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, OCH2CH2 18F, 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 presente invención está relacionada con un compuesto con la formula general (ver Fórmula) en el que R1 es fenilo, sustituido opcionalmente por uno o dos sustituyentes, seleccionados de 3H, halógeno, alquilo inferior, dimetilamino, NHC(O)-alquilo inferior, C(O)Oalquilo inferior, alcoxi inferior, OC(3H)3, O11CH3, OCH2CH218F, alcoxi inferior sustituido por halógeno, hidroxi, alquilo inferior sustituido por hidroxi, S-alquilo inferior, o por un grupo heterociclilo o es benzo[d] [1,3ldioxol-5-ilo, 2,3-dihidrobenzo[bl [1,4] dioxin-6-ilo, indolin-2-ona, o
