Nitrogen, heterocyclic compounds pyrrole

As discussed in Chapter 6, nitro compounds are converted into amines, oximes, or carbonyl compounds. They serve as usefid starting materials for the preparation of various heterocyclic compounds. Especially, five-membered nitrogen heterocycles, such as pyrroles, indoles, ind pyrrolidines, are frequently prepared from nitro compounds. Syntheses of heterocyclic compounds using nitro compounds are described partially in Chapters 4, 6 and 9. This chapter focuses on synthesis of hetero-aromadcs fmainly pyrroles ind indolesi ind saturated nitrogen heterocycles such as pyrrolidines ind their derivadves. 

An efficient synthesis of rigid tricyclic (5 5 5) nitrogen heterocycles 64 has been achieved via sequential and tandem Ugi/intramolecular Diels-Alder (IMDA) cycloaddition of pyrrole derivatives <2004JOC1207> and the trienes 477 were prepared by the acylaton of amines 475 with the anhydride 476. The amines 475 were in turn prepared starting from pyrrole-2-carbaldehyde. The triene 477 on heating in toluene at 80 °C for 15 h underwent the IMDA to afford the tricyclic compound 64 as a single diastereomer in quantitative yield. The sterically bulky N-substitutent on the triene 477 promoted cycloaddition under milder condition at 65 °C in toluene to provide the tricyclic compound 64 in quantitative yield (Scheme 108). 

In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two electrons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp hybridized, and one sp orbital on each is bonded to hydrogen. To complete the six 7i-electron system the non-hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of electrons, and the compound cannot function as a base without losing its aromatic character. 

Pyrrole A nitrogen-containing, heterocyclic compound found at low concentrations in crude. Pyrrole can readily polymerize in the presence of light to form dark-brown compounds. Pyrrole derivatives, such as indole, have been identified as sources which can lead to darkening of fuel color and eventual deposit formation. 

Pyrroles, indoles, pyridines, quinolines, and pyrazines are examples of N-hetero-cycles that are produced as fragrance and flavor substances. Thiazoles and dithiazines are examples of nitrogen- and sulfur-containing heterocycles. These heterocyclic compounds are mainly used in aroma compositions, exceptions are indoles and quinolines, which are important fragrance substances. 

Since proline already contains a pyrrolidine ring it provides a potential source of nitrogen heterocyclics in the MaiUard reaction, and a number of proline-con-taining model systems have been examined. Tressl et al. [32] identified more than 120 proline-specific compounds in the reaction of proline or hydroxypro-line with various sugars. These include pyrrolines, pyrroles, pyridines, indolines, pyrrolizines and azepines, but relatively few of the compounds have been identified among food volatiles. 

Paal-Knorr synthesis It is a useful and straightforward method for the synthesis of five-membered heterocyclic compounds, e.g. pyrrole, furan and thiophene. However, necessary precursors, e.g. dicarbonyl compounds, are not readily available. Ammonia, primary amines, hydroxylamines or hydrazines are used as the nitrogen component for the synthesis of pyrrole. 

When pyrroles, triazoles and other nitrogen containing heterocyclic compounds react with dichlorocarbene, no addition to their carbon-carbon double bonds has been observed. In some cases the formation of cyclopropane adducts has been suspected however, alternate ways to explain the reaction products cannot be excluded [228]. 

Kinetic results concerning catalytic hydrogenative destruction of heterocyclic nitrogen compounds have been published by Cox and Berg.21 In their operating condition (370°C, 17 atm, space velocity 0.5-20 h 1, which does not affect the rate of HDN, wt% N < 2.5%), they found that most of the five membered ring compounds (pyrrole derivatives and indole) follow a second-order law with respect to the amine pressure, while a first-order law is shown for six membered ring compounds (pyridine and derivatives). 

Food, flavors consist of numerous compounds, none of which alone is characteristic of specific food. Classes of compounds which emcompass food flavors are - hydrocarbons (aliphatic, ali-cyclic, aromatic) carbonyls (aldehydes, ketones) carboxylic acids, esters, imides, anhydrides alcohols, phenols, ethers alkylamines, alkylimines aliphatic sulfur compounds (thiols, mono-, di- and tri-sulfides) nitrogen heterocyclics (pyrroles, pyrazines, pyridines) sulfur heterocylics (thiophenes, thiazoles, trithiolane, thialidine) and oxygen-heterocyclics (lactone, pyrone, furan). Discussion will be limited to striking developments in heterocyclics. 

A heterocyclic compound is one that has an atom other than carbon as one of the ring atoms. The compound with a five-membered ring that has two double bonds and a nitrogen atom is called pyrrole. 

The nitrogen in fuels consists of complex, mostly heterocyclic compounds. In petroleum crudes, these include pyrroles, indoles, isoquinolines, acridines, and porphyrins. During refining most of these concentrate in the heavy resin and asphaltene fractions, which might suggest their relatively late release in the... 

Heterocyclic compounds containing a nitrogen atom commonly undergo N-alkylation or C-alkylation. N-Methyl pyrrole can be prepared by interaction of methyl iodide with potassium pyrrole (40%). N-Carbethoxy pyrrole is made from chloroformic ester and potassium pyrrole. The C-alkylation of pyrroles has been discussed. 3-Alkylindoles are made by the alkylation and decarboxylation of indole-2-carboxylic acid. The conditions for alkylation of pyrrolidine are analogous to those employed for the alkylation of a secondary amine. Thus, pyrrolidine on treatment with n-butyl bromide and potassium hydroxide in boiling benzene is con-... 

In another study, ammonia fixation of N-labeled ammonium hydroxide with Suwannee River fiilvic acid, IHHS peat and leonardite humic acid were examined by solution NMR with the application of INEPT and DEPT pulse sequences.(23) Similar reaction of ammonia with all three samples is reported. Most of the nitrogen incorporated seems to be in the form of indole and pyrrole followed by pyridine, pyrazine, amide and aminohydroquinone nitrogen. The authors also suggest a possible reaction mechanism to explain the formation of the heterocyclic compounds identified in the spectra. They also claimed that these results need to be substantiated through further work with model compounds and experiments with the reaction conditions, i.e., in which phenols will undergo oxidation to quinones when reacted with ammonia. 

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