Conformational Analysis and NMR Data Assignment of Taraxerone Isolated From Cnidoscolus aconitifolius (Chaya)

We report, for the first time, the isolation of taraxerone from an isopropanol extract of chaya (Cnidoscolus aconitifolius), a traditional plant used in Mayan culture. Although this natural product has been previously identified in other plant species, the complete assignment of its NMR data had not been accomplished until now. To achieve this, we employed 2D NMR experiments to assign the ¹H and ¹³C chemical shifts of taraxerone. Several approaches were used to determine the spin-spin coupling constants (J_H,H), including GIAO calculations, spin simulation, and dihedral angle analysis via a Karplus-type HLA equation. Conformational analysis revealed the presence of two conformations of Ring A: a chair and a twisted boat. To better understand the behavior of these conformers, a variable-temperature ¹H NMR experiment was performed, in which Δδ/°C values for selected protons were monitored. These changes are attributed to stereoelectronic effects, such as the Perlin effect, as evidenced by ¹J_C,H values obtained from HSQC nondecoupled experiments. Isotopomeric shifts (¹Δ^12/13C(¹H)) were also investigated. All findings were further supported by natural bond orbital (NBO) calculations, which helped explain the observed hyperconjugative interactions.