(+)/(−)-Alexanoid A, a pair of enantiomeric sesquiterpenoid with a novel 5/6/6 tricyclic backbone from the stem of Syringa pinnatifolia

•A pair of enantiomeric sesquiterpenoid with an unprecedented 4,11,11-trimethyltricyclo[6.3.1.01,5] dodecane skeleton was discovered from Syringa pinnatifolia Hemsl (SP).•The structures were elucidated by the spectroscopic data analysis including UV, IR, optical rotation, and electronic circular dichroism calculation.•The discovery of new skeleton sesquiterpenoid not only enrich the terpenoids of SP, but also suggests that more biogenetic pathways exit in SP, and new bioactive sesquiterpenoids could be discovered in future.