The Alkaloids Volume 13

A Review of the Literature Published between July 1981 and June 1982

By M. F. Grundon

The Royal Society of Chemistry

Copyright © 1983 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-367-2

Contents

Chapter 1 Biosynthesis By R.B. Herbert, 1,
Chapter 2 Pyrrolidine, Piperidine, and Pyridine Alkaloids By A.R. Pinder, 44,
Chapter 3 Tropane Alkaloids By G. Fodor and R. Dharanipragada, 55,
Chapter 4 Pyrrolizidine Alkaloids By D.J. Robins, 65,
Chapter 5 Indolizidine Alkaloids By J.A. Lamberton, 82,
Chapter 6 Quinolizidine Alkaloids By M.F. Grundon, 87,
Chapter 7 Quinoline and Acridone Alkaloids By M.F. Grun1on, 99,
Chapter 8 S-Phenylethylamines and the Isoquinoline Alkaloids By K.W. Bentley, 122,
Chapter 9 Aporphinoid Alkaloids By M. Shamma and H. Guinc1udec1u, 172,
Chapter 10 Amaryllidaceae Alkaloids By M.F. Grundon, 187,
Chapter 11 Erythrina and Related Alkaloids By A.S. Chawla and A.H. Jackson, 196,
Chapter 12 Indole Alkaloids By J.E. Saxton, 205,
Chapter 13 Lycopodium Alkaloids By W.A. Ayer, 277,
Chapter 14 Diterpenoid Alkaloids By S.W. Pelletier and S.W. Page, 281,
Chapter 15 Steroidal Alkaloids By D.M. Harrison, 309,
Chapter 16 Miscellaneous Alkaloids By J.R. Lewis, 322,

CHAPTER 1

Biosynthesis

BY R. B. HERBERT

Continuity with previous Reports in this series is maintained. Background information for new work appearing here is, as usual, obtainable through earlier Reports to which reference is given; two comprehensive reviews are also cited.

1 Pyrrolidine and Piperidine Alkaloids

1.1 Nicotine. — The biosynthesis of nicotine (6) is well established to be from ornithine (1), sequentially through putrescine (2), its N-methyl derivative (3), and (4) (cf. Vol. 12, p.1 ; Vol. 11, p.1). New results very usefully allow deduction of stereochemistry involved in each of the biosynthetic steps.

(R)-[1-2H]Putrescine (2) was well incorporated into nicotine (6) in tobacco plants. The nicotine [see (5)] showed deuterium n.m.r. signals, of similar height, corresonding to the 2′-proton and the 5′-pro-R proton in (6). If the putrescine labels are traced through Scheme 1 , the reader will see that this observation accords with stereospecific removal of the pro-S proton from a -CH2NH2 group of putrescine (7) in the oxidation of (3). (Removal of the 1-pro-R hydrogen atom of putrescine {see (7)} would have given nicotine labelled only at C-5′]. This stereochemistry is the same as that found for other reactions catalysed by diamine oxidase (cf. Vol. 10, p.9).

Levels of tritium retention in nicotine (6) using DL- and L-[(R S)-5-3H]ornithine samples as precursors showed that L-ornithine (1) rather than the D-isomer is the preferred precursor. Tritium retention was measured relative to DL-(5-14c]-ornithine administered at the same time. (For earlier application of the method used here, see Vol. 5, p.7.)

DL-[2-3H, 5-14C]Ornithine gave nicotine with loss of half of the tritium present in the precursor. This is consistent with decarboxylation of (1) occuring with retention of tritium and subsequent loss of half of it during the conversion of (3) into (4). Since the result with the [2H] is that the oxidation of (3) results in loss of the 1-pro-S proton of putrescine (7), the putrescine derived from L-[2-3H]ornithine (the usable part of the DL-precursor) must have had tritium in the (S)-configuration. Therefore decarboxylation of L-ornithine proceeds with retention of configuration, as is the case with bacterial ornithine decarboxylase. Indeed all the amino-acio decarboxylases so far studied catalyse decarboxylation in the same stereochemical sense (see ref. 6 and refs. cited in ref. 5).

The stereochemistry of the last step in nicotine biosynthesis (Scheme 1) follows from the known stereochemistry of the alkaloid.

It has been found that treating callus of Nicotiana tabacum with urea leads to an increase in the level of nicotine production. By correlation with this, the content of ornithine, citrulline, and arginine, which are urea cycle intermediates and nicotine precursors, was higher in treated callus than in untreated callus.

Good evidence has been obtained that N’-isoproylnornicotine is produced during air-curing of tobacco leaves and is not formed in intact plants. It was deduced to be formed from nornicotine.

1.2 Cocaine and Cuscohygrine. — Preliminary results (cf. Vol. 12, p.3), which showed that cocaine (8) is derived in part from ornithine (1) in Erythroxylon coca, have been published in fu11. New, and most interesting, information is that the label from DL-[S-14C]ornithine appeared equally divided between the two bridgehead carbon atoms (C-1 and C-5) in cocaine (8). This indicates that this alkaloid, in contrast to the structurally similar tropane alkaloids, is derived from ornithine by way of a symmetrical intermediate (putrescine). It is interesting to note that [S-14C]ornithine incorporation into cuscohygrine (9) in E. coca was also by way of a symmetrical intermediate, by contrast with the biosynthesis of cuscohygrine in other plants which does not involve any syrrunetrical intermediate (cf. Vol. 12, p.3). The key intermediate in the biosynthesis of these pyrrolidine alkaloids is (4). It appears quite simply that its biosynthesis from ornithine in some plants (e.g. E. coca and Nicotiana species) is via putrescine (7); in others it is not.

1.3 β-Pyrazol-1-ylalanine. The biosynthesis of β-pyrazol-1-yl–L-alanine (10), which contains an unusual N-N linkage, has been studied. The clear evidence, using whole plants and cell-free extracts of cucumber (Cucumi sativus), is that 1,3-diaminopropane is a precursor for the heterocyclic ring of (10). Pyrazole can also act as a precursor; it is enzymically condensed with O-acetylserine to give (10).

1.4 Pyrrolizidine Alkaloids. — The origins of the pyrrolizidine ring system [as (11)] which is found in these alkaloids has been receiving recent, well-merited attention (cf. Vol. 12, p.4; Vol. 11, p.2; Vol. 10, p.13). Work relating to the incorporation principally of [14C]ornithine and [14C]putrescine, which was previously published in preliminary form, (cf. Vol. 10, p.13) is now available in a full paper.

A careful analysis of the difficulties of unravelling pyrrolizidine alkaloid biosynthesis without ambiguity is contained in a full paper which is now available from one of two groups to use putrescine as a precursor labelled with 13C and 15N (cf. Vol. 12, p.5). Additional results, which relate to the incorporation of radioactive ornithine, putrescine, and spermidine, support the most recent results; Δ1-pyrroline, a possible alkaloid precursor, was not incorporated.

There is a cautionary tale relating to the use of mixed 3H and 14C labels. It was found with some samples of retronecine (11), where tritium was on carbon next to nitrogen, that there could be enough of a difference in pKa compared to when protium was present for partial separation of 14C-labelle and 3H-species to occur on chromatography, with consequent disastrous change in isotope ratio.

1.5 Anabasine. The specific incorporation of lysine (13) into the piperidine ring of anabasine (12) has been re-examined, with confirmation of earlier results. A mixture of DL-[4,5-13C2]- and DL-[6-14C]-lysine was used as precursor. The 13C n.m.r. spectrum of the derived anabasine (12) showed satellites for C-4′ and C-5′ due to the presence of the two contiguous 13C lysine labels in the alkaloid. This neatly confirms the previously deduced orientation of the lysine skeleton in the piperidine ring of (12). (For other applications of the approach used here, see Vol. 12, p.1 ; Vol. 11, pp.1 and 19.)

Degradation of the anabasine gave results showing that over 98% of the [6-14C] lysine label was located at C-6′. This confirms that lysine is incorporated into (12) without the intervention of any symmetrical intermediate. This excludes cadaverine (17) as an intermediate formed from lysine (13). In the above experiment, inactive cadaverine was added during isolation. When reisolated it was found to be essentially devoid of radioactivity, which indicates that cadaverine was not formed from the radioactive lysine fed. (For further discussion, see Section 1.6 below).

1.6 The Early Stages of Alkaloid Biosynthesis. — It is well established that L-lysine (13) is incorporated into some piperidine alkaloids by way of a sym_metrical intermediate; it is accepted that this symmetrical intermediate is cadaverine (17), which is also an alkaloid precursor. Lysine, however, is incorporated into other alkaloids, e.g. anabasine (12) (Section 1.5) and sedamine (18), without the intervention of any syrnmetrical intermediate. Cadaverine (17), although able to act as an alkaloid precursor, cannot be an intermediate formed from the lysine fed, because of its symmetry (cf. Vol. 10, p.9; refs. 1 and 2).

An ingenious model has been developed which accounts for the biosynthesis of all piperidine alkaloids (cf. Vol. 10, p. 9). The key idea is that those alkaloids which are formed with syrrunetrization of a lysine label are biosynthesized by way of free cadaverine (17); those which are formed without symmetrization of label are biosynthesized by way of cadaverine, which remains unsymmetrical by being (co)enzyme-bound [as (15)] until oxidation occurs to give (16) (Scheme 2).

The decarboxylation by lysine decarboxylase of L-lysine (13) to give cadaverine (17) occurs with retention of configuration [protonation occurs on the a-face of the imine (14)]. The oxidation of cadaverine (17) occurs with loss of the 1-pro-S proton, which is the proton originally sited at C-2 in L-lysine (13) (cf. Section 1.1). It follows that L-[2-3H]lysine should give alkaloids such as sedamine (18) with loss of the tritium label. But such a label is known to be retained on formation of sedamine (18). This observation is difficult to reconcile with the model shown in Scheme 2 (·Vol. 10, p. 9) but may be accommodated in a modified version.

If instead of protonation of the imine function in (14), in the lysine decarboxylase reaction, nucleophilic attack by the δ-amino-group of lysine [see (19)] occurs, (16) is obtained directly and independently of cadaverine, and without loss of the C-2 proton of lysine. This modified” decarboxylase would function, it is suggested, for the biosynthesis of alkaloids such as sedamine (18), see Scheme 3; lysine and cadaverine serve, on this model, as essentially independent alkaloid precursors (cf. Section 1.5 for supporting evidence). For those alkaloids, which are biosynthesized from lysine via cadaverine, the course of biosynthesis is straightforward [with normal protonation on the α-face of (14)) (Scheme 4). If the hypothesis is correct then it follows that all alkaloids formed unsymmetrically from lysine would retain the C-2 proton of the amino-acid. Those formed symmetrically would lose half of the label from a L-[2-3H]lysine precursor [complete retention with the formation of (17) and loss of half the label on oxidation of (17)). This has not yet been tested but in the analogous case of nicotine biosynthesis (cf. Schemes 1 and 4) half of the tritium from [2-3H]ornithine is lost on formation of the alkaloid (Section 1.1). It may be noted that in the original hypothesis (Scheme 2) a similar fate would be predicted for the lysine C-2 proton in the biosynthesis of both groups of piperidine alkaloids.

2 Phenethylamine and Isoquinoline Alkaloids

The seminal ideas relating to the oxidative coupling of phenols have found very widespread application, and yet the actual mechanism of coupling in vivo remains obscure. In new work, it has been found that extracts of Papaver somniferum which had peroxidase activity were unable to catalyse the conversion, by phenol oxidative coupling, of reticuline into salutaridine.

The establishment of alkaloid — producing tissue cultures of BerEeris species, of Papaver bracteatum of Corydalis ophiocarpa, and of P. somnifrum have been reported. Some alkaloid interconversions with cultures of the last two species were also reported.

All of the twenty-seven known spirobenzylisoquinoline alkaloids, e.g. (20), contain a methylenedioxy-group on ring D. It has been suggested that the formation of these groups from an ortho-methoxy-phenol is encouraged by steric compression around a C-9 methoxy-substit uent which is relieved very substantially upon the formation of a methylenedioxy-group in vivo.

2.1 Norlaudanosoline Synthase. Strong evidence, including some arising from the use of enzyme preoarations, has been obtained by several groups of workers that isoquinoline alkaloids are formed from a phenethylamine [as (21)] and an α-keto-acid [as (22)]: condensation affords the amino-acid [as (24)], which upon decarboxylation and reduction gives a typical isoquinoline base, exemplified by norlaudanosoline (25) (cf. Vol. 2, p.10: Vol. 6, p. 17: Vol. 7, p.10: Vol. 9, p.8: Vol. 10, p.15). An analogous pathway has been established for β-carboline alkaloids (this Report, Section 3.1). New evidence from another reputable group of workers is a stark contradiction.

An enzyme that synthesises norlaudanosoline (25) has been isolated, and purified, from several plant species which normally produce isoquinoline alkaloids. Substrates for the enzyme were dopamine (21) and, most surprisingly, 3,4-dihydroxyphenylacet-aldehyde (23), and not 3,4-dihydroxyphenylpyruvic acid (22). 4-Hydroxyphenylacetaldehyde was a substrate for the enzyme but not 4-hydroxyphenylpyruvic acid or phenylpyruvic acid or phenylpyruvic acid. The product of the clearly enzyme-catalysed reaction between (21) and (23) was norlaudanosoline (25) [predominantly the (S)-isomer]. No doubt the question of the normal intermediacy of (22) (23) in isoquinoline biosynthesis will receive urgent attention. In particular, it would be useful to discover if amino-acid precursors [as (24)) are required to have a particular chirality. If so, one could conclude that their utilization in biosynthesis is by a normal enzyme-catalysed reaction.

2.2 Hordenine and Normacromerine. — The metabolism of hordenine (25) in Hordeum vulgare plants has been studied. The alkaloid is ultimately degraded to C6-Cl intermediates that are incorporated into polymeric material.

Further information on the biosynthesis of normacromerine (27) (cf. Vol. 11, p.8; Vol. 10, p.15; Vol. 9, p.7) is that it can be “formed in Coryphantha macromeris from normetanephrine (28), which is in turn formed from norepinephrine (29). Normetanephrine was shown to be a natural constituent of the cactus. Octopamine (30) was a poor normacromerine precursor.

2.3 Hasubanonine, Protostephanine, and Laurifinine. — Full papers on the biosynthesis of hasubanonine (31) and protostephanine (32), which are most interesting benzylisoquinoline variants, have been published: an epic piece of research. (Preliminary accounts were reviewed in Vol. 8, p.8; Vol. 6, p.26).

Protostephanine (32) and laurifinine (33) have closely related structures, and the latter has been shown to derive from (+}-N-nor-protosinomenine (34) in Cocculus laurifolius. [The biosynthesis of (32) stands in marked contrast.] The precursor (34) and norlaudanosoline (25} were utilized for biosynthesis, but not three other isoquinolines with rnethylation patterns different from those of (34). ([+ or -])-N-Norprotosinomenine [as (34)) was incorporated without loss of its Q-methyl groups or the proton at C-1 (that is, of the (+)-isomer, the (-)-isomer not being utilized) .

C. Laurifolius also produces coccuvine (35), an alkaloid of the Erythrina type. The biosynthesis of this alkaloid had previously been deduced to proceed from (34) by way of the N-desmethyl derivative of laurifinine (33) (cf. Vol. 11 , p.14). The new results are complementary then to the old ones.

2.4 Aporphine Alkaloids. — The biosynthesis of N-methylcrotsparine (36), N-methylcrotsparinine (38), and N-methylsparsiflorine (37) in Croton sparsiflorus has been studied. The key precursor is N-methylcoclaurine (39), each enantiomer serving specifically as a precursor for either (36) and (37), on the one hand, or (38) on the other. The specific incorporation of (40), dopamine, and 4-hydroxyphenylpyruvic acid is to be noted. The results are entirely complementary to others obtained with this plant (cf. Vol. 6, p.19; Vol. 11 , p.10).

2.5 Bisbenzylisoquinoline Alkaloids. — The biosynthesis of a number of bisbenzylisoquinoline alkaloids has been investigated (cf. Vol. 12, p.11; Vol. 10, p.16; Vol. 9, p.11). One of these investigations concerned tiliacorine and tiliacorinine. The results, previously published in preliminary form, and reviewed in Vol. 9, are now available in a full paper (with the loss of two coworkers).

Preliminary results using Thalictrum minus which showed that thalicarpine (41) was formed from two molecules of reticuline (42) (cf. Vol. 12, p.13) have been included in a full paper. The results are supported by those of other workers obtained with Cocculus laurifolius. Of several related isoquinolines tested, reticuline (42) was the best precursor; norreticuline (43) was also satisfactorily incorporated, and it is specifically the (S)-isomer of reticuline which is used in biosynthesis. The expectation that the aporphine moiety is formed before elaboration of the bisbenzylisoquinoline skeleton is supported by the incorporation into (41) of tritiated isoboldine (44) and, at a lower level, boldine (45). Both isoboldine (44) and reticuline (42) were found to be present in T. minu. However, norreticuline (43) was transformed into (41) with loss of the 4′-Q-methyl which is expected to appear in the aporphine half of (41), but with retention of the 4′-Q-methyl group which should be present in the other half. It follows that a demethylation occurs at some point in the course of biosynthesis involving the aporphine half. This could occur to provide a free hydroxy-group necessary for phenol oxidative coupling within a bisbenzylisoquinoline precursor. This would mean that isoboldine (44) is not a normal intermediate in the biosynthesis of thalicarpine (41).

(Continues…)Excerpted from The Alkaloids Volume 13 by M. F. Grundon. Copyright © 1983 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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