Structure elucidation of a process-related impurity of dapoxetine

•A novel dapoxetine by-product has been identified using NMR and MS.•The mechanism for the impurity formation was proposed and also confirmed by synthesis.•The separation of dapoxetine and the process-related impurities was accomplished by HPLC. Unknown by-product associated with the synthesis of dapoxetine was isolated. The structure elucidation of this new compound using accurate mass data and NMR spectroscopy is presented herein. The unambiguous resonance assignment concluded to the formation of a tricyclic compound 4-phenyl-2H,3H,4H-naphtho[1,2-b]pyran, a new impurity of dapoxetine which has never been reported previously. A proposed mechanism for the formation of the new carbon–carbon bond is discussed. For the separation of dapoxetine and the process-related impurities, a gradient HPLC method was developed.