Dihydropyrazines (DHPs), which exhibit DNA strand-breaking activity and other biological activities associated with the generation of radical species, reacted with thiourea to give tetraazaindene (TAI) derivatives, despite thiourea is a well known radical scavenger. The structural determination of the TAIs was carried out and formation mechanism of TAI was investigated.
1,2-diamines, e.g. ethylenediamine and cis-(and trans-)1,2-diaminocyclohexane, caused [4+2] cyclocondensation to give the corresponding dihydropyrazine derivatives (compounds 1-6). They exhibited stronger DNA strand-breakage activity than that of dihydropyrazines, which has already been reported in previous papers.