Isolation, Structural Characterization and Antidiabetic Activity of New Diketopiperazine Alkaloids from Mangrove Endophytic Fungus Aspergillus sp. 16-5c

Six new DIKETOPIPERAZINE alkaloids aspergiamides A-F (1-6), together with ten known alkaloids (7-16), were isolated from the mangrove endophytic fungus Aspergillus sp. 16-5c. The structures of the new compounds were elucidated based on 1D/2D NMR spectroscopic and HR-ESIMS data analyses. The absolute configurations of aspergiamides A-F were established based on the experimental and calculated ECD data. All the compounds were evaluated for the antidiabetic activity against alpha-glucosidase and PTP1B enzyme. The bioassay results disclosed compounds 1 and 9 exhibited significant alpha-glucosidase inhibitory with IC50 values of 18.2 and 7.6 mu M, respectively; compounds 3, 10, 11, and 15 exhibited moderate alpha-glucosidase inhibition with IC50 values ranging from 40.7 to 83.9 mu M; while no compounds showed obvious PTP1B enzyme inhibition activity.