BOC-L-Asparagine (Nα-tert-butyloxycarbonyl-L-asparagine, CAS 7536-55-2) enters peptide API manufacture almost exclusively in Boc-strategy solid-phase peptide synthesis where the tert-butyl carbamate protects the α-amino group under TFA-mediated deprotection cycles. On a 0.4–0.8 mmol/g PAM or MBHA resin, the compound is dissolved in anhydrous DMF at 0.15–0.30 M and charged at 3.0–4.0 mol equiv relative to resin-bound free amine. Activation is carried out with HBTU or PyBOP in the presence of HOBt or Oxyma at 1.0–1.2 equiv relative to the carboxyl group, with DIPEA at 2.0–2.5 equiv; pre-activation is limited to 2–5 min at 0–5 °C before transfer to the reaction vessel. Prolonged pre-activation with carbodiimide reagents in the absence of a coupling additive converts the unprotected Asn side-chain amide into a β-cyano-L-alanine impurity, which cannot be separated from the target peptide by standard reversed-phase C18 chromatography without modifier adjustment. Coupling proceeds for 90–150 min at 20 ± 2 °C under nitrogen with intermittent overhead stirring; a Kaiser or chloranil test on a 2–5 mg resin sample confirms residual free amine below the detection threshold, and a second coupling is performed when the threshold is not met. Filtration through a sintered-glass or PTFE-fritted 50–200 L peptide reactor removes spent DMF; batch-to-batch resin fines can reduce drain velocity and leave stagnant pockets that lower the effective washing volume by 10–15%. The assembled peptide is cleaved from the PAM or MBHA linker with liquid hydrogen fluoride containing 5–10% p-cresol and 2–5% p-thiocresol at −5 °C to 0 °C for 45–60 min, followed by precipitation in cold diethyl ether. Terminal outputs include peptide fragments for subsequent conjugation to polyethylene glycol, lipids, or carrier proteins, with the Asn residue contributing a neutral polar side chain that controls aqueous solubility and hydrogen-bonding at receptor interfaces.

When solution-phase segment condensation is preferred over SPPS for short Asn-containing fragments

Solution-phase coupling of BOC-L-Asparagine to a C-terminal free amine fragment is selected when the target peptide is shorter than five residues and the production scale does not justify the resin cost and solvent intensity of solid-phase assembly. The protected acid is dissolved in a 1:1 v/v mixture of dichloromethane and dimethylformamide at 0.10–0.25 M, cooled to −5 °C to 0 °C, and activated with EDC·HCl at 1.05 mol equiv together with HOBt·H2O at 1.10 mol equiv. N-Methylmorpholine is added in portions to maintain the aqueous-layer pH at 8.0–8.5; full dissolution of the amine salt is confirmed before mixing. The reaction is stirred for 6–18 h at 20 ± 2 °C, with LC-MS sampling at 2 h intervals; conversion below 97% at 18 h triggers addition of a second 0.5 mol equiv of activated ester prepared separately at 0 °C. During workup the mixture is washed with 0.5 M citric acid, 1.0 M sodium bicarbonate, and saturated sodium chloride, each at 5 °C to minimize hydrolysis of the tert-butyl carbamate. The organic phase is dried over anhydrous sodium sulfate, filtered through a 0.45 µm PTFE membrane, and solvent-switched to methyl tert-butyl ether; the protected fragment is isolated by vacuum filtration and dried under reduced pressure at 30 ± 2 °C. Residual DMF is controlled below 880 ppm under ICH Q3C Class 2 Option 2 because the fragment enters downstream peptide API processing. Since the Asn side chain is not orthogonally protected, the pH during bicarbonate washes is maintained below 9.0 to avoid succinimide formation; the biphasic workup is rapid when the bulk solution temperature exceeds 15 °C.

During scale-up of an Asn-rich antimicrobial peptide sequence, Boc-SPPS is selected over Fmoc-strategy synthesis because TFA-mediated N-terminal deprotection avoids the repeated piperidine exposure that drives aspartimide formation at Asn-Gly or Asn-Ser junctions. The trade-off is the requirement for hydrogen fluoride cleavage, which restricts reactor construction to polychlorotrifluoroethylene or Hastelloy C-22 lined vessels. BOC-L-Asparagine is coupled after deprotection with 30–50% TFA in dichloromethane and neutralization with 5% DIPEA; the resin is washed with DMF until the wash effluent is neutral. Coupling uses 3.0–5.0 mol equiv of BOC-L-Asparagine, 3.0–5.0 mol equiv of HBTU, and 6.0–10.0 mol equiv of DIPEA in DMF at 0.05–0.10 M for 1–2 h. A quantitative ninhydrin test on 10 resin beads gives an optical density reading at 570 nm below 0.05 AU as the acceptance criterion; higher values trigger a repeat coupling with 2.0 equiv of pre-activated monomer. The absence of piperidine in the cycle allows the Asn side-chain carboxamide to remain intact across 10–30 amino-acid couplings. Cleavage from the resin with HF containing 10% anisole at 0 °C for 60 min yields the crude peptide; the Asn residue is then available for folding in buffer systems containing 0.5–2.0 M guanidine hydrochloride or urea. Published data for this specific configuration is limited when the sequence contains internal His-Asn or Met-Asn motifs, and pilot batches are required to establish the residue-specific coupling program.

What controls batch-to-batch coupling efficiency of BOC-L-Asparagine at production scale?

At production scale, coupling yield for BOC-L-Asparagine is governed less by the intrinsic reactivity of the carbamate monomer than by trace water content, solvent amine contamination, and the particle-size distribution of the starting material. The monomer is sieved through a 40-mesh stainless steel screen and charged to the peptide reactor only after the bulk solid is pre-dried in a vacuum oven at 35–40 °C for at least 12 h when the relative humidity exceeds 60%; water content by Karl Fischer coulometric titration according to USP <921> or ASTM E203 remains below 0.5% before use. In a 100 L jacketed peptide reactor, the agitator is set to 80–120 rpm to avoid vortexing but maintain resin suspension; coupling efficiency decreases by 3–7% when the DMF used for dissolution contains more than 0.01% w/w dimethylamine or formic acid impurities. Solvent quality is monitored by GC headspace injection for dimethylamine with a limit of 0.01%. Batch-to-batch variability in the bulk density of BOC-L-Asparagine affects the accuracy of gravimetric charging; the material is therefore charged as a pre-dissolved 0.25 M solution in DMF rather than as a solid. Process development reports from multipurpose peptide CDMO campaigns indicate that resin swelling in DMF can vary by ±10% between lots of the same PAM resin, so the coupling volume is adjusted after a 30 min swelling measurement. The target concentration of the activated ester is 0.10–0.28 M; below 0.05 M the coupling rate is too slow for a 2 h cycle, and above 0.35 M the exotherm can exceed the 25 °C upper control limit during HBTU activation. Analytical release of the coupled peptide resin uses quantitative chloranil or TNBS testing, with a residual amine threshold of ≤1.5% relative to the total available sites.

Regulatory control points for BOC-L-Asparagine in peptide DMF documentation

For a Type II drug master file covering a peptide API intermediate, BOC-L-Asparagine is controlled as a starting material when it contributes the Asn residue to the protected peptide backbone before global deprotection. The control strategy is based on the risk that residual solvents from recrystallization, palladium from reduction steps upstream, and enantiomeric impurity from incomplete chiral resolution pass into the peptide API. The following control points are applied in regulatory submissions under ICH Q6A and ICH Q3C, with acceptance criteria harmonized across Ph. Eur. and USP general chapters.

The starting material specification is applied to each lot before use; when a vendor certificate of analysis already includes residual solvent and elemental impurity data, the peptide manufacturer may rely on ICH Q7 qualification of the vendor with periodic confirmatory testing at a reduced frequency.

A route to orthogonally protected L-2,4-diaminobutyric acid via side-chain nitrile reduction

Conversion of the unprotected Asn side chain into a β-nitrile proceeds through controlled dehydration with trifluoroacetic anhydride in anhydrous tetrahydrofuran; the resulting Boc-β-cyano-L-alanine is then reduced to Nα-Boc-L-2,4-diaminobutyric acid, an orthogonally protected building block for peptide chain branching and side-chain cyclization. The dehydration is performed at 0–5 °C with 2.0–2.5 mol equiv of trifluoroacetic anhydride and 2.0–3.0 mol equiv of pyridine as an acid scavenger; conversion is monitored by TLC or LC-MS until the starting material is below 2.0 area%. Direct reduction of the nitrile uses hydrogen at 0.3–0.5 MPa over Raney nickel in ethanol containing 5.0–10.0 mol equiv of ammonia at 25 ± 2 °C for 4–8 h; the ammonia suppresses secondary amine dimerization that would otherwise convert the desired primary amine into a symmetric imine. The reaction mass is filtered through a 0.45 µm membrane under nitrogen and concentrated below 30 °C to avoid partial loss of the tert-butyl carbamate. Purification by cation-exchange chromatography on a sulfonic acid resin loads the crude amine and allows elution with 0.25–0.50 M ammonium hydroxide in a water-methanol gradient. The isolated product is lyophilized and checked by chiral HPLC and 1H NMR; typical impurity profiles contain unreacted nitrile and over-reduced 2-aminobutyric acid derivative. Because this route involves pyrophoric Raney nickel and hydrogen pressure, the scale-up is limited to multipurpose hydrogenation equipment with inert gas purging and pressure relief rated for 0.7 MPa; published data for this specific configuration is limited below pilot scale.