A convenient multigram preparation of functionalized 2-azido-2-deoxy-D-mannose as a useful orthogonally protected building block for oligosaccharide synthesis

Article abstract of DOI:10.1081/CAR-100108658

Multigram preparation of strategically protected 2-azido-2-deoxy mannose 5 from pentaacetyl glucose 1 is described. This manno-derivative was obtained in a straightforward manner and in high overall yield and represents a flexible building-block for complex oligosaccharide synthesis.

Full text of DOI:10.1081/CAR-100108658

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A CONVENIENT MULTIGRAM PREPARATION OF
FUNCTIONALIZED 2-AZIDO-2-DEOXY-D-MANNOSE AS
A USEFUL ORTHOGONALLY PROTECTED BUILDING
BLOCK FOR OLIGOSACCHARIDE SYNTHESIS
Veronica Draghetti a , Laura Poletti a , Davide Prosperi a & Luigi Lay a
a Università degli Studi di Milano , Via Venezian, Milano, 21-20131, Italy
Published online: 19 Aug 2006.
To cite this article: Veronica Draghetti , Laura Poletti , Davide Prosperi & Luigi Lay (2001) A CONVENIENT MULTIGRAM
PREPARATION OF FUNCTIONALIZED 2-AZIDO-2-DEOXY-D-MANNOSE AS A USEFUL ORTHOGONALLY PROTECTED BUILDING
BLOCK FOR OLIGOSACCHARIDE SYNTHESIS, Journal of Carbohydrate Chemistry, 20:9, 813-819
To link to this article: http://dx.doi.org/10.1081/CAR-100108658
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J. CARBOHYDRATE CHEMISTRY, 20(9), 813–819 (2001)
A CONVENIENT MULTIGRAM PREPARATION
OF FUNCTIONALIZED 2-AZIDO-2-DEOXY-D-
MANNOSE AS A USEFUL ORTHOGONALLY
PROTECTED BUILDING BLOCK FOR
OLIGOSACCHARIDE SYNTHESIS
Veronica Draghetti, Laura Poletti, Davide Prosperi,
and Luigi Lay*
Università degli Studi di Milano, Dip. di Chimica Organica e
Industriale, Via Venezian, 21–20131 Milano, Italy
ABSTRACT
Multigram preparation of strategically protected 2-azido-2-deoxy mannose 5
from pentaacetyl glucose 1 is described. This manno-derivative was obtained
in a straightforward manner and in high overall yield and represents a flexible
building-block for complex oligosaccharide synthesis.
INTRODUCTION
Mannosamine is a 2-deoxy-2-aminosugar playing different biological func-
tions. It is an intermediate in the biosynthesis of N-acetyl D-neuraminic acid,1 and
it acts as inhibitor of GPI biosynthesis2 and platelet aggregation factors in mam-
malian cells.3 It is also, inter alia, a component of cell walls and of the capsular
polysaccharides of different bacteria.4 Due to its role in recognition phenomena, it
represents a crucial building block in the preparation of natural and modified car-
bohydrate-based therapeutics. Complex carbohydrate synthesis often needs large
amounts of strategically protected monosaccharides as intermediates. It is there-
*Corresponding author. Fax ꢀ39 02 2664952; E-mail: luigi.lay@unimi.it
813
Copyright © 2002 by Marcel Dekker, Inc.
www.dekker.com

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DRAGHETTI ET AL.
fore highly desirable to have a quick, simple and straightforward procedure to pro-
vide a properly protected mannosamine precursor in multi-gram quantities, and
suitable for utilisation in the synthesis of N-acetyl mannosamine-containing
oligosaccharides.
In a project aimed at synthesising analogues of bacterial capsular polysac-
charides, we required a synthetic way of providing multi-gram amounts of pro-
tected mannosamine precursor 5 (Scheme 1), to be used as a building block in fur-
ther steps of the synthesis. To date, only two methods have been applied to the
preparation of a large amount of functionalised 2-azido-2-deoxy-D-mannosamine.
Paulsen et al.5b adopted a poorly stereoselective azido nitration5 on D-glucal, yield-
ing a 2:1 manno:gluco mixture. Augè et al.6 obtained 2-azido-2-deoxy-D-mannose
in three steps from glucose, using N,Nꢁ-sulfuryl diimidazole as a key reagent. Nev-
ertheless, this preparation furnishes very low overall yields due to the abundant for-
mation of by-products in the early steps. Other procedures leading to mannosamine
have also been reported, such as azido phenylselenylation7 of D-glucal and elec-
trophilic azidation of aldonolactones.8 Unfortunately, they show some disadvan-
tages such as low yields or poor stereoselectivities, use of undesired toxic reagents,
or reaction conditions which make it difficult to scale-up for a multigram prepara-
tion. Finally, Ishido et al.9 reported the nucleophilic displacement of a 2-O-triflate
of D-glucose as a method to obtain 2-azido-2-deoxy-D-manno derivatives with high
selectivity and yields. Therefore, we envisaged this reaction as the key step to pre-
pare our target molecule on a large scale.
RESULTS
In this endeavour, we developed a very simple and efficient procedure lead-
ing in eight steps to protected mannosamine precursor 5 in 39% overall yield from
1,2,3,4,6-penta-O-acetyl-ꢂ-D-glucopyranose 1 (Scheme 1). In compound 5 the
Scheme 1. a) 33% HBr in AcOH, CH2Cl2, 0°C to rt, 2 h; b) sym-collidine,5 eq MeOH dry, 0.5 eq
TBAB, 20 h, (92%o.y.); c) 0.2 eq MeONa 1M, MeOH, 14 min.; d) 9 eq BnBr, 4.5 eq NaH, DMF dry,
24 h (77%) from 2); e) Allylic alcohol, 0.22 eq TMSOTf, 0°C , 15ꢁ; f) 0.2 eq MeONa, MeOH, 2 h
(85% from 3); g) 4 eq Py, 3 eq Tf2O, CH2Cl2 dry, 0°C, 2 h; h) 3 eq Bu4NꢀN3ꢃ, dry toluene, 55–60°C,
24 h (65% from 4).

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2-AZIDO-2-DEOXY-D-NANNOSE
815
Scheme 2. a) HOAc/H2O ꢄ 20:1 v/v, 2.4 eq NaOAc, 1.1 eq PdCl2, 76%; b) Ac2O/HOAc 2:1 v/v,
3 eq ZnCl2, 84%.
amino group is masked by an azido group, widely used in the precursors of 2-de-
oxy-2-aminosugars for its poor steric hindrance and easy reconversion to an amino
group. Our procedure utilises inexpensive reagents, requires little chromatographic
purification and leads to the strategically protected azidomannose 5 on a multigram
scale. In addition, compound 5 is a versatile building block, as it can be elongated
from either its reducing or nonreducing end (Scheme 2).
1,2,3,4,6-Penta-O-acetyl-ꢂ-D-glucopyranose 1 was obtained by conventional
O-acetylation of D-glucose. Compound 1 was transformed into the known or-
thoester 2 (10) (33% HBr in acetic acid, then sym-collidine and MeOH in
dichloromethane) in 92% overall yield. After Zemplèn11 deacetylation, reaction
with benzyl bromide and sodium hydride yielded tribenzyl intermediate 3 (77%
from 2). The orthoester was then opened in allyl alcohol with trimethylsilyl triflate
as a catalyst; subsequent 2-O deprotection with sodium methylate in methanol af-
forded allyl glycoside 412 in 85% over two steps. The 2-OH was then activated as
a triflate with triflic anhydride in dichloromethane, followed by nucleophilic dis-
placement with tetrabutylammonium azide, providing target azidomannose 5 in
65% yield from 4 (39% o.y. from 1). Compound 5 is a flexible monosaccharidic
building block for oligosaccharide synthesis. In fact, it was easily deprotected at
the anomeric position with PdCl2 affording 2-azido-3,4,6-tri-O-benzyl-2-deoxy-D-
mannopyranose (6, 76%), which can be employed as a glycosyl donor upon proper
activation of the anomeric hydroxy group. On the other hand, chemoselective ace-
tolysis of 5 with acetic acid and ZnCl2 provided 6-O-acetyl azidomannose 7 (84%),
allowing the elongation from the nonreducing end of the sugar.
CONCLUSIONS
In conclusion, we reported herein an easy, quick and straightforward syn-
thetic route giving access to a strategically protected azidomannose in a multigram
scale and in high overall yield. This compound is a versatile, useful tool for
oligosaccharide synthesis, as demonstrated by selective deblocking of either the
anomeric or the 6-O-position.

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EXPERIMENTAL SECTION
General Methods. 1H NMR spectra were recorded on Bruker AC 300 and
Varian Gemini 200 spectrometers. Solvents were dried by standard procedures; al-
lyl alcohol was dried over 4 Å m.s. Melting points were determined with a Büchi
apparatus and are not corrected. Optical rotations were measured at room temper-
ature (23°C) with a Perkin-Elmer 241 polarimeter. TLC was carried out on Merck
Silica-gel 60 F254 plates (0.25 mm thickness), and spots were visualized by spray-
ing with a solution containing H2SO4 (31 mL), ammonium molybdate (21 g) and
Ce(SO4)2 (1 g) in 500 mL water, followed by heating at 110°C for 5 min Column
chromatography was performed by the flash procedure using Merck Silica-gel 60
(230–400 mesh). Elemental analyses were performed using the Carlo Erba ele-
mental analyzer 1108. Allyl alcohol was dried over m.s. 4 Å.
3,4,6-Tri-O-acetyl-[1,2-O-(1-methoxyethylidene)]-ꢂ-D-glucopyranose
(2). A solution of 1,2,3,4,6-penta-O-acetyl-D-glucopyranose 1 (50.5 g, 129.4
mmol) in 50 mL of dichloromethane was cooled to 0°C and hydrobromic acid 33%
in acetic acid (50 mL, 252 mmol) was added dropwise. The reaction was monitored
by TLC (eluent hexane/EtOAc ꢄ 7:3 v/v), and after 1.5 h the mixture was neu-
tralised with ice-cold satd NaHCO3, the organic layer was washed with water
(2 ꢅ 200 mL) and dried (Na2SO4). Removal of the solvent afforded the bromide
intermediate as a white glassy solid (53 g). Crude bromide (53 g, 129 mmol) was
dissolved under a nitrogen atmosphere in 70 mL of sym-collidine and warmed to
45°C. TBAB (20.7 g, 64.5 mmol) and dry methanol (26 mL, 645 mmol) were
added. The reaction was monitored by TLC (eluent hexane/EtOAc ꢄ 6:4 v/v). Af-
ter 16 h the solution was diluted with dichloromethane, the organic layer was
washed with water (2 ꢅ 200 mL), dried over Na2SO4 and concentrated under re-
duced pressure. The residue was suspended in diethyl ether (60 mL), the excess of
TBAB was filtered off, and the ethereal solution was concentrated. Chromato-
graphic purification (eluent hexane/EtOAc ꢄ 8:2 v/v containing 1% of TEA) af-
forded pure compound 2 as a white syrup (43.15 g, 92%), whose spectroscopic data
were in agreement with those reported in literature.10
3,4,6-Tri-O-benzyl-[1,2-O-(1-methoxyethylidene)]-ꢂ-D-glucopyranose
(3). Compound 2 (43.15 g, 119 mmol) was dissolved under a nitrogen atmo-
sphere in dry methanol (60 mL) and a 1M solution of CH3ONa in dry methanol
(23.8 mL) was added dropwise. The reaction was monitored by TLC (eluent hex-
ane/EtOAc ꢄ 7:3 v/v and CH2Cl2/MeOH ꢄ 85:15 v/v), and after 1.5 h the mix-
ture was concentrated affording 28 g of the orthoester intermediate as a yellow oil.
The crude compound (28 g, 119 mmol) was dissolved in dry DMF (100 mL) under
a nitrogen atmosphere. After the dropwise addition of BnBr (63.6 mL, 535 mmol),
NaH (25.70 g, 1.071 mol) was added portionwise. The reaction was stirred at room
temperature (TLC: hexane/EtOAc 7:3 v/v), and after 20 h the excess of NaH
was quenched by slow addition of methanol under vigorous stirring. The reaction
mixture was diluted with diethyl ether, the organic layer was washed with water

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817
(2 ꢅ 100 mL), dried over Na2SO4, filtered and concentrated under reduced pres-
sure. A quick filtration on silica gel (eluent hexane/EtOAc ꢄ 9:1 v/v containing
1% of TEA) afforded compound 3 as a yellow syrup (46.53 g, 77%), whose spec-
troscopic data were in agreement with those reported in literature.11
Allyl (3,4,6-Tri-O-benzyl)-ꢆ-D-glucopyranoside (4). Compound 3 (46.53
g, 92 mmol) was dissolved in 100 mL of dry allyl alcohol under a nitrogen atmo-
sphere. The solution was cooled to 0°C, and TMSOTf (3.2 mL, 20.2 mmol) was
added dropwise. The reaction was monitored by TLC (eluent hexane/EtOAc ꢄ 7:3
v/v). After 20 min the reaction gave two products, the 2-O-acetyl and the 2-hy-
droxy derivatives respectively, in a ratio of about 1:1. The solution was neutralised
with TEA and diluted with dichloromethane, the organic layer was washed with
water (2 ꢅ 100 mL), dried (Na2SO4), filtered and concentrated under reduced pres-
sure. The residue was dissolved in 80 mL of dry methanol and a 1M solution of
CH3ONa in methanol (18.4 mL, 18.4 mmol) was added. The reaction was stirred
at room temperature (TLC: hexane/EtOAc 7:3 v/v). After 20 h the mixture was
neutralised with Amberlite IR-120 (Hꢀ form), filtered and concentrated. The
residue was purified on silica gel (eluent hexane/EtOAc ꢄ 85:15 v/v) providing
compound 4 (38.3 g, 85%), whose spectroscopic data were in agreement with those
reported in literature.12
Allyl (2-Azido-3,4,6-tri-O-benzyl-2-deoxy)-ꢆ-D-mannopyranoside (5).
To a solution of compound 4 (38.3 g, 78.2 mmol) in dry dichloromethane (150 mL)
under a nitrogen atmosphere, pyridine (25.3 mL, 313 mmol) was added. The mix-
ture was cooled to 0°C, and after 15 min triflic anhydride (38.7 mL, 234.6 mmol)
was added dropwise. After stirring 1.5 h the reaction was diluted with
dichloromethane, the organic layer was washed with HCl 5% until acidic pH, then
with satd NaHCO3 until neutralisation, dried over Na2SO4, filtered and concen-
trated under reduced pressure. The crude 2-O-triflate intermediate was dissolved
under a nitrogen atmosphere in dry toluene (200 mL), Bu4NꢀNꢃ3 (55.46 mL,
195 mmol) was added quickly, and the solution was stirred at 55°C (TLC: hex-
ane/EtOAc 7:3 v/v). After 24 h the reaction mixture was concentrated and chro-
matographic purification (eluent hexane/EtOAc ꢄ 9:1 v/v) afforded compound 5
as a yellow syrup (26.17 g, 65%). [ꢂ]D20 ꢄ ꢃ36.8° (c 1, chloroform); 1H NMR (300
MHz, CDCl3) ꢇ7.43ꢃ7.04 (m, 15H, HAr), 5.89ꢃ5.76 (m, 1H, CHBCH2),
5.28ꢃ5.22 (m, 1H, CHBCHH), 5.04ꢃ5.01 (m, 1H, CHBCHH), 4.89 (d, 1H, J ꢄ
11.1 Hz, CHHPh), 4.56ꢃ4.40 (m, 4H, 3/2 CH2Ph, H-1), 4.38ꢃ4.14 (m, 4H,
CH2Ph, CHall, H-2), 4.01 (t, 1H, J3,4 ꢄ J4,5 ꢄ 9.3 Hz, H-4), 3.93ꢃ3.85 (m, 1H,
CHall), 3.71 (d, 2H, H-6, H-6ꢁ), 3.48 (dd, 1H, J3,2 ꢄ J3,4 ꢄ 8.8 Hz, H-3), 3.32 (dt,
1H, J5,4 ꢄ 9.6 Hz, J5,6 ꢄ J5,6ꢁ ꢄ 3.3 Hz, H-5).
Anal. Calcd for C30H33O5N3 (515.61): C, 69.88; H, 6.45; N, 8.15. Found:
C, 69.73; H, 6.48; N, 8.10.
2-Azido-3,4,6-tri-O-benzyl-2-deoxy-ꢂ,ꢆ-D-mannopyranose (6). Com-
pound 5 (26.17 g, 50.83 mmol) was dissolved in a HOAc/H2O ꢄ 20:1 v/v mixture

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DRAGHETTI ET AL.
(100 mL), then NaOAc (16.6 g, 122 mmol) and PdCl2 (6.56 g, 56 mmol) were
added. The reaction was monitored by TLC (eluent hexane/EtOAc ꢄ 7:3 v/v). Af-
ter 24 h the reaction mixture was diluted with EtOAc and filtered over celite,
washed with satd NaHCO3 until neutralisation, dried (Na2SO4), filtered and con-
centrated. Purification by flash chromatography (eluent hexane/EtOAc 9:1 v/v) af-
forded pure compound 6 as a yellow syrup (18.37g, 76%) as a mixture ofꢂ/ꢆ
anomers. The deprotection of the anomeric hydroxyl was ascertained by the disap-
pearance of the allyl signals detected in 1H NMR spectrum (multiplets at 5.8, 5.2
and 5.0 ppm).
Anal. Calcd for C27H29O5N3 (475.54): C, 68.19; H, 6.14; N, 8.83. Found:
C, 68.31; H, 6.12; N, 8.81.
Allyl 6-O-Acetyl-2-azido-3,4-di-O-benzyl-2-deoxy-ꢆ-D-mannopyra-
noside (7). Compound 5 (768 mg, 1.49 mmol) was dissolved in a mixture of
Ac2O/HOAc 2:1 v/v containing freshly melted ZnCl2 (610 mg, 4.47 mmol). After
6 h the reaction was diluted with water and EtOAc, washed with satd NaHCO3 un-
til neutralisation, dried (Na2SO4), filtered and concentrated. Flash chromatography
of the crude compound (eluent hexane/EtOAc ꢄ 9:1 v/v) afforded mannopyra-
noside 7 as a yellow syrup (585 mg, 84%). [ꢂ]D20 ꢄ ꢃ13.8° (c 1, chloroform); 1H
NMR (200 MHz, CDCl3) ꢇ7.50ꢃ7.10 (m, 10H, HAr), 6.01ꢃ5.81 (m, 1H,
CHBCH2), 5.34ꢃ5.20 (m, 2H, CHBCH2), 4.91 (d, 1H, J ꢄ 10.8 Hz, CHHPh),
4.80ꢃ4.66 (2H, AB system, CH2Ph), 4.59 (d, 1H, J ꢄ 10.8 Hz, CHHPh), 4.52 (s,
1H, H-1), 4.43ꢃ4.32 (m, 2H, H-6, H-6ꢁ), 4.28ꢃ4.19 (m, 1H, CHall), 4.12ꢃ4.02
(m, 1H, CHall), 3.95 (d, 1H, J2,3 ꢄ 3.2 Hz, H-2), 3.77 (t, 1H, J3,4 ꢄ J4,5 ꢄ 9.2 Hz,
H-4), 3.67 (dd, 1H, J3,4 ꢄ 9.2 Hz, J3,2 ꢄ 3.2 Hz, H-3), 3.41 (ddd, 1H, J5,4 ꢄ 9.2 Hz,
J
5,6 ꢄ 2.6 Hz, J5,6ꢁ ꢄ 5.2 Hz, H-5), 2.09 (s, 3H, OAc).
Anal. Calcd for C25H29O6N3 (467.52): C, 64.23; H, 6.25; N, 8.99. Found:
C, 64.14; H, 6.26; N, 9.00.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from EU (VACNET project,
CT 96-0158), CNR (Centro di Studio sulle Sostanze Organiche Naturali) and
MURST (COFIN 2000, prot. MM03155477).
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Received April 9, 2001
Accepted October 11, 2001

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