| HS Code | 432266 |
| Product Name | CBZ-D-Proline |
| Cas Number | 6404-31-5 |
| Iupac Name | (2R)-1-[(benzyloxy)carbonyl]pyrrolidine-2-carboxylic acid |
| Molecular Formula | C13H15NO4 |
| Molecular Weight | 249.26 g/mol |
| Appearance | White to off-white powder or solid |
| Melting Point | 77-80 °C |
| Optical Rotation | [α]D20 = +40° (c=1, ethanol) |
| Smiles | O=C(O)[C@H]1CCCN1C(=O)OCc2ccccc2 |
| Inchi | InChI=1S/C13H15NO4/c15-12(16)11-7-4-8-14(11)13(17)18-9-10-5-2-1-3-6-10/h1-6,11H,7-9H2,(H,15,16)/t11-/m1/s1 |
| Solubility | Soluble in ethanol, methanol, DMSO, and chloroform; sparingly soluble in water |
| Storage Condition | Store at 2-8 °C, protected from light and moisture |
| Mdl Number | MFCD00037384 |
As an accredited CBZ-D-Proline factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | CBZ-D-Proline is packaged in a sealed amber glass bottle with tamper-evident closure, containing 5 g, labeled with safety information. |
| Container Loading (20′ FCL) | CBZ-D-Proline is packed in drums, palletized, and loaded as a 20′ FCL, ensuring safe, efficient, and secure transport. |
| Shipping | CBZ-D-Proline ships at ambient temperature in a sealed, light-protected container with desiccant. Ensure compliance with local regulations; not classified as dangerous goods under standard conditions. Keep away from moisture, strong oxidizers, and heat during transit and storage. |
| Storage | Store CBZ-D-Proline in a tightly sealed container, protected from light and moisture, in a cool, dry, well-ventilated area. Room temperature is generally acceptable, but refrigeration may extend stability. Avoid exposure to strong acids, bases, or oxidizing agents. Keep away from ignition sources and incompatibles. Ensure proper labeling and minimize air contact after opening. |
| Shelf Life | Store tightly sealed, desiccated, and protected from light at -20°C; shelf life is typically 2-3 years. |
CBZ-D-Proline (CAS 6404-31-5, C13H15NO4, MW 249.26) is introduced in solution-phase fragment condensation for peptide APIs in which a D-proline residue must be inserted into a backbone that is sensitive to Fmoc deprotection conditions. In a jacketed glass reactor with nitrogen purge, 1.00 mol of CBZ-D-Proline and 0.95–1.00 mol of amino component are dissolved in anhydrous N,N-dimethylformamide or dichloromethane at 0–5 °C. A mixture of 1.05–1.20 mol 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 1.10–1.25 mol 1-hydroxybenzotriazole monohydrate is added in one portion, and N-methylmorpholine is dosed until the pH reaches 8.5–9.0. The batch is maintained at 0–15 °C for 2–8 h; conversion is tracked by high-performance liquid chromatography and the reaction is stopped when residual CBZ-D-Proline is below 0.5 area-%. The mixture is diluted with ethyl acetate, washed sequentially with 0.5 M citric acid, 5 % sodium bicarbonate, and 10 % sodium chloride solution. The organic layer is dried over anhydrous sodium sulfate and concentrated. The resulting N-Cbz-protected D-prolyl dipeptide fragments are carried into hydrogenolysis or maintained as protected intermediates for subsequent fragment coupling. Terminal products include D-proline-containing linear and cyclic peptide APIs in which the D-configuration impairs degradation by aminopeptidases. In cGMP peptide intermediate production, the process is governed by ICH Q7; residual organic solvent and moisture are controlled according to Ph. Eur. 5.4 and Ph. Eur. 2.2.32. The operational boundary is pH 9.0 because tert-butyl ester protecting groups on the amino partner undergo saponification in stronger bases, and coupling should be avoided when the amino component carries free aniline or secondary amine sites unless those sites are protected.
Carbodiimide activation of CBZ-D-Proline proceeds through O-acylisourea formation, which is intercepted by 1-hydroxybenzotriazole to furnish a benzotriazolyl active ester that undergoes aminolysis at the D-proline carbonyl. Racemization in proline residues is generally slower than in acyclic amino acids because the pyrrolidine ring restricts conformational freedom around the α-carbon, but loss of configuration can still occur through azlactone-type or base-assisted reversible deprotonation pathways when excess tertiary amine is present. In preparative coupling work, the critical processing conflict is temperature: below −10 °C, the coupling rate falls sharply and residual active ester persists after aqueous workup, whereas above 15 °C the O-acylisourea side product rearranges to N-acylurea and background racemization accelerates. The preferred window is therefore 0–10 °C for the first 2 h, with slow warming to 15 °C if conversion stalls. Process development batches evaluated by chiral HPLC on a Chiralpak IA column with n-hexane/ethanol/formic acid mobile phase typically resolve residual L-proline at below 0.1 area-% when the coupling is controlled within this window. Equipment configuration influences reproducibility: a jacketed reactor with a calibrated pH probe and split-range dosing of N-methylmorpholine is preferred over uncontrolled manual addition because local base concentration above 9.5 pH can deprotonate the α-carbon even at low bulk temperature. This reaction is incompatible with unprotected thiol substrates because the benzotriazolyl ester can be intercepted by free sulfhydryl groups, and it is incompatible with water contents above 0.5 % w/w in the solvent because the active ester hydrolyzes before aminolysis. Where the amino partner is available only as a hydrochloride salt, an equimolar quantity of N-methylmorpholine relative to the hydrochloride is pre-added to liberate the free amine before CBZ-D-Proline activation.
Reduction of the carboxyl terminus of CBZ-D-Proline to the corresponding primary alcohol provides N-Cbz-D-prolinol, a chiral building block for oxazaborolidine-based asymmetric reduction catalysts. A solution of 100 g CBZ-D-Proline in 800 mL anhydrous tetrahydrofuran is cooled to 0–5 °C under inert atmosphere. Borane-dimethyl sulfide complex (1.2–1.5 eq, 10 M) is added slowly over 30–60 min, maintaining controlled gas evolution. The cooling bath is removed and the batch is stirred at 20–25 °C for 12–16 h. Excess borane is quenched by slow methanol addition at 0 °C, followed by 1 M hydrochloric acid hydrolysis. The product is extracted with ethyl acetate, washed with 5 % sodium bicarbonate and brine, dried, and concentrated. The Cbz protective group is then removed by hydrogenolysis over 10 % palladium on carbon, 5–10 % w/w, at 1–3 bar hydrogen in methanol at 25–30 °C, yielding D-prolinol. D-prolinol is condensed with phenylboronic acid or substituted arylboronic acids to generate the oxazaborolidine catalyst used in asymmetric reduction of acetophenone derivatives with borane-THF. The finished chiral secondary alcohol is controlled by gas chromatography on a Chiraldex B-DM column, with target enantiomeric excess above 99 % area. The D-configuration of the prolinol gives the opposite enantioselectivity relative to L-prolinol-derived catalysts, and the reduction is typically run at 1–5 mol % catalyst loading in toluene at −10 to 30 °C. Residual palladium in the D-prolinol after hydrogenolysis is a critical limit and is measured by inductively coupled plasma mass spectrometry; ICH Q3D Option 1 classification for parenteral finished products requires palladium concentrations below 10 µg/g unless otherwise justified.
The organocatalytic activity of D-proline requires a free secondary amine, so the N-benzyloxycarbonyl substituent is normally removed by heterogeneous hydrogenolysis before the amino acid is charged to the aldol reaction vessel. In a typical batch, 1.0 kg of CBZ-D-Proline is dissolved in 10 L of methanol and hydrogenated over 50 g of 10 % palladium on carbon at 1–3 bar hydrogen and 25–30 °C for 3–6 h. The catalyst is removed by filtration through a 0.45 µm polytetrafluoroethylene membrane, and the filtrate is concentrated under vacuum below 40 °C to avoid partial lactamization of D-proline. The resulting D-proline is then used as a 10–30 mol % organocatalyst in DMSO or N,N-dimethylformamide for aldol addition of acetone to substituted benzaldehydes. The enantiocontrol is configured by the proline stereocenter, so D-proline yields products of opposite absolute configuration relative to L-proline under otherwise identical conditions. Transfer hydrogenation with ammonium formate can be used when high-pressure equipment is unavailable, but formate residues interfere with catalyst turnover in aldol applications and must be removed by recrystallization from water/2-propanol. This route is incompatible with reducing agents and carbonyl-containing co-solvents during hydrogenolysis because acetone and ethyl acetate undergo competitive reduction, and it is operationally limited at relative humidity above 60 % because the free amino acid absorbs moisture and forms a sticky solid that complicates filtration. Precise enantioselectivity data for D-proline aldol systems require confirmation on the specific substrate because published data for this specific configuration in industrial solvent systems is limited.
Conversion of CBZ-D-Proline to the corresponding amino alcohol and subsequent reaction with a halophosphine provides a protected P,N-ligand precursor after Cbz removal. This transformation is carried out on D-prolinol obtained by borane reduction, followed by treatment with chlorodiphenylphosphine in anhydrous tetrahydrofuran at 0–5 °C in the presence of triethylamine. The cooling is maintained for 2–4 h, after which the mixture is warmed to 20–25 °C and subjected to hydrogenolysis over palladium on carbon to remove the N-benzyloxycarbonyl group. The resulting D-proline-derived phosphinite ligand is used in palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate in dichloromethane at 0–25 °C. The catalytic cycle for palladium allylic alkylation with prolinol-derived P,N ligands is well documented, although published data specifically for CBZ-D-Proline-derived phosphinite ligands in multiplexed library synthesis is limited. Process controls include 31P NMR assay of the ligand before use, Karl Fischer titration of the reaction solvent below 0.1 % water, and gas chromatographic determination of the product enantiomeric ratio on a β-Cyclodextrin chiral column. The operational boundary is moisture ingress: hydrolysis of the phosphinite ligand produces D-prolinol and diphenylphosphine oxide, neither of which sustains high asymmetric induction. The finished chiral branched ester or amide products serve as intermediates in pharmaceutical candidate synthesis, and residual palladium in isolated intermediates must be controlled to ICH Q3D limits before further cGMP processing.
In CDMO analytical control, CBZ-D-Proline is retained as a batch-specific working standard for chiral impurity profiling of N-Cbz-D-proline input and its downstream prolinol and peptide fragments. The chiral high-performance liquid chromatographic method uses a Chiralpak IA-3 column (250 × 4.6 mm, 3 µm) with an n-hexane/ethanol/trifluoroacetic acid mobile phase at 80:20:0.1 (v/v/v), a flow rate of 0.8 mL/min, and ultraviolet detection at 210 nm. System suitability requires resolution between the D and L enantiomers of at least 1.5 per USP <621>, with the L-isomer limit set at 0.1 area-% unless data support a wider specification. Optical rotation is measured at 589 nm in methanol at 25 °C. Water content by Karl Fischer titration is controlled below 0.5 % w/w because residual moisture accelerates hydrolysis of the benzyl carbamate during storage. The material is stored at 2–8 °C under nitrogen, with a retest interval of 24 months when kept in closed, light-protected containers. For comparative release, the Cbz protective group is distinguished from Fmoc and Boc by its orthogonal cleavage chemistry as shown in the table below.
| Protective group | Cleavage | Acid wash stability | Base stability | Typical use |
|---|---|---|---|---|
| Cbz | H2/Pd/C, 1–3 bar, 25–30 °C | stable to 0.5 M citric acid | stable to 5 % sodium bicarbonate | solution-phase fragment condensation |
| Fmoc | 20 % piperidine in DMF, 20–25 °C | stable | labile | solid-phase peptide synthesis |
| Boc | 4 M hydrochloric acid in dioxane, 20–25 °C | labile | stable to mild aqueous base | solid-phase peptide synthesis |
When N-Cbz-D-proline is converted to the corresponding acid chloride and quenched with a D-amino acid methyl ester, the resulting protected dipeptide fragment serves as a rigid chiral building block for subsequent medicinally active peptide analogues. In a dried glass reactor, 1.00 mol of CBZ-D-Proline is suspended in dichloromethane, and 1.10 mol of oxalyl chloride is added dropwise at 0–5 °C in the presence of 0.02 mol N,N-dimethylformamide under nitrogen. Gas evolution is maintained below controlled vent capacity, and the mixture is stirred until a clear solution forms. Volatiles are removed under vacuum below 30 °C, and the acid chloride is redissolved in dichloromethane. The solution is added slowly to a pre-cooled mixture of 1.00 mol D-amino acid methyl ester hydrochloride and 1.20 mol N-methylmorpholine in dichloromethane at −10 to 0 °C. The coupling is held for 2–4 h, then quenched with 1 M hydrochloric acid and washed. The terminal product is the N-Cbz-D-prolyl-D-amino acid methyl ester, which is isolated and may be carried forward after selective ester hydrolysis with lithium hydroxide in tetrahydrofuran/water. This acid chloride route is incompatible with substrates containing acid-labile protecting groups such as Boc or tert-butyl esters within the same molecule. It is also incompatible with unprotected primary alcohols in the amino partner because esterification competes with amidation. The process is controlled by thin-layer chromatography with UV visualization and by liquid chromatography-mass spectrometry for the expected molecular ion. In production-scale batches, the external cooling system must maintain the quench step below 0 °C, because exothermic overshoot above 10 °C leads to racemization of the D-amino acid methyl ester and to formation of the diketopiperazine from intramolecular aminolysis.
Competitive CBZ-D-Proline prices that fit your budget—flexible terms and customized quotes for every order.
For samples, pricing, or more information, please contact us at +8615365186327 or mail to admin@ascent-chem.com.
We will respond to you as soon as possible.
Tel: +8615365186327
Email: admin@ascent-chem.com
Flexible payment, competitive price, premium service - Inquire now!
CBZ-D-Proline, also listed as N-benzyloxycarbonyl-D-proline, N-Cbz-D-proline, or Z-D-Pro-OH in peptide-synthesis supply catalogs, is supplied as the free carboxylic acid under CAS 6404-31-0, molecular formula C13H15NO4, and relative molecular mass 249.26 g mol−1. The carboxybenzyl group blocks the pyrrolidine nitrogen and leaves the C-terminal carboxyl available for activation, while the D absolute configuration inverts the spatial arrangement of the carboxyl substituent relative to the proteinogenic L-series. The free acid form is preferred over the dicyclohexylamine salt in automated and solution-phase peptide synthesis because no counterion liberation step is required before coupling. Acceptance specifications for research and peptide-synthesis grades typically require chromatographic assay not less than 98.0%, chiral purity not less than 99.0% enantiomeric excess, and loss on drying not greater than 0.50%.
| Release parameter | Acceptance value | Method |
|---|---|---|
| Assay by reversed-phase HPLC | ≥ 98.0% area | USP 621, C18 column, UV 210 nm |
| Chiral purity | ≥ 99.0% ee | HPLC on polysaccharide chiral stationary phase, n-hexane/ethanol 70:30 |
| Specific rotation | +60.0° ± 2.0°, c = 1.0, ethanol | USP 781 |
| Loss on drying | ≤ 0.50% | USP 731, vacuum 60 °C, 4 h |
| Residue on ignition | ≤ 0.10% | USP 281 |
| Storage temperature | 2–8 °C | airtight container, inert headspace |
Batch release under GMP peptide-synthesis supply often adds headspace gas chromatography for residual solvents according to USP 467 and palladium content below 50 ppm where the building block will be used in hydrogenolytic deprotection. In automated peptide synthesizers and solution-phase fragment assembly, the free acid is activated in situ with uranium salts or carbodiimides in anhydrous dimethylformamide at 0–4 °C. The product is added to the resin-bound amine or amino-acid ester as a protected building block. Because the pyrrolidine nitrogen is blocked, no free secondary amine is available for base-catalyzed side reactions until deprotection is performed. In multi-kilogram pilot campaigns, pre-drying under vacuum at 40 °C for 4 h is applied when Karl Fischer titration exceeds 0.5%, because residual water in the solid can quench active ester intermediates and reduce coupling efficiency.
By comparison with CBZ-L-Proline, the D-enantiomer induces opposite torsional constraints in folded peptides. Published solution NMR and X-ray data in proline-containing model peptides indicate that L-proline commonly occupies the i+1 position of a type II β-turn with backbone dihedral angles near φ −60° and ψ +120°, whereas D-proline shifts the turn to the mirror-image type II′ geometry with φ near +60° and ψ near −120°. This inversion changes the spatial presentation of the carbonyl and side-chain vectors, which is used in macrocyclic peptide design when a binding epitope requires a reverse turn that cannot be accommodated by L-amino acids. The difference is not limited to optical rotation; it affects cyclization yields, resin coupling rates, and chromatographic retention on polysaccharide chiral stationary phases.
In carbodiimide- or uranium-mediated couplings, the protected proline derivative behaves differently from primary amino acid derivatives because the pyrrolidine nitrogen lacks an amide N–H that could participate in classical oxazolone formation. Coupling to sterically hindered amines is nevertheless slower than coupling of glycine or alanine derivatives, and pre-activation time must be limited to avoid conversion to the less reactive symmetrical anhydride. When the target is a D-Pro-containing dipeptide ester, hydrogenolysis of the Cbz group creates a secondary amine that can undergo intramolecular diketopiperazine formation if the C-terminal ester is a methyl or ethyl ester and the mixture is warmed above 25 °C in the presence of base. This process conflict is avoided by keeping the ester as tert-butyl or benzyl, or by neutralizing the deprotection mixture before concentration.
Analytical release of CBZ-D-Proline is complicated by the weak chromophore of the Cbz group and the high polarity of the free carboxylate. Reversed-phase HPLC is run on a C18 column with an acidic aqueous acetonitrile mobile phase at 210 nm; the low wavelength is selected because the carbonyl absorption of the carbamate is weak and the target analyte lacks a conjugated chromophore. Enantiomeric purity is determined on polysaccharide-based chiral stationary phases such as Chiralpak IA or IG under normal-phase conditions with n-hexane/ethanol 70:30 and trifluoroacetic acid. Injection of the D and L reference mixture must produce baseline resolution with a separation factor not less than 1.2 for unambiguous integration. When peak tailing occurs due to free carboxylate interaction with silica, the mobile phase is acidified with 0.1% trifluoroacetic acid or the acid is derivatized to the methyl ester with trimethylsilyldiazomethane before injection. The limit of detection for the unwanted L-enantiomer under optimized conditions is typically below 0.1% area, but published data for this specific configuration is limited and must be verified on the installed instrument.
Fourier-transform infrared spectroscopy and melting point are used for identity but do not detect low-level enantiomeric contamination. Therefore, an optical rotation measurement alone is insufficient for release in GMP applications; it must be combined with chiral HPLC and, where the building block is used in an NDA route, with X-ray powder diffraction if crystal form consistency is critical. Batch records for production-scale campaigns frequently record optical rotation in ethanol, loss on drying, residue on ignition, and chiral purity in the same certificate because the free acid can retain solvent and the solvent mass affects both specific rotation and assay calculated on the as-is basis.
The free acid is stored in airtight containers at 2–8 °C; prolonged exposure to ambient humidity increases residual water and can produce a sticky crystalline cake when relative humidity exceeds 60%. Before use in moisture-sensitive activation, the solid is dried under vacuum at 40 °C for 4 h and transferred under nitrogen. The neat solid is stable for at least 24 months under the specified storage in supplier stability protocols, but the isolated N-hydroxysuccinimide ester is not: hydrolysis occurs within hours in moist dimethylformamide, and the active ester should be prepared fresh. In solution, CBZ-D-Proline free acid is soluble in ethyl acetate, dichloromethane, dimethylformamide, and methanol, and sparingly soluble in water. Aqueous solubility increases with pH as the carboxylate salt forms, but prolonged exposure to pH above 10 at elevated temperature slowly hydrolyzes the carbamate to D-proline and benzyl alcohol. These boundaries are operationally relevant for liquid-liquid extraction: the free acid remains in the organic layer at acidic pH, while the carboxylate salt partitions into aqueous sodium bicarbonate.
In solid-phase synthesis of D-Pro-containing sequences, CBZ-D-Proline is not removed under the standard Fmoc-deprotection condition of 20% piperidine in DMF, making it suitable for orthogonal protection of a selected D-proline residue in fragments that carry Fmoc on other amine sites. The Cbz group is then cleaved in batch mode by hydrogenation over 5% palladium on carbon in methanol/water mixtures at 0.2–0.5 MPa hydrogen pressure and 20–30 °C. Production-scale hydrogenation in stirred stainless-steel autoclaves fitted with gas-entrainment impellers is exothermic; uncontrolled temperature excursions above 35 °C have been associated with over-reduction of reducible side chains such as aryl iodides or nitro groups. If the substrate contains an alkene, alkyne, benzyl ester, or sulfur-containing heterocycle that poisons palladium, hydrogenolytic removal is contraindicated and the Fmoc-protected D-proline should be selected instead.
In preparative purification, the free acid is recrystallized from ethyl acetate/heptane, which raises chiral purity by rejecting the L-enantiomer in the mother liquor. Batch-to-batch variation in optical rotation is most often traced to residual ethyl acetate or water rather than to racemization; polarimetric measurement is therefore performed under thermostatted conditions at 20 °C after drying, and the result is read against the USP 781 method. In reversed-phase HPLC, the D and L enantiomers of Cbz-proline co-elute; separation requires a chiral stationary phase or a chiral derivatizing agent such as 1-(9-fluorenyl)ethyl chloroformate. On polysaccharide-based columns, the D-isomer may elute before or after the L-isomer depending on the selector and mobile phase; therefore, a single retention-time assumption without co-injection of a racemic standard is not reliable.
| Building block | Protecting group removal | Stability under Fmoc deprotection | Main restriction |
|---|---|---|---|
| CBZ-D-Proline | Hydrogenolysis over Pd/C or HBr/acetic acid | Stable to piperidine/DMF | Not compatible with reducible or catalyst-poisoning functional groups |
| Boc-D-Proline | Acidolysis with TFA/DCM | Stable to piperidine/DMF; removed by TFA | Avoid acid-labile resin linkers |
| Fmoc-D-Proline | Base-mediated elimination with 20% piperidine/DMF | Removed by standard Fmoc chemistry | UV chromophore complicates detection; not appropriate for hydrogenation-sensitive routes |
| CBZ-L-Proline | Hydrogenolysis over Pd/C | Stable to piperidine/DMF | Opposite optical rotation; yields L-proline turns |
If the target molecule contains an alkene, benzyl ester, nitro group, or sulfur-containing heterocycle that poisons palladium, CBZ-D-Proline must be replaced with Fmoc-D-Proline or Boc-D-Proline before assembly. The Cbz group is removed by hydrogenation over 5% palladium on carbon at 0.2–0.5 MPa hydrogen pressure; under these conditions an unprotected aryl iodide may be reduced to the dehalogenated product and a benzyl ester may cleave to the carboxylic acid. Sulfur-containing substrates can bind to the catalyst and require unreasonably high catalyst loadings, which increases the risk of heavy-metal contamination in the final drug-linker conjugate. Where hydrogenation is acceptable, residual palladium is controlled by treatment with trimercaptotriazine or activated carbon filtration, and the final peptide is analyzed for Pd by ICP-MS with an acceptance threshold commonly set at 10 ppm or lower for phase-appropriate material.
In solution-phase routes, the free acid is typically coupled as the pentafluorophenyl or N-hydroxysuccinimide ester rather than as the unprotected carboxyl when the downstream amine is highly hindered. The active ester is generated with dicyclohexylcarbodiimide in ethyl acetate at 0 °C and used without isolation because the isolated active ester hydrolyzes on extended storage. For large-scale batches, the acid chloride is avoided because the strongly acidic by-products promote partial loss of the Cbz group and darken the reaction mixture. In peptide synthesis, the replacement of L-proline with CBZ-D-Proline in a turn-forming sequence often changes the major cyclization product from a monomeric cyclic peptide to a dimer or vice versa, because the geometry of the activated ring junction alters ring-chain equilibria. This effect is observed in macrocyclization reactions performed at 0.001 M substrate concentration with pentafluorophenyl ester activation; the D-epimer can give a different monomer-to-dimer ratio by more than 10-fold in conformationally constrained tetrapeptides.
On pilot-scale batches, the main deviation observed is not enantiomeric contamination but residual solvent retention in the crystalline lattice. Ethyl acetate used for recrystallization can be retained at levels above 0.5% when drying time is shortened or vacuum below 25 mbar is not reached. The retained solvent suppresses the apparent assay and alters the optical rotation; therefore, production-scale vacuum trays are run with a minimum surface depth of 2 cm and monitored by headspace gas chromatography. In hydrogenation deprotection, exothermic temperature rise above 35 °C has been linked to excessive pressure drop and catalyst attrition in gas-entrainment impeller autoclaves; the control measure is staged hydrogen addition at 0.2 MPa until uptake ceases, followed by repressurization to 0.5 MPa for completion. When the substrate contains sulfur, the palladium catalyst is poisoned and hydrogenation stops; the batch is then reworked by switching to Fmoc-D-Proline, because fouled catalyst cannot be regenerated economically at small scale.
