Chapter 27: Q 41. (page 1101)

Question:What starting materials are needed to synthesize each compound by a thermal [4 + 2] cycloaddition?

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Answer

Starting material needed:

Step by step solution

Cycloaddition reactions are practically reversible reactions. The reverse cycloaddition reaction is known as retro cycloaddition reaction.

The simplest way to find the starting material is to “carry out a disconnection approach,” i.e., retro cycloaddition.

Diels–Alder reactions follow the cycloaddition mechanism. Therefore, the starting material can be determined by reverse Diels–Alder reaction (retro cycloaddition).

For the disconnection approach, draw three arrows going around the cyclohexene ring, starting from the first arrow in the middle of the double bond.

The diene has two sets of substituents: inside substituents and outside substituents. Their symmetry should not be changed.

Both reactants have symmetry; therefore, the product must retain this symmetry because the reaction is concerted.

(a)

Bond disconnection in (a)

Synthesis of the desired product by Diels–Alder reaction:

Here, on diene, the inside substituents are hydrogen and outside substituents are ethyl groups, i.e., cis-cis diene.
The ethyl groups end up beneath the new six-membered ring and hydrogens end up above the ring.

(b)

Bond disconnection of (b)

Synthesis of the desired product by Diels–Alder reaction:

Here, on diene, the inside substituents are hydrogen and ethyl group, i.e., cis-trans diene.
The two substituents end up on the opposite sides of the new six-membered ring.