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A C-Glycosylflavone from Piper ossanum, a Compound Conformationally Controlled by CH/π and Other Weak Intramolecular
María Larionova1, Iraida Spengler, Clara Nogueiras
1Instituto Superior de Medicina Militar Dr. Luis Díaz Soto, Laboratorio de Medicina Herbaria, Avenida Monumental y Carretera del Asilo, Habana del Este, 11700, Ciudad de la Habana, Cuba.
Abstract:
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.
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