BOC-L-Phenylalaninol, also designated N-Boc-L-phenylalaninol or (S)-tert-butyl (1-hydroxy-3-phenylpropan-2-yl)carbamate, is a single-enantiomer protected amino alcohol with CAS number 66605-57-0, molecular formula C₁₄H₂₁NO₃, and relative molecular mass 251.32 g/mol. It is supplied as a white to off-white crystalline powder with a lot-specific melting range generally reported between 94 °C and 98 °C. The tert-butoxycarbonyl group blocks the primary amine, leaving the primary hydroxyl available for selective esterification, sulfonate formation, oxidation, or displacement. Catalogue grades differ mainly by analytical documentation and purity tier. A routine research grade is released against HPLC purity not less than 98.0%; higher-tier lots may be released with HPLC purity not less than 99.0%, enantiomeric excess not less than 99.5%, and a custom certificate of analysis for GMP starting material use. Product model designations commonly include the synonym Boc-Phe-ol, but ordering should identify the L-stereochemistry explicitly because the D-enantiomer and racemic variant are separately catalogued. The product is not a finished pharmaceutical substance, and no monograph for this protected amino alcohol exists in USP–NF or Ph. Eur. Its functional value is retention of the L-configuration while the C-terminal side chain is transformed.

What release criteria generally govern enantiomeric purity for this amino alcohol?

Enantiomeric purity is controlled by chiral HPLC, not by optical rotation alone. Typical release documentation sets enantiomeric excess at ≥ 99.0%, using a polysaccharide-based chiral stationary phase and the D-enantiomer as the reference standard. The acceptance window for specific rotation under USP <781> is commonly −27° to −31° in methanol at 20 °C and c=1, but this value is influenced by residual water and solvent composition. HPLC area-normalisation purity is usually set at ≥ 98.0% with detection at 210 nm or 254 nm, because the phenyl chromophore gives strong absorbance and the carbamate absorbs at lower wavelengths. Karl Fischer coulometric titration limits water to ≤ 0.5%; residual moisture is a critical parameter because the alcohol is often used with sulfonyl chlorides, oxalyl chloride, or lithium aluminium hydride. Residual solvents are assigned against ICH Q3C by headspace GC-FID. Loss on drying at 60 °C under vacuum may be used as a supporting test, but for water-sensitive downstream reactions the coulometric water value is the main release criterion. Chiral method suitability often requires that a solution spiked with 0.5% D-enantiomer gives baseline separation from the L-form; if the peak area of the D-enantiomer is below 0.05%, the result is reported as not detected. The table below summarises typical release criteria and analytical methods from supplier dossiers; it does not replace a lot-specific certificate of analysis.

At pilot scale, the compound is charged into glass-lined reactors for conversion to the methanesulfonate or tosylate. Selective O-activation is performed with methanesulfonyl chloride at −10 °C to 0 °C in dichloromethane or tetrahydrofuran. The internal temperature during reagent addition is a processing window of roughly 5 °C; higher exotherms can precipitate triethylamine hydrochloride and increase agitator torque. A tertiary amine base is used at 2.0–2.2 equivalents to neutralise the liberated HCl, while methanesulfonyl chloride is held at 1.0–1.05 equivalents to limit sulfone formation and aggressive quench exotherms. After the reaction, the batch is quenched into cold water at 0–5 °C, washed with sodium bicarbonate to pH 7.5–8.0, and concentrated below 30 °C. If the free alcohol is not pre-dried at relative humidity above 60%, water reacts with sulfonyl chloride and reduces the available reagent. The Boc protecting group remains intact during sulfonylation but is rapidly removed by trifluoroacetic acid or HCl; therefore, acid deprotection is positioned after O-functionalisation. Storage of solid material in tightly closed containers at 2–8 °C with nitrogen purge after each opening is standard, because moisture ingress can produce caked solids and shift optical rotation through partial solvate formation. Published production-scale yield data for this specific configuration is limited, and process descriptions vary with the downstream reaction sequence.

When hydrogenolytic cleavage of Cbz protection is incompatible with unsaturated or sulfur-containing intermediates

The Boc group is removed by acidolysis, whereas the corresponding N-Cbz-L-phenylalaninol requires palladium-catalysed hydrogenation in ethanol or ethyl acetate. If the downstream intermediate contains alkenes, nitriles, nitro groups, or sulfur-bearing residues, hydrogenolysis can cause reduction or catalyst poisoning; in such cases BOC-L-Phenylalaninol is preferred. Acidolysis is carried out with 20% trifluoroacetic acid in dichloromethane at 0–25 °C or with 4 M HCl in 1,4-dioxane. The liberated tert-butyl cation must be scavenged with triisopropylsilane to prevent alkylation of electron-rich aromatic rings. If the substrate contains acid-labile acetal, silyl ether, or glycosidic bonds, the hydrogenolytic Cbz route may be preferred because it is pH-neutral. This difference in deprotection chemistry is a primary selection criterion and is more important than melting-point or chromatographic differences. BOC-L-Phenylalaninol also differs from N-Boc-L-phenylalanine in that the alcohol does not participate directly in amide coupling; this avoids carboxyl activation steps when a reduced amide or hydroxyethylamine motif is the synthetic target. The alcohol can be oxidised to N-Boc-L-phenylalaninal for reductive amination or converted to a sulfonate electrophile for nucleophilic displacement. In both cases, the Boc group provides temporary amine masking without a hydrogenation step.

Compared with unprotected L-phenylalaninol, the Boc-protected derivative has a higher molecular mass and a blocked primary amine. This blocking suppresses intramolecular aziridine formation when the hydroxyl is converted to a tosylate or mesylate; in the unprotected amino alcohol, the free amine can displace the leaving group and consume the intermediate. Chiral HPLC separation of BOC-L-Phenylalaninol from its D-enantiomer is typically achieved on a 250 mm × 4.6 mm polysaccharide carbamate column with 5 µm particles and an alcohol-hydrocarbon or polar organic mobile phase. Column temperature is usually held at 25 °C for repeatability; injection mass is limited to maintain baseline resolution. Detection at 210 nm can be affected by mobile-phase background, so 254 nm is used when the impurity profile is dominated by phenyl-containing species. The corresponding N-Boc-L-phenylalaninal is a more reactive intermediate in reductive aminations but is less stable on storage and more prone to base-catalysed racemisation. The alcohol form is therefore selected for transport and multi-step campaigns where the aldehyde is generated as a transient intermediate. Solubility of the alcohol in dichloromethane, tetrahydrofuran, ethyl acetate, and methanol is sufficient for common reaction concentrations of 0.1–0.5 M; heptane and other aliphatic hydrocarbons are used as antisolvents for crystallisation. Water solubility is limited, and aqueous workup losses increase if the organic phase is dispersed as fine droplets rather than a continuous layer.

Comparative properties of N-protected phenylalaninols

The following comparison is based on standard protecting-group chemistry and supplier technical dossiers, not on a single certificate of analysis.

Continuous flow processing of BOC-L-Phenylalaninol solutions is feasible when the feed is filtered through a 0.45 µm membrane to protect microreactor channels from crystalline fines. In a flow oxidation using Dess-Martin periodinane or trichloroisocyanuric acid-TEMPO, the residence time is governed by the alcohol-to-oxidant ratio and the back-pressure regulator setting; if the back-pressure is below 1.0 bar, gas evolution from dissolved by-products can create slug flow and reduce conversion. The reaction stream is quenched in-line with aqueous sodium thiosulfate to destroy excess oxidant before extraction; a pH of 7.5–8.0 is maintained during quench to avoid premature Boc cleavage. Process analytical technology may include near-infrared monitoring of the hydroxyl O-H stretch at approximately 3500 cm⁻¹, but calibration requires a representative set of process mixtures because water and solvent bands overlap. Batch-to-batch variance in residual water and solvent composition has a larger effect on reaction conversion than minor differences in particle size; this is why release criteria prioritise water and residual solvent limits. In automated solid-charging systems, caked material with poor flow can create dosing errors; if the powder has been stored under humid conditions, it should be dried under vacuum at 40 °C before use, but only if the certificate of analysis permits thermal drying for the intended application.

Supply-chain documentation for GMP-compliant use includes residual solvent testing under ICH Q3C, elemental impurities under ICH Q3D when the material is intended for late-phase API synthesis, and optical rotation under USP <781>. No pharmacopoeial monograph is published for BOC-L-Phenylalaninol in USP–NF or Ph. Eur.; therefore, compendial methods do not apply directly, and release specifications are buyer-specific. The material is often classified under customs tariff codes for amino-alcohol derivatives; shipping is generally regulated by the residual solvent profile rather than the solid itself. Safety data sheets from major suppliers commonly assign no GHS hazard classification, but this is not universal and must be verified for the actual lot. For research quantities, the product is packed under nitrogen in glass or fluoropolymer bottles; for larger quantities, double polyethylene liners inside sealed drums are used to limit moisture and oxygen ingress. Operators should use local exhaust ventilation when transferring dry powder, because fine organic solids can generate combustible dust clouds under conditions of sufficient ignition energy and particle size distribution.