2-Bromomelatonin: synthesis and characterization of a potent melatonin agonist

A practical synthesis of N-[2-(2-bromo-5-methoxy-1H-indol-3-yl)ethyl]- acetamide (2-bromomelatonin) was achieved by direct bromination of melatonin with N-bromosuccinimide (NBS) in anhydrous acetic acid at room temperature under nitrogen, followed by flash-chromatography. 1H-NMR and mass spectra showed the bromine to be incorporated at the C-2 position of the indole moiety. Tests performed in vitro with isolated melatonin receptors from rabbit parietal cortex demonstrated that the relative binding affinity of 2-bromomelatonin was about ten times higher than that of melatonin and close to that of 2-iodomelatonin. 2-Bromomelatonin behaved as a potent agonist in the physiological studies. It showed enhanced activity in inhibiting the spontaneous firing activity of cortical neurons and similarly to melatonin and 2-iodomelatonin potentiated significantly the inhibitory effect of GABA. 2-Bromomelatonin was also an extremely effective agonist in the tests performed in vivo in the Syrian hamster gonadal regression model.