Amino-pyridines, acylation

More stable iV-acylpyridinium salts can be formed by using 4-(Ar,Ar-dialkyl-amino)pyridines. The salts are less readily hydrolyzed and are effective in the acylation of sterically hindered alcohols (72S619) and in the formation of iV-f-butoxycarbonyl derivatives of a-amino acids in aqueous alkali, for use in peptide synthesis (71CC267). 

A-Aminopyridinium cations can be acylated or sulfonylated (with acid halides) and nitrated (H2S04 - HN03) to give the corresponding -(substituted amino)pyridines (972), often isolated as the imides (973 R = COR, S02R or N02). 

One particular amino-pyridine has a special role as a more effective acylation catalyst than pyridine itself. This is DMAP (DiMethylAminoPyridine) in which the amino group is placed to reinforce the nucleophilic nature of the... 

Instead of the U-4CR, a 3CR takes the place of its secondary reaction when 2-amino-pyridine or pyrazine, oxocompounds and isocyanides form their products, so that its carbonic acid components do not enter this product [124, 125]. In all probability, the intermediate protonated Schiff base reacts with its isocyanides so that, ultimately, the bicyclic 3-aminoimidazole [1,2a] pyridine is cyclized. Some further diversifications can proceed, if secondary amines can be acylized. No other simple way of producing a comparable collection of such a great variety of complex heterocycles can be accomplished [126]. 

If the 2-position bears hydrogen or methyl, ring-opening reactions of 4//-pyrido[3,4-r/] [1,3]oxazin-4-ones with ammonia, hydroxylamine, hydrazine, or aniline to give the 3-(acyl-amino)pyridine-4-carboxamides are directly followed by ring closure to yield the corresponding pyrido[3,4-ri]pyrimidin-4(3//)-ones.463,484... 

In order to understand the effect of these catalysts, it is important to observe that they do not act themselves as enhancers (no effect on light output is observed, when used alone). An explanation is proposed by considering in more detail the reaction steps from diazaquinone (L) to the luminol peroxide intermediate. This reaction involves nucleophilic attack on one of the diazaquinone carbonyls by hydrogen peroxide monoanion. A hydroperoxide species is formed, which can rearrange to the endoperoxide. Either compound is very unstable and collapses to 3-amino-phthalate in its excited state. Perhaps pyridine acylation catalysts could facilitate hydrogen peroxide attack by converting L into a more reactive intermediate. 

Natriumcarboxylate reagieren mit Acylchloriden in waBr. Medium in Gegenwart kataly-tischer Mengen Pyridin, 3-Amino-pyridin oder Pyridin-l-oxid93. Fur die Acyl-Gruppen-Obertragung ist wahrscheinlich ein intermediar auftretendes 1-Acyl-pyridinium-Salz ver-antwortlich. 

Sixty-two nicotine alkaloids have been identified in tobacco. Fifty-four have been identified in tobacco smoke and twenty-one of the ninety-five nicotine alkaloids are found in both tobacco and tobacco smoke. The vast majority of the nicotine alkaloids (fifty-four of sixty-two) contain a pyrrolidine ring connected to a pyridine ring. The most common functionalities associated with the pyrrolidine-containing nicotine alkaloids are alkyl, nitroso, carboxyl, amino, and acyl groups. 

Benzoyl chloride and derivatives acylate 2-amino-4-aryithiazoles in dioxane in yields of 80 to 90% (249, 250). The location of the acyl group on the exocyclic N has been demonstrated by the fact that the benzoyla-tion product is identical to the benzamidothiazole synthesized from benzamide and 2-bromothiazole (251). 3-Indolyl acetic acid chloride (89) acylates 2-aminothiazole in pyridine (Scheme 62) (81). 

Acylation. Reaction conditions employed to acylate an aminophenol (using acetic anhydride in alkaU or pyridine, acetyl chloride and pyridine in toluene, or ketene in ethanol) usually lead to involvement of the amino function. If an excess of reagent is used, however, especially with 2-aminophenol, 0,A/-diacylated products are formed. Aminophenol carboxylates (0-acylated aminophenols) normally are prepared by the reduction of the corresponding nitrophenyl carboxylates, which is of particular importance with the 4-aminophenol derivatives. A migration of the acyl group from the O to the N position is known to occur for some 2- and 4-aminophenol acylated products. Whereas ethyl 4-aminophenyl carbonate is relatively stable in dilute acid, the 2-derivative has been shown to rearrange slowly to give ethyl 2-hydroxyphenyl carbamate [35580-89-3] (26). 

Imidazo[4,5-6]pyridine, 2-acetonyl-3-phenyl-synthesis, 5, 637 Imidazo[4,5-6]pyridine, amino-acylation, 5, 619... 

In their acidity, basicity, and the directive influence exerted on electrophilic substitution reactions in benzenoid nuclei, acylamino groups show properties which are intermediate between those of free amino and hydroxyl groups, and, therefore, it is at first surprising to find that the tautomeric behavior of acylaminopyridines closely resembles that of the aminopyridines instead of being intermediate between that of the amino- and hydroxy-pyridines. The basicities of the acylaminopyridines are, indeed, closer to those of the methoxy-pyridines than to those of the aminopyridines, the position of the tautomeric equilibrium being determined by the fact that the acyl-iminopyridones are strong bases like the iminopyridones and unlike the pyridones themselves. Thus, relative to the conversion of an... 

The presence of the aliphatic amino group complicates the course of the reaction. Thus, the oxidative coupling of 4-ethynyl-1,3-dimethyl-5-aminomethylpyrazole in mild conditions (20°C, CuCl, pyridine, O2) leads to only 20% of butadiyne. However, acylic protection eliminates these complications, and 4-ethynyl-1,3-dimethyl-5-(acetyl)aminomethylpyrazole forms a dehydrodimer in 95% yield (Scheme 66) (86TH1). 

Piperazine NH group of 9-fluoro-10-(l-piperazinyl)-7-oxo-2,3-dihydro-7//-pyrido[l,2,3-<7e]-l,4-benzothiazine-6-carboxylate was reacted with 4-nitrophenylsulfonyl chloride, 2,6-dichloropyrazine, 2,6-dichloropyridine in DMF in the presence of pyridine, and with 4-nitrophenyl isothiocyanate in aqueous acetone in the presence of KOH (01MIP13). A side chain amino group on a 2,3-dihydro-7//-pyrido[l,2,3- fe]-l,4-benzothiazin-7-one skeleton was acylated (OOMIPIO). 

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