Other Substituted Pyrroles

The tautomeric equilibrium between 112 and 113 has been studied by Treibs and his associates. Most pyrromethenes take form 

113 unless both rings carry electron-withdrawing substituents in which case structure 112 is predominant. Common cations of type 

Chemical evidence has been advanced for the formulation of p nitrosoindoles as either 115 or 116 (sec reference 119 and references therein), but ultraviolet spectral comparison with both methylated forms clearly indicated that 116 was favored. Infrared spectral data are also considered to support structure 116. ° 

Prototropic Tautomerism of Heteroaromatic Compounds IV. Five-Membered Rings with Two or More Hetero Atoms  

Genetics Foundation, The University of Texas, Austin, Texas 

Tautomerism Involving Only Annular Nitrogen Atoms. .. 28 

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Other Methods. Newer methods for forming pyrrole and related heterocyctic rings iaclude the formatioa of substituted pyrrole 2-carboxylate esters by coadeasatioa of P-dicarboayl compouads with glyciaate esters (25). 

VIII. Other Substituted Furans, Thiophenes, Pyrroles, and TTieir... 

VIII. Other Substituted Furaus, Thiopheues, Pyrroles, and Their Benzo Derivatives... 

Although pyrrole appears to be both an amine and a conjugated diene, its chemical properties are not consistent with either of these structural features. Unlike most other amines, pyrrole is not basic—the pKa of the pyrrolin-ium ion is 0.4 unlike most other conjugated dienes, pyrrole undergoes electrophilic substitution reactions rather than additions. The reason for both these properties, as noted previously in Section 15.5, is that pyrrole has six 77 electrons and is aromatic. Each of the four carbons contributes one... 

As mentioned in CHEC-II(1996) <1996CHEC-II(8)16>, 3//-pyrrolizin-3-one 2 and many other substituted analogues were synthesized by FVP of Meldrum s acid derivatives 189 via the in. ( ////-generated pyrrol-2-ylmethylidene ketenes 190 which cyclize by an intramolecular A-acylation (Scheme 43) <1997J(P1)2195, 2000J(P1)3584>. 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Other examples of N-substituted 3-bromopyrroles that have been successfully derivatized at the 3-position include either 4- or 5-substituted-3-bromo-l-tosylpyrroles 5 (90TL6785 91T7615) 3-bromo-l-tritylpyrrole 6 (91UP1) and the bromo-azafulvene dimer , which leads to 4-substituted pyrrole-2-carboxaldehydes 8 after reaction with electrophiles and hydrolysis (88TL3215). 

Deangelis et al.259 have used the method with other five-membered rings, obtaining good yields (40-70%) with substituted pyrroles 164 and imidazoles... 

HydroxycarbazoIes react with citral to form either pyrano[3,2-a]carbazoles (132) or pyrano[2,3]carbazoles (133), whilst 1-hydroxycarbazoles produce pyrano[2,3-a]-carbazoles (81SC823, 81H(16)1445). Other cyclization reactions involving substituted pyrroles or indoles and aldehydes or ketones in which the substituent becomes part of the new ring system are discussed under the relevant sections relating to the reactivity of the substituent. 

Nitro-substituted pyrroles have usually been prepared by electrophilic substitution on preconstructed pyrrole rings. Like other electrophilic substitutions, nitration gives primarily... 

The mass spectra of pyrrole, four C-alkyl, and two A-alkyl derivatives are to be found in the American Petroleum Institute catalog of mass spectral data.318 Other publications report the most abundant fragment peaks of some N- and C-substituted pyrroles.313,314... 

Thermal addition (105°-142°) of DMAD to other N-substituted pyrroles (19a-d)80 83 gave azabicycloheptadienes (20) in yields of 30-45% lower temperatures sufficed for 19e-h. The reversibility of the addition was demonstrated for 20f, 60% of which after 9 hours at 65° in carbon tetrachloride was converted back into 19f compound 20h, however, was unaffected. The azabicycloheptadiene 20 (R = C02Et) undergoes cycloaddition with 1 mole of 2,4,6-trimethylbenzonitrile oxide at both double bonds to give 26 and 27, and both these products... 

Examples of experimental voltammograms are shown in Figs 6.22-6.24. These are all from our current research on the oxidation of substituted pyrroles [39,42]. Numerous other examples may be found in the literature [1, 2]. The voltammograms shown are recorded with a potentiostat without electronic compensation of the solution resistance in order to illustrate better some of the problems caused by the solution resistance. The peak potentials below are given with a precision of 10 mV, which is typical for values reported in the literature. These values originate from the data files and cannot be determined with this precision directly from the figures reproduced below. 

On the other hand, the specific electron configuration of the nitrogen allows a chemistry comparable to that of the ubiquitous cyclopentadienyl ligand and this offers an opportunity to get within distance of the all-powerful lanthanide alkoxide chemistry. The introduction of unsaturated monoanionic ligands like substituted pyrrols is currently in progress. In the last ten years there has also been a steadily growing interest in polycyclic amides, e.g., porphyrins and phthalocyanines. 

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