Both base-assisted non-concerted metallation-deprotonation (nCMD) and concerted metallation-deprotonation (CMD) have been identified as two potent operating mechanisms in palladium-catalysed direct C-H coupling of oxazole and thiazole-4-carboxylate esters with halides through base- and solvent-effect experiments. Novel C2- and C5-selective CMD direct arylation procedures in oxazole- and thiazole-4-carboxylate series were then designed by controlling the balance between electronic and steric factors. Notably, charge interactions between the palladium catalyst and substrate were identified as a parameter for controlling selectivity and reducing the impact of steric factors in the CMD reaction.

This work describes the synthesis of a series of novel macrocyclic taxoids 3 and 3(H) designed to mimic the docetaxel solid-state ("nonpolar") conformation. These compounds, bearing 18-, 20-, 21-, and 22-membered rings connecting the C-2 OH and C-3' NH moieties, were constructed by ring-closing olefin metathesis of the taxoid-omega,omega'-dienes 4. Biological evaluation of these new taxoids showed that activity is dependent on the ring size, and only the 22-membered ring taxoid 3d exhibits significant tubulin binding. Synthesis of the open-chain analogues 7 and 7(H) and comparison of their biological activities with macrocyclic taxoids show that the carbon tether between C-2 OH and C-3' NH does not hamper tubulin binding. Computational studies of the conformational behavior of the macrocyclic taxoids 3 indicate that the 18-, 20-, and 21-membered-ring 3a-c adopt mainly conformations that are not recognized by tubulin. The most active taxoid 3d appears to adopt a conformation that is between the "nonpolar" and T-shaped forms.

Novel C2-C3'N-linked macrocyclic taxoids 4 bearing an aromatic ring at position C2 were synthesized. These compounds, tethered between N3' and the C2-aromatic ring at the ortho, meta, or para position, were constructed by ring-closing metathesis. The para-substituted derivatives were unable to stabilize microtubules, whereas the ortho- and meta-substituted compounds show significant activity in cold-induced microtubule disassembly assay. The meta derivative 4c is the first C2-C3'-linked cyclic analogue to be equipotent to paclitaxel in this assay and to show significant cytotoxicity. Computational studies of the conformational behavior of these compounds indicate that they can adopt several conformations including mainly the "T-shaped" forms. Docking experiments have shown that the "T-shaped" form is preferred for a good interaction of these compounds with the beta-tubulin binding pocket.