Illicium sesquiterpenes: divergent synthetic strategy and neurotrophic activity studies

Majucin-type sesquiterpenes from Illicium sp., such as jiadifenolide (2), jiadifenin (3), and (1R,10S)-2-oxo-3,4-dehydroxyneomajucin (4, ODNM), possess a complex caged chemical architecture and remarkable neurotrophic activities. As such, they represent attractive small-molecule leads against various neurodegenerative diseases. We present an efficient, enantioselective, and unified synthesis of 2, 3, and 4 and designed analogues that diverge from tetracyclic key intermediate 7. The synthesis of 7 is highlighted by the use of an enantioselective Robinson annulation reaction (construction of the AB rings), a Pd-mediated carbomethoxylation reaction (construction of the C ring), and a one-pot oxidative reaction cascade (construction of the D ring). Evaluation of the neurotrophic activity of these compounds led to the identification of several highly potent small molecules that significantly enhanced the activity of nerve growth factor (NGF) in PC-12 cells. Moreover, efforts to define the common pharmacophoric motif suggest that substitution at the C-10 center significantly affects bioactivity, while the hemiketal moiety of 2 and 3 and the C-1 substitution might not be critical to the neurotrophic activity.