Thus, complex β-carbolines, e.g. the terpenoid indole alkaloid ajmalicine, are produced by a pathway using an aldehyde such as secologanin. Simpler structures employ keto acids, e.g. harmine (Figure 74) incorporates two extra carbons from pyruvate. In such a case, an acid is an intermediate, and oxidative decarboxylation gives the dihydro-β-carboline, from which reduced tetrahydro-β-carboline structures, e.g. elaeagninefrom Elaeagnus angustifolia (Elaeagnaceae), or fully aromatic β-carboline structures, e.g. harman and harmine from Peganum harmala (Zygophyllaceae) are derived (Figure 74). The methoxy substitution in the indole system of harmine is introduced at some stage in the pathway by successive hydroxylation and methylation reactions. A sequence from 6-hydroxytryptamine is also feasible. The reported psychoactive properties of the plants Peganum harmala and Banisteriopsis caapi(Malpighiaceae) is due to the β-carboline alkaloids such as harmine, harmaline, and tetrahydroharmine (Figure 74).
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