Isosteroidal alkaloids of Fritillaria taipaiensis and their implication to Alzheimer's disease: Isolation, structural elucidation and biological activity.

Four undescribed and five known isosteroidal alkaloids were isolated from the bulbs of Fritillaria taipaiensis and their structures were elucidated on the basis of HR-ESI-MS, 1D and 2D NMR spectroscopic data analyses. The undescribed compounds were designated taipainines A-D. Of these, taipainine D presented a unique structure having the D/E trans (H-13α/H-17ß) and E/F cis (ß-axial lone pair of the N atom/H-22ß) ring junctions. Possible biosynthetic pathway to taipainine D is proposed. Four compounds showed significant BChE inhibitory activities similar or better than the positive control galantamine. In addition, the preliminary structure-activity relationships (SARs) of these isosteroidal alkaloids were also investigated.