Isolation, modification, and NMR assignments of a series of cellulose oligomers

A homologous series of cellulose oligomers from two to eight repeating subu nits have been isolated and size-fractionated from the hydrolysis products of microcrystalline cellulose. Chemical modification of cellotriose (1), ce llotetraose (2), cellopentaose (3), and cellohexaose (4) to the correspondi ng beta-methyl glycosides 13-16 proceeded in three steps in overall yields of 16-46%. Peracetylation produced oligomers 5-8 in 70-75% yield, and subse quent formation of the P-methyl glycosides gave 9-12 in 42-89% yield. Remov al of the acetate-protecting groups employing guanidine provided 13-16 in 7 3-79% yield. This modification eliminated anomeric equilibration and permit ted a detailed NMR solution study of these oligomers. The complete H-1 and C-13 chemical shift assignments of each peracetylated and deprotected oligo mer were obtained through a combination of DQF-COSY, HMQC, HMBC, and HMQC-T OCSY experiments. All the resonances in methyl cellotriose (13) and methyl cellotetraose (14) were readily distinguishable from one another and direct ly assignable. Severe overlap of the resonances for the inner pyranose ring s of methyl cellopentaose (15) and methyl cellohexaose (16) was observed an d could only be resolved and assigned using a comprehensive battery of 3D p ulse sequences. These results demonstrate the utility of multidimensional N MR experiments in assigning the signals from a repeating polysaccharide and represent the first necessary step in a comprehensive, systematic study of cellulose oligomers in solution.