Synthesis of model chromophores related to the gold fluorescent protein (GdFP)

The two model chromophores 2 and 3 for the core 1 of the gold fluorescent protein (GdFP) were synthesized from commercially available 2-methyl-3- nitroaniline (4) in six synthetic steps and overall yields of 13% and 8%, respectively. The key step of the sequence is the chemoselective, reductive introduction of the amino group after assembly of the Z-configured 5-(indol-3-ylmethylene)imidazolin-4-one skeleton of the chromophore. Compound (Z)-2 was shown to undergo a light-initiated E/Z-isomerization, which allows access also to its E-isomer.