Efficient and concise synthesis of beta Man1-4GlcN of pentasaccharide core is established; direct beta-mannosylations of 4-OH group of 2-deoxy-2-phthaloyl and 2-azide-2-deoxy glucose derivatives by using 6-nitro-2-benzothiazolyl alpha-mannoside proceeded smoothly to afford the desired beta-mannosides in high yields.