D-Leucine methyl ester hydrochloride (C₇H₁₅NO₂·HCl; CAS 5845-53-4; molecular weight 181.66 g/mol) is supplied as a white to off-white crystalline solid. The compound is the hydrochloride salt of methyl (2R)-2-amino-4-methylpentanoate and is commonly catalogued as D-Leu-OMe·HCl or H-D-Leu-OMe·HCl. Standard research-grade packaging includes 5 g, 25 g, and 100 g quantities. The product is an unprotected C-terminal protected amino acid building block: the α-amino group is present as the protonated free amine, while the carboxylic acid is masked as the methyl ester. This structural arrangement allows the salt to be stored as a crystalline solid and then neutralized in situ with a tertiary amine immediately before coupling.

Commercial lot release criteria generally include chromatographic purity of ≥98.0% by HPLC at 210–220 nm, water content of ≤0.50% by Karl Fischer titration, and specific rotation [α]_D²⁰ in methanol of approximately −14° to −16° at c = 1. The hydrochloride form avoids the rapid amine oxidation and carbonate salt formation observed with the free amino ester. The compound is freely soluble in methanol, water, and dimethylformamide, but practically insoluble in hexane and toluene. These solubility boundaries define the available solvent systems for coupling and work-up.

What Distinguishes the Hydrochloride Salt from the Free Amino Ester and N-Protected Derivatives?

Free leucine methyl ester is a mobile liquid or low-melting solid with an unprotected primary amine. That free base is susceptible to atmospheric carbon dioxide uptake and progressive discoloration, whereas the hydrochloride salt protonates the α-amine and produces a stable crystalline matrix. Relative to N-protected derivatives such as Fmoc-D-Leu-OMe or Boc-D-Leu-OMe, the unprotected hydrochloride adds 36.46 g/mol of HCl and removes the requirement for an initial deprotection step. However, the salt must be neutralized with 1.0–1.2 equivalents of N-methylmorpholine or diisopropylethylamine in dimethylformamide at 0 °C before the free amine can participate in acylation. This in situ release creates an operational distinction: N-protected derivatives can be stored and then deprotected under acid or base, while the hydrochloride salt requires precise base accounting to avoid an acidic coupling environment.

For multi-kilogram handling, the product is typically received in high-density polyethylene drums with aluminum-foil laminate liners. Transfer into a low-humidity suite at 20–25 °C and relative humidity below 40% is recommended because the crystalline salt can cake at the drum headspace after repeated opening. Nitrogen purge at 0.1–0.2 bar and immediate resealing after dispensing reduce moisture uptake. Milling through a 0.5 mm sieve under dry nitrogen can restore free-flowing surface area before downstream weighing; static charge should be controlled by grounding equipment to ≤10⁶ Ω. Pre-drying at 40 °C under vacuum for 12 h is used before anhydrous coupling when water content exceeds the 0.5% release limit. Drying above 60 °C is avoided because methyl ester and hydrochloride salts can undergo thermal degradation or partial ester hydrolysis.

When Enantiopurity Becomes Critical in Bioactive Peptide Mimetics

D-Leucine methyl ester hydrochloride is selected primarily for the incorporation of D-leucine into peptide sequences where inversion of the side-chain orientation modifies protease resistance, secondary-structure propensity, or receptor contact geometry. Enantiomeric control is critical because residual L-leucine methyl ester at 0.5 mol% can propagate into diastereomeric impurities that are difficult to remove by conventional reversed-phase chromatography. Chiral HPLC methods using zwitterionic or crown ether chiral stationary phases with 0.1% formic acid in methanol/acetonitrile are typical for release testing. Limits of quantitation for the L-enantiomer are frequently set at 0.10 area%. Esterification of D-leucine with HCl-methanol below 0 °C generally preserves a D/L ratio of ≥99:1; the greater risk is prolonged exposure of the free amine to alkaline solution, where α-proton abstraction can generate a planar enolate and erode optical purity. Neutralized coupling solutions should therefore be used within 2 h at 0–4 °C. Published data for the specific racemization half-life of this compound in aqueous carbonate buffers is limited, so process-specific chiral HPLC verification is required rather than reliance on literature values.

In solution-phase fragment coupling, D-Leu-OMe·HCl is suspended in dry dichloromethane or dimethylformamide and neutralized with 1.05 equivalents of diisopropylethylamine at −5 °C before the incoming carboxylic acid component is activated. Coupling with HATU at 1.1 equivalents in 0.2 M N-methylmorpholine typically gives a reaction half-life of 8–15 minutes at 25 °C. Carbodiimide methods with 1-hydroxybenzotriazole require longer activation and can expose the free amine to racemization. The methyl ester chromophore is weak beyond 210 nm; process analytics therefore rely on Q-TOF LC/MS or charged aerosol detection. Work-up uses a 5% aqueous citric acid quench at 0 °C, followed by a carbonate wash at pH 8.0–8.5. The ester is stable to the acidic quench but hydrolyzes under strongly basic extraction, so the wash sequence is acid first and weak base second.

C-Terminal Methyl Ester Protection Suppresses Diketopiperazine Formation During Deprotection

When a dipeptide on a 2-chlorotrityl resin carries a C-terminal leucine, premature cleavage or Fmoc deprotection can generate 2,5-diketopiperazine if the C-terminal carboxyl is unprotected. The methyl ester reduces this intramolecular cyclo-dimerization by keeping the C-terminal electrophile as an ester carbonyl. The methyl ester is not a permanent cap in solid-phase synthesis; final peptide acid can be obtained by saponification with 0.5 M lithium hydroxide in tetrahydrofuran/water 3:1 at 0 °C for 30–45 minutes. Under these conditions, loss of the D-Leu residue by β-elimination is undetectable within 2 h by LC-MS. Hydrolysis is slower than for glycine methyl ester because the isobutyl side chain shields the ester carbonyl. pH is maintained below 11.5 because higher alkalinity promotes amide bond hydrolysis at the adjacent residue. Use of free D-Leu-OH avoids the saponification stage but increases the risk of 2,5-diketopiperazine release on acid-labile resins. This mechanistic distinction is the primary reason for selecting the methyl ester hydrochloride in medicinal chemistry campaigns requiring a protected C-terminal D-Leu building block.

The compound also serves as a precursor to N-sulfonyl D-leucine methyl esters for diastereomeric salt resolution and chiral auxiliary studies. Condensation with aromatic sulfonyl chlorides in dichloromethane at 0–5 °C generates derivatives with characteristic ¹H NMR splitting patterns. In preparative reversed-phase chromatography, the isobutyl side chain increases retention relative to D-alanine methyl ester hydrochloride. Replacement of the methyl ester with a benzyl ester increases hydrophobicity and permits hydrogenolytic cleavage over 5% Pd/C at 1 atm H₂, but the benzylic ester is incompatible with oxidizing coupling reagents that generate reactive halides. The methyl ester is therefore preferred when the protecting group must survive acidic washes and Fmoc deprotection while retaining acceptable aqueous solubility during work-up.

Crystallinity, Hygroscopicity, and Bulk Handling on Peptide Production Lines

Batch-to-batch variance in this product class is most commonly observed in residual water and particle size distribution. Coarse crystalline lots wet more slowly than micronized material but are easier to filter from non-aqueous suspensions. Micronized lots can achieve faster dissolution in dimethylformamide but may agglomerate under humid transfer. For pilot-scale peptide synthesis in 50 L reactors, the hydrochloride salt is typically dissolved in dimethylformamide at 0–5 °C before base is added. The neutralization exotherm is controlled by slow addition of N-methylmorpholine at a rate that keeps the internal temperature below 5 °C. Under conditions of incomplete neutralization, the residual hydrochloride can reduce the effective coupling base concentration and lead to incomplete acylation. The process boundary is therefore set by base stoichiometry and mixing efficiency rather than by the intrinsic reactivity of the methyl ester.

The table below summarizes the main distinctions between D-Leu-OMe·HCl, the L-enantiomer hydrochloride salt, and free D-leucine. The L-enantiomer is used for proteinogenic peptide synthesis, while the D-isomer is selected for mirror-image peptide fragments or proteolytically stabilized sequences. Free D-leucine is used in direct coupling without C-terminal ester cleavage, but it lacks the crystalline salt handling advantages and requires a separate activation strategy for carboxyl activation.

In anhydrous coupling reactions, the limiting moisture specification is ≤0.5%. Water above this threshold quenches active ester formation and reduces the yield of carbodiimide-mediated couplings. In a research-scale peptide synthesizer, the salt is charged as a 0.4 M dimethylformamide solution after pre-drying; the dissolution vessel is blanketed with nitrogen because the protonated amine exchange is reversible and the free ester can volatilize slowly under vacuum. Incompatibilities include strong aqueous bases above pH 9, acid chlorides, acid anhydrides, and oxidizing agents such as sodium hypochlorite. Contact with hypochlorite can generate chloramine derivatives. The compound is handled in a fume hood with local exhaust ventilation, nitrile gloves, and eye protection. The operational boundary is defined by the combination of moisture exclusion, temperature control below 60 °C, and neutralization stoichiometry rather than by a single storage or melting parameter.