Reversal of Pericyclic Selection Rules

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically allowed suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

For an excellent discussion of photochromism, see the lUPAC Technical Report on Organic Photochromism prepared by H. Bouas-Laurent and H. Diirr, available at http / /www.iupac.org/publications/pac/2001/ pdf/7304x0639.pdf. 

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. 

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