Synthesis of 2,6-dimethoxy-9-phenyl-1H-phenalen-1-one and structural revision of the benzoindenone from Eichhornia crassipes

The total synthesis of the natural phytoalexin 2,6-dimethoxy-9-phenyl-1H-phenalent-one (3) isolated from Eichhornia crassipes was accomplished in seven steps. The synthetic strategy included a Friedel-Crafts acylation and a Jacobsen-Katsuki epoxidation as the key reactions to obtain the target compound. Based on a comparison of the NMR data and a crystal structure of 3, the previously proposed structure of 2,5-dimethoxy-4-phenyl-benzoindenone (2) isolated from the same plant has been revised.