Question

A)

B)

Complete the drawing of the product of the Diels-Alder reaction, ignoring stereochemistry. The Diels- Alder reaction between butadiene and dimethyl maleatc yields a ring structure, as shown in the product below Complete the structure by drawing any missing bonds and indicating the stereochemistry of the new stereocenters.

Answer 1

Concepts and reason

The concept used to solve this question is to draw the structure of the product in the Diels-Alder reaction of the given substrates. The cycloaddition reaction of diene with a dienophile to give a cyclic adduct is known as Diels-Alder reaction. The stereo chemistry of the product in the Diels-Alder reaction depends on the stereochemistry of the transition state.

Fundamentals

Diels-Alder reaction is the [4+2] cycloaddition reaction of conjugated diene with an alkene or an alkyne with electron withdrawing group to give six-membered rings. The product is formed by the concerted mechanism in which movement of three pairs of electrons takes place. For example,

The stereochemical requirements of Diels-Alder transition state:

Diels-Alder reaction is a concerted process so the transition state determines the stereochemistry of the product. So the transition state depends on the following requirements.

1) s-cis conformation of the diene is required for Diels-Alder reaction because it allows the maximum overlap between end p-orbitals of diene with those of dienophile.

2) Syn addition takes place in Diels-Alder reaction so the substituents on diene and dienophile which are on the same side will be cis in the product.

3) Endo rule: The electron-withdrawing in dienophile with p-orbitals approach one of central carbon atoms (C2 or C3) of diene, which is called as secondary overlap, it stabilizes the transition state. Due to this reason, the electron-withdrawing groups occupy the position which is closest to the central carbon atoms (C2 or C3) of diene and the position is called as endo position. This rule is called as endo rule.

A)

The structures of given diene and dienophile are as follows:

In the above structurtes, diene is in s-cis form which is required condition for Diels-Alder reaction. The dienophile is an alkyne substrate with as electron withdrawing group. Thus, the [4+2] cycloaddition of the above diene and dienophile gives substituted cyclohexa-1,4-diene as product. The product of the Diels-Alder reaction is shown below.

B)

The structures of given diene and dienophile are as follows:

In the above structurtes, diene is in s-cis form which is required condition for Diels-Alder reaction. The electron withdrawing group in dienophile is group so it will occupy the endo position in the product according endo rule. Therefore, Diels-Alder reaction of the given staring materials gives substituted cyclohexene as product which is shown below with stereochemistry.

Ans: Part A

The structure of the product of the Diels-Alder reaction is shown below.

Part B

The structure of the product of the Diels-Alder reaction along with the stereochemistry is shown below.