From anhydrovinblastine oxidation

Whether leurosine, Catharine, and their congeners are true alkaloids, or artefacts derived from anhydrovinblastine, the fact remains that the aerial oxidation of anhydrovinblastine is a facile process which does not need to be enzyme-mediated, and a further examination of this reaction has revealed that all the alkaloids of the vinblastine group are produced. The oxidations were performed in acetonitrile solution, and in one experiment, conducted at 26 for 48 hours, the composition of the alkaloid mixture obtained was roughly similar to the relative abundances of the dimeric alkaloids isolated from Catharanthus species. In the oxidation the lone electrons on Nb are presumably involved, since anhydrovinblastine Nb-oxide is inert towards oxidation by air, and while the presence of moisture promotes the reaction, oxygen from the water is not incorporated into the oxidized alkaloids. On the basis of the available evidence, a mechanism, shown in truncated form in Scheme 37, was proposed for the oxidative transformation of anhydrovinblastine into the various alkaloids iso-lated. "°... 

The synthesis of Catharine (250), " to which catharinine was initially believed to be closely related, has in fact been achieved by a process which involves the fission of ring D of the velbanamine component of leurosine (249). This conversion was first reported as a result of the accidental over-oxidation that occurred in the preparation of leurosine from anhydrovinblastine by means of t-butyl hydroperoxide in the presence of trifluoroacetic acid. The by-product in this reaction was initially regarded as the 21-lactam related to leurosine, but it has now been recognised as Catharine, and can be prepared equally well by oxidation in the absence of acid (Scheme 41) a radical mechanism appears to be involved. In view of this facile conversion under oxidising conditions, the status of Catharine as a bona fide natural product is open to question. Indeed, the status of leurosine itself as an alkaloid has been questioned, in view of the ease with which anhydrovinblastine is oxidised to leurosine, even in the absence of specific oxidising agents. For example, anhydrovinblastine is oxidised to leurosine if not stored in an inert atmosphere, and the conversion is even more rapid in solution, particularly in the presence of adsorbents such as silica or alumina. A conversion of 40% has been observed after only 72 hours at room temperature. In view of these results it is perhaps not surprising that anhydrovinblastine has not been isolated from any Catharanthus species examined to date. 

The isolation of Catharine (10), C45H54N40,o, mp 271-275°C, an on-colytically inactive alkaloid, has been reported from several Catharan-thus species C. roseus (29-31), C. ovalis (32), and C. longifolius (33). The structure of Catharine (10) has been elucidated by X-ray crystallography (89-91) of its acetone solvate. Catharine (10) can be obtained by mild oxidation of either leurosine (11) (30) or anhydrovinblastine (8) (92-93). In view of the ease of this oxidation, Catharine (10) may be considered as an artifact of the isolation process. 

More recently, Kutney and co-workers (220) have investigated whether the same dihydropyridinium intermediate 109 is involved in the enzymatic conversion of catharanthine (4) and vindoline (3) to anhydrovinblastine (8) as is involved in the chemical conversion. Use of a cell-free preparation from a 5-day culture of the AC3 cell line gave 18% of the bisindole alkaloids leurosine (11), Catharine (10), vinamidine (25), and hydroxy-vinamidine (110), with 10 predominating. When the incubations were carried out for only 5-10 min, the dihydropyridinium intermediate was detected followed by conversion to the other bisindole alkaloids, with FAD and MnClj required as cofactors. Clearly a multienzyme complex is present in the supernatant, but further purification led to substantial loss of enzymatic activity. The chemical formation of anhydrovinblastine (3) is carried out with catharanthine A-oxide (107), but when this compound was used in the enzyme preparation described, no condensation with vindoline (3) occurred to give bisindole alkaloids. This has led Kutney and co-workers to suggest (220) that the A-oxide 108 is not an intermediate in the biosynthetic pathway, but rather that a 7-hydroperoxyindolenine... 

The extremely low yield of vincristine (2) from intact plants has made pursuit of its biosynthesis a very challenging problem, which at this point in time remains unsolved. Kutney et al. have used both anhydrovinblastine (8) (227) and catharanthine N-oxide (107) (233) as precursors to vincristine (2) in a cell-free preparation, but incorporation levels were extremely low. Therefore, the question of whether vinblastine (1) is an in vivo, as well as an in vitro, precursor remains to be answered. Several possibilities exist for the overall oxidation of vinblastine (1) to vincristine (2), including a direct oxidation of the A-methyl group or oxidative loss of the N-methyl group followed by N-formylation. 

Coupling of other vindoline derivatives with ring D or E modified oxidation levels (92-96) to catharanthine N-oxide provided new binary products for biological evaluation 39, 95-97). The two diastereomeric C-16 -C-14 PARE anhydrovinblastines 42 and 97 were obtained in a 46 54 ratio (50% yield) from racemic catharanthine (98), and the corresponding 20 -desethyl compounds 98 and 99 were generated at - 20°C in a 1 1 ratio (16% yield each), and at -76°C in lower yields, together with the corre-... 

The chemical reactivity of N-6 (or N ), directed entirely by the basicity of this atom, is controlled by the nature and stereochemistry of the substituents at C-4 (vide supra). Oxidation of N-6 occurs under mild conditions in several naturally occurring bisindole alkaloids. Thus, treatment of a dichloromethane solution of leurosine (4) with m-chloroperben-zoic acid at -20°C for 4 hr gives the N -oxide (15) in greater than 90% yield after preparative reversed-phase chromatography (46). Leurosine A/ -oxide has also been isolated from Catharanthus roseus and should therefore be considered a naturally occurring bisindole (50). The analogous conversion of vinblastine (1) to its A/ -oxide (16) proceeds under similar conditions but requires longer exposure to the peraeid (24 hr) (5/) 3, 4 -anhydrovinblastine is converted to its N -oxide (17) in 10 min at 0°C... 

The product from the reaction of anhydrovinblastine Nfc-oxide 1 with trifluoroacetic anhydride in methylene chloride was treated, after evaporation of the excess of the reagent, with a mixture of water and THF. This gave a mixture of products, only one of which could be obtained pure (27%), namely 5 -noranhydrovinblastine 2. 

If the C-15, C-16 bond is oxidatively cleaved, the secodine skeleton results (the proposed progenitor of the Aspidosperma and the iboga systems) through alternative Diels-Alder type cyclizations to afford tabersonine and catharanthine. The bisindole alkaloids of Catharanthus roseus reflect the union of vindoline and catharanthine to afford anhydrovinblastine modification affords the clinically significant alkaloids, vinblastine (VLB) and vincristine (VCR Fig. 39). The alkaloids, particularly VCR, are important as anticancer agents and have led to the development of the semisynthetic derivatives vindesine and vinorelbine (Fig. 40). Synthetic approaches are available to join the monomeric precursors. The enzymatically controlled sequence of reactions from tabersonine to vindoline has been elucidated. 

In a special case of tertiary amine oxidations, the electrogenerated diiminium ion obtained from the indole alkaloid cantharanthine couples in the 16-position with the electron-rich aromatic subunit of vindoline in the 10-position to give the highly cytostatic anhydrovinblastine. In the presence of methanol, the methoxy group is introduced in the 16-position, yielding, after a follow-up reduction, 16-methoxyclea-vamine [18]. 

It would thus appear that the presence of an a-acetoxy-group at C-15 severely inhibits the fission of the 16,21-bond in the coupling reaction, since the isovinblastine O-acetate (258) was obtained in yields of only 6 and 4%, respectively, from (257) and (260). The effect of a /3 -acetoxy-group is less well defined Honma and Ban " report the formation of anhydrovinblastine (255), but only as the minor product of the reaction, whereas Kutney and Worth report the formation of (253) and (254), but in unspecified yield. For the synthesis of vinblastine derivatives the absence of a C-15 substituent, as in catharanthine and dihydro-catharanthine, seems preferable for example, catharanthine N-oxide was... 

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