The Alkaloids Volume 9

A Review of the Literature Published between July 1977 and June 1978

By M. F. Grundon

The Royal Society of Chemistry

Copyright © 1979 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-660-4

Contents

Chapter 1 Biosynthesis By R. B. Herbert, 1,
Chapter 2 Pyrrolidine, Piperidine, and Pyridine Alkaloids By A. R. Pinder, 35,
Chapter 3 Tropane Alkaloids By G. Fodor and J. Butterick, 46,
Chapter 4 Pyrrolizidine Alkaloids By D. J. Robins, 55,
Chapter 5 lndolizidine Alkaloids By J. A. Lamberton, 67,
Chapter 6 Quinolizidine Alkaloids By M. F. Grundon, 69,
Chapter 7 Quinoline, Quinazoline, and Acridone Alkaloids By M. F. Grundon, 78,
Chapter 8 β-Phenethylamines and lsoquinoline Alkaloids By K. W. Bentley, 89,
Chapter 9 Aporphinoid Alkaloids By M. Shamma, 126,
Chapter 10 Amaryllidaceae Alkaloids By M. F. Grundon, 137,
Chapter 11 Erythrina and Related Alkaloids By A. H. Jackson, 144,
Chapter 12 Indole Alkaloids By J. E. Saxton, 151,
Chapter 13 Diterpenoid Alkaloids By S. W. Pelletier and S. W. Page, 221,
Chapter 14 Steroidal Alkaloids By D. M. Harrison, 238,
Chapter 15 Miscellaneous Alkaloids By J. R. Lewis, 251,

CHAPTER 1

Biosynthesis

BY R. B. HERBERT

1 Introduction

As before, previous Reports in this series appear as the first references, and extensive reference is made to them in the text. A new review on alkaloid biosynthesis and a wide-ranging book on alkaloid biology and metabolism have been published.

2 Piperidine, Pyridine, and Pyrrolidine Alkaloids

Dioscorine. — The most interesting observation, that the piperidine fragment (heavy bonding) of dioscorine (1) arises not from lysine (or acetate) but nicotinic acid, previously published in preliminary form, is now available in full. A similar finding for the piperidine ring of anatabine (2) is to be noted.

Coniine. — Further experiments on the enzyme-catalysed conversion of 5-oxo-octanal (3) plus alanine into y-coniceine (4) plus pyruvate have led to the isolation, from Conium maculatum, of two enzymes which will carry out this reaction. Their properties have been explored and it is suggested, from their different rates of reaction with (3) and differing inhibitions by pyruvate and (3), that they act together in mediating this reaction in the plant.

Quinolizidine Alkaloids. — Cadaverine is known to be a precursor for quinolizidine alkaloids. (For discussion of the biosynthesis of these alkaloids see also previous Reports). Recent experiments have shown that cadaverine is a precursor for alkaloids (anagyrine, pachycarpine, ammodendrine, N-methylcytisine, and cytisine) in Ammodendron karelinu too. Metabolism of lupanine, anagyrine, ammodendrine, and pachycarpine in the plant was also studied.

It has been found that isophoridine and allomatrine are not biosynthetic intermediates in Sophora alopecuroides. Curiously, alkaloids of the sparteine (5) type were found to be precursors of those with the matrine (6) skeleton. Methylation of cytisine (7) was observed to be reversible.

Matrine-type alkaloids [as (6)] were found to be labelled by radioactive lysine and cadaverine in Goebelia pachycarpa and to be interconvertible.

Quinolizidine alkaloids of the sparteine type [as (5)] are known to arise from three molecules of lysine via a symmetrical intermediate (cadaverine). Aphylline (8) also arises from three molecules of lysine in Anabasis aphylla, but without the participation of a symmetrical intermediate.

Pyrindicin. — The pattern of 13C n.m.r. signal enhancement observed on incorporation of [1-13C]- and [2-13C]-acetate and [l-13C]propionate into pyrindicin (9) in Streptomyces griseoflavus var. pyrindicus indicates that the metabolite is formed from five acetate units and one propionate unit, as shown in (9). Some labelling by acetate of the propionate unit was observed, which was interpreted as being the result of metabolism via both the tricarboxylic acid and glyoxalate cycles.

Tenellin. — Results of a study on the biosynthesis of tenellin (10), previously reported in a preliminary communication, are now available in a full paper. New information is that tyrosine is a less efficient precursor than phenylalanine, which indicates that benzene-ring hydroxylation occurs after the phenylalanine has undergone further modification. It is to be noted that incorporation of phenylalanine into tenellin (10) (benzene ring plus carbons 4, 5, and 6) involves an intramolecular rearrangement of the phenylalanine skeleton.

Tropane Alkaloids. — Hygrine [as (11)] is a proven precursor for tropane alkaloids, e.g. hyoscyamine (12). It has now been shown further that (-f-)-hygrine (11) is much preferred over its enantiomer as a substrate for elaboration of tropane alkaloids, e.g. (12), in Datura innoxia. On the other hand (-f-)-hygrine was only slightly preferred for the formation of cuscohygrine (14).

Tiglic acid is found as the esterifying acid in a number of tropane alkaloids, e.g. 3α-tigloyloxytropane (13). It is known to be derived from L-isoleucine via 2-methylbutanoic acid.28 Alternative formation of tiglic acid in D. meteloides from C1/C2/C3 sources, passing through 3rhydroxy-2-methylbutanoic acid, has been examined, with negative results. The hydroxy-acid was tested as an alkaloid precursor but was found not to be incorporated, whereas isoleucine was found to be incorporated in a parallel experiment.

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)].

Attention was then directed31 to determining changes in oxidation level at C-4 and C-6 of isoleucine on conversion into senecic acid. [6-3H, 6-14C]Isoleucine gave senecionine (15) in which the 14C activity was expected to be confined to C-14 and C-18, and equally distributed between them (see ref. 30). Approximately five-sixths of the tritium label was retained, and, although the probable interference of a tritium isotope effect would distort the results, it is clear that one, probably two, hydrogen atoms from C-6 of isoleucine are retained at C-14 of (15).

Half the tritium label from L-[4-3H]isoleucine was retained in the formation of the necic acid fragment of retrorsine (16) (at C-13 and C-20). This limits the isoleucine C-4 oxidation level to that of a carbinol or vinylic methine group, and excludes carbonyl formation at this centre. Corroboration is thereby provided for the negative results referred to above with 2-methyl-3-oxobutanoic acid.

Following up the possibility that C-4 of isoleucine becomes part of a vinyl system, supported by mechanistic considerations, (18) was tested as a precursor. Preliminary results indicate that it is incorporated into the senecic acid fragment of senecionine (15). The significance of this, however, must await more detailed investigation.

Phenanthroindolizidine Alkaloids. — Previous results establish that the phenacyl-pyrrolidines (19), (20), and (21) are important precursors for phenanthroindolizidine alkaloids, e.g. tylophorinine (28), in Tylophora asthmatica. These results and a consideration of the oxygenation pattern of the T. asthmatica bases in relation to possible biosynthesis by phenol oxidative coupling, as well as the structure (22) for the alkaloid septicine, pointed to (26) as a key intermediate.

Samples of (26), (23), and (25), doubly labelled as shown ([??]=14C), were examined as precursors for tylophorine (30), tylophorinidine (29), and tylophorinine (28) and were found to be incorporated into each alkaloid at a similar level, indicating a close biosynthetic relationship. The changes in isotope ratio were consistent with the necessary tritium loss from sites in (23) and (25) which became hydroxylated in the course of biosynthesis, and from C-6′ in (26) during phenol coupling. It follows that (23), (25), and (26) are intact precursors for (28), (29), and (30), but the much lower incorporation observed for (23) compared to the other two compounds indicated that it was utilized along a minor pathway. The major route to (26) must be (19) -> (20) -> (21) -> (25) -> (26). The hexahydroindolizine (26) can only give (28), (29), and (30) via the dienone (27), alternative courses of rearrangement, i.e. path b in Scheme 1, and reduction and rearrangement (path a) affording the three alkaloids after further minor modification. In the rearrangement of (27) (and the dienol derived from it), the unique opportunity, in alkaloid biosynthesis, of styryl (as against aryl) migration must be taken in affording (28) and (29), and it may well be taken in the formation of (30); isotylocrebrine (24), a minor base of T. asthmatica, may arise by alternative aryl shift within (27).

The base (26) can, on paper, give almost all the other known phenanthro-indolizidine alkaloids. It will be of interest to see whether this is so or not.

Nicotine. — Nicotine (33) is assembled in Nicotiana species from nicotinic acid (31) and N-methyl-Δ1-pyrroline (32). Administration to Nicotiana plants of 5-fluoronicotinic acid and derivatives of (32) methylated at C-2 and C-3 has resulted in the formation in vivo of unnatural nicotine analogues. In contrast, 4-methylnicotinic acid has been found not to be transformed in vivo into 4-methylnicotine, presumably because this particular methyl group interferes sterically with the appropriate enzyme reactions involved in nicotine biosynthesis.

3 Phenethylamine and Isoquinoline Alkaloids

A stimulating review on unsolved problems in isoquinoline biosynthesis has been published.

Normacromerine. — The β-hydroxy-phenethylamines, e.g. normacromerine (36), are closely related biosynthetically to phenethylamines. Both tyrosine and tyramine serve as precursors for (36) in Coryphantha macromeris var. runyonii and so do norepinephrine (34) and epinephrine (35). A role for these compounds as intermediates in normacromerine biosynthesis is supported by their detection as normal constituents of the plant. A two-fold difference in the level of incorporation of (34) and (35) was interpreted as indicating separate pathways via these bases to (36), but firm conclusions must await further work.

Reticuline. — A biosynthetic study of the important benzylisoquinoline reticuline (41), published in preliminary form, has appeared in full. Important new information is that 4-hydroxyphenylpyruvic acid (37) and 3,4-dihydroxyphenyl-pyruvic acid (38) are incorporated, like tyrosine, into both C6–C2 units of recticuline (41), in contrast to the transamination product of (38), i.e. dopa (39), which, curiously, is only used for the elaboration of one of these units (ring A and attached ethanamine residue), being incorporated via dopamine (40). Condensation of dopamine with (38) affords (43), which has been shown to be a benzylisoquinoline precursor followed in sequence by (42). The new results confirm these findings by showing that (42) and (43) are reticuline precursors too.

Morphinan Alkaloids. — Extensive research on the biosynthesis of morphine (51) and related alkaloids in Papaver somniferum has allowed a detailed description of the pathway from the amino-acid tyrosine through reticuline (44), thebaine (46), and codeine (50) to morphine (51) (Scheme 2). The incorporation of (R)- and (S’)-reticuline (44) occurs with extensive loss of tritium from C-l, consistent with equilibration of (44) and 1,2-dehydroreticuline (47) prior to their use in biosynthesis. This is strongly supported by the observation that (47) is an alkaloid precursor, a result which has been confirmed recently. Further conviction that (47) must be a biosynthetic intermediate follows from its isolation by radio-isotope dilution from P. somniferum after assimilation of 14CO2. The pool size of (47) appeared to be one-fifth of that of reticuline. (The use of a double-labelling technique to ensure the purity of material isolated is to be noted)

Biotransformation of thebaine (46) into codeine (50) (Scheme 2) occurs by way of neopinone (49) and codeinone (48), and involves 6-0- demethylation as a first step. The mechanism of this reaction has been explored with thebaine (46) labelled with an lsO-label at C-6. The codeine (50) and morphine (51) isolated in the experiment showed retention of one-third of the label. In a parallel experiment, however, codeinone (48) was shown to lose two-thirds of 18O-label from C-6 (by exchange). It follows that the conversion of thebaine (46) into neopinone occurs without loss of the oxygen at C-6. This means that a mechanism related to the chemical hydrolysis of an enol ether does not operate, and an alternative has been tentatively suggested (Scheme 3).

The conversion of reticuline (44) into morphinan alkaloids, which occurs with loss of tritium from C-l in P. somniferum (see above), has been observed also for the formation of thebaine (46) in P. bracteatum, a plant which produces this alkaloid but not codeine or morphine. Radioactive 1,2-dehydroreticuline (47) labelled both reticuline (44) and thebaine (46), whilst radioactive reticuline again labelled thebaine (46). Codeinone (48) and codeine (50) are biosynthetic intermediates between thebaine (46) and morphine (51) in P. somniferum, and it was shown that (48) was efficiently reduced to (50) in P. bracteatum. It is apparent that alkaloid biosynthesis in the two plants is similar, with the important difference that in P. bracteatum the enzymes which effect demethylation of (46) are missing, and so biosynthesis goes no further than thebaine (46).

Callus tissue of P. somniferum has been reported not to produce morphinan alkaloids but benzophenanthridine, protopine, and aporphine bases. Recent experiments have shown that (S‘)-reticuline from (R,S)-reticuline (41) administered to tissue cultures was transformed into (S)-scoulerine (52) and (S)-cheilan-thifoline (53) [(R)-reticuline was recovered unchanged]. Morphine, codeine, and thebaine were not metabolized by the culture but (–)-codeinone (48) was converted stereospecifically and in high yield into (–)-codeine (50), both by the culture and by a crude enzyme preparation from it.

Other workers have obtained a callus tissue culture from the same plant which does produce morphinan alkaloids. The production of alkaloids was found to be stimulated by tyrosine and ascorbic acid.

The latex of P. somniferum is known to metabolize morphine in vivo. Incubation of morphine with latex in vitro also results in its metabolism. One of the products was identified as morphine N-oxide, which also occurs naturally in the plant.

The efficient absorption through the surface of P. bracteatum and P. somniferum plants of radioactive glycine, phenylalanine, and urea, in the presence of detergent, and the appearance of radioactivity in morphine alkaloids, has been noted.

The morphinandienone alkaloid flavinantine (56) is biosynthesized via reticuline (44) in Croton flavens. Its O-methyl ether, sebiferine (55), has now been shown also to arise from reticuline and nor-reticuline (54) (in Cocculus laurifolius)’, two other possible isoquinoline precursors (nororientaline and laudanosine) were excluded by experiment. As in the case of morphine biosynthesis (above), both (R)- and (S)-reticuline (44) were equally good precursors, suggesting that (47) is again involved in biosynthesis. It appears that the 4′-O-methyl group in nor-reticuline (54) is retained on formation of sebiferine, although the results are not unambiguous. A further finding is that flavinantine (56) is converted into sebiferine (55) in C. laurifolius.

Bisbenzylisoquinoline Alkaloids. — Until recently, the biosyntheis of only one of the many bisbenzylisoquinoline alkaloids had been studied. This was epistephanine (57), shown to derive from two units of coclaurine (59). New results establish that several other bisbenzylisoquinolines are variations on the coclaurine theme too.

(Continues…)Excerpted from The Alkaloids Volume 9 by M. F. Grundon. Copyright © 1979 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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