Pyrazines, reviews

Author(s) (Total References Cited) Year Published Reduced Pyrazines Reviewed Refs. 

The number of simple alkylated pyrazines and the food products with which they are associated are enormous and a few examples are listed in Table 5. Fuller reviews on this subject are available (73MI21400). 

The final chapter by Istvan Hermecz (Chinoin, Ltd., Budapest, Hungary) deals with bicyclic systems containing one ring junction nitrogen and one heteroatom and their benzologs, i.e. pyrido-oxazines, pyrido-thiazines, pyrido-pyridazines, pyrido-pyrazines, pyrido-pyrimidines and their analogs. Much of this material has not been reviewed for forty years, during which time immense advances have occurred. 

Substituted pyrimidine N-oxides such as 891 are converted analogously into their corresponding 4-substituted 2-cyano pyrimidines 892 and 4-substituted 6-cya-no pyrimidines 893 [18]. Likewise 2,4-substituted pyrimidine N-oxides 894 afford the 2,4-substituted 6-cyano pyrimidines 895 whereas the 2,6-dimethylpyrimidine-N-oxide 896 gives the 2,6-dimethyl-4-cyanopyrimidine 897 [18, 19] (Scheme 7.6). The 4,5-disubstituted pyridine N-oxides 898 are converted into 2-cyano-4,5-disubsti-tuted pyrimidines 899 and 4,5-disubstituted-6-cyano pyrimidines 900 [19] (Scheme 7.6). Whereas with most of the 4,5-substituents in 898 the 6-cyano pyrimidines 900 are formed nearly exclusively, combination of a 4-methoxy substituent with a 5-methoxy, 5-phenyl, 5-methyl, or 5-halo substituent gives rise to the exclusive formation of the 2-cyanopyrimidines 899 [19] (Scheme 7.6). The chemistry of pyrimidine N-oxides has been reviewed [20]. In the pyrazine series, 3-aminopyrazine N-ox-ide 901 affords, with TCS 14, NaCN, and triethylamine in DMF, 3-amino-2-cyano-pyrazine 902 in 80% yield and 5% amidine 903 [21, 22] which is apparently formed by reaction of the amino group in 902 with DMF in the presence of TCS 14 [23] (Scheme 7.7) (cf. also Section 4.2.2). Other 3-substituted pyrazine N-oxides react with 18 under a variety of conditions, e.g. in the presence of ZnBr2 [22]. 

Undheim and Benneche reviewed the Pd-catalyzed reactions of pyrimidines, among other ji-deficient azaheterocycles including pyridines, quinolines and pyrazines, in 1990 [1] and 1995 [2], A review by Kalinin also contains some early examples in which C—C formation on the pyrimidine ring is accomplished using Pd-catalyzed reactions [3]. In this chapter, we will systematically survey the palladium chemistry involving pyrimidines. 

The Neber rearrangement has been used as a valuable synthetic tool to introduce an a-amino group relative to a ketone and it has been used as a key step in the synthesis of a large array of heterocycles, including imidazoles, oxazoles, isoquinolines and pyrazines and has been reviewed long ago °° . ... 

Aminotriazoles which are appropriately substituted at the C(5)-position are important intermediates for the synthesis of 8-azapurines. These reactions have been reviewed <86AHC(39)ll7>. The pharmaceutically useful acyclonucleosides bearing 1,2,3-triazolines and 8-azapurines have been synthesized <888879). 4,5-Diaminotriazoles react with 1,2-dicarbonyl reagents to give 1,2,3-triazolo[4,5- )]pyrazines. 4,5-Diamino-2-phenyltriazole and sulfur monochloride afford the triazolo[4,5-c][l,2,5]thiadiazole (855) <86AHC(40)129>. The synthesis of triazolopyridines from triazoles has been described in a review <83AHC(34)79>. For further applications of substituted triazoles in preparations of complex heterocycles, see Section 4.01.4. 

Efforts in the 1990s toward the lithiation/trapping sequence for pyrazines and quinoxalines have been reviewed <2001T4489, B-2002MI2>. 

Bioactivity and pharmaceutical use for naturally occurring as well as synthetic pyrazines (Section 8.03.12) have been reviewed <2006CLY959>. 

Structural and spectroscopic studies of pyrazino[2,3- ]pyrazine derivates were discussed in CHEC(1984) <1984CHEC(3)329>, and more recent data were reported in CHEC-II(1996) <1996CHEC-II(7)737>. Theoretical studies of the parent molecule, as for pyridazino[4,5- ]pyridazine, lead to the description of the nonbonding orbitals on nitrogen as perturbed naphthalene cr-orbitals, with contributions from cr -orbitals <1999PCA3089>. Infrared and Raman data for the parent have been reviewed and reassigned based on theoretical calculations <2000PCA7386>. 

Raw potato possesses little aroma. Approximately 50 compounds have been reported to contribute to raw potato aroma. Raw potatoes have a high content of LOX, which catalyses the oxidation of unsaturated fatty acids into volatile degradation products (Scheme 7.2) [187]. These reactions occur as the cells are disrupted, e.g. during peeling or cutting. Freshly cut, raw potatoes contain ( ,Z)-2,4-decadienal, ( ,Z)-2,6-nonadienal, ( )-2-octenal and hexanal, which are all products of LOX-initiated reactions of unsaturated fatty acids [188,189]. It is reported that two compounds represent typical potato aroma in raw potato methional and ( ,Z)-2,6-nonadienal [189]. Other important volatiles in raw potatoes produced via the LOX pathway are l-penten-3-one, heptanal, 2-pen-tyl furan, 1-pentanol and ( , )-2,4-heptadienal [189]. Pyrazines such as 3-iso-propyl-2-methoxypyrazine could be responsible for the earthy aroma of potato [35]. Some of the most important character-impact compounds of raw potatoes are summarised in Table 7.8. Aroma compounds from cooked, fried and baked potatoes have previously been reviewed [35]. 

In this section, we present the application of the PT to problems in the time domain, when fused with the theory of pseudomodes [47,49,50], The result of this approach is the EMDE method [40 3] it is a computationally powerful tool for studying multidimensional quantum dynamics, including the effects of external electromagnetic fields of arbitrary coupling strength on the system under study. Below, we review the EMDE methodology and present its application to the IC process in 24-mode pyrazine, as well as to the quantum dynamics of the three-dimensional strong-held dissociation of the //+ molecular ion. 

In this review, we have discussed the Feshbach-Lowdin PT as a tool for studying multidimensional quantum dynamics of (molecular) systems. The central element in this approach is the emergence of overlapping resonances through the application of the PT on the Hilbert space of the system under study, and the possibility that such resonances ultimately interfere. The TOR, which is the result of this approach, provides a fruitful method to understand and conceptually link diverse physical phenomena and processes. We have tried to demonstrate this by discussing various examples, as FIT and ORIT, the suppression of spontaneous decay in atoms and molecules, and the CC of IC in pyrazine and / -carotene, as well as of IVR in the OCS molecule. 

Early publications on [VO(/3-diketonato)2] have been reviewed.355 More recently, complexes with benzoyl m-nitroacetanilide, benzoyl acetanilide545 and l,l -(l,3-phenylene)-bis(butane-1,3-dione546 have been synthesized. Other [VO(/S-dik)2] adducts have been isolated, for example [VO(acac)2] adducts with a series of pyridine N-oxides547 and several pyridine carboxamides, 48 and [VO(bzac)2] adducts with pyridine, methylamine, isoquinoline and 4-picoline.549 Equilibrium constants of 1 1 and 2 1 adducts of pyrazine with [VO(tfacac)2] have been determined (equation 38).550 In the 2 1 complex, the pyrazine bridge between two equatorial sites of adjacent vanadium atoms promotes a weak exchange interaction. The nitroxide radical 2,2,6,6-tetramethylpiperidinyl N-oxide also forms an adduct with [VO(hfacac)2] in which there is a strong interaction between the electrons on the metal and nitroxide.551... 

The chemistry of the pyrrolopyridazines <73HC(27)761>, the pyrrolo[6]pyrazines (79HC(35)348) and the pyrroloquinoxalines (79HC(35)598> has been reviewed. 

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