Structural revisions of natural products by Computer Assisted Structure Elucidation (CASE) Systems

This review considers the application of CASE systems to a series of examples in which the original structures were later revised. We demonstrate how the chemical structure could be correctly elucidated if 2D NMR data were available and the expert system Structure Elucidator was employed. We will also demonstrate that if only 1D NMR spectra from the published articles were used then simply the empirical calculation of 13C chemical shifts for the hypothetical structures frequently enables a researcher to realize that the structural hypothesis is likely incorrect. We also analyze a number of erroneous structural suggestions made by highly qualified and skilled chemists. The investigation of these mistakes is very instructive and has facilitated a deeper understanding of the complicated logical-combinatorial process for deducing chemical structures.
The multiple examples of the application of Structure Elucidator for resolving mis-assigned structures has shown that the program can serve as a flexible scientific tool which assists chemists in avoiding pitfalls and obtaining the correct solution to a structural problem in an efficient manner. Chemical synthesis clearly still plays an important role in molecular structure elucidation. The multi-step process requires the structure elucidation of all intermediate structures at each step, for which spectroscopic methods are commonly used. Consequently, the application of a CASE system would be very helpful even in those cases when chemical synthesis is the crucial evidence to identify the correct structure. We also believe that the utilization of CASE systems will frequently reduce the number of compounds requiring synthesis.