The products of aldol reactions often undergo a subsequent elimination of water, made up of an alpha-hydrogen and the beta-hydroxyl group. When performing both reactions together always consider the aldol product first then convert to the enone. write detailed mechanisms for mixed Claisen reactions and reactions that are related to the mixed Claisen reaction, including those in which both reacting moieties are present in the same compound. 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. En fonction de la nature énolisable ou non des réactifs, il existe différents types de condensation de Claisen. During the reaction a new carbon-carbon bond is formed. Have questions or comments? A Claisen condensation between two different esters is called a crossed Claisen condensation.There are two main types of crossed Claisen that we will go over in this post. Dans la condensation de Claisen « classique », seul un ester énolisable est utilisé : Dans la condensation de Claisen « croisée », on utilise un ester ou une cétone énolisable et un ester non énolisable : Dans la condensation de Dieckmann, on utilise une molécule portant deux groupes esters qui réagissent intramoléculairement en formant un β-céto ester cyclique. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. On utilise pour ça des esters de méthyle ou d'éthyle, ce qui forme des groupes méthonalate ou éthonalate. Ti-crossed-Claisen condensation between a 1:1 mixture of carboxylic esters and acid chlorides promoted by TiCl4−Bu3N-N-methylimidazole proceeded successfully to give various β-keto esters in good yields with excellent selectivities (19 examples, ∼48−95% yield; cross/self-selectivity = ∼96/4−99/1). The additional stability provided by the conjugated carbonyl system of the product makes some aldol reactions thermodynamically and mixtures of stereoisomers (E & Z) are obtained from some reactions. Overall the general reaction involves a dehydration of an aldol product to form an alkene: Figure: General reaction for an aldol condensation. Because of this most mixed aldol reactions are usually not performed unless one reactant has no alpha hydrogens. The Dieckmann condensation reaction, in which a molecule featuring two ester … Again, by carefully selecting our substrates we can obtain a good yield of the desired product and minimize the number of possible by‑products. La base ne doit pas non plus interférer par une réaction de substitution nucléophile ou d'addition nucléophile sur le carbone du groupe carbonyle. 19.13: Dehydration of Aldol Addition Products Form α, β-Unsaturated Aldehydes and Ketones, 19.15: A Claisen Condensation Forms a β-Keto Ester, Aldol Condensation: the dehydration of Aldol products to synthesize α, β unsaturated carbonyl (enones). The success of these mixed aldol reactions is due to two factors. In most cases two sets of $$\alpha$$ hydrogens need to be considered. Crossed Claisen Condensation: eg: mechanism: Step 1: The alkoxide ion deprotanates the enolizable ester reversibly. En fonction de la nature énolisable ou non des réactifs, il existe différents types de condensation de Claisen. Have questions or comments? Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Because of this most Crossed Claisen reactions are usually not performed unless one reactant has no alpha hydrogens. Example 23.8.1: Crossed Claisen Condensation, Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University), Prof. Steven Farmer (Sonoma State University). Crossed Claisen reactions in which both reactants can serve as donors and acceptors generally give complex mixtures. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. The following abbreviated formulas illustrate the possible products in such a case, red letters representing the acceptor component and blue the donor. For this reaction to occur at least one of the reactants must have α hydrogens. 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. Separation and purification of the components of such a mixture would be difficult. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Molecules which contain two carbonyl functionalities have the possibility of forming a ring through an intramolecular aldol reaction. Have questions or comments? The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739.