Suitability of the Claisen ring expansion protocol for crenulide diterpene construction
作者:Leo A. Paquette、Jesus Ezquerra、Wei He
DOI:10.1021/jo00110a053
日期:1995.3
An enantiospecific synthesis of optically pure 2 a molecule possessing the ring system characteristic of the crenulide diterpenes, is reported. The nine-step sequence began with the previously described R lactone 6. The early bond-forming reactions include an aldol condensation with crotonaldehyde and intramolecular selenonium ion-promoted cyclization with participation by the neighboring hydroxyl group. Selenoxide elimination made possible the acquisition of allyl vinyl ether 4. Thermal activation of the latter in hot mesitylene proceeded via a Claisen ring expansion pathway to give 36a. This key intermediate was then ketalized, subjected to Simmons-Smith cyclopropanation, and carried through a selenoxide elimination step to arrive ultimately at 2. Selected transformations of the medium-ring bicyclic lactones are described, accompanied by X-ray crystallographic studies to substantiate the stereochemical assignments advanced throughout the investigation.