Butadienes electrocyclic ring closing

Under photochemical conditions, the electrocyclic ring closing of butadienes always proceeds by the disrotatory pathway, which is the opposite of the result under thermal conditions. The FMOs make the stereochemical result easy to understand. Under photochemical conditions, an electron is promoted from the HOMO i]ii to the LUMO 1//7, so i//i becomes the HOMO. Molecular orbital i//i has an antibonding interaction between the termini of the 77 system in the conrotatory TS but a bonding interaction between the termini of the 77 system in the disrotatory TS, so the reaction proceeds in a disrotatory fashion. 

The cyclobutene-butadiene interconversion involves four v electrons and is designated a process. Note that by the principle of microscopic reversibility, the number of tt electrons involved in the transformation is the same for ring opening as for ring closing. Once we know the number of tt electrons involved in an electrocyclic reaction and the method of activation, the stereochemistry of the process is fixed according to the rules outlined in Table 6.1. 

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