Facile synthesis of nitrophenyl 2-acetamido-2-deoxy-α-D- mannopyranosides from ManNAc-oxazoline

Article abstract of DOI:10.3762/bjoc.8.48

The synthetic procedures for a large-scale preparation of o- and p-nitrophenyl 2-acetamido-2-deoxy-α-D-mannopyranoside are described. The synthetic pathway employs the glycosylation of phenol with ManNAc oxazoline, followed by nitration of the aromatic moiety yielding a separable mixture of the o- and p-nitrophenyl derivative in a 2:3 ratio.

Full text of DOI:10.3762/bjoc.8.48

Facile synthesis of nitrophenyl 2-acetamido-2-deoxy-
α-D-mannopyranosides from ManNAc-oxazoline
Karel Křenek1, Petr Šimon1, Lenka Weignerová1, Barbora Fliedrová1,2,
Marek Kuzma1 and Vladimír Křen*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 428–432.
1Institute of Microbiology, Academy of Sciences of the Czech
Republic, Vídeňská 1083, CZ – 142 20, Prague, Czech Republic and
2Department of Biochemistry, Charles University in Prague, Hlavova
8, CZ – 128 40, Prague, Czech Republic
doi:10.3762/bjoc.8.48
Received: 20 January 2012
Accepted: 06 March 2012
Published: 20 March 2012
Email:
Vladimír Křen* - kren@biomed.cas.cz
This article is part of the Thematic Series "Synthesis in the
glycosciences II".
* Corresponding author
Guest Editor: T. K. Lindhorst
Keywords:
α-ManNAc; glycosidase; glycosylation; nitrophenyl; oxazoline
© 2012 Křenek et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The synthetic procedures for a large-scale preparation of o- and p-nitrophenyl 2-acetamido-2-deoxy-α-D-mannopyranoside are
described. The synthetic pathway employs the glycosylation of phenol with ManNAc oxazoline, followed by nitration of the
aromatic moiety yielding a separable mixture of the o- and p-nitrophenyl derivative in a 2:3 ratio.
Introduction
Hexosamines are fundamental structural elements and precur- strains occurring in their R-forms (R stands for rough: virulent,
sors of the peptidoglycan and membrane lipopolysaccharide containing ManNAc in the capsular structures) and S-forms (S
layer as well as of capsular polysaccharides in Gram-negative stands for smooth: nonvirulent, lower level of ManNAc in the
bacteria. N-Acetyl-D-mannosamine (ManNAc) has been found capsules) is related partly to the content of ManNAc. Thus,
to be, presumably, the strongest monosaccharidic ligand for the ManNAc units play a significant role in bacterial pathogenicity
natural killer cells (NK-cells) activating protein NKR-P1 [1], and virulence (e.g., Streptococcus pneumoniae) [4,5]. Surpris-
and some ManNAc-containing oligosaccharides (e.g., ingly, glycosidases active upon β-ManpNAc and α-ManpNAc
GlcpNAc-β-(1→4)ManpNAc) have been identified to be strong glycosides are not known so far. Therefore, the building blocks
immunoactivators [2]. Detection of ManNAc by the immune for the chemical synthesis of ManNAc-containing compounds,
system is probably very important for recognizing bacterial as well as the substrates for the hypothetical α-N-acetyl-
infection, as this carbohydrate is an unambiguous signal of a mannopyranosidase, are required.
microbial invader [3]. In the somatic cells (vertebrates) there are
no structures composed of ManNAc. This carbohydrate is a This paper describes new robust and effective methods for the
precursor of sialic acid(s). The pathogenicity of some bacterial synthesis of both o-nitrophenyl 2-acetamido-2-deoxy-α-D-
428

Beilstein J. Org. Chem. 2012, 8, 428–432.
mannopyranoside (7) and p-nitrophenyl 2-acetamido-2-deoxy- ManNAc), which was converted to its peracetate 2 (Ac2O/py/
α-D-mannopyranoside (8) usable as practical chromogenic DMAP (cat.), 90%). A crude mixture of peracetylated ManNAc
substrates for the screening of such enzymes.
2 was treated with trifluoromethanesulfonic acid [14] affording
oxazoline 3 in 75% yield.
Results and Discussion
The first reported preparation of p-nitrophenyl 2-acetamido-2- The glycosylation of phenol and p-nitrophenol with oxazoline 3
deoxy-α-D-mannopyranoside (pNP-α-ManNAc, 8) was was extensively tested. We tested a large array of reaction
described in [6], starting from 2-acetamido-2-deoxy-D-glucose conditions, including variation of catalyst (copper(II) chloride
and providing the product in 2% overall yield. We have previ- [12,15], 2,2-diphenyl-1-picrylhydrazyl, zinc chloride, tin(IV)
ously published [7] a 9-step synthesis of pNP-α-ManNAc (8) chloride) and solvent (CH2Cl2, THF, toluene, benzene), all of
from commercially available methyl α-D-glucopyranoside with which were not successful. Finally, we found that trimethylsilyl
an overall yield of 1.5%. Later, Popelová et al. [8] published trifluoromethanesulfonate in dichloromethane [11] was capable
a concise synthesis starting either from D-glucose of catalyzing the oxazoline ring opening with phenol to afford
(5 steps, 1.5% yield) or from methyl 4,6-O-benzylidene-α-D- α-phenyl glycoside 4 in a reasonable 56% yield. This reaction
glucopyranoside (5 steps, 23%). Unfortunately, in our hands was also tested with p-nitrophenol; however, the required
this synthetic procedure did not afford the published p-nitrophenyl glycoside 6 was produced under these conditions
yields, namely in the triflate and consequent azide substitution only in trace amounts (observed by TLC). Our and previously
steps.
published results [11] indicate that the deactivation of phenol by
the electron-withdrawing nitro group substantially decreases its
Oxazolines, such as 3, have been used as glycosylation agents reactivity in the glycosylation reaction with oxazoline and, on
in the preparation of glycoproteins [9,10], various alcohol the other hand, the presence of electron-donating groups
glycosides [11,12], phosphates [13] and oligosaccharides [12]. increases the yields.
To the best of our knowledge, glycosylation of phenolic OH
with oxazoline has not been accomplished so far. Therefore, we The resulting phenyl glycoside 4 was treated with a solution of
decided to test oxazoline 3 [14] glycosylation in our synthetic red fuming nitric acid in acetic acid [16], producing a mixture
approach.
of o-nitrophenyl glycoside 5 (22% yield) and p-nitrophenyl
glycoside 6 (34% yield) in approximately 2:3 ratio, which was
separated by flash chromatography. Zemplén deacetylation of 4
Synthetic pathway
The synthesis (see Scheme 1) was started from commercially and 5 afforded the title compounds 7 and 8 in almost quantita-
available 2-acetamido-2-deoxy-D-mannopyranose (1, tive yields.
Scheme 1: Synthetic pathway.
429

Beilstein J. Org. Chem. 2012, 8, 428–432.
Conclusion
experiment 0.4 μL of sample dissolved in 50% acetonitrile was
In conclusion, a simple and robust procedure for the synthesis allowed to dry at ambient temperature on the target and over-
of o- and p-nitrophenyl 2-acetamido-2-deoxy-α-D-mannopyra- laid with matrix solution (either 2,5-dihydroxybenzoic acid,
noside (7 and 8) from commercially available ManNAc, via its DHB or α-cyano-4-hydroxycinnamic acid, CCA). The spectra
oxazoline, is described affording an overall total yield of both 7 were collected in reflectron mode.
and 8 of over 21%, including the purification steps.
Phenyl 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-α-D-
Experimental
mannopyranoside (4): 2-Methyl-(3,4,6-tri-O-acetyl-1,2-
General methods
dideoxy-β-D-mannopyrano)-[2,1:4’,5’]-2-oxazoline [18] 3 (280
All chemicals were purchased from Sigma-Aldrich, except for mg, 0.85 mmol) was dissolved in 10 mL of dry
2-acetamido-2-deoxy-D-mannose, which was purchased from dichloromethane. To this solution, phenol (250 mg, 2.66 mmol,
BIOSYNTH AG, Staad, CH. The reactions were monitored by 3 equiv) and trimethylsilyl trifluoromethanesulfonate (0.2 mL,
TLC with precoated silica gel 60 F254 aluminium sheets from 0.73 mmol, 1 equiv) were added. The reaction mixture was
Merck, detected with UV light and/or charred with sulfuric stirred at room temperature for 3 days (monitored by TLC on
acid (5% in EtOH). The compounds were purified either by silica gel, chloroform/acetone 8:1). The reaction mixture was
column flash chromatography with silica gel 60 (230–240 evaporated to dryness in vacuo. The product was isolated by
mesh, Merck) or by gel permeation chromatography with column chromatography (silica gel, chloroform/acetone 20:1) as
Sephadex LH20 from Sigma Aldrich. The solvents were a white solid (200 mg, 56%). 1H NMR (400 MHz, CDCl3) δ
distilled and dried according to the standard procedures 2.015 (s, 3H, 6-Ac), 2.022 (s, 3H, 3-Ac), 2.048 (s, 3H, 4-Ac),
before use.
2.080 (s, 3H, 2-Ac), 4.022 (dd, J = 2.4, 12.2 Hz, 1H, H-6u),
4.104 (ddd, J = 2.4, 5.6, 10.2 Hz, 1H, H-5), 4.265 (dd, J = 5.6,
12.2 Hz, 1H, H-6d), 4.821 (ddd, J = 1.6, 4.7, 8.9 Hz, 1H, H-2),
NMR spectroscopy
NMR spectra were recorded on a Bruker Avance III 400 MHz 5.168 (dd, J = 10.2, 10.2 Hz, 1H, H-4), 5.475 (d, J = 1.6 Hz,
spectrometer (400.13 MHz for 1H, 100.55 MHz for 13C at 1H, H-1), 5.564 (dd, J = 4.7, 10.2 Hz, 1H, H-3), 5.919 (d, J =
30 °C) and a Bruker Avance III 600 MHz spectrometer 8.9 Hz, 1H, 2-NH), 7.047 (m, 1H, H-para), 7.075 (m, 2H,
(600.23 MHz for 1H, 150.93 MHz for 13C at 30 °C) in CD3OD H-ortho), 7.291 (m, 2H, H-meta); 13C NMR (100 MHz, CDCl3)
or CDCl3 (Sigma-Aldrich). Residual signals of the deuterated δ 20.60, 20.70 (3 x q, 3-Ac, 4-Ac, 6-Ac), 23.29 (q, 2-Ac), 50.51
solvents were used as internal standards (for CD3OD δH (d, C-2), 62.14 (t, C-6), 65.99 (d, C-4), 68.67 (d, C-5), 68.89 (d,
3.330 ppm, δC 49.30 ppm, for CDCl3 δH 7.265 ppm, δC 77.23 C-3), 97.12 (d, C-1), 116.49 (d, C-ortho), 122.99 (d, C-para),
ppm). NMR experiments 1H NMR, 13C NMR, gCOSY, 129.56 (d, C-meta), 155.60 (s, C-ipso), 169.82 (s, 3-CO),
gHSQC, and gHMBC were performed using the manufacturer’s 169.94 (s, 4-CO), 170.15 (s, 2-CO), 170.40 (s, 6-CO).
software. 1H NMR and 13C NMR spectra were zero filled to
fourfold data points and multiplied by a window function before o-Nitrophenyl 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-α-D-
Fourier transformation. A two-parameter double-exponential mannopyranoside (5) and p-nitrophenyl 2-acetamido-2-
Lorentz–Gauss function was applied for 1H to improve resolu- deoxy-3,4,6-tri-O-acetyl-α-D-mannopyranoside (6): Com-
tion, and line broadening (1 Hz) was applied to get a better 13C pound 4 (1.1 g, 2.59 mmol) was dissolved in a mixture of acetic
signal-to-noise ratio. Chemical shifts are given on a δ-scale with anhydride (10 mL) and glacial acetic acid (2 mL). The solution
the digital resolution justifying the reported values to three (δH) was cooled to 0 °C in an ice bath, and red fuming nitric acid
or two (δC) decimal places.
(≥99.5%, 0.5 mL) was added in one portion under stirring. The
reaction mixture was stirred overnight, allowing the ice bath to
The anomeric configuration of manno-structures was deter- melt and warm up to ambient temperature. The reaction mix-
mined based on the value of 1J (C-1, H-1) [17], which was ture was diluted by an ice–water mixture (50 mL), stirred for an
measured by using gHMQC or gHSQC with retained direct additional 30 min, and neutralized by the saturated aqueous
coupling constants between directly bonded carbon and solution of NaHCO3. After extraction with methylene dichlo-
hydrogen.
ride (3 × 50 mL), the combined organic layers were washed
with 50 mL of sodium bicarbonate solution and 50 mL of water.
After drying (Na2SO4) the organic phase was evaporated and
Mass spectrometry
Mass spectra were measured on MALDI–TOF/TOF ultraFLEX chromatographed (silica gel, chloroform/acetone 5:1). o-Nitro-
III mass spectrometers (Bruker-Daltonics). Positive spectra phenyl derivative 5 was isolated as a white solid (270 mg, 22%)
were calibrated externally by using the monoisotopic [M + H]+ and p-nitrophenyl derivative 6 was isolated as a white solid
ions of PepMixII calibrant (Bruker-Daltonics). For the MALDI (410 mg, 34%).
430

Beilstein J. Org. Chem. 2012, 8, 428–432.
(5) 1H NMR (400 MHz, CDCl3) δ 2.032 (s, 3H, 3-Ac), 2.039 (s, 174.61 (s, 2-CO); MS–MALDI–TOF (m/z): 365.1 [M + Na]+
3H, 6-Ac), 2.080 (s, 3H, 4-Ac), 2.104 (s, 3H, 2-Ac), 4.042 (dd, (DHB), 365.1 [M + Na]+ (CCA).
J = 2.2, 12.3 Hz, 1H, H-6u), 4.212 (ddd, J = 2.2, 5.2, 10.2 Hz,
1H, H-5), 4.286 (dd, J = 5.2, 12.3 Hz, 1H, H-6d), 4.812 (ddd, J p-Nitrophenyl 2-acetamido-2-deoxy-α-D-mannopyranoside
= 1.8, 4.8, 7,7 Hz, 1H, H-2), 5.213 (dd, J = 10.2, 10.2 Hz, 1H, (8): Compound 6 (400 mg, 0.85 mmol) was deacetylated and
H-4), 5.593 (dd, J = 4.8, 10.2 Hz, 1H, H-3), 5.750 (d, J = 1.8 purified as described for compound 7 yielding 8 as a white solid
Hz, 1H, H-1), 5.909 (d, J = 7.7 Hz, 1H, 2-NH), 7.170 (ddd, J = (290 mg, 99%). 1H NMR (400 MHz, CD3OD) δ 2.078 (s,
1.2, 7.4, 8.2 Hz, 1H, H-4´), 7.329 (dd, J = 1.2, 8.5 Hz, 1H, 2-Ac3H, ), 3.562 (ddd, J = 2.4, 4.7, 9.9 Hz, 1H, H-5), 3.746 (dd,
H-6´), 7.544 (ddd, J = 1.7, 7.4, 8.5 Hz, 1H, H-5´), 7.905 (dd, J = J = 2.4, 12.0 Hz, 1H, H-6u), 3.752 (dd, J = 9.7, 9.9 Hz, 1H,
1.7, 8.2 Hz, 1H, H-3´); 13C NMR (100 MHz, CDCl3) δ 20.62, H-4), 3.812 (dd, J = 4.7, 12.0 Hz, 1H, H-6d), 4.164 (dd, J = 4.9,
20.63, 20.65 (3 x q, 3-Ac, 4-Ac, 6-Ac), 23.28 (q, 2-Ac), 50.68 9.7 Hz, 1H, H-3), 4.544 (dd, J = 1.7, 4.9 Hz, 1H, H-2), 5.657 (d,
(d, C-2), 61.97 (t, C-6), 65.52 (d, C-4), 68.33 (d, C-3), 69.68 (d, J = 1.7 Hz, 1H, H-1), 7.300 (m, 2H, H-ortho), 8.230 (m, 2H,
C-5), 97.34 (d, C-1), 117.24 (d, C-6´), 122.82 (d, C-4´), 125.88 H-meta). 13C NMR (100 MHz, CD3OD) δ 22.93 (q, 2-Ac),
(d, C-3´), 134.09 (d, C-5´), 140.58 (s, C-2´), 148.66 (s, C-1´), 54.28 (d, C-2), 62.23 (t, C-6), 68.20 (d, C-4), 70.51 (d, C-3),
169.20 (s, 3-CO), 170.07 (s, 4-CO), 170.32 (s, 6-CO), 170.47 75.79 (d, C-5), 99.09 (d, C-1), 118.05 (d, C-ortho), 127.00 (d,
(s, 2-CO).
C-meta), 144.21 (s, C-para), 162.80 (s, C-ipso), 174.66 (s,
2-CO); MS–MALDI–TOF (m/z): 365.1 [M + Na]+ (DHB),
(6) 1H NMR (400 MHz, CDCl3) δ 2.028 (s, 3H, 6-Ac), 2.047 (s, 365.1 [M + Na]+ (CCA).
3H, 3-Ac), 2.065 (s, 3H, 4-Ac), 2.109 (s, 3H, 2-Ac), 4.02* (m,
1H, H-5), 4.03* (m, 1H, H-6u), 4.273 (dd, J = 5.6, 12.4 Hz, 1H, Phenyl 2-acetamido-2-deoxy-α-D-mannopyranoside (9):
H-6d), 4.818 (ddd, J = 1.6, 4.8, 8.4 Hz, 1H, H-2), 5.196 (dd, J = Compound 4 (50 mg, 0.12 mmol) was deacetylated and puri-
10.2, 10.3 Hz, 1H, H-4), 5.555 (dd, J = 4.8, 10.3 Hz, 1H, H-3), fied as described for compound 7, yielding 9 as a white solid
5.629 (d, J = 1.6 Hz, 1H, H-1), 5.905 (d, J = 8.4 Hz, 1H, 2-NH), (35 mg, 99%). 1H NMR (400 MHz, CD3OD) δ 2.067 (s, 3H,
7.205 (m, 2H, H-ortho), 8.225 (m, 2H, H-meta) (* = HSQC 2-Ac), 3.654 (m, 1H, H-5), 3.73* (m, 1H, H-6u), 3.75* (m, 1H,
readout); 13C NMR (100 MHz, CDCl3) δ 20.59, 20.62, 20.67 (3 H-4), 3.838 (dd, J = 4.2, 11.9 Hz, 1H, H-6d), 4.178 (dd, J = 4.8,
x q, 3-Ac, 4-Ac, 6-Ac), 23.28 (q, 2-Ac), 50.42 (d, C-2), 61.94 (t, 9.6 Hz, 1H, H-3), 4.525 (dd, J = 1.7, 4.8 Hz, 1H, H-2), 5.466 (d,
C-6), 65.58 (d, C-4), 68.35 (d, C-3), 69.31 (d, C-5), 96.89 (d, J = 1.7 Hz, 1H, H-1), 7.023 (m, 1H, H-para), 7.110 (m, 2H,
C-1), 116.47 (d, C-ortho), 125.82 (d, C-meta), 143.19 (s, H-ortho), 7.298 (m, 2H, H-meta) (* - HSQC readouts); 13C
C-para), 160.22 (s, C-ipso), 169.68 (s, 3-CO), 169.86 (s, 4-CO), NMR (100 MHz, CD3OD) δ 22.95 (q, 2-Ac), 54.64 (d, C-2),
170.26 (s, 6-CO), 170.36 (s, 2-CO).
62.22 (t, C-6), 68.35 (d, C-4), 70.81 (d, C-3), 75.06 (d, C-5),
99.22 (d, C-1), 118.00 (d, C-ortho), 123.79 (d, C-para), 130.83
o-Nitrophenyl 2-acetamido-2-deoxy-α-D-mannopyranoside (d, C-meta), 157.95 (s, C-ipso), 174.53 (s, 2-CO);
(7): Compound 5 (270 mg, 0.58 mmol) was dissolved in dry MS–MALDI–TOF (m/z): 320.1 [M + Na]+ (DHB), 320.1 [M +
methanol (2 mL) and three drops of NaOMe in MeOH (35%, Na]+ (CCA).
w/w) were added. The reaction mixture was stirred at ambient
temperature for 20 minutes. The solution was directly loaded Acknowledgements
onto the gel permeation chromatography column (Sephadex Support from the Czech Science Foundation grant P207/10/
LH-20) with methanol as a mobile phase (2 mL/min, UV detec- 0321, EU project NOVOSIDES FP7-KBBE-2010-4-265854
tion 254 nm). The product was isolated as a white solid (MŠMT 7E11011), and ESF COST Chemistry CM1102 project
(197 mg, 99%). 1H NMR (400 MHz, CD3OD) δ 2.070 (s, 3H, is acknowledged.
2-Ac), 3.660 (ddd, J = 2.3, 4.5, 10.0 Hz, 1H, H-5), 3.761 (dd, J
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