Stereocontrolled synthesis and reactivity of sugar acetylenes
C-Glycosidation is of great significance in the organic synthesis of optically 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 carbon source. Silylacetylenes are sufficiently reactive to form ‘sugar acetylenes’ for the selective introduction of various acetylenic groups in an alpha-axial manner at the anomeric position of D-hexopyranose rings. 1,4-Anti induction, on the other hand, gives a different stereochemical outcome in the 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. Bis-C-glycosidation allows the introduction of sugars at both ends of some bis(trimethylsilyl)acetylenes. A 2,3-dideoxyglucose derivative provides the corresponding C-1 α-acetylenic compounds, which would increase the scope of C-glycosidation with silylacetylenes. In sugar acetylenes, the alkynyl group at the anomeric position of a pyranose ring is epimerized via a hexacarbonyldicobalt complex by treatment with trifluoromethanesulfonic acid. The three steps—cobalt complexation, acidic transformation and decomplexation—afford overall epimerization and thus one can obtain either the α- or β-alkynyl C-glycoside as desired. Ring opening of a dihydropyran derivative using Nicholas-type cation intermediates is also part of this study. Several sets of decomplexation conditions for endo-type acetylene–cobalt complexes provide various olefins possessing potential utility for synthesis. These methodologies have been utilized for the synthesis of polyoxygenated natural products and derivatives.