Stereocontrolled Synthesis of the Spiroketal Unit of 22,23-Dihydroavermectin B1b.

A diastereocontrolled synthesis of the spiroketal sub-unit of 22,23-dihydroavermectin B1b has been developed using a formal double condensation at both ends of pentane 2,4-dione 3 with intermediate formation of the ketal 5.