Biogenetical Related Highly Oxygenated Macrocyclic Diterpenes from Sea Spurge Euphorbia paralias

A new tricyclic diterpenoid, named pre-segetanin (1), and two new diterpenes, named segetanin A and B (2 and 3), the latter based on the rare segetane skeleton, have been identified from the whole plant of sea spurge along with four known segetane diterpenes (4-7). Among them, pre-segetanin (1) has an unprecedented carbon skeleton, whose isolation provides a first insight into the biosynthesis of diterpenoids with a segetane skeleton. The stereostructure elucidation of the isolated metabolites was determined by extensive spectroscopic analysis, including 1D and 2D NMR (COSY, TOCSY, HSQC, HMBC, and ROESY) and HRFABMS experiments.