Synthesis and biological evaluation of (−)-dictyostatin and stereoisomers
Graphical abstract
Introduction
Microtubule stabilization by small molecule natural products and analogs1 is a clinically proven chemotherapeutic approach for the treatment of solid tumors, and microtubule stabilizers exhibit a diverse assortment of molecular scaffolds. Dictyostatin and discodermolide (Fig. 1), the taxanes paclitaxel and docetaxel, the epothilones, the sarcodictyins and eleutherobin, and the ketosteroids 2-ethoxy-7-keto-17β-estradiol and the taccalonolides all bind with varying affinities to the paclitaxel binding site on β-tubulin within microtubules.2, 2(a), 2(b), 3, 4, 5, 6, 7, 7(a), 7(b) The tau neuronal protein binds onto microtubules in the vicinity of the paclitaxel site, and this site is well-described due to high resolution cryoelectron microscopic analyses of zinc-induced sheets of tubulin polymer stabilized by taxanes or epothilones.8, 9 Drugs binding to the site interact with amino acid residues on the M-loop (for example, Phe270, Thr274, and Arg276) and the H7 alpha helix (for example, Ala231 and His227) of β-tubulin.
Interest in the dictyostatin family of microtubule-stabilizing agents has increased significantly over the last several years. Pettit initially isolated dictyostatin 1 from a marine sponge, suggested the correct two-dimensional structure (constitution), and showed that the compound had potent activity against cancer cells.10 Subsequently, Wright and co-workers isolated 1 from a different sponge species and showed that the compound displays potent microtubule-stabilizing actions.11, 11(a), 11(b)
Dictyostatin is a structural cousin of the important microtubule-stabilizing agent discodermolide 2.12, 12(a), 12(b) Dictyostatin's 26-carbon backbone is two carbons longer than discodermolide's, and it is joined by a 22-membered macrolactone formed between the carboxyl group on C1 and the alcohol on C21. (The longer chain length means that carbon numbers of dictyostatin are two higher than the comparable carbons of discodermolide.) Dictyostatin also differs from discodermolide by possessing a Z/E diene (C2–C5) instead of a γ-lactone, and it lacks a double bond at C15,16 and a methyl group at C18. Nonetheless, 10 of dictyostatin's 11 stereocenters are also present in the discodermolide structure.
The three-dimensional structure (configuration) of dictyostatin was uncertain for almost a decade. As an outgrowth of our interest in discodermolide analogs,13, 13(a), 13(b), 13(c) we made ‘dictyostatin/discodermolide’ hybrids like 3 in 2002,14, 14(a), 14(b), 14(c) and these compounds exhibited high biological activities in both tubulin and cell assays. An underlying tenet of this work was that a hypothetical cyclization of the carbamate nitrogen of discodermolide 2 onto its lactone carbonyl (C1) would provide a 22-membered ring, the same size as the macrocycle of dictyostatin 1. At this juncture, the lack of knowledge of the complete structure of dictyostatin and the tiny quantities available from isolation were serious impediments to further medicinal chemistry research. With a solid foundation in place from the synthesis of compound 3 and related molecules, we decided to address the dictyostatin structure and supply problems by total synthesis.
In a 1995 patent, Pettit and co-workers suggested a partial configuration for dictyostatin,15 and we selected the absolute configuration depicted for compound 4 because this enantiomer is more closely related to discodermolide. In 4, seven stereocenters have the same configurations as discodermolide, two are different (C6 and C14), and two are not assigned (C16 and C19). In 2004, Paterson and co-workers suggested structure 1 for dictyostatin based on detailed NMR studies;16 in addition to assigning the two missing stereocenters, the configurations of two other centers were inverted. Structure 1 was promptly proved by a pair of total syntheses that appeared in simultaneous communications from Paterson's group17 and ours,18 and recently Phillips and Ramachandran have also reported the total syntheses of 1.19, 19(a), 19(b), 20, 20(a), 20(b), 20(c), 20(d), 20(e)
In this paper, we provide details of our total synthesis of dictyostatin 1. Along the way, we made several stereoisomers of the natural product as we drew gradually closer to the correct structure. All of these compounds have been characterized by a battery of biological assays. These results, combined with the additional results for new analogs described elsewhere21, 22, 23 and with the known SAR for discodermolide, provide for the first time a good outline of the SAR of dictyostatin. Over 5 mg of synthetic dictyostatin was provided by this work, and this was used for the detailed biological characterization of this molecule. The results of these studies have fully validated the high level of interest in dictyostatin.2, 2(a), 2(b)
Section snippets
Synthesis of 6,16-bis-epi-dictyostatin 5
Though differing in relative configuration from Pettit's structure 4 in the middle fragment, we initially decided to make compound 5 because a number of key early intermediates were already in hand. We selected the (R) configuration at C16 arbitrarily, and though this proved to be wrong, the molecule—6,16-bis-epi-dictyostatin—ultimately turned out to be structurally much closer to dictyostatin than we had initially thought.
The strategy for the synthesis of this molecule is summarized at a high
Conclusions
In summary, we have provided here the full details of synthesis of dictyostatin 1, its open-chain methyl ester analog, assorted C6, C16, and C19 epimers, and two macrolactone isomers with (E,E) geometry at C2–C5. The biological evaluation of the compounds revealed a wide range of activities resulting from these very small structural changes. The SAR determined includes the following. The macrolactone is important but not a full requisite for microtubule stabilization and antiproliferative
Ethyl (4R,5S,2E)-5,7-bis(tert-butyldimethylsilyloxy)-4-methylhept-2-enoate (83)
From 82: a solution of triethyl phosphonoacetate (3.5 mL, 17.6 mmol) was added to a cooled (0 °C), stirred suspension of NaH (0.43 g, 17.0 mmol, 95% dispersion in mineral oil) in THF (46 mL) dropwise over 10 min. The mixture was brought to room temperature with a water bath (30 min) and then cooled back to −78 °C, and the aldehyde (2.73 g, 7.58 mmol) in THF (5 mL) was added. The resulting mixture was stirred for 1 h at 0 °C, and then pH≈7 phosphate buffer solution (10 mL) and Et2O (50 mL) were added. The
Acknowledgements
We thank the National Institutes of Health for support through grant CA078390. We thank Drs. Kenneth Bair and Fred Kinder for the gift of discodermolide, Prof. Ian Paterson and Dr. Robert Britton for conducting some of the 1H NMR and TLC comparison experiments of the dictyostatin samples, and Mr. W.-H. Jung for helpful suggestions on the manuscript.
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