Thermodynamic control of the electrocyclic ring opening of cyclobutenes: C=X substituents at C-3 mask the kinetic torquoselectivity.

The thermal ring-opening reactions of 4-phenyl-1,3,3-triethoxycarbonylcyclobutene and 4-methyl-1,3,3-triethoxycarbonylcyclobutene yield dienes that result from an unexpected selectivity for "inward" rotation of the phenyl and methyl groups. With 1-ethoxycarbonyl-4-phenylcyclobutene, "outward" rotation of the phenyl group occurs exclusively. Density functional theory was used to investigate the role of the 3,3-geminal diester groups and the origin of torquoselectivity in these electrocyclic reactions. The rules of torquoselectivity still hold, with a calculated 6-8 kcal/mol preference for outward rotation of the methyl and phenyl substituents. However, cyclization of the "out" dienes to pyran intermediates allows for isomerization and thermodynamic control of stereoselectivity.