Conjugated dienes sigmatropic shifts

The outcome of all this for photochemical sigmatropic shifts is that those most commonly observed are of order (1.3) or (1.7) these involve 4 or 8 electrons, respectively, and occur in a suprafacial manner. Examples of photochemical 1.3-shifts of hydrogen are found for monoalkenes (2.25) and for conjugated dienes 2.26). In the case of dienes a 1,3-shift is favoured over a 1,5-shift, because for the latter to occur photochemically it would have to take place in an antarafacial manner. Note that in both examples the direction of... 

There are five classes of pericyclic reactions, only two of which are covered in this book, cycloadditions and sigmatropic shifts. The most common, cycloaddition involves the reaction of a conjugated diene with an alkene, although we also examine the reaction of an alkene with an alkene. Hence, we are examining what are commonly called 4 + 2 and 2 + 2 cycloadditions, respectively, to keep track of the number of 77 electrons involved in the reaction. 

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