SYNTHESIS OF ALPHA-C-GLYCOPYRANOSIDES OF D-GALACTOSAMINE AND D-GLUCOSAMINE VIA IODOCYCLIZATION OF CORRESPONDING GLYCALS AND SILVER TETRAFLUOROBORANUIDE-PROMOTED ALKYNYLATION AT THE ANOMERIC CENTER

Iodocyclization of O-stannylated D-galactal 1, followed by azide ion displacement, gave 1,6-anhydro-2-azido-2-deoxy-beta-D-galactopyranose 9 in an expeditious way. Transformation into bromide 12 allowed coupling of various alkynyltributylstannanes in the presence of silver tetrafluoroboranuide (silver tetrafluoroborate), thus affording the corresponding alpha,beta-C-(D-galactopyranosyl)alkynes 13-17. Application of this methodology to the D-gluco isomeric bromide 24 gave a C-(D-glucopyranosyl)octyne 25 with total alpha-stereoselectivity. Conventional deprotection and saturation of the acetylenic linkage led to C-octyl-alpha-glycopyranosides of D-galactosamine 20 and D-glucosamine 28.