Pyrazinium salts addition

In the secondary addition of methanol to the monomethoxy adducts 42-44, their reactivities vary to the greater extent. It has been established by 1H-NMR spectroscopy that diaddition of sodium methoxide in methanol-d4 occurs at 20°C with the 1,4-diazinium salts 36e, 37a,c,f, and 40d (Scheme 40) (86KGS1380). Monosubstituted pyrazinium salts 36c,d as well as the pteri-dinium cations 40a,b do not produce any detectable quantities of the dimethoxy adducts 45. 

Scheme 50 Addition of dialkyl phosphonates 1-ethyl pyrazinium salts...

Acyloxy- and acylthiopyrazines are convenient acylation reagents for benzylamine, aniline, pyrrolidine, and related compounds (see Section 6.03.8.3). The acylthio compounds are also useful for acylation of alcohols and phenols. Pyrazinyl alkyl sulfoxides (see Section 6.03.8.4) give aldehydes and ketones by treatment with TFAA <91H(32)937>. The pyrazinium chlorochromate salt (181) is formed by addition of chromium(VI) oxide to a solution of pyrazine in hydrochloric acid, and it has proved to be more effective for the oxidation of aliphatic and allyl alcohols to the aldehydes than pcc <83H(20)2029>. 

The data on the Sn reactions of phosphorus-centered nucleophiles with electro-phihc nitroarenes and azines are scarcely available in the literature [11, 159] Although a number of o -adducts derived from the addition of P-nucleophiles to isoquinoline [140], phthalazine [160], 4,7-phenanthroline [161, 162], A -alkyl-pyrazinium, and quinoxalinium salts [163] have been isolated and identified by NMR and X-ray crystallography, no attempts to convert these o -adducts into the corresponding Sn products have been done. For instance, Af-ethylpyrazinium tetrafluoroborate reacts with dialkyl and diaryl phosphonates under very mild conditicHis (room temperature, acetonitrile) to give stable dialkyl 3-phenyl-5,6-dicyano-l-ethyl-l,2-dihydropyrazin-2-yl phosphonates in good yields (Scheme 50) [163],... 

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