Alkaloids derived from nicotinic acid contain a pyridine nucleus. Nicotinic acid itself is synthesized from L-tryptophan via N-formylkynurenine, L-kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid.
The pyridine nucleus is passed to alkaloids via dihydronicotinic acid, moving from dihydropyridine to nicotine and nornicotine (Figure 47). Dihydronicotinic acid is synthesized by the enzymatic activity of NADP and subsequently becomes 1,2-dihydropyridine by a reduction reaction. At the next stage, nicotine is synthesized by the reactions of ionization and enzyme NADP+. Nornicotine synthesis is achieved by hydroxylation, NADPH activity and, in its final stage, by non-enzymatic decomposition. Interestingly, during the ionization reaction the residue cation from putrescine (a derivate of L-ornithine) appears. The synthesis of other alkaloids derived from nicotinic acid is illustrated in Figure 48. Anabasine is synthesized from nicotinic acid using the A1-piperidinium cation (from L-lysine), and anatabine from nicotinic acid (as with nicotine) via dihydronicotinic acid and 1,2-dihydropyridine. However, the synthesis of ricine from nicotinic acid is accomplished through nicotinamide.
The other alkaloids derived from nicotinic acid, with pyridine nucleus such as arecoline, arecaidine and guvacoline, are tetrahydronicotinic acid (guvacine) derivatives.
Nicotinic acid also forms part of the sesquiterpene pyridine nucleus. As is well known, aliphatic and aromatic acids esterify hydroxyl groups of fundamental sesquiterpene. Some sesquiterpene forms bridges with pyridine rings of nicotine acid derivatives, and then the sesquiterpene pyridine nucleus appears. More than 220 sesquiterpene pyridine nucleus alkaloids have been documented58. Cathedulin alkaloids, isoevoniate, hydroxyisoevoniate, epimeric, norevoniate, wilfordate, hydroxywilfordate, isowilfordate, benzoyloxy- and furanoyloxywol-fordate, edulinate and cassinate constitute the groups of these reported alkaloids. In nature, sesqiterpene pyridine alkaloid formation involves a mixed biosynthetic route. The sesquiterpene moiety originates from the acetate metabolism via the mevalonic acid pathway. Sesquiterpene pyridine alkaloids are synthesized when nicotinic acid and (3S)-isoleucine form evoninic acid, which is configured with the sesquiterpene moiety.
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