfer-line blockages have been reported when cold manual neutralization is performed without sufficient co-solvent. For larger lots, the hydrochloride is therefore neutralized in the same solvent system used for the subsequent coupling, avoiding an isolated free base stage. Filtration and drying of the hydrochloride itself are simpler than for the free base; the crystalline solid can be vacuum-dried at 40 °C, while the free base requires inert-atmosphere handling. These operational boundaries define the safe handling envelope for peptide synthesis and medicinal chemistry applications.