L-Leucine methyl ester hydrochloride is supplied as a white to off-white crystalline solid, assigned CAS Registry Number 7517-19-3 and molecular formula C7H16ClNO2. The formula mass is 181.66 g mol⁻¹. Commercial product models generally appear as H-Leu-OMe·HCl, L-Leu-OMe·HCl, or methyl L-leucinate hydrochloride; these strings are supplier model codes and should be verified against the CAS number and batch certificate. Bulk packaging commonly includes 1 kg and 25 kg HDPE drums with inner polyethylene liners. The salt is freely soluble in water and methanol, soluble in ethanol, and practically insoluble in diethyl ether. The protonated amine and methyl-ester-protected carboxylic acid distinguish it from the parent L-leucine zwitterion and from N-protected leucine derivatives such as Boc-Leu-OH or Cbz-Leu-OH. Its use is confined to synthetic intermediate operations; the material is not a formulated pharmaceutical product.

The principal purpose of the methyl ester protection is to block the carboxyl group while leaving the amino group available for neutralization and subsequent reaction. In comparison with free L-leucine, the hydrochloride salt dissolves readily in water as a salt, and after neutralization the resulting methyl ester partitions into ethyl acetate or dichloromethane. This behavior is used in liquid-liquid extraction processes where the free amino acid would remain in the aqueous phase. The methyl ester group is smaller and less sterically demanding than the ethyl or tert-butyl ester, and this affects hydrolysis kinetics, crystallization, and downstream removal.

What Limits Direct Coupling of the Hydrochloride Salt?

The protonated amine prevents direct amide formation in carbodiimide-mediated coupling. Free-basing is therefore required before the material can be used as the amine component. In non-aqueous systems, triethylamine or N-methylmorpholine is added at 1.0–1.05 equivalents relative to the salt. The neutralization is exothermic and is controlled at 0–5 °C in anhydrous dichloromethane or tetrahydrofuran. The free base is not isolated; the resulting suspension is filtered to remove triethylamine hydrochloride and the filtrate is used immediately. In aqueous workup, sodium bicarbonate or potassium carbonate at pH 8.5–9.0 releases the free base into ethyl acetate. Recovery is improved by 3 countercurrent extractions; the organic layer is dried over sodium sulfate and concentrated below 30 °C to limit ester hydrolysis.

The protonated α-ammonium group of amino acid methyl ester hydrochlorides has an approximate pKa of 7.5–8.0, requiring a neutralization base with pKa above 10. Incomplete neutralization leaves hydrochloride salt in the organic phase and reduces coupling efficiency. At pilot scale, the precipitated triethylamine hydrochloride is a crystalline solid that can blind filter media; agitated pressure filters with 5–10 μm polypropylene cloth are used to maintain filtration rate. Liquid-liquid separation is performed in cylindrical vessels with bottom outlet and sight glass; emulsion formation is minimized by avoiding strong alkalinity above pH 9.5, which also accelerates saponification of the methyl ester.

The tabulated values are representative compilation criteria for commercial release; individual batch certificates govern actual acceptance limits. The HPLC procedure typically employs a C18 column with UV detection at 210 nm and a mobile-phase gradient of 0.1% trifluoroacetic acid in water and acetonitrile. Peak purity is evaluated by diode-array detection, with a single main peak accounting for ≥98.0% of total area. Chloride content is determined by potentiometric argentometric titration with 0.1 M silver nitrate. Specific rotation is measured in a 1 dm polarimeter cell at 20 °C at the sodium D-line. These methods distinguish the salt from L-leucine and from the free leucine methyl ester during incoming inspection.

In the preparation of L-leucine N-carboxyanhydride, the hydrochloride is slurried in anhydrous tetrahydrofuran at 0–5 °C under nitrogen, and triethylamine is added over 20–30 min. Triphosgene at 0.35–0.40 equivalents is then introduced as a tetrahydrofuran solution while maintaining internal temperature below 25 °C. The reaction is monitored by FTIR for appearance of NCA carbonyl bands at approximately 1830 cm⁻¹ and 1790 cm⁻¹. The byproduct triethylamine hydrochloride is removed by filtration under inert atmosphere. The filtrate is concentrated at ≤30 °C under reduced pressure, and the NCA is crystallized from ethyl acetate/n-heptane. Residual water above 0.1% in the starting salt reduces NCA yield because water hydrolyzes both the NCA product and phosgene equivalents.

For ring-opening polymerization, Leu NCA is dissolved in anhydrous N,N-dimethylformamide at 0.1–1.0 M. Initiation with n-hexylamine at monomer/initiator ratios between 20:1 and 100:1 produces poly(L-leucine) blocks. The reaction is followed by disappearance of the NCA carbonyl bands; final dispersity depends on residual water, solvent dryness, and initiator purity. For manufacturing campaigns where NCA stability controls molecular weight distribution, the hydrochloride is therefore purchased as a non-polymerizable dry solid and converted to the NCA immediately before polymerization.

Solution-Phase Peptide Coupling and Ester Saponification Routes

After neutralization, the methyl ester of L-leucine is used as the amine component in solution-phase peptide synthesis. Typical coupling reagents include N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride with 1-hydroxybenzotriazole in N,N-dimethylformamide at 0–5 °C, or O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate with N,N-diisopropylethylamine for hindered substrates. The hydrochloride counterion of the amino component is neutralized before coupling; otherwise the acidic proton consumes the coupling reagent. After chain extension, the methyl ester is removed with lithium hydroxide in tetrahydrofuran/water 1:1 at 0 °C or with sodium hydroxide in methanol. These conditions are selected to limit α-carbon racemization. At process scale, the saponification is conducted in a jacketed reactor with pH probe; the endpoint is indicated by stabilization at pH 10.5–11.0 and disappearance of the ester carbonyl stretch at approximately 1735 cm⁻¹ by process FTIR.

The methyl ester cleavage step is faster than the corresponding ethyl ester cleavage, which is exploited when a C-terminal acid must be revealed under mild base. If the C-terminus must remain blocked during repeated base treatments, the tert-butyl ester is preferred; if it must be removed by hydrogenolysis under neutral conditions, the benzyl ester is preferred. The hydrochloride salt of the methyl ester is not normally used in solid-phase peptide synthesis because the C-terminus is not attached to the resin in its protected methyl ester form, but it is used to prepare leucine building blocks and NCA monomers for subsequent polymerizations.

When the Methyl Ester Is Preferred Over the Ethyl or tert-Butyl Ester

The methyl ester is removed under basic or nucleophilic conditions more rapidly than the ethyl ester, and it remains intact under the acid conditions that cleave the tert-butyl ester. The tert-butyl ester is selected when the C-terminus must survive repeated basic transformations; the benzyl ester is selected when the C-terminus must be released by hydrogenolysis without disturbing acid- or base-labile groups elsewhere. The hydrochloride salt rather than the free base is preferred for storage because the protonated amine reduces volatile free-amine loss and improves batch-to-batch consistency in dry solid handling.

Compared with the free amino acid, the hydrochloride salt offers organic-phase extractability after neutralization and a protected carboxyl group for regioselective amine chemistry. Compared with the ethyl ester hydrochloride, the methyl ester generally hydrolyzes faster and has a smaller process mass. Compared with the tert-butyl ester hydrochloride, the methyl ester tolerates acidic conditions but is removed by base; this is the key process difference when designing a synthetic route with multiple ester or acid-labile groups.

Quality agreements for cGMP intermediates typically require the supplier to report residual solvents under USP <467> Option 2, elemental impurities under USP <233>/ICH Q3D, and water content by ISO 760. The material is manufactured under ISO 9001:2015 and can be supplied with a certificate of analysis that includes batch-specific chromatographic and titration data. EU REACH registration is required for EU manufacturing or import at ≥1 t/a; downstream users should confirm that their use is covered by the registered dossier. The substance is not generally classified as dangerous goods for transport in dry powder form; however, the safety data sheet should be checked for local hazard classifications and dust control requirements.

The Material Turns Pasty Above 60% Relative Humidity

The salt is hygroscopic and should be stored at 2–8 °C in tightly closed containers. At relative humidity above 60%, the powder can form a crust or paste. In production suites, drums should be opened under nitrogen or in a dry room; after each withdrawal, the container should be purged and resealed. Prior to use in NCA synthesis, the material may be dried at 40 °C under vacuum for 12 h; this reduces free water but does not remove bound HCl. Heating above 60 °C should be avoided because the methyl ester can hydrolyze or release methanol in the presence of moisture. Aqueous solutions should be prepared just before use and held at 0–5 °C where possible.

Incompatibilities include strong bases, which will saponify the methyl ester if added abruptly, and strong oxidizing agents. The hydrochloride salt should not be milled in ungrounded equipment without dust control; fine dust can be irritating to the respiratory tract. Storage beyond the retest date should be supported by batch-specific stability data and re-qualification against the original release specification.