Extending the possibility of an N-Troc-protected sialic acid donor toward variant sialo-glycoside synthesis

Article abstract of DOI:10.1016/S0040-4039(03)01707-6

The potential of an N-Troc-protected sialic acid donor, equipped with phenylsulfenyl functionality as a leaving group, has been explored. As a result, the entitled donor was proven to be highly reactive and to have broad applicability toward the synthesis of variant sialo-glycans, which have N-glycolyl, de-N-acetyl, 1,5-lactam and 8-O-sulfo sialic acid analogs.

Full text of DOI:10.1016/S0040-4039(03)01707-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6883–6886
Extending the possibility of an N-Troc-protected sialic acid
donor toward variant sialo-glycoside synthesisꢀ
Hiromune Ando,a,* Yusuke Koike,a Hideharu Ishidaa,b and Makoto Kisoa,b,
*
aDepartment of Applied Bioorganic Chemistry, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
bCREST, Japan Science and Technology Corporation (JST), 1-1 Yanagido, Gifu 501-1193, Japan
Received 6 June 2003; revised 9 July 2003; accepted 10 July 2003
Abstract—The potential of an N-Troc-protected sialic acid donor, equipped with phenylsulfenyl functionality as a leaving group,
has been explored. As a result, the entitled donor was proven to be highly reactive and to have broad applicability toward the
synthesis of variant sialo-glycans, which have N-glycolyl, de-N-acetyl, 1,5-lactam and 8-O-sulfo sialic acid analogs.
© 2003 Elsevier Ltd. All rights reserved.
Efficient chemical synthesis of fine sialic acid-containing
glycoconjugates is a central subject in carbohydrate
science, due to their diverse biological roles1 relevant to
cell–cell interaction including those such as pathogen–
host recognition, tumor metastasis, toxin–receptor
interaction, cell differentiation and proliferation, malig-
nant alteration, neural network formation and so on.
The focal point of sialo-glycoside synthesis is the con-
struction of stereoelectronically disfavored a-glycoside
adjacent to its deoxy structure. Efforts to solve this
problematic issue over a few decades gave rise to sev-
eral practical methodologies for the construction of
a-sialosides, including our approach2 that employs a
combination of an alkylthioglycoside donor of sialic
acid with a nitrile solvent effect, by which numerous
sialo-oligosaccharides were successfully synthesized so
far.3 However, in respect of the diversity of sialic acids
found in nature, such as N-glycolyl, de-N-acetyl, 1,5-
lactam, O-acetyl and O-sulfo analogues, which also
correlate to the polymorphous functions of sialo-glyco-
conjugates,4 most of these approaches that built up
N-acetyl sialic acid-containing molecules are not impec-
cable. In this context, a single expedient synthetic
approach to oligosaccharides having various analogs of
sialic acids is essential for scrutiny of the biological
significance of the entire sialo-glycoconjugates.5 Here
we report a novel versatile synthetic approach toward
glycans having N-glycolyl, de-N-acetyl, 1,5-lactam, 8-
O-sulfo sialic acid analogs.
We envisioned that an N-2,2,2-trichloroethoxycarbonyl
(Troc) protected-sialic acid donor would be feasible for
the pivotal approach to variant sialosides, considering
the acid-resistant nature and selective cleavability under
mild condition of the Troc group. Conversion of the
reported a-phenylthioglycoside of N-acetyl sialic acid
derivative (1)6 into the corresponding N-Troc derivative
(2) was easily accomplished by following Higuchi’s
one-pot methodology5e (68% in three steps) (Scheme 1).
In a comparison between the 1H NMR spectra of
compound 2 and its b-isomer,† it was observed that the
chemical shift of H-3eq of the a-isomer (l 2.88) was at
Scheme 1. Preparation of a-phenylthioglycoside of N-Troc
sialic acid 2. Reagents and conditions: (a) MsOH/MeOH,
reflux, 24 h; (b) TrocCl, Et3N/MeOH, 0°C“rt 30 min; (c)
Ac2O, pyr./rt, 16 h.
Keywords: N-Troc group; sialidation; sialic acid analogs.
ꢀ Part 134 of the series: Synthetic studies on sialoglycoconjugates.
For part 133, see: Hara-Yokoyama, M.; Ito, H.; Ueno-Noto, K.;
Takano, K; Ishida, H.; Kiso, M. Bioorg. Med. Chem. Lett.,
accepted.
† The corresponding b-isomer of 2 has been first reported by Wu and
co-workers, which was utilized for Neu5Ac-a(2“5)-Neu5Gc link-
age, but they did not refer to the availability of N-Troc donor for
the synthesis of diverse sialyl oligosaccharides. Ref. 3g.
* Corresponding authors. Tel./fax: +81-58-293-2918; e-mail:
hando@cc.gifu-u.ac.jp; kiso@cc.gifu-u.ac.jp
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01707-6

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H. Ando et al. / Tetrahedron Letters 44 (2003) 6883–6886
lower field than that of the b-isomer (l 2.73), and H-4
of the a-isomer (l 4.98) was shifted higher than that of
the b-isomer (l 5.45), and the coupling constant of J7,8
of the a-isomer (8.0 Hz) was bigger than that of the
b-isomer (2.5 Hz), which were in accordance with the
empirical rule to define the anomeric configuration of
N-acetyl sialic acid glycosides.7
Donor 2, in hand, was then subjected to coupling
reaction with a range of acceptors in order to evaluate
its efficacy as a glycosyl donor. All condensations were
promoted by the N-iodosuccinimide–acid8 system with
the assistance of the nitrile solvent effect,2 and stereo-
chemistry of the newly-formed glycosides was assigned
according to the empirical rule and, for some com-
pounds, further confirmed by the reported method
using HMBC technique.3f In entries 1, 2, 3 and 4, it was
noted that donor 2 was rapidly glycosidated to selec-
tively give a-anomeric outcomes in relatively high
yields, whereas condensation of the conventional N-
acetyl donor 1 with diol galactose acceptor 6 was
sluggish and afforded a-sialoside 12 in poor yield.
Encouraged by these interesting results, we next exam-
ined the reactivity of the N-Troc donor 2 by competi-
tive coupling reaction. As competitors, the
corresponding N,N-diacetyl and N-trifluoroacetyl
(TFAc) sialic acid donors (3 and 4) were selected based
on Boon’s reports,3e,9 and then coupled with acceptor 5,
respectively. Coupling reaction of both donors was as
rapid as that of donor 2 to give sialosides in high yield,
but with unexpected low a-selectivity in the case of
N,N-diacetyl donor 3 (entries 5 and 6). Interestingly,
the competitive coupling of donors 2, 3 and 4 with
acceptor 5 yielded 9 (N-Troc) in 40% yield, 13 (N,N-
diacetyl) in 22% yield and 14 (N-TFAc) in 15% yield,
respectively, suggesting the hierarchy of reactivity
among these sialyl donors is N-Troc>N,N-diacetyl>N-
TFAc (entry 7). Thereby, in entry 8, armed–disarmed
like coupling reaction10 could be performed between the
potent 2 and less reactive phenylthioglycoside of N-ace-
tyl sialic acid derivative 8 to furnish sialyl-(2“9)sialic
acid dimer 15 in 43% (a/b=24/19). In most of the
events, a/b ratios of the sialylated products were
around 4/1 and independent of the anomeric configura-
tion of donor 2 (data not shown). Isolation of a-prod-
ucts from concomitantly produced b-isomer by silica
gel column chromatography, which is usually an ardu-
ous procedure, was readily accomplished in all cases
because of bigger DRf (ca. 0.2) between the a- and
b-isomers, compensating for the medium a-selectivity.
Thus, the practical utility of compound 2 as a sialyl
donor has been demonstrated (Figs. 1 and 2 and Table
1).
Figure 1. Donors and acceptors used in this study. Arrows
point at the glycosylation site.
Figure 2. Sialylated products.
also transformed into 1,5-lacatamized bicyclo form
under basic conditions in refluxing methanol, which
was isolated as acetylated form (19) in moderate yield.
The precursor 19 was confirmed to be deacetylated in
almost quantitative yield via two-step manipulation:
N-Acetyl deblocking by NH2NH2·AcOH in THF and
de-O-acetylation under the Zemple´n condition. In addi-
tion, it was found that the selective cleavage of Troc
protection effected by zinc in 10% acetic acid solution
in 1,4-dioxane enhanced regioselective migration of the
acetyl group to the generating free amino group. This
treatment of compound 16 led to producing the 8-
hydroxyl-N-acetyl derivative (20) in high yield, which
was subjected to 8-O-sulfation and levulinoylation11 to
give the corresponding 8-O-sulfo and 8-O-levulinoyl
derivative in high yields, 21 and 22, respectively.
Removal of the levulinoyl group of 22 effected by
NH2NH2·AcOH11 was also highly efficient, suggesting
that capping of 8-hydroxyl with levulinoyl group will
work well for the 8-O-sulfated-sialyl oligosaccharide
synthesis (Scheme 2).
As expected, the selective deblocking of the Troc group
of sialyl galactoside (16) with zinc in acetic acid pro-
ceeded smoothly to give a free amino derivative, which,
on successive treatment with acylating reagents, ace-
toxyacetyl chloride and trifluoroacetic anhydride,
afforded the corresponding N-acetylglycolyl derivative
(17) as N-glycolyl sialoside precursor5d and N-TFAc
derivative (18) as 5-amino sialoside precursor5h in high
yields, respectively. The amino intermediate could be

H. Ando et al. / Tetrahedron Letters 44 (2003) 6883–6886
Table 1. Coupling reaction of sialyl donors with various acceptorsa
6885
Entry
Donor
Acceptor
Promoterb
Temp. (°C)
Time
Product
Yield (a/b)(%)c
1
2
3
4
5
6
7f
2
2
2
1
3
4
5
6
7
6
5
5
5
A
B
A
B
A
A
A
−30
−50
−30
−25
−30
−30
−30
10 min
20 min
5 min
13 h
10 min
10 min
16 h
9
76/19
10
11
12
13
14
9
13
14
15
54/<10d
35/<8d
23 (a)
42/42e
74/16e
32/8e
2, 3, 4
(1.0 equiv. each)
11/11e
12/3e
8
2
8
A
−40
3 h
24/19
a All reactions were conducted in CH3CN–CH2Cl2(5/1) except entry 2 (EtCN), and the mole ratio of donor to acceptor was 1.2 to 1 unless
otherwise specified.
b (A) NIS (1.5 equiv. of donor) and TESOTf (0.15 equiv. of donor); (B) NIS (1.5 equiv. of donor) and TfOH (0.15 equiv. of donor).
c Isolated yield.
d Including inseparable impurity.
e Ratio was calculated from relative signal intensity in 1H NMR spectra.
f Competitive coupling reaction. Yields are mean value of three trials.
Scheme 2. Conversion of 16 into variant sialosides. Reagents and conditions: (a) Zn/AcOH, rt, 1ꢀ2 h; (b) acetoxyacetyl chloride,
Et3N/THF, rt, 5 min; (c) TFAc2O, Et3N/THF, rt, 5 min; (d) NaOMe/MeOH, Drierite®, reflux, 2 days; (e) Ac2O, pyr., DMAP/rt;
(f) Zn/10% AcOH in 1,4-dioxane, rt, 24 h; (g) SO3·pyr./pyr., rt, 1 h; (h) levulinic acid, DCC, DMAP/CH2Cl2, rt, 17 h; (i)
NH2NH2·AcOH/ EtOH, rt, 4 h.
In conclusion, we have discovered the high potential
of N-Troc protected sialyl donor 2, and broadened its
utility toward variant sialosides having N-glycolyl, de-
N-acetyl, 1,5-lactam, and 8-O-sulfo sialic acid deriva-
tives.12 The reason underlying the enhanced reactivity
of 2 by Troc group cannot be rationalized yet. Now
at least, the possibility that an electron-withdrawing
property rules the sialyl donor reactivity was elimi-
nated by the coupling of the corresponding sialyl
under similar conditions as mentioned above that
took around 2 h to completion. This suggests that the
reasons for the enhancement are plural. Since the syn-
thesized sialyl-a(2“3)galactosides, 17, 18, 19, and 22,
can be converted into the corresponding glycosyl
donor blocks by the established protocol, this strategy
will be integrated with various convergent approaches
toward oligosaccharides.
donor having
a potent electron-withdrawal dini-
trobenzenesulfonyl group‡ and acceptors 6 and 7
‡ For 2,4-dinitrobenzenesulfonyl group for amine protection: see:
Fukuyama, T.; Cheung, M.; Jow, C.-K.; Hidai, Y.; Kan, T. Tetra-
hedron Lett. 1997, 38, 5831.

6886
H. Ando et al. / Tetrahedron Letters 44 (2003) 6883–6886
Scherman, A. A.; Yudina, O. N.; Shashkov, A. S.; Men-
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This work was partly supported by the Ministry of
Education, Culture, Sports, Science, and Technology
(MEXT) of Japan (Grant-in-Aid for Scientific Research
to M.K., No. 12306007 and Grant-in-Aid for JSPS
Fellows to H.A.) and CREST of JST (Japan Science
and Technology Corporation.). We thank Ms. Kiyoko
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12. NMR data of selected compounds: For compound a-
1
form of 9: H NMR (500 MHz, CDCl3) l 1.42–2.23 (m,
14H, Adamantane unit), 2.01, 2.04, 2.14 and 2.14 (4s,
12H, 4Ac), 2.70 (dd, 1H, Jgem=12.5, J3,4=4.5 Hz, H-
3eqNeu), 3.65 (q, 1H, J5,NH=J4,5=J5,6=10.3 Hz, H-5Neu),
3.76 (s, 3H, COOMe), 4.09 (dd, 1H, Jgem=10.7, J8,9a=1.6
Hz, H-9aNeu), 4.13 (dd, 1H, J6,7=1.6 Hz, H-6Neu), 4.27
(dd, 1H, Jgem=10.7, J8,9b=2.0 Hz, H-9bNeu) 4.47 and
4.89 (2 d, 2H, Cl3CCH2O), 4.93 (m, 1H, H-4Neu), 5.32 (m,
1H, H-8Neu), 5.37 (dd, 1H, J7,8=8.4 Hz, H-7Neu); 13C
NMR (100 MHz, CDCl3) l 20.79, 20.84, 20.88, 21.10,
26.95, 27.28, 31.41, 31.55, 33.04, 34.33, 36.35, 36.71,
37.50, 38.61, 51.64, 52.45, 62.20, 67.57, 68.64, 68.95,
71.82, 74.48, 76.75, 98.52, 154.16, 168.81, 169.91, 170.15,
170.47, 170.72. For compound b-form of 9: 1H NMR
(500 MHz, CDCl3) l 1.45–2.11 (m, 14H, Adamantane
unit), 2.01, 2.04, 2.06 and 2.14 (4s, 12H, 4Ac), 2.65 (dd,
1H, Jgem=13.0, J3,4=4.8 Hz, H-3eqNeu), 3.71 (q, 1H,
J
5,NH=J4,5=J5,6=10.3 Hz, H-5Neu), 3.75 (s, 3H,
COOMe), 4.09 (dd, 1H, J6,7=2.3 Hz, H-6Neu), 4.09 (dd,
1H, Jgem=12.3, J8,9a=4.3 Hz, H-9aNeu), 4.46 (d, 1H,
Cl3CCH2O), 4.89 (m, 2H, H-9bNeu and Cl3CCH2O), 5.12
(m, 1H, H-8Neu), 5.34 (m, 1H, H-4Neu), 5.37 (dd, 1H,
J
7,8=2.7 Hz, H-7Neu); 13C NMR (100 MHz, CDCl3) l
20.79, 20.81, 20.95, 21.09, 26.88, 27.07, 31.28, 31.64,
32.27, 34.29, 36.45, 36.72, 37.24, 38.14, 51.93, 52.53,
62.73, 68.77 69.18, 72.04, 72.89, 74.53, 76.58, 97.52,
154.24, 168.15, 170.15, 170.49, 170.61, 170.80.