Chrobisiamone A, a new bischromone from Cassia siamea and a biomimetic transformation of 5-acetonyl-7-hydroxy-2-methylchromone into cassiarin A.

A new bischromone, chrobisiamone A (1) with an antiplasmodial activity has been isolated from the leaves of Cassia siamea and the structure was elucidated by 2D NMR analysis. Cyclization of 5-acetonyl-7-hydroxy-2-methylchromone (2) in the presence of ammonium acetate resulted in generation of cassiarin A (3) with an unprecedented tricyclic skeleton, supporting a biogenetic pathway proposed for 3.