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 Chapter 18: Enols and Enolates 
Ch 18 contents
Advanced Aldol Condensations

The more advanced topics are : Dehydration, Mixed and Intramolecular Aldol reactions.

Dehydration of Aldol products

The dehydration of the initial Aldol products can occur, it is often favoured by the following factors:

note the conjugation in the product

Mixed Aldols

A mixed Aldol reaction means that the enolate and the carbonyl are not from the same aldehyde or ketone.

The most useful mixed Aldols are those where:

Otherwise there are selectivity problems that give rise to complex mixtures of products.
 
examples of aldehydes that have no alpha hydrogens   Neither of these aldehydes has α-hydrogens and therefore they cannot form an enolate.
a mixed aldol reaction   Only one of this pair of aldehydes has an α-hydrogen

QUESTIONS

 
  • In the above example, which aldehyde forms the enolate ?
  • ANSWER
     
  • Why can't the aldol product dehydrate ?
  • ANSWER

    Intramolecular Aldols

    an intramolecular Aldol
    How can you deduce this product ?
    1. The aldehyde will be the electrophile and the ketone the enolate (see above)
    2. There two possible enolates (at C5 or C7)
    3. The more favourable ring size will the 5-membered ring.
    Study Tip:
    When doing intramolecular reactions, it is a good idea to count carbons to avoid silly errors.

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    © Dr. Ian Hunt, Department of Chemistry, University of Calgary