Phomoidrides E-G, three dimeric anhydrides from the fungus Pleosporales sp. give new insight to the biosynthesis of phomoidrides

Three phomoidride-type dimeric anhydrides, phomoidrides E-G (1-3), were isolated from the fungus Pleosporales sp. Their structures were elucidated by comprehensive spectroscopic analyses. Phomoidride E (1) possesses a totally new caged tricyclo[5.3.2.0(2;8)]dodecane carbon skeleton with a newly formed five-membered bridged ring through a C-C bond. Phomoidride F (2) features a unique caged ring system containing a bicyclo[4.3.1]deca-2,6-diene fused with an additional gamma-lactone ring. The identification of 1-3 offers new insight into phomoidride biosynthesis, where a new dimerization strategy with a key Claisen-like intermediate M1 instead of the previously proposed aldol-like product could be involved. 1 shows potent inhibitory activity against the HeLa cell line with an IC50 value of 1.41 mu M.