Several transition states from (4)C(1) to skew conformations of beta-D-glucopyranose.

The interconversion pathways in the ring distortion of beta-D-glucopyranose were investigated using density functional calculations. We examined the energies of several conformers of beta-D-glucopyranose and tried to obtain the transition-state conformation and determine the pathway between a (4)C(1) chair and some distorted ring conformers. The results showed that two E(3)/(2)H(3) conformations and one E(3)/(4)H(3) conformation were transition states in such ring puckering. The transition state with the lowest energy conformation is the E(3)/(2)H(3) ring conformation with the side-chain conformation of r-ggG(+). Intrinsic reaction coordinate calculations indicated that the E(3)/(2)H(3) conformation with the lowest conformational energy is a transition state of the ring interconversion path between the conformations of (4)C(1) and (2)S(O)/B(3,O). The energy barrier of this interconversion was 6.13 kcal/mol. As far as we know, this is the first example of finding pathways for an interconversion of glucopyranose ring puckering at the level of a quantum chemical calculation.