Catalyst-Controlled Formal [4+3] Cycloaddition Applied to the Total Synthesis of (+)-Barekoxide and (-)-Barekol

The tandem cyclopropanation/Cope rearrangement between bicyclic dienes and siloxyvinyldiazoacetate, catalyzed by the dirhodium catalyst Rh-2(R-PTAD)(4), effectively accomplishes enantiodivergent [4 + 3] cycloadditions. The reaction proceeds by a cyclopropanation followed by a Cope rearrangement of the resulting divinylcyclopropane. This methodology was applied to the synthesis of (+)-barekoxide (1) and (-)-barekol (2).