Compounds of the formula in which R1 is Y or Y-C1-6 alkyl, where Y is carboxy, tetrazolyl, -SO2H, -SO3H, -OSO3H, -CONHOH, or -P(OH)OR', -PO(OH)OR', -OP(OH)OR' or -OPO(OH)OR' where R' is hydrogen, C1-6 alkyl, C2-6 alkenyl or optionally substituted phenyl-C1-6 alkyl, and R2, R3, R4, X and Z are as defined in the specification, possess affinity for metabotropic glutamate receptors and are useful in the treatment of disorders of the central nervous system.
2-amino-2-(3-substituted cyclobutyl) acetic acid derivatives