An unexpected Lewis acid-mediated structural conversion of a Euphorbia Diterpene: From a Lathyrane skeleton to diterpene pseudo-alkaloids.

Three types of new Euphorbia diterpene pseudo-alkaloids possessing 5/6/7/3 (1), 5/6/6/4 (2-5), and 5/7/7/4 (6-7) fused ring skeletons were obtained through an unexpected BF·EtO/CHCN-mediated structural conversion and amination of lathyrane diterpene (Euphorbia factor L), in which the solution acetonitrile had been introduced into the Euphorbia diterpene as a nitrogen source and tandem amination/oxirane-opening (cyclopropane-opening)/oxa-Michael addition reaction was involved in the conversion. The structures of new Euphorbia diterpene pseudo-alkaloids were elucidated by a combination of spectroscopic data and single crystal X-ray diffraction analysis. The basic skeletons of Euphorbia diterpene pseudo-alkaloids 1 and 2-5 could fall into the structural types of euphoractine B and euphoractine A diterpenes, respectively, suggesting the possible biogenetic pathway relationship between lathyrane diterpene with euphoractines A and B types diterpenes. Pseudo-alkaloids 1-7 did not show any potential cytotoxicity against several tumor cell lines.