Synthesis of α-C-glycopyranosides ofD-galactosamine andD-glucosamine via iodocyclization of corresponding glycals and silver tetrafluoroboranuidepromoted alkynylation at the anomeric centre

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Abstract
Iodocyclization of O-stannylated D-galactal 1, followed by azide ion displacement, gave 1,6-anhydro-2-azido-2-deoxy-β-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 αβ-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 α-stereoselectivity. Conventional deprotection and saturation of the acetylenic linkage led to C-octyl-α-glycopyranosides of D-galactosamine 20 and D-glucosamine 28.