Fully acetylated 1,5-anhydro-2-deoxypent-1-enitols and 1,5-anhydro-2,6-dideoxyhex-1-enitols in DFT level theory conformational studies

Abstract

Geometry optimizations at the B3LYP level of density functional theory (DFT) are reported for the 4H5 and 5H4 conformations of four glycals: 3,4-di-O-acetyl-1,5-anhydro-2-deoxy-d-erythro-pent-1-enitol (3,4-di-O-acetyl-d-arabinal), its d-threo isomer (3,4-di-O-acetyl-d-xylal), 3,4-di-O-acetyl-1,5-anhydro-2,6-dideoxy-d-arabino-hex-1-enitol (3,4-di-O-acetyl-d-rhamnal), and its d-lyxo isomer (3,4-di-O-acetyl-d-fucal). The Gibbs free energies, relative Gibbs free energies and geometry parameters are presented for all the optimized structures. Conformational analysis of both the acetoxy groups and 1,2-unsaturated pyranoid ring is performed. It is demonstrated that the acetoxy group is planar and adopts two conformations with regard to the Ac-O bond rotations, one of which is strongly preferred. One of the hydrogen atoms of the methyl group is always oriented synperiplanarly whereas the remaining two anticlinally with respect to the carbonyl oxygen. With regard to the AcO–R bond rotations some of the orientations are forbidden. The calculations indicate that 3,4-di-O-acetyl-d-arabinal adopts mainly the 4H5 conformation, whereas 3,4-di-O-acetyl-d-xylal prefers the 5H4 form, owing to the vinylogous anomeric effect (VAE). Competition between the VAE and quasi 1,3-diaxial interactions means that both 3,4-di-O-acetyl-d-rhamnal and 3,4-di-O-acetyl-d-fucal remain in the 4H5⇄5H4 conformational equilibrium, shifted in the 4H5 direction. It seems that the orientation of the 4-OAc group influences the quasi 1,3-diaxial interactions between 3-OAc and 5-CH3 groups. Theoretical results are compared with assignments based on 1H NMR studies.