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Chapter 32: Problem 744

Write a mechanism for the reaction of glycine with N- carboethoxyphthalimide involving a sequence of carbony1 addition, ring opening, ring closure and elimination. Would you expect a similar reaction with L-phenylalanine to yield the \(L\), the \(D\), or the D,L-product?

Short Answer

Expert verified
The reaction of glycine with N-carboethoxyphthalimide involves the following steps: 1. Carbonyl addition: The nitrogen atom from glycine's amino group attacks the carbonyl carbon, forming an alkoxide ion intermediate. 2. Ring opening: Carboxylate group attacks the oxygen atom from the middle of the five-membered ring, forming a linear ester intermediate. 3. Ring closure: Negative charged oxygen atom attacks the carboxylate carbon, forming a cyclic ester ring intermediate. 4. Elimination: Deprotonating the nitrogen atom with a base, collapsing the ester ring and restoring aromaticity in the phthalimide product, and forming N-carboxylate glycine. For L-phenylalanine, the stereochemistry should be retained, yielding the L-product, because there are no racemization or inversion steps in the mechanism.

Step by step solution

01

Carbonyl Addition

First, the nitrogen atom from the amino group of glycine will attack the carbonyl carbon from the N-carboethoxyphthalimide, forming a new bond and breaking one of the double bonds in the carbonyl group. This will result in an alkoxide ion intermediate and a positive charged nitrogen.
02

Ring Opening

After carbonyl addition, the negative charge on the oxygen will prompt the carboxylate group to attack the oxygen atom from the middle of the five-membered ring in N-carboethoxyphthalimide. This opens the ring and forms a linear ester intermediate.
03

Ring Closure

Now, a ring closure will occur when the negative charged oxygen atom that started as the carbonyl of N-carboethoxyphthalimide attacks the carboxylate carbon that was part of the glycine molecule. This step forms a cyclic ester ring intermediate.
04

Elimination

Finally, elimination will occur by deprotonating the nitrogen atom in the intermediate using a base, which will promote the collapse of the ester ring and restore aromaticity in the resulting phthalimide product. Alongside, it forms N-carboxylate glycine as the other product. Now, let's predict whether a similar reaction with L-phenylalanine would yield the L, the D, or the D, L-product.
05

Prediction

L-Phenylalanine is an enantiomerically pure compound, and the reaction mechanism would follow the same steps as described for glycine. Since no racemization or inversion of stereochemistry occurs at any step in the mechanism, the configuration of L-phenylalanine should be retained, yielding the L-product.

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