The present invention relates to compounds of general formula (I) wherein R1 is phenyl, optionally substituted by one or two substituents, selected from 3H, halogen, lower alkyl, di-methyl-amino, NHC(O)-lower alkyl, C(O)O-lower alkyl, lower alkoxy, OC(3H)3, O11CH3, OCH2CH218F, lower alkoxy substituted by halogen, hydroxy, lower alkyl substituted by hydroxy, S-lower alkyl, or by a heterocyclyl group or is benzo[d][l,3]dioxol-5-yl, 2,3-dihydrobenzo[b][l,4]dioxin-6-yl, indolin-2-one, or is heteroaryl, selected from the group consisting of thiophenyl, benzofuranyl, benzothiophenyl, pyrazinyl, or benzothiazolyl R2 is hydrogen, lower alkyl or lower alkyl substituted by halogen R3 is lower alkyl, C(3H)3, 11CH3, lower alkyl substituted by halogen, -(CH2)2-O-lower alkyl substituted by halogen or cycloalkyl or R2 and R3 form together with the N-atom to which they are attach a ring containing -CH2CH2CHRCH2CH2-, -CH2CH2CH2CH2-, -CH2CH2CH2CH2-, -CH2CH2-NR-CH2CH2-, -CH2CH2-O-CH2CH2-, -CH2CH2CHRCH2-, -CH2CHRCH2- or (A) R is hydrogen, halogen, lower alkyl substituted by halogen or lower alkoxy or to a pharmaceutically acceptable acid addition salt, to a racemic mixture or to its corresponding enantiomer and/or optical isomers thereof. The compounds are suitable as imaging tool, which will improve diagnosis by identifying potential patients with excess of tau aggregates in the brain, which may be likely to develop Alzheimer s disease.La présente invention concerne des composés de formule générale (I) dans laquelle R1 est un phényle, éventuellement substitué par un ou deux substituants choisi(s) parmi 3H, halogène, alkyle inférieur, diméthylamino, NHC(O)-(alkyle inférieur), C(O)O-(alkyle inférieur), alcoxy inférieur, OC(3H)3, O11CH3, OCH2CH218F, alcoxy inférieur substitué par un halogène, hydroxy, alkyle inférieur substitué par un hydroxy, S-(alkyle inférieur), ou par un groupe hétérocyclyle ou est un benzo[d][l,3]dioxol-5-yle, un 2,3-dihydrobenzo[b][l,4]dioxin-6-yle, une indolin-2-o
