Non-imidazole histamine H₃ ligands: part V. synthesis and preliminary pharmacological investigation of 1-[2-thiazol-4-yl- and 1-[2-thiazol-5-yl-(2-aminoethyl)]-4- n-propylpiperazine derivatives

Series of 1-[2-thiazol-4-yl-(2-aminoethyl)]- and 1-[2-thiazol-5-yl-(2-aminoethyl)]-4-n-propylpiperazine derivatives have been prepared and in vitro tested as H₃-receptor antagonists (the electrically evoked contraction of the guinea-pig jejunum). It appeared that by comparison of homologous pairs, the 1-[2-thiazol-5-yl-(2-aminoethyl)]-4-n-propylpiperazines (3a,b and 4a-d) have much higher potency than their analogous 1-[2-thiazol-4-yl-(2-aminoethyl)]-4-n-propylpiperazines (2a-k). Based on the obtained results, we observed the 5-position of 2-methyl-2-R-aminoethyl substituents in the thiazole ring is favourable for histamine H₃ receptor antagonist activity, whereas its presence in position 4 leads, almost in each case, to strong decrease of activity.