Direct glycosylation of unprotected and unactivated sugars using bismuth nitrate pentahydrate

Article abstract of DOI:10.1080/07328303.2015.1028585

Bi(NO₃)₃, a low-cost, mild, and environmentally green catalyst, has been successfully utilized for Fischer glycosylation for the synthesis of alkyl/aryl glycopyranosides by reacting unprotected sugars, namely, D-glucose, L-rhamnose, D-galactose, D-arabinose, and N-acetyl-D-glucosamine with various alcohols in good to excellent yields. The glycosides were formed with high α-selectivity. Further, an expedient separation of α- and β-glycosides using silver nitrate-impregnated silica gel flash liquid chromatography has been developed.

Full text of DOI:10.1080/07328303.2015.1028585

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Direct Glycosylation of Unprotected and
Unactivated Sugars Using Bismuth Nitrate
Pentahydrate
Innaiah K. Polankiab, Siva H. Kurmaab & Asish K. Bhattacharyaab
a Division of Organic Chemistry, CSIR-National Chemical Laboratory
(CSIR-NCL), Pune 411 008, India
b Academy of Scientific and Innovative Research (AcSIR),CSIR-NCL,
Pune 411 008, India
Published online: 26 May 2015.
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To cite this article: Innaiah K. Polanki, Siva H. Kurma & Asish K. Bhattacharya (2015) Direct
Glycosylation of Unprotected and Unactivated Sugars Using Bismuth Nitrate Pentahydrate , Journal of
Carbohydrate Chemistry, 34:4, 196-205, DOI: 10.1080/07328303.2015.1028585
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Journal of Carbohydrate Chemistry, 34:196–205, 2015
ꢀ
C
Copyright CSIR-NCL
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2015.1028585
Direct Glycosylation of
Unprotected and Unactivated
Sugars Using Bismuth Nitrate
Pentahydrate
Innaiah K. Polanki,1,2 Siva H. Kurma,1,2
and Asish K. Bhattacharya1,2
1Division of Organic Chemistry, CSIR-National Chemical Laboratory (CSIR-NCL),
Pune 411 008, India
2Academy of Scientific and Innovative Research (AcSIR), CSIR-NCL,
Pune 411 008, India
GRAPHICAL ABSTRACT
Bi(NO3)3, a low-cost, mild, and environmentally green catalyst, has been successfully
utilized for Fischer glycosylation for the synthesis of alkyl/aryl glycopyranosides by re-
acting unprotected sugars, namely, D-glucose, L-rhamnose, D-galactose, D-arabinose,
and N-acetyl-D-glucosamine with various alcohols in good to excellent yields. The gly-
cosides were formed with high α-selectivity. Further, an expedient separation of α- and
β-glycosides using silver nitrate–impregnated silica gel flash liquid chromatography
has been developed.
Keywords
Fischer glycosylation; Bismuth nitrate; Carbohydrates; Flash
chromatography; Separation
Received January 30, 2015; accepted March 9, 2015.
Address correspondence to Asish K. Bhattacharya, Division of Organic Chemistry,
CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune 411
008, India. E-mail: ak.bhattacharya@ncl.res.in
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/lcar.
196

Direct Glycosylation of Sugars
197
INTRODUCTION
Carbohydrates play an important role in various biological processes (e.g., cell
adhesion, cell–cell interaction) and in disease processes such as pathogen in-
fection, tumor metastasis, and inflammation in the precinct as glycosides and
glycoconjugates.[1–5] Considering their biological importance, several glycosy-
lation methods have been developed over the years. Beginning with the clas-
sical Koenig-Knorr method,[6] several excellent glycosidation methods have
been reported, including Schmidt’s trichloroacetamidates,[7] glycosyl halides,[8]
n-pentenyl glycosides,[9] thioglycosides,[10] glycols,[11] phosphates,[12] and phos-
phites,[13] to name a few. However, Fischer glycosidation,[14] developed in
1893, still remains a favorite for the simple glycoside bond formation as it
can be carried out in a single step without bothering with protecting group
strategies. Moreover, Fischer-type glycosylation is the preferred method for
the synthesis of simple glycosides, namely, viz. allyl, methyl, ethyl, benzyl,
and p-methoxybenzyl glycosides as these temporary protecting groups of the
anomeric hydroxyl group can easily undergo deprotection for further utiliza-
tion in oligosaccharide synthesis.[15] Several synthetic methodologies[] have
been explored to improve conventional Fischer glycosylation; however, in gen-
eral, some of these methodologies suffer from a number of shortcomings: use
of strong mineral acids, longer reaction time, use of excess of alcohols, prepa-
ration of specific catalysts, and decomposition of the product under reaction
conditions.[15] Since Fischer glycosylation is an important reaction for glyco-
side bond formation, we reasoned that synthesis of glycosides using a green
catalyst would still be desirable.
RESULTS AND DISCUSSION
In our quest for an alternative procedure for Fischer glycosylation, we opined
that bismuth nitrate could act as an efficient catalyst for the desired glycosy-
lation. In recent times, bismuth salts, especially bismuth nitrate, have been
found to be an efficient catalyst[17a–e] in various organic transformations due
to their low toxicity,[17f-g] easy availability, and low cost. Herein, we report bis-
muth nitrate–catalyzed Fischer-type glycosylation of various unprotected and
unactivated sugars with a diverse range of alcohols to afford alkyl/aryl glyco-
sides (Sch. 1).
Initially, D-glucose was reacted with a varied quantity of propargyl alco-
hol (4–10 equiv.) at 60◦C to 100◦C under solvent-free reaction conditions cat-
alyzed by Bi(NO3)3 (5 mol% and 10 mol%). We observed that D-glucose on
reaction with propargyl alcohol (5 equiv.) and Bi(NO3)3 (10 mol%) at 60◦C
afforded the desired propargyl D-glucoside in an 83% yield in 4 h as a mixture
of both anomers (α:β = 10:1). Thus, taking the alcohol (5 equiv.) and Bi(NO3)3

I. K. Polanki et al.
198
Scheme 1: Synthesis of alkyl/aryl glycosides.

Direct Glycosylation of Sugars
199
Table 1: Comparison of Bi(NO3)3·5H2O with other catalysts for the Fischer-type
glycosylation
Temp.
(◦C)
Time
(h)
Yielda
(%)
α:β
Ratio
Glycoside
Catalyst
Sulfamic acid
H2SO4-silica gel
Bi(NO3)3·5H2O
80
65
60
4
6
4
78[16d]
79[16b]
83
8:1
10:1
10:1
Sulfamic acid
H2SO4-silica gel
CF3SO3H
Bi(NO3)3·5H2O
80
65
80
60
4
6
48
3
82[16d]
79[16b]
67(16e)
81
5:1
10:1
-
15:1
Sulfamic acid
H2SO4-silica gel
Bi(NO3)3·5H2O
80
65
60
5
7
2.5
81[16d]
78[16b]
76
6:1
10:1
19:1
Sulfamic acid
H2SO4- silica
Bi(NO3)3·5H2O
80
65
60
4
6
3.5
78[15d]
79[15b]
83
8:1
10:1
13.2:1
a References for reported methods.
(10 mol%) at 60◦C as the ideal reaction condition, we embarked on generaliza-
tion of the reaction by reacting various sugars with alcohols to afford a series
of alkyl/aryl glycosides in excellent yields. It is pertinent to mention here that
in all of the examples, the thermodynamically more stable glycopyranosides[18]
were obtained. All of the alkyl/aryl glycoside products formed by the reaction
of unprotected and unactivated monosaccharides with varied alcohols are de-
picted with yield, α/β ratio, and the time taken for completion of the reaction
in parentheses. Also, we have carried out a comparison of our method with
some of the reported procedures by taking selected substrates (Table 1). The
comparative study was carried out with reference to the catalyst used, reaction
temperature and time, yield, and the ratio of α- and β-glycosides. The higher
selectivity for the formation of α-glycosides was observed in our method as
compared to the previous procedures.
In general, all of the reactions proceeded smoothly and afforded the de-
sired alkyl glycopyranosides in good to excellent yields with anomeric selectiv-
ity. In order to carry out the detailed characterization of the products formed,

I. K. Polanki et al.
200
the crude products were acetylated (see Experimental). The ratio of α- and
1
β-anomers formed was deduced by H NMR spectral analysis of the acety-
lated products by comparing the integration values of the peaks and coupling
constant. In all cases, the product was formed as a mixture of α/β-anomers,
but the thermodynamically more stable α-anomer was found to be predom-
inant. The ratio of anomers formed was also determined by HPLC profil-
˚
ing (SunFire C18 column, 100 A, 3.5 μm, 4.6 mm × 150 mm, mobile phase:
MeOH: H2O, 9:1, UV set at 254 or 220 nm) of all the synthesized compounds
(see supplementary data).
In all the cases, glycosides are formed in a mixture of α- and β-anomers
and the ratio of anomeric mixtures thus formed was practically determined
1
by H NMR spectroscopy of the products by comparing anomeric peaks and
coupling constants. The isolation of α- and β-glycosides was seldom carried
out, as isolation by column chromatography is tedious as well as time con-
suming. We were interested in the development of a method for the isolation
of pure α- and β-glycosides. In this endeavor, we turned our attention to us-
ing an automated flash chromatography system equipped with a UV detec-
tor and ELSD. The RediSep column cartridge (12 g) was filled with AgNO3-
impregnated flash silica gel.[19] The crude reaction mixture was loaded onto
the CombiFlash using a solid sample cartridge with slurry made with SiO2
(230–400 mesh). The detection of peaks was performed with the UV detector
set at 254 or 220 nm and ELSD and eluted in gradient mode with petroleum
ether/EtOAc at a 20-mL/min flow rate. All glycosides of rhamnose and arabi-
nose were eluted in 8% to 15% EtOAc/petroleum ether, whereas glycosides of
glucose, galactose, and N-acetyl glucosamine were found to be polar and eluted
with 15% to 20% EtOAc/petroleum ether mobile phase. A representative sepa-
ration of compounds 3l and 3t is depicted in Figure 1. All of the aryl/alkyl gly-
cosides were satisfactorily separated to furnish pure α- and β-glycosides using
the automated flash chromatography system with AgNO3-impregnated flash
silica gel except for the propargyl glycosides, as these formed complex mix-
tures with AgNO3 and could not be separated. These were separated by flash
chromatography/re–flash chromatography (230–400 mesh) under the identical
conditions. All of the isolated α- and β-glycosides were identified by their spec-
tral data (see supplementary data).
CONCLUSIONS
In conclusion, we have demonstrated that Fischer glycosylation of unpro-
tected and unactivated sugars with a diverse range of alcohols catalyzed by
environmentally benign catalyst Bi(NO3)3 afforded alkyl/aryl glycosides in
good to excellent yields. It is pertinent to mention here that in all of the

Direct Glycosylation of Sugars
201
Figure 1: Separation of α- and β-glycosides by AgNO3-impregnated silica gel using
automated flash chromatography system equipped with UV detector and ELSD: (a)
compound 3l; (b) compound 3t.
cases, the glycosides were formed with high α-selectivity. Also, we have de-
veloped an expedient separation method for α- and β-glycosides using silver
nitrate–impregnated silica gel flash liquid chromatography, which could be
used for the large-scale separations.

I. K. Polanki et al.
202
EXPERIMENTAL
Typical Experimental Procedure
To a mixture of sugar (1 mmol) and alcohol (5 mmol), Bi(NO3)3·5H2O
(10 mol%) was added with stirring at rt in a round-bottom flask fitted with
a condenser, and the reaction mixture was then allowed to stir at 60◦C under
nitrogen for an appropriate time until the completion (TLC) of the reaction.
The alcohol was removed in vacuo and the resulting crude product was passed
through a short bed of Celite and eluted with DCM-MeOH (95:5) to furnish
corresponding alkyl/aryl glycosides. Alternatively, the crude reaction product
was acetylated with Ac2O and pyridine (2:1), and usual workup resulted in
the crude acetylate product, which was purified by flash chromatography. All
of the products were characterized by spectral analysis. The spectral data of
all new compounds are given below, and the spectral data of selected known
compounds are given as supplementary data.
Spectral Data of New Compounds
Compound 3lα: Semisolid; υmax (CHCl3)/cm−1 2934, 1744, 1369, 1216,
1071, 734; δH (400 MHz, CDCl3, TMS): 5.33 (dd, J = 3.4, 3.4 Hz, 1H), 5.18
(dd, J = 1.7, 1.4 Hz, 1H), 5.05 (t, J = 9.8 Hz, 1H), 4.87 (brs, 1H), 3.99–3.92 (m,
1H), 3.62–3.55 (m, 1H), 2.15, 2.05, 1.98, 1.88–1.81 (m, 2H), 1.74–1.68 (m, 2H),
1.53–1.41 (m, 2H), 1.34–1.24 (m, 4H), 1.21 (d, J = 6.4 Hz, 3H); δC (100 MHz,
CDCl3, TMS): 170.1, 169.9, 169.4, 95.5, 75.9, 71.4, 70.5, 69.2, 66.2, 33.1, 31.3,
25.5, 24.0, 23.7, 20.9, 20.7, 20.7, 17.3; HRMS (ESI): calcd for C18H28O8Na
395.1676, found 395.1669.
Compound 3lβ: Semisolid; δH (500 MHz, CDCl3, TMS) 5.41 (brs, 1H),
5.07–5.01 (m, 2H), 4.73 (brs, 1H), 3.66 (brs, 1H), 3.52 (d, J = 6.1 Hz, 1H),
2.18, 2.05, 1.98, 1.86–1.81 (m, 2H), 1.70 (brs, 2H), 1.50–1.40 (m, 2H), 1.29–1.26
(m, 7H); δC (125 MHz, CDCl3, TMS) 170.6, 170.2, 169.8, 96.1, 77.0, 71.3, 70.8,
70.4, 69.8, 33.0, 31.4, 25.5, 23.8, 23.7, 20.9, 20.8, 20.6, 17.5.
Compound 3tα: Semisolid; υmax (CHCl3)/cm−1 2932, 1742, 1445, 1216,
1050, 747; δH (400 MHz, CDCl3, TMS) 5.41–5.35 (m, 1H), 5.23–5.11 (m, 1H),
4.99–4.94 (m, 1H), 4.44–4.39 (m, 1H), 4.26–4.21 (m, 1H), 4.10–4.04 (m, 1H),
3.66–3.56 (m, 1H), 2.10, 2.09, 2.07, 1.88–1.81 (m, 2H), 1.74–1.68 (m, 2H),
1.54–1.48 (m, 2H), 1.33–1.22 (m, 4H); δC (100 MHz, CDCl3, TMS) 170.7, 170.2,
169.7, 103.7, 78.4, 69.3, 67.4, 65.9, 63.4, 33.3, 31.5, 25.5, 23.9, 23.6, 21.0, 20.8,
20.6;HRMS (ESI): calcd for C17H26O8Na 381.1520, found 395.1512.
Compound (3tβ): Semisolid; δH (400 MHz, CDCl3, TMS) 5.25 (brs, 1H), 5.17
(dd, J = 7.6, 7.1 Hz, 1H), 5.04 (dd, J = 3.4, 3.4 Hz, 1H), 4.51 (d, J = 7.1 Hz,
1H), 4.03 (dd, J = 2.9, 3.2 Hz, 1H), 3.62 (dd, J = 1.2, 1.2 Hz, 2H), 2.14, 2.06,
2.02, 1.88–1.78 (m, 2H), 1.72–1.68 (m, 2H), 1.50–1.46 (m, 2H), 1.33–1.24 (m,

Direct Glycosylation of Sugars
203
4H); δC (100 MHz, CDCl3, TMS) 170.4, 170.2, 169.4, 99.5, 77.4, 70.4, 69.4, 67.9,
63.2, 33.3, 31.5, 25.5, 23.7, 23.5, 20.9, 20.8, 20.7.
Compound 3uα: Semisolid; υmax (CHCl3)/cm−1 2975, 1742, 1433, 1216,
1047, 746; δH (500 MHz, CDCl3, TMS) 5.25 (brs, 1H), 5.16 (dd, J = 7.2, 7.2 Hz,
1H), 5.04 (dd, J = 3.4, 3.8 Hz, 1H), 4.46 (d, J = 7.3 Hz, 1H), 4.03 (dd, J = 2.7,
3.1 Hz, 1H), 3.94–3.90 (m, 1H), 3.62 (dd, J = 1.5, 1.5 Hz, 1H), 2.14, 2.06, 2.02,
1.24 (d, J = 6.5 Hz, 3H), 1.15 (d, J = 6.1 Hz, 3H); δC (125 MHz, CDCl3, TMS)
170.4, 170.2, 169.4, 99.9, 72.4, 70.4, 69.4, 67.9, 63.3, 23.3, 21.8, 20.9, 20.8, 20.7;
HRMS (ESI): calcd for C14H22O8Na 341.1207, found 341.1200.
Compound 3uβ: Solid; m.p. 130–132◦C; δH (500 MHz, CDCl3, TMS)
5.37–5.35 (m, 2H), 5.20 (d, J = 3.4 Hz, 1H), 5.12 (dd, J = 3.1, 3.8 Hz, 1H),
4.05 (d, J = 12.9 Hz, 1H), 3.89–3.84 (m, 1H), 3.66 (d, J = 13.4 Hz, 1H), 2.16,
2.08, 2.02, 1.24 (d, J = 6.1 Hz, 3H), 1.12 (d, J = 6.1 Hz, 3H); δC (125 MHz,
CDCl3, TMS) 170.5, 170.4, 170.1, 94.7, 70.6, 69.3, 68.6, 67.3, 60.4, 23.2, 21.5,
21.0, 20.8, 20.7.
ACKNOWLEDGMENTS
The authors are grateful to Dr. Arun K. Shaw, Senior Principle Scientist,
Medicinal Chemistry Division, CSIR-Central Drug Research Institute, Luc-
know, for critical evaluation of the manuscript and fruitful suggestions.
FUNDING
This work was supported by the Council of Scientific and Industrial Research
(CSIR), New Delhi–sponsored network project (NaPAHA, CSC0130). IKP is
grateful to CSIR, New Delhi, for the award of a Senior Research Fellowship
(SRF).
SUPPLEMENTAL MATERIAL
Electronic Supplementary Information (ESI) available: Full experimental pro-
cedures; spectral data of all new and selected known compounds; copies of 1H,
13C NMR, and DEPT spectra; LCMS and HRMS spectra of all the compounds;
and HPLC chromatograms. These materials are available on request from the
corresponding author.
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19. Preparation[20] of AgNO3-impregnated flash silica gel: The AgNO3-impregnated
flash silica gel (230–400 mesh) for column chromatography was prepared by adding the
silica gel to a methanolic silver nitrate solution (5%, w/v). The slurry was then dried
under reduced pressure on a rotary evaporator to remove MeOH followed by drying in
a hot-air oven at 80ºC for approximately 1 h to afford dry AgNO3-impregnated flash
silica gel for flash column chromatography.
20. Daramwar, P.P.; Srivastava, P.L.; Priyadarshini, B.; Thulasiram, H.V. Analyst
2012, 137, 4564, and references cited therein.