Compounds of formula (I) have activity in alleviating the effects of impaired dopaminergic signaling, for example in the treatment of Parkinsons Disease: wherein: R1 is a carboxyl, carboxyl ester, or carboxamide group; R2 and R3 are independently hydrogen, or a group -C(-O)R6 or -C(-O)OR6 wherein Re is C1-C6 alkyl, or a group -CH2Q wherein Q is an optionally substituted monocyclic carbocyclic or heterocyclyl ring of 3 to 6 ring atoms; R4 and R5 are independently (a) the side chain of a natural amino acid, or (b) optionally substituted C1-C4 alkyl, C2-C4 alkenyl, or C2-C4 alkynyl, or (c) -CH2XCH3, -CH2CH2XCH3, or -CH2XCH2CH3, wherein X is -O-, S, or -NR7 wherein R7 is hydrogen, methyl or ethyl; or (d) -CH2Q or CH2OQ wherein Q is as defined in relation to R6; or R4 and R5 taken together with the carbon atom to which they are attached form an optionally substituted cycloalkyl or heterocyclic ring of 3 to 8 ring atoms, optionally fused to a second, optionally substituted, carbocyclic or heterocyclic ring
Amino Acid Derivatives