Stereocontrolled preparation of the C1-C14 polyene fragment of benzenic ansamycin antibiotics ansatrienin A and B

A practical synthesis of the C1-C14 polyene unit 5 in ansatrienin A (mycotr ien I), la, and other ansamycin antibiotics is described which involves ela boration of the chiral unsaturated aldehyde 6 and phosphonate 7, followed b y coupling of the building blocks by Horner-Emmons-olefination and Duthaler 's aldolation. The synthesis of 6 successfully applies two consecutive Evan s-aldol reactions for constructing the carbon backbone.