Gram-scale synthesis of (+)-discodermolide

A second-generation total synthesis of (+)-discodermolide: the development of a practical route using solely substrate-based stereocontrol

A novel total synthesis of the complex polyketide (+)-discodermolide, a promising anticancer agent of sponge origin, has been completed in 7.8% overall yield over 24 linear steps, with 35 steps altogether. This second-generation approach was designed to rely solely on substrate control for introduction of the required stereochemistry, eliminating the use of all chiral reagents or auxiliaries. The common 1,2-anti-2,3-syn stereotriad found in each of three subunits, aldehyde 9 (C(1)-C(5)), ester 40 (C(9)-C(16)), and aldehyde 13 (C(17)-C(24)), was established via a boron-mediated aldol reaction of ethyl ketone 15 and formaldehyde, followed by hydroxyl-directed reduction to give 1,3-diol 14. Alternatively, a surrogate aldehyde 22 was employed for formaldehyde in this aldol reaction, leading to the beta-hydroxy aldehyde 20 as a common building block, corresponding to the discodermolide stereotriad. Key fragment unions were achieved by a lithium-mediated anti aldol reaction of ester 40 and aldehyde 13 under Felkin-Anh control to provide (16S,17S)-adduct 51 and a boron-mediated aldol reaction between enone 10 and aldehyde 9, exploiting unprecedented remote 1,6-stereoinduction, to give the (5S)-adduct 57.

A practical improvement, enhancing the large-scale synthesis of (+)-discodermolide: a third-generation approach

[reaction: see text] A significant improvement to the Penn one-gram synthesis of (+)-discodermolide (1) has been achieved. Specifically, reduction of the steric bulk of the C(11) hydroxyl protecting group permits formation of the requisite AB Wittig salt at the expense of the undesired intramolecular cyclization upon treatment with PPh(3) at ambient pressure.

Design, synthesis and cytotoxicity of 7-deoxy aryl discodermolide analogues

A series of 7-deoxy discodermolide analogues in which the lactone fragment 'C' was replaced by aryl substituents were designed, synthesized, and evaluated for cytotoxicity.

Fluorous Mixture Synthesis of Stereoisomer Libraries: Total Syntheses of (+)-Murisolin and Fifteen Diastereoisomers

The synthesis of a stereoisomer library of 16 murisolins in individual pure form by fluorous mixture synthesis is reported. Four stereoisomeric precursors are tagged with different fluorous tags, and the resulting mixture is taken through the synthesis with four splits and late stage demixing and detagging to give all 16 products. These products exhibit only six different sets of NMR spectra, but all can be differentiated by chiral HPLC. The structure of murisolin is confirmed, but the structures of murisolin A and 16,19-cis-murisolin may never be known with certainty because insufficient data were collected on natural samples to differentiate each of them from one other isomer.

Novel biologically active natural and unnatural products

Natural products have been used as medicinal agents for many years. In addition, these compounds have served as the starting points for semisynthetic analogs with improved properties. This review highlights work on several classes of natural products and their derivatives, including both well established and emerging structural classes that are in, or nearing, clinical use for a variety of important indications.

Total synthesis of the potent microtubule-stabilizing agent (+)-discodermolide

The total synthesis of the potent microtubule-stabilizing, antimitotic agent (+)-discodermolide is described. The convergent synthetic strategy takes advantage of the diastereoselective alkylation of a ketone enolate to establish the key C15-C16 bond. The synthesis is amenable to preparation of gram-scale quantities of (+)-discodermolide and analogues.

Synthesis and biological assessment of simplified analogues of the potent microtubule stabilizer (+)-discodermolide

An efficient, convergent and stereocontrolled synthesis of simplified analogues of the potent antimitotic agent (+)-discodermolide has been achieved and several small libraries have been prepared. In all the libraries, the discodermolide methyl groups at C14 and C16 and the C7 hydroxy group were removed and the lactone was replaced by simple esters. Other modifications introduced in each series of analogues were related to C11, C17 and C19 of the natural product. Key elements of the synthetic strategy included (a) elaboration of the main subunits from a common intermediate and (b) fragment couplings using Wittig reactions to install the (Z)-olefins. Library components were analyzed for microtubule-stabilizing actions in vitro, for displacement of [3H]paclitaxel from its binding site on tubulin, for antiproliferative activity against human carcinoma cells, and for cell signaling and mitotic spindle alterations by a multiparameter fluorescence cell-based screening technique. The results show that even significant structural simplification can lead to analogues with actions related to microtubule targeting.

1,6-asymmetric induction in boron-mediated aldol reactions: application to a practical total synthesis of (+)-discodermolide

By relying solely on substrate-based stereocontrol, a practical total synthesis of the microtubule-stabilizing anticancer agent (+)-discodermolide has been realized. This exploits a novel aldol bond construction with 1,6-stereoinduction from the boron enolate of (Z)-enone 3 in addition to aldehyde 2. The 1,3-diol 7 is employed as a common building block for the C(1)-C(5), C(9)-C(16), and C(17)-C(24) subunits. [reaction--see text]

Total synthesis of bafilomycin V(1): a methanolysis product of the macrolide bafilomycin C(2)

A synthesis of bafilomycin V(1), a methanolysis product of the macrolide natural product bafilomycin C(2), is described. The route utilizes chiral nonracemic allenylzinc reagents, prepared in situ from propargylic mesylates, to access key segments of this methyl ester. The acetylenic moieties of the derived homopropargylic alcohol adducts play an important role in further elaboration of these subunits. Final assemblage of the 25-carbon chain, containing 12 stereocenters, an alpha-methoxy Z,E 1,3-dienic ester, and an additional E,E 1,3-diene, was achieved through Stille coupling of an acetylene-derived vinyl stannane and vinyl iodide of approximate equal complexity. Attempted cyclization of several C15 hydroxy acid derivatives to the 16-membered lactone bafilomycin A(1), a potent inhibitor of vacuolar ATPases, could not be achieved.

Efficient strategy for the synthesis of stereopentad subunits of scytophycin, rifamycin S, and discodermolide

[reaction: see text] An efficient, simple method has been developed for the stereocontrolled synthesis of polypropionate stereopentads in high enantio- and diastereomeric purities.

Coupling of alkynylTMS derivatives with vinylic iodides. An efficient route to 1,3-enynes and dienes

[reaction: see text] CuCl-promoted coupling of alkynylTMS derivatives with vinyl iodides leads to 1,3-enynes in high yield. Enynes prepared from homopropargylic alcohols undergo intramolecular hydrosilylation and subsequent silyl cleavage with TBAF to afford 1,3-dienes.

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.

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.

A practical synthesis of (+)-discodermolide and analogues: fragment union by complex aldol reactions

A practical stereocontrolled synthesis of (+)-discodermolide (1) has been completed in 10.3% overall yield (23 steps longest linear sequence). The absolute stereochemistry of the C(1)-C(6) (7), C(9)-C(16) (8), and C(17)-C(24) (9) subunits was established via substrate-controlled, boron-mediated, aldol reactions of the chiral ethyl ketones 10, 11, and 12. Key fragment coupling reactions were a lithium-mediated, anti-selective, aldol reaction of aryl ester 8 (under Felkin-Anh induction from the aldehyde component 9), followed by in situ reduction to produce the 1,3-diol 40, and a (+)-diisopinocampheylboron chloride-mediated aldol reaction of methyl ketone 7 (overturning the inherent substrate induction from the aldehyde component 52) to give the (7S)-adduct 58. The flexibility of our overall strategy is illustrated by the synthesis of a number of diastereomers and structural analogues of discodermolide, which should serve as valuable probes for structure-activity studies.

The relationship between Taxol and (+)-discodermolide: synthetic analogs and modeling studies

BACKGROUND

During the past decade, Taxol has assumed an important role in cancer chemotherapy. The search for novel compounds with a mechanism of action similar to that of Taxol, but with greater efficacy particularly in Taxol-resistant cells, has led to the isolation of new natural products. One such compound, (+)-discodermolide, although structurally distinct from Taxol, has a similar ability to stabilize microtubules. In addition, (+)-discodermolide is active in Taxol-resistant cell lines that overexpress P-glycoprotein, the multidrug-resistant transporter. Interestingly, (+)-discodermolide demonstrates a profound enhancement of the initiation process of microtubule polymerization compared to Taxol.

RESULTS

The synthesis of (+)-discodermolide analogs exploiting our highly efficient, triply convergent approach has permitted structure-activity relationship (SAR) studies. Small changes to the (+)-discodermolide structure resulted in a dramatic decrease in the ability of all four discodermolide analogs to initiate tubulin polymerization. Two of the analogs also demonstrated a decrease in total tubulin polymerization, while a change in the olefin geometry at the C8 position produced a significant decrease in cytotoxic activity.

CONCLUSIONS

The availability of (+)-discodermolide and the analogs, and the resultant SAR analysis, have permitted an exploration of the similarities and differences between (+)-discodermolide and Taxol. Docking of the X-ray/solution structure of (+)-discodermolide into the Taxol binding site of beta-tubulin revealed two possible binding modes (models I and II). The preferred pharmacophore model (I), in which the C19 side chain of (+)-discodermolide matches with the C2 benzoyl group of Taxol and the delta-lactone ring of (+)-discodermolide overlays with the C13 side chain of Taxol, concurred with the results of the SAR analysis.

Solution structure of (+)-discodermolide

[structure: see text]. The solution structure of (+)-discodermolide (1) has been determined via 1- and 2-D NMR techniques in conjunction with Monte Carlo conformational analysis. Taken together, the results demonstrate that in solution (+)-discodermolide occupies a helical conformation remarkably similar to the solid state conformation.