Heterocyclic studies. Part 40. Synthesis and unusual tautomerism of some dihydro-imidazo- and -pyrimido-[1,2-a]pyrimidines

Abstract

Syntheses of a number of 7-(substituted amino)-2,3-dihydro-5-methyl-6-nitroimidazo[1,2-a]pyrimidine hydrochlorides and 8-(substituted amino)-3,4-dihydro-6-methyl-7-nitro-2H-pyrimido[1,2-a]pyrimidine hydrochlorides are described. Free bases obtained from these compounds exhibit unusual tautomerism. The imidazopyrimidines with a 7-(monosubstituted amino)-group exist as 7-(substituted amino)-1,2,3,5-tetrahydro-5-methylene-6-nitroimidazo[1,2-a]pyrimidines in several solvent systems; those with a 7-(disubstituted amino)-group show solvent-dependent equilibria between the 5-methyl and 1,5-dihydro-5-methylene tautomers. The behaviour of pyrimidopyrimidines closely follows that of analogous imidazopyrimidines. Other closely related imidazo-[1,2-a]-and -[1,2-c]-pyrimidines have normal methyl groups. ¹H N.m.r. spectra of many of the compounds are reported.