he alkaloids found in the antiasthmatic plant Lobelia inflata
* (Campanulaceae) contain piperidine rings with alternative C6
side-chains derived from phenylalanine via cinnamic acid. These alkaloids are produced as in Figure 23, in which benzoylacetyl-CoA, an intermediate in the β-oxidation of cinnamic acid provides the nucleophile for the Mannich reaction. Oxidation in the piperidine ring gives a new iminium species, and this can react further with a second molecule of benzoylacetyl-CoA, againvia a Mannich reaction. Naturally, because of the nature of the side-chain, the second intramolecular Mannich reaction, as involved in pseudopelletierine biosynthesis, is not feasible. Alkaloids such as lobeline
from Lobelia inflata
, or sedamine
from Sedum acre
(Crassulaceae), are products from further N-methylation and/or carbonyl reduction reactions (Figure 23).
The simple piperidine alkaloid coniine from poison hemlock is not derived from lysine, but originates by an amination process and is discussed on page 381.