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Aldehyde Cyclocondensation Reaction in Natural Product Synthesis

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Aldehyde Cyclocondensation Reaction in Natural Product Synthesis Utility of the Catalytic, Asymmetric Acyl Halide- Aldehyde Cyclocondensation Reaction in Natural Product Synthesis by Andrew S. Wasmuth B.Sc., University of Rochester, 2001 Submitted to the Graduate Faculty of the Department of Chemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2006 UNIVERSITY OF PITTSBURGH ARTS AND SCIENCES This dissertation was presented by Andrew S. Wasmuth It was defended on September 6, 2006 and approved by Scott G. Nelson, Ph.D., Associate Professor Craig S. Wilcox, Ph.D., Professor Peter Wipf, Ph.D., University Professor Billy Day, Ph.D., Associate Professor Dissertation Advisor: Scott G. Nelson, Ph.D., Associate Professor ii Copyright © by Andrew S. Wasmuth 2006 iii Utility of the Catalytic, Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reaction in Natural Product Synthesis Andrew S. Wasmuth, Ph. D. University of Pittsburgh, 2006 Abstract The ability of the catalytic, asymmetric acyl halide-aldehyde cyclocondensation (AAC) reaction to produce stereoenriched β-lactone products has found extensive utility in natural product synthesis. The asymmetric Al(III)-catalyzed AAC-SN2’ ring opening sequence was exploited in synthetic efforts towards the enantioselective total synthesis of the aspidospermane alkaloid (−)- rhazinilam (1). The synthetic sequence features an enantioselective cyclization of a tethered pyrrole moiety onto an optically-active allene to set the quaternary carbon stereocenter while concomitantly forming rhazinilam’s tetrahydroindolizine core. In addition, attempts at forming the requisite biaryl bond via a Pd-catalyzed cross-coupling reaction are also discussed. N O N O R · Et Et N H O 18 CO2H Et (−)-Rhazinilam (1) Recently, it was found that the Cinchona alkaloids quinine and quinidine can catalyze the AAC reaction to produce disubstituted β-lactones in high yield and in essentially enantiomerically and diastereomerically pure form. Reaction conditions were developed which allowed for the iv effective formation of masked polypropionate units by employing the Cinchona alkaloid- catalyzed AAC reaction. Based on the pseudoenantiomer of the Cinchona alkaloid used, different stereoarrays of polypropionate units are obtained. A variety of optically active aldehydes are viable in this transformation as reaction conditions can be optimized for a specific substrate. A matched/mismatched phenomenon was observed where the matched case produced the desired polypropionate unit in good yield and high diastereoselectivity and the mismatched case afforded an unexpected β-lactone product in diminished yield and diastereoselectivity. H H N N TMSO TMSO H H MeO MeO N N TMS-quinine (TMS-QN) TMS-quinidine (TMS-QD) O O O OTMS O OTMS O OTMS TMS-QN TMS-QD H Ph Me Ph Me Ph Me Me Me 63 57 61 68% yield 86% yield 19:1/trans:cis single diastereomer (+)-Discodermolide (81) is a marine, microtubule-stabilizing polyketide that can only be isolated in scarce amounts from nature. Due to our inability to harvest it in supple amounts, the total synthesis of (+)-discodermolide has been the focus of many research groups. Application of the cinchona alkaloid-catalyzed AAC reaction towards the catalytic, asymmetric total synthesis of an analogue of (+)-discodermolide (100) is discussed. v OP 100 Me P = TBS O OP OP OP OP MeO Me Me N OP OP Me Me Me Me Me Me O OP OP O OP I OP OP MeO Me N H ++OP Me Me Me Me Me I Me Me Me 99 84 85 vi TABLE OF CONTENTS 1.0 EFFORTS TOWARDS THE TOTAL SYNTHESIS OF (-)-RHAZINILAM........ 1 1.1 ISOLATION AND BIOACTIVITY................................................................... 1 1.2 PAST SYNTHESES............................................................................................. 3 1.3 RETROSYNTHETIC ANALYSIS AND AAC TECHNOLOGY................... 7 1.4 RESULTS AND DISCUSSION........................................................................ 10 1.4.1 Background ................................................................................................. 10 1.4.2 Synthesis of 1-silyloxy-5,6,7,8-tetrahydroindolizine 28 ........................... 12 1.4.3 Synthesis of 1-benzyloxy-5,6,7,8-tetrahydroindolizine 35....................... 17 1.4.4 Formation of triflate precursor 45 ............................................................ 21 1.4.5 Attempts at forming the biaryl bond ........................................................ 27 1.4.5.1 Exploring siloxanes as cross-coupling partners............................... 27 1.4.5.2 Attempts at using a Suzuki cross-coupling reaction........................ 29 1.5 EXPERIMENTAL............................................................................................. 31 2.0 EFFICIENT FORMATION OF POLYPROPIONATE UNITS VIA THE CINCHONA ALKALOID-CATALYZED AAC REACTION............................................... 53 2.1 BACKGROUND................................................................................................ 53 2.2 RESULTS AND DISCUSSION........................................................................ 59 2.2.1 Synthesis and reactivity of model aldehyde 57......................................... 59 2.2.2 Synthesis and reactivity of model aldehyde 64......................................... 63 2.2.3 Synthesis and reactivity of aldehyde ent-64b ........................................... 71 2.2.4 Synthesis and reactivity of aldehyde 72 .................................................... 73 2.2.5 Iterative polypropionate unit formation................................................... 75 2.3 EXPERIMENTAL............................................................................................. 77 vii 3.0 EFFORTS TOWARDS A CATALYTIC, ASYMMETRIC SYNTHESIS OF AN ANALOGUE OF (+)-DISCODERMOLIDE............................................................................ 98 3.1 ISOLATION AND BIOACTIVITY................................................................. 98 3.2 PAST SYNTHESES AND RETROSYNTHETIC ANALYSIS..................... 99 3.3 RESULTS AND DISCUSSION...................................................................... 103 3.3.1 Synthesis of aldehyde 83........................................................................... 103 3.3.2 Synthesis of aldehyde 99........................................................................... 107 3.3.3 Synthesis of iodide 85................................................................................ 109 3.3.4 Synthesis of the model alkyne 108........................................................... 111 3.3.5 First attempt at the synthesis of alkyne 84 ............................................. 114 3.3.6 Revised synthesis of alkyne 84 ................................................................. 117 3.4 EXPERIMENTAL........................................................................................... 119 APPENDIX A............................................................................................................................ 150 BIBLIOGRAPHY..................................................................................................................... 152 viii LIST OF TABLES Table 1: Deprotection of model 3-benzyloxy pyrrole 41.............................................................. 24 Table 2: Protecting group effect on aldehyde 64 in the AAC....................................................... 65 Table 3: Co-solvent effect on the AAC reaction of aldehydes 64a,b ........................................... 66 Table 4: Temperature effect on the AAC reaction of aldehyde 64b............................................. 67 Table 5: Addition of model alkyne 108 into aldehyde 99 .......................................................... 113 ix LIST OF FIGURES Figure 1: (-)-Rhazinilam (1) and (+)-1,2-dehydroaspidospermidine (2) ........................................ 2 Figure 2: Key steps in Smith’s synthesis of rhazinilam.................................................................. 4 Figure 3: Key steps in Sames’ syntheses of rhazinilam.................................................................. 5 Figure 4: Key steps in Magnus’ synthesis of rhazinilam................................................................ 6 Figure 5: Key step in Trauner’s synthesis of rhazinilam ................................................................ 6 Figure 6: The Nelson Group’s retrosynthetic analysis of (-)-rhazinilam........................................ 8 Figure 7: The AAC-SN2' ring opening sequence and (-)-malyngolide (19) ................................. 10 Figure 8: Restrictions for Grignard precursor............................................................................... 11 Figure 9: 3-Alkoxy/silyloxy substituted pyrroles as Grignard precursors.................................... 12 Figure 10: Synthetic analysis of 3-silyloxy pyrrole 20................................................................. 13 Figure 11: Proposed mechanism for Pd(II)-catalyzed cyclization (M = Pd)................................ 16 Figure 12: Synthetic analysis of pyrrole 29 and synthesis of pyrrolone 30.................................. 18 Figure 13: Erythromycin (55) and (−)-dictyostatin (56)............................................................... 53 Figure 14: Formation of propionate units via enzymatic catalysis............................................... 54 Figure 15: Formation of polypropionate units via the alkaloid-catalyzed AAC .......................... 58 Figure 16: Explanation of beta-lactone products from aldehyde 57............................................
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