Stereodivergent Approach to the Avermectins Based on “Super Silyl” Directed Aldol Reactions

A stereodivergent approach to the spiroketal fragment of the avermectins is described. The strategy utilizes a sequence of three aldol reactions directed by the <i>tris</i>(trimethylsilyl)­silyl “super silyl” group. Central to this strategy is that each aldol reaction can be controlled to allow access to either diastereomer in high stereoselectivity, thereby affording 16 stereoisomers along the same linear skeleton. The aldol products can be transformed into spiroketals, including an advanced intermediate in the total synthesis of avermectin A1a