Reaction of esters with Grignard reagents

The reaction of esters with Grignard reagents and with lithium aluminum hydride, both useful in the synthesis of alcohols, were described earlier. They are reviewed in Table... 

Conversion of esters to secondary alcohols. The reaction of esters with Grignard reagents results mainly in tertiary alcohols, which arc formed by way of an intermediate ketone. Direct conversion of an ester to a secondary alcohol is possible by reaction with a Grignard reagent (2 equiv.) and lithium boroh dridc (0.5 equiv.). which reduces the intermediate ketone much more rapidly than it does the ester. 

Esters, under appropriate conditions, undergo hydrolysis. This reaction is the reverse of esterification as shown in the equation on page 207. Esters react with aqueous or alcoholic ammonia to form amides. A number of reactions of esters are due to activity of the carbonyl group and the adjacent a-hydrogen atoms. The reaction of esters with Grignard reagents to form ketones or tertiary alcohols, and the various condensations, are examples of this type of reaction. 

Reaction of Esters with Grignard Reagents Esters and lactone react with 2 equivalents of a Grignard reagent to yield a tertiary alcohol in which two of the substituents are identical (Section 17.6). The reactioo occurs by the usual nucleophilic substitution mechanism to give an intermediate ketone, which reacts further with the Grignard reagent to yield a tertiary alcohol. ... 

Some transformations constitute examples of more than one of these classes. For example, the reaction of esters with Grignard reagents gives alcohols in what is a substitution and an addition reaction. Remember that classification schemes are created by people. A compound does not stop and worry about whether a particular reaction fits within the boundaries of human classification schemes before it undergoes the reaction. 

Reaction of Esters with Grignard Reagents Synthesis of Tertiary Alcohols 836... 

TABLE 19.5 Reactions of Esters with Grignard Reagents and with Lithium Aluminum Hydride... 

As we have seen, most reactions of carboxylic acids, esters, and related compounds involve, as the first step, nucleophilic attack on the carbonyl carbon atom. Examples are Fischer esterification, saponification and ammonolysis of esters, and the first stage of the reaction of esters with Grignard reagents or lithium aluminum hydride. All of these reactions can be summarized by a single mechanistic equation ... 

Provide a mechanism for the following reaction, based on your knowledge of the reaction of esters with Grignard reagents. 

We begin by noticing that a lactone is merely a cyclic ester, and therefore we may refer to the reaction of esters with Grignard reagents described in Section 20-4 as a gnide for approaching this question. 

---