Formation of orthoester-linked d-arabinofuranose oligosaccharides and their isomerization into the corresponding glycosides

Article abstract of DOI:10.1016/j.tetlet.2011.02.019

Orthoester-linked d-arabinofuranose oligosaccharides were isolated in high yield for the first time via glycosylations with 2-O-chloroacetyl-substituded d-arabinofuranose thioglycosides promoted by NIS-AgOTf in the presence of 4 molecular sieves in CH

Full text of DOI:10.1016/j.tetlet.2011.02.019

Tetrahedron Letters 52 (2011) 1794–1796
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Formation of orthoester-linked D-arabinofuranose oligosaccharides
and their isomerization into the corresponding glycosides
Polina I. Abronina a, Sergey L. Sedinkin a, Nikita M. Podvalnyy a, Ksenia G. Fedina a,b, Alexander I. Zinin a,
Vladimir I. Torgov a, Leonid O. Kononov a,
⇑
a N.D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences, Leninsky Prosp., 47, Moscow 119991, Russian Federation
b The Higher Chemical College of the Russian Academy of Sciences, Miyuskaya pl. 9, Moscow 125047, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Orthoester-linked
D-arabinofuranose oligosaccharides were isolated in high yield for the first time via
Received 3 December 2010
Revised 21 January 2011
Accepted 4 February 2011
Available online 25 February 2011
glycosylations with 2-O-chloroacetyl-substituded
D
-arabinofuranose thioglycosides promoted by NIS–
AgOTf in the presence of 4 Å molecular sieves in CH2Cl2. These orthoesters can be rearranged into the iso-
meric glycosidically-linked oligosaccharides by treatment with TMSOTf in CH2Cl2, or in situ by extension
of the reaction time.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Glycosylation
Arabinofuranose
Orthoester
Glycoside
Isomerization
There has been growing interest in the construction of
D
-arabin-
The presence of a chloroethyl aglycon in the derived oligosaccha-
rides would enable the preparation of neoglycoconjugates.5
The obvious way to prepare oligosaccharides 6 and 7 would
involve bis-glycosylation of the corresponding mono- and
disaccharide 3,5-diols 26 and 37 with ethyl (or phenyl) 3,5-di-O-
ofuranose oligosaccharides related to the mycobacterial cell wall
components, arabinogalactan, lipoarabinomannan, and arabino-
mannan from Mycobacterium tuberculosis, over recent years1 due
to the necessity of creating means for diagnostics, prevention,
and treatment of human tuberculosis,2 a major worldwide health
problem. Although various strategies have been reported for the
preparation of arabinofuranose oligosaccharides,1 the develop-
ment of novel approaches to stereoselective oligosaccharide syn-
thesis remains an important task.
benzoyl-2-O-chloroacetyl-1-thio-a-D
-arabinofuranosides (1a,b)7
(Scheme 1). Similar thioglycoside glycosyl donors with a 2-O-chlo-
roacetyl participating protecting group, such as tolyl (or ethyl) 3,
5-di-O-benzyl-2-O-chloroacetyl-1-thio-a-D
-arabinofuranosides,1f,k
have been successfully used for the preparation of
a-linked-
We have attempted to develop a synthesis of oligosaccharide
fragments of mycobacterial arabinans by making use of a strategy
that has never been used for the assembly of oligoarabinofurano-
sides and relies on the extensive use of the combination of ‘perma-
nent’ O-benzoyl and ‘temporary’ O-chloroacetyl protecting groups.
During this work, we needed tri- and tetrasaccharides 6 and 7,
selectively protected with chloroacetyl (CA) groups at O-2 of the
terminal arabinofuranose residues (Scheme 1). After selective
cleavage of the O-chloroacetyl groups, the corresponding diols
could be used as glycosyl acceptors for the subsequent introduc-
tion of two b-(1?2)-arabinofuranosyl linkages to give access to a
hexasaccharide epitope3,4 (and its pentasaccharide fragment)
found at the non-reducing end of these mycobacterial arabinans.
arabinofuranose oligosaccharides.
However, we were surprised to find that coupling of disaccha-
ride diol 3 with thioglycoside 1a under commonly used glycosyla-
tion conditions [CH2Cl2, 4 Å molecular sieves (MS), NIS–AgOTf
(added at ꢀ40 °C),ꢀ20 °C to +10 °C (over 1 h)]1a resulted in clean
formation of tetrasaccharide bis-orthoester 58,9 as a single isomer,
isolated by silica gel chromatography in 90% yield, rather than the
expected tetrasaccharide 7. Prolonged treatment at ꢀ20 °C to
+10 °C (up to 2 h) resulted in the formation of bis-orthoester 5 as
a mixture of isomers at the two orthoester quaternary centers,
the exact ratio being dependent on the reaction time. Similarly,
the reaction of monosaccharide diol 2 with thioglycoside 1b at
ꢀ20 °C to +10 °C (over 2 h) gave only the trisaccharide bis-orthoes-
ter 48,9 as a mixture of isomers in 57% yield.
Identification of sugar-sugar orthoesters 4 and 5 was carried out
by a combination of chemical and spectrometric methods. The
orthoesters were easily detected on TLC by their ready decomposi-
tion under acidic conditions [treatment with 90% TFA (aq)–CHCl3
⇑
Corresponding author. Tel.: +7 495 943 2946, fax: +7 499 135 5328.
E-mail addresses: kononov@ioc.ac.ru, leonid.kononov@gmail.com (L.O. Kono-
nov).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.019

P. I. Abronina et al. / Tetrahedron Letters 52 (2011) 1794–1796
1795
our knowledge. Moreover, the orthoesters 4 and 5 are the first
examples of oligosaccharides with an orthochloroacetyl bridge be-
tween the furanose residues. Similar derivatives of pyranoses with
orthochloroacetyl bridges have been described.11
It is important to stress that no base (except for molecular
sieves) was added to the reaction mixture in our case. Formation
of intersaccharidic orthoesters is highly unusual considering the
high anomeric reactivity of arabinofuranose derivatives.1 We be-
lieve it is the presence of both 2-O-chloroacetyl and O-benzoyl
electron-withdrawing groups that makes the orthoesters 4 and 5
so stable that their isolation by silica gel column chromatography
becomes possible.
Oligosaccharides with intersaccharidic orthoacetyl10 or ortho-
chloroacetyl11a,11b linkages between the pyranose residues of var-
ious sugars are known to rearrange under acid catalysis into the
corresponding isomeric glycosidically-linked oligosaccharides with
a 2-O-acetyl or 2-O-chloroacetyl group, respectively (however, this
is not always possible to perform10,11c,d). We were pleased to find
that application of a similar approach [CH2Cl2, 4 Å MS, TMSOTf
(addition at ꢀ40 °C), +20 °C, 2 h]13 to arabinofuranose orthoesters
4 and 5 afforded the target glycosidically-linked tri- and tetrasac-
charides 6 and 7 in good yields (71% and 80%, respectively). This
process can be monitored conveniently by 13C NMR spectroscopy
since the resonances of the chloromethyl groups in orthoesters 4
and 5 (dC ꢁ45), and in glycosides 6 and 7 with 2-O-chloroacetyl
groups (dC ꢁ40) are clearly distinguishable from each other and
from the signals of the CH2Cl groups in the aglycon (dC ꢁ42). It is
important that isomerization proceeded stereoselectively10a and
no cleavage or anomerization of the acid-sensitive arabinofuranose
glycosidic bonds in tri- and tetrasaccharides 6 and 7 occurred un-
der the conditions used.
To simplify the glycosylation process and enable large scale
preparation, we developed one-pot conditions in which the ini-
tially formed orthoesters would rearrange in situ to the required
glycosides. It is known,10 that orthoesters can be transformed into
the corresponding glycosides in situ by the addition of an extra
catalyst or by extension of the reaction time. In our case, the best
results were obtained by gradually increasing the reaction temper-
ature from ꢀ17 °C to +4 °C over 4 h.14 Under these conditions the
yields of glycosidically-linked tri- and tetrasaccharides 6 and 7
were 97% and 84%, respectively.
Scheme 1. Reagents and conditions: (a) 1a + 3: CH2Cl2, 4 Å MS, NIS–AgOTf (added
at ꢀ40 °C), ꢀ20 °C to +10 °C (over 1 h) (90% of 5); 1b + 2: CH2Cl2, 4 Å MS, NIS–AgOTf
(added at ꢀ40 °C), ꢀ20 °C to +10 °C (over 2 h) (57% of 4); (b) 4 or 5, CH2Cl2, 4 Å MS,
TMSOTf (added at ꢀ40 °C), 20 °C, 2 h (71% of 6, 80% of 7); (c) 1b + 3, 1b + 2: CH2Cl2,
4 Å MS, NIS–AgOTf (added at ꢀ40 °C), ꢀ17 °C to +4 °C (over 4 h) (97 % of 6, 84% of 7).
(1:99, v/v) before elution]. Additional evidence for the orthoester
structure of 5 came from the low-field position of the respective
anomeric protons and relatively large vicinal coupling constants
[dH 6.06 (1 H, d, J = 3.9 Hz, H-1III) and 6.15 (1 H, d, J = 3.8 Hz,
H-1IV)], the latter being indicative of the 1,2-cis-arrangement
expected in such orthoesters. The presence of 1,3-intersaccharide
linkages between AraIII and AraII residues followed from the obser-
vation of a H-3II/C-1III correlation in the HMBC and the following
H/H correlations in the ROESY spectrum of 5: H-1III/H-3II, H-2III/
H-3II, H-3III/CCH2ClIII, H-2II/CCH2ClIII. The observation of H-1IV/
H-5II, H-2IV/H-5II correlations in the ROESY spectrum of 5 sug-
gested that AraIV and AraII were (1-5)-linked. The 13C NMR spectra
of both 4 and 5 clearly showed signals of the orthoester quaternary
carbons at dC 121–123.
The formation of orthoester-linked oligosaccharides during gly-
cosylation with glycosyl donors possessing participating acyl
groups at O-2 is well known in the pyranose series,10,11 and to a
lesser extent in the furanose series12 (intersaccharidic orthoace-
tates were only reported for oligosaccharides containing ribo- or
galactofuranose residues). These acid-labile orthoesters are usually
formed when insufficient acid is present in the reaction mixture
(e.g., in the presence of a base) and are believed to be intermedi-
ates on the way to their more stable glycosidically-linked isomers,
the final products of the glycosylation reaction. No reports describ-
ing the formation of similar oligosaccharide orthoesters compris-
ing arabinofuranose residues have been published, to the best of
The results obtained in this study clearly indicate that forma-
tion of oligosaccharides by glycosylation of the corresponding
glycosyl acceptors with a 2-O-chloroacetyl-substituted D-arabinof-
uranose thioglycoside proceeds through the formation of intersac-
charidic orthoesters. In some cases, these orthoesters can be
isolated and the orthoester moiety used as a protecting group
orthogonal to O-acyl groups. The latter could be removed at this
stage under basic conditions without affecting the orthoester moi-
ety, as was demonstrated in the pyranose series.11c,d
This finding is very important from a practical point of view
since it creates a basis for reliable glycosylation with arabinofura-
nose glycosyl donors possessing a 2-O-chloroacetyl protecting
group. Thus, if a complex mixture results from a glycosylation
reaction, it may be indicative of orthoester formation. Such a mix-
ture must first be analyzed by 13C NMR spectroscopy, and if signals
characteristic of orthoesters are found, it can then be subjected to
TMSOTf-catalyzed isomerization. Only after this should any at-
tempts at product isolation be undertaken.
In summary, for the first time, orthoester-linked D-arabinofura-
nose oligosaccharides 4 and 5 were isolated and characterized. It
was demonstrated that these orthoesters serve as intermediates
in efficient one-pot, two-stage bis-glycosylation of the correspond-
ing glycosyl acceptors with ethyl (or phenyl) 3,5-di-O-benzoyl-2-
O-chloroacetyl-1-thio-
NIS–AgOTf, leading to formation of protected
a
-
D
-arabinofuranosides 1a,b, promoted by
-linked tri- and
a-D

1796
P. I. Abronina et al. / Tetrahedron Letters 52 (2011) 1794–1796
OCHb, ClH2CC), 4.00 (2H, s, ClH2CC), 4.03 (1H, dd, J = 11.5 Hz, J = 4.4 Hz, H-5IIb),
4.37–4.40 (1H, m, H-3II), 4.40–4.44 (1H, m, H-4II), 4.53–4.62 (4H, m, H-4III, H-
4IV, H-5IIIa, H-5IVa), 4.68 (1H, dd, J = 11.9 Hz, J = 3.5 Hz, H-5IIIb), 4.75 (1H, dd,
J = 11.9 Hz, J = 3.5 Hz, H-5IVb), 5.24 (2H, s, H-1II, H-1IV), 5.36–5.39 (3H, m, H-2II,
H-2III, H-3IV), 5.43 (1H, d, J = 4.0 Hz, H-3III), 5.47 (1H, br s, H-1III), 5.49 (1H, br s,
H-2III), 7.12–7.65 (16H, m), 7.90–8.10 (8H, m, Ph). 13C NMR (125.37 MHz) dC
40.27, 40.31 (2ꢃ COCH2Cl), 42.6 (OCH2CH2Cl), 63.2 (2C, C-5III, C-5IV), 65.8 (C-
5II), 67.4 (OCH2CH2Cl), 77.4 (2C, C-3III, C-3IV), 81.1 (C-4II), 81.3, 81.4 (C-4III, C-
4IV), 81.6 (C-3II), 82.5 (2C, C-2III, C-2IV), 82.60 (C-3II), 105.1 (C-1IV), 105.5 (C-1II),
105.8 (C-1III), 128.3, 128.40, 128.45, 128.50, 129.67, 129.73, 129.85, 133.2,
133.46, 133.5, 133.6 (Ph), 165.50, 165.55, 165.90, 165.95, 166.0 (CO).
tetraarabinofuranosides 6 and 7, which are useful building blocks
for the synthesis of larger oligosaccharides related to mycobacte-
rial cell wall components.
Acknowledgments
The authors are grateful to A.S. Shashkov for the acquisition of
NMR spectra on Bruker AVANCE 500 and 600 spectrometers, to
Dr. A.O. Chizhov for recording mass spectra. This work was sup-
ported financially by the Russian Foundation for Basic Research
(project No. 07-03-00830).
Tetrasaccharide 7: Rf 0.52 (toluene–EtOAc 10:1), ESIMS found m/z 1511.2773
[M+Na+]. Calcd. for C75H67Cl3NaO26 1511.2878. ½a 2D4
ꢂ
+53.4 (c 1.0, CHCl3). 1H
NMR (600.13 MHz) dH 3.67–3.74 (2H, m, OCH2CH2Cl), 3.82 (1H, dt, J = 11.1 Hz,
J = 5.6 Hz, OCHa), 3.86–3.95 (6H, m, H-5Ia, H-5II , 2 ꢃ ClH2CC), 3.99 (1H, dt,
a
J = 11.1 Hz, J = 5.6 Hz,OCHb), 4.05 (1H, dd, J = 11.7, 4.2 Hz, H-5IIb), 4.11–4.16
(1H, m, H-5Ib), 4.43 (1H, d, J = 5.8 Hz, H-3II), 4.45–4.49 (1H, m, H-4I), 4.49–4.60
(5H, m, H-5IIIa, H-5IVa, H-4II, H-4III, H-4IV), 4.65–4.70 (1H, m, H-5IIIb), 4.73 (1H,
dd, J = 12.0 Hz, J = 3.5 Hz, H-5IVb), 5.27 (2H, s, H-1I, H-1II), 5.32–5.36 (2H, m, H-
2IV, H-3III), 5.36–5.39 (3H, m, H-1IV, H-3IV, H-2III), 5.44 (1H, s, H-1III), 5.47 (2H, s,
H-2I, H-2II), 5.64 (1H, d, J = 5.0 Hz, H-3I), 7.30–7.58 (21H, m, Ph), 7.88–7.90 (4H,
m, Ph), 7.94–7.98 (4H, m, Ph), 8.01–8.07 (6H, m, Ph). 13C NMR (125.37 MHz) dC
40.3 (OCOCH2Cl), 42.8 (OCH2CH2Cl), 63.3 (2C, C-5III, C-5IV), 65.6 (2C, C-5I, C-5II),
67.4 (CH2O), 76.8 (C-3I), 77.3 (C-3IV), 77.5 (C-3III), 80.9 (C-4III), 81.3 (C-4IV), 81.5
(C-3II), 81.7 (C-4II), 81.9C-2I), 82.1 (C-4I), 82.4 (C-2IV), 82.7 (C-2II, C-2III), 105.3
(C-1III), 105.47, 105.51 (C-1II, C-1IV), 105.57 (C-1I), 128.08, 128.11, 128.17,
128.21, 128.35, 128.50, 128.82, 128.86, 129.00, 129.42, 129.4, 129.51, 129.60,
129.64, 132.89, 132.93, 133.16, 133.23, 133.30 (Ph), 165.16, 165.18, 165.24,
165.34, 165.39, 165.55, 165.61, 165.74 (CO).
References
1. (a) Liang, X.-Y.; Deng, L.-M.; Liu, X.; Yang, J.-S. Tetrahedron 2010, 66, 87–93; (b)
Joe, M.; Bai, Y.; Nacario, R. C.; Lowary, T. L. J. Am. Chem. Soc. 2007, 129, 9885–
9901; (c) Crich, D.; Pedersen, C. M.; Bowers, A. A.; Wink, D. J. J. Org. Chem. 2007,
72, 1553–1565; (d) Fraser-Reid, B.; Lu, J.; Jayaprakash, K. N.; Lopez, J. C.
Tetrahedron: Asymmetry 2006, 17, 2449–2463; (e) Zhu, X.; Kawatkar, S.; Rao, Y.;
Boons, G.-J. J. Am. Chem. Soc. 2006, 128, 11948–11957; (f) Ishiwata, A.; Akao, H.;
Ito, Y. Org. Lett. 2006, 8, 5525–5528; (g) Lee, Y. J.; Lee, K.; Jung, E. H.; Jeon, H. B.;
Kim, K. S. Org. Lett. 2005, 7, 3263–3266; (h) Gadikota, R. R.; Callam, C. S.;
Appelmelk, B. J.; Lowary, T. L. J. Carbohydr. Chem. 2003, 22, 459–480; (j)
Marotte, K.; Sanchez, S.; Bamhaoud, T.; Prandi, J. Eur. J. Org. Chem. 2003, 3587–
3598; (k) Yin, H.; D’Souza, F. W.; Lowary, T. L. J. Org. Chem. 2002, 67, 892–903;
(l) Sanchez, S.; Bamhaoud, T.; Prandi, J. Eur. J. Org. Chem. 2002, 3864–3873. and
references cited therein.
2. http://www.who.int/topics/tuberculosis.
3. Kaur, D.; Lowary, T. L.; Vissa, V. D.; Crick, D. C.; Brennan, P. J. Microbiology 2002,
148, 3049–3057.
4. Murase, T.; Zheng, R. B.; Joe, M.; Bai, Y.; Marcus, S. L.; Lowary, T. L.; Ng, K. K. S. J.
Mol. Biol. 2009, 392, 381–392.
5. Magnusson, G.; Chernyak, A. Ya.; Kihlberg, J.; Kononov, L. O. In
Neoglycoconjugates Preparation and Application; Lee, Y. C., Lee, R. T., Eds.;
Academic Press Inc: San Diego, California, 1994. pp 53–143.
6. Podvalnyy, N. M.; Sedinkin, S. L.; Abronina, P. I.; Zinin, A. I.; Fedina, K. G.;
Torgov, V. I.; Kononov, L. O. Carbohydr. Res. 2011, 346, 7–15.
7. Abronina, P. I.; Podvalnyy, N. M.; Melnikova, T. M.; Zinin, A. I.; Fedina, K. G.;
Kachala, V. I.; Torgov, V. I.; Kononov, L. O.; Panferzev, E. A.; Baranova, E. V.;
Mochalov, V. V.; Dyatlova, V. I.; Biketov, S. F. Izv. AN, Ser. Khim. 2010, 2276–
2280. Russ. Chem. Bull. 2010, 59.
8. Reactions sensitive to air and/or moisture were performed under an argon
atmosphere with the use of anhydrous solvents purified according to standard
procedures, and commercial reagents (Aldrich and Fluka). All compounds were
fully characterized by 1D-(1H, 13C, APT) and 2D-NMR (COSY, HSQC, HMBC,
HMQC-TOCSY, ROESY) spectroscopy as solutions in CDCl3 [the 1H NMR
chemical shifts are referred to the residual signal of CHCl3 (dH 7.27), and the
13C NMR chemical shifts to the CDCl3 signal (dC 77.0)].
9. Preparation of bis-orthoester
5 (Typical procedure). A mixture of the
thioglycoside 1a (80 mg, 0.167 mmol), diol 3 (46 mg, 0.07 mmol), and freshly
activated 4 Å MS (200 mg) in dry CH2Cl2 (2.5 mL) was stirred for 2 h at ꢁ20 °C.
N-Iodosuccinimide (47 mg, 0.21 mmol) and AgOTf (5.4 mg, 0.021 mmol) were
added at ꢀ40 °C, the reaction mixture was stirred for 15 min at ꢀ20 °C,
then allowed to reach +10 °C over 1 h, the course of the reaction being
monitored by TLC. The reaction was quenched by the addition of saturated
aqueous NaHCO3 (0.1 mL). The mixture was diluted with CH2Cl2 (50 mL),
filtered through Celite, and washed with saturated aqueous Na2S2O3 (50 mL)
and saturated aqueous NaHCO3 (50 mL). The organic layer was dried (Na2SO4)
and concentrated in vacuo. The residue was purified by column
chromatography (gradient toluene to toluene–EtOAc, 10:1) to afford the bis-
orthoester 5 (94 mg, 90%).
10. (a) Kong, F. Carbohydr. Res. 2007, 342, 345–373; Fraser-Reid, B.; Lopez, J. C. In
Handbook of Chemical Glycosylation, Advances in Stereoselectivity and
Therapeutic Relevance; Demchenko, A. V., Ed.; WILEY-VCH GmbH and Co
KGaA: Weinheim, 2008; pp 381–415.
11. (a) Wang, G.; Zhang, W.; Lu, Z.; Wang, P.; Zhang, X.; Li, Y. J. Org. Chem. 2009, 74,
2508–2515; (b) Fürstner, A.; Jeanjean, F.; Razon, P.; Wirtz, C.; Mynott, R. Chem.
Eur. J. 2003, 9, 320–326; (c) Shen, X.-Q.; Xie, L.; Gao, L.; He, L.-L.; Yang, Q.; Yang,
J.-S. Carbohydr. Res. 2009, 344, 2063–2068; (d) Orgueira, H. A.; Bartolozzi, A.;
Schell, P.; Litjens, R. E. J. N.; Palmacci, E. R.; Seeberger, P. H. Chem. Eur. J. 2003, 9,
140–169; (e) Ye, X.-S.; Wong, C.-H. J. Org. Chem. 2000, 65, 2410–2431.
12. (a) Hanessian, S.; Adhikari, S.; Szychowski, J.; Pachamuthu, K.; Wang, X.;
Migawa, M. T.; Griffey, R. H.; Swayze, E. E. Tetrahedron 2007, 63, 827–846; (b)
Johnston, B. D.; Pinto, B. M. Carbohydr. Res. 1999, 315, 355–360; (c) Gorin, P. A. J.
Can. J. Chem. 1962, 40, 275–282.
Trisaccharide bis-orthoester 4 (mixture of isomers): Rf 0.25 (toluene–EtOAc
10:1). 1H NMR (500.13 MHz, selected signals) dH 6.05 (1H, d, J = 3.7 Hz), 6.16
(1H, d, J = 4.3 Hz). 13C NMR (125.37 MHz, selected signals) dC 42.5, 42.6
(CH2CH2Cl), 45.65, 45.68 (ClH2CC), 104.5, 104.78, 104.99, 105.8, 105.95 (C-1),
122.7, 123.0 (ClH2CC).
13. Rearrangement of orthoester
solution of bis-orthoester 5 (67 mg, 0.05 mmol)) in CH2Cl2 (2 mL) was stirred
L,
5 into tetrasaccharide 7 (Typical procedure). A
Tetrasaccharide bis-orthoester 5: Rf 0.52 (toluene–EtOAc 10:1). ESIMS found m/z
1506.3313 [M+NH4þ]. Calcd for C75H71Cl3O26
N
1506.3324. 1H NMR
with freshly activated 4 Å MS (0.1 g) for 2 h at ꢁ20 °C. TMSOTf (45
l
(500.13 MHz) dH 3.68–3.75 (4H, m, OCH2CH2Cl, ClH2CCIII), 3.77 (1H, d,
J = 3.9 Hz, ClH2CCIV), 3.84 (1H, dt, J = 10.7 Hz, J = 5.4 Hz, OCHa), 3.89 (2H, br s,
0.003 mmol) was added to the reaction mixture at ꢀ40 °C. After 2 h at 20 °C the
reaction was quenched by the addition of saturated aqueous NaHCO3 (0.1 mL).
The mixture was diluted with CH2Cl2 (50 mL), filtered through Celite, washed
with H2O (50 mL) and saturated aqueous NaHCO3 (50 mL). The organic layer
was dried (Na2SO4) and concentrated in vacuo. The residue was purified by
column chromatography (gradient toluene to toluene–EtOAc, 10:1) to afford
tetrasaccharide 7 (24 mg, 80%).
H-5II
,
H-5IIb), 3.92 (1H, dd, J = 4.2 Hz, J = 11.2 Hz, H-5Ib), 4.03 (1H, dt,
a
J = 10.7 Hz, J = 5.4 Hz, OCHb), 4.16 (1H, dd, J = 4.2 Hz, J = 11.2 Hz, H-5Ib), 4.40–
4.44 (1H, m, H-4III), 4.45–4.51 (3H, m, H-4IV, H-4II, H-3II), 4.52–4.58 (1H, m, H-
4I), 4.58–4.62 (4H, m, H-5IIIa, H-5IIIb, H-5IVa, H-5IVb), 5.04 (1H, d, J = 3.9 Hz,
H-2IV), 5.22 (1H, d, J = 3.9 Hz, H-2III), 5.28 (1H, s, H-1II), 5.31 (1H, d, J = 3.9 Hz, H-
3IV), 5.32 (1H, s, H-1I), 5.43 (1H, d, J = 4.4 Hz, H-3III), 5.47 (1H, s, H-2II), 5.54 (2H,
br s, H-2I), 5.65 (1H, d, J = 4.9 Hz, H-3I), 6.05 (1H, d, J = 3.9 Hz, H-1III), 6.14 (1H,
d, J = 3.8 Hz, H-1IV), 7.29–7.65 (20H, m, Ph), 7.90–8.16 (16H, m, Ph). 13C NMR
(125.37 MHz) dC 42.7 (OCH2CH2Cl), 45.3 (ClH2CIII), 45.5 (ClH2CIV), 60.6 (C-5II),
63.65 (C-5III), 63.75 (C-5IV), 65.7 (C-5I), 67.4 (OCH2), 75.6 (C-3II), 76.9 (C-3I, C-
3III, C-3IV), 81.0 (C-4II), 81.6 (C-4III), 81.9 (C-2I,C-4I), 82.2 (C-4IV), 82.8 (C-2II),
86.0 (C-2IV), 86.2 (C-2III), 104.8 (C-1III), 105.1 (C-1IV), 105.5 (C-1II), 105.6 (C-1I),
122.7, 123.1 (ClH2CCIII, ClH2CCIV), 128.5, 129.8, 130.0, 133.3,133.5, 133.6, 133.7
(Ph), 165.2, 165.6, 165.8, 166.1 (CO).
14. Preparation of tetrasaccharide
7 without isolation of orthoester 5 (Typical
procedure). A mixture of the thioglycoside 1b (381 mg, 0.72 mmol), diol 3
(198 mg, 0.30 mmol), and freshly activated 4 Å MS (0.5 g) in dry CH2Cl2 (5 mL)
was stirred for 2 h at ꢁ20 °C. N-Iodosuccinimide (203 mg, 0.90 mmol) and
AgOTf (23 mg, 0.09 mmol) were added at ꢀ45 °C, and the reaction mixture was
stirred for 1 h at ꢀ17 °C, then allowed to reach +4 °C over 3 h, the course of the
reaction being monitored by TLC. The reaction was quenched by addition of
saturated aqueous NaHCO3 (0.1 mL). The mixture was diluted with CH2Cl2
(100 mL), filtered through Celite, and washed with a 1:1 mixture of saturated
aqueous Na2S2O3ꢀNaHCO3 (100 mL). The organic layer was dried (Na2SO4) and
concentrated in vacuo. The residue was purified by column chromatography
(gradient toluene to toluene–EtOAc, 10:1) to afford tetrasaccharide 7 (377 mg,
84%).
Trisaccharide 6: Rf 0.25 (toluene–EtOAc 10:1). ½a D23
ꢂ
+49.0 (c 1.0, CHCl3). ESIMS
found m/z 1171.1963 [M+Na+]. Calcd. for C56H51Cl3NaO20 1171.1937. 1H NMR
(500.13 MHz) dH 3.63 (2H, t, J = 5.6 Hz, OCH2CH2Cl), 3.79 (1H, dt, J = 11.2 Hz,
J = 5.7 Hz, OCHa), 3.87 (1H, dd, J = 11.6 Hz, J = 2.6 Hz, H-5IIa), 3.90–3.98 (3H, m,