Catalyzed Condensation Reactions

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Model studies directed toward the synthesis of Ecteinascidin 743 employed an elegant Pictet-Spengler cyclization of phenethylamine 54 and the 1,2-dicarbonyl compound 55 to assemble the spiro tetrahydroisoquinoline 56 in a stereospecific fashion. " The silica-catalyzed condensation reaction provided 56 in excellent yield. 

Besides the oxidative and transition-metal-catalyzed condensation reactions discussed above, several other syntheses were developed to generate PPP and PPP derivatives. 

Glycosides may also be prepared by enzyme-catalyzed condensation reactions utilizing a glycosyl fluoride. Thus 6-0-a-maltosylcyclodextrins were prepared enzymically from a-maltosyl fluoride (obtained from the corresponding heptaacetate by Zemplen deacetylation) and cyclodex-trins. " ... 

C13-0108. The first step of glucose metabolism is an enzyme-catalyzed condensation reaction between phosphoric acid and the CH2 OH hydroxyl on glucose. Draw the structure of this glucose phosphate. 

Scheme 2.13. Amine-Catalyzed Condensation Reactions of the Knoevenagel Type...

While the Olin efforts on the production of high-temperature polymers were centered mainly on FeCl3-catalyzed condensation reactions, subsequent efforts from Union Carbide, Inc. utilized an aminosilane route and a ureidosilane route (Fig. 3) to synthesize carboranylenesiloxane polymers of high molecular weights. Using the former route, the polymer was produced by a condensation reaction between a carborane-disilanol and bis(dimethylamino)dimethylsilane, during which an expulsion of a... 

The original FeCl3-catalyzed condensation reaction strategy has been exploited recently by Patel and co-workers for the synthesis of poly(m-carborane-siloxane) rubbers (103) (Fig. 63) in the reactions between dimethoxy-m-carborane terminated monomers and dichlorodimethylsilane.131 They have also synthesized similar polymers... 

Certain catalytic modes have been well exploited in flame retardant systems, namely the dehydrating action of compounds which yield strong acids under flaming or smoldering conditions. Friedel-Crafts and other acid catalyzed condensation reactions have been exploited to increase char. These mechanisms don t work very well for polymers of mainly hydrocarbon character. Are there other modes of catalysis which might work better ... 

Base catalyzed condensation reactions of esters and ketones have an additional factor of importance in determining the product, and this is the fact that the overall reaction, as well as the intermediate steps, is highly reversible. The final product may be rate or equilibrium determined, and in the latter case the result may depend on the relative acidity of the various possible products. In a highly basic medium the product will be partly in the form of a salt and the stability of the salt is then a product-determining factor. Failure of a condensation to take place may be due either to an insufficiently high concentration of carbanions or to the instability of the product. The reactions of ethyl isobutyrate will illustrate both points.419... 

M. Yamada, J. Sun, Y. Suda, and T. Nakaya, Synthesis of fluorenebisphenoxy derivatives by acid-sulfur compound catalyzed condensation reaction, Chem. Lett., 28 1055-1056, 1998. 

M. S. Silver, S. L. T. James, Surprising Consequence of the Tendency of Pepsin to Catalyze Condensation Reaction of Small Peptides , Biochim. Biophys. Acta 1983, 743, 13-22. 

Because of their ring-chain tautomeric character, tetrahydro-l,3-oxazines can be used as aldehyde sources in acid-catalyzed condensation reactions involving the carbon transfer via the open forms. This approach is especially advantageous in those cases where the aldehydes required for the condensations are unstable or difficult to access <2003EJ03025>. 

Isowa, Y., Ohmori, M., Ichikawa, T., Mori, K., Nonaka, Y. andKihara, K. (1979) The thermolysin-catalyzed condensation reactions of N-substituted aspartic and glutamic acids with phenylalanine alkyl esters. Tetrahedron Lett., 28, 2611-2612. 

Position 5 is also the most reactive in the nitration of guaiacyl propane units containing a free phenolic hydroxyl group and in analogous model compounds (7). By contrast, acid-catalyzed condensation reactions appear to proceed more rapidly at position 6 (15) and in this respect resemble the protodedeuteration reactions. 

K. Oyama, K. Kihara, and Y. Nonaka, On the mechanism of the action of thermolysin kinetic study of the thermolysin-catalyzed condensation reaction of N-benzyloxy-carbonyl-t-aspartic add with L-phenyl-alanine methyl ester, J. Chem. Soc. Perkin Trans. 2 1981a, 356-360. 

Similar oxygen stabilized ethylene dications were proposed in several types of superacid-catalyzed condensation reactions involving 1,2-dicarbonyl compounds. For example, 2,3-butanedione condenses in high yield with benzene and the superelectrophile (35) is considered to be the key intermediate, because the monoprotonated species (78) is not sufficiently electrophilic to react with benzene (eq 17).35 Several biologically important a-ketoacids were also found to generate superelectrophiles... 

Chenevert R, Dasser M (2000) Chemoenzymatic synthesis of the microbial elicitor (—)-syringolide via a fructose 1,6-diphosphate aldolase-catalyzed condensation reaction. J Org Chem 65 4529 1531... 

Scheme 21 Rhenium-catalyzed condensation reaction of furfural with glycerol to obtain the corresponding 1,3-dioxolane and 1,3-dioxane...

In 2007, Wiles et al. (2007c) demonstrated the ability to employ solid-supported catalysts in series within an EOF-based microreactor. As summarized in Scheme 39, the model reaction sequence involved combining a previously investigated acid-catalyzed deprotection with a base-catalyzed condensation reaction to enable the synthesis of a,(3-unsaturated carbonyl compounds from dimethyl acetals. 

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