N-heterocyclic carbene catalyzed stereoselective glycosylation of 2-nitrogalactals

Article abstract of DOI:10.1021/acs.orglett.7b02543

An efficient N-heterocyclic carbene catalyzed glycosylation of 2-nitrogalactals with alcohols and phenol has been developed for the first time. A wide variety of 1,2-cis-2-nitroglycosides can be obtained with good to excellent yields and high to excellent

Full text of DOI:10.1021/acs.orglett.7b02543

Letter
pubs.acs.org/OrgLett
N‑Heterocyclic Carbene Catalyzed Stereoselective Glycosylation of
2‑Nitrogalactals
Jia-Lin Liu, Yu-Tong Zhang, Hang-Fan Liu, Ling Zhou,* and Jie Chen*
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry &
Materials Science, Northwest University, Xi’an 710127, P. R. China
S
* Supporting Information
ABSTRACT: An efficient N-heterocyclic carbene catalyzed
glycosylation of 2-nitrogalactals with alcohols and phenol has
been developed for the first time. A wide variety of 1,2-cis-2-
nitroglycosides can be obtained with good to excellent yields
and high to excellent α-selectivities.
lycosides are widely spread in nature and play numerous
important roles in living organisms. 2-Amino-2-deoxy-
anosides as the major products.8 Meanwhile, Yoshida, Takao,
and co-workers developed the amino-thiourea catalyed glyco-
sylation of 2-nitrogalactal with phenols to give α-galactopyrano-
sides as the major products.9 Although organocatalytic strategies
have been utilized for glycosylation,10 the control of stereo-
selectivity and limited substrate scope remain as challenges.
The N-heterocyclic carbene (NHC) catalyzed reaction has
emerged as a powerful organocatalytic strategy in organic
chemistry.11 Important advances in NHC-catalyzed reactions
have been achieved via the “Breslow intermediate” generated by
the nucleophilic addition of NHC to aldehydes.11 Additionally,
NHC-catalyzed reactions through noncovalent bonding inter-
actions have also been developed.12 Recently, we have developed
several unconventional NHC catalyzed reactions.13 To continue
our interest in the development and application of NHC
catalyzed reactions, herein, we report the first stereoselective
glycosylation of 2-nitrogalactals with alcohols and phenol
catalyzed by NHC, which leads to the formation of α-2-
nitrogalactopyranosides as the major products.
G
glycosides are an integral part of many sugar peptides, sugar
esters, protein polysaccharides, nucleosides, and aminoglycosidic
antibiotics, many of which play important roles in many
biological processes such as cancer metastasis, inflammation,
immune defense, fertilization, signal transduction, cell growth,
cell−cell adhesion, and cell recognition.1 Among them, 2-
acetamido-2-deoxy-D-O-galactopyranosides are the most com-
mon units in a number of proteins such as mucins, cell membrane
glycoproteins, immunoglobulins, and antifreeze glycoproteins.2
Because of the many pharmaceutical applications of glycosides,
many remarkable endeavors have been made to develop the
glycosylation method over the past decades.3
The stereoselective synthesis of 2-acetamido-2-deoxy-D-O-
galactopyranosides has also been developed by using 2-N-
protected amido, 2-azido galactopyranoses, and 2-nitroglycals as
glycosyl donors.4,5 2-Nitroglycals are efficient glycosyl donors for
the formation of glycosidic bonds, offering excellent yields and
high stereoselectivities (Scheme 1).5 For instance, Schmidt and
Inspired by Schmidt’s base-catalyzed glycosylation reactions
and the hydrogen-bonding activation model of NHC with a
hydroxyl group,6,13 we envisioned that a glycosyl acceptor or
other OH group coordinated to a bulky NHC would attack a 2-
nitrogalactal stereoselectively. To test this hypothesis, we
examined the reaction using 2-nitrogalactal (1a) and methanol
(2a) in a ratio of 1:2 in acetonitrile at room temperature, and the
chiral triazolium salt 4a was used as the catalyst. To our delight,
the desired product 3a was obtained in 65% yield after 120 h,
albeit as a 1:1.6 α/β anomeric mixture (Table 1, entry 1). The
chiral triazolium salt 4b resulted in a better yield with a poorer
stereoselectivity (Table 1, entry 2). Thiazolium salt 4c gave a
slight excess of α-selectivity (Table 1, entry 3). We then tested
imidazolium salts 4d−f. Surprisingly, both the yields and the
stereoselectivities were improved (Table 1, entries 4−6). A 4:1
α:β ratio was obtained when the imidazolium salt 4f with two
bulky 2,6-diisopropylphenyl substituents was used as the catalyst
Scheme 1. Stereoselective Glycosylation of 2-Nitrogalactal
t
co-workers developed strong base (e.g., BuOK) catalyzed
glycosylation of 2-nitrogalactals with various acceptors to
construct O-/S-/N-glycosidic bonds with excellent stereo-
selectivities.6 Sun, Yu, and co-workers developed DMAP- or
PPY-catalyzed glycosylation of 2-nitrogalactals with alcohols to
give β-galactopyranosides as the major products.7 Recently,
Galan and co-workers developed chiral cinchona thiourea
catalyed glycosylation of 2-nitrogalactals to obtain α-galactopyr-
Received: August 16, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02543
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
stereoselectivities (Scheme 2). Functionalized ethanols offered
excellent yields (90−97%) and high to excellent α-anomers (8:1
Table 1. Optimization of the Reaction Conditions
Scheme 2. NHC-Catalyzed Stereoselective Glycosylation of 2-
a
Nitrogalactals with Alcohols and Phenol
b
entry catalyst
solvent
time (h) yield (%)
α/β
1
4a
4b
4c
4d
4e
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
4f
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
120
144
60
65
77
59
93
99
93
84
98
98
93
92
97
97
90
97
98
98
1:1.6
1:1.2
2
3
1.4:1
4
40
3:1
5
120
60
1:1
6
4:1
7
1,4-dioxane
144
10
1:1.1
8
CCl4
20:1
9
CH2Cl2
3.5
19:1
16:1
13:1
26:1
29:1
3:1
10
11
12
13
14
15
16
17
toluene
10
1
n-hexane
CCl4/n-hexane = 1:1
CCl4/n-hexane = 3:2
CCl4/n-hexane = 3:2
CCl4/n-hexane = 3:2
CCl4/n-hexane = 3:2
CCl4/n-hexane = 3:2
1.5
1
3
c
d
e
4
28:1
69:1
2:1
28
70
a
Reactions were carried out with compounds 1a (0.15 mmol), 2a
(0.30 mmol), catalyst (0.023 mmol, 15 mol %), Cs2CO3 (0.015 mmol,
10 mol %) in solvent (1.5 mL) at room temperature under Ar; yields
b
c
shown are for the isolated products. Determined by 1H NMR. At 40
°C. At 0 °C. At −30 °C.
d
e
(Table 1, entry 6), which confirmed that steric bulk in the
aromatic substituent in NHCs plays an important role in
determining glycosylation stereoselectivity (Table 1, entry 6 vs
entries 4, 5).
Next, we attempted to optimize the reaction by varying
different parameters systematically by using 4f as the optimal
catalyst. Interestingly, a dramatic increase in selectivity toward
the desired α-anomer was achieved when a nonpolar solvent like
CCl4, CH2Cl2, toluene, or n-hexane was used (α:β 13:1−20:1;
Table 1, entries 8−11). Notably, an increase in selectivity was
observed when a mixed solvent was used (α/β 26:1, Table 1,
entry 12). Further optimization of this reaction by varying
different temperature allowed us to discover the optimal
conditions: a 98% yield and 69:1 α:β ratio of 3a was obtained
when the reaction was conducted in CCl4/n-hexane (3:2, 0.1 M)
at −30 °C (Table 1, entry 16). A control experiment in the
absence of 4f was conducted, and the desired product 3a was
obtained in excellent yield with very poor stereoselectivity,
suggesting that Cs2CO3 can also catalyze the reaction but with
poor stereoselectivity, while the NHC plays an important role in
the stereoselectivity (Table 1, entry 17).
a
Reactions were carried out with compounds 1 (0.15 mmol), 2 (0.30
mmol), 4f (0.023 mmol), Cs2CO3 (0.015 mmol) in CCl4/n-hexane =
3:2 (1.5 mL) at room temperature under Ar; yields shown are for the
isolated products; both isomers were separated by flash column
b
c
d
chromatography. At −30 °C. 1/2 = 1:1.2. 1/2 = 2:1.
to only α; 3b−ea). Notably, substrates bearing halogen
functionalities could also undergo the desired transformation
with very impressive performance (3c, 3d), and without any
elimination side reactions, thus offering handles for further
synthetic transformations. Propargyl alcohol, allyl alcohol,
substituted allyl alcohols, benzyl alcohol, p-methoxybenzyl
alcohol, and furfuryl alcohol all resulted in excellent yields
(90−96%) and high to excellent α-stereoselectivities (8:1−25:1;
With the optimized conditions in hand, the scope of the
reaction was investigated by varying the acceptors. A diverse
acceptor with a variety of functional groups performed well in
this approach, and the corresponding products 3 were produced
with good to excellent yields and high to excellent α-
B
DOI: 10.1021/acs.orglett.7b02543
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Organic Letters
Letter
3fa−m). Reaction of C-6 silyl protected galactal and some
alcohols afforded the desired products with clearly improved α:β
ratio (3eb, 3fb, 3kb vs 3ea, 3fa, 3ka), probably for steric reasons.
Besides simple alcohols described above, N-Boc- and Fmoc-
protected L-serine and L-threonine were also selected as
substrates to test this new glycosylation reaction. Satisfactorily,
all of the desired 2-nitroglyco-conjugates 3n−q were obtained in
high yields (78−96%) with exclusive α-selectivities under the
optimal conditions. Notably, the nitro group of these compounds
could be transformed to an N-acetylamino group smoothly (vide
infra), offering valuable building blocks for O-glycopeptide
synthesis. For example the mucin class of glycopetides, which
contain an N-acetylaminogalactose motif and are α-linked to L-
serine and L-threonine.1h,5a,14 Furthermore, the successful
reaction with L-threonine derivative indicated secondary alcohols
are also suitable substrates for this methodology (see also 3u).
Additionally, reactions with 6-O-unprotected glycosyl acceptors
bearing electron-donating or electron-withdrawing protecting
groups afforded disaccharides 3r−t in moderate to good yields
with excellent α-selectivities. Reaction with 3-O-unprotected
glucoside 2u as acceptors also furnished the α-product 3u in
good yield (85%). Compounds 3t and 3u bearing double bonds
are potential building blocks for further nitration and Michael
addition reactions. Finally, the value of this method was further
amplified by the successful replacement of alcohols by phenol
(3v, 80% yield, only α), a challenging glycosylation acceptor
because of the weak nucleophilicity.6e,9
Scheme 4. Proposed Mechanism
the Michael addition of the hydroxyl group to the 2-nitrogalactal
from the sterically favored side, generating α-anomer bearing C-2
carbanion intermediate stereoselectively, together with 4f. Then
protonation of the C-2 carbanion intermediate with 4f at the β-
side yields the 1,2-cis glycoside product and regenerates the NHC
organocatalyst. Interestingly, 2-nitroglucal returned excellent β-
selectivities under the optimized conditions, indicating that the
C-4 stereocenter is also responsible for the stereocontrol in this
glycosylation reaction.15
In summary, we have developed the first NHC-catalyzed
stereoselective glycosylation of 2-nitrogalactals, providing a new
method for the construction of O-glycosidic bonds under green,
economic, and environmental friendly conditions. Both alcohols
and phenol are suitable acceptors for this reaction, and various 2-
nitroglycosides were obtained with high yields and excellent α-
selectivities. This NHC-catalyzed system has the potential to be a
valuable platform for the development of a wide range of broadly
useful stereocontrolled synthesis of glycopeptides, 2-amino-
sugars, and other bioactive natural or non-natural products.
Further investigations to better understand the mechanism and
to extend the scope and application of this glycosylation reaction
are underway.
To test the synthetic practicability of this method, the
stereoselective glycosylation was conducted on a gram scale,
and 3q was produced in comparable yield and selectivity (95%
yield, only α, Scheme 3). To further demonstrate the synthetic
Scheme 3. Gram-Scale Preparation of 3q and Synthesis of
Compound 5
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02543.
Experimental procedures, characterization data for all new
compounds, and 1H, 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
utility of the newly developed methodology, the preparation of
1,2-cis-2-acetamido-2-deoxy-D-galactopyanoside 5 was carried
out. As shown in Scheme 3, reduction of the nitro group of α-
glycoside 3q using Zn/HCl in a mixture of AcOH/THF/H2O,
followed by N-acetylation with acetic anhydride in pyridine,
furnished the target 5 in 87% yield over two steps without any
loss of stereochemistry.6g
While a precise understanding of the origin of the stereo-
selectivity needs further studies, a plausible mechanistic cycle is
proposed in Scheme 4. The deprotonation of imidazolium salt 4f
by Cs2CO3 forms carbene A, a carbon-centered Brønsted base.
Next, A interacts with the hydroxyl group via hydrogen bonding
to give an NHC−HOR complex B.12a,15 This complex facilitates
*E-mail: zhoul@nwu.edu.cn.
*E-mail: chmchenj@nwu.edu.cn.
ORCID
Ling Zhou: 0000-0002-6805-2961
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(NSFC-21672170, 21302151), NFFTBS (No. J1210057), and
Northwest University for financial support.
C
DOI: 10.1021/acs.orglett.7b02543
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Letter
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