The present invention provides compounds of formula I: wherein R1 is an alkyl, phenyl, heterocyclyl, cycloalkyl, alkoxy, ester, amino or amide group; R2 is a phenyl, heterocyclyl, alkyl, cycloalkyl or cycloalkylalkyl group; R3 is an alkyl, cycloalkyl, cycloalkylalkyl, amino or heterocyclyl group; R4 and R5 are hydrogen or alkyl or form a cycloalkyl ring; A is O or N; and m is zero or one; as inhibitors of GlyT1 and thus as useful for treating or preventing diseases such as schizophrenia; with the provision of pharmaceutical compositions, first and second medical uses and methods of treatment.