Acylating powerful

The rate of acetylation of 0-(hydrox3Tnethyl)cellulose (and other hydroxy compounds) by mixtures of carboxylic acids and their anhydrides has been found to increase greatly in the presence of trifluoroacetic acid. The acceleration is very much less with mono- and tri-chloroacetic acids, presumably because they form unsymmetrical anhydrides which are less effective acylating agents than acyl trifluoroacetates. The exceptional acylating power of the latter anhydrides is shown by their use in the synthesis of alkyl aryl ketones from polyalkylbenzenes, phenyl ethers, furan, and thiophene under mild conditions. The principle has been extended to include acids... 

The biological activity of P-lactam antibiotics has been often correlated with the chemical reactivity of the amide bond [11]. Nevertheless, very few studies have dealt with modifications of the P-lactam acylating power , by replacing the amide-carbonyl with unsaturated groups of variable electrophilic character. 

Other imides that mimic more closely the penicillins were synthesised [176] (Scheme 102). The good acylating power of 329 and 330 was demonstrated by their very fast reaction with hydroxylamine to give hydroxamic acids. Com-... 

The acylating power of acylpyridinium salts has been mentioned already (p. 195). It is probable that in the formation of phenol esters, arylation of the 2-position in the pyridinium salt is an essential step ... 

The use, in this reaction, of a system containing an active methylene group, but devoid of acylating power (c.g. 3-phenyl-A -isoxazolin-5-one), was also studied. Attack takes place again at the 2-position of the 2-alkyl-A -thiazolines, and terminates with the production of thiols corresponding to (144). The configuration of these products about the alkylidene group is not known with certainty. ... 

The order of reactivity of carboxylic acid derivatives toward nucleophilic acyl sub stitution can be explained on the basis of the electron donating properties of sub stituent X The greater the electron donating powers of X the slower the rate... 

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

The O acylation of phenols with carboxylic acid anhydrides can be conveniently catalyzed m either of two ways One method involves converting the acid anhydride to a more powerful acylatmg agent by protonation of one of its carbonyl oxygens Addi tion of a few drops of sulfuric acid is usually sufficient... 

Alkylamino-5-nitrosopyrimidines undergo acylation in their isonitroso forms to yield acyloxyimino derivatives, such as the 5-benzoyloxyimino-6-methylimino-5,6-dihy-dropyrimidine-2,4(l//,3//)-dione (277) (70CB900), which are powerful acylating agents in their own right. 

Oxaziridines are powerful oxidizing agents. Free halogen is formed from hydrobromic acid (B-67MI50800). Reduction by iodide in acidic media generally yields a carbonyl compound, an amine and two equivalents of iodine from an oxaziridine (1). With 2-alkyl-, 2-acyl and with N-unsubstituted oxaziridines the reaction proceeds practically quantitatively and has been used in characterization. Owing to fast competing reactions, iodide reduction of 2-aryloxaziridines does not proceed quantitatively but may serve as a hint to their presence. 

Another well-established process to generate fluoro ketones proceeds via acylation ofenolates [68, 69] or activated methylene compounds [70 71] as well as by Claisen type condensation reactions [72] Because of the electrophilic power of the acylating agents, there is usually no need tor a catalyst [68]... 

From semiamidals, the correspondmg tnfluoromethyl substituted N-acyl mines [25, 28], 1,3-diambutadienes [27], N-sulfonyl mines [5, 29], N sulfinyl mines [30], and N-phosphoryl mines [31] can be obtamed in high yields on reaction with powerful dehydratmg realms like POC -pyndine or tnfluoroacetic anhydnde-pyndme [2, 5]... 

Step 2 Structurally, O-acylisoureas resemble carboxylic acid anhydrides and are powerful acylating agents. In the reaction s second stage the amine adds to the car bonyl group of the O-acylisourea to give a tetrahedral intermediate. 

Enzymatic desymmetrization of prochiral or meso-alcohols to yield enantiopure building blocks is a powerful tool in the synthesis of natural products. For example, a synthesis ofconagenin, an immunomodulator isolated from a Streptomyces, involved two enzymatic desymmetrizations [149]. The syn-syn triad of the add moiety was prepared via a stereoselective acylation of a meso-diol, whereas the amine fragment was obtained by the PLE-catalyzed hydrolysis of a prochiral malonate (Figure 6.56). 

The power ofbiocatalysts for the production of chiral compounds can be elevated to a second stage when proper use of the process leads to a larger number of different enantioenriched products from the same reaction. Some efforts have been made in this direction because the enzyme can be selective to either nucleophile or acyl donor. The elegancy of the reaction is increased when the process is carried out for the resolution of both. Scheme 7.20 depicts this possibility [38]. 

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