Structure Revision of Plakotenin Based on Computational Investigation of Transition States and Spectroscopic Properties

We show that the previously [Tetrahedron Lett. 1992, 33, 2579] proposed structure of natural plakotenin must be revised. Recently, the total synthesis of plakotenin was achieved via an intramolecular Diels-Alder reaction from a (E,E,Z,E)-tetraene as linear precursor. Using density functional theory, the computation of the four possible transition states for this reaction shows that the previously proposed structure could only have been formed via an energetically high-lying transition state, which is very unlikely. Instead, we suggest that the structure of plakotenin corresponds to the product formed via the lowest transition state. A comparison of experimental and theoretical optical rotation, circular dichroism, and two-dimensional nuclear Overhauser enhancement spectra conclusively proves that the structure of plakotenin is the one that is suggested by the transition state computations. Moreover, the simulation of the nuclear Overhauser enhancement spectra suggests that it is most likely that the misassignment of the H-1 chemical shifts of two methyl groups has led to the wrong structure prediction in the 1992 work. The previously proposed structure of iso-plakotenin remains unaffected by our structure revision, but the structures of homo- and nor-plakotenin must also be revised. The present work shows how the total synthesis of a natural product, together with the theoretical determination of the barrier heights of the reactions involved, can be of great help to assign its structure. It appears that intramolecular Diels-Alder reactions can be modeled accurately by today's first-principles methods of quantum chemistry.