Synthesis and NMR-driven conformational analysis of taxol analogues conformationally constrained on the C13 side chain

Analogues of Taxol (paclitaxel) with the side chain conformationally restri cted by insertion of a carbon linker between the 2 ' -carbon and the ortho- position of the 3 ' -phenyl ring were synthesized. Biological evaluation of these new taxoids showed that activity was dependent on the length of the linker and the configuration at C2 ' and C3 '. Two analogues in the home se ries, 9a and 24a, showed tubulin binding and cytotoxicity comparable to tha t of Taxol. NAMFIS (NMR analysis of molecular flexibility in solution) deco nvolution of the averaged 2-D NMR spectra for 9a yields seven conformations . Within the latter set, the hydrophobically collapsed "nonpolar" and "pola r" classes are represented by one conformation each with predicted populati ons of 12-15%. The five remaining conformers, however, are extended, two of which correspond to the T-conformation (47% of the total population). The latter superimpose well with the recently proposed T-Taxol binding conforme r in beta -tubulin. The results provide evidence for the existence of two p reviously unrecognized structural features that support Taxol-like activity : (1) a reduced torsion angle between C2 ' and C3 ' and (2) an orthogonal a rrangement of the mean plane through C1 ', C2 ' and the 2 ' -hydroxyl and t he 3 ' -phenyl plane, the latter ring bisected by the former plane. By cont rast, epimerization at 2 ' ,3 ' and homologation of the tether to CH2-CH2 w ere both detrimental for activity. The decreased activity of these analogue s is apparently due to configurational and steric factors, respectively.