β-Selective One-Pot Synthesis of Acyl-C-Glycosides via Corey-Seebach Umpolung Reaction

Article abstract of DOI:10.1055/s-0039-1690158

C-Glycosides are commonly used as carbohydrate mimics in drug development due to their stability against enzymatic and chemical hydrolysis. In this Synpacts article we elaborate on our fast and efficient β-selective approach towards protected and unprotected acyl glycosides. Application of a Corey-Seebach umpolung reaction enables the exclusive formation of the β-Anomer of aromatic acyl-C-glycosides in good to excellent yields. 1 Introduction 2 C-Glycosylation of Benzylated Glycosyl Donors 3 C-Glycosylation of Silylated Glycosyl Donors 4 Conclusion.

Full text of DOI:10.1055/s-0039-1690158

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
synpacts
A
en
G. J. Boehlich, N. Schützenmeister
Synpacts
Synlett
-Selective One-Pot Synthesis of Acyl-C-Glycosides via Corey–
Seebach Umpolung Reaction
G. Jacob Boehlich
Nina Schützenmeister*
0
0
0
0-0
0
0
1-
9
6
0
8-0
0
5
7
S
S
R'
R'
R'
PIFA or I2
Li
MeTHF
Ar
S
S
O
Universität Hamburg, Institut für Pharmazie, Bundesstraße 45,
20146 Hamburg, Germany
Nina.Schuetzenmeister@chemie.uni-hamburg.de
O
R
O
O
PGO
PGO
PGO
PGO
BnO/HO
BnO/HO
H2O/cosolvent, r.t
R
R
Ar
Ar
–95 °C 0 °C
X
Dedicated to Prof. Armin de Meijere on the occasion of his 80th
birthday
PG = Bn or TMS
Glc, Gal, Xyl or 2dGlc
Ar = Ph, 4-anisyl or 2-furyl
31–99% yield
b only
one-pot (unless Ar = 2-furyl)
R = H or OH
R' = H or CH2OH
Received: 05.07.2019
Accepted after revision: 25.07.2019
Published online: 13.08.2019
DOI: 10.1055/s-0039-1690158; Art ID: st-2019-p0352-sp
Abstract C-Glycosides are commonly used as carbohydrate mimics in
drug development due to their stability against enzymatic and chemical
hydrolysis. In this Synpacts article we elaborate on our fast and efficient
-selective approach towards protected and unprotected acyl glyco-
sides. Application of a Corey–Seebach umpolung reaction enables the
exclusive formation of the -anomer of aromatic acyl-C-glycosides in
good to excellent yields.
1
2
3
4
Introduction
C-Glycosylation of Benzylated Glycosyl Donors
C-Glycosylation of Silylated Glycosyl Donors
Conclusion
Nina Schützenmeister (left) was born in Reinbek, Germany and re-
ceived her diploma in 2006 from the Georg-August-University Göttin-
gen after an external diploma thesis in the group of Prof. P. H.
Seeberger and her PhD in 2012 from the Georg-August-University
Göttingen under guidance of Prof. L. F. Tietze. After a postdoctoral stay
at the Max-Planck-Institute for biophysical chemistry in Göttingen in
the group of Prof. C. Griesinger (2012–2013) she moved to Bristol (UK)
for a postdoctoral stay at the University of Bristol in the group of Prof. V.
K. Aggarwal F.R.S. (2013–2015). Since 2015 she is Juniorprofessor at
the Institute for Pharmacy at the Universität Hamburg with research in-
terest in the efficient synthesis of natural products and the develop-
ment of novel anti-infective compounds.
Key words carbohydrates, C-glycosylation, umpolung, Corey–Seebach
reaction, natural products, scleropentasides
1 Introduction
Scleropentasides are furanoyl--C-glucosidic com-
pounds that have been obtained from the leaves and twigs
of the sandalwoods Scleropyrum pentadrum1 and from the
stems of Dendrotrophe frutescens.2
The structural motif of acyl-C-glycosidic was so far un-
precedented in plant natural products. The structurally
simplest compound is scleropentaside A (1a, Figure 1).
G. Jacob Boehlich (right) was born in Hamburg, Germany and received his
BSc (2015) and MSc (2017) under the guidance of Prof. N. Schützenmeister.
Since 2018 he is a PhD student working on the synthesis of -C-glyco-
sides and biologically active natural products.
OH
To further investigate the biological activities of these
substances we thought a synthetic approach towards these
compounds might be useful. Employed methods3 for the
synthesis of C-glycosides seemed unsuitable due to the 1,2-
difunctionalized anomeric bond requiring an umpolung re-
action.4 Acyl-C-glycosides had been synthesized by addition
of organometallic compounds to nitrile 2.5 A Corey–Seebach
umpolung reaction6 that generated lactol 6, which had to be
reduced subsequently, was also reported.7 A cross-coupling
HO
RO
HO
O
O
HO
HO
HO
O
HO
O
HO
O
O
O
O
R'O
A 1a R = H
HO
HO
O
O
B 1b R = α-rhamnopyranosyl
C 1c R = β-xyloopyranosyl
F 1f R = galloyl
D 1d R' = H
R' = galloyl
E 1e
Figure 1 Scleropentasidse A–F 1
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
G. J. Boehlich, N. Schützenmeister
Synpacts
Synlett
of glycosylstannanes 3 with retention of stereochemistry
using large amounts of copper salts was described recently
by Walczak et al. (Scheme 1).8
2 C-Glycosylation of Benzylated Glycosyl Do-
nors
Because of their widespread use and the high stability
of benzyl groups we investigated the use of glycosyl donors
13a/b, which were synthesized according to reported pro-
cedures.11 The donors 14a/b were obtained by anomeric
bromination of 13a/b and used after solvent removal with-
out further purification. The aromatic dithianes 15a/b were
obtained in one step from benzaldehyde or 4-methoxy-
benzaldehyde,12 and lithiated by t-BuLi at –95 °C. Reaction
of these lithiated dithianes 15a/b with D-glucosyl or D-ga-
lactosyl donors 14a/b provided the -C-glycosides 16a–d.
The reaction failed when the corresponding D-mannosyl or
D-allosyl donors were employed. In case of the D-mannosyl
donor the crude spectrum after quenching only showed the
starting materials dithianes 15a/b and benzylated manno-
syl bromide, indicating that no reaction occurred at all. The
D-allosyl donor, however, decomposed and gave a complex
mixture due to 1,3-diaxial interactions.
R–M
M = Li, Mg R' = H
or M = Zn R' = Bn
BnO
O
BnO
CN
BnO
R'O
O
2
X
R
X = SePh, SPh
BnO
BnO
BnO
BnO
BnO
O
O
O
Pd0, CuI
SnBu3
BnO
BnO
R
R'O
3
7
2 steps
S
S
Li
R
BnO
BnO
BnO
BnO
BnO
S
S
O
5
O
BnO
BnO
BnO
R
O
OH
4
6
After solvent exchange to MeCN/H2O, the crude perben-
zylated dithianes 16a–d were converted into the ketones
7a–d under oxidative conditions with (bis(trifluoroace-
toxy)iodo) benzene (PIFA)13 to give the acyl--C-glycosides
7a–d in 57–68% yield over three steps (Scheme 4). This
three-step one-pot reaction exclusively yielded acyl--C-
glycosides, which was proven by NMR spectroscopic analy-
sis. The major side reaction was elimination of HBr from
donor 13a/b, which yielded glycals 17a/b (Figure 2). This
side reaction, which is common for reactions of glycosyl ha-
lides with organometallics,14 was partially suppressed by
Scheme 1 Previous syntheses of acyl--C-glycosidic compounds
Most of these methods require multiple steps. Thus, we
investigated a more direct SN2 approach between a suitable
glucosyl donor 8 and a carbonyl anion equivalent 9 of furfu-
ral (Scheme 2).
OH
OH
O
O
O
HO
HO
O
HO
HO
O
+
HO
O
HO
X
(BrCO)2 (1.25 equiv.)
0 °C → r.t., 1 h, DCM
1a
8
9
BnO
BnO
BnO
BnO
O
O
BnO
BnO
Scheme 2 Basic idea behind our approach
BnO
BnO
OH
Br
13a/b
14a/b
We chose the masked carbonyl compound 5 as a car-
bonyl anion equivalent to react with glycosyl donor 10 in a
Corey–Seebach reaction (Scheme 3).9 The desired -C-gly-
cosidic bond can be generated by a SN2′-type nucleophilic
attack on the -glycosyl donor 10, which can be selectively
obtained because of the anomeric effect.10
Dithiane 15a/b (2.00 equiv.)
t-BuLi (2.00 equiv.)
–95 °C → 0 °C
2 h, 2-MeTHF
PIFA (3.00 equiv.)
r.t., 30 min
MeCN/H2O
BnO
BnO
BnO
BnO
S
O
S
O
O
BnO
BnO
BnO
BnO
16a–d
Ar
Ar
7a–d
OBn
OBn
S
S
O
O
O
O
BnO
BnO
BnO
BnO
Li
Ar
PGO
PGO
PGO
PGO
HO
HO
S
O
S
5
BnO
BnO
O
O
O
PGO
PGO
HO
PGO
11
HO
12
PGO
Ar
Ar
X
7a, 57%
7b, 68%
OMe
OMe
10
OBn
OBn
BnO
BnO
BnO
BnO
Scheme 3 Approach for the synthesis of acyl--C-glycosidic com-
O
O
O
O
pounds
BnO
BnO
7c, 62%
7d, 60%
Scheme 4 -Selective one-pot syntheses of acyl-glycosides 7a–d
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

C
G. J. Boehlich, N. Schützenmeister
Synpacts
Synlett
use of 2-MeTHF instead of THF as solvent. In the synthesis
of 7a, small amounts of the glucal 18 formed, which is
probably caused by elimination of benzyl alcohol from the
acyl glucoside. Guillarme et al. also observed this product
when they reacted Li and Mg organometallics with perben-
zylated glycosyl cyanides (Figure 2).5
Furthermore, the -configuration of furoyl-C-glycoside
7e was proven beyond doubt by single-crystal X-ray diffrac-
tion analysis (Figure 3).18
OBn
O
OBn
O
O
BnO
BnO
BnO
BnO
BnO
17a/b
18
Figure 2 Observed side products in the synthesis of acyl--clycosides
The synthesis of scleropentaside A (1a) required furyl-
dithiane 15c as building block, which can be synthesized
from furfural in one step.15 However, the furan substituent
was more challenging to introduce. First of all, the lithiated
dithiane 5c is more unstable compared to the lithiated dith-
ianes 5a/b as it readily decomposes even at lower tempera-
tures.16 Secondly, the resulting glycosyl dithianes 16e/f also
proved to be unstable employing the hydrolysis method us-
ing PIFA. Thus, the glycosyl dithianes 16e/f had to be puri-
fied prior to hydrolysis of the dithiane moiety under milder
conditions. While the Corey–Seebach umpolung reaction of
D-glucose donor 14a and dithiane 5c provided 16e in an ex-
cellent yield of 73%, a significant reduction in yield was ob-
served using the D-galactose derivative 16f (49%).
Figure 3 Single crystal X-ray structure of 7e
Removal of the benzyl groups proved to be difficult.
Even though most methods to synthesize acyl-C-glycosides
use benzyl-protective groups they do not offer an efficient
way to remove these. In our case, standard hydrogenation
procedures showed no conversion at all. Walzcak et al. re-
ported hydrogenolytic conditions that cleaved the benzyl
groups, but also reduced the ketone to the alkane.8 We had
moderate success by using FeCl3 in DCM19 but the workup
was tedious and the yield mediocre (Scheme 6).
BnO
BnO
HO
HO
FeCl3 (8.00 equiv.)
r.t., 1 h, DCM
O
O
O
O
BnO
HO
As before, in both cases the exclusive formation of -C-
glycosidic anomers was achieved. The hydrolyses of the
dithianes 16e/f were performed using I2 and H2O2 in
DCM/H2O17 to yield acyl--C-glycosides 7e/f in good to ex-
cellent yields (Scheme 5).
BnO
7e
O
HO
1a
O
41%
Scheme 6 Deprotection of benzylated -acyl-glycoside 7e
3 C-Glycosylation of Silylated Glycosyl Donors
BnO
BnO
BnO
BnO
(BrCO)2 (1.25 equiv.)
0 °C, 1 h, DCM
O
O
To overcome this problem, we investigated the use of
TMS as protective group. The easily synthesized persilylat-
ed monosaccharides 19a–d were converted into the corre-
sponding donors 20a–d by a literature-known procedure.20
Solvent exchange to 2-MeTHF after donor synthesis and
subsequent Corey–Seebach umpolung reaction between the
donors 20a–d and the lithiated species 5a/b of the dithianes
15a/b provided the -C-glycosides 21a–h.
Again, use of THF instead of 2-MeTHF as the solvent led
to a significant reduction in yield. Removal of the TMS
groups by NaOMe in MeOH and oxidative hydrolysis of the
respective dithianes delivered the ketones 12a–h in excel-
lent to quantitative yields for the four-step one-pot proce-
dure. Again, in all cases only the acyl--C-glycosides 12a–h
were formed (Scheme 7).
BnO
BnO
BnO
BnO
OH
Br
13a/b
14a/b
Dithiane 15c (2.00 equiv.)
t-BuLi (2.00 equiv.)
–95 °C → 0 °C
2 h, 2-MeTHF
I2, H2O2
r.t., 30 min
DCM/H2O
BnO
BnO
O
BnO
BnO
O
S
S
O
BnO
BnO
BnO
O
S
BnO
O
7e/f
16e/f
OBn
OBn
S
O
O
O
O
BnO
BnO
BnO
BnO
BnO
O
O
BnO
O
16e, 73%
7e, 82%
OBn
OBn
The 2-deoxyglucosyl iodide 20d proved to be unstable
in DCM. However, unlike other glycosyl iodides it was possi-
ble to generate the iodide in 2-MeTHF. Employing this do-
nor in the following key step only generated the desired C-
glycosides with excellent -selectivity. This observation
BnO
BnO
BnO
BnO
S
S
O
O
BnO
O
BnO
16f, 49%
7f, 62%
Scheme 5 -Selective syntheses of acyl-glycosides 7e/f
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

D
G. J. Boehlich, N. Schützenmeister
Synpacts
Synlett
groups by NaOMe in MeOH. Finally, oxidative hydrolysis of
dithianes 21i–l by I2 and H2O2 in H2O yielded scleropenta-
side A (1a), its D-galactose analogue 1g, its D-xylose ana-
logue 1h, and 2-deoxy scleropentaside A (1i) in very good
yields (Scheme 8).
R
R
TMSI (1.10 equiv.)
0 °C, 15 min, DCM
O
O
TMSO
TMSO
TMSO
TMSO
R'
R'
OTMS
I
19a–d
20a–d
Dithiane 15a/b (2.00 equiv.)
t-BuLi (2.00 equiv.)
–95 °C → 0 °C
R
2 h, 2-MeTHF
NaOMe (1.00 equiv.)
r.t., MeOH, 10 min
R
TMSI (1.10 equiv.)
0 °C, 15 min, DCM
O
O
TMSO
TMSO
TMSO
TMSO
R'
OTMS
R'
I
19a–d
20a–d
PIFA (3.00 equiv.)
r.t., 30 min
MeOH/H2O
R
R
Dithiane 14c (2.00 equiv.)
t-BuLi (2.00 equiv.)
–95 °C → 0 °C, 2 h, THF
NaOMe (1.00 equiv.)
r.t., MeOH, 10 min
S
O
S
O
R'
O
HO
HO
HO
HO
R'
21a–h
Ar
Ar
12a–h
OH
OH
O
O
I2 (5.00 mol%),
H2O2 (4.40 equiv.)
r.t., 30 min, H2O
O
O
HO
HO
HO
R
R
HO
S
O
S
O
R'
O
HO
HO
HO
HO
HO
HO
O
R'
21i–l
O
12a, 85%
OH
12b, 62%
OH
1a–d
OMe
HO
HO
HO
HO
O
O
OH
OH
O
O
S
O
S
S
HO
12c, 73%
HO
12d, 48%
O
O
HO
HO
HO
HO
HO
21i, 60%
O
O
HO
1a, 78%
O
O
OMe
OMe
O
O
O
O
HO
HO
HO
HO
OH
OH
OH
HO
OH
HO
HO
12e, 84%
OH
HO
S
O
O
O
O
O
12f, quant.
HO
HO
OH
21j, 40%
1g, 77%
O
O
O
O
HO
HO
HO
HO
S
S
S
O
O
HO
HO
HO
HO
HO
O
O
HO
O
O
12g, 77%*
12h, 52%*
OMe
21k, 74%
1h, 70%
*2-MeTHF used instead of DCM in step 1.
OH
OH
Scheme 7 -Selective one-pot syntheses of acyl-glycosides 12a–h18
S
O
O
HO
HO
HO
HO
supports the assumption of an SN2-controlled mechanism
during nucleophilic attack at the anomeric center due to a
lack of a shielding group in 2-position.
21l, 67%*
1i, 92%
*2-MeTHF used instead of DCM or THF in step 1 and 2.
In most cases, decrease in yield was observed when D-
galactosyl donors were employed. This is probably caused
by the shielding effect of the axial substituent. This as-
sumption is in agreement with the observation that -man-
nosyl donors, which have their axial substituent closer to
the anomeric center, did not react at all. -Allosyl donors
were too unstable due to 1,3-diaxial interactions and de-
composed under the reaction conditions.
Scheme 8 -Selective syntheses of furoyl-glycosides 1
As before, single-crystal X-ray diffraction analysis con-
firmed the -C-glycosidic linkage beyond doubt (Figure 4).21
4 Conclusion
Again, the furyl substituent required special treatment
to be introduced. When 2-MeTHF was used as solvent, all
donors except 2-deoxyglucosyl donor 20d only yielded trac-
es of product. Solvent exchange to THF after donor synthe-
sis and Corey–Seebach umpolung reaction of donor 20a–d
and the lithiated species 5c of dithiane 15c provided exclu-
sively the -C-glycosides 21i–l after cleavage of the silyl
In conclusion, we have developed a fast and exclusively
-selective approach towards benzylated and unprotected
acyl--C glycosides. The applicability of the method was
shown on the dramatically short synthesis of scleropenta-
side A (1a), which was synthesized in only four steps in
with an excellent overall yield of 47%. Furthermore, the
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

E
G. J. Boehlich, N. Schützenmeister
Synpacts
Synlett
H.; Schmidt, R. R. Liebigs Ann. Chem. 1994, 975. (m) Panigot, M.
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Figure 4 Single-crystal X-ray structure of 21l21
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Funding Information
Generous financial support of this research by the Fonds der Che-
mischen Industrie is gratefully acknowledged. G.J.B. thanks the Uni-
versität Hamburg for a PhD fellowship (Hamburg Stipendium).()
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Acknowledgment
We thank Isabelle Nevoigt for technical assistance (X-ray).
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(18) CCDC 1880770 contains the supplementary crystallographic
data for this paper. The data can be obtained free of charge from
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data for this paper. The data can be obtained free of charge from
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