1-(9H-Fluorenylmethoxycarbonyloxy)-2,5-Pyrrolidinedione

Why Does Solid-Phase Peptide Synthesis Demand Anhydrous Fmoc-OSu Activation?

In cGMP manufacture of therapeutic peptides exceeding 30 residues, the optical and chemical purity of the Fmoc-amino acid monomer directly governs crude product homogeneity and final purification yield. Fmoc-OSu is the preferred acylating agent to convert free L-amino acids into their N-α-Fmoc derivatives because the by-product, N-hydroxysuccinimide, remains water-soluble and does not generate the dipeptide or tripeptide contaminants observed with Fmoc-Cl-mediated reactions. A typical process loads 1.05–1.10 molar equivalents of Fmoc-OSu relative to the amino acid dissolved in a 1:1 (v/v) water–1,4-dioxane mixture. Sodium carbonate is maintained at 1.2 eq to keep the α-amine nucleophilic. The dioxane solution of Fmoc-OSu is metered into the chilled amino acid slurry over 45–60 minutes while the jacket temperature is held at 0–5 °C to suppress racemization at the Cα center. At-line TLC (silica gel 60 F₂₅₄, eluent: chloroform/methanol/acetic acid 90:8:2) confirms consumption of the amino acid before the batch is warmed to 20±2 °C and stirred for a further 2 h. The aqueous phase is washed with methyl tert-butyl ether (2 × 1.5 L/kg amino acid), acidified to pH 2.0–2.5 with 6 M HCl, and the precipitated Fmoc-amino acid is isolated on a pressure filter–dryer under nitrogen. Both the filter-dryer (pore size 10–20 µm) and all transfer lines are purged to maintain residual moisture below 0.2% by Karl Fischer titration, critical for downstream DIC/HOBt activation where free water initiates diketopiperazine formation. The product is vacuum-dried at 40 °C and ≤5 mbar until loss on drying is <0.5%. Chiral HPLC according to USP 〈621〉 on a Chiralpak® IA column (mobile phase: n-hexane/2-propanol/trifluoroacetic acid 80:20:0.1) routinely records <0.3% D-enantiomer. Identification is cross-validated by specific optical rotation against Ph. Eur. 5.2.34 monograph limits for protected amino acids. Residual dioxane and MTBE are quantified by headspace GC per ICH Q3C and must fall below 380 ppm and 500 ppm, respectively, when the derivative is destined for injectable peptide API production. The terminal product is milled to D₅₀ <100 µm using a jet mill with chilled nitrogen to avoid ring-opening of the Fmoc group. These monomers are directly loaded onto automated peptide synthesizers such as the CSBio II with 5–50 mmol scale reactors to manufacture GLP-1 agonists, teriparatide, and calcitonin analogues, where batch records reference FDA 21 CFR Part 11-compliant electronic signatures at each acylation cycle.

In cGMP production of therapeutic peptides exceeding 30 residues, the optical and chemical purity of the Fmoc-amino acid monomer directly governs crude product homogeneity and final purification yield. This reality drives the selection of Fmoc-OSu over chloroformate reagents when the Cα center must retain L-configuration above 99.5% as documented under Ph. Eur. 5.2.34. A charge of L-leucine (1.00 mol) suspended in a 1:1 (v/v) water–1,4-dioxane mixture with 1.20 mol sodium carbonate is kept below 3 °C while a dioxane solution of Fmoc-OSu (1.08 mol) is diluted to 3 volumes and metered through a mass flow controller over 75 min. The low-temperature window prevents oxazolone formation that would otherwise raise the D-isomer content above 0.5%. After acidification to pH 2.2 with 6 N HCl, the precipitated Fmoc-Leu-OH is collected on a centrifuge with 10 µm cloth, washed with water until conductivity of the filtrate drops below 50 µS/cm, and dried in a double-cone vacuum dryer at 35 °C/5 mbar. Release tests include residual N-hydroxysuccinimide by ion-exchange chromatography with conductivity detection (LOD 50 ppm) and residual dioxane by headspace GC–FID per USP 〈467〉 Class 2 solvent limits. The milled powder is filled into laminated aluminium bags under ≤5% RH dry nitrogen, directly feeding 96-well solid-phase synthesis blocks producing corticorelin acetate, where the first-residue attachment to Wang resin requires a precisely controlled 1.0 eq Fmoc-AA/DIC/HOBt coupling to avoid diastereomeric enrichment.

When a non-proteinogenic amino acid is synthesized in-house because commercial sources lack batch-to-batch reproducibility, Fmoc-OSu remains the only reagent able to install the base-labile Fmoc group without deuterating the side-chain protecting groups. For example, Fmoc-β-cyclohexyl-L-alanine is prepared by dissolving the free amino acid in 10% aqueous Na₂CO₃ and tetrahydrofuran (1:2 v/v) and adding 1.10 eq Fmoc-OSu at 22±2 °C over 30 min. After 4 h the THF is stripped on a rotary evaporator at ≤40 °C and the pH adjusted to 2.8, precipitating the product which is recrystallized from ethyl acetate/n-heptane to remove the Fmoc-β-Ala-OH impurity formed by β-elimination. 1H NMR (400 MHz, DMSO‑d₆) integration of the Fmoc methylene signal versus the Cα proton serves as the release criterion; any deviation >±0.5% from theoretical triggers a re-slurry campaign. The final crystallized solid feeds fragment condensation strategies for constrained cyclic peptides, where the steric bulk of the cyclohexyl ring narrows the disulfide bond isomerization window to pH 8.0–8.3, a processing constraint that demands Fmoc removal via 20% piperidine/DMF within 4 min to avoid diketopiperazine elimination. Terminal products include selective CXCR4 antagonists currently in Phase II trials.

At commercial kilogram scale, the exotherm during Fmoc-OSu addition on non-polar amino acids such as phenylalanine or tryptophan must be managed with jacket cooling capable of removing −85 kJ/mol within the first 15 min. A 500 L glass-lined reactor with a retreat-curve impeller set to 85 rpm is charged with the aqueous alkaline amino acid solution. The Fmoc-OSu dioxane stream is fed through a 0.5 µm inline filter to remove any particulate Fmoc-B-OH that could nucleate premature crystallization. On-line FTIR monitoring of the carbonyl stretch at 1820 cm⁻¹ (cyclic imide) and 1720 cm⁻¹ (carbamate formation) generates a real-time conversion curve; the feed is terminated when the 1720 cm⁻¹ absorbance plateaus for 10 min. Work-up skips aqueous extraction and instead relies on crystallization from a 2:3 ethanol/water mixture under a controlled cooling ramp of 0.3 °C/min to obtain a cubic crystal habit with a mean aspect ratio of 1.2, which improves filterability by 40% relative to needle-like aggregates. The filtered crystal cake is washed with pre-cooled 5% ethanol and dried under a nitrogen sweep while the jacket is ramped from 30 °C to 45 °C at 2 °C/h. This tightly defined drying profile prevents the glass transition that amorphizes the surface layer, preserving free-flowing properties for automated solid-phase synthesis dispensing. Trace endotoxins, relevant only when the monomer flows into injectable drug substance lines, are controlled below 0.06 EU/mg as determined by LAL kinetic chromogenic assay per USP 〈85〉. The purified material is assigned a retest period of 36 months at −20±5 °C based on accelerated stability studies at 40 °C/75% RH tracking Fmoc cleaved free amine by OPA derivatization and HPLC fluorescence.

In liquid-phase peptide fragment condensation, the requirement to temporarily protect the N-terminus of a C-terminal methyl ester intermediate without disturbing an acid-labile side-chain protecting group forces the use of Fmoc-OSu when the corresponding Fmoc-amino acid building block is not commercially available. The fragment H-Gly-Phe-OMe·HCl (1.0 eq) is dissolved in anhydrous DMF and treated with diisopropylethylamine (2.5 eq) to neutralize the hydrochloride and generate the free amine. An anhydrous DMF solution of Fmoc-OSu (1.02 eq) is added dropwise at 0–5 °C and the mixture is stirred for 60 min before being allowed to reach 20 °C. After 2 h total reaction time, the conversion is checked by HPLC on a C18 column (gradient 30–90% acetonitrile in 0.1% TFA over 15 min). The absence of the starting fragment at Rt 4.2 min triggers work-up. The batch is diluted with 10 volumes of ethyl acetate, washed with 1 N HCl (to remove excess DIEA), 5% NaHCO₃ (to strip N-hydroxysuccinimide), and brine, then dried over Na₂SO₄. The organic phase is concentrated on a rotary evaporator with bath temperature held below 35 °C to avoid diketopiperazine rearrangement. The residue is purified by flash chromatography on silica gel (230–400 mesh) eluting with 2% methanol in dichloromethane. Fractions containing the product are combined and evaporated to a foam, which is redissolved in dioxane and lyophilized to give a fluffy white powder. The isolated Fmoc-Gly-Phe-OMe is characterized by LC–MS ([M+H]⁺ 473.2) and the purity specification is set at ≥98.5 area% with any single impurity <0.5%. This fragment is subsequently used for 3+2 segment coupling in a total synthesis of a macrocyclic depsipeptide, where the C-terminal methyl ester is saponified with LiOH in THF/water 3:1 at 0 °C to yield the acid segment without β-elimination of the protected serine residue. The entire process is performed under ICH Q7 guidelines for active pharmaceutical ingredients when the final compound exceeds 10 g batch size for toxicology supply.

Fmoc Protection of Non-Peptide Heterocyclic Amines for cGMP Intermediate Synthesis

In the route to a hepatitis C NS5A inhibitor, the key intermediate 6-amino-2-phenylbenzoxazole requires temporary masking of the primary aromatic amine to allow Pd-catalyzed Suzuki coupling at the 5-bromo position. The free amine strongly coordinates palladium, shutting down catalytic turnover, thus a bulky, base-labile protecting group that does not poison the catalyst is mandatory. Fmoc-OSu is added in one portion (1.25 eq) to a vigorously stirred suspension of 6-amino-2-phenylbenzoxazole (1.0 eq) and NaHCO₃ (1.5 eq) in 1:1 acetonitrile/water at 22±3 °C. After 6 h, HPLC monitoring (XBridge C18 3.5 µm, 4.6×100 mm, detection at 254 nm) indicates >97% conversion to the mono-Fmoc adduct. The acetonitrile is distilled off under reduced pressure and the aqueous suspension is extracted with ethyl acetate. The organic layer is washed with 5% NaHCO₃ and brine, dried with anhydrous Na₂SO₄, and concentrated to ~2 volumes. While hot, n-heptane is added to crystallize the product as an off-white crystalline solid. The crystals are filtered, washed with 1:5 ethyl acetate/n-heptane, and vacuum-dried at 50 °C to yield the Fmoc-protected benzoxazole with a purity of >99% by HPLC and an isolated yield of 88%. Crucially, 0.5 mol% of Pd(PPh₃)₄ subsequently loaded in the Suzuki step maintains full activity after 4 h reflux in toluene/water, confirming that residual N-hydroxysuccinimide and related species are absent. The Fmoc group is removed later with 5% piperidine/DMF at room temperature, regenerating the free amine without opening the benzoxazole ring. The entire manufacture is conducted in an ISO 7 cleanroom under FDA 21 CFR 210/211 cGMP because the downstream API is filed in an IND. A process validation batch demonstrates that paddle speed of 180 rpm in the acetonitrile/water slurry step produces a crystal-size distribution with D₉₀ <250 µm that ensures uniform heating in the vacuum oven. Residual acetonitrile is controlled below 410 ppm compliant with ICH Q3C, and nitrosamine risk assessment per EMA/CHMP/428186/2019 is completed with no nitrosating agents in the process. The final des-Fmoc intermediate is the direct precursor to daclatasvir, a blockbuster antiviral.

When a linker-payload conjugate for an antibody–drug conjugate (ADC) requires a monoprotected polyethylene glycol diamine, Fmoc-OSu is the reagent of choice to differentiate the two amino termini. An α-amino-ω-carboxy PEG₂₄ (average Mn 1,100 Da, polydispersity index <1.05) is dissolved in anhydrous dichloromethane containing DIEA (2.0 eq) and cooled to 0 °C. A solution of Fmoc-OSu (1.03 eq) in dry DCM is added dropwise over 30 min while the temperature is maintained below 5 °C. The mixture is allowed to warm to 20 °C and stirred for 12 h. The reaction is quenched with 0.1 N HCl, the organic phase separated, washed with water until neutral, and concentrated. The crude viscous oil is precipitated from cold MTBE (−20 °C) to give a waxy solid that is collected by filtration and vacuum-dried at 25 °C for 18 h. ¹H NMR integration of the 9-fluorenylmethoxy CH₂ singlet at δ 4.40 against the PEG backbone signal at δ 3.64 calculates a functionality of 92–95%. Additional Fmoc-OSu treatment to force the reaction to >98% typically generates 5–10% di-Fmoc impurity, which cannot be removed by flash chromatography because of the near-identical polarity; the specification thus allows the monofunctional product to contain <8% di-Fmoc-PEG-carboxylic acid, which does not interfere with the subsequent maleimidocaproyl conjugation since only the free amine participates in NHS ester-mediated amidation. The Fmoc-PEG₂₄-COOH intermediate is stored at −20 °C under argon and used within 7 days because the Fmoc group undergoes slow elimination catalyzed by residual basic sites in the PEG backbone. The eventual Fmoc deprotection with 2% DBU/DMF liberates the ω-amine for reaction with the maleimidocaproic acid NHS ester, after which the linker-payload is conjugated to a cysteine-engineered trastuzumab scaffold under pH 7.0 borate buffer. The ADC drug substance is controlled to a drug-to-antibody ratio of 3.4–3.8 as measured by hydrophobic interaction chromatography per Ph. Eur. 2.2.46. The entire linker synthesis is performed in dedicated stainless-steel reactors previously qualified for Trace Grade C residual metals per EMA guideline on metal catalysts, because even 1 ppm palladium would deactivate the maleimide coupling step.

When PEG₈‑Diamine Must Be Selectively Monofunctionalized with Fmoc for Solid‑Phase Linker Construction

Short-chain PEG diamine (Mn ~400 Da) poses a selectivity challenge because both termini possess near-identical nucleophilicity; the mono-Fmoc adduct is obtained only under kinetic control. The diamine (1.0 eq) is dissolved in dichloromethane at a concentration of 0.2 M and cooled to −15 °C. A dilute solution of Fmoc-OSu (0.95 eq) in DCM is added through a syringe pump at 0.5 mL/min with vigorous stirring. The substoichiometric amount and low temperature force the reaction to stop after the first acylation event, leaving unreacted diamine that is removed via extraction with 10% citric acid after the reaction. The organic phase is then washed with 0.1 M NaOH to hydrolyze any unreacted Fmoc-OSu, dried, and evaporated. The residue is purified by chromatography on neutral alumina (activity Grade I) to separate the mono-Fmoc from the di-Fmoc by-product. The pure fraction is concentrated to an oil that solidifies at 4 °C. The mono-Fmoc-PEG₈-amine is then loaded onto 2-chlorotrityl chloride resin (1.2 mmol/g) in DCM with DIEA, establishing a cleavable linker platform for the synthesis of branched peptide dendrimers. After Fmoc removal with 20% piperidine/DMF, the free amine is coupled with a trimeric lysine core, and subsequent cycles build a multimeric vaccine epitope. The entire solid-phase protocol is executed on a Symphony X synthesizer, and the linker-loading efficiency is quantified by spectrophotometric measurement of the Fmoc dibenzofulvene adduct at 301 nm (ε = 7,800 M⁻¹cm⁻¹). Should the mono-Fmoc intermediate contain over 5% di-Fmoc, the resin capacity drops proportionally, leading to a final peptide-dendrimer with unacceptable deletion sequences. Quality control of the mono-Fmoc-PEG₈-amine thus mandates NMR purity >96% and amine value titration per ASTM D2074-07 to confirm ≥1.05 mmol/g primary amine. The terminal product is a four-arm peptide vaccine candidate administered in a Phase I oncology trial, triggering the requirement for ICH Q8 design-space verification on the mono-Fmoc PEG linker step, where safe operating ranges are established for temperature (−18 to −10 °C) and Fmoc-OSu addition rate (0.4–0.7 mL/min).

Fmoc-Hydrazine Synthesis via Nucleophilic Substitution—Stoichiometric Boundaries and By-Product Control

Hydrazine monohydrate reacts with Fmoc-OSu at nearly diffusion-controlled rates, generating Fmoc-hydrazine as the kinetic product along with significant di-Fmoc-hydrazine if the local concentration of Fmoc-OSu spikes. To bias the reaction toward mono-protection, a 2.0:1 molar ratio of hydrazine to Fmoc-OSu is employed. Hydrazine hydrate (64% N₂H₄, 2.0 mol) is dissolved in 4 volumes of water and cooled to 5 °C. A solution of Fmoc-OSu (1.0 mol) in tetrahydrofuran (2 volumes) is added dropwise so that the internal temperature does not exceed 8 °C. After 30 min of addition, the ice bath is removed and the mixture stirred for 1 h at room temperature. Tetrahydrofuran is removed by rotary evaporation and the aqueous residue is extracted with ethyl acetate (3 × 2 volumes). The combined organic extracts are washed with brine, dried over Na₂SO₄, and concentrated under reduced pressure to give a viscous oil that slowly crystallizes upon trituration with cold n-hexane. The crude solid is recrystallized from 1:3 ethanol/water to afford white needles of Fmoc-hydrazine with 95–97% purity. The mother liquor enriches in di-Fmoc-hydrazine, which can be isolated and treated with hydrazine in a recycling loop. ¹H NMR (400 MHz, CDCl₃) reveals a characteristic singlet at δ 6.20 (NHNH₂) that integrates for 1.9–2.1 protons relative to the Fmoc CH₂ signal, confirming monofunctionality. Residual hydrazine content is determined by iodometric titration per USP 〈541〉 and must be <0.1% before the material is released for bioconjugation. The final Fmoc-hydrazine is utilized in the preparation of glyoxylyl peptide aldehydes for native chemical ligation: the hydrazide is activated with NaNO₂ at −15 °C in pH 3.0 buffer to generate the acyl azide in situ, which is then condensed with a C-terminal thioester peptide to form a native amide bond. The entire ligation protocol is performed under ISO 5 laminar flow when the conjugated product is a sterile injectable. Regulatory agencies require that the Fmoc-hydrazine reagent be tested for genomic impurities via Ames II assay (OECD 471) with allowable impurity limits set below the threshold of toxicological concern (1.5 µg/day) per ICH M7(R2).