The use of the o-nitrophenyl group as a protecting/activating group for 2-acetamido-2-deoxyglucose

Article abstract of DOI:10.1016/S0008-6215(02)00470-6

The o-nitrophenyl group, a protecting group with latent activation potential, was used as a protecting group for the glycosidic position. It is stable to common conditions used in synthesis and can be activated for displacement and glycoside formation by an alcohol, using zinc chloride as a catalyst. Good to excellent yields of β-glycosides of the important amino sugar N-acetylglucosamine were obtained. A mechanism for the reaction is proposed.

Full text of DOI:10.1016/S0008-6215(02)00470-6

Carbohydrate Research 338 (2003) 455–458
www.elsevier.com/locate/carres
Note
The use of the o-nitrophenyl group as a protecting/activating
group for 2-acetamido-2-deoxyglucose
Gabriela Pistia-Brueggeman, Rawle I. Hollingsworth*
Department of Chemistry, Michigan State Uni6ersity, East Lansing, MI 48824, USA
Received 1 February 2002; accepted 28 October 2002
Abstract
The o-nitrophenyl group, a protecting group with latent activation potential, was used as a protecting group for the glycosidic
position. It is stable to common conditions used in synthesis and can be activated for displacement and glycoside formation by
an alcohol, using zinc chloride as a catalyst. Good to excellent yields of b-glycosides of the important amino sugar N-acetylglu-
cosamine were obtained. A mechanism for the reaction is proposed. © 2003 Elsevier Science Ltd. All rights reserved.
Keywords: N-Acetylglucosamine; Glycosylation
1. Introduction
Glycosides of 2-acetamido-2-deoxy-D-glucose (Glc-
NAc) are widely distributed in living organisms. They
are found in human milk, in blood group substances, in
bacterial lipopolysaccharide antigens where they consti-
tute the dominant epitopes, and in plants.22 Of special
interest is the occurrence of amino sugars in a variety of
antibiotics, such as kanamycin C, paromomycin, hy-
droxymycin, trehalosamine and zygomycin A.23 The
preparation of glycosides of GlcNAc is problematic.
Since the first glycoside synthesis performed by Fis-
cher,1 numerous glycosyl donors have been designed
and used. Some of them require direct activation of the
anomeric center, such as glycosyl halides,2–6 thiogly-
cosides,7–9
glycosyl
phosphates,10–12
glycosyl
phosphites13,14 and glycosyl 2-propenyl carbonates.15
Other glycosidic leaving groups can be activated re-
motely by the interaction of a promoter with a site on
the leaving group that is not directly attached to the
anomeric center. This is the case with glycosyl
trichloroacetimidates,16 n-pentenyl glycosides,17 2-
pyridylthioglycosides,18 and 3-methoxy-2-pyridyloxy
glycosides.19–21 The vast majority of the activating
groups described above cannot survive most of the
typical transformations carried out on carbohydrate
molecules. These include acylations, alkylations, oxida-
tions, reductions, epoxidations, sulfations, phosphory-
lations, halogenations and hydroxylations. The
development of activating groups that are stable to
these conditions but that can still readily promote
coupling is therefore of extreme interest.
2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-a-D-glucopy-
ranosyl bromide is too reactive to be of much practical
use. The corresponding chloro derivative is also reactive
and tends to give oxazolines as a major product. The
use of non-participating protecting groups on the
amino nitrogen is often necessary in order to avoid this.
Because of their chemical lability, the use of halo
groups for activation of the anomeric position pre-
cludes transformations elsewhere on the pyranose ring
before the coupling step. This is a major obstacle to be
overcome in the design of synthetic strategies.
The o-nitrophenyl group is a very inert structure
susceptible only to reductions with hydrogen, ferric
chloride, and strong metal hydride reductants such as
lithium aluminum hydride. It is resistant to acids, bases,
oxidizing agents, acylating agents and alkylating agents,
and such commonly used reductants as sodium borohy-
dride. Because the strongly electron-withdrawing nitro
group is in direct conjugation with the glycosidic oxy-
* Corresponding author. Tel.: +1-517-353-0613; fax: +1-
517-353-9334
E-mail
address:
rih@argus.cem.msu.edu
(R.I.
Hollingsworth).
0008-6215/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(02)00470-6

456
G. Pistia-Brueggeman, R.I. Hollingsworth / Carbohydrate Research 338 (2003) 455–458
gen, the o-nitrophenyl group can, in principle, function
as a latent activating group for glycosides. This would
allow the use of this group in vast majority of routine
transformations typical in carbohydrate chemistry, but
would make the coupling of another group to the
anomeric position possible without installing a different
activating group. Here we present the preparation of
this stable new glycosyl donor and its use in glycosyla-
tion reactions
and ethyl lactate (Scheme 2) at 70 °C in the presence of
anhydrous zinc chloride. This afforded high yields of
isopropyl 2-acetamido-2-deoxy-b-
D
-glucoside (5) and 1-
(ethoxycarbonyl)ethyl 2-acetamido-2-deoxy-b-
D-glu-
coside (6). The b anomer was formed exclusively.
Because of the acidic nature of the reaction conditions,
no racemization of the ethyl lactyl group was observed.
The proposed mechanism for the transglycosylation
reaction is thought to involve the coordination of zinc
to both the nitro group and the glycosidic oxygen atom,
mediating the elimination of the o-nitrophenyl group
and the generation of the positive charge at C-1. This
can be stabilized by the acetamido group and by the
ring oxygen. An oxonium ion is therefore produced
through a concerted process. Since the reaction condi-
tions are acidic, no oxazoline byproducts were formed.
The participation of the acetamido group in glycosyla-
tion reactions involving GlcNAc shields the bottom
face of the oxonium intermediate, permitting attack of
the alcohol from the equatorial position only, thus
affording exclusively the trans glycoside in this case.
In summary, we have developed a new glycosylation
procedure for N-acetylglucosamine, using the o-nitro-
phenyl group as a protecting/activating function. This
aglycon provides good protection of the anomeric cen-
ter against a variety of agents and can be activated with
ZnCl2 in the presence of alcohols to give b-glycosides of
N-acetylglucosamine in high overall yields and total
stereoselectivity. The common and unwanted formation
of oxazoline by-products is avoided, as is the necessity
to use other protecting groups on the amino nitrogen to
avoid this. One major drawback of the present method
is that relatively high temperatures are needed to effect
reaction in a reasonable time. This is an especially
serious shortcoming when complex glycosides or
oligosaccharides are to be prepared.
2. Results and discussion
The o-nitrophenyl glycoside of GlcNAc, 3, was readily
synthesized by reacting 2-acetamido-2-deoxy-3,4,6-tri-
O-acetyl-a- -glucopyranosyl chloride (1) with o-nitro-
D
phenol in the presence of KH and 18-crown-6 (Scheme
1). The facile displacement of the chloro group of 1
under these conditions has previously been described.24
Under standard deacetylation conditions, 3 was con-
verted into o-nitrophenyl 2-acetamido-2-deoxy-D-glu-
coside (4). This product is stable at room temperature
for indefinite time. The suitability of the o-nitrophenyl
group as a leaving group was evaluated by reacting
glycoside 4 with two secondary alcohols: 2-propanol
Scheme 1.
3. Experimental
3.1. General methods
Melting points were measured on a Fisher–Johns melt-
ing point apparatus. Optical rotations were measured
(u=589 nm) at room temperature (rt) using a Perkin–
1
Elmer 341 polarimeter in a 1 mL cell. The H and 13C
NMR spectra were recorded at 500 MHz on a Varian
VXR spectrometer. The HRMS FAB mass spectra
were obtained using a Jeol HX-110 double focusing
mass spectrometer operating in positive ion mode. All
solvents were distilled in order to remove traces of
water. The solid reactants were dried in the vacuum
oven at rt. Potassium hydride was washed with hexane
before weighing.
Scheme 2.

G. Pistia-Brueggeman, R.I. Hollingsworth / Carbohydrate Research 338 (2003) 455–458
457
3.2. Preparation of o-nitrophenyl 2-acetamido-2-deoxy-
3,4,6-tri-O-acetyl-b- -glucopyranoside (3)
formed. The product 5 (0.068 g) was recovered in 95%
1
D
yield: [h]2D3 −5.0° (c 0.48, CH3OH); H NMR (D2O): l
1.03 (d, 3 H, J 6.1 Hz, CH3), 1.09 (d, 3 H, CH3), 1.94
(s, 3 H, NHAc), 3.33 (m, 1 H, H-5), 3.34 (m, 1 H, H-2),
3.45 (dd, 1 H, J6a,6b 10.3, J6b,5 8.1 Hz, H-6b), 3.52 (dd,
1 H, J6a,5 8.3 Hz, H-6a), 3.163 (dd, 1 H, J2,3 5.52, J3,4
12.3 Hz, H-3), 3.82 (dd, 1 H, J3,4 12.3, J4,5 1.9 Hz, H-4),
3.92 (m, 1 H, J 6.1, i-propyl), 4.49 (d, 1 H, J1,2 8.1 Hz,
H-1); 13C NMR (D2O): l 21.3, 22.3, 22.4, 56.06, 61.0,
70.1, 73.8, 74.1, 76.0, 99.8, 174.7; IR (film): 3323 (b),
1648 (s), 1374 (m), 1071 (s) cm−1; HRFABMS [M+
H+] calcd 264.1447, found 264.1451.
In a two-necked, round-bottomed flask equipped with a
dropping funnel and purged with N2, KH (12.85 g, 0.2
mol) was added to CH2Cl2 (80 mL) at 0 °C. To this, a
soln of 18-crown-6 (26.5 g, 0.1 mol) and o-nitrophenol
(28 g, 0.2 mol) in CH2Cl2 (100 mL) was added through
a dropping funnel during 1 h. The suspension was
stirred for another 2 h, after which time 2-acetamido-
3,4,6-tri-O-acetyl-2-deoxy-a-D-glucopyranosyl chloride
(36.6 g, 0.1 mol) was added and stirring was continued
at rt for a further 2 h. The reaction was quenched with
glacial AcOH (12 mL). The soln was washed with 5%
aq NaHCO3 (1 L) and then with water. The combined
organic layers were dried with Na2SO4 and then rotary
evaporated, to afford crude product 3 (45 g), which was
used in the next step without further purification
3.5. 1-(Ethoxycarbonyl)ethyl 2-acetamido-2-deoxy-b-D-
glucopyranoside (6)
A suspension of compound 4 (0.1 g, 0.29 mmol) and
ZnCl2 (0.1 g, 0.7 mmol) in dry ethyl lactate (2 mL) was
heated at 70 °C for 20 h. The solvent was removed and
the crude reaction mixture was dissolved in water and
rotary evaporated to distill off the o-nitrophenol. In
order to remove the traces of ZnCl2 and separate the
product from the ethyl lactate dimer, column chro-
matography with a 7:1 CHCl3–MeOH eluent was per-
formed. The product 6 (0.06 g) was recovered in 63%
yield: mp 199–200 °C, [h]2D3 −50.1° (c 0.46, CH3OH);
1H NMR (D2O): l 1.05 (t, 3 H, J 7.0 Hz, CH3ꢀethyl),
1.14 (d, 3 H, J 6.8 Hz, CH3), 1.78 (s, 3 H, NHAc), 3.03
(m, 1 H, H-5), 3.08, (m, 1 H, J2,1 8.06, JNH,2 1.71 Hz,
H-2), 3.26 (dd, 1 H, J2,3 8.06, J3,4 10.25 Hz, H-3), 3.37
(dd, 1 H, H-4), 3.43 (dd, 1 H, J6a,b 11.97, J6a,5 5.61 Hz,
H-6a), 3.65 (dd, 1 H, H-6b), 3.96 (d, 2 H, J 7.0 Hz,
CH2ꢀethyl), 4.26 (d, 1 H, H-1), 4.28 (d, 1 H, J 6.8 Hz,
CHꢀlactyl); 13C NMR (D2O): l 8.7, 13.3, 17.7, 50.9,
56.0, 57.8, 65.2, 68.8, 69.0, 71.3, 95.6, 169.9, 170.0; IR
(film): 3439 (m), 3367 (s), 1740 (s), 1666 (s), 1138 (m)
cm−1; HRFABMS [M+H+] calcd 322.1502, found
322.1493.
3.3. Preparation of o-nitrophenyl 2-acetamido-2-deoxy-
b-D-glucopyranoside (4)
To a soln of 3 (45 g) in MeOH (1050 mL) and water (70
mL) K2CO3 (55 g, 0.4 mol) was added and the suspen-
sion was stirred overnight at rt. After filtration of the
salts, the soln was rotary evaporated. More MeOH was
added to precipitate the excess salt, which was then
filtered off. Water was added to the soln and o-nitro-
phenol removed by rotary evaporation at 40 °C. Re-
crystallization from hot EtOH gave 4 (18.4 g) in 54%
overall yield starting from 1; mp 176–177 °C; [h]D23
1
+30.4° (c 0.77, CH3OH); H NMR (CD3OD): l 1.99
(s, 3 H, NHAc), 3.40 (dd, 1 H, J3,4 8.6, J4,5 9.9 Hz,
H-4), 3.48 (ddd, 1 H, J4,5 9.7, J5,6b 5.9, J5,6a 2.2 Hz,
H-5), 3.57 (dd, 1 H, J2,3 10.3, J3,4 8.61 Hz, H-3), 3.72
(dd, 1 H, J6a,6b 12.1, J6b,5 5.9 Hz, H-6b), 3.88 (dd, 1 H,
J2,3, J1,2 8.4 Hz, H-2), 3.93 (dd, 1 H, J6a,5 2.2 Hz, H-6a),
5.14 (d, 1 H, H-1), 7.17 (ddd, 1 H, Ja,b 8.1, Jb,c 7.2, Jb,d
1.3 Hz, HꢀArb), 7.50 (dd, 1 H, Jc,d 8.3 Hz, HꢀArd), 7.56
(ddd, 1 H, Ja,c 1.5 Hz, HꢀArc), 7.70 (dd, 1 H, HꢀAra);
13C NMR (CD3OD): l 22.9, 57.2, 62.6, 71.8, 75.6, 78.7,
101.7, 119.6, 123.8, 125.3, 134.6, 142.7, 151.1, 174.0; IR
(film): 3279 (b), 1642 (s), 1527 (s), 1351 (m), 1351 (m),
1068 (s) cm−1; HRFABMS [M+H+] calcd 343.1141,
found 343.1139.
References
1. Fischer, E. Ber. 1893, 26, 2400–2412.
2. Koenigns, W.; Knorr, E. Ber. 1901, 34, 957–982.
3. Mukaiyama, T.; Murai, Y.; Shoda, S. Chem. Lett. 1981,
431–432.
4. Kunz, H. Hel6. Chim. Acta 1985, 68, 283–287.
5. Hashimoto, S.; Hayashi, M.; Noyori, T. Tetrahedron
Lett. 1984, 25, 1379–1382.
3.4. Isopropyl 2-acetamido-2-deoxy-b-D-glucopyranoside
(5)
6. Suzuki, K.; Maeta, T.; Matsumoto, T.; Tsuchihashi, G.
Tetrahedron Lett. 1988, 29, 3571–3574.
7. Fugedi, P.; Garegg, P. J.; Lonn, H.; Norberg, T. Glyco-
conjugate J. 1987, 4, 97–108.
8. Lonn, H. Carbohydr. Res. 1989, 139, 105–113.
9. Geurtsen, R.; Holmes, D. S.; Boons, G. J. J. Org. Chem.
1997, 62, 8145–8154.
10. Hashimoto, S.; Honda, T.; Ikegami, S. J. Chem. Soc.,
Chem. Commun. 1989, 11, 685–687.
A suspension of compound 4 (0.1 g, 0.29 mmol) and
ZnCl2 (0.1 g, 0.7 mmol) in dry 2-propanol (5 mL) was
heated at 70 °C for 22 h. The solvent was removed by
evaporation and the crude reaction mixture dissolved in
water and rotary evaporated to distill off the o-nitro-
phenol. In order to remove the traces of ZnCl2, column
chromatography with 6:1 CHCl3–MeOH was per-

458
G. Pistia-Brueggeman, R.I. Hollingsworth / Carbohydrate Research 338 (2003) 455–458
11. Schmid, U.; Waldemann, H. Tetrahedron Lett. 1996, 37,
3837–3840.
12. Plante, O. J.; Andrade, R. B.; Seeberger, P. H. Org. Lett.
1999, 1, 211–214.
13. Martin, T. J.; Schmidt, R. R. Tetrahedron Lett. 1992, 33,
6123–6126.
14. Sim, M. M.; Kondo, H.; Wong, C.-H. J. Am. Chem. Soc.
1993, 115, 2260–2267.
15. Marra, A.; Esnault, J.; Veyrieres, A.; Sinay, P. J. Am.
Chem. Soc. 1992, 114, 6354–6360.
16. Schmidt, R. R.; Kinzy, W. Ad6. Carbohydr. Chem.
Biochem. 1994, 50, 21–123.
17. Fraser-Reid, B.; Madsen, R. In Preparati6e Carbohydrate
Chemistry; Hanessian, S., Ed.; Dekker: New York, 1996;
p 339.
18. Hanessian, S.; Bacquet, C.; LeHong, N. Carbohydr. Res.
1980, 80, C17–C22.
19. Hanessian, S. In Preparati6e Carbohydrate Chemistry;
Hanessian, S., Ed.; Dekker: New York, 1997; p 381.
20. Lou, B.; Reddy, G. V.; Wang, H.; Hanessian, S. In
Preparati6e Carbohydrate Chemistry; Hanessian, S., Ed.;
Dekker: New York, 1997; p 389.
21. Hindsgaul, O.; Schweizer, F. Curr. Opin. Chem. Biol.
1999, 3, 291–297.
22. Schmidt, R. Angew. Chem., Int. Ed. Engl. 1986, 25,
212–234.
23. Banoub, J.; Boullanger, P.; Lafont, D. Chem. Re6. 1992,
92, 1167–1195.
24. Kim, K.; Hollingsworth, R. I. Tetrahedron Lett. 1994, 35
(7), 1031–1032.