Progress towards the total synthesis of guanacastepene A. Approaches to the construction of quaternary carbons and the 5-7-6 tricyclic carbon skeleton

The construction of the 5-7-6 tricyclic skeleton of guanacastepene A, including the quaternary carbons, has been achieved. The C-11 and C-8 quaternary carbons were constructed by cuprate addition and carbene insertion, respectively. The 5-7-6 tricyclic core was generated by a selective aldol and SmI2-induced intramolecular aldehyde–alkene coupling.