During related substance analysis of montelukast bulk drug, a potential unknown impurity was detected in routine reverse phase impurity profiles by high performance liquid chromatography (HPLC). This impurity was identified by LC-MS and characterized by (1H NMR, 13C NMR, gDQCOSY, gHSQC, LC/MS/MS, elemental analysis and FTIR) after isolation from montelukast drug substance exposed to sunlight. Based on spectral data, the impurity was unambiguously named as (E)-2-(2-(3-(3-(2-(7-chloroquinolin-2-yl)vinyl)phenyl)-3-((2-methylenebutyl)thio)propyl)phenyl)propan-2-ol. To the best of our knowledge, this impurity has not been reported elsewhere. Structural elucidation of the impurity by spectral data is discussed in detail.
