Structural Revision of Tinotufolins from <i>Tinospora crispa</i> Leaves Guided by Empirical Rules and DFT Calculations
Yuto Nishidono, Ken Tanaka
Abstract
Clerodane diterpenes are a class of secondary metabolites that can be classified into four types according to the configuration of the H 3 -19/H-10–H 3 -17/H 3 -20 fragment, i.e., trans – cis (TC), trans – trans (TT), cis – cis (CC), and cis – trans (CT). Tinotufolins A–C and E ( 1a – 3a and 5a ), isolated from the leaves of Tinospora crispa, were previously elucidated as CT-type clerodanes; however, our established 13 C NMR-based empirical rules and density functional theory calculations suggested that these clerodanes belong to the CC type. Therefore, tinotufolins A–F ( 1 – 6 ) were reisolated from the leaves of T. crispa, along with an undescribed compound 7 and known compounds 8 – 11, and their structures were established by extensive spectroscopic analyses. The structures of tinotufolins A–C and E were revised to CC-type 1 – 3 and 5, and undescribed compound 7 was established as a CC-type clerodane. The present study demonstrates that empirical rules and calculations can efficiently identify and revise erroneous structures in clerodane diterpenes.