Formal Synthesis of Palmerolide a Using Fragmentation Methodology Marilda Pereira Lisboa
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THE FLORIDA STATE UNIVERSITY
COLLEGE OF ARTS AND SCIENCES
FORMAL SYNTHESIS OF PALMEROLIDE A
USING FRAGMENTATION METHODOLOGY
By
MARILDA PEREIRA LISBOA
A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy
Degree Awarded: Spring Semester, 2013
Marilda Pereira Lisboa defended this dissertation on April 1, 2013.
The members of the supervisory committee were:
Gregory B. Dudley
Professor Directing Dissertation
Akash Gunjan
University Representative
Igor V. Alabugin
Committee member
Lei Zhu
Committee member
Michael Shatruk
Committee member
The Graduate School has verified and approved the above-named committee members, and certifies that the dissertation has been approved in accordance with university requirements.
ii
ACKNOWLEDGEMENTS
The PhD is definitely a life changing experience. I gained knowledge, I learned how to build knowledge and I exchanged knowledge with various people during this journey. Without a doubt, the person that helped me the most during the process was my advisor Dr Gregory
Dudley. I would like to thank Dr Dudley for all his support. In the beginning, I had no idea of how much I would learn from him, not only in the scientific level, but also in the personal level. I am grateful because I had the opportunity to work with a role model like Dr Dudley.
There are countless other people that help me during the last five years. Without listing their names, I just want to thank everyone for their contributions and friendship:
my friends from the Dudley lab,
my friends from the other labs,
my professors,
my students,
the staff from the Chemistry and Biochemistry Department,
the janitors,
my friends from UFMG in Brazil,
Fulbright and Capes,
my committee members,
and my family…
iii
TABLE OF CONTENTS
LIST OF TABLES ...... v
LIST OF FIGURES ...... vi
LIST OF ABBREVIATIONS ...... x
ABSTRACT ...... xiv
CHAPTER 1 - PALMEROLIDE A ...... 1
CHAPTER 2 - TOTAL SYNTHESES AND APPROACHES TOWARDS PALMEROLIDE A . 7
CHAPTER 3 - RING OPENING OF CYCLIC VINYLOGOUS ACYL TRIFLATES USING STABILIZED CARBANION NUCLEOPHILES ...... 41
CHAPTER 4 - FORMAL SYNTHESIS OF PALMEROLIDE A ...... 67
CHAPTER 5 - ENDEAVORS IN THE SYNTHESIS OF PALMEROLIDE A ...... 91
CHATER 6 - FUTURE DIRECTIONS ...... 112
APPENDIX A ...... 114
APPENDIX B ...... 121
APPENDIX C ...... 138
REFERENCES ...... 147
BIOGRAPHICAL SKETCH ...... 155
iv
LIST OF TABLES
Table 1 - V-ATPase inhibition versus cytotoxicity of palmerolides A, D, E, F and G...... 3
Table 2 - Cytotoxicity of palmerolide A and some analogues...... 6
Table 3 - Overview of the synthetic sequences to palmerolide A...... 39
Table 4 - Reaction of vinylogous acyl triflates with 1.1 equiv of phosphonate ...... 55
v
LIST OF FIGURES
Figure 1 - Structure of palmerolides A-G...... 2
Figure 2 - First-generation of palmerolide A analogues...... 4
Figure 3 - Second-generation of palmerolide A analogues...... 6
Figure 4 - Retrosynthesis of proposed structure of palmerolide A...... 8
Figure 5 - Synthesis of fragment 29...... 8
Figure 6 - Preparation of fragments 30 and 31...... 9
Figure 7 - De Brabander synthesis of proposed structure 28 and ent-1...... 10
Figure 8 - Retrosynthesis of proposed structure of palmerolide A...... 11
Figure 9 - Synthesis of fragment 40...... 11
Figure 10 - Preparation of fragment 41...... 12
Figure 11 - Preparation of fragment 42...... 12
Figure 12 - Synthesis of proposed (28) and revised structure of palmerolide A (1)...... 13
Figure 13 - Improvements in the synthesis reported by Nicolaou/Chen...... 14
Figure 14 - Alternative macrocyclization strategies for the synthesis of palmerolide A...... 15
Figure 15 - Organoboron methodologies developed in Hall lab...... 16
Figure 16 - Retrosynthesis of palmerolide A by Hall and coworkers...... 17
Figure 17 - Synthesis of fragment 56...... 18
Figure 18 - Synthesis of fragment 57...... 19
Figure 19 - Completion of the synthesis of palmerolide A...... 19
Figure 20 - Retrosynthesis of palmerolide A by Maier group...... 20
Figure 21 - Preparation of fragments 81 and 82...... 21
Figure 22 - Preparation of fragment 69...... 21
Figure 23 - Completion of Maier formal synthesis...... 22
vi
Figure 24 - Retrosyntheses of palmerolide A according to Prasad group...... 23
Figure 25 - Synthesis of fragment 40...... 24
Figure 26 - Synthesis of fragment 89...... 24
Figure 27 - Synthesis of the late intermediate 88 in Nicolaou/Chen synthesis...... 25
Figure 28 - Synthesis of fragment 90...... 25
Figure 29 - Synthesis of fragment 91...... 26
Figure 30 - Completion of the formal synthesis of palmerolide A...... 26
Figure 31 - Retrosynthesis of palmerolide A as reported by Kaliappan group...... 28
Figure 32 - Synthesis of fragment 111...... 28
Figure 33 - Synthesis of fragment 112...... 29
Figure 34 - Synthesis of fragment 113...... 30
Figure 35 - Completion of Kaliappan formal synthesis...... 30
Figure 36 - The C1-C14 fragment of palmerolide A by Chandrasekhar group...... 30
Figure 37 - Synthesis of the C1-C14 fragment of palmerolide A...... 31
Figure 38 - Retrosynthesis of palmerolide A according to Meyer/Cossy group...... 32
Figure 39 - Preparation of fragment 137...... 32
Figure 40 - Preparation of fragment 138...... 33
Figure 41 - Preparation of fragment 40...... 33
Figure 42 - Preparation of fragment C3-C15...... 33
Figure 43 - Fragment C3-C15 of palmerolide A...... 34
Figure 44 - Preparation of fragment C13-C21...... 34
Figure 45 - The C3-C14 fragment of palmerolide A by Baker group...... 35
Figure 46 - Preparation of fragment C3-C14...... 35
Figure 47 - Retrosynthetic analysis of palmerolide C and revised structure...... 36
Figure 48 - Preparation of fragments 165 and 167...... 37
vii
Figure 49 - Preparation of fragment 159...... 37
Figure 50 - Synthesis of the proposed structure of palmerolide C...... 38
Figure 51 - Prospective application to palmerolide A...... 41
Figure 52 - Mechanism of the classical Claisen condensation of ethyl acetate...... 42
Figure 53 - Fragmentation reactions...... 43
Figure 54 - Synthesis of VAT from a symmetrical dione...... 43
Figure 55 - VAT-Claisen reaction...... 44
Figure 56 - VAT-Claisen Reaction using lithium enolate of acetophenone...... 46
Figure 57 - Possible mechanistic pathways and deuterium-labeling experiment...... 47
Figure 58 - First report Claisen-type condensation reactions of VAT 177a...... 49
Figure 59 - Preparation of Horner-Wittig reagent 176f...... 50
Figure 60 - Proposed fragmentation reaction pathway between 177a and 192...... 50
Figure 61 - Crossover experiment with lithium enolate 195...... 52
Figure 62 - Proposed mechanism of the reaction between VAT 177a and 191...... 54
Figure 63 - Common methods for the preparation of phosphonates...... 54
Figure 64 - Byproducts 198b and 198c...... 56
Figure 65 - Synthesis of phosphonate 176i...... 57
Figure 66 - Synthesis of phosphonate 176m, an analogue of phosphonate 176i...... 58
Figure 67 - Proposed mechanism of the reaction between VAT 177 and phosphonates...... 59
Figure 68 - Retrosynthetic analysis: Identification of three key subunits for assembly to palmerolideA...... 68
Figure 69 - Synthesis of phosphonate 202...... 68
Figure 70 - Synthesis of aldehyde 203...... 69
Figure 71 - Synthesis of iodide 204...... 70
Figure 72 - Assembly of subunits 202, 203 and 204 to complete the formal synthesis of palmerolide A...... 71
viii
Figure 73 - Synthesis of Horner-Wadsworth-Emmons reagent 176a...... 91
Figure 74 - Synthesis of fragments 215 and 217...... 92
Figure 75 - Synthesis of fragment C1-C15 of palmerolide A...... 93
Figure 76 - Olefination reaction between phosphine oxide 215 and aldehyde 217...... 93
Figure 77 - Synthesis of new aldehyde fragment 203...... 94
Figure 78 - Synthesis of phosphonate 202 and enone 211...... 95
Figure 79 - (R)-CBS reduction of the C1-C15 fragment of palmerolide A...... 96
Figure 80 - CBS reduction of macrolactone 222 in the synthesis of palmerolide A...... 97
Figure 81 - Palmerolide A and targeted fragment C16-C24...... 97
Figure 82 - Convergent Negishi coupling for the preparation of aldehyde 229...... 98
Figure 83 - Homopropargylation by Grignard addition and Singaram’s method...... 99
Figure 84 - Unexpected carbometalation difficulties (top), and an unusual substitution reaction under identical conditions...... 100
Figure 85 - Synthesis of target C16-C24 fragment...... 100
Figure 86 - Attempts to convert aldehyde 238 to dienamide 239...... 101
Figure 87 - Model study applied to the macrocyclization event...... 102
Figure 88 - Alternative vinylogous aldol reaction...... 112
Figure 89 - Alternative macrocyclization by RCM...... 113
ix
LIST OF ABBREVIATIONS
µM micromolar
Ac acetyl
AD-mix asymmetric dihydroxylation mixture
BBN borabicyclononane
BH3.THF borane tetrahydrofuran complex
Bn benzyl
Bz benzoyl
CBS Corey-Bakshi-Shibata
Cp cyclopentadienyl
CSA camphorsulfonic acid dba dibenzylideneacetone de diastereomeric excess
DEAD diethyl azodicarboxylate
DIPT diisopropyl tartrate
DMAP dimethylaminopyridine
DMEDA N, N-dimethylethylenediamine
DMP Dess-Martin periodinane
DMS dimethylsulfide dr diastereomeric ratio
EDC 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
x
ee enantiomeric excess
EI electron impact ent enantiomer equiv equivalent
ESI electrospray ionization
EWG electron withdrawing group
FT fourier transform g gram
GI50 growth inhibition, 50%
HF.py hydrogen fluoride pyridine complex
HMDS hexamethyldisilazane
HMPA hexamethylphosphoramide
HRMS high-resolution mass spectroscopy
HWE Horner-Wadsworth-Emmons
IBX iodoxybenzoic acid
IC50 inhibitory concentration, 50%
Imid imidazole iPr isopropyl
IR infrared
KAPA potassium aminopropylamine
K-Selectride potassium tri-sec-butylborohydride
LAH lithium aluminum hydride
LC50 lethal concentration, 50%
xi
LDA lithium diisopropylamide
M molar m-CPBA meta-chloroperbenzoic acid mg milligram
MHz megahertz mL milliliter mmol milimol
MOM methoxymethyl ether
MS molecular sieves
NBS N-bromosuccinimide
NCI National Cancer Institute nM nanomolar
NMR nuclear magnetic resonance
NOE nuclear overhauser effect
Ph phenyl
PKS polyketide synthase
PMB para-methoxybenzyl
PMP para-methoxyphenyl ppm parts per million
PPTS pyridinium para-toluenesulfonate
RCM ring-closing metathesis
Red-Al sodium bis(2-methoxyethoxy)aluminum hydride
ROESY rotating frame overhauser effect spectroscopy
xii
SI selectivity index
TBAF tetrabutylammonium fluoride
TBDPS tert-butyldiphenyl silyl
TBS tert-butyldimethylsilyl tBu tert-butyl
TCBC trichlorobenzoyl chloride
TEMPO tetramethylpiperidyl oxide
TES triethyl silyl
Tf triflate
THP tetrahydropyran
TIPS triisopropyl silyl
TLC thin-layer chromathography
TMS trimethylsilyl
Tr trityl or triphenylmethyl
Ts tosyl
VAT vinylogous acyl triflate
V-ATPase vacuolar adenosine triphosphatase
xiii
ABSTRACT
In this dissertation is described the formal synthesis of the natural product palmerolide A, using an addition/ fragmentation reaction of vinylogous acyl triflates developed in the Dudley
Lab. In the first chapter, the isolation and biological activity of palmerolide A and analogues are presented. Palmerolide A has promising biological activity relevant to melanoma research, and chemical synthesis is the only reliable source of the material. The second chapter contains a brief description of the reported syntheses of palmerolide A, as well as the various approaches towards this molecule and congeners. In chapter three, the fragmentation of vinylogous acyl triflates using stabilized carbanion nucleophiles is discussed. The mechanistic studies and the optimization that resulted in a practical synthesis of one of the fragments of palmerolide A are detailed. The formal synthesis of palmerolide A developed by the Dudley Lab and some tactical challenges are illustrated in chapter four. The next chapter includes several obstacles, efforts and the solutions that made possible the formal synthesis of this natural product. Future directions of the project are presented in chapter six. The synthesis of palmerolide A described in this thesis is the highest yielding synthesis reported to date.
xiv
CHAPTER 1
PALMEROLIDE A
Antarctic marine natural products offer exciting and underexplored opportunities for drug discovery. Antarctica is secluded from other continents by large distances, deep waters and marine currents, such that its ecosystem is isolated from the rest of the globe.1 The Antarctic marine fauna had to adapt to extreme local and cold environmental conditions.2 One way of adaptation is chemical defense. Sessile marine invertebrates, for instance, protect themselves from mobile predators and microorganisms via secondary metabolites.3 In this context, the study of this secluded location can lead to the isolation of unique compounds.4
The palmerolides (Figure 1) are a group of natural products isolated from the tunicate
Synoicum adareanum encountered in the cold shallow waters of Antarctica near Anvers Island.5,6
Palmerolide A (1) is a twenty-membered ring macrolide bearing an enamide side chain.
Palmerolide A is the most prominent member of this group, which is mainly due to its cytotoxicity. In the National Cancer Institute (NCI) sixty cell panel, palmerolide A targets melanoma (UACC-62, LC50 = 18 nM), colon cancer (HCC-2998, LC50 = 6.5 µM) and renal cancer cell lines (RXF 393, LC50 = 6.5 µM) selectively by three orders of magnitude when compared to the other cell lines in the panel.5 Moreover, palmerolide A is a vacuolar proton
7 ATP-ase (V-ATPase) inhibitor (IC50 = 2 nM) and is active in the NCI-s hollow fiber assay.
Some interesting structural features of palmerolide A, such as the carbamate and the vinyl amide moieties, led the Baker group to hypothesize a bacterial origin for this polyketide. The microbial community of the tunicate Synoicum adareanum was characterized using a single colony, however further studies are necessary to discriminate the free-living bacteria and the
1
associated populations. One finding that can support the possibility of bacteria producing palmerolide was the identification of polyketide synthase (PKS) genes, similar to bryostatin biosynthetic genes.8
Figure 1 - Structure of palmerolides A-G.
The cytotoxicity and selectivity of palmerolide A are important, because melanoma is an aggressive type of cancer, for which current chemotherapies are lacking.9 Melanoma occurs in the melanocytes, mainly in the skin, but can also appear in other pigmented tissues, like eye and intestines.10 The cancer of the skin is the most common type of cancer, and melanoma accounts for only 5% of the skin cancer cases. However, melanoma is responsible for the majority of skin cancer deaths.11 The incidence rates for melanoma have been rising for more than thirty years. In
2013, 76690 new cases and 9480 deaths from melanoma are estimated in the United States.10
Palmerolide A is a potent V-ATPase inhibitor. Some V-ATPase inhibitors like salicylihalamide A12 and the bafilomycins13 are known to possess cytotoxic activity. V-ATPases
2
are proton pumps present in the membranes of endosomes, lysosomes and secretory vesicles.
These enzymes play a major role in cancer metastasis because they are responsible for microenviroment acidification. Data show that the acidity of the tumor microenviroment is correlated to cancer progression and metastasis.14-17
Since the isolation of palmerolide A by Dr. Bill Baker and coworkers in 2006, the mechanism of action for the cytotoxicity of this natural product is not entirely understood. The discrepancy between V-ATPase inhibition and cytotoxicity among the palmerolides suggests an additional mechanism of action. Considering the biological activity among the palmerolides, palmerolide D is less potent towards V-ATPase (IC50 = 25 nM) than palmerolide A, but it presents greater cytotoxicity (UACC-62 LC50 = 2 nM). Also, palmerolide G is a very potent V-
ATPase inhibitor (IC50 = 7 nM), but it presents lower cytotoxicity (UACC-62 LC50 = 1200 nM) when compared to palmerolide A (Table 1).
Table 1 - V-ATPase inhibition versus cytotoxicity of palmerolides A, D, E, F and G. Palmerolide A Palmerolide D Palmerolide E Palmerolide F Palmerolide G V-ATPase 0.002 0.025 >10.000 0.063 0.007 (IC50, µM) UACC-62 0.024 0.002 5.000 0.758 1.207 (LC50, µM)
Additional biological studies are crucial to identify the molecular targets of palmerolide
A. For that, chemical synthesis is the best option to make available large amounts of palmerolide.
The accessibility of palmerolide A from natural sources is limited because it was isolated from the remote Antarctic continent. Moreover, Antarctica is protected by the Antarctic Treaty, which prohibits the commercial exploitation of the region.18
3
To date, three total syntheses19-22 and four formal syntheses23-28 of palmerolide A have been reported, as well as various synthetic approaches.29-35 One formal synthesis28 and two approaches come from the Dudley research group.36,37 Besides that, one synthesis of the proposed structure of palmerolide C was recently reported38. The earliest syntheses developed by
De Brabander19 and by the Nicolaou/Chen20 groups led to the stereochemistry reassignment of the natural product at the C7, C10 and C11 positions from R to S. Later in the same year, the
Baker group reported degradative studies on natural palmerolide A corroborating the reassignment.39
Figure 2 - First-generation of palmerolide A analogues.
In addition to that, the syntheses of various analogues were reported by the Nicolaou and
Chen groups and their structure-activity relationship was investigated.40,41 In the first report on the synthesis and biological evaluation of palmerolide A analogues (Figure 2), Nicolaou and
Chen observed the in vitro activity of palmerolide A derivatives against various cancer cell lines.
Natural palmerolide A, synthetic palmerolide A, taxol and doxorubicin were used as controls.
4
Based on previous findings on the correlation of enamide containing molecules and V-ATPase inhibitory activity,12 the enamide portion of the molecule was the main modification site (2-6).
Besides that, a series of diastereomers (8-10), compounds with deletions of carbamate (11-15) or hydroxyl groups (16 and 17), and the enantiomer of palmerolide A were synthesized and their biological activities assessed.
The results showed that natural (UACC-62 GI50 = 57 nM) and synthetic (UACC-62 GI50
= 62 nm) palmerolide A have comparable activity, while the enantiomer of palmerolide A showed a decrease of activity by 100-fold (UACC-62 GI50 = 8077 nM). Modifications in the side chain usually led to a decrease in activity, with the exception of the benzamide analogue 5 that was more potent (UACC-62 LC50 = 9 nM) and the saturated analogue 3 (UACC-62 GI50 = 67 nM) which showed similar activity when compared to palmerolide A. The diastereomers presented a large decrease in activity or no activity. The deletion of the carbamate group (12,
UACC-62 GI50 = 322 nM) or hydroxyl group at C10 (16, UACC-62 GI50 = 6979 nM) also decreased the activity. However, deletion of the hydroxyl group at C7 does not change the
40 activity significantly, (17, UACC-62 GI50 = 63 nM).
In 2011, the Chen group reported the synthesis and biological evaluation of a second- generation of analogues. Structural variations in the enamide side chain and in the C1-C8 fragment of various palmerolide derivatives (Figure 3), without the hydroxyl group at C7, were tested against a series of cancer cells and human melanocytes (HM-2). The human melanocytes cells were used to calculate the selectivity index (SI = GI50 HM-2/ GI50 UACC-62).
The results have shown that deletion of the enamide side chain afforded compounds with no activity, substitutions in the aromatic ring of the benzamide (18-22) and modifications in the
C1-C8 fragments decreased the activity (24-27). The benzamide analogue 23 (no OH at C7)
5
showed a small decrease in activity (UACC-62 GI50 = 55 nM) when compared to analogue 5
(UACC-62 GI50 = 9 nM). None of the analogues tested were more selective and potent than the benzamide analogue 5 (SI = 256.6) (Table 2).41
Figure 3 - Second-generation of palmerolide A analogues.
Table 2 - Cytotoxicity of palmerolide A and some analogues. UACC-62 (µM) HM-2 (µM) SI 1 0.061 ± 0.011 6.177 ± 1.272 101.3 5 0.009 ± 0.000 2.309 ± 1.269 256.6 17 0.076 ± 0.010 14.006 ± 4.589 184.3 23 0.055 ± 0.011 8.256 ± 2.222 150.1
In summary, the natural product palmerolide A and congeners are promising anticancer agents, and further biological studies are necessary to comprehend the cytotoxicity and selectivity of this group. However, the availability of material from natural sources is limited as a result of the remote isolation area and the Antarctic treaty. Accordingly, chemical synthesis is the best alternative to afford the palmerolides, and improvements are required for a more efficient synthetic sequence.
6
CHAPTER 2
TOTAL SYNTHESES AND APPROACHES TOWARDS PALMEROLIDE A
Palmerolide A was isolated in 2006. Since then, three total syntheses and four formal syntheses were achieved. Additionally, several synthetic approaches targeting this natural product, including one towards the synthesis of palmerolide C have been published. In this chapter, I present a review of the synthetic sequences reported to date, and categorized by research groups, excluding the ones developed within the Dudley Lab. I then conclude by contrasting the various strategies underlying the synthesis of palmerolide A.
The De Brabander group
One year after the disclosure of palmerolide A, the De Brabander group published the first synthesis of the proposed structure along with the enantiomer of this natural product.19 The proposed structure 28 was synthesized, but its physical properties and NMR spectra diverged from the data for natural palmerolide A. Noteworthy, a reevaluation of the NMR spectra and
Mosher ester analysis, led to the synthesis of the enantiomer of palmerolide A.
Specifically, the synthesis was accomplished by the coupling of three fragments (i.e. the vinyl iodide 29, the boronate 30 and the phosphonate 31). The fragments were connected by a
Suzuki cross-coupling, followed by a Yamaguchi esterification and finalized by a Horner-
Wadsworth-Emmons olefination to give the macrocyclic core. The enamide side chain was installed at the end of the sequence by a Curtius rearrangement (Figure 4).19
7
Figure 4 - Retrosynthesis of proposed structure of palmerolide A.
Vinyl iodide 29 was prepared using a vinylogous aldol reaction between silyl enol ether
32 and vinyl iodide 33. The anti-aldol product 34 was obtained in 80% yield and diastereomeric ratio (dr) of 13:1. In this reaction, the alkene at C21-C22 and the stereocenter at C20 were installed. The stereochemistry of the hydroxyl at C19 was inverted by Mitsunobu protocol in
66% yield. Thus, chiral auxiliary cleavage using DIBAL, followed by olefination, gave fragment
29 in 92% over the 2 steps (Figure 5).19
Figure 5 - Synthesis of fragment 29.
Fragment 30 was synthesized from D-arabitol, taking advantage of the chirality of the starting material to set stereocenters C10 and C11. The synthesis was achieved in eight steps in a sequence of protecting group manipulations and reduction-oxidation reactions. Fragment 31 was prepared in five steps from δ-valerolactone (Figure 6).19
8
Figure 6 - Preparation of fragments 30 and 31.
The macrocyclic core of the proposed structure of palmerolide A commenced with an intricate Suzuki coupling between vinyl iodide 29 and boronate 30 using Pd(PPh3)4 and Tl2CO3.
The diene product was obtained in 79% yield and connected to fragment 31 using Yamaguchi conditions (69% yield). Protecting group manipulations and selective oxidation gave an aldehyde. Subsequently, macrocyclization using Horner-Wadsworth.-Emmons olefination afforded product 36 in 70% yield over two steps. The alcohol at C11 was protected as a TMS silyl ether and (S)-CBS reduction of C7 gave an alcohol in 99% yield and 4:1 dr. Following the reduction, TBS ether installation and saponification gave a carboxylic acid, which was converted to an acyl azide in 92% yield. Curtius rearrangement followed by addition of 2-Me- propenylmagnesium bromide gave dienamide 39. Installation of the carbamate and deprotection afforded the proposed structure 28 (Figure 7).19
Unfortunately, the NMR spectra revealed that the proposed structure differed from the natural product. Accordingly, reanalysis of the original data suggested that C19 and C20 stereocenters might have been misassigned. Mosher ester analysis was used to assign the stereochemistry of the triad C7, C10 and C11, but C19 and C20 were assigned based on coupling constants, NOE and ROESY experiments. The spectra clearly showed that C19 and C20 have a syn relative configuration. However, the stereochemistry might have been assigned erroneously.
Thus, the De Brabander group decided to synthesize the C19 and C20 epimer of the proposed structure (i.e. the enantiomer of palmerolide A).
9
The enantiomer of palmerolide A was synthesized using the same sequence of reactions through the enantiomer of vinyl iodide 29 (yields in red/parentheses in figure 7). Notably, the
(S)-CBS reduction of C7 favored the opposite diastereomer and the product was obtained in 98% yield and 1:3 dr (mismatched case). Reduction with NaBH4 also favored the wrong stereochemistry at C7 (1:10 dr) confirming the substrate bias.19
Figure 7 - De Brabander synthesis of proposed structure 28 and ent-1.
The spectroscopic data for natural palmerolide A and the synthesized enantiomer were identical, except for the mirror image CD-spectra. The Mosher ester analysis of C7, C10 and C11 described in the isolation of palmerolide A was incorrect, the reagents were switched.39 As a result, the structure of the natural product was corrected.19,20,39
Nicolaou and Chen groups
10
The second total synthesis of palmerolide A was reported by the Nicolaou/Chen group in
2007.20 Nicolaou and Chen synthesized the proposed and revised structures of palmerolide A.
The retrosynthesis was also based in the assembling of three fragments (Figure 8). The fragments vinyl iodide 40, stannane 41, and acid 42 were coupled using a Stille coupling, followed by a
Yamaguchi esterification, then, ring-closing metathesis afforded the macrocyclic core. In the final step, the enamide in the side chain was installed.
Figure 8 - Retrosynthesis of proposed structure of palmerolide A.
The first fragment 40 was synthesized from oxazolidinone 43 and vinyl iodide 33 in a syn-selective aldol reaction in 46% and 95% de. Then, removal of the chiral auxiliary, olefination and protecting group manipulations gave vinyl iodide 40 (Figure 9).
Figure 9 - Synthesis of fragment 40.
The synthesis of the second fragment 41 began with the reaction of aldehyde 45 and an organoborane to give a monoprotected diol (>95% de and > 90% ee). After removal of TMS
11
protecting group, alcohol 46 was obtained in 74% yield over two steps. Hence, the carbamate and the vinyl stannane moieties were installed in three steps (62% yield) to afford fragment 41
(Figure 10).20
Figure 10 - Preparation of fragment 41.
Fragment 42 was prepared from alkene 47. A Jacobsen kinetic resolution converted the
+ - alkene into epoxide 48 in 42% yield (2 steps) and 99% ee. Epoxide opening with Me3S I afforded alcohol 49, setting the hydroxyl stereochemistry at C7. Five more steps converted alcohol 49 to carboxylic acid 42 (Figure 11).20
Figure 11 - Preparation of fragment 42.
Subsequently, fragments 40 and 41 were connected in a Stille coupling using Pd(dba)2 and AsPh3 in 67%. The third fragment 42 was coupled with alcohol 50 in a Yamaguchi esterification in 61% yield. Two steps later, a Takai olefination installed a vinyl iodide in the side chain in preparation for the enamide coupling at the end of the synthesis. The MOM protecting groups were removed using acidic conditions. Grubbs II catalyst promoted the cyclization of structure 52 in 76% yield and the enamide was installed using Buchwald protocol in 44% yield to give the proposed structure of palmerolide A.
12
Figure 12 - Synthesis of proposed (28) and revised structure of palmerolide A (1).
While the Nicolaou/ Chen team was completing the synthesis, they observed difference between the natural product and the synthetic material by NMR. Meanwhile, De Brabander reported a synthesis of palmerolide A. Grounded in this foundation work, the Nicolaou/Chen group synthesized the revised structure of palmerolide A using the same synthetic sequence presented in Figure 12 using ent-41 and ent-42 (yields in red/parentheses). This time, the synthetic and natural palmerolide A were identical.
After the first report, Nicolaou and Chen published a paper detailing the synthesis of the proposed structure, the 19-epimer, the 20-epimer, the 19,20-epimer and the revised structure of palmerolide A.21 A similar sequence of reactions, as described above in Figure 12, was used to prepare all the compounds along with some improvements. For example, a vinylogous anti-aldol reaction was used to prepare the vinyl iodide fragment 40 (Figure 13) replacing the previous aldol reaction (43→44, Figure 9). This modification decreased the number of steps and allowed for the preparation of other isomers. Besides that, the order of fragments coupling was alternated.
Initially, fragments 40 and 41 were connected in a Stille coupling followed by Yamaguchi esterification with fragment 42 (32% over 2 steps for the revised structure of palmerolide A).
13
Alternating the order of the reactions (1- Yamaguchi; 2- Stille), the yield increased to 55% over the 2 steps (Figure 13). The exchange of Cs2CO3 to K2CO3 was the last alteration performed in an attempt to increase the yield of the enamide coupling. However, only a small improvement was observed (46% to 51% yield).
Figure 13 - Improvements in the synthesis reported by Nicolaou/Chen.
Next, Nicolaou/Chen team explored different alternatives for the macrocyclization of the revised structure of palmerolide A (Figure 14). The options for the macrocyclization were: 1) a
Horner-Wadsworth-Emmons olefination (C2-C3 olefin) achieved in 73% yield; 2) a Stille coupling to connect C14-C15, which was not successful, giving a crude mixture that could not be purified; 3) a Yamaguchi macrolactonization accomplished in 81% yield; and 4) a Mitsunobu protocol, that gave the product in only 40% yield. Nicolaou/Chen group concluded that the
14
Yamaguchi lactonization and the Horner-Wadsworth-Emmons olefination are valuable options to the RCM in the construction of the macrocycle of palmerolide A.21
Later in 2008, Nicolaou and Chen published the synthesis and biological activity of various analogues of palmerolide A.40 Moreover, a second-generation of palmerolide A analogues were synthesized in 2011, and their biological activities studied by Chen group.41 The methods utilized for the synthesis of the analogues were equivalent to the previous reports.
Figure 14 - Alternative macrocyclization strategies for the synthesis of palmerolide A.
Hall group
The third total synthesis of palmerolide A was developed by the Hall group. Organoboron methods evolved by Hall and coworkers were used in the synthesis of this natural product.22
The catalytic enantioselective crotylboration of aldehydes was the first method utilized.
To this extent, Hall and colleagues published a series of articles on the theme.42-44 Various catalysts were designed to improve the yields and enantioselectivity of the crotyl- or allylboration. After optimization, the best results were achieved with p-F-Vivol (A - Figure 15).
A cycloheptyl derivative of this catalyst was used in the synthesis of palmerolide A to install the stereocenters C19 and C20 in the side chain. The second strategy used by Hall team in the
15
synthesis of palmerolide A was a [4+2] cycloaddition/allylboration sequence (B - Figure 15).45,46
The Jacobsen’s chromium (III) complex is the catalyst and the product can react with an aldehyde to afford α-hydroxyalkyl pyrans in good yields and high stereoselectivity. This methodology along with a Claisen rearrangement was used to set the stereocenters at C7, C10 and C11.
Figure 15 - Organoboron methodologies developed in Hall lab.
The Hall retrosynthetic approach to palmerolide A is depicted in Figure 16. The last step would be the enamide coupling in the side chain, following De Brabander’s end game. The coupling of two fragments 56 and 57, by Suzuki coupling and Yamaguchi macrolactonization, was thought to give the core of palmerolide A. Fragments 56 and 57 were envisioned based on asymmetric organoboron methodologies. Fragment 56 was prepared from the ring opening of pyran 58, which was synthesized from 59 in a Curtius rearrangement. The α-hydroxy pyran 59 was prepared using a [4+2] cycloaddition/allylboration strategy (Figure 16).22
The synthesis of fragment 56 started with an E-crotylboration of aldehyde 33 catalyzed by p-F-vivol[7].SnCl4 to give vinyl iodide 61 in 95% yield and 95:5 dr, with anti stereochemistry. The next step was the inversion of the stereochemistry at C19 (MeSO2Cl, Et3N;
16
then CsOAc). Then, a series of eight steps converted acetate 62 into fragment 56 in 44% yield
(Figure 17).
Figure 16 - Retrosynthesis of palmerolide A by Hall and coworkers.
The second fragment 57 was prepared from a [4+2]-cycloaddition between 3- boronoacrolein pinacolate 63 and ethyl vinyl ether 64. The Diels-Alder product 65 undergoes allylboration with aldehyde 63 to give α-hydroxyl pyrane 66 in 84% yield and 96% ee. The hydroxyl is acylated to give 67 in preparation for a Claisen-Ireland rearrangement. Then, reaction with LDA and TMSCl afforded an enolate, which gives the rearranged product through the chair-like transition state 68. In this process, the hydroxyl groups at C10 and C11 could be differentiated. Oxidation of C10 with retention of configuration and esterification gave pyran 69 in 55% over 4 steps. Differentially protected diol 70 was obtained in five further steps. Thus, ring opening and installation of the α,β-unsaturated ester gave fragment 57 (Figure 18).
17
Figure 17 - Synthesis of fragment 56.
Fragments 56 and 57 were coupled via Suzuki coupling in 50-77% yield. Subsequently, saponification and Yamaguchi macrolactonization gave the core of palmerolide in 66% yield over two steps. The synthesis was completed by a Curtius rearrangement to install the dienamide, carbamate formation and deprotection (6 steps, 11% yield) to give the natural product palmerolide A (Figure 19).
18
Figure 18 - Synthesis of fragment 57.
Figure 19 - Completion of the synthesis of palmerolide A.
The Maier Group
In 2007, the Maier group published a synthetic approach to the C3-C23 fragment of the proposed structure of palmerolide A.31 Two years later, in 2009, a formal synthesis of palmerolide A was reported using the same strategy.23 The Maier team prepared a late intermediate in Nicolaou/Chen synthesis. The retrosynthetic analysis relied in the coupling of
19
two fragments 73 and 74, which can be connected by a Horner-Wadsworth-Emmons olefination and a metal mediated cross-coupling reaction (Figure 20).
Figure 20 - Retrosynthesis of palmerolide A by Maier group.
The aldehyde fragment 74 was prepared from δ-valerolactone, the same starting material as in De Brabander synthesis. δ-Valerolactone underwent ring opening and alkynylation to give product 75. A Noyori asymmetric hydrogenation set the stereocenter at C7, then deprotection of the alkyne and protection of the hydroxyl group afforded 76. Reaction with acrolein gave the substrate for a Johnson-Claisen rearrangement 77. The rearrangement took place in the presence of triethyl orthoacetate and propionic acid to give, two steps later, enyne 78. Next, the stereocenters at C10 and C11 were installed in a Sharpless dihydroxylation and the alcohol at
C10 could be selectively protected as a TBDPS group providing alcohol 79. The carbamate was installed and deprotection/oxidation gave aldehyde 80, which was converted in four steps to alkene 81 or to tributylstannane 82 (Figure 21).
20
Figure 21 - Preparation of fragments 81 and 82.
The second fragment 73 was prepared using an aldol reaction between oxazolidinone 83 and vinyl iodide 33, strategy equivalent to Nicolaou/Chen synthesis, though, with increased yield of 72% (26% higher yield). Then, a sequence of seven steps resulted in fragment 73 (Figure 22).
Figure 22 - Preparation of fragment 69.
The vinyl iodide fragment 73 was coupled with fragment 81 to give terminal alkene 85, which could be converted to macrocycle 87 in a Heck reaction in 81% yield (only the E alkene was observed). Alternatively, fragment 73 was connected to aldehyde 82 to give tributylstannane
86, which could be used in a Stille coupling to afford macrocycle 87. However, the product was
21
obtained in 3:1 E/Z mixture. Isomerization of the double bond with iodine gave the E-alkene in
68-77% yield. Hence, the Heck reaction was more efficient than the Stille coupling in the macrocyclization. Macrolactone 87 was converted to the vinyl iodide intermediate 88 in the
Nicolaou/Chen synthesis in five more steps and 40% overall yield (Figure 23).
Figure 23 - Completion of Maier formal synthesis.
The Prasad Group
The first report by Prasad group was the synthesis of the C1-C18 fragment palmerolide A from tartaric acid.34 After that, the group published two formal syntheses of palmerolide A featuring different synthetic strategies.26,27 The first formal synthesis, released in 2011, was accomplished by the coupling of two fragments (40 and 89) using Yamaguchi and Heck macrocyclization. Fragment 40 was prepared by a Jung reaction. In the second formal synthesis, three fragments were assembled to form the macrocycle core. Fragments 40, 90 and 91 were
22
coupled by a Stille reaction, followed by Yamaguchi esterification and ring-closing metathesis.
Fragments 90 and 91 were prepared taking advantage of the chiral pool tartaric acid (Figure 24).
Figure 24 - Retrosyntheses of palmerolide A according to Prasad group.
Fragment 40 was common to the two formal syntheses reported by the group. The same fragment was used in the Nicolaou/Chen synthesis of palmerolide A, however, the Prasad group utilized a different strategy towards its preparation. The synthesis started with olefination of vinyl iodide 33 to give alcohol 92 in 68% yield over two steps. Then, Sharpless protocol gave epoxide 93, which was treated with TESOTf in the present of Hünigs base using Jung non-aldol conditions.47,48 The reaction set stereocenters C19 and C20. Olefination of the resulting aldehyde gave ester 94 in 60% over the two steps. Three more steps gave the desired fragment 40 (Figure
25).
23
Figure 25 - Synthesis of fragment 40.
Figure 26 - Synthesis of fragment 89.
In the initial formal synthesis of palmerolide A developed by Prasad team, the preparation of the fragment 89 started with TBS protection and NBS oxidation of furyl carbinol
95 to give aldehyde 96 in 47% over the two steps. Thus, a six-step sequence afforded enone 97 in
53% yield. Then, (R)-CBS reduction gave a mixture of separable diastereomers (7:3). The minor undesired diastereomer 98 was converted to the desired alcohol 99 under Mitsunobu conditions in 64% yield. Otherwise, reaction with NaBH4 gave a 1:1 mixture of diastereomers. The second fragment 89 was obtained five steps later in 65% yield (Figure 26).
Both fragments 40 and 89 were coupled in a Yamaguchi reaction in 91% yield and a
Heck macrocyclization in 60%. The Heck macrocyclization was used earlier in Maier formal
24
synthesis. Six more steps converted macrolide 97 into the late intermediate vinyl iodide 88 in
Nicolaou/Chen synthesis (Figure 27).26
The second formal synthesis involved the preparation of fragment 90. The known alcohol
102 derived from L-tartaric acid was converted to alkenol 103 in three steps. First, there was formation of a triflate, followed by reaction with 3-butenylmagnesium bromide and deprotection of the TBS ether using TBAF. Then, a sequence of four steps gave iodide 90 in 40% yield
(Figure 28).27
Figure 27 - Synthesis of the late intermediate 88 in Nicolaou/Chen synthesis.
Figure 28 - Synthesis of fragment 90.
25
The second fragment 91 started with a reaction between the bis-Weinreb amide of tartaric acid 104 with 3-butenylmagnesium bromide and reduction of the resulting ketone by K-
Selectride in 82% over two steps. The group utilized the same sequence in a previous publication.49 Protection of the hydroxyl group as a TBS ether gave alkene 105. The Weinreb amide was reduced using NaBH4 and installation of an alkyne using Bestmann-ohira reagent gave compound 106, which could be converted to tributylstannane 91 (Figure 29).
Figure 29 - Synthesis of fragment 91.
Figure 30 - Completion of the formal synthesis of palmerolide A.
With the three fragments in hand, they could couple vinyl iodide 40 and stannane 91 in a
Stille reaction in 71% yield (Figure 30). The primary alcohol was selectively converted to an iodide in two steps and Yamaguchi esterification with fragment 90 gave diiodide 108. Boord
26
elimination50 with zinc dust originated the diene 109 in 70% yield. Ring-closing metathesis using
Grubbs II, same strategy employed in Nicolaou/Chen synthesis, gave macrocycle 110 in 62% yield. Protection with MOMCl gave the macrolactone 101, which was published by Prasad team in their first formal synthesis of palmerolide A. With the synthesis of intermediate 101, the group accomplished the second formal synthesis of this natural product.
The Kaliappan group
In 2007, the Kaliappan group reported an approach to C1-C9 and C15-C21 fragments of the proposed structure of palmerolide A.29 Then, in 2010, Kaliappan and coworkers published a formal synthesis of palmerolide A.24 One year later, the group disclosed a full article detailing their findings and discussing the challenges related to the synthesis.25 The retrosynthetic analysis is presented in figure 31, and illustrates a ring-closing metathesis as the last event in the synthesis, which was also employed by Nicolaou/Chen and Prasad groups. The coupling of three fragments 111, 112 and 113 would afford the macrocyclic core of palmerolide A by a Julia-
Kocienski olefination, a Yamaguchi esterification and a ring-closing metathesis. The stereocenters at C19 and C20 were set by a Shimizu protocol; C10 and C11 were installed by a
Sharpless dihydroxylation and C7 by Jacobsen kinetic resolution.
The fragment 111 was synthesized from alcohol 114. Oxidation using IBX to form an aldehyde followed by olefination gave ester 115 in 85% yield. A sequence of reduction, oxidation, olefination and epoxidation using Sharpless conditions gave epoxide 116 in high ee.
The epoxide was opened by palladium-catalyzed hydrogenolysis to set the stereocenters C19 and
C20 in alcohol 117. This protocol, developed by Shimizu51 and coworkers, was employed as an alternative to the aldol reactions utilized in previous syntheses. Three steps further, addition of
27
vinyl Grignard to carbonyl compound 118, followed by acetylation gave diacetate 119, which was used in allylic rearrangement to provide alkene 120 as a mixture of isomers 1:7.5. The desired isomer could be purified after the selective deprotection of the primary alcohol. Then, oxidation gave aldehyde 111, precursor for the Julia-Kocienski olefination (Figure 32).
Figure 31 - Retrosynthesis of palmerolide A as reported by Kaliappan group.
Figure 32 - Synthesis of fragment 111.
The synthesis of the coupling partner for the Julia-Kocienski olefination, sulfone 112, started with mono-protected diol 121. Chlorination, Sharpless dihydroxylation and formation of the epoxide gave 122 in 62% yield over the three steps. The hydroxyl group was protected and
28
reaction with dimethylsulfonium ylide gave alcohol 123 in 75% yield. In this way, the diol C10 and C11 could be differentiated. The same approach with a TBS group at C11 was not successful. Four extra steps converted alcohol 123 to the sulfone fragment 112 (Figure 33).
Figure 33 - Synthesis of fragment 112.
The third fragment was prepared from Sharpless kinetic resolution of alcohol 124 to give the enantiomerically enriched alcohol 125 (95% ee) in 42% yield. In five additional steps the third fragment 113 was obtained (Figure 34).
The Julia-Kocienski olefination between fragments 111 and 112 was successful giving the diene product 126 in 80%. Alternative olefinations using inverted coupling partners (111 sulfone and 112 aldehyde) or with a ketone analogue of 111 were ineffective. The vinyl iodide in the side chain was inserted by a Takai olefination according to Nicolaou/Chen precedent. Next,
Yamaguchi conditions with fragment 113 gave ester 128. The carbamate was installed, the TIPS protecting groups were removed and ring-closing metathesis gave macrolactone 88.
Noteworthy, the ring-closing metathesis could not be accomplished with the TIPS protected starting material. Also, when PMB protecting groups were used to replace TIPS groups, the substrate could not be deprotected and the reaction resulted in decomposition of the starting material. Macrolide 88 is an advanced intermediate in the Nicolaou/Chen synthesis, concluding the formal synthesis of palmerolide A (Figure 35).
29
Figure 34 - Synthesis of fragment 113.
Figure 35 - Completion of Kaliappan formal synthesis.
The Chandrasekhar group
The Chandrasekhar group reported an approach to the C1-C14 portion of palmerolide A30
(Figure 36) showcasing of a methodology developed in the group, a deoxygenative rearrangement via allene to form a diene.52
Figure 36 - The C1-C14 fragment of palmerolide A by Chandrasekhar group.
30
Figure 37 - Synthesis of the C1-C14 fragment of palmerolide A.
The synthesis started with the alkynylation of aldehyde 131 producing the alcohol 132.
Then, reaction with triphenylphosphine produced diene 133 via an allene intermediate in 73% yield. Sharpless dihydroxylation gave a diol that was protected as an acetonide. Later, the stereocenter at C7 was installed using (R)-CBS in 90 % yield and 97% de. Some additional steps afforded the desired alcohol 130, which corresponds to the C1-C14 fragment of palmerolide A
(Figure 37).
The Meyer/Cossy Group
The Meyer/Cossy group developed an approach to the C3-C15 and C16-C23 fragments of palmerolide A.32 In their retrosynthetic analysis, the natural product could be obtained by the coupling of three fragments 40, 137 and 138. The three portions of the molecule would be combined by a nucleophilic addition of alkyne 138 to aldehyde 137, then a palladium catalyzed cross-coupling with fragment 40 and a ring-closing metathesis to form the C2-C3 bond (Figure
38).
31
The first fragment 137 was prepared from alcohol 139. Iodination and alkyne insertion afforded acetonide 140. Deprotection followed by selective protection of the primary alcohol gave 141 in 88% yield. The secondary alcohol was protected as a PMB and the primary alcohol converted to an aldehyde giving fragment 137 (Figure 39).
Figure 38 - Retrosynthesis of palmerolide A according to Meyer/Cossy group.
Figure 39 - Preparation of fragment 137.
The synthesis of the second fragment 138 started with acid 142, which was converted to a
Weinreb amide and alkynylated to form ketone 143. Noyori reduction and protecting group manipulations gave the alkyne fragment 138 (Figure 40).
The vinyl iodide fragment 40 (C16-C23) was also present in Nicolaou/Chen and Prasad syntheses of palmerolide A. Meyer/Cossy used similar synthetic strategy as Nicolaou/Chen.
However, the Meyer/Cossy fragment was obtained in two fewer steps due to the installation of a
32
Weinreb amide replacing the chiral auxiliary in vinyl iodide 44. The amide could be reduced to aldehyde 144, avoiding protection/deprotection of the hydroxyl group at C19.
Figure 40 - Preparation of fragment 138.
Fragments 137 and 138 were connected by nucleophilic addition of an alkynyl anion to aldehyde 137 producing diyne 145. Nevertheless, the alcohol group was formed during the addition in a 7:3 mixture of diastereomers. Subsequently, Dess-Martin periodinane oxidation gave a ketone that could be reduced by Noyori protocol in high diastereoselectivity. Next, the internal alkyne was reduced with Red-Al, the alcohol protected and the terminal alkyne uncovered, completing the synthesis of the second fragment C3-C15, 147.
Figure 41 - Preparation of fragment 40.
Figure 42 - Preparation of fragment C3-C15.
33
The Loh group
In 2012, the Loh group disclosed a synthesis of the C13-C21 fragment of palmerolide A
(Figure 43).35 A methodology developed by the Loh Lab was used to produce a 1,3-butadiene from the coupling of alkyl substituted alkenes and acrylates. Diene 148 was prepared from aldehyde 149 in five steps. The last step was the coupling between n-butyl acrylate and alkene
151. The yields and E/Z selectivity were improved by changing the protecting group in the primary alcohol to give acetate 148c in 69% yield and 80:20 E/Z (Figure 44).
Figure 43 - Fragment C3-C15 of palmerolide A.
Figure 44 - Preparation of fragment C13-C21.
The Baker group
34
The Baker Lab also contributed with an approach towards the synthesis of the natural product palmerolide A. The group reported the synthesis of C3-C14 fragment (152) using chiral pool (Figure 45).33
Figure 45 - The C3-C14 fragment of palmerolide A by Baker group.
Figure 46 - Preparation of fragment C3-C14.
The synthesis started with sugar derivatives 153 and 156. Alcohol 153 was monoprotected with a benzyl group, and then oxidation/olefination gave ester 154. Next, reduction, oxidation and olefination resulted in alkene 155. Alcohol 156 was protected with a benzyl group and the acetonide removed. The secondary alcohol was converted to an acetate group, and oxidation/olefination gave alkene 158. Alkenes 155 and 156 were coupled by a
Grubbs II-catalyzed cross-metathesis to give fragment 152 in 40% yield (Figure 46). Attempts to use Julia-Kocienski olefination to form the C8-C9 double bond were ineffective.
35
The Florence group
Florence and coworkers reported a total synthesis of the proposed structure of the natural product palmerolide C, a constitutional isomer of palmerolide A.38 After the synthesis, the NMR data of the synthesized material and the natural product were distinct and a revised structure was suggested. The synthetic approach involved the installation of the enamide in the side chain by
Buchwald protocol as the last event. Prior to that, two main fragments 159 and 160 afforded the macrolactone core by a Julia-Kocienski olefination and Yamaguchi macrolactonization. The fragments 159 and 160 were designed based in various aldol strategies (Figure 47).
The synthesis of the fragment 160 started with a (S)-proline catalyzed aldol reaction between acetonide 161 and aldehyde 162, giving the aldol adduct 163 in 44% yield and 96% ee.
Ester 157 was generated in seven steps and 39% yield. The ester was converted to a methyl ketone 158 via Weinreb amide in 93% yield. The C1-C6 fragment 167 was synthesized from monoprotected diol 166 in four steps and 53% overall yield (Figure 48).
Figure 47 - Retrosynthetic analysis of palmerolide C and revised structure.
36
Fragment 159 was synthesized from a Mosher ester derivative aldehyde 168. Olefination, reduction and oxidation gave aldehyde 169 that could be converted to vinyl iodide 170 by Takai olefination. Then, a vinylogous Mukaiyama aldol reaction converted aldehyde 170 into alcohol
171 in 70% and 6:1 dr. After that, four additional steps generated sulfone 159 (Figure 49).
Figure 48 - Preparation of fragments 165 and 167.
Figure 49 - Preparation of fragment 159.
Compounds 165 and 167 were connected by an aldol reaction to form alcohol 172, which could be eliminated to form the double bond between C6 and C7. The ketone was reduced under
Luche conditions to afford alcohol 173 in 94% yield and high diastereoselectivity. Few steps
37
later, aldehyde fragment 160 was coupled with fragment 159 in a Julia-Kocienski olefination to give 174 in 78% yield and 7:1 E/Z (Figure 50). Previously, Julia-Kocienski olefination of similar substrates in Kaliappan formal synthesis of palmerolide A was unsuccessful.
To conclude the synthesis of the proposed structure of palmerolide C, the Florence team used a Yamaguchi macrolactonization and Buchwald protocol to install the enamide in the side chain following Nicolaou/Chen tactic (Figure 50). However, the NMR data of the synthesized material and the natural product were dissimilar, suggesting the structural assignment of the natural product was wrong. Reassessment of the NMR spectra suggests that the stereochemistry at C9 and C10 were assigned erroneously. Specifically, the stereotriad C8, C9 and C10 observed in the natural product were most likely to be 8S, 9R, 10R, as a replacement for 8S, 9S, 10S in the initially proposed structure.
Figure 50 - Synthesis of the proposed structure of palmerolide C.
38
Summary
Collectively, the retrosynthetic strategy to disconnect the macrocyclic core of palmerolide A is similar throughout the total and formal syntheses reported for this natural product. The main features of the syntheses were summarized in Table 3. The formal synthesis developed by Dudley and coworkers (Chapter 4) is included.
Usually, three fragments were coupled to form the C8-C9 double bond, the C1-O ester and the C14- C15 or C15-C16 double bonds, except by Hall and Maier syntheses where two main fragments were used to assemble the macrocycle. Another variation pertains to the Horner-
Wadsworth-Emmons olefination, which is used by Hall group to form the C2-C3 double bond
(Table 3).
Table 3 - Overview of the synthetic sequences to palmerolide A. De Nicolaou/ Hall Maier Kaliappan Prasad Dudley Brabander Chen C1-C8 C1-C8 C1-C13 C3-C15 C1-C8 C1-C8 C1-C8 Fragments C9-C15 C9-C15 C14-C24 C16-C23-C2 C9-C14 C9-C15 C9-C15 C16-C24 C16-23 C15-23 C16-23 C16-C24 Suzuki Yamaguchi Suzuki HWE Julia Stille HWE Fragment (81%) (66%) (50-77%) (92%) (80%) (71%) (96%) coupling Yamaguchi Stille Yamaguchi Yamaguchi Yamaguchi (69%) (84%) (68%) (74%) (91%)
- Bond C8-C9 C8-C9 C1-O C15-C16 C8-C9 C8-C9 C15-C16 li HWE RCM Yamaguchi Heck RCM RCM Heck
tion Name 58% 81% 90% 81% 70% 62% 60% Cyc za Yield C7 (S)-CBS Jacobsen res B-Claisen Noyori red Jacobsen res tartaric acid NaBH4 C10 D-arabitol B-crotyl B-Claisen AD-mix-α AD-mix-α tartaric acid AD-mix-α C11 D-arabitol B-crotyl B-Claisen AD-mix-α AD-mix-α tartaric acid AD-mix-α Stereo centers C19 vinyl. aldol aldol B-crotyl aldol Shimizu Jung vinyl. aldol C20 vinyl. aldol aldol B-crotyl aldol Shimizu Jung vinyl. aldol Steps 22 18 26 27 26 24 22 Yield 0.06% 0.6% 0.7% 1% 0.4% 0.4% 1.4% Total
The most common macrocyclization events were a ring-closing metathesis (C8-C9) or
Heck reaction (C15-C16). Additionally, De Brabander used a Horner-Wadsworth-Emmons
39
macrocyclization (C8-C9) and Hall a Yamaguchi macrolactonization (Table 3). Nicolaou/Chen groups studied several tactics for the macrocyclization and they observed that the ring-closing metathesis, the Yamaguchi and the Horner-Wadsworth-Emmons were the highest yielding macrocyclization reactions.
The stereocenters at C19 and C20 were mainly installed by aldol or vinylogous aldol reactions. Other approaches used were a boron-crotylation by Hall, a Shimizu protocol by
Kaliappan and a Jung non-aldol reaction by Prasad. The diol at C10 and C11 were primarily installed by Sharpless dihydroxylation, organoboron methodologies or chiral pool (tartaric acid and D-arabitol). A variety of strategies were used to install C7 hydroxyl group, some reduction reactions (e.g. (S)-CBS, NaBH4, Noyori), Jacobsen kinetic resolution, organoboron/Claisen rearrangement and tartaric acid (Table 3).
The sensitive dienamide in the side chain was installed at a late stage. Two alternatives were used to synthesize the enamide, a Curtius rearrangement, first used by De Brabander and a
Buchwald protocol introduced by Nicolaou/Chen groups.
Overall, several synthetic approaches were reported on palmerolide A using similar disconnections. The unique features of the syntheses rely on the preparation of the fragments to assemble the macrolactone. However, the syntheses were achieved in low yield and can produce only limited amounts of palmerolide A and derivatives for biological studies. Therefore, taking into account the low availability of palmerolide A from natural sources, a more efficient synthesis is still needed. The synthesis developed in the Dudley Lab and ways to circumvent the current drawbacks and improve the effectiveness of the synthesis will be discussed in the following chapters.
40
CHAPTER 3
RING OPENING OF CYCLIC VINYLOGOUS ACYL TRIFLATES USING
STABILIZED CARBANION NUCLEOPHILES
Introduction
Since the isolation of palmerolide A, many laboratories became interested in the synthesis of this natural product. A methodology developed in the Dudley Lab was one of the reasons to pursue the synthesis of the palmerolide A. The Horner-Wadsworth-Emmons reagent
176a produced by the fragmentation of a vinylogous acyl triflate (177a) would be ideal for the synthesis of the C1-C8 fragment (Figure 51).53 In this chapter, the Claisen type condensation of vinylogous acyl triflates with carbanion nucleophiles will be described, as well as the efforts to optimize the reaction to afford -ketophosphonate 176a and analogues.54
Figure 51 - Prospective application to palmerolide A.
The Claisen condensation has been an important reaction in synthetic organic chemistry for over a century.55 In the classic Claisen condensation, an ester enolate nucleophile reacts with a carboxylic ester electrophile in a reversible addition and elimination sequence. The resulting