L-Norvaline methyl ester hydrochloride, model designation NVA-OMe·HCl-98, is the hydrochloride salt of methyl (2S)-2-aminopentanoate. The registry identity is CAS 56558-30-6; the molecular formula is C6H14ClNO2, and the formula weight is 167.63 g/mol. The molecular structure comprises a linear n-propyl side chain attached to the chiral Cα carbon, a primary amino group, and a methyl ester-protected carboxyl. In its supplied form the product is a white to off-white crystalline powder with a minimum assay of 98.0% by reversed-phase HPLC and a typical enantiomeric excess of ≥98.0%. It is classified as a non-proteinogenic amino acid derivative and is used as a C-terminal protected building block in peptide and peptidomimetic synthesis. Because the amino group is protonated, the material is less prone to ambient carbon dioxide absorption than the free amino ester; however, the salt must be neutralized before acylation. The product is differentiated from L-valine methyl ester hydrochloride by the linear rather than β-branched side-chain topology and from L-leucine methyl ester hydrochloride by a one-methylene shorter aliphatic chain.

What Distinguishes the Hydrochloride Salt from the Free Amino Ester?

The free base of L-norvaline methyl ester is commonly encountered as a low-melting liquid or semi-crystalline mass that can absorb atmospheric carbon dioxide to form carbamate, whereas the hydrochloride is a stable crystalline salt with better handling characteristics. Salt formation also permits long-term storage without polymerization or discoloration. In peptide coupling, however, the protonated amine must be released by a tertiary amine. In a jacketed glass reactor equipped with overhead stirring, the hydrochloride is suspended in anhydrous dichloromethane or dimethylformamide and cooled to 0–5 °C. Diisopropylethylamine or N-methylmorpholine is added at 1.0–2.5 mol per mole of substrate. The base is added slowly because neutralization is exothermic; on production scale, the temperature rise is controlled within ±2 °C of the set point by adjusting addition rate. The resulting tertiary amine hydrochloride may remain in solution or precipitate; in dichloromethane, precipitation is common and can be removed by filtration before activation if a clear solution is required. Failure to complete neutralization leaves residual protonated amino ester that is unreactive toward acylation and consumes active coupling species via acid-base side reactions.

In solution-phase peptide synthesis, the methyl ester of L-norvaline serves as a temporary C-terminal protecting group that can survive coupling steps and be removed later by saponification. A typical laboratory-scale acylation is carried out in a 250 mL jacketed glass reactor under nitrogen. The hydrochloride is neutralized with 1.2–2.0 equivalents of diisopropylethylamine in anhydrous dimethylformamide at 0–5 °C. The acylating amino acid, protected as the Fmoc or Cbz derivative, is activated separately with 1.0–1.5 equivalents of a carbodiimide or uronium reagent in the presence of ethyl cyano(hydroxyimino)acetate or 1-hydroxybenzotriazole. The activated carboxyl component is then added to the neutralized ester solution. Coupling progress is monitored by removing aliquots and analyzing by HPLC; when the substrate peak area falls below 2% of its initial value, the reaction is quenched. After aqueous workup, the methyl ester is saponified with lithium hydroxide in tetrahydrofuran/water at 0–5 °C to regenerate the carboxylic acid. In solid-phase protocols, the hydrochloride is neutralized and the free base loaded onto 2-chlorotrityl chloride resin in dichloromethane; loading capacity is typically determined by Fmoc release at 290 nm. Published data for production-scale solid-phase loading of this specific ester is limited; most reported procedures are on the 0.1–2 mol scale.

Specification Profile and Identity Verification

The certificate of analysis for the NVA-OMe·HCl-98 grade reports the following specification matrix. Identity is confirmed by 1H NMR and FTIR; the ester carbonyl band near 1740 cm⁻¹ and the ammonium N–H stretch are used as confirmatory signals. Assay is determined by reversed-phase HPLC on a C18 column, 150 mm × 4.6 mm, 5 µm particle size, with UV detection at 210 nm and a phosphate-buffered acetonitrile gradient. The table lists parameters, method designations, and typical acceptance limits.

When Is L-Norvaline Methyl Ester Hydrochloride Preferred Over L-Valine or L-Leucine Methyl Esters?

The selection between norvaline, valine, and leucine methyl ester hydrochlorides is governed by side-chain topology. L-Norvaline methyl ester hydrochloride and L-valine methyl ester hydrochloride share the same molecular formula and formula weight, C6H14ClNO2, but the norvaline side chain is a linear n-propyl group. The absence of β-branching reduces steric hindrance at the peptide backbone and can be advantageous when a residue with a linear aliphatic chain is required. L-Leucine methyl ester hydrochloride has one additional methylene and a branching pattern at the γ-carbon; L-isoleucine methyl ester hydrochloride introduces a sec-butyl group with restricted rotation. The table below summarizes the structural identifiers for adjacent amino acid methyl ester hydrochlorides. The choice of norvaline ester is not merely a molecular-weight substitution; it changes the side-chain accessible surface area, hydrophobic contact potential, and conformational sampling in the final peptide. Analysts should note that the norvaline and valine esters are isobaric isomers and cannot be distinguished by unit-mass liquid chromatography–mass spectrometry alone; fragmentation patterns or chiral separation are required.

Bulk handling of the powder exhibits hygroscopic behavior that influences feeder performance and batch uniformity. Bulk containers are opened only in a dry nitrogen purged dispensing suite. The product is charged to a polished stainless-steel hopper equipped with mechanical agitation to prevent bridging; screw feeder speed is adjusted to maintain a feed rate appropriate for the downstream reaction. In campaigns where relative humidity exceeds 60%, caking can occur after short exposure; open handling is therefore limited to 15 min per transfer unless the material is under nitrogen flow. If residual water of the incoming lot exceeds 0.5% by Karl Fischer titration, the powder is dried in a vacuum tray dryer at 40 °C with a shelf temperature variation of ±1 °C for at least 4 h. The drying endpoint is confirmed by repeated Karl Fischer sampling. Batch-to-batch variation in residual solvent is controlled by recrystallization from methanol/ethyl acetate or methanol/diethyl ether; the certificate of analysis reports residual solvent levels against ICH Q3C. Because the hydrochloride is incompatible with strong aqueous alkali, cleaning of process equipment after saponification steps should use acidic or neutral wash sequences. Avoid holding neutralized aqueous solutions of the ester above 5 °C for extended periods, as ester hydrolysis increases.

Interpreting Residual Water and Enantiomeric Purity Under Coupling Conditions

Residual water controls activation stoichiometry in water-sensitive coupling reagents and limits premature hydrolysis of the activated ester. Incoming lots are sampled in triplicate for Karl Fischer titration. A titration relative standard deviation below 5% is considered acceptable for lot release. Enantiomeric purity is measured by chiral HPLC on an amylose-based chiral stationary phase using hexane/ethanol mobile phase; the method is verified to resolve the L- and D-enantiomers with a resolution factor of at least 1.5, as described in USP <621> system suitability. Under coupling conditions, base-induced racemization at Cα is minimized by keeping the reaction temperature at 0–5 °C and adding the base after the acylating reagent is activated. In a jacketed glass reactor, temperature excursions above 10 °C during mixed anhydride formation with isobutyl chloroformate can promote oxazolone formation and epimerization. The processing window is narrow; a deviation of ±2 °C from the set point is controlled through a recirculating chiller and thermocouple feedback. At the completion of the reaction, the mixture is quenched with cold aqueous citric acid, and the organic layer is washed with sodium bicarbonate to remove residual acid. The methyl ester is then extracted into ethyl acetate, dried over sodium sulfate, and concentrated under reduced pressure at a bath temperature not exceeding 30 °C to avoid thermal degradation.

Hydrolytic stability of L-norvaline methyl ester hydrochloride depends on the ionization state. In the solid protonated form, the methyl ester is stable under recommended storage at 2–8 °C in a sealed, desiccated container under nitrogen. When the salt is dissolved in neutral or basic aqueous media, the free amino ester is generated, and hydroxide-catalyzed saponification competes with acylation. At pH 10–12 and temperatures above 25 °C, ester cleavage is accelerated; saponification for carboxyl recovery is therefore conducted at 0–5 °C with controlled lithium hydroxide addition. The product is not recommended for aqueous coupling procedures unless a polar aprotic co-solvent is present at sufficient volume to suppress hydrolysis. Long-term exposure to ambient light is avoided by storage in amber glass or opaque bulk containers. Equipment surfaces should be cleaned with water or dilute acetic acid after product contact; the hydrochloride is corrosive to carbon steel under humid conditions, and 316L stainless steel or glass-lined equipment is preferred for process vessels. In pharmaceutical intermediate applications, residual solvent and elemental impurity documentation should align with ICH Q3C and ICH Q3D. Published toxicological data for this specific hydrochloride is limited; local exhaust ventilation, nitrile gloves, and chemical safety goggles are used during handling. When the product is shipped in fiber drums with polyethylene liners, desiccant packets are included to maintain the low-humidity environment.