1,5-Lactamized sialyl acceptors for various disialoside syntheses: Novel method for the synthesis of glycan portions of Hp-s6 and HLG-2 gangliosides

Article abstract of DOI:10.1002/anie.200501608

(Chemical Equation Presented) A dramatic enhancement of the reactivity of the C4- and C8-hydroxy groups of sialic acid has been demonstrated by 1,5-lactam bridging. Sialyl-α(2→4)sialoside and sialyl-α(2→8) sialoside were made available in high yields thro

Full text of DOI:10.1002/anie.200501608

Angewandte
Chemie
Sialylation
DOI: 10.1002/anie.200501608
1,5-Lactamized Sialyl Acceptors for Various
Disialoside Syntheses: Novel Method for the
Synthesis of Glycan Portions of Hp-s6 and
HLG-2 Gangliosides**
Hiromune Ando,* Yusuke Koike, Sachiko Koizumi,
Hideharu Ishida, and Makoto Kiso*
The ongoing studies on oligosaccharide synthesis have
resulted in the development of precise synthetic methods by
which a large portion of the complex natural oligosaccharides
can be duplicated.[1] Although the synthesis of the sequence
Neu5Aca(2!8)Neu5Ac (a(2!8)disialic acid; Neu5Ac = N-
acetylneuraminic acid) has been a major difficulty, the
emergence of several exquisite methods[2] that employ
indirect coupling by using a C3-functionalized N-acetyl
sialyl donor and direct coupling by using an N-trifluoroacetyl
(TFAc)-protected sialic acid donor with the help of the nitrile
solvent effect have paved the way for the successful synthesis
of a(2!8)disialic acid containing oligosaccharides, such as
those with GD3[2c] and GQ1b[3] glycan portions. However, it is
obvious that the synthesis of new congeners of disialic acid,
such as 8-O-sulfo-Neu5Aca(2!8)Neu5Ac in ganglioside Hp-
s6[4] and Neu5Gca(2!4)Neu5Ac in ganglioside HLG-2[5]
(Scheme 1), is still difficult because of the diverse modifica-
tions possible at the functionality level. On the basis of the
predicted biological functions of the disialic acid congener
containing oligosaccharides relevant to functions such as
neural network formation and fertilization, the establishment
of an expedient synthetic method that includes the entire
disialic acid family seems essential not only for the progress of
glycochemistry but also for studying in detail the molecular
Scheme 1. Structures of novel disialyl gangliosides Hp-s6 and HLG-2.
basis underlying the biological functions of these compounds.
In this study, we report a novel synthetic method for the
synthesis of disialic acid congener containing glycans that uses
highly reactive lactamized sialyl acceptors and an N-2,2,2-
trichloroethoxycarbonyl (Troc)-protected sialyl donor.
Recently, we reported an N-Troc-protected sialyl donor
(N-Troc donor 1) that shows elevated reactivity and a high
degree of accessibility for various sialic acid congeners such as
N-glycolylneuraminic acid (Neu5Gc), 8-O-sulfo-Neu5Ac, and
1,5-lactam-Neu.[6] Initially, we anticipated that use of the N-
Troc donor would enable the design of HLG-2 and Hp-s6
glycan sequences in an expedient manner. However, as
depicted in Scheme 2, the results of the condensations with
4-OH and 8-OH sialyl acceptors, 2 and 3, respectively, did not
meet expectations with regard to yields and stereoselectivity.
Even in the case of 2, which showed the relatively higher
reactivity, a-disialyl glycoside was obtained in less than 5%.
We hypothesized that the poor results were mainly due to
unfavorable hydrogen bonding with the amide moiety at C5,
as proposed previously by Tsvetkov and Schmidt.[7] This
hypothesis was the basis of the idea that the conformational
transformation from the 2C5 chair form to the fixed boat form
with the 1,5-lactam bridge would result in increased reactivity
of both the C4- and C8-hydroxy groups.[8]
[*] Dr. H. Ando
Division of Instrumental Analysis
Life Science Research Center
Gifu University
1-1 Yanagido, Gifu-shi, Gifu 501-1193 (Japan)
Fax: (+81)58-293-2617
E-mail: hando@cc.gifu-u.ac.jp
To form the 1,5-lactam bridge in the sialoside, the
previously reported N-TFAc-sialic acid derivative 6[9] was
used as the key precursor (Scheme 3). After the coupling
reaction of 6 and tribenzylated glucosyl acceptor 7, the
resulting sialyl-a(2!6)Glc disaccharide, 8, was subjected to
1,5-lactamization. First, we attempted a carbodiimide-medi-
ated intramolecular amide formation after the complete
deacylation and saponification of 8, but this reaction yielded a
complex mixture. The optimum yield was obtained when 8
was treated with methanolic sodium methoxide in the
presence of Drierite under reflux to provide the 1,5-lactam-
sialyl glucoside 9 in 85% yield; through regioselective
benzoylation of the C9-hydroxy group of 9 with benzoyl
chloride and pyridine, under kinetic control, triol acceptor 10
was produced. For the synthesis of the 8-hydroxy-1,5-lacta-
Y. Koike, S. Koizumi, Dr. H. Ishida, Dr. M. Kiso
Department of Applied Bioorganic Chemistry
Faculty of Applied Biological Sciences
Gifu University
1-1 Yanagido, Gifu-shi, Gifu 501-1193 (Japan)
Fax: (+81)58-293-2918
E-mail: kiso@cc.gifu-u.ac.jp
[**] Synthetic Studies on Sialoglycoconjugates, Part 139. This work was
financially supported by CRESTof JST (M.K.), MEXTof Japan (Grant-
in-Aid for Scientific Research; no. 16780083 to H.A., no. 16580086
to H.I., and no. 17101007 to M.K.), and the Mitsubishi Chemical
Corporation Fund (H.A.). We thank Ms. Kiyoko Ito for technical
assistance. For Part 138, see: M. Yamaguchi, H. Ishida, A.
Kanamori, R. Kannagi, M. Kiso, Glycoconjugate J. 2005, 22, 83–96.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Next, we carried out the glycosylation of the
lactam acceptors 10 and 15 with N-Troc- and N-
TFAc-sialyl donors to evaluate their properties as
glycosyl acceptors (Scheme 4). First, the triol
acceptor 10 was treated with N-Troc donor 1 in the
presence of NIS, TfOH, and a molecular sieve in
EtCN[11] at ꢀ408C to provide the Neua(2!4)Neua-
(2!6)Glc sequence 17, along with the correspond-
ing b isomer. The anomeric configuration of the new
ketosidic linkage was determined on the basis of
previous reports[12] by measuring the long-range
3JC1,H3ax coupling constants. For compound 17 this
coupling constant was 5.4 Hz, whereas for the
b isomer it was less than 1.0 Hz, a fact indicating
that the amomeric configuration of 17 was a.
Similarly, the coupling reaction with the N-TFAc
donor 6 and the complete acetylation that followed
yielded the corresponding Neua(2!4)Neua(2!
6)Glc sequence 17 in 41% yield, along with the
b isomer (10%) and the Neu(2!8){[Neu(2!4)]
Neua}(2!6)Glc sequences as an anomeric mixture
(8%).
Scheme 2. Unsuccessful sialylation to 4-OH and 8-OH sialyl acceptors. Bn=benzyl,
MS=molecular sieves, NIS=N-iodosuccinimide, SE=2-(trimethylsilyl)ethyl, TfOH=tri-
fluoromethanesulfonic acid.
Next, we attempted to fashion the purest form of
mized sialyl acceptor, 8,9-O-benzylidenation was required
prior to the lactamization because 8,9-O-acetalization of the
lactamized derivative was unsuccessful. Thus, compound 6
was de-O-acetylated and this was followed by the conven-
tional 8,9-O-bezylidenation with benzaldehyde dimethyl
acetal and camphorsulfonic acid to produce 11, which was
then subjected to the one-pot 1,5-lactam formation men-
tioned earlier to yield bicyclo-sialoside 12 in 89% yield. Next,
12 was completely acetylated with Ac2O in the presence of
pyridine and the product was successively de-N-acetylated
with hydrazinium acetate in a chemoselective manner to
produce compound 14. Finally, reductive ring opening of the
benzylidene acetal moiety, influenced by BH3·NMe3 and
AlCl3 in THF,[10] produced the 8-OH lactam acceptor 15.
Neua(2!8)Neu sequence (Scheme 4). As initially expected,
the glycosylation reactions of the lactam acceptor 15 with N-
Troc and N-TFAc donors (1 and 6) yielded the corresponding
Neua(2!8)Neu sequences. Thus, N-Troc donor 1 and N-
TFAc donor 6 were incorporated, in the presence of NIS,
TfOH, and a molecular sieve in EtCN, at ꢀ808C to yield
a(2!8)disialosides 18 and 19 in 49 and 71% yield, respec-
tively; no corresponding b form was generated in either
event. To the best of our knowledge, the yield of addition to
the C8-hydroxy group of sialic acid (71%) during the
sialylation process was the highest value obtained by direct
coupling methods[2] In keeping with the results of the previous
experiments, the anomeric configuration of the new linkages
was determined to be a from 3JC1,H3ax coupling constants that
Scheme 3. a) 7, NIS, TfOH, CH3CN/CH2Cl2, MS (3Š), ꢀ308C, 5 min, 74%; b) NaOMe, MeOH, Drierite, reflux, 44 h, 85%; c) BzCl, py/CH2Cl2,
ꢀ408C, 90 min, 79%; d) NaOMe, MeOH, room temperature, 29 h; e) PhC(OMe)2, CSA, DMF, 408C, 2 h, 88% (2 steps); f) NaOMe, MeOH,
Drierite, reflux, 5 d, 89%; g) Ac2O, py, DMAP, room temperature, 3 h; h) NH2NH2·AcOH, THF, room temperature, 80 min, 94% (2 steps);
i) BH3·NMe3, AlCl3, THF, MS (4Š), 08C!RT, 6 h, 74%. Bz=benzoyl, CSA=(ꢁ)-10-camphorsulfonic acid, DMF=N,N-dimethylformamide,
DMAP=4-dimethylaminopyridine, py=pyridine, THF=tetrahydrofuran.
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Scheme 4. a) 1 (2.0 equiv), NIS (3.0 equiv), TfOH (0.3 equiv), EtCN, MS (3Š),
ꢀ408C, 6 h, 84% (a/b 66:18); b) 1. 6 (2.0 equiv), NIS, TfOH, EtCN, MS (3Š),
ꢀ408C, 6 h; 2. Ac2O, py, DMAP, 408C, 17 h, 51%; c) 1 (3.0 equiv), NIS, TfOH,
EtCN, MS (3Š), ꢀ808C, 5 h, 49% (a only); d) 6 (3.0 equiv), NIS, TfOH, EtCN,
MS (3Š), ꢀ808C, 3 h, 71% (a only).
ranged from 6.7 to 6.9 Hz. Furthermore, the phenylsulfenyl
group at the bridgehead anomeric center of acceptor 15
remained unaffected during the coupling reactions. This result
confirmed our initial hypothesis, based on Bredtꢀs rule,
suggesting the basis of a novel method for the complete
deactivation of a sialyl donor.
On the basis of the results obtained with regard to the
performance of 1,5-lactamized sialic acid acceptors 10 and 15
in the sialylation reactions, we focused on the synthesis of the
glycan portions of HLG-2 and Hp-s6 gangliosides of 16 and
18, respectively, in order to demonstrate the practical efficacy
of the synergic strategy for synthesizing variant disialosides
from the 1,5-lactam-sialyl acceptor and N-Troc-sialyl donor.
In the initial stages of the synthesis of the HLG-2 glycan
portion (Scheme 5), the trisaccharide 16 was O-acetylated to
provide 20, to which the N-glycolyl moiety was introduced by
our reported method,[6] thereby providing 21 in a relatively
high yield (66% from 16). Next, we attempted to recover the
2C5 conformation of the inner sialic acid unit. The following
reaction sequences supplied HLG-2 glycan frame 23 in a high
yield: N-benzyloxycarbonylation, basic hydrolysis, and ensu-
ing methylation of the carboxy group. Debenzylation and
acetylation to replaced the Cbz group of 23 by the acetyl
group and full deprotection of the product 24 yielded the
HLG-2 glycan structure 25.
In the case of the Hp-s6 glycan frame, the “locked-up”
phenylsulfenyl group at the bridgehead carbon atom of 18 was
converted into an active state in the initial stages (Scheme 6).
To be precise, the reaction sequences mentioned earlier
yielded 2C5 conformer 27 in 62% overall yield. For the
purpose of 8-O-sulfonylation in the final stages of the
synthesis, 27 was further transformed into the 8-hydroxy
derivative 28 by our regioselective acetyl-transfer method,[6]
and the C8 hydroxy group was capped with a levulinoyl group
Scheme 5. a) Ac2O, py, room temperature, 10 h, 89%; b) 1. Zn, AcOH,
room temperature, 2 h; 2. BnOAcCl, THF, room temperature, 1 h, 74%
(2 steps); c) Cbz2O, DMAP, py, 408C, 26 h, 95%; d) 1. Et3N, H2O/
CH3CN, room temperature, 2 d; 2. MeI, K2CO3, DMF, room temper-
ature, 30 min, 74% (2 steps); e) 1. H2, 10% Pd(OH)2/C, NH3, EtOH,
room temperature, 2 h; 2. AcCl, room temperature, 1 h, 68%
(2 steps); f) 1. H2, 10% Pd(OH)2/C, EtOH; 2. Ac2O, py, room temper-
ature, 54% (2 steps). Cbz=benzyloxycarbonyl.
to produce high yields (89%) of the suitably protected disialic
acid donor 29 in two steps. Compound 29 was then treated
with glucosyl acceptor 7, influenced by the NIS/TfOH
activator system in EtCN at ꢀ80!ꢀ608C, to provide Neua-
(2!8)Neua(2!6)Glc sequence 30 in 66% yield, predom-
inantly in the a configuration. Next, replacement of the Cbz
and benzyl groups of trisaccharide 30 by the acetyl group,
followed by chemoselective deblocking of the levulinoyl
group with hydrazinium acetate[13] and sulfonylation with
SO3·pyridine resulted in the formation of a completely
1
protected Hp-s6 glycan frame, 33.[14] The H NMR signal for
the C8 proton of the outer sialic acid appeared in compound
33 at lower magnetic field (d = 4.92 ppm) than in compound
32 (d = 4.22 ppm), and the heteronuclear multiple-bond
coherence (HMBC) spectrum of compound 33 contained
cross-coupling signals between carbonyl carbon atoms of
acetyl groups at C7 and C9, and H7 (d = 5.40 ppm) and H9
Angew. Chem. Int. Ed. 2005, 44, 6759 –6763
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Communications
based on the N-Troc donor and the lactamized acceptors as
the main units, has been demonstrated by the novel method
for the synthesis of the HLG-2 and Hp-s6 glycan chains. On
the basis of these results, we are now investigating the
synthesis of a(2!8)-linked oligosialic acids.
Received: May 11, 2005
Revised: July 13, 2005
Published online: September 27, 2005
Keywords: gangliosides · glycosylation · lactams · sialic acids
.
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ature, 28 h, 94%; d) LevOH, DCC, DMAP, CH2Cl2, room temperature,
2 h, 95%; e) 7, NIS, TfOH, EtCN, MS (3Š), ꢀ80!ꢀ608C, 4 d, 66%;
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In conclusion, we have discovered that 1,5-lactam bridg-
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hydroxy groups, thereby leading to the supply of a(2!4)- and
a(2!8)disialic acid sequences in high yields. Furthermore,
the practical efficacy of the synergic synthetic approach
toward diverse disialic acid containing oligosaccharides,
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