Studies directed toward the total synthesis of azaspiracid:: Stereoselective construction of C1-C12, C13-C19, and C21-C25 fragments

The efficient entry to the C-1-C-12, C-13-C-19, and C-21-C-25 fragments of azaspiracid is outlined. The C-1-C-12 portion is constructed using a key asymmetric allenyl borane addition to the corresponding alpha,beta -unsaturated aldehyde. The synthesis of the C-13-C-19 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.