Isolation, Modification, and NMR Assignments of a Series of Cellulose Oligomers

Abstract

A homologous series of cellulose oligomers from two to eight repeating subunits have been isolated and size-fractionated from the hydrolysis products of microcrystalline cellulose. Chemical modification of cellotriose (1), cellotetraose (2), cellopentaose (3), and cellohexaose (4) to the corresponding β-methyl glycosides 13−16 proceeded in three steps in overall yields of 16−46%. Peracetylation produced oligomers 5−8 in 70−75% yield, and subsequent formation of the β-methyl glycosides gave 9−12 in 42−89% yield. Removal of the acetate-protecting groups employing guanidine provided 13−16 in 73−79% yield. This modification eliminated anomeric equilibration and permitted a detailed NMR solution study of these oligomers. The complete 1H and 13C chemical shift assignments of each peracetylated and deprotected oligomer were obtained through a combination of DQF−COSY, HMQC, HMBC, and HMQC-TOCSY experiments. All the resonances in methyl cellotriose (13) and methyl cellotetraose (14) were readily distinguishable fr...