Brazilian Conference on Natural Products
7th Brazilian Conference on Natural Product/ XXXIII RESEM Proceedings

In 1997, a compound presenting a novel benzoxocane ether skeleton, (+)-Helianane, was isolate from the marine sponge Haliclona fascigera. Its structure was elucidated by extensive NMR studies. The discovery of the presence of this new type of eight-membered cyclic skeleton caught the attention synthetic groups by all over the world. Pettus’s group turned to an elegant ring closure metathesis strategy, succeeding in the elaboration of the originally proposed structure. However, the NMR data of the synthetic samples did not match with those obtained for the isolated natural product. Thereby, they got to the inescapable conclusion that the synthetic and natural compounds had different structures and there was a need to review the originally proposed structures as curcudiol and settled the repudiation of the helianane family. However, several groups defending that originally proposed structure cannot be ruled out immediately, rather a further experimentation is necessary. This structural problem remains unsolved. This way, according to our continued interest in the application and development of new tools for structural elucidation of complex natural products by means of quantum chemical calculations of NMR chemical shifts, and taking into account the remote possibility of unraveling the structures of helianane by reisolation, we decided to approach this problem computationally.
The 13C NMR chemical shift calculations of both helianane (eight membered ring system) and curcudiol were performed by a Gauge Included Atomic Orbitals-Hybrid Density Functional Theory (GIAO-HDFT) calculation procedure at the GIAO/mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level of theory using the Gaussian 09 software package. Our analysis suggests that the synthetic and the isolated natural compounds really presents different structures, which is compatible with the amendment made by Pettus.