Named reactions Claisen condensation

Ludwig Claisen was a Ger man chemist who worked during the last two decades of the nineteenth century and the first two decades of the twentieth His name is associated with three reac tions The Claisen-Schmidt reaction was presented in Section 18 10 the Claisen condensation is discussed in this section and the C/a/sen rearrangement will be intro duced in Section 24 13... 

Ba.se Catalyzed. Depending on the nature of the hydrocarbon groups attached to the carbonyl, ketones can either undergo self-condensation, or condense with other activated reagents, in the presence of base. Name reactions which describe these conditions include the aldol reaction, the Darzens-Claisen condensation, the Claisen-Schmidt condensation, and the Michael reaction. 

In the presence of a strong base, the a carbon of a carboxylic ester can condense with the carbonyl carbon of an aldehyde or ketone to give a J3-hydroxy ester,556 which may or may not be dehydrated to the a,J3-unsaturated ester. This reaction is sometimes called the Claisen condensation,557 an unfortunate usage since that name is more firmly connected to 0-108. It is also possible for the a carbon of an aldehyde or ketone to add to the carbonyl carbon of a carboxylic ester, but this is a different reaction (0-109) involving nucleophilic substitution and not addition to a 0=0 bond. It can, however, be a side reaction if the aldehyde or ketone has an a hydrogen. 

If, however, it is necessary to generate a crossed product by the reaction of an enolate derived from one carbonyl compound with a second carbonyl compound as the electrophile, tilings can go bad rapidly. Because both carbonyl groups must be present in solution at the same time and each can form etiolates to some extent, there can be four possible products from the various combinations of etiolates and carbonyl compounds. This problem was illustrated for the crossed-Claisen condensation above. The number of products can be minimized if one carbonyl component lacks a protons and cannot form an enolate and is also a more reactive electrophile than the second carbonyl component. If these conditions are met, then crossed condensations can be carried out successfully using alkoxide bases. Many of the named reactions were developed so that product mixtures could be avoided. 

Enzymes that catalyze such Claisen condensations (and some related reactions, see helow) are all members of the thiolase superfamily (named for the first member of the family which was described, the degradative thiolase I from S. cerevisiae), as they all share a common fold despite a lack of sequence similarity and a large... 

B.ii. The Dieckmann Condensation. Just as there is an intramolecular version of the aldol condensation, there is an intramolecular version of the Claisen condensation but it has been given a different name, the Dieckmann condensation. 109,110 reaction involves intramolecular cyclization of an a, co-diester such as... 

In carbonyl condensation reactions the enolate or enol of one carbonyl compound reacts with the carbonyl group of another to join the two reactants. As part of the process, a new molecule that is derived from them condenses (forms). Often this molecule is that of an alcohol or water. The main types of condensation reactions we shall study are the Claisen condensation and the aldol condensation. Aldol condensations are preceded mechanistically by aldol additions, which we shall also study. The name aldol derives from the fact that aldehyde and alcohol functional groups are present in the products of many aldol reactions. 

In this section, we examine the formation of an enolate anion from one ester, followed by the nucleophilic acyl substitution of the enolate anion at the carbonyl carbon of another ester. One of the first of these reactions discovered was the Claisen condensation, named after its discoverer, German chemist Ludwig Qaisen (1851—1930). We illustrate a Claisen condensation by the reaction between two molecules of ethyl acetate in the presence of sodium ethoxide, followed by acidification, to give ethyl acetoacetate (note that, in this and many of the equations that follow, we abbreviate the ethyl group as Et) ... 

The crossed aldol reaction is important enough to have been given its own name, the Claisen-Schmidt condensation (Ludwig Claisen, 1851—1930). 

Notice once again that the product is just a P-keto ester. This reaction has its own name (the Dieckmann condensation). But it is really just an intramolecular Claisen condensation. Therefore, the steps of this mechanism are identical to the steps of a regular Claisen condensation. Propose a mechanism for the Dieckmann condensation. Try to do it without looking back at your previous work. You will need a separate piece of paper to record your answer. 

An aldol-like reaction involving an ester is called a Claisen reaction (or a Claisen condensation) named for German chemist Ludwig Claisen, 1851-1930. The final product is a resonance-stabilized anion. 

Place 100 g. of adipic acid in a 750 ml. round-bottomed flask and add successively 100 g. (127 ml.) of absolute ethyl alcohol, 250 ml. of sodium-dried benzene and 40 g. (22 ml.) of concentrated sulphuric acid (the last-named cautiously and with gentle swirling of the contents of the flask). Attach a reflux condenser and reflux the mixture gently for 5-6 hours. Pour the reaction mixture into excess of water (2-3 volumes), separate the benzene layer (1), wash it with saturated sodium bicarbonate solution until eflfervescence ceases, then with water, and dry with anhydrous magnesium or calcium sulphate. Remove most of the benzene by distillation under normal pressure until the temperature rises to 100° using the apparatus of Fig. II, 13, 4 but substituting a 250 ml. Claisen flask for the distilling flask then distil under reduced pressure and collect the ethyl adipate at 134-135°/17 mm. The yield is 130 g. 

The mechanism of the reaction between aromatic aldehydes and esters probably involves the intermediate formation of an aldol (hence the name— Claisen aldol condensation) ... 

Condensation reactions of the aldol type play an important part in heterocyclic chemistry. There are a large number of condensation reactions that are closely related to the aldol condensation. Each of these reactions has its own name Claisen, Dieckmann, Doebner, Knoevenagel, Perkin, to mention a few, but the chemistry is essentially the same as that of the aldol condensation. 

---