Regioselective introduction of 2-propynyl groups into the C-2 or C-3 position of furanoside ring

The reaction of methyl 2,3-anhydro-alpha-D-ribofuranoside or methyl 2,3-anhydro-5,6-O-cyclohexylidene-alpha-D-allofuranoside with 2-propynyl metallic reagents exclusively gave the corresponding 2-C-(2-propynyl)-arabino-pentofuranosides and 2-C-(2-propynyl)-altro-hexofuranosides, respectively. In contrast, the beta-anomers were selectively attacked at the position 3 to afford the corresponding 3-C-(2-propynyl)furanosides.