Structure elucidation of degradation products of Z-ligustilide by UPLC-QTOF-MS and NMR spectroscopy

Z-Ligustilide, a major phthalide isolated from a widely used traditional Chinese medicine Ligusticum chuanxiong, possesses various pharmacological activities including neuroprotective, anti-inflammatory, anti- proliferative and vasorelaxing effects.  However, it is unstable and inclined to degrade in natural conditions, which limits its study and application greatly.  In this study, degradation behavior of Z-ligustilide and its degradation products stored at room temperature under direct sunlight were investigated and structure elucidated by HPLC-UV, UPLC-QTOF-MS and NMR.  Z-ligustilide degradation and total five degradation products were generated and detected.  Two degradation products were unequivocally identified as senkyunolide I and senkyunolide H by comparison with reference compounds.  Another two degradation products were further isolated by semi-preparative HPLC and structure elucidated as (E)-6, 7-trans-dihydroxyligustilide and (Z)-6, 7-epoxyligustilide by ¹H and ¹³C NMR, respectively.  The degradation pathways of Z-ligustilide were finally proposed.  Oxidation, hydrolysis and isomerization are the major degradation reactions.