Synergistic Combination of NAPROC-13 and NMR 13 C DFT Calculations: A Powerful Approach for Revising the Structure of Natural Products.

This article describes the structure revision of nine triterpenoids that have been reported corresponding to the same 13 C NMR data set. In addition, 13 C NMR calculation shows that some chemical shift assignments must be swapped. Our analysis improves the fit between the experimental and calculated data. Correcting misassigned structures and correctly assigning each signal is essential for elucidating new structurally related compounds. Furthermore, the ambiguity of several compounds, the structure of which differs in the literature and the Sci-Finder database, has been eliminated. Misassigned structures were found by chemical shift searches in NAPROC-13, and the results provide two or more different compounds with the same 13 C NMR data. The process to determine the correct, most likely structural proposal in agreement with the experimental 13 C NMR data was carried out by DFT calculations.