Immunomodulatory Constituents of Conocephalum conicum (Snake Liverwort) and the Relationship of Isolepidozenes to Germacranes and Humulanes.

Abstract

Structural elucidation of three new sesquiterpenoids, namely, (1Z,4E)-lepidoza-1(10),4-dien-14-ol (1), rel-(1(10)Z,4S,5E,7R)-germacra-1(10),6 diene-11,14-diol (2), and rel-(1(10)Z,4S,5E,7R)-humula-1(10),5-diene-7,14-diol (3), isolated from the liverwort Conocephalum conicum, was accomplished by a combination of extensive NMR experiments, 1H NMR simulation, and other means. Additionally, the change of the identity of bicyclogermacren-14-al, previously reported as a C. conicum constituent, to isolepidozen-14-al is proposed. Compounds 2 and 3 appear to be related to 1 via hydration involving a shared intermediate, a substituted cyclopropylmethyl cation, formed by a highly regio- and stereoselective protonation of 1, followed by a stereospecific fission of the three-membered ring. In other words, an isolepidozene derivative might be a branchpoint to humulanes and germacranes; this transformation could be of, up to now, unknown, biosynthetic and/or synthetic relevance. Multivariate statistical analysis of the compositional data of C. conicum extract constituents was used to probe the hypothesized biochemical relations. The immunomodulatory effect of 1-3 and conocephalenol (4) was evaluated in an in vitro model on both nonstimulated and mitogen-stimulated rat splenocytes. The compounds displayed varying degrees of cytotoxicity to nonstimulated splenocytes, whereas 2 and 3 were found to exert immunosuppressive effects on concanavalin A-stimulated splenocytes while not being cytotoxic at the same concentrations.