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-α-rhamnosyl-4''-O-methylvitexin (apigenin-8-C-α-rhamnosyl-(1→2)-β-4-O-methylglucopyranoside), isolated from the leaves of Piper ossanum, was revised to acacetin-8-C-neohesperidoside (acacetin-8-C-α-rhamnosyl-(1→2)-β-glucopyranoside or 2''-O-α-rhamnosyl-4'-O-methylvitexin) (1). The NMR data and theoretical calculations established the preferred conformation of 1, which is controlled by CH/π 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 Δ2 effect, and several hydrogen bonds.