Introduction:

One of the most influential tools for the formation of cyclic molecules is the Diels-Alder reaction. The reaction usually comprises the combination of a diene by a 'dienophile'. There are amazingly some limitations on the character of either fragment; however the presence of electron withdrawing substituents on the dienophile improves the reaction rate.

The number of illustrations of this reaction that have been studied is vast, and the methods given here are typical.

Reaction mechanism:

The Diels-Alder reaction is a conjugate addition reaction of the conjugated diene to an alkene (that is, the dienophile) to generate a cyclohexene. The simplest illustration is the reaction of 1,3-butadiene with ethene to make cyclohexene:

Fig: Diels-Alder reaction mechanism

The analogous reaction of 1,3-butadiene by ethyne to make 1,4-cyclohexadiene is as well recognized:

Fig: Analogous reaction of 1,3-butadiene

As the reaction makes a cyclic product, through a cyclic transition state, it can as well be illustrated as 'cycloaddition'. The reaction is a concerted method:

Fig: Cycloaddition

Because of the high degree of regio- and stereoselectivity (because of the concerted mechanism), the Diels-Alder reaction is a very influential reaction and is broadly employed in the synthetic organic chemistry.

Dienes and Dienophiles:

The reaction is generally thermodynamically favorable because of the conversion of 2 π-bonds into 2 new stronger σ-bonds. The two reactions illustrated above need harsh reaction conditions, however the normal Diels-Alder reaction is favored via electron withdrawing groups on the electrophilic dienophile and by electron donating groups on the nucleophilic diene. Some of the common illustrations of the components are illustrated below:

Examples of Dienes:

Fig: Examples of Dienes

Examples of dienophiles:

Fig: Examples of dienophiles

Stereoselectivity:

The Diels-Alder reaction is Stereospecific with respect to both the diene and dienophile.  Addition is syn on both the components (that is, bonds form from similar species at similar time). This is described by the illustrations below:

Fig: Stereoselectivity

A cis-dienophile provides cis-substituents in the product as illustrated by the two ester groups in the product above. On the other hand, a trans-dienophile provides trans-substituents in the product as described by the two ester groups in the below product:

Fig: Trans-dienophile gives trans-substituents

Whenever the diene substituents encompass the similar stereochemistry (that is, in the illustration which follows they are both E), then both the diene substituents end up on the similar face of the product. Consider the two methyl groups in the reaction product which follows:

Whenever the diene substituents encompass opposite stereochemistry (as in the example which follows one is E and one Z), then the diene substituents end up on opposite faces of the product (that is, notice the two methyl groups in the product).

Fig: Diene substituents have opposite stereochemistry

Cyclic dienes can provide stereoisomeric products based on whether the dienophile lies under or away from the diene in the transition state.  The endo product is generally the main product (because of the kinetic control).

Fig: Cyclic dienes give stereoisomeric products

Synthetic applications:

A) Synthesis of steroids:

One of the most basic and most significant illustrations of the Diels-Alder reaction in total synthesis was in syntheses of the steroids cortisone and cholesterol.

Fig: Synthesis of steroids

B) Synthesis of tetracycline:

The Synthesis of linear tetracyclic core of the antibiotic was accomplished by Diels-Alder reaction. Thermally initiated, conrotatory opening of the benzocyclobutene produced the o-quinodimethane that reacted intermolecularly to provide the tetracycline skeleton.

Fig: Synthesis of tetracycline

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