Pyrrol-2-ones

Judging by the predominance of a-substitution in electrophilic attack of pyrrole, one would say that of the cations (13) and (14), the former would greatly predominate. In support of this there is the argument that the charge in (13) is more delocalized (3 canonical... 

Although V-protected 2-lithiopyrroles are readily generated and many types are known [6, 26], these intermediates have not generally been employed to synthesize halogenated pyrroles. One exception is the synthesis of the two natural seabird hexahalogenated bipyrroles 15 and 17,... 

There are two principal routes to pyrroles. One is called the Paal-Knorr synthesis, in which pyrroles are formed by the interaction of 1,4-dicarbonyl compounds and ammonia. No intermediates have ever been isolated, so the mechanism shown in Scheme 6.13 is speculative. 

Color test for pyrroles. One drop of an aqueous, ethereal, or alkaline test solution is mixed on a spot plate with one or two drops of a 5% solution of p-dimethylamino-benzaldehyde. If pyrrole or a substance capable of forming pyrrole is present, a violet color will appear. The aldehyde condenses with a tautomeric form of pyrrole (3) to give (4), which is converted by hydrogen ion into the colored quinonoid ion... 

Another class of indole syntheses involve annelations of pyrroles. One new example of this type of indole synthesis involved the electrocyelization of a 2-alkenyl-3-allenylpyrrole intermediate <04H(63)1765>. This was exploited for the synthesis of indole-4,7-quinones. 

Figure 5.9 Photograph from a microscope of a composite membrane prepared from NEOSEPTA CM-1 and pyrrole. One surface of a ferric ion form cation exchange membrane, NEOSEPTA CM-1, was contacted with an aqueous 0.745 N pyrrole solution for 4 h.

The most simple method of obtaining a porphyrin on the gram scale is to reflux a dilute benzaldehyde-pyrrole mixture in propionic acid (141°C) for 30 minutes, cool, and filter. A 20% yield of crystalline porphyrin is easy to achieve (Scheme 6.3.1) (Lindsey et al., 1987, 1994 Prathapan et al., 1993). Even pentamers have been obtained by such a reaction in one step from a porphyrin benzaldehyde building block and pyrrole. One just has to carry out the primary formation of the porphyrinogen at a relatively low concentration (10 M) and under nonoxidative conditions, which allow the rearrangement of undesired polymers. Up to 50% of the colorless porphinogens are obtained by acid-catalyzed condensation of pyrrole and aromatic aldehydes under nitrogen. 

An interesting aspect of thiophene chemistry is the differences in reactivity between thiophene and its more aromatic isostere, benzene, and its less aromatic isosteres, furan and pyrrole. One interesting facet of this contrast is that metal cation-exchanged clay catalyzed Diels-Alder reactions work for fiiran and pyrrole to produce reaction with a,p-unsaturated carbonyl compounds the thiophene examples do not react <94JCS(P1)761>. 

Several methods for making pyrroles involve in situ generation of azomethine ylides and anions. In the presence of electrophilic acetylenes these can be trapped and converted to pyrroles. One source of azomethine ylides are A -(a-trimethylsilylmethyl) derivatives of imidates and thioimidates. Azomethine ylides are generated by desilylation with fluoride ion. The salts are obtained by N-alkylation of imidate esters <83TL4303> or S-alkylation of thioacetamides <86JOCi997>. The alkoxy or alkylthio groups are eliminated after cycloaddition. Electrophilic alkenes such as dimethyl maleate, dimethyl fumarate, maleonitrile and methyl acrylate give A -pyrrolenines (Scheme 80). 

In the last years, only some examples of multicomponent pyrrole syntheses have been published, but those are highly interesting in terms of reaction conditions and substitution patterns of the obtained pyrroles. One example is the conversion of a-iodoketones 594 in a Hantzsch pyrrole synthesis under high-speed vibration milling conditions (HSVM) and CAN catalysis, with silver nitrate as scavenger for the produced hydroiodic acid (Scheme 13.149) [269]. 

Knowledge of the porphyrins has been reached by the convergence of work on chlorophylls, chiefly by the Willstatter school, and on the blood pigments, by Hans Fischer and important contributions have been made by Stoll, Conant, Marchlewski, Kuhn, and Kiister. The properties of the porphyrins as a class suggests a ring structure composed of four stable units. On oxidation of a porphyrin, these units are obtained in the form of di-substituted pyrroles, one substituent of which is always a methyl group. 

If one heats acetone and pyrrole in the presence of catalytic amounts of acid, so-called acetone pyrrole is formed in over 80%i yield. This colorless, macrocyclic compound contains four pyrrole units which are connected by dimethylmethylene bridges, ft is formed by electrophilic-a-substitution of pyrrole by acetone, acid-catalyzed oligomerization, and spontaneous. 

Pd(II) salts promote the carbonylation of organomercury compounds. Reaction of phenylmercury chloride and PdCh under CO pressure affords benzophenone (429)[387]. Both esters and ketones are obtained by the carbonylation of furylmercury(Il) chloride in alcohol[388]. Although the yields are not satisfactory, esters are obtained by the carbonylation of aryl- and alkylmercuryfll) chlorides[389,390]. One-pot catalytic carbonylation of thiophene, furan, and pyrrole (430) takes place at the 2-position via mercuration and transmetallation by the use of PdCb, Hg(N03), and CuCl2[391]. 

These two methods are closely related but differ in the point of initial attachment of the substituent from which the carbocyclic indole ring is constructed. One strategy for building up 2-substituted pyrroles capable of... 

This category corresponds to the construction of the carbocyclic ring by 2 + 4 cycloaddition with pyrrole-2,3-quinodimethane intermediates. Such reactions can be particularly useful in the synthesis of 5,6-disubstituted indoles. Although there are a few cases where a pyrrolequinodimethane intermediate is generated, the most useful procedures involve more stable surrogates. Both 1,5-di-hydropyrano[3,4-b]pyrrol-5(lf/)-ones[l] and l,6-dihyropyrano[4,3-b]pyrrol-6-(in)-ones[2] can serve as pyrrole-2,3-quinodimethane equivalents. The adducts undergo elimination of CO2. 

Ethyl-4-methyl-l-(phenylsulfonyl)indole 4-Ethyl-7-methyl-l-phenylsulfonyl-l,5-dihydropyrano-[3,4-b]pyrrol-5-one phenyl vinyl sulfoxide 60 [3]... 

Methoxycarbonyl)-7-methyl-l-(phenylsulfonyl)- 5-trimethylsilylindole 7-Methyl-l-phenylsulfonyi-l,5-dihydropyrano[3,4-b]pyrrol-5-one ethyl 3-trimethyIsilyIpropynoate 53 [1]... 

Besides being useful precursors to pyrroles pyridine-2-ones -4-ones, -4-thiones. and -4-imines 4-alkylidene-dihydropyridines thiophenes 1,2,4-triazoles thiapyrane-2-thiones, isoquinoline-3-ones isoben-zothiophenes and 4-mercaptoimidazolium hydroxide inner salts, mesoionic thiazoles are potentially useful in the construction of molecules with herbicidic (39). central nerve stimulating, and antiinflammatory properties (40,41). Application in dye synthesis has likewise been reported (42). 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

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