Condensed Heteroaromatic Rings

In addition, some Stille adducts have been further manipulated to form condensed heteroaromatic ring systems. The coupling of 4-acetylamino-5-bromopyrimidine 69 and ( )-1-ethoxy-2-(tributylstannyl)ethene resulted in ( )-4-acetylamino-5-(2-ethoxyethenyl)pyrimidine 70, which then cyclized under acidic conditions to furnish pyrrolo[2,3-d]pyrimidine 71. Pyrrolo[3,2-d]pyrimidines were also synthesized in a similar fashion by using 5-acetylamino-4-bromopyrimidine [40]. 

Photochemically induced ring closure may give rise to larger condensed heteroaromatic ring systems. For example, irradiation (>300nm) of imine 53 for 6-8h gave the tetracyclic product 54 in modest yield (30%) (Equation 1) <2006JOC7165>. 

One of the main drawbacks of the intermolecular reaction is that to achieve good yields and selectivity it is frequently necessary to run reactions to low conversion rates or use the heteroaromatic base in vast excess. Such a limitation does not apply when conducting reactions intramolecularly and in recent times much attention has been focussed on such variants. A number of synthetically useful orr/jo-cyclisation and j so-substitution reactions have been uncovered providing new routes condensed heteroaromatic ring systems and substituted pyridines. Reactions can be effected at neutral pH and may occur to each of the carbon centres in the pyridine ring system. As these recent advances have not been summarised previously, much of this review will be devoted to the intramolecular reaction. 

Sakamoto, T., Yasuhara, A., Kondo, Y., Yamanaka, H. Condensed heteroaromatic ring systems. XXIII. A concise synthesis of hippadine. Heterocycles 1993, 36, 2597-2600. 

Kondo,Y, Shiga, F Murata, N., Sakamoto, T, and Yamanaka, H. (1994) Condensed heteroaromatic ring systems. XXIV. Palladium-catalyzed cydization of 2-substituted phenylacetylenes in the presence of carbon monoxide. Tetrahedron, 50, 11803-11812. 

Sakamoto, T, Kondo, Y, Sato, S., and Yamanaka, H. (1996) Condensed heteroaromatic ring systems. Part 24. Synthesis of rigidin, a pyrrolo[2,3-djpyrimidine marine alkaloid. J. Chem. Soc. Perkin Trans. I, 459-464. 

The main chain of these polymers contains, as the principal component, five- or six-membered heteroaromatic rings, ie, imides, which are usually present as condensed aromatic systems, such as with benzene (phthalimides, 3) and naphthalene (naphthalimides, 4) rings. 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

Ya. L. Gol dfarb, I. P. Konyaeva, and V. P. Litvinov, Condensed heteroaromatic systems including the thiophene ring. 27. Synthesis of -bromosubstituted thieno-[2,3-6]thiophene and selenopheno[2,3-6]thiophene. Izv. Akad. Nauk SSSR, Ser. Khim. 1570 (1974). 

V. P. Litvinov, T. V. Shchedrinskaya, P. A. Konstantinov, and Ya. L. Gol dfarb, Condensed heteroaromatic sytems including the thiophene ring. 33. Catalytic liquid-phase oxidation of 3-methyl-substituted thieno[2,3-h]thiophene and thieno[3,2-h]thiophene. Khim. Geterotsikl. Soedin., 492 (1975). 

In condensed heteroaromatic systems with a bridge-head pyrrolic nitrogen atom, tr-electron density is always shifted from the electron-rich six-membered ring (formally contains 7 Tr-electrons) towards the five-membered ring (formally has 6 Tr-electrons). As a result electrophiles are directed to carbon atoms of the latter. Thus, imidazo[ 1,2-a]pyridines (140) unsubstituted at C-3 almost always react with electrophiles at that position. 

The most frequent methods for the preparation of 1,5-diheteropentalene systems involve construction of the second heterocyclic ring onto an existing heterocycle. Three new syntheses of iso-condensed heteroaromatic pyrroles and their derivatives... 

Other members of the cyanine dye family that are readily accessible by condensation strategies are rhodacyanines 12. In general, the rhodacyanine dyes 12 possess two different conjugated systems, a neutral merocyanine and a cationic cyanine moiety, consisting of three heterocyclic components, in which two end heteroaromatic rings, a and y, flank a rhodanine moiety (> (Fig. 5.5). 

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