Synthesis of the tetrasaccharide repeating unit of the O-glycan from the polar flagellum flagellin of Azospirillum brasilense Sp7

Article abstract of DOI:10.1016/j.carres.2014.06.019

Chemical synthesis of the tetrasaccharide repeating unit of the O-glycan from the polar flagellum flagellin of Azospirillum brasilense Sp7 in the form of its p-methoxyphenyl glycoside is reported. The required glycosidic linkages have been accomplished by

Full text of DOI:10.1016/j.carres.2014.06.019

Accepted Manuscript
Synthesis of the tetrasaccharide repeating unit of the O-glycan from the polar
flagellum flagellin of Azospirillum brasilense Sp7
Kumar Bhaskar Pal, Balaram Mukhopadhyay
PII:
S0008-6215(14)00257-2
DOI:
http://dx.doi.org/10.1016/j.carres.2014.06.019
CAR 6780
Reference:
Carbohydrate Research
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 May 2014
24 May 2014
18 June 2014
Please cite this article as: Pal, K.B., Mukhopadhyay, B., Synthesis of the tetrasaccharide repeating unit of the O-
glycan from the polar flagellum flagellin of Azospirillum brasilense Sp7, Carbohydrate Research (2014), doi: http://
dx.doi.org/10.1016/j.carres.2014.06.019
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Synthesis of the tetrasaccharide repeating unit of the O-glycan from the polar
flagellum flagellin of Azospirillum brasilense Sp7
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Kumar Bhaskar Pal, Balaram Mukhopadhyay*
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Indian Institute of Science Education and Research Kolkata, Mohanpur, Nadia 741252, India
Abstract: Chemical synthesis of the tetrasaccharide repeating unit of the O-glycan from the polar
flagellum flagellin of Azospirillum brasiliens Sp7 in the form of its p-methoxyphenyl glycoside is
reported. The required glycosidic linkages have been accomplished by activation of thioglycosides with
N-iodosuccinimide in the presence of H2SO4-silica. H2SO4-silica was found to be an effective alternative
to the classical acid promoters like TfOH or TMSOTf and it can lead to the formation of both 1,2-cis and
1,2-trans glycosidic linkage formation depending on the protecting group manipulation and control of the
reaction condition.
Keywords: Flagellin O-glycan, total synthesis, H2SO4-silica
1. Introduction
Bacterial flagellum, consisting of the protein flagellin, is important for the movement of the
bacteria in space and adapting to the environmental conditions in water, soil, plant and animal tissues.
The C- and N-terminus of flagellin is conserved but the middle portion varies and it is responsible for the
adhesive functions and antigenic properties of the bacteria concerned.1,2 It is evident that the presence of
carbohydrates in the middle region of flagellin affects the antigenic property of the bacteria.3 The flagellin
glycosylation occurs either independently4 or in association with the LPS biosynthesis.5 It is observed that
 Corresponding author’s E.mail: sugarnet73@hotmail.com; Tel: +91 9748 261742, Fax:
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the bacterial species having glycosylated flagellin actively interact with other microorganisms.6 The
bacteria of the genus Azospirillum is well known for the associative plant-microbe interactions and it has
a polar flagellum with glycosylated flagellin.7 Recently Belyakov et. al.8 have reported the structure of
the O-linked repeatitive glycan chain of the polar flagellum flagellin isolated from Azospirillum
brasiliense Sp7. Herein, we report the chemical synthesis of the tetrasaccharide repeating unit in the form
of its p-methoxyphenyl glycoside (1, Figure 1). Chemical synthesis of the repeating unit will provide the
route to have the target tetrasaccharide in pure form and reasonable quantity to enable better insight of the
biosynthesis and antigenic behaviour of this important post translational modifications occurred in
bacterial flagellum.
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Figure 1. Structure of the target tetrasaccharide related to the O-linked glycan from the polar flagellum
flagellin of Azospirillum brasilense Sp7
2. Results and discussion
The synthesis of the desired tetrasaccharide was planned in the form of it p-methoxyphenyl
glycoside as this can be cleaved selectively from the per-O-acetylated derivative and allows further
glycoconjugate formation through trichloroacetimidate chemistry. Therefore, known p-tolyl 3-O-(4-
methoxybenzyl)-1-thio--D-galactopyranoside (2)9 was acetylated using Ac2O in pyridine to afford the
corresponding tri-O-acetyl derivative 3 in 96% yield. Glycosylation of the donor 3 with known acceptor,
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p-methoxyphenyl 4,6-O-benzylidene-2-deoxy-2-phthalimido--D-glucopyranoside (4)10 using N-
iodosuccinimide (NIS) in the presence of H2SO4-silica11,12,13 afforded the disaccharide 5 in 89% yield.
Use of N-phthalimido instead of the desired N-acetamido group was required as the acceptor containing
NHAc becomes too unreactive to react in the glycosylation reaction.14 The p-methoxybenzyl group was
selectively removed by the oxidative cleavage using DDQ15 to furnish the disaccharide acceptor 6 in 81%
yield. This disaccharide acceptor was subsequently glycosylated with known donor, p-tolyl 2,3,4-tri-O-
acetyl-1-thio--L-rhamnopyranoside (7).16 NIS promoted activation of the thioglycoside gave the
trisaccharide 8 in 85% yield. Finally, the reductive opening of the benzylidene acetal using triethylsilane
in the presence of BF3*Et2O17 afforded the trisaccharide acceptor 9 in 83% yield (Scheme 1).
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Scheme 1:Synthesis of the trisaccharide acceptor 9
In a separate experiment, known p-tolyl 3,4-O-isopropylidene-1-thio--L-fucopyranoside (10)14
was benzylated using BnBr in the presence of NaH18 to produce the fully protected derivative 11 in 92%
yield. The isopropylidene group was hydrolyzed by 80% AcOH at 80 C19 and the free hydroxyl groups
were subsequently acetylated to afford the compound 12 in 86% yield over two steps. Further, donor 12
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was coupled with the trisaccharide acceptor 9 using NIS in the presence of H2SO4-silica to give the
protected tetrasaccharide 13 in 81% yield. It is important to note that the fucosyl donor 11 is also suitable
for the required cis-glycosylation. However, it is found to be too reactive and failed to produce the desired
tetrasaccharide in good yield. Introduction of two acetate groups instead of the isopropylidene moiety
gave the donor optimum reactivity and the yield of the final glycosylation was acceptable. Once the
protected tetrasaccharide is in hand, the NPhth group was cleaved using ethylene diamine20 and converted
to the corresponding acetamido group by subsequent acetylation to furnish the tetrasaccharide 14 in 75%
yield. The benzyl groups were removed by catalytic hydrogenolysis using 10% Pd-C on a Thales Nano H-
cube flow hydrogenation assembly with constant flow of hydrogen. Finally, the acetate groups were
cleaved by transesterification using NaOMe in MeOH21 to afford the target tetrasaccharide 1 in 68% over
two steps (Scheme 2).
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Scheme 2: Synthesis of the target tetrasaccharide 1
3. Conclusion
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In conclusion, the total synthesis of the tetrasaccharide repeating unit of the O-glycan from the
polar flagellum flagellin of Azospirillum brasiliens Sp7 is achieved in the form of its p-methoxyphenyl
glycoside. All required glycosidic linkages have been formed by the activation of thioglycosides using
NIS in the presence of H2SO4-silica. All glycosylation reactions were resulted in formation of single
desired isomer in good to excellent yield. The p-methoxyphenyl group at the reducing end of the target
tetrasachharide is of particular interest as it can be cleaved selectively from the per-O-acetylated
derivative and opens up the scope for further glycoconjugate formation using trichloroacetimidate
chemistry. It is worth mentioning that during the course of our synthetic experiments we have seen a
report by Mandal et al22 on the synthesis of the same repaeting unit structure but using a completely
different synthetic strategy.
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4. Experimental
4.1. General: All reagents and solvents were dried prior to use according to standard methods.23
Commercial reagents were used without further purification unless otherwise stated. Analytical TLC was
performed on Silica Gel 60-F254 (Merck or Whatman) with detection by fluorescence and/or by charring
following immersion in a 10 % ethanolic solution of sulfuric acid. An orcinol dip carefully prepared by
the addition of concentrated sulfuric acid (20 mL) to an ice-cold solution of 3,5-dihydroxytoluene (360
mg) in EtOH (150 mL) and H2O (10 mL) was used to detect deprotected oligosaccharides by charring.
Flash chromatography was performed with Silica Gel 230-400 mesh (Merck, India). Optical rotations
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were measured at the sodium D-line at ambient temperature, with a Perkin Elmer 141 polarimeter. H
NMR and 13C NMR spectra were recorded on Bruker Avance spectrometer at 500 and 125 MHz
respectively, using Me4Si or CH3OH as internal standards, as appropriate.
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4.2. p-Tolyl 2,4,6-tri-O-acetyl-3-O-(4-methoxybenzyl)-1-thio-β-D-galactopyranoside (3): To a solution
of compound 2 (2.5 g, 6.2 mmol) in dry pyridine (20 mL), Ac2O (5 mL) was added and the solution was
stirred at room temperature for 2 hours when TLC (n-hexane–EtOAc, 3:1) showed complete conversion
of the starting material to a faster moving spot. The solvents were evaporated in vacuo and co-evaporated
with toluene to remove residual pyridine. The syrupy residue thus obtained was purified by flash
chromatography using n-hexane–EtOAc (3:1) as the eluent to afford pure compound 3 (3.1 g, 96%) as a
yellowish gel. [α]D25 +108 (c 1.1, CHCl3). 1H NMR (CDCl3, 500 MHz) : 7.17-6.84 (4d, 8H, ArH), 5.49
(dd, 1H, J3,4 3.5 Hz, J4,5 1.0 Hz, H-4), 5.10 (t, 1H, J1,2, J2,3 9.5 Hz, H-2), 4.59, 4.32 (ABq, 2H, JAB 11.5
Hz, OCH2C6H4OCH3), 4.54 (d, 1H, J1,2 9.5 Hz, H-1), 4.15 (m, 2H, H-6a, H-6b), 3.80 (m, 1H, H-5), 3.79
(s, 3H, OCH2C6H4OCH3), 3.52 (dd, 1H, J2,3 9.5 Hz, J3,4 3.5 Hz, H-3), 2.32 (s, 3H, S-C6H4-CH3), 2.11,
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2.06, 2.05 (3s, 9H, 3×COCH3). C NMR (CDCl3, 125 MHz) : 170.5, 170.4, 170.3 (3×COCH3), 159.4,
138.1, 132.9(2), 129.5(4), 129.4, 129.2, 113.8(2) (ArC), 86.9 (C-1), 77.1, 74.6, 70.9, 68.9, 66.1, 62.3,
55.2 (C6H4OCH3), 21.1 (S-C6H4-CH3). HRMS calcd. for C27H32O9SNa (M+Na)+: 555.1665, found:
555.1668.
4.3. p-Methoxyphenyl 2,4,6-tri-O-acetyl-3-O-(4-methoxybenzyl)-β-D-galactopyranosyl-(13)-4,6-O-
benzylidene-2-deoxy-2-N-phthalimido-β-D-glucopyranoside (5): A mixture of compound 3 (2.1 g, 3.9
mmol), compound 4 (1.5 g, 3.0 mmol) and MS 4Å (2.0 g) in dry CH2Cl2 (25 mL) was stirred under
nitrogen for 45 min. NIS (1.1 g, 5.0 mmol) was added and the mixture was cooled to 0 C followed by the
addition of H2SO4-silica (100 mg) and the mixture was allowed to stir at the same temperature for 15
minutes when all acceptor monosaccharide 4 was consumed as evident by TLC (n-hexane-EtOAc; 3:2).
The mixture was immediately filtered through a pad of Celite® and the filtrate was washed successively
with aq. Na2S2O3 (2×30 mL), saturated NaHCO3 (2×30 mL) H2O (30 mL). The organic layer was
separated, dried (Na2SO4), filtered and concentrated in vacuo. The crude product thus obtained was
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purified by flash chromatography using n-hexane–EtOAc (1:1) to give the pure disaccharide 5 (1.6 g,
89%) as white foam. [α]D25 +78 (c 1.0, CHCl3). 1H NMR (CDCl3, 500 MHz) : 7.86-6.69 (m, 17H, ArH),
5.65 (d, 1H, J1,2 8.5 Hz, H-1), 5.59 (s, 1H, CHPh), 5.32 (bd, 1H, J3,4 3.0 Hz, H-4), 4.89 (dd, 1H, J1,2 8.0
Hz, J2,3 10.0 Hz, H-2), 4.77 (dd, 1H, J2,3 10.5 Hz, J3,4 9.0 Hz, H-3), 4.55 (dd, 1H, J1,2 8.5 Hz, J2,3 10.5
Hz, H-2), 4.49 (d, 1H, J1,2 8.0 Hz, H-1), 4.48, 4.15 (ABq, 2H, JAB 12.0 Hz, OCH2C6H4OCH3), 4.37 (dd,
1H, J5,6a 5.0 Hz, J6a,6b 10.5 Hz, H-6a), 4.07 (dd, 1H, J5,6a 8.0 Hz, J6a,6b 11.0 Hz, H-6a), 3.91-3.76 (m, 4H,
H-4, H-5, H-6b, H-6b), 3.75 (s, 3H, OCH2C6H4OCH3), 3.69 (s, 3H, OC6H4OCH3), 3.41 (m, 1H, H-5),
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3.28 (dd, 1H, J2,3 10.0 Hz, J3,4 3.0 Hz, H-3), 2.08, 1.95, 1.51 (3s, 9H, 3×COCH3). C NMR (CDCl3,
125 MHz) : 170.3, 170.1, 168.7 (3×COCH3), 167.7, 167.4 (2×Phthalimido C=O), 159.1, 155.5, 150.4,
137.0, 134.2, 131.5, 129.4, 129.1, 129.0(3), 128.2(2), 126.0(2), 123.5(2), 118.4(3), 114.4(2), 113.5(2)
(ArC), 101.4 (CHPh), 100.5 (C-1), 97.9 (C-1), 80.8, 76.4, 75.5, 70.8, 70.7, 70.5, 68.6, 66.4, 65.4, 61.3,
55.5 (OCH2C6H4OCH3), 55.3 (C-2, OC6H4OCH3), 20.7, 20.5, 20.2 (3×COCH3). HRMS calcd. for
C48H49NO17Na (M+Na)+: 934.2898, found: 934.2901.
4.4. p-Methoxyphenyl 2,4,6-tri-O-acetyl-β-D-galactopyranosyl-(13)-4,6-O-benzylidene-2-deoxy-2-
N-phthalimido-β-D-glucopyranoside (6): To a solution of compound 5 (1.6 g, 1.8 mmol) in CH2Cl2–
H2O (4:1, 25 mL), DDQ (450 mg, 2.0 mmol) was added and the mixture was allowed to stir at room
temperature for 2 hours when TLC (n-hexane-EtOAc, 1:1) showed complete conversion of the starting
material to a slower moving spot. The mixture was diluted with CH2Cl2 (20 mL) and washed successively
with H2O (3×30 mL). The organic layer was collected, dried (Na2SO4), filtered and evaporated in vacuo.
The crude residue was purified by flash chromatography using n-hexane–EtOAc (1:1) as the eluent to
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afford the pure disaccharide acceptor 6 (1.5 g, 81%) as colourless foam. [α]D25 +103 (c 1.1, CHCl3). H
NMR (CDCl3, 500 MHz) : 7.92-6.74 (m, 13H, ArH), 5.69 (d, 1H, J1,2 8.5 Hz, H-1), 5.64 (s, 1H, CHPh),
5.18 (dd, 1H, J3,4 3.5 Hz, J4,5 1.0 Hz, H-4), 4.86 (dd, 1H, J1,2 8.0 Hz, J2,3 10.0 Hz, H-2), 4.84 (dd, 1H,
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J2,3 10.5 Hz, J3,4 9.0 Hz, H-3), 4.59 (dd, 1H, J1,2 8.5 Hz, J2,3 10.5 Hz, H-2), 4.49 (d, 1H, J1,2 8.0 Hz, H-1),
4.41 (dd, 1H, J5,6a 5.0 Hz, J6a,6b 10.5 Hz, H-6a), 4.09 (dd, 1H, J5,6a 7.5 Hz, J6a,6b 11.0 Hz, H-6a), 3.92 (m,
2H, H-4, H-6b), 3.83 (dd, 1H, J5,6b 5.5 Hz, J6a,6b 11.0 Hz, H-6b), 3.76 (dd, 1H, J2,3 10.0 Hz, J3,4 3.5 Hz,
H-3), 3.74 (s, 3H, OC6H4OCH3), 3.59 (m, 1H, H-5), 3.53 (m, 1H, H-5), 2.14, 1.98, 1.68 (3s, 9H,
3×COCH3). 13C NMR (CDCl3, 125 MHz) : 170.9, 170.7, 170.2 (3×COCH3), 168.7, 168.4
(2×Phthalimido C=O), 155.7, 150.5, 137.0, 134.4, 131.5, 129.2, 128.3(3), 126.1(3), 123.7(2), 118.5(2),
114.5(2) (ArC), 101.4 (CHPh), 100.3 (C-1), 98.1 (C-1), 80.6, 75.5, 72.8, 71.5, 70.6, 69.4, 68.6, 66.5,
61.3, 60.4, 55.6 (OC6H4OCH3), 55.4 (C-2), 20.7, 20.6, 20.3 (3×COCH3). HRMS calcd. for C40H41NO16Na
(M+Na)+: 814.2323, found: 814.2328.
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4.5.
p-Methoxyphenyl-2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl-(1→3)-2,4,6-tri-O-acetyl-β-D-
galactopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-2-N-phthalimido-β-D-glucopyranoside (8): A
mixture of acceptor 6 (1.5 g, 1.9 mmol), donor 7 ( 1.0 g, 2.6 mmol) and MS 4Å (2.0 g) in dry CH2Cl2 (20
mL) was stirred under nitrogen atmosphere for 10 min at 0 C. NIS (720 mg, 3.2 mmol) was added
followed by H2SO4–silica (100 mg) and the mixture was stirred at 0 C for another 30 min when TLC (n-
hexane-EtOAc, 3:2) showed complete consumption of the acceptor. The mixture was filtered through a
42 pad of Celite® and the filtrate was washed successively with aq. Na2S2O3 (2×30 mL), saturated NaHCO3
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(2×30 mL) and H2O (30 mL). The organic layer was collected, dried (Na2SO4), filtered and evaporated in
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vacuo and the residue was purified by flash chromatography using n-hexane-EtOAc (1:1) as eluent to
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afford pure trisaccharide 8 ( 1.8 g, 89%) as white amorphous mass. [α]D25 +73 (c 1.0, CHCl3). H NMR
(500 MHz, CDCl3) δ: 7.75-7.35 (m, 9H, ArH), 6.76, 6.69 (2d, 4H, J 9.5 Hz, OC6H4OCH3), 5.63 (d, 1H,
J1,2 8.5 Hz, H-1), 5.59 (s, 1H, CHPh), 5.17 (bd, 1H, J3,4 3.0 Hz, H-4), 5.00 (dd, 1H, J1,2 8.0 Hz, J2,3
10.0 Hz, H-2), 4.98 (m, 1H, H-3), 4.97 (t, 1H, J3,4, J4,5 8.5 Hz, H-4), 4.79 (m, 1H, H-2), 4.77 (dd,
1H, J2,3 10.0 Hz, J3,4 8.5 Hz, H-3), 4.68 (d, 1H, J1,2 1.5 Hz, H-1), 4.54 (dd, 1H, J1,2 8.5 Hz, J2,3 10.0 Hz,
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H-2), 4.50 (d, 1H, J1,2 8.0 Hz, H-1), 4.35 (dd, 1H, J5,6a 5.0 Hz, J6a,6b 10.5 Hz, H-6a), 4.00 (dd, 1H, J5,6a
7.5 Hz, J6a,6b 11.0 Hz, H-6a), 3.90 (m, 3H, H-4, H-5, H-6b), 3.78 (dd, 1H, J5,6b 5.5 Hz, J6a,6b 11.0 Hz,
H-6b), 3.70 (m,1H, H-5), 3.68 (s, 3H, OC6H4OCH3), 3.60 (dd, 1H, J2,3 10.0 Hz, J3,4 3.0 Hz, H-3), 3.40
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(m, 1H, H-5), 2.14, 2.04, 2.02, 2.00, 1.91, 1.57 (6s, 18H, 6×COCH3), 1.11 (d, 3H, J5,6 6.0 Hz, C-CH3).
13C NMR (CDCl3, 125 MHz) δ: 170.4, 170.1(2), 169.9, 169.4, 168.8 (6×COCH3), 167.9, 167.7
(2×phthalimido CO), 155.5, 150.4, 137.0, 134.1(2), 131.7(2), 129.2, 128.3(2), 126.0(2), 123.6, 123.3,
118.4(2), 114.4(2) (ArC), 101.4 (COPh), 100.2 (C-1), 98.2 (C-1), 97.9 (C-1), 80.8, 76.0, 75.3, 71.1,
70.5, 70.4, 70.0, 68.6, 68.0, 67.9, 67.2, 66.3, 61.3, 55.4 (OC6H4OCH3), 55.2, 20.7(2), 20.6, 20.5, 20.4,
19.9 (6×COCH3), 17.2 (C-CH3). HRMS calcd. for C52H57O23NNa (M+Na)+: 1086.3219, found:
1086.3223.
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4.6.
p-Methoxyphenyl-2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl-(1→3)-2,4,6-tri-O-acetyl-β-D-
galactopyranosyl-(1→3)-6-O-benzyl-2-deoxy-2-N-phthalimido-β-D-glucopyranoside (9): To a stirred
solution of compound 8 (1.8 g, 1.7 mmol) and Et3SiH ( 3.2 mL, 20 mmol) in dry CH2Cl2 (20 mL),
BF3*Et2O ( 0.4 mL, 3.4 mmol) was added at 0 C and the mixture was allowed to stir for 2 min when
TLC (n-hexane–EtOAc; 1:1) showed complete conversion of the starting material to a slower moving
spot. The mixture was washed successively with H2O (2×30 mL), saturated NaHCO3 (2×30 mL) and
brine (30 mL). The organic layer was collected, dried (Na2SO4), filtered and evaporated in vacuo. The
residue was purified by flash chromatography using n-hexane–EtOAc (3:4) as eluent to afford pure
trisaccharide acceptor 9 (1.5 g, 83%) as colourless foam. [α]D25 +111 (c 1.0, CHCl3). 1H NMR (500 MHz,
CDCl3) δ: 7.77-7.26 (m, 9H, ArH), 6.80, 6.65 (2d, 4H, J 9.5 Hz, OC6H4OCH3), 5.47 (d, 1H, J1,2 8.0 Hz,
H-1), 5.26 (bd, 1H, J3,4 3.5 Hz, H-4), 5.18 (dd, 1H, J1,2 8.0 Hz, J2,3 10.0 Hz, H-2), 5.02 (dd, 1H, J2,3
3.0 Hz, J3,4 10.0 Hz, H-3), 4.98 (t, 1H, J3,4, J4,5 10.0 Hz, H-4), 4.85 (m, 1H, H-2), 4.67 (d, 1H,
J1,2 1.5 Hz, H-1), 4.61, 4.58 (ABq, 2H, JAB 12.0 Hz, CH2Ph), 4.54 (m, 2H, H-2, H-3), 4.35 (d, 1H, J1,2
9

8.0 Hz, H-1), 4.18 (dd, 1H, J5,6a 3.5 Hz, J6a,6b 12.0 Hz, H-6a), 3.98 (m, 2H, H-6a, H-6b), 3.89 (dd, 1H,
J2,3 10.0 Hz, J3,4 3.5 Hz, H-3), 3.84 (m,1H, H-5), 3.71 (m, 4H, H-4, H-5, H-5, H-6b), 3.67 (s, 3H,
OC6H4OCH3), 2.19, 2.06, 2.05, 2.02, 1.93, 1.52 (6s, 18H, 6×COCH3), 1.13 (d, 3H, J5,6 6.5 Hz, C-CH3).
13C NMR (CDCl3, 125 MHz) δ: 170.4, 170.3, 170.0, 169.9, 169.4, 168.8 (6×COCH3), 168.0, 167.8
(2×phthalimido CO), 155.3, 150.7, 138.3, 134.4(2), 131.4(2), 128.2(2), 127.5(2), 127.4, 123.7, 123.5,
118.3(2), 114.3(2) (ArC), 101.2 (C-1), 98.4 (C-1), 97.4 (C-1), 82.4, 75.9, 75.8, 73.4, 71.6, 70.4, 70.2,
69.9, 69.8, 69.5, 68.2, 67.9, 67.5, 62.0, 55.5 (OC6H4OCH3), 54.6, 20.7, 20.5(2), 20.4(2), 19.7
(6×COCH3), 17.3 (C-CH3). HRMS calcd. for C52H59NO23Na (M+Na)+: 1088.3376, found: 1088.3380.
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4.7. p-tolyl 2-benzyl-3,4-O-isopropylidene-1-thio-β-D-fucopyranoside (11): To a solution of known
compound 10 (2.0 g, 6.4 mmol) in dry DMF (20 mL) was added NaH (620 mg, 25.8 mmol, 50% in
mineral wax) followed by benzyl bromide (1.0 mL, 8.4 mmol) and the mixture was stirred at room
temperature for 2 hours. Excess NaH was neutralized by careful addition of MeOH (2.0 mL) and the
mixture was evaporated in vacuo. The residue was dissolved in diethyl ether (30 mL) and washed
successively with H2O (30 mL) and brine (30 mL). The diethyl ether layer was collected, dried (Na2SO4),
filtered and evaporated. The residue was purified by flash chromatography using n-hexane–EtOAc, 6:1 to
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1
afford pure compound 11 (2.4 g, 92%) as a pale yellow solid. [α]D -34 (c 1.0, CHCl3). H NMR (500
MHz, CDCl3) δ: 7.45-7.08 (m, 9H, ArH), 4.82, 4.67 (ABq, 2H, JAB 11.5 Hz, CH2Ph), 4.52 (d, 1H, J1,2 9.5
Hz, H-1), 4.21 (t, 1H, J2,3,J3,4 6.0 Hz, H-3), 4.04 (dd, 1H, J3,4 6.0 Hz, J4,5 2.0 Hz, H-4), 3.79 (qd, J4,5 2.0
Hz, J5,6 6.5 Hz, 1H, H-5), 3.48 (dd, 1H, J1,2 9.5 Hz, J2,3 6.0 Hz, H-2), 2.33 (s, 3H, SC6H4CH3), 1.41, 1.36
13
(2s, 6H, 2×isopropylidene CH3), 1.39 (d, 3H, J5,6 6.5 Hz, C-CH3). C NMR (CDCl3, 125 MHz) δ: 138,
137.5, 132.7 (2), 129.8, 129.5 (2), 128.5, 128.3, 128.2 (2), 127.7 (ArC), 109.6 [C(CH3)2], 86.5 (C-1),
79.9, 78.2, 73.5, 72.4, 27.9, 26.4 (2×isopropylidene CH3), 21.1 (SC6H4CH3), 16.9 (C-CH3). HRMS calcd.
for C23H28O4SNa (M+Na)+: 423.1606, found: 423.1611.
10

4.8. p-tolyl 2-benzyl-3,4-di-O-acetyl-1-thio-β-D-fucopyranoside (12): A suspension of compound 11
(2.4 g, 5.9 mmol) in 80% AcOH (20 mL) was stirred at 80 C for 2 hours when TLC (n-hexane–EtOAc,
2:1) showed complete convertion of the starting material to a slower moving spot. The solvents were
evaporated in vacuo and co-evaporated with toluene to remove residual AcOH. The syrupy residue thus
obtained was dissolved in 15 mL pyridine and 10 mL of Ac2O as added. The solution was stirred at room
temperature for 2 hours when TLC (n-hexane–EtOAc, 4:1) showed complete conversion of the starting
material to a faster moving spot. The solvents were evaporated in vacuo and co-evaporated with toluene
to remove residual pyridine. The syrupy residue thus obtained was purified by flash chromatography
using n-hexane–EtOAc (4:1) as the eluent to afford pure compound 12 (2.3 g, 86%) as a white amorphous
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mass. [α]D -48 (c 1.0, CHCl3). H NMR (500 MHz, CDCl3) δ: 7.32-7.12 (m, 9H, ArH), 5.24 (bd, 1H,
J3,4 3.0 Hz, H-4), 5.02 (dd, 1H, J2,3 9.5 Hz, J3,4 3.0 Hz, H-3), 4.85, 4.58 (ABq, 2H, JAB 11.0 Hz, CH2Ph),
4.65 (d, 1H, J1,2 9.5 Hz, H-1), 3.72 (t, 1H, J1,2, J2,3 9.5 Hz, H-2), 3.71 (m, 1H, H-5), 2.34 (s, 3H,
13
SC6H4CH3), 2.14, 1.93 (2s, 6H, 2×COCH3), 1.20 (d, 3H, J5,6 6.5 Hz, C-CH3). C NMR (CDCl3, 125
MHz) δ: 170.2, 169.7 (2×COCH3), 137.8, 137.5, 132.4(2), 129.7(2), 128.1(2), 127.9(2), 127.7, 127.6
(ArC), 87.5 (C-1), 75.1, 74.9, 74.4, 72.4, 70.6, 20.9, 20.5 (2×COCH3), 20.4 (SC6H4CH3), 16.3 (C-CH3).
HRMS calcd. for C24H28O6SNa (M+Na)+: 467.1504, found: 467.1509.
4.9. p-Methoxyphenyl-4-O-(2,3-di-O-acetyl-2-O-benzyl-α-L-fucopyranosyl)-2,3,4-tri-O-acetyl-α-L-
rhamnopyranosyl-(1→3)-2,4,6-tri-O-acetyl-β-D-galactopyranosyl-(1→3)-6-O-benzyl-2-deoxy-2-N-
phthalimido-β-D-glucopyranoside (13): A mixture of acceptor 9 (1.5 g, 1.4 mmol), donor 12 ( 810 mg,
1.8 mmol) and MS 4Å (2.0 g) in dry CH2Cl2 (20 mL) was stirred under nitrogen atmosphere for 10 min at
0 C. NIS (720 mg, 3.2 mmol) was added and the solution was kept at -45 C for 15 minutes followed by
the addition of H2SO4–silica (100 mg) and the mixture was stirred at -45 C for another 30 min when
TLC (n-hexane-EtOAc; 1:1) showed complete consumption of the acceptor. The mixture was filtered
through a pad of Celite® and the filtrate was washed successively with aq. Na2S2O3 (2×30 mL), saturated
11

NaHCO3 (2×30 mL) and H2O (30 mL). The organic layer was collected, dried (Na2SO4), filtered and
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3
evaporated in vacuo and the residue was purified by flash chromatography using n-hexane-EtOAc (3:4)
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25
as eluent to afford pure tetrasaccharide 13 (1.6 g, 81%) as white amorphous mass. [α]D +138 (c 0.9,
6
7
8
1
CHCl3). H NMR (500 MHz, CDCl3) δ: 7.89-7.26 (m, 14H, ArH), 6.73, 6.64 (2d, 4H, J 9.0 Hz,
9
OC6H4OCH3), 5.41 (d, 1H, J1,2 8.5 Hz, H-1), 5.26 (bd, 1H, J3,4 2.5 Hz, H-4), 5.24 (m, 3H, H-3, H-4,
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12
13
14
H-1), 5.06-5.00 (m, 2H, H-2, H-5), 4.97 (ABq, 2H, JAB 10.0 Hz, CH2Ph), 4.85 (m, 2H, H-2, J3,4
,
15 J4,5 7.5 Hz, H-4), 4.64 (ABq, 1H, JAB 11.5 Hz, CH2Ph), 4.61 (d, 1H, J1,2 1.5 Hz, H-1), 4.51 (m,
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18
4H, H-2, H-3, H-3, CH2Ph), 4.35 (m, 2H, H-6a, H-6b), 4.14 (d, 1H, J1,2 8.0 Hz, H-1), 4.07 (t, 1H, J3,4
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9.5 Hz, H-4), 3.91 (m, 3H, H-6a, H-3, H-5), 3.69 (s, 3H, C6H4OCH3), 3.67 (m, 3H, H-5, H-5,H-6b),
3.38 (dd,1H, J1,2 4.0 Hz, J2,3 3.5 Hz, H-2), 2.18, 2.14, 2.10, 2.09, 2.07, 2.02, 1.94(2) (7s, 24H,
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13
8×COCH3), 1.28 (d, 3H, J5,6 6.5 Hz, H-6), 1.13 (d, 3H, J5,6 6.5 Hz, H-6). C NMR (CDCl3, 125
MHz) δ: 170.9, 170.7, 170.6, 170.2, 169.7(2), 169.5, 169.4 (8×COCH3), 168.0, 167.8 (2×phthalimido
CO), 155.5, 150.8, 138.1, 137.9, 134.7(2), 131.3(2), 128.4(2), 128.3(2), 127.9, 127.7(2), 127.5, 123.8(2),
118.8(2), 114.4(2) (ArC), 100.5 (C-1), 98.1 (C-1), 97.8 (C-1), 97.2 (C-1), 75.7, 75.5, 74.2, 73.8, 73.7,
73.3, 73.0, 72.2, 71.4, 70.9, 70.6, 70.2, 68.1, 67.9, 67.3, 67.1, 64.5, 61.6, 56.3, 55.6 (OC6H4OCH3),
20.9(2), 20.8(2), 20.7, 20.6(2), 20.5 (8×COCH3), 17.4 (C-CH3), 15.9 (C-CH3). HRMS calcd. for
C69H79O29NNa (M+Na)+: 1408.4635, found: 1408.4639.
46 4.10. p-Methoxyphenyl-4-O-(2,3-di-O-acetyl-2-O-benzyl-α-L-fucopyranosyl)-2,3,4-tri-O-acetyl-α-L-
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48
49
rhamnopyranosyl-(1→3)-2,4,6-tri-O-acetyl-β-D-galactopyranosyl-(1→3)-6-O-benzyl-2-acetamido-2-
50
deoxy-β-D-glucopyranoside (14): To a solution of the compound 13 (1.6 g, 1.1 mmol) in n-butanol (15
mL) was added ethylene diamine (0.15 mL, 1.3 mmol) and the solution was stirred at 110 C for 24
hours. After evaporating the solvents in vacuo, the residue was dissolved in pyridine (10 mL) followed by
Ac2O (10 mL) and the solution was stirred at room temperature for 4 hours when TLC (n-hexane–EtOAc,
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12

1:2) showed complete conversion of the starting material to a slower moving spot. The solvents were
evaporated in vacuo and coevaporated with toluene to remove residual pyridine. The crude material thus
obtained was purified by flash chromatography using n-hexane–EtOAc (1:4) to give pure compound 14
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(1.1 g, 75%) as a colourless foam. [α]D +122 (c 1.0, CHCl3). 1H NMR (500 MHz, CDCl3) δ: 7.30-7.23
9
(m, 10H, ArH), 6.98, 6.78 (2d, 4H, J 9.0 Hz, OC6H4OCH3), 6.51 (d, 1H, J2,NH 9.0 Hz, NHAc), 5.40 (d,
1H, J3,4 3.5 Hz, H-4), 5.25 (m, 5H, H-1, H-2, H-1, H-3, H-4), 5.12 (dd, 1H, J2,3 3.0 Hz, J3,4
10.0 Hz, H-3), 5.05 (t, 1H, J3,4, J4,5 10.0 Hz, H-4), 4.98 (dd, 1H, J1,2 2.0 Hz, J2,3 3.0 Hz, H-2),
4.93 (d, 1H, J1,2 1.5 Hz, H-1), 4.82(d, 1H, J 8.0 Hz, H-1), 4.57 (ABq, 2H, JAB 11.5 Hz, CH2Ph), 4.46,
4.38 (ABq, 2H, JAB 12.0 Hz, CH2Ph), 4.18 (m, 4H, H-2, H-3, H-6a, H-6b), 4.03 (m, 3H, H-6a, H-5, H-
5), 3.90 (m, 3H, H-4, H-6b, H-3), 3.83 (m, 1H, H-5), 3.76 (m, 1H, H-4, H-2), 3.75 (s, 3H, C6H4OCH3),
2.14, 2.13, 2.11, 2.05(3), 2.00, 1.95 (7s, 24H, 8×COCH3), 1.75 (s, 3H, NHCOCH3), 1.21 (d, 3H, J5,6 6.0
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Hz, H-6), 1.18 (d, 3H, J5,6 7.0 Hz, H-6). C NMR (CDCl3, 125 MHz) δ: 170.5(3), 170.2, 170.1,
170.0(2), 169.6(2) (8×OCOCH3, NHCOCH3), 155.0, 150.9, 138.2, 137.3, 128.5(2), 128.3(2), 128.2,
128.1(2), 127.7(2), 127.5, 118.0(2), 114.5(2) (ArC), 99.7 (C-1), 98.3 (C-1, C-1), 93.5 (C-1), 75.5,
75.3, 73.5, 73.4, 73.2, 72.1, 71.5, 71.4, 70.8, 70.7, 70.3, 70.2, 69.9, 69.3, 68.5, 68.1, 67.4, 65.1, 61.5, 55.6
(OC6H4OCH3), 20.9(3), 20.8, 20.7(2), 20.6(2), 20.5 (8×OCOCH3, NHCOCH3), 17.4 (C-CH3), 15.9 (C-
CH3). HRMS calcd. for C63H79O28NNa (M+Na)+: 1320.4686, found: 1320.4689.
46 4.11. p-Methoxyphenyl-4-O-α-L-fucopyranosyl-α-L-rhamnopyranosyl-(1→3)-β-D-galactopyranosyl-
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49
(1→3)-2-acetamido-2-deoxy-β-D-glucopyranoside (1): A methanolic solution (200 mL) of the
50
tetrasaccharide 14 (1.1 g, 0.8 mmol) was passed through an H-cube flow hydrogenation assembly using a
10% Pd-C cartridge with a flow rate of 1 mL/min. Complete debenzylation was achieved after three
cycles, as confirmed by the mass spectrum. Solvents were evaporated in vacuo and the residue was
purified by flash column using CH2Cl2–MeOH (12:1). The compound thus obtained was dissolved in
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13

MeOH (20 mL). NaOMe (2.0 mL, 0.5 M in MeOH) was added and the solution was stirred at room
temperature for 24 hours. The solution was neutralized with DOWEX 50W H+ resin, filtered and the
solvents were evaporated under reduced pressure and the residue was purified by a short flash column
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4
5
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7
8
1
using CH2Cl2–MeOH (3:1) to afford the target tetrasaccharide 1 (450 mg, 68%). H NMR (CD3OD, 500
9
MHz, selected data) δ: 6.99, 6.85 (2d, 4H, J 9.0 Hz, OC6H4OCH3), 5.12 (bs, 1H, H-1), 5.09 (d, 1H,
J1,2 4.0 Hz, H-1), 4.98 (d, 1H, J1,2 8.0 Hz, H-1), 4.47 (d, 1H, J1,2 7.5 Hz, H-1), 3.77 (s, 3H,
OC6H4OCH3), 2.03 (s, 3H, NHCOCH3), 1.28 (d, 3H, J 6.0 Hz, C-CH3), 1.22 (d, 3H, J 6.5 Hz, C-CH3).
13C NMR (CD3OD, 125 MHz) δ: 174.3 (NHCOCH3), 156.8, 153.1, 119.2(2), 115.6(2) (ArC), 105.1 (C-
1), 103.7 (C-1), 101.7 (C-1), 99.6 (C-1), 81.1, 78.6, 77.6, 76.7, 74.1, 73.8, 73.4, 72.1(2), 72.0, 71.2,
70.2, 70.0, 69.6, 67.7, 62.6, 61.3, 57.4, 56.1 (OC6H4OCH3), 23.2 (NHCOCH3), 18.0 (C-CH3), 16.6 (C-
CH3). HRMS calcd. for C33H51O20NNa (M+Na)+: 804.2902, found: 804.2905.
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Acknowledgement
KBP is thankful to UGC, New Delhi for Junior Research Fellowship. The work is supported by the
research grant 01(2370)/10/EMR-II from CSIR, New Delhi to BM.
Reference:
1.
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15

Synthesis of the tetrasaccharide repeating unit
of the O-glycan from the polar flagellum
flagellin of Azospirillum brasilense Sp7
Kumar Bhaskar Pal, Balaram Mukhopadhyay

Highlights
1. Total synthesis of the tetrasaccharide repeating unit of the flagellin O-glycan
2. Activation of thioglycoside using NIS in the presence of H2SO4-silica
3. Use of OPMP glycoside for further glycoconjugate formation