Glycosidation of 2,5-anhydro-3,4-di-O-benzyl-D-mannitol with different glucopyranosyl donors. A comparative study

Article abstract of DOI:10.1016/j.carres.2006.01.028

2,5-Anhydro-3,4-di-O-benzyl-d-mannitol was glycosylated using different donors such as tetra-O-acetyl-α-d-glucopyranosyl bromide in the presence of Hg(CN)₂, the corresponding β-thiophenylglycoside in the presence of NIS and TfOH as well as the

Full text of DOI:10.1016/j.carres.2006.01.028

Carbohydrate Research 341 (2006) 776–781
Note
Glycosidation of 2,5-anhydro-3,4-di-O-benzyl-D-mannitol
with different glucopyranosyl donors. A comparative study
*
´
´
´
´
Aniko Tegdes, Gabor Medgyes, Sandor Boros and Janos Kuszmann
IVAX Drug Research Institute, PO Box 82, 1325 Budapest, Hungary
Received 19 December 2005; accepted 30 January 2006
Available online 13 February 2006
Abstract—2,5-Anhydro-3,4-di-O-benzyl-D-mannitol was glycosylated using different donors such as tetra-O-acetyl-a-D-glucopyr-
anosyl bromide in the presence of Hg(CN)2, the corresponding b-thiophenylglycoside in the presence of NIS and TfOH as well
as the a- and b-trichloroimidate with TMSOTf as promoter. The resulting mixtures were analyzed by HPLC and the following main
components were isolated and characterized: 2,5-anhydro-3,4-di-O-benzyl-1-O-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-D-man-
nitol; 6-O-acetyl-2,5-anhydro-3,4-di-O-benzyl-1-O-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-D-mannitol; 2,5-anhydro-3,4-di-O-
benzyl-1,6-bis-O-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-D-mannitol; 2,5-anhydro-3,4-di-O-benzyl-1-O-[-2-O-(2,3,4,6-tetra-O-
acetyl-b-D-glucopyranosyl)-3,4,6-tri-O-acetyl-b-D-glucopyranosyl]-6-O-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-D-mannitol and
2,5-anhydro-3,4-di-O-benzyl-1,6-bis-O-(3,4,6-tri-O-acetyl-1,2-O-ethylidene-20-yl-a-D-glucopyranosyl)-D-mannitol. The latter com-
pound representing a bis-orthoester might be a common intermediate in all the investigated reactions, as its rearrangement and/
or decomposition can yield all of the isolated compounds.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: 2,5-Anhydro-D-mannitol derivatives; Mono-, di- and triglycosylated derivatives; Orthoesters and their decomposition products
Recently, we reported1 the synthesis of several 1,6-di-O-
glycosylated 2,5-anhydro-D-mannitol derivatives using
2,5-anhydro-3,4-di-O-benzoyl-D-mannitol 3 as acceptor
and the following donors: acetobromo derivatives in
the presence of Hg(CN)2 or thioglycosides in the pres-
ence of NIS and TfOH, respectively. In every case com-
plex mixtures were formed from which the 1,5-di-O-
glycosylated trisaccharides could be obtained after col-
umn chromatography in disappointingly low yields
only. This is more inconceivable as the primary OH
groups of 3 should react more readily than the second-
ary OH groups of 2,5-anhydro-1,6-di-O-benzoyl-D-man-
nitol, which afforded the corresponding 3-O-glycosides
in high yields.2 For getting a better insight into the reac-
tion mentioned above, a systematic investigation was
started. To avoid a possible interference of the O-ben-
zoyl groups of 3, the corresponding 3,4-di-O-benzyl ana-
logue 2 was used as acceptor and acetobromo a-D-
glucopyranose 5 in the presence of Hg(CN)2, the corre-
sponding b-thiophenylglycoside 6 in the presence of NIS
and TfOH as donors as well as the a- and b-trichloroim-
idate 7 and 8 with TMSOTf as promoter. The molar
ratio of the acceptor and donor was 1:2.2 and the reac-
tions were quenched after the donor had been consumed
(TLC). The crude mixtures obtained after the usual pro-
cessing were analyzed by HPLC and submitted thereaf-
ter to preparative column chromatography. In most
cases the multi-component mixtures could only be sepa-
rated partially in one run and the main components of
the so obtained fractions were isolated usually by
repeated column chromatography. The structures of
the purified components were established by NMR
spectroscopy and the corresponding data, including
the conditions of the reactions are listed in Tables 1–5.
As can be seen from Table 1, the 1,6-di-O-glycosyl-
ated compound 11 was formed in highest yield (44%),
when bromide 5 was used as donor, but a substantial
amount (9%) of the O-monoglycosylated compound 9
*
Corresponding author. Tel.: +36 1 399 3441; fax: +36 1 399 3356;
e-mail: janos.kuszmann@idri.hu
0008-6215/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.01.028

A. Tegdes et al. / Carbohydrate Research 341 (2006) 776–781
Table 1. Glycosidation of 4 with different donors applied in a molar ratio of 1:2.2
777
Run
Donor
Temp
Time
Promoter
% Of products according to HPLC
9
10
11
12
13
1
2
3
4
5
5
6
7
7
8
rt
20 h
2 h
25 min
10 min
10 min
Hg(CN)2
9
15
26
2
9
10
44
21
18
24
25
—
15
—
24
23
—
—
21
1
ꢀ40 ꢁC
ꢀ40 ꢁC
ꢀ40 ꢁC
ꢀ40 ꢁC
NIS; TfOH
TMSOTfa
TMSOTfb
TMSOTfb
—
—
—
—
1
a 10 mol %.
b 20 mol %.
Table 2. Data of the isolated compounds
9
10
+2
0.7
11
12
13
[a]D (CHCl3)
Rf (TLC)a
tR (HPLC) min
Anal. Calcd for
C
0
0.4
3.6
+5
0.5
8.0
+15
+40
0.4
8.8
0.6
9.6
6.6
C34H42O14
60.53
6.27
C36H44O15
60.33
6.19
C48H60O23
57.37
6.02
C60H76O31
55.73
5.92
C48H60O23
57.37
6.02
H
Found: C
Found: H
60.37
6.42
60.21
6.30
57.13
6.14
55.40
6.09
57.24
6.26
a Solvent: EtOAc–hexane 2:1.
Table 3. Characteristic 1H NMR chemical shifts
2,5-Anhydro-D-mannitol unit
Glucopyranosyl unit(s)
H-1a
H-1b
H-2
H-3
H-4
H-5
H-6a
3.64
4.15–4.18
3.61
3.56
H-6b
3.70
H-1
H-2
H-3
H-4
H-5
H-6a
4.10
4.11
4.10
4.07
4.11
4.11
H-6b
9
3.63
3.63
3.61
3.65
3.92
3.94
3.91
3.93
4.18
4.19
4.12
4.12
4.01
4.03
3.99
4.11a
4.03
3.96
3.99
4.04a
4.08
4.21
4.12
4.12
4.54
4.54
4.52
4.79
4.44
4.47
5.57
5.00
5.02
5.00
3.71
4.96
5.00
4.34
5.19
5.20
5.20
5.16
5.13
5.18
5.17
5.08
5.08
5.07
4.96
5.02
5.08
4.89
3.66
3.67
3.66
3.63
3.70
3.67
3.94
4.24
4.25
4.24
6.03
6.03
6.04
10
11
12
3.91
3.92
0
00
000
13
3.55
3.58
4.12
3.96
3.96
4.12
3.55
3.58
4.19
a Interchangeable assignation.
Table 4. Characteristic JH,H couplings
2,5-Anhydro-D-mannitol unit
2J1a,1b 3J1a,2 3J1b,2 3J2,3 3J3,4 3J4,5 3J5,6a 3J5,6b 2J6a,6b
Glucopyranosyl unit(s)
3J1,2 3J2,3 3J3,4 3J4,5 3J5,6a 3J5,6b 2J6a,6b
9
10.6
6.3
5.8
5.9
5.5
6.1
5.7
3.7
3.6
7.0
3.0
3.6
4.2
3.2
7.0
ꢁ6.0
3.7
5.7
11.9
8.0
8.0
8.0
7.5
8.0
7.9
5.2
9.5
9.6
9.6
9.3
9.6
9.6
2.0
9.7
9.6
9.6
9.6
9.6
9.6
2.0
9.7
9.7
9.7
9.7
9.7
9.7
9.5
2.8
2.4
2.3
4.7
4.6
4.7
12.3
12.3
12.3
10 10.5
11 10.6
12 10.6
5.9
5.5
10.6
10.4
0
00
000
13 10.1
6.1
6.2
4.1
4.1
6.1
6.2
10.1
and its 6-O-acetylated derivative 10 (15%) could also be
detected in the reaction mixture. Formation of the latter
can be explained by the formation of an orthoacetate 14,
the decomposition of which can lead according to the
mechanism suggested by Garegg et al.3 either to the
O-acetylated derivative 10, to the O-glycoside 11 or to
an intermediate, in which a free OH group is present
at C-2 of the glycosyl moiety (15).4–7 Although this lat-
ter could not be detected in the aforementioned reaction,
when the thioglycoside 6 was used as donor and NIS as
promoter (Table 1, Run 2) the 1,6-bis-glycosylated
anhydro derivative 12 could be isolated from the result-
ing mixture carrying a 2-O-glycosido-glycosyl unit at
one of the terminal positions of the anhydro-mannitol

778
A. Tegdes et al. / Carbohydrate Research 341 (2006) 776–781
Table 5. Characteristic 13C NMR chemical shifts
2,5-Anhydro-D-mannitol unit
Glycopyranosyl unit(s)
C-1
C-2
C-3
C-4
C-5
C-6
C-1
C-2
C-3
C-4
C-5
C-6
9
69.4
69.4
69.4
69.6
81.6
81.8
81.7
81.5a
84.3
84.4
84.5
84.5b
84.0
84.4
84.5
84.0b
83.3
80.7
81.7
81.4a
62.6
64.1
69.4
69.3
100.9
101.0
100.9
101.5
100.2
100.9
97.0
71.2
71.2
71.2
77.6
71.7
71.1
73.2
72.7
72.8
72.8
74.1
73.0
72.8
70.1
68.3
68.4
68.4
68.6
68.3
68.4
68.2
71.8
71.9
71.8
71.5
72.0
71.8
67.1
61.9
61.9
61.9
61.8
62.0
62.0
63.1
10
11
12
0
00
000
13
63.5
81.6
84.6
84.6
81.6
63.5
a,bArbitrary assignments.
residue. That means that, in this case, the orthoester 14
must have been formed as an intermediate, which col-
lapsed to the 2-OH glycoside 15 and this reacted further
with the donor affording 12. It should be mentioned that
because of the C2 symmetry of the acceptor molecule 4,
the two terminal positions (C-1 and C-6) are equal and
can be distinguished from each other by NMR spectro-
scopy only after an asymmetric substitution. The con-
nection of the additional glucose unit to O-2 in case of
compound 12 was proven by the anomalous chemical
shifts of H-20, C-20 and the H-100 signals (see Tables 3–
5) as well as the heteronuclear long-range couplings H-
100/C-20, H-20/C-100 detected by HMBC (Scheme 1).
When the a-imidate 7 was used as donor and a cata-
lytic amount (10 mol %) of TMSOTf as promoter in
the presence of a relatively large amount of a molecule
sieve (Table 1, Run 3), besides the 1,6-di-O-glycopyr-
anoside 11, the symmetric bis-orthoester 13 was formed
O
O
AcO
OH
HO
OTr
TrO
O
X
AcO
RO
OR
RO
1 R = H
OR
3 R = Bz
4 R = Bn
OAc
AcO
2 R = Bn
5 X = α−Br
AcO
6 X = β−SPh
1
O
1'
6
O
7 X = α-OC(=NH)CCl3
8 X = β-OC(=NH)CCl3
AcO
O
OR
BnO
OBn
OAc
AcO
OAc
OAc
AcO
9 R = H
10 R = Ac
O
O
O
AcO
O
O
AcO
BnO
OBn
OAc
OAc
AcO
AcO
AcO
AcO
11
OAc
OAc
O
1'
6
1
O
O
O
1'''
O
O
BnO
OBn
OAc
AcO
1''
AcO
AcO
O
12
OAc
OAc
OAc
AcO
OAc
O
O
O
AcO
O
O
O
O
O
OAc
O
AcO
Me
Me
BnO
OBn
13
Scheme 1.

A. Tegdes et al. / Carbohydrate Research 341 (2006) 776–781
779
in almost equal amount. The orthoester structure of 13
is evident from its NMR spectra as proved by the anom-
alous chemical shifts and coupling pattern of the glucose
units, the characteristic chemical shifts of the quaternary
carbon signal at 121.3 ppm, and anomalous methyl sig-
nal at 1.69 ppm (see Tables 3–5). The couplings H-10/
Corthoacetate and H2-1,6/Corthoacetate detected by the
HMBC also prove this structure. The formation of 13
is a direct proof of the hypothesis3 according to which
orthoesters are formed in the Koenigs–Knorr type gly-
cosidation reactions as intermediates, which are rela-
tively stable but collapse in the presence of strong
acids. The large amount of the used molecular sieve
absorbed obviously a substantial part of the promoter,
therefore the rearrangement of the orthoester into the
normal glycoside was a slow process. This could be
proved indirectly, as when the amount of the promoter
was raised to 20 mol %, and simultaneously the amount
of the molecular sieve lowered (see Table 1, Run 4), the
orthoester could be detected in traces only, but instead
of 11 the triglucosyl derivative 12 was formed unexpect-
edly in amounts almost equal to 11. On the other hand,
a substantial amount of the monoglycosylated deriva-
tive 9 with a free terminal OH group could also be
detected, which is probably formed via collapse of the
mono-orthoester 14. Change of the anomeric configura-
tion of the donor molecule, using the b-anomer 8 instead
of 7 did not change substantially the product distribu-
tion (Scheme 2).
product 11 is a result of the complexicity of this
reaction.
1. Experimental
1.1. General methods
Organic solns were dried over MgSO4 and concentrated
under diminished pressure at or below 40 ꢁC. TLC used
E. Merck precoated Silica Gel 60 F254 plates, with 1:19
EtOAc–hexane and detection by spraying the plates
with a 0.02 M soln of I2 and a 0.3 M soln of KI in
10% aq H2SO4 followed by heating at ca. 200 ꢁC. For
column chromatography, Kieselgel 60 was used. The
mp are uncorrected. Optical rotations were determined
on 1.0% solns in CHCl3 at 20 ꢁC. The NMR spectra
were recorded on a Bruker Avance 500 spectrometer
at 500 (1H) and 125 (13C) MHz, respectively, in CDCl3
soln at ambient temperature. The chemical shifts were
referenced to dTMS 0 ppm. For structure determination,
1H,1H-COSY, TOCSY, HMQC, HMBC as well as
selective 1D TOCSY and NOESY spectra were
recorded. For HPLC, Eurospher-100 C18 column
(5 lm, 125 · 4 mm I.D., Knauer, Berlin) was used with
a Waters 600 E pump and system controller and a
Waters 996 PDA detector (at 256 nm). For elution, 3:2
acetonitrile–0.1% ammonium acetate (pH 4.5) was used
at a speed of 1 mL/minꢀ1. The retention time for the
isolated compounds is given in Table 2.
As a conclusion, It is likely that in all the investigated
reactions the bis-orthoester 13 was formed as a common
intermediate, as the rearrangement and/or decomposi-
tion of it can result in all of the isolated derivatives
(9–12). Furthermore, other anomers and related com-
pounds are probably also formed via the same path-
ways3 as by-products, but they could not be isolated in
pure state from the multi-component mixture. The dif-
ferent donors and promoters can shift the composition
of the resulting mixtures by influencing the reaction
mechanism but the relatively low yield of the expected
1.2. 2,5-Anhydro-3,4-di-O-benzyl-1,6-di-O-trityl-D-
mannitol (2)
(a) To a stirred soln of 11 (12.7 g, 20 mmol) in Me2SO
(50 mL), a soln of NaOH (5 g) in water (50 mL) and
benzyl chloride (5 mL, 60 mmol) were added simulta-
neously. Stirring was continued at 50 ꢁC for 3 h. There-
after the mixture was cooled, poured into water and
extracted with CHCl3. The residue obtained after
OAc
AcO
O
O
O
OAc
O
AcO
O
O
9, 10, 11
O
OAc
Me
BnO
OBn
AcO
OAc
14
AcO
AcO
OAc
O
O
O
+ donor
O
O
OAc
12
BnO
OBn
AcO
OAc
OAc
HO
15
Scheme 2.

780
A. Tegdes et al. / Carbohydrate Research 341 (2006) 776–781
concentration gave, after column chromatography (1:4
EtOAc–hexane) 2 (12 g, 72%); [a]D +10 (c 1, CHCl3).
Anal. Calcd for C58H52O5: C, 84.03; H, 6.32. Found:
C, 83.97; H, 6.50.
1.6. Glycosidation of 4 using imidate 7 as donor
(a) A soln of 4 (7.94 g, 23.1 mmol) and 7 (25 g,
50.7 mmol) in dry CH2Cl2 (400 mL) was stirred in the
˚
(b) 10 g NaH (0.4 mol) was dissolved in DMF
(150 mL) and a soln of 1 (64 g, 0.1 mol) was added gradu-
ally with stirring. Thereafter benzyl chloride (26 mL,
0.3 mol) was added and the mixture was stirred at
90 ꢁC for 2 h. Thereafter MeOH (20 mL) was added
and the mixture was kept at rt overnight. The residue
of the concentrated mixture was dissolved in CHCl3,
washed with water, dried and concentrated to give crude
2 (83 g, ꢁ100%) as a brown oil. This was used without
purification in the next step.
presence of molecular sieves (4 A) (20 g) for 30 min at
rt. Thereafter the mixture was cooled to ꢀ40 ꢁC and
TMSOTf (0.4 mL, 2.2 mmol) was added. Stirring was
continued at ꢀ40 ꢁC for 25 min. Thereafter Et3N
(2 mL) was added and the temperature was raised to
rt. The filtered mixture was washed with water, dried
and concentrated. The residue was investigated by
HPLC. For results, see Table 1, Run 3.
(b) A soln of 4 (1.72 g, 5 mmol) and 7 (5.42 g,
11 mmol) in dry CH2Cl2 (60 mL) was stirred in the pres-
˚
ence of molecular sieves (4 A) (5 g) for 30 min at rt.
Thereafter the mixture was cooled to ꢀ40 ꢁC and
TMSOTf (0.2 mL, 1.1 mmol) was added. The mixture
was stirred at ꢀ40 ꢁC for 10 min and processed as
described for a. The residue was investigated by HPLC.
For results see, Table 1, Run 4.
1.3. 2,5-Anhydro-3,4-di-O-benzyl-D-mannitol (4)
A soln of crude 2 (56.5 g, 68 mmol) in AcOH (300 mL)
was cooled to +10 ꢁC and treated with 33% HBr in
AcOH (50 mL). The resulting precipitate was filtered
off after 1 min and the filtrate was diluted with ice-water
(1 L). The precipitated oil was extracted with CHCl3,
washed with cold water, 5% NaHCO3 and water, dried
and concentrated. The residue was separated by column
chromatography (2:1 EtOAc–hexane). The fractions
having Rf 0.2 afforded on concentration crystalline 4
(1.4 g, 30%); mp 76–78 ꢁC (ether–hexane); [a]D +40 (c
1, CHCl3). Anal. Calcd for C20H24O5: C, 69.75; H,
7.02. Found: C, 69.70; H, 7.12.
1.7. Glycosidation of 4 using imidate 8 as donor
The process described above in method b was repeated
using 8 as donor. The obtained residue was investigated
by HPLC. For results see Table 1, Run 5.
The following compounds were isolated from the dif-
ferent glycosidation reactions in pure state:
No improvement was obtained when 80% aq AcOH
was used for the hydrolysis of 2 at 80 ꢁC for 1 h.
1.8 2,5-anhydro-3,4-di-O-benzyl-1-O-(2,3,4,6-tetra-O-
acetyl-b-D-glucopyranosyl)-D-mannitol (9);
1.9 6-O-acetyl-2,5-anhydro-3,4-di-O-benzyl-1-O-(2,3,4,
6-tetra-O-acetyl-b-D-glucopyranosyl)-D-mannitol
(10);
1.4. Glycosidation of 4 using bromide 5 as donor
1.10 2,5-anhydro-3,4-di-O-benzyl-1,6-bis-O-(2,3,4,6-
tetra-O-acetyl-b-D-glucopyranosyl)-D-mannitol (11);
1.11 2,5-anhydro-3,4-di-O-benzyl-1-O-[-2-O-(2,3,4,6-
tetra-O-acetyl-b-D-glucopyranosyl)-3,4,6-tri-O-
acetyl-b-D-glucopyranosyl]-6-O-(2,3,4,6-tetra-O-
acetyl-b-glucopyranosyl)-D-mannitol (12);
1.12 2,5-anhydro-3,4-di-O-benzyl-1,6-bis-O-(3,4,6-tri-
O-acetyl-1,2-O-ethylidene-2-yl-a-D-glucopyrano-
syl)-D-mannitol (13).
A soln of 4 (6.88 g, 20 mmol) in dry MeCN (100 mL)
was stirred in the presence of molecular sieves (4 A)
˚
(7 g) for 30 min at rt. Thereafter 5 (18.1 g, 44 mmol)
and Hg(CN)2 (12.6 g, 50 mmol) were added and stirring
was continued at rt for 20 h. The filtered mixture was
diluted with 3-fold CHCl3, washed with 5% NaHCO3
and a 10% aq soln of KBr, dried and concentrated.
The resulting oily residue was investigated by HPLC.
For results, see Table 1, Run 1.
Compounds 9 and 12 had the same Rf values, but they
were never present simultaneously in the different reac-
tion mixtures. For further structural identification, an
aliquot part of the reaction mixtures was acetylated:
when 9 was converted into its 6-O-acetate 10, 12
remained unchanged.
1.5. Glycosidation of 4 using thioglycoside 6 as donor
A soln of 4 (4.64 g, 13.5 mmol) in dry MeCN (100 mL)
˚
was stirred in the presence of molecular sieves (4 A) (5 g)
for 30 min at rt. Thereafter 6 (13.5 g, 29.6 mmol), and
after cooling to ꢀ40 ꢁC, NIS (9.51 g, 42.3 mmol) and
TfOH (0.25 mL, 2.8 mmol) were added. After 2 h the
temperature was raised to rt, the mixture was filtered,
washed with 5% aq Na2S2O3, 5% NaHCO3 and water,
dried and concentrated. The residue was investigated
by HPLC. For results see Table 1, Run 2.
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