The Computational Study of the Mechanism of the Acid-Promoted Pyranoside-into-Furanoside Rearrangement.

The pyranoside-into-furanoside (PIF) rearrangement is an uncommon but important process in carbohydrate chemistry. The quantum chemical investigation of the driving force of the recently discovered TfOH-catalyzed ring contraction revealed that it stemmed from the π-π interactions of the phenyl rings in benzoyl-protecting groups. In this study, we focused on the kinetic aspects, which included preliminary 2-O-benzoyl group rotation followed by the protonation of the endo-cyclic O5 atom. Using a combination of DFT and DLPNO-CCSDT methods, we found that in some cases, DFT may not produce adequate energies at the rate-limiting stage of the pyranoside ring opening, presumably due to inadequate modeling of Van der Waals interactions. Predicted rate constants for the PIF rearrangement of the β-O-methyl and β-S-ethyl galactosides were in agreement with NMR kinetic experiments, as the latter reacts significantly slower. The estimated constant for the β-O-phenyl galactoside supports its inability to undergo ring contraction and suggests temperatures of over 400 K for such transformation. The proposed mechanism was additionally confirmed by substituting the triflic acid with the much weaker trifluoroacetic one, which led to a drastic decrease of the reaction rate both in computations and in the experiment.