Total Synthesis of Oxazole- and Cyclopropane-Containing Epothilone A Analogues by the Olefin Metathesis Approach

Perspectives from nearly five decades of total synthesis of natural products and their analogues for biology and medicine

Covering: 1970 to 2020By definition total synthesis is the art and science of making the molecules of living Nature in the laboratory, and by extension, their analogues. Although obvious, its application to the synthesis of molecules for biology and medicine was not always the purpose of total synthesis. In recent years, however, the field has acquired momentum as its power to reach higher molecular complexity and diversity is increasing, and as the demand for rare bioactive natural products and their analogues is expanding due to their recognised potential to facilitate biology and drug discovery and development. Today this component of total synthesis endeavors is considered highly desirable, and could be part of interdisciplinary academic and/or industrial partnerships, providing further inspiration and momentum to the field. In this review we provide a brief historical background of the emergence of the field of total synthesis as it relates to making molecules for biology and medicine. We then discuss specific examples of this practice from our laboratories as they developed over the years. The review ends with a conclusion and future perspectives for natural products chemistry and its applications to biology and medicine and other added-value contributions to science and society.

Joys of Molecules. 1. Campaigns in Total Synthesis

Our bodies are made of molecules, and it is from molecules that we derive our strength and joys. The joys of molecules manifest themselves in many ways. These include beautiful colors, exquisite aromas, distinct tastes, psychological ups and downs, and intellectual inspirations, among other forms of stimulation, material or spiritual. In this Perspective, written on the occasion of the 2005 American Chemical Society Arthur C. Cope Award address, I recount some of the joys I have experienced and shared with my students during campaigns to synthesize some of Nature's most intriguing and complex molecules.

The 12,13-diol cyclization approach for a truly stereocontrolled total synthesis of epothilone B and the synthesis of a conformationally restrained analogue

A highly convergent and stereocontrolled synthesis of epothilone B (1) has been developed. The epoxide moiety in 1 was generated by regioselective mesylation and base treatment of the 12,13-diol 30 which was formed by a chelate Cram controlled Grignard addition of 14 and methyl ketone 13. Both fragments were synthesized from the chiral carbon pool precursors (S)-citronellol and (S)-lactic acid, respectively. A highly diastereoselective aldol addition of epoxy-aldehyde 7 and the known Southern hemisphere ketone 8 delivered the full carbon skeleton, containing all the stereogenic centers of 1. Functional group manipulation, macrolactonization and removal of two protecting groups then yielded 1. The spatial closeness of the C4-beta-methyl and C6-methyl group in the crystal structure of 1 inspired us to connect them through a methylene bridge to give a cyclohexanone derivative. Thus, the Northern hemisphere aldehyde 7 was added to the enolate of the cyclohexanone 47. Further manipulations and macrolactonization delivered the conformationally restrained epothilone derivative 42.

Catalytic Antibody Route to the Naturally Occurring Epothilones: Total Synthesis of Epothilones A-F

Naturally occurring epothilones have been synthesized starting from enantiomerically pure aldol compounds 9-11, which were obtained by antibody catalysis. Aldolase antibody 38C2 catalyzed the resolution of (+/-)-9 by enantioselective retro-aldol reaction to afford 9 in 90% ee at 50 % conversion. Compounds 10 and 11 were obtained in more than 99% ee at 50% conversion by resolution of their racemic mixtures using newly developed aldolase antibodies 84G3, 85H6 or 93F3. Compounds 9, 10 and 11 were resolved in multigram quantities and then converted to the epothilones by metathesis processes, which were catalyzed by Grubbs' catalysts.

Total synthesis of epothilone E and related side-chain modified analogues via a Stille coupling based strategy

A Stille coupling strategy has been utilized to complete a total synthesis of epothilone E from vinyl iodide 7 and thiazole-stannane 8h. The central core fragment 7 and its trans-isomer 11 were prepared from triene 15 using ring-closing metathesis (RCM), and were subsequently coupled to a variety of alternative stannanes to provide a library of epothilone analogues 18a-o and 19a-o. The Stille coupling approach was then used to prepare epothilone B analogues from the key macrolactone intermediate 24 which was itself synthesized by a macrolactonization based strategy.

The antibody catalysis route to the total synthesis of epothilones

An efficient monoclonal aldolase antibody that proceeds by an enamine mechanism was generated by reactive immunization. Here, this catalyst has been used in the total synthesis of epothilones A (1) and C (3). The starting materials for the synthesis of these molecules have been obtained by using antibody-catalyzed aldol and retro-aldol reactions. These precursors were then converted to epothilones A (1) and C (3) to complete the total synthesis.

Oxidative and reductive transformations of epothilone A

The C7 hydroxy group of cytotoxic epothilone A was selectively oxidized using PDC. A selective oxidation of the C3 hydroxy group was accomplished with Me2S/(PhCO2)2 after in situ protection of C7-OH. Reduction of epothilone A or of a C5, C7 dioxo derivative with NaBH4 proceeded at the C5 carbonyl group. Oxidation and hydrogenation of the C16-C17 double bond proved to be difficult but it was easily cleaved with ozone and the resulting keto derivative was transformed to epothilone analogs with different side chains.

Synthesis and biological properties of C12,13-cyclopropyl-epothilone A and related epothilones

BACKGROUND

The epothilones are natural substances that are potently cytotoxic, having an almost identical mode of action to Taxoltrade mark as tubulin-polymerization and microtubule-stabilizing agents. The development of detailed structure-activity relationships for these compounds and the further elucidation of their mechanism of action is of high priority.

RESULTS

The chemical synthesis of the C12,13-cyclopropyl analog of epothilone A and its C12,13-trans-diastereoisomer has been accomplished. These compounds and several other epothilone analogs have been screened for their ability to induce tubulin polymerization and death of a number of tumor cells. Several interesting structure-activity trends within this family of compounds were identified.

CONCLUSIONS

The results of the biological tests conducted in this study have demonstrated that, although a number of positions on the epothilone skeleton are amenable to modification without significant loss of biological activity, the replacement of the epoxide moiety of epothilone A with a cyclopropyl group leads to total loss of activity.