CBZ-L-asparagine (Nα-carbobenzyloxy-L-asparagine; synonym Z-Asn-OH) is a protected amino acid used as an N-terminal building block in solution-phase peptide synthesis. Its CAS registry number is 2304-96-3, molecular formula C₁₂H₁₄N₂O₅, and molar mass 266.25 g mol⁻¹. The material is supplied as a white to off-white crystalline powder; typical release specifications include an HPLC area-percent assay of ≥98.0%, a melting range of 160–166 °C, and specific rotation [α]D20 of −5.0° to −7.0° at c=1 in dimethylformamide, measured against the optical rotation method class Ph. Eur. 2.2.7. No harmonized commercial model number exists across suppliers; the product is commonly ordered by the code Z-Asn-OH with a purity suffix, for example “Z-Asn-OH, ≥98%, peptide synthesis grade.” Storage is specified at 2–8 °C in a tightly closed container under inert gas, with desiccant, to limit hydrolytic degradation and moisture uptake. The α-amino group carries the benzyloxycarbonyl urethane, while the β-carboxamide side chain remains unprotected. This pattern makes the product stable to anhydrous basic conditions and to short trifluoroacetic acid exposure, but removable by catalytic hydrogenation or by hydrogen bromide in acetic acid.

Lot-to-lot variability in commercial CBZ-L-asparagine is largely determined by residual solvent content, particle size distribution, and optical purity. Materials dried in vacuum tray dryers at 40 °C under 1 kPa can show residual dimethylformamide levels from 0.01% to 0.05%; when residual solvent exceeds the agreed limit, downstream crystallization is complicated by solvent carry-over, and the resulting protected peptide may require additional reslurry operations. This variability is handled in production by dry-weight correction and by confirmation of chromatographic retention time against a qualified reference standard.

What Limits Solubility and Handling of Cbz-L-Asparagine in Anhydrous Coupling Media?

The compound dissolves readily in dimethylformamide and dimethyl sulfoxide at 0.1–0.5 M at 20–25 °C; solubility in methanol and ethyl acetate is lower, and aqueous solubility is poor unless the pH is raised to form the carboxylate salt. For carbodiimide-mediated couplings, moisture is the primary handling variable. Karl Fischer water content above 0.5% alters activation stoichiometry, consumes N,N′-dicyclohexylcarbodiimide, and can reduce coupling yield by 5–15% in batch runs. Pre-drying is therefore performed under vacuum at 40 °C for 12–18 h over phosphorus pentoxide or silica gel desiccant when anhydrous activation is required.

On pilot-scale peptide lines, dissolution in dimethylformamide is commonly carried out in jacketed glass reactors with nitrogen blanketing. Filtration through a 0.45 µm PTFE filter is applied to remove trace insoluble particulates before transfer to the activation vessel. Residual solvent analysis by headspace gas chromatography, method class Ph. Eur. 2.2.28, is used to confirm that dimethylformamide and toluene levels in the final protected peptide remain below process-specific limits; however, published data for this specific configuration is limited and release limits must be set against the final peptide monograph. Moisture ingress during extended production campaigns is a recognized bottleneck, because hygroscopic uptake can occur even through partly sealed drum liners in humid manufacturing areas.

In typical solution-phase assembly, Cbz-L-asparagine is activated at the carboxyl group for reaction with an amino-terminal peptide or amine. Two activation routes are industrially common: mixed anhydride formation with isobutyl chloroformate and N-methylmorpholine in tetrahydrofuran at −15 °C to 0 °C, and carbodiimide activation with 1-hydroxybenzotriazole as racemization suppressant. The unprotected β-carboxamide side chain introduces a defined side reaction. Prolonged activation with N,N′-dicyclohexylcarbodiimide at temperatures above 0 °C can dehydrate the side-chain amide to β-cyanoalanine; process definitions therefore limit DCC activation time to 20–40 min before addition of the amine nucleophile and maintain the jacket at −10 °C to 0 °C. Coupling completion is monitored by thin-layer chromatography or by HPLC using method class Ph. Eur. 2.2.29, with UV detection at 214 nm where the Cbz chromophore and peptide bond absorb. The product is not a polymer or resin; it is a low-molecular-weight crystalline intermediate, and melt processing, extrusion, or molding terminology does not apply.

A representative solution-phase protocol uses mixed anhydride formation with isobutyl chloroformate at −10 °C to −5 °C in tetrahydrofuran, followed by coupling to the amino component over 2–4 h while the pH is maintained at 8–8.5 with N-methylmorpholine. After coupling, the protected peptide is extracted, washed with sodium bicarbonate and citric acid, dried over sodium sulfate, and concentrated by rotary evaporation. Column chromatography or recrystallization from ethyl acetate/hexane mixtures is used for purification. The Cbz group prevents racemization at the activated carboxyl and masks the α-amino group during activation.

Hydrogenation Equipment Operating Windows for Cbz Removal

Deblocking of the Cbz group is performed by catalytic hydrogenolysis over palladium on carbon, or by acidolysis with hydrogen bromide in acetic acid. For hydrogenolysis, the protected peptide is dissolved in methanol, methanol-water, or dimethylformamide-water, and charged to a stainless-steel hydrogenation autoclave with a hydrogen partial pressure of 0.2–0.4 MPa and a jacket temperature of 20–40 °C. Palladium on carbon, typically 5–10 wt% dry basis relative to substrate, is slurried under nitrogen before hydrogen is introduced. Reaction time typically falls between 2 h and 8 h; reaction progress is followed by disappearance of the starting protected peptide by HPLC method class Ph. Eur. 2.2.29. The Cbz group is converted to toluene and carbamic acid, the latter decarboxylating to the free amine. Filtration through a depth-media pad or Celite is required to reduce palladium fines; batches that bypass catalyst filtration show elevated palladium residues in the isolated peptide and irregular deprotection at scale.

Equipment-specific failure modes include catalyst poisoning by residual sulfur compounds and incomplete hydrogenation at low hydrogen mass transfer. For production vessels without a gas-dispersing impeller, hydrogen uptake may be limited by the gas-liquid interface; in such vessels the hydrogen partial pressure is raised stepwise and the agitator speed is set to maintain a stable gas-liquid dispersion. Acidolytic removal with hydrogen bromide in acetic acid at 0–25 °C is an alternative when substrate unsaturation or other reducible groups cannot tolerate hydrogenation. The acidolytic route generates benzyl bromide as a side stream and requires corrosion-resistant glass-lined or fluoropolymer-lined equipment. Batch-to-batch variance in hydrogenation time often correlates with residual moisture in the protected substrate and with catalyst age; used palladium catalyst may require re-slurry or additional charge.

Release testing for CBZ-L-asparagine follows supplier-specific specifications because no dedicated monograph exists in the major pharmacopeias. The table below compiles the commonly reported acceptance criteria and the corresponding general method classes used by peptide-grade suppliers. Limits are not to be read as official compendial standards, but they represent the purchase-specification range seen across catalog listings and certificates of analysis.

When Fmoc-L-Asparagine Is Replaced by the Cbz Congener in a Fragment Condensation Scheme

The Cbz derivative differs from Fmoc-L-asparagine and Boc-L-asparagine in the chemistry of its α-amino protecting group. Fmoc is removed rapidly by secondary amines such as piperidine, making Fmoc-L-asparagine the default for solid-phase peptide synthesis; Cbz is stable to piperidine and therefore unsuitable for Fmoc-type SPPS cycles. Boc is removed by trifluoroacetic acid, while Cbz survives short trifluoroacetic acid exposure and is removed by hydrogenolysis or strong acid. This orthogonality allows Cbz-L-asparagine to be used in fragment-condensation schemes in which a Boc or tert-butyl ester protecting group must remain intact during Cbz removal, or in which hydrogenation of the Cbz group is carried out on a substrate that is not compatible with piperidine.

The table below summarizes the comparative behavior of the main protected asparagine building blocks. The information is compiled from standard solid- and solution-phase peptide synthesis practice; the user must verify the specific stability window for the target sequence because neighboring side-chain groups can shift deprotection rates.

A direct substitution of Fmoc-L-asparagine with Cbz-L-asparagine in the same solid-phase reactor is not feasible because the Cbz group is not removed by the piperidine cycle. Substitution is instead considered for solution-phase or hybrid fragment approaches. The benzyloxycarbonyl group is smaller than the fluorenylmethoxycarbonyl group and does not exhibit the strong dibenzofulvene UV signal used for Fmoc loading; Fmoc removal is commonly monitored at 301 nm, whereas Cbz-protected intermediates are more commonly followed at 214 nm by amide absorption. The standard Cbz-L-asparagine has a free β-carboxamide side chain; compared with side-chain-protected derivatives such as trityl- or xanthyl-protected asparagine, the unprotected side chain simplifies final deprotection but increases the risk of β-cyanoalanine formation under prolonged carbodiimide activation. If the target sequence requires long activation times or higher coupling temperatures, a side-chain-protected congener is typically selected.

Operational boundaries must include the β-amide side chain and the Cbz group stability limits. The product should not be exposed to hydrogenation conditions when the target molecule contains sulfur-containing residues that poison palladium; benzyl ether or benzyl ester protecting groups will also be removed or reduced under the same hydrogenolysis conditions. Avoid combination with strong nucleophiles that can open the urethane, and avoid prolonged storage above 25 °C or at relative humidity above 60%. For sequences containing methionine or cysteine, acidolytic removal with hydrogen bromide is often selected to avoid palladium catalyst deactivation. Residual water must be controlled before carbodiimide activation, and free amines should not be stored in the same container because carbamate exchange can occur at elevated temperature. Published data for this specific configuration is limited for long-term humidity stress; therefore, retest after 24 months is recommended when stored under the specified conditions.