Compounds having the structural formula I or a pharmaceutically acceptable salt thereof, wherein R is optionally substituted phenyl, furanyl, thienyl, pyridyl, pyridyl N-oxide, oxazolyl or pyrrolyl, or cycloalkenyl R1, R2, R3, R4 and R5 are H, alkyl or alkoxyalkyl; and Z is optionally substituted aryl or heteroaryl are disclosed. Also disclosed is the use of compounds of formula I in the treatment of central nervous system diseases, in particular Parkinson's disease, alone or in combination with other agents for treating Parkinson's disease, and pharmaceutical compositions comprising them.