Structural Diversity of Chemical Constituents from Okinawan Plants

In our continuing search for secondary metabolites from plants, we investigated the chemical constituents of various Okinawan plants. From the stems and leaves of Croton cascarilloides (Euphorbiaceae), a great number of rare diterpenoids: crotofolanes; crotocascarins A-Q, and new diterpenoids; rearranged nor-crotofolanes: crotocascarins alpha-gamma; a rearranged crotofolane: neocrotocascarin; and a new skeletal diterpenoid: isocrotofolane glucoside were isolated. Their structures were mainly elucidated from NMR spectroscopic evidence. In addition, absolute configurations of crotocascarins A, B, K and O, crotocascarin alpha and neocrotocascarin were determined by X-ray crystallographic analyses, chiral HPLC analyses of 2-methylbutyric acid in their molecules, application of the circular dichroism rule for the alpha,beta-unsaturated gamma-lactone ring, and the modified Mosher's method. The absolute structures of crotofolanes were reported for the first time, and a biosynthetic mechanism for the formation of crotocascarin alpha and neocrotocascarin from crotocascarin B was proposed. Other interesting compounds, namely diosmarioside H: a dimeric ent-kaurane glycoside from the leaves of Diospyros maritima, and melionosides A-C: megastigmane glucosides with a spiro-ring structure from the leaves of Meliosma lepidota ssp. squamulata were also isolated.