8-Coniceine from -coniine

Earlier results with C02 in hemlock have been refined and a primary role for y-coniceine in the formation of the other Conium alkaloids seems clear. A biosynthetic sequence from y-coniceine (3)— coniine (2)— iV-methylconiine is consistent with the findings. 

This base does not occur in hemlock but is obtained from coniine. In a new synthesis of this substance, l-(2-pyridyl)-3-bromopropanol prepared from ethylene oxide, picolyllithium, and bromobenzene, is hydrogenated catalytically. The product is 8-coniceine identified through its picrate, m.p. 224-227°, its chloroaurate, m.p. 185-188°, its chlor-platinate, m.p. 209-211°, and its mercuric salt, m.p. 233-237° (40). 

The y-coniceine and coniine are generally the most abundant, and they account for most of the plant s acute and chronic toxicity. These alkaloids are synthesized by the plant from eight acetate units from the metabolic pool, forming a polyketoacid which cyclizes through an aminotransferase and forms gamma-coniceine as the parent alkaloid via reductiOTi by an NADPH-dependent reductase. 

Leete E, Adityachaudhury N (1967) Biosynthesis of hemlock alkaloids II conversion of y-coniceine to coniine and l/-conhydrine. Phytochemistry 6 219-223 Leete E (1970) The biosynthesis of coniine from octanoic acid in hemlock plants (Conium maculatum). J Am Chem Soc 92 3835... 

The first paper of the series (Fairbaim and Challen, 1959) documented marked changes of alkaloid content during the period from the development of flowers up to the production of the mature fruits. Most striking were the variations in content of y-coniceine and coniine, which provided a hypothesis of actual interconversion of these two alkaloids (Fairbaim and... 

Recently Roberts (1975) isolated a y-coniceine reductase from leaves and fruit of a number of Conium maculatum cultivars which are NADH-dependent (Figure 6.32). The ready interconversion of y-coniceine to (+)-coniine may have an important role in the regulation of alkaloid metabolism. The reduction of y-coniceine requires NADPH rather than NADH (the reaction proceeds approximately seven times faster with NADPH), and experiments indicate that the stereospecific removal of hydride is from the B(pros) side of NADPH. The availability of precursors and products may have an effect on the substrate concentration in the in vivo systems. 

The most famous compounds of this group are probably the piperidine alkaloids of poison hemlock Conium maculatum), which were used to execute the Greek philosopher Socrates. While piperidine alkaloids are usually synthesized from the amino acid L-Iysine, the carbon skeleton of the piperidine alkaloids in C. maculatum originates from four acetate units (Leete, 1963, 1964). Only the nitrogen is derived from L-alanine by transamination (Roberts, 1971). Hemlock alkaloids are accumulated in all plant parts, however, highest levels are found in unripe fruits (1.6%) (Dewick, 2002). The two major hemlock alkaloids are y-coniceine and coniine (Fig. 25). Piperidine alkaloids like coniine occur... 

Figure 2.2 Three piperidine alkaloid teratogens from Conium maculatum (poison-hemlock) (a) coniine, (b) y-coniceine, and (c) A-methyl coniine, with accompanying LD50 as determined in a mouse bioassay.

The acid-catalysed decomposition of tertiary azides derived from cyclopentane to give a-substituted piperideines has been applied to the synthesis of certain piperidine alkaloids such as y-coniceine, ( )-coniine, and ( )-dihydropinidine. 

Phytochemistry The roots contain up to 0.042 % total alkaloids, with the stems up to 0.065 % and leaves up to 0.1 %. The fruits contain up to 1 % total alkaloids, but sometimes unripe fruits contain up to 2 % (with 50 % of it being coniine). Other alkaloids include conhydrine, pseudoconhydrine, y-coniceine, and methyl-coniine. The above parts also contain essential oils (mainly terpenes), vitamin C, carotene, and caflfeic acid. Quercetin and kaempferol have been isolated from the flowers (Khalmatov 1964 Lopez et al. 1999). 

Although the alkaloids coniine (48) and y-coniceine (51) bear a structural resemblance to the piperidine alkaloids, these compounds are derived from a polyketide pathway (Fig. 29.17). Lysine is a poor precursor, and early attempts to show incorporation of this compound resulted in failure. Acetate is a much better precursor. Coniine is a highly toxic alkaloid and is one of the toxic components of poison hemlock (Conium maculatum, Apiaceae) (Cutler, 1992). Otherwise, alkaloids are very uncommon in the Apiaceae. Coniine does occur in several other plants, for example, Sarracenia (Sarracenia). y-Coniceine is found in several species of Aloe (Liliaceae) (Dring et al., 1984). Coniine is toxic to the aquatic plant Lemna (Wink, 1993). The LDioo p.o. in the... 

Conium alkaloids simple piperidine alkaloids found only in poison hemlock, Conium maculatum. The main alkaloids are Coniine (see) and y-coniceine (M, 125.22, b.p. 168°C) the secondary alkaloids are A -methyl and hydroxy derivatives of coniine. In contrast to other piperidine alkaloids, the ring system of C. a. is synthesized from acetate rather than from lysine (Fig.). 

A fatty acid precursor octanoic acid (capric acid) is employed, which is subsequently transformed into the ketoaldehyde through successive oxidation and reduction steps (Fig. 26.2). The resulting ketoaldehyde acts as a substrate for a transamination reaction the amino moiety is derived from L-alanine [1, 8, 21, 30, 33, 45,47,49, 50,55,68,70,72, 74, 75]. The ultimate transformation leads to the formation of imine, giving the heterocyclic ring present in y-coniceine, and then reduction of the coniine, as shown in Fig. 26.2. 

Biosynthesis of these compounds proceeds through a completely different route from that of the other piperidine alkaloids (Fig. 26.3). Early work pointed to y-coniceine as the first alkaloid to be formed, followed by transformation to coniine, conhydrine, and M-methylconiine. Levels of these compounds varied greatly within the plant and under different environmental conditions and between varieties. Variations were even found between 2- or 4-h periods of a day and between days. This can be surprising, when, for instance, Leete and coworkers usually fotmd coniine and conhydrine in equal amounts in their plants but on a subsequent occasion when the plants were grown in a greenhouse in the early autumn, coniine and pseudoconhydrine were the major components [44, 50-52, 68, 88]. 

Uniformly labeled L-lysine- " C administered to hemlock plants afforded radioactive coniine, but no degradative studies were produced by these authors to prove that labeling was present in the piperidine ring. Instead, feeding sodium acetate-to 2-year-old C. maculatum plants indicated that alkaloids obtained from the plant (coniine, y-coniceine, and conhydrine) presented labeling. Chemical degradation of the obtained coniine showed that activity was mostly and evenly distributed in even-numbered C-atoms, which allowed to speculate that hemlock alkaloids are derived from an 8-C aliphatic polyketo chain produced by the linear attachment of four acetate units probably through the condensation of 4 molecules of acetyl-CoA. The results of may be due to metabolization of lysine- C to radioactive acetate units, then incorporated to coniine. 

Later experimental work provided evidence that the 8-carbon polyketoacid intermediate in the synthesis of y-coniceine is derived from octanoic acid since this acid was shown to be readily incorporated into coniine. Further work indicated that 5-keto-octanoic acid and 5-keto-octanal were produced during the biosynthesis of y-coniceine. A transaminase (L-alanine 5-keto-octanal aminotransferase) was obtained from C. maculatum. This transaminase catalyzes the reaction between 5-keto-octanal with L-alanine as the amino group donor to form the piperidine ring and the propyl side chain. Another C. maculatum alkaloid, A-methylconiine, was shown to be produced by another enzyme from the plant a coniine methyltransferase which acts as a transmethylator utilizing 5-adenosyl-L-methionine as a methyl group donor. 

The entire skeleton of coniine 6.14) derives from acetate, with labelling of C-2, C-2, C-4, and C-6 by [l- C]acetate. A polyketide pathway immediately seems likely and 5-oxo-octanoic acid 6.11) and the corresponding aldehyde 6.12), with the necessary functionality for inclusion of the nitrogen atom, were shown, by tracer and enzymic evidence, to be implicated in coniine biosynthesis, as was y-coniceine 6.13) which is also a hemlock alkaloid. These results [6, 7] lead in a straightforward way to the pathway shown in Scheme 6.5. Most curiously, however, it has been found that octanoic acid... 

Another reversible reaction is the interconversion of (-i-)-coniine (20) and y-coniceine (21) which occurs in hemlock (Conium maculatum). The reaction is stereospecific, only the natural (-t-)-coniine being dehydrogenated to y-coniceine. y-Coniceine is formed in hemlock from 5-oxo5ctanal by a transamination involving L-alanine. By administering [l - C, N]coniine and [r- C, N]-y-coniceine to hemlock, and then re-isolating the alkaloids... 

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