Oligosaccharide analogues of polysaccharides - Part 26 - Mimics of cellulose I and cellulose II: Di- and monoalkynyl C-cellosides of 1,8-disubstituted anthraquinones

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 1. 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 1, 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 (D-6)DMSO were assigned on the basis of DQFCOSY, HSQC, and TOCSY (only of T-4. T-44, 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 H-1-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 C-13-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 C-13-NMR spectrum of the C-1-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 I-beta. 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 1). 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 C-13-NMR spectra of A-4 and A-4-4 to the one of CTA 1, and of A-8 and A-8-8 to the one of CTA If 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 deacelylation. 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 beta-cellobiosylethyne and beta-cellobiosyl-buta-1,3-diyne 13 and 17, respectively, with the cellobiosyl donor 2 and the cellohexaosyl donor 47.