Biosynthesis

BY R. B. HERBERT

1 Piperidine, Pyridine, and Pyrrolidine Alkaloids

Lycopodine. — Pelletierine (1) serves as a specific precursor for cemuine (2) and lycopodine (3). The reasonable hypothesis that these alkaloids were modified dimers of pelletierine had to be abandoned when it was discovered that pelletierine gave only one each of the two C8N units (heavy bonding) of (2) and (3).

The results were surprising as lysine (4), cadaverine (5), and Δ1-piperideine (6) had all been built equally into each of the two C8N units of lycopodine and cernuine. It followed, therefore, that if lycopodine is formed from two pelletierine-like units, these units must either be identical or, if different, be derived from Δ1-piperideine (6) with the same overall dilution by non-labelled pools. Two models were proposed which could account for the incorporation of pelletierine, one (Scheme l) in which it could substitute for one of the two identical units, i.e. (8), and the other where it was an obligatory intermediate and precursor for one of the C8N units (Scheme 2); it is a consequence of the equal labelling of both halves of lycopodine by Δ1-piperideine, lysine, and cadaverine that in this scheme the steady-state concentration of pelletierine and (10) be small compared with (9).

The failure of [2-14C]-2-allylpiperidine [as (7)] to label C-5 of lycopodine (3) specifically in Lycopodium tristachyum provided evidence against the first hypothesis. Evidence was then adduced which showed that pelletierine (1) was very probably a normal intermediate in lycopodine biosynthesis. Firstly, the presence of pelletierine (1) in L. tristachyum was demonstrated. [2-14C]-Δ1-Piperideine [as (6)] together with a large quantity of inactive pelletierine was administered to the plant; a similar experiment was carried out with [1,5-14C2]-cadaverine [as (5)]. Radioactive pelletierine was isolated at the end of each experiment.

The presence and synthesis of pelletierine from lycopodine precursors was thus demonstrated but its obligatory participation in lycopodine biosynthesis was not proven. Secondly, therefore, the ability was examined of inactive pelletierine to dilute radioactivity from one of the C8N units following simultaneous administration of labelled Δ1-piperideine or cadaverine. Difficulties normally experienced with this type of experiment in plants were obviated since labelling of the C8N unit not derived from pelletierine served as an internal standard. It was found that pelletierine very effectively diluted activity in both experiments from the C8N unit (heavy bonding) of (3), consonant with its being an obligatory intermediate in lycopodine biosynthesis. All the experimental evidence is accommodated in Scheme 2, which also allows for the non-intact incorporation of β-hydroxybutyrate and acetoacetate.

Dioscorine. — Full details of the mode of incorporation of acetate into dioscorine (11) have been published. The results are consistent with either variant, a6 and b,4 of a pathway involving condensation of four acetate units with a lysine-derived unit, plausibly Δ1-piperideine (Scheme 3); as indicated in path b, pelletierine may be involved. Administration of [2-14C]lysine [as (4)] to the tropical yam, Dioscorea hispida, however, gave dioscorine (11) with little radioactivity; whilst [6-14C]-Δ1-piperideine [as (6)] was better used, the labelling pattern was essentially the same as that from [1-14C]acetate, arising presumably by catabolism of the radioactive Δ1-piperideine to acetate. These poor incorporations were rationalized by suggesting that, at the time of feeding, some compound, derivable from lysine, was not being actively synthesized but that it was available for condensation with acetate. This hypothetical compound could not be pelletierine for [1-14C]acetate incorporation would result in labelling of C-10 and C-12 but not C-5, and in fact almost equal labelling of these positions is observed.

It is worth noting that incorporation of acetate into dioscorine was only achieved with considerable difficulty and it seems possible that the administered lysine and Δ1-piperideine are not reaching the site of alkaloid synthesis, in which case pelletierine (1) may yet prove to be a precursor for dioscorine (11).

Anabasine. — Study of the biosynthesis of the piperidine ring of alkaloids like anabasine (14), N-methylpelletierine (16), and sedamine (17) has proved most interesting. Two pathways (Scheme 4, paths a and b) have been proposed which are consistent with the experimental results, in particular the incorporation of L-lysine without intervention of symmetrical intermediates or loss of hydrogen from C-2 or C-6. N-Methyl-Δ1-piperideine (13) is implicated as an intermediate in path a, and its role in the biosynthesis of anabasine (14) in Nicotiana glauca and N. tabacum has been examined by administration of [2-14C]-labelled material. Similar results were obtained for both plants. Thus efficient incorporations into (-)-anabasine (14) and (-)-N-methylanabasine (15) were recorded and without randomization of the label. However, the specific activity of the N-methylanabasine, unlike that of the anabasine, was the same as the specific activity of the precursor, i.e., no dilution by inactive alkaloid had occurred in the plant. The failure of these plants to produce N-methylanabasine normally was confirmed by g.l.c. analysis of crude alkaloid extracts and also by the failure to dilute it out of the plants after administration of DL-[2-14C]lysine. The formation of N-methylanabasine (15) from N-methyl-Δ1-piperideine (13) in Nicotiana, therefore, is an aberrant one. The synthesis of unnatural alkaloids by Nicotiana had been reported earlier and this...