title: Synthesis, conformational analysis, and the glycosidic coupling reaction of substituted 2,7-dioxabicyclo[4.1.0]heptanes: 1,2-anhydro-3,4-di-O-benzyl-beta-L- and beta-D-rhamnopyranoses.
abstract: 1,2-Anhydro-3,4-di-O-benzyl-alpha-L-rhamnopyranose was synthesized from L-rhamnose, while the D-enantiomer was synthesized from methyl 6-deoxy-2,3-O-isopropylidene-alpha-D-mannopyranoside. For both of the syntheses, the key intermediates were 2-O-acetyl-3,4-di-O-benzyl-alpha-D- and -alpha-L-rhamnopyranosyl chlorides that were quantitatively prepared from the corresponding diacetates by chlorination. Ring closure of the chlorides was carried out readily with potassium tert-butoxide in oxolane, and crystalline 1,2-anhydro-3,4-di-O-benzyl-beta-D- and beta-L-rhamnopyranose were obtained in high yields. Conformational calculations, which were carried out using vicinal proton-proton coupling constants by the modified Karplus equation, suggested that the conformations of the pyranose rings of the title compounds were basically a half chair (4H5) with some flattening at C-4. Force-field calculations (MMP2) confirmed the experimental conformation with good agreement. The coupling reaction of the 1,2-anhydro-L-rhamnose ether with 1,2;3,4-di-O-isopropylidene-alpha-D-galactopyranose was effected in oxolane by catalysis by a Lewis acid, and only the alpha-linked disaccharide was obtained.
authors.affiliation: Research Center for Eco-Environmental Sciences, Academia Sinica, Beijing, People's Republic of China.
authors.forename: Q
authors.initials: Q
authors.lastname: Chen
authors.forename: F
authors.initials: F
authors.lastname: Kong
authors.forename: L
authors.initials: L
authors.lastname: Cao
mesh_terms: D000942:Antigens, Bacterial / Q000737:chemistry; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D004852:Epoxy Compounds / Q000737:chemistry*; D006027:Glycosides / Q000138:chemical synthesis*; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D011135:Polysaccharides, Bacterial / Q000737:chemistry; D012210:Rhamnose / Q000031:analogs & derivatives* / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D000942:Antigens, Bacterial; D004852:Epoxy Compounds; D006027:Glycosides; D011135:Polysaccharides, Bacterial; C080447:1,2-anhydro-3,4-di-O-benzylrhamnopyranose; D012210:Rhamnose