16. Conjugated Systems

In each Diels–Alder reaction shown, predict the product that will result.

(c)

Predict the product of the following reactions. [When all of the reactions from this chapter are shown together, you must first decide which type of reaction each is. Is it a Diels–Alder, an electrocyclic, or a sigmatropic rearrangement? Drawing the product will be easier once this determination is made.]

(a)

Show the steps involved in the following reaction:

An important variation of the Diels–Alder reaction is intramolecular, in which the diene and the dienophile are connected. This type of Diels–Alder reaction makes two new rings. Draw the compound produced in each of these examples; try to predict stereochemistry (using models will help). In some cases, Lewis acid catalysts are used; that can be ignored for this problem.

(a)

(b)

Predict the products of the following proposed Diels–Alder reactions.

(f)

What stereochemical result would you expect if the Diels–Alder reaction with the free α,β- unsaturated ketone was faster than reaction with the iminium-bound alkene?

We predicted that the products would have a 1,2- or 1,4-relationship of the proper substituents. Draw the charge-separated resonance forms of the reactants to support these predictions.

(a) (b)

An important variation of the Diels–Alder reaction is intramolecular, in which the diene and the dienophile are connected. This type of Diels–Alder reaction makes two new rings. Draw the compound produced in each of these examples; try to predict stereochemistry (using models will help). In some cases, Lewis acid catalysts are used; that can be ignored for this problem.

(c)

An important variation of the Diels–Alder reaction is intramolecular, in which the diene and the dienophile are connected. This type of Diels–Alder reaction makes two new rings. Draw the compound produced in each of these examples; try to predict stereochemistry (using models will help). In some cases, Lewis acid catalysts are used; that can be ignored for this problem.

(d)

What is the major product when the methoxy substituent in the preceding reaction is bonded to C-2 of the diene rather than to C-1?

Explain why the following compounds are not optically active:

a. the product obtained from the reaction of 1,3-butadiene with cis-1,2-dichloroethene

Explain why the following compounds are not optically active:

b. the product obtained from the reaction of 1,3-butadiene with trans-1,2-dichloroethene

In each Diels–Alder reaction shown, predict the product that will result.

(a)

By forming a Lewis acid–Lewis base complex with the dienophile, Lewis acids are able to increase the rate of Diels–Alder reactions. Why might this be true?

Which of the following concerted reactions would have a more stable transition state? Why?