Oligosaccharide Analogues of Polysaccharides. Part 26
Authors: Murty Kadiyala V. S. N. , Xie Tong , Bernet Bruno , Vasella Andrea 
Published: April 2006

The anthraquinone derivatives T‐x‐x (x = 2, 4, and 8), possessing two cellobiosyl, cellotetraosyl, and cellooctaosyl chains, respectively, C‐glycosidically bonded at C(1) and C(8) were synthesised as potential mimics of cellulose I. The anthraquinone template enforces a parallel orientation of the cellodextrin chains at a distance corresponding to the one between the crystallographically independent chains of cellulose I, and the ethynyl and buta‐1,3‐diynyl linker units ensure an appropriate phase shift between them. The H‐bonding of the T‐x‐x mimics was analysed and compared to the one of the mono‐chained analogues T‐x and of the known cellulose II mimics N‐x‐x and N‐x where one or two cellodextrin chains are O‐glycosidically bonded to naphthalene‐1,8‐diethanol, or to naphthalene‐1‐ethanol. The OH signals of T‐x and T‐x‐x in solution in (D6)DMSO were assigned on the basis of DQFCOSY, HSQC, and TOCSY (only of T‐4, T‐4‐4, and T‐8‐8) spectra and on a comparison with the spectra of N‐x and N‐x‐x. Hydrogen bonding was analysed on the basis of the chemical shift of OH groups and its temperature dependence, coupling constants, SIMPLE 1H‐NMR experiments, and ROESY spectra. T‐4‐4 and T‐8‐8 in (D6)DMSO appear to adopt a V‐shape arrangement of the cellosyl chains, avoiding inter‐chain H‐bond interactions. The well‐resolved solid‐state CP/MAS 13C‐NMR spectra of the mono‐chained T‐x (x = 1, 2, 4, and 8) show that only T‐8 is a close mimic of cellulose II. While the solid‐state CP/MAS 13C‐NMR spectrum of the C1‐symmetric diglucoside T‐1‐1 is well‐resolved, the spectra of T‐2‐2 and T‐4‐4 show broad signals, and that of T‐8‐8 is rather well resolved. The spectrum of T‐8‐8 resembles that of cellulose Iβ. A comparison of the X‐ray powder‐diffraction spectra of T‐8‐8 and T‐8 with those of celluloses confirms that T‐8‐8 is a H‐bond mimic of cellulose I and T‐8 one of cellulose II.

Surprisingly, there is little difference between the CP/MAS 13C‐NMR spectra of the acetyl protected mono‐chained C‐glycosylated anthraquinone derivatives A‐x and the double‐chained A‐x‐x (x = 2, 4, and 8). The spectra of A‐4 and A‐4‐4 resemble strongly the one of cellulose triacetate I (CTA I). The (less well‐resolved) spectra of the cellooctaosides A‐8 and A‐8‐8, however, resemble the one of CTA II. The similarity between the solid‐state CP/MAS 13C‐NMR spectra of A‐4 and A‐4‐4 to the one of CTA I, and of A‐8 and A‐8‐8 to the one of CTA II is opposite to the observations in the acetylated cellodextrin series.

The mono‐chained A‐x cellulose triacetate mimics 21 (A‐2), 32 (A‐4), and 55 (A‐8) were synthesised by Sonogashira coupling of the cellooligosyl‐ethynes 15, 28, and 50, followed by selective deacetylation. Complete deacetylation provided the corresponding T‐x mimics. The double‐chained A‐x‐x mimics 24 (A‐2‐2), 35 (A‐4‐4), and 58 (A‐8‐8) were prepared from A‐x by triflation and Sonogashira coupling with the cellosyl‐buta‐1,3‐diynes 19, 31, and 53. Their deacetylation provided the corresponding T‐x‐x mimics 25, 36, and 59. The cellooligosyl‐ethynes and cellooligosyl‐buta‐1,3‐diynes required for the Sonogashira coupling were prepared by stepwise glycosylation of the partially O‐benzylated β‐cellobiosyl‐ethyne and β‐cellobiosyl‐buta‐1,3‐diyne 13 and 17, respectively, with the cellobiosyl donor 2 and the cellohexaosyl donor 47.