Conformational analyses of model structures for intramolecular glycoside bond formation
✍ Scribed by Armin Geyer; Ursula Huchel; Richard R. Schmidt
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 121 KB
- Volume
- 37
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Cyclic disaccharides with m-xylylene bridges were synthesized via an intramolecular glycosylation reaction. 3J(HH) coupling constants and average interproton distances from ROESY spectra describe the relative spatial orientation of a m-xylylene spacer and the D-Glcb(1 ] 4)D-Glc disaccharide unit within the ring structures. The m-xylylene moiety bridges a 14-membered ring including 3-O of pyranose a and 6-O of pyranose b in compound 1 and it spans a 15-membered ring including both 6-Os in 2. The glycosidic torsions are similar for both compounds ; the di †ering cyclization geometry is compensated for by a switch of the exocyclic torsion of ring b from the gg rotamer in 1 to the gt rotamer in 2. A single conformation is populated by 1, where NOEs and coupling constants are in accordance with one structure. The exocyclic torsions of the m-xylylene moiety are restricted to values of 0¡ and 60¡, respectively. The m-xylylene spacer exhibits mobility within the slightly larger 15-membered ring of 2. Fast conformational averaging leads to an averaging of NOEs within the m-xylylene moiety.