Semi-Synthesis of Chloroxaloterpin A and B and Their Antifungal Activity against Botrytis cinerea

Chloroxaloterpin A (1) and B (2) are two diterpenoids with potent inhibitory activities against spore germination of Botrytis cinerea, which were identified from Streptomyces sp. SN194 as minor products previously. In order to overcome the poor yields, 1 and 2 were synthesized using viguiepinol (3), the major metabolite of Streptomyces sp. SN194, as the precursor compound. Scanning electron microscope and transmission electron microscope observations revealed that after treating B. cinerea spores with 1 and 2, spores were obviously aberrant, the cytoplasm appeared withdrawn, and plasma membranes were blurred. Propidium iodide fluorescence assay indicated that 1 and 2 damaged plasma membranes of B. cinerea spores. In vivo assays showed that compounds 1 and 2 could effectively inhibit the sporulation of B. cinerea on tomato fruits, with sporulation inhibitory rates reaching 98.8% at 100 μg/mL. These findings provide new insights into chloroxaloterpin A and B and demonstrate their potential as lead candidates for novel fungicides.