Computational Studies of the Arabinofuranose Ring: Conformational Preferences of Fully Relaxed Methyl α-d-arabinofuranoside

Abstract

Two approaches for identifying the minimum energy conformers of methyl α-d-arabinofuranoside 1, in the gas phase have been explored and compared. In the first approach (the constrained envelope method), 30 previously reported envelope geometries of 1 were allowed to fully optimize at the B3LYP/6-31G* level. B3LYP/6-31+G** single-point energies of these optimized structures were also determined, which led to the identification of the 3T4 and 2T1 ring conformers as the Northern (N) and Southern (S) minima, respectively, with the latter being the global minimum. The importance of intramolecular hydrogen bonding was probed by optimizing another set of 30 envelope geometries with initial geometries biased against the formation of these stabilizing interactions. These calculations led to the same two families of low-energy ring conformers (3T4 and 2T1); however, the N, and not the S, conformer was the global minimum without hydrogen bonding. The second approach involved the identification of conformers for 1 through the use of a Monte Carlo search coupled with molecular mechanics and then further optimization of these structures at higher levels of theory (HF/6-31G* and B3LYP/6-31G*). Subsequent B3LYP/6-31+G** single-point energy calculations afforded results that are similar to the constrained envelope method, but the stochastic approach led to more low-energy conformers, and to a new global minimum. A comparison of these computational results with the experimentally determined solution conformation of 1 is also presented.