Synthesis of Hexp-(1→4)-β-D-GlcpNAc-(1→2)-α-D-Manp-(1→O)(CH2)7CH3 probes for exploration of the substrate specificity of glycosyltransferases: Part I, Hex = β-D-Gal, 4-deoxy-β-D-Gal, 4-O-methyl-β-D-Gal, 4-deoxy-4-fluoro-β-D-Gal, or β-D-Glc