Studies Directed toward the Total Synthesis of Azaspiracid:  Stereoselective Construction of C₁−C₁₂, C₁₃−C₁₉, and C₂₁−C₂₅ Fragments

The efficient entry to the C₁−C₁₂, C₁₃−C₁₉, and C₂₁−C₂₅ fragments of azaspiracid is outlined. The C₁−C₁₂ portion is constructed using a key asymmetric allenyl borane addition to the corresponding α,β-unsaturated aldehyde. The synthesis of the C₁₃−C₁₉ portion utilizes an Evans asymmetric alkylation followed by Sharpless asymmetric dihydroxylation. In addition, a novel solution to the mismatched effects of a neighboring chiral oxazolidinone during a Sharpless dihydroxylation is detailed.