Convergent synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin, marine polycyclic ether toxins causative of diarrheic shellfish poisoning, has been accomplished. The A-ring fragment was constructed by coupling of an appropriately functionalized sulfonyl-stabilized oxiranyl anion and a triflate prepared from an erythritol derivative. An iterative protocol of the oxiranyl anion strategy was also applied for the construction of the DEF-ring fragment. The triflate derivatives of the A-ring and the DEF-ring fragments were connected with lithium acetylide. The resulting acetylene derivative was further transformed into the hexacyclic ABCDEF fragment via oxidation of the acetylene unit to 1,2-diketone, double methyl acetal formation, and reductive etherification.