Elsevier

Tetrahedron

Volume 61, Issue 8, 21 February 2005, Pages 2123-2139
Tetrahedron

Synthesis of novel, simplified, C-7 substituted eleutheside analogues with potent microtubule-stabilizing activity

https://doi.org/10.1016/j.tet.2004.12.039Get rights and content

Abstract

The synthesis of a number of novel, simplified, C-7 substituted eleutheside analogues with potent tubulin-assembling and microtubule-stabilizing properties is described, using ring closing metathesis as the key-step for obtaining the 6–10 fused bicyclic ring system. The RCM precursors were synthesized starting from aldehyde 3 [prepared in six steps on a multigram scale from R-(−)-carvone in 30% overall yield] via multiple stereoselective Hafner–Duthaler allyltitanations and/or Brown allylborations. ‘Second generation’ RCM-catalyst 15 gave the desired ring closed ten-membered carbocycles as single Z stereoisomers in good yields. The RCM stereochemical course (100% Z) is likely to reflect thermodynamic control. Molecular mechanics and semi-empirical calculations also show that the Z stereoisomers of these ten-membered carbocycles are consistently more stable than the E. The crucial role of the homoallylic and allylic substituents and of their protecting groups for the efficiency of the RCM reactions is discussed. In particular, we have found that p-methoxyphenyl (PMP) protected allylic alcohols, the products of a stereoselective oxyallylation, are compatible with the RCM reaction and give better yields than the corresponding free allylic alcohols. One of the simplified analogues of the natural product (44, lacking inter alia the C-4/C-7 ether bridge) retains potent microtubule-stabilizing activity. However, the cytotoxicity tests did not parallel the potent tubulin-assembling and microtubule-stabilizing properties: limited cytotoxicity was observed against three common tumor cell lines (human ovarian carcinoma, human colon carcinoma and human leukemia cell lines, IC50 in the μM range), approximately two orders of magnitude less than paclitaxel (IC50 in the nM range). The mechanism of cell cycle arrest induced by compound 44 is similar to that obtained with paclitaxel.

Introduction

Sarcodictyins1, 2 A (1a) and B (1b) and eleutherobin3, 4 (2) (the ‘eleutheside’ family of microtubule-stabilizing drugs, Fig. 1) are active against paclitaxel resistant tumor cell lines and therefore hold potential as second generation microtubule-stabilizing anticancer agents.4, 5 The scarce availability of 12 from natural sources makes their total syntheses vital for further biological investigations.5 To date, sarcodictyins A and B have been synthesized successfully by Nicolaou et al.6 who have also exploited a similar route for accessing eleutherobin.7 A subsequent report by Danishefsky and co-workers details an elegant alternative access to eleutherobin.8 A number of partial syntheses and approaches have also been described.9, 10

The total syntheses of the eleuthesides have generated very limited diversity in the diterpenoid core, with major variations reported only in the C-15 functionality and C-8 side-chain.5, 6, 7, 8 We previously described the synthesis of a number of eleutheside analogues with potent tubulin-assembling and microtubule-stabilizing activity.9h,m,n However, the cytotoxicity assays did not parallel the potent tubulin-polymerizing properties: limited cytotoxicity was observed against three common tumor cell lines (human ovarian carcinoma and human colon carcinoma cell lines, IC50 in the μM range), two-to-three orders of magnitude less than paclitaxel (IC50 in the nM range).9n These results were attributed to an easy esterase-mediated hydrolytic cleavage of the N-methylurocanic ester side-chain in living cells (it is known that the natural eleuthesides are devoid of any cytotoxicity when the N-methylurocanic ester side-chain is lacking in position 8).5 The simplified analogues, in fact, had an unsubstituted –CH2– in position 7, while natural eleuthesides have a fully substituted quaternary carbon, which is likely to hinder the hydrolysis of the adjacent ester at C-8.

In this paper, we describe the synthesis of a number of eleutheside analogues substituted at C-7, using ring closing metathesis (RCM) as the key-step for obtaining the 6–10 fused bicyclic ring. We also report the tubulin-polymerizing activities (ED50 and ED90 values) and the cytotoxicity tests (IC50 values) performed on these compounds using several different tumor cell lines.

Section snippets

Synthesis of the eleutheside analogues substituted at C-7

Aldehyde 3 (prepared in six steps on a multigram scale from R-(−)-carvone in 30% overall yield)9a,g was submitted to a stereoselective titanium-mediated Hafner–Duthaler crotylation,11 generating alcohol 5 (Scheme 1).

The crotylation reaction proceeded in high yield (85%) and with complete stereocontrol in favor of the desired stereoisomer (diastereomeric purity >95% by 1H and 13C NMR).12 After standard alcohol protection, an efficient and well established sequence of steps8c led to the

General procedures

All reactions were carried out in flame-dried glassware under argon atmosphere. All commercially available reagents were used as received. The solvents were dried by distillation over the following drying agents and were transferred under nitrogen: CH3CN (CaH2), CH2Cl2 (CaH2), (CH2Cl)2 (CaH2), MeOH (CaH2), Et3N (CaH2), iPr2EtN (CaH2), HN(TMS)2 (CaH2), THF (Na), Et2O (Na), benzene (Na), toluene (Na), n-hexane (Na). Organic extracts were dried over anhydrous Na2SO4. Reactions were monitored by

Acknowledgements

We thank the European Commission for financial support (IHP Network grant ‘Design and synthesis of microtubule stabilizing anticancer agents’ HPRN-CT-2000-00018) and for postdoctoral fellowships to L. C. (HPRN-CT-2000-00018), P. B. (‘Marie Curie’ HPMF-CT-2000-00838) and O. S. (‘Marie Curie’ MEIF-CT-2003-500880). We also gratefully acknowledge the Spanish ‘Ministerio de Ciencia y Tecnologia’ for a ‘Ramon y Cajal’ fellowship to A. M. C., ‘Merck Research Laboratories’ (Merck's Academic Development

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    Present address: Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, UK.

    Present address: Dept. de Química, Unitat Química Orgànica, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain.

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