Potent Antiviral Potamogetonyde and Potamogetonol, New Furanoid Labdane Diterpenes from Potamogeton malaianus

New furanoid labdane diterpenes, potamogetonyde (3) and potamogetonol (4), together with two known compounds, potamogetonin (1) and 15,16-epoxy-12-oxo-8(17),13(16),14-labdatrien-20,19-olide (2), were isolated from the CH2Cl2 extract of Potamogeton malaianus. The chemical structures of 1−4 were elucidated by the analyses of their spectral data, mainly by 1D and 2D NMR techniques. Potamogetonyde (3) and potamogetonol (4) exhibited potent antiviral (HSV-1) activity with respective IC50 values of 8 and 3 μg/mL. Compounds 1−4 possessed cytotoxicity toward insect cells (fall armyworm and mosquito larvae, IC50 of 11−72 μg/mL). Furanoid diterpenes 3 and 4 also exhibited cytotoxicity against the Vero cell line with respective IC50's of 31 and 28 μg/mL, while 1 and 2 were inactive at 50 μg/mL. Compounds 1−4 were inactive (at 20 μg/mL) against KB and BC cell lines and showed only weak antimycobacterial activity against Mycobacterium tuberculosis H37Ra with minimum inhibitory concentrations of 50−100 μg/mL.