Quinazoline Alkaloids

Peganine (Figure 107) is a quinazoline alkaloid found in Peganum harmala (Zygophyllaceae), where it co-occurs with the β-carboline alkaloid harmine. It is also responsible for the bronchodilator activity of Justicia adhatoda(Adhatoda vasica) (Acanthaceae), a plant used in the treatment of respiratory ailments. As a result, the alternative name vasicine is also sometimes used for peganine. Studies in Peganum harmalahave clearly demonstrated peganine to be derived from anthranilic acid, the remaining part of the structure being a pyrrolidine ring supplied by ornithine (compare Figure 1, page 292). The peganine skeleton is readily rationalized as a result of nucleophilic attack from the anthranilate nitrogen on to the pyrrolinium cation, followed by amide formation (Figure 107). Remarkably, this pathway is not operative in Justicia adhatoda, and a much less predictable sequence from Nacetylanthranilic acid and aspartic acid is observed (Figure 107).

Figure 107