Synthesis of methyl 3-O-alpha-D-galactopyranosyl-6-O-alpha-D-mannopyranosyl- alpha-D-mannopyranoside, methyl 3-O-alpha-D-glucopyranosyl-6-O-alpha-D- mannopyranosyl-alpha-D-mannopyranoside, methyl 6-O-alpha-D-galactopyranosyl-3- O-alpha-D-mannopyranosyl-alpha-D-mannopyranoside, and methyl 6-O-alpha-D