Total synthesis of the antiparasitic agent avermectin B(1a)

The total convergent synthesis of the antiparasitic agent avermectin B1a involves the preparation of three principal subunits containing a spiroketal segment, a hexahydrobenzofuran unit and an acyclic portion. A 21-step synthetic sequence helps assemble these subunits to design avermectin B1a aglycon and an average yield of 73% is obtained per step. The final step involves the attachment of the disaccharide L-oleandrosyl-L-oleandrose to avermectin B1a aglycon.