Pyrazines iV-oxidation

Ring nitrogens in pyrazines and the benzo derivatives react with electrophiles to form quaternary ammonium species such as iV-alkylpyrazinium salts and pyrazine iV-oxides. N-Alkylation has generally been performed by treatment with a reactive alkyl iodide. The N-1 nitrogen in 2(l//)-pyrazinone 5 is methylated using chloro(chloromethyl)dimethyl-silane followed by desilylation with cesium fluoride to yield l-methyl-2(l//)-pyrazinone <2000TL4933>. 

Monocyclic oxadiazepines have not been isolated in reactions of this type but it has been proposed that the 1,2,6-system (508) is an intermediate in the formation of (509) from the pyrimidine IV-oxide (507) (69TL4899), and that the 1,2,5-systems (511) and (512) are intermediates in the formation of the imidazoles (513) and (514) from the pyrazine IV-oxide (510) (67TL1197). 

With the exception of those halogenopyrazines made by primary synthesis (see Chapters 1 and 2), most chloropyrazines have been made recently by the reaction of tautomeric pyrazinones with a phosphorus chloride or by the reaction of pyrazine iV-oxides with phosphoryl chloride in contrast, most other halogenopyrazines have been made by direct halogenation or by transhalogenation of chloropyrazines. A single interesting example of the conversion of a methoxy- into a chloropyrazine is included at the end of Section 4.1.1. 

Nuclear Halogenopyrazines by Deoxidative Halogenadon of Pyrazine iV-Oxides (H105)... 

Both pyrazine iV-oxides and 2,3-dihydropyrazines rearrange photochemically to imidazoles. Irradiation of the 2,5-disubstituted oxides (187) gives mixtures of the imidazole products (188, 189), probably through the intermediacy of oxaziridine intermediates (Scheme 109). 

This volume summarizes published pyrazine chemistry with emphasis on syntheses, properties, and reactions of pyrazines and pyrazine iV-oxides (Chapters 1-X). Treatment of theoretical aspects is minimal. Although not strictly relevant. Chapter XI is presented as a summary of earlier reviews and more recent literature of reduced pyrazines (including piperazines). The literature recorded in Beilstein to 1929 and Chemical Abstracts through 1978 (Volume 89) has been covered together with selected references to 1980. Whereas every reasonable effort has been made to incorporate most significant material, no attempt has been made to include all relevant data. Tables have been incorporated in the text to extend the range of examples. 

Methylpyridinc 2,6-Dimethylpyridine Isoniazid Cyanoacetate Pyrazine iV-oxide... 

Although pyridazine and pyrazine iV-oxides can be readily prepared by oxidation of the parent heterocycles, pyrimidine iV-oxides are more difficult to obtain in this way but they can conveniently be prepared by ring synthesis. ... 

This methodology has also been applied to the Ar-H arylation of diazine IV-oxides such as pyrazine IV-oxides (eq 20) and pyridazine A -oxides (eq 21). ... 

Pyrazine- iV-oxide Pyrimidine (9) Quinoxaline (10) Dimethyl sulfoxide Thiourea Thioacetamide Dithiooxamide... 

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