(R)-4-Phenyl-2-oxazolidinone, identified by CAS 90319-52-1, is a pharma-grade chiral oxazolidinone intermediate supplied as a white to off-white crystalline powder. Molecular formula is C9H9NO2 and molecular mass is 163.17 g/mol. The commercial model designation is not a harmonized pharmacopeial identifier; it is referenced as a manufacturer-specific article code on the certificate of analysis, for example a grade code linked to lot-level release for downstream synthetic use. The material is used as a chirality-controlling building block in the synthesis of active pharmaceutical ingredients subsequently formulated into tablets, capsules, granules, oral liquids, and injectable preparations. It is not itself directly compressed into final dosage forms; instead, it is consumed in acylation, alkylation, or aldol chemistry before the final API is isolated, purified, and released for formulation.

Residual solvent removal is constrained by crystalline lattice stability. Methanol is frequently used as the recrystallization solvent, and drying is carried out in rotary vacuum dryers at 40–45 °C jacket temperature with reduced pressure until loss on drying and headspace gas chromatography meet the acceptance limits. If dichloromethane is used during extraction or purification, the Class 2 limit of 600 ppm becomes the release control, and extended drying under vacuum may be required for batches above 50 kg. Storage is normally in double low-density polyethylene bags inside a fiber drum under nitrogen headspace at 2–8 °C in a dry environment. Exposure to relative humidity above 60% requires re-drying before use due to surface moisture uptake that can affect gravimetric feeding accuracy.

What Process Specifications Apply to (R)-4-Phenyl-2-Oxazolidinone as a Chiral Auxiliary Intermediate?

Because no harmonized USP or Ph. Eur. monograph exists for this intermediate, the material is released under supplier specifications aligned with ICH Q7 and ICH Q6A for critical starting materials and intermediates. The control strategy includes enantiomeric purity, assay, residual solvents, water content, and elemental impurities. In downstream API synthesis, the C4 stereocenter directs enolate alkylation and aldol addition; enantiomeric excess above 99.0% is generally required for subsequent diastereoselective transformations. Analytical release of the intermediate typically uses high-performance liquid chromatography with chiral stationary phases, often with hexane/2-propanol mobile-phase systems and UV detection at 254 nm.

Specifications for this intermediate should not be confused with final dosage form release. For tablet and capsule APIs, ICH Q6A decision trees for particle size, polymorphism, and dissolution apply only after the final drug substance is manufactured. For injectable APIs, bacterial endotoxin and particulate matter are controlled on the final drug substance, not on this synthetic intermediate. The compound is not depyrogenated for sterile use because it is introduced at a synthetic stage where subsequent purification, filtration, and final sterilization are required.

In production-scale chiral auxiliary chemistry, the material is typically dried to a free-flowing crystalline state after recrystallization. Residual chiral counterpart content is monitored by chiral HPLC because even 0.5% of the (S)-enantiomer can alter downstream diastereomeric ratios. The analytical certificate of analysis therefore includes the enantiomeric purity as a release criterion. Process deviations in enantiomeric purity are managed by recrystallization or re-crystallization from selected solvent systems; published data for this specific configuration is limited to supplier process development reports rather than independent peer-reviewed studies.

Solid-State Handling, Milling, and Blending Behaviour

The phenyl substituent increases crystallinity relative to 4-benzyl-2-oxazolidinone analogues, which reduces solvent retention but also lowers aqueous solubility. The dry powder is handled as a crystalline solid; particle size distribution is controlled by sieving or laser diffraction after milling. Jet milling under nitrogen is used when fine particle size is required for solubility of the subsequent acylated intermediate, but high-energy milling can generate amorphous surface domains and must be followed by controlled recrystallization or equilibration. No USP powder flow classification is assigned to the intermediate, and it is not used as a direct compression excipient. Bulk and tapped density measurements per USP 616 may be reported on supplier certificates, but published data for this specific configuration is limited.

Incompatibilities include strong aqueous alkali, mineral acids, and oxidizing agents. Aqueous hydroxide can open the oxazolidinone ring under uncontrolled conditions; strong acid can hydrolyze the carbamate functionality. Storage should be separate from oxidizing agents and from moisture-sensitive reagents. If the material is exposed to high humidity or elevated temperature above 40 °C, re-testing for water content and chiral purity is required before release.

When the Intermediate Is Transferred into Oral Solid Dose and Injectable Synthetic Routes

In oral solid dose synthesis, (R)-4-phenyl-2-oxazolidinone is typically N-acylated with an acid chloride or mixed anhydride in anhydrous tetrahydrofuran. Enolate formation is conducted with lithium diisopropylamide or n-butyllithium at temperatures between −78 °C and −65 °C, followed by addition of an alkylating agent. The resulting diastereomeric product is separated by chromatography or crystallization. The chiral auxiliary is then cleaved by lithium hydroperoxide or lithium hydroxide hydrolysis; because the C4 phenyl group is not removed by hydrogenolysis, the intact auxiliary is recovered as a recyclable cleavage stream. This differs from 4-benzyl-2-oxazolidinone auxiliaries, where the benzyl substituent can be removed by catalytic hydrogenolysis over palladium on carbon.

For APIs intended for injectable use, the synthetic route must purge auxiliary-derived byproducts to below ICH Q3C and ICH Q3D thresholds. The final injectable API is then tested for bacterial endotoxin per Ph. Eur. 2.6.14 or USP 85, and for particulate matter per Ph. Eur. 2.9.19 or USP 788. The intermediate itself is not subjected to these injectable release tests because it does not appear in the final formulation. For tablet, capsule, and granule APIs, the key control is the absence of auxiliary-derived impurities in the final drug substance; the synthetic route must include a cleaning step capable of removing or reducing the auxiliary to levels justified by ICH Q3A or pharmacopeial thresholds.

Model-specific documentation for the intermediate should include the chemical name, CAS number, supplier article code, batch number, retest date, storage conditions, and approved specification reference. In pharmaceutical quality systems, the material is controlled as an intermediate or starting material under ICH Q7 where designated as an API starting material; otherwise, it is managed under supplier qualification and vendor change control. The product is not interchangeable with the (S)-enantiomer or with non-chiral 2-oxazolidinone in routes requiring stereocontrol.

Comparing the (S)-Enantiomer and 4-Benzyl Analogues for Chiral Induction Strength

The (R)-phenyl analogue is selected when the synthetic route cannot tolerate hydrogenolysis conditions or when phenyl substitution provides better crystallization of downstream diastereomers. Its higher crystallinity can improve isolation of acylated intermediates, but it also presents slower filtration during workup if fine crystals are generated. The 4-benzyl analogue is preferred when catalytic hydrogenolysis is already present in the route and when auxiliary recovery by hydrogenolysis is desirable. The non-chiral 2-oxazolidinone cannot be used as a chiral auxiliary; it has no C4 stereocenter and therefore does not exert stereochemical control in Evans enolate chemistry.

Operational boundaries for the (R)-4-phenyl intermediate are defined by the stability of the oxazolidinone ring and the chiral center. The material should not be stored in the presence of strong aqueous base or mineral acid at ambient temperature. In synthesis, enolate formation should be maintained below −65 °C to limit racemization and auxiliary ring opening. When acylating the auxiliary, moisture must be excluded because residual water consumes the organolithium base and reduces conversion. After cleavage, the recovered auxiliary may be contaminated with lithium salts and must be re-crystallized before reuse. Published data for repeated auxiliary recycling under good manufacturing practice conditions is limited; each reuse must be qualified by chiral purity and residual solvent testing.