Vinylation-Electrophilic Cyclization of Aldopentoses: Easy and Stereoselective Access to C-Glycopyranosides of Rare Sugars

A new C-glycosylation procedure, which involves vinylation of a properly protected aldopentose and mercuriocyclization of the obtained glycoenitol, is described; it allows C-glycopyranosides of rare sugars to be obtained from easily available pentoses. 2,3,5-Tri-O-benzyl-D-arabinose (1a) reacted with vinylmagnesium bromide to afford a mixture of D-gluco and D-manno 1,2-dideoxy-4,5,7-tri-O-benzyl-1-heptenitols 2a and 2b and with divinylzinc to afford only 2a. 2,3,5-Tri-O-benzyl-D-ribose (1b) afforded mainly 1,2-dideoxy-4,5,7-tri-O-benzyl-D-altro-1-heptenitol (2c) with both vinylmetallic reagents, whereas 5-O-trityl-2,3-O-isopropylidene-D-ribose (1e) afforded 1,2-dideoxy-7-O-trityl-4,5-O-isopropylidene-D-allo-l-heptenitol (2h). Vinylation of 2,3,5-tri-O-benzyl-D-xylose (1c) afforded a mixture of D-iodo and D-gulo 1,2-dideoxy-4,5,7-tri-O-benzyl-1-heptenitols 2d and 2e, the first one being largely predominant when divinylzinc was employed. 2,3,5-Tri-O-benzyl-D-lyxose (1d) reacted only with vinylmagnesium bromide to afford a mixture of D-talo and D-galacto 1,2-dideoxy-4,5,7-tri-O-benzyl-1-heptenitols 2f and 2g. The enitols 2a-h where cyclized with mercuric acetate and then processed with potassium chloride to afford the corresponding (D-glycopyranosylmethyl)mercurium chlorides 3a-h. The cyclization was stereoselective except for 2e, and in all cases, except 3d, the anomeric substituent of the C-glycopyranoside was cis related with the alkoxy substituent at the adjacent carbon atom. So α-gluco, β-manno, β-altro, β-ido, α- and β-gulo, β-talo, α-galacto, and α-allo C-glycopyranosides were obtained.