(R)-2-[3-(Diisopropylamino)-1-phenylpropyl]-4-(hydroxymethyl)phenol fumarate, with CAS number 380636-50-0, is a chemical compound primarily known as the active pharmaceutical ingredient (API) Solifenacin succinate. It is a fumarate salt form of the (R)-enantiomer of solifenacin. The molecule features a chiral center and contains key structural motifs including a phenylpropylamine core, a diisopropylamino group, and a hydroxymethylphenol moiety. This compound is a potent and selective muscarinic M3 receptor antagonist. Its primary and well-established application is in the treatment of overactive bladder (OAB) syndrome, where it reduces symptoms such as urinary urgency, frequency, and incontinence. It is marketed under brand names like Vesicare. The (R)-enantiomer is the therapeutically active form, highlighting the importance of its chiral synthesis. As a final drug substance, it is categorized as an API. Furthermore, due to its chiral nature and role in the synthetic pathway of solifenacin, intermediates related to this structure, particularly those containing the chiral phenylpropylamine backbone, are considered valuable chiral building blocks or intermediates in pharmaceutical synthesis.