Summary of Diels-Alder thus far:

For dienophiles having substituents attached:

-       If the substituent has no pi-bonds, sterics will govern the stereochemistry and the exo product will be favored.

-       If the substituent has pi-bonds, then electronic properties overcome sterics, and the endo product will be favored. (2ndary orbital effects)

What do we do if we want an exo-carbonyl compound:

There are times when the exo product of a Diels Alder reaction is required in a system when 2ndary orbital overlap would seem to dictate that the endo product would be favored.  How can the exo product be formed when the endo product is generally thought to be favored.  In other words, how can electronic factors be overcome, or evaded, in the Diels-Alder reaction?

Remember that carbonyl compounds can be protected if we do not want them to interact during the course of a reaction.  We have protected carbonyl groups when working under anionic conditions so they would not act as electrophiles under the conditions.  In this case, the carbonyl will interact via 2ndary orbital overlap, and we wish for that overlap not to occur.  Protecting the carbonyl as an acetal or ketal will eliminate the possibility for 2ndary orbital overlap. 

With the carbonyl protected, there are no p-orbitals present to participate in 2ndary orbital interactions.  As a result, sterics will dictate the energy pathway that is favored in this reaction and the exo product will be formed.  The orbital picture for exo formation of the protected ketone is shown below:

Upon completion of the Diels-Alder reaction, the carbonyl compound may be deprotected.  The overall result of this reaction is that an exo product has been formed when an endo product would be predicted.