Total Synthesis of (−)-Gambierol

The first total synthesis of (−)-gambierol (1), a marine polycyclic ether toxin, has been achieved. Key features of the successful synthesis include (1) a convergent union of the ABC and EFGH ring fragments (5 and 6, respectively) via our developed B-alkyl Suzuki−Miyaura cross-coupling strategy leading to the octacyclic polyether core 4 and (2) a late-stage introduction of the sensitive triene side chain by use of Pd(PPh3)4/CuCl/LiCl-promoted Stille coupling. The ABC ring fragment 5 was synthesized in a linear manner (B → AB → ABC), wherein the A ring was formed by intramolecular hetero-Michael reaction and the C ring was constructed via 6-endo cyclization of hydroxy epoxide 7. An improved synthetic entry to the EFGH ring fragment 6 is also described, in which SmI2-induced reductive cyclization methodology was applied to the stereoselective construction of the F and H rings, leading to 6 with remarkable overall efficiency. Stereoselective hydroboration of 5 and subsequent Suzuki−Miyaura coupling with 6 provided endocyclic enol ether 45 in high yield, which was then converted to octacyclic polyether core 4. Careful choice of the global deprotection stage was a key element for the successful total synthesis. Functionalization of the H ring and global desilylation gave (Z)-vinyl bromide 2. Finally, cross-coupling of 2 with (Z)-vinyl stannane 3 under Corey's Pd(PPh3)4/CuCl/LiCl-promoted Stille conditions completed the total synthesis of (−)-gambierol (1).