Abstract
The effect of substitution on intermolecular interactions was investigated in a series of 1,6-anhydro-2,3-epimino-hexopyranoses. The study focused on the qualitative evaluation of intermolecular interactions using DFT calculations and the comparison of molecular arrangements in the crystal lattice. Altogether, ten crystal structures were compared, including two structures of C4-deoxygenated, four C4-deoxyfluorinated and four parent epimino pyranoses. It was found that the substitution of the original hydroxy group by hydrogen or fluorine leads to a weakening of the intermolecular interaction by approximately 4 kcal/mol. The strength of the intermolecular interactions was found to be in the following descending order: hydrogen bonding of hydroxy groups, hydrogen bonding of the amino group, interactions with fluorine and weak electrostatic interactions. The intermolecular interactions that involved fluorine atom were rather weak; however, they were often supported by other weak interactions. The fluorine atom was not able to substitute the role of the hydroxy group in molecular packing and the fluorine atoms interacted only weakly with the hydrogen atoms located at electropositive regions of the carbohydrate molecules. However, the fluorine interaction was not restricted to a single molecule but was spread over at least three other molecules. This feature is a base for similar molecule arrangements in the structures of related compounds, as we found for the C4-Fax and C4-Feq epimines presented here.
Highlights
Fluorinated carbohydrates are members of a large family of glycomimetics—chemically modified carbohydrates that structurally and functionally mimic natural carbohydrate ligands [1,2,3,4]
Epitopes based on fluorinated carbohydrate are structurally similar to natural epitopes
Deoxyfluorinated carbohydrates can serve as probes in studies focused on protein–carbohydrate interactions in which 19F NMR spectroscopy plays a prominent role [9,10,11]
Summary
Fluorinated carbohydrates are members of a large family of glycomimetics—chemically modified carbohydrates that structurally and functionally mimic natural carbohydrate ligands [1,2,3,4]. We focus on intermolecular interactions in the crystal structures of C4deoxyfluorinated 1,6-anhydro-2,3-epimino-hexopyranoses and their comparison to the parent and deoxygenated molecules These compounds contain an annulated aziridine (or epimine) ring, a three-membered saturated nitrogen heterocycle. In the field of carbohydrate chemistry, pyranosides and furanosides with annulated aziridine rings have served mainly as intermediates in the synthesis of amino sugar analogues, the typical transformation being the nucleophilic opening of the three-membered ring [28,29]. In this respect, they function as nitrogen analogues of sugar epoxides. The effect of fluorine substitution on intermolecular interactions in crystal packing is evaluated through a comparison with the structures of the corresponding hydroxyl [31] and dihydrogen [32] analogues at the same C4 position, 10 crystal structures in total
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