The present invention relates to compounds of formula I wherein R1' is CH3； R1 is methyl, ethyl, CF3, cyclopropyl, CH2OH or R1' and R1 may form together with the carbon atom to which they are attached a 1,1- dioxo-thiolan-3-yl ring； R2 is hydrogen, methyl, ethyl, isopropyl, tertbutyl, cyclopropyl, CH2OH or C(CH3)2OH； R3 is C1, F, CF3, cyano, methyl or cyclopropyl； R4 is hydrogen, methyl or F； or to a pharmaceutically acceptable salt or acid addition salt, to a racemic mixture, or to its corresponding enantiomer and/or optical isomer and/or stereoisomer thereof for use in the treatment of psychiatric disorders such as schizophrenia, bipolar disorder, obsessive-compulsive disorder or autism spectrum disorder.
