Synthesis and biological activity of A-nor-paclitaxel analogues

The Chemistry of Nonclassical Taxane Diterpene

The taxane diterpenes are a pharmaceutically vital family of natural products, consisting of more than 550 congeners. All taxane diterpenes are isolated from slow growing evergreen shrubs (genus Taxus) commonly known as "yews" and have a history of over 50 years as potent anticancer compounds. The most prominent congener, taxol (paclitaxel = PTX), has been used in clinics for more than 25 years and is one of the top-selling anticancer drugs worldwide, with annual sales reaching 1.5 billion USD in 1999. Within the taxane diterpene family 11 different scaffolds originating from rearrangements, fragmentations, or transannular C-C bond formations of the "classical taxane core" are known. Among them, five different scaffolds alone belong to the so-called complex or cyclotaxane subfamily, their signature structural feature bearing different types and numbers of transannular C-C bonds across the classical taxane backbone. For synthetic chemists, these five scaffolds represent by far the most challenging of all and have thus evaded total synthesis as well as detailed pharmaceutical evaluation-the latter due to extremely poor sourcing from natural producers. The cousinship of complex taxanes to taxol renders them potentially interesting compounds for drug research in the fight against cancer.This Account specifically summarizes the work on nonclassical taxanes from a biosynthetic, as well as a synthetic, point and provides a synthetic perspective on complex taxanes. Special attention is given to the biosynthetic relationship of complex taxanes and their biological emergence from classical taxanes. The transannular C-C bond forming events in the biosynthesis leading to the five individual scaffolds within this subfamily are structured on the basis of the exact type and number of these specific C-C bond formations. Since functionalization of the classical taxane core in the "oxidase phase" of the biosynthesis precedes the formation of complex taxanes, and is in part prerequisite for these transannular cyclization events, a detailed discussion of these oxidations of the classical taxane backbone is provided. Synthetic efforts toward nonclassical taxanes are scarce in literature and are thus presented in a comprehensive manner for abeotaxanes and complex taxanes. The last part of this Account deals with a synthetic perspective on the synthesis of complex taxanes (cyclotaxanes) and how these most intricate scaffolds can potentially be obtained via a deconvolution strategy. This discussion involves in part unpublished results by our group and is based upon synthetic studies in the literature. The deconvolution strategy we advocate aims for selective fragmentations of the signature transannular C-C bonds of the most intricate scaffold represented by the natural product canataxpropellane, which has recently been synthesized by our group. This strategy represents the converse process of the biosynthesis of complex taxanes (e.g., transannular cyclizations) and is enabled and feasible due to our approach to the canataxpropellane scaffold. We show that, by following this deconvolution strategy, all five scaffolds of complex taxanes can thereby be accessed.

Dual-functional abeo-taxane derivatives destabilizing microtubule equilibrium and inhibiting NF-κB activation

Taxchinin A, with a 11(15→1)-abeo-taxane skeleton, is a major, but inactive taxoid contained in leaves of Taxus chinensis . In our design of dual-functional antitumor abeo-taxane derivatives, two fragments from antitumor agents with different molecular targets (the N-acyl-3'-phenylisoserine side chain from the antimitotic agent paclitaxel and an α,β-unsaturated carbonyl system from NF-κB inhibitors) were incorporated into the scaffold of taxchinin A. The resulting compounds displayed broad inhibitory effects against proliferation of tumor cell lines, with notable selectivity toward colon cancer, melanoma, and renal cancer, when evaluated in the NCI-60 human tumor cell line screening panel. On the basis of the NCI-60 assay data, structure-activity relationship (SAR) correlations were elucidated. Mechanistic studies indicated that this new compound type can both destabilize microtubules and inhibit NF-κB activation, thereby inducing tumor cell apoptosis. This first report of the dual-functional taxoid-core compounds thus provides new opportunities for future drug development based on natural axoid scaffolds.

Bridging converts a noncytotoxic nor-paclitaxel derivative to a cytotoxic analogue by constraining it to the T-Taxol conformation

The synthesis of the bridged A-nor-paclitaxel 4 has been achieved from paclitaxel in a key test of the T-Taxol conformational hypothesis. Although the unbridged A-nor-paclitaxel 3 is essentially noncytotoxic, the bridged analogue 4 is strongly cytotoxic. This result provides strong evidence for the T-Taxol conformation as the bioactive tubulin-binding conformation of paclitaxel.

Concise means for accessing an advanced precursor to 1-deoxypaclitaxel

A program directed toward a total synthesis of 1-deoxypaclitaxel is described. The most direct route consists of six steps from the previously described diketone 12 and proceeds in 18% overall yield. The transformations involved in reaching the target molecule 22 consist of stereoselective alpha-ketol generation, an EtAlCl(2)-catalyzed transannular hydride shift, regioselective monomesylation, and a Wagner-Meerwein 1,2-shift. The central issue of this synthesis is the sequential deployment of these highly controlled steps along the perimeter and across the interior gap of a nine-membered ring.

A spectroscopic and thermodynamic study of Taxol nucleic acid complexes

The interactions of natural and synthetic polynucleotide double strands with the antitumor agent paclitaxel and the oncological product "Taxol for Injection Concentrate" (abbreviated as taxol) were examined in diluted aqueous solutions by thermal denaturation profiles (Tm), CD spectra and UV-absorption measurements. Furthermore, DNA-paclitaxel and -taxol complexes in condensed nucleic acid solutions were studied by differential scanning calorimetry. As polynucleotides alternating and homologous poly[d(AT)] and poly[d(GC)] and calf thymus DNA were used. The results point to stabilizing interactions of paclitaxel to AT nucleotides, whereas in the presence of GC base pairings no interaction took place. Thereby the interaction to homologous (dA).(dT)-tracts seems to be preferred.