On the total synthesis and determination of the absolute configuration of rishirilide B: Exploitation of subtle effects to control the sense of cycloaddition of o-quinodimethides

The total synthesis of racemic rishirilide B has been accomplished. The synthesis serves to define the relative relationships of its stereogenic centers. Also, starting with readily available chiral pool, ent-rishirilide B was synthesized, thereby demonstrating that natural configuration of rishirilide B. The defining step in our total synthesis is the facile cycloreversion of the bis(siloxy)-benzocyclobutane and the intermolecular o-quinodimethide Diels-Alder cycloaddition. We believe that the tight regiochemical guidance in this step arises from a meshing of the electron-donating effects of the symmetry-perturbing aromatic OTBS group of the o-quinodimethide diene with the reactivity differential of the dienophile (enedione), modulated by the hydroxyl group at the α-position. The validity of the hypothesis of hydroxy-directed activation of its vicinal ketone function in the context of the enedione dienophile warrants further study. This type of activation may find broader applications in distinguishing reactivity profiles of key closely related functional groups in organic substrates.