A selective electrochemical method of glycosylation of 3β-hydroxy-Δ5-steroids

Article abstract of DOI:10.1016/j.carres.2010.03.018

A new electrochemical glycosylation method is presented. According to the method cholesterol and other 3β-hydroxy-Δ⁵-steroids can be selectively transformed to glycosides using non-activated sugars. The method is also useful for the synthesis of glycoconjugates with sugar linked to a steroid moiety by an ether bond.

Full text of DOI:10.1016/j.carres.2010.03.018

Carbohydrate Research 345 (2010) 1051–1055
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
A selective electrochemical method of glycosylation of 3b-hydroxy-D5-steroids
a,*
a
a
a
a
Jacek W. Morzycki , Zenon Łotowski , Leszek Siergiejczyk , Piotr Wałejko , Stanisław Witkowski ,
b
b
b
Jan Kowalski , Jolanta Płoszy n´ ska , Andrzej Sobkowiak
a
Institute of Chemistry, University of Białystok, Piłsudskiego 11/4, 15-443 Białystok, Poland
Faculty of Chemistry, Rzeszów University of Technology, PO Box 85, 35-959 Rzeszów, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new electrochemical glycosylation method is presented. According to the method cholesterol and other
Received 1 February 2010
Received in revised form 9 March 2010
Accepted 16 March 2010
Available online 20 March 2010
5
3b-hydroxy-D -steroids can be selectively transformed to glycosides using non-activated sugars. The
method is also useful for the synthesis of glycoconjugates with sugar linked to a steroid moiety by an
ether bond.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Glycosylation
Cholesterol
Electrochemical oxidation
Electrochemical reactions, when compared to other methods of
organic synthesis, are cheap and environmentally friendly. They of-
ten become synthetically useful, because the initial reaction takes
place at the electrode surface and the process is controllable by
electrolysis conditions including the potential applied.1–4 Electro-
chemical oxidation of cholesterol 1 was intensively studied during
(Fig. 2). This mesomerically stabilized carbocation is well known
in steroid chemistry. It reacts with nucleophiles preferably at
1
3
25
the last two decades. Most reports concern indirect oxidation with
5
mediators such as Mn(III)/IV–porphyrin–O
2
,
Ti(II)/(III)–hemato-
6
ꢀ
ꢀ 7
porphyrin–O
selective radical oxidation of the cholesterol side chain at C-25
Fig. 1) to the corresponding tertiary alcohol, while the latter one
2
, and Br /BrO . The first two systems caused the
7
HO
3
5
1
6
(
led to oxidation of the C-5–C-6 double bond and the allylic C-7 po-
sition. Also electrochemical oxidation of cholesteryl acetate in-
duced by dioxygen and iron picolinate complexes afforded
Figure 1. Cholesterol electrooxidation sites.
8
mostly 7-hydroxylated products. Recently described direct elec-
trooxidation of cholesterol on carbon electrode in acetonitrile pro-
duced cholesta-4,6-dien-3-one.9
,10
We have previously reported
direct electrochemical oxidation of cholesterol on platinum elec-
trode in glacial acetic acid. The reaction took place at the allylic po-
1
1
sition yielding 7a- and 7b-acetoxy derivatives of cholesterol.
HO
Nu
Cholesterol electrooxidation reactions were usually carried out in
polar nucleophilic solvents. Our recent study on direct electrooxi-
dation of cholesterol in dichloromethane, which stabilizes cation-
radicals and is devoid of nucleophilic properties, led to quite differ-
ent results. Surprisingly, the reaction occurred exclusively at the
hydroxyl group at C-3. The initially formed cation-radical under-
went cleavage to homoallylic carbocation and hydroxyl radical
1
-
e-
NuH, -H+
1
2
.
-
HO
.
+
(
+)
HO
(
+)
*
Corresponding author. Tel.: +48 85 7457585; fax: +48 85 7457581.
Figure 2. Electrooxidation of cholesterol in dichloromethane in the presence of a
E-mail address: morzycki@uwb.edu.pl (J.W. Morzycki).
nucleophile.
0
008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2010.03.018

1
052
J. W. Morzycki et al. / Carbohydrate Research 345 (2010) 1051–1055
C-6 under buffered conditions because the reaction is faster at this
position than at C-3 due to the higher positive charge at C-6 in the
carbocation. However, if the reaction is thermodynamically con-
trolled (it becomes reversible in the absence of a buffer) only prod-
ucts of nucleophilic attack at C-3 are formed. For stereoelectronic
reasons attack of a nucleophile at this position occurs exclusively
from the upper side (b) of the steroid molecule. Formation of a
non-classical homoallylic carbocation during cholesterol electro-
oxidation was convincingly proved. Such carbocations may react
with any nucleophile present in the reaction mixture providing
various products of substitution at C-3 with retention of
configuration.
from the presence of a broad signal of axial 3a-H at d 3.43 ppm in
1
their H NMR spectra. The detailed analysis of the reaction mixture
led to isolation and identification of the minor reaction products —
3, 4, 5, and 6. Dicholesteryl ether 3 has been isolated in the previ-
1
2
ous study as the major product in the absence of a sugar in the
reaction mixture. Cholesteryl acetate 4 has also been isolated pre-
viously as a result of reaction of the carbocation with acetate leak-
ing from the electrolytic bridge. However, in the present study the
only source of acetate could be tetraAc-Glu-OH. Compounds 5 and
6 are the direct oxidation products of cholesterol; they have also
12
9
,10
been reported previously.
The electrooxidation was repeated
on glassy carbon electrode but the results were similar. In the sep-
arate experiment cholestan-3b-ol was subjected to the same reac-
tion conditions instead of cholesterol. However, as it was expected,
no reaction was observed.
To avoid the formation of cholesteryl acetate 4 in the next
experiment 2,3,4,6-tetra-O-benzyl-D-glucopyranose was used as a
glycosyl donor. The cholesterol anodic oxidation was carried out
on a platinum electrode under the same conditions as those de-
scribed for the reaction with tetraAc-Glu-OH. The TLC of the reac-
tion mixture looked slightly cleaner but the yield of glycoside 7
was similar to that obtained in the previous case. Again the prod-
uct proved to be a mixture of anomers in the ratio 1:1 (of course,
the reaction proceeded with complete 3b-stereoselectivity with re-
spect to cholesterol). When a mixture of equal amounts of choles-
terol and cholestan-3b-ol was used for the electrochemical
reaction only cholesterol was transformed into glycoside 7, while
cholestan-3b-ol remained intact.
It must be stressed that other 3b-hydroxy-D5-steroids may fol-
low the above-described cholesterol electrooxidation pathway but
it is not valid for saturated steroids (stanols). The purpose of the
present study was to check if sugar molecules may serve as nucle-
ophiles. Should this be the case, various glycoconjugates could be
produced in an electrochemical way from non-activated sugars
and sterols.
Despite considerable progress in carbohydrate chemistry in the
past few years, the stereoselective formation of O-glycosidic bonds
between carbohydrates and steroids is frequently a time-consum-
ing process, with relatively low yields of the newly formed glyco-
sides. This is due to the low reactivity of secondary alcohol
functions in the steroid moiety and the necessity to activate the
1
4
glycosyl donors. Various methods of glycosylation of steroids
have been described. Glycosyl donors used among others were:
1
5
16,17
18
glycosyl halides, trihalogenoacetimidates,
thioglycosides,
1
9
20
1
-O-sulfonyl glycosides, glycals, 1-O-acyl sugars,21 orthoesters
or ethers, phosphate derivatives, 1-O-silylated glycosides.
Since the primary hydroxyl group is more accessible than that
at the anomeric position, it was reasonable to expect that the reac-
2
2
23
24
There are also known electrochemical methods of glycosylation,
though they were not used for the preparation of steroidal glyco-
sides. In the Noyori method,25 aryl glycoside (Gly-OAr) is oxidized
tion is better suited for the construction of glycoconjugates of 3b-
5
hydroxy-
D
-steroids with sugar linked to the aglycone by an ether
bond. As an example of such reactions, electrochemical glycosyla-
tion of cholesterol with 1,2:3,4-di-O-diisopropylidene- -galacto-
pyranose was chosen. The reaction was completely 3b-
stereoselective (a broad signal of axial 3 -H appeared at d
3.35 ppm in the H NMR spectrum) and afforded pure cholesteryl
diisopropylidene- -galactos-6-yl ether 8 in 20% yield in addition
Å+
in a one-electron-transfer process to a cation-radical (Gly-OAr ),
a-D
+
Å
+
which undergoes cleavage to Gly and ArO . In the final step Gly
reacts with alcohol (R-OH) to afford glycoside (Gly-OR). This meth-
od was also adapted to aryl thioglycosides, but then glycosylation
a
1
26–29
occurred at a lower oxidation potential.
The method presented
D
in this paper is completely different from the previous ones.
In the preliminary step of this study, it was checked that sugars
used in this study (tetraAc-Glu-OH, tetraBn-Glu-OH, and diisopro-
pylidene-Gal-OH) are not oxidized at the potential of cholesterol
oxidation. Since all sugars remained intact until the decomposition
to recovered cholesterol (36%).
In summary, a new electrochemical method has been developed
for selective glycosylation of 3b-hydroxy-
tage of the method is that non-activated sugars can be used. The
method can be applied for the synthesis of glycosides but it is bet-
ter suited for the synthesis of glycoconjugates with sugar via an
ether bond to the steroid. Further study on applications of this pro-
tocol to a wider range of substrates is under way.
5
D
-steroids. The advan-
of the supporting electrolyte at +1.9 V (vs Ag/0.1 M AgNO
3
in
MeCN), while the oxidation of cholesterol under the same condi-
1
1
tions occurred at about 1.5 V, it was confirmed that under poten-
tiostatic conditions generation of the non-classical carbocation
from cholesterol in the presence of a sugar is possible. The reac-
tions were carried out using excess of a sugar to avoid a competing
reaction of the carbocation with other nucleophiles (e.g., choles-
terol). In this preliminary study are presented the reactions of cho-
lesterol with sugars containing only one free hydroxyl group either
at the anomeric position or at the primary carbon atom (C-6). The
reactions were carried out on platinum or glassy carbon electrodes
in dichloromethane using tetrabutylammonium tetrafluoroborate
or tetrabutylammonium hexafluorophosphate as a supporting
electrolyte. The results obtained are summarized in Scheme 1.
The anodic oxidation of cholesterol in the presence of 2,3,4,6-
1. Experimental
1.1. General methods
The preparative electrolyses were performed with a potentio-
stat/galvanostat (Princeton Applied Research Model 273 A) under
galvanostatic conditions using current equal to 9 ± 0.1 mA during
3 h unless otherwise indicated. The current applied was the maxi-
mum current available for the used electrolysis setup (power sup-
ply and ohmic resistance). After 3 h of electrolysis no further
increase in the concentration of glycosylation products was ob-
served by TLC. A divided H-cell was used, in which the cathodic
and anodic compartments (15 mL each) were separated by glass
tetra-O-acetyl-D-glucopyranose on platinum electrode afforded
glycoside 2 (as the 1:1 mixture of
a
- and b-anomers) in 28% yield.
The reaction appeared to be not stereoselective. This is so, because
6 4
frit. In all measurements 0.1 M solution of TBAPF or TBABF (both
the highly reactive carbocation does not differentiates between
from Aldrich) in dichloromethane was used as a supporting elec-
trolyte. Cholesterol (from Aldrich, 99+%) and a sugar substrate
were introduced into the anodic compartment whereas anionite
(2.5 g, Dowex 2x8, 200–400 mesh, perchlorate form) was placed
in the cathodic compartment to eliminate chloride ions formed
nucleophiles — the
a- and b-anomers of tetraacetyl-D-glucopyra-
nose react with similar rates. On the other hand, complete stere-
oselectivity was observed with respect to the steroidal moiety.
Both anomers of 2 showed the b-configuration at C-3 as it follows

J. W. Morzycki et al. / Carbohydrate Research 345 (2010) 1051–1055
1053
OAc
tetraAc-Glu-OH
anodic oxidation
O
+
AcO
AcO
HO
O
OAc
1
2
(28%; α/β 1:1)
+
+
+
+
O
AcO
O
O
3
(4%)
4 (10%)
5 (6%)
6 (9%)
OBn
recovered
tetraBn-Glu-OH
anodic oxidation
O
+
cholesterol (28%)
BnO
BnO
O
HO
OBn
1
7 (21%; α/β 1:1)
O
recovered
diisopropylidene-Gal-OH
anodic oxidation
O
O
+
cholesterol (36%)
O
8
(20%)
HO
1
O
O
Scheme 1. Electrochemical glycosylation of cholesterol.
by the reduction of dichloromethane.12 The solutions in both com-
partments were stirred during electrolysis and additionally a con-
tinuous flow of argon was applied in the anodic compartment.
Platinum mesh was used as a cathode and platinum plate
1.2. Electrolysis of cholesterol and 2,3,4,6-tetra-O-acetyl-D-
glucopyranose on platinum electrode
6
A 0.1 M solution of TBAPF in dichloromethane was used as a
(
3 ꢁ 1.5 cm) or glassy carbon plate (2.7 ꢁ 1.5 cm) was used as an-
supporting electrolyte. Platinum plate was applied as an anode.
The anolyte contained cholesterol (100 mg; 0.26 mmol) and glu-
cose tetraacetate (200 mg; 0.57 mmol). After completion of elec-
trolysis the solvent was evaporated in vacuo. The dry residue
was dissolved in 10 mL of benzene–hexane (8:2) mixture and
subjected to a silica gel column chromatography. Elution with a
benzene–hexane (1:1) mixture afforded dicholesteryl ether 3
(4 mg; 4%). Pure benzene eluted cholesteryl acetate 4 (11 mg;
10%). Further elution with a benzene–ethyl acetate (96:4) mix-
odes. All measurements were preformed at room temperature. The
3
0
sugar substrates (2,3,4,6-tetra-O-acetyl-
D
-glucopyranose, 2,3,4,6-
-glucopyranose, and 1,2:3,4-di-O-diisopropylid-
-galactopyranose32) were prepared according to the known
procedures.
Melting points were determined on a Kofler apparatus of the
31
tetra-O-benzyl-
D
ene-
a-D
1
13
Boetius type. H and C NMR (400 and 100 MHz, respectively)
spectra were recorded on the Bruker Avance II spectrometer in
CDCl
33
3
solutions with TMS as the internal standard (only selected
ture gave only partially separated cholest-4-en-3-one (5; 6%)
1
34
signals in the H NMR spectra are reported). Infrared spectra were
recorded on the Nicolet series II Magna-IR 550 FTIR spectrometer
in chloroform solutions. Mass spectra were obtained at 70 eV with
an AMD-604 spectrometer. Optical rotations were measured on a
Perkin–Elmer 141 polarimeter at the sodium D line (589 nm).
Merck Silica Gel 60, F 256 TLC aluminum sheets were applied for
thin layer chromatographic analysis. For visualization of products,
a 5% solution of phosphomolybdic acid in ethanol was used. The
reaction products were separated by column chromatography per-
formed on 70–230 mesh silica gel (J. T. Baker).
and a bit more polar cholesta-4,6-dien-3-one (6; 9%). The major
reaction products, cholesteryl
a- and b-D
-glucosides 235 (52 mg;
28%), were eluted with benzene–ethyl acetate (85:15, v/v) as a
1:1 mixture. The mixture was subjected to another slow chro-
matographic separation with benzene–ethyl acetate (9:1, v/v)
elution (the
a-anomer was eluted first). The unreacted D-glucose
tetraacetate was recovered (54 mg) with benzene–ethyl acetate
(1:1) mixture.
All reaction products were previously described in the literature
and their characterization data matched that already reported.3
3–35

1
054
J. W. Morzycki et al. / Carbohydrate Research 345 (2010) 1051–1055
1
.3. Electrolysis of cholesterol and 2,3,4,6-tetra-O-acetyl-
D
-
The anolyte contained cholesterol (100 mg; 0.26 mmol) and diiso-
propylidene- -galactose (168 mg; 0.27 mmol). The electrolysis
glucopyranose on a glassy carbon electrode
D
was performed within 1.5 h. After completion of electrolysis the
solvent was evaporated in vacuo. The dry residue was dissolved
in benzene (10 mL) and subjected to a silica gel column chroma-
tography. Elution with benzene–ethyl acetate (96:4) mixture affor-
4
A 0.1 M solution of TBABF in dichloromethane was used as a
supporting electrolyte. Glassy carbon was applied as an anode.
The anolyte contained cholesterol (100 mg; 0.26 mmol) and glu-
cose tetraacetate (200 mg; 0.57 mmol). After completion of elec-
trolysis the solvent was evaporated in vacuo. The dry residue
was dissolved in 10 mL of a benzene–hexane (8:2) mixture and
subjected to a silica gel column chromatography.
Elution with benzene–hexane (1:1) mixture afforded dicho-
lesteryl ether 3 (2 mg; 2%). Benzene elution gave cholesteryl ace-
tate 4 (15 mg; 14%). With benzene–ethyl acetate (95:5, v/v) a
mixture of cholest-4-en-3-one (5) and cholesta-4,6-dien-3-one
ded cholesteryl diisopropylidene-D-galactos-6-yl ether 8 (32 mg;
20%) followed by the unreacted cholesterol (36 mg; 36%).
0
0
0
0
0
3-O-(1 ,2 :3 ,4 -di-O-isopropylidene-
a-D-galactopyranos-6 -yl)-
cholest-5-en-3b-ol (8): colorless crystals, mp 145–148 °C (hexane–
2
D
5
dichloromethane); ½
a
ꢂ
ꢀ52.3 (c 0.5, CHCl
3
); IR, mmax: 1083,
ꢀ
1
1
0
1064 cm
(dd, 1H, J
;
H NMR, d: 5.37 (s, 1H, H-1 and m, 1H, H-6), 4.50
0
1
= 5.8 Hz, J
2
= 5.5 Hz, H-5 ), 4.46 (dd, 1H, J
1
= 7.0 Hz,
0
0
J
2
= 5.8 Hz, H-4 ), 4.15 (d, 1H, J = 7.0 Hz, H-3 ), 4.10 (d, 1H,
J = 7.6 Hz, H-6 a), 3.62 (s, 1H, H-2 ), 3.58 (dd, 1H, J
= 7.6 Hz, H-6 b), 3.35 (m, 1H, H-3
0
0
(
6) in the ratio 1.1:1 (8 mg) was eluted. Further elution with ben-
zene–ethyl acetate (85:15, v/v) afforded a 1:1 mixture of and b-
glucosides 2 (24 mg; 28%). The unreacted glucose tetraacetate
32 mg) was eluted with benzene–ethyl acetate (1:1) mixture.
1
= 5.5 Hz,
0
a
D
-
J
2
a), 1.54 (s, 6H, H-isopropyli-
dene), 1.36 (s, 6H, H-isopropylidene), 1.01 (s, 3H, H-19), 0.92 (d,
(
3H, J = 6.5 Hz, H-21), 0.88 (d, 3H, J = 1.8 Hz, H-26), 0.86 (d, 3H,
1
3
J = 1.8 Hz, H-27), 0.69 (s, 3H, H-18); C NMR, d: 140.4 (C), 122.1
(CH), 108.4 (2 ꢁ C), 100.7 (CH), 79.4 (CH), 75.5 (CH), 75.4 (CH),
1
.4. Electrolysis of cholesterol and 2,3,4,6-tetra-O-benzyl-
D-
glucopyranose on a platinum electrode
2
72.0 (CH), 69.4 (CH), 63.1 (CH ), 56.8 (CH), 56.2 (CH), 50.2 (CH),
4
2.3 (C), 39.8 (CH
2
), 39.5 (CH
), 35.8 (CH), 31.94 (CH
), 24.4 (CH
), 22.6 (2 ꢁ CH ), 21.1 (CH
2
), 39.2 (CH
), 31.90 (CH), 29.0 (CH
), 24.3 (CH ), 23.8 (CH
), 19.4 (CH ), 18.7 (CH
2
), 37.2 (CH
2
), 36.8 (C),
A 0.1 M solution of TBABF
4
in dichloromethane was used as a
36.2 (CH
2
2
2
), 28.2
supporting electrolyte. Platinum plate was applied as an anode.
The anolyte contained cholesterol (100 mg; 0.26 mmol) and tet-
(CH
22.8 (2 ꢁ CH
11.9 (CH
593.5 [(Mꢀacetone+Na) , 100%].
2
), 28.0 (CH), 25.8 (CH
3
3
2
2
),
),
3
3
2
3
3
+
rabenzyl-
D
-glucose (270 mg; 0.50 mmol). After completion of elec-
3
); ESI MS, m/z: 1164.0 [(2 ꢁ (Mꢀacetone)+Na) , 20%],
+
trolysis the solvent was evaporated in vacuo. The dry residue was
dissolved in benzene (10 mL) and subjected to a silica gel column
chromatography. Cholesteryl tetrabenzyl-
lated as a mixture (1:1) of - and b-anomers (49 mg; 21%) by elu-
tion with benzene–ethyl acetate (98:2, v/v). The unreacted starting
materials, cholesterol (28 mg) and tetrabenzyl- -glucose (76 mg),
were consecutively eluted with benzene–ethyl acetate (94:6) mix-
ture, respectively. The mixture of cholesteryl tetrabenzyl- -gluco-
sides 7 was separated by another column chromatography with
slow elution with benzene containing 0.01% of methanol (the
anomer was eluted first). The characterization data obtained for
D-glucosides 7 were iso-
Acknowledgments
a
The authors thank Mrs. J. Maj for her skillful technical assis-
tance. Financial support from the University of Białystok and
Rzeszów University of Technology is gratefully acknowledged.
D
D
a
-
References
3
6
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a- and b-anomers matched those reported previously.
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1
1
lesteryl tetrabenzyl-D-glucosides 7 isolated as a 1:1 mixture of a-
and b-anomers (14 mg; 12%) by elution with benzene–ethyl ace-
tate (98:2, v/v). In addition to that the unreacted starting materials
were recovered; cholesterol (39 mg; 76%) and cholestan-3b-ol
1
1
(
47 mg; 94%) were eluted with benzene–ethyl acetate (96:4) mix-
ture followed by tetrabenzyl- -glucose (46 mg; 65%) eluted with
benzene–ethyl acetate (94:6) mixture.
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D
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