Toward an understanding of the mechanisms of the intramolecular [5+2] cycloaddition reaction of gamma-pyrones bearing tethered alkenes. A theoreticalstudy

The molecular mechanism for the intramolecular [5 + 2] cycloaddition reacti on of beta-silyloxy-gamma-pyrones bearing tethered alkenes has been charact erized using ab initio methods. A comparative study for this sort of cyclad dition carried out at different computational levels points out that the B3 LYP/6-31G* calculations give similar barriers to those obtained with the MP 3/6-31G* level. Analysis of the energetic results shows that the reaction t akes place along a stepwise process: first, the migration of the neighborin g silyl group to the carbonyl group of the gamma-pyrone takes place to give a weak oxidopyrylium ylide intermediate, which by a subsequent concerted i ntramolecular [5 + 2] cycloaddition affords the final cycloadduct. The cycl oaddition process is very stereoselective due to the constraints imposed by the tether. The [5 + 2] cycloaddition reaction has a large barrier, and th e presence of the silyloxy group and the intramolecular character of the pr ocess are necessary to ensure the thermodynamic and kinetic feasibility of these cycloadditions.