Direct imine acylation: synthesis of the proposed structures of 'upenamide.

The synthesis of the two proposed structures of macrocyclic marine natural product 'upenamide is reported. The key step utilizes direct imine acylation (DIA) with a protected β-hydroxy acid to construct the key tricyclic ABC ring system. The macrocyclization was completed in the final step using a Stille cross-coupling reaction.