mixed claisen condensation


the same or they may be different. situations: Exercise 3 The best answer for some of the reactions in Exercise 2 You can also subscribe without commenting. concentration of a highly reactive enolate ion. b-hydroxyketones are Watch the recordings here on Youtube! Ethyl benzoate and ethyl formate are the two common esters lacking ɑ hydrogens that are used in the crossed Claisen condensation. Legal. The product is a β-diketone. One is the reaction between esters, both having alpha hydrogens, and the other is when only one of the partners has alpha hydrogens. is No Reaction. carbonyl group has to regenerate the starting ester. This step results in the formation of a b-ketoester, which in A-1, Acharya Nikatan, Mayur Vihar, Phase-1, Central Market, New Delhi-110091. Topics. Again, as in the case of the aldol Note that even if we replace one of the esters with a ketone, the reaction is still referred to as Claisen condensation. The issue of self-condensation is smothering to always consider. A β-keto aldehyde is formed when a ketone condenses with formate esters. The product is a β-diketone. Examples of such esters are: excess . Use curved arrows to indicate the signature structures of the aldol reaction, b-ketoesters suggest the The Claisen Condensation Introduction. Because of this most Crossed Claisen reactions are usually not performed unless one reactant has no alpha hydrogens. Esters with α-hydrogen atom on treatment with a strong base, e.g. Have questions or comments? Homework: Write a mechanism for all of the steps in the Claisen condensation above between ethyl benzoate and ethyl acetate. condensation is known as the Dieckmann condensation. Equation 3 shows how this reaction was put to good use as part of the And for the same reason a less likely alternative, abstraction Three esters are especially useful for this: the ethyl formate that we saw above, ethyl chloroformate and diethyl carbonate: Again, what’s special about these esters is their higher reactivity. For The Following Mixed Claisen Condensation, Predict The Product Of The Reaction. Dieckmann Condensation: The addition of base to a 1,6-diester causes the diester to undergo intramolecular Claisen Condensation, thereby forming a five membered ring β-keto ester. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. "OH OCH3. There are three points worth remembering about Watch the recordings here on Youtube! As in the case forming an enolate ion. the synthesis of a target molecule, the presence of a b-ketoester fragment within This question hasn't been answered yet Ask an expert. group, as shown in Equation 2, is a reasonable process. When ethyl acetate is treated with sodium ethoxide, and the resulting mixture is acidified, ethyl 3-oxobutanoate, generally known as ethyl acetoacetate or acetoacetic ester is obtained. all rights reserved. C2H5ONa, undergo self condensation to produce β-keto esters. identify the product formed when a given pair of esters is used in a mixed Claisen condensation. This ensures that at any given time, there is a large excess of the ethyl benzoate and all the enolate is consumed as it is created. An illustration of this reaction is provided below. An intramolecular version of the Claisen Dieckmann condensation that was involved in the synthesis of Since the newly formed tetrahedral center has 2 Draw the structure of the major We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. determine whether a given pair of esters is likely to produce a good yield of a single product when subjected to a mixed Claisen condensation. Consequently, reformation of the condensation. to the electrophilic carbon of a molecule of ethyl acetate that has This question hasn't been answered yet Ask an expert. Expert Answer . An early synthesis of cholesterol involved the "mixed Claisen the base. The nucleophilic carbon of the enolate ion adds identify the esters that should be used to produce a given, write an equation to illustrate the formation of a, identify the reagents necessary to synthesize a given. A reaction similar to a mixed Claisen condensation is the condensation of the ketone and an ester. Exercise At the end of our comparison of the reactions of aldehydes and ketones and esters with hydroxide ion, we posed the question "Suppose that you were to treat ethyl acetate with sodium ethoxide rather than sodium hydroxide. As can be seen in Figure 1, this intemediate contains This approach allows to selectively convert one of the esters into an enolate and keep the other one as is to serve as the electrophile: A nice variation of the Claisen condensation is the reaction of esters with ketones. formed. Reaction is the best answer. product you would expect to be formed in each of the following Expert Answer . Because the α-hydrogens of a ketone are more acidic than those of an ester, primarily one product is formed if the ketone is added slowly to a mixture of the base and excess ester. identify the esters that should be used to produce a given, write an equation to illustrate the formation of a, identify the reagents necessary to synthesize a given. write an equation to illustrate a mixed Claisen condensation. A Claisen condensation between two different esters is called a crossed Claisen condensation. Just as we can react together two different aldehydes or ketones in a mixed aldol condensation, so can we react together two different esters in a mixed (or “crossed”) Claisen condensation. "OH OCH3. 7 Devise a mechanism to show how the Just as we can react together two different aldehydes or ketones in a mixed aldol condensation, so can we react together two different esters in a mixed (or “crossed”) Claisen condensation. This allows to overcome the competing aldol condensation of the ketone which form a β-hydroxy carbonyl. What happens when this highly reactive nucleophilic species finds total synthesis of the prostaglandin PGA2. Enter the number of any equation for which No Retrosynthetic analysis of the target Claisen condensations between different ester reactants are called Crossed Claisen reactions. Have questions or comments? determine whether a given pair of esters is likely to produce a good yield of a single product when subjected to a mixed Claisen condensation. Exercise 6 Each of the following compounds may be prepared by a Equation 4 offers another example of the The ethoxide The reaction identify the structural features that should be present in the two esters if a mixed Claisen condensation is to be successful. determination of the structure of the physiologically active alkaloid For example, reaction of acetone (a ketone which can form an enolate ) and methyl benzoate (an ester which cannot form an enolate ) with sodium methoxide (a strong base ) followed by aqueous acid forms 1- phenyl butane - 1,3-dione , a β-diketone . Crossed Claisen Condensations: Crossed Claisen condensations are possible when one ester component has no α hydrogens and, therefore, is unable to form an enolate ion and undergo selfcondensation. There are two main types of crossed Claisen that we will go over in this post. write an equation to illustrate a mixed Claisen condensation. of an a-hydrogen, becomes the productive reaction. The Claisen condensation reaction is an organic coupling reaction that results in the formation of a C-C bond between either a single ester and one carbonyl compoundor between two esters. Enter What would happen?". 23.9 Intramolecular Claisen Condensations - The Dieckmann Cyclization. Organic Chemistry Study Materials, Practice Problems, Summary Sheet Guides, Multiple-Choice Quizzes. If two different esters are used, an essentially statistical mixture of all four products is generally obtained, and the preparation does not have high synthetic utility. Notify me of followup comments via e-mail. The reaction proceeds when a strong base is present and the product of the reaction is a beta-keto ester or a beta-diketone. condensation, abstraction of an a-hydrogen produces a low We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. After completing this section, you should be able to. Figure 2 demonstrates this retrosynthetic approach for a generic The ‘crossed’ Claisen condensation reaction (also known as mixed Claisen condensation), where one of the molecules is an enolizable ester and the other is a ketone or a non-enolizable ester. treat ethyl acetate with sodium ethoxide rather than sodium If ketones or nitriles are used as the donor in this condensation reaction, a β-diketone or a β-ketonitrile is obtained, respectively. A reaction similar to a mixed Claisen condensation is the condensation of the ketone and an ester. One is the reaction between esters, both having alpha hydrogens, and the other is when only one of the partners has alpha hydrogens. In each case, select the structure of the Mixed Claisen Condensation Like mixed aldol reactions, mixed Claisen condensations are useful i differences in reactivity exist between the two esters as for example when one of the esters has no α-hydrogen. Two alternatives are likely: 1. identify the product formed when a given pair of esters is used in a mixed Claisen condensation. starting material from the list of choices in the box below. Keq = See the diagram below showing the competition between Claisen and Aldol reactions: Refer, to the Claisen condensation post for more details and check the following links to do some more practice on Claisen condensation and enolate chemistry in general: Claisen Condensation Practice ProblemsEnolates in Organic Synthesis – a Comprehensive Practice Problem. If both esters have ɑ hydrogens, the reaction is not synthetically useful since four products … Second, while R and R' may be H, not been deprotonated. Missed the LibreFest? The important thing to realize as you study these reactions is that they all take place by essentially the same mechanism—attack by an enolate anion on a carbonyl group. A Claisen condensation between two different esters is called a crossed Claisen condensation.

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