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THE SYNTHESIS OF DIAZOBENZOFLUORENE ANTITUMOR ANTIBIOTICS: LOMAIVITICINS A, B AND KINAMYCIN C

Final Report Summary - LOMAIVITICINS (The synthesis of diazobenzofluorene antitumor antibiotics: lomaiviticins A, B and kinamycin C)

Project context and objectives

This project entails the development of a methodology for the synthesis of the potent anti-tumour antibiotics Lomaiviticins A, B and Kinamycin C. Lomaiviticins represent a family of structurally complex natural products containing unique dimeric diazobenzo[b]fluorene glycoside structures. They are highly active DNA cleaving agents that exhibit potent activity against a broad spectrum of cancer cell lines. The structurally related kinamycins contain a monomeric diazobenzo[b]fluorene moiety and a highly oxygenated cycloxehene ring. Although isolated in the 1970s, the potential of kinamycins as novel anti-cancer agents was discovered only recently with the isolation of lomaiviticins in 2001. The biological activity of kinamycins and lomaiviticins is thought to arise from the reductive cleavage of DNA involving the diazonaphthoquinone function. The fully deacetylated kinamycin F, detected as a metabolite in culture broths of Streptomyces murayamaensis, was proposed to be generated in cells by the activity of esterases on O-acetylated forms of kinamycin and may be the active metabolite responsible for the observed inhibition of cancer cell growth.

Work performed

Our initial model studies on lomaiviticins (Period 1 report, Publishable Summary) did not come to fruition and therefore we had to re-evaluate our project goals and priorities. Although other approaches could be envisioned for lomaiviticins, in period 2 we directed our efforts towards the synthesis of the structurally related kinamycins given the short timeframe of the project, as well as the limited manpower. Our strategy for the synthesis of kinamycin F involves the construction of benzofluorenone 8 from 7, the product of a metal-catalysed coupling of a-iodocyclohexenone 2 and AB ring sub-unit bromonaphthaldehyde 6. Cyclohexenone 2, a highly oxygenated D-ring precursor, which contains in place the latent C2, C3 and C4 kinamycin chiral centres, was prepared in ten steps from commercially available 3-methylcyclohexen-2-one 1. Cleavage of the PMB group on 2 followed by a hydroxy-directed ketone reduction resulted in trans-diol 3 and diacetate 4 after acetylation, thus completing the stereoselective construction of the highly oxygenated D-ring of the kinamycin family of anti-tumour antibiotics. It is important to note that this strategy can be amenable to asymmetric synthesis and provide access to chiral material via the asymmetric reduction of 1.

Main results

Iodocyclohexenes 3 and 4 constitute ideal candidates for participation in a Suzuki or Stille coupling with an AB ring sub-unit to assemble the kinamycin ABD ring system. However, iodocyclohexenone 2 seemed to be an attractive candidate for a metal-catalysed coupling with AB ring sub-unit bromonaphthaldehyde 6. Compound 6 was prepared in five steps from known bromojuglone 5, but its synthesis proved quite capricious, despite the presence of published reports describing the preparation of similar compounds. Indeed, compound 6 underwent a metal-catalysed coupling with enone 2, which provided access to a-naphthylcyclohexenone 7, a kinamycin ABD intermediate with the appropriate functionality in place to enable the formation of the C5-C4a bond and C-ring closure. Although access to kinamycin ABCD tetracycle benzofluorenone 8 has not yet been achieved, studies towards the conversion of 7 to benzofluorenone 8 are currently in progress. Once access to compound 8 is ensured, it could be converted to kinamycin F as well as other acetylated analogues via a series of previously described transformations.

Although the syntheses of kinamycins and lomaiviticin aglycon have been recently reported in the literature, research in the field is currently ongoing given the limited availability of these structurally complex cytotoxic agents isolated by fermentation. Thus, access to significant quantities of kinamycin F, as well as its acetylated analogues via a convergent strategy, would enable:

a) the study of their mechanism of action;
b) the design of new and more potent kinamycin analogues;
c) the design and synthesis of novel kinamycin-peptide cytotoxic conjugates for the selective targeting of tumours and cancer cells that over-express peptide receptors in order to enhance the efficacy of kinamycins; and ultimately
d) the development of novel cancer therapeutics to improve the health of European citizens.

The described research is fully within the spirit of current EU and Greek government policies in advancing citizens' quality of life through innovation and discovery. Although Greece has made substantial progress in the basic biomedical sciences over the past 20 years, the translation of this knowledge to products that lead to economic growth in the fields of biotechnology and drug discovery has been limited. We expect further research on the design and synthesis of new analogues based on kinamycins and lomaiviticins to ultimately lead to the development of novel cancer therapeutics that may improve the health of EU citizens. The described research has the potential to steer the industrial sector (pharmaceutical / biotechnology companies) in the direction of innovation and provide career opportunities for dedicated scientists and professionals.

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