Stereoselective β-Mannosylation with 2,6-Lactone-bridged Thiomannosyl Donor by Remote Acyl Group Participation

Article abstract of DOI:10.1002/asia.201801740

Stereoselective β-mannosylation has been recognized as one of the greatest challenges of carbohydrate chemistry. Herein, we described a practical method for stereoselective construction of β-mannosides by using a 2,6-lactone-bridged thiomannosyl donor through the remote acyl-group participation as well as the steric effect of O-4 substituent. The two effects are enabled through the conversion of a regular mannopyranosyl ⁴C₁ conformation into a 2,6-lactone bridged conformation. The lactone donor could be readily prepared in three steps on a gram scale and the β-mannosylation proceeded smoothly with high stereoselectivity for primary, secondary and tertiary alcohol acceptors. In addition, this strategy was successfully applied to the synthesis of a naturally occurring trisaccharide.

Full text of DOI:10.1002/asia.201801740

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: β-Stereoselective Mannosylation with a 2,6-Lactone-bridged
Thiomannosyl Donor by Remote Acyl Group Participation
Authors: Huanfang Xu, Long Chen, Qi Zhang, Yingle Feng, Yujia Zu,
and Yonghai Chai
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To be cited as: Chem. Asian J. 10.1002/asia.201801740
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
-Stereoselective Mannosylation with a 2,6-Lactone-bridged
Thiomannosyl Donor by Remote Acyl Group Participation
‡
‡
Huanfang Xu, Long Chen, Qi Zhang, Yingle Feng, Yujia Zu and Yonghai Chai*
Abstract: Stereoselective -mannosylation has been recognized as
one of the greatest challenges of carbohydrate chemistry. Herein,
we described a practical method for stereoselective construction of
switch the C-4 hydroxyl group, which is at the equatorial position
4
in a regular mannopyranosyl
1
C conformation, to the axial or
pseudo-axial position. Furthermore, the bulky substituent at O-4
position might sterically disfavour the -face attack by a
nucleophilic acceptor, resulting in higher -selectivity.

-mannosides by using a 2,6-lactone-bridged thiomannosyl donor
via the remote acyl group participation as well as the steric effect of
O-4 substituent. The two effects are enabled through the conversion
4
of a regular mannopyranosyl
1
C conformation into a 2,6-lactone
bridged conformation. The lactone donor could be readily prepared
in 3 steps on a gram scale and the -mannosylation proceeded
smoothly with high stereoselectivity for primary, secondary and
tertiary alcohol acceptors. In addition, this strategy was successfully
applied to the synthesis of a naturally occurring trisaccharide.
In recent decades, great efforts have been devoted to the
development of efficient and stereoselective glycosylation
methodologies due to the biological significance of
[
1]
glycoconjugates. However, the stereoselective construction of
-mannosides is still challenging since both the anomeric effect
and the steric effect of the C-2 axial substituent on the

[
2]
Scheme 1. Our strategy for -mannosylation using a 4-O-acyl 2,6-lactone-
bridged thiomannosyl lactone as donor: 1). Upon activation, thiomannoside
forms the oxocarbenium cation A, which is then attacked by an acceptor from
-face (-attack) or from -face (-attack). The -attack is disfavored due to
the steric repulsion between O-4 substituent and the nucleophilic acceptor; 2).
mannopyranosyl ring favor the formation of -mannosides. To
[
3]
overcome this obstacle, different strategies
have been
developed. The 4,6-O-benzylidene method, established by Crich
[4]
and co-workers, is one of the most powerful tools for the
-attack is blocked due to the formation of dioxocarbenium ions B via O-4 acyl
installation of -mannosides. The updated 4,6-O-benzylidene
group remote participation.
[
5]
[6]
versions and 4,6-O-silylene version have also been proven to
be effective for -mannosylation. Another impactful protocol for
the synthesis of -mannosides is the intramolecular aglycone
delivery (IAD) method, which was introduced by Hindsgaul and
Notably, at our final stage of manuscript preparation, Sasaki
and co-workers reported their novel method for -mannosylation
by using 2,6-lactone donors without acyl group in the O-4
position. The stereoselectivity is believed to arise mainly from
[
7]
[8]
co-works and further extensively studied by Stork, Ito and
[
9]
Ogawa et al. Recently, some innovative strategies have also
been introduced into the construction of -mannosyl linkages
including hydrogen-mediated aglycone delivery (HAD)
S 2(-like) displacement of the -nucleofuge at the anomeric
N
position with the glycosyl cation generated from the donor being
sterically -directing. In their case, the highest -stereoselectivity
was obtained when -trichloroacetimidates were used as donors.
But unfortunately, the synthetic yields of this kind of -
trichloroacetimidate donors were quite low (32% from the
hemiacetal precursor, 4-di-O-benzyl-D-mannopyranurono-2,6-
[
10]
method,
method
glycosyl-acceptor-derived boronic ester-promoted
[
11]
1
and
C
4
conformation-based neighboring-group
[
12]
participation (NGP) method. Although significant progress has
been made in the construction of β-mannoside, practical and
highly selective approaches are still in great demand.
In this context, we developed an efficient protocol for -
[14]
lactone; 5% from the starting material, allyl mannoside).
mannosylation
with
the
use
of
2,6-lactone-bridged
Our study commenced with the synthesis of various 2,6-
lactone-bridged mannoside donors bearing different O-4 acyl
group and C-1 leaving group. Trichloroacetimidates 5a and 5b
thiomannosides as donors predominantly via the O-4 remote
acyl group participation, which proved to play an important role
[
13]
in the glycosylation , as well as the steric effect of O-4
substitutes (Scheme 1). Considering the axial or pseudo-axial
hydroxy group at C-4 position, when acylated, could provide a
remote acyl participation, we employed a 2,6-lactone bridge to
were first chosen as the target donors (Scheme 2a). Using
[15]
Stahl's oxidative protocol
,
3-O-benzyl protected allyl
[14]
mannoside 1 was converted into lactone 2, in which the C-4
hydroxyl group locates at our expected axial or pseudo-axial
position. After acylation, followed by deprotection of allyl group,
hemiacetals 4a and 4b were prepared successfully. However,
the final trichloroacetimidation was in the grip of unacceptable
yields, and the desired donor 5a or 5b was only obtained in 13%
or 10%, respectively. The low yields may be attributed to two
reasons. First, during the reaction, considerable side-products
[
a]
H. Xu, L. Chen, Dr. Q. Zhang, Dr. Y. Feng, Y. Zu, Prof. Dr. Y. Chai
Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi
Normal University, 620 West Chang'an Avenue, Xi’an, Shaanxi,
710119 (China)
E-mail: ychai@snnu.edu.cn
6a or 6b was always formed although various reaction
‡
These authors contributed equally.
conditions were examined. Second, trichloroacetimidates
containing the 2,6-lactone moiety were found to be very fragile
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - An Asian Journal
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COMMUNICATION
(
>20:1)
and would partially decompose during workup and column
chromatography. Actually, when R is benzyl group, the similar
2
o
54%
(>20:1)
3
4
NIS/AgOTf
NIS/AgOTf
DCM
0 C/1 h
unsatisfied result was received (34%, from 4-Di-O-benzyl-D-
mannopyranurono-2,6-lactone, see Supporting Information, 5c
o
[
13]
Toluene
THF
0 C/4 h
NR
and 6c), which is consistent with data reported by Sasaki et al
.
o
To solve the problem, we then turned our effort to the synthesis
of donors 10a-h, in which the C-1 leaving group is ethylthio ether
5
6
NIS/AgOTf
NIS/AgOTf
0 C/4 h
Trace
Trace
o
CH
3
CN
0 C/4 h
(
Scheme 2b). Starting from the commercially available ethyl-1-
[
d]
o
68%
thio--D-mannopyranoside 7, several 2,6-lactone-bridged
thiomannosyl donors were easily prepared in practical total
yields on a gram scale by a 3-steps strategy. With the treatment
of Bu
7
8
9
NIS/AgOTf
NIS/AgOTf
NIS/AgClO
DCM
DCM
DCM
0 C/50 min
(>20:1)
[e]
o
88%
(>20:1)
0 C/40 min
n
2
SnO and benzyl bromide, 7 was first regioselectively
[
16]
benzylated
to give 3-O-benzylmannoside 8, which was then
[
e]
o
74%
(>20:1)
[
15]
4
0 C/2 h
oxidized to lactone 9 by using Stahl's protocol . The C-1
hydroxyl group in 9 was readily to be modified with different
protecting agents, affording the desired 2,6-lactone-bridged
thiomannosyl donors bearing various O-4 substituents.
[
a] Reaction of 10a (1 equiv), 11a (1.5 equiv), NIS (1 equiv), catalyst (0.2
-1
equiv) and 4Å MS in
a mentioned solvent (c = 0.1 mol·L ) under Ar
1
atmosphere. [b] Yield of the isolated product. [c] Ratios determined by H NMR
spectroscopy of the crude reaction mixture. [d] Reaction of 10a (1.2 equiv),
1
1
1a (1 equiv), NIS (1.2 equiv), AgOTf (0.3 equiv) and 4Å MS. [e] Reaction of
0a (1.5 equiv), 11a (1 equivl), NIS (1.5 equiv), Ag(I) salt (0.5 equiv) and 4Å
MS.
Figure 1. NOE correlation between H
disaccharide -12aa.
1 3
-H and X-ray crystal structure of the
The efficiency and stereoselectivity of our synthesized 2,6-
lactone-bridged thiomannosyl donors 10a-g were then evaluated
using 11b as acceptor under the above-mentioned optimized
condition (Table 2). The glycosylation of 4-O-TBS donor 10a
with 11b gave the corresponding mannoside with a 2:1 ratio of
Scheme 2. Synthesis of donnors 5a-b and 10a-h.

-anomer (Table 2, entry 1). Enhanced -selectivity was
obtained when bulkier donor 10b bearing a TBDPS group in the
O-4 position was employed ( = 4:1, Table 2, entry 2), which
indicates that increasing the -face (namely O-4 positional)
steric hindrance of the donor may sterically disfavor -face
nucleophilic attack and thus resulted in higher -selectivity. To
our delight, with the use of O-4 acyl protected donors, such as
With the donors in hand, we explored the -mannosylation
using donor 10a as model reaction. As shown in Table 1, the
glycosylation of 10a with acceptor 11a led to the expected -
mannoside product in excellent yield with high stereoselectivity
under the optimized reaction condition (Table 1, entry 8). The-
anomeric configuration of the product was determined by the
10d-g, the -selectivity could be further improved ( 7-10:1 vs
NOE correlation between H
1 3
and H and further unequivocally
2-4:1,Table 2, entries 4-7 vs 1-3). This outcome implies that the
confirmed by X-ray single crystal diffraction analysis (Figure 1).
remote participation of 4-O-acyl group may play a significant -
directing role and contributes extra -selectivity in the
mannosylation. Furthermore, when bulky Piv group was
introduced into the 4-O position of the donor, the best result was
achieved ( = 10:1, Table 2, entry 7), which provides evidence
that both the remote participation and the steric effect of O-4
substitute contribute to the -selectivity of mannosylation.
[
a]
Table 1 Optimization of the reaction conditions with 10a and 11a
[
b]
Yield
[
a]
Entry
Promoter
Solvent
Temp./Time
[c]
Table 2 Optimization of the reaction conditions with 10a and 11a
(/)
o
32%
1
2
NIS/TfOH
DCM
DCM
0 C/13 h
(>20:1)
o
NIS/TMSOTf
0 C/13 h
40%
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COMMUNICATION
4
5
6
7
8
11e
11f
12ge
12gf
12gg
12gh
12gi
72
81
82
78
58
>20:1
>20:1
>20:1
>20:1
>20:1
11g
11h
11i
[
b]
c]
Entry
Donor (R)
Product
Yield
1
0a (4-O-
TBS)
0b (4-O-
TBDPS)
0c (4-O-
Bn)
1
2
3
4
5
6
7
12ab
12bb
12cb
12db
12eb
12fb
78 %
82 %
86 %
76 %
68 %
57 %
73 %
2:1
4:1
3:1
7:1
9:1
8:1
10:1
9
11j
11k
11l
12gj
12gk
12gl
74
74
70
>20:1
20:1
1
10
1
1
2
11:1
1
1
11m
11n
12gm
12gn
77
65
16:1
12:1
10d (4-O-
Ac)
13
10e (4-O-
[a] Reaction of 10e (1.5 equiv), acceptor (1 equiv), NIS (1.5 equiv) and AgOTf
0.5 equiv). [b] Yield of the isolated product. [c] Ratios determined by H NMR
spectroscopy of the crude reaction mixture.
Bz)
1
(
10f (4-O-1-
Naphthoyl)
Opening the lactone disaccharides by a catalytic amount of
10g (4-O-
12gb
Piv)
NaOMe or by reduction with LiBH
4
generated mannuronic
methyl esters 13 or mannosides 14 with the restoration of the
[
(
a] Reaction of donor (1.5 equiv), 11b (1 equiv), NIS (1.5 equiv) and AgOTf
1
4
0.5 equiv). [b] Yield of the isolated product. [c] Ratios determined by H NMR
conformation to C1 (Table 4). The -configuration of the
spectroscopy of the crude reaction mixture.
4
disaccharides with restored C
1
conformation were confirmed by
[
17]
Next, different alcohol acceptors were subjected to the
glycosylation with the optimal donor 10g. As summarized in
Table 3, the glycosylation of 10g with most of secondary
alcohols proceeded smoothly, providing the corresponding
disaccharides in good yield with excellent -selectivity (Table 3,
entries 1, 3-13). The -mannosylation of primary alcohol 11c
their JC-1,H-1 coupling values which are smaller than 160 Hz.
[a]
Table 4 Opening of the lactone dissacharides
[b]
[c]
Product
Yield
Product
Yield
Entry
Lactone
(
Table 3, entry 2) and tertiary alcohol 11n (Table 3, entry 13)
also delivered the desired disaccharides with commendable 
selectivity.
1
[
a]
Table 3 Glycosylations of Donor 10e with Acceptors 11a−n
2
3
4
5
[
a] Yield of the isolated product. [b] Deprotection by NaOMe (0.05 equiv). [c]
[
b]
[c]
Reduction by LiBH (4 equiv).
4
Entry
Acceptor
Product
Yield (%)
/
After the development of this new donor for -mannosylation,
we set out to synthesize trisaccharide 18, which is a component
1
2
3
11a
11c
11d
12ga
12gc
12gd
88
62
65
>20:1
10:1
[
18]
of the Hyriopsis schlegelii glycosphingolipid.
As depicted in
Scheme 4, -mannosylation of donor 10g with acceptor 11i
provided the disaccharide 12gi in 58% yield with excellent -
selectivity. The trisaccharide 16 was obtained via the base-
>20:1
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COMMUNICATION
catalyzed opening of the lactone disaccharide with NaOMe,
followed by glycosylation with imidate 15. Reduction of 17 with
LiAlH and subsequent global debenzylation in the presence of
4
Pd/C under hydrogen atmosphere accomplish the synthesis of
the target molecule 18, the characterization data of which are in
[19]
accordance with those reported in the literature.
Entry
1
Donor
R
LG
Conditions
NIS,
Result
removal
of Boc
[a]
AgOTf
BSP, Tf
TTBP
2
O,
no
reaction
1
0h
0i
SEt
2
3
4
[b]
BSP,
removal
of Boc
Boc
[c]
2
Tf O
BSP,
no
reaction
[c]
2
Tf O
1
Tol
[
d]
removal
of Boc
5
6
AuCl
3
BSP,
no
reaction
10j
SEt
[c]
2
Tf O
Scheme 4. Synthesis of trisaccharide 18.
tricloroacet
imidoyl
trichloroimi
date
[e]
20
(73%)
7
10k
TMSOTf
Several trapping experiments were also carried out to further
understand the factors influencing the stereoselectivity,
especially the -directing effect by remote participation of O-4
acyl group in the donors (Table 5). We selected O-4 Boc and
tricloroacetimidoyl protected lactone donors to trap anomeric
oxocarbenium ion intermediates by the intramolecular
nucleophilic attack of the Boc group or the tricloroacetimidoyl
group. The trapping of intramolecular electrophilic centers by the
[
(
a] Donor (1 equiv), NIS (1 equiv), AgOTf (0.5 equiv). [b] Donor (1 equiv), BSP
1.1 equiv), TTBP (2.5 equiv), Tf O (1.1 equiv). [c] Donor (1 equiv), BSP (1.1
equiv), Tf O (1.1 equiv). [d] Donor (1 equiv), AuCl (0.5 equiv). [e] Donor (1
equiv), TMSOTf (0.3 equiv).
2
2
3
[
20],[21]
Boc group results in the formation of cyclic carbonates
and
by the tricloroacetimidoyl group results in the formation of cyclic
[
13a],[13d],[22]
oxazolines or oxazines.
This strategy was well
established and used for providing support for the neighboring-
group participation and remote participation of acyl group in the
glycosyl donors. In our trapping experiments, although no cyclic
carbonate 19 was obtained in the reaction of Boc protected
mannosyl thioether 10h and triflate 10i with different promoters
Figure 2. X-ray crystal structure of the key intermediate.
(
Table 5, entries 1-5), we were capable of attaining stable
In conclusion, we have developed an efficient -
mannosylation method using a 2,6-lactone-bridged thiomannosyl
donor via remote acyl group participation as well as O-4 steric
effect. The lactone donor could be rapidly prepared on a gram
scale in 3 steps and the glycosylation of the lactone donor with
primary, secondary and tertiary alcohol acceptors proceeds
smoothly with high -selectivity. The easy availability of the
donor and the high -selectivity for mannosides enable this
method to be an alternative to those reported in the literatures
for the construction of -mannosides. The utility of the method
has been demonstrated by the synthesis of the trisaccharide, a
component of the Hyriopsis schlegelii glycosphingolipid.
tricyclic product 20, the structure of which was firmly confirmed
by NMR, HRMS and single-crystal X-ray analysis (Figure 2),
when using 4-O-tricloroacetimidoyl-mannosyl trichloroimidate
1
5
strong proof for the remote participation of O-4 acyl group.
Moreover, the concentration-depending experiment shows that
the -selectivity of this mannosylation reaction only improved
slightly as the concentration increased [/ 8.5:1 (c 0.05 M) vs
0k under the conditions of catalytic amounts of TMSOTf (Table
, entry 7). The result of these trapping experiments serves as
1
N
0:1 (c 0.1 M) vs 12:1 (c 1 M)], indicating that the S 2(-like)
pathway is not the predominant pathway for our 2,6-lactone-
bridged thiomannosyl donors with the treatment of NIS/AgOTf
(
see Supporting Information, Table S-1).
Acknowledgements
Table 5 Trapping experiments.
This work was supported by the National Natural Science
Foundation of China (No. 21472119), Ministry of Science and
Technology (No. 2012ZX09502-001-001) and the Fundamental
Research Funds for the Central Universities (GK201603036,
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Chemistry - An Asian Journal
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COMMUNICATION
GK201701002). All the financial support are gratefully
acknowledged.
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[
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Chemistry - An Asian Journal
10.1002/asia.201801740
COMMUNICATION
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COMMUNICATION
Stereoselective -mannosylation was
achieved using a 2,6-lactone-bridged
thiomannosyl donor via O-4 remote
acyl group participation and the
assistance of O-4 substituent's steric
effect. The glycosylation proceeded
smoothly with high stereoselectivity for
primary, secondary and tertiary
alcohol acceptors. The novel donor
could be readily prepared in 3 steps
on a gram scale.
Huanfang Xu, Long Chen, Qi Zhang,
Yingle Feng, Yujia Zu and Yonghai
Chai*
Page No. – Page No.

-Stereoselective Mannosylation with
a 2,6-Lactone-bridged Thiomannosyl
Donor by Remote Acyl Group
Participation
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