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3.4 Result and Discussion
It was found that the cyclocondensation of 2-alkylaminopyrimidines with αbromoacetophenones
at 80 °C first led to stable 2-hydroxy-2,3-dihydroimidazo[1,2a]pyrimidinium
bromides. The bicyclic nature of the structures clearly followed from
1 H NMR data, indicating the doublet of doublets of the methylene fragments, as well
as from 13 C NMR spectra.
The 2-hydroxy-2,3-dihydroimidazo[1,2-a]pyrimidinium salts were obtained in good
yields, and only for sterically hindered 2-alkylaminopyrimidines the yields slightly
decreased. Dehydration of the 2-hydroxy-2,3-dihydroimidazo[1,2-a]pyrimidinium
salts was successfully achieved by short (15 min) heating in polyphosphoric acid
(PPA) at 150 °C. The corresponding imidazo[1,2-a]pyrimidinium salts were isolated
and characterized as perchlorates, and in most cases the yields were almost
quantitative. The structures of aromatic salts were completely consistent with their 1 H,
and 13 C NMR data.
The cleavage step was performed under microwave irradiation at 100 °C, using 7
equivalents of hydrazine hydrate. Resulting 2-amino-1H-imidazoles were obtained in
high yields. Using this protocol we have synthesized library of substituted 2aminoimidazole
(Table-1)
No.
Compound
Code
H 2N
N
N
R 1
R 2
R1 R2 Yield % m.p. o C
1 BS-042 cyclododecane H 53 228-230
2 BS-045 cyclooctane H 53 152-154
3 BS-048 cycloheptane H 54 180-182
4 BS-078 cyclohexane H 64 153-155