Synthesis of the principal chain of the 0-antigenic polysaccharides ofShigella flexneri Communication 1. Synthesis of 2,4- and 3,4-di-o-benzoyl-?-L-rhamnopyranosides by a combination of selective acylation and deacetylation, and the synthesis of 1,2-di-o-acetyl-3,4-di-o-benzoyl-L-rhamnopyranose via 1,2-o-benzylidene-3,4-di-o-benzoyl-?-L-rhamnopyranose