Pyrazine carbonation

The precise numerical values of the calculated electron densities are unimportant, as the most important feature is the relative electron density thus, the electron density at the pyrazine carbon atom is similar to that at an a-position in pyridine and this is manifest in the comparable reactivities of these positions in the two rings. In the case of quinoxaline, electron densities at N-1 and C-2 are proportionately lower, with the highest electron density appearing at position 5(8), which is in line with the observation that electrophilic substitution occurs at this position. 

The NMR spectrum of 26 showed well-dispersed signals of 76 protons, which included seven methyl singlets and two methyl doublets. The 13C NMR spectrum revealed a ketonic carbonyl carbon (5 221.2), two acetal/ hemiacetal carbons (8 108.9,119.8), four oxygenated quaternary carbons, four oxygenated methines, and an oxygenated methylene carbon. In addition to the pyrazine carbons, two sp2 carbon signals resonating at 8 121.2... 

The mechanism leading to thiophene formation is presumed to involve dissociation of one sulfur atom of a dithiolene ehelate followed by sulfur attack on a pyrazine carbon, cyclization and oxidation to form a thiophene ring (A in Scheme 2.21). When the dithiolene-forming reaction in Scheme 2.20 employed a pterinyl alkyne, no tris-pterinyldithiolene complex could be isolated. However, pterinyl dithiolene formation was implied by the isolation of a pterin thiophene whose identity was eonfirmed by X-ray crystallographic determination (B in Scheme 2.21). This observation was an early confirmation of the correctness of the proposed pterin-dithiolene in Moco prior to any available X-ray protein structures. These thiophene products were reminiscent of Moco oxidation products 1 and the natural metabolite of Moeo, urothione 2 (see Figure 2.2), studied by Rajagopalan (C in Seheme 2.21). 

Bia-[4-methoiy-phenyl]-pyrazin-carbon" s6uie-(2)-amid 26, 200. 

For heavy molecules with very small rotational state spacing, this limit on AJ puts severe upper limits on the amount of energy that can be taken up in the rotations of a heavy molecule during a collision. Despite these limitations, P(E, E ) distributions have been obtained by inverting data of the type described here for values of AE in the range -1500 cm > AE > -8000 cnD for the two donor molecules pyrazine and hexafluorobenzene with carbon dioxide as a bath acceptor molecule [15,16]. Figure C3.3.11 shows these experimentally derived... 

Figure C3.3.11. The energy transfer probability distribution function P(E, E ) (see figure C3.3.2) for two molecules, pyrazine and hexafluorobenzene, excited at 248 nm, arising from collisions with carbon dioxide...

Figure C3.3.12. The energy-transfer-probability-distribution function P(E, E ) (see figure C3.3.2 and figure C3.3.11) for two molecules, pyrazine and hexafluorobenzene, excited at 248 nm, arising from collisions with carbon dioxide molecules. Both collisions that leave the carbon dioxide bath molecule in its ground vibrationless state, OO O, and those that excite the 00 1 vibrational state (2349 cm ), have been included in computing this probability. The spikes in the distribution arise from excitation of the carbon dioxide bath 00 1 vibrational mode.

Michaels C A, Mullin A S, Park J, Chou J Z and Flynn G W 1998 The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide excitation of the infrared inactive (10°0), (02°0), and (02 0) bath vibrational modes J. Chem. Phys. 108 2744-55... 

Substitution of two carbon atoms of a benzene ring by tervalent nitrogen atoms may occur in three ways, giving rise to pyridazines (see Chapter 2.12), the pyrimidines (see Chapter 2.13) and the pyrazines, with the nitrogen atoms occupying a 1,2-, 1,3- or 1,4-disposition respectively. 

Structural parameters and interatomic distances derived from electron diffraction (7) (77JST(42)l2i) and X-ray diffraction (8) studies (76AX(B)3178) provide unequivocal evidence that pyrazine is planar with >2a symmetry. There is an increased localization of electron density in the carbon-nitrogen bonds, with carbon-carbon bonds being similar in length to those in benzene. ... 

The electron density at nitrogen in phenazine is intermediate between those of pyrazine and quinoxaline and the highest electron density on the carbon atoms of the benzene rings is at C-1 (with positions 4, 6 and 9 being equivalent). ... 

Af-Oxidation of pyrazines appears to result in increased shielding of the a and a carbon resonances by 6-11 p.p.m., whereas the /3 and /3 carbon atoms are deshielded by 3-4 p.p.m., a trend similar to that observed with substituted pyridines. These results have been qualitatively explained in terms of resonance polar effects (80OMR(l3)l72). 

Phenacylpyridinium bromide (155) with aqueous sodium carbonate yields the chloroform soluble zwitterion (156) which, with dimethyl acetylenedicarboxylate in the presence of palladium on charcoal, cyclized to the indolizine (157) in ca. 20% yield. In a similar way the pyrazine (158) gave a mixture of (159) and (160) through loss of the benzoyl group. The last compound was also ob-... 

Alkyl pyrazines lose RCN from their molecular ions when the alkyl group is attached to the carbon adjacent to the nitrogen atom. 

It has been discovered that direct chlorination of pyrazines can be accomplished and this has also been used to make candidate drugs. For example, when 2-methylpyrazine (120) is heated with chlorine in carbon tetrachloride, a mixture of the 3-chloro (121) and the 6-chloro derivatives result. After separation, 121 is heated with piperidine to give modaline (122), an antidepressant. 6... 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

This may suggest that fully conjugated charge centers are an important aspect in directing nucleophilic attack to the terminal carbon. Thus, the superacid promoted reactions of the olefinic pyrazines (and related systems) may be viewed as the superelectrophilic version of Michael addition. 

Electron-spin resonance (e.s.r.) spectra with characteristic hyperfine structure have been recorded during the initial stages of the Maillard reaction between various sugar and amino compounds. The products responsible for the spectra appear to be IV, Af -disubstituted pyrazine radical cations. The pyrazine derivatives are assumed to be formed by the bimolecular condensation of two- and three-carbon enaminol compo-... 

Derivatives of pyrido[2,3-f>]pyrazine were prepared from pyridyl-substituted AAs. Reductive cyclization afforded 115 (79CZ387), whereas cyclization with potassium carbonate gave the dione 116 (87TL6363). 

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