Supplied as a white to off-white crystalline powder, Amino Orotic Acid is identified in chemical inventories as 5-amino-2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid and is registered under CAS number 7164-43-4. The molecular formula is C₅H₅N₃O₄, corresponding to a formula weight of 171.11 g·mol⁻¹. Commercial product models are differentiated by assay and impurity profile rather than by a harmonized pharmacopoeial grade designation. Laboratory reagent grade is typically released at ≥98.0% HPLC area purity, while fine-chemical intermediate grade is released at ≥99.0% HPLC area purity with reduced orotic acid content. Micronized lots with defined particle-size distribution are supplied when downstream processing requires dispersion in a solid formulation or viscous reaction medium. The C-5 primary amino group is the key structural distinction from the parent orotic acid. This substituent provides a reactive handle for diazotization, acylation, Schiff base formation, and transition-metal coordination without eliminating the 2,4-dione tautomeric system. The powder is sparingly soluble in water, dissolves in dilute aqueous alkali, and can be charged to reactors as an alkaline solution or as a damp filter cake after isolation. Because the free carboxylic acid is part of the pyrimidinedione ring, pH adjustment controls precipitation and crystal habit. The material should not be confused with mineral orotate salts such as magnesium orotate or zinc orotate, which are salts of unsubstituted orotic acid and are used in dietary supplement applications.

What Process-Control Limits Apply During Pilot-Scale Isolation?

During pilot-scale campaigns in a 100 L glass-lined reactor with centrifuge discharge, the isolation of 5-aminoorotic acid is governed by the solubility difference between the alkaline sodium salt and the free acid. Acidification of an alkaline liquor with dilute mineral acid is maintained at 10–15 °C; uncontrolled acid addition generates high-aspect-ratio crystals smaller than 20 µm that blind filter cloths and extend filtration time. The slurry is therefore held below 25 °C and anti-solvent is added over 45–60 min to maintain filterability. Filtration is followed by a chilled deionized-water wash at 5–10 °C to remove residual inorganic salts, with wash volume controlled by filtrate conductivity. Vacuum tray drying is conducted at 60 °C and −0.08 MPa until loss on drying falls below 0.5%. Dry powder handling can generate static charge; equipment grounding and local exhaust ventilation are specified for open transfers. The primary aromatic amine ring is incompatible with nitrosating agents, strong oxidizers, and concentrated mineral acids at elevated temperature. Contact with nitrite under acidic conditions generates the diazonium intermediate, which is thermally unstable above 5 °C. Published full-scale filtration data for this specific compound are limited; the above operating limits are derived from toll-manufacturing batch records and should be re-established for each reactor geometry.

In synthetic use, Amino Orotic Acid serves as an intermediate for 5-substituted pyrimidines. Diazotization in 3 M hydrochloric acid at 0–5 °C with 1.05 mol sodium nitrite per mol of substrate, followed by addition to potassium iodide at −5 °C, yields the 5-iodo derivative; the same diazonium intermediate is used to prepare 5-cyano and 5-chloro analogs via Sandmeyer-type chemistry. Acylation with acetyl chloride in pyridine at 0–5 °C protects the C-5 amino group before esterification of the C-4 carboxylic acid. Condensation with substituted benzaldehydes in refluxing ethanol produces Schiff bases that can be isolated by filtration after cooling; these ligands are subsequently used in coordination chemistry with cobalt(II), copper(II), and zinc(II) salts. For metal-binding studies, the ligand-to-metal stoichiometry is confirmed by elemental analysis and infrared spectroscopy. The carboxylate oxygen and the C-5 amino nitrogen may both participate in coordination, but the binding mode must be established by single-crystal X-ray diffraction for each complex. Preparative-scale reactions are monitored by reversed-phase HPLC with UV detection at 254 nm to track consumption of the starting material and formation of the more retained 5-substituted orotate product. Published procedures for analogous 5-aminopyrimidines describe these ranges; when applying them to Amino Orotic Acid, pilot reactions should verify conversion and impurity profiles.

Solution stability is pH- and temperature-dependent. Alkaline solutions of Amino Orotic Acid are used for reactor charging because the carboxylate salt increases solubility, but the pyrimidinedione ring can undergo slow hydrolytic opening under strongly basic conditions. Prepared alkaline solutions are therefore used within the working shift. Acidic solutions should not be stored because the protonated amino group can form the corresponding diazonium ion if nitrite is present. For long-term storage, the dry solid is more stable than solution forms. Thermal desolvation of any hydrate or solvate should be verified by thermogravimetric analysis; published thermograms for this specific compound are limited, so each new source of material should be profiled before routine release.

Solid-State Specification Limits and Certificate-of-Analysis Parameters

Release testing for high-purity lots is performed against supplier specifications that combine pharmacopoeial general chapters with purchaser-specific impurity limits. The following table is representative of a typical certificate of analysis for a fine-chemical intermediate grade; exact values must be confirmed with the supplier because no monograph for 5-aminoorotic acid is published in the United States Pharmacopeia or the European Pharmacopoeia.

For chromatographic purity, a reversed-phase method with an end-capped octadecylsilane column, 25 cm × 4.6 mm, 5 µm particle size, and a mobile-phase gradient of 0.1% phosphoric acid and acetonitrile is used. System suitability criteria typically include a tailing factor for the amino orotic acid peak of not more than 2.0 and a resolution between orotic acid and the main peak of not less than 1.5. Method precision should be generated by the receiving laboratory; a relative standard deviation of ≤1.0% for six replicate injections is commonly used as a validation target, but this is method performance rather than a product property. The C-5 amino group gives the product a stronger UV chromophore at 254 nm than the corresponding unsubstituted orotic acid, so HPLC area percent is a conservative estimate of mass purity if non-UV-absorbing impurities are present. Residual solvent levels are addressed by gas chromatography using reference solvent mixtures. Purchasers should specify limits for methanol, ethanol, or acetonitrile if the product is isolated from those solvents. When the product enters a pharmaceutical supply chain, ICH Q3C residual solvent classifications are applied; methanol, if used in isolation, is controlled to ≤3000 ppm, and class 3 solvents such as ethanol are controlled to ≤5000 ppm. Thermal analysis is not generally a release criterion, but it is used during reference standard qualification. If polymorphic consistency is required, X-ray powder diffraction is preferred because visual appearance and FTIR alone may not distinguish crystal forms.

When 5-Aminoorotic Acid Replaces Unsubstituted Orotic Acid in Pyrimidine Syntheses

Replacement of orotic acid with Amino Orotic Acid is warranted when a C-5 functional group is required in the target structure. The amino substituent increases the electron density of the pyrimidinedione ring and changes the regioselectivity of electrophilic substitution relative to the parent acid. In comparison with unsubstituted orotic acid, the product is more reactive toward nitrous acid and acylating agents but requires stricter temperature control to avoid diazonium decomposition. Compared with 5-fluoroorotic acid, which is primarily used as a counterselection agent in yeast molecular genetics, Amino Orotic Acid is used as a synthetic intermediate rather than as a biological selection tool. Compared with mineral orotate salts, Amino Orotic Acid is not a salt form and does not provide a mineral cation carrier function. The table below summarizes the key distinctions for common materials in the same product family.

Regulatory and safety documentation for Amino Orotic Acid is mixture- and supplier-specific. Downstream users should confirm TSCA, REACH, and CLP inventory status using CAS 7164-43-4, request a current safety data sheet, and evaluate residual solvent and elemental impurity data before use in regulated synthesis. The material is stored in tightly closed containers at room temperature in a dry area; exposure to high relative humidity promotes particle agglomeration. There is no harmonized pharmacopoeial monograph, so buyer-seller agreements define the certified limits for each lot. When the product is intended for pharmaceutical intermediate use, reference standards are qualified by NMR, FTIR, and HPLC retention time, and a retest date is usually assigned from stability data rather than from a fixed expiry rule. Handling of the crystalline powder requires the same engineering controls used for other fine organic powders with primary aromatic amine character; open transfer is conducted under local exhaust ventilation, and prolonged contact with nitrosating agents is avoided.