BOC-L-tyrosine Methyl Ester (CAS 4326-36-7; N-α-tert-butoxycarbonyl-L-tyrosine methyl ester; molecular formula C₁₅H₂₁NO₅; nominal molecular mass 295.33 g/mol) is supplied as a white to off-white crystalline powder. The compound carries an acid-labile tert-butoxycarbonyl group on the α-amine and a methyl ester on the C-terminal carboxyl, leaving the phenolic side chain unprotected. This substitution pattern is used in solution-phase peptide synthesis and in the preparation of C-terminal methyl ester tyrosine intermediates where the carboxyl must remain masked during N-protection manipulation.

Catalog model numbers for this material are generally differentiated only by fill size—10 g, 25 g, 100 g, 500 g, and 1 kg—and by packaging configuration rather than by chemical grade. A peptide synthesis grade designation indicates that the release panel includes HPLC purity, specific optical rotation, water content by Karl Fischer titration, residual solvent analysis, and chiral impurity control. The material is not intended for direct therapeutic use; it is a chemical intermediate subject to downstream qualification according to the user’s process.

What Release Specifications Govern a Peptide Synthesis Grade Lot?

Release protocols for BOC-L-tyrosine methyl ester are built around compendial analytical methods. HPLC purity is measured by reversed-phase C18 separation with UV detection at 220 nm; a typical acceptance limit is ≥98.0% area. The monoisotopic mass is 295.142 Da, and positive-ion electrospray ionization shows [M+H]⁺ at 296.149 Da and [M+Na]⁺ at 318.131 Da. Specific optical rotation is release-controlled by polarimetry at the sodium D-line, with typical lot values falling at [α]_D²⁰ = -7.0° ± 1.0° (c = 1, CH₃OH). Karl Fischer water limit is ≤0.5% w/w per USP <921> or Ph. Eur. 2.5.12. Residual solvents are quantified by headspace gas chromatography against USP <467>; common controls are methanol ≤1000 ppm and dichloromethane ≤600 ppm. Melting range is 93–98°C by capillary method. Enantiomeric impurity, where the D-tyrosine derivative is separated on a chiral stationary phase, is controlled to ≤0.5% area.

The HPLC method uses a C18 column, 150 mm × 4.6 mm, 5 µm particle size, with mobile phase A consisting of 0.1% TFA in water and mobile phase B consisting of 0.1% TFA in acetonitrile. Gradient elution from 20% B to 90% B over 20 min separates BOC-L-tyrosine methyl ester from the free amine and from more polar process impurities. UV detection at 220 nm monitors the tyrosine aryl chromophore. Residual solvent analysis uses a DB-624 column, 30 m × 0.32 mm, 1.8 µm film, with headspace equilibration at 80°C for 30 min. Method quantification limits are typically 50 ppm for methanol and 100 ppm for dichloromethane.

In solution-phase assembly, BOC-L-tyrosine methyl ester is used as a C-terminal blocked tyrosine source. The Boc group is removed before coupling by acidolysis with 20–30% TFA in dichloromethane at 0–25°C for 1–2 h, or with 4 M HCl in dioxane to precipitate the HCl salt. The methyl ester remains intact under these conditions when water is excluded; therefore anhydrous TFA or HCl/dioxane is preferred. After deprotection, the amine salt is neutralized with N-methylmorpholine or diisopropylethylamine in an ice-jacketed vessel at -5°C to 5°C and coupled immediately with a carboxyl-activated peptide fragment. Carbodiimide reagents such as DIC/HOBt or EDC/HOBt in DMF or DCM are used at 0–25°C; preactivation of 1-hydroxybenzotriazole esters reduces racemization at the tyrosine α-carbon because the N-protected urethane suppresses oxazolone formation relative to free amine conditions.

During acidolytic deprotection, the tert-butyl cation generated from the Boc group can alkylate the unprotected tyrosine phenol. To suppress this, 2–5% triisopropylsilane or 5–10% anisole is added to the TFA/DCM cleavage cocktail. The resulting H-Tyr-OMe is isolated as the TFA salt and used immediately. Failure to include a scavenger results in tert-butylated tyrosine impurities detectable by RP-HPLC at relative retention times of approximately 1.1–1.3 relative to the main product.

On a production-scale line, a glass-lined reactor fitted with a high-efficiency condenser and nitrogen sweep is charged with the Boc-protected building block. TFA is added below liquid surface with stirring, and the exotherm is controlled below 20°C. After deprotection, neutralization is conducted under a nitrogen blanket, and the resulting H-Tyr-OMe is transferred through a jacketed PTFE-lined hose to the coupling reactor to avoid moisture uptake. In campaigns producing 1–5 kg of oligopeptide intermediate, solution clarity is maintained by filtering through a 0.45 µm polypropylene cartridge before acylation. Coupling progression is monitored by RP-HPLC at 220 nm, with a target residual free amine below 2.0% area before the next acylation or wash cycle. Published kinetic data for this specific configuration is limited; therefore, for non-routine solvent systems, stability-indicating HPLC is performed at pre-determined time points to assess methyl ester hydrolysis and epimerization.

In a typical synthesis of BOC-L-tyrosine methyl ester, L-tyrosine methyl ester is acylated with di-tert-butyl dicarbonate in the presence of triethylamine or sodium bicarbonate in aqueous tetrahydrofuran. The reaction is cooled to 0–5°C to reduce racemization, and the product is extracted into ethyl acetate, washed with 1 M citric acid, saturated sodium bicarbonate, and brine, then dried over sodium sulfate. Crystallization from ethyl acetate/heptane or methyl tert-butyl ether/heptane gives a white solid with typical yields of 85–92% before recrystallization. Identity is confirmed by ¹H NMR and ¹³C NMR; characteristic signals include a tert-butyl singlet near 1.42 ppm and a methyl ester singlet near 3.75 ppm in CDCl₃, with the Boc carbonyl near 155.0 ppm and ester carbonyl near 171.5 ppm in ¹³C NMR. FTIR typically shows ester carbonyl absorption near 1740 cm^-1, carbamate carbonyl near 1685 cm^-1, and phenolic O–H stretch near 3300 cm^-1.

When Methyl Ester Protection Is Preferred Over Free Acid in Fragment Coupling Routes

Compared with BOC-L-tyrosine, the methyl ester form has materially different solubility and reactivity. BOC-L-tyrosine is a free acid that requires separate carboxyl activation, whereas the methyl ester is already masked at the C-terminus and therefore avoids mixed anhydride side reactions during N-deprotection. DCM and ethyl acetate solubility are higher for the methyl ester than for the free acid, which supports homogeneous coupling in low-polarity solvents and simplifies aqueous workup. The methyl ester also avoids diketopiperazine formation during dipeptide cyclization because the C-terminal carboxyl is not available for intramolecular aminolysis under mildly basic neutralization. This difference is important when preparing tyrosine-containing dipeptide fragments by solution-phase methods.

The methyl ester group can be removed later by saponification with LiOH or NaOH in aqueous tetrahydrofuran, but saponification is pH- and temperature-dependent. In neutral anhydrous DMF at 25°C, methyl ester loss is typically below 2% over 24 h; in aqueous alkali above pH 10, hydrolysis proceeds rapidly. For this reason, the compound is not held in aqueous alkaline media, and neutralization of HCl or TFA salts is performed immediately before coupling. The free phenol of tyrosine is normally not protected in standard coupling; however, if a downstream sequence requires prolonged reaction at temperatures above 50°C or exposure to strongly nucleophilic bases, temporary protection of the phenol as a tert-butyl ether or silyl ether should be evaluated by stability-indicating HPLC.

Compared with the ethyl ester analogue, the methyl ester of BOC-L-tyrosine has a lower molar mass and typically higher crystallinity. Saponification to the free carboxylic acid proceeds more readily in 0.5 M LiOH in THF/water 3:1 at 0°C. The ethyl ester may be selected when the ester is retained through a later synthetic stage because it can reduce volatility loss during solvent evaporation. Published kinetic data for the specific saponification of BOC-L-tyrosine methyl ester in all solvent systems is limited; a stability-indicating HPLC check is standard before scaling any hydrolysis step.

Storage Stability, Moisture Uptake, and Incompatibility Boundaries

Desiccated storage at 2–8°C is recommended. The bulk solid should be kept in amber glass or fluoropolymer-lined aluminum foil under argon or nitrogen. When relative humidity exceeds 60%, handling should occur under dry nitrogen, and the material should be pre-dried in a vacuum desiccator over phosphorus pentoxide at room temperature for 12 h if water uptake is suspected. Adsorbed moisture does not immediately hydrolyze the methyl ester in the solid state, but it may reduce the quality of the subsequent acidolytic deprotection by generating aqueous acid microenvironments that promote ester cleavage. Avoid storage with open containers of concentrated hydrochloric acid because HCl vapor can cleave the Boc group on the powder surface.

BOC-L-tyrosine methyl ester is incompatible with strong oxidizing agents, strong reducing agents such as LiAlH₄, and strong aqueous acids or bases. The Boc group is removed by TFA or HCl/dioxane, not by piperidine or morpholine, which distinguishes it from Fmoc derivatives. The methyl ester is reduced by hydride reagents to the corresponding alcohol, and the carbamate is reduced or cleaved under forcing hydride conditions. The product is for research and development use; no USP or EP monograph exists specifically for this intermediate, so release is governed by the manufacturer’s internal specifications and the user’s qualified analytical methods.

Differences from Fmoc-L-tyrosine methyl ester are defined by the orthogonal protecting group. Fmoc is base-labile and is removed with 20% piperidine in DMF in 5–20 min, while Boc is acid-labile. A synthetic route using Boc-Tyr-OMe can be carried through basic aqueous washes that would deprotect an Fmoc amine. Conversely, a route using Fmoc-Tyr-OMe can tolerate acidic conditions that would remove Boc. Cbz-L-tyrosine methyl ester is removed by catalytic hydrogenation over palladium on carbon at 1–4 atm H₂ and 20–25°C, or by HBr/AcOH; this is less compatible with substrates containing reducible groups. H-L-Tyr-OMe hydrochloride lacks an N-protecting group and must be neutralized before use; its DCM solubility is low without an added base, and direct acylation may be accompanied by higher epimerization risk than the Boc-protected derivative.

The unprotected phenolic hydroxyl of BOC-L-tyrosine methyl ester remains compatible with carbodiimide-mediated coupling and does not require protection for standard acylation at 0–25°C. The product is generally not combined with amine-based condensation additives in the absence of a carboxyl component, because such mixtures can promote N-acyl transfer or accelerate decomposition of activated esters. When the title material is converted to H-Tyr-OMe hydrochloride, immediate coupling is recommended; isolated amine salts stored beyond 24 h at room temperature may show increased moisture uptake and should be re-analyzed by HPLC before use.

The material is less suited to solid-phase peptide synthesis as a resin-loaded monomer because the C-terminal methyl ester is not reactive with aminomethyl or hydroxymethyl resin handles. In solution-phase fragment assembly, however, this ester masking allows selective N-deprotection and coupling without premature C-terminal activation. The absence of a free carboxyl also avoids charge formation that can complicate phase transfer in dichloromethane/water systems. These features differentiate BOC-L-tyrosine methyl ester from both the free acid and the unprotected methyl ester hydrochloride, and account for its selection in stepwise peptide chain extension where C-terminal retention is required until a specific deprotection point.

When the methyl ester and Boc group are both present, aqueous workup must avoid pH above 9 and strong acid above 1 M unless intentional deprotection is desired. The phenolic hydroxyl can form phenolate under basic conditions, increasing aqueous solubility and causing slight red coloration in strongly alkaline solution. Acidic washes must be brief because the Boc group is progressively cleaved in aqueous TFA or concentrated HCl. These operational boundaries are incorporated into standard batch records as pH controls and hold-time limits. If the product is exposed to a non-validated solvent mixture or heated above 40°C during drying, HPLC purity and water content should be rechecked before release for a subsequent coupling step.