CBZ-D-alanine (N-α-carbobenzoxy-D-alanine, N-benzyloxycarbonyl-D-alanine, Cbz-D-Ala-OH, CAS 26607-51-2) is a carbamate-protected D-amino acid employed as a masked D-alanine building block in solution-phase peptide synthesis, chiral auxiliary preparation, and active pharmaceutical ingredient intermediate manufacturing. The empirical formula C₁₁H₁₃NO₄ corresponds to a formula weight of 223.23 g·mol⁻¹. Commercial product grades include research grade, peptide synthesis grade, high-purity grade, and GMP-intermediate grade; these grades do not differ in covalent structure but in residual solvent profiles, water content, chiral impurity limits, heavy metal reporting, and documentation. Typical peptide synthesis grade material appears as a white to off-white crystalline powder with a specific rotation of −14.0° to −15.0° at 20 °C, measured at 1 g/100 mL in methanol, an HPLC area purity of ≥ 98.5%, and an enantiomeric impurity limit for Cbz-L-alanine of ≤ 0.5%.

What Analytical Release Parameters Govern Peptide Synthesis Grade Acceptance?

The acceptance profile for CBZ-D-alanine is governed by identity, purity, chiral integrity, and solvent mass balance. Because the D-configuration is the functional requirement, enantiomeric purity is treated as a release parameter rather than a periodic audit parameter. Reverse-phase HPLC with a C18 column and a phosphate-buffered acetonitrile gradient at 210 nm is a common assay platform; chiral HPLC on a ligand-exchange or macrocyclic glycopeptide stationary phase separates Cbz-D-alanine from the L-enantiomer without derivatization. Karl Fischer titration, loss on drying, and headspace gas chromatography are used to close the mass balance for the crystalline powder.

Representative specification parameters for peptide synthesis grade are listed below. Acceptance ranges vary by supplier and intended use; compendial monographs for this specific protected amino acid are not uniformly harmonized, so the following table reflects typical industrial certificate-of-analysis parameters rather than a single pharmacopoeial monograph.

Residual solvent limits require particular attention when the material is used in final API steps. Methanol, ethyl acetate, dichloromethane, and toluene may be retained in sub-percent amounts after recrystallization, and their concentrations are batch-specific rather than fixed by the molecular formula. Azeotropic drying with toluene or vacuum drying at 25–30 °C for 4–6 h is used to reduce residual methanol and water below release thresholds.

When Cbz Protection Replaces Boc or Fmoc in Chiral Intermediate Routes

The protective group chemistry of CBZ-D-alanine differs from that of Boc-D-alanine and Fmoc-D-alanine primarily at the deprotection step. Cbz is removed by catalytic hydrogenolysis over palladium on carbon, by transfer hydrogenation with ammonium formate, or by acidolysis with HBr in acetic acid. Boc is removed under acid conditions such as TFA in dichloromethane or HCl in dioxane. Fmoc is removed by secondary amines, typically 20% piperidine in DMF or DBU in DMF. These differences define the synthetic sequence in which the protected D-alanine can be introduced.

Selection of CBZ-D-alanine over the L-enantiomer is dictated solely by the required D-configuration at the α-carbon. The two enantiomers share the same molecular weight and achiral reactivity but exhibit opposite optical rotation. Substitution of Cbz with Boc changes the deprotection trigger from hydrogenation to acidolysis, while substitution with Fmoc changes the trigger to base-mediated cleavage through the fluorenylmethyl cation pathway. In routes that require an acid-stable, base-stable N-protected D-alanine that can be removed at a late stage by hydrogenolysis, Cbz provides a defined orthogonality window; however, the hydrogenolysis step is incompatible with substrates containing reducible ketones, aryl chlorides, or alkenes unless transfer hydrogenation conditions are tuned.

In a carbodiimide-mediated coupling sequence, CBZ-D-alanine can form both the symmetric anhydride and the 2-alkyl-5(4H)-oxazolone intermediate. The oxazolone α-position is labile to abstraction by tertiary amine, which can produce enantiomeric erosion of the D-alanyl residue. Suppression requires activation at 0–5 °C in the presence of HOBt or HOAt, with preactivation times typically limited to 2–5 min before addition of the amine nucleophile. Extended preactivation beyond 15 min, particularly in DMF above 10 °C, can produce measurable D/L exchange even in the presence of an auxiliary nucleophile. Published kinetic data specific to Cbz-D-alanine oxazolone racemization under every solvent combination are limited; chiral HPLC validation is therefore required before scale-up.

Thermal and Storage Boundaries for Carbobenzoxy-Protected Amino Acid Powders

On production-scale Schotten-Baumann acylation of D-alanine with benzyl chloroformate, the reaction is exothermic and pH-sensitive. Maintaining the aqueous layer at pH 8.5–9.0 with sodium hydroxide while the jacket is held at 0–5 °C limits the hydrolysis of benzyl chloroformate to benzyl alcohol and carbonate by-products. pH-stat-controlled feeding of benzyl chloroformate, typically over 45–60 min per charge, prevents foaming from carbon dioxide evolution. Equipment fouling of the pH probe by precipitated sodium chloride or potassium bicarbonate can degrade control loop response; in glass-lined reactors, periodic probe rinsing is required to maintain the setpoint. The crude product is extracted into ethyl acetate or dichloromethane, washed with dilute acid and brine, dried over sodium sulfate, and concentrated. Recrystallization from ethyl acetate/hexane or ethyl acetate/heptane yields the crystalline solid, but residual solvent retention can vary with cooling rate and drying time.

Storage is recommended at 2–8 °C in a tightly closed container under inert gas. Powder exposed to relative humidity above 60% can form agglomerates and should be dried in a vacuum oven at 25–30 °C for 4–6 h before use in water-sensitive coupling or esterification reactions. The compound is incompatible with strong mineral acids, hydrogen bromide in acetic acid, boron tribromide, iodotrimethylsilane, trichlorosilane, and catalytic palladium under hydrogen; these agents cleave the carbobenzoxy group. Contact with primary amines in polar aprotic solvents under prolonged storage is not required for deprotection but can lead to slow benzyl carbamate exchange under forcing thermal conditions.

For peptide bond formation, CBZ-D-alanine is commonly converted to the corresponding active ester or mixed anhydride before aminolysis. Mixed anhydride activation with isobutyl chloroformate and N-methylmorpholine in tetrahydrofuran at −15 °C to −10 °C provides efficient coupling to hindered amines while minimizing racemization. Active ester formation with N-hydroxysuccinimide and dicyclohexylcarbodiimide gives Cbz-D-alanine N-hydroxysuccinimide ester, which is a shelf-stable electrophile used for aqueous or anhydrous acylation. Coupling to solid-phase resins is possible, but Cbz-deprotection by palladium/hydrogen is impractical on conventional cross-linked polystyrene resins; therefore the compound is more frequently applied in solution-phase routes or as a protected intermediate for subsequent fragment condensation.

The product differentiates from Cbz-L-alanine only by the absolute configuration of the alanine α-carbon. It differentiates from Boc-D-alanine in that the Cbz group remains intact under TFA conditions, allowing acid-labile side-chain protecting groups such as tert-butyl esters or Boc groups to be removed while the N-terminal Cbz remains in place. It differentiates from Fmoc-D-alanine in that Cbz is stable to piperidine and DBU, which allows base-mediated ester hydrolysis or amide coupling without premature N-deprotection. These differences make CBZ-D-alanine a selective building block in convergent synthesis of peptide antibiotics, glycopeptide intermediates, and protease inhibitor scaffolds that demand a hydrogenolytically removable D-alanyl terminus.

GMP-intermediate grades manufactured under ICH Q7 include full audit trail, specification verification against a qualified reference standard, and extended stability evaluation at 25 °C/60% RH and 2–8 °C. For applications in the European Union, documentation should address ICH Q3C residual solvent classes and any local authorization requirements for benzyl chloroformate-derived impurities. Published toxicological data for this specific protected amino acid configuration is limited; standard chemical hygiene practices for synthetic amino acid derivatives apply, including local exhaust ventilation, nitrile gloves, and eye protection.