Analysis of anomeric effects in some oxa diaza spiro decan derivatives by DFT molecular orbital calculations

The conformational behavior of oxa diaza spiro decan derivative compounds has been investigated using DFT calculations at the B3LYP/cc-pVDZ level of theory. Natural bond orbital (NBO) analysis of the total energy behavior yielded the orbital-interaction factors contributing to the conformational equilibria. The relative energies, NBO analysis and structural parameters predicted that steric and anomeric effects determine the stabilization of the axial and/or equatorial conformers. The dipole moments of the optimized systems were used to estimate the electrostatic contributions to the anomeric effect. The results of calculations indicated that the axial conformers are the most stable in all of the studied compounds. Finally, the strain energy barriers for the most stable conformers were calculated at HF/6-311G*//HF/6-311G* level of theory. Interconversion between chair and twist conformations takes place via the half-chair as transition state. The lowest calculated strain energy for this process is 42.41 kJ mol−1.