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The Leading Question:

 

• What exactly is a Norrish Type II photochemical reaction?

 

-The Norrish Type II reaction is similair in concept to mass spectroscopy, but serves an entirely different purpose. In this experiment, this reaction results in the conversion of a carbonyl into a 4-carbon ring and an alchohol.When the oxygen of the carbonyl and the adjacent carbon are excited by the photochemical energy, both become radicals. The radical oxygen then becomes protanated by a nearby carbon atom (4 bonds away) in a McLafferty-like rearrangement, which leaves the deprotanated carbon as a radical. The radical deprotanated carbon then bonds with the radical carbonyl carbon and forms a stable 4-membered ring.

In this reaction, this is extremely important, as the carbonyl on carbon 18 is so sterically hindered that it will not react with any sizeable reagent. This reaction also drastically changes the shape of the molecule and adds a stereocenter,and overall results in a much smoother synthesis of the final product.

It is also worthy of note that the Norrish Type II was not originally planned for this reaction. The reaction was initially planned to use a Norrish Type I reaction, but the experiment was changed after the researchers found that the Norrish Type II led to better yields.