Conversion into Pyrazinethiones

This conversion is often done indirectly via an halogenopyrazine although direct thiation of pyrazinones has usually been successful when Lawesson s reagent (43) or good quality phosphorus pentasulfide has been employed. The following examples indicate typical conditions used and yields to be expected  

3-Phenyl-2( 1//)-pyrazinonc (45, X = O) gave 3-phenyl-2(l//)-pyrazincthione (45, X = S) (P2S5, pyridine, reflux, 2 h 65% the 5-phenyl isomer was made similarly.1033 

Irrespective of the type of reagent or the conditions used, alkylation of a tautomeric 2(l//)-pyrazinone usually gives an N-alkylated pyrazinone, sometimes accompanied by a smaller amount of the isomeric alkoxypyrazine. Occasionally, the alkoxypyrazine may predominate when a diazoalkane or trialkyloxonium tertafluo-roborate is used, when the steric and/or electronic factors associated with the reagent or substrate are favorable, or when the substrate s ring is partially reduced. 

The following alkylations illustrate the results to be expected from various types of tautomeric pyrazinones and a variety of reagents and conditions. The examples are grouped according to the type of substrate and the given percentages represent isolate yields except when stated otherwise. 

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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... 

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