Stereocontrolled synthesis and reactivity of sugar acetylenes

C-Glycosidation is of great significance in the organic synthesis of optica lly active materials, since it allows the introduction of carbon chains to sugar chirons and the use of sugar nuclei as a chiral pool as well as a car bon source, Silylacetylenes are sufficiently reactive to form 'sugar acetyl enes' for the selective introduction of various acetylenic groups in an alp ha-axial manner at the anomeric position of D-hexopyranose rings, 1,4-Anti induction, on the other hand, gives a different stereochemical outcome in t he case of C-glycosidation of pentopyranose glycals, The mechanism of these reactions includes oxonium cation intermediates in which stereoelectronic and/or steric factors drive the direction of the incoming silylacetylene. B is-C-glycosidation allows the introduction of sugars at both ends of some b is(trimethylsilyl)acetylenes. A 2,3-dideoxyglucose derivative provides the corresponding C-1 a-acetylenic compounds, which would increase the scope of C-glycosidation with silylacetylenes. In sugar acetylenes, the alkynyl gro up at the anomeric position of a pyranose ring is epimerized via a hexacarb onyldicobalt complex by treatment with trifluoromethanesulfonic acid. The t hree steps-cobalt complexation, acidic transformation and decomplexation-af ford overall epimerization and thus one can obtain either the alpha- or bet a-alkynyl C-glycoside as desired. Ring opening of a dihydropyran derivative using Nicholas-type cation intermediates is also part of this study. Sever al sets of decomplexation conditions for endo-type acetylene-cobalt complex es provide various olefins possessing potential utility for synthesis, Thes e methodologies have been utilized for the synthesis of polyoxygenated natu ral products and derivatives.