Total Synthesis and Structural Reassignment of (±)-Cereoanhydride

The first total synthesis of (±)-cereoanhydride has been achieved under the inspiration of its biosynthetic hypothesis. The tricyclic skeleton of trypethelone, the proposed biosynthetic precursor of cereoanhydride, was constructed by an interesting ring expansion of cyclobutenone in one step in which the introduced acetyl group is pivotal to avoid the following aerial oxidation. On the basis of X-ray crystallographic analysis, the structure of cereoanhydride is reassigned to a spiroketal structure 2, which should be formed through an isomerization of the originally proposed structure 1.