Potential anti-diabetic isoprenoids and a long-chain δ-lactone from frangipani (Plumeria rubra).

A decoction of Plumeria rubra flowers has been used traditionally for the treatment of diabetes in China and Mexico. Chemical investigations on the bioactive constituents of these flowers led to the isolation of 30 compounds, including the four new compounds, one iridoiod (1), two triterpenoids (4, 5), and a long-chain δ-lactone (16). In addition, 26 known compounds (2, 3, 6-15, 17-30) are also reported. All of these compounds were identified on the basis of spectroscopic data interpretation and the absolute configurations of compound 4, 5, 16 were determined by Mosher's method. Compounds 1-4, 7, 8 and 16 showed moderate to significant inhibitory activities against α-glucosidase and protein tyrosine phosphatase 1B, with 4 having IC50 values of 19.45 μM and 0.21 μM, respectively.