Merremoside D: De novo synthesis of the purported structure, nmr analysis, and comparison of spectral data

Article abstract of DOI:10.1021/ol403369h

The first synthesis of the purported structure of Merremoside D has been achieved in 22 longest linear steps. The de novo asymmetric synthesis relied on the use of asymmetric catalysis to selectively install all 21 stereocenters in the final compounds fro

Full text of DOI:10.1021/ol403369h

Letter
pubs.acs.org/OrgLett
Merremoside D: De Novo Synthesis of the Purported Structure, NMR
Analysis, and Comparison of Spectral Data
Ehesan U. Sharif,†,∥ Hua-Yu Leo Wang,†,∥ Novruz G. Akhmedov,§ and George A. O’Doherty*,†
†Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States
§Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
S
* Supporting Information
ABSTRACT: The first synthesis of the purported structure of Merremoside D has been achieved in 22 longest linear steps. The
de novo asymmetric synthesis relied on the use of asymmetric catalysis to selectively install all 21 stereocenters in the final
compounds from commercially available achiral starting materials. Adiabatic gradient 2D NMR techniques (gHSQCAD,
gHMBCAD, gH2BCAD, gHSQCTOXYAD, ROESYAD) were used to completely assign the structure of synthetic Merremoside
D. Comparison of our assignments with the limited NMR data reported for natural Merremoside D allows for the tentative
confirmation of its structure.
he merremoside family of resin glycoside type natural
membranes.3 While several resin glycoside natural products
have been synthesized,4 no member of the merremoside family
has succumbed to total synthesis.5 Despite their interesting
biological activities, no detailed structure−activity relationship
(SAR) has been carried out.
T
products was isolated from the fresh tuber of Merremia
mammosa (Lour.) Hall. f. (convolvulaceae) by Kitagawa.1
These structurally complex oligosaccharides possess a macro-
lactone (20- or 21-membered) which consisted of a bis-
rhamnose disaccharide bridged at the C-1 and C-2′ or C-3′ by a
jalapinolic acid (Figure 1). The tuber of the Merremia mammosa
To gain a better understanding of the promising and diverse
biological activities associated with this novel set of natural
products, we became interested in a synthesis led SAR-study of
the merremosides. In this vein, we targeted for the synthesis of
merremoside D. Intrigued by the possibility that enantiomeric
analogues of these target compounds would possess the ion
transport properties, yet would lack the same target protein
interactions, we decided to develop a de novo asymmetric
approach to the merremosides. We have demonstrated that a de
novo approach to carbohydrates6 can be used for the assembly
and medicinal chemistry study of oligosaccharides.7,8 The
approach combines the use of asymmetric synthesis of
pyranone glycosyl donor 5, a Pd(0)-catalyzed glycosylation
and post-glycosylation transformation, which allow the enone
functionality of the pyranones to serve as atom-less protecting
groups for the C-2 to C-4 triol portion of the target rhamno-
pyranose. Because the route uses asymmetric synthesis, it offers
equal efficiency to access to various all D-, all L-, and mixed D/L-
isomers.9 Herein, we disclose our successful efforts in the
synthesis of the purported structure of merremoside D.
Figure 1. Structures of merremoside family of resin glycosides.
plant has been traditionally used for the treatment of illnesses
associated with the throat and respiratory systems.1a This
tradition continues to the present day, where the powder of the
tuber is sold as an herbal medicine for treating patients with
various maladies (e.g., cancers, appendicitis, swollen veins,
typhus).2 The amphiphilic nature of the resin glycosides has
been suggested to be the source of its ionophoretic activity (i.e.,
membrane transporter) as observed in human erythrocyte
Received: November 21, 2013
© XXXX American Chemical Society
A
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Letter
Retrosynthetically we envisioned that construction of all the
stereocenters in the merremoside D 1 could be accomplished
from achiral starting materials utilizing our de novo approach to
carbohydrates (Scheme 1). The tetrasaccharide 2 would be
oxidative alkyne cleavage, and esterification resulting in the
desired jalapinolic ester (i.e., 13 to 7).10
The synthesis of the macrolactone disaccharide began with
the de novo asymmetric synthesis of the key pyranone building
block 5 (Scheme 3). The three-step synthesis of 5 involves a
Scheme 1. Retrosynthetic Analysis: De Novo Approach
Scheme 3. Synthesis of Macrolactone Disaccharide
Noyori reduction of acylfuran 611 followed by a subsequent
Achmatowicz rearrangement13 and diastereoselective tert-butyl
carbonate (Boc)-protection of the axial anomeric alcohol. A Pd-
catalyzed stereoselective glycosylation of 5 with jalapinolic ester
7 gave 15 (at this stage the minor diastereomer was removed by
flash chromatography). The enone 15 was converted to
glycosyl acceptor 18 via Luche reduction,14 Upjohn dihydrox-
ylation,15 and syn-diol protection. A second Pd-catalyzed
glycosylation with pyranone 5 followed by enone reduction
gave allylic alcohol 19. Benzylation of the allylic alcohol
followed by alkene dihydroxylation and a subsequent methyl
ester saponification resulted in the diol-acid 8.
With the macrolactone precursor 8 in hand, we investigated
the macrolactonization utilizing conditions reported by Yang et
al.5 Macrolactonization of 8 under Corey−Nicolaou con-
ditions16 gave a mixture of regioisomeric products 20 and 21
in a 4.7:1 ratio (Scheme 4). Extensive NMR analysis of the
constructed by a convergent glycosylation between macro-
lactone disaccharide 9 and the imidate disaccharide 3, with a C-
2 chloroacetate serving as an anomeric directing group. The
desired C-3 esterification on 3 would be obtained by tin-
mediated regioselective esterification of diol 4, which in turn
would be obtained from pyranone 5. The macrolactone
disaccharide 9 would be obtained from diol-acid 8, the bis-
rhamnose unit of which would be obtained from the same
building block pyranone 5. This key building block pyranone 5
would be obtained from acetylfuran 6 in three steps.7a,b
A
Scheme 4. Macrolactonization/Synthesis of Glycosyl
Acceptors
highly stereoselective Pd-catalyzed glycosylation would install
all the desired 1,4-α-linkage in the tetra-rhamnose backbone.
The aglycon 7 (jalapinolic ester) would be obtained from
alkynone 10, using a modified asymmetric variant of a route
reported by Heathcock.10 Thus as devised, all the chiral centers
in 1 would have their absolute stereochemistry resulting from
the Noyori asymmetric reduction of furan 6 and ynone 10.11
The asymmetric approach to aglycon 7 began with the
synthesis of alkynone 10 from undecyne 11 and hexanal 12
(Scheme 2).10 Enantioselective (S,S)-Noyori reduction of
alkynone 10 gave alkynol 13 (89% ee).11a,b An alkyne-zipper
reaction was used to isomerize the internal alkyne 13 into the
terminal alkyne,12 which was followed by hydroxyl protection,
Scheme 2. Synthesis of Jalapinolic Ester
macrolactones confirmed that the major product was a C-2′
macrolactone 20, and the minor product was a C-3′
macrolactone 21. This observation was opposite to what was
reported by Yang et al. (see Supporting Information).5
To our delight, we found that the major C-2′ regioisomeric
macrolactone 20 could be isomerized to the C-3′ macrolactone
B
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Letter
21. Under the isomerization conditions (1 equiv of DBU/
toluene), the two macrolactones 20 and 21 reached a
thermodynamic equilibrium (2:1) in 12 h. The remaining
hydroxyl group on the macrolactones (20 and 21) was
protected as chloroacetic ester, and the benzyl ether was
removed to form the glycosyl acceptors (20 to 22 and 21 to 9)
respectively.
Scheme 6. Total Synthesis of Merremoside D
The donor disaccharide portion was also prepared using our
de novo asymmetric strategy. At the outset, we hoped to be able
to carry the C-2/C-3 alkene functionality throughout the
synthesis, thus serving as an atom-less protecting group
(Scheme 5).7e The pyranone 5 was converted to protected
Scheme 5. Glycosyl Donor Synthesis/Attempted
Glycosylation
comparison of the spectral data was more complicated (see SI).
The structural identity of synthetic 1 was confirmed by detailed
1
1D and 2D NMR analysis (see SI). The H and 13C NMR
spectra of the synthetic merremoside D (1) were acquired in
CDCl3, pyridine-d5, and pyridine-d5/D2O (5:1) and used for
comparison with the limited NMR data reported for the isolated
merremoside D. For instance, only 21 signals were reported for
1
the H NMR (pyridine-d5/D2O) and 7 signals for the 13C
NMR data (pyridine-d5).
While it is likely that these two materials are the same, the ill-
defined ratio of the pyridine-d5/D2O mixture used to record the
1H NMR spectra of the natural material rendered it difficult to
match the chemical shifts between the two samples with
absolute certainty (e.g., depending on D2O/H2O concentration
the Dppm for various signals varied from ≤0.78 ppm to 0.55
ppm in pyridine-d5). For instance, seven of the 21 signals had
Dmax greater than 0.1 ppm with three of those being greater
than 0.2 ppm. Similar spectral inconsistencies due to
concentration/solvent effects have been observed in related
oligosaccharides.21 Our efforts to find the identical solvent ratio
used in the isolation were further confounded by the
observation of ester migration/hydrolysis in synthetic 1 upon
standing in pyridine-d5/D2O (see SI). In contrast to the
chemical shifts, there was excellent agreement between the
1H−1H coupling constants for the 14 multiplets reported for
natural merremoside D and the multiplets for synthetic 1,
regardless of the ratio of the pyridine-d5/D2O mixture.
Similarly, there was significantly better agreement between
the limited 13C NMR data (seven signals) reported for natural
1 in pyridine-d5 and those found for synthetic 1 (see SI). For
instance, five of the seven signals were within 0.4 ppm and two
are within 0.7 ppm, which is consistent with the known effects
associated with small amounts of D2O on carbohydrates.1a,3,21
In conclusion, the first total synthesis of the purported
structure of merremoside D was achieved in 22 longest linear
steps with a 3% overall yield. The route demonstrates the
power of a de novo asymmetric approach to a stereochemically
complex (21 stereocenters) oligosaccharide natural product.
The approach provided a sufficient quantity of material (29
mg) for both structural and biological evaluation, enabling the
screening against an array of organisms. In addition, the
approach exposes some practical limitations of the use of atom-
less protecting groups (i.e., C-2/3 alkene) with traditional
glycosylation technology, in contrast to the Pd-catalyzed
glycosylation. Detailed NMR analysis was used to confirm the
rhamno-pyranosides 23a/b in four steps (Pd-catalyzed
glycosylation, Luche reduction, Upjohn dihydroxylation, and
acetonation).17,18 A subsequent Pd-catalyzed glycosylation with
pyranone 5 followed by acetonide removal gave syn-diol 4/24.
This set the platform for the regioselective introduction of the
required C-3 iba-ester, which was accomplished via a tin-
mediated esterification with isobutyryl chloride (ibaCl).19 The
C-2 hydroxyl of 25a/b was then protected as chloroacetic ester.
Subsequent Luche reduction/esterification gave allylic esters
(26a/b). At this stage disaccharide 26b was converted into our
desired disaccharide donor 28 with the key C-2/C-3 double
bond (DDQ; Cl3CCN/NaH). Unfortunately we were not able
to find a promoter for glycosylation that did not cleave the
allylic glycosidic bond in 28. Thus suggesting a limit to the use
of C-2/3-alkenes in traditional glycosylations.
To overcome this problem, glycosyl donor 3 was synthesized
from 26a by functionalizing the double bond and anomeric
activation (OsO4/NMO; 2,2-DMP/H+; Pd/C, H2; Cl3CCN/
NaH). As expected, the anomeric disaccharide bond in 3 was
stable under Schmidt’s glycosylation conditions.20 Thus
exposing a 2:1 mixture of disaccharide glycosyl donor 3 and
disaccharide glycosyl acceptor 9 to 12% TMSOTf (CF3SO3Si-
(CH3)3) afforded tetrasaccharide 2 in 71% yield with complete
α-selectivity via the anchimeric assistance of the C-2
chloroacetate (Scheme 6). Deprotection of the tetrasaccharide
2 began with the removal of both acetonide groups to furnish
tetraol 29. Finally the two chloroacetates in 29 were removed
to provide merremoside D (1) with thiourea without any
deleterious effects to the macrolactone and the ester
substituents. While the physio-chemical properties (specific
rotation, melting point, and HRMS) of the synthetic 1 match
those reported for the natural material, unfortunately the
C
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structural identity of the synthetic material, which was
consistent with the data reported for the natural 1. However,
1
the lack of complete and reliable H and 13C NMR data
precludes a conclusive confirmation of the structural assign-
ment for merremoside D. Further efforts to elucidate the full
biological structure−activity relationships of the merremoside
family of natural products are ongoing.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and spectral data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226.
(15) VanRheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett. 1976,
17, 1973.
AUTHOR INFORMATION
■
Corresponding Author
(16) Corey, E. J.; Nicolaou, K. C. J. Am. Chem. Soc. 1974, 96, 5614.
(17) (a) Hinds, J. W.; McKenna, S. B.; Sharif, E. U.; Wang, H. L.;
Akhmedov, N. G.; O’Doherty, G. A. ChemMedChem 2013, 8, 63.
(b) Shan, M.; Sharif, E. U.; O’Doherty, G. A. Angew. Chem., Int. Ed.
2010, 49, 9492. (c) Sharif, E. U.; O’Doherty, G. A. Eur. J. Org. Chem.
2012, 2095.
(18) Bedini, E.; Parrilli, M.; Unverzagt, C. Tetrahedron Lett. 2002, 43,
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(19) David, S.; Hanessian, S. Tetrahedron 1985, 41, 663.
(20) Schmidt, R. R.; Michel, J. Angew. Chem., Int. Ed. Engl. 1980, 19,
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*E-mail: G.ODoherty@neu.edu.
Author Contributions
∥E.U.S. and H.L.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the NIH (GM088839) and NSF (CHE-
0749451) for their generous support of our research program.
(21) (a) Nagano, T.; Pospísil, J.; Chollet, G.; Schulthoff, S.;
Hickmann, V.; Moulin, E.; Herrmann, J.; Muller, R.; Furstner, A.
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Chem.Eur. J. 2009, 15, 9697. (b) Molinaro, A.; De Castro, C.;
Lanzetta, R.; Manzo, E.; Parrilli, M. J. Am. Chem. Soc. 2001, 123,
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