Modifications on the basic skeletons of vinblastine and vincristine.

The synthetic investigation of biologically active natural compounds serves two main purposes: (i) the total synthesis of alkaloids and their analogues; (ii) modification of the structures for producing more selective, more effective, or less toxic derivatives. In the chemistry of dimeric Vinca alkaloids enormous efforts have been directed towards synthesizing new derivatives of the antitumor agents vinblastine and vincristine so as to obtain novel compounds with improved therapeutic properties.


Introduction
Vinblastine (1) and vincristine (2) are dimeric alkaloids (Figure 1) isolated from the Madagaskar periwinkle plant (Catharantus roseus), exhibit significant cytotoxic activity and are used in the antitumor therapy as antineoplastic agents.
In the course of cell proliferation they act as inhibitors during the metaphase of the cell cycle and by binding to the microtubules inhibit the development of the mitotic spindle. In tumor cells these agents inhibit the DNA repair and the RNA synthesis mechanisms, blocking the DNA-dependent RNA polymerase.  (1) and vincristine (2). The 17-O-acetyl group of vinblastine was selectively hydrolysed by Thimmaiah and co-workers [5] using a phosphate buffer in methanol; thus 17-desacetylvinblastine (6) was successfully prepared in 95% yield (Scheme 2). Compound 6 can be considered as the active metabolite of vinblastine [6], because its activity is substantially higher than that of the prodrug vinblastine. Scheme 2. Selective desacetylation of vinblastine (1).
Numerous derivatives of vinblastine on the catharanthine part (compounds 14-20, Scheme 7) were prepared by the same research group from 12′-iodovinblastine (10), primarily by coupling reactions catalyzed by palladium (e.g., Stille, Songashira, Negishi) [8]. SAR of derivatives were investigated on HeLa (cervical cancer) and MCF-7 (breast cancer) cell lines and several derivatives showed promising anticancer activity in the P388 murine leukemia model.
Rao and co-workers synthesized 17-desacetylvinblastine-16-hydrazide (8) from vinblastine (1), then the azide 27 was prepared and without isolation of the intermediate azide, vinblastine (1) was coupled with different amino acids through their amino group giving compounds 28 (Scheme 11). Finally, in most cases, the 17-hydroxy substituent was acetylated again to give 29 [14]. Most of the new compounds showed cytotoxic activity against P388 and L1210 leukemia, melanoma, breast cancer and small cell lung cancer [15]. D-and L-Tryptophan derivatives of the 16-position of desacetylvinblastine were conjugated through the carboxyl group with oligoarginine octapeptide as a carrier peptide at the N-terminus (compounds 30,31) by Bánóczi et al. [16] (Scheme 12). One of the stereoisomers 30 or 31 showed a selective cytotoxic effect against the HL-60 human leukemia cells of higher proliferation rate. Ngo et al. [17] synthesized vinorelbine (33) in two steps from anhydrovinblastine (32), prepared by the coupling of vindoline (4) and catharanthine (3) (Scheme 13). This derivative is a potent antitumor agent in the treatment of non-small cell lung cancer [18]. In addition vinorelbine (33) and anhydrovinblastine (32) were hybridized through the 17-hydroxy group of the vindoline part with colchicine, podophyllotoxine and baccatine III to investigate the effect of the new molecules on the polymerisation of tubuline [19].

Scheme 13. Synthesis of vinorelbine (33).
Ngo and his research group hybridized the cleavamine moiety of anhydrovinblastine (32) and vinorelbine (33) on the tertiary amine part with the antimitotic cyclopeptide phomopsin-A and obtained some potent inhibitors of the microtubules assembly and derivatives of good cytotoxicity against KB human cell lines [17]. Vinblastine (1) was also conjugated with a folic acid unit by the azide coupling method presented by Vlahov et al. [20].
A number of vinblastine congeners were synthesized by Kuehne and co-workers, first of all by changing the piperidine ring of catharanthine (3), which were potent against leukemia and colon cancer cell lines. Vinblastine (1) was successfully oxidized to vincristine (2) (Scheme 14) by the same research group, using potassium permanganate and 18-crown-6 phase transfer catalyst [21].
Szántay et al. prepared cyclovinblastine (34) by oxidation of vinblastine (1). The ring transformation reaction of cyclovinblastine (Scheme 15) and cyclovincristine was also investigated and the structure of the products was identified [22].  [23].

Scheme 16. Preparation of vinamidine (35).
Bornmann and Kuehne elaborated the total synthesis of vinamidine (35) and some similar alkaloids [24]. In the course of the reaction procedure a new tetracyclic key intermediate was synthesized, which could be used for preparation of other similar alkaloids.
The key step of the reaction procedure was the reaction of the dimers 36 with isocyanates. In the course of the synthesis compounds were obtained with excellent cytotoxic activity and the new derivatives were active against leukemia. Scheme 17. Spiro-substituted dimer alkaloids.

Scheme 18. Deacetylation of vincristine (2).
The reduced derivative of vincristine (43) was prepared by Szántay and his co-workers (Scheme 19). The reduction was carried out with sodium borohydride in a mixture of alcohols in 63% yield [26].
Superacids were used in some cases to carry out the fluorination reactions [32], and the mechanism of the superacidic fluorination was investigated in detail [33]. The preparation of the vinflunine (58) was also elaborated directly by fluorination of vinorelbine (33) or by fluorination of anhydrovinblastine (32) and then with C′-ring contraction [34].

Scheme 24. Preparation and reaction of 9-chlorocatharanthine (59).
Catharanthine 63 containing an oxirane ring (Scheme 25) was prepared in the reaction of compound 62 with tert-butyl peroxide by Hardouin and his co-workers [36] who used this intermediate in the synthesis of leurosine. The latter was converted to anhydrovinblastine (32) which can be considered as the key intermediate for the synthesis of vinorelbine (33).

Scheme 27. New derivatives of catharanthine.
In the course of the mentioned research project the ester group in position 18 of catharanthine was converted to other acid derivatives [38], e.g., amide, nitrile, and to different substituents such as aldehyde, hydroxy, alkyl, etc. by standard methods (Scheme 27). The aim of this work was to obtain new dimer alkaloids substituted in the catharanthine monomer.
Numerous derivatives of vinblastine and vincristine were prepared by Boger [39]. These compounds were synthesized by the coupling of catharanthine substituted at the aromatic ring in position 12′ and vindoline (4) (Scheme 28). The new derivatives contained different substituents (nitro, amino, halogene, nitrile, alkyl, alkoxy, and thioalkyl), but compounds 76 and 77 proved to be the best antitumor molecules in the case of both sensitive and resistent human colon cancer cells.

Derivatizations of Vindoline
According to the literature the monomer Vinca alkaloid vindoline (4) was generally presumed and found to be ineffective against cell proliferations. Reactions and derivatives of vindoline for the synthesis of vinblastine were scarcely investigated. The reactivity of the aromatic ring of vindoline was presented by Szántay et al. [40], but the new vindoline derivatives were not used for the preparation of dimer alkaloids.

Coupling Reactions
In this section only some tipical example resulting in new derivatives of dimeric alkaloids are mentioned from the many coupling reactions presented in the literature. Tam et al. [38] successfully coupled in two reaction steps the vindoline part with catharanthine substituted in the position 18′ shown before (cf. Scheme 27). The anhydro intermediates 90 were then oxidized to the expected target molecules, which are vinblastines 91 substituted in the catharanthine monomer (Scheme 34). (4)  Ishikawa and his research group [42] elaborated the preparation of vinblastine by coupling catharanthine and vindoline in one step (Scheme 35). Depending the kind of Fe salt and the solvents used, products [vinblastine (1), anhydrovinblastine (32), leurosidine (65) and desoxyleurosidine (92), respectively] and their yields could be changed Scheme 35. Coupling in a one-step reaction.

Conclusions
Over the years a number of research teams have performed extensive and valuable work to synthesize new derivatives of vinblastine and vincristine. Modifications in the vindoline skeleton or in the catharanthine moiety resulted in a number of new antitumor agents with more selectivity or less toxic properties. The mechanism of activity of Vinca alkaloids was investigated using these new derivatives and some new important results were found in connection with the tubulin polymerisation system. Currently, the structure of these dimers still seems to be an inexhaustible source of further research in this field of chemistry and therapy.