Six derivatives (1–6) of moronic acid were semi-synthesized and their in vitro protein tyrosine phosphatase 1B (PTP-1B) inhibition activity assessed. Derivatives 2 (IC50 = 10.8 ± 0.5 μM) and 6 (IC50 = 7.5 ± 0.1 μM) displayed the most potent inhibitory activity. Therefore, they (50 mg/Kg) were tested for their antidiabetic effect in vivo using a non-insulin dependent diabetes mellitus rat model. The