Triterpenes, Phenols, and Other Constituents from the leaves of Ochroma pyramidale (Balsa Wood, Bombacaceae): Preferred Conformations of 8-C-β-D-Glucopyranosyl-apigenin (vitexin)

Lupeol, oleanolic acid, stigmasterol, β-sitosterol, β-sitosteryl-β-D-glucopyranoside, catechin, epicatechin, and 8-C-β-D-glucopyranosylapigenin (vitexin) were isolated from the acetonic extract of the leaves of Ochroma pyramidale (balsa wood, Bombacaceae), a tree noted by its exceedingly light wood. ¹H and 13C NMR of 8-C-β-D-glucopyranosyl-apigenin (vitexin) at room temperature exhibited doubling of some signals, suggesting the presence of atropisomers. ¹H NMR spectra at 70 °C showed one set of signals, confirming the presence of rotational isomers at room temperature. Molecular modelling using AM1 methods, established the two conformations of minimum energy for vitexin, which correspond to the -ac (ω = -94 °) and +ac (ω = + 96 °) arrangement along the C(2")-C(1")-C(8)-C(7) bonds.