Progress Toward the Asymmetric de Novo Synthesis of Lanostanes: A Counter Biomimetic Cucurbitane-to-Lanostane Type Transformation.

An oxidative rearrangement has been established that enables a cucurbitane-to-lanostane type rearrangement that is counter to known biomimetic transformations that proceed in an opposite direction by way of a lanostane-to-cucurbitane transformation. Here, an oxidative dearomatization/Wagner-Meerwein rearrangement with a substrate bearing the characteristic cucurbitane triad of quaternary centers at C9, C13 and C14, and possessing an alkene at C11-C12, proceeds in a manner that selectively shifts the methyl group at C9 to C10 in concert with the establishment of a sterically hindered allylic cation. The major product isolated from this transformation is formed by trapping of the allylic cation by addition of acetate to C12, rather than termination of the cascade by loss of a proton at C8. While proceeding by way of a unique sequence of bond-forming reactions that begins by oxidative dearomatization, this process achieves what we believe is an unprecedented cucurbitane-to-lanostane transformation, generating a product that contains the characteristic lantostane triad of quaternary centers at C10, C13 and C14 while also delivering a functionalized C-ring.