Question

Following is a retrosynthetic scheme for the synthesis of the tricyclic diene on the left. Show how to accomplish this synthesis from 2 -bromopropane, cyclopentadiene, and 2-cyclohexenone.

Short answer

Question: Outline the synthesis of a tricyclic diene from 2-bromopropane, cyclopentadiene, and 2-cyclohexenone using retrosynthetic analysis and detail the major steps, reagents, and conditions required for each step.

Step-by-step solution

Retrosynthetic Analysis

Begin by analyzing the tricyclic diene product and identifying potential disconnections that would lead back to the starting materials. Upon analysis, it is apparent that we can break the diene system into three components: 2-cyclohexenone, cyclopentadiene, and 2-bromopropane joined together sequentially.

Synthesis of the Intermediate

We can begin the forward synthesis by reacting 2-bromopropane with cyclopentadiene. This reaction involves the use of a Grignard reagent. First, convert 2-bromopropane into the corresponding Grignard reagent: isopropylmagnesium bromide. This conversion can be achieved by reacting 2-bromopropane with magnesium in an ether solvent, such as diethyl ether. \( 2-Bromopropane + Mg \xrightarrow{ether} IsopropylMgBr \) Now, react the isopropylmagnesium bromide with cyclopentadiene to obtain the intermediate. The Grignard reagent will act as a nucleophile and attack the diene, forming a new carbon-carbon bond. This step will yield a substituted cyclopentadiene. \( Cyclopentadiene + IsopropylMgBr \rightarrow Substituted\,cyclopentadiene \)

Addition of 2-Cyclohexenone

In the final step, perform a Diels-Alder reaction between the substituted cyclopentadiene (which acts as diene in this reaction) and 2-cyclohexenone (which acts as a dienophile). The Diels-Alder reaction is a [4+2] cycloaddition which forms new carbon-carbon bonds and generates a tricyclic compound. The reaction occurs under a mild heat condition: \( Substituted\,cyclopentadiene + 2-Cyclohexenone \xrightarrow{\Delta} Tricyclic\,diene \) Finally, we have accomplished the synthesis of the tricyclic diene from 2-bromopropane, cyclopentadiene, and 2-cyclohexenone using Grignard reaction and Diels-Alder reaction.