The present invention relates to compounds of formula IwhereinR1′； is CH3 R1 is methyl, ethyl, CF3, CH2OH, cyclopropyl or cyano, or R1′； and R1 may form together a 1,1-dioxo-tetrahydro-thiophen-3-yl ring；R2 is hydrogen, methyl, ethyl, isopropyl, tertbutyl, cyclopropyl or CF3；R3 is Cl, F, CF3, methyl, methoxy, isopropyl or cyclopropyl；R4 is hydrogen, methyl, F or Cl；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 schizophrenia, bipolar disorder, obsessive-compulsive disorder or autism spectrum disorder.
