We recently developed a novel acyclic stereocontrol based on the stereospecific methylation of γ,δ-epoxy acrylates with trimethylaluminum in the presence of water by which both anti and syn compounds can be highly stereoselectively synthesized from trans- and cis-γ,δ-epoxy acrylates, respectively. We report here regio- and stereoselective methylation of terminal epoxides from the inside and synthetic studies toward total synthesis of Discodermolide and Streptolydigin based on the above methodology. Regio- and Stereoselective Alkylation of Terminal Epoxides from the Inside It has been known that nucleophilic opening of terminal epoxides by organocopper reagents and alkyllithiums occurs regioselectively at the terminal position resulting in the formation of secondary alcohols. Contrary regio- and stereoselective alkylation of terminal epoxides from the inside have little been known. We designed such a reaction as extension of the above methylation of γ,δ-epoxy acrylates with trimethylaluminum. Indeed reaction of terminal γ,δ-epoxy acrylates 1 and 2, prepared from D-mannitol, with excess trimethylaluminum in the presence of water proceeded regiospecifically to give 3 and 4 as a single product, respectively, with maintenance of optical integrity. Similarly reaction of 1 with triethylaluminum under the same conditions afforded exclusively the ethylation product at the γ-position. This type of reaction is stereospecific and demonstrates that the above method is applicable to terminal epoxides. The products thus obtained serve as useful chiral synthons for natural product synthesis. Synthetic Studies on Discodermolide Discodermolide is a marine natural product isolated from Discodermia dissoluta and has elicited considerable attention from synthetic chemists due to its extremely potent immunosuppressive activity. We set out synthesis of Discodermolide based on the above stereospecific methylation reaction. Discodermolide has thirteen chiral centers of which segment A having four contiguous chiral centers and segment C possessing five chiral centers have been highly stereoselectively synthesized as shown in Schemes 4 and 5, respectively. Synthetic Studies on Streptolydigin Stereoselective synthesis of oxabicyclo segment 31 of a unique tetramic acid antibiotic Streptolydigin was carried out as shown in Schemes 6 in which four asymmetric centers was highly stereoselectively constructed by repeating the above methylation reaction. These results demonstrate the usefulness of the new acyclic stereocontrol in natural product synthesis.
