## 5 Trepenes
### Phytosterols  
Phytosterols or plant sterols are a group of terpenoids that fall into the triterpene family. The core structure of phytosterols consists of a 29-carbon structure formed from the joining of six isoprene monomers (Figure 8). The core structure consists of four fused rings, three hexacyclic and one pentacyclic, all with _trans_ ring junctions forming a planar structure, and an 8-carbon side chain emanating from C-17. There are three substitutions on the planar ring system, all with stereochemistry above the cyclic plane, a methyl group at C-18, a methyl group at C-19, and a hydroxy group at C-3 [116]. Variation in phytosterol structure can be found at C-4 which may or may not feature methyl or di-methyl substitution, the presence of a double bond at C-5, and/or C-7, methyl or ethyl substitution at C-24 with variable stereochemistry, and a double bond at C-22 with variable stereochemistry. The most abundant type of phytosterols found in nature feature no methyl substitution at C-4, a double bond between C-5, and a methyl or ethyl substitution at position C-24, similar to those found in _U. tometosa_[117].  
An important chemical attribute of sterols is their amphiphilicity due to the large planar non-polar regions interacting with hydrophobic membranes and the small, polar hydroxy substitution. Phytosterols play an important role in the regulation of plant cell membranes, affecting the fluidity and permeability [116]. In addition, phytosterols play roles in plant cell proliferation and development, as well as defense against biotic and abiotic stressors [118].  
The types of phytosterols found in _U. tometosa_ consist of three of the most common phytosterols; \(\beta\)-sitosterol **59**, stigmasterrol **60**, and campesterrol **62** (Figure 8) [119]. These sterols are all 4-desmethyl sterols that feature a double bond at position C-5. Variation to the structure can be seen in substitutions to C-22 of the side chain. Compounds **59** and **60** feature an ethyl substitution, while compound **62** features a methyl substitution. These compounds also differ by the double bond at position C-22, with compound **60** possessing a double bond in the _trans_ configuration and **59** and **62** being fully saturated at this position. The characterization of these compounds was determined via \({}^{1}\)H NMR spectroscopy and mass spectrometry [119].  
Figure 8: Structures of phytosterols (**59–**62**). Sugar in red is \(\beta\)-D-glucose.  
Minor plant sterols found in _U. tomentosa_ include a sterol glycoside. In this compound, the hydroxyl group at C-3 becomes a target for glycosylation. Compound **61**, or \(\beta\)-sitosterol-D-glucopyranoside, has a D-glucose moiety, which has been attached via the hydroxyl at C-3. The core structure of compound **59** is identical to \(\beta\)-stigmasterol. These compounds make up a small percentage of the steroidal fraction of _U. tomentosa_[120].  
Phytosterols are bioactive compounds, with the most prominent activity related to their cholesterol lowering effect, due to the structural similarity between these compounds and cholesterol [118]. Phytosterols also exhibit antibacterial activity. Compound **59** exhibited antibacterial activity against _S. aureus_ and _E. coli_ producing zones of inhibition \(17.83\pm 0.58\) mm and 14.5 \(\pm\) 1.84 mm [121], and 13 mm and 14 mm, respectively [122]. Compound **60** exhibited antibacterial activity against seven bacterial strains with MIC values between 12.5 and 25 \(\mathrm{\SIUnitSymbolMicro mL}\) and minimum bactericidal concentration (MBC) values between 25 and 50 \(\mathrm{\SIUnitSymbolMicro mL}\)[123]. Compound **61** indicated antibacterial activity against a variety of microbes with MIC and MBC values reported [124].