The reaction of organometallic derivatives of monosaccharides with aldehydes catalyzed with BF3·OEt2 was studied. A significant difference in reactivity between the pyranosidic and furanosidic allyltins was noted. The former reacted readily with aldehydes affording precursors of higher carbon sugars with very high stereoselectivity, while the latter underwent rearrangement with elimination of the stannyl