Carboxylic condensation with aldehydes

Reactions. Malonie acid is a useful tool lor synthesizing a-unsaturaled carboxylic acids because of its ability lo undergo decarboxylation and condensation with aldehydes or ketones at the methylene group. Cinnamic... 

Aldol-type reactions. In the presence of I r. h, these ketals condense with aldehydes to form p-hydroxy carboxylic acids (equation 1) ir with imines to form P-lactams (equation 11). 

The 2-methyl group of selenazoles and benzoselenazoles, as well as the selenazolium derivatives (see Section 3.08.12) undergoes condensation with aldehydes to give the unsaturated product <92MI 308-01). The 2-methylselenazoles are also oxidized to the carboxylic acid with Se02 <79HC(34)217>. 

Conducted in an intramolecular sense, Mannich reactions have been much used for the construction of tetrahydro-P-carbolines. ° Tryptamines carrying a 2-carboxylic acid group, which can be conveniently prepared (20.16.6.3), but are not easily decarboxylated as such, undergo cyclising Mannich condensation with aldehydes and ketones, with loss of the carbon dioxide in a final step. ° ... 

It has been found that carboxylic acid enolates, generated under thermodynamic control (25 C for three days), condense with aldehydes to form a preponderance of... 

The f/ireo-3-hydroxycarboxylic acids (45) are readily obtained in high dia-stereoisomeric purity on condensation of dilithio-salts of carboxylic acids with aldehydes. Further purification is achieved on recrystallization from chloroform. The pure condensate (45) may serve as a common precursor to both E- and Z-... 

Closely related chemistry has permitted the development of an enantioselective synthesis of certain classes of carboxylic acids.The key reagent is the oxazoline 7, which is commercially available in optically active form. The oxazoline condenses with aldehydes to give the alkylidene derivatives 8. The alkylidene intermediates can undergo conjugate addition of organolithium reagents in a highly stereoselective... 

A variety of terminal functional groups and their chemical transformations on SAMs have been examined for example, (i) olefins—oxidation [23,24,131,132], hydroboration, and halogenation [23,24] (ii) amines—silyla-tion [145,146], coupling with carboxylic acids [22,146], and condensation with aldehydes [22,147] (iii) hydroxyl groups—reactions with anhydrides [148,149], isocyanates [150], epichlorohydrin [151], and chlorosilanes [152] (iv) carboxylic acids—formation of acyl chlorides [153], mixed anhydrides [154], and activated esters [148,155] (v) carboxylic esters—reduction and hydrolysis [156] (vi) thiols and sulfides—oxidation to generate disulfides [157-159] and sulfoxides [160] and (vii) aldehydes—condensation with active amines [161], Nucleophilic... 

In the presence of a strong base, the ot carbon of a carboxylic ester can condense with the carbonyl carbon of an aldehyde or ketone to give a P-hydroxy ester, which may or may not be dehydrated to the a,P-unsaturated ester. This reaction is sometimes called the Claisen reaction,an unfortunate usage since that name is more firmly connected to 10-118. In a modem example of how the reaction is used, addition of tert-butyl acetate to LDA in hexane at -78°C gives the lithium salt of ferf-butyl acetate, " (12-21) an enolate anion. Subsequent reaction a ketone provides a simple rapid alternative to the Reformatsky reaction (16-31) as a means of preparing P-hydroxy erf-butyl esters. 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 (10-119) involving nucleophilic substitution and not addition to a C=0 bond. It can, however, be a side reaction if the aldehyde or ketone has an a hydrogen. 

This is another reaction that involves carbanions derived from esters, e.g. (Ill), but this time adding to the carbonyl carbon atom of another ester molecule. The reason for considering it here rather than under carboxylic derivatives (p. 237) is that it can, in its initiation, be regarded as something of an analogue, for esters, of the aldol condensation on aldehydes (cf. p. 224), e.g. with ethyl ethanoate (acetate, 112) ... 

A variety of methods have been developed for the preparation of substituted benzimidazoles. Of these, one of the most traditional methods involves the condensation of an o-phenylenediamine with carboxylic acid or its derivatives. Subsequently, several improved protocols have been developed for the synthesis of benzimidazoles via the condensation of o-phenylenediamines with aldehydes in the presence of acid catalysts under various reaction conditions. However, many of these methods suffer from certain drawbacks, including longer reaction times, unsatisfactory yields, harsh reaction conditions, expensive reagents, tedious work-up procedures, co-occurrence of several side reactions, and poor selectivity. Bismuth triflate provides a handy alternative to the conventional methods. It catalyzes the reaction of mono- and disubstituted aryl 1,2-diamines with aromatic aldehydes bearing either electron-rich or electron-deficient substituents on the aromatic ring in the presence of Bi(OTf)3 (10 mol%) in water, resulting in the formation of benzimidazole [119] (Fig. 29). Furthermore, the reaction also works well with heteroaromatic aldehydes. 

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