Synthesis and isolation of bromo‐β‐carbolines obtained by bromination of β‐carboline alkaloids

Abstract

Abstractβ‐Carbolines (1–5) undergo electrophilic aromatic substitution with N‐bromosuccinimide under different experimental conditions. Although 6‐bromo‐nor‐harmane (la) obtained by bromination of nor‐harmane (1) was isolated and fully characterized sometime ago, the other bromoderivatives of nor‐harmane (1b‐1e) and harmane (2a‐2e) were partially described as part of the reaction mixtures. The preparation and subsequent isolation, purification and full characterization of 1b, 1c, 1d, 1e, 2a, 2b, 2c, 2d, 2e are reported (mp, Rf, 1H‐nmr, 13C‐nmr and ms) together with the preparation, isolation and charaterization, for the first time, of the bromoderivatives obtained from harmine (3a‐3e), harmol (4a, 4b) and 7‐acetylharmol (5a‐5c). As brominating reagent N‐bromosuccinimide and N‐bromosuccinimide‐silica gel in dichloromethane and in chloroform as well as the β‐carboline ‐ N‐bomosuccinimide solid mixture have been used and their uses have been compared. Semiempirical AMI and PM3 calculations have been performed in order to predict reactivity in terms of the energies of HOMO, HOMO‐LUMO difference and in terms of the charge density of β‐carbolines (1–5) and bromo‐β‐carbolines (1a‐1e, 2a‐2e, 3a‐3e, 4a, 4b, 5a, 5b and 5c) (Scheme 1). Theoretical and experimental results are discussed briefly.