Investigation of Glycosyl Nitrates as Building Blocks for Chemical Glycosylation

Article abstract of DOI:10.1002/ejoc.201801272

Glycosyl nitrates are important synthetic intermediates in the synthesis of 2-amino sugars, 1,2-orthoesters or, more recently, 2-OH glucose. However, glycosyl nitrates have never been glycosidated. Presented herein is our first attempt to use glycosyl nitrates as glycosyl donors for O-glycosylation. Lanthanide triflates showed good affinity to activate the nitrate leaving group.

Full text of DOI:10.1002/ejoc.201801272

A Journal of
Accepted Article
Title: Investigation of Glycosyl Nitrates as Building Blocks for Chemical
Glycosylation
Authors: Tinghua Wang, Yashapal Singh, Keith Stine, and Alexei V.
Demchenko
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10.1002/ejoc.201801272
European Journal of Organic Chemistry
Investigation of Glycosyl Nitrates as Building Blocks for Chemical Glycosylation
Tinghua Wang, Yashapal Singh, Keith J. Stine, and Alexei V. Demchenko*
Abstract
Glycosyl nitrates are important synthetic intermediates in the synthesis of 2-aminosugars, 1,2-
orthoesters or, more recently, 2-OH glucose. However, glycosyl nitrates have never been
glycosidated. Presented herein is our first attempt to use glycosyl nitrates as glycosyl donors for
O-glycosylation. Lanthanide triflates showed good affinity to activate the nitrate leaving group.
Keywords: glycosylation, leaving group, nitrate, oligosaccharides, activation
Key Topic: Glycosylation or Carbohydrates
Address: Tinghua Wang, Dr. Yashapal Singh, Prof. Keith J. Stine, and Prof. Alexei V.
Demchenko*
Department of Chemistry and Biochemistry, University of Missouri – St. Louis
One University Boulevard, St. Louis, Missouri 63121, USA
e-mail: demchenkoa@umsl.edu
www: http://www.umsl.edu/chemistry/Faculty/demchenko.html
1
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Introduction
With recent advances in the area of glycomics,[1-9] we now know that half of the proteins in the
human body are glycosylated[10] and cells present a multitude of glycostructures.[11] Glycan and
glycoconjugate biomarkers are present in all body fluids, able to transmit a plethora of biological
information, and hence offer fantastic opportunities for diagnostics. Changes in the level and
distribution of glycans as well as changes in glycosylation and branching patterns can indicate
presence and progression of a disease.[12-19] With improved understanding of the functions of
carbohydrates the demand for the development of new glycosylation reactions that will offer new
capabilities for obtaining complex glycan biomarkers has increased.
Recently we reported the synthesis of 1,3,4,6-tetra-O-acetyl-α-D-glucopyranose (2-OH
glucose) wherein glycosyl nitrates were found to be the key intermediates.[20] Glycosyl nitrates
have been known for years, but their application was limited to their use as precursors for other
leaving groups after azidonitration reaction of glycals.[21-22] Another, less known application, is
the synthesis of 1,2-orthoesters.[23] However, glycosyl nitrates have never been glycosidated
beyond the synthesis of aliphatic glycosides of aminosugars with charged nucleophiles[24-25] or
under microwave irradiation conditions.[22] The ability to form 2-OH glucose from glycosyl
nitrates without additional reagents implies that the anomeric nitro group can act as a suitable
leaving group in glycosylation. Reported herein is our first attempt to study glycosyl nitrates as
glycosyl donors in chemical glycosylation reactions with a variety of sugar alcohols as glycosyl
acceptors.
2
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Results and Discussion
A preliminary series of experiments involved reactions promoted with different Lewis acids,
common in many glycosylation reactions with reactive donors. However, reactions in the
presence of FeCl3, ZrCl4, SnCl4 or TMSOTf were sluggish, and some resulted in the preferential
formation of the corresponding 1,2-orthoester side product albeit in moderate yields. See the
supporting information (SI) for further details on preliminary screening of promoters. A
preliminary set of experiments with heavy metal triflates was far more successful. Whereas
AgOTf and Cu(OTf)2 still led to the formation of substantial amounts of the 1,2-orthoester along
with some glycoside, Bi(OTf)3 and Ba(OTf)2 provided the highest conversion rates and yields of
the desired O-glycoside product. Thus, when benzoylated glucosyl nitrate 1[20] and primary
acceptor 2[26] were coupled in the presence of sub-stoichiometric Bi(OTf)3 (0.5 equiv) in
acetonitrile, disaccharide 3 was obtained in 32% yield, albeit without any detectable 1,2-
orthoester by-product formation (entry 1, Table 1). When excess Bi(OTf)3 (1.1 or 1.5 equiv) was
used, the yield of disaccharide 3 increased to 38% and 45% (entries 2 and 3, respectively).
However, the formation of the product was accompanied by the formation of the nitrate-transfer
by-product, methyl 2,3,4-tri-O-benzyl-6-O-nitro-α-D-glucopyranoside (see the SI for details). In
reactions between donor 1 and acceptor 2 in presence of Ba(OTf)2 (1.1 or 1.5 equiv) in CH3CN
disaccharide 3 was obtained in 65% and 63% yield (entries 4 and 5). However, the corresponding
1,2-orthoester was also formed as the side product (see the SI for details). We have also
investigated 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl nitrate[20] as glycosyl donor. These
glycosylations, however, were much less efficient due to a number of side reactions leading to
the predominance of the donor-acceptor acetyl transfer products (no data shown).
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European Journal of Organic Chemistry
Table 1. The establishment and optimization of reaction conditions for the glycosidation of per-
benzoylated glucosyl donor 1 with glycosyl acceptor 2
Entry Equiv of 1 Promoter (equiv)
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Et2O
Yield of 3
32%
1
2
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.2
1.2
1.2
1.5
Bi(OTf)3 (0.5)
Bi(OTf)3 (1.1)
Bi(OTf)3 (1.5)
Ba(OTf)2 (1.1)
Ba(OTf)2 (1.5)
Yb(OTf)3 (1.1)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.1)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
38%a
45%a
65%b
63%b
57%
3
4
5
6
7
62%
8
74%
9
Et2O
78%
10
11
12
13
14
Et2O
85%
CH3CN
CH2Cl2
1,2-DCE
Et2O
63%
55%
53%
91%
Et2O/CH2Cl2,
4/1, v/v
Et2O/CH2Cl2,
1/1, v/v
Et2O/CH2Cl2,
1/4, v/v
Et2O/1,2-DCE,
5/1, v/v
Et2O/1,2-DCE,
1/1, v/v
Et2O/1,2-DCE,
1/5, v/v
Et2O/1,2-DCE,
1/1, v/v
Et2O/1,2-DCE,
1/1, v/v
15
16
17
18
19
20
21
22
23
24
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Yb(OTf)3 (1.5)
Er(OTf)3 (1.5)
Gd(OTf)3 (1.5)
Sm(OTf)3 (1.5)
Ce(OTf)3 (1.5)
76%
83%
75%
78%
88%
79%
77%
76%
61%
60%
Et2O/1,2-DCE,
1/1, v/v
Et2O/1,2-DCE,
1/1, v/v
a Methyl 2,3,4-tri-O-benzyl-6-O-nitro-α-D-glucopyranoside by-product was also formed.
b The corresponding 1,2-orthoester byproduct was also formed.
4
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Having established that glycosyl nitrates can indeed serve as potential glycosyl donors for
O-glycosylation, we were still somewhat disappointed by the conversion yields and the
formation of a number of by-products. In order to address these issues, we turned the attention to
investigating lanthanide(III) triflates that are known to act as nitrate capture reagents.[27-28] We
assumed that the application of lanthanides would be beneficial for our reaction in order to
prevent the nitrate-transfer products observed in some preliminary experiments. The
glycosylation reaction typically begins by the formation of the activated donor-promoter
complex as a result of the interaction of the leaving group and the promoter. In the presence of
the lanthanide ions the nitrate leaving group will be activated via the formation of a stable bi-
dentate complex as shown in Scheme 1.[29] This anticipated pathway differentiates the glycosyl
nitrate leaving group from other leaving groups wherein the activation takes place in a
monodentate manner.[30]
Scheme 1. Proposed mechanism of the nitrate activation with lanthanides
Upon the subsequent dissociation of the glycosyl donor, and expulsion of the activated
leaving group in the rate-determining step (RDS), oxacarbenium ion is typically formed. Other
intermediates may also form at this stage with or without a counter-anion or the reaction solvent
involvement. As a consequence of the sp2-hybridization of the anomeric carbon the subsequent
5
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European Journal of Organic Chemistry
glycosyl acceptor (ROH) is possible either from the bottom or the top face of the ring leading to
the formation of a mixture of diastereomers. Reactions with glycosyl donors equipped with the
neighboring participating acyl groups are typically 1,2-trans-selective due to the intermediacy of
the acyloxonium ion. Once the proton transfer occurs, the formation of the glycosidic bond is
irreversible (the termination step).[31]
Lanthanide ions with higher charge-to-size ratio, such as Yb(III), are expected to bind the
nitrate group more strongly than those with smaller ratios. Hence, we selected Yb(OTf)3 as the
promoter for the subsequent study. To our delight, a clean reaction between nitrate donor 1 and
acceptor 2 in presence of Yb(OTf)3 (1.1 or 1.5 equiv) was observed, and no side products were
detected. However, the reaction was still fairly sluggish and disaccharide 3 was obtained in 57%
and 62% yield in 18 h (entry 6 and 7, respectively). Nevertheless, we felt that this result was
sufficient to begin further investigations in order to refine the reaction conditions. The next step
to improve the reaction rate and enhance the yield of the glycosylation reaction was to
investigate different solvents in Yb(OTf)3-promoted reactions. The yield of disaccharide 3 was
increased to 74% when glycosylation reaction between donor 1 and acceptor 2 was performed in
the presence of Yb(OTf)3 (1.1 equiv) in diethyl ether as the solvent (entry 8). Increasing the
amount of the promoter to 1.5 equiv led to a further increase in the yield of disaccharide 3 (78%,
entry 9).
At this stage we switched to investigating the amount of the glycosyl donor on the
outcome of this reaction. The use of donor 1 in a higher excess, 1.2 equiv vs. previously used 1.1
equiv, led to an increase in the yield of disaccharide 3 to a very respectable 85% (entry 10). The
solvent effect should be particularly noted in these reactions because reactions performed in
CH3CN, CH2Cl2 or ClCH2CH2Cl (1,2-DCE) afforded disaccharide 3 in much lower yields of
6
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European Journal of Organic Chemistry
63%, 55% or 53%, respectively (entries 11-13). When the amount of donor 1 was further
increased to 1.5 equiv, the reaction in the presence of Yb(OTf)3 (1.5 equiv) afforded disaccharide
3 in an excellent yield of 91% (entry 13). However, we felt that this increase was not sufficiently
substantial to justify the use of such a large excess of the donor.
Although the best result was achieved in Et2O as the reaction solvent, the use of ethereal
solvents in general is not ideal for glycosylation reactions due to moderate solubility of many
sugar building blocks. Considering the excellent solubility of protected carbohydrates in
halogenated hydrocarbons, and the fact that the yields of 3 in neat CH2Cl2 or 1,2-DCE were only
moderate (53-55%) vs. the reaction in neat ether (85%, entry 10) we next endeavored studying
Et2O in combination with CH2Cl2 or 1,2-DCE as reaction solvents. Using Et2O/CH2Cl2 as the
reaction solvent in different ratios (entries 15-17) brought us to the realization that the best yield
of disaccharide 3 (83%, entry 16) can be achieved in Et2O/CH2Cl2, 1/1, v/v). Similarly, using
Et2O/1,2-DCE as the reaction solvent in different ratios (entries 18-20) allowed us to achieve the
best yield of disaccharide 3 (88%, entry 19) in Et2O/1,2-DCE, 1/1, v/v). A series of other
lanthanide (III) triflates were screened and while Er(OTf)3 and Gd(OTf)3 were found to have
comparable activity to that of Yb(OTf)3, Sm(OTf)3 and Ce(OTf)3 were found to be somewhat
less reactive (entries 21-24).
In a further attempt to enhance the yield of glycosylation reactions, we performed the
reaction using a large excess of donor 1 (1.5 equiv.) using the optimized solvent Et2O/1,2-DCE
(1/1, v/v). As a result, disaccharide 3 was obtained in 80% yield (Table 2, entry 1). A very similar
outcome (18 h, 81%) was achieved when the electronically deactivated methyl 2,3,4-tri-O-
benzoyl-α-D-glucopyranoside was used as the glycosyl acceptor. Although the use of donor
excess was unnecessary for glycosylation of primary acceptor 2, in further experiments with the
7
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European Journal of Organic Chemistry
secondary acceptors and other less reactive series of glycosyl nitrates (vide infra) the donor
excess (1.5 equiv) was found beneficial for obtaining practical yields. With the optimized
reaction conditions, per-benzoylated glycosyl donors of the D-gluco, D-manno and D-galacto
series were then investigated with both primary and secondary glycosyl acceptors. The
glycosylation reaction between donor 1 and secondary 2-OH acceptor 4[26] produced
disaccharide 5 in 88% yield (entry 2). When 3-OH acceptor 6[26] was glycosylated with donor 1,
disaccharide 7 was obtained in 94% yield (entry 3). 4-OH acceptor 8[26] is less reactive than other
secondary acceptors tested herein, and this was reflected in a lower yield of disaccharide 9 (67%,
entry 4).
Table 2. Glycosidation of per-benzoylated glycosyl donors of the D-gluco, D-manno and D-
galacto series with various glycosyl acceptors
Entry
Donor
Acceptor
Product (Yield)
1
1
1
2
3 (80%)
2
4
6
5 (88%)
7 (94%)
9 (67%)
3
4
1
1
8
2
5
10
11 (71%)
8
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European Journal of Organic Chemistry
Table 2 (contd). Glycosidation of per-benzoylated glycosyl donors of the D-gluco, D-manno and
D-galacto series with various glycosyl acceptors
Entry
Donor
10
Acceptor
4
Product (Yield)
6
12 (43%)
13 (25%)
14 (12%)
16 (79%)
7
8
10
10
6
8
9
2
4
15
15
10
17 (79%)
18 (72%)
19 (71%)
11
12
15
15
6
8
We then turned our attention to studying per-benzoylated mannosyl nitrate 10 that was
found to be fairly unreactive. Although a good yield for disaccharide 11 was achieved for the
reaction of donor 10 with the primary 6-OH acceptor 2 (71%, entry 5), reactions with the
secondary acceptors gave yields that were below a practical value. Thus, disaccharides 12-14
were obtained in modest yields of 12-43% that were in line with the relative reactivity of
glycosyl acceptors (entries 6-8). In contrast, per-benzoylated galactosyl nitrate 15 was sufficently
reactive, similar to that of glucosyl nitrate 1. This was translated to good yields (71-79%) of
9
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European Journal of Organic Chemistry
disaccharides 16-19 irrespectively of the nature of the glycosyl acceptor used (entries 9-12).
Based on successful attempts with per-benzoylated glycosyl nitrate donors we
endeavored to investigate glycosyl donors with a non-participating benzyl group at C-2.
Glycosidations of per-benzylated (armed) glucosyl nitrate 20 proceeded much more rapidly than
those with per-benzoylated (disarmed) nitrate 1. Thus, glycosylation reaction between donor 20
and 6-OH acceptor 2 in acetonitrile smoothly produced disaccaride 21 in 96% yield within 1 h
(Table 3, entry 1). The use of acetonitrile as the solvent favored the formation of the β-isomer of
21 (α/β = 1/3.7). When diethyl ether was used as the solvent instead, a nearly quantitave yield
(99% yield, entry 2) was acheived in 3 h. The stereoselectivity was inverted toward the
preferential formation of the α-linked disaccharide 21 (α/β = 1.6/1). Also glycosylation in
Et2O/1,2-DCE (1/1, v/v) was slightly α-selective (85%, α/β = 1.3/1, entry 3). These results follow
the general trend of known solvent effects on the stereoselectivity of glycosylation reactions.[32]
Although glycosidation of donor 20 with secondary acceptors 4, 6 and 8 requred longer
reaction time (18 h), disaccharides 22-24 were produced in excellent yields of 90-96% albeit
with low selectivity (entries 4-6). We also investigated glycosidation of nitrate 25 equipped with
the superdisarming, 3,4,6-tri-O-acetyl-2-O-benzyl protecting group pattern.[33] All reactions with
this donor were slower, which was also reflected in a higher stereocontrol albeit lower yields
ranging from 68% (α/β = 2.3/1) for reactive primary acceptor 2 to 36% (α/β > 20/1) for the least
reactive 4-OH acceptor 8. The results of these reactions, the synthesis of disaccharides 26-29 are
summarized in entries 7-11. We note that the yield of these glycosylations can be improved by
performing the reactions at elevated temperatures (see the footnote for Table 3).
10
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Table 3. Glycosidation of donors 20 and 25 with a non-participating benzyl group at C-2 with
various glycosyl acceptors in different solvents
Entry Donor Acceptor Solvent (time) Product (yield, ratio α/β)
1
20
2
CH3CN (1 h)
21 (96%, α/β = 1/3.7)
21 (99%, α/β = 1.6/1)
2
3
20
20
2
2
Et2O (3 h)
Et2O/1,2-DCE,
1/1, v/v (4 h)
21 (85%, α/β = 1.3/1)
Et2O/1,2-DCE,
1/1, v/v (18 h)
4
20
4
22 (96%, α/β = 1.1/1)
23 (94%, α/β = 1.1/1)
Et2O/1,2-DCE,
1/1, v/v (18 h)
5
6
20
20
6
8
Et2O/1,2-DCE,
1/1, v/v (18 h)
24 (90%, α/β = 1.3/1)
7
25
2
CH3CN (18 h)a
26 (44%, α/β = 1/3.1)
26 (68%, α/β = 2.3/1)
Et2O/1,2-DCE,
1/1, v/v (18 h)
8
9
25
25
2
4
Et2O/1,2-DCE,
1/1, v/v (18 h)
27 (37%, α/β = 5.6/1)
28 (40%, α/β = 4.3/1)
29 (36%, α/β > 20/1)
Et2O/1,2-DCE,
1/1, v/v (18 h)
10
11
25
25
6
8
Et2O/1,2-DCE,
1/1, v/v (18 h)
a The yield was increased to 60% when the reaction was performed at 50 °C.
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Conclusions
Presented herein is our first attempt to employ glycosyl nitrates as donors in O-glycosylation
reactions. Lanthanide ions with higher charge-to-size ratios showed good affinity to bind the
nitrate leaving group. High reaction yields were achieved with per-benzylated and per-
benzoylated donors of the D-glucose and D-galactose series with both primary and secondary
glycosyl acceptors in various solvents. On the other hand, 3,4,6-tri-O-acetyl-2-O-benzyl-
protected glucosyl nitrate and per-benzoylated D-mannosyl nitrate were found to be much less
reactive, which resulted in fair yields in reactions with secondary acceptors. The latter results
imply that further investigations of more reactive promoters maybe of interest.
Experimental
General. The reactions were performed using commercial reagents and the ACS grade solvents
used for reactions were purified and dried in accordance with standard procedures. Column
chromatography was performed on silica gel 60 (70-230 mesh), reactions were monitored by
TLC on Kieselgel 60 F254. The compounds were detected by examination under UV light and by
charring with 10% sulfuric acid in methanol. Solvents were removed under reduced pressure at
<40 °C. CH2Cl2 and 1,2-dichloromethane (1,2-DCE) were distilled from CaH2, and Et2O was
distilled from Na directly prior to application. Molecular sieves (3 Å), used for reactions, were
crushed and activated in vacuo at 390 °C for 8 h in the first instance and then for 2-3 h at 390 °C
directly prior to application. Optical rotations were measured using a ‘Jasco P-1020’ polarimeter.
1H NMR spectra were recorded at 300 MHz, 13C NMR spectra were recorded at 75 MHz. The 1H
1
NMR chemical shifts are referenced to tetramethylsilane (TMS, δH = 0 ppm) for H NMR
spectra for solutions in CDCl3. The 13C NMR chemical shifts are referenced to the central signal
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European Journal of Organic Chemistry
of CDCl3 (δC = 77.16 ppm) for solutions in CDCl3. Mass analysis was performed using an
Agilent 6230 ESI TOF LC/MS mass spectrometer.
Synthesis of glycosyl nitrate donors
2,3,4,6-Tetra-O-benzoyl-β-D-glucopyranosyl nitrate (1) was obtained from 2,3,4,6-tetra-O-
benzoyl-α-D-glucopyranosyl bromide[34] as described previously.[20]
2,3,4,6-Tetra-O-benzoyl-α-D-mannopyranosyl nitrate (10). AgNO3 was added to a solution of
2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl bromide[35] (5.85 g, 8.87 mmol) in dry acetonitrile
(15 mL) and the resulting mixture was stirred for 5 min at rt. After that, the solids were filtered
off through a pad of Celite and the filtrate was concentrated under reduced pressure. The residue
was dissolved in CH2Cl2 (∼500 mL) and washed with water (100 mL), 1% aq. NaOH (100 mL),
and water (3 × 100 mL). The organic layer was separated, dried over MgSO4, and concentrated
in vacuo. The residue was purified by column chromatography on silica gel (ethyl acetate–
hexane gradient elution) to afford the title compound (4.05 g, 71% yield) as a white amorphous
22
solid. Analytical data for 10: Rf = 0.48 (ethyl acetate/hexanes, 3/7, v/v); [α]D -31.8 (c = 1,
CHCl3); 1H-n.m.r. (300 MHz): δ, 4.50 (dd, 1H, J6a,6b = 12.3 Hz, H-6a), 4.60 (dt, 1H, J5,6a = 3.8
Hz, J5,6b = 2.5 Hz, H-5), 4.72 (d, 1H, H-6b), 5.78 (dd, 1H, J3,4 = 10.0, H-3), 5.84 (dd, 1H, J2,3
=
3.4 Hz, H-2), 6.24 (dd, 1H, J4,5 = 12.7 Hz, H-4), 6.49 (d, 1H, J1,2 = 1.9 Hz, H-1), 7.22-7.69 (m,
12H, aromatic), 7.85 (d, J = 7.1 Hz, 2H, aromatic), 7.96 (d, J = 7.1 Hz, 2H, aromatic), 8.07 (t, J
13
= 7.9 Hz, 4H, aromatic) ppm; C-n.m.r. (75 MHz): δ, 62.1, 65.7, 67.8, 69.6, 71.5, 96.5, 128.5
(×2), 128.6 (×7), 128.8 (×2), 129.8, 129.8 (×2), 129.9 (×2), 129.9 (×2), 130.0 (×2), 133.3, 133.6,
133.8, 134.0, 165.1, 165.3, 165.4, 166.0 ppm; HR-ESI MS [M+Na]+ calcd for C34H27NO12Na
13
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European Journal of Organic Chemistry
calcd: 664.1431, found 664.1424.
2,3,4,6-Tetra-O-benzoyl-β-D-galactopyranosyl nitrate (15). AgNO3 was added to a solution of
2,3,4,6-tetra-O-benzoyl-α-D-galactopyranosyl bromide[36] (2.79 g, 4.23 mmol) in dry acetonitrile
(15 mL) and the resulting mixture was stirred for 5 min at rt. After that, the solids were filtered
off through a pad of Celite and the filtrate was concentrated under reduced pressure. The residue
was dissolved in CH2Cl2 (~250 mL) and washed with water (50 mL), 1% aq. NaOH (50 mL),
and water (3 × 50 mL). The organic layer was separated, dried over MgSO4, and concentrated in
vacuo to afford the title compound (2.58 g, 95% yield) as a white amorphous solid. Analytical
22
1
data for 10: Rf = 0.74 (ethyl acetate/hexanes, 3/7, v/v); [α]D +105.8 (c = 1, CHCl3); H-n.m.r.
(300 MHz): δ, 4.46 (dd, 1H, J6a,6b = 10.6 Hz, J5,6a = 5.7 Hz, H-6a), 4.59-4.51 (m, 1H, H-5), 4.65
(dd, 1H, J5,6b = 6.4 Hz, H-6b), 5.74 (dd, 1H, J3,4 = 3.3 Hz, H-3), 5.91 (dd, 1H, J2,3 = 10.1 Hz, H-
2), 6.05 (dd, 1H, J4,5 = 1.2 Hz, H-4), 6.14 (d, 1H, J1,2 = 8.4 Hz, H-1), 7.22-7.70 (m, 12H,
aromatic), 7.83-7.74 (m, 2H, aromatic), 8.05-7.92 (m, 4H, aromatic), 8.13-8.05 (m, 2H,
13
aromatic). C-n.m.r. (75 MHz): δ, 61.8, 66.3, 67.6, 71.7, 72.7, 97.8, 128.4, 128.5 (×2), 128.7
(×5), 128.7, 128.9 (×2), 129.2, 129.9 (×4), 130.0 (×2), 130.1 (×2), 133.5, 133.7, 133.9, 134.0,
164.9, 165.4, 165.5, 166.1 ppm; HR-ESI MS [M+Na]+ calcd for C34H27NO12Na calcd: 664.1431,
found 664.1419.
2,3,4,6-Tetra-O-benzyl-D-glucopyranosyl nitrate (20). AgNO3 was added to a solution of
2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl chloride[37-38] (0.45 g, 0.80 mmol) in dry acetonitrile
(5.0 mL) and the resulting mixture was stirred for 5 min at rt. After that, the solids were filtered
off through a pad of Celite and the filtrate was concentrated under reduced pressure. The residue
14
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was dissolved in CH2Cl2 (∼100 mL) and washed with water (20 mL), 1% aq. NaOH (20 mL),
and water (3 × 20 mL). The organic layer was separated, dried over MgSO4, and concentrated in
vacuo to afford the title compound (α/β = 1/10, 451.3 mg, 96% yield) as a clear syrup. Analytical
1
data for β-20: Rf = 0.70 (ethyl acetate/hexanes, 3/7,v/v): H-n.m.r. (300 MHz): δ, 3.50-3.64 (m,
2H, H-2, 4), 3.80-3.65 (m, 4H, H-3, 5, 6a, 6b), 5.00-4.41 (m, 8H, 4 × CH2Ph), 5.76 (d, 1H, J1,2
=
13
8.2 Hz, H-1), 7.13 (dd, 3H, J = 2.9, 6.6 Hz, aromatic), 7.43-7.22 (m, 17H, aromatic) ppm; C-
n.m.r. (75 MHz): δ, 68.0, 73.7, 75.2, 75.4, 75.8, 76.0, 76.9, 78.9, 84.8, 100.2, 127.9 (×2), 128.0
(×6), 128.1, 128.2, 128.3 (×2), 128.6 (×6), 128.7 (×2), 137.4, 137.8, 137.9, 138.2; HR-ESI MS
[M+Na]+ calcd for C34H35NO8Na calcd: 608.2260, found 608.2252.
3,4,6-Tri-O-acetyl-2-O-benzyl-D-glucopyranosyl nitrate (25). AgNO3 was added to a solution
of 3,4,6-tri-O-acetyl-2-O-benzyl-α-D-glucopyranosyl bromide[39] (653.4 mg, 1.42 mmol) in dry
acetonitrile (6.0 mL) and the resulting mixture was stirred for 5 min at rt. After that, the solids
were filtered off through a pad of Celite and the filtrate was concentrated under reduced pressure.
The residue was dissolved in CH2Cl2 (∼100 mL) and washed with water (20 mL), 1% aq. NaOH
(20 mL), and water (3 × 20 mL). The organic layer was separated, dried over MgSO4, and
concentrated in vacuo to afford the title compound (501.3 mg, 86% yield, α/β = 1/25) as a clear
syrup. Analytical data for β-25: Rf = 0.73 (ethyl acetate/hexanes, 1/1, v/v); 1H-n.m.r. (300 MHz):
δ, 1.92, 2.02, 2.07 (3 s, 9H, 3 x COCH3), 3.61 (dd, 1H, J2,3 = 9.2 Hz, H-2), 3.86 (ddd, 1H, J5, 6a
=
2.2 Hz, J5,6b = 4.6 Hz, H-5), 4.10 (dd, 1H, J6a,6b = 12.5 Hz, H-6a), 4.30 (dd, 1H, H-6b), 4.67 (dd,
2H, 2J = 11.7 Hz, CH2Ph), 5.01 (dd, 1H, J4,5 = 10.0 Hz, H-4), 5.27 (dd, 1H, J3,4 = 9.4 Hz, H-3),
5.74 (d, 1H, J1,2 = 8.3 Hz, H-1), 7.46-7.03 (m, 5H, aromatic); 13C n.m.r. (75 MHz): δ, 20.7, 20.8
15
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(×2), 61.4, 67.7, 72.7, 73.8, 75.0, 75.6, 99.6, 128.2 (×2), 128.4, 128.7 (×2), 136.9, 169.7, 170.0,
170.7 ppm; HR-ESI MS [M+Na]+ calcd for C19H23NO11Na calcd: 464.1169, found 464.1164.
Synthesis of disaccharides
A typical glycosylation procedure. A mixture of glycosyl donor (0.0675 mmol or as indicated in
tables), glycosyl acceptor (0.045 mmol), and freshly activated molecular sieves (3 Å, 150 mg) in
acetonitirle (1.0 mL, or other solvents as indicated in tables) was stirred under argon for 1 h at rt.
Promoter (0.05–0.0675 mmol) was added, and the resulting mixture was stirred at rt for the time
indicated in tables. The solids were filtered off through a pad of Celite and rinsed successively
with CH2Cl2. The combined filtrate (~40 mL) was washed with water (10 mL), sat. aq. Na2CO3
(10 mL) and water (2 × 10 mL). The organic phase was separated, dried with MgSO4, and
concentrated in vacuo. The residue was purified by column chromatography on silica gel (ethyl
acetate–hexane gradient elution) to afford a disaccharide in yields listed below and in tables.
Methyl 6-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (3) was obtained from donor 1[20] and acceptor 2[26] as a clear syrup in yields
listed in tables. Analytical data for 3 was in accordance with that reported previously.[40]
Methyl 2-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (5) was obtained from donor 1[20] and acceptor 4[26] as a clear syrup in 88%
yield. Analytical data for 5 was in accordance with that reported previously.[41]
16
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Methyl 3-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (7) was obtained from donor 1[20] and acceptor 6[26] as a clear syrup in 94%
yield. Analytical data for 7 was in accordance with that reported previously.[26]
Methyl 4-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (9) was obtained from donor 1[20] and acceptor 8[26] as a clear syrup in 67%
yield. Analytical data for 9 was in accordance with that reported previously.[40]
Methyl 6-O-(2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (11) was obtained from donor 10 and acceptor 2[26] as a clear syrup in 71%
yield. Analytical data for 11 was in accordance with that reported previously.[41]
Methyl 2-O-(2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (12) was obtained from donor 10 and acceptor 4[26] as a clear syrup in 43%
yield. Analytical data for 12 was in accordance with that reported previously.[42]
Methyl 3-O-(2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (13) was obtained from donor 10 and acceptor 6[26] as a clear syrup in 25%
22
yield. Analytical data for 13: Rf = 0.50 (ethyl acetate/toluene, 15/85, v/v); [α]D -0.97 (c = 1,
CHCl3); 1H-n.m.r. (300 MHz): δ, 3.39 (s, 3H, OCH3), 3.62-3.79 (m, 4H, H-2, 5, 6a, 6b), 3.83 (dd,
1H, J4,5 = 9.6 Hz, H-4), 3.98 (dd, 1H, J5’,6a’ = 3.7 Hz, J6a’,6b’ = 12.4 Hz, H-6a’), 4.31 (dd, 1H, J3,4
= 9.2 Hz, H-3), 4.45 (dd, 1H, J5’,6b’ = 2.0 Hz, H-6b’), 4.52 (dd, 2H, 2J = 12.0 Hz, CH2Ph), 4.62-
4.80 (m, 4H, 2 × CH2Ph), 4.81-4.92 (m, 2H, H-1, 5’), 5.57 (d, 1H, J1’,2’ = 1.3 Hz, H-1’), 5.82 (dd,
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1H, J2’,3’ = 3.1 Hz, H-2’), 5.94 (dd, 1H, J3’,4’ = 10.2 Hz, H-3’), 6.05 (dd, 1H, J4’,5’ = 10.1 Hz, H-
4’), 6.96-8.18 (m, 35H, aromatic); 13C-n.m.r. (75 MHz): δ, 55.1, 62.5, 66.4, 68.1, 68.4, 69.7,
70.3, 70.4, 72.3, 73.5, 74.6, 76.2, 77.8, 79.1, 97.2, 97.7, 127.4, 127.6 (×3), 127.8, 128.0 (×2),
128.1 (×3), 128.3 (×7), 128.4 (×3), 128.5 (×2), 129.1 (×2), 129.3, 129.6 (×2), 129.7 (×2), 129.8
(×5), 130.1, 132.7, 133.1, 133.2 (×2), 137.5 (×3), 165.1, 165.2, 165.7, 166.2 ppm; HR-ESI MS
[M+Na]+ calcd for C62H58NO15Na calcd: 1065.3673, found 1065.3662.
Methyl 4-O-(2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (14) was obtained from donor 10 and acceptor 8[26] as a clear syrup in 12%
yield. Analytical data for 14 was in accordance with that reported previously.[43]
Methyl 6-O-(2,3,4,6-tetra-O-benzoyl-β-D-galactopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (16) was obtained from donor 15 and acceptor 2[26] as a clear syrup in 79%
yield. Analytical data for 16 was in accordance with that reported previously.[44]
Methyl 2-O-(2,3,4,6-tetra-O-benzoyl-β-D-galactopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (17) was obtained from donor 15 and acceptor 4[26] as a clear syrup in 79%
yield. Analytical data for 17 was in accordance with that reported previously.[45]
Methyl 3-O-(2,3,4,6-tetra-O-benzoyl-β-D-galactopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (18) was obtained from donor 15 and acceptor 6[26] as a clear syrup in 72%
yield. Analytical data for 18 was in accordance with that reported previously.[45]
18
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Methyl 4-O-(2,3,4,6-tetra-O-benzoyl-β-D-galactopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (19) was obtained from donor 15 and acceptor 8[26] as a clear syrup in 71%
yield. Analytical data for 19 was in accordance with that reported previously.[41]
Methyl 2,3,4-tri-O-benzyl-6-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-α-D-
glucopyranoside (21) was obtained from donor 20 and acceptor 2[26] as a clear syrup in yields
and α/β ratios listed in tables. Analytical data for 21 was in accordance with that reported
previously.[40]
Methyl 3,4,6-tri-O-benzyl-2-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-α-D-
glucopyranoside (22) was obtained from donor 20 and acceptor 4[26] as a clear syrup in 96%
yield with α/β = 1.1/1. Analytical data for 22 was in accordance with that reported previously.[41]
Methyl 2,4,6-tri-O-benzyl-3-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-α-D-
glucopyranoside (23) was obtained from donor 20 and acceptor 6[26] as a clear syrup in 94%
yield with α/β = 1.1/1. Analytical data for 23 was in accordance with that reported previously.[46]
Methyl 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-D-glucopyranosyl)-α-D-
glucopyranoside (24) was obtained from donor 20 and acceptor 8[26] as a clear syrup in 90%
yield with α/β = 1.3/1. Analytical data for 24 was in accordance with that reported previously.[40]
Methyl 6-O-(3,4,6-tri-O-acetyl-2-O-benzyl-D-glucopyranosyl)-2,3,4-tri-O-benzyl-α-D-
glucopyranoside (26) was obtained from donor 25 and acceptor 2[26] as a clear syrup in yields
19
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and α/β ratios listed in tables. Analytical data for 26 was in accordance with that reported
previously.[47]
Methyl 2-O-(3,4,6-tri-O-acetyl-2-O-benzyl-D-glucopyranosyl)-3,4,6-tri-O-benzyl-α-D-
glucopyranoside (27) was obtained from donor 25 and acceptor 4[26] as a clear syrup in 37%
yield (α/β = 5.6/1). Analytical data for 27 was in accordance with that reported previously.[41]
Methyl 3-O-(3,4,6-tri-O-acetyl-2-O-benzyl-D-glucopyranosyl)-2,4,6-tri-O-benzyl-α-D-
glucopyranoside (28) was obtained from donor 25 and acceptor 6[26] earlier as a clear syrup in
40% yield (α/β = 4.3/1). Analytical data for 28 was in accordance with that reported
previously.[48]
Methyl 4-O-(3,4,6-tri-O-acetyl-2-O-benzyl-D-glucopyranosyl)-2,3,6-tri-O-benzyl-α-D-
glucopyranoside (29) was obtained from donor 25 and acceptor 8[26] as a clear syrup in 90%
yield (α/β > 20/1). Analytical data for 29 was in accordance with that reported previously.[41]
Methyl
2,3,4-tri-O-benzoyl-6-O-(2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl)-α-D-
glucopyranoside (31) was obtained from donor 1 and methyl 2,3,4-tri-O-benzoyl-α-D-
glucopyranoside (30)[49] as a clear syrup in 81% yield. Analytical data for 31 was in accordance
with that reported previously.[50]
20
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ASSOCIATED CONTENT
1
13
Supporting Information. Additional experimental details, H and C NMR spectra for all new
and selected known compounds have been supplied as the Supporting Information. This material
is available free of charge via the Internet.
AUTHOR INFORMATION
Corresponding Author
* Alexei V. Demchenko. E-mail: demchenkoa@umsl.edu
Author Contributions
The manuscript was written through contributions of all authors. / All authors have given
approval to the final version of the manuscript.
Funding Sources
This work was supported by grants from the National Institute of General Medical Sciences
(GM120673) and the National Science Foundation (CHE-1800350) .
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