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Synthetic Efforts Towards Ingenol Cross-Conjugated Trienes and Their Application to Rapid Increases in Molecular Complexity

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Synthetic Efforts Towards Ingenol Cross-Conjugated Trienes and Their Application to Rapid Increases in Molecular Complexity SYNTHETIC EFFORTS TOWARDS INGENOL CROSS-CONJUGATED TRIENES AND THEIR APPLICATION TO RAPID INCREASES IN MOLECULAR COMPLEXITY by Lingfeng You B.Sc., University of Science and Technology of China, 1998 Submitted to the Graduate Faculty of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2005 UNIVERSITY OF PITTSBURGH FACULTY OF ARTS AND SCIENCES This dissertation was presented by Lingfeng You It was defended on Aug 2nd, 2005 and approved by Professor Peter Wipf, Department of Chemistry Professor Paul Floreancig, Department of Chemistry Professor Billy Day, Department of Pharmaceutical Sciences Professor Kay Brummond, Department of Chemistry Dissertation Director ii Abstract SYNTHETIC EFFORTS TOWARDS INGENOL CROSS-CONJUGATED TRIENES AND THEIR APPLICATION TO RAPID INCREASES IN MOLECULAR COMPLEXITY Lingfeng You University of Pittsburgh, 2005 Synthesis of the ingenol skeleton using a Pauson-Khand reaction as the key step was investigated. Although the Pauson-Khand reaction failed to provide the highly strained ingenol skeleton, several relatively complex Pauson-Khand precursors were prepared in about ten steps. The scope and limitation of the Pauson-Khand reaction in accessing highly strained molecules were studied. A rhodium(I)-catalyzed allenic Alder-ene reaction was developed that provides cross-conjugated trienes in good yields. This method shows enticing functional group compatibility, and progress has been made to increase the stereoselectivity of the olefinic side chain via iridium(I) catalysis. A consecutive one-pot Alder-ene/[4+2]/[4+2] reaction has been developed to demonstrate the potential of the cross-conjugated triene for accessing polycyclic compounds. The reaction sequence is highly selective with the Alder-ene and the first Diels-Alder reaction only providing a single isomer and the intermolecular [4+2] cycloaddition giving two endo addition products resulting from addition of the dienophile to either face of the diene. The transformation is highly atom- efficient with all the atoms of the starting dialkynyl allenes and the dienophiles appearing in the products. iii Acknowledgements I would like to thank my advisor, Professor Kay M. Brummond, whose guidance, dedication and enthusiasm made my graduate study possible. From WVU to University of Pittsburgh, she has always been there giving me support and encouragement. Without her help, none of what I have achieved would be possible. I would like to extend my appreciation to Professors Wipf, Floreancig, Day, Wilcox, Nelson, and Koide for their encouragement and helpful discussions concerning my proposal examination and dissertation studies. Working in close harmony with the members of the Brummond group has been a wonderful experience. I would also like to thank them for their help and encouragement. In particular, I would like to thank Dr. Angela Kerekes and Dr. Jianliang Lu for their help when I started in Brummond group; Dr. Peter Sill and Dr. Hongfeng Chen for their collaboration and support in ingenol project and rhodium(I)- catalyzed allenic Alder-ene project. I would like to thank Dr. Steven Geib for x-ray diffraction analysis, Jamie McCabe and Drs. Fu-Tyan and Fu-Mei Lin for assistance with high-field NMR spectroscopy, Dr. Kasi Somayajula for mass spectral analysis and the University of Pittsburgh for financial support. Finally, I would like to thank my family for a lifetime of encouragement and support. iv List of Abbreviations Ac...................acetyl BDPP..............2,4-bis-(diphenylphosphino)pentane Bn...................benzyl Bu...................butyl Bz ...................benzoyl COD ...............1,5-cyclooctadiene COE................cyclooctene Cp...................cyclopentadienyl Cy...................cyclohexyl DBU ...............1,8-diazobicyclo[5.4.0]undec-7-ene DCC ...............dicyclohexylcarbodiimide DEAD ............diethylazodicarboxylate DIOP ..............2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane DMAP............4-dimethylaminopyridine DME...............1,2-dimethoxyethane DMF...............N,N-dimethylformamide DMS...............dimethyl sulfide DMSO............dimethyl sulfoxide dppb................bis(diphenylphosphino)butane dppe................bis(diphenylphosphino)ethane dppm ..............bis(diphenylphosphino)methane dppp................bis(diphenylphosphino)propane DUPHOS........1,2-bis-2,5-(dimethylphospholano)benzene HMDS............hexamethyldisilazane HMPA............hexamethylphosphoramide Imid................imidazole m-CPBA.........3-chloroperoxybenzoic acid Ms ..................methanesulfonyl PDC................pyridinium dichromate PG ..................protective group Ph ...................phenyl TBAF .............tetra-n-butylammonium fluoride TBS ................tert-butyldimethylsilyl TES ................triethylsilyl Tf....................trifluoromethanesulfonyl TFE ................trifluoroethanol THF................tetrahydrofuran TIPS ...............triisopropylsilyl TMS ...............trimethylsilyl Tol..................p-tolyl Ts....................p-toluenesulfonyl v Table of Contents Abstract..........................................................................................................................................iii Acknowledgements........................................................................................................................ iv List of Abbreviations ...................................................................................................................... v List of Tables ...............................................................................................................................viii List of Schemes.............................................................................................................................. ix List of Figures............................................................................................................................... xii 1. Synthetic Efforts Towards Ingenol ..................................................................................... 1 1.1 Introduction to Ingenol ................................................................................................... 1 1.1.1 Isolation and Structure Determination.................................................................... 1 1.1.2 Mechanism of Action.............................................................................................. 3 1.1.3 Prior Synthetic Strategies to Ingenol ...................................................................... 6 1.1.3.1 Winkler’s Ring Expansion Strategy.................................................................... 7 1.1.3.2 Kuwajima’s Cyclization-Rearrangement Strategy ............................................. 9 1.1.3.3 Funk’s Ring Contraction Strategy .................................................................... 10 1.1.3.4 Wood’s Ring-Closing Metathesis Strategy....................................................... 12 1.1.3.5 Rigby’s 1,5-H Sigmatropic Approach .............................................................. 13 1.1.4 Retrosynthetic Analysis Utilizing Pauson-Khand Approach................................ 15 1.1.4.1 Winkler’s Pauson-Khand Approach to Ingenol................................................ 15 1.1.4.2 Our Synthetic Strategy Using a Pauson-Khand Reaction................................. 15 1.1.4.3 Pauson-Khand Reaction in Preparation Bridged Compounds Synthesis.......... 18 1.1.4.4 Pauson-Khand Reaction Involving Electron-Deficient Olefins........................ 19 1.2 Results and Discussion ................................................................................................. 22 1.2.1 Preparation of Pauson-Khand Reaction Precursors .............................................. 22 1.2.2 Attempts to Construct Ingenol Skeleton Utilizing Pauson-Khand Reaction........ 40 1.3 Conclusions................................................................................................................... 45 1.4 Experimental Section.................................................................................................... 46 Reference ...................................................................................................................... 65 2. Rhodium(I)-Catalyzed Allenic Alder-ene Reaction ......................................................... 70 2.1 Transition Metal Catalyzed Alder-ene Reaction........................................................... 70 2.2 Results and Discussion of Rhodium(I)-Catalyzed Alder-ene Reaction to be Used in Subsequent Cycloaddition Reactions .......................................................................... 78 2.2.1 Synthesis of Allenyne Substrates.......................................................................... 78 2.2.2 The Alder-ene Reaction of 2.40 & 2.42 and E/Z Selectivity of 2.42.................... 83 2.2.3 Study of Five- and Seven-Membered Cross-Conjugated Triene Formation Through Rhodium(I)-Catalyzed Alder-ene Reactions.......................................................... 86 2.3 Conclusions................................................................................................................... 89 Reference ...................................................................................................................... 91 vi 3. Cross-Conjugated Trienes and Their Application to Rapid Increases in Molecular Complexity...................................................................................................................................
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