Isomerizations Involving Enols and Enolates

FIGURE 17.8 Proton NMR spectroscopic data for dimedone in CDCIj. 

In Problem 17.1, we saw that 3-cyclohexenone was converted to 2-cyclohexenone in this section, we examine that type of reaction more closely. The formation of an enol, which we describe as tautomerization, is, strictly speaking, an isomerization. However, once we have 

If there is a chiral center bearing a hydrogen atom adjacent to a carbonyl group, racemization is often easy and rapid. Consider the enolization of compound 17.19. The enol is planar, and cannot remember which enantiomer it used to be, so it may be reprotonated on either face of the double bond to give either enantiomer. If this compound is subjected to acidic or basic conditions, it will be rapidly racemized. This type of racemization will be of concern to us again when we come to look at peptide synthesis. 

Explain why R-2-methylcyclohexanone is racemized in aqueous base, but l -3-methylcyclohexanone is not. 

We first need to draw both the molecules and consider how they might enolize. Enolization of 2-methylcyclohexanone toward the 6-position has no effect on stereochemistry, but enolization 

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