Synthesis of epothilones A and B in solid and solution phase

Studies towards the synthesis of epothilone A via organoboranes

Studies towards the synthesis of epothilone A via organoboranes have been described. A modified procedure for the large-scale preparation of B-gamma,gamma-dimethylallyldiisopinocampheylborane from prenyl alcohol has been developed. This reagent, upon reaction with various aldehydes, provides the corresponding alpha,alpha-dimethylhomoallylic alcohols in high enantioselectivities. The application of this reagent for the synthesis of the C1-C6 subunit of epothilone has been demonstrated. Alternatively, inter- and intramolecular asymmetric reduction protocols have also been utilized for the synthesis of the C1-C6 subunit of epothilone A. The synthesis of the C7-C21 fragment of epothilone A involving asymmetric alkoxyallyl- and crotylboration using alpha-pinene-derived reagents has also been described.

Chemoselective Construction of Substituted Conjugated Dienes Using an Olefin Cross-Metathesis Protocol

[Reaction: see text] Various substituted conjugated dienes have been made by olefin cross-metathesis. Using either electronic or steric "protection," one of the olefins of the conjugated diene was deactivated relative to the other for cross-metathesis. The reactions proceed with very high chemoselectivity and, when steric deactivation is used, very high diastereoselectivity.

Interaction of epothilone analogs with the paclitaxel binding site: relationship between binding affinity, microtubule stabilization, and cytotoxicity

The interactions of epothilone analogs with the paclitaxel binding site of microtubules were studied. The influence of chemical modifications in the C15 side chain and in C12 on binding affinity and microtubule elongation was characterized. Modifications favorable for binding affinity are (1). a thiomethyl group at C21 of the thiazole side chain, (2). a methyl group at C12 in S configuration, (3). a pyridine side chain with C15 in S configuration, and (4). a cyclopropyl moiety between C12 and C13. The same modification in different ligands has similar effect on affinity, allowing good structure-affinity characterization. The correlation between binding, microtubule stabilization, and cytotoxicity of the compounds has been determined, showing differential effects of the modifications. The binding constants correlate well with IC(50) values, demonstrating that affinity measurements are a useful tool for drug design.

Total Syntheses of Epothilones B and D

A convergent, total synthesis of epothilones B (2) and D (4) is described. The key steps are Normant coupling to establish the desired (Z)-stereochemistry at C12-C13, Wadsworth-Emmons olefination of methyl ketone 28 with the phosphonate ester 8, diastereoselective aldol condensation of aldehyde 5 with the enolate of keto acid derivatives to form the C6-C7 bond, selective deprotection of acid 52, and macrolactonization.

Discovery of (E)-9,10-dehydroepothilones through chemical synthesis: on the emergence of 26-trifluoro-(E)-9,10-dehydro-12,13-desoxyepothilone B as a promising anticancer drug candidate

We provide a full account of the discovery of the (E)-9,10-dehydro derivatives of 12,13-desoxyepothilone B (dEpoB), a new class of antitumor agents with promising in vivo preclinical properties. The compounds, which are to date not available by modification of any of the naturally occurring epothilones, were discovered through total chemical synthesis. We describe how our investigations of ring-closing metathesis reactions in epothilone settings led to the first and second generation syntheses of (E)-9,10-dehydro-12,13-desoxyepothilone congener 6. With further modifications, the synthesis was applied to reach a 26-trifluoro derivative compound (see compound 7). To conduct such studies and in anticipation of future development needs, the total synthesis which led to the initial discovery of compound 7 was simplified significantly. The total synthesis methodology used to reach compound 7 was then applied to reach more readily formulated compounds, bearing hydroxy and amino functionality on the 21-position (see compounds 45, 62, and 63). Following extensive in vitro evaluations of these new congeners, compound 7 was nominated for in vivo evaluations in xenograft models. The data provided herein demonstrate a promising therapeutic efficacy, activity against large tumors, nonrelapseability, and oral activity. These results have identified compound 7 as a particularly promising compound for clinical development. The excellent, totally synthetic, route to 7 makes such a program quite feasible.

Rational design of azepane-glycoside antibiotics targeting the bacterial ribosome

RNA recognition by natural aminoglycoside antibiotics depends on the 2-deoxystreptamine (2-DOS) scaffold which participates in specific hydrogen bonds with the ribosomal decoding-site target. Three-dimensional structure information has been used for the design of azepane-monoglycosides, building blocks for novel antibiotics in which 2-DOS is replaced by a heterocyclic scaffold. Azepane-glycosides showed target binding and translation inhibition in the low micromolar range and inhibited growth of Staphylococcus aureus, including aminoglycoside-resistant strains.

The essence of total synthesis

For the past century, the total synthesis of natural products has served as the flagship of chemical synthesis and the principal driving force for discovering new chemical reactivity, evaluating physical organic theories, testing the power of existing synthetic methods, and enabling biology and medicine. This perspective article seeks to examine this time-honored and highly demanding art, distilling its essence in an effort to ascertain its power and future potential.

Ketone-catalyzed asymmetric epoxidation reactions

This Account summarizes our efforts in developing organic ketone catalysts for enantioselective and diastereoselective epoxidation of olefins. We have developed an efficient and general protocol for epoxidation of olefins using dioxiranes generated in situ from activated ketones and Oxone (2KHSO5.KHSO4.K2SO4) in a homogeneous aqueous solvent system. We have also developed chiral ketone catalysts for highly enantioselective epoxidation of unfunctionalized trans-olefins and trisubstituted olefins. Excellent diastereoselectivities have been achieved for epoxidation of substituted cyclohexenes with ketone catalysts.

Epothilones: Mechanism of Action and Biologic Activity

Drugs that target microtubules are among the most commonly prescribed anticancer therapies. Although the mechanisms by which perturbation of microtubule function leads to selective death of cancer cells remain unclear, several new microtubule-targeting compounds are undergoing clinical testing. In part, these efforts focus on overcoming some of the problems associated with taxane-based therapies, including formulation and administration difficulties and susceptibility to resistance conferred by P-glycoprotein. Epothilones have emerged from these efforts as a promising new class of anticancer drugs. Preclinical studies indicate that epothilones bind to and stabilize microtubules in a manner similar but not identical to that of paclitaxel and that epothilones are effective in paclitaxel-resistant tumor models. Clinical phase I and early phase II data are available for BMS-247550, BMS-310705, EPO906, and KOS-862. The results suggest that these compounds have a broad range of antitumor activity at doses and schedules associated with tolerable side effects.

Expedient Protocol for Solid-Phase Synthesis of Secondary and Tertiary Amines

[reaction: see text] An expedient solid-phase synthetic approach to secondary and tertiary amines was developed. The protocol employs conversion of resin-bound amino alcohols to the corresponding iodides, followed by iodide displacement with primary or secondary amines or with unprotected amino alcohols. This two-step procedure, affording products in good to excellent yields, is suitable for solid-phase synthesis of polyamines.

Phase I Clinical and Pharmacokinetic Study of BMS-247550, a Novel Derivative of Epothilone B, in Solid Tumors

PURPOSE

The purpose of this study was to determine the maximum tolerated dose, toxicity, and pharmacokinetics of BMS-247550 administered as a 1-h i.v. infusion every 3 weeks.

EXPERIMENTAL DESIGN

Patients with advanced solid malignancies were premedicated and treated with escalating doses of BMS-247550. Blood sampling was performed to characterize the pharmacodynamics and pharmacokinetics of BMS-247550.

RESULTS

Twenty-five patients were treated at six dose levels ranging from 7.4 to 59.2 mg/m(2). At 50 mg/m(2), 4 of 9 patients (44.4%) had dose-limiting toxicity (neutropenia, abdominal pain/nausea). At 40 mg/m(2) (the recommended Phase II dose), 2 of 12 patients (16.7%) had dose-limiting neutropenia. Overall, the most common nonhematological toxicity was fatigue/generalized weakness (grade 3-4 seen in 9.0% of patients), followed by neurosensory deficits manifested as peripheral neuropathy and by gastrointestinal discomfort. At 40 mg/m(2), the incidence of grade 3 fatigue, abdominal pain, diarrhea, and neuropathy was 7.7%. Grade 1-2 neuropathy was observed in all patients enrolled and treated at 40 mg/m(2). Two patients with paclitaxel-refractory ovarian cancer, one patient with taxane-naïve breast cancer, and another patient with docetaxel-refractory breast cancer had objective partial responses (lasting 6.0, 5.3, 3.0, and 4.5 months, respectively). The mean pharmacokinetic parameter values during course 1 for clearance, volume of distribution, and apparent terminal elimination half-life at the 40 mg/m(2) (recommended Phase II dose) dose level were 21 liters/h/m(2), 826 liters/m(2), and 35 h (excluding one outlier of 516 h), respectively. Values during course 1 and course 2 were similar.

CONCLUSIONS

The recommended dose for Phase II evaluation of BMS-247550 is 40 mg/m(2), although more long-term observations are needed. BMS-247550 has advantages over taxanes in relation to drug resistance and warrants further study.

Ring-closing metathesis: development of a cyclisation-cleavage strategy for the solid-phase synthesis of cyclic sulfonamides

A series of novel 7-membered cyclic sulfonamides have been synthesised using a solid-phase cyclisation-cleavage RCM strategy. Model solution studies indicated the sulfonamides were suitable substrates for RCM using the Grubbs' catalyst 2. Starting from either 2-carboxyethyl polystyrene (21) or Merrifield resin, various seven-membered sulfonamides were prepared in good to excellent yields at low catalyst loadings (2.5-5 mol%) using a flexible spacer between the polymer and the substrate. In addition, a novel double-armed linker was shown to allow efficient RCM cleavage of sulfonamides with as little as 1 mol% of the ruthenium alkylidene complex 2.

Ring-Opening Metathesis/Oxy-Cope Rearrangement: A New Strategy for the Synthesis of Bicyclic Medium Ring-Containing Compounds

Ring-opening/ring-closing metathesis on cyclobutene-containing substrates with angular oxygen functionality provides a stereospecific introduction of 1,5-bis-dienes required for an anion-accelerated oxy-Cope rearrangement. The reaction sequence offers generally a stereocontrolled preparation of a variety of medium ring-containing bicyclic ring systems, while rearrangement to the bicyclo[7,3,0]dodecane (9-5) system leads to a mixture of olefin isomers.

Preparation, physical properties, on-bead binding assay and spectroscopic reliability of 25 barcoded polystyrene-poly(ethylene glycol) graft copolymers

Here we describe the preparation of 25 beaded polystyrene-poly(ethylene glycol) graft copolymers from six spectroscopically active styrene monomers: styrene, 2,5-dimethylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 4-tert-butylstyrene, and 3-methylstyrene. These polymers were thoroughly characterized by Raman, infrared, and (1)H/(13)C NMR spectroscopies, and differential scanning calorimetry. Determination of the swelling properties, peptide synthesis, and on-bead streptavidin-alkaline phosphatase (SAP) binding assay further established that their physical and chemical properties where not significantly altered by the diversity of their encoded polystyrene core. Each of the 25 resins displayed a unique Raman and infrared vibrational fingerprint, which was converted into a "spectroscopic barcode". The position of each bar matches the peak wavenumber in the corresponding spectrum but is independent of its intensity. From this simplified representation similarity maps comparing 35 000 resin pairs were generated to establish the spectroscopic barcoding as a reliable encoding methodology. In effect, in 99% of the cases, the highest similarity coefficients were obtained for resin pairs prepared from the same styrene derivatives even after SAP binding assay. We have also shown that a small but unique combination of a resin's vibrations (30-40%) is sufficient for its identification. However, in rare cases where a resin's vibrational signature has been severely compromised, both the Raman and infrared barcodes were synergistically and reliably utilized to unequivocally identify its chemical make up.

Treatment of recurrent cervical adenocarcinoma with BMS-247550, an epothilone B analog

OBJECTIVE

The incidence of recurrent cervical adenocarcinoma is rising relative to the squamous subtype. There are limited therapeutic options for women with advanced cervical adenocarcinoma. Only a few chemotherapy agents have demonstrated activity in this disease. This report describes results with BMS-247550, an epothilone B analog that stabilizes microtubules, with activity in previously treated adenocarcinoma of the cervix.

METHOD

We present two women with recurrent cervical adenocarcinoma with metastases to the lung. Both women were treated previously with paclitaxel and were enrolled in a phase I study with BMS-247550. Both women had partial responses to BMS-247550 with a decrease in tumor size and CA-125 levels.

CONCLUSIONS

The demonstration of a response to BMS-247550, especially after additional chemotherapy had been administered, is encouraging, albeit preliminary. The ultimate role of BMS-247550 and multiagent chemotherapy in the treatment of adenocarcinoma of the cervix should be further investigated.

Chemoinformatics methods for systematic comparison of molecules from natural and synthetic sources and design of hybrid libraries

Until recently, the field of diversity and library design has more or less ignored natural products as a compound source. This is probably due to at least two reasons. First, combinatorial and reaction-based approaches have been major focal points in the early days of computational library design. In addition, a widespread view is that natural products are often highly complex and not amenable to medicinal chemistry efforts. This contribution introduces recent computational approaches to systematically analyze natural molecules and bridge the gap between natural products and synthetic chemistry programs. Large scale comparisons of natural and synthetic molecules are discussed as well as studies designed to identify 'synthetic mimics' of natural products with specific activity. In addition, a concept for the design of natural/synthetic hybrid libraries is introduced. Although research in this area is still in its early stages, an important lesson to be learned from computational analyses is that there is no need to a priori 'shy away' from natural products as a source for molecular design.

Phase I trial and pharmacokinetic study of BMS-247550, an epothilone B analog, administered intravenously on a daily schedule for five days

PURPOSE

The epothilones are a novel class of nontaxane microtubule-stabilizing agents. BMS-247550 is a semisynthetic analog of the natural product epothilone B. We conducted a phase I study administering BMS-247550 as a 1-hour intravenous infusion daily for 5 consecutive days every 21 days.

PATIENTS AND METHODS

Twenty-one patients received BMS-247550 without filgrastim in the first cycle. An additional six patients were enrolled at a starting dose of 8 mg/m2/d with filgrastim support. Twenty-one of the 27 patients had received prior paclitaxel, docetaxel, or both.

RESULTS

One hundred seven cycles were administered to 27 patients. The maximum-tolerated dose was 6 mg/m2 of BMS-247550 administered as a 1-hour intravenous infusion daily for 5 consecutive days every 21 days. Dose-limiting toxicity at a dose of 8 mg/m2/d was neutropenia with or without filgrastim support. Nonhematologic grade 3 toxicities included fatigue (seven cycles), stomatitis (two cycles), and anorexia (one cycle). The mean terminal half-life of BMS-247550 was 16.8 +/- 6.0 hours, the volume of distribution at steady-state was 798 +/- 375 L, and the clearance was 712 +/- 247 mL/min. Objective responses were observed in patients with breast, cervical, and basal cell cancer. Reductions in CA-125 levels were noted in patients with ovarian cancer.

CONCLUSION

The recommended phase II dose of BMS-247550 on the daily schedule for 5 days is 6 mg/m2/d. Neutropenia was dose limiting, but higher doses were tolerated by a large fraction of patients with filgrastim support. Peripheral neuropathy was mild, even after multiple cycles of therapy, and was not dose limiting.

The role of the microtubules in tumor necrosis factor-alpha-induced endothelial cell permeability

Tumor necrosis factor (TNF)-alpha, a major proinflammatory cytokine, triggers endothelial cell activation and barrier dysfunction which are implicated in the pathogenesis of pulmonary edema associated with acute lung injury syndromes. The mechanisms of TNF-alpha-induced vascular permeability are not completely understood. Our initial experiments demonstrated that TNF-alpha-induced decreases in transendothelial electrical resistance across human pulmonary artery endothelial cells are independent of myosin light chain phosphorylation catalyzed by either myosin light chain kinase or Rho kinase. We next assessed the involvement of another cytoskeletal component, the tubulin-based microtubule network, and found TNF-alpha to induce a decrease in stable tubulin content and partial dissolution of peripheral microtubule network as evidenced by anti-acetylated tubulin and anti-beta-tubulin immunofluorescent staining, respectively. Microtubule-stabilizing agents, paclitaxel and epothilone B, significantly attenuated TNF-alpha-induced decreases in transendothelial electrical resistance, inhibited the cytokine-induced increases in actin stress fibers, formation of intercellular gap, and restored the TNF-alpha-compromised vascular endothelial (VE)-cadherin-based cell-cell junctions. Importantly, neither TNF-alpha nor paclitaxel treatment was associated with endothelial cell apoptosis. Inhibition of p38 mitogen-activated protein kinase by SB203580 significantly attenuated TNF-alpha-induced microtubule destabilization, actin rearrangement, and endothelial barrier dysfunction. These results strongly suggest the involvement of microtubule rearrangement in TNF-alpha-induced endothelial cell permeability via p38 mitogen-activated protein kinase activation.

Parallel synthesis of H-pin polyamides by alkene metathesis on solid phase

A small library of H-pin polyamides with variable aliphatic bridge lengths (CH(2))(n)(), where n = 4-8, connecting a central Py/Py pair was prepared via parallel synthesis with Ru-catalyzed alkene metathesis on solid phase as a complexity-generating cross-linking reaction. DNA binding affinities and sequence specificities were analyzed for each member of the library to determine the optimum linker length. An H-pin polyamide with a six-methylene bridge was found to have the highest affinity to its match site with high selectivity over a 1-bp mismatch site. The relationship between the number of methylenes in the linker (CH(2))(n)() and affinity is n = 6 > 4 > 7 > 5 > 8. These results indicate that 6 followed by 4 methylene-bridged polyamides represent the optimum spacer length for the H-pin motif in the DNA minor groove. Importantly, the H-pin is competitive with hairpin polyamides with respect to affinity and specificity. The metathesis-based convergent synthetic route to H-pin polyamides expands the scope of readily available DNA recognition motifs for small molecule-based gene regulation studies.

Solid-phase synthesis of dysidiolide-derived protein phosphatase inhibitors

Biologically active natural products can be regarded as evolutionary selected and biologically validated starting points in structural space for the development of compound libraries. For libraries designed and synthesized around a given natural product, a higher hit rate and the identification of biologically relevant hits can be expected, justifying a probably higher investment in the development of the corresponding syntheses. This approach requires the development of complex multistep reaction sequences on the solid phase. Employing the protein phosphatase Cdc25 inhibitor dysidiolide as an example, we demonstrate that this goal can be achieved successfully. The reaction sequences developed led to dysidiolide analogues in overall 8-12 linear steps with the longest sequence on the solid support amounting to up to 11 sequential transformations. The desired products were obtained in overall yields ranging from 6% to 27% and in multimilligram amounts starting from 100 mg of resin. The transformations applied include a variety of very different reaction types widely used in organic synthesis (i.e., an asymmetric cycloaddition employing a removable chiral auxiliary, different organometallic transformations, olefination reactions, different oxidation reactions, acidic hydrolyses, and a nucleophilic substitution). Biological investigation of the eight dysidiolide analogues synthesized showed that they inhibit Cdc25C in the low micromolar range with the IC(50) value varying by a factor of 20 and that they display considerable and differing biological activities in cytotoxicity assays employing different cancer cell lines.

Total syntheses of [17]- and [18]dehydrodesoxyepothilones B via a concise ring-closing metathesis-based strategy: correlation of ring size with biological activity in the epothilone series

A convergent ring-closing metathesis strategy has been employed for the highly concise syntheses of 10,11-dehydro-13,14-[17]desoxyepothilone B ([17]ddEpoB) and 10,11-dehydro-14,15-[18]desoxyepothilone B ([18]ddEpoB), which are 17- and 18-membered ring homologues of 10,11-dehydro-12,13-desoxyepothilone B ([16]ddEpoB or epothilone 490). We have demonstrated that the ring-closing metathesis (RCM) provides [17]ddEpoB or [18]ddEpoB with a high level of stereocontrol in the generation of the desired olefin in the products. These analogues were evaluated for antitumor activity. The results from the in vitro assays revealed that the [17]ddEpoB analogue is highly active against various tumor cell lines with a potency comparable to that of [16]ddEpoB. This is the first example of a 17-membered ring macrolactone epothilone that has retained its antitumor activity. In contrast, the biological data revealed that [18]ddEpoB is significantly less active than either [17]ddEpoB or the parent [16]ddEpoB.

SAR and pH stability of cyano-substituted epothilones

[formula: see text] 3-Cyano epothilones 15-18 are the only examples of non-hydroxy C-3-substituted analogues. Their tubulin binding affinity and cytotoxicity provide meaningful structure-activity relationship information on the dependence of C-1/C-3 conformation upon activity. 12-Cyano epothilone 24 has improved pH stability over epothilone B, and its activity further supports the hypothesis that C-12 stereochemistry is not critical for tubulin affinity.

Synthesis of cyclic peptidomimetics from aldol building blocks

Aldol products (3-hydroxy acids) with an allyl-protected hydroxy group were converted to amino alcohols by Curtius rearrangement. Combination of the carboxylic acid with the amino alcohols gave the amides 10. Ring-closing metathesis led to the 12-membered lactams 12 as mixtures of E/Z-isomers. The scheme was also transferred to the solid-phase. In this case the macrolactams are formed via cyclorelease. For a pair of E/Z-isomers the solution conformation was determined by ROESY spectroscopy.

Highly concise routes to epothilones: the total synthesis and evaluation of epothilone 490

A concise modular laboratory construction of the epothilone class of promising antitumor agents has been accomplished. For the first time in the epothilone area, the new synthesis exploits the power of ring-closing olefin metathesis (RCM) in a stereospecific way. Previous attempts at applying RCM to epothilone syntheses have been repeatedly plagued by complete lack of stereocontrol in the generation of the desired 12,13-olefin geometry in the products. The isolation of epothilone 490 (3) prompted us to reevaluate the utility of the RCM procedure for fashioning the 10,11-olefin, with the Z-12,13-olefin geometry already in place. Olefin metathesis of the triene substrate 12 afforded the product diene macrolide in stereoselective fashion. For purposes of greater synthetic convergency, the C3-(S)-alcohol was fashioned late in the synthesis, using chiral titanium-mediated aldol conditions with the entire O-alkyl fragment as a C15 acetate as the enolate component. Examination of the effects of protecting groups on the RCM process showed that deprotection of the C7 alcohol has a beneficial effect on the reaction yield. Performing the RCM as the last synthetic step in the sequence afforded a 64% yield of only the desired E-olefin. Selective diimide reduction of the new 10,11-olefin yielded 12,13-desoxyepothilone B, our current clinical candidate, demonstrating the utility of this new RCM-reduction protocol in efficiently generating the epothilone framework. Furthermore, the new olefin was selectively funtionalized to demonstrate the advantage conferred by this route for the construction of new analogues for SAR studies, in cytoxicity and microtubule affinity screens. Also described is the surprisingly poor in vivo performance of epothilone 490 in xenografts in the light of very promising in vitro data. This disappointing outcome was traced to unfavorable pharmacokinetic features of the drug in murine plasma. By the pharmacokinetic criteria, the prognosis for the effectiveness of 3 in humans is, in principle, much more promising.

Modern methods to produce natural-product libraries

Natural sources offer a wealth of chemically diverse compounds that have been evolutionary preselected to modulate biochemical pathways. Several industrial and academic groups are accessing this source using advanced technology platforms. Methods have been reported to generate large and diverse natural-product libraries optimised for high-throughput screening and for a fast discovery process. In addition to prefractionated and pure natural-product libraries, parallel synthesis gives access to synthetic, semi-synthetic and natural-product-like libraries. Natural-product chemistry and organic synthesis are powerful tools for optimising natural leads and for generating new diversity from natural scaffolds. The amalgamation of both may be expected to become an important strategy in future drug design.

Chemoinformatics methods for systematic comparison of molecules from natural and synthetic sources and design of hybrid libraries

Until recently, the field of diversity and library design has more or less ignored natural products as a compound source. This is probably due to at least two reasons. First, combinatorial and reaction-based approaches have been major focal points in the early days of computational library design. In addition, a widespread view is that natural products are often highly complex and not amenable to medicinal chemistry efforts. This contribution introduces recent computational approaches to systematically analyze natural molecules and bridge the gap between natural products and synthetic chemistry programs. Large scale comparisons of natural and synthetic molecules are discussed as well as studies designed to identify 'synthetic mimics' of natural products with specific activity. In addition, a concept for the design of natural/synthetic hybrid libraries is introduced. Although research in this area is still in its early stages, an important lesson to be learned from computational analyses is that there is no need to a priori 'shy away' from natural products as a source for molecular design.

Solid phase synthesis of complex natural products and libraries thereof

Natural products have served as an important source of medicinal compounds and pharmaceutical leads over the last century. Within the last 10 years, significant interest has developed in applying combinatorial chemistry techniques to the study of natural products and their biological activities. In this review, we examine several representative efforts wherein natural product skeletons have been constructed or immobilized on solid support and subsequently derivatized, giving rise to analog libraries useful in understanding the structure-activity relationships of the parent natural product. Issues such as target selection, library design, linker development, automation, and library characterization are addressed.

Alkyne metathesis: development of a novel molybdenum-based catalyst system and its application to the total synthesis of epothilone A and C

Sterically hindered molybdenum(III) amido complexes of the general type [Mo[(tBu)(Ar)N]3] (1), upon treatment with CH2Cl2 or other halogen donors, have been converted into highly effective catalysts for all kinds of alkyne metathesis reactions. Although the actual nature of the propagating species formed in situ is still elusive, halogen transfer to the Mo center of 1 plays a decisive role in the activation of such precatalysts. It was possible to isolate and characterize by X-ray crystallography some of the resulting molybdenum halide derivatives such as 15, 16 and 20 which themselves were shown to be catalytically active. Numerous applications illustrate the performance of the catalytic system 1/CH2Cl2 which operates under mild conditions and tolerates an array of polar functional groups. The wide scope allows the method to be implemented into the total synthesis of sensitive and polyfunctional natural products. Most notable among them is a concise entry into the potent anticancer agents epothilone A (86) and C (88). The macrolide core of these targets is forged by ring closing alkyne metathesis (RCAM) of diyne 113, followed by Lindlar hydrogenation of cycloalkyne 114 thus formed. Since this strategy opens a stereoselective entry into (Z)-alkene 115, the approach is inherently more efficient than previous syntheses based on conventional RCM.

Anticancer therapy with novel tubulin-interacting drugs

Antimitotic agents that target tubulin, including the taxanes and vinca alkaloids, are important components of current anticancer therapy. Whilst these antimitotic drugs are highly effective in the treatment of a number of cancers, both acquired and intrinsic resistance to these agents is a major clinical problem. Furthermore, the systemic toxicity, and in some cases lack of oral availability, make these agents less than ideal. Recently much effort has been directed on the isolation and synthesis of new antimitotic drugs that target the tubulin/microtubule system and display efficacy against drug-refractory carcinomas. Newly described compounds include structurally diverse natural products, such as dolastatin, epothilones and discodermolide, derivatives and structural analogues of traditional antimitotics, and novel synthetic molecules. Additionally, new developments in drug targeting are improving efficacy and therapeutic indices of traditional agents. A number of promising 'new generation' antimitotics are now undergoing clinical testing. These new agents are reviewed here in terms of their mechanism(s) of action on microtubules, effectiveness against drug-resistant tumour cells and clinical potential.

Total synthesis of epothilone B

[structure-see text] A convergent and stereoselective total synthesis of epothilone B (2) is described. The key steps are Normant reaction, Wadsworth-Emmons reaction of a methyl ketone 14 with the phosphonate reagent 7, diastereoselective aldol condensation of aldehyde 3 with enolate 4 to form the C6-C7 bond, and macrolactonization.

Novel molecules that interact with microtubules and have functional activity similar to Taxol

Taxol is an antitumor drug approved by the FDA for the treatment of ovarian, breast and non-small-cell lung carcinomas. Originally isolated from the bark of the Pacific yew, Taxus brevifolia, it was the first natural product described that stabilized microtubules. In the past five years, a group of novel natural products, including the epothilones, discodermolide, eleutherobin, sarcodictyins and the laulimalides, all of which have biological activities similar to those of Taxol, has been discovered. In this review, we discuss each of these novel microtubule-stabilizing agents and the search for a common pharmacophore among them, taking into consideration recent advances in our understanding of the taxanes and tubulin.

Combinatorial synthesis through disulfide exchange: discovery of potent psammaplin A type antibacterial agents active against methicillin-resistant Staphylococcus aureus (MRSA)

Psammaplin A is a symmetrical bromotyrosine-derived disulfide natural product isolated from the Psammaplysilla sponge, which exhibits in vitro antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA). Inspired by the structure of this marine natural product, a combinatorial scrambling strategy for the construction of heterodimeric disulfide analogues was developed and applied to the construction of a 3828-membered library starting from 88 homodimeric disulfides. These psammaplin A analogues were screened directly against various gram positive bacterial strains leading to the discovery of a series of potent antibacterial agents active against methicillin-resistant Staphylococcus aureus (MRSA). Among the most active leads derived from these studies are compounds 104, 105, 113, 115, 123, and 128. The present, catalytically-induced, disulfide exchange strategy may be extendable to other types of building blocks bearing thiol groups facilitating the construction of diverse discovery-oriented combinatorial libraries.

Chemical synthesis and biological evaluation of cis- and trans-12,13-cyclopropyl and 12,13-cyclobutyl epothilones and related pyridine side chain analogues

The design, chemical synthesis, and biological evaluation of a series of cyclopropyl and cyclobutyl epothilone analogues (3-12, Figure 1) are described. The synthetic strategies toward these epothilones involved a Nozaki-Hiyama-Kishi coupling to form the C15-C16 carbon-carbon bond, an aldol reaction to construct the C6-C7 carbon-carbon bond, and a Yamaguchi macrolactonization to complete the required skeletal framework. Biological studies with the synthesized compounds led to the identification of epothilone analogues 3, 4, 7, 8, 9, and 11 as potent tubulin polymerization promoters and cytotoxic agents with (12R,13S,15S)-cyclopropyl 5-methylpyridine epothilone A (11) as the most powerful compound whose potencies (e.g. IC(50) = 0.6 nM against the 1A9 ovarian carcinoma cell line) approach those of epothilone B. These investigations led to a number of important structure-activity relationships, including the conclusion that neither the epoxide nor the stereochemistry at C12 are essential, while the stereochemistry at both C13 and C15 are crucial for biological activity. These studies also confirmed the importance of both the cyclopropyl and 5-methylpyridine moieties in conferring potent and potentially clinically useful biological properties to the epothilone scaffold.

Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents that stabilize microtubules. Although 12,13-epoxide derivatives are potent antiproliferative agents, the activities of the corresponding 12,13-olefin analogs are significantly decreased. These data were confirmed for two new analogs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the natural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpressing NCI/Adr cells. By using tritiated pEB/pED, compound uptake, release, and nuclear accumulation were investigated in A431 and NCI/Adr cells. In these cells, epothilones can principally be recognized and exported by Verapamil-sensitive efflux pumps, which are not identical to MDR1. The degree of export depends on the structure, olefin vs. epoxide-analog, and also on the intracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 microM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCI/Adr (2-fold) and not in A431 cells. In addition, strong nuclear accumulation was observed for pEB (40-50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study suggests that differences in growth inhibitory efficacy between epoxide and olefin analogs may be based on different mechanisms of drug accumulation and subcellular distribution.

Solid- and solution-phase synthesis of vancomycin and vancomycin analogues with activity against vancomycin-resistant bacteria

Vancomycin, the prototypical member of the glycopeptide family of antibiotics, is a clinically used antibiotic employed against a variety of drug-resistant bacterial strains including methicillin-resistant Staphylococcus aureus (MRSA). The recent emergence of vancomycin resistance, viewed as a growing threat to public health, prompted us to initiate a program aimed at restoring the potency of this important antibiotic through chemical manipulation of the vancomycin structure. Herein, we describe the development of synthetic technology based on the design of a novel selenium safety catch linker, application of this technology to a solid-phase semisynthesis of vancomycin, and the solid- and solution-phase synthesis of vancomycin libraries. Biological evaluation of these compound libraries led to the identification of a number of in vitro highly potent antibacterial agents effective against vancomycin-resistant bacteria. In addition to aiding these investigations, the solid-phase chemistry described herein is expected to enhance the power of combinatorial chemistry and facilitate chemical biology and medicinal chemistry studies.