Synthetic Applications of the BHQ Reaction: Towards the Total Synthesis of Plumbagin A thesis submitted to the University of Manchester for the degree of Master of Philosophy in the Faculty of Engineering and Physical Sciences 2014 Michael Wong Supervisor: Dr. Peter Quayle School of Chemistry Table of Contents Abstract 4 Declaration 5 Copyright 6 Acknowledgements 7 Abbreviations 8 Section 1: Introduction 1.1 Plumbagin 9 1.2 Properties of Plumbagin 10 1.2.1 Anticancer Properties 10 1.2.2 Agricultural Applications 12 1.2.3 Anthelmintic Properties 13 1.3 Extraction Methods 14 1.4 Synthetic Routes to Plumbagin 16 1.5 Derivatisations 19 1.6 Atom Transfer Radical Cyclisations (ATRC’s) 23 1.7.1 The BHQ Reaction 24 1.7.2 Targeted Syntheses 27 1.8 Aims and Objectives 29 Section 2: Results and Discussion 2.1 Synthesis of Dimethyl Ether 39 30 2.2 Formylation of the Aromatic Ring 32 2.3 Dakin-West Oxidation 33 2.4 Preparation of the Allyl Phenyl Ether 35 2.5 The ortho-Claisen Rearrangement 36 2.6 Esterification 38 2.7 The BHQ Reaction 39 2.8 Oxidation of Dimethyl Ether 45 to Quinone 46 42 2.9 Displacement Reactions 43 2.10 Synthesis of Aryl Bromide 48 46 2.11 Lithium-Halogen Exchange 47 2.12 Final Steps to Plumbagin 50 Section 3: Conclusions and Further Work 56 Section 4: Experimental General Considerations 60 39 - 1-4-dimethoxy-2-methylbenzene 61 40 - 2,5-dimethoxy-4-methylbenzaldehyde 62 41 - 2,5-dimethoxy-4-methylphenol 63 42 - 1-(Allyloxy)-2,5-dimethoxy-4-methylbenzene 64 43 - 2-allyl-3,6-dimethoxy-4-methylphenol 65 44 - 2-allyl-3,6-dimethoxy-4-methylphenol-2,2,2-trichloroacetate 66 45 - 5-chloro-1,4-dimethoxy-2-methylnaphthalene 67 46 - 5-chloro-2-methyl-1,4-naphthoquinone 68 47 - 2-allyl-3,6-dimethoxy-4-methylphenol-2,2,2-tribromoacetate 69 48 - 5-bromo-1,4-dimethoxy-2-methylnaphthalene 70 49 - 2-(5,8-dimethoxy-6-methylnaphthalen-1-yl)-4,4,5,5-tetramethyl- 1,3,2-dioxaborolane 71 50 - 2-methyl-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) naphthalene-1,4-dione 72 1 – 5-hydroxy-2-methyl-1,4-dione 73 References 74 Appendix 78 3 Abstract Plumbagin is a naturally occurring quinone which is well documented for having a plethora of beneficial medicinal properties. This report explores a synthetic preparation of the natural product through the use of the BHQ reaction, a unique and efficient benzannulation method which regiospecifically installs a halogen on the 4-position on the newly formed six- membered ring, during the course of a ten-step total synthesis. The synthetic route commences with 2-methyl hydroquinone and after subjugation to several chemical transformations 5-chloro-1,4-dimethoxy-2-methyl naphthalene was afforded supported by evidence from X-ray crystal diffraction analysis. Although the installed aryl chloride proved unsuitable for further chemical manipulation an alternative substrate, an aryl bromide, was produced and successfully displaced with a boronic ester providing a suitable functional precursor leading to the target compound however further attempts to isolate plumbagin from the by-products in the last step did not come to fruition. 4 Declaration I declare that no portion of the work referred to in the dissertation has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. 5 Copyright The author of this dissertation (including any appendices and/or schedules to this dissertation) owns any copyright in it (the “Copyright”) and s/he has given The University of Manchester the right to use such Copyright for any administrative, promotional, educational and/or teaching purposes. Copies of this dissertation, either in full or in extracts, may be made only in accordance with the regulations of the John Rylands University Library of Manchester. Details of these regulations may be obtained from the Librarian. This page must form part of any such copies made. The ownership of any patents, designs, trademarks and any and all other intellectual property rights except for the Copyright (the “Intellectual Property Rights”) and any reproductions of copyright works, for example graphs and tables (“Reproductions”), which may be described in this dissertation, may not be owned by the author and may be owned by third parties. Such Intellectual Property Rights and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property Rights and/or Reproductions. Further information on the conditions under which disclosure, publication and exploitation of this dissertation, the Copyright and any Intellectual Property Rights and/or Reproductions described in it may take place is available from the Head of School of the School of Chemistry. 6 Acknowledgements I would like to thank everybody involved in providing help throughout the course of this project. Firstly, my family and friends for their unfaltering support and comfort during harder times. A special thank you to Dr. Peter Quayle for his infinite knowledge into the finer intricacies of Chemistry and persistently cheerful disposition as well as the members of the Quayle group in particular Drs Mark Little and Gregory Price for the countless first-to-three’s, practical advice, blunt encouragement, personality quirks and infinite patience, in both professional and recreational settings, during the course of this MPhil for they have taught me everything I know. A personal inclusion of Brohammad Izharul Albakhri for being a conscientious student, taking on board my unorthodox approach to practical Chemistry and, above all, the nicest guy in town. It has been my utmost pleasure and a privilege to work with all of them. Additional mentions to the analytical skills of Dr. Jim Raftery in X-ray Crystallography, Gareth Smith in Mass Spectrometry and the ever varying denizens passing through the door of office 2.32. 7 Abbreviations AcOH – Acetic acid ATP – Adenosine triphosphate ATRC – Atom transfer radical cyclisation Bipy – 2,2’-Bypyridine BHQ – Bull-Hutchings-Quayle B-pin– 4,4,5,5-tetramethyl-1,3-2-dioxaborolane iPrO-B-pin - 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane CAN – Ceric ammonium nitrate CH3CN – Acetonitrile DCM - Dichloromethane DIBAL-H – Diisobutylaluminium hydride DMF – N,N-Dimethylformamide DMSO – Dimethyl sulfoxide EtOAc – Ethyl acetate iPr - Isopropyl MeI – Methyl iodide m-CPBA – meta-Chloroperoxybenzoic acid µW – Microwave reactor Na2S2O4 – Sodium dithionite NMR – Nuclear Magnetic Resonance n-BuLi – n-Butyllithium PIFA – [Bis(trifluoroacetoxy)iodo]benzene 8 Section 1: Introduction 1.1 Plumbagin Fig. 1 - 5-hydroxy-2-methyl-1,4-naphthoquinone (plumbagin). Plants produce a wide variety of metabolites which are divided into two major categories; primary and secondary metabolites. Primary metabolites comprise of compounds directly affecting the plants life cycle such as chlorophyll.1 Secondary metabolites are indirectly involved in the same development processes such as growth, development and reproduction however also have a more unique and interesting ecological functionhelping particular species survive the stresses endured during their life cycles whether that is deterring predators, inhibiting the growth of pathogens or defending against exposure to environmental factors.2,3 Plumbagin (Fig. 1) falls into the second category and is one of the simplest secondary metabolites found in the Plumbagenaeace, Droseraceae and Ebenenaceae families.2 Its structure consists of a 1,4-naphthoquinone core accompanied by a methyl subsitutent in the 2- position and a hydroxyl group in the 5-position appearing visually as a strongly yellow pigment.2 Examples of other naphthoquinone derivatives used in herbal preparations include menadione and juglone (Fig. 2). Fig. 2 - Structures of menadione, 2, and juglone, 3. 9 It is well documented that the species Plumbago zeylanica and Plumbago scandens L. are known for containing high levels of the quinone and is suggested to be the pharmacologically active constituent since its first isolation by Dulong D’Astafort in 1828.2 Since then plumbagin had been neglected for a century until being revisited in 1928 when Roy and Dutt recognised the quinone and hydroxyl characteristics were present. They further deduced an empirical formula which was proven to be incorrect however Madinaveitia and Gallego3 went on to elucidate the correct formula. 1.2 Properties of Plumbagin As with a majority of naturally occurring compounds isolated from plant extracts, extensive documentation shows plumbagin exhibits a broad range of biological activity including, but not limited to, antifungal, antibacterial, antioxidant, anticancer and anti-inflammatory effects.2,3 1.2.1 Anticancer Properties A relevant property to today’s medical concerns of plumbagin is its ability to affect and disrupt cancerous entities, in particular non-small cancers found in the lung.4 Gomathinayagam et al. reported in their recent publication that chemotherapy and radiotherapy have limitations on non-small cell lung cancers (NSCLC) due to their ability to develop resistance against conventional forms of treatment. Previous research carried out highlights plumbagin has effects on various cancer cell lines and induce apoptosis to in vitro cell cultures and in vivo tumour cultures in mice.4 Of the two cell lines used in this study, H460 and A549, the H460 line was significantly more sensitised than the A549 line suggesting that plumbagin targets EGFR (epidermal growth factor receptor) mediated Akt signalling causing a G2/M arrest inducing apoptosis. Down-regulating, or inhibiting, cyclin B1 and Cdc25B expression is suggested to be the mechanism responsible for G2/M arrest supported by different agents (ionizing radiation, doxorubicin and sulforaphane) instigating apoptosis to a variety of cell lines in the same way. Moreover this is 10 not exclusive to NSCLC’s; ovarian and breast cancer cells are also susceptible.5,6,7 Observing Bcl-2, another protein present in cancer cells, was consistently down-regulated in the H460 line was an important discovery as it opens up the possibility of synergy with modern chemotherapeutic methods and, therefore, increased therapeutic successes.