Isolation and structural elucidation of bioactive obovatol dimeric neolignans from the bark of Magnolia officinalis var. biloba

(+)/(−)-Diobolignans A–F, six pairs of enantiomeric obovatol dimeric neolignans, were isolated from the bark of Magnolia officinalis var. biloba. (+)/(−)-Diobolignans A to C possessed a dioxane ring between the benzene ring and the propyl side chain, while (+)/(−)-diobolignans D to F possessed a furan ring. Meanwhile, (+)/(−)-diobolignans B and C, as well as (+)/(−)-diobolignans E and F, were two epimeric pairs of enantiomers, respectively. Their structures were determined by extensive analyses of HRESIMS, UV, IR, NMR and electronic circular dichroism (ECD) calculations. The bioassay showed (+)-diobolignan A displayed cytotoxic activities against human cancer cell lines HGC27 and HT29 with the IC50 values of 9.43 and 9.45 μM, respectively. In addition, (−)-diobolignan A and (±)-diobolignan E showed neuroprotective effect on glutamic acid-induced cellular damage in SK-N-SH cell.