Revisiting the Case of an Intramolecular Hydrogen Bond Network Forming Four- and Five-Membered Rings in d-Glucose.

Abstract

The conformational behavior of cyclic monosaccharides has been widely studied over the past years, but there is no general agreement about which effects are in fact responsible for the observed conformational preferences. A recent microwave spectroscopy study determined the conformational equilibrium of d-glucose in the gas phase with a preference for a counterclockwise arrangement of the hydroxyl groups. Nevertheless, the effects that control this orientation are still uncertain because the role of intramolecular hydrogen bonds (IHBs), electrostatic and steric repulsions is not clear. This work reports a density functional theory approach based on the conformational energies of d-glucose and of some derivatives in which the anomeric hydroxyl is replaced with hydrogen (H, small and not prone to participate in proton transfer), fluorine (F, small, electronegative, and as capable as OH of forming hydrogen bonds as a proton acceptor), and chlorine (Cl, big and not anticipated to be involved in effective hydrogen bond formation) to obtain insights into the effects of the substituent at the anomeric carbon on the arrangement of the hydroxyl groups in d-glucose. The nature of the substituents at this position is crucial to determine the orientation of the remaining hydroxyl groups. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses, in addition to NMR chemical shift calculations, have been provided to support the conformational energy outcomes. Overall, the results agree with the lack of IHBs forming four- and five-membered rings in d-glucose and emphasize that steric and electrostatic repulsions involving the hydroxyl groups in the clockwise orientation are driving forces of the conformational behavior.