4-(4-Methylbenzenesulfonate)-4-isoquinolinol

4-(4-Methylbenzenesulfonate)-4-isoquinolinol is an impurity of 1-Phenyl-2,3-dihydroisoquinolin-4(1H)-one Hydrochloride (P309050), which is a useful synthetic intermediate in the synthesis of cis-Hydroxy Solifenacin (H953525) and cis-Hydroxy Solifenacin N-Oxide (H953485); both metabolites of Solifenacin (S676700, HCl) which is a muscarinic M3 receptor antagonist and is used in treatment of urinary incontinence.