Conformation taxoids

Syntheses, Biological Activity, and Conformational Analysis of Fluorine-Containing Taxoids Ojima, I. Kuduk, S.D. Slater, J.C. Gimi, R. H. Sun, C.M Chakravarty, S. Ourevitch, M. Abouabdellah, A. Bonnet-Delpon. D. in Biomedical Frontiers of Fluorine Chemistry Ojima, I. McCarthy, J. R. Welch, J. T., Eds. ACS Symposium Series 639 American Chemical Society Washington, DC, 1996 pp 228-243. 

In this chapter, we describe an account of our research on the chemistry and biology of paclitaxel and taxoid anticancer agents (taxoid = taxol-like compound). The topics covered in this chapter include (i) the development of a practical and efficient method for the semisynthesis of paclitaxel and docetaxel using chiral 3-hydroxy-P-lactams as synthetic intermediates, (ii) structure-activity relationship (SAR) studies of various taxoids that led to the discovery of the extremely potent second-generation taxoids, and (iii) biological and conformational studies with the use of fluorine-containing taxoids as probes. ... 

The requirement for a certain level of rigidity of the C-2 and C-3 substituents led us to investigate the solution conformation of taxoid 48 (nonataxel) in aqueous media by 2-D NMR and molecular modeling techniques. We found that the two 2-methylprop-l-enyl moieties of this molecule orient themselves with respect to each other quite specifically to form a strong hydrophobic clustering (see Section IV.C).73... 

IV. FLUORINE-CONTAINING TAXOIDS SAR STUDY AND FLUORINE PROBE APPROACH TO THE IDENTIFICATION OF BIOACTIVE CONFORMATIONS... 

In the course of our study on the rational design, synthesis and SAR of new antitumor taxoids, we became interested in incorporating fluorine(s) into paclitaxel and taxoids in order to investigate the effects of fluorine on cytotoxicity, blocking of known metabolic pathways, and its use as a probe for identifying bioactive conformation(s) of taxane antitumor agents. 

The use of 19F NMR for a variable temperature (VT) NMR study of fluorinated taxoids is obviously advantageous over the use of H NMR because of the wide dispersion of the l9F chemical shifts that allows fast dynamic processes to be frozen out. Accordingly, F2-paclitaxel 65 and F-docetaxel 66 were selected as probes for the study of the solution structures and dynamic behavior of paclitaxel and docetaxel, respectively, in protic and aprotic solvent systems.77 The inactive 2, 10-diacetyldocetaxel (73) was also prepared to investigate the role of the 2 -hydroxyl moiety in the conformational dynamics.89 While molecular modeling and NMR analyses (at room temperature) of 73 indicate that there is no significant conformational changes as compared to paclitaxel, the 19F NMR VT study clearly indicates that this modification exerts marked effects on the dynamic behavior of the molecule.77... 

Thus, the fluorine probe approach has proved useful for the conformational analysis of paclitaxel and taxoids in connection with the determination of possible bioactive conformations.77 The previously unrecognized conformer C might be the molecular structure first recognized by the P-tubulin binding site on microtubules. 

C. Determination of the Binding Conformation of Taxoids to Microtubules Using Fluorine Probes... 

It has been noted that the global shape of other taxoid site ligands, such as PTX, DTX or epothilones, can be described as consisting of a cyclic core and a side-chain tail [124], Although DDM structure is not cyclic and hence its conformation is less restricted, it seems that DDM folds in such a way to resemble the shape of the other drugs. 

Bourchard et al. attempted to reduce the 7,19-cyclopropane docetaxel analog 92a electrochemically. Besides the major 10-deoxy product 92b, C-ring expanded 19-nor analog 93 was obtained as a minor product. Taxoid 93 did not exhibit any activity in both cytotoxicity and tubulin binding assays, although its conformation is analogous to that of docetaxel. 

Metaferia et al. reported on the synthesis and cytotoxicity of C3 -C4 linked macrocyclic taxoids in 2001. The synthetic strategy was also based on RCM. Similar to Ojima et al. s finding, these compounds (100) are less active than paclitaxel in both cytotoxicity and tubulin assays.However, after careful inspection of the bioactive conformations of taxoids with computer simulation and nuclear magnetic resonance (NMR) experiments, they designed several C-4 and C3 -Ph orf/io-position linked taxoids (101) that were highly active, several times better than paclitaxel. ... 

It should be noted that in 2004, Ganesh et al. and in 2000, Snyder et claimed in their articles that only the T-conformation was the conformation to be adopted by paclitaxel when binding to tubulin, whereas collapsed polar and nonpolar conformations do not work. Their conclusion was supported by computer simulation and NMR experiments of semisynthetic taxoids, in which the C-4 alkyl terminal and the C3 -Ph ortfio-position were linked. ... 

In another application, 19F NMR studies have provided critical information on the bioactive conformation of taxoids. Fluorine-containing taxoids have been used as probes for NMR analysis of the conformational dynamics of paclitaxel in conjunction with molecular modeling [166], The dependence of the 19F chemical shifts and the Jm--ny values of these fluorinated analogues is examined through 19F and H variable-temperature (VT) NMR measurements. The experiments clearly indicate highly dynamic behavior of these molecules and the existence of equilibrium between conformers. The analysis of the VT NMR data in combination with molecular modeling, including restrained molecular dynamics (RMD), has identified three key conformers, which were further confirmed by the 19F- H heteronuclear NOE measurements. 

Figure 1.29 Solid-state NMR studies on the microtubule-bound conformation of taxoids using fluoro-taxoid probes.

Ojima, I., Kuduk, S. D., Slater, J. C., et al. (1996) Syntheses, biological activity, and conformational analysis of fluorine-containing taxoids, in Biomedical Frontiers of Fluorine Chemistry, ACS Symposium Series 639 (eds I. Ojima, J. R. McCarthy, J. T. and Welch.), American Chemical Society, Washington, D.C., pp. 228-243. 

Two taxane alkaloids have been investigated by diffraction techniques taxine A (la) [23] and 2 deacetoxyaustrospicatine (41) [49], The results gave important information on the orientation of the side chain relatively to the terpenoid core. In both cases the aromatic ring is close to the terpenoid core, but in 4f the phenyl and the catbonyl are antiperiplanar, whereas in taxine A they are synclinal (cf. C and A in Scheme 3). As a result, the protons closer to the phenyl ring are H(6a) in taxine A and H(14a) in 4f. This is in accordance with the results of the H-NMR analysis [46], In 4f, the conformation with C-l and the phenyl antiperiplanar is stabilized by hydrophobic interactions with the methyl of the 13-acetate [49], The X-ray features of the terpenoid core of 4f and taxine A show linear strain at several carbon-carbon bonds, as generally observed in taxoids [55]. 

Shigemori, and J. Kobayashi, Crystal and Solution State Conformations of Two Taxoids, Taxinine and Taxinine B Tetrahedron, 1997,53 (13), 4621 -... 

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