Synthesis of methyl 3-O-α-d-galactopyranosyl-6-O-α-d-mannopyranosyl-α-d-mannopyranoside, methyl 3-O-α-d-glucopyranosyl-6-O-α-d-mannopyranosyl-α-d-mannopyranoside, methyl 6-O-α-d-galactopyranosyl-3-O-α-d-mannopyranosyl-α-d-mannopyranoside, and methyl 6-O-α-d-glucopyranosyl-3-O-α-d-mannopyranosyl-α-d-