Total Facial Discrimination of 1,3-Dipolar Cycloadditions in a d‑Erythrose 1,3-Dioxane Template: Computational Studies of a Concerted Mechanism

A new d-erythrose 1,3-dioxane
derivative was synthesized
from d-glucose and found to be a highly stereoselective template
as a dipolarophile. Different 1,3-dipoles of allenyl-type were employed,
giving different regioselectivities, depending on its nature; the
regioselectivity is complete with alkyl azides and phenyl­diazomethane,
but is inexistence with nitrile oxides. Computational studies were
performed to understand the mechanisms of cycloadditions. All the
studied cycloadditions were found to be concerted involving small
free activation energies and are all exoenergonic. The stereoselectivity
is due to a combined result of the steric effect H-8a and the hyperconjugative
effect of the *C–O to the incoming 1,3-dipole. The regioselectivity
observed in alkyl azides and phenyldiazomethane is mostly dependent
on the distortion effect during the cycloaddition process. This distortion
effect is however higher in the alkyl azide compounds than in phenyldiazomethane.