3.5.2. Tricycloalterfurenes

Tricycloalterfurenes A–D (**100**–**103**) were isolated in 2017 from the culture extract of an *Alternaria alternata* strain (k21-1) isolated from the surface of the marine red alga *Lomentaria hakodatensis* [91]. These meroterpenes present activity against three phytoplankton (*Chattonella marina*, *Heterosigma akashiwo*, and *Prorocentrum donghaiense*) and one marine zooplankton (*Artemia salina*). The higher activity of tricycloalterfurene A (64, 37, 46%, respectively, against the phytoplankton species) indicates that hydroxylation at C2 or C3 negatively influences the activity against these organisms. Later, Oh and Shin reported the isolation, from a marine-derived *Stemphylium* sp. fungus, of tricycloalterfurenes E–G (**104**–**106**) [92]. Very recently, Fraga proposed some structural revisions [93]. Regarding tricycloalterfurenes A–C, the correct configuration of the hydroxyl group would be *4R* (Figure 25), comparing the NMR data with that of guignardone T [94]. With respect to trycicloalterfurene D, the correct configuration was proposed to be 6-β-OH (*6R*), also by NMR data comparison to similar systems [95]. To date, no synthetic approaches have been reported.

**Figure 25.** Structure of tricycloalterfurenes and proposed structural revisions (in pink) by Fraga.
