Chapter 10

Show, by construction of both a TS orbital array and an orbital symmetry correlation diagram, which of the following electrocyclizations are allowed. a. disrotatory cyclization of the pentadienyl cation to the cyclopent-2-enyl cation. b. disrotatory cyclization of the pentadienyl anion to the 3 -cyclopentenyl anion. c. disrotatory cyclization of the heptatrienyl anion to the cyclohepta-3,5-dienyl anion.

It has been found that 3-substituted methyl 3-hydroxy-2-methylene alkanoates give rise to a preference for the \(Z\)-isomer if \(\mathrm{R}\) is alkyl, but for the \(E\)-isomer if \(R\) is aryl under the conditions of the thermal orthoester Claisen rearrangement. Analyze the transition structure for the reaction in terms of steric interactions and suggest a reason for the difference in stereoselectivity.

On treatment with potassium metal, cis-bicyclo[6.1.0]nona-2,4,6-triene gives a monocyclic aromatic dianion. The trans isomer under similar conditions give a bicyclic radical anion that does not undergo further reduction. Explain how the stereochemistry of the ring junction can control the course of these reductions.