3.2.2. Sesterterpenoids

Sesterterpenoids are a relatively small and rare group of terpenoids found in widespread sources. They always possess interesting carbon skeletons, including linear, monocyclic, polycyclic, and miscellaneous. In addition, they exhibit diverse biological activities such as antimicrobial, cytotoxicity, anti-inflammatory, and protein tyrosine phosphatase B inhibition.

The group of She has been dedicated to the search for structurally unique and biologically active compounds from the mangrove plant-derived fungal endophytes. Five sesterterpenoids of three kinds of carbon skeletons, asperterpenoid A (**70**) (Figure 19), asperterpenols A and B (**71** and **72**), and aspterpenacids A and B (**73** and **74**), have been isolated and characterized from *Aspergillus* sp. Among them, asperterpenoid A (**70**), possessing a new 5/7/(3)6/5 pentacyclic carbon skeleton, exhibited potent inhibitory activity against *Mycobacterium tuberculosis* protein tyrosine phosphatase B (mPTPB) with an IC50 value of 2.2 *μ*M [80].

**Figure 19.** Structures of compounds **70**–**74**.

In addition, asperterpenol A (**71**) and asperterpenol B (**72**), two sesterterpenoids with an unusual 5/8/6/6 tetracyclic ring skeleton, showed inhibitory effects on acetylcholinesterase (AChE) with IC50 values of 2.3 *μ*M and 3.0 *μ*M, respectively [81]. Furthermore, the two unusual pentacarbocyclic sesterterpenoids, aspterpenacids A (**73**) and B (**74**), with an unusual carbocyclic skeleton containing an unprecedented 5/3/7/6/5 ring system, showed no antibacterial and cytotoxic activities [82]. The structures of **69**–**73** were elucidated based on spectroscopic methods, and the absolute configurations of **70**–**73** were determined by single-crystal X-ray diffraction analysis. She et al. proposed a hypothetical biosynthetic pathway for **70**–**74** [80–82]. In brief, they are derived from geranylfarnesyl pyrophosphate (GFPP), followed by cyclization rearrangemen<sup>t</sup> and redox reactions (Scheme 11).
