Development of highly stereoselective GalN3 donors and their application in the chemical synthesis of precursors of Tn antigen

Article abstract of DOI:10.1039/c1ob05893b

Two GalN3 thioglycoside donors were designed and prepared based on protecting group-stereoselectivity relationship study for optimum alpha selectivity. These donors showed excellent stereoselectivity in test reactions with various acceptors and were successfully applied in the synthesis of precursors for Tn antigen and a core structure of O-glycan.

Full text of DOI:10.1039/c1ob05893b

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PAPER
Development of highly stereoselective GalN3 donors and their application in
the chemical synthesis of precursors of Tn antigen†
George Ngoje, Janet Addae, Harpreet Kaur and Zhitao Li*
Received 3rd June 2011, Accepted 6th July 2011
DOI: 10.1039/c1ob05893b
many of these factors are not well understood and it is hard
to predict their influence on the stereoselectivity. Development
Introduction
a-GalNAc linkage is one of the most important glycosidic linkages
in mucin-type O-glycans,1 because all eight core structures are built
upon the a-GalNAc-Ser(Thr) residues(Fig. 1).2 Since abnormal
expression of O-glycans is often associated with different stages of
cancer, study of the physiologic role of these carbohydrates is very
important. Many tumor associated carbohydrate antigen (TACA)
have the common structure of a-GalNAc-Ser (or Thr), such as
TF antigen, T antigen and Sialyl-T antigen. These antigens are
interesting targets for developing new therapeutic and diagnostic
tools for cancer treatments. Due to the difficulty of obtaining these
molecules from natural sources in homogenous form and large
quantity, development of highly selective reaction protocols to
these structures is of particular interest for carbohydrate chemists
and glycobiologists.3–5
of robust protocol that can give the desired stereoselectivity
(a-selectivity) without intensive optimization of the reaction
conditions is of particular interest to carbohydrate chemists.
Our research group has been especially interested in the
stereochemistry effect of protecting groups, because they are an
indispensible part of carbohydrate chemistry and at the same time
have a profound influence on the stereoselectivity of donors.15–22
Our recent study indicated that the protecting groups, especially
the acyl group, play a critical role for the stereoselectivity of GalN3
donors.23 More specifically, acetyl protecting groups at the 3- and
4-position of GalN3 are most beneficial to the a-selectivity. The
acetyl group at 6-position, on the other hand, has only very little
or even opposite effect for the a-selectivity. Based on our theo-
retical study, this selectivity is potentially associated with remote
participating effect of 3- and 4- acetyl groups, which has also
been observed in Galactose donors before.24 Another discovery is
that the reaction temperature also affects the selectivity, i.e. higher
temperature favors the a-selectivity. Based on this study, the ideal
combination of protecting groups would be therefore acetyl groups
at 3- and 4-positions and an electron-donating group at 6-position.
In this paper, we report the rational design of two new GalN3
donors that showed excellent a-selectivity and their applications
in the synthesis of Tn antigen derivatives.
Fig. 1 Core structures of mucin type O-glycans.
Results and discussion
Synthesis of a-GalNAc linkages, like many other 1,2-cis glyco-
sidic linkages, is very challenging due to the lack of control on
the stereochemistry outcome. General approach to form 1,2-cis
glycosyidic linkage is through using non-participating group at 2-
position to avoid anchimeric effect, which will favor the formation
1,2-trans linkages. In the case of forming a-GalNAc linkages,
one of the most commonly used methods is to use 2-azido-2-
deoxy-galactosyl (GalN3) donors. However, in general, to achieve
reasonable stereoselectivity, extensive optimization of the reaction
conditions is necessary, because many factors are known to affect
the ratio of a/b anomers and only the right combination of all
factors can afford the optimum selectivity.6–14 This optimization
process can be very time-consuming and labor intensive, because
Based on our study, we designed a new GalN3 thioglycoside
donor (compound 1a, Scheme 1). Acetyl groups are used as
protecting group at 3 and 4-positions. TBDPS group was used
at the 6-position. This combination would afford the optimum
a-selectivity in glycosylation reaction. The desired donor was
then prepared from known compound 2 in four step through
standard protecting group manipulations. Known compound 2
was synthesized from galactal through literature procedures.25
∑
Reaction of compound 2 with thiocresol with BF3 Et2O afforded
the thioglycoside 3, which was then deacetylated to afford
intermediate 4.26 Selective protection of 6-OH with TBDPSCl gave
compound 5, which upon acetylation gave desired donor 1a as an
inseparable mixture of a/b anomers (close to 1 : 1 ratio).
Donor 1awas thentestedinglycosylationreactionwithacceptor
6 using donor 3 as control (Scheme 2). The reactions were carried
out in DCM, with TMSOTf/NIS as the promoter at 0 ◦C. When
Department of Chemistry, Binghamton University, Binghamton, NY, 13902
† Electronic supplementary information (ESI) available: 1H and 13C
NMR of compounds 7, 8, 15, 16, 17, 19, 20, 21 and 23. See DOI:
10.1039/c1ob05893b
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Scheme 1 Synthesis of donor 1a and 1b.
Scheme 2 Comparison between donor 1a and 3.
donor 3 was used, the reaction gave a 3 : 1 mixture of anomers
in 86% yield, favoring a-product. When donor 1a was used,
the reaction gave almost exclusively a-product (> 20 : 1, with
small amount of impurity that could be b-product but cannot be
confirmed due to the low concentration) in high yield (99%). This
experiment confirms the superior a-selectivity of this rationally
designed donor.
This donor was then tested in a series of glycosylation reactions
with various glycosyl acceptors (Table 1). In reaction with
acceptor 9, a secondary glucose acceptor with benzyl protection,
a disaccharide was also isolated as the predominant product
(> 15 : 1) in 77% yield (entry 2). The reaction with acceptor 10
gave only a-product at 68% yield (entry 3). The low reaction
yields of acceptor 9 and 10 (entry 2 and 3) are believed to be
associated with the lower reactivity of these acceptors, because
unreacted acceptors were recovered from both reactions. Reaction
with acceptor 11 also gave just a anomer (at 99% yield, entry 4). In
all reactions with glycosyl acceptors, donor 1a showed excellent
stereoselectivity. It is noticeable that the reaction condition was
arbitrarily set without any optimization, yet the stereoselectivity
was excellent and the yields were also generally good.
afforded a mixture of both anomers. Even though the reaction
highly favors the a product (5 : 1, entry 5), it was not satisfactory
to us. Reaction at room temperature improved the selectivity (to
7 : 1, entry 6), but still less than perfect. To further improve the
selectivity, a second donor was designed and prepared (compound
1b, Scheme 1), which has benzoyl protecting groups at 3 and
4-positions, with the expectation that the benzoyl group could
participate better than acetyl group and therefore improve the a-
selectivity. In glycosylation with donor 1b, the L-serine acceptor
12 did give only a product, which showed much satisfactory
stereoselectivity (entry 7). A L-threonine acceptor (compound 13)
was also tested in the glycosylation, excellent a-selectivity (> 15 : 1)
was obtained (entry 8). Since Fmoc is another commonly used
protecting group for amino acid, an L-serine acceptor (compound
14) with Fmoc protection was tested in the glycosylation. The
reaction also afford predominantly a-product (> 15 : 1, entry 9).
The product formed from donor 1b and serine acceptors can be
easily converted to Tn antigen. They are also useful intermediates
for the synthesis of the core structures of mucin type O-glycans,
like core 6 and core 7. To demonstrate the application of these
intermediates in synthesis of core structures, the TBDPS group
of compound 21 was selectively removed to afford compound 22,
which gave a core 7 precursor (compound 23) (Scheme 3)after
glycosylation reaction with donor 1a in good yield (78%) and high
stereoselectivity (only a-product).
This donor was then tested in reaction with an L-serine
acceptor 12 (Cbz protection on amino group, benzyl protection on
carboxylic acid) to test the stereoselectivity in making Tn antigen
derivatives. Under the general reaction conditions, the reaction
Scheme 3 Synthesis of a precursor to core 7.
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Table 1 Results of glycosylations
b
Entry
1
R–OH
Products
Yielda
99%
a : b
20 : 1c
2
77%
15 : 1c
3
68%
a Only
4
99%
a Only
5
90%
5 : 1
6
90%
7 : 1d
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Table 1 (Contd.)
b
Entry
7
R–OH
Products
Yielda
70%
a : b
a only
15 : 1c
15 : 1c
8
65%
9
71%
Note:a Isolated yield. b Determined by 1H NMR. c Small amount of impurity was observed, which is tentatively assigned as b anomer, but not confirmed
due to limited quantity. d At room temperature.
In conclusion, two highly stereoselective GalN3 donors were
designed and synthesized based on a rational analysis of the
protecting group-selectivity relationship. The glycosylation tests
demonstrated that these donors are highly selective. Donor 1a
affords only a product in glycosylation reactions with glycosyl
acceptors and also showed very high a-selectivity in glycosylation
with serine acceptor. Donor 1b gives only a product even in
reactions with serine acceptor. This study not only provides a
very accessible and highly selective donor for future research on
stereoselective synthesis of a-GalNAc linkages, but also proves the
concept that thorough study of the protecting group-selectivity
relationship is extremely useful for the development of more
efficient chemical synthesis of oligosaccharides.
and/or by charring with p-anisaldehyde stain. All NMR data
ware recorded on Bruker 360 MHz spectrometer. All H NMR
1
data was obtained at 360 MHz, all 13C NMR data was obtained at
90 MHz. Proton and carbon chemical shifts are reported in parts
per million (ppm) using CDCl3 as an internal reference unless
otherwise noted. Coupling constants (J) are reported in (Hz) and
multiplicities are abbreviated as follows: (s), doublet (d), triplet (t),
quartet (q), multiplet (m) and broadened (br).
Typical procedure for the glycosylation reaction: donor
(0.11 mmol, 1.1 equiv.) and acceptor (0.1 mmol, 1 equiv.) were
dissolved in anhydrous dichloromethane (4 mL). Flame-dried
molecular sieves was added. The mixture was stirred at room
temperature for 30 mins and then cooled to 0 ◦C in ice bath. NIS
(0.11 mmol, 1.1 equiv.) and TMSOTf (5 mL) was added. After
30–60 mins, the reaction was quenched with triethylamine (20 mL)
and purified by flash chromatography.
Experimental
Materials and methods
2-Azido-6-O-(tert-butyldiphenylsilyl)-2-deoxy-D-galactopyranose-
1-O-methylthioglycoside (5)
Unless otherwise noted, reagents and materials were obtained
from commercial suppliers and were used without further pu-
rification. TLC was performed on pre-coated glass plates (Silica
Gel F254). Spots were detected by visualization under UV lamp
27
Compound 4
(0.41 g, 1.32 mmoles, 1.0eq) was dissolved
in dry DMF (15 ml) and imidazole(0.134 g, 2.0 mmoles,
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1.5 mmoles) was added. While stirring at room temperature, tert-
butyldiphenylchlorosilane was slowly added in duration of 15 min.
The reaction mixture was then stirred overnight to completion.
The reaction was then quenched with water (30 ml), extracted with
dichloromethane(100 ml) and the resulting organic layer washed
3 ¥ 50 ml distilled water, 1 ¥ 50 ml brine, dried over MgSO4 and
then concentrated to give the crude oily product which was then
purified through flash chromatography (0–50% EtOAc/hexanes)
to give 0.58 g (80% yield) of 5 as a mixture of anomers. Rf 0.56 (1 : 1
2.31(s, 3H), 1.00(s, 18H); 13C NMR (90 MHz, CDCl3): d 172.1,
165.3, 165.2, 165.1, 164.9, 138.9, 138.2, 135.5, 135.4, 135.3, 134.9,
134.4, 133.7, 133.2, 133.1, 130.2, 129.8, 129.7, 129.6, 128.8, 128.5,
128.4, 128.3, 127.8, 127.6, 87.9, 85.9, 77.5, 73.9, 70.9, 70.1, 68.1,
67.2, 61.2, 59.9, 59.3, 26.6, 21.1, 18.9. IR (cm-1), 2931, 2114, 1731,
1521, 1428, 1215, 1113.
Methyl 3,4-di-O-acetyl-2-azido-6-O-(tert-butyldiphenylsilyl)-2-
deoxy-a-galactopyranosyl-(1→6)-2,3,4-tri-O-benzyl-a-
glucopyranoside (8)
1
EtOAc/hexanes:). H NMR (360 MHz, CDCl3): d 7.82–7.66(m,
9H), 7.50–7.30(d, J = 7.7 Hz, 2H), 7.49–7.33(m, 16H), 7.09(d, J =
7.9 Hz, 2H), 7.05(d, J = 7.7 Hz, 2H), 5.6(d, J = 5.5 Hz, 1 Hz),
4.45–4.36(m, 2H), 4.25–4.16(m, 2H), 4.09(s, 1H), 4.05–3.80(m,
5H), 3.64–3.42(m, 4H), 3.32(br, 1H), 3.13(br, 1H), 2.34(s, 3H),
2.33(s, 3H), 1.12(s, 18H); 13C NMR (90 MHz, CDCl3): d 138.3,
137.6, 135.6, 135.5, 135.4, 133.4, 132.9, 132.7, 132.6, 130.0, 129.8,
129.7, 128.1, 127.9, 127.8, 88.9, 86.9, 77.8, 74.4, 70.4, 69.9, 69.1,
64.2, 64.1, 62.9, 61.3, 26.8, 21.1, 19.1. IR (cm-1), 3450, 2932, 2112,
1428, 1258, 1113, 908.
1
Rf 0.64 (1 : 2 EtOAc/Hexanes), H NMR (360 MHz, CDCl3): d
7.62–7.25 (m, 25 H) 5.54 (d, J = 2.9 Hz, 1 H), 5.33 (dd, J = 11.2,
3.2 Hz, 1 H), 5.04–4.76 (m, 5 H), 4.67–4.60 (m, 3 H), 4.09–3.98
(m, 2 H), 3.80–3.47 (m, 8 H), 3.39 (s, 3 H), 2.05 (s, 3 H), 1.99 (s, 3
H), 1.02 (s, 9 H); 13C NMR (90 MHz, CDCl3): d 169.8, 138.2,
138.1, 135.49, 135.47, 129.8, 129.7, 128.4, 128.3, 128.0, 127.9,
127.82, 127.80, 127.7, 127.6, 127.5, 98.0, 97.9, 82.0, 80.0, 77.7, 75.7,
75.0, 73.3, 70.1, 69.1, 68.3, 67.7, 66.2, 61.6, 57.8, 55.1, 26.6, 20.6,
20.4, 19.0. HRMS (ESI-TOF) [MNa+] calcd for C54H63N3O12SiNa
996.4073, found 996.4072. IR (cm-1), 2933, 2111, 1749, 1371, 1215,
1073, 701. UV/VIS, l; 231 nm.
2-Azido-3,4-di-O-acetyl-6-O-(tert-butyldiphenylsilyl)-2-deoxy-D-
galactopyranose-1-O-methylthioglycoside (1a)
To a stirred solution of the diol 5 (1.16 g, 2.12 mmoles, 1.0eq)
in dichlomethane (15 ml) was added acetic anhydride (0.4 ml,
4.2 mmoles, 2.0eq), triethylamine(0.6 ml, 4.2 mmoles, 2.0eq) and
catalytic DMAP and the reaction allowed to proceed for 1 h
after which the the mixture was concentrated and the residue
purified on flash chromatography(0–30% EtOAc/hexanes) to give
the 3,4-di-O-Acetyl derivative 1a (1.28 g, 75% yield). Rf 0.5 (1 : 5
Methyl 3,4-di-O-acetyl-2-azido-6-O-(tert-butyldiphenylsilyl)-2-
deoxy-a- D-galactopyranosyl-(1→4)-2,3,6-tri-O-benzyl-a-
D-glucopyranoside (15)
1
Rf 0.67 (1 : 2 EtOAc/Hexanes), H NMR (360 MHz, CDCl3): d
7.62–7.20 (m, 25 H), 5.71 (d, J = 4.0 Hz, 1 H), 5.57 (d, J = 1.8 Hz,
1 H), 5.33 (dd, J = 11.5, 2.9 Hz, 1 H), 5.10 (d, J = 10.8 Hz, 1 H),
4.84 (d, J = 11.1 Hz, 1 H), 4.69 (d, J = 11.9 Hz, 1 H), 4.60–4.57 (m,
2 H), 4.43–4.41 (m, 2 H), 4.08–4.03 (m, 2 H), 3.82–3.80 (m, 2 H),
3.68–3.48 (m, 5 H), 3.38 (s, 3 H), 2.05 (s, 3 H), 1.95 (s, 3 H), 1.00 (s,
9 H); 13C NMR (90 MHz, CDCl3): d 169.71, 169.67, 138.8, 137.9,
137.8, 135.6, 135.5, 132.9, 132.7, 129.72, 129.65, 128.4, 128.3,
128.22, 128.17, 128.04, 127.90, 127.86, 127.78, 127.75, 127.70,
127.64, 127.61, 127.53, 127.48, 127.44, 127.28, 127.21, 97.9, 97.5,
81.6, 80.5, 74.6, 74.4, 73.1, 69.4, 69.2, 69.1, 68.4, 67.4, 61.3, 57.6,
55.2, 26.7, 20.6, 20.5, 19.0. HRMS (ESI-TOF) [MH+] calcd for
C54H64N3O12Si 974.4254, found 974.4244. IR (cm-1), 2932, 2111,
1749, 1369, 1045, 701. UV/VIS, l; 229 nm.
1
EtOAc/hexanes). H NMR (360 MHz, CDCl3): d 7.68–7.59(m,
9H), 7.53–7.28(m, 17H), 7.10(d, J = 8.1 Hz, 2H), 5.67(d, J = 3.0 Hz,
1 Hz), 5.59–5.48(m, 2H), 5.20(dd, J = 3.0, 10.7 Hz, 1H),4.91(dd,
J = 3.4, 10.7 Hz, 1H), 4.65(t, J = 6.4 Hz, 1H), 4.45(d, J = 10.2 Hz,
1H), 4.25(dd, J = 5.1, 11.0 Hz, 1H) 3.85–3.70(m, 2H), 3.68–3.56(m,
4H), 2.34(s,3H), 2.29(s, 3H), 2.07(s, 3H), 2.04(s, 3H), 2.03(s, 3H),
2.00(s, 3H), 1.05(s, 18H); 13C NMR (90 MHz, CDCl3): d 169.6,
169.5, 169.4, 138.5, 138.1, 135.5, 133.6, 133.1, 129.8, 129.8, 129.6,
127.7, 87.7,86.8, 76.6, 73.3, 70.3, 69.8, 67.5, 66.6, 61.5, 61.3, 59.6,
58.4, 26.7, 21.0, 20.5, 20.4, 18.9. IR (cm-1), 2932, 2113, 1750, 1368,
1214, 1081.
2-Azido-3,4-di-O-benzoyl-6-O-(tert-butyldiphenylsilyl)-2-deoxy-D-
galactopyranose-1-O-methylthioglycoside (1b)
Methyl 3,4-di-O-acetyl-2-azido-6-O-(tert-butyldiphenylsilyl)-2-
deoxy-a- D-galactopyranosyl-(1→4)-2,3-di-O-acetyl-6-O-
benzyl-a- D-glucopyranoside (16)
The diol 5 (0.54 g, 0.98 mmoles, 1.0eq) was dissolved in 10 ml
of pyridine. Benzoyl chloride (0.34 ml, 2.95 mmoles, 3.0eq) was
then added dropwise and the reaction stirred overnight. After
dilution with 100 ml of dichloromethane, the reaction mixture
was washed 2 ¥ 50 ml distilled water, 2 ¥ 50 ml 10% hydrochloric
acid, 1 ¥ 50 saturated NaHCO3 then dried and purified on
column chromatography to give 1b (0.58 g, 78%yield). Rf 0.55 (1 : 3
EtOAc/hexanes). 1H NMR (360 MHz, CDCl3): d 7.9(dd, J = 7.7,
15.8 Hz, 4H), 7.8(t, J = 8.5 Hz, 3H), 7.7–7.2(m, 34H),7.15(d, J =
8.1 Hz, 1H), 7.10(t, J = 7.3 Hz, 3H), 7.05(d, J = 8.1 Hz, 2H),
6.10(d, J = 3.0, 1H), 5.97(d, J = 3.0, 1H), 5.69(d, J = 5.9 Hz, 1H),
5.63(dd, J = 3.4, 11.1 Hz, 1H) 5.32(dd, J = 3.0, 10.7 Hz, 1H),
4.9(t, J = 7.3 Hz, 1H), 4.59–4.47(m, 2H), 3.90–3.82(m, 1H), 3.81–
3.71(m, 1H)), 3.70–3.62(m, 2H), 3.72–3.64(m, 1H), 2.42(s, 3H),
1
Rf 0.41 (1 : 2 EtOAc/Hexanes), H NMR (360 MHz, CDCl3): d
7.60–7.13 (m, 15 H), 5.60 (br, 1 H), 5.54 (t, J = 9.7 Hz, 1 H),
5.28 (dd, J = 7.2, 3.6 Hz, 1 H), 5.15 (d, J = 4.0 Hz, 1 H), 4.84
(d, J = 3.2 Hz, 1 H), 4.79 (dd, J = 5.8, 3.2 Hz, 1 h), 4.36 (m,
2 H), 4.11 (m, 1 H), 3.95 (t, J = 9.0 Hz, 1 H), 3.81 (br, 1 H),
3.78–3.49 (m, 5 H), 3.40 (s, 3 H), 2.04–1.97 (12 H), 1.00 (s, 9
H); 13C NMR (90 MHz, CDCl3): d 170.3, 169.5, 137.8, 135.52,
135.48, 133.7, 132.8, 130.0, 129.8, 128.2, 127.70, 127.68, 127.44,
127.37, 99.3, 96.7, 75.8, 73.2, 72.0, 69.6, 69.4, 68.4, 68.3, 67.3,
61.1, 57.8, 55.2, 28.5, 26.6, 20.8, 20.7, 20.6, 20.4. HRMS (ESI-
TOF) [MH+] calcd for C44H56N3O14Si 877.3526, found 877.3518.
IR (cm-1), 2933, 2112, 1751, 1646, 1238, 1044, 703. UV/VIS, l;
230 nm.
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Methyl 3,4-di-O-acetyl-2-azido-6-O-(tert-butyldiphenylsilyl)-2-
deoxy-a-D-galactopyranosyl-(1→3)-2,4,6-tri-O-benzyl-a-
D-galactopyranoside (17)
5.6 (dd, J = 3.2, 12 Hz, 1 H), 5.3–5.1 (m, 6H), 5.1 (s, 1 H), 5.0(d,
J = 3.6 Hz, 1 H), 4.5 (br, 2H), 3.8 (dd, J = 3.2, 11 Hz, 1 H), 3.7 (d,
J = 7.3 Hz, 2 H), 1.28 (d, J = 3.0 Hz, 3 H), 0.99(s, 9 H); 13C NMR
(90 MHz, CDCl3): d, 170.0, 165.3, 165.0, 156.8, 136.2, 135.4,
135.3, 135.0, 134.9, 134.7, 133.2, 132.8, 132.7, 130.1, 129.7, 129.2,
128.6, 128.5, 128.2, 128.1, 128.0, 127.9, 127.7, 127.6, 124.8, 99.2,
69.8, 69.3, 69.1, 68.8, 68.1, 67.8, 67.5, 67.2, 61.5, 59.9, 58.7, 29.6,
26.5, 25.7, 25.6, 18.9, 18.7; HRMS (ESI-TOF) [MH+] calcd for
C55H57N4O11Si 977.3787, found 977.3778. IR (cm-1), 2932, 2112,
1728, 1513, 1276, 1094, 909. UV/VIS, l; 231 nm.
1
Rf 0.69 (1 : 2 EtOAc/Hexanes), H NMR (360 MHz, CDCl3):
d7.62–7.20 (m, 25 H), 5.61–5.57 (m, 2 H), 5.24 (d, J = 2.9 Hz, 1 H),
5.02 (d, J = 10.8 Hz, 1 H), 4.72 (d, J = 11.5 Hz, 1 H), 4.62–4.39 (m,
6 H), 4.15 (dd, J = 10.0, 2.5 Hz, 1 H), 4.00–3.96 (m, 2 H), 3.90–3.81
(m, 2 H), 3.68–3.64 (m, 1 H), 3.59–3.50 (m, 3 H), 3.27 (s, 3 H), 2.08
(s, 3 H), 2.03 (s, 3 H), 0.98 (s, 9 H); 13C NMR (90 MHz, CDCl3): d
169.8, 169.6, 138.5, 138.2, 135.5, 133.0, 132.9, 129.6, 129.5, 128.4,
128.3, 128.2, 128.0, 127.9, 127.73, 127.70, 127.6, 127.52, 127.47,
98.4, 95.2, 75.7, 75.0, 74.9, 73.9, 73.40, 73.36, 69.0, 68.9, 68.81,
68.76, 68.74, 67.7, 61.1, 58.5, 55.1, 26.7, 26.6, 20.7, 20.6, 19.0.
HRMS (ESI-TOF) [MNa+] calcd for C54H63N3O12SiNa 996.4073,
found 996.4070. IR (cm-1), 2931, 2115, 1752, 1428, 1110, 667.
UV/VIS, l; 228 nm.
N-(9-Fluorenylmethoxycarbonyl)-O-[3,4-di-O-benzoyl-2-azido-6-
O-(tert-butyldiphenylsilyl)-2-deoxy-a-D-galactopyranosyl]-L-
serine benzyl ester (21)
Rf 0.54 (1 : 2 EtOAc/hexanes) 1H NMR (360 MHz, CDCl3): d7.9
(2d, J = 7.7, 7.9 Hz, each 2 H), 7.8 (d, J = 7.5 Hz, 3 H), 7.7–7.5
(m, 7 H), 7.4–7.3, (m, 17 H), 7.1 (t, J = 7.2 Hz, 2 H), 4.5 (br, 2
H), 6.0 (d, J = 2.3 Hz, 1 H), 5.9–5.8 (m, 1 H), 5.7 (dd, J = 3.2,
11 Hz, 1 H), 5.4–5.1 (m, 4 H), 5.0(br, 1 H), 4.6 (br, 1 H), 4.5–4.3
(m, 3 H), 4.3–4.1 (m, 3 H), 4.0 (br, 1 H), 3.9–3.7(m, 4 H), 1.0(s,
9 H), 13C NMR (90 MHz, CDCl3) d, 169.6, 165.2, 165.0, 156.0,
143.7, 141.2, 135.5, 135.3, 135.0, 134.8, 133.2, 132.7, 132.5, 130.2,
129.8, 129.7, 129.5, 129.2, 128.6, 128.5, 128.2, 127.7, 127.6, 127.5,
127.0, 125.2, 119.8, 99.5, 69.9, 69.4, 68.8, 68.0, 67.7, 67.4, 61.3,
58.4, 54.5, 47.0, 29.6, 26.5, 25.6, 18.9; HRMS (ESI-TOF) [MH+]
calcd for C61H59N4O11Si 1051.3944, found 1051.3941. IR (cm-1),
2932, 2113, 1728, 1512, 1276, 1106, 909. UV/VIS, l; 230 nm.
N-Benzyloxy carbonyl-O-(2-azido-3,4-di-O-acetyl-6-O-(tert-
butyldiphenylsilyl)-2-deoxy-galactopyranosyl)-L-serine benzyl
ester (18)
1
Rf 0.50 (1 : 2 EtOAc/Hexanes), H NMR (360 MHz, CDCl3): d
7.7–7.6 (m, 4H), 7.5–7.3, (m, 12H), 5.7 (d, J = 8.1, 1H) 5.7–5.6
(m, 1H), 5.55 (br, 1H), 5.50–5.52 (m, 1H), 5.48 (d, J = 2.9, 1H),
5.28 (d, J = 0.6, 2H), 5.26–5.18(m, 1H), 5.2–5.1 (m, 1H), 5.1
(s, 1H), 4.8 (d, J = 3.4 Hz, 1H), 4.76–4.74 (m, 1H), 4.6–4.5(m,
1H), 4.4–4.3(m, 1H),4.23(d, J = 7.88, 1H), 3.94–3.84(m, 1H), 3.7–
3.6(m, 1H), 2.04(s,3H), 2.0(s, 3H), 1.03(s, 9H); 13C NMR (90 MHz,
CDCl3): d, 169.7, 169.6, 169.5, 169.3, 156.0, 136.0, 135.0, 132.8,
128.6, 128.5, 128.4, 128.1, 127.7, 102.5, 99.3, 73.3, 71.2, 69.7, 69.5,
68.2, 67.7, 67.3, 67.1, 67.0, 66.2, 61.2, 60.8, 57.6, 54.4, 34.6, 34.5,
31.5, 29.6, 29.0, 26.6, 23.5, 22.6, 20.6, 20.4, 18.9, 14.0, 11.34. IR
(cm-1), 2959, 2113, 1750, 1510, 1215, 1113. UV/VIS, l; 229 nm.
N-(Benzyloxycarbonyl)-O-[3,4-di-O-acetyl-2-azido-6-O-(tert-
butyldiphenylsilyl)-2-deoxy-a-D-galactopyranosyl-(1→6)-
3,4-di-O-benzoyl-2-azido-2-deoxy-a-D-galactopyranosyl]-
L- serine benzyl ester (23)
Compound 19 (0.12 g, 0.125 mmoles, 1.0eq) was dissolved in 5 ml
of dry THF, followed by addition of TBAF in THF (140 mL,
0.48 mmoles, 3.8eq) and acetic acid (4.7 mL, 0.003 mmoles,
0.0833 mmoles, 0.66eq) at room temperature and the reaction
stirred overnight. Upon completion, the reaction mixture was
diluted with 50 ml of DCM washed with water (50 ml), NaHCO3
(50 ml) and then water again (50 ml) then dried over MgSO4,
concentrated then purified on flash chromatography (10–35%
EtOAc/hexanes) to afford 0.077 g (85% yield) of the alcohol 22. Rf
0.31 (1 : 2 EtOAc/hexanes) 1H NMR (360 MHz, CDCl3) d, 8.0, (d,
J = 7.3 Hz, 2 H), 7.9 (d, J = 8.1 Hz, 2 H), 7.7–7.3 (m, 16 H), 6.0 (d,
J = 7.7 Hz, 1 H), 5.7 (br, 1 H), 5.6 (dd, J = 3.0, 11 Hz, 1 H), 5.3–5.2
(m, 2 H), 5.1(s, 2 H), 5.0 (d, J = 3.0 Hz, 1 H), 4.6 (br, 1 H), 4.3–4.2
(m, 1 H), 4.1–4.0 (m, 2 H), 3.83(dd, J = 3.4, 11 Hz, 1 H), 3.7–3.6,
(m, 1 H), 3.6–3.4, (m, 1 H). Acceptor 22 was glycosylated with
donor 1a under standard reaction conditions to afford compound
N-(Benzyloxycarbonyl)-O-[3,4-di-O-benzoyl-2-azido-6-O-(tert-
butyldiphenylsilyl)-2-deoxy-a-D-galactopyranosyl]-L- serine benzyl
ester (19)
Rf 0.52 (1 : 2 EtOAc/Hexanes), 1H NMR (360 MHz, CDCl3): d8.0
(d, J = 8.0, 2 H), 7.9 (d, J = 7.3 Hz, 2 H), 7.7–7.6 (m, 4H), 7.6–7.4,
(m, 5 H), 7.4–7.3 (m, 17 H), 7.1 (t, J = 7.3 Hz, 2 H), 6.0 (d, J =
2.6 Hz, 1 H), 5.7 (dd, J = 3.0,11 Hz, 1 H), 5.3–5.2 (m, 3 H), 5.1
(s, 1 H), 5.0(d, J = 3.4 Hz, 1 H), 4.7 (d, J = 8.1 Hz, 1 H), 4.2 (t,
J = 6.8 Hz, 1 H), 4.2–4.1 (m, 1 H), 4.0 (dd, J = 3.0, 11.1 Hz, 1 H),
3.8–3.7(m, 3 H), 1.0(s, 9 H); 13C NMR (90 MHz, CDCl3): d, 169.6,
165.2, 165.0, 156.0, 136.0, 135.5, 135.4, 135.0, 133.4, 133.3, 133.2,
132.7, 132.5, 129.8, 129.7, 129.6, 129.2, 128.6, 128.5, 128.4, 128.2,
128.0, 127.7, 99.5, 69.9, 69.6, 68.9, 68.0, 67.0, 61.3, 58.3, 54.5,
26.6, 18.9; HRMS (ESI-TOF) [MH+] calcd for C54H55N4O11Si
963.3631, found 963.3621. IR (cm-1), 2931, 2112.42, 1729, 1215,
1068. UV/VIS, l; 231 nm.
1
23. Rf 0.63 (1 : 2 EtOAc/hexanes) H NMR (360 MHz, CDCl3):
d8.0 (d, J = 7.3 Hz, 2 H), 7.9 (d, J = 7.5 Hz, 3 H), 7.6 (t, J = 5.8 Hz,
4 H), 7.5 (t, J = 7.9 Hz, 1 H), 7.4–7.3 (m, 21 H), 4.5 (br, 2 H), 5.9
(d, J = 8.4 Hz, 1 H), 5.7 (d, J = 3.0 Hz, 1 H), 5.6 (s, 1 H), 5.3 (dd,
J = 3.0, 11 Hz, 1 H), 5.3–5.2 (m, 2 H), 5.2 (d, J = 4.5 Hz, 2 H),
5.0 (d, J = 3.4 Hz, 1 H), 4.8 (d, J = 3.4 Hz, 1 H), 4.7 (br, 1 H),
4.3–4.1 (m, 5 H), 3.8–3.7(m, 2 H), 3.7–3.5 (m, 4 H), 2.0 (s, 3 H),
1.9 (s, 3 H), 0.97 (s, 9 H), 13C NMR (90 MHz, CDCl3) d, 169.7,
165.2, 165.0, 156.0, 136.2, 135.5, 135.2, 133.5, 133.2, 132.9, 132.8,
N-(Benzyloxycarbonyl)-O-[3,4-di-O-benzoyl-2-azido-6-O-(tert-
butyldiphenylsilyl)-2-deoxy-a-D-galactopyranosyl]-L- threonine
benzyl ester (20)
Rf 0.58 (1 : 2 EtoAc/Hexanes), 1H NMR (360 MHz, CDCl3): d7.9
(2d, J = 7.0, 6.9 Hz, each 2 H), 7.7–7.6 (m, 4 H), 7.–7.4, (m, 4 H),
7.4–7.3 (m, 22 H), 7.2 (t, J = 7.5 Hz, 2 H), 6.0 (d, J = 2.8 Hz, 1 H),
6830 | Org. Biomol. Chem., 2011, 9, 6825–6831
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129.7, 129.0, 128.6, 128.5, 128.4, 128.2, 128.0, 127.6, 99.0, 97.3,
69.3, 68.9, 68.8, 68.7, 68.6, 67.8, 67.6, 64.5, 67.1, 66.1, 61.3, 58.1,
57.6, 54.2, 29.6, 26.6, 20.6, 18.9; HRMS (ESI-TOF) [MH+] calcd
for C64H68N7O17Si 1234.4435, found 1234.4446. IR (cm-1), 2931,
2112, 1726, 1511, 1068, 909. UV/VIS, l; 232 nm.
9 S. Chamberland, J. W. Ziller and K. A. Woerpel, J. Am. Chem. Soc.,
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Financial support was provided by Binghamton University Re-
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