The present invention relates to compounds of general formula IA or IB ##STR00001## wherein X.sup.1 and X.sup.2 are each independently N or C--R'' and R.sup.1, R.sup.2,R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are as defined in the specification and to pharmaceutically acceptable acid addition salts thereof. The compounds can be used for the treatment of diseases related to the glycine transporter inhibitor, such as schizophrenia and Alzheimer's disease.