STUDIES IN MACROLIDE SYNTHESIS - A STEREOCONTROLLED SYNTHESIS OF OLEANDOLIDE EMPLOYING REAGENT-CONTROLLED AND SUBSTRATE-CONTROLLED ALDOL REACTIONS OF (S)-1-(BENZYLOXY)-2-METHYLPENTAN-3-ONE

A highly stereocontrolled total synthesis of oleandolide (2), the aglycon of the macrolide antibiotic oleandomycin (1), has been completed in 8% overall yield (20 steps longest Linear sequence, 26 steps in total) with 90% overall diastereoselectivity. Initially, reagent-controlled syn aldol reactions of (S)-1-(benzyloxy)-2-methylpentan-3-one ((S)-8) were employed to prepare adducts 6 (SS) and 7 (SA), which were elaborated to provide the two advanced fragments 33 and 27, respectively. Coupling of these fragments followed by functional group manipulation and macrolactonization gave the macrocyclic ketone 42, possessing S configuration at C-9. Elaboration of 42 to oleandolide, however, proved troublesome. Substrate-controlled syn and anti aldol reactions of ketone (S)-8, meanwhile, provided the adducts 6 (SS) and 7 (AA), which enabled synthesis, via fragments 64 and 60, of the key macrocyclic ketone intermediate 69, having R configuration at C-9. Stereoselective epoxidation of ketone 69, by reaction with dimethylsulfonium methylide under macrocyclic stereocontrol, provided the (8R)-epoxide 83; subsequent elaboration then gave oleandolide (2).