The present invention relates to compounds of formula IwhereinR1′ 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 Cl, 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.
