Pyrazines special

Unlabelled 2-methoxy-3-alkylpyrazines are commercially available (Pyrazine Specialities, Inc., Atlanta, GA, USA). They can also be synthesized by the method of Murray and Whitfield (1975). The chloro compounds (8) and (9), prepared by the method of Murray and Whitfield (1975), are converted to trideuterated methoxypyrazines (6) and (7) by the action of sodium methoxide- Hj (Fig. 2 Harris et d. 1987 Lacey et d. 1991). 

A special influence on the course of a reaction by a neighboring group is shown in the reactions of 5-acetyl-leuco(iso)alloxazines - with diazomethane. The methylation occurs neither in the pyrimidine nor in the pyrazine ring, but on the hydroxyl group of a newly formed oxazoline ring. For example,... 

Sato et al. carried out detailed studies on the possibilities of transformation of tetrazolo[l,5-tf]pyrazines 54 to 2-aminopyrazines 56 < 1994S931 >. These authors found that the generally used methods for this conversion fail because the starting compound exists in the stable bicyclic form 54, whereas partial formation of the azide valence bond isomer 55 would be necessary for the success of the transformation. Application of special reaction conditions succeeded, however hydrogenation over palladium catalyst in the presence of ammonium hydroxide or treatment with stannous chloride in a mixture of methanol and hydrochloric acid solved this problem. Thus, a great number of derivatives of 54 was reduced to the corresponding 2-aminopyrazine 56 in medium to high yields (45-100%). 

Thus, ketone enolates easily substitute chlorine in position 2 of the electrophilic nucleus of pyrazine (1,4-diazabenzene), and even in the dark, the reaction proceeds via the Sj l mechanism (Carver et al. 1981). It is expected that the introduction of the second chlorine in the ortho position to 4-nitrogen in the electrophilic nucleus of pyrazine promotes the ion-radical pathway even more effectively. However, 2,6-dichloropyrazine in the dark or subjected to light reacts with the same nucleophiles by Sr.,2 and not S nI mechanism (Carver et al. 1983). The authors are of the opinion that two halogens in the pyrazine cycle facilitate the formation of a-complex to the extent that deha-logenation of anion-radicals in solution and a subsequent nucleophilic attack of free pyrazine radical become virtually impossible. Thus, the reaction may either involve or exclude the intermediate a-complex, and only special identification experiments can tell which is the true one. 

A recent communication is based on the nucleophilic substitution of N-silylated iminophosphoranes (Scheme 8), where activated chloro- (12) and nitro heterocycles (13) (triazines, pyrazines, and pyridines) are transformed into monosubstituted iminophosphoranes (14). Of special advantage are the mild reaction conditions and the preferential formation of monosubstituted... 

All discussed Ksem s are related to the separately solvated members of a two step redox system. In solvents of low polarity the charged forms may form ion pairs. Especially prone to this association are anionic redox systems of Type C (RED + OX = 2 SEM ) since the often used gegenions K , Na and Li tend to form ion pairs with the anions. These exhibit special UV/VIS-, NMR- and ESR-spectra as well as g-values Dimeres of the type (SEM M )2 may also be formed, as demonstrated with the anion radicals of pyrazines ° heptafulvalene and tetracyano-quinodimethanes. Corresponding associations are reported for dian-... 

A special case, demonstrating the high versatility of the bis-lactimether method , utilizes the symmetrical derivative obtained from L-leucine, i.e, (2S,5S)-2,5-dihydro-3,6-dimethoxy-2,5-bis(2-methylpropyl)pyrazine (1)14. 

Fig. 17. The emission spectra of pyrazine in benzene at 4.2°K. The upper tracing is fluorescence and the lower one is phosphorescence. Of special interest is the similarity in the activity of totally symmetric vibrations, and the difference in activity for the nontotally symmetric vibrations.

Fig. 18. The phosphorescence of pyrazine in /j-dichlorobenzene at 4.2°K. Of special interest is the progression in the 915 cm-1 (6ls) mode that is not strong in a benzene host.

Cyclotene is the precursor of dihydrocyclopentapyrazines after having reacted with ammonia respectively amino acids to form cyclotene imine. Figure 2 demonstrates not only the isomerization and transformation of this compound but also the formula of bis-dihydro-cyclopentapyrazines as special reaction products. They are formed by condensation of cyclotene-imine and represent a mixture of 4 di-astereomeric compounds. These compounds produce identical mass spectra (Shibamoto et a 1,16). We have carried out corresponding experiments by heating Furaneol with phenylalanine or serine in an autoclave at 180 0 C. As Figure 3 demonstrates some monocyclic pyrazines are formed by reaction of the degradation products from Furaneol. 

Pteridine synthesis from pyrazine precursors are usually applied in such cases where the formation of special derivatives cannot be achieved easily by the pyrimidine approach. Methyl 3-amino-2-pyrazinecarboxylate (362) reacts with thiophosgene to give the corresponding 3-isothiocyanato derivative (363) which cyclizes with primary amines, arylalkylamines, aminoalcohols, amino acids,... 

A lot of experimental data have been reported in CHEC-I. The x-ray crystal structure of pyra-zino[2,3-6]pyrazine and some special spectroscopic investigations were reported. More experimental... 

B—1) Special products such as for instance thiadiazoles, isoxazo-les, pyridazinones, pyrazines, pyrazoles, etc. 

Considering the special bioactive and biocatalytic functions of copper(II) carboxylate complexes,[9] along with the magnetic properties and various coordination fashions of Cu(II) ion,[10-12] we focused our work on the Cu(II) and H4btec system. Based on the layer structure of [Cu2(btec)(H20) -4H20] (1),[13] in which the Cu(ll) atom with bi-pyramidal configuration is coordinated by two monodentate carboxyl O atoms and three aqua molecules, part of the coordinated aqua molecules of Cu(II) is possible to be replaced by some bridging spacers, such as pyrazine, 4,4 -bipyridine (4,4 -bpy) and the related species, and to construct 2-D and 3-D open-frameworks with variable cavities and channels. 

The reactivity of substituents dealt with in this section is limited to those on the heteroaromatic ring in pyrazines and quinoxalines. In Section 6.03.9 the reactions of the benzo-fused rings (regarded as a special type of ring-carbon substituent for the purposes of this review) are covered. 

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