Morphine alkaloids, from reticuline

Barton(29) devised a biomimetic route to thebaine (3), dihydrothebainone (13), and thus morphine following observations of Battersby.(30,3l) The alkaloid salutaridine (19) was derived from (+)-reticuline (18) by a low-yielding (0.03%) regioselective para-ortho phenolic coupling reaction according to Scheme 2.3. By the use of thallium tristrifluoracetate, Schwartz(32) improved yields to up to 23%. 

The capsules and stems of Papaver somniferum contain opiate alkaloids essential in medicine. They are classified into two groups, phenanthrene types (morphine, codeine, thebaine) and benzylisoquinoline types [papaverine and noscapine(narcotine)]. These two types of alkaloids show sharply specific pharmacological properties. It is noteworthy that morphinane alkaloids are formed from (-)-(/ )-reticuline, whereas most other alkaloids derive from (-l-)-(5)-re-ticuline 11). 

The first reported biotransformation by means of cell cultures of P. somniferum was the conversion of thebaine to codeine (453). As P. som-nifemm cell cultures only produced alkaloids derived from (5)-reticuline (e.g., sanguinarine. Fig. 9) but none of the alkaloids derived from (/ )-reticuline, Furuya et al. 461) administered (R,S)-reticuline to the cell suspension cultures. After 3 days the alkaloids were isolated. Two alkaloids derived from (5)-reticuline were identified cheilathifoline and scoulerine. A third alkaloid isolated was identified as pure (/ )-reticuline. Thebaine, morphine, and codeine were not metabolized by these cell cultures. However, the cells were capable of stereospecifically reducing codeinone to codeine. Tam et al. 462) also found the same bioconversion. Furthermore, the conversion of thebaine to neopine was reported. Codeine, neopine, papaverine, and D,L-laudanosoline were not metabolized. Enzymatic reduction of codeinone to codeine was also achieved with cell-free preparations of whole plants of both P. somniferum and P. bracteatum 464). Yeoman and co-workers 108,465) reported the use of in reticulate polyurethane immobilized P. somniferum cells for this bioconversion. 

Aporphine-type alkaloids are formed by the intramoleculer oxidative coupling of the benzyhsoquinoline alkaloid, S-reticuline. Thus, from the ortho, ortho -, and ortho, para -intramolecular coupling of the biradicals formed from S-reticuline, bulbocapnine- and isoboldine-type aporphine alkaloids are formed, respectively. When para, ort/zo -intramolecular coupling of the biradicals formed from S-reticuline occurs, the morphine... 

Phylica rogersii (Rhamnaceae). In the opium poppy, Pa-paver somniferum, both enantiomers of reticuline occur with the (+ )-enantiomer predominating. Morphine alkaloids are derived from (- )-reticuline (21). 

The in vivo conversion of (/ )-reticuline to morphinandie-nones such as salutaridine (52) (promorphinanes) has been demonstrated (Santavy, 1979) (Fig. 32.18). An enz)mie from a cell-free extract of Papaver somniferum utilized H2O2 to produce salutaridine in high yield. Morphine alkaloids (see below) are derived from the morphinandienone alkaloid salutaridine. 

Poppies (Papaver somnifenim, Papaveraceae) have long been used as medicinal plants, food plants, and drugs of abuse. Morphine alkaloids are fotmd in many members of the genus Papaver, but codeine and morphine are only found in P. somnifemm. Another species, Papaver bracteatum contains thebaine. Morphine alkaloids are derived from ( )- or (R)-reticuline by series of reactions involving an oxidative coupling reaction. Radioactive labelling experiments have estabhshed the series of reactions from thebaine to codeine to morphine. The most important compotmd from a biosynthetic... 

The first biomimetic synthesis of morphine was described by Sir Derek Barton (Imperial College in London). Barton showed that oxidation of reticuline (31) with potassium ferricyanide gave salutaridine (32) in 0.015% yield. Since salutaridine had been converted to morphine, this constituted a synthesis of the target alkaloid. How does one isolate a product in 0.015% yield. Not easily. In this case, the isotope dilution method was used to establish yield. Reticuline was prepared in radioactive hot form (tritiation in the aromatic ring). The oxidation reaction was run and a known amount of cold salutaridine was added to the reaction mixture. The salutaridine was then isolated and purified to a constant level of radioactivity. The amount of hot salutaridine derived from reticuline was calculated based on the radioactivity that had been incorporated, and the percentage yield of salutaridine (from reticuline) was thus determined. The details are given on Morphine-6. The bottom line is that this demonstrated the feasability of this route to morphine, but it did not provide a practical route to the natural product. 

Morphinans present by far the most important class of reticuline-derived natural products. In contrast to alkaloids of the protoberberine and aporphine skeleton, opium alkaloids are biosynthesized from (/ )-reticuline (20) via an ortho-para conpling reaction. For better comprehension, the structure of (f )-reticuhne (20) has been redrawn to highlight the position of the oxidative conpling reaction and to clearly picture the emerging morphine skeleton (24). Because of the pronounced medicinal and economic importance, opium alkaloids are discussed in detail in the following. 

Papaver Alkaloids.—Biosynthesis of morphine (36) occurs, in Papaver somniferum, through reticuline (33) by way of thebaine (35). The sequence from (35) to (36) involves, inter alia, two O-demethylations, with that at the methoxy-group at C-6 occurring first.1,2 Confirmation that the other methoxy-group is not demethylated first in this Papaver species obtains from the failure to detect oripavine (37), which is found in other Papaver species, as a natural constituent of P. somniferum. The experiment involved attempted isolation of radioactive (37), using inactive alkaloid as carrier, following a feeding experiment with radioactive reticuline (33).37... 

S)-Reticuline is a branch-point intermediate in the biosynthesis of most BAs. Most work has focused on branch pathways leading to the benzophenanthridine (e.g., sanguinarine), protoberberine (e.g., berberine), and morphinan (e.g., morphine and codeine) alkaloids.19 Most enzymes involved have been isolated, many have been purified, and four corresponding cDNAs have been cloned.19 The first committed step in benzophenanthridine and protoberberine alkaloid biosynthesis involves the conversion of (S)-reticuline to (5)-scoulerine by the berberine bridge enzyme (BBE) (Fig.7.2). BBE was purified from Berberis beaniana,20 corresponding cDNAs were cloned from E. californica and B. stolonifera,21 22 and BBE genes have been isolated from P. somniferum and E. californica.23,24... 

Conversion of (S)-reticuline to its ( )-epimer is the first committed step in morphinan alkaloid biosynthesis in certain species. 1,2-Dehydroreticuline reductase catalyzes the stereospecific reduction of 1,2-dehydroreticuline to (7 )-reticuline.39 Intramolecular carbon-carbon phenol coupling of (if)-reticuline by the P450-dependent enzyme salutaridine synthase (STS) results in the formation of salutaridine.40 The cytosolic enzyme, salutaridine NADPH 7-oxidoreductase (SOR), found in Papaver bracteatum and P. somniferum, reduces salutaridine to (7S)-salutaridinol.41 Conversion of (7S)-salutaridinol into thebaine requires closure of an oxide bridge between C-4 and C-5 by acetyl coenzyme A salutaridinol-7-0-acetyltransferase (SAT). The enzyme was purified from opium poppy cultures and the corresponding gene recently isolated (Fig.7.2).42,43 In the last steps of morphine... 

The role of reticuline as an intermediate in the biosynthesis of the mor-phinan alkaloids (Fig. 2.8) was demonstrated by the isolation both of (S)-and (f )-reticuline from the opium poppy. An excess of the (S)-reticuline over the (f )-isomer was found in opium (poppy latex) obtained from the mature plant, in contrast to the roughly equal amounts of these two isomers that occur in poppy seedlings. Both isomers were found to be incorporated into morphine, the major alkaloid isolated from opium, although incorporation of the (f )-isomer was slightly more efficient. (f )-Reticuline is firmly established in P. somniferum as the precursor of the morphinan-type alkaloids (Loefer and Zenk, 1990). (S)-Reticuline, however, is the central intermediate in isoquinoline alkaloid biosynthesis. It has been postulated that (R)-reticuline is formed from (S)-reticuline by isomerization. This inversion of configuration can be explained by the intermediate formation of the 1,2-dehydroreticulinium ion originating from (S)-reticuline, followed by stereospecific reduction to yield the (R) counterpart. The 1,2-dehydroreticulinium ion is efficiently incorporated into opium alkaloids and its role as a precursor of the morphinan-t)q)e alkaloids has been unequivocally established (De-Eknamkul and Zenk, 1990, 1992). 

Morphine and related alkaloids are specific to the genus Papaver (Berberidaceae), although the antipodal series of alkaloids is distributed in the Menisperma-ceae. Early in the biosynthesis of morphine, an inversion at C-1 of (5)-reticuline occurs, followed by ortho-para benzylic coupling to afford salutaridine. Stereospecific reduction and cyclization-elimination affords the 4,5-Ether bridge and thebaine. The dominant pathway from this point involves neopinone, codeinone, codeine, and morphine. Again, most of the enzymes in this sequence were isolated and characterized by Zenk s group (Fig. 30). 

Figure 12.13. The biosynthetic pathway for L-tyrosine derived tetrahydrobenzylisoquinoline alkaloids. The core production of (5)-reticuline and the branch points for berberine, macarpine, and morphine production. Reprinted with permission from Chou and Kutchan (I998)" .

An early key intermediate in benzylisoquinoline biosynthesis is (57), which by decarboxylation affords (59) this in turn leads to (61) and on to alkaloids (Scheme 2). Confirmation of this pathway has come from a study using cell-free preparations of P. somniferum stems and seed capsules. It was found that this preparation catalysed the formation of (57), (59), and (61) from dopamine (54) plus 3,4-dihydroxyphenylpyruvic acid (55) without the addition of 5-adenosyl-methionine, NADPH, and pyridoxal phosphate, the reaction stopped at (57). The formation of the alkaloids reticuline, thebaine, codeine, and morphine, produced by whole plants, could not be detected with this cell-free system. The results confirm not only the intermediacy of (57) and (59) in benzylisoquinoline biosynthesis, but also the involvement of (54) and (55). 

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 The incorporation of (R)-... 

The conversion of reticuline (44) into morphinan alkaloids, which occurs with loss of tritium from C-1 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. bracteatumf 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 (5)-reticuline from (R,S)-reticuline (41) administered to tissue cultures was transformed into (5)-scoulerine (52) and (5)-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 examples of alkaloids possessing the morphinan skeleton include sinomenine [6—8] isolated from the roots of Sinomenium acutum (Menisper-maceae), and metaphanine [9—14] isolated from the stems of Stephania japonica (Menispermaceae). Sinomenine possesses the mirror image skeleton to that of morphine, and is derived from (S)-reticuline. On the other hand, metaphanine possesses the hasubanan skeleton. Total syntheses of metaphanine have been reported [15,16]. 

Hasubanan alkaloids (about 40 in total), in which the nitrogen is attached at position G-14 (rather than C-9), are derived from sinoacutine (Guinaudeau and Bruneton, 1993). (-I- )-Reticuline (20) gives rise to sinoacutine (53). In Sino-menium acutum, sinoacutine is the precursor of sinomenine (54), a compound enantiomeric to morphine. 

Alkaloids of the morphinane group. If the tetrahydroisoquinoline alkaloid norlaudanosoline is written in such a way that part of the molecule is rotated around the dotted line (Fig. 280), the relationship to the morphinane-type alkaloids becomes obviously. The actual precursor of these compounds, however, is (R)-reticuline. It is probably attacked by a phenol oxidase (C 2.3.1) yielding a biradical which is stabilized by the formation of the dienone (- -)-salutaridine. After reduction of (-j-)-salutaridine closure of a new 0-heterocyclic ring results in the formation of thebaine. The alkaloids codeine and morphine are synthesized from thebaine in Papaver somniferum,... 

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