Hydroxylamine-O-sulfonic acid, commonly abbreviated as HOSA, is an inorganic compound with the molecular formula H3NO4S and CAS number 2950-43-8. Also known by synonyms such as amino hydrogen sulfate and aminosulfuric acid, HOSA is a remarkably versatile and indispensable reagent in modern organic synthesis. Its unique chemical structure, existing as a zwitterion (⁺H3NOSO3^-) in its solid state, underpins its dual reactivity as both an electrophile under basic conditions and a nucleophile under neutral or acidic conditions, making it a cornerstone for numerous transformations across various chemical industries.

Presented as an off-white crystalline powder, Hydroxylamine-O-sulfonic acid exhibits a melting point of approximately 210 °C (with decomposition) and a molecular weight of 113.093 g/mol. It is readily soluble in cold water but decomposes slowly at room temperature and more rapidly at higher temperatures, necessitating storage in a cool, ventilated, and dry environment to maintain its integrity and efficacy. Its hygroscopic nature further underscores the importance of proper handling and storage to prevent degradation and ensure product purity.

One of the most significant applications of Hydroxylamine-O-sulfonic acid is its role as a crucial intermediate in the synthesis of pharmaceuticals. Notably, it is a key precursor for Brinzolamide (CAS: 138890-62-7), an important carbonic anhydrase inhibitor used in the treatment of glaucoma. The availability of high-quality HOSA is therefore vital for the efficient and reliable production of this essential ophthalmic drug.

Beyond its pharmaceutical intermediate role, HOSA's broad utility stems from its powerful ability to introduce amine groups (–NH2) into various organic molecules. This amination capability is central to the synthesis of a wide array of nitrogen-containing compounds. For instance, it reacts with tertiary amines to form trisubstituted hydrazinium salts and converts pyridine into 1-aminopyridinium salt. It is also instrumental in the N-amination of electron-deficient heterocycles, such as tetrazoles and benzotriazoles, yielding valuable amino-substituted derivatives. Furthermore, HOSA facilitates the amination of sulfur compounds, producing sulfinimines, and phosphorus compounds, leading to phosphine imides, showcasing its broad applicability in diverse chemical classes.

Hydroxylamine-O-sulfonic acid is equally adept at reacting with carbonyl compounds, leading to a variety of useful products. It efficiently converts aldehydes into nitriles, a transformation that is highly valued in organic synthesis. Alicyclic ketones can be transformed into lactams (cyclic amides), a key reaction in the production of various nylon precursors and other industrial chemicals. Under specific conditions, it can also react with aldehydes and ketones in the presence of primary amines to form diaziridines, which are versatile intermediates in organic chemistry. Moreover, HOSA plays a critical role in the synthesis of 1,2-Benzisoxazole, a structural motif found in several antipsychotic medications like risperidone and paliperidone, and the anticonvulsant zonisamide. Its participation in the one-pot synthesis of N-aryl[3,4-d]pyrazolopyrimidines highlights its importance in creating purine analogs for diagnostic and therapeutic applications.

Other notable applications of HOSA include its use as a polymerization catalyst, its involvement in inhibiting the formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated biphenyls (PCB) during co-combustion processes, contributing to environmental safety. It also enhances the chemiluminescence of systems like luminol/cobalt(II) chloride, indicating its utility in analytical chemistry.

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