ABSTRACT
TOTAL SYNTHESIS OF THE BIDENSYNEOSIDES; REMARKABLE PROTECTING GROUP EFFECTS IN GLYCOSYLATION AND SYNTHETIC EFFORTS TOWARDS THE TOTAL SYNTHESIS OF A PENTAACETYLENIC GLUCOSIDE
By: Ryan Michael Fox
This document describes the work towards the synthesis of polyacetylene glucosides isolated from nature. Theses syntheses represent the first total synthesis of the bidensyneosides (1-5) isolated from Bidens parviflora. Biological assays have shown that the bidensyneosides effectively regulate histamine release and nitric oxide production. Additionally, a remarkable protecting group effect was observed during glycosylations, effectively showing that the nature of the protecting group can lead to a differentiation between the orthoester and anomeric product. Furthermore, an attempt to assemble a pentaacetylenic glucoside 34 isolated from Microglossa pyrifolia was undertaken. The assembly consists of a glycosylation followed by a triply convergent unsymmetrical cross coupling to give a pentaacetylene. To this end, the establishment of conditions capable of producing a pentaacetylene without subsequent addition of base to the electrophilic pentaacetylene side chain has been unsuccessful. Further studies of these conditions should lead to the desired product, which after deprotection would give the natural product.
TOTAL SYNTHESIS OF THE BIDENSYNEOSIDES; REMARKABLE PROTECTING GROUP EFFECTS IN GLYCOSYLATION AND SYNTHETIC EFFORTS TOWARDS THE TOTAL SYNTHESIS OF A PENTAACETYLENIC GLUCOSIDE
A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the Requirements in the degree of Masters of Science Department of Chemistry and Biochemistry By Ryan Fox Miami University Oxford, Ohio 2004
Advisor ______Dr. Benjamin W. Gung
Reader ______Dr. Michael W. Crowder
Reader ______Dr. Richard T. Taylor
Reader ______Dr. Christopher A. Makaroff
Table of contents Page
List of Abbreviations iv List of Figures v List of Schemes vi List of Tables vii List of Structures viii Acknowledgments xix Introduction: The importance of natural product isolation and synthesis 1
Chapter 1: Total Synthesis of Bidensyneoside A2, C, and 3-deoxybidensyneoside C,: Remarkable Protecting Group Effects in Glycosylation 1.1 Introduction 6 1.2 Results and Discussion 10 1.2.1 Preparation of 3-Deoxybidensyneoside C 10 1.2.2 Synthesis of Bidensyneoside C 12
1.2.3 Synthesis of Bidensyneoside A2 21 1.3 Conclusion 23 1.4 Experimental 24
Chapter 2: Total Synthesis of Bidensyneoside A1 and Bidensyneoside B 46 2.1 Introduction 47 2.2 Results and discussion 48 2.2.1 Synthesis of Bidensyneoside B 48
2.2.2 Synthesis of Bidensyneoside A1 49 2.3 Conclusion 51 2.4 Experimental 52 Chapter 3: The Attempted Total synthesis of a pentacetylenic glucoside isolated 58 from Microglossa pyrifolia 3.1 Introduction 59 3.2 Results and discussion 62 3.3 Conclusion 73 3.4 Experimental 74
ii Chapter 4: References 86 Chapter 5: Spectra for select compounds 90
iii List of Abbreviations Ac Acetyl
Ac2O Acetic anhydride Bz Benzoyl DMAP 4-Dimethylaminopyridine DMF Dimethylformamide ee Enantiomeric Excess DMTST dimethyl(methylthio) sulfonium triflate Et Ethyl Me Methyl NBS N-Bromosuccinimide PPTS Pyridinium p-Toluenesulfonate Pyr Pyridine TBAF Tetra-n-butylammonium fluoride TBDMS t-Butyldimethylsilyl TEA Triethylamine Tf Triflate THF Tetrahydrofuran THP Tetrahydropyran COSY Correlation Spectroscopy EI/MS Electron Impact Mass Spectroscopy HRMS High resolution Mass spectroscopy IR Infrared Spectroscopy UV Ultraviolet spectroscopy
iv List of Figures page Figure 1 Structures of Callypentayne and 2-Deoxydiplyne D sulfate 2 Figure 2 Bioactive sucrose esters from bidens parviflora 3 Figure 3 An Antimalarial compound isolated from Bidens pilosa 3 Figure 4 Triterpeoids from the roots of Microglossa pyrifolia 5 Figure 5 Pentaacetylenic glucoside from Microglossa pyrifolia 5 Figure 1.1 Structures of the bidensyneosides from Bidens parviflora 8 Figure 3.1 Polyacetylenic glucosides from Microglossa pyrifolia 59 Figure 3.2 Polyacetylenic aglucones 60
v List of Schemes page Scheme 1.1 retrosynthetic analysis of the bidensyneosides 9 Scheme 1.2 Synthesis of 3-deoxybidensyneoside C 10 Scheme 1.3 Synthesis of Alcohol 12 12 Scheme 1.4 TBS protection of thioglycoside 15 Scheme 1.5 Acylation of thioglycosides 16 Scheme 1.6 Orthoester formation 17 Scheme 1.7 Methoxy orthoester 18 Scheme 1.8 Synthesis of Bidensyneoside C 19 Scheme 1.9 Synthesis of C3 inverted Bidensyneoside C 21
Scheme 1.10 Synthesis of Bidensyneoside A2 22 Scheme 2.1 Synthesis of Bidensyneoside B 49 Scheme 2.2 Synthesis of E-3-pentene-1-yn 50
Scheme 2.3 Synthesis of Bidensyneoside A1 50 Scheme 3.1 Retrosynthetic analysis of 34 61 Scheme 3.2 Glycosylation model for bromoglucoside 62 Scheme 3.3 Selective protection of primary alcohols; compounds 38 and 39 62 Scheme 3.4 Benzoylation of compounds 13 and 14 63 Scheme 3.5 Glycosylations forming compounds 43-46 64 Scheme 3.6 Formation enamine 47 and 49 66 Scheme 3.7 Symmetric alkynes 50 and 51 67 Scheme 3.8 Synthesis of 52 and 53 69 Scheme 3.9 Kinetic model compounds 70 Scheme 3.10 Kinetic study, compounds 54, 55, 56, 57 73 Scheme 3.11 Kinetic study, compounds 58, 59, 60 74
vi List of Tables page Table 3.1 Product distribution in three component Cadiot-Chodkiewicz 68 Conditions with a variation of ethylamine concentration
vii List of Structures
No. Structure OH
HO O HO O OH HO 1
Bidensyneoside A1 OH
HO O HO O OH HO 2
Bidensyneoside A2 OH
HO O HO O OH HO 3 Bidensyneoside B OH OH HO O HO O OH HO 4 Bidensyneoside C OH OH HO O HO O OH 5 3-Deoxybidensyneoside C
viii OAc
AcO O AcO O OAc 6 4-Pentynyl tetra-acetyl-β-D-glucopyranoside Br
OTBS 7 Silane, (1,1-dimethylethyl)dimethyl[5-bromo-(2E)-2-penten-4-ynyloxy] OH OTBS HO O HO O OH 8 [10-tert-Butyldimethylsilyloxy-8-decen-4,6-diynyl]-β-D-glucopyranoside OH
TBSO 9 (+)-3R-5-tert-butyldimethylsilyloxy-1-pentyn-3-ol O Ph O OMe TBSO 10 O-Methyl mandelate ester of 9 OAc
TBSO 11 (+)-3R-3-Acetoxy-1-tert-butyldimethylsilyl-4-pentyn-1-ol OAc
HO 12 (+)-3(R)-3-Acetoxy-4-pentyn-1-ol
ix OTBS
HO O HO STol OH 13 p-Tolyl 6-O-(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside OTBS
HO O TBSO STol OH 14 p-Tolyl 3,6-O-bis(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside OTBS
TBSO O HO STol OH 15 p-Tolyl4,6-O-bis(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside OTBS
HO O HO STol OTBS 16 p-Tolyl 2,6-O-bis(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside OTBS
HO O TBSO STol OTBS 17 p-Tolyl 2,3,6-O-tris(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside OTBS
AcO O AcO STol OAc 18 p-Tolyl 2,3,4-O-tris(acetyl)-6-O-(tert-butyldimethylsilyl)-1-thio-β-D- glucopyranoside
x OTBS
AcO O TBSO STol OAc 19 p-Tolyl 2,4-O-bis(acetyl)-3,6-O-bis(tert-butyldimethylsilyl)-1-thio-β-D- glucopyranoside
AcO H1 TBSO O H H 2 OAc 4 O H 3 O OAc O 20 (S)-Orthoester
AcO H1 TBSO O H H 2 OAc 4 O H 3 O OAc O 21 (R)-Orthoester OTBS
AcO O AcO O OAc OAc 22 (3S)-3-Acetoxy-4-pentynyl 2',3’,4'-O-tris(acetyl)-6'-O-(tert-butyldimethylsilyl)- β-D-glucopyranoside HO TBSO O O O O OH 23 Methoxy Orthoester OTBS AcO O TBSO O OAc AcO
xi 24 (3R)-3-Acetoxy-4-pentynyl 2',4'-O-bis(acetyl)-3',6'-O-bis(tert- butyldimethylsilyl)-β-D-glucopyranoside Br
OH 25 5-Bromo-2-penten-4-yn-1-ol OTBS OH AcO O TBSO O OAc HO 26 (3R)-3',6'-O-Bis(tert-butyldimethylsilyl)-2',4'-O-bis(acetyl) bidensyneoside C OTBS AcO O TBSO O OAc AcO 27 (3S)-3-Acetoxy-4-pentynyl 2',4'-O-bis(acetyl)-3',6'-O-bis(tert- butyldimethylsilyl)- β-D-glucopyranoside OTBS OH AcO O TBSO O OAc HO 28 (3S)-3',6'-O-bis(tert-butyldimethylsilyl)-2',4'-O-bis(acetyl) bidensyneoside C OH OH HO O HO O OH HO 29 3-(S)-bidensyneoside C
xii OTBS AcO O TBSO O OAc Br AcO 30 (3R)-3’-Acetoxy-5’-bromo-4’-pentynyl 2,4-O-bis(acetyl) 3,6-O-bis(tert-butyldimethylsilyl)-β-D-glucopyranoside OTBS
AcO O TBSO O OAc HO 31 (8Z)-(3’R)-3’,Hydroxy-8’-decen-4’,6’-diynyl 3,6-O-bis(tert-butyldimethylsilyl)- 2,4-O-bis(acetyl)-β-D-glucopyranoside OTBS
AcO O TBSO O OAc HO 32 (3’R)-3’,Hydroxy-4’,6’,8’-triynyl 3,6-O-bis(tert-butyldimethylsilyl) -2,4-O-bis(acetyl)-β-D-glucopyranoside OTBS
AcO O TBSO O OAc HO 33 (8E)-(3’R)-3’,Hydroxy-8’-decen-4’,6’-diynyl 3,6-O-bis(tert-butyldimethylsilyl) -2,4-O-bis(acetyl)-β-D-glucopyranoside OH
HO O HO O OH
OH 34
xiii 2-β-D-Glucopyranosyloxy-1-hydroxy-trideca-3,5,7,9,11-pentayne HO
HO 35 1,2-dihyrdoxy-trideca-3,5,7,9,11-pentayne E/Z HO
HO 36
1,2-dihydroxy-3(E/Z)-tridecene-5,7,9,11-tetrayne
HO OH 37 1,3-dihydroxy-6(E)-tetradecene-8,10,12-triyne OH TBSO
Br 38 Silane, (1,1-dimethylethyl)dimethyl[4-bromo-2-S-hydroxy-3-pentyne] OH BzO
Br 39 1-bromo-4-benzyol-3(R)-1-butyne-3-ol OTBS
BzO O BzO STol OBz 40 p-Tolyl 2,3,4-O-tris(benzoyl)-6-O-(tert-butyldimethylsilyl)- 1-thio-β-D-glucopyranoside
xiv OTBS
BzO O TBSO STol OBz 41 p-Tolyl 2,4-O-bis(benzoyl)-3,6-O-bis(tert-butyldimethylsilyl)- 1-thio-β-D-glucopyranoside OBz
BzO O TBSO STol OBz
42 p-Tolyl 2,4,6-O-tris(benzoyl)-3-O-(tert-butyldimethylsilyl)- 1-thio-β-D-glucopyranoside OTBS
BzO O BzO O OBz Br
OTBS 43 6-O-(tert-butyldimethylsilyl)-2,3,4-O-tris(benzoyl)-2-β-D-Glucopyranosyloxy-4- bromo-1-tert-butyldimethylsilyl-2(S)-3-butyne OTBS
BzO O BzO O OBz Br
OBz 44 6-O-(tert-butyldimethylsilyl)-2,3,4-O-tris(benzoyl)-2-β-D-Glucopyranosyloxy- 4-bromo-1-benzoyl-2(S)-3-butyne OTBS
BzO O TBSO O OBz Br
OTBS
xv 45 3,6-O-Di(tert-butyldimethylsilyl)-2,4-O-bis(benzoyl)-2-β-D-Glucopyranosyloxy- 4-bromo-1- tert-butyldimethylsilyl -2(S)-3-butyne OTBS
AcO O TBSO O OAc Br
OTBS 46 3,6-O-bis(tert-butyldimethylsilyl)-2,4-O-bis(acetyl)-2-β-D-Glucopyranosyloxy- 4-bromo-1-tert-butyldimethylsilyl-2(S)-3-butyne OTBS H AcO O N TBSO O OAc OTBS
47 Enamine OTBS
AcO O OTBS TBSO O OAc
H 48 3,6-O-bis(tert-butyldimethylsilyl)-2,4-O-bis(acetyl)-2-β-D-Glucopyranosyloxy- 1-tert- butyldimethylsilyl-2(S)-3-butyne TBSO OTBS OTBS TBSO O O O AcO O N OAc AcO TBSO H OAc OTBS 49
xvi Triyne-enamine OTBS TBSO AcO O TBSO OTBS O O TBSO O OAc OAc OTBS AcO 50 Symmetric diyne OTBS TBSO AcO O TBSO OTBS O O TBSO O OAc OAc OTBS AcO 51 Symmetric tetrayne OH
BzO O BzO O OBz Br
OH 52 2,3,4-O-tris(benzoyl)-2’-β-D-Glucopyranosyloxy-4’-bromo-2’(S)-3’-butyne OTBS
HO O HO O OH Br
OTBS 53 6-O-(tert-butyldimethylsilyl)-2’-β-D-Glucopyranosyloxy-4’-bromo-1’- tert-butyldimethylsilyl-2’(S)-3’-butyne
54 Deca-2,4,6,8-tetrayne
HO 55 Deca-4,6,8-triyn-1-ol
xvii HO
HO 56 Nona-3,5,7-triyne-1,2(S)-diol OH HO
HO 57 Nona-3,5-diyne-1,2(S),9-triol
HO OH
HO 58 Octa-3,5-diyne-1,2(S),8-triol OH
HO 59 Nona-3,5-diyne-1,9-diol OH
HO 60 Octa-3,5-diyne-1,8-diol
xviii Acknowledgements
I would like to thank Dr. Benjamin Gung for his insightful knowledge in the area of synthetic organic chemistry and more importantly, for his support throughout my research career here at Miami University. I would also like to thank my family, especially my mother and father for their support throughout my stay at Miami. I wish to thank all of the wonderful individuals who have worked in Dr. Gung’s lab over the last three years. Their work ethic and presence made the long hours required in lab a more enjoyable experience.
xix
Introduction Polyacetylenic natural products are intriguing molecules that have caught the attention of research groups throughout the scientific community. From the synthetic organic standpoint, the compounds resemble a chemical masterpiece sculpted by living systems due to their uniqueness and high degree of unsaturation, which is difficult to achieve in a classical organic synthesis. Although many of the polyacetylenic compounds found in Nature have interesting structures, there is additionally an underlying curiosity regarding the biological activity of these compounds. Furthermore, being able to envision an elegant synthetic pathway towards a natural product and the successful execution of it is a satisfying experience. The work to be presented in the following chapters depicts the difficulties encountered in the synthesis of polyacetylenic natural products. Although the difficulties were a common reality, persistence and creativity of the human mind made it possible for these syntheses to be successful. As with most areas of science, there are many directions that can be taken when undertaking a project with the objective of synthesizing a natural product, specifically, a highly unsaturated natural product. The diversity with respect to a variety of functionality in a target is essentially up to the individual looking for an appealing target. With regards to looking for a natural product containing a high degree of unsaturation, the possibilities range from simple hydrocarbons such as callypentayne, isolated from the marine sponge Callyspongia sp.1 to the slightly more complicated structure of 2-deoxydiplyne D sulfate isolated from the Philippines sponge Diplastrella sp.,2 (Figure 1) to dozens of interesting compounds that have been characterized over the years by natural product chemists and are waiting to be tackled by synthetic organic chemists.
1
Figure 1
callypentayne
Br OH
OSO3H
2-Deoxydiplyne D Sulfate The ways in which Nature produces molecular structures that are critical to the existence of life to say, the least are elegant, interesting, and incredible. Certain key components continually arise when examining biological systems and are absolutely critical to maintain the proper function of these systems. One such class of compounds are carbohydrates. Carbohydrates are the primary energy source of life and play an important role in molecular recognition and thus have become a focus of many organic chemists and companies. Although carbohydrates are perceived as being a source of food, they additionally function as critical structural features in drugs, which assist the body in warding off unwanted infections. Some of these carbohydrate-containing compounds include erythromycin A, Oleandomycin,
Concanamycin A, Azalomycin B, Digitoxin, Olivomycin A, and Esperamicin A1 to mention a few.3,4 In selecting targets for the research to be presented herein, there was an underlying interest to find natural products that contain both carbohydrate subunits and highly unsaturated subunits. This is an area of chemistry that has not been explored to a great extent, which is consequently why only a small number of natural products have been isolated that contain a high degree of unsaturation as well as a sugar motif. In the end, the search came down to a handful of polyacetylenic glucosides encompassing the bidensyneosides, isolated from Bidens parviflora WILLD (1 – 5), and a pentacetylenic glucoside isolated from Microglossa pyrifolia. Bidens parviflora is a type of plant that has been used in traditional Chinese medicine as an antipyretic, anti-inflammatory, and antirheumatic remedy.5 Bidens parviflora contains a wide
2 variety of bioactive compounds including sterols, monoterpenes, flavonones, flavonoids, polyacetylene glucosides, chalcones, aurones, and flavonol glycosides.5 Over the years, several species of the Bidens family have been investigated by chemical methods.6-10 Based on these findings, it is not surprising that these plants have influenced traditional medicinal practices in indigenous regions. Figures 2 and 3 illustrate some of the compounds that Figure 2 OH O OH HO O O HO HO OH HO O OH O O O O OH O HO OH HO O
O HO O H HO O HO O H O HO HO HO HO O OH OH O O O H O O HO H O HO O HO OH HO OH HO Bioactive sucrose esters from bidens parviflora Figure 3
O O
An antimalarial compound isolated from Bidens pilosa have been isolated from the Bidens species. The sucrose esters in Figure 2 share the same ability as the bidensyneosides in the inhibition of histamine release in rat mast cells, and additionally show inhibitory activity of PGE2 production by macrophages. The compound illustrated in Figure 3 exhibited antimalarial properties, which would clearly be of use in underdeveloped areas of the world where malaria is an unpleasant potentially fatal illness that is common in these regions.
3 The leaves of Microglossa pyrifolia are used in traditional medicine of Papua New Guinea for the treatment of spear wounds and sore eyes. The ability of the leaves of Microglossa pyrifolia to be of medicinal utility is not the only useful aspect of this plant. There is a wide spectrum of medicinal applications attributed to the contents of the roots of Microglossa pyrifolia. Recently, a wide range of dihydrobenzofurans and triterpeoids (Figure 4) were isolated and characterized, although there are no reports on the biological activity of these compounds.11 Figure 4
H O O Me Me Me Me H OH O Me Me O CO Me 2 AcO O
Of the many compounds that have been isolated from Microglossa pyrifolia, a pentacetylene glucoside 34 (figure 5) captured our interest. From a retro synthetic breakdown of 34, the framework should be able to be assembled with relative ease. Thus, 34 became the focus of a total synthesis project In the end, the knowledge provided to natural product chemists by individuals indigenous to an area regarding the uses of certain plants as medical remedies led to the isolation of natural products that can be shown to regulate or inhibit various biological functions.5,7,11-16 In analyzing how these compounds interact in biological environments, a great deal of knowledge can be obtained that has potential use in other areas of science. Additionally, knowing the structure of a natural product and knowing how it interacts with biological pathways provides building blocks in the synthesis of different analogs and thus increases the probability of pharmaceutical companies’ chances of getting a good hit. Figure 5 OH
HO O HO O OH
OH 34
4 This thesis describes the synthetic work towards the synthesis of polyacetylene glucosides isolated from nature. Theses syntheses represent the first total synthesis of the bidensyneosides (1-5) isolated from Bidens parviflora.5 Biological assays have shown that the bidensyneosides effectively regulate histamine release and nitric oxide production. Additionally, a remarkable protecting group effect was observed during glycosylations, effectively showing that the nature of the protecting group can lead to a differentiation between the orthoester and anomeric product. Furthermore, an attempt to assemble a pentaacetylenic glucoside 34 isolated from Microglossa pyrifolia15 was undertaken. The assembly consists of a glycosylation followed by a triply convergent unsymmetrical cross coupling to give a pentaacetylene. To this end, the establishment of conditions capable of producing a pentaacetylene without subsequent addition of base to the electrophilic pentaacetylene side chain has been unsuccessful. However, the reactivity of various bromoalkynes was investigated and the information gained will provide a basis for further studies of these conditions and should lead to the desired product in the future.
5 Chapter 1
Total Synthesis of Bidensyneoside A2, C, and 3-deoxybidensyneoside C,: Remarkable Protecting Group Effects in Glycosylation
6 1.1 Introduction The bidensyneosides are a group of five new polyacetylenic glycosides (see Figure 1.1) isolated from the traditional Chinese medicinal plant Bidens parviflora WILLD,5 which contains rich bioactive natural products12. It was shown that bidensyneosides inhibit both histamine release from rat mast cells and nitric oxide production by the murine macrophage-like cell line RAW264.7 activated by lipopolysaccharide and recombinant mouse interferon-γ.5 The structure of the bidensyneosides has been assigned based on spectroscopic analyses, physicochemical properties, and application of the Mosher ester method.17 Assays have been performed to identify the biological activity of bidensyneosides, but no attempt at their synthesis has been reported.
7 OH
HO O HO O OH HO
Bidensyneoside A1 1 OH
HO O HO O OH HO
Bidensyneoside A2 2 OH
HO O HO O OH HO Bidensyneoside B 3 OH OH HO O HO O OH HO Bidensyneoside C 4 OH OH HO O HO O OH
3'-Deoxybidensyneoside C 5 Figure 1.1. Structures of bidensyneosides from Bidens parviflora WILLD.
The bidensyneosides are glucosides with a 10-carbon polyacetylenic side chain. These five natural products differ from one another primarily in the degree of oxidation of this side chain. In the most potent antiallergic agent, bidensyneoside C (4),5 the side chain contains hydroxyl groups at C3 and C10, while bidensyneosides A1 (1), A2 (2), and B (3) lack a C10 hydroxyl group. Herein, the synthetic methodology explored in the polyacetylene glucoside area
is described, which has led to the first total synthesis of bidensyneoside A1 (1), A2 (2), B (3), C (4), and 3-deoxybidensyneoside C (5). These syntheses establish a synthetic entry to the bidensyneosides and furthermore confirm the stereochemistry at C3.
8 Because the bidensyneosides differ among themselves mainly in the aglycon side chain, the development of a strategy allowing for the convergent assembly of different side chain analogs was important in developing a successful synthesis. The consideration of a mild glycosylation step, which would allow the attachment of a functionalized side chain, led to the retrosynthetic intermediates I, II, and III (Scheme 1.1).
Scheme 1.1 OR OR OH O 1 - 4 RO + + RO STol HO Br OR I II III
Thioglycosides were chosen as the glycosylation donors because it is known that these donors are stable, and they couple with a variety of acceptors under mild conditions.18,19 The endiyne side chain was envisioned to arise from a copper-catalyzed coupling of intermediates II and III, and the chiral acetylenic diol II was envisioned to arise from an enzymatic resolution of a racemate, which could be prepared from a 3-alkoxy propanal and ethynylmagnesium bromide. The enzymatic resolution of acetylenic alcohols has been studied17,20 and was predicted to provide both enantiomers, thereby allowing verification of the C3 stereochemistry. Moreover, applying the Mosher ester method17 to the acetylenic diol II allows unambiguous determination of the nature of the enantiomer of interest. .
9 1.2 Results and Discussion 1.2.1 Preparation of 3-Deoxybidensyneoside C 5 As a preliminary study, we first carried out the glycosylation reaction between glucose 21 pentacetate and 4-pentyn-1-ol in the presence of BF3•OEt2. The desired product 6 was obtained in 30% yield (Scheme 1.2). The low yield of the desired product 6 is possibly due to achimerically-assisted deglycosylation, as reported for 4-pentenyl glucosides22. However, because of the relatively low cost and availability of the starting materials, 6 could be synthesized in adequate amounts allowing for continuation and completion of the synthesis.
Scheme 1.2 OAc OAc HO O AcO O AcO AcO O AcO OAc BF3OEt2 OAc OAc 30% 6 Br
OTBS OH 7 OTBS HO O CuCl, EtNH2, MeOH HO O OH 31% 8
OH HFPyr, THF OH HO O HO O 51% OH 5
With regards to the Lewis acid mediated deglycosylation, the BF3•Et2O conditions were the most favorable among the methods explored, although the yield was only 30%. The use of
SnCl4 led to a complex mixture of products with the desired product 6 only isolatable in 5 – 8% yield, while a α-chloro-glycoside was isolated in nearly 30% yield. The latter is presumably formed via the pathway described by Magnusson and coworkers.23 It is known that allyl 15 5 alcohol and 4-pentene-1-ol can both be introduced via SnCl4-promoted glycosylation in high yields. Unfortunately the presence of an alkyne moiety in place of the alkene seems to hinder the
10 formation of the desired product and presumably leads to the deglycosylation product and eventually the chloro glucoside. The use of thioglycoside 18 under DMTST conditions yielded a complex mixture of products, which exhibited nearly impossible chromatographic separation and consequently the inability to isolate any single compound exclusively for identification. Thus, the DMTST methodology was abandoned throughout the synthesis of 5, although thoiglycoside 18 later exhibited some profound properties attributed to the nature of the protecting groups used on the glycoside and the donor used in the glycosylation. Bromo enyne 7 was easily prepared from commercially available (E)-2-penten-4-yn-1-ol via (1) hydroxyl protection as the t-butyldimethylsilyl (TBS) ether and (2) bromination with 24 NBS in the presence of AgNO3. The coupling of bromo alkyne 7 and glucoside 6 was carried out under Cadiot-Chodkiewicz conditions.25 The copper-promoted coupling reaction, which was carried out in a mixture of EtNH2 and MeOH, proceeded concurrently with the removal of
acetate groups by EtNH2, leading to the polar glucoside 8 in 31% yield. The Cadiot- Chodkiewicz coupling has been utilized in our labs in the synthesis of a variety of natural products26-28 because the method is one of the most efficient ways to avoid the homocoupling of alkynes and has been exploited by many other labartories.29-33 Although this is a superior method in the unsymmetrical coupling of alkynes, it has some disadvantages. The low yield observed here can be attributed to the aliphatic carbon chain flanking the bromoalkyne, which we have found in our lab to result in poor yields, typically with a large amount of homocoupled bromoalkyne forming in the reaction. Alternatively, if the bromoalkyne has a propargylic substitute, the reactivity greatly increases and results in a better overall yield of the reaction with little or no homocoupling side products. Attempted coupling with the unprotected bromo enyne 7 resulted in loss of product during work-up due to difficulties in separating the extremely polar 5 from solvent. Using a TBS ether protecting group alleviated the problem. Diyne 8 could be extracted from the aqueous solution and purified on a silica gel column. Removal of the TBS protecting group was chosen to take place in THF with the HF•pyridine complex, a relatively mild reaction condition compared to that of TBAF or aqueous HF.34 To avoid the loss of the
product, the work-up consisted of adding solid NaHCO3 and evaporating the solvent to a slurry, which was transferred to a silica gel column and eluted with a mixed solvent system
(MeOH/CHCl3, 10:90) to give 51% of 5 as a white solid.
11 The synthetic sample gave identical 1H and 13C NMR spectra as the reported natural product.5 Five UV absorptions were reported for 3-deoxybidensyneoside C (328, 283, 267, 252, and 239 nm),5 but we observed only four (282, 266, 252, and 240 nm). Considering reported UV spectra for other endiynes and those reported for the other four bidensyneosides,5 we believe that the 328 nm absorption the spectrum of the natural product was a result of an impurity .
1.2.2 Synthesis of Bidensyneoside C 4 The rest of the bidensyneosides presented a slightly more difficult challenge due to the stereochemical nature at the C3 position on the side chain. In the end, it was determined that the best way to introduce the stereocenter on the side chain was to utilize an enzymatic resolution approach (Scheme 1.3), which has been exploited frequently in our lab and typically leads to desirable results. The THP and TBS protected alcohols were prepared from commercially available 1,3-propanediol. While the THP group could be introduced in 95% yield with little or no di-protected product, the TBS group methodology suffered from a significant amount of di- protection, leading to only a 47% yield.
Scheme 1.3
1. (COCl)2, DMSO Mono-protection Et3N, CH2Cl2 HO OH RO OH 47%-95% 2. HC CMgBr R = THP 61% R = TBS
OH OH OAc Lipase AK + RO vinyl acetate RO RO rac-9 9 S-11 80% R = THP < 15%ee <15%ee R = TBS > 95%ee >95%ee OH OAc separation 1. Ac2O RO HO 9 2. HF-Pyr 12 55%
Oxidation of the mono-protected diols under the conditions of Swern35 and in situ addition of ethynylmagnesium bromide to the resulting aldehyde afforded the racemic propargyl
12 alcohol 9 in 61% yield.36 The above preparation of 9 utilizes less expensive reagents and requires fewer steps than a previous reported procedure.37 The hydroxyl protecting group at C1 of rac-9 has a profound influence on the lipase- mediated kinetic resolution that can be rationalized in the following ways. The TBS ether group, unlike the THP, provides the steric bulk that is critical for an efficient resolution. Furthermore, the introduction of the THP group creates an additional chiral center that leads to two sets of enantiomers. By replacing the THP with TBS, greater than 95% ee was obtained when this ether was treated with Amano lipase AK in the presence of vinyl acetate in hexanes.17,20 This is in contrast to a less than 15% ee when using the THP protected derivative. The progress of the enzymatic resolution was monitored carefully by 1H NMR spectroscopy, and the reaction was terminated after 50% of rac-9 was consumed. After separation of the alcohol R-9 from the acetate S-11 by column chromatography, both enantiomers were obtained in high optical purity as confirmed by the 1H NMR spectra of their O-methyl mandelic esters.38 No diastereomeric isomer was detectable by 500 MHz 1H NMR analyses. The final two steps in the preparation of the side chain involved the acylation of the secondary OH group in R-9 and the removal of the TBS ether with HF•pyridine complex to afford the optically pure alcohol (R)-12. With the successful preparation of the chiral subunit present in the remaining four targets, the next challenge was to design a suitable acceptor for the glycosylation that would allow for successful glycosylation with the chiral subunit, while preserving the functionality of the side chain and then being able to undergo the subsequent coupling reaction necessary for the diyne functionality. A final point realized in the synthesis of 5 was that the protection sequence needed to be designed such that a successful assembly could be achieved with relatively simple purification so that the final deprotection results in a minimal amount of side products, allowing for a clean isolation and identification of each compound. The thioglycoside in scheme 1.4 with four acetate groups is considered to be "disarmed" or inert in the presence of the promoter dimethyl(methylthio) sulfonium triflate (DMTST).39 Although DMTST is less active than other thiophiles, such as NIS/TfOH, it does not produce a nucleophile as a byproduct.18 This was important so that the structural properties of both the acceptor and the donor were preserved in full throughout the reaction. These goals were not 40 achieved when the glycosylation was attempted under NIS/TfOH or SnCl2 in the presence of a silver salt41,42. Each resulted in a complex mixture of products that were unidentifiable.
13 Additionally, we explored a rather different methodology involving Tris(4- bromophenyl)aminium hexachloroantimonate (TBPA),43 which has been reported to mediate glycosylations of selenoglycosides and thioglycosides via a single electron transfer pathway. Although thioglycosides are less reactive than selenoglycosides, we speculated that there should be some formation of the desired product when subjecting the tetra-acetyl thioglucoside and R- 12 to TBPA conditions. Although both starting materials were consumed during the reaction, the complexity of the resulting mixture made product identification impossible. We thus decided to increase the reactivity of the glucoside donor by replacing some of the acetate protecting groups with TBS ethers. The tetra-acetyl thioglycoside was deacetylated
with K2CO3 in MeOH (Scheme 1.4) and the crude tetraol was treated with either one or two equivalents of TBSCl and imidazole in DMF. It is known that primary alcohols react considerably faster than secondary alcohols when in the presence of TBSCl, thus leading to the ability to selectively protect and introduce functionality in molecules44. With that in mind, the selective protection of the primary alcohol at carbon 6 after the deacylation had the potential of presenting us with a challenge (Scheme 1.4). Although the primary alcohol at carbon six as predicted to react first, giving a mono-TBS protected glycoside, the question was where a second and third TBS group would react.
14 Scheme 1.4
OAc OH O MeOH, K2CO3 AcO HO O AcO STol HO STol OAc OH
OTBS TBSCl, 1.1eq, Imid HO O STol 96% HO OH 13 OTBS OTBS
HO O TBSO O TBSO STol HO STol OH OH TBSCl, 2.2eq, Imid 14, 65% 15, 17% 99% OTBS OTBS
HO O HO O TBSO STol HO STol OTBS OTBS 17, 4% 16, 13% Although our original target was simply either the 6-TBS ether 13 or the 3,6-di TBS ether 14, respectively, it was worth the trouble to identify each of the products and try to pinpoint the preference of reactivity on the glucose framework. When treating 13 with 1.1 eq of TBSCl, we were pleased to get almost exclusive formation of the primary TBS ether 14 was observed. When subjecting 13 to the same conditions and doubling the amount of TBSCl to 2.2 eq, we found the following trend was observed in the diprotected products; the desired 3,6-di TBS ether 14 was formed in a modest 65% yield, while the 2,6-di TBS ether 16 formed in a 13% yield and the 4,6,-di TBS ether 15 formed in a 17% yield. Additionally, there was a small amount (4%) of the tri-TBS ether isolated 17. Unfortunately, the 2,6 and 4,6 products were difficult to separate on silica gel, but the protons of each compound could be assigned by careful analysis of a COSY NMR spectra, and the relative chemical shifts of each proton shed light on whether there was a free hydroxyl group present or a silyl ether. These compounds opened the door to more options with regard to selectively protecting the glucose moiety, allowing for more site selective and asymmetric control.
15 All of the bidensyneosides have the β-configuration at the anomeric carbon. To prevent the formation of the α-configuration, a C2-acetate group is necessary to provide neighboring group assistance in order to control the stereochemistry of the glycosylation.45 Therefore, the free hydroxyl groups of 13 and 14 were acetylated (Scheme 1.5). According to a study by Wong, replacing a C6 acetate group with TBS increases the glycosyl donor reactivity by a factor of 3 to 5 times.46 This is consistent with the ability of the sill ethers ability to donate electron density during the glycosylation reaction and stabilize the oxycarbinium ion. The ability of different substituents on carbohydrates and the ability to stabilize the intermediate oxycarbinium ion has been the focus of many publications and have led to insightful results.47,48 As for the other acceptors isolated in Scheme 5, 16 and 17 were useless due to the presence of the TBS ether at carbon two. Although 15 has a free hydroxyl group at C2, the low yield of the 4,6-di TBS ether makes it an unattractive synthone, and thus the synthesis focused exclusively on the 3,6-di TBS ether, which in the end proved to be advantageous, specifically in the chromatographic challenges that were to lie ahead. Scheme 1.5 OTBS
Ac2O, Pyr O 13 AcO DMAPcat AcO STol 87% OAc 18
OTBS Ac2O, Pyr 14 AcO O DMAPcat TBSO STol 87% OAc 19
Now that we had a collection of thioglycosides along with the chiral donor alcohol, it was time to explore the glycosylation reaction. When thioglycoside donor 18 and (R)-12 were allowed to react in the presence of DMTST for one hour at room temperature, the orthoester 20 was isolated in 74% yield, rather than the normal anomeric coupling product (Scheme 1.6). Assigning the structure of 20 began as a puzzling endeavor but after careful analysis and utilization of NMR spectroscopic methods, including 1H, 13C, DEPT 13C, and 2D COSY, HSQC and HMBC techniques, the structure of 20 was assigned. The small coupling constants between
16 H2 and H3 (J23 = 2.8 Hz) and between H3 and H4 (J34 = 2.8 Hz) are consistent with a twist boat or "skew" conformation for 20.49 Previous studies on carbohydrates containing orthoesters suggested a similar conformation.50,51 At first glance, one might propose alternative structures of a bicyclic nature that could form rather than the orthoester observed below. After taking structures of that nature into account, a series of HMBC NMR experiments were performed while changing the default coupling constant on the instrument, thus allowing one to see longer range couplings or shorter couplings between the protons and carbons. In doing this, it was clear that the acetate carbonyl carbons coupled to protons H3 and H4. Furthermore, the quaternary carbon making up the orthoester showed coupling to proton H1, H2, the terminal methyl group,
and the aliphatic CH2 on the side chain. Unfortunately, all attempts to investigate NOE enhancements failed due to the molecular weight of the orthoester. Although there were some additional experiments that could have been investigated such as a ROSY, we were confident with the assignment and continued with the synthesis. Scheme 1.6
OAc
OTBS HO AcO H1 12 TBSO O H AcO O 2 OAc DMTST, CH2Cl2 H4 STol O AcO H OAc 0 to rt, 1.5 hr 3 O OAc O 18 74% 20 AcO H1 TBSO O OAc H2 H OAc 4 O H OTBS HO 3 O S-12 OAc O AcO O DMTST, CH2Cl2 21 AcO STol OAc 0 to rt, 1.5 hr 18 26% OTBS
21:22 = 46:54 AcO O AcO O OAc OAc 22 To further prove that the structure of 20 and explore the reactive properties of orthoester,
20 was treated with K2CO3 in MeOH to remove the acetates (Scheme 1.7). Based on the chemical shifts of protons at the C3 and C4 positions prior to the deacylation and after the
17 deacylation, it was consistent with our assignment of the orthoester configuration. Additionally, the methoxide ion created in the reaction caused a considerable amount of the orthoester to lose the chiral side chain and replace it with a methoxy substitute 23.
Scheme 1.7
AcO H1 HO TBSO O TBSO O H H 2 OAc MeOH, K CO 4 O 2 3 O H 3 O O OAc O OH O 20 23
The exclusive formation of the orthoester 20 was reproducible from 18 and (R)-12, and there was no sign of decomposition during the reaction even by letting the reaction stir for more than three hours. Although the orthoester was not the synthetic focus, its exclusive formation was intriguing, which prompted a further investigation of the same reaction using the enantiomer of 12. When the S-enantiomer of 12 was used as the acceptor, we were surprised to find that, not only was the yield of the reaction considerably lower (26%), but the normal glycosylation product 22 was also isolated in addition to the orthoester. The difference between R- and S-12 appears to be an effect of matching and mismatching pairs with regard to the glucoside donor 18. This ability of the glycoside to discriminate between the chiralities of the alcohol is remarkable due to the displacement between the chiral center and the reacting alcohol. With these profound properties confirmed, we decided to use the more reactive donor 19 to couple with alcohol 12, although with it being more reactive. The position of the TBS ether at C3 might promote a normal chair conformation, allowing for NMR spectra that are easier to interpret. Although it should be possible to transform orthoester 20 to the diastereomer of 22 under acidic conditions.13 we decided not to pursue a synthetic pathway of that nature due to the inability to successively isolate any products while trying to convert 20 and 21 into their corresponding normal anomeric products.
18
Scheme 1.8 OAc OTBS OTBS HO O 12 AcO AcO O DMTST, CH2Cl2 TBSO O TBSO STol OAc OAc 68% AcO 19 24
Br
OH OTBS OH 25 AcO O TBSO O CuCl, EtNH2, MeOH OAc HO 56% 26
OH 1. HFPyr, AcOH OH HO O HO O 2. K2CO3, MeOH OH 50% HO 4
The reaction was essentially instantaneous when glycosyl donor 19 was allowed to react with the chiral alcohol R-12 in the presence of DMTST (Scheme 1.8). Within 5 min at 0 °C, TLC analysis indicated that all the starting material had been consumed, and the glycosylation product 24 was isolated in 68% yield with no orthoester formation. The dramatic difference in glycosylation results observed between donor 18 and 19 originates from a single protecting group at C3. Although the formation of orthoesters is quite common in glycosylation reactions, we believe this is the first time that a single hydroxyl protecting group has been shown to alter the outcome of the glycosylation reaction effectively. Currently, we believe this difference is related to the relative stability of the glycosylation intermediate, the oxycarbenium ion. It appears that donor 18 with a C3 acetate group produces a less stable oxycarbenium ion and requires neighboring group stabilization and thus the formation of the orthoester. Donor 19 with a TBS ether group should yield a more stable oxycarbenium ion that then reacts directly with the acceptor. An alternative explanation is that both reactions proceed through the initial formation of the orthoester, which rearranges more rapidly in the case of 19 to the glycoside.
19 With the successful glycosylation, the only remaining major step was the introduction of the side chain to create the enediyne framework. The copper promoted coupling reaction (Cadiot-Chokwicz reaction)27,52 between the terminal alkyne 24 and the bromoalkyne 25 was carried out under the conditions discussed earlier (Scheme 1.2). This reaction furnished bidensyneoside C (26) in its protected form. Interestingly, only the C3 acetate group was removed during the reaction; the two acetate groups on the glucose ring remained intact.
Apparently the steric bulk of the TBS ethers in 24 prevent EtNH2 from attacking these esters. Removal of the protecting groups was undertaken in the sequence shown in Scheme 1.8. The removal of the protecting groups had to be done in a specific sequence. Trying to remove the acetates with the TBS ethers on the molecule resulted in essentially no reaction, even in the presence of 10 eq K2CO3. The use of HF•Pyridine led to little or no reaction as well. However, the addition of acetic acid made it possible for the TBS groups to be removed. The reaction was monitored by TLC, and after the completion of the reaction, only one spot was present. Workup and subsequent removal of the acetate groups furnished the desired product. All of the spectroscopic data on the synthetic sample were consistent with that reported
for the natural product, except for the optical rotation, which was reported to be [α]D -71.6°, compared to our [α]D-50.84°. Although the compound has six chiral centers, only one was really questionable, that being the chiral center at carbon 3 on the side chain. Thus, we investigated the possibility that previous reports were in error, or possibly in taking the data for the natural product. Therefore we synthesized an analog of bidensyneoside C with inverted stereochemistry at C3 (Scheme 1.9). To investigate this inconsistency, the (S) enantiomer of 12 was used in the
20 Scheme 1.9 OAc OTBS OTBS HO O 12 AcO AcO O DMTST, CH2Cl2 TBSO O TBSO STol OAc OAc 51% AcO 19 27
Br
OH OTBS OH 25 AcO O TBSO O CuCl, EtNH2, MeOH OAc HO 32% 28
OH 1. HFPyr, AcOH OH HO O HO O 2. K2CO3, MeOH OH 50% HO 29 glycosylation with 19. And as was observed in the reactions with 18, the use of the (S) enantiomer of 12 resulted in a low yield, 51%. The subsequent copper catalyzed coupling gave 26 and following deprotection, gave the desired product with the inverted stereochemistry at C3 on the side chain. The NMR data for the isolated compound was almost identical to that of its natural occurring counterpart, but the optical rotation significantly deviated from that of the natural product being [α]D +13.0° again, compared to the [α]D -71.6° as mentioned before. With these findings in mind, the assignments of both the reporter structures and our synthetic work was deemed correct, and the synthesis of the remaining natural products was continued. .
1.2.3 Synthesis of Bidensyneoside A2 2 With the key intermediate 24 in hand, we were eager to expand our synthetic route to other members of the bidensyneosides. The commercial availability of (Z)-3-penten-1-yne
prompted us to prepare bidensyneoside A2 (2) as a test for our methodology. It turns out that the more practical route for compound 2 is to prepare bromoalkyne 30 because of the volatility of (Z)-3-penten-1-yne and the difficulty in making its bromoalkyne analog. (Scheme 1.10).
21 Scheme 1.10
OTBS NBS, AgNO3 CuCl, EtNH2, MeOH 24 AcO O DMF TBSO O 63% OAc Br 86% AcO 30 OTBS OH 1. HFPyr, AcOH AcO O HO O TBSO O HO O OAc 2. MeOH, K2CO3 OH HO HO 31 85% Bidensyneoside A2, 2
In the bromination reaction of the terminal alkyne 24, the preferred solvent was DMF, instead of acetone, to minimize the breakage of the glycosyl bond and the formation of several different unidentifiable side products. The copper-catalyzed coupling of bromoalkyne 30 with
(Z)-3-penten-1-yne proceeded smoothly to produce the protected bidensyneoside A2 in 86% yield. This is consistent with our recent observation that a propargylic oxygen substitution in the bromoalkyne enhances the rate of the copper-catalyzed cross coupling reaction to produce conjugated diynes.27 Without a propargylic oxygen substitution, homocoupling of the bromoalkyne often dominates. The removal of the TBS ether and the acetate protecting group was performed in order affording compound 2 in 85% yield for two steps. The synthetic sample was identified by spectroscopic methods, and the results were consistent with the reported natural product.
22 1.3 Conclusions
The total synthesis of bidensyneoside A2 (2), and C (4) was achieved in 7 linear steps starting from glucose pentacetate with an overall yield of 22% and 9%, respectively, and 3- deoxybidensyneoside C (5) in three steps and 5% overall yield. These syntheses represent the first synthetic entry to the bidensyneosides and confirm the stereochemistry at C3. Glycoside formation was found to be highly dependent on the nature of the protecting groups on the glucose. Exclusive formation of the normal glycosylation product or the orthoester was observed, respectively, depending on the characteristics of the protecting group at C3 of the glucoside. The electron-withdrawing acetate group led to the formation of the orthoester, while an electron- releasing TBS group led to the normal glycosylation product. Based on this study, a change of protecting groups effectively alters the outcome of the glycosylation reaction. In accord with our recent observations, a propargylic oxygen substitution in the bromoalkyne enhances the copper- catalyzed cross coupling reaction to produce conjugated diynes.
23 1.4 Experimental section
General Experimental Procedures All reactions were carried out under an atmosphere of nitrogen in oven-dried glassware with magnetic stirring. Reagents were purchased from commercial sources and used directly without
further purification. Methylene chloride was dried over P2O5 and freshly distilled before use. Purification of reaction products was carried out by flash chromatography using silica gel 40 – 63 µm (230 – 400 mesh) unless otherwise stated. Reactions were monitored by 1H NMR and/or thin-layer chromatography. Visualization was accomplished with UV light, staining with 5%
KMnO4 solution followed by heating or with p-anisaldehyde (200 mL of 95% EtOH, 10 mL of
H2SO4, and 10 mL of p-anisaldehyde). Chemical shifts were recorded in ppm (δ) using tetramethylsilane (H, C) as the internal reference. Data are reported as: (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet; integration; coupling constant(s) in Hz). Optical rotations were measured using Autopol III. Melting points were measured with a Gallenkamp melting point apparatus. Infrared spectra were recorded on a Perkin Elmer 1600 series FTIR for liquids, and on a Perkin Elmer Spectrum 2000 FTIR for siolds. High-resolution mass spectra were recorded at The Ohio State University.
24 4-Pentynyl tetra-acetyl-β-D-glucopyranoside (6) OAc OAc O HO AcO AcO O AcO OAc AcO O OAc BF3OEt2 OAc 6
A solution of (1.95 g, 5 mmol) glucose pentaacetate in 10 ml CH2Cl2 was stirred with 6 BF3•Et2O (5.5 mmol) at room temperature for 1 hr. Next, 0.63 g of 4-pentyne-1-ol (7.5 mmol) was added, and the mixture was allowed to stir for an additional 4 h. After consumption of the
starting material, the mixture was cooled to 0° C and stirred with saturated aq. NaHCO3 for 30 min. The resulting mixture was extracted three times with diethyl ether. Then the combined extracts were washed with water and brine and dried over MgSO4. The resulting solution was filtered, concentrated and purified over silica gel. (50% EtOAc/Hex) to give 632 mg (30%) as a yellow syrup. 1 [α]D 10.60° (MeOH, c = 6.6), H-NMR (200 MHz, CDCl3): δ 1.67 (2H, m), 1.87 (1H, t, J = 2.56 Hz), 1.91-1.99 (12H, 4s), 2.16 (2H, dt, J = 7.12, 2.55 Hz), 3.56 (2H, m), 3.87 (1H, dt, J = 9.66, 5.40 Hz), 4.00 (1H, m), 4.18 (1H, dd, J = 12.29, 4.62 Hz), 4.42 (d, 1H, J = 7.90 Hz), 4.89 (1H, t, 13 J = 9.28 Hz), 4.99 (1H, t, J = 9.57 Hz), 5.23 (1H, t, J = 9.36 Hz). C-NMR (50 MHz, CDCl3): δ
51.11 (CH2), 20.97-21.10 (CH3), 28.53 (CH2), 62.28 (CH2), 68.70 (CH2), 68.75 (CH), 69.27, 71.62 (CH), 72.10 (CH), 73.11 (CH), 83.73, 101.37 (CH), 169.74, 169.78, 170.63, 171.03. IR: ν -1 cm 3282, 2117, 1755, HRMS: calcd for C19H26O10 + Na, 437.1424, found M + Na, 437.1446.
Silane, (1,1-dimethylethyl)dimethyl[5-bromo-(2E)-2-penten-4-ynyloxy] (7) Br NBS, AgNO3
OTBS Acetone OTBS 7 To a suspension of NBS (541 mg, 3.04 mmol) and the TBS ether of 2-penten-4-yn-1-ol,
(510 mg 2.6 mmol) in 20 ml acetone was added AgNO3 (44 mg, 0.26 mmol). The mixture was allowed to stir for one hour, and then it was diluted with 50 ml Et2O and 50 ml NaHCO3. The solution was extracted three times with Et2O, and the combined organic layers were dried over
MgSO4. The product was purified over silica gel, 2% EtOAc/Hex giving 411 mg (58%) of a yellow oil.
25 1 H-NMR (300 MHz, CDCl3): δ 0.05 (6H, s), 0.86 (9H), 4.17 (2H, ddd, J = 14.07, 4.92, 2.22 Hz), 13 5.70 (1H, dt, J = 15.78, 2.19 Hz), 6.23 (1H, dt, J = 15.65, 4.10 Hz). C-NMR (75 MHz, CDCl3):
δ -4.98 (CH3), 18.73, 26.25 (CH3), 49.53, 63.06 (CH2), 78.84, 108.55 (CH), 144.52 (CH). IR: ν cm-1 3400, 3039, 2167, 1638, 1133
[10-tert-Butyldimethylsilyloxy-8-decen-4,6-diynyl]-β-D-glucopyranoside (8)
OAc OH O 7, CuCl, EtNH OTBS AcO 2 HO O AcO O MeOH HO O OAc OH 6 8 To a 10 ml flask charged with nitrogen and a stirbar was added 24 mg (0.34 mmol)
NH2OH•HCl, 1 ml methanol, 200 mg (0.48 mmol) of 6, and 1 ml EtNH2 solution. The mixture was cooled to 0° C and CuCl (2.4 mg, 0.24 mmol) was added followed by slow addition of 146 mg (0.53 mmol) of 7. The reaction was allowed to stir for 30 minutes and then allowed to warm
to room temperature. A KCN/NH4Cl solution was added to the reaction flask. The resulting mixture was then extracted 7 times with EtOAc. The combined extracts were then concentrated and purified over silica gel (10%MeOH/CHCl3) giving 66 mg (31%) of a yellow oil. 1 [α]D -7.7° (MeOH, c = 0.9). H-NMR (300 MHz, CDCl3): δ 0.10 (2xCH3, s), 0.94 (3xCH3, s), 1.85 (2H, m), 2.50 (2H, t, J = 7.08 Hz), 3.18 (1H, t, J = 8.62 Hz), 3.28 (2H, m), 3.37 (1H, m), 3.66 (2H, m), 3.88 (1H, dd, J = 11.67, 1.75 Hz), 3.97 (1H, dt, J = 10.03, 6.05 Hz), 4.24 (3H, dd, J = 4.49, 2.89 Hz, and d, J = 7.38 Hz), 5.78 (1H, dm, J = 15.70 Hz), 6.34 (1H, dt, J = 15.72, 4.23 13 Hz). C-NMR (75 MHz, CDCl3): δ -5.31 (CH3), 16.82 (CH2), 19.16, 26.31 (CH3), 29.81 (CH2),
62.74 (CH2), 63.85 (CH2), 66.17, 69.21 (CH2), 71.62 (CH), 74.06, 75.11 (CH), 75.33, 77.91 (CH), 78.05 (CH), 84.30, 104.46 (CH), 108.46 (CH), 146.81 (CH). IR: ν cm-1 3402, 2930, 1654,
1077, HRMS: calcd for C22H36O7Si + Na, 463.2128, found M + Na: 463.2155.
3-Deoxybidensyneoside B (5) OH OTBS HFPyr, THF HO O 3-Deoxybidensyneoside C, 5 HO O OH 8
26 To a solution of 24 mg (0.055 mmol) 8 in 4 ml THF and at 0° C was added 55 µl HF•pyridine. After the addition, the mixture was allowed too warm to room temperature and stir
for an additional 16 hours. To the mixture was added solid NaHCO3, and the solvent was evaporated to ~1 ml. The solution was directly purified over silica gel (30%MeOH/CHCl3) giving 9.5 mg, (51%) of a white solid (m.p 158-161°C).
20 ° 1 = −15.15 (MeOH, c = 0.138), H-NMR (500 MHz, Methanol-d4): δ 1.86 (2H, m), 2.50 [α]D (2H, J = 7.12 Hz), 3.19 (1H, dd, J = 9.06, 7.82 Hz), 3.29 (2H, m), 3.36 (1H, t, J = 8.81 Hz), 3.66(1H, dt, J = 10.01, 6.22 Hz), 3.69, (1H, dd, J = 11.81, 5.35 Hz), 3.88 (1H, dd, 11.81, 2.05 Hz), 3.98 (1H, dt, J = 10.02, 6.06 Hz), 4.14 ( 2H, dd, J = 4.81, 2.05 Hz), 4.27 (1H, d, J = 7.77 Hz), 5.78 (1H, d, J = 15.95 Hz), 6.36 (1H, dt, J = 15.95, 4.78 Hz). 13C-NMR (75 MHz,
Methanol-d4): δ 16.82 (CH2), 29.81 (CH2), 62.69 (CH2), 62.75 (CH2), 66.13 (C), 69.21 (CH2), 71.64 (CH), 74.06, 75.13 (CH), 75.24, 77.93 (CH), 78.07 (CH), 84.33, 104.45 (CH), 109.13 -1 (CH), 147.07 (CH). IR: ν cm 3213 2228 2140 1627 1159 1030 1009, UV/VIS: λ(max)nm: 282,
266, 252, 240. HRMS: Calculated C16H22O7+Na = 349.1263, found C16H22O7+Na = 349.1252. Reported Data:
20 = -67.7° (MeOH, c = 0.5), 1H-NMR for side chain and the anomeric carbon, (500 MHz, [α]D
Methanol-d4): δ 3.96 (1H,dt, J = 9.8, 5.8 Hz), 3.72 (1H, dt, J = 9.8, 6.6 Hz), 1.83 (2H, m), 2.47 (2H, t J = 7.3 Hz), 5.76 (1H, dq, J = 15.9, 1.9 Hz), 6.34 (1H, dq, J = 15.9, 4.6 Hz), 4.13 (2H, dd, J 13 = 4.6. 1.9 Hz), 4.24 (1H, d, J = 8.0 Hz). C-NMR (125 MHz, Methanol-d4): δ 16.9 (CH2), 29.8
(CH2), 62.2 (CH2), 62.8 (CH2), 66.2 (C), 69.2 (CH2), 71.7 (CH), 74.1, 75.2 (CH), 75.3, 77.9 (CH), 78.1 (CH), 84.3, 104.5 (CH), 109.2 (CH), 147.1 (CH). IR: ν cm-1 3330, 2927, 2231, 1627,
1160 1074. UV/VIS: λ(max)nm: 328, 283, 267, 252, 239. HR-EI-MS m/z: 326.13568 + (Calculated for C16H22O7 [M] = 326.13655).
(+)-3R-5-tert-butyldimethylsilyloxy-1-pentyn-3-ol (R-9) OH OH OAc Lipase AK, vinyl acetate + TBSO TBSO Hexanes TBSO rac-9 9 S-11
27 A solution of racemic 9 (2.62 g, 12.19 mmol) in 100 ml of hexanes was added 3 g of molecular sieves (4Å) and 6.29 g of vinyl acetate (73.17 mmol). Next, 2.62 g lipase AK was added, and the mixture was allowed to stir at room temperature. When 1H NMR analysis indicated that the ratio of acylated alcohol to free alcohol was approximately 1:1, the mixture was filtered over a pad of Celite, washed with Hex, and purified over silica gel (15 % EtOAc/Hex) to give 1.06 g (81 %) of (R)-9 as a yellow oil and 1.25 g (80 %) of (-)-11 as a light yellow oil.
20 20 1 (R)-9 = +14.8 (CHCl3, C = 8.25), (S)-11: = -51.4 (MeOH, C = 1.52) H-NMR (200 [α]D [α]D
MHz, CDCl3): δ 0.06 (3H), 0.07 (3H), 0.88 (9H), 1.90 (2H, m), 2.44 (1H, d, J = 2.10 Hz), 3.55 13 (1H, d, J = 6.16 Hz), 3.84 (1H, m), 4.03 (1H, m), 4.60 (1H, m). C-NMR (200 MHz, CDCl3): δ -1 -5.14 (CH3), 18.5, 26.25 (CH3), 38.67 (CH2), 61.51 (CH2), 62.28 (CH), 73.25, 84.79. IR: ν cm
3437, 3033, 2111, 1654. MS, (ESI): calcd for C11H22O2Si + Na, 237.1, found M + Na, 237.1.
O-Methyl mandelate of R (9) O Ph OH O (R)-(-)-α−Methoxyphenyl acetic acid OMe TBSO TBSO DCC, DMAP, CH Cl 9 2 2 10
To a 25 ml round bottom flask was added 114 mg (0.53 mmol) of R-9 and 6 ml CH2Cl2. Next, (88 mg, 0.53 mmol) of (R)-(-)-α-methoxyphenyl acetic acid was added followed by DCC (110 mg 0.53 mmol) and DMAP (6.6 mg, 0.053 mmol). The reaction was monitored by TLC, and after 20 hr, the mixture was diluted with 50 ml hexanes. The solution was washed with 25 ml 1N HCl, 25 ml saturated NaHCO3, brine, and then dried over Na2SO4. The solvents were removed under reduced pressure, and the residue purified over silica gel (20% EtOAc/Hex) giving the product as a colorless oil. 1 H-NMR (200 MHz, CDCl3): δ -0.04 (3H, s), -0.02 (3H, s), 0.8 (9H, s), 1.95 (2H, m) 2.35 (1H, d, J = 2.12 Hz), 3.40 (3H, s), 3.62 (2H, m), 4.77 (1H, s), 5.52 (dt J = 2.06, 7.24 Hz), 7.31 (3H, m), 13 7.41 (2H, m). C-NMR (200 MHz, CDCl3): δ -5.10, 18.64, 26.27, 37.94, 57.86, 58.80, 62.21, 74.52, 80.34, 83.05, 127.60, 129.01, 129.18, 136.19, 170.01.
(+)-3R-3-Acetoxy-1-tert-butyldimethylsilyl-4-pentyn-1-ol (R)-(+) (11)
28 OH OAc Ac2O, DMAP TBSO TBSO Pyridine 9 11 To a solution of 1.06 g of R-9 in 5 ml pyridine was added 5 mg of DMAP and a solution of 0.74 ml of acetic anhydride (7.40 mmol) in 3 ml pyridine. The mixture was heated to 60° C and allowed to stir for 4 hrs.19 After TLC analysis indicated the completion of the reaction, the solution was diluted with 5 ml of 2N HCl and 5 ml of a 50:50 Et2O/Hex solution. The mixture was extracted three times with a mixture of Et2O/Hex, and the combined organic extracts were washed three times with water and once with brine. The organic layer was dried over Na2SO4. The solvents were removed under reduced pressure, and the residue was purified over silica gel (15% EtOAc/Hex) to give 1.08 g (85 %) of a light yellow oil. 1 [α]D +55.3 (MeOH, C = 1.25), H-NMR (200 MHz, CDCl3): δ 0.00 (6H, s), 0.84 (9H, s), 1.95 (2H, m), 2.03 (3H, s), 2.41 (1H, d, J = 2.14 Hz), 3.69 (2H, m), 5.43 (1H, dt, J = 2.10, 6.92), 13C-
NMR (50 MHz, CDCl3): δ -5.08 (CH3), 18.64, 21.34 (CH3), 26.26 (CH3), 37.95 (CH2), 58.93 -1 (CH2), 61.40 (CH), 73.99, 81.58, 170.13. IR: ν cm 3447, 3311, 2123, 1747, 1231, MS (ESI): calcd for C13H24O3Si + Na, 279.1, found M + Na, 279.2.
(+)-3(R)-3-Acetoxy-4-pentyn-1-ol (R)-(+) (12) OAc OAc HFPyr, THF TBSO HO 11 12 To a solution of (R)-11 (689 mg, 2.68 mmol) in 40 ml THF was added at 0° 2.68 ml HF•pyridine. The mixture was allowed to warm to room temperature and stirred for 18 hours.
After completion, the solution was diluted with Et2O and washed with NaHCO3, brine, and dried over Na2SO4. The solution was concentrated and purified over silica gel (50% EtOAc/Hex) to give 320 mg (84 %) as a clear oil. 1 [α]D +103.6 (MeOH, C = 2.70), H-NMR (200 MHz, CDCl3): δ 2.00 (2H, m), 2.05 (3H, s), 2.32 (1H, s), 2.47 (1H, d, J = 2.11 Hz), 3.69 (2H, m), 5.49 (1H, dt, J = 2.09, 6.67 Hz). 13C-NMR (50
MHz, CDCl3): δ 21.36 (CH3), 37.86 (CH2), 58.68 (CH2), 61.70 (CH), 74.56, 81.18, 170.67. IR: ν -1 cm 3479, 3303, 2118, 1752, 1631, HRMS: calcd for C7H10O3 + Na, 165.0528, Found M + Na, 165.0536,
Deprotection of (-)-11 gave (-)-12 with [α]D -96.6 (MeOH, C = 40)
29
p-Tolyl 6-O-(tert-butyldimethylsilyl)-1-thio-β-D-glucopyranoside (13) OAc OTBS 1) MeOH, K CO AcO O 2 3 HO O AcO STol HO STol OAc 2) TBSCl, Imid, DMF OH 13 To a dry 25 ml round bottom flask was added p-tolyl-2,3,4,6-tetra-acetyl-1-thio-β-D-
glucopyranoside 0.66g (1.45 mmol), 14 ml MeOH, and K2CO3 10 mg (0.072 mmol). The reaction was allowed to stir at room temperature for 2 hrs at which point the methanol was removed by vacuum, and the white residue was subject to high vacuum overnight. The mass of the flask indicated complete removal of the acetates and methanol. Then, the residue was dissolved in 5 ml DMF, and TBSCl was added (285 mg, 1.89 mmol) at 0°C. Next, imidazole (130 mg 1.90 mmol) was added in three portions over 10 min. The solution was allowed to stir
at room temperature for 1.5 hrs, diluted with 5 ml 0.5N HCl, and extracted three times with Et2O.
The combined organic extracts were thne washed with NaHCO3 and brine and dried over MgSO4. After removing the solvents under reduced pressure, the residue was purified over silica gel (70% EtOAc/Hex) to give 203 mg (35%) as a clear syrup. 1 [α]D -36.1º (MeOH, C = 1.14), H-NMR (200 MHz, Methanol-d4): δ 0.10 (3H, s), 0.11 (3H, s), 0.95 (9H, s), 2.33 (3H, s), 3.18 – 3.34 (4H, m), 3.79 (1H, dt, J = 11.3, 2.34 Hz), 3.98 (1H, d, J = 11.0 Hz), 4.54 (1H, d, J = 9.60 Hz), 7.12 (2H, d, J = 8.07 Hz), 7.48 (2H, d, J = 8.16 Hz). 13C-
NMR (50 MHz, Methanol-d4): δ -5.87 (CH3), -5.81 (CH3), 18.35, 20.34 (CH3), 25.62 (CH3),
63.30 (CH2), 70.13 (CH), 72.70 (CH), 78.74 (CH), 81.22 (CH), 88.62 (CH), 129.56 (CH), 130.47, 132.46 (CH), 137.51. IR: ν cm-1 3392, 2928, 1641, 1493, 1071, LCMS (ESI): calc