| HS Code | 614835 |
| Product Name | D-Tryptophan Methyl Ester Hydrochloride |
| Cas Number | 14907-27-8 |
| Molecular Formula | C12H15ClN2O2 |
| Molecular Weight | 254.71 g/mol |
| Appearance | White to off-white crystalline powder |
| Melting Point | 214-216 °C |
| Purity | ≥98% (TLC) |
| Optical Rotation | [α]D20 = -13.5° (c=1, methanol) |
| Solubility | Soluble in water, methanol, DMF, DMSO |
| Storage Conditions | Store at -20 °C, protected from moisture and light, under inert atmosphere |
| Mdl Number | MFCD00077653 |
| Smiles | COC(=O)[C@H](Cc1c[nH]c2ccccc12)N.Cl |
As an accredited D-Tryptophan Methyl Ester Hydrochloride factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Packaged as 25 g in a sealed amber glass bottle, stored under inert gas, protected from light and moisture. |
| Container Loading (20′ FCL) | D-Tryptophan Methyl Ester Hydrochloride is packed in sealed drums, palletized, and securely loaded into a 20-foot FCL container for safe transport. |
| Shipping | D-Tryptophan Methyl Ester Hydrochloride is typically shipped at ambient temperature in airtight, light-resistant packaging. Upon receipt, store refrigerated (2–8°C), protected from moisture and light. Keep container tightly sealed with desiccant. Avoid contact with oxidizing agents. Use appropriate PPE during handling. |
| Storage | Store D-Tryptophan Methyl Ester Hydrochloride in a tightly sealed container under inert gas, protected from moisture, light, and heat. Keep in a cool, dry environment, ideally refrigerated at 2–8°C. Ensure the container is clearly labeled and stored away from incompatible substances. Minimize exposure to air to preserve purity and stability. |
| Shelf Life | Shelf life is typically 2 years when stored sealed, dry, protected from light, at room temperature. |
In the production of tadalafil API, D-tryptophan methyl ester hydrochloride functions as the chirality-bearing indole fragment in the Pictet–Spengler condensation with piperonal. The hydrochloride is charged in a nitrogen-overlaid, glass-lined 6,300 L reactor equipped with a retreat-curve impeller and jacket circulation capable of maintaining the reaction mass at 0–5 °C with a deviation of ±2.0 °C; the condensation is run in dichloromethane under an acid catalyst and held for 8–14 h until chiral HPLC integration shows the residual D-tryptophan methyl ester peak at less than 1.0 area% relative to the main tetrahydro-β-carboline intermediate. The molar charge ratio of D-tryptophan methyl ester hydrochloride to piperonal is controlled at 0.98–1.08 mol/mol, which corresponds to approximately 14–19 wt% of the initial reaction mass depending on dichloromethane loading; an excess beyond 1.10 mol/mol is avoided because residual free amine can form undesired acylation by-products after chloroacetyl chloride addition. The downstream sequence includes phase separation with aqueous sodium carbonate at pH 8.0–8.5, N-acylation with chloroacetyl chloride in the same reactor train, then cyclization with methylamine in methanol at 25–35 °C; the crude tadalafil is solvent-switched into ethyl acetate, carbon-treated, hot-filtered through a 0.5–1.0 µm membrane, crystallized from an alcohol-water or ethyl acetate–alkane mixture, and vacuum-dried at 50 °C ± 2 °C to a moisture limit below 0.5%. Compliance is anchored to ICH Q7 Section 7.4 for solvent recovery, 21 CFR 211.160 for laboratory controls, ICH Q3C Table 2 for residual dichloromethane at 600 ppm, ICH Q3D Table A.2.2 for oral elemental impurity limits, and ICH M7 for mutagenic impurity control at the 1.5 µg/day threshold. Terminal product types include tadalafil API powder, directly compressible tadalafil blends, and finished tablets in 2.5 mg, 5 mg, 10 mg, and 20 mg strengths. Operational boundaries include pre-drying the hydrochloride under vacuum at 40 °C if storage relative humidity has exceeded 60%, because absorbed water changes the charged mass and alters the stoichiometric ratio; contact with strong aqueous alkali prior to condensation also leads to partial methyl ester hydrolysis and should be avoided until the intermediate is deliberately released for peptide-grade D-tryptophan.
| Compliance standard | Specific clause or method | Threshold/control |
|---|---|---|
| ICH Q3C Table 2 | Dichloromethane | 600 ppm (Class 2) |
| ICH Q3C Table 2 | Methanol | 3,000 ppm (Class 2) |
| ICH Q3D Table A.2.2 | Oral elemental impurities | As 15 µg/day; Cd 5 µg/day; Pb 5 µg/day; Hg 30 µg/day |
| ICH M7 | Mutagenic impurities | TTC 1.5 µg/day |
| USP Tadalafil monograph | HPLC assay | 98.0–102.0% on anhydrous basis |
In triptorelin acetate and related D-Trp-containing peptide APIs, the methyl ester hydrochloride is not loaded directly onto solid-phase supports; it is first converted into an N-protected building block through saponification and N-protection because the methyl ester is prone to premature hydrolysis under coupling conditions. The conversion is executed in a jacketed glass reactor at 0–5 °C using 1.0–1.2 equivalents of lithium hydroxide in a tetrahydrofuran-water mixture, followed by Boc protection with di-tert-butyl dicarbonate in the presence of sodium bicarbonate; after extraction and lyophilization, the resulting N-protected D-Trp derivative is tested by chiral HPLC and should present an enantiomer excess of not less than 99.5 area% before release. In solution-phase fragment coupling for triptorelin acetate, the activated D-Trp building block is charged at 1.05–1.20 mol per mol of deprotected peptide segment, with activation carried out by HOBt/HBTU or DIC/Oxyma in dimethylformamide at −5 °C to +5 °C; the coupling is stirred for 12–24 h and monitored by ninhydrin test and UPLC-MS until residual free amine is below 0.5% of the target peptide area. Compliance for this segment is derived from the Ph. Eur. Triptorelin monograph, 21 CFR 210.1 and 21 CFR 211.160, ICH Q7 Sections 5.0 and 6.0 for process equipment and materials, ICH Q3C Table 2 for tetrahydrofuran at 720 ppm and methanol at 3,000 ppm, and ICH Q3D Table A.2.2 for elemental impurities in injectable dosage forms. Terminal product types include triptorelin acetate lyophilized injection powder, triptorelin pamoate microsphere depot kits, and peptide intermediate batches intended for regulatory filing support. The main process bottleneck is racemization during saponification if the temperature rises above 8 °C or if aqueous hydroxide is charged too rapidly; such excursions produce L-Trp-related peptide diastereomers that co-elute in reverse-phase HPLC and trigger batch rejection under compendial peptide purity limits.
In analytical laboratories supporting tadalafil and triptorelin supply chains, D-tryptophan methyl ester hydrochloride is employed as a chiral purity reference material and as a spiked impurity for HPLC method validation. The reference substance is dissolved in methanol–water 80:20 v/v at 0.1 mg/mL and filtered through a 0.22 µm PTFE membrane; the validation workflow follows ICH Q2(R2) and USP <621>, with linearity spikes at 0.05% w/w, 0.10% w/w, 0.30% w/w, and 0.50% w/w relative to the API sample weight, and accuracy preparations at the same levels in triplicate. The separation is typically performed on a chiral polysaccharide column—4.6 mm × 250 mm, 5 µm particle size—with a mobile phase of n-hexane/ethanol/trifluoroacetic acid in a 90:10:0.1 v/v/v ratio, pump flow of 1.0 mL/min, column temperature at 25 °C, and UV detection at 220 nm; retention times for D-tryptophan methyl ester hydrochloride and its enantiomeric counterpart are baseline-resolved with a resolution factor not less than 2.0. The laboratory compliance framework includes ISO/IEC 17025 for method validation records, 21 CFR 211.160 for laboratory controls, and ICH Q7 Section 11 for laboratories in API supply chains. Terminal products from this application include certified reference standard vials, impurity marker solutions, and HPLC method validation kits used by QC laboratories. The operational limit is that the hydrochloride reference material must be stored over desiccant at 2–8 °C; exposure to ambient humidity above 60% for more than 2 h causes deliquescence and changes gravimetric accuracy, requiring Karl Fischer correction before use.
In the production of 1,2,3,4-tetrahydro-β-carboline scaffolds for central nervous system and oncology discovery libraries, D-tryptophan methyl ester hydrochloride undergoes Pictet–Spengler condensation with substituted benzaldehydes to produce a chiral tetrahydro-β-carboline framework. The reaction is carried out in a 1,000–2,000 L glass-lined reactor with a pH-stat probe and a moisture trap; the feedstock is pre-dried under vacuum at 40 °C for 3 h until Karl Fischer analysis shows less than 0.5% water, because residual water shifts the imine/iminium equilibrium and narrows the diastereomer excess. The addition ratio is set at 1.05–1.20 mol aldehyde per mol of D-tryptophan methyl ester hydrochloride, with trifluoroacetic acid charged at 0.05–0.10 mol/mol as catalyst; the mass is held at −5 °C to +5 °C for 6–10 h, quenched with 10% aqueous sodium carbonate to pH 9.0, and extracted into ethyl acetate. The extract is concentrated under vacuum, and the product is isolated either as the free amine by crystallization or as the N-Boc derivative by reaction with di-tert-butyl dicarbonate, which is then purified by flash chromatography or recrystallization from ethyl acetate–n-heptane. Compliance for this downstream segment is governed by ICH Q11 for development-stage chemical manufacturing, ICH Q3C Table 2 for residual ethyl acetate at 5,000 ppm and n-heptane at 5,000 ppm, and ICH Q3D Table A.2.2 for heavy metals when organometallic catalysts are screened. Terminal product types include chiral tetrahydro-β-carboline free bases, N-Boc-protected carboline intermediates, diastereomer-resolved salts, and fee-for-service screening library batches. The most critical process incompatibility is the presence of strong aqueous alkalis before ring closure; early basification of the hydrochloride to pH > 8 converts the methyl ester to the carboxylate and produces a zwitterionic intermediate with poor extractability into the organic phase.
When custom synthesis projects require the parent D-tryptophan rather than an ester, D-tryptophan methyl ester hydrochloride is subjected to acid hydrolysis in 1.0–2.0 N hydrochloric acid at 70–80 °C for 6–10 h, a process that avoids the racemization risk associated with strongly basic saponification. The reaction is conducted in a glass-lined vessel with hydrochloric acid charged to a mass fraction of 10–20% w/w relative to the methyl ester hydrochloride; after hydrolysis, the pH is brought to 5.5–6.0 with 10% sodium hydroxide while maintaining the temperature below 40 °C, and crude D-tryptophan is recovered by vacuum filtration, washed with cold ethanol, and recrystallized from water. The resulting D-tryptophan is released only after chiral HPLC shows an enantiomer excess of at least 99.0 area% and chloride content is below the compendial limit; residual solvents are controlled according to ICH Q3C Table 2, and heavy metals according to ICH Q3D Table A.2.2. The segment is used when a customer requires D-tryptophan or D-tryptophanol as a chiral pool starting material for oxazaborolidine catalyst assembly, peptide fragment coupling, or natural product total synthesis. Terminal product types include D-tryptophan powder, D-tryptophanol hydrochloride, and N-protected D-tryptophan derivatives. Operational boundaries include avoiding hydrochloric acid concentrations above 2.0 N because prolonged exposure at reflux slowly degrades the indole ring, visible as an increase in brown by-products and a drop in HPLC purity below 98.0 area%; similarly, neutralization above pH 6.5 causes precipitation of inorganic salts that complicate filtration.
The conversion of D-tryptophan methyl ester hydrochloride into Fmoc-D-Trp(Boc)-OH is a dedicated downstream operation that supplies protected amino acid feedstock to automated solid-phase peptide synthesizers. In this route, the methyl ester hydrochloride is hydrolyzed under mild lithium hydroxide conditions at 0–5 °C, and the resulting D-tryptophan is Nα-protected with Fmoc-OSu at 1.10–1.20 mol per mol of D-tryptophan in aqueous dioxane with sodium carbonate buffer; the indole nitrogen is then protected with di-tert-butyl dicarbonate at 1.05–1.15 mol per mol of intermediate in the presence of a catalytic base, and the crude material is extracted into ethyl acetate, washed with sodium chloride solution, and crystallized from ethyl acetate–n-heptane. The process is run in a 500–1,000 L reactor with cooling and distillation skids, and the final material is vacuum-dried at 35–40 °C until residual solvent levels meet ICH Q3C Table 2 limits. Compliance includes ICH Q7 for GMP-grade protected amino acids, Ph. Eur. 2.2.46 and USP <621> for chromatographic purity, ICH Q3C Table 2 for residual ethyl acetate and n-heptane, and ICH Q3D Table A.2.2 for elemental impurities. Terminal product types include bottled Fmoc-D-Trp(Boc)-OH powder, pre-weighed solid-phase synthesis vials, and custom peptide synthesis feedstocks supplied with certificate-of-analysis data for enantiomer excess and residual water. A specific process failure mode observed at production scale is the formation of an emulsion during ethyl acetate extraction when the aqueous phase is not kept above pH 8.0; this emulsion reduces extraction yield and can entrain inorganic salts into the organic phase, requiring additional brine washes and prolonging batch processing time.
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D-Tryptophan Methyl Ester Hydrochloride is identified by CAS RN 14907-27-8 and molecular formula C12H15ClN2O2; its molecular weight is 254.71 g/mol. The IUPAC designation is methyl (2R)-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride. The material is provided as a white to off-white crystalline powder with a single chiral center at the α-carbon. In peptide synthesis, the hydrochloride salt acts as a masked C-terminal carboxylic acid, allowing selective amide bond formation at the α-amino group after in situ neutralization with a tertiary amine. This compound differs from D-tryptophan free acid because the carboxylic acid is already protected as the methyl ester, and it differs from N-protected methyl ester derivatives because the α-amino group remains available for direct acylation. The unprotected indole side chain absorbs at 280 nm, which permits direct UV monitoring during preparative HPLC purification.
Representative release acceptance for reagent-grade lots includes reversed-phase HPLC purity of ≥98.0% area, chiral HPLC enantiomeric purity with the L-isomer limited to ≤0.5% area, Karl Fischer water content of ≤1.0%, and residue on ignition of ≤0.1%. Specific rotation is measured at c = 1.0 in methanol by Ph. Eur. 2.2.7; because optical rotation can vary in sign and magnitude between salt forms, batch acceptance is based on chiral HPLC identity rather than polarimetry alone. The product is soluble in DMF, methanol, and water; dried DCM solubility is lower and may limit coupling-solvent selection.
| Parameter | Method | Representative acceptance |
|---|---|---|
| Appearance | Visual inspection | White to off-white crystalline powder |
| Chemical purity | RP-HPLC, USP <621> | ≥98.0% area |
| Enantiomeric purity | Chiral HPLC, amylose-based chiral stationary phase | L-isomer ≤0.5% area |
| Specific rotation | Ph. Eur. 2.2.7 | Reported per lot, c = 1.0 in methanol |
| Water content | Karl Fischer, USP <921> | ≤1.0% |
| Residue on ignition | USP <281> | ≤0.1% |
| Chloride content | Ion chromatography or argentometric titration | Theoretical chloride 13.92%; typical assay 13.5%–14.3% |
In solution-phase peptide coupling, the hydrochloride is dissolved in anhydrous DMF at 0.2 M to 0.5 M and cooled to 0–5 °C before addition of 1.05 eq of N-methylmorpholine or 1.1 eq of N,N-diisopropylethylamine. The neutralized free amine is transferred into a pre-activated acid solution containing HATU or EDC/HOBt in a jacketed glass-lined reactor; slow addition over 30 min prevents localized pH excursions that would saponify the methyl ester. Reaction monitoring at 280 nm by HPLC is used to confirm consumption of the starting ester. Typical isolated yields for pilot-scale acylation of this substrate are process-dependent; published data for this specific configuration under all coupling-agent combinations is limited, so laboratory verification with the selected acid and solvent is required before scale-up.
For solid-phase peptide synthesis, the compound is used primarily in solution-phase segment condensation or as a C-terminal building block because the unprotected α-amino group prevents standard on-resin Fmoc strategy unless the amino group is first protected. In fragment coupling routes, D-tryptophan methyl ester hydrochloride is neutralized and coupled to the terminal acid of a protected peptide segment. The indole side chain does not require protection under standard carbodiimide or aminium coupling conditions, but in TFA-mediated deprotection the indole ring can scavenge tert-butyl cations; a scavenger such as water or triisopropylsilane is therefore required to limit indole alkylation. The methyl ester remains stable during anhydrous coupling but is removed after segment assembly by saponification with lithium hydroxide in THF/water at 0 °C; the hydrolysis endpoint is confirmed by TLC or HPLC.
Unlike the L-isomer, D-tryptophan methyl ester hydrochloride introduces the non-proteinogenic D-configuration at the α-carbon, which alters proteolytic stability and receptor recognition of the final peptide. The two enantiomers have identical molecular weight and formula but opposite configuration; they are differentiated by chiral HPLC retention time and by inversion of specific rotation. The free acid D-tryptophan has the same indole side chain but lacks C-terminal protection and is zwitterionic, reducing solubility in DCM and requiring separate carboxyl activation before coupling. The hydrochloride salt is generally easier to handle than the free base because the salt is crystalline and less prone to discoloration from free-amine oxidation; however, the hydrochloride counterion must be neutralized before carbodiimide-mediated coupling to avoid formation of an unreactive protonated amine.
| Product | CAS RN | Molecular weight | Distinguishing feature |
|---|---|---|---|
| D-Tryptophan methyl ester hydrochloride | 14907-27-8 | 254.71 g/mol | Crystalline HCl salt; unprotected amino group; C-terminal methyl ester |
| L-Tryptophan methyl ester hydrochloride | 7524-52-9 | 254.71 g/mol | Opposite α-carbon configuration; same protecting-group pattern |
| D-Tryptophan | 153-94-6 | 204.23 g/mol | Free zwitterionic acid; requires C-terminal activation |
For process development, the methyl ester protecting group creates a defined C-terminal boundary. Under carbodiimide coupling with EDC/HOBt, the active ester is formed at the carboxylic acid component; the neutralized D-tryptophan methyl ester then attacks the activated acyl group. The reaction is typically run at 0–5 °C because the α-proton of D-amino acid esters is susceptible to base-mediated deprotonation at higher temperatures, leading to configurational drift. Chiral HPLC retention time for the L-isomer is used to set the enantiomeric purity limit. When HATU is used, the base is added after pre-activation to minimize unreactive guanidinium byproducts. The addition sequence has a direct effect on isolated yield; reverse addition of base to the ester before activation can reduce conversion by protonating the amine and promoting methyl ester solvolysis.
After acylation, the reaction mass is quenched with aqueous ammonium chloride while maintaining temperature below 10 °C to avoid saponification. Extraction with ethyl acetate or DCM, followed by washing with dilute sodium bicarbonate, removes residual coupling reagents. The indole-containing product is monitored by TLC with ninhydrin staining and by HPLC at 280 nm. Silica gel chromatography using dichloromethane/methanol gradients is typical; the hydrochloride salt may require neutralization or addition of 0.1% triethylamine to the mobile phase to reduce streaking. Rotary evaporation at 25–30 °C under reduced pressure avoids thermal decomposition of the methyl ester. Crystallization from ethyl acetate/hexane or methanol/diethyl ether is used to isolate the acylated methyl ester; the exact solvent ratio depends on the N-acyl substituent.
Compared with the corresponding ethyl ester, the methyl ester is less sterically hindered and generally hydrolyzes faster under alkaline conditions, which is an advantage when final C-terminal acid deprotection is required after peptide assembly. In contrast, the benzyl ester would require hydrogenolysis or strong acid for removal and can introduce byproducts if the indole ring is present. The hydrochloride salt form is selected because it is easier to store than the free base, which may darken upon exposure to air; however, the salt form adds a neutralization step in coupling protocols. N-Boc- and N-Fmoc-protected D-tryptophan methyl esters are available and can be coupled without prior neutralization of the α-amino group, but their use introduces additional protection/deprotection steps and cost. The choice among these derivatives is governed by whether the α-amino group is destined for acylation or for subsequent solid-phase elongation.
The compound also serves as a chiral intermediate for the preparation of D-tryptophan-derived building blocks. The indole NH is relatively non-nucleophilic under typical coupling conditions; acylation at the α-amine proceeds selectively. For N-acyl derivatives, the methyl ester can be converted to the corresponding hydrazide, hydroxamic acid, or amide by treatment with hydrazine, hydroxylamine, or amines in methanol. These transformations require careful temperature control because the indole system can undergo acid-catalyzed dimerization under strongly acidic conditions. Published data for the specific dimerization threshold in D-tryptophan methyl ester hydrochloride is limited; process development should include forced-degradation studies under the intended reaction matrix.
In fragment coupling, the processing window is constrained by two competing side reactions. At temperatures above 10 °C, base-mediated epimerization of the D-α-carbon can occur in the presence of tertiary amine bases, while below 0 °C, the coupling rate may drop and unreacted starting material can remain. The recommended addition temperature of 0–5 °C is therefore not arbitrary but reflects the intersection of sufficient acylation rate and acceptable configurational stability. In batch records, the temperature is controlled by a jacketed reactor with a chiller capable of maintaining set point ± 2 °C. Addition of N-methylmorpholine is performed over 15–20 min with the reaction mass under nitrogen; rapid addition has been observed in pilot-plant batches to cause local pH rise and methyl ester saponification at the addition point.
The hydrochloride counterion is neutralized with a stoichiometric amount of an auxiliary base. Use of inorganic bases such as sodium bicarbonate in aqueous media is less suitable for anhydrous coupling because water can hydrolyze the active ester; however, a two-phase Schotten-Baumann protocol can be used with dichloromethane and aqueous sodium bicarbonate at 0–5 °C for acylation with acid chlorides. Under these conditions, the methyl ester is retained, but the aqueous phase must be kept slightly above pH 7.0 and below pH 8.5 to balance neutralization and ester hydrolysis. This method is preferred when the acylating agent is water-sensitive but the product is stable to aqueous workup.
The product is stored in sealed containers under inert gas at 2–8 °C. Because the hydrochloride is hygroscopic, exposure to ambient humidity above 60% RH can cause caking and increase water content; containers should be allowed to equilibrate to room temperature before opening to reduce condensation. Pre-drying under vacuum at 40 °C is recommended when water content exceeds 1.0% prior to anhydrous coupling. The material is incompatible with strong bases such as sodium hydroxide and DBU under aqueous or alcoholic conditions because the methyl ester undergoes base-catalyzed hydrolysis; coupling reaction pH after neutralization is maintained below 8.5. Contact with strong oxidizing agents, acid chlorides, or electrophilic halogenating agents should be avoided because the indole ring can undergo substitution. Waste disposal must follow site permits and applicable REACH or local chemical-hygiene regulations; no USP monograph for this specific salt was identified at the time of writing.
Quality control of incoming lots includes identity confirmation by FTIR and HPLC retention time relative to a certified standard. Chiral HPLC methods typically use an amylose tris(3,5-dimethylphenylcarbamate) stationary phase with hexane/ethanol or heptane/isopropanol mobile phases; the L-isomer elutes either before or after the D-isomer depending on the column and mobile-phase composition. The method must be qualified according to ICH Q2(R1) for specificity, linearity, and precision if the product is used in GMP peptide manufacture. Typical limit of quantitation for the L-isomer is below 0.05% area in qualified methods; this protects enantiomeric purity claims. The product should be retested according to the supplier-assigned interval; published data for specific long-term stability at 25 °C is limited, so refrigerated storage remains the default requirement.
For use in pharmaceutical intermediates, the product is controlled under ICH Q7 for GMP starting materials when sourced from qualified suppliers. Raw material specifications include residual solvents by USP <467>, elemental impurities by USP <232>/<233>, and residue on ignition by USP <281> if the material enters registered synthesis routes. Documentation should include certificate of analysis, safety data sheet, and REACH registration status where applicable. The absence of a pharmacopeial monograph for this specific hydrochloride means that acceptance criteria are supplier- or process-specific rather than compendial.
