Pyrazine, 1,4-dihydro-, structure

Derivatives of 1,2-, 1,4-, 2,3-, and 2,5-dihydropyrazine are known, but the structures of these compounds are not easy to establish because of the tendency of dihydropyrazines to isomerize, dimerize, and oxidize. Dihydro structures are stabilized by groups such as alkoxycarbonyl and phenyl 1,4-dihydro-2,3,5,6-tetraethoxycarbonyl-pyrazine and 2,3-dihydro-5,6-diphenylpyrazine are relatively stable compounds. 

These compounds are formed by addition of lithium alkyls to alkylpyrazines, followed by hydrolysis (see Section V,A). They readily oxidize to pyrazines on exposure to air. Some early examples of 1,4-dihydropyrazines have recently been shown to have 1,2-dihydro structures.384 ... 

Thieno[3,4-d]oxazole-3a(4H)-carboxylic acid, dihydro-2-methyl-synthesis, 6, 1020 Thieno[2,3-d Joxazoles synthesis, 6, 990 Thieno[3,2-g]pteridine structure, 3, 284 lH-Thieno[3,4-c]pyran-2-ones synthesis, 4, 1032 Thienopyrazines synthesis, 4, 1022-1024 Thieno[2,3-6]pyrazines, 4, 1023 electrophilic substitution, 4, 1024 Thieno[3,4-6]pyrazines, 4, 1024 Thieno[3,4-c]pyrazole, 4,6-dihydro-3-hydroxy-carbamates... 

Similar structures appear to be intermediates in the photolysis of dihydro-pyrazines (Scheme 16), - and are also formed from the base treatment of certain iV-methyl-2i/-imidazolium salts (Section IV,D,2). ... 

Pyrazino[2,3-fc]pyrazines365 and pyrazino[2,3-h]quinoxalines366 (230) are reduced similarly to pyrazines and quinoxalines to the 1,4- and 5,10-dihydro derivatives, respectively [Eq. (127)], with which they form reversible systems. 5,10-Dihydropyrazinoquinoxaline gives an ill-defined wave near the background discharge, but the structure of the product is unknown. 

One such epidioxypiperazinedione has been reduced to a regular pyrazine. Thus l,4-dibenzyl-2,3-dioxa-5,7-diazabicyclo[2.2.2]octane-6,8-dione (86) underwent reduction by sodium borohydride in ethanol at 20°C during 1 h to afford 3,6-dibenzyl-3,6-dihydroxy-3,6-dihydro-2,5( l//, 4//)-pyrazincdione (86a) in 65% yield, confirmed in structure by dehydration to 3,6-dibenzylidene-3,6-dihydro-2,5(1 H, 4//)-pyrazincdionc (86b).5... 

Since modern structural determination depends heavily upon H and C NMR analysis, information from IR and UV spectroscopies is less abundant than formerly. However, these spectroscopies are still useful to study photophysical properties, chemical behavior, and electronic orientation of the substituents in heteroaromatics. The pH dependence of both electronic and IR spectra of pyrazine has been examined to obtained p. values and information about the solute-solvent interactions, respectively <84JST(i 14)367). Electronic absorption taken in conjunction with fluorescence spectra indicate that 1,4-dihydro-2,3-quinoxalinedione exists in its diketo form irrespective of the nature of the solvent, by comparison with the spectra of the parent quinoxaline and 1,4-dimethyl-2,3-quinoxalinedione <87MI 603-01). New measurements of the singlet-triplet absorption spectra of pyrazine and pyrazine-d 4 at longer pathlengths and higher vapor pressures have been recorded <93aci537). 

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