Pyrazine quinones

The general term oxypyrazine is used here to include derivatives such as the cy-cloamidic tautomeric pyrazinones (1), the alcoholic hydroxyalkylpyrazines (2), the etherial alkoxypyrazines (3-5), the cycloamidic nontautomeric pyrazinones (6), and pyrazine A-oxides (7, 8) in addition, related types like diketopiperazines, acy-loxypyrazines, pyrazine quinones, and endoperoxypyrazines are covered as appropriate. Some brief ancillary information on trivial names, natural occurrence, and biological activities of pyrazines (mainly oxy derivatives) is collected in a final Appendix section. 

Different azanthraquinones 390-392 were prepared from 3-amino-4-imino-4//-pyrido[l,2-a]pyrazines 373 with 1,4-quinones in one pot reactions via [4-1-2] cycloaddition and the subsequent ring transformation (Scheme 9) (97T5455). 

In Eq. (6), DEexp (cf. ref. 2) is given in kcal/mole and DEth in /3 units. Data for non-alternant hydrocarbons and quinones have not been included. The data for pyridine2 and quinoline2,33 obey the equation satisfactorily. On the other hand, the theory predicts too high values for the diazines (pyridazine, pyrimidine, and pyrazine) which have recently been studied experimentally.33... 

Stepwise pyrazine ring-formation using 5-nitropyrimidine was applied to the synthesis of 4a-hydroxytetrahydrobiopterin (95), which is an interesting intermediate in the metabolism of aromatic amino acids (see Sect. 5.2). As illustrated in Scheme 18, the 5-aminopyrimidine 97 prepared from chloroni-tropyrimidine 96 by nucleophilic substitution followed by catalytic hydrogenation was oxidized under acidic conditions to o-quinone derivative 98. 

In another study, ammonia fixation of N-labeled ammonium hydroxide with Suwannee River fiilvic acid, IHHS peat and leonardite humic acid were examined by solution NMR with the application of INEPT and DEPT pulse sequences.(23) Similar reaction of ammonia with all three samples is reported. Most of the nitrogen incorporated seems to be in the form of indole and pyrrole followed by pyridine, pyrazine, amide and aminohydroquinone nitrogen. The authors also suggest a possible reaction mechanism to explain the formation of the heterocyclic compounds identified in the spectra. They also claimed that these results need to be substantiated through further work with model compounds and experiments with the reaction conditions, i.e., in which phenols will undergo oxidation to quinones when reacted with ammonia. 

Although the o-quinone (30.10) condenses with a 1,2-diaminobenzene (30.11) as expected, some o-quinones do not always react at these functional groups for example, in methanol, the two otherwise unsubstituted reactants (with an excess of quinone) at ambient temperature give, on treatment with acetic anhydride-pyridine, the phenazine diacetate (30.12), but in diethyl ether, the unsubstituted phenazine Is formed [2305]. Heating a 1,5-diketone with ammonium acetate leads to the formation of a fused pyrazine in high yield [2819]. 

S02,403 on the photoreduction of quinones,404 on acetyl and acetyl peroxide radicals in gas-phase reactions,405 on fluoroalkyl radicals in solution,408 on the photochemistry of 2,2-dimethoxy-2-phenylacetophenone,407 on pyridine-3,5-dicarboxylic acid,408 on photolytically reduced pyrazine,409 410 on the dimerization... 

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