Conjugated Dienes and Ultraviolet Light

Diels-Alder reactions have been used widely in the laboratory syntheses of steroids. The key Diels-Alder reactions used to prepare the C ring of estrone and the B ring of cortisone are as follows  

Problem 16.25 Draw the product (A) of the following Diels-Alder reaction. A was a key intermediate in the synthesis of the addicting pain reliever morphine, isolated from the opium poppy. 

Recall from Chapter 13 that the absorption of infrared energy can promote a molecule from a lower vibrational state to a higher one. In a similar fashion, the absorption of ultraviolet (UV) light can promote an electron from a lower electronic state to a higher one. Ultraviolet light has 

When electrons in a lower energy state (the ground state) absorb hght having the appropriate energy, an electron is promoted to a higher electronic state (the excited state). 

This energy difference depends on the location of the electron. 

As the number of conjugated n bonds increases, the energy difference between the ground and excited abg i velengths. 

Problem 16.29 Which compound in each pair absorbs UV light at longer wavelength  

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This synthesis method can be utilised by any alkene or alkyne, but steric hindrance on internal double bonds can cause these reactions to be quite slow. Conjugated dienes and aromatic alkenes are not suited for the ultraviolet light-initiated process. The use of other free-radical initiators is required in free-radical-initiated reactions involving these species. 

Formation of cyclic peroxides by conjugated dienes is a general reaction and although ultraviolet light often initiates the reaction, better results are achieved by carrying out the... 

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