Oligosaccharide Analogues of Polysaccharides, Part 17, Synthesis and Characterization of a Hexadecamer: Evidence for the Essential Structural Role of the Intramolecular, Interresidue C(3)−OH⋅⋅⋅O−C(5) Hydrogen Bond in Celluloses

Abstract

The hexadecamer 63 dissolves readily in DMSO and shows no sign of association, in agreement with the essential structural role of the intramolecular, interresidue C(3)−OH⋅⋅⋅O−C(5) H-bond of celluloses. MM3 Calculations predict a rod-like shape for the cello-oligosaccharides. In agreement with this, nanocrystalline domains of parallel chains are observed by high-resolution electron microscopy (HREM) for 54 and 63 on carbon-coated copper grids. For the synthesis of the hexadecamer 54, we studied the influence of protecting groups on the cross-coupling of acetyleno-oligosaccharides and on their solubility (Scheme 2). The octamer 12 derived from the minimally O-protected monomer 2 is insoluble, while the methoxymethyl- and triisopropylsilyl-protected hexadecamer 54, prepared in twelve steps from the dimer 41 (Scheme 4), is readily soluble in a variety of solvents. O- and C-deprotection of 54 led in four steps and in 76% yield to the hexadecamer 63 (Schemes 5 and 6). There is only a small difference of spectroscopic properties between the protected octamer 50 and hexadecamer 54, and, similarly, between the unprotected octamer 64 and hexadecamer 63 (Fig. 3).