Nitrogenous cyclonerane sesquiterpenes from an algicolous strain of Trichoderma asperellum†
Cyclonerins A (1) and B (2) along with seven new congeners, deoxycyclonerins A–D (3–6), cyclonerinal (7), cyclonerizole (8), and cyclonerpyranol (9), were isolated from the culture of a marine algicolous strain (A-YMD-9-2) of Trichoderma asperellum. Their structures and absolute configurations were established by spectroscopic techniques, such as 1D/2D NMR, MS, ECD, and X-ray diffraction. Compounds 1–8, highlighted by the presence of an isoxazole ring in sesquiterpene 8, represent the first occurrence of nitrogenous cyclonerane derivatives, with 1 and 2 belonging to an unprecedented family of hydroxamic acids. The new isolates 1–9 exhibited potent inhibition of one or more marine phytoplankton species tested, and it is interesting that the ferric complexes of 1 and 2 feature higher activities than themselves.