Electrocyclic reactions Ring opening, Rearrangements, sigmatropic

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

This ring opening is clearly pericyclic—the electrons go round in a ring, and the curly arrows could t>e drawn either way—but it is neither a cycloaddition (only one 7t system is involved) nor a sigmatropic rearrangement (a a bond is broken rather than moved). It is, in fact, a member of the third and last kind of pericyclic reaction, an electrocyclic reaction. 

The first step is an electrocyclic ring-opening reaction, and the second step is a [1,7] sigmatropic rearrangement. This sequence of steps not only occurs in our bodies but is also utilized commercially to enrich milk with vitamin D3. All milk sold in the United States is irradiated with UV light, which converts the 7-dehydrocholesterol present in the milk into vitamin D3. 

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