2-Amino-3-mercaptopyrazine

By using thiourea in 2N sulfuric acid, Cullen and Harrison (905) were able to convert 2-chloro-3-methylpyrazine at reflux for 30 minutes to 2-mercapto-3-methylpyrazine (55%), and 3-chloro-2,5-dimethylpyrazine similarly treated gave 3-mercapto-2,5-dimethylpyrazine together with another higher molecular weight compound, postulated as either (60) or (61). 2-Amino-3-chloropyrazine refluxed with thiourea in alcohol, and the thiouronium salt refluxed with aqueous sodium hydroxide, produced 2-amino-3-mercaptopyrazine (535). 2-Chloropyrazine failed to react with thiourea (821) under the mild conditions that converted 2-chloro-quinoxaline to 2-mercaptoquinoxaline (1014). 

Hydroxypyrazine with phosphorus pentasulfide in refluxing pyridine for 45 minutes was readily converted to 2-mercaptopyrazine (46%) (55) (its 1-methyl analogue was prepared similarly) (821,1100) and 5-mercapto-2,3-diphenylpyrazine was prepared likewise (834, 1008). 2-Amino-3-mercaptopyrazine was prepared from 2-amino-3-hydroxypyrazine and phosphorus pentasulflde in refluxing 0-picoline(llOl). 

Mercaptopyrazine in aqueous hydrochloric or acetic acids was chlorinated to give the sulfonyl chloride which with excess liquid ammonia gave 2-sulfamoyl-pyrazine (1006, 1153). Direct sulfonation of the pyrazine ring has never been reported (819) but 2-sulfopyrazine (17) has been prepared from 2-chloropyrazine and aqueous sodium sulfite at 150° (819), and from 2-fluoropyrazine and aqueous sodium sulfite at reflux for 2 hours (882, 884). 2-Amino-3-mercaptopyrazine in aqueous sodium hydroxide with concentrated ammonia and sodium hypochlorite has been shown to give 2-amino-3-sulfamoylpyrazine (1101). 

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