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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.