The invention relates to compounds of formulas I and II wherein R1 is hydrogen, lower alkyl, lower alkoxy, halogen, S-lower alkyl, lower alkoxy substituted by halogen, di-lower alkyl amino, C(O)O-lower alkyl, lower alkyl substituted by hydroxy or hydroxy R2 is hydrogen, lower alkyl, halogen, lower alkoxy, S-lower alkyl, lower alkoxy substituted by halogen, O(CH2)2-lower alkoxy substituted by halogen, di-lower alkyl amino, alkyl amino, NH-lower alkyl substituted by halogen, N(lower alkyl)-benzyl, lower alkyl substituted by hydroxy, heterocycloalkyl optionally substituted by halogen, CH2-lower alkoxy, CH2-lower alkoxy substituted by halogen or hydroxy or R1 and R2 form together with the carbon atoms to which they are attached a ring containing —OCH2CH2O—, OCH2O—, OCH2CH2CH2O— or —NHC(O)CH2O— R3 is hydrogen or lower alkoxy R4 is hydrogen or lower alkyl R5 is lower alkyl, cycloalkyl, lower alkyl substituted by hydroxy or lower alkyl substituted by halogen or R4 and R5 form together with the nitrogen atom to which they are attached a ring containing —CH2CH2CHRCH2CH2—, —CH2CHRCH2CH2—, —CH2CH2OCH2CH2—CH2CH2NR′CH2CH2—, CH2CHR— or —CH2CH2CH2— wherein R is hydrogen, halogen, lower alkyl, lower alkoxy or lower alkyl substituted by halogen R′ is lower alkyl substituted by halogen Ra is hydrogen or 3H Rb is hydrogen, hydroxy or 3H R6 is hydrogen, halogen or lower alkyl HetAr is selected from the group consisting of thiophenyl, furanyl, thiozolyl, benzofuranyl, pyrazolyl, benzoimidazolyl or pyridinyl n is 1 or 2 or to a pharmaceutically acceptable acid addition salt, to a racemic mixture or to its corresponding enantiomer and/or optical isomers thereof. The present 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 Alzheimers disease.
