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The Total Synthesis of Aigialomycin D and Analogues

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The Total Synthesis of Aigialomycin D and Analogues THE TOTAL SYNTHESIS OF AIGIALOMYCIN D AND ANALOGUES by Lynton James Baird VICTORIA UNIVERSITY OF WELLINGTON Te Whare Wananga o te Upoko o te Ika a Maui A thesis submitted to Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry Victoria University of Wellington 2009 Abstract In 2002 a new family of 14-membered resorcylic macrolides, the aigialomycins, were isolated from the mangrove fungus Aigialus parvus BCC 5311. Subsequent biological testing of these new natural products found aigialomycin D (Am D) to be the most biologically active member of the family, exhibiting moderate activity against malaria (Plasmodium falciparum K1, IC50 19.7 μM) and modest cytotoxicity towards certain cancer cells (KB cells: IC50 9.0 μM and BC-1 cells: 53.8 μM). More recently, Am D has been shown to inhibit the kinases CDK1/5 and GSK at low μM concentrations. At the onset of this research project, with only one total synthesis of Am D reported in the literature, there remained a need for an efficient synthesis of Am D that would be amenable to the synthesis of a range of analogues. This thesis reports two synthetic approaches to Am D that differ primarily in the chemistry utilised to install the (E)- olefins at C1′–C2′ and C7′–C8′: a Horner-Wadsworth-Emmons (HWE) strategy and a Ramberg-Bäcklund (RB) strategy. The Ramberg-Bäcklund strategy ultimately proved to be successful, providing Am D in 16 steps with 9% overall yield. A retrosynthetic analysis of Am D disconnects the molecule into three major fragments: an aromatic fragment, a C2′–C7′ carbohydrate-derived fragment and a C8′–C11′ alcohol fragment. 11' 8' O HO P lactonisation EtO OEt OH O OH O 11' or 11' HWE or 8' OH O RCM HO 1' 8' 7' HO HO X Y 7' X 2' OH 2' X=P(O)(OEt)2 or Br HWE or OH OH RB OH aigialomycin D Y=O;X=O Y=SH,H;X=CH2 The synthesis of the three fragments for each strategy is described and the attempts made to couple the fragments together, first with HWE methodology and then successfully with ring-closing metathesis (RCM) and RB reactions, are discussed. The synthesis of several Am D analogues and their preliminary biological testing is also described. i Acknowledgements What is a PhD like? To what shall I compare it? It is like a long journey; an adventure that takes you through the well trod streets, the green lush meadows and the barren wildernesses. With the vision of the end goals fixed in mind, you press on for the prize. Once you reach the end and look back, you realize that the whole process was the prize. The small successes, the frustrating failures, the sudden crystallisations, the imparted chemical wisdom, the laboratory camaraderie; are all important parts of a good PhD. But you can’t succeed in this journey without the guidance and support of many people. First and foremost, I must thank my supervisors. You willingly took on an inorganic chemist and thrust him into the uncharted world of total synthesis. I wouldn’t have survived without your constant support and encouragement. Joanne, thank you for all your guidance over the past three or so years. Your encouragement and suggestions during the many wilderness wanderings kept me going even when I was sure I couldn’t get to the end. And Paul, thank you for the new angles, new insights and extra guidance you oft provided, particularly during my all too brief forays into biology and computer modelling. Not many students get to hang out and snorkel with their supervisors in Hawaii. A huge thank you is due to my editorial team – Joanne, Paul, Shivs, Mattie and Emma the Champion. I think you managed to pick up all my spelling mistackes! Thank you to all the technical and admin staff in SCPS. Thanks especially to Sally Wisheart (hunting down my chemical needs), John Ryan (fixing all my NMR bads), Alan Rennie (always happy to drill), Darren Alexander (so efficient and friendly), Teresa Gen, Lisa Meredith, Gordon Healey, Grant Franklin, Scott Forbes, Jenny Hall and Celia Falchi. Also, big thanks are due to all the academic staff – especially Prof. Spencer, Dr. Brendan Burkett (the MW-man), Assoc. Prof. Peter Northcote, and the ever encouraging Assoc. Prof. Kate McGrath. Mattie, thank you for your excellent suggestions and the interest you showed in my project, especially when Joanne was away on leave. Your ability to manage so many projects is phenomenal. ii I have been privileged to work alongside many great chemists and friends over the past 3.5 years. Thank you all for your support, encouragement, friendship, the laughs and lab sing-alongs (thank you Alanis). In particular, thank you Emma for all the chats at our hoods, coffees at the staff club and conferences abroad. We have spent so much time together that we must be family by now. Shivs, thanks for all the late nights, coaching, lively dancing and for “bringing sexy back” to the lab. Little Emma, there were many times you managed to perk me up out of the doldrums. Thank you for always remaining positive, sharing words of hope and giving effective engagement advice. Russ the Muss, you have entertained us all with your enthusiasm and responsiveness to chemistry, “Yessss!” Thank you for not complaining when I stole all your clean NMR tubes, often without asking. You are a generous soul. Thank you to the rest of my 209 labmates who made it a great place to hang out and work: Rhys, Anna, Shi, Birthe. Thank you to all the inhabitants of 208 for all the great debates we oft overheard and for allowing me to use your lab as a thoroughfare. Thanks to the Natural Products group in 210 – Wendy, Jo, Mina, Jono and Katie – for your help on the freeze drier, HPLC and with my other purification needs. Thanks Wendy for your words of support during the writing-up period and for organising fantastic conference accommodation. Thanks to all those who have put up with my mess in the grad write-up room. Most especially to Jeremy, Benjamin, and Ashna. And thanks to my flatmates, friends and brothers of 4B – whose prayers and cheers got me through. Finally, I must thank my family, without whom I wouldn’t be here. Thank you Mum and Father, Christie, Matthew, Anne-Marie, Ross, Richard, Nana, and my extended family and in-laws. Your love, support and encouragement has meant so much. Thank you Father for always being interested in my research and for supplying me with chemicals (KNO3, S8 and C) whilst I was still at school. To Anna, my love, thank you. You have been at my side for most of this journey, always willing to listen to my chemical frustrations and celebrations. You have had to deal with all the tears, late nights and stress. Your patience, unconditional love and smile have brought me to this place. May I grow to be the husband you deserve. Thank you Jesus, for in You there is life, abundant life. iii Table of Contents Abstract ..............................................................................................................................i Acknowledgments............................................................................................................ ii Table of Contents ............................................................................................................ iii List of Figures ............................................................................................................... viii List of Schemes.............................................................................................................. xii List of Tables .............................................................................................................. xviii List of Abbreviations .....................................................................................................xix Chapter One: Introduction ..................................................................................................................1 1.1 Synthesis of Natural Products .................................................................................1 1.2 Resorcylic Acid Lactones (RALs) ..........................................................................2 1.2.1 Biosynthesis of RALs ..............................................................................3 1.2.2 The Family of 14-Membered RALs.........................................................4 1.3 Aigialomycin D.......................................................................................................8 1.3.1 Discovery and Isolation of Aigialomycin D ............................................8 1.3.2 Biological Testing of Aigialomycin D...................................................10 1.3.3 Previous Total Syntheses of Aigialomycin D ........................................11 1.3.3.1 Danishefsky’s synthesis..................................................................11 1.3.3.2 Pan’s synthesis................................................................................16 1.3.3.3 Winssinger’s Synthesis...................................................................18 1.3.3.4 Chen’s Synthesis.............................................................................23 1.3.3.5 Montgomery’s Synthesis.................................................................27 1.3.3.6 Zhang’s Synthesis of the C2′–C7′ Segment of Am D.....................29 1.3.3.7 Jennings’ Synthesis of 6′-epi-Am D ...............................................31 1.3.3.8 Summary of Total and Partial Syntheses of Am D and Analogues 35 1.4 Research Objectives..............................................................................................36
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