Nitrogen-Bridgehead Compounds; 401. Cyclization in Phosphoryl Chloride-Polyphosphoric Acid Mixture

Synthesis, in vitro antiplatelet activity and molecular modelling studies of 10-substituted 2-(1-piperazinyl)pyrimido[1,2-a]benzimidazol-4(10H)-ones

The multistep preparation of the new 10-substituted 2-(1-piperazinyl)pyrimido[1,2-a]benzimidazol-4(10H)-ones 6a-o, and of the two isomers 10-ethyl-2-(diethylamino)pyrimido[1,2-a]benzimidazol-4(10H)-one 6p and 10-ethyl-4-(diethylamino)pyrimido[1,2-a]benzimidazol-2(10H)-one 13, as well as the in vitro evaluation of their inhibitory activity on human platelet aggregation induced in platelet-rich plasma by ADP, collagen or the Ca(2+) ionophore A23187 were here described. Nine out of fifteen 2-(1-piperazinyl)derivatives (6g-o) showed good inhibitory properties towards all the platelet aggregation agonists used. Moreover, a molecular modelling study has been performed on two of the best compounds of this series (6i and 6o) to confirm in silico their interactions with the catalytic site of human platelet PDE3, using the X-ray data of the PDE3B isoform in complex with an inhibitor.

Synthesis, antiplatelet activity and comparative molecular field analysis of substituted 2-amino-4H-pyrido[1,2-a]pyrimidin-4-ones, their congeners and isosteric analogues

2-(1-Piperazinyl)-4H-pyrido[1,2-a]pyrimidin-4-one (5a) is a recently described in vitro inhibitor of human platelet aggregation which specifically inhibits the activity of high affinity cAMP phosphodiesterase. A number of substitution derivatives, isosteres, and analogues of 5a were now synthesized and tested in vitro for their inhibitory activity on human platelet aggregation induced in platelet-rich plasma by ADP, collagen, or the Ca2+ ionophore A23187. Among the most effective compounds, the 6-methyl, 8-methyl and 6,8-dimethyl derivatives of 5a resulted nearly as active as the lead when platelet aggregation was induced by ADP or A23187, but less active when collagen was the inducer. On the basis of present results and those previously obtained by us in this and 2-aminochromone structural fields, we have developed a statistically significant 3-D QSAR model, using comparative molecular field analysis (CoMFA), describing the variation of the antiplatelet activity in terms of molecular steric and electrostatic potential changes.