Cantharidin (1) and norcantharidin (2) are potent protein phosphatase 1 and 2A inhibitors that also display high levels of anticancer activity against a broad range of tumor cells lines. Surprisingly, Δ-5, 6-ethyl norcantharidin (3, cis-tetrahydrofurano [3, 4-c] furan-1, 3-dione) displays neither phosphatase inhibition nor anticancer activity. This suggests that the 5, 6- ethyl bridge is pivotal to both anti-cancer and protein phosphatase activity. Additionally ...