D-valine methyl ester hydrochloride, CAS 7146-15-8, molecular formula C6H14ClNO2, molar mass 167.63 g/mol, is the hydrochloride salt of the methyl ester of D-valine. Commercial lots for peptide synthesis are supplied as white to off-white crystalline powder with a typical assay of ≥98.0% on the anhydrous basis and enantiomeric excess of ≥99.0% by chiral GC or HPLC. No harmonized model number exists; product codes are supplier-specific, and procurement specifications are therefore anchored to the CAS registry number and the lot certificate, not to a proprietary model designation. The compound is used as a chiral C-terminal building block in solution-phase peptide synthesis and as an intermediate for N-protected D-valine derivatives. Unlike the free amino ester, the hydrochloride form is a non-volatile crystalline solid that can be handled for short periods in dry ambient conditions and stored without polymerization risk.

Chiral purity determination for this compound is typically performed by gas chromatography after N-trifluoroacetylation or by chiral HPLC with a derivatizing chromophore. Optical rotation alone is not sufficient for release of pharmaceutical intermediates because low-level L-isomer contamination may not shift the observed rotation beyond an acceptance range. The free amine has a weak UV chromophore; therefore HPLC methods use pre-column derivatization with Marfey's reagent or another chiral derivatizing agent, or a charged aerosol detector. This analytical constraint explains why enantiomeric excess is a separate release parameter from assay.

Why Is the Hydrochloride Salt Form Preferred Over the Free Amino Ester?

Free D-valine methyl ester is a low-molecular-weight amine that can exist as a clear liquid or low-melting solid and may absorb carbon dioxide during ambient storage. Conversion to the hydrochloride gives a crystalline lattice that restricts molecular motion and slows ester hydrolysis. The salt also permits exact stoichiometric control in peptide coupling because the active amine content is determined by non-aqueous titration. In practice, D-valine methyl ester hydrochloride is charged with a tertiary base such as N-methylmorpholine or diisopropylethylamine in 1.00–1.10 molar equivalents relative to the hydrochloride. Insufficient base leaves protonated amine unavailable for acylation; excess base above 1.5 equivalents can promote α-carbon deprotonation and enantiomeric erosion. Solubility in DMF and NMP is adequate at 0.05–0.30 g/mL; in dichloromethane the salt is less soluble, and pre-dissolution in DMF followed by dilution is used as a standard workaround.

Storage and Drying Limits Are Defined by Ester Hydrolysis and Moisture Sensitivity

Commercial material should be stored in a tightly closed container at 2–8°C. At ambient conditions above 60% relative humidity the crystalline solid becomes tacky within several hours, and free D-valine may be detected after 24 h as the methyl ester hydrolyzes. Pre-drying is therefore required for production campaigns where water content must remain below 0.5% before coupling. A vacuum tray dryer operated at 40°C and ≤10 mmHg for 8–12 h is used; bed thickness should not exceed 5 cm to avoid entrapped methanol. A static desiccator containing activated molecular sieves can reduce surface moisture for laboratory lots. The hydrochloride is incompatible with strong aqueous alkali, concentrated sulfuric acid, and strong oxidizers. Contact with sodium hydroxide solution at pH above 10 rapidly liberates the free base and accelerates ester saponification. Material removed from cold storage should be allowed to reach room temperature before opening to prevent condensation.

When Chiral Integrity Is Maintained During Multikilogram Coupling Campaigns

In a jacketed glass-lined reactor equipped with an internal PT100 probe and recirculation chiller, the neutralization and coupling sequence is controlled to keep the reaction mass below 5°C during reagent addition. D-valine methyl ester hydrochloride is charged as a DMF solution, followed by 1.05 equivalents of N-methylmorpholine. The resulting exotherm is typically less than 8°C when the base is added over 30–45 min. The N-protected amino acid or peptide acid is pre-activated with HATU and DIPEA in DMF at 0°C for 5–10 min, then transferred into the reactor under nitrogen. Process samples taken at 2 h and 18 h are quenched into acetonitrile and analyzed by chiral HPLC. If the D-valine epimer peak exceeds 0.5% area by 18 h, the batch is diverted to rework because downstream salt removal does not remove the epimeric impurity. The epimer is not resolved by ordinary silica chromatography; only chiral stationary phases or diastereomeric recrystallization are effective. Published data for this specific peptide configuration is limited; therefore the acceptable epimer threshold must be established for each coupling target.

In solution-phase couplings, typical solvent systems are DMF, NMP, or DMF–dichloromethane mixtures. Water-miscible solvents allow direct extractive workup: the crude methyl ester is diluted with ethyl acetate or methyl tert-butyl ether, washed sequentially with 5% sodium carbonate solution, 1 mol/L hydrochloric acid, and brine, then concentrated under reduced pressure at ≤35°C. Methyl ester cleavage to the free acid is performed with lithium hydroxide in tetrahydrofuran–water at 0–5°C; aqueous sodium hydroxide at room temperature can reduce enantiomeric excess and is not recommended. For reduction to D-valinol, the ester is treated with lithium aluminum hydride in tetrahydrofuran under inert atmosphere; the standard quench sequence uses water, 15% aqueous sodium hydroxide, and water in a 1:1:3 volume ratio. The hydrochloride salt is not directly reduced; the free base is liberated before hydride addition.

Peptide-Structural Consequences of D-Valine Insertion and Coupling Reagent Selection

Insertion of D-valine into a peptide backbone alters backbone torsional preferences and can stabilize type II' β-turn geometries when paired with a D-amino acid at the i+1 position. The methyl ester hydrochloride is used as the C-terminal electrophile-protected monomer in solution synthesis; the hydrochloride is neutralized to the free amine before coupling. Carbodiimide reagents such as EDC·HCl with HOBt or HOAt achieve reliable activation of N-protected amino acids, while uronium reagents HATU and HBTU provide faster conversion in polar aprotic media. The coupling reaction is typically run at 0–5°C for the first 2 h and then warmed to 20–25°C for 12–24 h. Conversion is monitored by HPLC after derivatization or by LC-MS. Incomplete coupling below 95% conversion usually requires fresh activation rather than prolonged reaction because extended exposure to tertiary base is a known source of epimerization. The D-valine methyl ester peptide is isolated as a foam or crystalline solid depending on sequence.

What Distinguishes D-Valine Methyl Ester Hydrochloride from L-Valine, Racemic Valine, and N-Protected Derivatives?

Configuration at the α-carbon determines whether a valine ester is compatible with a given enzymatic resolution or chiral synthesis. The D-isomer is the mirror-image form of proteinogenic L-valine and is not incorporated into ribosomal peptides; it is used in non-ribosomal peptide synthesis, antimicrobial peptide analogs, and chiral auxiliaries. The hydrochloride methyl ester differs from N-protected derivatives because the amino group is free and must be neutralized before coupling. This limits direct use in automated solid-phase peptide synthesis, where Fmoc-protected monomers are standard, but provides a lower-cost C-terminal fragment for solution-phase campaigns. The racemic ester hydrochloride offers no enantiomeric specificity and is not substitutable for chiral syntheses without preparative resolution. Table 2 summarizes the structural and functional differences.

Documentation for incoming lots should include a certificate of analysis with batch-specific assay, enantiomeric excess, loss on drying, residue on ignition, and residual solvent profile. Vendors exporting to the EU should provide REACH registration status; pharmaceutical intermediate purchasers may request residual solvent data according to ICH Q3C, elemental impurities data according to USP <232>/ICH Q3D, and quality management system certification under ISO 9001:2015. For GMP clinical campaigns, full batch records, change control, and stability data under 25°C/60% RH storage may be required. The methyl ester is not typically supplied as a sterile product; if sterile filtration is required, it is performed after dissolution in the receiving process solvent because the solid itself cannot be sterile-filtered.

Common impurities controlled in release testing are D-valine hydrochloride from ester hydrolysis, the corresponding L-enantiomer from chiral contamination, and residual methanol from the esterification process. Trace chloride cannot be used as an identity marker alone because other chloride salts may co-elute. Liquid chromatography with charged aerosol detection or refractive index detection is used for non-UV-absorbing impurities; D-valine methyl ester lacks a strong chromophore, so derivatization or alternative detection is required.