A highly efficient PASE approach to a new class of polycyclic purine derivatives has been proposed. The strategy includes a consecutive reduction, auto-aromatization, and heterocyclization of the initial nitrobenzimidazopyrimidines obtained by a three-component condensation. It was shown that reduction of nitrobenzimidazopyrimidines by metals in acidic media was more efficient than heterogeneous hydrogenation. Novel derivatives of benz[4,5]imidazo[1,2-a]purines were obtained in good yields and the proposed structure was confirmed by X-ray crystal structure analysis. The obtained convergent benzimidazopurines combine two relevant medicinal chemistry scaffolds – benzimidazole and purine.
The
nitration of azolo[1,5-
a
]pyrimidin-7-amines
with several nitration agents (such as acetic nitric anhydride, nitronium
tetrafluoroborate, and a mixture of concentrated nitric acid and sulfuric
acid) has been investigated. It has been shown that, depending on
the conditions, the nitration of pyrazolopyrimidin-7-amines bearing
electron-withdrawing groups in the pyrazole ring leads to nitration
products in the pyrimidine and/or pyrazole ring. The nitration of
triazolo[1,5-
a
]pyrimidin-7-amines with “nitrating
mixture” has been optimized, thus allowing us to obtain a series
of 6-nitro[1,2,4]triazolo[1,5-
a
]pyrimidin-7-amines,
followed by their reduction into the corresponding [1,2,4]triazolo[1,5-
a
]pyrimidin-6,7-diamines (yields 86–89%). The latter
have been subjected to heterocyclization by a variety of electrophilic
compounds (such as CS
2
, glyoxal, triethyl orthoformate)
with the formation of five- or six-membered annulated cycles.
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