The Exocyclic Olefin Geometry Control via Ireland–Claisen Rearrangement: Stereoselective Total Syntheses of Barmumycin and Limazepine E

Stereoselective total syntheses of Limazepine E and Barmumycin, potent, naturally occurring antitumor agents, are described. The total syntheses control the olefin geometry via a highly selective chelation-controlled Ireland–Claisen rearrangement of a seven-membered lactone-derived boron enolate for the synthesis of (E)-4-ethylidene proline, a crucial building block for a number of natural products.