Carboxyl anions, acyl transfer

A similar long arm is apparent in biotin (7.9), again enzyme bound through a lysine residue. The coenzyme (7.9) assists in the carboxylation of acetyl-CoA 1.8) with carbon dioxide yielding malonyl-CoA 1.14). Exchange of both acetyl-CoA and malonyl-CoA occurs with acyl carrier proteins (AGP) having free thiol groupings. Condensation then occurs between acetyl-S-ACP and malonyl-S-ACP with simultaneous decarboxylation the carboxylate anion is transferred into the new bond (Scheme 1.2) [6]. Subsequent steps involve reduction, dehydration and double-bond saturation. They... 

The direct reaction of indoles with carbon dioxide at atmospheric pressure to give the 3-carboxylated derivatives has been reported. The reaction occurs in dimethyl for-mamide in the presence of a large excess in lithium t-butoxide whose function is to deprotonate the indole. Although the N-H proton is more acidic, the C(3) anion yields the thermodynamically more stable carboxylated product. Acyl transfer from amides, such as A-(4-nitrophenyl)acetamide to benzene may occur in triflic acid. The mechanism is likely to involve diprotonation of the amide, as shown in (39), and dissociation of the C-N bond to give an acyl cation. The methodology allows the formation of aromatic ketones by inter- or intra-molecular reaction. The acylation of arenes may also be... 

The intermediate N-acylpyridinium salt is highly stabilized by the electron donating ability of the dimethylamino group. The increased stability of the N-acylpyridinium ion has been postulated to lead to increased separation of the ion pair resulting in an easier attack by the nucleophile with general base catalysis provided by the loosely bound carboxylate anion. Dialkylamino-pyridines have been shown to be excellent catalysts for acylation (of amines, alcohols, phenols, enolates), tritylation, silylation, lactonization, phosphonylation, and carbomylation and as transfer agents of cyano, arylsulfonyl, and arylsulfinyl groups (lj-3 ). 

The mechanism of amide hydrolysis in base has the usual two steps of the general mechanism for nucleophilic acyl substitution—addition of the nucleophUe followed by loss of a leaving group— plus an additional proton transfer. The initially formed carboxylic acid reacts further under basic conditions to form the resonance-stabilized carboxylate anion, and this drives the reaction to completion. Mechanism 22.10 is written for a 1° amide. 

This is because carboxylic acids are converted to their corresponding carboxylate anions under these conditions, and these anions are incapable of acyl transfer to alcohols. 

Acid chlorides are noxious chemicals formed in the reaction of a carboxylic acid and reagents such as PCI3 or SOCI2. Acid anhydrides are formed by the combination of an acid chloride and a carboxylate anion. Acid anhydrides can react with an alcohol to produce an ester and a carboxylic acid. This is an example of an acyl group transfer reaction. 

STEPS 1-2 OF FIGURE 23.3 ACYL ENZYME FORMATION The first nucleophilic acyl substitution step—reaction of the triacylglycerol with the active-site serine to give an acyl enzyme— begins with deprotonation of the serine alcohol by histidine to form the more strongly nucleophilic alkoxide ion. This proton transfer is facilitated by a nearby side-chain carboxylate anion of aspartic... 

At high pH the possibility of O N acyl-transfer to imidazole anion occurs via a nucleophilic mechanism. Subsequent hydrolysis of the acyl-imidazole intermediate takes place because the rate of hydrolysis exceeds the thermodynamically unfavorable N O transfer. This model does have some analogy with serine proteases mode of action although only the neutral pH mechanism is relevant. The introduction of a carboxylate group, however, provides a more accurate assessment of the charge-relay system ... 

Dimsyl anion 88 reacts with esters of aromatic carboxylic acids and aliphatic acids which do not have a readily transferable proton, to give /5-ketosulfoxides114,133,138-141. There are not many cases in which acyl chlorides were used142,143. However, the reaction... 

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