Stereoselectivity electrocyclic rearrangements

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

An electrocyclic reaction is an intramolecular reaction in which the rearrangement of tt electrons leads to a cyclic product that has one fewer tt bond than the reactant. An electrocyclic reaction is completely stereoselective—it preferentially forms one stereoisomer. For example, when (2 , 4Z,6 )-octatriene undergoes an electrocyclic reaction under thermal conditions, only the cis product is formed when (2 , 4 6Z)-octatriene undergoes an electrocyclic reaction under thermal conditions, only the trans product is formed. Recall that E means the high-priority groups are on opposite sides of the double bond, and Z means the high-priority groups are on the same side of the double bond (Section 3.5). 

A review detailing computational methods for determining transition-state geometry in stereoselective cycloaddition reactions has been presented. Diels-Alder reactions, 1,3-dipolar cycloadditions, sigmatropic rearrangements, electrocyclic reactions, and ene reactions have been extensively reviewed. Pressure-induced cycloadditions to strained arenes have been reviewed. ... 

An electrocycUc reaction is an intramolecular reaction in which the formation of a a bond between the ends of the tt system and the rearrangement of the ir electrons leads to a cyclic product that has one less tt bond than the reactant. An electrocyclic reaction is completely stereoselective it is also stereospecific. 

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