| HS Code | 834437 |
| Product Name | BOC-D-tryptophan |
| Cas Number | 5241-64-5 |
| Molecular Formula | C16H20N2O4 |
| Molecular Weight | 304.34 g/mol |
| Iupac Name | (2R)-2-[(tert-butoxycarbonyl)amino]-3-(1H-indol-3-yl)propanoic acid |
| Appearance | White to off-white crystalline powder |
| Melting Point | 136-139 °C |
| Optical Rotation | [α]D^20 = -19.0° (c=1, DMF) |
| Solubility | Soluble in DMF, DMSO, methanol, ethanol, ethyl acetate; sparingly soluble in water |
| Storage Condition | Store at 2-8 °C, under inert atmosphere, protected from moisture |
| Smiles | CC(C)(C)OC(=O)N[C@H](Cc1c[nH]c2ccccc12)C(=O)O |
| Synonyms | Boc-D-Trp-OH; N-Boc-D-tryptophan; (R)-2-[(tert-butoxycarbonyl)amino]-3-(1H-indol-3-yl)propanoic acid |
As an accredited BOC-D-tryptophan factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 25 g of BOC-D-tryptophan in a glass bottle, sealed under nitrogen, stored cool and dry, protected from light. |
| Container Loading (20′ FCL) | BOC-D-tryptophan loaded in 20′ FCL: drums palletized and secured, avoiding moisture and heat, ensuring safe transport. |
| Shipping | BOC-D-tryptophan is shipped as a white crystalline powder in sealed, light-protected containers. Transport at ambient temperature, away from moisture and strong oxidizers. Ensure upright handling to prevent breakage. Not classified as dangerous goods under normal shipping conditions, but avoid inhalation and skin contact during handling. |
| Storage | Store BOC-D-tryptophan in a tightly sealed, light-protected container at –20°C, preferably under inert gas. Keep away from moisture and heat, and use a desiccant if necessary. Avoid repeated freeze-thaw cycles; allow the vial to warm to room temperature before opening to prevent condensation. Ensure the container is clearly labeled and compatible for long-term storage. |
| Shelf Life | Store at -20°C, desiccated, and protected from light; shelf life is typically up to 2 years. |
Solution-phase fragment condensation campaigns for D-tryptophan-containing peptide drugs rely on Boc-D-tryptophan (CAS 5241-64-5, MW 304.34 g/mol) as a pre-protected chiral building block with a free C-terminal carboxy group. Process-scale batches are pre-dried at 25–30 °C under vacuum for 4–8 h when ambient relative humidity exceeds 60%, because residual water alters carbodiimide activation. The acid is dissolved in anhydrous DMF with water content ≤0.05% by Karl Fischer, cooled to 0–5 °C, and activated with 1.0–1.2 eq of DIC or EDC in the presence of 1.0–1.2 eq of HOBt or 6-Cl-HOBt and 1.0–1.15 eq of N-methylmorpholine. Preactivation is held for 10–15 min at −5 to 0 °C to limit oxazolone formation before addition to a protected amino acid ester such as D-Lys(Boc)-OBzl or Lys(Boc)-OSu in DMF. The batch is warmed to 20–25 °C over 30–60 min and maintained under nitrogen for 12–24 h. This sequence is used when the D-Trp residue must be placed adjacent to a hindered lysine or ornithine residue in somatostatin analog fragments, where late-stage racemization at the D-Trp α-carbon would generate the L-Trp diastereomer and alter receptor-binding performance. Disappearance of Boc-D-tryptophan is followed by HPLC at 280 nm; before workup, conversion is typically greater than 98.5%. The reaction is quenched with 10% w/w citric acid at 0–5 °C, extracted into ethyl acetate, washed with 5% w/v sodium bicarbonate and 15% w/v sodium chloride, then concentrated. Solvent exchange into methyl tert-butyl ether followed by n-heptane addition precipitates the protected dipeptide as a crystalline solid, which is dried under vacuum at 30–35 °C until loss on drying is ≤0.50% by USP ⟨731⟩. Chiral purity is checked by HPLC on an amylose-based chiral stationary phase and is expected to be ≥99.0 area%; residual Boc-D-tryptophan in the isolated fragment is limited to ≤0.10 area%. The output is an advanced peptide intermediate under ICH Q7 control and is not released into the next coupling step until optical rotation and assay are confirmed.
In Boc-chemistry solid-phase synthesis, Boc-D-tryptophan is used when the target peptide is assembled on acid-stable benzyl-based side-chain protection and when Fmoc/tBu chemistry would create an incompatible protection scheme. The resin is either a phenylacetamidomethyl resin for peptide acids at 0.3–0.8 mmol/g loading or a 4-methylbenzhydrylamine resin for C-terminal peptide amides at 0.4–0.7 mmol/g. Dry resin is loaded into a jacketed solid-phase reactor with a sintered PTFE frit and swelled in DCM at 3–5 mL per gram for 30 min, followed by DMF for 20 min at 20–25 °C. Nα-Boc deprotection is performed with 50% v/v TFA in DCM containing 1% v/v anisole and 2% v/v dimethyl sulfide, using two short flow washes of 1 min and one batch treatment of 30 min; the scavenger combination reduces tert-butyl cation alkylation of the indole ring. Neutralization with 5% v/v DIPEA in DCM is followed by DMF washes. Boc-D-tryptophan is coupled at 2.0–4.0 eq relative to free amine using PyBOP/HOBt or HATU with 4.0–8.0 eq DIPEA in NMP or DMF for 45–90 min. Hindered preceding residues require double coupling and acetic anhydride/pyridine capping to terminate unreacted chains. The Kaiser ninhydrin test is run after each coupling; residual blue color above background indicates free primary amine and triggers an additional fast coupling with 2.0 eq of the activated amino acid. Cleavage from the resin is carried out with liquid HF containing 10% v/v p-cresol and 5% v/v thioanisole at −5 to 0 °C for 60–90 min. Alternative TFMSA/TFA/DMS mixtures are used only when HF equipment is unavailable, but they produce higher indole alkylation byproducts unless the scavenger ratio is increased. Crude peptide is precipitated in cold diethyl ether, filtered, and purified by preparative RP-HPLC on a C18 column with aqueous 0.1% TFA/acetonitrile gradients. Fractions containing the D-Trp-bearing peptide are pooled and lyophilized. Production failure modes observed on manufacturing lines include incomplete MBHA resin swelling after long storage, TFA dilution drift in deprotection cycles, and racemization when DIPEA is added too rapidly during activation of the D-amino acid. The lyophilized peptide is released under ICH Q7 when destined as a peptide API intermediate.
The indole nitrogen of Boc-D-tryptophan has a pKa near 17 and is not appreciably acylated under carbodiimide/HOBt coupling conditions at pH 7.5–8.5, so N-acylation of the indole is rarely the primary side reaction. The main side-chain risk in downstream processing is electrophilic attack on the electron-rich indole ring by the tert-butyl carbocation released during Nα-Boc removal. In TFA/DCM deprotection without scavenger, tert-butyl alkylation at C2 or C5 of the indole produces a characteristic set of late-eluting impurities detected by HPLC at 280 nm. For this reason, solution-phase deprotection of Boc-D-Trp intermediates is performed in acidolysis cocktails containing 1–5% v/v triisopropylsilane, anisole, thioanisole, or 1,2-dimethoxybenzene; dodecanethiol is added when strong trifluoromethanesulfonic acid or HF is present. The indole ring is also sensitive to autoxidation in peroxide-containing ether solvents and to photodegradation under uncontrolled light. Manufacturing batches of the protected amino acid are therefore stored at 2–8 °C under argon in amber containers. During coupling, bases stronger than tertiary amines are avoided because the D-configuration at the α-carbon can undergo base-catalyzed enolization; sodium hydroxide during aqueous workup of activated esters is replaced by sodium bicarbonate or potassium hydrogen sulfate to maintain pH below 8.5. DMF and NMP are preferred coupling solvents because DMSO can slowly oxidize the indole ring at elevated temperature, and chlorinated solvents do not provide adequate solubility for long peptide fragments. Low-moisture DMF, minimal base load, and short preactivation times reduce oxazolone formation to a level at which chiral HPLC typically shows ≤0.20% of the unwanted L-Trp diastereomer in fragment condensations.
Acid-catalyzed methyl esterification of Boc-D-tryptophan is performed by dissolving the free acid in methanol at −5 to 0 °C and adding thionyl chloride 2.0–3.0 eq dropwise while maintaining the internal temperature below 5 °C. The mixture is warmed to 20–25 °C and stirred for 12–16 h, after which TLC or HPLC confirms complete consumption of starting material. Methanol is removed under vacuum at 30 °C, and the residue is partitioned between ethyl acetate and water; the organic layer is washed with 5% sodium bicarbonate and brine, then concentrated to give Boc-D-Trp-OMe as a pale oil or low-melting solid. This ester is prepared as an entry point to D-tryptophan methyl ester hydrochloride, which is a chiral building block for peptide drug candidates and is released only after Nα-Boc deprotection with 4 M HCl in dioxane at 20 °C for 2–3 h. Amidation is performed by converting Boc-D-tryptophan to a mixed anhydride with isobutyl chloroformate and N-methylmorpholine in THF/DMF at −15 °C for 15–20 min, followed by addition of aqueous ammonia or the target primary amine. The residual isobutyl carbonate byproduct is removed by acid–base extraction, and the protected amide is crystallized from ethyl acetate/heptane. Both sequences preserve the D-configuration if the mixed anhydride temperature does not exceed −10 °C during formation; at higher temperatures the oxazolone can rearrange and cause measurable L-tryptophan contamination in the final intermediate.
For release of Boc-D-tryptophan as a regulatory starting material under ICH Q11, a full analytical profile controls low-level process solvents that would otherwise carry into the first synthetic step of the peptide API and increase downstream purification burden. Residual methanol, if used in crystallization or esterification, is controlled to ≤3000 ppm as a Class 2 solvent under USP ⟨467⟩; ethyl acetate is controlled to ≤5000 ppm as a Class 3 solvent; toluene and n-hexane are controlled to ≤890 ppm and ≤290 ppm respectively when present. The analytical method is headspace gas chromatography with flame-ionization detection on a DB-624 column using water or DMSO as matrix diluent. Loss on drying is typically ≤0.50% by USP ⟨731⟩, and residue on ignition is ≤0.10% by USP ⟨281⟩. Heavy metals are reported according to ICH Q3D risk assessment; when required by older monographs, the limit is ≤10 μg/g using EP 2.4.8. Optical rotation is measured at 589 nm in methanol and compared to the lot-specific reference value; a broad acceptance range may be +17.0° to +19.0° depending on solvent and concentration, but the exact range is fixed in the vendor specification. HPLC assay on an anhydrous, solvent-free basis is usually 98.0–101.5%; chiral purity is ≥99.0% by HPLC using a chiral stationary phase suitable for amino acid enantiomers. These controls are embedded into the batch release and APQR review for peptide API manufacture and are used to reject lots showing drifting optical rotation or elevated residual DMF before the material enters the peptide bond-forming step.
| Attribute | Method | Typical acceptance criterion |
|---|---|---|
| Appearance | Visual inspection | White to off-white crystalline powder |
| Identification | IR / EP 2.2.24 | Matches reference spectrum |
| Specific rotation | EP 2.2.7, 589 nm, methanol | +17.0° to +19.0° lot-specific |
| HPLC purity | UV detection at 280 nm | ≥98.5 area% |
| Chiral purity | Chiral HPLC | ≥99.0% |
| Loss on drying | USP ⟨731⟩ | ≤0.50% |
| Residue on ignition | USP ⟨281⟩ | ≤0.10% |
| Residual solvents | Headspace GC / USP ⟨467⟩ | Methanol ≤3000 ppm; ethyl acetate ≤5000 ppm |
| Heavy metals | EP 2.4.8 / ICH Q3D | ≤10 μg/g if required |
| Assay | HPLC external standard, anhydrous basis | 98.0–101.5% |
During peptidomimetic protease inhibitor synthesis, Boc-D-tryptophan is used as a masked α-amino aldehyde precursor because the Boc group tolerates basic and reductive conditions but can be removed orthogonally in acid without cleaving the indole. The free acid is first converted to the Weinreb amide by treatment with N,O-dimethylhydroxylamine hydrochloride, EDC or DIC, HOBt, and DIPEA in DMF at 0–25 °C for 8–16 h. The resulting Boc-D-Trp-N(OMe)Me is isolated by ethyl acetate extraction and is stable enough for long storage at −20 °C. Reduction with DIBAL-H in THF or toluene at −78 °C for 30–60 min yields Boc-D-tryptophanal; this aldehyde must be used immediately or stored under argon at −70 °C for no more than 48 h because it undergoes oligomerization and epimerization at room temperature. Temperature control during DIBAL reduction is the main process parameter: excursions above −65 °C lead to over-reduction to the corresponding alcohol and lower the yield of the chiral aldehyde to below 80%. In downstream Horner-Wadsworth-Emmons olefination, the aldehyde is coupled to a phosphonate at −78 °C with tetramethylguanidine or DBU; the resulting α-amino acid-derived olefin is used to construct hydroxyethylene isosteres for aspartyl protease inhibitors. Batches processed in this way are monitored by chiral HPLC and by aldehyde content through derivatization with 2,4-dinitrophenylhydrazine, because the chiral aldehyde itself is poorly retained on reversed-phase columns.
Boc-D-tryptophan is supplied to automated peptide synthesizers as a 0.2 M solution in DMF after filtration through a 0.22 μm membrane and degassing under reduced pressure. In parallel synthesis of D-Trp-containing peptide libraries for receptor-binding screens, the building block is delivered through the arm lines of a multi-channel synthesizer and activated in situ with HOBt/DIC or HATU/DIPEA. Coupling efficiency is measured by quantitative ninhydrin or by subsequent loading when the synthesizer is run in hybrid Boc/Fmoc mode; a target coupling efficiency of ≥99.0% per step is expected for Boc-D-tryptophan to avoid deletion peptides that cannot be separated from the full-length product on C18 preparative HPLC. Throughput bottlenecks arise from the low solubility of Boc-D-tryptophan in DCM at high concentration, which forces DMF/NMP as the sole coupling solvent and slows drainage from resin frits. The material should be pre-dried under vacuum at room temperature before dissolution because residual water reacts with HATU and lowers activation efficiency; when relative humidity exceeds 60%, bottle open time should be limited to 15 min. For discovery-scale libraries, the terminal products are D-Trp-containing peptides of 7–15 residues, cleaved with TFA/scavengers and used directly after precipitation without HPLC purification for preliminary binding assays. For scale-up of confirmed hits, the same building block is re-qualified under GMP guidelines because the discovery-scale purity profile may contain 0.3–1.0% of deletion sequences that are not detected by fast liquid chromatography-mass spectrometry gradients.
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BOC-D-tryptophan, catalogued as Nα-(tert-butoxycarbonyl)-D-tryptophan and commonly ordered as Boc-D-Trp-OH, is assigned CAS Registry Number 5241-64-5. The molecular formula is C16H20N2O4, and the molecular mass is 304.34 g mol−1. The product is supplied as a white to off-white crystalline powder with supplier release purity typically between 98.5% and 99.5%. No harmonized industry model number exists; catalogue codes are supplier-specific and generally append purity descriptors such as “peptide grade” or “reagent grade” to the Boc-D-Trp-OH abbreviation. The D-configuration at the α-carbon is the defining feature that separates this material from the more common L-isomer. BOC-D-tryptophan functions as a protected amino acid building block for solid-phase peptide synthesis, particularly in sequences requiring a D-tryptophan residue for structural or biological studies. Storage labels ordinarily specify a desiccated environment at 2–8°C; long-term retention under nitrogen below −18°C is used when anhydrous integrity must be maintained. The unprotected indole ring is sensitive to photochemical and oxidative discoloration, so amber glass and inert-gas packaging are standard.
The α-amino protection is a tert-butoxycarbonyl group removed by acidolysis. In Boc-benzyl solid-phase peptide synthesis, deprotection is most commonly performed with 40–50% trifluoroacetic acid in dichloromethane. Because the side-chain indole remains unprotected in this product, deprotection and cleavage cocktails must include scavengers to intercept tert-butyl cations and other electrophiles that otherwise alkylate the indole C-2 position. This requirement distinguishes BOC-D-tryptophan from base-labile Fmoc-protected tryptophan building blocks and from side-chain-protected variants such as formyl derivatives.
The product is generally classified as non-hazardous for transport under DOT, IATA, and ADR for the unmodified solid; however, local classifications and safety data sheets should be consulted because the material is a fine organic powder and may form combustible dust. Local exhaust ventilation is required in weigh-out operations at scales above 100 g.
Because no standalone pharmacopeial monograph covers BOC-D-tryptophan, certificate-of-analysis parameters are assembled from general compendial methods and validated supplier procedures. Typical release parameters are summarized below. These values are not universal; they represent commonly published acceptance windows in peptide-grade supplier documentation and should be confirmed against the specific lot required.
| Parameter | Typical acceptance criterion | Reference method |
|---|---|---|
| Appearance | White to off-white crystalline powder | Visual inspection under diffuse daylight |
| Identity by NMR | 1H NMR spectrum consistent with structure | 400 MHz spectrometer, DMSO-d6 |
| Assay by HPLC | ≥98.5% or ≥99.0% area at 280 nm | C18 column, acetonitrile/water gradient with 0.1% trifluoroacetic acid |
| Enantiomeric purity | ≥99.5% D-isomer | Chiral HPLC, polysaccharide-based CSP, 280 nm |
| Water content | ≤0.5% | Karl Fischer titration, USP <921> |
| Residual solvents | Dichloromethane, methanol, N,N-dimethylformamide within limits | USP <467> gas chromatography-headspace |
| Sulfated ash | ≤0.1% | USP <281> |
| Elemental impurities | ≤10 ppm where specified | ICH Q3D for GMP requests |
| Specific optical rotation | Concentration- and solvent-dependent | Polarimeter at 589 nm, supplier-defined solvent |
A representative reversed-phase method uses a C18 column of 150 × 4.6 mm packed with 5 μm particles, operated at 1.0 mL min−1 and 30°C. Mobile phase A is water with 0.1% trifluoroacetic acid; mobile phase B is acetonitrile with 0.1% trifluoroacetic acid. Detection at 280 nm is preferred to monitor the indole chromophore and to reduce background from solvent additives. The specific optical rotation is not a reliable identification test without reference to solvent, concentration, and temperature because the value is pH- and matrix-dependent.
Pre-drying is required when water content exceeds 0.5% or when the product has been exposed to relative humidity above 60%. A vacuum oven at 40°C for at least 12 h over phosphorus pentoxide is typical. Higher temperatures are avoided because prolonged thermal stress above 45°C can produce discoloration and partial deprotection. Packaging under argon in amber glass reduces oxidative degradation.
In Boc-benzyl SPPS on chloromethylated polystyrene-divinylbenzene resin, BOC-D-tryptophan is coupled after repetitive N-terminal deprotection steps. The processing conflict is the unprotected indole ring. Under acidolytic conditions—often 2 × 5 min with 40% trifluoroacetic acid in dichloromethane, or a single 20 min deprotection in microwave-assisted vessels—the indole C-2 position can capture tert-butyl cations released from the Boc group. This reaction generates tert-butyl indole adducts that persist through final cleavage. Scavengers such as anisole, thioanisole, or indole are therefore used at 2–5% v/v in the deprotection cocktail. Quantitative kinetic data for this specific side reaction under all TFA concentrations are limited, so process development should confirm protected peptide purity by LC-MS rather than rely solely on the building block specification.
Oxidation is a second boundary. Tryptophan residues can form oxindole and other oxidative by-products during repeated TFA exposure, neutralization, and storage of the resin-bound peptide. Manual fritted-glass vessels with nitrogen agitation reduce headspace oxygen but do not eliminate oxidant ingress during wash cycles. The D-configuration itself does not alter indole reactivity; it only changes the orientation of the side chain in the assembled peptide. Discoloration of the solid reagent from white to tan is a late indicator of breakdown, but off-white lots may still pass HPLC assay. Color cannot replace chromatographic purity testing for this material.
For sequences longer than 30 residues, cumulative TFA exposure during iterative coupling can convert trace indole adducts into deletion or termination products. Process engineers therefore limit the total number of unprotected Boc-D-tryptophan residues in a single sequence or select side-chain-protected tryptophan derivatives when multiple tryptophan residues must tolerate extended acid cycles. This is an operational boundary specific to unprotected BOC-D-tryptophan.
In preparative chromatographic comparison, BOC-D-tryptophan and BOC-L-tryptophan are not resolved by conventional reversed-phase HPLC. Chiral recognition requires a polysaccharide-based chiral stationary phase operated under normal-phase or polar organic conditions. A mobile phase containing 0.1% trifluoroacetic acid or ammonium trifluoroacetate is common, with detection at 280 nm. Validated methods can determine the enantiomeric ratio to 0.1% area percent for the minor isomer. The structural difference between the enantiomers is not limited to retention: in peptide synthesis, D-tryptophan changes the side-chain torsional angle and hydrogen-bonding direction in the final sequence. Substitution of BOC-L-tryptophan in a sequence designed for the D-isomer may reduce target binding or alter protease susceptibility, but the magnitude is sequence-dependent and no universal test standard applies.
Compared with N-acetyl-D-tryptophan, which is used in microbial metabolism and enzymatic study, BOC-D-tryptophan is a protected amino acid for chain assembly rather than a substrate analogue. Compared with formyl-protected variants such as BOC-D-Trp(For)-OH, the unprotected product avoids the post-cleavage formyl removal step but requires scavenger management during acid deprotection. The selection between unprotected and formyl-protected tryptophan is therefore governed by the number of acid cycles, the cleavage reagent, and the tolerance of the target peptide for indole modification. The racemic mixture, BOC-DL-tryptophan, is sometimes used for method development or racemic crystallography; it differs in melting range and chiral purity and should not be substituted for the D-isomer without chiral analysis. In peptide synthesis, the racemate introduces both enantiomers and is generally unsuitable for sequence-defined peptide assembly.
In automated microwave-assisted synthesizers, Fmoc-D-tryptophan is preferred when deprotection with 20% piperidine in N,N-dimethylformamide is used, because dibenzofulvene release can be monitored by UV absorbance at 301 nm. BOC-D-tryptophan is not transferred directly into Fmoc schedules because the Boc group remains intact under piperidine and the final TFA cleavage releases tert-butyl cations in the presence of the fully assembled peptide. Conversely, in Boc protocols using hydrofluoric acid or trifluoromethanesulfonic acid cleavage, Fmoc chemistry is incompatible because the entire protection scheme is designed for acid deprotection and base-labile Fmoc removal has no place in the cycle.
The solvent compatibility envelope also differs. BOC-D-tryptophan is commonly coupled in dichloromethane or N-methylpyrrolidone, whereas Fmoc-D-tryptophan is coupled in N,N-dimethylformamide after activation with HBTU or DIC/Oxyma. For microwave peptide synthesis, coupling of Fmoc-D-tryptophan is often performed at 50°C for 10 min with a fivefold molar excess of activated amino acid relative to resin loading. BOC-D-tryptophan has no such widely accepted microwave protocol because TFA deprotection in microwave instruments requires specialized vessel headspace management. Published data for microwave-assisted Boc solid-phase synthesis with unprotected tryptophan is limited.
The equipment boundary matters. Glass reactor vessels with PTFE frits used for Boc chemistry are not always certified for repeated base exposure required by Fmoc deprotection, whereas automated Fmoc synthesizers may lack acid-resistant wetted materials for repeated TFA handling. Substitution of one building block without adjusting the entire orthogonal protection scheme is not feasible because the N-terminal protection determines the deprotection schedule, scavenger selection, resin linker chemistry, and final cleavage condition. For this reason, BOC-D-tryptophan is selected only when the entire synthesizer and downstream cleavage train are configured for Boc-benzyl chemistry.
At pilot scale, batch-to-batch variability in N-terminal coupling yield has been observed when the feedstock particle size distribution exceeds the range specified for manual solid-phase vessels. Milling or sieving through a 60 mesh screen reduces dissolution lag in dichloromethane; otherwise, activation with HBTU may be incomplete and the first coupling step may require a double-coupling protocol. In production peptide campaigns, cumulative D-isomer content is monitored against the enantiomeric ratio in the crude peptide, because an L-isomer impurity of 0.5% can propagate into epimerized sequences that co-elute with the target peptide. The material is therefore specified at ≥99.5% chiral purity for sequences longer than 30 residues or for impurity-profiled registered products. Shorter sequences may use lower chiral purity, but this should be connected to a crude peptide enantiomeric ratio specification and confirmed by chiral amino acid analysis after hydrolysis.
Residual solvent profiles are also monitored at scales above 1 kg, because dichloromethane and methanol can concentrate during recrystallization. Final acceptance of the building block is tied to the intended peptide release specification; the building block alone does not guarantee downstream peptide purity unless the coupling and cleavage sequence is controlled.
