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Conformational analyses of model structures for intramolecular glycoside bond formation

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‐Glcβ(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 differing 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. Copyright © 1999 John Wiley & Sons, Ltd.