Synthesis of the principal chain of the o-antigenic polysaccharides ofShigella flexneri. Communication 4. Synthesis of 1,3-di-o-acetyl-4,6-di-o-benzoyl-2-desoxy-2-phthalimido-D-glucopyranose and 1-o-acetyl-2-o-(3-o-acetyl-4,6-di-o-benzoyl-2-desoxy-2-phthalimido-β-D-glucopyranosyl)-3,4-di-o-benzoyl-L