Vindoline intermediates, formation

The enzyme-catalyzed formation of anhydrovinblastine (8) from catharanthine (4) and vindoline (3) was first examined by Kutney and co-workers (170,219) using a cell-free preparation. [ao f- H]Catharanthine (4) and [acety/- C]vindoline (3) were incubated for 3-8 hr, both separately and jointly with a preparation from C. roseus, which led to the isolation of labeled anhydrovinblastine (8) and leurosine (11) incorporations were of the order of 0.54 and 0.36%, respectively. On this basis, anhydrovinblastine (8) was proposed as the key biosynthetic intermediate en route to vinblastine (1) and vincristine (2). 

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... 

A rationalization of the generally selective formation of the undesired C-16 -C-14 PREF relative stereochemistry on reaction of the chloro-imine-chloroindoline alkene 17a,b with vindoline (3) may be found in a preferred conformation, 25a or 25b, of the cationic intermediate 25, where the C-3 methylene group hinders approach to one face of the nine-mem-bered ring (Fig. 2). A conformationally more flexible nine-membered ring cationic intermediate 29a,b would be expected to be formed from the chloro derivative of a D-secocleavamine 30. From reaction of such a compound with vindoline (3), under the usual protic acid conditions for cou-... 

Rosazza et al. (52) have proposed the mechanism shown in Fig. 6 for the formation of both dihydrovindoline ether and its dimer from vindoline. This mechanism, which proposes an intermediate enamine-immonium ion species 54-55 produced by N-oxidation, followed by intramolecular attack of the C-l 6 hydroxyl oxygen, also accounts for the formation of 3-acetonyldihydro-vindoline ether 42 from vindoline by Streptomyces albogriseolus (49). Reduction of the immonium ion 55 thus leads directly to dihydrovindoline ether, whereas its capture by acetoacetate or the enamine 54 (79) leads to 42 and 45, respectively. 

The dimerization of catharanthine and vindoline is believed to proceed via the formation of an unininm intermediate with catharanthine (Fig. 2e). This unininm intermediate is reduced to form anhydrovinblastine, a naturally occurring compound in C. roseus plants (115). In support of this mechanism, anhydrovinblastine is incorporated into vinblastine and vincristine in feeding studies (116-119). 

Sundberg, R. J., Gadamasetti, K. G., Hunt, P. J. Mechanistic aspects of the formation of anhydrovinblastine by Potier-Polonovski oxidative coupling of catharanthine and vindoline. Spectroscopic observation and chemical reactions of intermediates. Tetrahedron 1992,48, 277-296. 

The partial synthesis of vinblastine itself makes use of 20-acetoxycatharanthine (223), prepared from catharanthine as described earlier. The modified Polonovski reaction in the presence of vindoline gave an intermediate immonium ion, which was reduced to 20 -acetylvinblastine (Scheme 37). Mild alkaline hydrolysis afforded deacetylvinblastine, the secondary hydroxy-group of which could be re-acetylated preferentially, with formation of vinblastine (271). ... 

The tetracyclic indoline (525) is a key intermediate in the synthesis of vindoline (101), and the corresponding demethoxy analog is important in the formation of vindorosine (99). Takano and co-workers have now reported two improved routes to the intermediate 525 (309). 

It has been observed that labeled strictosidine [3] geis-soschizine [4] (80,85,86) stemmadenine [7] (86,87) and tabersonine [7b,9] (86-88) were all incorporated into both catharanthine [8] and vindoline [10] in Catharanthus roseus plants, indicating that these are the main precursors in the biosynthetic pathway to the Aspidosperma-lboga alkaloids. Other intermediates such as geissoschizine oxindole [5], preakuammicine [6] have been detected 28-40 hours after germination of c. roseus seeds (85,87,89) provided strong evidence for the formation of catharanthine [8] and vindoline [10] as presented in schemes I and ll. 

Preakuammicine (19) serves as an efficient precursor of akuammicine (20) (a class 1 Strychnos alkaloid) and vindoline (9) (an Aspidosperma alkaloid). When labeled stemmadenine (21) (a Strychnos alkaloid) (class 1) was fed to Catharanthus roseus plants, labeled tabersonine (24) (class 1), vindoline (9) (class 3), and catharanthine (10) (class 5) were produced. Thus, preakuammacine (19) and stemmadenine (21) serve as key intermediates of both class 3 and 5 alkaloids. It has been proposed that incorporation of stemmadenine (21) into vindoline (9) and catharanthine (10) proceeds via dehydrosecodine (25). A Diels-Alder reaction could lead to the formation of tabersonine (24) and catharanthine (10), respectively (Fig. 34.5). 

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