2- pyrazine with thiourea

The conversion of halogenopyrazines into pyrazinethiones is usually done either with sodium hydrogen sulfide solution or by initial treatment with thiourea and subsequent hydrolysis of the intermediate isothiouronium salt (frequently unisolated). A third method, involving treatment of the halogeno substrate with thiosulfate, has proven promising in some heterocyclic series but not so far in the pyrazines 2-chloropyrazine did so give 2(1 //)-pyrazinethionc but only in 20% yield.1358... 

Dicyanopyrazine with sodium methoxide in methanol with ammonia at reflux gave 5,7-diimino-6//-pyrrolo[3,4-i)]pyrazine (65) (1442) and 2,3-dicyano-5,6-dimethyl(and 5,6-diphenyl)pyrazine with hydrazine by the procedure of Patel and Castle (1339) gave 2,3-dimethyl(and 2,3-diphenyl)-5,8-diaminopyrazino[2,3-rfJ-pyridazine(s) (66) (1435). Fusion of 2-chloro-3-cyano-5,6-diphenylpyrazine with urea and thiourea was reported to give 4-amino-2-hydroxy(and mercapto)-6,7-diphenylpteridine (848). 

Pyrazine- and quinoxalinethiones have been synthesized by treatment of halogeno compounds with sodium or potassium hydrogen sulfide, sodium polysulfide, phosphorus pentasulfide, or thiourea . Pyrazinethiones are directly prepared by treating 2(177)-pyrazinones with Lawesson s reagent (2,4-bis(4-methoxyphenyl)-l,3,2,4-dithiadiphosphetane 2,4-disulfide) <83H(20)797>. They are also obtained by treatment of (4-methoxybenzylthio)pyrazines with mer-cury(II) acetate and successive reduction <93JCS(P1)15>. 

The isothiazolo[4,5-6 4, 5 -e]pyrazine system 51 has been obtained by cyclizing the isothiazole 50 <97MC97> and the reaction of 2,3-dichloro-5,8-dimethoxyquinoxaline with thiourea yields the pentacyclic system 52 <96JCS(P1)2443>. 

High density brine completion fluids also often require the use of corrosion inhibitors (8,9). Blends of thioglycolates and thiourea alkyl, alkenyl, or alkynyl phosphonium salts thiocyanate salts mercaptoacetic acid and its salts and the reaction products of pyridine or pyrazine derivatives with dicarboxylic acid monoanhydrides have been used as high density brine corrosion inhibitors. 

The bromine of l-bromo-3-methylimidazo[l,5-a]pyrazine could not be displaced by a variety of nucleophiles. This contrasts with the reactivity of the 8-chloro substituent of (219 R = H or Me), prepared by the usual chlorination of the imidazo[l,5-a]pyrazin-8(7i/)-ones (218), which was readily displaced by thiourea to give the thiones (220). 

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