Dihydrovindoline ether

Scheme 14. The structures of vindoline (46) and derivatives, vinblastine and vincristine (52 and 53), and dihydrovindoline ether (54).

Recent work in our laboratories has confirmed the existence of a similar pathway in the oxidation of vindoline in mammals (777). The availability of compounds such as 59 as analytical standards, along with published mass spectral and NMR spectral properties of this compound, served to facilitate identification of metabolites formed in mammalian liver microsome incubations. Two compounds are produced during incubations with mouse liver microsome preparations 17-deacetylvindoline, and the dihydrovindoline ether dimer 59. Both compounds were isolated and completely characterized by spectral comparison to authentic standards. This work emphasizes the prospective value of microbial and enzymatic transformation studies in predicting pathways of metabolism in mammalian systems. This work would also suggest the involvement of cytochrome P-450 enzyme system(s) in the oxidation process. Whether the first steps involve direct introduction of molecular oxygen at position 3 of vindoline or an initial abstraction of electrons, as in Scheme 15, remains unknown. The establishment of a metabolic pathway in mammals, identical to those found in Strep-tomycetes, with copper oxidases and peroxidases again confirms the prospective value of the microbial models of mammalian metabolism concept. 

The Eli Lilly workers found that incubation of vindoline with Streptomyces sp. A17000 gave a complex mixture of products, from which they were able to isolate dihydrovindoline ether (40) and 16-dehydroxy-14,15-dihydro-15,16-epoxy-14-oxo-3-norvindoline (43) in low yield (49). The structure of 40 was determined by comparison of its PMR and mass spectral data with those of the previously described deacctyidihydrovindoline ether (41) and the alkaloid cathanneine (cathoclavine) (53) (73 - 75). Confirmation of the... 

Fig 5. Mass spectral fragmentation of dihydrovindoline ether dimer (45) (52). 

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. 

Many Actinomycetes and Streptomycetes demonstrated the ability to remove the O-acetyl group, but very few carried out other transformations (188). A Streptomyces sp. A17000, when incubated with vindoline (101) for five days afforded several compounds of which two were characterized. One of these, termed dihydrovindoline ether or more correctly 11-methoxycathovaline (104), was identified by comparison of the spectral data, particularly the PMR and mass spectra with those of cathovaline (100). A minor product proved to have a novel skeleton. 

The microbial transformation of vindoline by means of Streptomyces griseus gives dihydrovindoline ether (previously obtained) and a novel vindoline dimer (265). ... 

Reaction of dihydrovindoline (534) with mercuric acetate in refluxing dioxane gave in one step the ether lactam 536. The intermediates in this interesting reaction are the six-membered lactam and the a,( -unsaturated lactam, which then undergoes Michael attack at the /8 -position to afford 536. The ether bridge was opened by removal of a proton alpha to the lactam carbonyl with the anion of triphenylmethane, and the tertiary hydroxyl group was acetylated to afford 537. The lactam carbonyl was removed by conversion to the imino ether followed by borohydride reduction. Treatment of the product with moist silica gel gave vindoline (101 Scheme 31). 

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