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

Abstract

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. α‐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 β‐anomers prevailed. The presence of the hexacarbonyldicobalt–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 α‐stereoselectivity. Furthermore, a bis(indolyl) linear compound was obtained from the reaction of a hexacarbonyldicobalt–C‐3‐alkynylglycal with 2 equiv. of indole.