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■ Density Functional Theory Study of the Preferred Conformation of Geissoschizine

Juriffah Ariffin, Hiromitsu Takayama, Mariko Kitajima, Norio Aimi,* and Chikara Kaneko

*Graduate School of Pharmaceutical Sciences, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Abstract

Density functional theory (DFT) study of the hitherto proposed structures of geissoschizine (1) has proved that the structure (1a) having hydrogen bond between the enol hydroxyl and Nb functions with a trans-quinolizidine-type C/D ring and twistboat D ring (proposed by van Tamelen and Wright) is the most stable one. A strong contribution of the zwitterionic species (1a') to 1a is suggested together with the reasons why 1a' was not found as a local minimum. Calculations of ¹H- and ¹³C- NMR chemical shifts based on this structure also support its structure.