A novel oxidatively removable linker and its application to α-selective solid-phase oligosaccharide synthesis on a macroporous polystyrene support

Article abstract of DOI:10.1055/s-1999-2779

A novel p-acylaminobenzyl-type linker and its application to the solid phase synthesis of oligosaccharides are described. The p-glutarylaminobenzyl ether of a monosaccharide was readily introduced to a polymer support by amide bond formation. The linker moiety was smoothly cleaved by DDQ oxidation. α-Selective glycosylation was accomplished by virtue of the solvent effect of diethyl ether on a solid support of a macroporous polystyrene. High α-selectivity was achieved by the use of benzylated donors possessing the bulky t-butyldiphenylsilyl group at the 6-position.

Full text of DOI:10.1055/s-1999-2779

1074
LETTER
A Novel Oxidatively Removable Linker and Its Application to a-Selective
Solid-Phase Oligosaccharide Synthesis on a Macroporous Polystyrene Support
Koichi Fukase,* Yoshihiko Nakai, Kenji Egusa, John A. Porco, Jr.#, Shoichi Kusumoto*
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
#Argonaut Technologies, 887 Industrial Road, Suite G, San Carlos, CA 94070, USA
Fax +81 6 6850 5419; E-mail: koichi@chem.sci.osaka-u.ac.jp
Received 7 April 1999
glycosylation in such a mixture was considerably reduced
Abstract: A novel p-acylaminobenzyl-type linker and its applica-
as compared to those in ether alone (data not shown).
tion to the solid phase synthesis of oligosaccharides are described.
Such restriction of solvents has been a general problem of
solid-phase organic synthesis. Recently, a new type of a
The p-glutarylaminobenzyl ether of a monosaccharide was readily
introduced to a polymer support by amide bond formation. The link-
er moiety was smoothly cleaved by DDQ oxidation. a-Selective polystyrene resin, ArgoPore™, was developed to over-
glycosylation was accomplished by virtue of the solvent effect of
diethyl ether on a solid support of a macroporous polystyrene. High
a-selectivity was achieved by the use of benzylated donors possess-
poly(styrene-co-divinylbenzene) resin and hence has con-
ing the bulky t-butyldiphenylsilyl group at the 6-position.
come the solvent restrictions of gel-type polymer sup-
ports.8) ArgoPore™ is a highly cross-linked macroporous
stant ability for uptake (swelling) of organic solvents. A
wide range of solvents typically employed for solution-
phase reactions, including protic solvents and even water,
Key words: solid-phase synthesis, glycosylation, thioglycoside,
macroporous polystyrene, acylaminobenzyl linker
can thus permeate easily into the pores and be used for sol-
id-phase synthesis on this resin. In addition, more rapid
access of reagents to reaction sites is expected than in the
case of gel-type resins, where the rate of diffusion may be
low. We therefore investigated a-selective glycosylation
using ether as a solvent on this macroporous polystyrene.
Oligosaccharides and glycoconjugates play important
roles in many biological recognition processes such as
cellular trafficking, cell-cell adhesion, chronic inflamma-
tion, viral and bacterial infection, and immunostimulation
by bacterial cell surface glycoconjugates. Solid-phase
synthesis of oligosaccharides has been one of the most im-
portant topics in organic chemistry, because it will enable
rapid and easy preparation of many oligosaccharides for
biological or functional studies. Since Danishefsky’s oli-
gosaccharide synthesis on solid-phase in 1993, extensive
efforts have been made and several successful studies
have already been reported.1-6) However, general and
practical methods for solid-phase synthesis have not been
established yet. There remain several crucial issues in-
cluding i) stereoselective glycosylation on solid supports
and ii) facile and selective cleavage of linkers.
Clear-cut linking/liberation procedures to/from polymer
supports are indispensable for successful solid-phase syn-
thesis. Acid-stable linkers are of particular importance for
oligosaccharides and glycoconjugates synthesis by the
solid-phase strategy since glycosylation reactions are nor-
mally carried out under acidic conditions. Wang-type
linkers having p-alkoxybenzyl ether linkages labile to
Lewis acids are not suitable in this. In addition, selective
cleavage of linkers, leaving other protecting groups intact,
followed by purification and final deprotection is a ratio-
nal strategy for oligosaccharide synthesis. This is an obvi-
ous difference from the solid-phase synthesis of peptides
where all the side chain protections and linkers may be re-
moved simultaneously. In the case of oligosaccharides pu-
rification in protected forms is generally more effective,
whereas completely deprotected peptides have been suc-
cessfully purified by HPLC. We recently reported several
new acid-stable benzyl-type protecting groups e.g., p-ni-
trobenzyl, p-acylaminobenzyl, and p-azidobenzyl groups
and their application to the synthesis of complex glyco-
conjugates.10) p-Acylaminobenzyl ethers among them,
i.e., p-acetamidobenzyl and p-pivaloylaminobenzyl ether,
are much more stable under acidic conditions than p-
methoxybenzyl (MPM) ether, though the former can be
readily cleaved by 2,3-dichloro-5,6-dicyanobenzo-
quinone (DDQ) oxidation at a rate comparable to the
cleavage of the latter MPM ether. This fact suggests that
the p-acylaminobenzyl group should be used as a versatile
acid-stable linker by using appropriate spacers.
In the present study, we have focused on achieving 1,2-
cis-a-selective glycosylation on the solid support, which
have not been reported. The stereoselective formation of
1,2-cis-glycosides such as a-glucosides is generally rather
difficult since assisting effects such as neighboring-group
participation are not available. The combination of diethyl
ether as a solvent and perchlorates as a counteranion for
oxocarbenium ion intermediates have been frequently
used for a-selective glycosylation.7) In previous solid-
phase oligosaccharide synthesis, 1 or 2% cross-linked gel-
type poly(styrene-co-divinylbenzene) resins have been
mainly used as polymer supports. Although swelling resin
in solvents is essential for solid-phase reactions, gel-type
polystyrene resins show limited swelling (approx. 3.5 ml/
g) in diethyl ether. In fact, glycosylation using gel-type
polystyrene resins in ether did not proceed in our prelimi-
nary experiments. A mixture of CH2Cl2 and diethyl ether
can swell a gel-type polystyrene resin, but a-selectivity of
Synlett 1999, No. 07, 1074–1078 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A Novel Oxidatively Removable Linker
1075
Glycosylation on solid support was then investigated by
the use of a benzylated glycosyl trichloroacetimidate do-
nor with a 6-O-trichloroethoxycarbonyl (Troc) 7,18) since
we previously found a-selectivity was enhanced by the in-
fluence of the 6-O-Troc function.7,14) The glycosylation
was carried out using 3 equiv. of donor 7 to solid support-
ed acceptor 5 and TMSOTf (0.2 equiv. relative to 7) in
ether (25 °C, 12 h) (Scheme 4).
Cl
Cl
O
O
NC
O
CN
OH
O
CH2
R
R
NH-CO(CH2)nCO
CH
NX
R
Scheme 1
The p-acylaminobenzyl ether linker can be easily ob-
tained from the corresponding p-nitrobenzyl ether by re-
duction of the nitro group and subsequent acylation with
bifunctional carboxylic acids, e.g., glutaric acid. Introduc-
tion of the linker at glycosidic position should be avoided,
since its cleavage usually gives an equilibrium mixture of
the anomeric hydroxy group and thus renders purification
of the product difficult. A synthesis of monosaccharides
possessing 2-O-bound p-acylaminobenzyl ether linker is
shown in Scheme 2 as an example. The 2-O-glutaryl-
aminobenzylated monosaccharide 411) was prepared from
2-O-p-nitrobenzyl glucose derivative 112) via regioselec-
tive ring opening with BH3•Me2NH and BF3•OEt2 in
CH2Cl2,13) reduction of the nitro function by Zn-Cu and
acetylacetone14,15), and final N-glutarylation (Scheme 2).
HO
O
TrocO
BnO
O
BnO
O
O
BnO
BnO
O
O C CCl3
NH
OMe
+
N
H
CH2
N
BnO
H
7
5
TrocO
O
BnO
BnO
TMSOTf
O
DDQ
BnO
O
O
+
5
BnO
BnO
Et2O 25°C
CH2Cl2-H2O (5:1)
O
O
CH2
OMe
N
N
H
H
8
TrocO
O
BnO
BnO
O
+
6
O
BnO
BnO
BnO
HO
9
OMe
Scheme 4
The monosaccharide 4 was readily introduced to aminom-
ethylated macroporous polystyrene resin ArgoPore™
amine by the use of 4 (1.2 equiv.), N,N’-diisopropylcarbo-
diimide (DIC), and 1-hydroxybenzotriazole (HOBt) in
CH2Cl2 quantitatively (Scheme 3).16) The linker of 5 was
smoothly cleaved by DDQ oxidation in CH2Cl2-H2O
(20:1) to give 6 in a good yield.17)
The glycosylation proceeded to give the desired disaccha-
ride 9 in 48% yield along with 25% of monosaccharide 6
derived from unreacted acceptor after cleavage from the
resin.19) a-Preferential glycosylation by virtue of the sol-
vent effect of ether was thus achieved though the selectiv-
ity was not satisfactory (a:b = 2:1). The chemical yield of
9 (47%) was not improved and unreacted monosaccharide
6 was recovered in 21% yield, even when the glycosyla-
tion reaction step was repeated twice. These results indi-
cated the existence of reactive and non-reactive
monosaccharide residues on the pore surface of the sup-
port (Table 1). Dry ArgoPore™ resin has an average pore
size of approximately 90 Å. Since the intermediates of
acylation reaction are non-ionic, they may readily reach
the amino groups even in smaller pores and thus acylation
reaction proceeds quantitatively. On the other hand, the
ionic reaction intermediates derived from the glycosyl do-
nor consists of oxocarbenium ions and a potentially large
counterions. Glycosylation therefore may not proceed in
small pores owing to steric hindrance of the resin itself but
may proceed efficiently in larger pores. This assumption
was confirmed by the following trisaccharide synthesis
(Scheme 5).
HO
Ph
O
O
BF3•Et2O
Me2NH•BH3
O
O
BnO
BnO
BnO
O
CH2
O
CH2
OMe
CH2Cl2
OMe
-20°C
96%
O2N
O2N
O
1
2
HO
O
O
BnO
O
Zn-Cu
BnO
O
Acetylacetone
OMe
H2N
CH2
THF
THF
3
92%
91%
HO
O
BnO
BnO
O
O
O
OMe
N
CH2
HO
H
4
Scheme 2
HO
BnO
After the second glucose residue was introduced as de-
scribed above, the 6-O-Troc group of the disaccharide 8
was removed by NaOMe in MeOH.20) Glycosylation with
the trichloroacetimidate 7 and TMSOTf was then repeated
at two times. The desired trisaccharide 10 was obtained in
42% yield after cleavage from the resin and the disaccha-
ride 9 was formed in only a tiny amount as indicated by
thin layer chromatography (TLC) analysis. This result
shows that ca. 90% of disaccharide acceptor on the sup-
O
H2N
ArgoPore™
BnO
O
O
O
CH2
OMe
4
N
N
DIC - HOBt
DMF
H
H
5
quant.
HO
DDQ
O
BnO
BnO
CH2Cl2-H2O (20:1)
91%
HO
OMe
6
Scheme 3
Synlett 1999, No. 07, 1074–1078 ISSN 0936-5214 © Thieme Stuttgart · New York

1076
K. Fukase et al.
LETTER
HO
By the use of a macroporous polystyrene, we succeeded in
glycosylation on solid support in ether in good yields
though the a-selectivity was not satisfactory.
O
BnO
7, TMSOTf
BnO
O
O
O
CH2
OMe
Et2O
25°C
N
H
N
H
5
a-Glycosylation on solid support was next investigated by
using thioglycosides as glycosyl donors on the basis of
our recent methodologies,7) since it is difficult to obtain
high a-selectivity by using glycosyl trichloroacetimi-
dates.21) The results using 6-O-Troc thioglycoside 11 as a
donor are summarized in Table 2. All the reactions were
carried out by using 3 equiv. of the thioglycoside relative
to the acceptor. PhSeNPhth-Mg(ClO4)2 which effectively
promotes glycosylation with thioglycosides under mild
conditions7a) in solution also promoted the reaction on sol-
id-support, but a considerable portion of the acceptor on
the solid-support was not glycosylated even though the
30% loaded resin was employed (entry 1). The a-selectiv-
ity was, however, improved to a:b = 84:16. This result
shows some of acceptor 5 was still loaded on less reactive
sites of the polymer, which may be accessible by other
strong activating reagents. Hypervalent iodine reagents
prepared from PhIO and strong Lewis acids were previ-
ously shown very high reactivity, where the a-selectivity
can be controlled by selecting Lewis acids.7b) The glyco-
12 h
two cycles
TrocO
O
BnO
BnO
O
BnO
O
O
7,
1M NaOMe
TMSOTf
BnO
BnO
O
O
CH2
MeOH
1h
Et2O
25°C
12 h
OMe
N
N
H
H
two cycles
8
TrocO
O
BnO
BnO
O
DDQ
BnO
O
O
BnO
BnO
CH2Cl2-H2O (5:1)
BnO
O
BnO
BnO
10
HO
OMe
Scheme 5
Scheme 5
port was glycosylated by the second glycosylation step to
give the trisaccharide whereas the monosaccharide accep-
tor on unreactive site was not glycosylated by the second
cycle of the reaction.
The above experiments indicate that the presence of unre- sylation with thioglycoside 11 was effected by the use of
active sites on the macroporous polystyrene resin there- either PhIO-TMSClO4 (entry 2) or PhIO-SnCl4-AgClO4
fore is not expected to cause serious problems in the total (entry 3) to give the disaccharide 9 in ca. 60% yields with
process of oligosaccharide synthesis.
trace amounts of monosaccharide 6 after cleavage from
the resin, with moderate a selectivity (a:b = 3:1).
Nevertheless, we tried to use only those amino functions
on the surface of large pores in order to improve the yield
of glycosylation at the first step. Selective loading of the
monosaccharide 4 on the reactive sites, i.e., the surface of
large pores was accomplished by decreasing the loading
levels (Table 1). Monosaccharide resins were prepared in
which 30 or 50% of the total amino groups were acylated
with the monosaccharide 4.15) The unreacted amino group
on the resin was blocked by acetylation. The glycosyla-
tion was carried out on the solid support in ether by using
3 equiv. of the imidate 7 and 0.2 equiv. of TMSOTf at 25
°C and the results are shown in Table 1.18) The yields of
the disaccharide increased as the loading ratio decreased
as expected. By using the 30% loading resin, the disaccha-
ride 9 was obtained in 82% yield with trace amounts of
monosaccharide 6. Subsequent glycosylation also pro-
ceeded smoothly using 30% loaded resin to give the
trisaccharide 10 in 70% yield from 5 (total 4 steps, aver-
age yield 92%).
HO
O
R-O
BnO
BnO
O
BnO
O
O
O
SPh
+
OMe
BnO
N
H
CH2
N
H
BnO
11 : R = Troc
12 : R = TBDPS
5
30% loading
R-O
O
BnO
BnO
reagents
DDQ
O
BnO
O
O
BnO
BnO
CH2Cl2-H2O (5:1)
Et2O 25°C
O
O
CH2
OMe
N
H
N
H
8 : R = Troc
13 : R = TBDPS
R-O
BnO
O
O
BnO
+
O
6
BnO
BnO
BnO
9 : R = Troc
14 : R = TBDPS
HO
OMe
Scheme 6
Table 2 Glycosylation with 6-O-Ttov thioglycoside 11 on ArgoPo-
reTM in ether
Table 1 Glycosylation on ArgoPore™ using trichloroacetimidate 7
in ether as the function of the loading level of the monosaccharide
yield (%)
reagents
(equiv. to donor)
entry
1
temp.
α : β
yield (%)
loading
6
9
entry
α : β
time
level (%)
9
6
PhSeNPhth-Mg(ClO4)2
(1.2-0.5)
84 : 16
35°C 43 30
12 h x 2
12 h x 2
12 h x 2
47 21
55 21
82 trace
1
2
3
100
50
65 : 35
66 : 34
65 : 35
2
3
PhIO-TMSClO4
(1.0-0.5)
76 : 24
75 : 25
25°C 61 trace
25°C 62 trace
30
PhIO-SnCl4-AgClO4
(1.0-0.5-0.5)
Synlett 1999, No. 07, 1074–1078 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A Novel Oxidatively Removable Linker
1077
We recently found a strong a-orienting effect of bulky tri- In summary, we have successfully accomplished solid-
tyl (Trt) and t-butyldimethylsilyl (TBDMS) groups at the phase synthesis of oligosaccharides by applying solution
6-position of glycosyl donors7a,22). Since Trt group is phase synthesis methodologies. a-Selective glycosylation
readily cleaved by acids, 6-O-TBDPS thioglycoside 12 on solid support was achieved by using a macroporous
was examined and the results are shown in Table 3. a-Se- polystyrene support, ArgoPore™, which permits the use
lectivity was dramatically improved by the use of 6-O- of diethyl ether as the solvent. We also demonstrated the
TBDPS donor 12 as expected. Activation by PhSeNPhth- versatility of an acylaminobenzyl linker which can be se-
Mg(ClO4)2 or NBS-LiNO37c) gave disaccharide 14 with lectively cleaved by DDQ oxidation. The strong a-orient-
high a-selectivity, though some of the monosaccharide 6 ing effect of the 6-O-TBDPS function is applicable to a-
was recovered (entry 1, 2). Combination of PhIO with selective glycosylation in general.
SnCl4-AgClO4 completed the reaction to give the disac-
charide 14 in 62% yield with substantially high a-selectiv-
Acknowledgement
ity (a:b = 9:1) (entry 3).23) This was the most practical
The present work was financially supported in part by Grants-in-
Aid for Scientific Research No. 07680630 and on Priority Areas
Nos. 06240105 and 08245229 from the Ministry of Education, Sci-
ence and Culture, Japan and by "Research for the Future" Program
No. 97L00502 from the Japan Society for the Promotion of Science.
result in terms of both yield and a-selectivity.
Table 3. Glycosylation with 6-O-TBDPS thioglycoside 12 on Argo-
Pore™ in ether.
We also thank Dr. Jeff W. Labadie (Argonaut Technologies) for
helpful discussions.
yield (%)
reagents
(equiv. to donor)
entry
1
temp.
α : β
6
14
PhSeNPhth-Mg(ClO4)2
(1.2-0.5)
100 : 0
35°C 48 22
References and Notes
(1) a) Danishefsky, S. J.; McClure, K. F.; Randolph, J. T.;
Ruggeri, P. B. Science 1993, 260, 1307. b) Randolph J. T.;
Danishefsky, S. J. Angew. Chem. Int. Ed. Engl. 1994, 33,
1470. c) Randolph, J. T.; McClure, K. F.; Danishefsky, S. J.
J. Am. Chem. Soc. 1995, 117, 5712. d) Zheng, C.; Seeberger,
P. H.; Danishefsky, S. J. J. Org. Chem. 1998, 63, 1126.
(2) a) Yan, L.; Taylor, C. M.; Goodnow Jr., R.; Kahne, D. J. Am.
Chem. Soc. 1994, 116, 6953. b) Liang, R.; Yan, L.; Loebach,
J.; Ge, M.; Uozumi, Y.; Sekanina, K.; Horan, N.; Gilderskeve,
J.; Thompson, C.; Smith, A.; Biswas, K.; Still, W. C.; Kahne,
D. Science 1996, 274, 1520.
(3) a) Rademann, J.; Schmidt, R. R. Tetrahedron Lett. 1996, 37,
3989.b) Rademann, J.; Schmidt, R. R. J. Org. Chem. 1997, 62,
3650. c) Heckel, A.; Mross, E.; Jung, K.-H.; Rademann, J.;
Schmidt, R. R. Synlett 1998, 171.
(4) Nicolaou, K. C.; Winssinger, N.; Pastor, J.; DeRoose, F.
J. Am. Chem. Soc. 1997, 119, 449.
2
3
NBS-LiNO3
(1.2-0.5)
97 : 3
25°C 41 20
25°C 62 trace
PhIO-SnCl4-AgClO4
(1.0-0.5-0.5)
90 : 10
The synthesis of trisaccharide 15 was investigated by use
of 6-O-TBDPS thioglycoside 12 as a donor and PhIO-
SnCl4-AgClO4 as promoter (Scheme 7). After the first
glycosylation step, the 6-O-TBDPS group was removed
by HF/Pyr in THF.24) The second glycosylation step was
then carried out in the same manner as the first one. At
each step, the glycosylation reaction was repeated two
times each with 3 equiv. of the donor. Finally, cleavage
from the resin by DDQ oxidation afforded the trisaccha-
ride 15 in 50% yield (total 4 steps, average yield 84%).
The a:b selectivity was 9:1 for each anomeric position as
judged by 1H NMR analysis.
(5) Douglas, S. P.; Whitfield, D. M.; Krepinsky, J. J. J. Am. Chem.
Soc. 1995, 117, 2116.
(6) a) Ito, Y.; Kanie, O.; Ogawa, T. Angew. Chem. Int. Ed. Engl.
1996, 35, 2510. b) Ito, Y.; Ogawa, T. J. Am. Chem. Soc. 1997,
119, 5562. c) Kononov, L. O.; Ito, Y.; Ogawa, T. Tetrahedron
Lett. 1997, 38, 1599.
(7) a) Fukase, K.; Nakai, Y.; Kanoh, T.; Kusumoto, S. Synlett
1998, 85. b) Fukase, K.; Kinoshita, I.; Kanoh, T.; Hasuoka, A.;
Kusumoto, S. Tetrahedron 1996, 52, 3897. c) Fukase, K.;
Hasuoka, A.; Kinoshita, I.; Aoki, Y.; Kusumoto, S.
Tetrahedron 1995, 51, 4923.
(8) Commercially available from Argonaut Technologies, San
Carlos, California: http://www.argotech.com. For
investigations of other highly crosslinked macroporous resins,
see: Hori, M.; Gravert, D.J.; Wentworth, P., Jr.; Janda, K.D.
Bioorganic Med. Chem. Lett. 1998, 8, 2363.
HO
BnO
BnO
PhIO-
SnCl4-AgClO4
O
12,
O
O
O
CH2
OMe
Et2O 25°C
12 h two cycles
N
N
H
H
5
TBDPSO
BnO
BnO
O
O
PhIO-
BnO
O
O
O
12,
HF/Pyr
THF
SnCl4-AgClO4
Et2O 25°C
12 h two cycles
BnO
BnO
O
OMe
O
N
H
CH2
N
H
(9) a) Kobayashi, S.; Tetrahedron Lett. 1998, 39 b) Hanessian, S.;
Xie, F. Tetrahedron Lett. 1998, 39, 737. c) Hanessian, S.; Xie,
F. Tetrahedron Lett. 1998, 39, 733.
13
TBDPSO
BnO
BnO
O
DDQ
(10) Fukase, K.; Egusa, K.; Nakai, Y.; Kusumoto, S. Molecular
Diversity 1997, 2, 182.
BnO
O
BnO
BnO
O
BnO
BnO
CH2Cl2-H2O (5:1)
(11) 4:1H NMR (500 MHz, CDCl3) d:7.69 (s, 1H, CONHPh), 7.47
(d, 2H, J = 8.24 Hz, o protons of CONHPhCH2O), 7.36-7.27
(m, 12H, Ph x 2, m protons of CONHPhCH2O), 4.95, (d, 2H,
Jgem = 10.99 Hz, CH2Ph), 4.83 (d, 2H, Jgem = 10.99 Hz,
BnO
O
15
HO
OMe
Scheme 7
Synlett 1999, No. 07, 1074–1078 ISSN 0936-5214 © Thieme Stuttgart · New York

1078
K. Fukase et al.
LETTER
concentrated in vacuo. The residue was purified with
preparative silica-gel TLC (toluene-EtOAc = 1:1) to give
colorless solid. Yield 19.6 mg (91%).
CH2Ph), 4.86 (d, 2H, Jgem = 10.99 Hz, CH2Ph), 4.63 (d, 2H,
Jgem = 10.99 Hz, CH2Ph), 4.73 (d, 2H, Jgem = 12.36 Hz,
CH2Ph), 4.60 (d, 2H, Jgem = 12.36 Hz, CH2Ph), 4.53 (d, 1H,
J1, 2 = 3.43 Hz, H-1), 3.99 (dd, 1H, J2,3 = 9.16 Hz, J3, 4 = 9.16
Hz, H-3), 3.77 (dd, 1H, J6, 5 = 2.52 Hz, J6, 6’= 11.90 Hz, H-6),
3.69 (dd, 1H, J6’, 5 = 3.89 Hz, J6’, 6 = 11.90 Hz, H-6’), 3.65-3.62
(m, 1H, H-5), 3.52 (dd, 1H, J3,4 = 9.16 Hz, J4, 5 = 9.62 Hz, H-
4), 3.45 (dd, 1H, J1,2 = 3.43 Hz, J2, 3 = 9.16 Hz H-2), 3.34 (s,
3H, OMe), 2.46-2.39 (m, 4H, HOCOCH2CH2CH2CONHPh),
2.06-2.00 (m, 2H, HOCOCH2CH2CH2CONHPh); FAB-MS
(positive) m/z 616.2 [(M+Na)+].
(18) The general procedure for solid-phase glycosylation using a
glycosyl trichloroacetimidate: The monosaccharide resin
(30% loading) (155 mg, 30.0 mmol) was washed with dry ether
(3 ml) three times. To the resin were added Molecular Sieves
4A beads, 8-12 mesh (200 mg), a solution of a glycosyl
trichloroacetimidate (69.3 mg, 90.0 mmol) in dry ether
(3.0ml), and TMSOTf (3.5 ml, 18.0 mmol), successively. The
reaction mixture was shaken with Rotator RT-50 (Taitech) at
room temperature for 12 h. The solution was removed by
filtration and the resin was washed with CH2Cl2 and ether.
After Molecular Sieves 4Å beads were removed by picking
with forceps, the resins were washed twice with ether and
dried under vacuum. After this procedure was repeated, the
resins were subjected to cleavage reaction by DDQ or
deprotection of the Troc group.
(19) All the yields were obtained after cleavage from the resin
followed by purification by silica-gel column chromatography
or preparative silica-gel TLC.
(20) Typical procedure for the removal of the Troc group: To the
disaccharide resin 8 (180 mg) was added a solution of MeONa
in MeOH (1 M, 3 ml). After the reaction mixture was shaken
for 1 h, the solution was removed by filtration and the resin
was washed with MeOH and CH2Cl2.
(12) Fukase, K.; Matsumoto, T.; Ito, N.; Yoshimura, T.; Kotani, S.;
Kusumoto, S. Bull. Chem. Soc. Jpn. 1992, 65, 2643.
(13) Reduction of 3-O-alkylated glucose or glucosamine
derivatives with BH3◊Me2NH and BF3◊OEt2 in CH2Cl2 gave
the corresponding 4-O-benzylated products, whereas
reduction of 3-O-acylated or alkoxycarbonylated compounds
in CH3CN gave the 6-O-benzylated products: Oikawa, M.;
Liu, W.-C.; Nakai, Y.; Koshida, S.; Fukase, K.; Kusumoto, S.
Synlett 1996, 1179: Fukase, K.; Fukase, Y.; Oikawa, M.; Liu,
W.-C.; Suda, Y.; Kusumoto, S. Tetrahedron 1998, 54, 4033.
Reduction of the benzylidene group with LiAlH4 and AlCl3
decomposes the p-nitrobenzyl group.
(14) Fukase, K.; Yoshimura, T.; Kotani, S.; Kusumoto, S. Bull.
Chem. Soc. Jpn. 1994, 67, 473.
(15) Reduction with NaBH4-NiCl2◊6H2O in MeOH also gave 3 in
good yields.
(21) a-Selectivity was improved to a:b = 4:1 by virtue of a-
orienting effect of the 6-O-TBDPS group by using 0.2 equiv.
of TBSClO4 as a catalyst.
(22) Wakao, M.; Nakai, Y.; Fukase, K.; Kusumoto, S. Chem. Lett.
1999, 27.
(16) A typical procedure for introduction of a monosaccharide on
solid support. ArgoPore resin (NH2:0.60 mmol/g) (1.41 g, 845
mmol) was placed in a polypropylene tube (Varian) fitted with
a filter, and washed with 5% diisopropylamine in CH2Cl2 and
then CH2Cl2. Compound 4 (251 mg, 423 mmol), HOBt (286
mg, 4.22 mmol), CH2Cl2, and DIC (132 ml, 1.69 mmol) were
added to the tube, successively. The reaction mixture was
shaken for 3 d and filtered. The resin was washed with CH2Cl2
and stained with Bromophenol Blue in order to check the
progress of N-acylation. The residual amino groups on the
resin were then capped with acetic anhydride (1.0 ml) and
triethylamine (500 ml) in CH2Cl2 (2.0 ml) by shaking for 10
min. The capping reaction was repeated until the color of the
resin turned from blue green to pale yellow. The resin was
then shaken with 1M NaOMe (15 ml) for 1 h and then washed
successively with MeOH, CH2Cl2, and diethyl ether. Yield of
the resin 1.68 g (Theoretical yield of the resin is 1.68 g.).
(17) A typical cleavage reaction of acylaminobenzyl linker:
ArgoPore resin linked with monosaccharide via
(23) The general procedure for solid-phase glycosylation using a
thioglycoside: The monosaccharide resin (30% loading) (129
mg, 25 mmol) was washed with dry ether (3 ml) three times.
To the resin (30% loading) were added Molecular Sieves 4Å
beads, 8-12 mesh (200 mg), a solution of a thioglycoside (75
mmol) in dry ether (3.0 ml), PhIO (16.5 mg, 54 mmol), SnCl4
(4.4 ml, 38 mmol), and AgClO4 (8 mg, 39 mmol), successively.
The reaction mixture was shaken with Rotator RT-50
(Taitech) (or Nautilus™ 2400, Argonaut Technologies8)) at
room temperature for 12 h. The solution was removed by
filtration and the resin was washed successively with ether and
CH2Cl2. After Molecular Sieves 4Å beads were removed by
picking with forceps, the resins were washed twice with ether
and the resins were dried under vacuum. After this procedure
was repeated two times, the resins were subjected to cleavage
reaction by DDQ or deprotection of the Troc or TBDPS group.
(24) The general procedure for cleavage of TBDPS group: To a
disaccharide resin23) was added a solution of hydrogen
fluoride-pyridine (Aldrich) (1ml) in THF (2 ml). The reaction
mixture was shaken at room temperature for 18 h. The
solution was removed by filtration and the resin was washed
successively with THF and CH2Cl2.
acylaminobenzyl linker (174 mg, 57.6 mmol of a
monosaccharide) was shaken with a mixture of DDQ (15. 7
mg, 69.1 mmol) in CH2Cl2 (2.5 ml) and water (125 ml) at room
temperature for 4 h. After the reaction mixture was filtered,
saturated ascorbic acid solution (10 ml) was added to the
filtrate to quench excess DDQ. The resin was shaken again
with DDQ (7.9 mg, 34.5 mmol) in CH2Cl2 (2.5 ml) and water
(125 ml) at room temperature for 4 h and worked up as the
same manner. The organic layer was combined, washed with
saturated NaHCO3 solution and brine, dried over MgSO4, and
Article Identifier:
1437-2096,E;1999,0,07,1074,1078,ftx,en;Y06599ST.pdf
Synlett 1999, No. 07, 1074–1078 ISSN 0936-5214 © Thieme Stuttgart · New York