Synthetic Studies on Gambieric Acids, Potent Antifungal Polycyclic Ether Natural Products: Reassignment of the Absolute Configuration of the Nonacyclic Polyether Core by NMR Analysis of Model Compounds

A highly stereocontrolled, convergent synthesis of the A/B-ring fragment of gambieric acids (GAs) has been developed on the basis of (i) a Suzuki-Miyaura coupling of the C1-C6 alkylborate and the C7-C17 vinyl iodide and (ii) a diastereoselective haloetherification for the construction of the A-ring tetrahydrofuran as key steps. Inspection of the H-1 and C-13 NMR chemical shifts of the synthesized A/B-ring model compounds led to a stereochemical reassignment of the absolute configuration of the polycyclic ether core of GAS. This structure revision was further supported by a synthesis of the A/BC-ring model compound of gambieric acid B and a comparison of its H-1 and C-13 NMR data with those of the natural product.