AN NMR, X-RAY CRYSTAL-STRUCTURE, AND MOLECULAR MECHANICS STUDY OF DI-(3-DEOXY-D-GLYCERO-PENTULOSE) 1,2'/2,1'-DIANHYDRIDE

The structure of di-(3-deoxy-D-glycero-pentulose), 1,2':2,1' dianhydride, C10H16O6, has been studied by NMR spectroscopy in D2O and Me(2)SO-d(6) solutions, by X-ray crystal-structure analysis at 123 K, and by molecular mechanics calculations. The NMR data show that the molecule is asymmetric with distinct resonances that can be assigned to all hydrogens and protonated carbon atoms. In the crystal, the central dioxane ring has a chair conformation. Both furanoid rings have envelope conformations, one is (3)E, the other E(O). The molecular mechanics calculations indicate a marked difference in the pseudorotational flexibility in the two furanoid rings. The (3)E (alpha-anomeric) ring is relatively restricted, with T-3(2)-(3)E-T-3(4) conformations within 1 kcal/mol from the energy minimum. The E(O) (beta-anomeric) ring is much more flexible, with conformations from T-5(4) through E(O) to T-4(3), within the 1 kcal/mol energy contour. This result is consistent with the NMR data. For the 3E ring, the NMR dihedral angles, calculated from coupling constants, are consistent with conformations from (3)E to T-5(4), while for the E(O) ring they range from T-3(4)-T-2(O) BY decomposing the calculated energy components, the difference in the pseudorotational flexibility of the two furanoid rings can be related to the difference in the torsion angle energies about the C-5-O-5 bonds in the two rings. The molecules in the crystal are linked by infinite chains of hydrogen bonds.