Ferrier–Nicholas Cations from C-3-Alkynylglycals: Access to C-3-Branched Allylic Glycosides and Ring-Opening Derivatives

Hexacarbonyldicobalt-C-3-alkynyl-substituted glycal derivatives, when treated with BF3.OEt2 give rise to Nicholas-stabilized Ferrier cation intermediates (Ferrier-Nicholas cations) that react with alcohols or C-nucleophiles to give C-3-branched 2,3-unsaturated glycosides or C-glycosides, respectively. -C-Glycosides were the sole compounds obtained when allyltrimethylsilane was used as the nucleophile. On the contrary, the reaction of C-3-alkynylglycals with heteroaryl or alcohol nucleophiles led to anomeric mixtures in which the beta-anomers prevailed. The presence of the hexacarbonyldicobalt alpha-C-3-alkynyl substituent seems to be of key importance in the stereoselectivity of these transformations, since the reaction of C-3-alkynylglycals - devoid of the hexacarbonyldicobalt moiety - showed a preferred -stereoselectivity. Furthermore, a bis(indolyl) linear compound was obtained from the reaction of a hexacarbonyldicobalt-C-3-alkynylglycal with 2 equiv. of indole.