Acetylpyrroles

Whereas under the conditions specified above JV-acetylpyrrole, like a typical acetamide, is not detectably hydrolyzed in neutral aqueous medium, the half-life of N-acetylpyrazole is 908 min, and that of A-acetylimidazole is reduced to 41 min for 1-acetyl-1,2,4-triazole and for the isomeric 1-acetyl-1,2,3-triazole, half-lives of 6.4 and 26.6 min, respectively, were observed (for an explanation of the different reactivities of the two pairs of isomers see above). Hydrolysis of TV-acetyltetrazole under the same conditions occurs too rapidly to be measured with conventional spectroscopic techniques. The reaction enthalpy AH was determined for A-acetylimidazole to be — 4.83 kcal/mol for the corresponding 1,2,4-triazolide the value was — 7.29, and for the tetra-zolide — 10.31 kcal/mol. 111... 

Aromatic heterocycles can also be mercurated. For example, iV-acetylpyrrole and Ar- p h e n y 1 s u 1 fo n y I p y no 1 e react with HgCl2/NaOAc to form the monomercurated products 79 and 80, respectively (Equations (30) and (31)). The amide or sulfonamide group provides an oxygen donor atom that coordinates with the incoming mercuric ion, ultimately directing mercuration at the 2-position.100... 

Scheme 19.—Mechanism for Formation of 2-Acetylpyrrole and 2-Methyl-3-pyridinol.

Wilde (9) generated N-acyl mtinchnones for the first time via the acylation and desilylation of 5-siloxyoxazoles. Thus, for example, exposure of 17 to acetyl chloride in the presence of DMAD affords A-acetylpyrrole (19) via A-acetyl-miinchnone 18. Other trapping agents (thiocarbonyls, A-phenylmaleimide) failed and the analogous trimethylsiloxyoxazoles were unusually labile and gave lower yields of pyrroles (13-15%). Ethyl chloroformate was also used in this sequence and gave, for example, pyrroles 20-21. 

For 2-acetylpyrrole, 2-acetylthiophene, and 2-acetylselenophene, the presence in solution of the preferred X,0-cis conformation, planar or nearly planar, is widely accepted (81RCR336). Low-temperature NMR measurements C C) of 2-CO-alkyl derivatives of thiophene have provided evidence (85JCS(P2)1839) that the S,0-trans conformer, if present, is not detectable (<5%), at least in CHF2CI solution. The LIS method at room temperature gave (85JCS(P2)1839) 15% of this conformer in the case of the COMe derivative and 25% for CO-i-Pr. Better agreement between experimental and calculated LIS values was obtained (85JCS(P2)1839) when planar structures were assumed even for the conformers of the 2-pivaloyl derivative. 

In N-formylcarbazole (82, R = H) a second benzene ring does not cause further depression of the energy barrier relative to IV-formylindole (see Table V), whereas a drastic decrease in the N-acetyl derivative is found in comparison with (V-acetylpyrrole. Consistent deviation from planarity of the acetyl group in the ground state of (V-acetylcarbazole may occur (79JCS(P2)1045). 

Pyrroles are found in the volatiles of most heated foods [29], although they have received less attention than some other classes of aroma volatiles. Some pyrroles may contribute desirable aromas, e.g. 2-acetylpyrrole has a caramel-like aroma, and pyrrole-2-carboxaldehyde is sweet and corn-like, but alkylpyrroles and ac-ylpyrroles have been reported to have unfavourable odours [22]. Many more volatile pyrroles have been found in coffee than in other foods [30], and they are common products of amino acid-sugar model systems. Pyrroles are closely related in structure to the furans, and they are probably formed in a related manner from the reaction of a 3-deoxyketose with ammonia or an amino compound followed by dehydration and ring closure (cf Scheme 12.2). 

Acetylpyrrole, Oxoethylpyrrole or Metbyl-pyrrylketone, C6 H7NO. One isomer is listed in Beil 20,165 and another in Beil 21,271,(280) [236]... 

By using the potent dienophile 1,3-diethoxycarbonylallene (201) in the cycloaddition reaction with N-acetylpyrrole, a cycloadduct (202) is formed which on treatment with potassium hydride undergoes a facile reorganization reaction to 4-ethoxycarbonyloxindole... 

Photoisomerization of bis(2-acetyl-l-pyrrolyl)methane yields, as the major product (12%), 5-(2-acetyl-l-pyrrolylmethyl)-2-acetylpyrrole, together with the corresponding... 

Protodeacetylation of 3-acetylpyrroles has also been reported via the dithioketal derivatives (81CC283). 

Predictably, nitrosation of 2-acetylpyrrole and pyrrole-2-carboxylic esters with alkyl nitrites or nitrous acid preferentially yields the relatively stable 4-nitroso derivatives, whilst 2,4-dialkyl- or -diaryl-pyrroles are nitrosated at the 5-position. Further reaction of the dialkyl and diaryl nitrosopyrroles with an excess of alkyl nitrite in the absence of a base can result in the formation of the nitropyrroles, whereas the reaction with nitrous acid converts the nitrosopyrroles into diazopyrroles (B-77MI30502). 

Diethyl penta-2,3-dienedioate reacts with 1-acetylpyrrole to give an oxindole via the base-catalyzed rearrangement of the initially formed [ 4 + 2] cycloadduct (Scheme 63) (77CC582, 78JOC2083). 

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