3.4.7. Teurilene

Teurilene (**92**) (Figure 21) is a triterpene polyether that was isolated in 1985 from the red alga *Laurencia obtusa* by Kurosawa and coworkers [82]. It has three linked 2,5-disubstituted THF rings and, even though it has eight stereogenic centres, it is an achiral compound due to its *meso*-symmetry (*Cs*). Compound **92** has a remarkable cytotoxic activity against KB cells (IC50 = 7.0 μg mL−1) [69]. Synthetic approaches and routes up to 2014 are commented on in a previous review [83].

**Figure 21.** Structure of teurilene **92**.

3.4.8. Thyrsiferol

Thyrsiferol (**93**) is a polyoxygenated triterpenoid ether that was isolated in 1978 from the red alga *Laurencia thyrsifera* [84]. Its absolute stereochemistry (Figure 22) was determined in 1986 when venustatriol was isolated, since the latter could be crystallized and characterized by X-ray diffraction [85]. During the next two decades, a plethora of thyrsiferol derivatives have been isolated. Their biological profiles (cytotoxic, anti-viral, anti-tumor) have attracted much attention and several syntheses were published and reviewed in 2008 [86].

**Figure 22.** Structure of thyrsiferol (**93)**.

Thyrsiferol has later been found to inhibit hypoxia-induced hypoxia-inducible factor-1 (HIF-1) activation in T47D human breast tumor cells, as well as to inhibit a mitochondrial ETC complex I and show tumor cell line-selective time-dependent inhibition of cell viability/proliferation [77]. In addition, Piñero and Fernández recently reported it to be active against *Acanthamoeba castellanii* trophozoites (IC50 = 13.97 μM). Its derivative, 22-hydroxydehydrotyrsiferol, was similarly active (IC50 = 17.00 μM) and both were not toxic against murine macrophages, which makes them potential leads for the discovery of novel amoebicidal substances [87].
