Synthesis of the tetrasaccharide lipid intermediate P1-Dolichyl P2-[O-α-D-mannopyranosyl-(1→6)-O-β-D-mannopyranosyl-(1→4)-O-(2-acetamido-2-deoxy-β-D-glucopyranosyl)- (1→4)-2-acetamido-2-deoxy-α-D-glucopyranosyl] disphosphate