Synthesis and characterization of 2-aminodihydropyrimidines

Abstract

Guanidine ring compounds of 2-aminodihydropyrimidine derivatives were effectively synthesized by the cyclocondensation of free guanidine derivatives with $\alpha,\beta$ unsaturated carbonyl compounds such as benzalacetophenone, methylacrylate, and methylmethacrylate in good yields under mild conditions. The dihydropyrimidine was so unstable at high temperature and in protic solvents even at low temperature that its yield might be seriously effected by solvent and the reaction temperature. The new derivatives of 2-aminodihydropyrimidine were successfully synthesized in good yields under mild conditions and the structure was determined by spectroscopic methods. The steric effect for the cyclocondensation reaction was observed by the alkyl substituents not only in the guanidine derivatives but also in the $\alpha,\beta$-unsaturated carbonyl compounds. Pure 2-amino-3,4-dihydropyrimidine derivatives were isolated and they were found to be unstable in protic solvents such as water, methanol, and ethanol. Their unstability in protic solvents was monitored by $^1H$ nmr and UV spectra. The preferable condition for the synthesis of 2-aminodihydropyrimidine was to keep the reaction temperature low and to use sterically hindered alcohols such as i-propanol and t-butanol.