Disrotatory photochemical ring

Figure 14-11 Disrotatory photochemical ring closure of cis,trans-and frans,frans-2,4-hexadiene.

Cyclopropyllithiums (49) undergo photochemical ring-opening at low temperatures by a net disrotatory pathway (equation 33). ... 

Figure 7.20. Comparison of the AO interactions in the photochemical chain abridgement in a polysilane (top) and in the disrotatory eiectrocyciic ring closure of butadiene (bottom) (by permission from Michl and Balaji, 1991).

Fig. 4.17 Orbital correlation diagrams for the photochemical ring-closure reaction conrotatory (forbidden, left) and disrotatory (allowed, right)...

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

An example of introducing strain using photochemistry is the synthesis of Dewar benzene. Dewar benzene represents a classic strained ring system that many chemists have studied. One convenient synthesis is shown in Eq. 16.26, the key step being a photochemical disrotatory electrocyclic ring closure. 

The primary product of a cycloaddition is often unstable in employed conditions. For example, the photochemical addition of ethyne to benzene gives an ortho-adduct, which undergoes thermal disrotatory electrocyclic ring opening of cyclohexadiene to give a cyclooctateraene. 

In photochemical reactions, we consider the LUMO (the light photon promotes electrons into the LUMO) the symmetry of this is invariably opposite to that of the HOMO, so 4e processes are disrotatory, and 6e processes are conrotatory, exactly the opposite of the thermal reaction. Figure 18.32 shows the LUMO of butadiene, and Figure 18.33 shows the photochemical ring closures of substituted butadienes. 

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