MOC-D-phenylglycine, designated N-methoxycarbonyl-D-phenylglycine or (R)-2-(methoxycarbonylamino)-2-phenylacetic acid, is distributed under CAS registry number 50890-96-5 with molecular formula C10H11NO4 and molar mass 209.20 g/mol. The molecule carries a compact carbamate on the α-amino position of the D-phenylglycine backbone and leaves the α-carboxylic acid available for amide formation. It is supplied as a white to off-white crystalline powder and is categorized as a protected chiral building block rather than a finished pharmaceutical substance. Because the methoxycarbonyl group is smaller and less lipophilic than fluorenylmethoxycarbonyl, process handling differs from Fmoc-protected phenylglycine derivatives in solubility, chromatographic behaviour, and deprotection by-products.

Available packaging models are supplier-specific and are not described by a universal model number. Research-grade aliquots are commonly offered in 1 g, 5 g, 25 g, and 100 g units, while pilot and commercial lots are supplied in 500 g, 1 kg, and 10 kg solid packs. The research-grade release typically requires normalized HPLC purity of not less than 98.0 % at 210 nm; bulk material adds residual solvent certification, sulfated ash, and trace metal limits. The solid is shipped in vacuum-sealed polyethylene liners inside fibre drums or high-density polyethylene containers with desiccant. Storage is recommended at 2–8 °C under nitrogen or argon, and opened containers should be re-purged because the free carboxylic acid can hydrogen-bond with atmospheric moisture and shift water content upward.

Pre-drying is warranted when Karl Fischer water content exceeds 0.5 % before moisture-sensitive coupling. Material is dried in a vacuum tray dryer at 40–50 °C for 4–12 h; higher temperatures are avoided because carbamate degradation and phenylglycine racemization may accelerate in the presence of trace base. The free acid is sparingly soluble in neutral water and freely soluble in dimethylformamide and dimethyl sulfoxide; solubility in dichloromethane and tetrahydrofuran is sufficient for solution-phase coupling after mild warming. Above approximately pH 8, the carboxylic acid forms an alkali-metal or ammonium salt and aqueous solubility increases sharply.

When MOC-D-phenylglycine Replaces Cbz-D-phenylglycine in Solution-Phase Peptide Bond Formation

Replacing Cbz-protected D-phenylglycine with MOC-D-phenylglycine alters the protection and deprotection sequence. The Cbz derivative requires hydrogenolysis over palladium on carbon or transfer hydrogenation; this limits sequences containing sulfur residues, thiophene heterocycles, or other catalyst poisons. The MOC route avoids catalytic hydrogenation but introduces a base-labile carbamate that must be orthogonal to other base-sensitive groups. In a standard coupling, MOC-D-phenylglycine is activated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 1-hydroxybenzotriazole in anhydrous dimethylformamide at 0–25 °C. Phenylglycine derivatives are more racemization-prone than alanine or valine, so tertiary amine loading is kept between 1.0 equiv and 1.2 equiv, and pre-activation time is limited to below 5 min before addition of the amine nucleophile. Aqueous workup uses 5 % citric acid followed by 1 M sodium bicarbonate; maintaining the aqueous phase below pH 8.5 prevents premature carbamate hydrolysis.

The mass overhead of MOC-D-phenylglycine is lower than that of the Cbz derivative by 76.10 g/mol, and the methoxycarbonyl group presents a less complex UV chromophore. Thin-layer visualization at 254 nm is possible but weaker than for Fmoc-protected analogues; HPLC with detection at 210 nm is therefore used for reaction monitoring. When the peptide chain contains a C-terminal tert-butyl ester, the MOC group can be removed under alkaline conditions that leave the tert-butyl ester intact, whereas Cbz hydrogenolysis is also compatible with tert-butyl esters. Published solubility parameters for this specific compound in mixed solvent systems are limited; pilot adjustments are generally based on lot-specific behaviour rather than a fixed phase diagram.

What Limits Deprotection Efficiency of the Methoxycarbonyl Group Compared with Boc and Cbz?

The methoxycarbonyl group is a compact carbamate that does not follow the acid-catalysed tert-butyl cation elimination pathway of Boc. Deprotection usually requires alkaline hydrolysis, often with lithium hydroxide in tetrahydrofuran-water or sodium hydroxide in methanol-water. Reaction progress must be monitored for competing hydrolysis of any methyl or ethyl ester on the peptide backbone. The removal generates methanol and carbon dioxide; in a sealed pilot-scale reactor, the gas evolution must be considered in vent design. Unlike Cbz removal, no palladium catalyst is required, so the process avoids spent-catalyst filtration and residual palladium limits in the final intermediate. However, methoxycarbonyl hydrolysis can be slower than Fmoc removal with piperidine, and the pH window must be balanced against epimerization of the liberated D-phenylglycine residue at higher base concentrations.

Boc-D-phenylglycine is removed by trifluoroacetic acid in dichloromethane, which is fast but incompatible with acid-labile resins and tert-butyl ester protection. Cbz-D-phenylglycine is often crystalline and readily separable, but hydrogenolysis consumes a heterogeneous catalyst and may leave trace metals. MOC-D-phenylglycine is suited to orthogonal base-sensitive intermediates where strong acid or hydrogen gas is excluded. The protective group does not generate dibenzofulvene, which is a major advantage over Fmoc in solution-phase work because no hydrocarbon scavenger extraction is required.

The selection between these forms depends on the functional-group orthogonality matrix of the target peptide or β-lactam intermediate. MOC-D-phenylglycine is not recommended for Fmoc-strategy solid-phase peptide synthesis because repeated piperidine exposure can prematurely cleave the methoxycarbonyl group. It is used in solution-phase routes or in orthogonal sites where Boc and Cbz are incompatible. The lower molecular mass of the MOC derivative reduces mass load per mole of active D-phenylglycine; however, the absence of a strong UV chromophore reduces detection sensitivity and may require evaporative light-scattering detection during preparative chromatography.

Chiral purity control is stricter for MOC-D-phenylglycine than for achiral N-protected amino acids. Release testing uses an HPLC purity method derived from the general monograph USP 621. A reversed-phase C18 column with a mobile phase of phosphate buffer at pH 2.5 and acetonitrile gradient is used to separate the free acid from phenylglycine-related impurities. Detection at 210 nm is selected because the carbamate carbonyl and phenyl ring absorb weakly at higher wavelengths. Chiral purity is determined on a polysaccharide-based chiral stationary phase with a normal-phase hexane/ethanol mobile phase containing 0.1 % trifluoroacetic acid. Under these conditions, the L-enantiomer should elute with a resolution not less than 1.5, and the limit of quantitation for the undesired enantiomer is typically 0.05 %. This detection level is necessary because enantiomeric contamination in a D-phenylglycine building block can propagate through amide coupling and crystallize as a diastereomeric salt in downstream β-lactam intermediates.

Specific rotation is measured at 20 °C on the anhydrous basis using the sodium D-line according to Pharmacopoeia method 2.2.7. Unprotected (R)-phenylglycine reference material has a specific rotation of approximately −157° in 1 N hydrochloric acid; the N-methoxycarbonyl derivative has a separate, batch-specific optical rotation that should not be compared directly with the free amino acid. Because optical rotation is affected by residual solvent, water, and pH, the certificate of analysis lists the solvent and concentration used. A loss on drying value above 0.5 % can bias the observed rotation, so pre-dried material is used for the measurement. Water content is measured by coulometric or volumetric Karl Fischer titration according to ASTM E203. Residual solvents are measured by headspace gas chromatography following USP 467; common residual solvents from recrystallization are ethyl acetate and heptane, which are controlled to class 3 limits. Sulfated ash is determined by USP 281, with an acceptance criterion of not more than 0.1 % for bulk material. Trace metal analysis by ICP-MS is added when the material enters a GMP campaign; copper and iron are monitored because both can catalyse oxidative discolouration of the unprotected phenylglycine core during storage.

Acceptance criteria and supplier certificate-of-analysis parameters

The table below consolidates representative acceptance criteria reported in supplier certificates of analysis. These values are not a compendial monograph; they are typical release criteria for the free acid form and vary between manufacturers. For GMP use, the individual batch certificate remains the controlling document.

MOC-D-phenylglycine is applied mainly as a protected chiral building block for D-phenylglycine residues in semisynthetic β-lactam antibiotics, non-ribosomal peptide fragments, and chiral auxiliary intermediates. In a typical side-chain coupling, the protected acid is converted to a mixed anhydride or activated ester and reacted with the amino function of 6-aminopenicillanic acid or a 7-aminocephalosporanic acid derivative in anhydrous solvent. The α-carbamate suppresses side reactions of the free amino acid zwitterion and allows the reaction to be run in dichloromethane or dimethylformamide at −10 °C to 5 °C. The temperature is held low because phenylglycine α-protons are particularly sensitive to base; with this substrate, racemization risk is higher than with leucine or proline derivatives. Process-scale reactors are typically glass-lined or stainless steel with a nitrogen purge, and the activated acid solution is added to the nucleophile in a controlled feed to avoid local pH excursions.

The main process conflict in this application is the lability of the β-lactam ring under the alkaline conditions required to remove the methoxycarbonyl group. Deprotection is therefore carried out in a two-phase mixture at pH 10–11 with short residence time, and the liberated D-phenylglycine intermediate is rapidly extracted into an organic phase. The deprotection step releases methanol and carbon dioxide; reactor vents must be sized accordingly. In batch records, base addition is controlled by an in-line pH probe, and the temperature is maintained below 5 °C when the β-lactam substrate is particularly sensitive.

MOC-D-phenylglycine should not be confused with unprotected D-phenylglycine. The free amino acid is a zwitterion with limited solubility in aprotic coupling solvents and can initiate side reactions through the free amino group. The MOC derivative adds a methyl carbamate that neutralizes the nucleophilic amino function, raises molecular mass by 58.04 g/mol, and changes crystal habit from glassy plates to a more compact powder. The D-enantiomer differs from MOC-L-phenylglycine in both optical rotation and chromatographic retention; a chiral HPLC method is mandatory because physical appearance and achiral HPLC retention are identical. Compared with the corresponding L-enantiomer, the D-form is used when downstream biological recognition requires the (R)-configuration, as in many β-lactam side-chain syntheses.

Stored as a dry powder at 2–8 °C, the material is generally assigned a supplier retest interval of 24 months when kept under inert gas; long-term stability data for this specific configuration should be requested. Exposure to relative humidity above 60 % can increase water content, and contact with primary or secondary amines in the presence of base may trigger premature carbamate displacement before the intended coupling step. Strong aqueous acid is not recommended for storage because the methoxycarbonyl group can undergo acid-catalysed hydrolysis under harsh conditions. Combination with lithium aluminum hydride or diborane is incompatible because the carboxylic acid and carbamate groups are reactive toward strong reducing agents.

At production scale, the largest batch-to-batch variance is not usually chemical purity but particle size and water content. MOC-D-phenylglycine crystallized from ethyl acetate/heptane often has a median particle size of 50–150 µm depending on cooling rate and stirring. A fine powder dissolves more quickly in dimethylformamide, but light particles can be displaced during vacuum charging; a nitrogen-purged glovebox or contained transfer system is therefore used. Drying in a vacuum tray dryer at 40 °C for 6–10 h usually reduces water to below 0.3 %. Before a critical coupling, the material is dissolved in anhydrous solvent and filtered to remove insoluble particulates. Visible brown or yellow tint suggests thermal or metal contamination, and the lot should be re-analysed by HPLC before use.