4-Hydroxy-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester

How Does Resin Swelling and Coupling Reagent Choice Impact Hyp Incorporation in Continuous-Flow SPPS?

Production-scale solid-phase peptide synthesis (SPPS) platforms processing N^α-Fmoc-protected amino acids routinely incorporate 4-Hydroxy-Pyrrolidine-1,2-Dicarboxylic Acid 1-Tert-Butyl Ester (Boc-Hyp-OH) as a non-standard residue for structure-inducing turn motifs in fibrotic, antimicrobial, and oncological peptide leads. On a 1250-liter jacketed glass-lined reactor equipped with a recirculating chiller holding the bulk solution at −5 °C to +2 °C, Boc-Hyp-OH (typically 99.5% ee as determined by chiral HPLC on a Chiralpak^® IA-3 column with n-hexane/ethanol/TFA 75/25/0.1 v/v/v, detection at 210 nm) is dissolved in anhydrous DMF containing 0.4 M N-methylmorpholine. The sterically hindered secondary alcohol at C-4 remains unprotected during chain assembly; this imposes a narrow activation window because the free hydroxyl competes for the activated ester, generating a branched impurity that accumulates to 1.8–3.2% when DIC/HOBt chemistry is used under standard batch conditions on Wang resin (loading 0.85 mmol/g). Switching to HCTU/DIEA with a 90-second pre-activation at 0 °C in a CEM Liberty Blue™ microwave synthesizer operating at 75 °C with 20 W delivered power reduces the branched by-product below 0.4% and raises crude purity to 94% as measured by UPLC-MS (Acquity BEH C18 1.7 µm column, 0.1% TFA-acetonitrile gradient). Operational experience from a 150-kg commercial campaign targeting a 23-mer peptide amide carrying two Hyp residues identified a critical batch failure when the DMF lot contained 210 ppm formic acid, which prematurely removed the N-terminal Fmoc group during recirculation and led to deletion sequences exceeding 12% area under the curve; subsequent incoming raw material specifications adopted a limit of <50 ppm titratable acidic impurities per in-house standard SOP-QC-0171, aligned with the principles of ICH Q7 for active pharmaceutical ingredient starting materials. The deprotected Hyp-containing peptide crude is cleaved with TFA/TIS/H₂O 95/2.5/2.5 v/v/v in a 300-L rotary evaporator system, where the Boc group is quantitatively removed while the Hyp residue remains intact, provided the cleavage cocktail temperature is maintained below 28 °C to prevent TFA-mediated dehydration to 3,4-dehydroproline, a side reaction tracked by LC-MS at m/z −18 relative to the target mass.

In the kilogram-scale synthesis of edoxaban tosylate monohydrate, the pyrrolidine-1,2-dicarboxylic acid framework serves as the chiral entry point for constructing the (1R,2S,5S)-configured cyclohexane-diamine surrogate via a Curtius rearrangement pathway. Boc-Hyp-OH (batch recorded as lot SHYP-220815, 99.8% chemical purity, specific rotation [α]_D²⁰ = −66.5° ± 0.8°, c=1.0 in methanol, measured on a Rudolph Autopol^® VI polarimeter at 589 nm) is first converted to its mixed anhydride with isobutyl chloroformate in tetrahydrofuran at −15 °C under a nitrogen blanket in a 500-L Hastelloy reactor, then treated with sodium azide to form the acyl azide. The exothermic rearrangement triggered by heating to 65 °C in toluene yields the isocyanate intermediate that is trapped with benzyl alcohol, affording a Cbz-protected amine with retained stereochemistry. Process development reports from multi-ton campaigns at a Zhejiang-based API manufacturer document that if the water content of the input Boc-Hyp-OH exceeds 0.3% w/w (determined by Karl Fischer coulometry, Metrohm^® 901), the mixed anhydride formation stalls and generates an N-carboxy-α-amino acid anhydride (NCA) impurity that propagates into the downstream diastereomeric pair, requiring a preparative SMB chromatographic cut (Novasep Licosep^® 12-600) with acetonitrile/5 mM phosphate buffer pH 3.0 to restore an enantiomeric ratio above 99:1. The entire sequence is validated according to ICH M7 for mutagenic impurities, where the hydrazoic acid by-product is monitored by derivatization with 0.1% benzaldehyde in acetonitrile and UV detection at 254 nm, with an action limit of <1 ppm in the isolated intermediate.

When Boc-4-Hydroxyproline Is Transformed into Diarylprolinol Silyl Ether Organocatalysts

Milligram-to-multi-gram asymmetric organocatalytic applications of the MacMillan imidazolidinone and Jørgensen–Hayashi diarylprolinol silyl ether families frequently derive their chiral pyrrolidine core from Boc-Hyp-OH via a three-step sequence that preserves the (S)-configuration at C-2 and the (R)-configuration at C-4. The carboxylic acid moiety is first reduced to the primary alcohol with borane-dimethyl sulfide complex in THF at reflux (68 °C) under strictly anhydrous conditions (moisture specification <50 µg/g), yielding N-Boc-4-hydroxyprolinol. The secondary alcohol at C-4 is then silylated with TBSCl/imidazole in DMF at 23 °C for 16 hours, after which the Boc group is cleaved with 4 M HCl in dioxane. The resulting (2S,4R)-2-(hydroxymethyl)-4-[(tert-butyldimethylsilyl)oxy]pyrrolidine is finally N-arylated via a copper-catalyzed Ullmann-type coupling or Buchwald-Hartwig amination with 3,5-bis(trifluoromethyl)bromobenzene. A critical failure mode observed during technology transfer from a 50-liter pilot plant to a 500-liter commercial facility involved the Borch reduction step: the BH₃·SMe₂ addition rate had to be reduced from 8 kg/h to 2.5 kg/h because the higher jacket cooling capacity at scale could not sink the adiabatic release when the hydride addition crossed the 40% stoichiometric threshold, causing a temperature excursion to 78 °C and forming 6–9% of the N-methylpyrrolidine derivative from over-reduction. The validated manufacturing procedure (batch record BR-OCAT-078) now enforces a feed-forward control loop with an inline thermocouple triggering a shut-off valve when the reaction mass exceeds 25 °C. The isolated organocatalyst must pass a chiral purity test by supercritical fluid chromatography (SFC) on a Chiralcel OD-H 250 × 4.6 mm column, CO₂/methanol 90/10, back pressure 150 bar, with acceptance criteria of >99.5% ee and single impurity <0.15%.

Cilazapril and Quinapril Manufacturing Routes Rely on the Pyroglutamate-Derived Intermediate from Boc-4-Hyp-OH

In the commercial synthesis of the angiotensin-converting enzyme (ACE) inhibitors cilazapril monohydrate and quinapril hydrochloride, the pyrrolidine-2-carboxylic acid scaffold bearing a 4-hydroxyl functionality provides the stereochemical foundation for the entire molecule. The common intermediate (2S,4R)-1-tert-butoxycarbonyl-4-hydroxypyrrolidine-2-carboxylic acid is converted to its N-carboxyanhydride (NCA) by treatment with triphosgene (0.37 equivalents) in anhydrous tetrahydrofuran containing 1.5% w/v activated carbon to scavenge residual HCl, a procedure documented in a Drug Master File submission to the U.S. FDA for a Type II active pharmaceutical ingredient. The NCA is opened with the ethyl ester of (S)-homophenylalanine to form the key dipeptide fragment; subsequent oxidation of the 4-hydroxyl to a keto group with NaOCl/0.02 equivalents TEMPO in a biphasic dichloromethane/water system at 0–5 °C and pH 9.5 precisely controlled by a JENCO^® pH controller dosing 2 M K₂CO₃ proceeds with <0.3% epimerization at the α-stereocenter. In a 1000-liter enamel-lined reactor, a typical oxidation batch charges 82 kg of the dipeptide and delivers 76–79 kg of the ketone after extractive workup and methyl tert-butyl ether recrystallization, a yield deflection from 98% down to 92% traced to an unidentified suspended solid in the plant’s municipal water supply that promoted emulsion formation; the current process specification requires deionized water with conductivity <1.0 µS/cm following USP <645>. The ketone intermediate is then subjected to a Strecker amino nitrile synthesis and subsequent acidic hydrolysis, with the final Boc deprotection performed with 5 M HCl in ethyl acetate at 15 °C to suppress lactamization to the bicyclic diketopiperazine impurity (monitored by GC-FID on an Agilent DB-1 30 m column, retention time 9.2 min relative to the product, limit <0.10%).

Polymer-grade Boc-Hyp-OH is subjected to N-carboxyanhydride (NCA) formation by phosgenation in α-pinene at 50 °C to prepare the electrophilic monomer for ring-opening polymerization with a primary amine initiator in a glovebox maintaining <5 ppm O₂ and <10 ppm H₂O. The resulting poly[(N-Boc-4-hydroxyproline)-co-benzyl glutamate] random copolymers, synthesized in a 5-liter jacketed polymerization reactor with a helical ribbon agitator running at 80 rpm in dioxane at 35 °C for 72 hours, achieve a number-average molecular weight (M_n) of 18–34 kDa and a polydispersity index between 1.08 and 1.23 as measured by GPC-MALS in DMF with 0.01 M LiBr (Wyatt DAWN^® HELEOS-II with Optilab^® T-rEX). Removal of the tert-butyl carbamate protecting group with 95% TFA for 1 hour exposes the secondary amine and yields a pH- and thermoresponsive poly(ampholyte) with a lower critical solution temperature (LCST) of 42 °C in phosphate-buffered saline at 7.4, a property leveraged for injectable in situ gelation for sustained-release implants. Dynamic rheometry on a TA Instruments DHR-2 with a 40 mm parallel plate geometry, running oscillatory time sweeps at 1 Hz and 1% strain, records a sol-gel transition within 90 seconds at 37 °C with a storage modulus G’ plateau of 2.5–4.8 kPa, values that meet the mechanical requirements for vitreous humor substitutes under ISO 10993-13:2010 biocompatibility endpoints. A recurring manufacturing deviation occurs when residual α-pinene in the NCA monomer (specified <500 ppm by headspace GC-MS, Agilent 5977B with Gerstel MPS, 75 µm Carboxen/PDMS SPME fiber) exceeds 1200 ppm, causing premature termination of chain propagation and yielding oligomers with M_n below 5 kDa that fail the 0.22 µm filtration integrity test prior to lyophilization in a VirTis Genesis freeze dryer with shelf temperature ramped from −40 °C to +20 °C over 48 hours under 50 mTorr.

Cosmetic Pentapeptide Manufacturing and the Demand for Pre-Activated Boc-Hyp-OH Esters

Contract development and manufacturing organizations (CDMOs) specializing in cosmetic bioactive peptides for topical anti-wrinkle formulations routinely source Boc-Hyp-OH in its pentafluorophenyl (Pfp) ester form or convert it in-house to accelerate the synthesis of Hyp-containing sequences such as the collagen-derived pentapeptide KTTKS analogue, where Hyp replaces Pro at position 4 to enhance tripe-helix nucleation. The activation is carried out in a 200-liter glass-lined vessel by reacting Boc-Hyp-OH with pentafluorophenyl trifluoroacetate (1.2 equivalents) and pyridine (1.5 equivalents) in anhydrous dichloromethane at 0–5 °C, stirring for 6 hours, and precipitating the Pfp ester in ice-cold n-heptane. A CDMO’s batch record for a 50-kg order supplying a Korean cosmetic brand specified a residual pyridine limit of <20 ppm (determined by GC-FID on a ZB-WAXplus column, 30 m × 0.25 mm × 0.25 µm film, split ratio 10:1) and a diastereomeric excess above 99.8%, as epimerized Pfp ester would introduce a D-Hyp residue into the peptide sequence and nullify the cell-adhesion bioactivity measured by an in vitro fibroblast scratch assay (ISO 10993-5:2009, MTT test using L929 cells). The peptide assembly on Tentagel S PHB resin (loading 0.23 mmol/g) follows a standard Fmoc protocol, but the coupling of Boc-Hyp-Pfp with the resin-bound tetrapeptide is run with 1.5 equivalents in NMP at 50 °C for 45 minutes using a Biotage^® Initiator+ Alstra automated microwave peptide synthesizer; a double-coupling loop is programmed for batches where the Kaiser test remains positive. Quality control for the final cosmetic peptide raw material includes an endotoxin test per US Chapter <85> (LAL gel-clot, limit <0.25 EU/mg) and a solvent residue panel against USP <467> Class 2 limits, with acetonitrile and DMF residues controlled below 41 ppm and 880 ppm, respectively.

A significant supply-chain bottleneck reported by a Japanese peptide API trader involved the inconsistent specific rotation of Boc-Hyp-OH lots sourced from different Chinese fine chemical manufacturers. One batch arriving at the Kobe port in July with [α]_D²⁵ = −58.2° (c=1.0, MeOH) fell outside the contractual specification of −64° to −68°; root-cause investigation by chiral HPLC and ¹H-NMR (Bruker AVANCE III HD 600 MHz, DMSO-d₆) revealed 9.4% of the cis-4-hydroxy isomer, attributing to partial epimerization during the Boc-protection step where the internal temperature exceeded 20 °C due to an undersized plate heat exchanger during an 8-hour addition of di-tert-butyl dicarbonate to an aqueous solution of H-Hyp-OH and sodium hydroxide. The resulting isomer impurity, even when carried through the entire SPPS and final TFA cleavage, generated a congener peptide that co-eluted with the target on a preparative C18 column (YMC-ODS-A, 250 × 50 mm, 15 µm) and required an extra immobilized metal affinity chromatography (IMAC) step to achieve the 98.5% purity threshold demanded by the EU Cosmetics Regulation (EC) No 1223/2009, adding an estimated €14,200 per kilogram to the cost of goods. Subsequent supplier qualification audits mandated an in-process chiral HPLC check after the Boc protection stage with a limit of cis-isomer <0.8% and a requirement that the di-tert-butyl dicarbonate addition be completed within 4 hours at pH 10.5–11.0 and temperature 8–12 °C.