Studies on oligosaccharides. XI. 6- and 6′- O-tritylmaltose peracetates

Abstract

6′- O-Trityl-α-maltose heptaacetate ( 1) was prepared by detritylation of 6,6′di- O-trityl-α-maltose hexaacetate ( 5) with 80% acetic acid at 100°, followed by retritylation with an equimolar ratio of reagent, and subsequent acetylation. When 5 was prepared by tritylation of maltose with a 3-molar equivalent of reagent followed by acetylation, the β- D anomer ( 6), of 5 was also isolated. A mixture of the six possible 6- and 6′-mono-, and 6,6′-di- O-tritylmaltose peracetates was obtained by direct tritylation of maltose with a 1:1 molar ratio of reagent and subsequent acetylation. All six products were isolated by chromatography on silica gel and by precise fractional crystallization; the yields of each were very different. The main product was 6′- O-trityl-β-maltose heptaacetate ( 2). The isomers having the trityl group on the nonreducing residue were much more dextrorotatory than those having the trityl group on the reducing residue. Studies on the molecular rotations suggested that changes in conformation at the glycosidic linkage, as expressed in torsional angles about the C—O and O—C bonds, may influence the magnitude of the optical rotations of these compounds. All of the six trityl derivatives gave positive, plain o.r.d. curves. An analog of 6′- O-trityl-α-maltose heptaacetate ( 1) having a trideuterioacetyl group at C-6, and an analog of 6- O-trityl-α-maltose heptaacetate ( 3) having a trideuterioacetyl group at C-6′, were synthesized to permit assignments of the 6-acetoxyl group signal in the n.m.r. spectrum of 1 and the 6′-acetoxyl group signal in the n.m.r. spectrum of 3. The discrepancy in the chemical shifts of these acetyl methyl protons substantiated the foregoing postulates based on optical rotation.