Bimodal Glycosyl Donors Protected by 2- O-(ortho-Tosylamido)benzyl Group

Article abstract of DOI:10.1021/acs.orglett.8b01922

A glucosyl donor equipped with C2-o-TsNHbenzyl ether was shown to provide both α- and β-glycosides stereoselectivity, by changing the reaction conditions. Namely, β-glycosides were selectively obtained when the trichloroacetimidate was activated by Tf₂NH. On the other hand, activation by TfOH in Et₂O provided α-glycosides as major products. This "single donor" approach was employed to assemble naturally occurring trisaccharide α-d-Glc-(1→2)-α-d-Glc-(1→6)-d-Glc and its anomers.

Full text of DOI:10.1021/acs.orglett.8b01922

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Bimodal Glycosyl Donors Protected by 2-O‑(ortho-
Tosylamido)benzyl Group
Feiqing Ding, Akihiro Ishiwata,* and Yukishige Ito*
Synthetic Cellular Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
*
S Supporting Information
ABSTRACT: A glucosyl donor equipped with C2-o-TsNHbenzyl ether was shown to provide both α- and β-glycosides
stereoselectivity, by changing the reaction conditions. Namely, β-glycosides were selectively obtained when the
trichloroacetimidate was activated by Tf NH. On the other hand, activation by TfOH in Et O provided α-glycosides as
2
2
major products. This “single donor” approach was employed to assemble naturally occurring trisaccharide α-D-Glc-(1→2)-α-D-
Glc-(1→6)-D-Glc and its anomers.
tereoselective O-glycosylation is essential in order for the
Scheme 1. Preparation of C2-o-TsNHbenzyl D-Glucosyl
S
efficient synthesis of biologically relevant oligosacchar-
ides. The performance of such reactions is influenced by a
Trichloroacetimidate 6b as Donor
1
variety of factors including the leaving group, the activator,
2
protective groups, and reaction conditions. In most cases, 1,2-
trans glycosylation has exploited neighboring-group partic-
3
ipation (NGP) of a 2-O-acyl group. On the other hand,
donors having 2-O-ether protection favor the formation of axial
4
glycosides for stereoelectronic reasons (Figure 1, donor 1).
Aimed at achieving stereocontrolled glycosylation, non-
classical NGP strategies involving C2 chiral auxiliaries (Figure
5
3a,6
1
, 2) or the 2-O-picolyl group (3) have been developed.
More recently, the use of 2-O-ortho-substituted benzyl groups
7
8
such as o-nitrobenzyl (4) and o-cyanobenzyl (5) have shown
to be effective for the formation of 1,2-trans-glycosides (Figure
1
, donor 4 and donor 5).
In spite of these, synthetic schemes for complex
oligosaccharides are usually complicated, because formation
of 1,2-cis and -trans glycosides usually requires distinct donors.
A generalized strategy for the stereoselective synthesis of both
Figure 1. Application of the ether-type participating groups in
glycosylation reactions.
Received: June 20, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01922
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Screening of Reaction Conditions
Scheme 2. Glycosylation with Various Acceptors
b
c
entry
cat.
solvent (M)
t (°C)
yield (%)
α:β
1
2
3
4
5
6
7
8
9
TMSOTf
TMSOTf
TMSOTf
TMSOTf
toluene (0.1)
MeCN (0.1)
DCM (0.1)
DCM (0.1)
EtCN (0.1)
DCM (0.1)
toluene (0.1)
0
0
0
−78
−78
−78
−78
−78
rt
98
95
99
93
91
87
85
84
90
94
92
96
92
88
14
74
95
35:65
13:87
33:67
2:98
d
d
Tf NH
β
2
d
Tf NH
β
2
d
Tf NH
β
2
d
Tf NH
Et O (0.1)
14:86
39:61
55:45
73:27
74:26
76:24
75:25
62:18
77:23
81:19
84:16
74:26
71:29
69:31
2
2
Tf NH
Et O (0.1)
2
2
10
11
12
13
14
15
16
17
18
19
20
21
TMSOTf
TMSOTf
TMSOTf
TfOH
Cu(OTf)2
AgOTf
ZnBr2
TfOH
TfOH
TfOH
TfOH
THF (0.1)
0
0
Et O (0.1)
2
Et O (0.1)
2
rt
rt
rt
rt
rt
rt
rt
0
Et O (0.1)
2
Et O (0.1)
2
Et O (0.1)
2
Et O (0.1)
2
Et O (0.01)
2
e
Et O (0.004)
2
71
Et O (0.004)
2
92
87
72
d
Et O (0.004)
2
−40
−78
d
TfOH
Et O (0.004)
2
a
Reaction conditions: donor 6b (1.0 equiv), cyclohexanol 8a (1.2
b
equiv), catalyst (0.1 equiv), MS 3 Å (100 mg/mL). Combined yield
of the anomeric mixture of corresponding glycoside. Determined by
the integration ratio obtained from H NMR of crude mixture. For 2
h. Isolated yield of corresponding α-glycoside.
c
1
d
e
isomers from a single donor has yet to be established. With the
goal of achieving stereocontrolled construction of both α- and
β-glycosidic bonds from a single donor, our attention was
turned to the 2-O-(ortho-tosylamido)benzyl (TAB) modified
a
Conditions A: entry 5, Table 1; conditions B: entry 18, Table 1.
Determined by the integration ratio obtained from H NMR of
b
1
9
crude mixture.
donor 6 (Figure 1). We expected that its behavior could be
tuned by modulating the internal hydrogen bonding with the
C-2 oxygen (A) or the incoming nucleophile (B).
a
Scheme 3. Stereocontrolled Galactosylation
As a prototypical donor, we designed the trichloroacetimi-
date 6b which was prepared, as depicted in Scheme 1. To
evaluate the stereodirecting ability of the 2-O-TAB group,
initial experiments were carried out using cyclohexanol (8a) as
an acceptor. In the presence of 10 mol % of TMSOTf (Table
1
, entries 1−5), the product 9a was obtained in modest to high
stereoselectivity which favored the β-isomer (α/β = 33/67 to
10
2
/98) (entries 1−4). In the presence of triflimide (Tf NH),
2
nearly complete selectivity was observed in EtCN, DCM, or
toluene (entries 5−7). Interestingly, the β-isomer was still
dominant in Et O at low temperature (entry 8, α/β = 14/86).
2
By contrast, variable α-selectivity was observed when other
Brønsted or Lewis acids were employed in ether (Table 1,
entries 11−21). Screening of the reaction conditions revealed
that (1) TfOH was most effective, (2) α-selectivity was
enhanced at lower concentration, and (3) lowering the
reaction temperature was not beneficial.
a
Conditions A: entry 5, Table 1; conditions B: entry 18, Table 1.
Determined by the integration ratio obtained from H NMR of
b
1
crude mixture.
The scope of the reaction was assessed under conditions A
Table 1, entry 5) and B (Table 1, entry 18), which are
1
1
12
(
protected amino acids (8e−g), and glycosides 8k−q were
examined as acceptors. β-Glucosylated products (9k−q) were
obtained in good yield and excellent stereoselectivity under
conditions A, whereas 9m and 9n were obtained as mixtures of
isomers. In the latter cases, conditions B exhibited excellent α-
selectivity. Similarly, α-glucosides were obtained from 8k
suitable for 1,2-cis and -trans selective glucosylations,
respectively (Scheme 2). Under conditions A, reactions with
benzyl alcohol (8b), 1-adamantanol (8c), and L-menthol (8h)
gave corresponding glycosides 9b and 9c with complete β-
stereoselectivity, whereas tert-butanol (8d) exhibited lower
selectivity. In contrast, conditions B predominantly gave α-
isomers (α/β = 85/15 to 92/8). Subsequently, steroids (8i−j),
O‑6
O‑4
O‑2
O‑2
(Glc ), 8l (Glc ), 8o (Glc ), 8p (Man ), and 8q (β-
O‑3
Gal -(1→4)-Glc), albeit in somewhat attenuated selectivity.
B
DOI: 10.1021/acs.orglett.8b01922
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. (A) Structures of Isomeric Trisaccharides D-Glc-
Scheme 6. Reactions of the 2-O-Benzylated Donor 1b
(
1→2)-D-Glc-(1→6)-D-Glc; (B) Selective Deprotection of
Bn and TsNHBn Ethers
Scheme 7. Proposed Mechanism for α- and β-Selective
Glycosylations
standard debenzylation conditions (Scheme 4). Conversely,
removal of benzyl groups was achieved by DDQ without
deteriorating the TAB group to give 15 (Scheme 4). Selective
cleavage of the TAB ether was achieved in three steps through
Boc protection, deprotection of the tosyl group by Mg
treatment, and treatment with DDQ to give 17 with complete
preservation of the benzyl groups (Scheme 4).
Our protocol for the selective cleavage of the TAB ether
could be completed in 24 h which was sufficient for the current
purpose. Indeed, disaccharides β-9k and α-9k were efficiently
converted to β-21 and α-21 via corresponding 18, 19, and 20,
in 46% and 53% overall yields, respectively (Scheme 5).
Subsequent glycosylation of β-21 with 6b under conditions A
produced the trisaccharide β-D-Glc-(1→2)-β-D-Glc-(1→6)-α-
D-Glc-OMe (22) in a highly stereoselective fashion. Coupling
under conditions B produced isomeric trisaccharide α-D-Glc-
(1→2)-β-D-Glc-(1→6)-α-D-Glc-OMe (23) in 61% yield.
Acid-labile groups such as t-Bu, Boc, and benzylidene acetal
were compatible with both conditions.
We also examined the effects of the 2-O-TAB group for
galactosylation. To this end, the galactose donor 6c was
prepared (see Supporting Information) and reacted with 8a, 8j,
8
k, and 8n (Scheme 3). Similarly to 6a, it produced the
corresponding β- (β-9r−9u) and α-galactosides (α-9r−9u)
under conditions A and B, respectively.
Subsequently, we planned to assemble all four isomers of
1
3
trisaccharides D-Glc-(1→2)-D-Glc-(1→6)-D-Glc (10−13)
(Scheme 4) using 6b as the common donor. To put our
plan into practice, selective deprotection of the TsNHBn group
was examined first. Initially, the glycosylated product α-9a was
subjected to hydrogenolysis (H , Pd/C) to produce the tetraol
2
1
4, confirming that the TAB group is removable under
Scheme 5. Synthesis of Four Isomers of D-Glc-(1→2)-D-Glc-(1→6)-D-Glc Using 6b as a Common Donor
C
DOI: 10.1021/acs.orglett.8b01922
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Similarly, α-21 obtained from α-9k led to the trisaccharides β-
D-Glc-(1→2)-α-D-Glc-(1→6)-α-D-Glc-OMe (24) (conditions
A: α/β = 8/92, 59% isolated yield of the β-isomer) and α-D-
Glc-(1→2)-α-D-Glc-(1→6)-α-D-Glc-OMe (25) (conditions B:
α/β = 90/10, 47% isolated yield of the α-isomer), respectively.
All four isomers of D-Glc-(1→2)-D-Glc-(1→6)-α-D-Glc-OMe
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Takemichi Nakamura and Dr. Hiroyuki Koshino
(RIKEN, CSRS) for measurement of mass spectra and low-
temperature NMR measurements, respectively. We also thank
Ms. Akemi Takahashi (RIKEN, SCCL) for technical
assistance. This work was partly supported by a Grant-in-Aid
for Specially Promoted Research (No. 16H06290 to YI) from
the Ministry of Education, Culture, Sports, Science, and
Technology, for Scientific Research (No. 26350966,
15H02443 to AI) from the Japan Society for the Promotion
of Science, and Special Postdoctoral Program (FD) from
RIKEN.
(
10−13) were obtained after conventional mild global
deprotections of the protected trisaccharides (Scheme 5).
These results indicate that the TAB group behaves
differently in ethereal and nonethereal solvents, serving as an
NGP active substituent in DCM, toluene, and EtCN.
Reactions using perbenzylated donor 1b provided correspond-
ing glycoside 26 with diminished stereoselectivity (α/β = 11/
8
9 compared to β-only for 6b), while the stereoselectivity was
similar between 1b and 6b (α/β = 83/17 and 84/16) under
conditions B (Scheme 6).
To explain the stereodirecting effects of the TAB group, we
postulate the contribution of a hydrogen bonding between
tosylamide and the benzylic oxygen, as NH signals of donor
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Org. Lett. XXXX, XXX, XXX−XXX