Toward an understanding of the mechanisms of the intramolecular
The molecular mechanism for the intramolecular [5 + 2] cycloaddition reaction of beta-silyloxy-gamma-pyrones bearing tethered alkenes has been characterized using ab initio methods. A comparative study for this sort of cycloaddition carried out at different computational levels points out that the B3LYP/6-31G calculations give similar barriers to those obtained with the MP3/6-31G level. Analysis of the energetic results shows that the reaction takes place along a stepwise process: first, the migration of the neighboring silyl group to the carbonyl group of the gamma-pyrone takes place to give a weak oxidopyrylium ylide intermediate, which by a subsequent concerted intramolecular [5 + 2] cycloaddition affords the final cycloadduct. The cycloaddition process is very stereoselective due to the constraints imposed by the tether. The [5 + 2] cycloaddition reaction has a large barrier, and the presence of the silyloxy group and the intramolecular character of the process are necessary to ensure the thermodynamic and kinetic feasibility of these cycloadditions.
Domingo
LR
Departamento de Quimica Organica, Universidad de Valencia, Dr Moliner 50, 46100-Burjassot, Valencia, Spain.
Zaragoza
RJ