Cytochalasins and an Abietane-Type Diterpenoid with Allelopathic Activities from the Endophytic Fungus Xylaria Species

Bioactivity-guided isolation of the cultures of the endophytic fugus Xylaria sp. XC-16 residing in a deciduous tree Toona sinensis led to the discovery of four new allelochemicals (1-4), including three cytochalasins, epoxycytochalasin Z₁₇ (1), epoxycytochalasin Z₈ (2), and epoxyrosellichalasin (3), and an abietane-type diterpenoid, hydroxyldecandrin G (4), along with four known analogues, 10-phenyl-[12]-cytochalasins Z₁₆ (5) and Z₁₇ (6), cytochalasin K (7), and cytochalasin E (8). The structures of these compounds were elucidated by comprehensive spectroscopic methods, and their absolute configurations were determined by electronic circular dichroism (CD) and X-ray diffraction. All of the chemicals were tested for their allelopathic effects on turnip ( Raphanus sativus) and wheat ( Triticum aestivum). Notably, compounds 3, 4, and 7 strongly inhibited wheat shoot elongation, and compounds 5, 7, and 8 inhibited wheat root elongation, showing comparable IC₅₀ values to the positive control glyphosate. Meanwhile, compound 8 was a potential inhibitor on turnip root elongation, with an IC₅₀ value of 1.57 ± 0.21 μM, which was 50-fold more potent than glyphosate. Nevertheless, compounds 5 and 7 stimulated turnip shoot elongation at lower concentrations.