Stereocontrolled synthesis and reactivity of sugar acetylenes

Abstract

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 a-acetylenic compounds, which would increase the scope of Cglycosidation 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 decomplexationafford overall epimerization and thus one can obtain either the aor b-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 pro-vide various olefins possessing potential utility for synthesis. These methodologies have been utilized for the synthesis of polyoxygenated natural products and derivatives.

Recent Advances in the Synthesis of C-oligosaccharides

“…Those alkynylated compounds are called "sugar acetylenes" [45]. The mechanism includes eliminative formation of cation to which silylacetylene can coordinate on the α-face (Scheme 40).…”

Total Synthesis of α-C-Mannosyltryptophan, a Naturally Occurring C-Glycosyl Amino Acid

“…Herein we describe an efficient stereocontrolled synthesis of a-C-mannosyltryptophan (2). 13 In our previous paper, a dehydrotryptophan intermediate was synthesized from C-mannosylindole 3, which was prepared from D-mannose derivative 5 through ethynylmannose 4 by stereoselective a-C-glycosidation 14 with tinacetylene and Castro indole synthesis as the key steps (Scheme 1). 15 However, hydrogenation of the dehydrotryptophan gave a diastereomeric mixture, whose absolute stereochemistry has not been determined.…”

Regioselectivity of Larock Indole Synthesis Using Functionalized Alkynes

“…[20][21][22] acetate, which was treated with a large amount of NaBH 4 in the presence of CeCl 3 to afford allylic alcohol 22 in 53% overall yield from 20. In the next epoxidation with MCPBA, we found that bis-TBS ether 23 derived from 22 was the best substrate to obtain high yield of -epoxide 24.…”