DISAL glycosyl donors for the synthesis of a linear hexasaccharide under mild conditions.

Article abstract of DOI:10.1021/ol034242q

[reaction: see text] The new class of glycosyl donors with a methyl 3,5-dinitrosalicylate (DISAL) anomeric leaving group has proved efficient for glycosylation under strictly neutral, mildly basic, or mildly acidic conditions. Here, we report the synthesi

Full text of DOI:10.1021/ol034242q

ORGANIC
LETTERS
2003
Vol. 5, No. 8
1309-1312
DISAL Glycosyl Donors for the
Synthesis of a Linear Hexasaccharide
under Mild Conditions
Lars Petersen,† Jane B. Laursen,† Kim Larsen,‡ M. Saddik Motawia,‡ and
,‡
Knud J. Jensen*
Department of Chemistry, The Royal Veterinary and Agricultural UniVersity,
40 ThorValdsensVej, DK-1871 Frederiksberg C, Denmark, Department of Chemistry,
Technical UniVersity of Denmark, Building 201, KemitorVet, DK-2800 Kgs. Lyngby,
Denmark, and Plant Biochemistry Laboratory and Center for Molecular Plant
Physiology (PlaCe), Department of Plant Biology, The Royal Veterinary and
Agricultural UniVersity, 40 ThorValdsensVej, DK-1871 Frederiksberg C, Denmark
kjj@kVl.dk
Received February 11, 2003
ABSTRACT
The new class of glycosyl donors with a methyl 3,5-dinitrosalicylate (DISAL) anomeric leaving group has proved efficient for glycosylation
under strictly neutral, mildly basic, or mildly acidic conditions. Here, we report the synthesis of novel DISAL disaccharide glycosyl donors
prepared by easy nucleophilic aromatic substitution. These DISAL donors proved efficient in the synthesis of a starch-related hexasaccharide
under very mild conditions. Glycosylations proceeded with r-selectivity and were compatible with Trt protecting groups.
Cell-surface oligosaccharides are involved in many biological
recognition processes including leukocyte-endothelial cell
adhesion (leukocyte recruitment in inflamed tissue), bacterial
and viral infection, and immunological recognition of tumor
cells and foreign tissue in xenotransplantation.1 Furthermore,
free oligosaccharides can have biological effects such as the
rhizobial lipochitin oligosaccharides, which function as
nodulation factors.2 Besides, well-defined linear and branched
oligosaccharides are important substrates for biosynthesis and
degradation studies.3
Established glycosylation methods, most notably those
employing glycosyl bromides, fluorides, trichloroacetimi-
dates, sulfoxides, and n-pentenyls, as well as thioglycosides
and glycals, all require activation by a Lewis acid (e.g.,
AgOTf, BF3‚Et2O, TMSOTf, or DMTST) for efficient
glycosylation of aliphatic alcohols. Waldmann and co-
workers have reported activation of glycosyl fluorides,
trichloroacetimidates, and phosphites using the very mild
† Technical University of Denmark.
‡ The Royal Veterinary and Agricultural University.
(1) (a) Varki, A. Glycobiology 1993, 3, 97-130. (b) Molecular Glyco-
biology; Fukuda, M., Hindsgaul, O., Eds.; IRL Press: Oxford, 1994. (c)
Yarema, K. J.; Bertozzi, C. R. Curr. Opin. Chem. Biol. 1998, 49-61.
(2) (a) Lerouge, P.; Roche, P.; Faucher, C.; Maillet, F.; Truchet, G.;
Prome´, J. C.; De´narie´, J. Nature 1990, 344, 781-784. (b) Lopez-Lara, I.
M.; van den Berg, J. D. J.; Thomas-Oates, J. E.; Glushka, J.; Lugtenberg,
B. J. J.; Spaink, H. P. Molecular Microbiol. 1995, 15, 627-638. (c) De´narie´,
J.; Debelle´, F.; Prome´, J.-C. Annu. ReV. Biochem. 1996, 65, 503-535.
(3) (a) Denyer, K.; Waite, D.; Motawia, S.; Møller, B. L.; Smith, A. M.
Biochem. J. 1999, 340, 183-197. (b) Damager, I.; Denyer, K.; Motawia,
M. S.; Møller, B. L.; Blennow, A. Eur. J. Biochem. 2001, 268, 4878-
4884. (c) Mori, H.; Bak-Jensen, K. S.; Gottschalk, T. E.; Motawia, M. S.;
Damager, I.; Møller, B. L.; Svensen, B. Eur. J. Biochem. 2001, 268, 6545-
6558. (d) Damager, I.; Olsen, C. E.; Blennow, A.; Dener, K.; Møller, B.
L.; Motawia, M. S. Carbohydr. Res. 2003, 338, 189-197.
10.1021/ol034242q CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/01/2003

Lewis acid LiClO4, which can exert a “special salt effect”.4
Lubineau et al. have described LiOTf as an alternative to
LiClO4 as a promoter in these glycosylation reactions.5
In three recent papers, we have described a new, efficient
method for glycosylation under strictly neutral, mildly basic,
or very mild acidic (LiClO4) conditions.6 In this glycosylation
technique, the anomeric leaving group on benzyl- or benzoyl-
protected donors is methyl 3,5-dinitrosalicylate (DISAL) or
its para regioisomer. The potential of DISAL glycosyl donors
was demonstrated in their successful application to solid-
phase oligosaccharide synthesis. The carboxyl moiety in
DISAL was furthermore utilized for linking two monosac-
charides together, followed by intramolecular glycosylation
in a 1,9-glycosyl shift to yield 1,4-linked disaccharides of
gluco- and mannopyranosides, otherwise accessible only with
considerable difficulty.
for example, activation of an “armed” Trt-protected thiogly-
coside with NIS/TfOH is compatible with Trt integrity.12 It
would thus be a test of the DISAL donor concept, whether
it would be compatible with the very versatile and highly
acid-labile Trt protecting group.
Benzyl-protected lactol 1 was reacted with 2-fluoro-3,5-
dinitrobenzoic acid (DISAL-F, 3) in the presence of 1,4-
dimethylpiperazine (DMP) and Li2CO3 in CH2Cl2. This
provided a (nonoptimized) 68% yield of the DISAL disac-
charide 4 with an R/â ratio of 1:3.3, a â-selectivity which
was lower than previously achieved for monosaccharides.6
Substituting DMP for DMAP gave a yield of 89% with an
R/â ratio of 2.4:1, in a faster reaction (Scheme 1). For the
Scheme 1. Donor Synthesis
Here, we report the first synthesis of disaccharide DISAL
glycosyl donors and their application for the synthesis of
longer oligosaccharides, i.e., a starch-related hexasaccharide.
The disaccharide precursor Bn4-R-Glcp-(1f4)-Bn3-D-
Glcp-OH (1) was readily available.7 The second disaccharide
precursor 6-O-Trt-Bn3-R-D-Glcp-(1f4)-Bn3-D-Glcp-OH (2)
was obtained from phenyl 2,3,6-tri-O-benzyl-4-O-(2,3-di-
O-benzyl-4,6-O-benzylidene-R-D-glucopyranosyl)-1-thio-â-
D-glucopyranoside8 via a regioselective reductive cleavage
of the benzylidene acetal function with a LiAlH4-AlCl3
reagent9 to give disaccharide acceptor 7. Conventional
tritylation of 7 followed by removal of the anomeric
phenylthio group with NBS/aq acetone10 gave the desired
precursor 2. The latter contained the very acid-labile Trt
protecting group, which would allow extension at O-6 after
the first glycosylation step (to form a tetrasaccharide).
However, it is well-documented that both primary and
secondary hydroxyls protected as their Trt ethers in some
cases can be glycosylated with concomitant Trt cleavage.11
Trityl protecting groups are labile to Lewis acids; for
example, TMSOTf can be used for trityl deprotection.
However, thioglycosides generally require soft Lewis acids
for their activation, and it has been demonstrated that the
Trt moiety can be stable to these glycosylation conditions;
transformation of Trt-protected lactol 2 into the correspond-
ing DISAL derivative 5, we thus relied on the DMAP
protocol, which after column chromatography of the reaction
mixture using EtOAc-toluene (1:15) on dry silica gel gave
an 89% yield with an R/â ratio of ∼3:1 (Scheme 1).
In previous DISAL studies, we established three sets of
standard conditions for glycosylation with DISAL donors
(1.5-3 equiv): (i) donor and acceptor in NMP at 40-60
°C; (ii) donor and acceptor in (CH2Cl)2, CH2Cl2, or CH3-
NO2 in the presence of LiClO4 (2.0-3.7 equiv), in some
cases also with Li2CO3 as an acid scavenger (2 equiv); (iii)
donor and acceptor in (CH2Cl)2 or CH2Cl2 in the presence
of LiClO4 and Bu4NI.
These operationally simple reactions were carried out in
plastic vials. The reactions were analyzed by HPLC, and the
(4) (a) Schmid, U.; Waldmann, H. Chem. Eur. J. 1998, 4, 494-501. (b)
Schene, H.; Waldmann, H. Chem. Commun. 1998, 2759-2760. (c) Schene,
H.; Waldmann, H. Eur. J. Org. Chem. 1998, 1227-1230. (d) Schene, H.;
Waldmann, H. Synthesis 1999, 1411-1422. Mukaiyama and co-workers
have reported the use of LiClO4 as an additive in TrtClO4 and TrtB(C5F5)4
promoted glycosylations: (e) Mukaiyama, T.; Kobayashi, S.; Shoda, S.
Chem. Lett. 1984, 907-910. (f) Uchiro, H.; Mukaiyama, T. Chem. Lett.
1996, 79-80.
Table 1. Model Glycosylations Using Cyclohexanol as Model
Acceptora
(5) Lubineau, A.; Drouillat, B. J. Carbohydr. Chem. 1997, 16, 1179-
1186.
entry
solvent
additives (equiv)
yield (R/â)b
1c
2
3
4
5
6
7
8
NMP
61% 6 (2.7:1)
d
(6) (a) Petersen, L.; Jensen, K. J. J. Org. Chem. 2001, 66, 6268-6275.
(b) Petersen, L.; Jensen, K. J. J. Chem. Soc., Perkin Trans. 1 2001, 2175-
2182. (c) Laursen, J. B.; Petersen, L.; Jensen, K. J. Org. Lett. 2001, 3,
687-690.
CH2Cl2
CH2Cl2
CH3CN
CH3NO2
Et2O
LiClO4 (2.5)
Bu4NI, LiClO4 (2.5)
Bu4NI, LiClO4 (2.5)
Bu4NI, LiClO4 (2.5)
Bu4NI, LiClO4 (2.5)
Bu4NI (2.5)
87% 6 (26.5:1)
66% 6 (9.4:1)
67% 6 (11.8:1)
40% 6 (8.9:1)
80% 6 (14.5:1)
d
(7) Motawia, M. S.; Olsen, C. E.; Møller, B. L.; Marcussen, J. Carbohydr.
Res. 1994, 252, 69-84.
(8) Motawia, M. S.; Larsen, K.; Olsen, C. E.; Møller, B. L. Synthesis
2000, 1547-1556.
CH3NO2
CH3NO2
(9) Liptak, A.; Imre, J.; Harangi, J.; Nanasi, P.; Neszmelyi, A. Tetra-
hedron 1982, 38, 3721-3727 and references therein.
(10) Motawia, M. S.; Marcussen, J.; Møller, B. L. J. Carbohydr. Chem.
1995, 14, 1279-1294.
Bu4NI (0.1)
a Reaction conditions were rt for 17 h, unless otherwise noted. b Yields
were determined by HPLC (area at 215 nm). c Reaction conditions were
40 °C for 4 h. d Incomplete conversion.
(11) (a) Tsvetkov, Y. E.; Kitov, P. I.; Backinowsky, L. V.; Kochetkov,
N. K. J. Carbohydr. Chem. 1996, 15, 1027-1050. (b) Demchenko, A. V.;
Boons, G.-J. J. Org. Chem. 2001, 66, 2547-2554.
1310
Org. Lett., Vol. 5, No. 8, 2003

yields in Table 1 were based on integration of relevant peaks
at 215 nm. The disaccharide donor proved efficient in these
glycosylations. The highest yield and R/â ratio of cyclohexyl
disaccharide 6 was obtained for glycosylations in CH2Cl2 in
the presence of LiClO4 and Bu4NI (Scheme 2, Table 1, entry
3).
Glycosylation of 7 with 6-O-Trt-protected donor 5 in CH3-
NO2 in the presence of LiClO4, Li2CO3, and Bu4NI gave
tetrasaccharide 9 with R selectivity in ∼63% isolated yield
after 5 days (Table 2, entry 6). A comparable yield but poorer
diastereoselectivity was obtained much faster at elevated
temperatures in (CH2Cl)2 with LiClO4 and Li2CO3 (Table 2,
entry 7). It was rewarding to see that the Trt protecting group
was completely stable to the very mild conditions used in
glycosylations with DISAL donors. Removal of the 6-O-Trt
protecting group in 9 was performed directly on the crude
product from the fast glycosylation reaction by applying mild
acidic conditions (AcOH/H2O/CH3CN, 90:7:3) at 60 °C. This
gave partially deprotected tetrasaccharide 10, ready for
further elongation, in 38% yield for the glycosylation and
deprotection (two steps).
Scheme 2. Model Glycosylations Using Cyclohexanol as
Acceptor
Detritylated tetrasaccharide 10 was glycosylated with
disaccharide donor 4 using the optimized conditions derived
from the synthesis of tetrasaccharides 8 and 9: LiClO4 and
Li2CO3 in (CH2Cl)2 (Scheme 4). As expected, the reactivity
Glycosylation of disaccharide acceptor 7 with DISAL
donor 4 to give the tetrasaccharide 8 proceeded under a
variety of conditions (Scheme 3, Table 2). Good yields and
Scheme 4. Hexasaccharide Synthesis
Scheme 3. 1,6-Glycosylations
the highest R/â ratio (pure R within our detection limit) was
obtained in the presence of LiClO4 and Bu4NI in CH2Cl2
(rt) or CH3NO2 (5 °C). At elevated temperatures (50 °C in
(CH2Cl)2), the glycosylation proceeded smoothly within 2 h
and with an acceptable R/â ratio of 3:1 (Table 2, entries 1-4
versus 5).
Table 2. Glycosylations of Benzylated and Tritylated Donors
Forming Tetrasaccharides
of tetrasaccharide glycosyl acceptor 10 was lower than for
disaccharide acceptor 4, and a larger excess of donor and
slightly elevated temperatures (35 °C) had to be applied.
Hexasaccharide 11 was isolated in 38% yield while 50%
of the acceptor was recovered. This glycosylation gave an
R/â ratio of 3:2. Higher temperatures (50 °C) led to fast
destruction of the glycosyl donor. However, the much faster
reaction at 50 °C gave an acceptable yield of 34% hexa-
saccharide 11. In addition, the long reaction time caused a
more complex reaction mixture by formation of more
byproducts.
entry donor
solvent
CH2Cl2
additives (equiv)
yielda (R/â)
1b
4
LiClO4, Li2CO3,
Bu4NI (2.5)
48% 8 (>20:1)
2b
3c
4d
5e
6d
4
4
4
4
5
CH2Cl2
LiClO4, Bu4NI (2.5) 51% 8 (>20:1)
CH3NO2 LiClO4, Bu4NI (2.5) 30% 8 (>20:1)
CH3NO2 LiClO4, Bu4NI (2.5) 60% 8 (>20:1)
(CH2Cl)2 LiClO4 (2.5)
CH3NO2 LiClO4, Li2CO3,
Bu4NI (4.0)
80% 8 (3:1)
63% 9 (>20:1)
7f
5
(CH2Cl)2 LiClO4, Li2CO3 (4.0) g (3:2)
In conclusion, novel disaccharide DISAL glycosyl donors
were prepared following procedures established for monosac-
a Isolated yields. b Reaction conditions were rt for 5 days. c Reaction
conditions were rt for 17 h. d Reaction conditions were 5 °C for 5 days.
e Reaction conditions were 50 °C for 2 h. f Reaction conditions were 40 °C
for 17 h. g The product was not isolated but taken on in the synthesis without
purification. Estimated yield of glycosylation by HPLC analysis: 60%.
(12) (a) Boons, G. J.; Bowers, S.; Coe, D. M. Tetrahedron Lett. 1997,
38, 3773-3776. (b) Alibe´s, R.; Bundle, D. R. J. Org. Chem. 1998, 63,
6288-6301.
Org. Lett., Vol. 5, No. 8, 2003
1311

charides. The â-selectivity in DMP-promoted reactions to
form DISAL glycosyl donors was lower than for monosac-
charides; DMAP-promoted reactions gave, as expected, the
DISAL donors with good R-selectivity. Two disaccharide
DISAL glycosyl donors proved efficient for the glycosylation
of model alcohol, cyclohexanol. Optimal glycosylations were
at room temperature in the presence of Bu4NI, and in some
cases LiClO4, in either CH2Cl2 or CH3NO2. These conditions
were transferred to the synthesis of tetrasaccharides and
a hexasaccharide. Glycosylations proceeded with good
R-selectivity; however, yields decreased with increasing size
of the glycosyl acceptor. This decrease in yield of the
intermolecular glycosylation has often been observed and
is probably due to the competing intramolecular elimination
to form the glycal. Glycosylations with DISAL donors were
compatible with the presence of thioglycosides and the very
acid-labile Trt protecting group. These glycosylations were
operationally simple and were carried out in standard plastic
vials; purification of preparative reactions was achieved by
preparative HPLC on a C4 reversed-phase column after the
reaction. However, further developments are required for this
new methodology to give yields comparable with the best
literature procedures for oligosaccharide synthesis. The
mildness and efficiency of this glycosylation protocol should
make it useful for glycosylation of a wide range of substrates,
including in oligosaccharide syntheses.
Acknowledgment. We thank the Technical University
of Denmark (L.P. and J.B.L.), The Danish National Research
Foundation (K.L. and M.S.M.), and the LEO foundation
(K.J.J.) for financial support of this work. We thank
Professors John Nielsen and Birger L. Møller for their
support of this work.
Supporting Information Available: General experimen-
tal procedures for synthesis of and analytical data for
compounds 4-6 and 8-11. NMR spectra of coumpounds
5, 8, and 11. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL034242Q
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Org. Lett., Vol. 5, No. 8, 2003