Pyrrole compounds

The disadvantages associated with the Clemmensen reduction of carbonyl compounds (see 3 above), viz., (a) the production of small amounts of carbinols and unsaturated compounds as by-products, (h) the poor results obtained with many compounds of high molecular weight, (c) the non-appUcability to furan and pyrrole compounds (owing to their sensitivity to acids), and (d) the sensitivity to steric hindrance, are absent in the modified Wolff-Kishner reduction. 

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

A further example of an azo coupling reaction with an activated methylene compound (12.91), followed by ring closure to give a pyridazine derivative (12.92) in good yield (66%) was decribed by Gewald and Hain (1984). The reductive treatments of 12.92 give the pyrrole compounds 12.93 and 12.94 in 70% yield (Scheme 12-45). 

Hansa yellow org chem Group of organic azo pigments with strong tinting power, but poor opacity in paints used where nontoxIcIty Is Important. han-sa yel o ) Hantzsch synthesis org chem The reaction whereby a pyrrole compound is formed when a p-ketoester, chloroacetone, and a primary amine condense. hansh, sin-tha-sas ... 

In addition, in fuel compositions or lubricating oil compositions, poly(isobutyl) N-substituted pyrrole compounds are particularly useful as a detergent-dispersants. The additives can be synthesized via a carbocationic polymerization (5). 

Long-range coupling has also been observed in other pyrrole compounds.134,458-460... 

Thus, with a general simple synthetic route to pyrrolyl indoles, a whole class of pyrrole compounds, completely inaccessible not long ago, has been worked out. 

Heated with phosphorus pentasulpliido it yields the corresponding thiophene compound, while tlio action of alcoholic ammonia solution forms a pyrrol compound ... 

Indole (2) undergoes electrophilic substitution preferentially at the b(C3)-position whereas pyrrole (1) reacts predominantly at the a(C2)-position [15]. The positional selectivity in these five-membered ring systems is well explained by the stability of the Wheland intermediates for electrophilic substitution. The intermediate cations from 3 (for indole, 2) and a (for pyrrole, 1) are the more stabilized. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure [15]. However, calculations of the frontier electron population for indole and pyrrole show that the HOMO of indole exhibits high electron density at the C3 while the HOMO of pyrrole is high at the C2 position [25-28] (Scheme 3). 

Rhazinilam 3 is an axially chiral phenyl-pyrrole compound which was first isolated from Melodinus australis (F. Muell.) Pierre [53]. It has also been found in other Apocynaceae such as Rhazya stricta Decaisne [54,55], Aspidosperma quebracho-bianco Schlecht. [56,57], Leuconitis eugenifolia A. DC [58,59] and Kopsia singapurensis [60]. More recently, (-)-rhazinilam was isolated from intergeneric somatic hybrid cell cultures of two Apocynaceae, Rauvolfia serpentina Benth. Ex Kurz and Rhazia stricta Decaisne [61]. 

Chemistry (CHEC-I), chemical literature references prior to 1980 are included where necessary. An attempt has been made to classify the compounds into categories containing pyridine, pyrrole, and pyran and thiopyran rings. Generally, a large volume of chemical literature is available for the pyridine and pyrrole compounds, but for most of the pyran and thiopyran compounds much less information is available. This is noted in the text where appropriate. 

D. C. Hodgkin, Fortschr. Chem. Org. Naturstoffe 15, 167-220 (1958). Other naturally occurring pyrrole compounds ... 

A further approach to the synthesis of pyrroles other than that shown in Scheme 47 involves formation of l-metalla-5-aza-l,3,5-trienes 123 (M = W) by insertion of an alkyne into the M=C bond of an (imino)carbene complex 121, leading to chain extension by two carbon atoms ( alkyne route to pyrroles). Compounds 123 cyclize spontaneously to 2//-pyrrole complexes 124, from which ///-pyrroles 122 are obtained by disengagement of the metal unit (Scheme 48).l2d 2H-Pyrrole complexes of type 124 have been characterized by X-ray analysis, and 2/Z-pyrroles could be eliminated from such complexes if substituents other than hydrogen are attached to C2 of the compound.136125 137 l2al2b... 

At 0°C, with HBr 4-bromo-4,5-dihydro-2-(2-pyrrolyl)furanium bromides 284 are seen in the NMR spectra, resulting from the addition of HBr to the protonated furan moiety (Scheme 63) <1998MI30>. Thus, it is demonstrated that under certain conditions pyrrole compounds can undergo electrophilic addition like nonaromatic unsaturated systems. 

Catalytic multicomponent synthesis of highly substituted pyrroles has been described. A one-pot reaction uses DBU with the commercially available thiazolium salt 513 to produce the necessary nucleophilic zwitterionic catalyst in situ, which promotes a conjugate addition of acylsilanes (sila-Stetter) and unsaturated ketones to generate 1,4-dicarbonyl compounds in situ. Subsequent addition of various amines promotes a Paal-Knorr reaction, affording the desired polysubstituted pyrrole compounds in a one-pot process in moderate to high yields (Scheme 129) <2004OL2465>. Microwave heating dramatically reduced the reaction time (from 16 h to 30 min), but offered no improvement in yields. 

Indole.— The second compound above, a condensed benzene and pyrrole compound, is known as indole and is a very important compound both physiologically and in its relation to indigo. While indigo itself is not a simple condensed hetero-cyclic compound it belongs here in our discussion because of its relation to indole. The constitution of indole is proven by several synthetic relationships. 

C-8 position of A-ring pyrrole compounds bearing the simplified DNA-binding groups. Bioorg. Med. Chem., 8, 381-391. 

Pyrroles are common products in the reaction of different lipid oxidation products with primary amino groups of amines, amino acid, and protein (Zamora et al., 2000). For example, when squid microsomes were oxidized with iron and ascorbate, TEARS increased simultaneously with the -value (yellowness) and pyrrole compounds, concomitantly with a decrease in free amines. Off-color formation in squid muscle could be due to the nonenzymatic browning reactions occurring between aldehydic lipid oxidation products and the amines on phospholipids head groups... 

A pyridine compound was found to storage degrade the Shale-I fuel faster than a pyrrole compound (Table IV, expt. 9, 11). 

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