Preparations of 2-hydroxy- and 2-mercapto-1H-imidazo[4,5-blpyrazines from pyrazine-2,3-diamines are described. Methylation of 1H-imidazo[4,5-b]pyrazine-2(3H)-thione with methyl iodide and diazomethane gave some S, N1, N3 and N4 methyl derivatives; and 1H-imidazo[4,5-b]pyrazin- 2(3H)-one with diazomethane gave products which involved O, N1, N3 and N4 methylation. These products showed slight activity as amplifiers of phleomycin but the benzothiazoles (10b) and (10c) (as hydrobromides) showed three and four star activity respectively.
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.