This allows formation of additional six- and five-membered oxygen heterocyclic rings, as seen with other systems, e.g. coumarins, isoflavonoids, etc. By an analogous series of reactions, the dimethylallyl derivative can act as a precursor of furoquinoline alkaloids such as dictamnine and skimmianine(Figure 108). These alkaloids are found in both Dictamnus albus and Skimmia japonica (Rutaceae). To simplify the mechanistic interpretation of these reactions, it is more convenient to consider the di-enol form of the quinolone system.
Should chain extension of anthranilyl-CoA (as the N-methyl derivative) incorporate three acetate/ malonate units, a polyketide would result (Figure 109). The acridine skeleton is then produced by sequential Claisen reaction and C–N linkage< by an addition reaction, dehydration, and enolization leading to the stable aromatic tautomer. Again, the acetate-derived ring, with its alternate oxygenation, is susceptible to electrophilic attack, and this can lead to alkylation (with dimethylallyl diphosphate) or further hydroxylation. Alkaloids melicopicine from Melicope fareana, acronycinefrom Acronychia baueri, and rutacridone from Ruta graveolens (Rutaceae) typify some of the structural variety that may then ensue (Figure 109).
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