Experimental and theoretical evaluation of trans-3-halo-2-hydroxy-tetrahydropyran conformational preferences. Beyond anomeric interaction

Conformational isomerism in trans-3-X-2-hydroxy-tetrahydropyrans (X = F, Cl, Br, I) was investigated by NMR spectroscopy and electronic structure calculations. The compounds were synthesized, purified and identified by H-1, C-13 and selective TOCSY NMR spectra and by HSQC, COSY and NOESY contour maps. The geometries and conformer energies for the most stable conformers in the isolated molecules were calculated using M06-2X hybrid functional (DFT) and MP2 (ab initio) methods with the aug-cc-pVTZ basis set. Theoretical calculations taking into account the solvent effect (CHCl3 and DMSO) were performed using the IEFPCM solvent model, M06-2X/aug-cc-pVTZ level of theory for C, H and O atoms and M06-2X/aug-cc-pVDZ-PP with pseudopotential for the iodine atom. NBO, QTAIM and NCI analyses were applied to identify which stereoelectronic interactions are responsible for their conformational preferences. The conformer stability changes in the presence of solvent. The anomeric effect does not appear to have a significant influence on the molecular conformations in these molecules.