A C-Glycosylflavone from Piper ossanum, a Compound Conformationally Controlled by CH/π and Other Weak Intramolecular Interactions

The structure of the known 2 ''-O-alpha-rhamnosyl-4 ''-O-methylvitexin (apigenin-8-C-alpha-rhamnosyl-(1 -> 2)-beta-4-O-methyl-glucopyranoside), isolated from the leaves of Piper ossanum, was revised to acacetin-8-C-neohesperidoside (acacetin-8-C-alpha-rhamnosyl-(1 -> 2)-beta-glucopyranoside or 2 ''-C-alpha-rhamnosy1-4'-O-methylvitexin) (1). The NMR data and theoretical calculations established the preferred conformation of 1, which is controlled by CH/pi interactions. This phenomenon explains the unusual chemical shifts of some protons in the molecule, besides other weak intramolecular interactions such as the anomeric effect, the Delta 2 effect, and several hydrogen bonds.