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20 -- Part 5: CARBOXYLIC ACIDS
A mechanism for the conjugate addition of a nucleophile to acrylonitrile six-membered rings is proposed.
You can use resonance forms to show the structure.
The double bond is activated by the female hormone estra nitro groups.
The following products could be made from suitable Michael donors and acceptors.
Adding a ketone enolate to an a,b@unsaturated ketone gives a d@diketone.
A new six-membered ring can be created if the d@diketone undergoes a conjugate addition under strongly basic or acidic conditions.
An example would be to use a substitution of cyclohexanone as the Michael donor and a substitution ofMVK as the Michael acceptor.
A d@diketone is formed by the Michael addition of the cyclohexanone enolate.
The d@diketone is ideally suited for the formation of a six-membered ring.
The cyclohexanone carbonyl is attacked by the enolate of the methyl ketone.
The product gives a cyclohexenone.
A six-membered ring is formed by Cyclic aldol.
If you remember that the Michael addition is first, followed by an aldol condensation with dehydration, you can draw the mechanisms for the Robinson annulation.
The system for proposing mechanisms summarized in Appendix 3A is used in this problem-solving example.
The problem is to come up with a mechanism for the base-catalyzed reaction.
Determine the type of mechanism first.
The use of a basic catalyst suggests that the reaction involves strong nucleophiles.
We expect to see anionic intermediates, but no strong acids, or free radicals.
The product must be made from ethyl acetoacetate.
The carbon from the C " C double bond" should be derived from the ethyl acetoacetate.
The structure can be seen in the four remaining carbons.
Consider if one of the reactants is strong enough to react without being activated.
Both reactants are strong enough to attack the other.
The enolate ion can be given by ethoxide ion because acetoacetate is more acidic.
The product of this bond formation can be drawn.
The carbonyl group ofMVK might be attacked by the enolate of acetoacetic ester.
One of the bonds needed in the product is a Michael addition.
The ethyl acetoacetate group needs to be converted to a C " C double bond in the a,b position.
This conversion is related to aldol condensation.
The enolate that is needed to give the observed product is formed by the removal of the most acidic protons.
Use curved arrows to draw out the steps.
Don't show more than one step at a time.
Combining the preceding equations gives the complete mechanism.
You can review the steps by writing out the mechanism.
The point of a mechanism problem is not that we can draw other mechanisms, but that we can't draw other products.
Even though other products are likely formed as well, and possibly in higher yields, the question asked for a mechanism to explain only this one product.
The approach shown will help you propose mechanisms for multistep condensations.
The mechanism for the reaction should be proposed.
There is a mechanism for the Perkin condensation.
Show how you would make the following compounds.
The cyclohexenone is the new ring and the double of Robinson is formed by dehydration, so you can usually spot a product Work backward.
The double has a new ring.
A full summary of additions and condensations would take a long time.
The major classes of condensations are covered in this summary.
The anion is the initial form of the product.
The carbanions are stable enough to exist in solution.
Many of the nucleophilic reaction types have been covered previously.
Enolates are strong bases and usually need an acidic workup to supply H+.
Dehydration often leads to the formation of aldol condensations.
Chapter 22 has reactions shown in red.
Reactions are shown in blue.
A ring is formed by Claisen condensation.
The carbon atom is deprotonated next to a carbonyl group.
Such a hydrogen may be lost and regained through tautomerism.
The acylation of malonic ester is followed by the hydrolysis and decarbox ylation.
Robinson is followed by an aldol condensation with dehydration.
The acylation of a ketone or aldehyde involves the use of an enamine derivative.
The alkylated or acylated ketone or aldehyde can be regenerated.
An isomerism involves the movement of a double bond and a protons.
tautomerism is related to the isomers.
condensations take on a wide variety of forms in this chapter.
To gain confidence in working out new variations of the standard mechanisms, you need to work enough problems.
Make sure you can propose condensations that form new rings.
Each skill is followed by problem numbers.
Predict the reactions of aldol before and after dehydration.
Acid-catalyzed and base-catalyzed reactions have mechanisms.
The compounds should be ranked in order of increasing acidity.
List the compounds that would be more than 99% deprotonated by a solution of sodium ethoxide.
There is a mixture of 8% keto and 92% enol forms.
Explain the stability of the stable enol tautomer.
In order to increase acid strength, rank these compounds.
In order to increase enol content, rank these compounds.
Draw the most stable enol.
Show how you would use Robinson to make these compounds.
You can show how you would make each compound with aldol, Claisen, or another type of condensation.
How would you accomplish the following conversions?
You can use any necessary reagents.
The following compounds would be made using the malonic ester synthesis.
The acetoacetic ester synthesis would be used to make the following compounds.
The following compounds can be made using aldol condensations.
In the case of the aldol condensation, an active methylene compound reacts with an aldehyde or ketone, in the presence of a secondary amine as a basic catalyst, to produce a new C.
The following enamine alkylation and acylation reactions have expected products.
After the iminium salts are hydrolysis, give the final products.
The following multistep conversions would be accomplished by showing how you would accomplish them.
You can use any additional reagents.
Many of the condensations we studied are not permanent.
There are mechanisms to account for the reactions.
The chemistry lab students added an excess of ethylmagnesium bromide to methyl furoate, expecting the Grignard reagent to add twice and form the tertiary alcohol.
The product was a mixture of two compounds.
The expected product had two ethyl groups, but the unexpected product had three.
There is a mechanism to explain the formation of the unexpected product.
The splitting of fructose-1,6-diphosphate to give glyceraldehyde- 3-phosphate is a reaction involved in the metabolism of sugars.
The base-catalyzed reaction can be proposed.
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