Stereoselective assembly of complex oligosaccharides using anomeric sulfonium ions as glycosyl donors

Article abstract of DOI:10.1021/ja3018187

The development of selectively protected monosaccharide building blocks that can reliably be glycosylated with a wide variety of acceptors is expected to make oligosaccharide synthesis a more routine operation. In particular, there is an urgent need for the development of modular building blocks that can readily be converted into glycosyl donors for glycosylations that give reliably high 1,2-cis-anomeric selectivity. We report here that 1,2-oxathiane ethers are stable under acidic, basic, and reductive conditions making it possible to conduct a wide range of protecting group manipulations and install selectively removable protecting groups such as levulinoyl (Lev) ester, fluorenylmethyloxy (Fmoc)- and allyloxy (Alloc)-carbonates, and 2-methyl naphthyl ethers (Nap). The 1,2-oxathiane ethers could easily be converted into bicyclic anomeric sulfonium ions by oxidization to sulfoxides and arylated with 1,3,5-trimethoxybenzene. The resulting sulfonium ions gave high 1,2-cis-anomeric selectivity when glycosylated with a wide variety of glycosyl acceptors including properly protected amino acids, primary and secondary sugar alcohols and partially protected thioglycosides. The selective protected 1,2-oxathianes were successfully employed in the preparation of a branched glucoside derived from a glycogen-like polysaccharide isolated form the fungus Pseudallescheria boydii, which is involved in fungal phagocytosis and activation of innate immune responses. The compound was assembled by a latent-active glycosylation strategy in which an oxathiane was employed as an acceptor in a glycosylation with a sulfoxide donor. The product of such a glycosylation was oxidized to a sulfoxide for a subsequent glycosylation. The use of Nap and Fmoc as temporary protecting groups made it possible to install branching points.

Full text of DOI:10.1021/ja3018187

Article
pubs.acs.org/JACS
Stereoselective Assembly of Complex Oligosaccharides Using
Anomeric Sulfonium Ions as Glycosyl Donors
Tao Fang,†,‡ Kai-For Mo,† and Geert-Jan Boons*,†,‡
‡
†Complex Carbohydrate Research Center and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United
States
S
* Supporting Information
ABSTRACT: The development of selectively protected
monosaccharide building blocks that can reliably be glycosy-
lated with a wide variety of acceptors is expected to make
oligosaccharide synthesis a more routine operation. In
particular, there is an urgent need for the development of
modular building blocks that can readily be converted into
glycosyl donors for glycosylations that give reliably high 1,2-cis-
anomeric selectivity. We report here that 1,2-oxathiane ethers
are stable under acidic, basic, and reductive conditions making
it possible to conduct a wide range of protecting group manipulations and install selectively removable protecting groups such as
levulinoyl (Lev) ester, fluorenylmethyloxy (Fmoc)- and allyloxy (Alloc)-carbonates, and 2-methyl naphthyl ethers (Nap). The
1,2-oxathiane ethers could easily be converted into bicyclic anomeric sulfonium ions by oxidization to sulfoxides and arylated
with 1,3,5-trimethoxybenzene. The resulting sulfonium ions gave high 1,2-cis-anomeric selectivity when glycosylated with a wide
variety of glycosyl acceptors including properly protected amino acids, primary and secondary sugar alcohols and partially
protected thioglycosides. The selective protected 1,2-oxathianes were successfully employed in the preparation of a branched
glucoside derived from a glycogen-like polysaccharide isolated form the fungus Pseudallescheria boydii, which is involved in fungal
phagocytosis and activation of innate immune responses. The compound was assembled by a latent-active glycosylation strategy
in which an oxathiane was employed as an acceptor in a glycosylation with a sulfoxide donor. The product of such a glycosylation
was oxidized to a sulfoxide for a subsequent glycosylation. The use of Nap and Fmoc as temporary protecting groups made it
possible to install branching points.
To speed up the process of oligosaccharide synthesis, efforts
are underway to identify limited numbers of monosaccharide
building blocks that can repeatedly be employed for the
synthesis of a wide range of target structures.7 Key features of
such building blocks include modification at key positions with
selectively removable protecting groups and providing reliably
glycoside products in high yield with excellent anomeric
control. In respect to the latter, 1,2-trans-glycosides can reliably
be introduced by employing a glycosyl donor modified at C-2
with a protecting group that can perform neighboring group
participation during glycosylation. On the other hand, conven-
tional methods to introduce 1,2-cis-glycosides require glycosyl
donors having a nonassisting functionality at C-2, and in these
cases, reaction conditions such as solvent, temperature, and
promoter as well as constitution of the glycosyl donor and
acceptor (e.g., type of saccharide, leaving group at the anomeric
center, protection, and substitution pattern) determine
anomeric selectivity.8 1,2-cis-Glycosylations give often mixtures
of anomers, which requires time-consuming purification
protocols resulting in loss of material, as well as limits the
use of one-pot multistep glycosylations9 and automated
INTRODUCTION
■
Complex carbohydrates are involved in a wide range of
biological processes such as cell−cell recognition, fertilization,
embryogenesis, neuronal development, hormone activities, the
proliferation of cells and their organization into specific tissues,
viral and bacterial infection, and tumor cell metastasis.1
Furthermore, carbohydrates are capable of inducing a
protective antibody response, which is a major contributor to
the survival of an organism during infection.2 Oligosaccharides
have also been found to control the development and defense
mechanisms of plants.3 The increased appreciation of the role
of carbohydrates in the biological and pharmaceutical sciences
has resulted in a revival of interest in carbohydrate chemistry.4
A major obstacle to advances in glycobiology and
glycomedicine is the lack of pure and structurally well-defined
carbohydrates and glycoconjugates. These compounds are
often found in low concentrations and in microheterogeneous
forms, greatly complicating their isolation and characterization.
In many cases, well-defined oligosaccharides can only be
obtained by chemical or enzymatic synthesis.5 Unfortunately,
no general method is available for the preparation of complex
carbohydrates of biological importance.6 As a result the
chemical synthesis of each target compound is a challenging
and time-consuming endeavor.
Received: March 2, 2012
Published: April 4, 2012
© 2012 American Chemical Society
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polymer-supported synthesis.10 Recent advances in anomeric
control include the use of protecting groups that sterically
shield the β-face of galactosyl donors11 or locking a glycosyl
donor in a conformation that allows nucleophilic attack from
only one face of an anomeric oxacarbenium ion.12
We have introduced a stereoselective glycosylation approach
based on neighboring group participation by a (S)-phenyl-
thiomethylbenzyl moiety at C-2 of a glycosyl donor (Scheme
1a).13−15 Upon formation of an oxacarbenium ion, the
traditional glycosylations or alkylation of thioglycosides can also
lead to the formation of anomeric sulfonium ions and enhance
α-anomeric selectivity (Scheme 1c).21,22
To advance auxiliary mediated glycosylations for building
block based oligosaccharide synthesis, it is critical to establish
convenient procedures for the preparation of selective
protected anomeric sulfonium ions, which can be employed
in glycosylations that allow rapid assembly of complex branched
compounds. We report here that 1,2-oxathiane ethers are stable
under acidic, basic, and reductive conditions making it possible
to conduct a wide variety of protecting group manipulations to
give panels of selectively protected compounds. The resulting
1,2-oxathiane ethers can easily be converted into bicyclic
anomeric sulfonium ions by oxidization to sulfoxides, followed
by arylation with 1,3,5-trimethoxybenzene. It is shown that the
latter compounds are appropriate glycosyl donors for highly
selective 1,2-cis-glycosylations provided that they are sufficiently
deactivated with electron withdrawing protecting groups. The
selective protected 1,2-oxathianes were successfully employed
for the preparation of a branched glucoside derived from
Pseudallescheria boydii,23 which is involved in fungal phag-
ocytosis and Toll-like receptor activation.
Scheme 1. Sulfonium-Ion Promoted 1,2-cis-Glycosylations
RESULTS AND DISCUSSION
■
Synthesis and Glycosylations of Oxathiane. We have
observed that the (S)-(phenylthiomethyl)benzyl ethers are
sensitive to moderately strong acidic conditions complicating a
number of important protecting group manipulations. We
envisaged that this problem could be addressed by using 1,2-
oxathiane ethers as precursors of anomeric sulfonium ions. 1,2-
Oxathiane ethers were expected to be stable under acidic
conditions because their ring structure prevents the sulfur atom
of participating in acid catalyzed cleavage of the benzylic ether
linkage, and thus should be compatible with a wide variety of
protecting group manipulations. These compounds can,
however, be readily converted into an anomeric sulfonium
ions by oxidative arylation, for stereoselective 1,2-cis-glyco-
sylations.15,20
a) Neighboring group participation by C-2 (S)-auxiliary leading to 1,2-
cis-glycosides; b) β-sulfonium ion formation from precyclized
oxathiane ketal. In these reactions, hydrolysis of the C-2 auxiliary is
a major side product; c) Sulfonium ion formation in the presence of a
thioether.
nucleophilic thiophenyl moiety of the C-2 functionality
participates leading to the formation of an intermediate
sulfonium ion. The formation of the trans-decalin stereoisomer
is strongly favored because of the absence of unfavorable
gauche interactions. In addition, the alternative cis-decalin
system places the phenyl-substituent in an axial position
inducing further unfavorable steric interactions. Displacement
of the equatorial anomeric sulfonium ion by a sugar alcohol
leads to the formation of a 1,2-cis-glycoside. We have shown
that the (S)-(phenylthiomethyl)benzyl moiety can readily be
introduced by reaction of a sugar alcohol with (S)-
(phenylthiomethyl)benzyl acetate in the presence of BF3−
OEt2 and removed by conversion into an acetyl ester by
treatment with BF3−OEt2 in acetic anhydride. The attractive-
ness of chiral auxiliary mediated glycosylations has been shown
by solid phase synthesis of several branched pentasaccharides
having only 1,2-cis-glycosidic linkages.16 The proposed
participation mechanism is supported by low temperature
NMR experiments, which unambiguously identified a β-
substituted sulfonium ion as a reaction intermediate.
Furthermore, the displacement mechanism is supported by
the observation that glycosylations with donors having a C-2
auxiliary with incorrect stereochemistry {(R)-(phenyl-
thiomethyl)benzyl} give mixtures of anomers.
To explore the compatibility of 1,2-oxathiane ethers with a
range of protecting group manipulations, compounds 9-11 and
13-16 were prepared which differ in the pattern of ether and
ester protecting groups (Scheme 2). Furthermore, several of
these derivatives are protected with Alloc, Lev or Fmoc
protecting groups, which can be selectively removed under mild
conditions providing opportunities for further functionalization.
The selectively protected 1,2-oxathianes can be oxidized to the
corresponding sulfoxides 17-23, which can then be used for the
generation of sulfonium ions for subsequent glycosylations.
Thus, oxathiane 2 was prepared on large scale by a one-pot,
two-step reaction involving treatment of 1 with TMS2O in the
presence of TMSOTf to give an intermediate trimethylsilyl
acetal, which was reduced by the addition of Et3SiH. The 4,6-
diol of 2 was protected as a benzylidene acetal by treatment
with (dimethoxymethyl)benzene in the presence of camphor-
sulfonic acid (CSA) in DMF under reduced pressure to give 3
in a yield of 87%. The C-3 hydroxyl of compound 3 was
protected as an acetyl- and levulinoyl- (Lev) ester,24
fluorenylmethyloxy- (Fmoc)25 and allyloxy- (Alloc)26 carbo-
nate, and benzyl ether using standard conditions to give fully
protected 4, 5, 6, and 7, respectively. The benzylidene acetal of
4 was reductively opened using Et3SiH and triflic acid (TfOH)
in DCM at −78 °C27 to give compound 8 having a C-4
hydroxyl which was acetylated with Ac2O in pyridine,
Several other studies have shown the usefulness of anomeric
sulfonium ions for stereoselective glycosylations. In this respect,
a number of donors having an achiral thioether at C-2 have
been examined and some of these derivatives gave remarkably
good α-anomeric selectivity.13,17 Another interesting approach
for forming bicyclic anomeric sulfonium ions involves arylation
of 1,2-oxathiane ketals, which can easily be prepared from a
thioglycoside (Scheme 1b).18−20 The addition of a thioether to
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a
Scheme 2. Synthesis of 1,4-Oxathiane Protected Donors
Table 1. Stereoselective Glycosylations between Donor 17
and Various Acceptors
a
Reagents and conditions: a) TMS2O, TMSOTf, 0 °C, 30 min, then
Et3SiH, 3 h (74%); b) PhCH(OMe)2, CSA, reduced pressure, DMF,
50 °C, 18 h (87%); c) Ac2O, Py; d) Levulinic acid, DCC, DMAP,
DCM (79%); e) AllocCl, TMEDA, DMAP, DCM (87%); f) BnBr,
NaH, DMF (79%); g) Et3SiH, TfOH, DCM, −78 °C; h) Et3SiH,
PhBCl2, −78 °C; i) Ac2O, Py(9: 85%, g, i 2 steps; 13: 78%, h, i 2 steps;
14: 79%, g, i 2 steps; 15: 92%, g, i 2 steps; 16: 84%, l, i 2 steps); j)
Ag2O, BnBr (10: 55%, g, j 2 steps); k) FmocCl, Py/DCM = 1/1 (v/v)
(11: 66%, g, k 2 steps); l) EtSH, TsOH, DCM; m) m-CPBA, DCM,
−78 °C (17: 92%; 18: 96%; 19: 98%; 20: 93%; 21: 98%; 22: 82%; 23:
96%).
a
1,3,5-Trimethoxybenzene, Tf2O, DTBMP, molecular sieves 4 Å, −10
°C, 30 min, then add acceptor, −40 °C to room temperature, 16 h.
b
c
Isolated yields of the α/β mixture of disaccharide products. The α/β
1
ratios were determined by the integration of key signals in the H
NMR spectra of the disaccharide products after purification by LH-20
size exclusion chromatography.
the presence of 1,3,5-trimethoxybenzene in DCM at −10 °C.
After completion of the electrophilic aromatic substitution and
formation of the intermediate sulfonium ion, alcohols 24−28
were added and after a reaction time of 16 h at room
temperature, the disaccharides 29−33 were isolated by size
exclusion column chromatography and anomeric selectivities
determined by careful analysis of 1H NMR spectra. Each
glycosylation proceeded with exceptional high α-anomeric
selectivity, however, in the case of acceptors 24, 25, and 27 a
trace amount of β-anomer was detected. The use of primary
and secondary sugar alcohols gave the corresponding
disaccharides in good to excellent yields. The glycosylation
protocol is also compatible with the use of appropriately
protected amino acids and for example glycosylated threonine
derivative 31 could be prepared in high yield as only the α-
anomer. Furthermore, it was also found that thioglycosides can
be employed as glycosyl acceptor and for example the use of 27
and 28 led to clean formation of disaccharides 32 and 33,
respectively. The latter type of glycosylation is attractive
because it is to be expected that the thioglycosyl products can
employed as glycosyl donors in subsequent glycosylations using
an appropriate thiophilic reagent thereby offer a rapid strategy
for oligosaccharide assembly.
benzylated under neutral conditions using benzyl bromide and
Ag2O in DMF, or treated with FmocCl in a mixture of pyridine
and CH2Cl2 to provide selectively protected oxathianes 9, 10,
and 11, respectively. Alternatively, the benzylidene acetal of 4
could be opened by treatment with Et3SiH and PhBCl2 at −78
°C27 to give 12 having a C-6 hydroxyl, which was acetylated to
give derivative 13. The benzylidene acetals of compounds 5 and
6 could also be selectively opened to give derivatives having a
C-4 alcohol, which were acetylated using standard conditions to
provide compounds 14 and 15, respectively. Finally, the
benzylidene acetal of 7 was removed by treatment with p-
toluenesulfonic acid in the presence of ethanethiol to give a
diol, which was acetylated to provide derivative 16. The
selectively protected oxathianes 9−11 and 13−16 were
oxidized by m-CPBA at −78 °C to give sulfoxides 17−23 as
mixtures of diastereoisomers. The successful preparation of
these compounds demonstrates that oxathianes can be
subjected to a wide variety of protecting group manipulation
and withstand acidic, basic and reductive conditions.
Having the sulfoxides 17−23 at hand, attention was focused
on their conversion into anomeric sulfonium ions for
subsequent glycosylations. In this respect, activation of the
sulfoxides by triflic anhydride in the presence of trimethox-
ybenzene will result in arylation of sulfur to give an
intermediate sulfonium ion.15,18 In the first instance, oxathiane
17 was explored as a glycosyl donor for glycosylations with a
variety of different glycosyl acceptors (Table 1). Thus, 17 was
activated with trifluoromethanesulfonic anhydride (Tf2O) in
Next, we explored whether protecting groups commonly
employed in oligosaccharide synthesis are compatible with the
glycosylation protocol. Thus, glycosyl donors 17, 18, 20−23,
34, and 35 were activated by triflic anhydride in the presence of
1,3,5-trimethoxybenzene to give intermediate sulfonium ions,
which were glycosylated with acceptors 36 and 37. As can be
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seen in Table 2, the expected disaccharides 38−54 were
the sulfonium ion was established by an HMBC experiment,
which allows the determination of three-bond proton-carbon
couplings. In particular, a strong correlation was observed
between C-1 and H8eq, confirming the presence of a trans-
decalin system. Furthermore, the smaller geminal-coupling
constant between H8eq and H8ax (12 Hz) indicates that the
trimethoxyphenyl substituent of sulfur adopts an equatorial
orientation. Comparing chemical shifts of the trimethoxyphenyl
(H8eq 3.98, H8ax 4.28 ppm) and phenyl (H8eq 4.32, H8ax
3.66 ppm) substituted sulfonium ion indicates that their
aromatic rings adopt different conformations and in particular
deshielding of H8ax of the trimethoxylbenzene substituted
system indicates that the trimethoxybenzene group is
perpendicular to the plane of the sugar ring (see SI for further
details).
Interestingly, sulfonium 55 was stable at room temperature
for 9 h, while subsequent incubation at 45 °C for 5 min resulted
in complete decomposition. It was also noticed that the excess
of trimethoxylbenzene was further consumed after complete
formation of the sulfonium ion, and was no longer present after
an incubation time of 9 h.
A number of additional sulfonium ions (56−61) were
generated and their structure and reactivity studied by NMR.
Thus, compounds 55−58 were prepared by arylation of the
corresponding sulfoxides and differ in the pattern of protecting
groups at C-3, C-4, and C-6. Compounds 59−61 were
prepared by methylation of the corresponding sulfoxides with
methyl triflate and also differ in hydroxyl protection. Finally,
acyclic sulfonium ion 62 was formed by methylation of the
corresponding methyl thioglycoside with methyl triflate.
As expected sulfonium ions 55−58 are stable at room
temperature but readily react with alcohols to give mainly or
exclusively α-glucosides. Interestingly, exposure of sulfonium
ion 59 to an excess of methanol did not lead to glucoside
formation and this compound was unaltered after a reaction
time of 24 h. Benzylated sulfonium ion 61 was slightly more
reactive and exposure to an excess of 24 gave the corresponding
glucoside in a low yield of 21% as a mixture of anomers. These
results clearly demonstrate that the nature of the sulfonium ion
is a major determinant of anomeric reactivity. It was also found
that the cyclic nature of sulfonium ions influences anomeric
reactivity and for example within 15 min acyclic sulfonium ion
62 reacted with excess methanol to give a methyl glycoside
whereas the corresponding cyclic sulfonium ion 59 is unreactive
under these conditions.22
It is well-known that carbon chemical shifts are mainly
determined by local electronic environment.29 Interestingly, it
was observed that the chemical shift of C-1 of the arylated and
alkylation sulfonium ions differ by approximately 2 ppm
indicating a lower electronic density at C-1 of alkylated
sulfonium ion, which surprisingly did not result a higher
reactivity toward alcohols. Probably, the higher reactivity of the
arylated sulfonium corresponds to its better leaving group
ability. In this respect, a weaker base constitutes in general a
better leaving group. Furthermore, it has been established that
the relative stabilities of sulfonium ions corresponds with the
relative basicities of the corresponding leaving group.30 The fact
that thioanisol is a weaker base than dimethyl sulfide30,31 would
indicate that arylated sulfonium ions are less stable and provide
a better leaving. This is in agreement with the observations
described above.
isolated in good to excellent yields. Furthermore, glycosyl
Table 2. Protecting Group Pattern Effect on Stereoselectivity
b
yield
(%)
α/β
c
entry donor R1
R2
R3
Ac
ROH product
ratio
1
34
34
34
20
20
17
17
22
22
21
21
23
23
18
18
35
35
Ac
Ac
Ac
Ac
Ac
Bn
Bn
Bn
Bn
Bn
Bn
Ac
Ac
Bn
Bn
Bn
Bn
Ac
Ac
Ac
Bn
Bn
Ac
Ac
Ac
Ac
Ac
Ac
Ac
Ac
Bn
Bn
Bn
Bn
36
24
37
36
37
36
37
36
24
36
37
36
37
36
37
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
80
59
48
84
59
94
67
67
78
85
64
98
46
95
66
98
57
α only
α only
α only
α only
>15:1
α only
>25:1
α only
>15:1
>15:1
>15:1
α only
7:1
2
Ac
3
Ac
4
Ac
5
Ac
6
Ac
7
Ac
8
Alloc
Alloc
Lev
Lev
Bn
9
10
11
12
13
14
15
16
17
Bn
Ac
>15:1
10:1
Ac
Bn
>15:1
1.5:1
Bn
a
1,3,5-Trimethoxybenzene, Tf2O, DTBMP, molecular sieves 4 Å, −10
b
°C, 30 min, then add acceptor, −40 °C to r.t., 16 h. Isolated yields of
c
the α/β mixture of disaccharide products. The α/β ratios were
determined by the integration of key signals in the 1H NMR spectra of
the disaccharide products after purification by LH-20 size exclusion
chromatography.
donors modified with electron-withdrawing protecting groups
at C-3, -4, and -6 gave the corresponding disaccharides as only
the α-anomer (entries 1−4). Glycosyl donors that had an ether
function at C-4 or C-6 (entries 5−11) gave exceptional high α-
anomeric selectivities (α:β > 15:1). Highly reactive donors
having multiple ether type protecting groups gave in some case
disaccharides with modest anomeric selectivities (entries 13 and
17). These observations are in agreement with previous
finding15 that highly reactive anomeric sulfonium ions can
react through an oxacarbenium ion resulting in loss of α-
anomeric selectivity.
Characterization of Sulfonium Ions by NMR. To
ascertain that glycosylations of oxathianes proceed through a
sulfonium ion intermediates, compound 17, trimethoxyben-
zene, and DTBMP were dissolved in CDCl3, activated with
1
triflic anhydride and H, gCOSY, gHSQC, and HMBC spectra
were recorded (Figure 1). As expected, the chemical shift of H-
1 of the newly formed product shifted downfield (δ = 4.31 and
δ = 4.15 to δ = 5.60) and exhibited J1,2 = 9 Hz, which is
consistent with a β-anomeric configuration. Full assignment of
proton and carbon signals could be made by gCOSY and
gHSQC. Careful analysis of proton coupling constants
indicated that no conformational distortion of the saccharide
ring had occurred. The presence of a carbon−sulfur linkage of
Chemical Synthesis of an Immunoactive Glycogen-
Like Component. Having established a range of selective
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Figure 1. NMR study of sulfonium ions. (a) Thermostability of sulfonium ion 55; (b) HMBC spectrum of sulfonium ion 55; (c) comparison of
electronic properties of different sulfonium ions by NMR chemical shift (all chemical shifts were referred to TMS).
Scheme 3. Latent-Active Glycosylation Strategy
a
The α/β ratios were determined by the integration of key signals in the 1H NMR spectra of the disaccharide products after purification by LH-20
size exclusion chromatography.
protected oxathiane building blocks suitable for selective 1,2-cis-
glycosylations, attention was focused on the preparation of
tetraglucoside 79, which is derived from the cell wall
polysaccharide of the fungus Pseudallescheria boydii.23 This
fungus is found in soil and polluted water and can cause
infections in immunocompromised and immunocompetent
hosts. Recent structural studies have shown that the cell wall of
P. boydii contains a glycogen-like component that is composed
of a α(1−4)-linked glucopyranoside backbone modified at C-6
with α-Glcp-moieties.32 A soluble form of the polysaccharide
could inhibit phagocytosis in a dose-dependent manner, and
furthermore enzymatic degradation of the glycogen-like
component of P. boydii could reduce the phagocytic index,
demonstrating that it plays a key role in phagocytosis. It has
also been shown that the α-glucan can induce cytokine
secretion in a TLR2-dependent manner. The minimal structural
motif that can induce these properties is difficult to establish
due to structural heterogeneity of the polysaccharide. However,
it is to be expected that this problem can be addressed by
chemically synthesizing a library of glucans differing in C-6
branching patterns.
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α-Glucans represent a considerable synthetic challenge due
to the presence of multiple 1,2-cis-glycosidic linkages, the low
reactivity of C-4 hydroxyls of glucosyl acceptors and the
branched nature of the compound. Therefore, this component
offered an exciting opportunity to examine the usefulness of
oxathianes for the preparation of complex and biological
important compounds.
We envisaged that α-glucans such as 79 can be assembled by
a latent-active glycosylation strategy33 in which a sulfoxide
donor is coupled with an oxathiane acceptor to give a product
that can be oxidized to a sulfoxide for further glycosylations
(Schemes 3 and 4). To establish the feasibility of such a latent-
orthogonal and will allow glucosylations at these positions to
give after deprotection the expected tetrasaccharide 79.
First, the C-2 auxiliary of 65 was converted into an acetyl
ester to give 67 using a standard procedure. This additional
step was required to avoid oxidation of the (2,4,6-
trimethoxyphenyl)sulfane moiety of the nonreducing glucoside
during the subsequent oxidation step to convert the oxathiane
moiety into a sulfoxides. As expected, compound 67 could be
cleanly oxidized to the corresponding sulfoxides, which was
activated with Tf2O/1,3,5-trimethoxybenzene to give an
anomeric sulfonium ion that was glycosylated with alcohol 69
to provide spacer-containing 70 as only the α-anomer. The C-2
auxiliary of 70 was converted into an acetyl ester and the Fmoc
group of the resulting compound 71 was cleaved by treatment
with N-methyl-2-pyrrolidone (NMP) in DMF to give glucosyl
acceptor 72. Next, glycosyl acceptor 72 was coupled with the
sulfonium ion derived from sulfoxide 17 using standard
conditions to give trisaccharide 73 as mainly the α-glucoside.
The trace amount of unwanted β-anomer could easily be
removed by silica gel column chromatography. The C-2
auxilairy of 73 was converted into an acetyl ester by standard
procedures to give compound 74, which was converted into
glycosyl acceptor 75 by oxidative removal of the Nap ether
using 5,6-dicyano-1,4-benzoquinone (DDQ) in a mixture of
DCM and water. A Tf2O/1,3,5-trimethoxybenzene mediated
glycosylation of 17 with 75 gave tetrasaccharide 77 as an 8/1
mixture of α/β anomers. In order to improve the anomeric
selectivity, the glycosylation was repeated with glycosyl donor
20 having an acetyl esters at C-3 and C-6 and a benzyl ether at
C-4 and gratifyingly the use of this compound gave
tetrasaccharide 76 in a yield of 70% as only the α-anomer.
Deprotection could easily be accomplished by a three-step
procedure involving treatment with TFA in DCM to remove
the C-2 auxiliary, saponification of the acetyl esters with sodium
methoxide in methanol and catalytic hydrogenolysis of the
benzyl ether and the reduction of azide using Pd(OH)2/C. The
structural identity of α-glucan 77 was confirmed by homo- and
heteronuclear two-dimensional NMR (gCOSY and gHSQC)
experiments and in particular the chemical shifts and coupling
constants of the anomeric protons confirmed (1→4)- and (1→
6)-linked α-glucosidic linkages.35
Scheme 4. Synthesis of the Glycogen-Like Glucan Isolated
a
from P. boyii
a
Reagents and conditions: a) i. 10% TFA in DCM, 0 °C, 1 h; ii. Ac2O,
CONCLUSIONS
■
Py (67: 89%, 2 steps; 71: 98%, two steps; 74: 76%, 2 steps); b) m-
CPBA, DCM, −78 °C (98%); c) general glycosylation condition (70:
86%, α only; 73: 65%, α/β > 15:1; 76: 70%, α only; 77: 73%, α/β =
8:1); d) 20% NMP in DMF, 30 min (91%); e) DDQ, H2O/DCM =
1:10 (92%); f) 10% TFA in DCM, 0 °C, 1 h (85%); g) i. NaOMe,
MeOH; ii. H2, Pd(OH)2/C, tBuOH/H2O/AcOH = 40:1:1 (83%, 2
steps).
Modular building blocks that can readily be converted into
glycosyl donors and acceptors have the potential to speedup the
process of oligosaccharide assembly. The potential of such a
strategy will, however, only be realized when building blocks
will be developed that reliably give high 1,2-cis-anomeric
selectivities. Previously, we introduced a stereoselective 1,2-cis-
glycosylation approach based on neighboring group partic-
ipation by a (S)-phenylthiomethylbenzyl moiety at C-2 of a
glycosyl donor to give a bicyclic β-sulfonium ion intermediate
that can readily be displaced by sugar alcohol leading to the
formation of 1,2-cis-glycosides. Although the auxiliary mediated
approach has many attractive features, the acid sensitivity of the
(S)-phenylthiomethylbenzyl moiety complicates certain pro-
tecting group manipulation, which in turn makes it difficult to
develop robust modular building blocks. We describe here that
monosaccharides protected as 1,2-oxathiane ethers are stable to
commonly employed protecting groups manipulations making
it possible to install a variety of selectively removable protecting
groups such as Lev ester, Fmoc and Alloc carbonates, and Nap
ether. Furthermore, the 1,2-oxathiane ether could easy be
active glycosylation strategy, sulfoxides 17, 19, and 20 were
treated with Tf2O in the presence of 1,3,5-trimethoxybenzene
to form an intermediate sulfonium ion, which was glycosylated
with oxathiane 63 to give disaccharide 64−66 as mainly α-
glucosides. These results demonstrate that oxathianes are stable
under the condition used for arylation of sulfoxides to give
sulfonium ion.
It was expected that disaccharide 65 would be a suitable
substrate for the preparation of target compound 79. In this
respect, the anomeric oxathiane can be oxidized to a sulfoxide
and then converted into a sulfonium ion for glycosylation with
a properly protected aminopropyl spacer. Furthermore, the
Nap34 ether at C-6 and Fmoc carbonate at C-4 are fully
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Journal of the American Chemical Society
Article
installed by a novel one-pot two-step procedure and it was
found that protection of the 1,2-diol facilitated modification of
the C-3, C-4, and C-6 alcohols. The resulting 1,2-oxathianes
could be employed as glycosyl donors for highly selective 1,2-
cis-glycosylation by oxidation to sulfoxides followed by arylation
with 1,3,5-trimethoxybenzene to give a bicyclic anomeric
sulfonium ion. The attractiveness of the new building blocks
has been demonstrated by the preparation of a biologically
important branched α-glucan, which was assembled by a latent-
active glycosylation strategy.
assignment and conformational analysis of diastereomeric
sulfonium ions, control glycosylations using donors without
C-2 auxiliary, copies of 1H and 13C NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
gjboons@ccrc.uga.edu
■
Notes
The authors declare no competing financial interest.
EXPERIMENTAL SECTION
Full experimental is available in Supporting Information. Compounds
1,18 24,36 25,37 26,38 27,39 34,15 35,15 and 3740 were prepared
following literature procedures.
ACKNOWLEDGMENTS
■
■
We thank Dr. John Glushka for assisting with NMR
experiments and Dr. Paul von Rague Schleyer for helpful
discussions. The research was supported by the National
Institute of General Medical Sciences (NIGMS) of the U.S.
National Institutes of Health (R01GM065248, G.-J.B.).
General Procedure for the Preparation of Sulfoxide Donors
17−23 from their Corresponding Oxathianes 9−11 and 13−
16. m-CPBA (≤77%, l.1 equiv) was dissolved in DCM and slowly
injected into a cooled (−78 °C) solution of oxathiane in DCM. The
mixture was stirred at −78 °C for 30 min, diluted with DCM (50 mL)
and then poured into 10% Na2S2O3 aqueous solution. The organic
layer was washed with saturated NaHCO3, dried (MgSO4), filtered,
and the filtrate was concentrated in vacuo. The residue was purified by
silica gel chromatography.
General Glycosylation Procedure for Oxathiane Donors with
Various Acceptors. A mixture of sulfoxide donor (1 equiv), 1,3,5-
trimethoxybenzene (1.5 equiv), 2,6-di-tert-butyl-4-methyl pyridine (2
equiv), and activated molecular sieves (4 Å) in DCM (adjusted donor
concentration to 0.15 M) was stirred for 1 h under an atmosphere of
argon. After cooling to −10 °C, trifluoromethanesulfonic anhydride
(1.1 equiv) was added. After 30 min, the reaction mixture was cooled
(−40 °C), and a solution of acceptor (0.8 equiv) in DCM (adjusted
donor concentration to 0.1 M) was added slowly. The temperature of
the reaction mixture was kept at −40 °C for another 60 min before
allowed to warm to room temperature. After 15 h, the reaction mixture
was diluted with DCM (10 mL), filtered, and the filtrate was
concentrated in vacuo. The residue was purified by silica gel column
chromatography or sephadex LH20 size exclusion chromatography
(DCM/MeOH = 1:1, 0.2 mL/min).
General Procedure for the Removal of C-2 Auxiliary.
Trifluoroacetic acid was added dropwise to a solution of glucoside
in DCM at 0 °C adjusting the final concentration to 10% (v/v). The
reaction mixture was stirred for 0.5−3 h until TLC indicated complete
consumption of starting material. The reaction mixture was diluted
with DCM and poured into saturated NaHCO3. The organic layer was
dried (MgSO4) and filtered, and the filtrate was concentrated in vacuo.
The residue was purified by silica gel column chromatography.
Procedures for NMR Study of Sulfonium ions. S-(2,3,5-
Trimethoxylphenyl)oxathianium triflate: a mixture of R/S sulfoxides
(10 μmol), 1,3,5-trimethoxybenzene (16 μmol), 2,6-di-tert-butyl-4-
methyl pyridine (21 μmol), and activated molecular sieves (4 Å,
pellets) in CDCl3 (1 mL) was shaken for 30 min under an atmosphere
of argon. Then 0.5 mL of the solution was transferred to a 5 mm NMR
tube and sealed. After cooling to 0 °C, a trifluoromethanesulfonic
anhydride stock solution (25 μL, 0.23 M in CDCl3) was added and
NMR spectra for 55−58 were recorded at room temperature. S-
Methyl-oxathianium triflate: Sulfide (10 μmol) was dissolved in dry
CDCl3 (1 mL), methyl triflate (0.1 mmol) was added under the
atmosphere of argon at room temperature. The reaction mixture was
stirred for 2−16 h and monitored by TLC. After the consumption of
sulfide, 0.5 mL of the reaction mixture was transferred to a 5 mm
NMR tube and sealed. NMR spectra for 59−61 were recorded at
room temperature.
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