A general method for N-glycosylation of nucleobases promoted by (p-Tol)2SO/Tf2O with thioglycoside as donor

Article abstract of DOI:10.1039/c5cc03617h

Based on a preactivation strategy using the (p-Tol)₂SO/Tf₂O system, a series of nucleosides were synthesized by coupling various thioglycosides with pyrimidines and purines under mild conditions. High yields and excellent β-stereoselectivities were obtained with either armed or disarmed N-glycosylation donors by tuning the amount of (p-Tol)₂SO additive.

Full text of DOI:10.1039/c5cc03617h

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A general method for N-glycosylation of
nucleobases promoted by (p-Tol)2SO/Tf2O
with thioglycoside as donor†
Cite this: Chem. Commun., 2015,
51, 12803
Received 30th April 2015,
Accepted 2nd July 2015
Guang-jian Liu, Xiao-tai Zhang and Guo-wen Xing*
DOI: 10.1039/c5cc03617h
www.rsc.org/chemcomm
Based on a preactivation strategy using the (p-Tol)2SO/Tf2O system, Through these advances, many novel nucleoside analogues
a series of nucleosides were synthesized by coupling various thio- have been successfully synthesized. However, it is still desired
glycosides with pyrimidines and purines under mild conditions. to develop new universal strategies for various armed/disarmed
High yields and excellent b-stereoselectivities were obtained with sugar donors and pyrimidine/purine acceptors to synthesize
either armed or disarmed N-glycosylation donors by tuning the nucleosides under mild conditions.
amount of (p-Tol)2SO additive.
Thioglycoside, one of the most useful donors for O-glycosylation,
is convenient to prepare and stable under many reaction
Nucleosides have been arguably one of the most pre-eminent conditions.15a However, few studies16 using thioglycoside for
compounds since the structure of DNA was elucidated by nucleoside synthesis have been reported. Moreover, the substrate
Watson and Crick.1 To date, modified nucleosides have been scope was mainly limited to pentose sugars and pyrimidine
used increasingly in expanding the genetic code,2 developing new nucleobases. In particular, low yields and poor selectivities were
antiviral and antitumor drugs,3 as well as creating biological probes.4 obtained with ambidentate and weakly nucleophilic purines as
Therefore, the study of nucleoside synthesis has continuously been a acceptors.16c–f Over the years, the preactivation protocol with the
topical subject, and a large number of studies have been reported.5,6 (p-Tol)2SO/Tf2O system, developed from the traditional Ph2SO/Tf2O
Among various methodologies to construct nucleosides, the pair,15b has been demonstrated to be a practical, straightforward,
N-glycosylation reaction involving the direct coupling of a sugar and high stereoselectivity method for O- and C-glycosylations
moiety with a nucleobase is the simplest synthetic strategy, and using N-acetyl-5-N,4-O-oxazolidione protected thiosialoside as
has been classified into three types: (1) acid-catalyzed fusion of donor.17 In continuation of our studies of this glycosylation
peracylated sugars with nucleobases;7 (2) reaction of a metal protocol, we have investigated whether this method is suitable
salt of heterocyclic systems with protected sugar halides;8 (3) for N-glycosylation, and finally establish one general method for
the silyl-Hilbert–Johnson method developed by Vorbru¨ggen et al.9 synthesis of various O-, C-, and N-glycoconjugates.
In particular, the Vorbru¨ggen reaction has been used widely and
Herein, we report a facile and general method, based on the
has been developed into the most prevailing route for nucleoside preactivation strategy promoted by ( p-Tol)2SO/Tf2O, for the
synthesis.10 More recently, further more efficient methods have production of nucleoside analogues from armed/disarmed
been devised to enhance the product yield and selectivity of the sugar donors and various nucleobases in high yields and with
coupling reaction. For example, Yu et al. coupled the glycosyl high stereoselectivities.
ortho-alkynylbenzoates with nucleobases using a gold(I) complex
With donor 1a in hand, we initially performed the model
as an activator in high regioselectivity11 and stereoselectivity.12 N-glycosylation of 1a with trimethylsilylated uracil 2a using the
Fraser-Reid13 and co-workers utilized a reverse synthesis strategy preactivation protocol. A solution of 1a and ( p-Tol)2SO (1.2 equiv.)
permitting prior extensive structural modification to the ribose was preactivated by Tf2O (1.2 equiv.) in dichloromethane at
unit. In addition, Jamison’s research group14 telescoped cumber- ꢀ70 1C for 0.5 h, followed by addition of 2a (3.0 equiv.) in CH3CN.
some nucleoside synthesis processes into a continuous one- The resulting mixture was stirred for 2 h, and then warmed up to
flow multistep sequence using pyridinium triflates as catalysts. ꢀ50 1C for another 2 h. The coupling product 3 (Table 1, entry 1)
was obtained in 60% yield with exclusively b-stereoselectivity.
Based on our previous findings17 that an excessive amount of
( p-Tol)2SO can raise the glycosylation yields by stabilizing the
Department of Chemistry, Beijing Normal University, Beijing 100875, China.
E-mail: gwxing@bnu.edu.cn
active intermediates derived from donors, we increased the
† Electronic supplementary information (ESI) available: Detailed experimental
procedures. See DOI: 10.1039/c5cc03617h
amount of ( p-Tol)2SO from 1.2 equiv. to 3.0 equiv., and found
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Table 1 Effect of the (p-Tol)2SO amount and preactivation temperature
on N-glycosylation with 1a as the donor
Entry
T (1C)
ꢀ70
( p-Tol)2SO (equiv.)
Yield (%)
a/b
1
2
3
4
5
6
1.2
2.0
3.0
1.2
2.0
3.0
60
57
79
66
91
99
b
b
b
b
b
b
ꢀ60
that the corresponding yields were not increased dramatically
(Table 1, entries 2 and 3). These inferior results, which we
attributed to the deactivating effect of the acetyl protecting
group on donor 1a, prompted us to optimize the reaction
conditions by modulating both the temperature of preactivation
and the amount of ( p-Tol)2SO at the same time. Surprisingly,
when the preactivation temperature was elevated to ꢀ60 1C, the
isolated product yield jumped significantly from 66% to 99% by
increasing the amount of ( p-Tol)2SO from 1.2 equiv. to 3.0 equiv.
(Table 1, entries 4–6).
During the above optimization studies, it was revealed that
the efficiency of the N-glycosylation was significantly dependent
on the preactivation temperature and the ( p-Tol)2SO amount,
which are closely related to the donor reactivity and the stability
of the glycosylation transition state, respectively. Higher pre-
activation temperature could benefit the unreactive donors,
and greater ( p-Tol)2SO amount would be helpful to trap the
less stable glycosylation intermediates.
Next, a series of frequently used thioglycosides 1a–f and
nucleobase derivatives 2a–f (Fig. 1) were prepared to explore the
substrate scope of the N-glycosylation promoted by ( p-Tol)2SO/
Tf2O. We first investigated the reaction of pyrimidine nucleobases
2a–c with donors 1a–f, derived from D-galactose, D-glucose and
D-ribose (Scheme 1). Under the optimized reaction conditions
(ꢀ60 1C - ꢀ40 1C, ( p-Tol)2SO (3.0 equiv.)), the coupling reactions
of 1a with TMS-thymine (2b) and TMS-N4-benzoylcytosine (2c)
provided nucleosides 4 and 5 in excellent yields (496%).
Under the same reaction conditions for 1a, the N-glycosylation
of 1b with 2a afforded product 6 in a higher yield (76%) than that
observed by Yu11 (67%) with glycosyl ortho-alkynylbenzoate as
donor. Moreover, upon repetition of the reaction of 1b and 2a by
warming up the preactivation temperature to ꢀ50 1C, a much
Scheme 1 N-Glycosylation of glycosyl donors with pyrimidines.
higher yield (84%, Scheme 1) was obtained, and similar results
were obtained for 2b and 2c. In addition, the peracylated
ribofuranose 1c reacted with TMS-pyrimidines (2a–c) to provide
the desired b nucleosides 9–11 in good yields.
Taking into account that the benzoyl protected donors are
less reactive than the corresponding acetyl protected donors,18
we performed the coupling reactions of 1d–f with nucleobases
(2a–c) at a higher preactivation temperature. Much to our
delight, among all these N-glycosylation reactions, satisfactory
yields of the coupling products 12–20 were provided with eight
out of nine examples ^83% (Scheme 1).
Then, we focused on the real challenge of N-glycosylation
with purines. Since Boc-protected purine nucleobases have
been prepared19a and used in N-glycosylations and other reactions
with good solubility and high N9/N7 regioselectivity,10,11,19b–d
we selected purine analogues, 2,6-dichloropurine (2d), N2-tert-
butoxycarbonyl-2-amino-6-chloropurine (2e) and N6-bis(tert-butoxy-
carbonyl)-adenine (2f), as acceptors to extend the N-glycosylations
with thioglycosides as donors. As expected, donor 1f coupled
with acceptor 2d smoothly and furnished exclusively product 21
quantitatively. Notably, the structure of the coupling product 21
was determined unequivocally on the basis of the precedent20 as
well as the combined analysis of 1D and 2D NMR (1H, 13C,
HSQC, HMBC) experiments. In particular, the strong HMBC
correlation between H-10 and C4 rather than H-10 and C5 was
telltale (see ESI,† for details) for confirming that compound 21
is an N9 product. Similarly, N2-tert-butoxycarbonyl-2-amino-6-
chloropurine (2e) also underwent N-glycosylation with 1f to
predominantly give the desired N9-nucleoside 22 in good yield
(75%). In the case of N6-bis(tert-butoxycarbonyl)adenine (2f ),
coupling product 23 was isolated along with the corresponding
mono-Boc-protected nucleoside as a side product (Scheme 2).
In a similar manner, the coupling reactions of the three
purine nucleobases (2d–f) with 1d and 1e were also examined.
N9-nucleosides 24–28 were obtained in moderate to excellent yields
(61–100%, Scheme 2), and the corresponding N7-nucleosides were
not detected. The structures of 22–28 were definitively corroborated
by NMR experiments as described for nucleoside 21.
Next, in an effort to ascertain the efficiency of the N-glycosylation
based on the ( p-Tol)2SO/Tf2O preactivation protocol completely, and
Fig. 1 Structures of glycosyl donors and acceptors for nucleosides
syntheses.
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Table 3 N-Glycosylation of perbenzyl protected donor 1h with 2a–c
under different preactivation conditions
Entry
Product
T (1C)
ꢀ70
( p-Tol)2SO (equiv.)
Yielda (%)
a/bb
1
2
3
4
5
6
7
8
32
4.0
6.0
2.0
4.0.
6.0
2.0
6.0
6.0
64
56
92
Quant.
99
93
99
80
1 : 4.3
1 : 4.6
1 : 5.6
1 : 8.7
1 : 17
1 : 3.6
1 : 25
1 : 20
ꢀ60
33
34
ꢀ60
ꢀ60
a
b
Isolated yields. Determined by NMR.
Scheme 2 N-Glycosylation of glycosyl donors with purines.
TMS-pyrimidines (2a–c) were performed using different amounts of
(p-Tol)2SO with the preactivation temperature elevated to ꢀ60 1C.
More efficient N-glycosylations were achieved (Table 3, entries 5, 7
and 8), indicating that the yield and stereoselectivity of the
N-glycosylations could be tuned to a certain degree by changing
the amount of (p-Tol)2SO and the preactivation temperature,
according to the characteristics of the used donors.
especially to probe the influence of different added amounts of
( p-Tol)2SO on the reaction, some complementary experiments
with perbenzyl protected thioglycosides (1g and 1h) as donors
were performed (Tables 2 and 3).
Exploratory reactions of 1g with 2a were undertaken with
different amounts of ( p-Tol)2SO. As shown in Table 2, with
increasing amount of ( p-Tol)2SO from 1.2 equiv. to 6.0 equiv.,
both the yield and stereoselectivity of the reaction increased
significantly (Table 2, entries 1–5). Remarkably, similar excellent
yields (B100%) and b-stereoselectivities were also obtained in the
syntheses of 30 and 31 with 6.0 equiv. of ( p-Tol)2SO used for the
N-glycosylations (Table 2, entries 6 and 7). To the best of our
knowledge, this is the first time that perbenzyl protected nucleo-
sides have been obtained with such high stereoselectivity and
yield simultaneously without the anchimeric assistance at the C2
position of glycosyl donors.21
Table 3 illustrates the results of our efforts to examine the
N-glycosylations between perbenzyl protected glucopyranoside (1h)
and nucleobases. Interestingly, when the preactivation temperature
was ꢀ70 1C, the results of the reaction were not satisfactory, even
when 6.0 equiv. of ( p-Tol)2SO was used (Table 3, entry 1, 2). This
somewhat surprising fact was comparable with previous observa-
tions (Table 1), and probably arose from the low reactivity of donor
1h at very low reaction temperature. Analogous reactions of 1h with
In addition, the above N-glycosylation protocol was also
applied to the syntheses of 2-deoxyribonucleosides with 2-deoxy-
b-thioriboside 1i and TMS-thymine (2b) as glycosyl donor and
acceptor, respectively (Table 4). Interestingly, when the amount of
(p-Tol)2SO additive was increased from 0.8 equiv. to 4.0 equiv., the
product yield of 2-deoxyribonucleoside 35 was elevated from 53%
to B100%, whereas the b-stereoselectivity of the reaction
decreased from b/a = 7.8 :1 to 2.8:1 (Table 4, entries 1–5). These
results are similar to our previous research findings for another
kind of deoxysugar, sialic acid, wherein the a-stereoselectivity
dropped down with increasing Ph2SO amount,17a,d indicating that
the detailed glycosylation mechanism could be different between
deoxysugar- and ordinary sugar-involving reactions. It is noted
that much lower b-stereoselectivity (b/a = 1.6 :1) and reaction yield
(43%) were obtained in the same N-glycosylation of 1i and 2b with
NIS/TfOH16e as promoter. Considering the influences of both
reaction yield and b-stereoselectivity on the coupling, 1.2 equiv.
of ( p-Tol)2SO was employed for the subsequent N-glycosylation of
TMS-uracil (2a) and TMS-N4-benzoylcytosine (2c). The expected
Table 2 N-Glycosylation of perbenzyl protected donor 1g with 2a–c
using different amounts of (p-Tol)2SO
Table 4 N-Glycosylation of perbenzoyl protected 2-deoxy-b-thioriboside
1i with nucleobases 2a–c
Entry
Product
( p-Tol)2SO (equiv.)
Yielda (%)
a/bb
Entry
Product
( p-Tol)2SO (equiv.)
Yielda (%)
a/bb
1
2
3
4
5
6
7
29
1.2
2.0
3.0
4.0
6.0
6.0
6.0
76
92
99
97
Quant.
Quant.
Quant.
1 : 2.5
1 : 2.7
1 : 3.7
1 : 7.9
41 : 30
b
1
2
3
4
5
6
7
35
0.8
1.2
1.6
2.0
4.0
1.2
1.2
53
75
86
99
Quant.
79
54
1 : 7.8
1 : 6.6
1 : 5.1
1 : 3.2
1 : 2.8
1 : 4.5
1 : 5.1
30
31
36
37
b
a
b
a
b
Isolated yields. Determined by NMR.
Isolated yields. Determined by NMR.
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This project was financially supported by the Natural
Science Foundation of China (21272027) and Beijing Municipal
Commission of Education.
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