2,4-Dimethoxybenzyl: An amide protecting group for 2-acetamido glycosyl donors 1

Article abstract of DOI:10.1081/CAR-100108272

2,4-Dimethoxybenzyl (Dmob) was used as an amide protecting group for 2-acetamido glycosyl donors. The N-Dmob group was introduced by imine formation between 2,4-dimethoxybenzaldehyde and d-glucosamine, followed by per-O-acylation, reduction to form the am

Full text of DOI:10.1081/CAR-100108272

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2,4-DIMETHOXYBENZYL: AN AMIDE PROTECTING
GROUP FOR 2-ACETAMIDO GLYCOSYL DONORS
Nicholas M. Kelly a & Knud J. Jensen b
a Department of Chemistry, Technical University of Denmark, Building 201, Kemitorvet
DK-2800 Kgs, Lyngby, Denmark
b Department of Chemistry, Technical University of Denmark, Building 201, Kemitorvet
DK-2800 Kgs, Lyngby, Denmark
Published online: 16 Aug 2006.
To cite this article: Nicholas M. Kelly & Knud J. Jensen (2001): 2,4-DIMETHOXYBENZYL: AN AMIDE PROTECTING GROUP
FOR 2-ACETAMIDO GLYCOSYL DONORS , Journal of Carbohydrate Chemistry, 20:7-8, 537-548
To link to this article: http://dx.doi.org/10.1081/CAR-100108272
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J. CARBOHYDRATE CHEMISTRY, 20(7&8), 537–548 (2001)
2,4-DIMETHOXYBENZYL: AN AMIDE
PROTECTING GROUP FOR 2-ACETAMIDO
GLYCOSYL DONORS1
Nicholas M. Kelly and Knud J. Jensen*
Department of Chemistry, Technical University of Denmark,
Building 201, Kemitorvet DK-2800 Kgs. Lyngby, Denmark
Dedicated to Professor Joachim Thiem.
ABSTRACT
2,4-Dimethoxybenzyl (Dmob) was used as an amide protecting group for 2-ac-
etamido glycosyl donors. The N-Dmob group was introduced by imine forma-
tion between 2,4-dimethoxybenzaldehyde and d-glucosamine, followed by per-
O-acylation, reduction to form the amine, and finally N-acetylation to give
1,3,4,6-tetra-O-acetyl-2-deoxy-2-(2,4-dimethoxybenzylacetamido)-ꢀ-D-glu-
copyranose. Selective 1-O-deacetylation and treatment with trichloroacetoni-
trile gave the corresponding trichloroacetimidate glycosyl donor. Lewis acid-
promoted glycosylations of the model substrate 3-nitrobenzyl alcohol gave
exclusively the ꢀ-glycoside product, either with or without the Dmob protecting
group remaining depending on the reagent and conditions employed. The N-
Dmob protected 1-O-acetate glucosyl donor gave higher glycosylation yields
than the corresponding 2-acetamido glucosyl donor without Dmob protection.
Key Words: Carbohydrates; Glycosides; Glycosylations; Stereoselective
synthesis; Dimethoxybenzyl
*Corresponding author.
537
Copyright © 2001 by Marcel Dekker, Inc.
www.dekker.com

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KELLY AND JENSEN
INTRODUCTION
Glycoconjugates play crucial roles in the development, growth, and proper
functions of an organism.2 Amino sugars are found in many biologically important
poly- and oligosaccharides, e.g., in glycans of O- and N-glycoproteins and -pep-
tides, lipochitin nodulation factors, and amino glycoside antibiotics such as
streptomycin. Synthesis of oligosaccharides provides an important tool for glyco-
biology, but the synthesis of glycosides of amino sugars such as D-glucosamine and
D-galactosamine involves special problems.3 Glycosyl donors with a 2-acylamino
group give, upon electrophilic activation of an anomeric (C-1) leaving group, an
oxocarbenium ion which by neighbouring group participation forms an oxazolin-
ium ion. Abstraction of the amide proton then gives a relatively stable oxazoline.
To avoid this stable and thus rather unreactive intermediate, the 2-amino group
should be protected with either one bivalent, or one or two monovalent protecting
groups. Bivalent protecting groups used include phthaloyl (Phth),3 tetrachloroph-
thaloyl (TCP),4 dithiasuccinoyl (Dts),5 dimethylmaleoyl (DMM),6 and thiodigly-
coloyl (TDG).7 Monovalent protecting groups used for single protection include
trichloroacetyl (TCA).8 trichloroethoxycarbonyl (Troc),9 trifluoroacetyl (TFA),10
and allyloxycarbonyl (Alloc),11 whereas double protection has been achieved with
diacetyl,12 Ac/Troc,9b and dibenzyl.13 Of these approaches, the Ac/Troc strategy
retains the acetamido functionality, whereas the TCA strategy relies on a masked
acetamido functionality. The protecting groups can then be cleaved by base, re-
duction, palladium catalysed transfer or thiolysis, sometimes requiring harsh
conditions for removal. Backbone amide protection with an acidolyzable 2,4-
dimethoxybenzyl (Dmob) moiety has been developed for peptide synthesis,14 and
the Dmob moiety can been removed by treatment with concd TFA.14 Herein, we
report preliminary results on the synthesis of novel N-Dmob protected 2-acetamido
glucosyl donors and their use in model glycosylations.
RESULTS AND DISCUSSION
Synthesis of N-Dmob protected glucosyl donors began with condensation of
D-glucosamine hydrochloride 1 with 2,4-dimethoxybenzaldehyde, 2, in the pres-
ence of aq sodium hydroxide15 to give imine 3 in 70% yield (Scheme 1). Imine 3
was then O-acetylated with acetic anhydride in pyridine to give the 1-O-ꢀ -acetate
4 exclusively. Washing imine 3 with ethanol and ensuring that the solid was thor-
oughly dried was essential to give consistent acetylation results. If this was not
done, then large amounts of 2-acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-ꢀ-D-glu-
copyranose, 5, were formed due to hydrolysis of the imine bond followed by acety-
lation of the amino group.
Next, reduction of imine 4 was investigated. Use of sodium borohydride re-
sulted in over-reduction whilst sodium triacetoxyborohydride did not fully reduce
the imine bond and often gave capricious results. However, use of sodium
cyanoborohydride in either THF-H2O or THF-MeOH gave the desired amine 6 in
a moderate yield with a small amount of an easily removed by-product. Finally,

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539
Scheme 1. Synthesis of N-Dmob protected 2-acetamido glycosyl donor 7.
N-acetylation with acetic anhydride gave the glycosyl donor 7 in quantitative yield.
Although the amide 7 was practically pure by HPLC and TLC, purification by
vacuum liquid chromatography (VLC) or preparative HPLC was carried out to
guarantee comparative results in the glycosylation experiments. The more reactive
trichloroacetimidate was accessed by a regioselective hydrazinolysis16 of the 1-O-
acetyl of 7 to give 8 in good yield and subsequent treatment with trichloroacetoni-
trile17 to give the trichloroacetimidate 9 (Scheme 2). An ꢀ1:1 mixture of the ꢁ-
and ꢀ-anomers of 9 was obtained in 51% after VLC purification. The pure
ꢁ-trichloroacetimidate 9ꢁ could be obtained by preparative HPLC. Rotamers were
observed in the 1H and 13C NMR spectra of all N-acetylated compounds, e.g., in 7.
However, purity and product type could easily be determined by analytical photo-
diode array (PDA) HPLC.
As a model glycosyl acceptor we chose 3-nitrobenzyl alcohol (10) for its
characteristic UV spectrum easily identified by PDA-HPLC. To provide a com-
parison for glycosylations with Dmob protected glycosyl donors, we first studied
the glycosylation of 10 with 2-acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-ꢀ-D-glu-
copyranose (5)18 and its ꢁ-anomer, 1119 (Scheme 3). Glycosylations were carried
out in either toluene or CH2Cl2 using equimolar amounts of donor and acceptor,
and BF3.OEt2 or TMSOTf as the Lewis acid (Table 1).
Glycosylations in CH2Cl2, at 40 °C, gave up to 56% yield, while higher yields
were obtained in toluene, at 100 °C. The ꢁ-anomer 11 gave slightly higher yields,
as did the use of BF3.OEt2 as Lewis acid, resulting in up to 69% yield of the gly-
Scheme 2. Synthesis of N-Dmob protected 2-acetamido trichloroacetimidate 9.

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KELLY AND JENSEN
Scheme 3. Glycosylations with 2-acetamido 1-O-acetates 5 and 11.
coside product 12 (Table 1, entry 5). No reaction was achieved using the ꢀ-anomer
5 and BF3.OEt2 in CH2Cl2 (Table 1, entry 3).
With the new Dmob protected glycosyl donors in hand, glycosylations with
N-Dmob protected 1-O-acetate 7 were then studied (Scheme 4). Using equimolar
amounts of donor and acceptor, and the same conditions as employed before for the
2-acetamido glycosyl donors 5 and 11, the Dmob group was cleaved and the ꢀ-gly-
coside 12 was isolated in good yield (Table 2, entries 1, 2, and 4).
As expected, the reaction proceeded with high stereoselectivity and only the
ꢀ-anomer was observed. In this case, the use of TMSOTf as the Lewis acid gave
higher yields of ꢀ-glycoside 12. A small amount of the 1-ꢁ-acetate 13 was also
formed when using TMSOTf in CH2Cl2 (Table 2, entry 4) which was probably due
to the attack of water on the oxazolinium intermediate. With BF3.OEt2 in CH2Cl2
(Table 2, entry 3), 12 was obtained together with ꢀ-glycoside 14 in which the
Dmob was still present. To increase the yield of the latter, 3 equiv of donor 7 were
used in the glycosylation reactions (Table 2, entries 5–8). Surprisingly, the yield of
the N-Dmob protected ꢀ-glycoside 14 did not change when using the same condi-
tions of BF3.OEt2 in CH2Cl2 (Table 2, entry 7 vs. 3). However, increasing the tem-
perature to 100 °C and lowering the amount of catalyst increased the yield of
ꢀ-glycoside 14 to an excellent 94% (Table 2, entry 5). Reversing the ratios and us-
ing 3 equiv of acceptor 10 to donor 7, gave a 45% yield of ꢀ-glycoside 14 (Table
2, entry 9).
Table 1. Glycosylation of 3-nitrobenzyl Alcohol (10) with Donors 5 and 11a
Equiv of
Lewis Acid
Entry
Donor
Lewis Acid
Solventb
Yieldc
1
2
3
4
5
6
7
8
5
5
5
BF3ꢂOEt2
TMSOTf
BF3ꢂOEt2
TMSOTf
BF3ꢂOEt2
TMSOTf
BF3ꢂOEt2
TMSOTf
0.1
0.1
1.0
1.0
0.1
0.1
1.0
1.0
PhMe
PhMe
CH2Cl2
CH2Cl2
PhMe
PhMe
CH2Cl2
CH2Cl2
51% 12
33% 12
NRd
56% 12
69% 12
63% 12
43% 12
13% 12
5
11
11
11
11
a The reactions were carried out in the presence of molecular sieves 3Å under argon for 18 h using 1
equiv of donor.
b Reactions in PhMe were run at 100°C and those in CH2Cl2 were run at 40°C.
c All yields are of isolated, pure products.
d No reaction.

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541
Scheme 4. Glycosylations with N-Dmob protected 2-acetamido glycosyl donor 7.
In a further effort to increase the yield of the glycoside product without con-
comitant cleavage of the Dmob group, glycosylations with the N-Dmob protected
amine 6 were attempted. Since the nitrogen was not acetylated, the Lewis acidic
conditions presumably would not cleave the Dmob group and so, a ‘safety-catch’
system was anticipated.20 However, the use of TMSOTf or BF3.OEt2 in toluene, as
described before, only gave decomposition of acetate 6, and in CH2Cl2 no reaction
was observed.
Glycosylation with trichloroacetimidate 9 was then attempted with an
equimolar amount of 3-nitrobenzyl alcohol 10 at ꢃ20 °C using 0.1 equiv of either
TMSOTf or BF3.OEt2 in CH2Cl2. However, no reaction was observed after 1 h at
this temperature. Warming to rt and using BF3.OEt2 as the catalyst, only 10% of
Table 2. Reaction of Glycosyl Donor 7 with 3-nitrobenzyl Alcohol 10a
Equiv
of Donor
Equiv of
Entry
Lewis Acid
Lewis Acidb
Solventc
Yieldd
1
2
3
1
1
1
BF3·OEt2
TMSOTf
BF3·OEt2
0.1
0.1
1.0
PhMe
PhMe
CH2Cl2
57% 12
64% 12
20% 12
24% 14
71% 12
21% 13
94% 14
5% 14
25% 14
9% 14
45% 14
27% 14
4
1
TMSOTf
1.0
CH2Cl2
5
6
7
8
9
3
3
3
3
0.33
0.33
BF3·OEt2
TMSOTf
BF3·OEt2
TMSOTf
BF3·OEt2
BF3·OEt2
0.1
0.1
1.0
1.0
0.1
1.0
PhMe
PhMe
CH2Cl2
CH2Cl2
PhMe
10
CH2Cl2
a The reactions were carried out in the presence of molecular sieves under argon for 18 h.
b Equiv relative to acceptor.
c Reactions in PhMe were run at 100°C and those in CH2Cl2 were run at 40°C.
d All yields are of isolated, pure products.

ORDER
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KELLY AND JENSEN
the desired glycoside 14 was obtained together with 33% of the rearranged acetate
13. However, using TMSOTf, 26% of the desired glycoside 14 was obtained to-
gether with 47% of acetate 13.
CONCLUSION
In conclusion, new N-Dmob protected 2-acetamido glucosamine acetate and
trichloroacetimidate glycosyl donors have been prepared. The N-Dmob protected
acetate 7 was found efficient for the glycosylation of the model acceptor 3-ni-
trobenzyl alcohol. Depending on the glycosylation conditions, the Dmob moiety
was either retained or cleaved.
EXPERIMENTAL
General Methods. 1H NMR spectra were recorded at 499.87 MHz and 13C
NMR spectra at 125.70 MHz on a Varian Inova 500 spectrometer equipped with a
zꢃ (single axis) PFG inverse detection CMHMP probe. Chemical shifts, ꢄ, were
measured in ppm and coupling constants, J, in Hz. For NMR spectra in deuterated
solvents, the solvent peak was used as reference (CDCl3: ꢄ 7.27 for 1H, 77.00 for
1
13C; acetone-d6: ꢄ 2.05 for H). When necessary, NMR data was assigned using
1HM1H correlated spectra. Melting points are uncorrected. Specific rotations were
measured on a Perkin-Elmer 241 polarimeter. IR spectra were recorded on a
Perkin-Elmer 1720 FTIR spectrometer. Mass spectra were recorded on Micromass
LCT or PE Sciex API 150EX instruments. TLC was performed on Merck 60F254
precoated silica plates and spots were detected by immersing the plate in sulphuric
acid (2 M) then charring. Vacuum liquid chromatography21 (VLC) was performed
with silica gel 60 H (Merck, 5–40 ꢅm). The product was pre-dried onto silica gel
60 (Merck, 40–63 ꢅm, 1 g of silica per gram substrate) then loaded onto the VLC
column before elution with an increasing concentration of ethyl acetate in hexanes.
Sinter funnel filtrations were performed with silica gel 60 (Merck, 40–63 ꢅm). An-
alytical HPLC was performed on a Waters system equipped with a 600E pump and
a 996 PDA detector on a Nova-Pak C18 (3.9ꢆ50 mm, 4 ꢅm, 60 Å) column. The
program used was run at 2 mL/min in water-acetonitrile with the following linear
gradient: initial 92:8; 1 min 92:8; 7 min 65:35; 12.5 min 5:95; 13 min 5:95; 13.5
min 92:8; 20 min 92:8. Preparative HPLC was performed on a Waters system with
a Delta 600 pump and 996 PDA detector on a stack of three Prep Nova-Pak H C18
(40ꢆ100 mm, 6 ꢅm, 60 Å) RCM units. Unless stated otherwise, the program used
water-acetonitrile with the following linear gradient: initial 95:5 at 0 mL/min;
1 min 95:5 at 30 mL/min; 69 min 20:80 at 30 mL/min; 79 min 20:80 at 30 mL/min;
80 min 20:80 at 0 mL/min. Fractions were collected every 40 s and the product was
extracted into ethyl acetate, dried over MgSO4, filtered and concentrated in vacuo.
All solvents for reactions were dried and distilled before use. Crystallizations were
from CH2Cl2-hexane.

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543
1,3,4,6-Tetra-O-acetyl-2-deoxy-2-(2,4-dimethoxybenzylidenamino)-ꢀ-D-
glucopyranose (4): D-Glucosamine (21.56 g, 100 mmol) and 2,4-dimethoxy-
benzaldehyde (16.62 g, 100 mmol) were shaken in aq sodium hydroxide (100 mL,
1.0 M, 100 mmol) for 18 h at rt.15,22 The white solid was filtered and washed with
ice cold water (100 mL), ethanol (100 mL) and ether (100 mL) then dried under
high vacuum for 4 h. The resultant white powder was crushed and then dried un-
der high vacuum for an additional 1 h to yield 2-deoxy-2-(2,4-dimethoxybenzyli-
denamino)-D-glucopyranose 3 (25.8 g, 79%) which was used in the next reaction.
A solution of crude 3 (6.54 g, 20.0 mmol) in pyridine (32 mL, 400 mmol),
cooled to 0 °C, and acetic anhydride (19 mL, 200 mmol) was added dropwise. The
reaction was stirred for 18 h at rt then ethyl acetate (100 mL) was added. The prod-
uct was washed with excess saturated aq NaHCO3 solution, brine (30 mL), dried
over MgSO4, filtered and concentrated in vacuo. Toluene (2ꢆ100 mL) was added
and the mixture concentrated in vacuo to give title compound 4 as a viscous pale
orange oil, which was contaminated with about 10% of 2,4-dimethoxybenzalde-
hyde; the product was used directly in the next reaction. A pure sample of the imine
22
could be prepared by VLC, eluting with ethyl acetate in hexanes (0:1 to 1:1): [ꢁ]D
ꢇ59.9° (c 1.0 CH2Cl2); 1H NMR (500 MHz, CDCl3): ꢄ 1.90 (s, 3H), 2.02 (s, 3H),
2.03 (s, 3H), 2.09 (s, 3H), 3.45 (dd, 1H, J ꢈ 9.6, 8.3 Hz, H-2), 3.82 (s, 3H), 3.84
(s, 3H), 3.96 (ddd, 1H, J ꢈ 10.0, 4.6, 2.1 Hz, H-5), 4.13 (dd, 1H, J ꢈ 12.4, 2.1 Hz,
H-6), 4.36 (dd, 1H, J ꢈ 12.4, 4.6 Hz, H-6ꢉ), 5.14 (dd, 1H, J ꢈ 10.0, 8.8 Hz, H-4),
5.40 (dd, 1H, J ꢈ 9.6, 8.8 Hz, H-3), 5.95 (d, 1H, J ꢈ 8.3 Hz, H-1), 6.42 (d, 1H, J
ꢈ 2.3 Hz), 6.49 (dd, 1H, J ꢈ 8.7, 2.3 Hz), 7.81 (d, 1H, J ꢈ 8.7 Hz), 8.56 (s, 1H,
NMH); 13C NMR (125 MHz, CDCl3): _ 20.23, 20.41, 20.46, 20.52, 55.25, 55.37,
61.65, 67.88, 72.52, 72.75, 73.05, 93.09, 97.67, 105.35, 116.99, 128.83, 160.14,
160.37, 163.52, 168.52, 169.31, 169.57, 170.34; HRMS (FAB): m/z calcd for
C23H30NO11 [MꢇH]ꢇ 496.1819; found 496.1819; analytical HPLC, 11.12 min.
1,3,4,6-Tetra-O-acetyl-2-deoxy-2-(2,4-dimethoxybenzylamino)-ꢀ-D-glu-
copyranose (6): Sodium cyanoborohydride (1.51 g, 24 mmol) was added to
crude 4, (from the previous reaction) in THF-H2O (50 mL, 20:1) and the reaction
was stirred at rt under argon for 60 min. Saturated aq ammonium chloride solution
(20 mL) was added and the reaction mixture stirred for 5 min, next 1.0 M aq HCl
was added until pH 5. The mixture was stirred for 5 min, then neutralised with sat-
urated aq NaHCO3 solution. The product was extracted with ethyl acetate (3ꢆ100
mL), and the organic phase washed with brine, then dried over MgSO4, filtered and
concentrated in vacuo to give a yellow oil. Purification by column chromatography
eluting with hexanes and ethyl acetate or by preparative HPLC (program used wa-
ter-acetonitrile with the following gradient: initial 92:8 at 0 mL/min; 1 min 92:8 at
30 mL/min; 79 min 20:80 at 30 mL/min; 80 min 20:80 at 0 mL/min) gave title com-
pound 6 as a white foam (5.91g, 59%, two steps): [ꢁ]D22 ꢇ29.5° (c 1.0 CH2Cl2);
1H NMR (CDCl3): ꢄ 1.94 (s, 3H), 1.98 (s, 3H), 2.04 (s, 3H), 2.14 (s, 3H), 2.88 (dd,
1H, J ꢈ 9.8, 8.5 Hz, H-2), 3.64 (d, 1H, J ꢈ 13.2 Hz, ChHꢉ-Ar), 3.75 (ddd, 1H,
J ꢈ 9.8, 4.7, 2.4 Hz, H-5), 3.76 (s, 3H), 3.78 (s, 3H), 4.04 (dd, 1H, J ꢈ 12.4, 2.1
Hz, H-6), 4.27 (dd, 1H, J ꢈ 12.4, 4.7 Hz, H-6ꢉ), 4.98 (m, 1H, H-4), 5.03 (m, 1H,

ORDER
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KELLY AND JENSEN
H-3), 5.55 (d, 1H, J ꢈ 8.5 Hz, H-1), 6.41 (2H, m), 7.05 (m, 1H); 13C NMR
(CDCl3): _ 20.49, 20.54, 20.59, 20.93, 47.38, 55.18, 55.26, 59.53, 61.72, 68.39,
72.31, 73.78, 95.26, 98.65, 103.75, 120.47, 130.47, 158.47, 160.21, 168.88,
169.54, 170.49, 170.52; IR _max (cmꢃ1) 3472, 2938, 1752, 1614, 1589, 1508, 1368,
1225, 1157, 1069, 1042; HRMS (FAB): m/z calcd for C23H30NO11 [MꢇH]ꢇ
498.1975; found 498.1960; analytical HPLC, 10.69 min.
1,3,4,6-Tetra-O-acetyl-2-deoxy-2-(N-(2,4-dimethoxybenzyl)-acetamido)-
ꢀ-D-glucopyranose (7). Compound 6 (6.30 g, 12.67 mmol) and acetic anhydride
(2.4 mL, 25 mmol) were stirred together in CH2Cl2 (25 mL) for 18 h at rt. The re-
action mixture was diluted with CH2Cl2 (25 mL), then washed with saturated aq
sodium bicarbonate solution (2ꢆ30 mL), brine (30 mL), dried over MgSO4, fil-
tered and concentrated in vacuo to give a white foam. The product was over 95%
pure by TLC and analytical HPLC, but purification by preparative HPLC was nor-
mally carried out to give title compound 7 as a white foam (6.20 g, 91%); mp
133–134.5 °C; [ꢁ]D25 ꢇ14.7° (c 1.0 CH2Cl2); 1H NMR (CDCl3): ꢄ 1.82 (s, 1H),
1.93 (br. s, 4H), 2.00 (s, 1H), 2.01 (s, 2H), 2.04 (s, 1H), 2.06 (s, 2H), 2.07 (s, 1H),
2.17 (br. s, 2H), 2.31 (s, 1H), 3.28 (br. s, 0.5H), 3.73 (ddd, 0.5H, J ꢈ 10.2, 4.5, 2.1
Hz), 3.77 (s, 1H), 3.81 (s, 1.5H), 3.81 (m, 0.5H), 3.82 (s, 2.5H), 3.88 (s, 1H), 3.92
(m, 0.5H), 4.04 (m, 1H), 4.23 (d, 0.67H, J ꢈ 16.2 Hz), 4.29 (m, 0.5H), 4.35 (dd,
0.5H, J ꢈ 12.6, 4.5 Hz), 4.46 (d, 0.67H, J ꢈ 16.2 Hz), 4.87 (d, 0.5H, 15.8 Hz), 5.01
(m, 0.5H), 5.10 (dd, 0.5H, Jꢈ10.2, 9.0 Hz), 5.45 (d, 0.5H, J ꢈ 8.7 Hz), 5.55 (dd,
0.5H, J ꢈ 10.7, 9.0 Hz), 6.16 (m, 0.67H), 6.43 (m, 2H), 6.65 (br. s, 0.5H), 7.11 (br.
s, 0.67H), 7.23 (m, 0.33H); HRMS (FAB): m/z calcd for C25H34NO12 [MꢇH]ꢇ
540.2081; found 540.2098; analytical HPLC: 10.36 min.
3,4,6-Tri-O-acetyl-2-deoxy-2-(N-(2,4-dimethoxybenzyl)-acetamido)-D-
glucopyranose (8). Compound 7 (10.0 mmol, 5.38 g) and hydrazinium acetate
(1.10 g, 12.0 mmol) were stirred together in DMF (20 mL) for 18 h, at rt, under ar-
gon. The reaction mixture was diluted with ethyl acetate (100 mL), then washed se-
quentially with water (2ꢆ100 mL), saturated aq NaHCO3 (30 mL) and brine (30
mL), then dried over MgSO4, filtered and concentrated in vacuo to give a white
foam (4.86 g, 98%). The product was pure apart from a small amount of DMF,
which could be removed by flash column chromatography, eluting with chloro-
form (100 mL) to give pure title compound 8 as a white foam (4.13g, 83%): mp
73–74.5 °C; [ꢁ]D25 ꢇ19.2° (c 1.0 CH2Cl2); 1H NMR (CDCl3): ꢄ 2.00 (br. s, 3H),
2.02 (s, 3H), 2.06 (s, 3H), 2.23 (br. s, 3H), 3.81 (s, 3H), 3.83 (s, 3H), 4.01 (dd, 1H
J ꢈ 12.0, 1.7 Hz), 4.07 (m, 1H), 4.21 (dd, 0.5H, J ꢈ 12.0, 4.9 Hz), 4.32 (m, 1.5H),
4.70 (d, 1H, J ꢈ 15.6 Hz), 4.80 (m, 1H), 5.03 (m, 1H), 6.15 (br. s, 1H), 6.47 (2H,
m), 7.07 (m, 1.5H), 7.23 (m, 0.5H); HRMS (FAB): m/z calcd for C25H33NO12
[MꢇH]ꢇ 498.1975; found 498.1975; analytical HPLC: 8.86 and 9.29 min (equili-
brating anomeric mixture)
3,4,6-Tri-O-acetyl-2-deoxy-2-(N-(2,4-dimethoxybenzyl)-acetamido)-ꢁ-
D-glucopyranosyl trichloroacetimidate (9). A solution of hemiacetal 8 (1.49 g,
3.0 mmol) and trichloroacetonitrile (3.0 mL, 30 mmol) in CH2Cl2 (15 mL) under ar-

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2,4-DIMETHOXYBENZYL
545
gon in the presence of molecular sieves, was cooled to ꢃ20 °C, and DBU (112 ꢅL,
0.75 mmol) was added dropwise. After stirring for for 30 min at ꢃ20 °C, the solu-
tion was warmed to rt over 30 min. The reaction was filtered through a pad of Celite
eluting with CH2Cl2 (20 mL) and concentrated in vacuo to give a dark brown oil
which was purified by column chromatography eluting with 10–50% ethyl acetate
in hexanes affording title compound 9 as a white powder (986 mg, 51%) with anꢁ/ꢀ
ratio of ꢀ1:1. Purification by preparative HPLC gave the ꢁ-anomer as the sole prod-
uct; mp 64.5–65.5 °C; [ꢁ]D25 ꢇ85.0° (c 1.0 CH2Cl2); 1H NMR (acetone-d6): ꢄ 1.74
(s, 3H), 1.92 (s, 3H), 1.95 (s, 3H), 1.99 (s, 3H), 3.77 (s, 3H), 3.81 (s, 3H), 4.09 (dd,
1H, J ꢈ 12.4, 2.1 Hz, H-6), 4.25 (dd, 1H, J ꢈ 12.4, 4.7 Hz, H-6ꢉ), 4.30 (ddd, 1H,
J ꢈ 10.2, 4.7, 2.1 Hz, H-5), 4.47 (d, 1H, J ꢈ 19.2 Hz), 4.61 (d, 1H, J ꢈ 19.2 Hz),
5.16 (dd, 1H, J ꢈ 10.2, 8.8 Hz, H-4), 5.34 (dd, 1H, J ꢈ 11.7, 3.4 Hz, H-2), 5.61 (dd,
1H, J ꢈ 11.7, 8.8 Hz, H-3), 6.51 (m, 2H), 6.53 (d, 1H, J ꢈ 3.4 Hz, H-1), 6.91 (m,
1H), 9.4 (s, NMH); calcd for C25H31Cl3N2O11 [M]: 640.10 (monoisotopic), 641.88
(average), [MMC2NCl3]: 497.1897 (monoisotopic), [MMC2HONCl3]: 480.1870
(monoisotopic); negative ion ESMS found m/z 641.0 [MMH]ꢃ; positive ion ESMS
found m/z: 498.23, 480.28; analytical HPLC: 11.73 min.
General procedure for glycosylation with acetates 5, 7, and 11 in CH2Cl2
or toluene. The glycosyl donor (0.5 mmol) and 3-nitrobenzyl alcohol (10, 0.5
mmol) were stirred together in CH2Cl2 or toluene (5 mL), respectively, at rt under
argon in the presence of molecular sieves. After 10 min, TMSOTf (0.5 mmol) or
BF3.OEt2 (0.5 mmol) was added, and the reaction was heated to 100 °C under ar-
gon for 18 h. The reaction was cooled to rt, and water (0.5 mL) and CH2Cl2 (25
mL) were added. The organic phase was dried over MgSO4, filtered, and concen-
trated in vacuo to give an oil. Purification was achieved by preparative HPLC. Ta-
bles 1 and 2 summarise the data.
General procedure for glycosylation with trichloroacetimidates in
CH2Cl2. The trichloroacetimidate donor 9 (128 mg, 0.20 mmol) and 3-nitroben-
zyl alcohol (10, 31 mg, 0.20 mmol) were dissolved in CH2Cl2 (5 mL) at ꢃ20 °C,
under argon, and TMSOTf (0.02 mmol) or BF3.OEt2 (0.02 mmol) in CH2Cl2 (0.1
mL) was added. The reaction was stirred at ꢃ20 °C to ꢃ10°C for 1 h (TLC indi-
cated no reaction at this temperature), then stirred at rt for 18 h. Water (1mL) was
added, and the product extracted with ethyl acetate (2ꢆ10mL), dried over MgSO4,
filtered, and concentrated in vacuo to give an oil. The product was purified by col-
umn chromatography eluting with 0–100% ethyl acetate in hexanes to give the
products as described in the text.
3-Nitrobenzyl 2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-ꢀ-D-glycopyra-
noside (12). Mp 182–183°C; [ꢁ]D25 ꢃ31.2° (c 1.0 CH2Cl2); 1H NMR (CDCl3):
ꢄ 1.98 (s, 3H), 2.03 (s, 3H), 2.05 (s, 3H), 2.11 (s, 3H), 2.17 (s, 3H), 3.73 (ddd, 1H,
J ꢈ 10.0, 4.7, 2.6 Hz, H-5), 4.09 (dt, 1H, J ꢈ 10.7, 8.5 Hz, H-2), 4.25 (dd, 1H,
J ꢈ 12.4, 2.6 Hz, H-6), 4.38 (dd, 1H, J ꢈ 12.4, 4.7 Hz, H-6ꢉ), 4.64 (d, 1H, J ꢈ 12.8
Hz), 4.82 (d, 1H, J ꢈ 8.5 Hz, H-1), 4.99 (d, 1H, J ꢈ 12.8 Hz), 5.16 (dd, 1H,
J ꢈ 10.0, 9.2 Hz, H-4), 5.25 (dd, 1H, J ꢈ 10.7, 9.2 Hz, H-3), 5.78 (s, 1H, J ꢈ 8.5

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546
KELLY AND JENSEN
Hz, NMH), 7.52 (m, 1H), 7.63 (m, 1H), 8.17 (m, 2H); 13C NMR (CDCl3): ꢄ 20.51,
20.56, 20.63, 23.12, 54.49, 62.01, 68.57, 69.11, 71.93, 72.12, 100.07, 122.07,
122.65, 129.34, 133.26, 139.49, 148.25, 169.29, 170.47, 170.58, 170.75; HRMS
(FAB): m/z calcd for C21H27N2O11 [MꢇH]ꢇ 483.1615; found 483.1622; analyti-
cal HPLC: 8.88 min.
1,3,4,6-Tetra-O-acetyl-2-deoxy-2-(2,4-dimethoxybenzylamino)-ꢀ-D-glu-
copyranose (13). Colourless oil; 1H NMR (CDCl3): ꢄ 1.97 (s, 3H), 1.98 (s, 3H),
1.99 (s, 3H), 2.16 (s, 3H), 2.95 (dd, 1H, J ꢈ 10.7, 3.6 Hz, H-2), 3.70 (d, 1H, J ꢈ
13.6 Hz), 3.73 (d, 1H, J ꢈ 13.6 Hz), 3.78 (s, 3H), 3.82 (s, 3H), 3.97 (dd,
1H, J ꢈ 12.4, 2.6 Hz, H-6), 4.06 (ddd, 1H, J ꢈ 10.2, 4.3, 2.6 Hz, H-5), 4.21 (dd,
1H, J ꢈ 12.4, 4.3 Hz, H-6ꢉ), 4.98 (dd, 1H, J ꢈ 10.2, 9.4 Hz, H-4), 5.16 (dd, 1H,
J ꢈ 10.7, 9.4 Hz, H-3), 6.13 (d, 1H, J ꢈ 3.6 Hz, H-1), 6.46 (m, 1H), 6.52 (m, 1H),
7.17 (m, 1H); analytical HPLC: 9.69 min.
3-Nitrobenzyl 3,4,6-Tri-O-acetyl-2-deoxy-2-(N-(2,4-dimethoxybenzyl)-
25
acetamido)-ꢄ-D-glucopyranoside (14). Colourless foam; [ꢁ]D ꢃ25.3° (c 1.0
CH2Cl2); 1H NMR (CDCl3): ꢄ 1.84 (br. s, 2H), 2.00 (br. s, 2H), 2.01 (s, 0.5H), 2.04
(s, 0.5H), 2.05 (s, 0.5H), 2.07 (s, 4H), 2.08 (s, 0.5H), 2.17 (s, 2H), 3.24 (br. s, 0.5H),
3.60 (ddd, 0.5H, J ꢈ 10.0, 4.7, 2.3 Hz), 3.65 (d, 0.5H, 11.7 Hz), 3.71 (s, 0.5H), 3.74
(s, 3H), 3.75 (br. s, 1.5H), 3.81 (m, 1H), 3.88 (s, 1H), 4.00 (dd, 0.5H, J ꢈ 10.7, 8.3
Hz), 4.08–4.5 (m, 4H), 4.58 (d, 0.5H, J ꢈ 11.5 Hz), 4.64 (br. s, 0.5H), 4.96 (m,
1.5H), 5.11 (dd, 0.5H, J ꢈ 10.0, 8.9 Hz), 5.62 (dd, 0.5H, J ꢈ 10.6, 8.9 Hz), 5.70
(br. s, 0.5H), 6.10 (br. s, 0.5H), 6.24 (m, 1H), 6.33 (br. s, 0.5H), 6.43 (d, 0.5H,
J ꢈ 2.4 Hz), 7.11 (m, 0.5H), 7.49 (m, 2H), 7.99 (0.5H, m), 8.14 (1.5H, m); HRMS
(FAB): m/z calcd for C30H37N2O13 [MꢇH]ꢇ 633.2296; found 633.2295; analyti-
cal HPLC: 11.85 min.
ACKNOWLEDGMENTS
We thank Dr. John Nielsen, Dr. Henrik Pedersen, and Mr. Peter Brøsen for
MS spectra, the Lundbeck Foundation (KJ), the Alfred Benzon Foundation (KJ),
and the Danish Technical Research Council (‘Talent Project’ grant to KJ) for fi-
nancial support.
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carrying out the reaction in a beaker and shaking the reagents for 18 h afforded both
efficient mixing and facilitation of the work-up procedure.
Received August 18, 2000
Accepted July 5, 2001

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