*3.2. Phytotoxic Activity*

Prehelminthosporol (**38**) and dihydroprehelminthosporol (**34**) along with compounds **35**–**37**, **39**, **40**, and **43** were separated by SiO2, flash CC, and preparative TLC from the EtOAc extract of the *Bipolaris* species, which is a *Sorghum halepnse* (Johnson grass) pathogen (Figure 3). These metabolites were assessed for their phytotoxic potential towards *Sorghum bicolor* (Sorghum) and *Sorghum balepense* (Johnson grass) in leaf spot assays [36,37]. Compounds **34**, **38**, and **39** produced similar lesions to those caused by the fungus in the field. The lesions appeared as a reddish-brown area (0.3–0.5 cm diameter) surrounded by a black circle with an outer chlorotic zone. Compounds **34** and **38** (concentration of 25 μg/5 μL) had comparable toxic effectiveness, while compound **38** maintained its effect at a lower concentration of 2.5 μg/5 μL; meanwhile, the other compounds were non-toxic [36,37]. Victoxinine was also toxic to cereals in the order of oats > rye and barley > wheat > sorghum in a root inhibition assay [37]. The phytotoxic influence of compounds **34** and **38**–**40** versus sorghum, corn, bent-grass, sickle-pod, and morning glory was also assessed in leaf spot assays. Moreover, victoxinine caused a water-soaked translucent appearance with defined irregular necrotic edges. It is worth mentioning that 3-deoxyanthocyanidins are sorghum stress response metabolites (phytoalexins), which were accountable for the red wound response. Compounds **34**, **38**, and **39** were elicitors of a very strong reddening compared with the wounding-produced reddening, but compound **40** did not elicit a sorghum phytoalexin response. In bent grass and corn, compounds **34** and **38**–**40** produced a light-brown area limited by a chlorotic region, whereas in sickle pod and morning glory, they showed necrotic lesions that extended at high concentrations beyond the under-drop area. It is noteworthy that compound **38** was the most toxic compound versus all tested plants except for the morning glory [37].

**Figure 2.** Structures of bicyclic bergamotane sesquiterpenoids (compounds **18**–**28**).

**Figure 3.** Structures of the bicyclic bergamotane sesquiterpenoids (compounds **29**–**37**)**.**

Helminthosporium victoriae, the causative agent of oats Victoria blight disease yielded phytotoxins, victoxinine (**40**) and victoxinine α-glycerophosphate (**41**), which were separated from its diethyl ether extract using Sephadex LH-20 and SiO2 CC and detected on the TLC plate by giving a blue color with 5% vanillin:H2SO4 [41] (Figure 4). The existence of α-glycerophosphate moiety was established by coupling between the phosphorous and carbon. Compound **40** completely prohibited the root growth of toxin-susceptible and toxin-resistant oats (concentration of 2.5 × <sup>10</sup>−<sup>4</sup> M); it was ≈ 7500 times more toxic for susceptible plants on a weight basis, while its toxicity for resistant plants was nearly similar, suggesting a role of the victoxinine moiety on the toxicity [38,39,41]. On the other side, compound **41** (concentration of 100 μg/mL) demonstrated little or no growth inhibition effectiveness on either susceptible or resistant oats [41].

**Figure 4.** Structures of tricyclic bergamotane sesquiterpenoids (compounds **38**–**43**)**.**
