Ab initio calculations on the barrier to pseudorotation of model 2′-deoxyfuranose and 2′-deoxy-2′-fluorofuranose rings

Model ring systems 2′-deoxy-2′-fluororibofuranose and deoxyribofuranose have been investigated using ab initio calculations with the 3–21G basis set. The energy barrier to pseudorotation between the N and S states has been evaluated for the three preferred orientations of the (3′)-OH group. Positions of the energy minima and the transition state have been optimized with respect to the (3′)-OH orientation. The barrier to pseudorotation of 2′-deoxy-2′-fluorofuranose is high and asymmetrical (ΔE_N→S ≈ 20, ΔE_N←S ≈ 8 kJ/mol), whereas the barrier of 2′-deoxyfuranose is lower and almost symmetrical (ΔE ≈ 11–12 kJ/mol). The results obtained show that the preferred configuration of the 2′-deoxy-2′-fluororibo-furanose (N state) is stabilized by an internal O(3′)-H…?F interaction in accord with the crystallo-graphic data.