Senecic Acid Biosynthesis

A more reasonable genesis of monocrotalic acid, at least as far as C-1, -2, -3, -6, and -7 are concerned, would be from isoleucine by analogy with senecic acid biosynthesis (see above) the C5 unit which these atoms constitute is a common feature of the necic acids. Accordingly satisfactory incorporations of l-[[/- Cjisoleucine and its precursor, L-[f/- C]threonine [as (50)], into (49) were observed and the activity was shown to be confined to the necic acid moiety. Moreover, the isoleucine was found to label C-1, -2, -3, -6, and -7 of monocrotalic... 

The mechanism by which these two isoleucine untis are coupled in senecic acid biosynthesis is not obvious. Crout et al. (25) tested a number of Cs-intermediates of isoleucine metabolism as possible precursors for this coupling process. Eventually, they found that 2-amino-[3- H2]-methylenepentanoic acid (98) was incorporated specifically into senecic acid (97) with about the same efficiency as for L-isoleucine. Unfortunately, the low radioactivity in the necic acid precluded further degradation to establish whether one or both halves of the senecic acid were labelled. There is also the possibility that the precursor (98) could be converted into isoleucine before incorporation. Thus, the status of (98) as an intermediate in senecic acid biosynthesis clearly requires further investigation. Nevertheless, Crout (73) has postulated a mechanism for activation of (98) into a pyridoxal bound mesomeric nucleophile (99), which could then attack an unspecified electrophilic metabolite of isoleucine to form senecic acid (Scheme 24). 

Further experiments with threonine and isoleucine have been carried out in Senecio magnificus Both [2- " C]- and [6- " C]-isoleucine were incorporated into senecic acid (21) and degradation gave a pattern of incorporation consistent with biosynthesis from two isoleucine units (Scheme 5). [ /- " CJ-L-Threonine was equally well incorporated into senecic acid, which was shown to contain essentially no activity at C-2 and C-8, consistent with the way in which threonine condenses with pyruvic acid to give isoleucine the route from threonine to senecic acid is illustrated (Scheme 6). In these three feeding experiments the incorporation of radioactivity into the main alkaloidal component of this plant, retronecine, was very low. 

Axillarine and axillaridine therefore contain Cjo necic acid components belonging to a new structural type. The four groups of necic aads previously known, of which senecic acid (63) may be cited, contain a common C5 structural component [indicated to the right of the dotted line in (63)], which suggests that in the biosynthesis of the necic acids this unit is coupled to various intermediates derived from common amino-acids. In the biosynthesis of axillarine and axillaridine, however, it would appear that this same C5 unit is involved, but with a different mode of coupling to the other 5-carbon unit. This adds credence to the earlier proposal that in the biosynthesis of the necic acids, the common C5 unit (derived from isoleucine ) is coupled as an intact unit. ... 

Pyrrolizidine Alkaloids.— The full account of work on the biosynthesis of the necic acids previously published in preliminary form and reviewed, has appeared.In essence only the reported incorporation of DL-[5- C]isoleucine into the senecic acid (181) moiety of senecionine with equal labelling of C-7 and C-9 is new. These results are in accord with those available earlier. 

Pyrrolizidine Alkaloids.— Two molecules of L-isoleucine are used for the biosynthesis of the senecic acid component of senecionine (15). In order to understand how the two isoleucine fragments are linked together (C-6 of one joins to C-4 of the other), further work has been undertaken. First, 2-methyl-3-oxobutanoic acid and the five-carbon intermediates in isoleucine metabolism, i.e. 2-methylbutanoic acid and angelic acid (17), were examined as precursors for the senecic acid fragment of senecionine (15), with negative results [angelic acid rather than the isomeric tiglic acid, see (13), was examined since its stereochemistry is the same as that around C-15-C-20 in (15)]. 

Similar findings were reported for the acidic part in the PA strigosine [69] and for angeloylic and tigloylic acid [70]. In the case of the acidic part in the PA monocrotaline, it was shown that not acetate, mevalonate, orglutarate (as reported earlier) are involved but it is only formed via isoleucine [71], Stereospecific aspects were also studied in the case of senecic and isatinecic acid [72, 73]. The biosynthesis of trichodesmic acid was studied and it was shown that one part of the Cio acid was formed by (25)-isoleucine or its biosynthetic precursor (25)-threonine and the other C5 unit from (25)-leucine or (25)-valine [74]. The complete labeling pattern of senecic acid (the acidic part in the PAs rosmarinine and senecionine) was studied by NMR experiments and it was stated that the biosynthesis of this acid is processed via two molecules of isoleucine (Scheme 13.2) [75]. 

Crout DHG, Davies NM, Smith EH, Whitehouse D (1972) Pyrrolizidine alkaloids. The biosynthesis senecic acid. J Chem Soc, Perkin Trans I 671-680... 

Seneciphyllic acid Scheme 6 Biosynthesis of senecic acid... 

During the conversion of L-isoleucine (96) into senecic acid (97), both isoleucine molecules lose one of the C-4 hydrogens. Cahill etal. (61) have shown that senecic acid is formed with loss of the C-4 pro-S hydrogens from both isoleucine molecules. This clearly rules out the possibility of a carbonyl function at C-4 in the right-hand Cs-unit (Scheme 23) acting as an electrophilic acceptor in senecic acic biosynthesis. 

In summary it is to be noted that the evidence available now on the biosynthesis of the necic acids points to branched-chain amino-acids as precursors. As discussed in part above, isoleucine has been shown to be a precursor for the necic acids of the senecic [as (47)] and monocrotalic (49) types, and of the angelic acid component of heliosupine. " Valine, on the other hand, is a specific precursor of the echimidinic acid (51) moiety in heliosupine. 

Cahill R, Crout DHG, Mitchell MB, Muller US (1980) Isoleucine biosynthesis and metabolism stereo-chemistry of the formation of L-isoleucine and of its conversion into senecic and isatinecic acid. J Chem Soc, Chem Comm 419-421... 

Bale, N. M., R. Cahill, N. M. Davies, M. B. Mitchell, E. H. Smith, and D. Whitehouse Biosynthesis of the Necic Acids of the Pyrrolizidine Alkaloids. Further Investigations of the Formation of Senecic and Isatinecic Acids in Senecio Species. J. Chem. Soc., Perkin Trans. I 1978, 101. 

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