The present invention provides compounds of formula (I), wherein both p's are one or two, R.sup.1 is generally heteroaryl or cycloalkyl, R.sup.2 is C.sub.3-6cycloalkyl or phenyl and R.sup.3 is heteroaryl, and pharmaceutically acceptable salts thereof, as GlyT1 inhibitors for treating schizophrenia, pharmaceutical compositions comprising the same and methods for their preparation. ##STR00001##