BOC-L-Isoleucine (synonym N-(tert-butoxycarbonyl)-L-isoleucine, Boc-Ile-OH, CAS 13139-16-7) is a protected chiral amino acid with molecular formula C₁₁H₂₁NO₄, molecular weight 231.29 g·mol⁻¹, and a free α-carboxylic acid. The substance is supplied as a white to off-white crystalline powder in research-grade and GMP-grade lots. Lot-specific certificates of analysis for research-grade material typically list HPLC assay ≥98.5%, loss on drying ≤0.5%, residue on ignition ≤0.1%, and specific rotation [α]_D²⁰ +2.0° to +3.0° (c = 1, ethanol). The tert-butoxycarbonyl group blocks the α-amino nitrogen during carboxyl activation, while the β-branched sec-butyl side chain controls steric hindrance and coupling kinetics in peptide assembly. In contrast to unprotected L-isoleucine, the Boc-protected monomer prevents premature N-acylation and allows selective C-terminal elongation.

When Acid-Labile N-Terminal Protection Is Required for β-Branched Amino Acid Residues

Peptide synthesis routes using Boc-L-isoleucine depend on TFA-mediated acidolysis. The carbamate bond is protonated, releases isobutylene and carbon dioxide, and leaves the amino group as a trifluoroacetate salt. Neutralization with 5–10% DIPEA in DCM or DMF restores the free amine. A common deprotection sequence uses 20–50% TFA in DCM in two stages: 1 × 5 min and 1 × 25 min at 20–25 °C. Incomplete neutralization or residual TFA salts reduces coupling efficiency in the subsequent step. The Boc group is stable under hydrogenation and standard basic washes, but it is rapidly removed by strong organic acids; the monomer is therefore not compatible with routes requiring acid-stable N-protection through the entire assembly.

Coupling of Boc-L-isoleucine to a resin-bound peptide normally uses 2–4 molar equivalents relative to free amino groups. Typical activation combinations include DIC/HOBt, HBTU/DIPEA, or PyBOP/HOAt in DMF or NMP. The reaction is maintained at 0–25 °C for 1–18 h, depending on resin loading, sequence context, and agitation. Because the β-branched side chain raises steric hindrance at the α-carbon, a single coupling may not drive the reaction to completion. Automated solid-phase peptide synthesis commonly introduces a second coupling or a capping step with acetic anhydride/pyridine after a positive Kaiser test or FDNB test indicates residual free amine. The β-branching effect is a kinetic property of the protected amino acid and is more pronounced when the preceding residue is another hindered amino acid or when the resin substitution exceeds 0.5 mmol/g.

These values are compiled from typical commercial certificates of analysis. Research-grade lots may contain residual recrystallization solvent and may not include full residual solvent data. GMP-grade lots are processed with documented change control and include additional residual solvent and heavy metal data. Both grades share the same chemical identity but differ in analytical compliance, not molecular structure. Published toxicological data for this specific derivative are limited, and occupational exposure controls for pharmaceutical intermediates should be applied.

In solid-phase peptide synthesis, Boc-L-isoleucine is commonly introduced on Merrifield polystyrene-divinylbenzene resin with 1% crosslinking and substitution between 0.3 mmol/g and 1.0 mmol/g. The protected amino acid is dissolved to 0.2–0.5 mol/L in DMF or NMP before activation. Pre-activation with DIC/HOBt in DMF at 0–5 °C for 2–5 min forms the HOBt active ester while limiting symmetrical anhydride formation. The activated mixture is transferred to the resin reactor within 15 min. Washing volumes of 5–10 mL/g of resin are used after deprotection and after coupling to remove trifluoroacetate salts and coupling by-products. Inadequate washing after TFA deprotection leaves acidic residues that protonate the free amine and reduce the apparent coupling yield.

On a 100 mmol synthesis scale, a jacketed glass or PTFE reactor with overhead stirring at 30–60 rpm is sufficient; magnetic stirring may grind the resin beads and generate fines that slow filtration. For automated synthesizers, the reaction vessel volume should be at least 5–10 times the swollen resin bed volume. Swelling in DCM followed by DMF washing produces a uniform reactive surface. If the resin bed is not fully swollen, the hindered Boc-L-isoleucine monomer accesses only the outer beads, and subsequent FDNB tests may falsely suggest complete coupling when interior sites remain unreacted. A monomer concentration of 0.2 mol/L is generally sufficient for substitution below 0.5 mmol/g; higher-loading resins require the upper end of the concentration range.

Batch-to-batch variance in the polymer resin itself affects the apparent coupling rate. Resins with high crosslink density swell less in DMF, reducing accessibility of the bulky Boc-L-isoleucine monomer. Resins with substitution below 0.3 mmol/g have greater inter-site spacing and show faster coupling, but lower peptide yield per batch. Pilot-scale reactions often use 0.4–0.6 mmol/g substitution as a compromise between yield and kinetic efficiency. If coupling is performed at the high end of the substitution range, the monomer concentration should be raised to 0.5 mol/L and the reaction time extended to 18–24 h with intermittent agitation.

Foaming during deprotection can occur from isobutylene release. Reactor headspace should be vented to an acid scrubber or local exhaust. The deprotection solution should be added slowly at 20–25 °C; adding TFA solution directly to a cold resin bed can cause thermal shock and bead fracture. Final cleavage of Boc-synthesized peptides from Merrifield resin typically uses liquid HF at 0 °C for 1 h in the presence of anisole and thioanisole scavengers, followed by cold diethyl ether precipitation. This process is performed in dedicated HF-resistant vacuum manifolds with Teflon/Kel-F wetted parts.

For solution-phase fragment coupling, Boc-L-isoleucine is activated as an active ester or mixed anhydride. A representative method uses ethyl chloroformate and N-methylmorpholine in THF at −15 °C to 0 °C, followed by addition of the amine nucleophile at 20–25 °C. Low-temperature activation suppresses racemization at the α-carbon; the absence of the unprotected α-amino group reduces base-catalyzed oxazolone formation. Moisture in the solvent or product consumes the activation reagent. When relative humidity exceeds 60%, pre-drying of the powder at 25–30 °C under ≤10 mbar for 12 h is recommended before weighing.

Model variants sold under the same CAS number include the free acid, N-hydroxysuccinimide ester, pentafluorophenyl ester, and N-methylamide forms. The free acid is the most common for solid-phase peptide synthesis; the active ester forms are used in acylation of amines without in situ activation. Selection among these forms depends on whether the carboxylic acid is to be activated immediately or isolated as a shelf-stable derivative. Boc-L-isoleucine differs from Boc-L-valine by an additional methylene in the side chain; the molecular weight of Boc-L-valine is 217.26. Both are β-branched, but the sec-butyl group of isoleucine contains an extra carbon and a second chiral center, which requires enantiopure starting material and creates additional steric hindrance during coupling. Boc-L-isoleucine therefore may require longer coupling times than Boc-L-valine for complete incorporation into hindered peptides.

What Distinguishes Boc-L-isoleucine from Fmoc-L-isoleucine and Isomeric Protected Amino Acids?

The decisive difference between Boc-L-isoleucine and Fmoc-L-isoleucine is the N-protecting group and its removal chemistry. Boc-L-isoleucine uses TFA-mediated acidolysis; Fmoc-L-isoleucine uses piperidine-mediated elimination. This determines the side-chain protection strategy and final cleavage conditions for the entire peptide route. Boc chemistry is generally paired with benzyl-type side-chain protection and cleavage by HF or trifluoromethanesulfonic acid; Fmoc chemistry is paired with tert-butyl-type side-chain protection and cleavage by TFA. The free α-carboxylic acid is present in both compounds, so carboxyl activation chemistry can be similar when the N-terminus remains protected.

Boc-L-isoleucine and Boc-L-leucine have identical molecular formula and molecular weight; they are constitutional isomers. They cannot be differentiated by molecular weight or by standard C18 HPLC under steep gradients. Resolution requires a shallow acetonitrile gradient with an ion-pairing modifier, an ion-exchange amino acid analysis system with ninhydrin post-column detection, or a chiral method using an authenticated reference standard. Use of one instead of the other changes the peptide side chain from a β-branched sec-butyl group to an isobutyl group and alters hydrophobic packing and proteolytic susceptibility.

Boc-D-isoleucine is the enantiomer of Boc-L-isoleucine and has an opposite specific rotation of approximately −2.0° to −3.0° under the same polarimetric conditions. In peptide synthesis, D-isoleucine produces diastereomeric peptides with reversed side-chain orientation at the incorporation site. Chiral purity is therefore critical for bioactivity. Lot-specific enantiomeric excess can be assessed by derivatization with a chiral amine and gas chromatography or by chiral HPLC; specifications should state enantiomeric excess ≥99.0% for GMP use.

Boc-L-isoleucine should not be substituted directly into an Fmoc solid-phase protocol unless the Boc group is intended to survive the piperidine cycles and be removed at the final TFA cleavage. The use of two different temporary protecting groups in one sequence introduces orthogonality but complicates coupling and final deprotection. The Kaiser test for primary amines can generate false positives if the resin contains residual DIPEA; a cleaved amino acid analysis by HPLC after acid hydrolysis is the most quantitative method for verifying isoleucine incorporation.

Storage stability is bounded by temperature, moisture, and acid vapor exposure. Closed containers should be stored at 2–8 °C under argon or nitrogen. Repeated opening under high humidity increases water content and changes stoichiometric weight. If loss on drying exceeds 0.5%, vacuum drying at 25–30 °C and ≤10 mbar for 12 h restores a suitable handling condition. Drying above 40 °C is not recommended because thermal decomposition near the melting edge can generate acidic sites that accelerate Boc deprotection. The product should not be stored with trifluoroacetic acid, hydrochloric acid, thionyl chloride, or other acid vapors. Direct contact with carbonate or bicarbonate solutions leads to carboxylate salt formation. On production lines, dedicated scoops and containers prevent cross-contamination with Fmoc-protected monomers; accidental mixing of Boc and Fmoc protection groups in a synthesis run produces heterogeneous sequences and premature termination.

Quality control for GMP-grade lots includes HPLC assay, water content, residual solvent, residue on ignition, and enantiomeric purity. HPLC is commonly performed on a reversed-phase C18 column with UV detection at 210 nm, using a mobile phase of 0.1% TFA in water and acetonitrile. Residual solvents are reported according to USP <467>; water content by Karl Fischer titration; residue on ignition by USP <281>. The receiving laboratory should qualify each supplier lot with an authenticated reference standard because published data for impurity identity and toxicological thresholds for this specific derivative are limited.