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Hydroxyketones The Aldol Condensation

The main problem that we face in aldol condensations is control—obviously we do not want to be limited to self-condensations in synthesis. We will discuss three main strategies for control in aldol condensations, which will also be applicable in other types of condensation reactions. These are the use of intramolecular processes forming five- or six-membered rings the use of nonenolizable, but highly electrophilic species and activation of the nucleophile by addition of another anion-stabilizing group. [Pg.950]

FIGURE 20.14 Intramolecular aldol reactions are usually selective. [Pg.951]

FIGURE 20.15 Intramolecular aldol reactions In synthesis. PROBLEM 20.4 [Pg.951]

4(a) The first step is ozonolysis of the double bond to give CH3C(=0)(CH2)4C(=0)CH3. This then undergoes an intramolecular aldol to yield a p-hydroxyketone, which may or may not dehydrate under the reaction conditions. The formation of the five-membered ring is more favorable than the formation of a seven-membered ring. [Pg.951]

FIGURE 20.17 Aldol reactions with nonenolizable components. [Pg.952]


We met in Chapter 17 the archetypal process for the preparation of p-hydroxyketones, the aldol condensation. This is one of the most important of all construction reactions in organic synthesis. Self-condensation reactions (Figure 20.13) are generally straightforward and work well. The first example is the self-condensation of acetophenone the product is readily dehydrated. The second example is shown as a disconnection—perhaps not completely obvious that this is a selfcondensation until it is disconnected. [Pg.950]


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