Fmoc-HomoArg(Boc)₂-OH (CAS 158478-81-6; molecular formula C₃₂H₄₂N₄O₈; molecular weight 610.70 g/mol) is a protected L-homoarginine derivative intended for Fmoc/tBu solid-phase peptide synthesis. The product designated as Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable functions as a pharmaceutical-grade building block in the manufacture of peptide active pharmaceutical ingredients; it is not formulated directly as a finished tablet, capsule, granule, or injectable. Synonyms include Fmoc-L-homoarginine(Nω,Nω'-di-Boc)-OH. The α-amino group is masked by the base-labile 9-fluorenylmethoxycarbonyl group, while the guanidino side chain carries two acid-labile tert-butoxycarbonyl groups. This orthogonal protection allows selective α-amino deprotection with piperidine without premature side-chain loss. Typical release data for pharmaceutical-grade material include chromatographic purity ≥98.0 area% by reverse-phase HPLC at 220 nm, water content ≤0.5%, and residual solvent control according to ICH Q3C. Manufacturing is conducted under ICH Q7 quality expectations, with elemental impurity risk assessed per ICH Q3D and change control aligned to 21 CFR 210/211 where applicable.

What Limits Orthogonal Deprotection Efficiency in Fmoc/tBu Solid-Phase Synthesis?

The Fmoc group is removed under basic conditions that must not cleave the two guanidino Boc groups. In standard SPPS, Fmoc deprotection is performed with 20% v/v piperidine in DMF at 18–25°C using two 5-minute treatments. Under these conditions, premature Boc loss is typically ≤0.5% over a 20-minute piperidine exposure as measured by RP-HPLC of the deprotection solution. The deprotection reaction proceeds through β-elimination to form dibenzofulvene; the dibenzofulvene-piperidine adduct is UV-active and can be monitored at 301 nm. Incomplete Fmoc removal leads to deletion peptides because the residual Fmoc-protected amine cannot participate in the next coupling step. After deprotection, washing with DMF is performed 6–8 times at 10–20 mL/g resin to remove piperidine; residual piperidine in the next coupling can remove Fmoc from the incoming monomer and cause premature chain extension.

Deprotection efficiency is influenced by resin swelling, solvent viscosity, and agitation. A PTFE-fritted reactor with overhead agitation at 150–300 rpm and a solvent volume of 10–20 mL/g resin provides consistent removal on polystyrene resins crosslinked with 1% divinylbenzene. Moisture in DMF above 0.1% w/w reduces active ester stability and is controlled by using fresh, amine-free DMF stored over activated molecular sieves. Coupling of Fmoc-HomoArg(Boc)₂-OH is sterically demanding because the two Boc groups on the guanidino side chain increase the volume of the activated species. Pre-activation with 1-hydroxybenzotriazole and diisopropylcarbodiimide or with ethyl (hydroxyimino)cyanoacetate and diisopropylcarbodiimide is conducted at 0–4°C for 3–5 minutes before transfer to the resin. Coupling onto hindered resin-bound amino groups may require a second coupling or an extended reaction time of 4–16 hours. Incomplete coupling leaves free amino groups that are capped with acetic anhydride/pyridine to terminate deletion sequences. The process conflict is that longer coupling times increase the risk of on-resin racemization and diketopiperazine formation at dipeptide stages; therefore, the coupling protocol is adjusted batch-to-batch based on Kaiser or chloranil test results. Published data for the exact kinetic parameters of this derivative on all resin types is limited; process development runs are recommended before scale-up.

Specification Profile and Release Limits for Fmoc-HomoArg(Boc)2-OH

A representative release specification for pharmaceutical-grade material includes the parameters shown in Table 1. The material is typically isolated by precipitation from ethyl acetate/heptane or by lyophilization from acetonitrile/water, then dried under vacuum at 40°C for 12–24 hours to achieve water and residual solvent limits. Batch sizes from 1 kg to 50 kg are processed in jacketed filter dryers with nitrogen inerting; vacuum levels below 10 kPa are applied to remove solvents. Variation between batches is controlled by in-process HPLC after Fmoc protection and Boc protection steps.

Identification is confirmed by electrospray ionization LC-MS in positive mode; the protonated molecular ion is observed at m/z 611.7 ± 0.5. The RP-HPLC method uses a C18 column with an acetonitrile/water gradient containing 0.1% v/v trifluoroacetic acid, and detection at 220 nm exploits the Fmoc chromophore. Residual solvents are measured by headspace gas chromatography with flame ionization detection. The material is tested against the applicable methods of Ph Eur and USP where methods exist; because this protected amino acid is not a pharmacopoeial substance, the specification is derived from the manufacturer’s filed drug master file and is aligned with ICH Q3A impurity expectations for starting materials. The substance is soluble in DMF, DMSO, and NMP at typical coupling concentrations of 0.2–0.5 M; its aqueous solubility under neutral pH is low, and direct aqueous processing is limited to the final peptide.

When the Guanidinium Side Chain Is Not Orthogonally Protected

If Fmoc-homoarginine without side-chain protection is used in SPPS, the free guanidino group can be acylated during carbodiimide-mediated coupling, generating branched or over-acylated impurities. The free guanidino function may also participate in hydrogen-bonded aggregation that reduces resin swelling and slows coupling kinetics. Fmoc-HomoArg(Boc)₂-OH suppresses these side reactions by masking the guanidino group as bis-Boc. The trade-off is increased steric hindrance during coupling and a slower acidolytic removal during final cleavage. Two Boc groups release two equivalents of tert-butyl cations and isobutylene; efficient scavenging with triisopropylsilane and water is required to prevent irreversible alkylation of the peptide.

Fmoc-Arg(Pbf)-OH is the standard arginine derivative for Fmoc/tBu SPPS; its side chain contains one fewer methylene than homoarginine. The additional methylene in Fmoc-HomoArg(Boc)₂-OH alters the positioning of the guanidino group and can influence peptide-protease recognition, receptor binding, and metabolic stability. Fmoc-HomoArg(Pbf)-OH is an alternative when Pbf protection is preferred; Pbf removal is generally faster than removal of two Boc groups, but Pbf cleavage can produce sulfonic acid byproducts that require scavenging. Fmoc-HomoArg(Mtr)-OH is another sulfonyl-protected variant with historically slower Mtr deprotection; its use has declined in favour of Pbf and Boc2 forms. Published comparative data for all homoarginine derivatives in the same peptide sequence is limited; the selection should be confirmed by small-scale coupling and cleavage studies. Cleavage with trifluoroacetic acid/triisopropylsilane/water 95:2.5:2.5 v/v/v for 2–4 hours is standard for peptides containing homoarginine(Boc)₂. If cleavage time is shorter than 2 hours, residual Boc protection may remain; if longer than 4 hours, acid-sensitive residues may degrade. For sequences containing tryptophan, methionine, or cysteine, scavengers such as 1,2-ethanedithiol or anisole are added at 2–5% v/v.

In downstream peptide manufacturing, the protected amino acid is not directly formulated. After chain assembly, the peptide-resin is treated with a cleavage cocktail of trifluoroacetic acid/triisopropylsilane/water 95:2.5:2.5 v/v/v for 2–4 hours at 18–25°C. This step removes the Boc groups, cleaves the peptide from the resin, and releases tert-butyl cations that are trapped by triisopropylsilane. The crude peptide is precipitated in cold methyl tert-butyl ether at -20°C, washed, and dried under vacuum. Purification is performed by preparative RP-HPLC on C18 or C8 columns with acetonitrile/water gradients containing 0.1% v/v trifluoroacetic acid. Preparative HPLC is typically run on columns of 100–300 mm internal diameter packed with C18 silica of 10–15 μm particle size at linear velocities of 100–150 cm/h; loading is 1–2% w/w crude peptide per gram of stationary phase. Fractions are pooled based on analytical purity ≥98.5%. Counterion exchange to acetate or hydrochloride may be performed for peptides intended for parenteral administration. The resulting peptide API can be lyophilized and formulated as a sterile injectable, or spray-dried with excipients for oral tablets, capsules, or granules. For injectable dosage forms, the finished peptide API must meet bacterial endotoxin limits in USP <85>, sterility in USP <71>, and particulate matter limits in USP <788>. Oral dosage units require uniformity of dosage units per USP <905> and dissolution testing per USP <711> when a compendial monograph applies.

Controlling Moisture, Amines, and Static During Dispensing

Fmoc-HomoArg(Boc)₂-OH is stored at -20°C ± 5°C under dry nitrogen in tightly sealed amber glass containers. Prolonged storage at 25°C and relative humidity above 60% leads to slow Fmoc hydrolysis and loss of chromatographic purity. The material is incompatible with primary and secondary amines, including piperidine, morpholine, and ammonia, because these reagents remove the Fmoc group. Contact with strong acids before SPPS is avoided because premature Boc removal generates tert-butyl cations and carbon dioxide. At relative humidity below 30%, electrostatic dispersion can cause the powder to adhere to polyethylene surfaces; a nitrogen-purged glovebox or dry-room is recommended for dispensing sub-100 g quantities. For production-scale dispensing, stainless steel scoops and glass containers with PTFE-lined closures are used to limit moisture uptake and static charge. After first opening, the material should be returned to -20°C storage within 1 hour and protected from moisture. The substance is not sterile and is not intended for direct injection; any use in parenteral manufacturing requires subsequent purification and sterilisation steps. Published data for the exact solid-state degradation profile of this derivative in all dosage form matrices is limited; forced degradation studies should be performed on the peptide API before formulation.