The present invention relates to compounds of general formula (I) wherein R1 is phenyl or pyridinyl, which are optionally substituted by one, two or three substituents, selected from halogen, lower alkyl substituted by halogen, lower alkoxy, lower alkoxy substituted by halogen, cyano or S(O)2-lower alkyl, or is morpholinyl, dihydropyranyl or piperidinyl, wherein piperidinyl is optionally substituted by halogen, or is C(O)O-lower alkyl; R2 is hydrogen; R3 is hydrogen, lower alkyl substituted by halogen, –(CH2)n-S(O)2-lower alkyl, –(CH2)n- cycloalkyl or –(CH2)n-lower alkoxy; R4 is hydrogen or lower alkyl; n is 0, 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 compounds may be used for the treatment of schizophrenia, obsessive-compulsive personality disorder, major depression, bipolar disorders, anxiety disorders, normal aging, epilepsy, retinal degeneration, traumatic brain injury, spinal cord injury, post-traumatic stress disorder, panic disorder, Parkinson's disease, dementia, Alzheimer's disease, cognitive impairment, chemotherapy-induced cognitive dysfunction ("chemobrain"), Down syndrome, autism spectrum disorders, hearing loss, tinnitus, spinocerebellar ataxia, amyotrophic lateral sclerosis, multiple sclerosis, Huntington's disease, stroke, and disturbances due to radiation therapy, chronic stress, optic neuropathy or macular degeneration, or abuse of neuro-active drugs, selected from alcohol, opiates, methamphetamine, phencyclidine and cocaine.