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Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes

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Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes I. The Asymmetric Total Synthesis of Apratoxin D II. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes III. Synthesis and Application of Multidentate LiGands Toward the Realization of Fluxional Mechanocatalysis by Bradley David Robertson Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Ross A. Widenhoefer, Supervisor ___________________________ Steven W. Baldwin ___________________________ Stephen L. CraiG ___________________________ Patrick Charbonneau Dissertation submitted in partial fulfillment of the requirements for the deGree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2015 ABSTRACT I. The Asymmetric Total Synthesis of Apratoxin D II. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes III. Synthesis and Application of Multidentate LiGands Toward the Realization of Fluxional Mechanocatalysis by Bradley David Robertson Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Ross A. Widenhoefer, Supervisor ___________________________ Steven W. Baldwin ___________________________ Stephen L. CraiG ___________________________ Patrick Charbonneau An abstract of a dissertation submitted in partial fulfillment of the requirements for the deGree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2015 CopyriGht by Bradley David Robertson 2015 Abstract Apratoxin D, recently isolated from two species of cyanobacteria, L. majuscula and L. sordida, exhibits hiGhly potent in vitro cytotoxicity aGainst H-460 human lunG cancer cells with an IC50 value of 2.6 nM. The potent biological activity exhibited by apratoxin D combined with its intriguinG molecular architecture has led to the pursuit of its asymmetric total synthesis. Studies toward and completion of the first asymmetric total synthesis of apratoxin D are reported. Key transformations include a Kelly thiazoline formation, Paterson anti-aldol and an Evans syn-aldol. The synthesis was completed in 2.1% total yield over 31 steps from (R)-citronellic acid. Cationic Gold (I) complexes are hiGhly efficient catalysts for the cycloisomerization of 1,6-enynes, a transformation capable of providinG a Great amount of structural complexity from simple startinG materials. The in situ spectroscopic analysis of the catalytic cycloisomerization of a 7-phenyl-1,6-enyne, as well as the tandem gold/silver-catalyzed cycloaddition/hydroarylation of 7-aryl-1,6-enynes is described. The cycloaddition/hydroarylation reaction provides 6,6- diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involvinG Gold-catalyzed cycloaddition followed by silver-catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate. The control of bond scission and formation by mechanocatalysis has potential in a variety of applications, includinG biomedical devices, mechanical sensors and self- iv healing materials. The synthesis and study of C2-symmetric bis(phosphine) liGands with applications toward mechanocatalysis is described. Additionally, the synthesis and study of a tetradentate ligand desiGned toward mechanochemical activation of a latent catalytic complex is reported. These studies have allowed further development in the desiGn of transition metal complexes capable of activation by mechanical force. v To my family and friends. vi Contents Abstract .......................................................................................................................................... iv List of Tables ................................................................................................................................. xi List of FiGures .............................................................................................................................. xii List of Schemes ........................................................................................................................... xiv AcknowledGements .................................................................................................................. xvii 1. The Asymmetric Total Synthesis of Apratoxin D ............................................................. 1 1.1. BackGround and Introduction .................................................................................... 1 1.1.1. Macrocyclic natural products ................................................................................ 1 1.1.2. Isolation and bioloGical activity of the apratoxins ............................................. 3 1.1.3. BioloGical activity of the apratoxins and structure-activity relationships ...... 5 1.1.4. Previous synthetic studies ..................................................................................... 8 1.1.4.1 Syntheses of the apratoxins and common strateGies ................................... 8 1.1.4.2 Forsyth’s synthesis of apratoxin A ................................................................. 9 1.1.4.3 The Takahashi/Doi synthesis of apratoxin A .............................................. 13 1.1.5. Apratoxin D ........................................................................................................... 17 1.1.5.1 BackGround ...................................................................................................... 17 1.1.5.2 Retrosynthetic analysis of apratoxin D ........................................................ 18 1.2. Results and Discussion .............................................................................................. 22 1.2.1. Synthesis of polyketide from 3-butenal ............................................................. 22 1.2.2. Synthesis of polyketide fragment via ACC alkylation of an advanced intermediate ........................................................................................................................ 25 vii 1.2.3. Synthesis of polyketide fragment from (R)-citronellic acid ............................ 26 1.2.4. Tripeptide incorporation, thiazoline formation and macrocyclization ......... 29 1.2.5. Summary and conclusions ................................................................................... 31 1.3. Experimental ............................................................................................................... 32 1.3.1. General methods ................................................................................................... 32 2. Studies in the Gold(I)-Catalyzed Cycloisomerization of 7-Aryl-1,6-enynes ............... 56 2.1. Introduction ................................................................................................................ 56 2.1.1. Homogeneous Gold catalysis ............................................................................... 56 2.1.2. Mechanistic considerations in the Gold(I)-catalyzed cycloisomerization of 1,6-enynes ............................................................................................................................ 60 2.1.2.1 Cyclobutenes in the cycloisomerization of 1,6-enynes .............................. 61 2.1.2.2 Molecular complexity from the trapping of carbenoid intermediates in the cycloisomerization of 1,n-enynes .................................................................................. 64 2.1.3. Project Goals and scope ........................................................................................ 65 2.2. Results and Discussion .............................................................................................. 67 2.2.1. In situ spectroscopic analysis of the conversion of 2.12 to 2.13. ..................... 67 2.2.2. Trapping of cyclobutene 2.25 via hydroarylation ............................................ 71 2.2.3. Nucleophile scope in the hydroarylation of 3.12. ............................................. 73 2.2.4. Mechanistic considerations in the hydroarylation of 2.12. ............................. 75 2.2.5. Summary and conclusions ................................................................................... 78 2.3. Experimental ............................................................................................................... 80 2.3.1. General methods ................................................................................................... 80 2.3.2. In situ spectroscopic analysis of the conversion of 2.12 to 2.13 ...................... 81 viii 2.3.2.1 Spectroscopic analysis of the Gold-catalyzed conversion of 2.12 to 2.13 81 2.3.2.2 In situ spectroscopic snalysis of the Gold/triflic acid catalyzed conversion of 2.12 to 2.13. ................................................................................................................. 82 2.3.3. Gold complexes ..................................................................................................... 82 2.3.4. 1,6-Enynes .............................................................................................................. 83 2.3.5. 6,6-Diaryl-bicyclo-[3.2.0]-heptanes ..................................................................... 87 2.3.6. Optimization and control experiments ............................................................ 104 2.3.7. X-ray crystal structure of 2.48 ............................................................................ 109 3. Synthesis and Application of Multidentate LiGands Toward Realization of Fluxional Mechanocatalysis
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