Imidazo -1,2,3,5-tetrazin-4 -ones

Thus, the recognition that nitrosoimidazoles and phenyl methylcarbazates can participate in an efficient condensation to give 2,8-dihydro-imidazo[5,l- 7][l,2,3,5]tetrazine-l-one derivatives 9 led to the elaboration of a new reaction pathway to this biologically active substance <2002J(P 1)1877> (Scheme 14). 

Similarly, it has been shown that diazoimidazoles 15 react with isocyanates to give imidazo-[5,l-t/ -l,2,3,5-tetrazin-4(3//)-ones 16 (Method A). The reverse process, fragmentation to form the isocyanate and 15 from the imidazo[5,l-r/]-l,2,3,5-tetrazin-4(3/i )-ones 16, can be achieved by heating in non-nucleophilic solvents such as acetonitrile or acetic acid (see Section I.4.2.2.).41 Since compounds 10 and 16 show strong antitumor activity, research in this area has increased enormously.6,31-40,46-50... 

An alternative procedure (Method B) for the synthesis of imidazo[5,l-reacted with isocyanates to yield the ureas 17 which cyclize by reaction with nitrous acid.36,47,48... 

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