L-Phenylalanine Methyl Ester Hydrochloride, CAS 7524-50-7, molecular formula C10H13NO2·HCl, and molecular weight 215.68 g/mol, is a protected L-phenylalanine derivative supplied as a white to off-white crystalline solid. Commercial product models are differentiated by chromatographic assay, typically 98.0%, 98.5%, and 99.0% area-normalised HPLC purity at 210 nm, rather than by formulation type. The product serves as a C-terminal methyl ester-protected amino acid salt for peptide coupling and as a chiral intermediate in pharmaceutical and dipeptide-sweetener routes. Unlike L-phenylalanine free acid, this derivative presents the carboxylate as an ester from the outset, which removes a selective esterification step and permits amide bond formation at the N-terminus after neutralization. The hydrochloride counterion also confers crystalline storage stability compared with the free amino ester, which is commonly generated in situ or held as a solution.

Which Specification Limits Are Applied to This Amino Acid Ester Hydrochloride?

Representative certificate-of-analysis limits for pharmaceutical-intermediate and peptide-synthesis grades are shown below. Individual supplier specifications differ, but the analytical methodology is typically aligned with pharmacopoeial general chapters. The specific rotation value is a key chiral identity check and should be compared against the supplier’s reference value because solvent selection influences the observed rotation.

For chiral purity, some suppliers report enantiomeric excess by chiral HPLC using a cellulose tris(3,5-dimethylphenylcarbamate) column with hexane/ethanol mobile phase, with a typical acceptance criterion of ≥99.0% ee. Published data for specific column-lot variability is limited. The methyl ester hydrochloride is generally not volatile and does not require gas chromatography for assay; HPLC with evaporative light scattering detection is also used when UV response is insufficient due to the weak chromophore.

In a neutralization and coupling workflow, the hydrochloride salt is suspended or dissolved in dimethylformamide or dichloromethane and treated with N-methylmorpholine or N,N-diisopropylethylamine at 0–5°C. The tertiary base liberates the free amino ester; the stoichiometric amount is usually 1.0–1.1 equivalents relative to the hydrochloride salt, with an additional 0.05–0.1 equivalent often omitted until pH is confirmed. After neutralization, the solution is added to a pre-activated acid of a carbodiimide coupling reagent such as N,N’-diisopropylcarbodiimide or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride with 1-hydroxybenzotriazole or ethyl 2-cyano-2-(hydroxyimino)acetate in the same solvent system. The reaction is typically maintained at 0–10°C for the first 30–60 minutes to suppress racemization, then allowed to reach 20–25°C. Completeness is assessed by reversed-phase UPLC with UV detection at 210 nm or 254 nm, depending on the chromophore of the acyl component.

Solution-Phase Amide Bond Formation with Carbodiimide Reagents

The ester-protected L-phenylalanine methyl ester participates as the amine component in peptide coupling because the methyl ester remains intact under neutral and mildly acidic conditions. Racemization during activation is a primary process risk; the presence of the methyl ester does not prevent base-catalyzed enolization of the activated acid, but the use of low-temperature pre-activation and additives such as ethyl 2-cyano-2-(hydroxyimino)acetate reduces L-to-D inversion. In pilot-scale batches, jacketed glass-lined reactors are used to maintain the stated temperature band. The neutralization exotherm is generally small, but local overheating at the point of base addition can hydrolyze the methyl ester; therefore, base is added as a dilute solution in dimethylformamide or dichloromethane over 15–30 minutes, and the pot temperature is held below 5°C during this addition. Local pH excursions above pH 8.0 accelerate methyl ester hydrolysis and should be avoided.

In addition to carbodiimide protocols, uronium salt reagents such as O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate are used with the same neutralization requirement. Activation of the acid partner is performed separately, and the hydrochloride salt is added as the neutralized amine to avoid direct contact between the free amine and the uronium reagent, which can form a guanidine by-product. This side reaction is minimised by maintaining stoichiometric base and by keeping the temperature at 0–5°C during the first 10 minutes of coupling. The product is typically charged at 1.0–1.2 equivalents relative to the acyl donor to compensate for minor moisture or amine salt losses.

After coupling, the reaction mixture is typically diluted with ethyl acetate or dichloromethane and washed with 10% aqueous citric acid, saturated sodium bicarbonate, and brine. The methyl ester remains stable in the organic layer under these mildly acidic and mildly basic wash conditions; the base wash is kept short and at 0–5°C to limit ester hydrolysis. Organic layers are dried over sodium sulfate and concentrated below 40°C under reduced pressure. This work-up sequence removes the tertiary amine hydrochloride and residual coupling reagents while retaining the C-terminal methyl ester.

As a building block for dipeptide and tripeptide synthesis, L-Phe-OMe·HCl is employed where a C-terminal methyl ester is required in the downstream intermediate. In aspartame-related routes, L-phenylalanine methyl ester is a direct condensation partner for N-protected L-aspartic acid derivatives; the hydrochloride salt is neutralized before coupling, and the methyl ester is retained until final deprotection or saponification. The product also appears in the synthesis of peptide esters for prodrug programs and other chiral amide intermediates where the C-terminus must remain blocked during repeated N-terminal modifications. Published yields for specific proprietary API routes are not available in the open literature; however, the use of C-terminal methyl ester protection is standard practice in solution-phase oligopeptide construction because it permits selective N-terminal deprotection without exposing the carboxylate to nucleophilic side reactions.

When the Hydrochloride Salt Is Compared with Free Base, D-Isomer, and N-Protected Methyl Esters

Process selection among phenylalanine derivatives turns on whether the N-terminus, C-terminus, or both require protection. L-Phenylalanine methyl ester hydrochloride differs from L-phenylalanine free acid in that the carboxylate is already protected; selective reaction at the N-terminus can therefore proceed without an esterification step. In comparison with N-Boc- or N-Fmoc-protected methyl esters, the hydrochloride salt leaves the N-terminus free for direct coupling or derivatization, eliminating the deprotection step but requiring neutralization before coupling. The counterion also alters solubility: the protonated salt is more readily handled in dry crystalline form, whereas the free amino ester is often generated in situ to avoid storage instability.

The D-isomer differs only in absolute configuration, but chiral identity is critical in peptide and pharmaceutical applications because enzymatic and receptor recognition are stereospecific; optical rotation and chiral HPLC are used to discriminate the isomers. The methyl ester hydrochloride also differs from the ethyl ester hydrochloride in ester cleavage kinetics and residual solvent profile, with the methyl ester generally hydrolysing faster under basic conditions. Process development should verify which ester remains compatible with downstream hydrogenation and deprotection steps. Salt form also influences the stoichiometry of base-sensitive steps: because the amine is protonated, reactions with acid chlorides or isocyanates require a tertiary base, whereas the free base would react directly. These differences affect both the order of addition and the localized heat load in batch reactors.

The compound is soluble in methanol and water, while the free amine liberated in situ partitions into ethyl acetate and dichloromethane. Solubility in nonpolar hydrocarbons is negligible. The salt’s melting range of 156–160°C is often reported with decomposition, so differential scanning calorimetry should be interpreted against the supplier’s reference thermogram rather than as a single sharp endotherm. Bulk density and tapped density are not standardized across suppliers but may be requested for automatic dispensing lines; typical values in powder handling equipment are not consistently published and should be measured under the intended machine conditions.

In solid-phase peptide synthesis, the methyl ester hydrochloride is not normally used as a direct resin-loading intermediate because the methyl ester is not cleaved by trifluoroacetic acid-based conditions. It is instead used in solution-phase segment condensation or to synthesize C-terminal methyl ester peptides that are later saponified with lithium hydroxide in aqueous tetrahydrofuran or methanol to generate the free acid. When saponification is performed, the reaction is carried out at 0–5°C with 1.0–1.5 equivalents of lithium hydroxide and monitored by UPLC to prevent hydrolysis of sensitive side-chain amide bonds. Published data for specific resin-bound configurations of L-Phe-OMe.HCl is limited; users should confirm compatibility of the methyl ester with the intended linker and cleavage cocktail before committing to SPPS routes.

What Process-Scale Handling Limits Are Encountered in Drying and Storage?

The hydrochloride salt is hygroscopic and should be stored in tightly sealed high-density polyethylene or glass containers with desiccant, under an inert nitrogen or argon atmosphere. Long-term supplier recommendations typically specify storage at 2–8°C, with protection from light and moisture; short-term transport at ambient temperature is acceptable when relative humidity is held below 40% and the material remains sealed. The ester linkage imposes operational boundaries: prolonged contact with aqueous alkaline solution above pH 8.0 leads to saponification, and contact with strong oxidizing agents or acid chlorides should be avoided. During final drying, vacuum tray dryers or rotary vacuum dryers should maintain product temperature below 40°C and vacuum below 10 kPa to avoid thermal discoloration. Residual moisture is best determined by Karl Fischer titration rather than loss on drying because the product contains no bound hydrate; specifications for water content are commonly set at ≤0.5%. Safety data sheets should be consulted for formulation-specific hazard classification and occupational exposure controls.

In pharmaceutical intermediate supply chains, the product is typically controlled under ICH Q7 as a starting material or intermediate depending on the route, and residual solvent limits follow ICH Q3C. Documentation should include a certificate of analysis, safety data sheet, and, where the material is invoiced as chiral, a chiral HPLC or optical rotation result to confirm enantiomeric purity.