Compounds of the general formula I characterized in that A represents a) hydroxyl, b) branched or unbranched hydroxy C 1 -C 5 alkoxy c) N(C 1 -C 5 alkyl) 2, d) NH 2, e) N -L, f) O-L or g) O-Z, G represents a) hydroxyl, b) branched or unbranched O-C 1 -C 5 alkyl, c) branched or unbranched O-C 2 -C 5 alkenyl, d) branched or unbranched O-C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl or e) branched or unbranched O-C 2 -C 5 alkynyl, R 1 and R 2 represent a) hydrogen, b) branched or unbranched 18 F- C 1 -C 5 alkoxy c) branched or unbranched 18 F- C 1 -C 5 alkyl, d) branched or unbranched 18 F-hydroxy- C 1 -C 5 alkyl, e) branched or unbranched 18 F-C 2 -C 5 alkenyl, or f) branched or unbranched 18 F-C 2 -C 5 alkynyl, g) hydroxyl, h) branched or unbranched C 1 -C 5 alkyl, or i) branched or unbranched C 1 -C 5 alkoxy, provided that one of the substituents R 1 or R 2 contains exactly one 18 F isotope and the other substituent in each case contains no 18 F isotope, L represents a) branched or unbranched C 1 -C 5 alkyl, b) branched or unbranched C 2 -C 5 alkenyl, c) branched or unbranched C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl, or d) branched or unbranched C 2 -C 5 alkynyl, and Z represents a metal cation equivalent which can be monovalent or divalent, where divalent metals may form an ionic bond with two residues of the proposed structures, characterized in that n = 0, 1, 2 or 3 and diastereomers and enantiomers. A represents a) hydroxyl, b) branched or unbranched hydroxy C 1 -C 5 alkoxy c) N(C 1 -C 5 alkyl) 2, d) NH 2, e) N -L, f) O-L or g) O-Z, a) hydroxyl, b) branched or unbranched hydroxy C 1 -C 5 alkoxy c) N(C 1 -C 5 alkyl) 2, d) NH 2, e) N -L, f) O-L or g) O-Z, G represents a) hydroxyl, b) branched or unbranched O-C 1 -C 5 alkyl, c) branched or unbranched O-C 2 -C 5 alkenyl, d) branched or unbranched O-C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl or e) branched or unbranched O-C 2 -C 5 alkynyl, a) hydroxyl, b) branched or unbranched O-C 1 -C 5 alkyl, c) branched or unbranched O-C 2 -C 5 alkenyl, d) branched or unbranched O-C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl or e) branched or unbranched O-C 2 -C 5 alkynyl, R 1 and R 2 represent a) hydrogen, b) branched or unbranched 18 F- C 1 -C 5 alkoxy c) branched or unbranched 18 F- C 1 -C 5 alkyl, d) branched or unbranched 18 F-hydroxy- C 1 -C 5 alkyl, e) branched or unbranched 18 F-C 2 -C 5 alkenyl, or f) branched or unbranched 18 F-C 2 -C 5 alkynyl, g) hydroxyl, h) branched or unbranched C 1 -C 5 alkyl, or i) branched or unbranched C 1 -C 5 alkoxy, provided that one of the substituents R 1 or R 2 contains exactly one 18 F isotope and the other substituent in each case contains no 18 F isotope, a) hydrogen, b) branched or unbranched 18 F- C 1 -C 5 alkoxy c) branched or unbranched 18 F- C 1 -C 5 alkyl, d) branched or unbranched 18 F-hydroxy- C 1 -C 5 alkyl, e) branched or unbranched 18 F-C 2 -C 5 alkenyl, or f) branched or unbranched 18 F-C 2 -C 5 alkynyl, g) hydroxyl, h) branched or unbranched C 1 -C 5 alkyl, or i) branched or unbranched C 1 -C 5 alkoxy, L represents a) branched or unbranched C 1 -C 5 alkyl, b) branched or unbranched C 2 -C 5 alkenyl, c) branched or unbranched C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl, or d) branched or unbranched C 2 -C 5 alkynyl, and a) branched or unbranched C 1 -C 5 alkyl, b) branched or unbranched C 2 -C 5 alkenyl, c) branched or unbranched C 1 -C 5 alkyl-(O-C 1 -C 4 alkyl) n -O-C 1 -C 4 alkyl, or d) branched or unbranched C 2 -C 5 alkynyl, and Z represents a metal cation equivalent which can be monovalent or divalent, where divalent metals may form an ionic bond with two residues of the proposed structures, characterized in that n = 0, 1, 2 or 3 and diastereomers and enantiomers. n = 0, 1, 2 or 3 and diastereomers and enantiomers.