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Chapter 20: Problem 45

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

Expert verified
Question: Outline the synthesis pathway for creating a tricyclic diene from 2-bromopropane, cyclopentadiene, and 2-cyclohexenone. Answer: The synthesis pathway involves the following steps: 1. Formation of the Grignard reagent (2-Propylmagnesium bromide) from 2-bromopropane and magnesium metal. 2. Addition of the Grignard reagent to 2-cyclohexenone, resulting in the formation of 3-(2-Propyl)hexan-3-ol followed by an acidic work-up. 3. Performing a Diels-Alder reaction between cyclopentadiene and 3-(2-Propyl)hexan-3-ol to form a Diels-Alder adduct. 4. Dehydration of the Diels-Alder adduct to form the desired tricyclic diene product.

Step by step solution

01

Formation of the Grignard reagent from 2-bromopropane

First, we need to prepare the Grignard reagent from 2-bromopropane. This reaction is carried out by treating 2-bromopropane with magnesium metal as follows: 2-Bromopropane + Mg ------> 2-Propylmagnesium bromide (Grignard reagent) The Grignard reagent (2-Propylmagnesium bromide) formation is carried out in the presence of an anhydrous ether solvent, such as diethyl ether or THF (tetrahydrofuran). It is crucial to keep the reaction environment anhydrous as the Grignard reagent is highly sensitive to moisture.
02

Addition of the Grignard reagent to 2-cyclohexenone

Next, we will use the Grignard reagent (2-Propylmagnesium bromide) to attack the carbonyl group in 2-cyclohexenone, which is a nucleophilic addition reaction. This will result in the formation of a new carbon-carbon bond and an alkoxide anion: 2-Cyclohexenone + 2-Propylmagnesium bromide --> 3-(2-Propyl)hexan-3-ol After this reaction step, we need to perform an acidic work-up using an aqueous acid (such as diluted hydrochloric acid) to protonate the alkoxide anion to generate the alcohol: 3-(2-Propyl)hexan-3-ol^- + H^+ --> 3-(2-Propyl)hexan-3-ol
03

Formation of the Diels-Alder Adduct

We will use cyclopentadiene and the alcohol product (3-(2-Propyl)hexan-3-ol) from Step 2 to perform a Diels-Alder reaction. The Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and an alkene (in this case, the double bond in 3-(2-Propyl)hexan-3-ol). The reaction forms a new six-membered ring, with the Diels-Alder adduct as the product: Cyclopentadiene + 3-(2-Propyl)hexan-3-ol --> Diels-Alder adduct
04

Dehydration of Diels-Alder Adduct to form the tricyclic diene

Finally, to obtain the desired tricyclic diene product, we need to perform a dehydration reaction on the Diels-Alder adduct. This reaction step involves the elimination of a water molecule (H2O) from the Diels-Alder adduct, forming a new carbon-carbon double bond. Diels-Alder adduct --> Tricyclic diene + H2O This dehydration reaction can be achieved by treating the Diels-Alder adduct with a strong acid catalyst (like sulfuric acid), under appropriate reaction conditions. In conclusion, through these four reaction steps, we have successfully synthesized the desired tricyclic diene using 2-bromopropane, cyclopentadiene, and 2-cyclohexenone as starting materials.

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Most popular questions from this chapter

The \(2+2\) cycloaddition with one suprafacial and one antarafacial interaction is allowed. Show this conclusion via a frontier molecular orbital analysis. Although the reaction is allowed, it is seldom seen. Can you think of a reason not based upon an orbital analysis of why this reaction is difficult?

Treatment of anthranilic acid with nitrous acid gives an intermediate, \(\mathbf{A}\), that contains a diazonium ion and a carboxylate group. When this intermediate is heated in the presence of furan, a bicyclic compound is formed. Propose a structural formula for compound \(\mathrm{A}\) and a mechanism for the formation of the bicyclic product.

1,3-Butadiene is a gas at room temperature and requires a gas-handling apparatus to use in a Diels-Alder reaction. Butadiene sulfone is a convenient substitute for gasheated above its boiling point of \(110^{\circ} \mathrm{C}\), decomposes by a reverse Diels-Alder reaction to give s-cis-1,3-butadiene and sulfur dioxide. Draw Lewis structures for butadiene sulfone and \(\mathrm{SO}_{2}\); then, show by curved arrows the path of this reaction, which resembles a reverse Diels-Alder reaction.

Which molecules can function as dienes in Diels-Alder reactions?

Estimate the stabilization gained as a result of conjugation when 1,4 -pentadiene is converted to trans-1,3-pentadiene. Note that the answer is not as simple as comparing the heats of hydrogenation of 1,4 -pentadiene and trans-1,3-pentadiene. Although the double bonds are moved from unconjugated to conjugated, the degree of substitution of one of the double bonds is also changed, in this case from a monosubstituted double bond to a trans disubstituted double bond. To answer this question, you must separate the effect that is the result of conjugation from that caused by a change in the degree of substitution.
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