Stereoselective Construction of an Unprecedented 7−8 Fused Ring System in Micrandilactone A by [3,3]-Sigmatropic Rearrangement

The 7−8 bicyclic ring system of micrandilactone A (1) with the required stereochemistry and functional groups was constructed by a Bu3Al-promoted Claisen rearrangement. Computational studies indicated that the exocyclic vinyl ether undergoes a [3,3] sigmatropic process via a
chairlike transition state to afford exclusively the Z-double bond in the newly generated 8-membered ring with a high level of chirality transfer.