The preparations of a series of thiazolo[4,5-b]pyrazines by ring closure of 3-aminopyrazine-2-thiols with orthoesters, and potassium ethyl xanthate are described. 2-Amino-5-bromopyrazine-3-thiol with potassium ethyl xanthate gave both 6-bromothiazolo[4,5-b]pyrazine-2-thiol and thiazolo[4,5-b]- pyrazine-2,6-dithiol. Various S-alkyl derivatives are reported. As amplifiers of phleomycin, 2-(5,6-dimethylthiazolo[4,5-b]pyrazin-2-yltho)- NN-dimethylethylamine, 2-6- (2-dimethylamino- ethylthio)thiazolo[4,5-b]pyrazin-2-ylthio)-N,N-dimethylethylamine, and N,N-dimethyl-2-(2-methyl- thiazolo[4,5-b]pyrazin-6-ylthio)ethylamine each showed moderate activity.
Derivatives of the thiazolo[4,5-b]pyrazine system are reported including those with 2-phenyl substituents or fused benzene rings (as in thiazolo[4,5-b]quinoxalines) together with strongly basic N,N-dimethylaminoethylthio or N,N-dimethylaminopropylthio side chains. Series of thiazolo[4,5-c]- pyridines, thiazolo[5,4-b]pyridines and quinoxalines are also described. A new process for the preparation of 3-N,N-dimethylaminopropylthio derivatives of heterocycles by reaction of the mercapto compound with 3-chloro-N,N-dimethylpropylamine in ethanolic ammonia has been shown to give more reliable and improved results. Of the compounds examined for amplification of the activity of phleomycin, N,N-dimethyl- 3-(2-methylthiazolo[4,5-pyrazin-6-ylthio)propylamine and 3-[3-(3-N,N-dimethylaminopropylthio)- quinoxalin-2-ylthiol-N,N-dimethylpropylamine were the best, and showed four star activity at 1 mM and 0.5 mM respectively. A 2-phenyl substituent in, or a benzene ring fused to, thiazolo[4,5-b]- pyrazines did not increase amplification. The 2-substituted thiazolo[4,5-c]pyridines showed activity comparable to that of the 2-substituted thiazolo[4,5-blpyrazines whereas that of the thiazolo[5,4-b]pyrazines was lower.