Heterocycle, five ring

The heterocyclic five-ring system that surrounds the Mg has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 19-41) the chlorophylls have unusually high molar extinction coefficients (see Box... 

Tautomerism of Heterocycles Five-Membered Rings ith One Heteroatom... 

The diazotization of heteroaromatic amines is basically analogous to that of aromatic amines. Among the five-membered systems the amino-azoles (pyrroles, diazoles, triazoles, tetrazoles, oxazoles, isooxazoles, thia-, selena-, and dithiazoles) have all been diazotized. In general, diazotization in dilute mineral acid is possible, but diazotization in concentrated sulfuric acid (nitrosylsulfuric acid, see Sec. 2.2) or in organic solvents using an ester of nitrous acid (ethyl or isopentyl nitrite) is often preferable. Amino derivatives of aromatic heterocycles without ring nitrogen (furan and thiophene) can also be diazotized. 

Mechanisms involving glycol bond fission have been proposed for the oxidation of vicinal diols, and hydride transfer for other diols in the oxidation of diols by bromine in acid solution.The kinetics of oxidation of some five-ring heterocyclic aldehydes by acidic bromate have been studied. The reaction of phenothiazin-5-ium 3-amino-7-dimethylamino-2-methyl chloride (toluidine blue) with acidic bromate has been studied. Kinetic studies revealed an initial induction period before the rapid consumption of substrate and this is accounted for by a mechanism in which bromide ion is converted into the active bromate and hyperbromous acid during induction and the substrate is converted into the demethylated sulfoxide. 

Oxidation of Aromatic Heterocycles Five-Membered Rings... 

Regioselactive g-metallation of ir-excessive five ring heterocycles is not a novel reaction. Oxazoline and pyridine as well as carboxylate- and carboxamide -substituted heterocycles have been lithiated. From the point of synthetic utility thiophenes have been shown to be useful substrates after careful optimization of reaction conditions furans have been of less utility. 

From this point of view, both the pyranoses and the furanoses may participate in the mutarotation reaction, and both a- and /3-d 1uco-pyranose will have similar mutarotation constants. We may expect a heterocyclic five-membered ring with two adjacent cis hydroxyl groups to have a A of about 1000 units for 0.5 M solutions. In the case of D-glucose it is between 93 and 80 hence, the concentration of a-D-glucofuranose is probably less than 10 per cent. This quantity is sufficient, however, to explain the formation of derivatives of the furanoses. 

Aromaticity indices based on a statistical evaluation of peripheral bond orders have been derived for five-ring heterocycles a value of 50 for 1,3,4-oxadiazole compares with values of 43 and 66 for furan and thiophene respectively <85T1409>. Somewhat in contrast, O—C, C—N, and N—N bond orders of 1.3124, 1.9062, and 1.3348 (MMX, EXE calculation) for 1,3,4-oxadiazole led to the conclusion that the molecule was not aromatic <91H(32)2023>. 

Quinacridone is the trivial name given to the five-ring heterocyclic system exemplified by the linear irani-quinacridone (2.69). The yellow-red to reddish-violet shade pigments based on this ring system show outstanding durability and are used in plastics, in industrial and automotive finishes and in exterior finishes. Like copper phthalocyanines the unsubstituted linear quinacridone exhibits polymorphism and two crystal forms, the reddish violet P-form and the red y-form, are commercially... 

Pyrolysis of some Ai-aiyl five-membered-ring heterocycles causes ring opening and rearrangement to the quinoline ring <99H(51)2377,99JOC3608>. 

Cytosine, thymine, and uracil are pyrimidines along with adenine and guanine they account for the five nucleic acid bases. Pyrimidines are heterocyclic single-ringed compounds based on the structure of pyrimidine. Cytosine, thymine, and uracil, like adenine and guanine, form nucleosides and nucleotides in RNA and DNA. When the bases combine with ribose, a ribo-nucleoside forms and when it attaches to deoxyribose, a deoxyribosenucleoside is formed. Names of the nucleoside are summarized in Table 29.1. These in turn combine with phospho-ryl groups, in a process called phosphorylation, to form their respective nucleotides that form nucleic acids. The nucleotides can be tri, di, and mono phosphate nucleotides similar to the way in which adenine forms ATP, ADP, and AMP. 

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