| I. Synthesis of the fucose-aglycon conjugate of the heptadecasaccharide natural products saccharomicins A and B has been accomplished. Schmidt glycosylation of an appropriately protected 3,4-dihydroxy-cinnamate ester with 2,3,4-triacetoxy-L-fucopyranosyl-1-trichloroacetimidate gives aryl glycoside in high yield and diastereoselectivity. C2'-sulfation of fucose, installation of taurine, and global deprotection of remaining protecting groups affords the fucose-aglycon conjugate. This synthesis, which arises from L-fucose, establishes the absolute configuration of the reducing terminus of the saccharomicin oligosaccharide.;II. 3,4-Isopropylidenedioxy-1-yne-5,6-diols undergo endo-selective cycloisomerization to the corresponding seven-membered ring glycals under tungsten carbonyl catalysis. Control experiments suggest that the unexpected regioselectivity for reaction of the distal 6-hydroxyl group rather than the proximal 5-hydroxyl is dependent on the presence of the cyclic dioxolane structure tethering the terminal alkyne and diol functional groups. Additional studies demonstrate viability of septanose glycals as glycosyl donors and acceptors for application in oligosaccharide synthesis.;III. Alkynyl carbamates undergo tungsten-promoted cycloisomerization to endocyclic enecarbamates in modest yield. A highly substituted, polyoxygenated alkynyl carbamate shows enhanced reactivity towards 6- endo-regioselective cycloisomerization, indicating a favorable substitution effect. |
