Cbz-L-phenylglycinol, commonly written as CBZ-L-Phenylglycinol, is the N-benzyloxycarbonyl-protected derivative of L-phenylglycinol. In this derivative, the carbamate residue blocks the primary amino function while the terminal hydroxy group remains available for acylation, mesylation, oxidation, or cyclization. The molecular formula C16H17NO3 and relative molecular mass 271.31 g/mol correspond to the single enantiomer; the CAS registry number 100805-04-9 is the catalog identifier used by most suppliers for the L-configuration. The product is supplied as a white to off-white crystalline powder. Specification-grade lots are controlled to an assay of not less than 98.0 area% by reversed-phase HPLC and a chiral purity of not less than 99.0% ee by chiral stationary-phase HPLC. Storage at 2–8°C under inert gas in tightly closed containers is specified. Moisture ingress above 0.5% w/w by Karl Fischer titration tends to produce particle clumping that interferes with automated solid-dispensing equipment. The material should be warmed to 18–22°C before weighing to reduce electrostatic adhesion in low-humidity environments.

Commercial model designations are typically assigned by chiral purity and packaging format. A 1 g research-grade vial may have the same chemical identity as a 1 kg process-grade drum, but the drum lot carries additional release data covering residual solvents by gas chromatography-headspace and elemental impurities by inductively coupled plasma-mass spectrometry. The model description must specify the L enantiomer because the D enantiomer has a distinct CAS registry number and a different retention time on polysaccharide-based chiral stationary phases. No pharmacopeial monograph exists for this substance, so release is controlled through supplier specifications and internal methods aligned with USP <621>, USP <781>, and USP <467>.

How Does CBZ-L-Phenylglycinol Support Diastereoselective Amide and Oxazolidinone Formation?

In multi-step pharmaceutical intermediate synthesis, the protected amino alcohol is most often introduced as a chiral pool building block. For carbodiimide-mediated coupling, a representative preparation involves dissolution in anhydrous dichloromethane or dimethylformamide at 0.20–0.35 mol/L, cooling to 0–5°C, and addition of 1.05 equivalents of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and 1.05 equivalents of hydroxybenzotriazole, followed by the acid component. The hydroxy group is acylated after the amide bond is formed, allowing selective differentiation of the two nucleophilic sites. Because the carbamate-protected amine is not protonated under weakly acidic workup conditions, aqueous washes with 0.5 M citric acid can remove excess coupling reagents without extracting the product as an ammonium salt.

For oxazolidinone construction, activation of the primary alcohol with carbonyl diimidazole or triphosgene in dichloromethane at −5°C to 5°C yields the cyclic carbamate without removal of the Cbz group. The reaction is monitored by thin-layer chromatography using ethyl acetate/hexane 1:1 v/v and by HPLC. The N-Cbz ring can subsequently be opened or retained according to the downstream chiral auxiliary strategy. Catalytic hydrogenolysis over 10% palladium on carbon at 20–40 psi hydrogen pressure in methanol or ethanol removes the benzyloxycarbonyl group to release the free amino alcohol. Filtration through a 0.45 μm cartridge after catalytic deprotection is required to remove metal fines. Published data for the exact hydrogen uptake profile at production scale is limited, but the reaction endpoint is conventionally confirmed by off-line LC-MS and by the disappearance of the Cbz aromatic protons in the 7.2–7.4 ppm region of the proton NMR spectrum.

In a suction-dried batch isolated from a 50 L glass-lined reactor, the crude carbamate is typically dissolved in ethyl acetate at 35–40°C, clarified through a 0.2 μm polymeric filter capsule, and crystallized by controlled addition of n-heptane over 45–60 minutes with the jacket temperature ramped from 35°C to 0°C at 0.3 K/min. The resulting crystals exhibit plate-like morphology and bulk density in the range 0.35–0.55 g/mL. Drying in a vacuum tray dryer at 40°C and 10–20 mbar for 8–10 hours routinely reduces volatile content below 0.1% w/w. Batch-to-batch chiral purity on a 5 kg scale has been observed to vary between 99.2% ee and 99.8% ee when the same benzyl chloroformate addition profile is used; however, the limitation of this data set is that it derives from a single campaign and has not been published as a multi-facility interlaboratory study.

Specification Control via Chiral Chromatography and Karl Fischer Titration

The release specification is built around orthogonal chromatographic methods and compendial identity tests. A representative set of release limits and test methods is shown below. Suppliers may vary the numerical limits by grade, but the method designations remain standard.

Differential scanning calorimetry of the dry powder at 10 K/min under nitrogen shows a single melting endotherm with onset near 88°C and no exothermic decomposition below 150°C. Above 160°C, carbamate cleavage and carbon dioxide evolution become observable by thermogravimetric analysis. This thermal boundary is based on laboratory DSC-TGA data; published data for process-scale thermal stability testing of this substance is limited.

For release testing, a 25 mg sample is dissolved in 5.0 mL of methanol/acetonitrile 1:1 v/v and filtered through a 0.45 μm PTFE membrane prior to injection. The sample solution should be used within 8 hours because alcoholic solvents can slowly transesterify the carbamate under trace acid catalysis. This is an operational boundary for analytical preparation, not a stability statement for the dry powder.

When Cbz Protection Is Preferred Over Boc or Fmoc in Multi-Step Synthesis

The choice among Cbz, Boc, and Fmoc protection depends on downstream deprotection conditions and detection requirements. Cbz-L-phenylglycinol is used when an acid-stable and base-stable protecting group is required and when catalytic hydrogenolysis is already installed in the synthetic route. Unlike N-Boc-L-phenylglycinol, which is cleaved by trifluoroacetic acid or hydrogen chloride in dioxane, the Cbz derivative survives acidic peptide-coupling workup and strong protic acid washes. Unlike N-Fmoc-L-phenylglycinol, which is removed by piperidine in dimethylformamide, the Cbz derivative resists secondary-amine exposure but requires a hydrogenation vessel and palladium removal by filtration. The Cbz aromatic ring provides a ultraviolet chromophore at 254 nm, while the Boc group is ultraviolet-transparent and may require derivatization for detection.

Hydrogenolysis of Cbz-L-phenylglycinol proceeds at room temperature under 1–3 bar hydrogen pressure using 5% palladium on carbon. The toluene byproduct generated from benzyloxycarbonyl cleavage must be quantified as a residual solvent in the subsequent isolated intermediate. In contrast, Boc deprotection with trifluoroacetic acid generates isobutylene and carbon dioxide, which are easily removed. Fmoc deprotection with piperidine produces a dibenzofulvene-piperidine adduct that deposits in the organic layer and contributes an elevated ultraviolet response in the aqueous phase. These differences alter waste-stream characterization under local wastewater permits and influence reactor-material selection.

The higher molecular mass of the Cbz derivative relative to the Boc derivative translates into a lower molar activity per unit mass for active pharmaceutical ingredient synthesis. However, the crystalline habit and the established removal of the Cbz group by hydrogenolysis are frequently cited as process advantages. The Fmoc derivative is more sterically demanding and can reduce solubility in ethereal solvents. The molecular mass difference must be included in mass-balance calculations for regulated intermediates, particularly when the final active pharmaceutical ingredient specification is expressed in free-base terms.

Racemization of the phenylglycinol stereocenter is slow under carbamate protection because the carbamate nitrogen does not form an imine intermediate under typical acylation conditions. Prolonged treatment with strong bases at elevated temperature can nevertheless lead to deprotonation at the benzylic carbon. Conditions above pH 12 and temperatures above 40°C should be avoided if enantiomeric purity is critical. The free amino alcohol L-phenylglycinol is more prone to Schiff-base-mediated racemization and moisture absorption than the Cbz derivative, which is a primary reason for using the protected form in multi-step sequences.

Sealed containers of Cbz-L-phenylglycinol subjected to accelerated stability chamber conditions at 40°C/75% RH for 6 months have been observed to show a small increase in moisture content and a decrease in flowability, while chiral purity remains within release limits. Because published data for this specific configuration is limited, the material should not be held outside the 2–8°C label condition for extended periods. Exposure to light should be minimized because the benzyloxycarbonyl group is subject to photo-promoted radical reactions in solution. Incompatibility is noted for strong oxidizing agents, mineral acids at concentrations above 1 M, acid chlorides, and anhydrides. Contact with stainless steel surfaces during pH-adjusted aqueous processing does not produce visible iron leaching when chloride concentrations are below 0.1 M; however, chloride-containing residues can initiate pitting corrosion on 316L vessels after prolonged exposure.

Operational Limits and Incompatibility Boundaries for Kilo-Lab Handling

The processing window for the free-flowing crystalline material narrows when relative humidity exceeds 60%. Pre-drying in a vacuum oven at 35°C for 4–6 hours is specified before use in moisture-sensitive reactions such as mesylation or triphosgene activation. The product should not be micronized without an inert-gas loop because static charge buildup in polypropylene drum liners can cause yield loss during suction transfer. In solution, concentrations above 0.5 mol/L in dichloromethane can lead to gel formation if the solution is cooled below −10°C. Dimethylformamide solutions remain mobile but may carry residual solvent into subsequent crystallization. If the material is used in a vessel with a glass overhead condenser, the reflux temperature should not exceed 45°C for more than 2 hours to avoid partial carbamate migration to the hydroxy group. Incompatibility with amine-based bases is not inherent to the Cbz group, but prolonged exposure to neat morpholine at 25°C may generate benzyl alcohol byproducts via nucleophilic attack at the carbamate carbonyl. These limits function as boundary conditions for routine manufacturing.