Practical synthesis of 4′-selenopurine nucleosides by combining chlorinated purines and ‘armed’ 4-selenosugar

Article abstract of DOI:10.1016/j.tet.2016.08.071

The synthesis of a variety of chemically modified oligonucleotides requires the development of a practical synthetic method for its building block, i.e., nucleoside analogs. The one-pot Pummerer-like reaction using hypervalent iodine in combination with 6-chloropurine and an ‘armed’ 4-selenosugar bearing an electron-donating group at the 2-position gave the desired 4′-seleno-6-chloropurine derivative in higher yield as compared to the previous method using a ‘disarmed’ 4-selenosugar bearing an electron-withdrawing group at the 2-position. In addition, the use of 2,6-dichloropurine as a nucleobase transformable into guanine skeleton enabled an effective Pummerer-like reaction followed by isomerization to the desired N9 isomer under the acidic conditions. This Pummerer-like reaction between chlorinated purine bases and an ‘armed’ 4-selenosugar is advantageous because it affords 4′-selenopurine nucleosides in one-pot without the need for isolation of the unstable selenoxide derivative.

Full text of DOI:10.1016/j.tet.2016.08.071

Tetrahedron 72 (2016) 6589e6594
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Tetrahedron
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Practical synthesis of 40-selenopurine nucleosides by combining
chlorinated purines and ‘armed’ 4-selenosugar
Kazuki Ishii a, Noriko Saito-Tarashima a, Masashi Ota a, Seigi Yamamoto a,
,
Yasuko Okamoto b, Yoshiyuki Tanaka b, Noriaki Minakawa a
*
a Graduate School of Pharmaceutical Science, Tokushima University, Shomachi 1-78-1, Tokushima 770-8505, Japan
b Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2016
Received in revised form 22 August 2016
Accepted 24 August 2016
Available online 26 August 2016
The synthesis of a variety of chemically modified oligonucleotides requires the development of a prac-
tical synthetic method for its building block, i.e., nucleoside analogs. The one-pot Pummerer-like reaction
using hypervalent iodine in combination with 6-chloropurine and an ‘armed’ 4-selenosugar bearing an
electron-donating group at the 2-position gave the desired 40-seleno-6-chloropurine derivative in higher
yield as compared to the previous method using a ‘disarmed’ 4-selenosugar bearing an electron-
withdrawing group at the 2-position. In addition, the use of 2,6-dichloropurine as a nucleobase trans-
formable into guanine skeleton enabled an effective Pummerer-like reaction followed by isomerization
to the desired N9 isomer under the acidic conditions. This Pummerer-like reaction between chlorinated
purine bases and an ‘armed’ 4-selenosugar is advantageous because it affords 40-selenopurine nucleo-
sides in one-pot without the need for isolation of the unstable selenoxide derivative.
Keywords:
40-Selenopurine nucleosides
Pummerer-like reaction
Hypervalent iodine
4-Selenosugar
Ó 2016 Elsevier Ltd. All rights reserved.
Chloropurine
1. Introduction
synthesis of 40-selenonucleic acids have not been performed
appropriately. In this sense, unlike its 40-thio congener, the effective
The synthesis of chemically modified oligonucleotides (ONs) is
still receiving much attention towards the utility of nucleic acid-
based therapeutics, which are expected to show higher hybridiza-
tion ability and superior nuclease resistance as compared to natural
ONs. Thus, a variety of modified nucleoside units have been syn-
thesized and incorporated into ONs.1,2 Our group has been working
on the development of chemically modified ONs having hybridiza-
tion ability and nuclease resistance while also being biocompatible
with natural DNA and RNA. To date, we have reported the synthesis
and properties of 40-thionucleic acids such as 40-thioDNA3e5 and 40-
thioRNA6e8 with a sulfur atom instead of a furanose-ring oxygen.
The 40-thionucleic acids exhibited biocompatibility with natural
DNA and RNA while also having superior hybridization ability and
nuclease resistance, and thus, were utilized in gene silencing via
RNAi technology9e13 and in the selection of aptamers by SELEX.14,15
These successful results prompted us to develop the novel ONs such
as 40-selenonucleic acids. Moreover, we have recently reported the
chemical synthesis of 40-selenoRNA16 and enzymatic synthesis of
40-selenoDNA using the corresponding nucleoside 50-tri-
phosphate.17 However, comprehensive investigations on the
synthesis of 40-selenonucleosides, especially 40-selenopurine
derivatives, has been unsuccessful thus far.18,19
To date, our group has reported the synthesis of 40-selenopyr-
imidine nucleosides via the practical Pummerer-like reaction using
hypervalent iodine (Fig. 1a).20 This method is advantageous because
the condensation reaction between 4-selenosugar 1 and uracil can
be achieved in one-pot, thereby avoiding the isolation of the
unstable selenoxide derivative. However, coupling reaction
of 6-chloropurine and 2-amino-6-chloropurine with 1 have been
unsuccessful. Although the moderate yield of undesired N7-6-
chloropurine isomer was observed in the reaction using
hypervalent iodine (>40%), isomerization to the desired N9-6-
chloropurine isomer was undetectable (unpublished results).
Recently, Jeong and co-workers reported the first synthesis of 40-
selenoadenosine and 40-selenoguanosine via the Vorbruggen
reaction between 1-acetoxyseleno sugar
4 (prepared from
4-selenosugar 3) and 6-chloropurine and 2-amino-6-chloropurine
(Fig. 1b).21 Although this synthesis pathway requires more steps
than our method and the yield of N9-2-amino-6-chloropurine
producing 40-selenoguanosine was still insufficient, these authors
remarkably achieved significantly higher coupling yields with
6-chloropurine (61%) while smoothly achieving the isomerization of
kinetic N7-6-chloropurine isomer 5 to the thermodynamic N9
* Corresponding author. Fax: þ81 88 633 7288; e-mail address: minakawa@
tokushima-u.ac.jp (N. Minakawa).
http://dx.doi.org/10.1016/j.tet.2016.08.071
0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

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K. Ishii et al. / Tetrahedron 72 (2016) 6589e6594
yields. In this paper, we report the details of the practical synthesis
of 40-selenopurine derivatives.
2. Results & discussion
The desired 4-selenosugar 3 was first prepared according to
reported methods.21,23 As we were planning to use the resulting 40-
selenonucleosides for further applications such as 40-selenoRNA
synthesis, large-scale synthesis avoiding tedious chromatographic
purification were explored. Accordingly, the desired product 3 was
obtained in 27% overall yield (8 steps) starting from 0.3 mol of D-
ribose and involving one chromatographic purification as the last
step (see Supplementary data).
With the resulting product 3 in hand, the Pummerer-like re-
action using hypervalent iodine in the presence of 6-chloropurine
was examined. Thus, a solution of silylated 6-chloropurine was
added to a mixture of 3 and iodosylbenzene in dichloroethane, and
2,6-lutidine was subsequently added to the reaction mixture. The
resulting mixture was subsequently heated (see Experimental
section), and the formation of N7-6-chloropurine isomer 5 fol-
lowed by isomerization to the desired N9 isomer 6 were both
monitored by TLC. Unlike the reaction involving 1, the desired
isomerization to N9 isomer 6 readily proceeded by heating the
reaction mixture at 85 ꢀC for 13 h. However, the formation of
a-
isomers (inseparable mixture by chromatographic purification)
was also observed for long reaction times (Table 1, entry 1). With
the aim to avoid the formation of
shortened to 2.5 h, which was immediately after the consumption
of starting 4-selenosugar 3. As expected, the formation of -isomers
a-isomers, the reaction time was
Fig. 1. Previous synthetic studies of 40-selenonucleosides.
a
was less, thereby the chemical yields of 5 and 6 were improved
(Table 1, entry 2). Conversely, consumption of 3 required longer
times and subsequent isomerization to 6 was insufficient at lower
temperature (Table 1, entry 3). Because the separated N7 isomer 5
was isomerized to the desired N9 isomer 6 in 53% yield upon
treatment with trimethylsilyl trifluoromethane sulfonate (TMSOTf)
in toluene at 90 ꢀC, we succeeded in obtaining 6 in 55% yield from 3.
This was much better than that of the previous report (55% in 2
steps vs 32% in 4 steps).21
Because the coupling reaction with 6-chloropurine proceeded
readily, we subsequently examined the reaction with 2-amino-6-
chloropurine, which is usually used for the synthesis of guano-
sine analogs.18 Thus, the Pummerer-like reaction of 3 using
hypervalent iodine in the presence of 2-amino-6-chloropurine was
conducted in a similar manner as described for 6 (Scheme 1). As
isomer 6 producing 40-selenoadenosine under their reaction con-
ditions. Based on these research backgrounds, we conceived that 4-
selenosugar 1 possessing an electron-withdrawing group (i.e., 2,4-
dimethoxybenzoyl; DMBz) at 2-position, known as ‘disarmed’
sugar,22 was difficult to couple with a nucleobase and to isomerize
to desired N9 isomer via selenium intermediate. The 4-selenosugar
3 used by the Jeong’s group can be recognized as an ‘armed’ sugar,22
because it has an electron-donating group at 2-position. Accord-
ingly, we envisioned the reaction between purine bases and ‘armed’
4-selenosugar 3 in the presence of hypervalent iodine (our one-pot
conditions). As a result, we found that 6-chloropurine and 2,6-
dichloropurine were suitable to produce the desired N9 isomers,
thereby successfully preparing 40-selenopurine nucleosides in good
Table 1
Pummerer-like reaction with 6-chloropurine
Entry
Conditions
Temp (ꢀC)
Yields (%)
Time (h)
5
6
a-N7/N9
1
2
3
85
85
50
13.5
2.5
9.5
18
31
40
27
39
24
26
8
7

K. Ishii et al. / Tetrahedron 72 (2016) 6589e6594
6591
Scheme 1. Synthesis of 40-selenoguanosine derivatives.
a result, the desired N9 isomer 8 (16%) was obtained along with N7
isomer 7 (31%) after heating for 8.5 h. The separated product 7 was
then treated with TMSOTf in toluene at 90 ꢀC. However, unlike 5, no
isomerization into the desired product 8 was observed. These re-
sults indicated that 4-selenosugar 3 was much better than 1 for the
Pummerer-like reaction using hypervalent iodine. But the reaction
between 4-selenosugar 3 and 2-amino-6-chloropurine was still
insufficient from the viewpoint of isomerization. Accordingly, we
examined the Pummerer-like reaction with a variety of nucleobases
transformable into guanine skeleton. As a result, no coupling
product was obtained when 2-N-acetyl-6-O-diphenylcarba-
moylguanine24 and 5-amino-1H-imidazole-4-carboxamine25 were
examined. Conversely, some coupling product was observed
(<20%) in the reaction with 6-chloro-2-methylthiopurine26,27 (data
not shown). These results reveal that the exocyclic amino group on
the nucleobase was unsuitable for both coupling and isomerization
reactions to the desired N9 isomer, and prompted us to examine the
Pummerer-like reaction over 2,6-dichloropurine.28 To our delight,
the coupling reaction proceeded smoothly to give a mixture of N7
isomer 9 and N9 isomer 10 in good yield (64%, 1:1 inseparable
mixture). The resulting mixture was subsequently treated with
TMSOTf in toluene at 90 ꢀC to exclusively afford the desired product
10 in 62% yield after chromatographic purification.
we reported that selenoxide 14 gradually deoxygenated in CDCl3 to
give 1 within 1 week.20 We also noticed that selenoxide 15
decomposed much faster than 14 in CDCl3 within 1 day. Further-
more, unlike the decomposition of 14 into 1, the 1H NMR spectrum
of the decomposed product of 15 revealed the presence of an al-
dehyde proton (see Fig. S1 in Supplementary data). Thus, structural
analysis of the decomposed product after chromatographic purifi-
cation revealed that selenoxide 15 was readily converted into
a known aldehyde 1932 in CDCl3. A plausible mechanism explaining
the formation of 19 is illustrated in Figure 2. Thus, the trans-
formation of 15 was probably initiated by selenoxide syn-elimina-
tion to give selenenic acid 16. The resulting product 16 was readily
converted into the aldehyde 19 via seleniranium 17 and acetal 18
intermediates. Similar transformations of the selenoxide involving
the formation of selenenic acid followed by the seleniranium
With the aim of converting 10 into its guanosine derivative, 10
was first treated with methanolic ammonia at 80 ꢀC in a steel
container to give 11 in 95% yield.28 After conversion of the 6-amino
group on 11 into a 6-hydroxy group via diazotization (85%),29 the
resulting product 12 was treated again with methanolic ammonia
at 150 ꢀC in a steel container to give the desired guanosine de-
rivative 13 (77%),30 whose analytical data was identical with the
reported data.21
As expected, the Pummerer-like reaction with 4-selenosugar 3
achieved significantly better coupling and isomerization results as
compared to 1. This result can be explained by the armed/disarmed
principle reported by Fraser-Reid et al.22 Although the reaction with
2-amino-6-chloropurine was still insufficient because its exocyclic
amino group may inhibit the acid catalyzed isomerization via
protonation at the N7 position of the nucleobase, this issue can be
overcome by using 2,6-dichloropurine.
As reported by our group and others,20,23,31 selenoxides 14 and
15, prepared from 1 and 3, respectively, are unstable. For example,
Fig. 2. Plausible mechanism giving 19.

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K. Ishii et al. / Tetrahedron 72 (2016) 6589e6594
intermediate were reported by Reich et al.,33 thereby supporting
our hypothesis. This different behavior of selenoxides 15 and 14
could be explained by their different sugar conformation. In the
case of 15, the silyloxymethyl substituent at 4-position should lo-
cate pseudoaxial because of the 2,3-isopropylidene protecting
group. Accordingly, the selenoxide oxygen and hydrogen atoms at
5-position are spatially close, thus syn-elimination preferentially
occurs. Conversely, the silyloxymethyl substituent at 4-position
should locate pseudoequatorial in the case of 14, thereby in-
creasing the distance between the selenoxide oxygen and hydrogen
atoms at 5-position. Accordingly, deoxygenation of 14 gradually
occurred without syn-elimination. As described above, our one-pot
Pummerer-like reaction does not require the formation of the un-
stable selenoxide derivative. Thus, we developed a practical syn-
thesis of 40-selenopurine nucleosides by combining chlorinated
purines with ‘armed’ 4-selenosugar 3 using hypervalent iodine.
and brine, dried (Na2SO4) and concentrated in vacuo. The residue
was purified by a silica gel column, eluted with hexane/EtOAc (6/
1e1/1), to give 5 (392 mg, 31% as a white foam) and the desired 6
(489 mg, 39% as a white foam). Then, to a solution of 5 (392 mg,
0.62 mmol) in toluene (5 mL) was added TMSOTf (56
mL,
0.31 mmol), and the reaction mixture was stirred for 30 min at
90 ꢀC. After being cooled to 0 ꢀC, the reaction was quenched by
addition of satd aqueous NaHCO3. The reaction mixture was par-
titioned between EtOAc and H2O, the organic layer was washed
with satd aqueous NaHCO3 and brine, dried (Na2SO4) and concen-
trated in vacuo. The residue was purified by a silica gel column,
eluted with hexane/EtOAc (6/1e1/1), to give 6 (207 mg, 53%; total
696 mg, total 55% in 2 steps from 3 as a white foam).
1H NMR data of 6;21 1H NMR (CDCl3)
d 8.66 (1H, s), 8.30 (1H, s),
7.68e7.61 (4H, m), 7.44e7.33 (6H, m), 6.27 (1H, d, J¼3.5 Hz), 5.04
(1H, dd, J¼3.5 and 5.5 Hz), 4.96 (1H, dd, J¼2.8 and 5.5 Hz), 4.13 (1H,
dt, J¼2.8 and 7.3 Hz), 4.07 (1H, dd, J¼7.3 and 10.3 Hz), 3.97 (1H, dd,
J¼7.3 and 10.3 Hz), 1.61 (3H, s), 1.30 (3H, s), 1.07 (9H, s).
3. Conclusion
1H NMR data of 5;21 1H NMR (CDCl3)
d 8.90 (1H, s), 8.64 (1H, s),
Here we prepared 4-selenosugar 3 in 27% overall yield (8 steps)
starting from 0.3 mol of D-ribose without chromatographic purifi-
7.69e7.65 (4 H, m), 7.46e7.37 (6H, m), 6.74 (1H, d, J¼3.8 Hz), 4.83
(1H, dd, J¼3.0 and 5.5 Hz), 4.78 (1H, dd, J¼3.8 and 5.5 Hz), 4.10 (1H,
dt, J¼3.0 and 7.0 Hz), 3.98 (1H, dd, J¼3.0 and 10.8 Hz), 3.92 (1H, dd,
J¼7.0 and 10.8 Hz), 1.60 (3H, s), 1.29 (3H, s), 1.10 (9H, s).
cation. The Pummerer-like reaction over 3 using hypervalent iodine
in the presence of 6-chloropurine and 2,6-dichloropurine, followed
by isomerization under acidic catalyst, afforded the desired N9
isomers 6 and 10, which are transformable to 40-selenoadenosine
and 40-selenoguanosine, in good yields, respectively. In the case of
coupling with uracil, the Pummerer-like reaction proceeded well in
‘disarmed’18,20 4-selenosugars 1 as well as ‘armed’34 3 in our re-
action conditions. Unlike the reaction with pyrimidine base, the
coupling reaction, followed by isomerization are required in the
case of coupling with purine bases. Thus, utilization of ‘armed’ 4-
selenosugar 3 and chlorinated purines would especially affect
isomerization to the desired N9 isomers. Thus, this route allows 40-
selenopurine and 40-selenopyrimidine nucleosides for the synthe-
sis of 40-selenoRNA. The biological applications of these compounds
are under investigation.
4.2. 2-Amino-6-chloro-7-(2,3-O-isopropylidene-5-O-tert-bu-
tyldiphenylsilyl-4-seleno-
2-amino-6-chloro-9-(2,3-O-isopropylidene-5-O-tert-butyldi-
phenylsilyl-4-seleno-
-ribofranosyl)-9H-purine (8)21
b-D-ribofranosyl)-7H-purine (7) and
b-D
In the similar manner as described for 6, 3 (476 mg, 1.0 mmol) in
1,2-dichloroethane (10 mL) was treated with silylated 2-amino-6-
chloropurine (339 mg, 2.0 mmol of 2-amino-6-chloropurine,
0.93 mL, 8.0 mmol of 2,6-lutidine, and 1.45 mL, 8.0 mmol of
TMSOTf) in the presence of iodosylbenzene (264 mg, 1.2 mmol) for
8.5 h at 85 ꢀC to give 7 (200 mg, 31% as a white foam) and the
desired 8 (103 mg, 16% as a white foam).
1H NMR data of 8;21 1H NMR (CDCl3)
d 7.90 (1H, s), 7.67e7.63
4. Experimental procedures
(4H, m), 7.43e7.35 (6H, m), 6.12 (1H, d, J¼3.5 Hz), 5.02 (1H, dd,
J¼3.5 and 5.5 Hz), 4.96 (2H, br s, exchangeable with D2O), 4.92 (1H,
dd, J¼2.8 and 5.5 Hz), 4.11e4.04 (2H, m), 3.94 (1H, dd, J¼2.8 and
5.8 Hz), 1.59 (3H, s), 1.30 (3H, s), 1.07 (9H, s).
Physical data were measured as follows: 1H and 13C NMR spectra
were recorded at 400 or 500 MHz and 100 or 125 MHz instruments
(Bruker FT-NMR AV400 or AV500) in CDCl3 with tetramethylsilane
as an internal standard or DMSO-d6 as the solvent. Chemical shifts
1H NMR data of 7;21 1H NMR (CDCl3)
d 8.34 (1H, s), 7.70e7.64
(4H, m), 7.47e7.38 (6H, m), 6.56 (1H, d, J¼3.5 Hz), 5.11 (2H, br s,
exchangeable with D2O), 4.81 (1H, dd, J¼3.3 and 5.8 Hz), 4.76 (1H,
dd, J¼3.5 and 5.8 Hz), 4.04 (1H, dt, J¼3.3 and 7.3 Hz), 3.94 (1H, dd,
J¼7.3 and 10.8 Hz), 3.88 (1H, dd, J¼7.3 and 10.8 Hz),1.58 (3H, s), 1.29
(3H, s), 1.09 (9H, s).
are reported in parts per million (d), and signals are expressed as s
(singlet), d (doublet), t (triplet), q (quartet), m (multiplet), or br
(broad). All exchangeable protons were detected by addition of
D2O. TLC was done on Merck Kieselgel F254 precoated plates. Silica
gel used for column chromatography was KANTO CHEMICAL Silica
Gel 60 (spherical) 63e210 mesh. Mass spectrum analysis was car-
ried out with SQD2 (Waters) and SYNAPT G2-Si HDMS (Waters).
4.3. 2,6-Dichloro-9-(2,3-O-isopropylidene-5-O-tert-butyldi-
phenylsilyl-4-seleno-b-D-ribofranosyl)-9H-purine (10)
4.1. 6-Chloro-9-(2,3-O-isopropylidene-5-O-tert-butyldiphe-
To a suspension of 2,6-dichloropurine (756 mg, 4.0 mmol) in
1,2-dichloroethane (20 mL) were added 2,6-lutidine (0.93 mL,
8.0 mmol) and TMSOTf (2.9 mL, 16 mmol), and the mixture was
stirred at room temperature until giving a clear solution. The
resulting solution containing silylated 2,6-dichloropurine was
added to another solution of 3 (951 mg, 2.0 mmol) in 1,2-
dichloroethane (20 mL) containing iodosylbenzene (528 mg,
2.4 mmol) dropwisely. Then, a solution of 2,6-lutidine (0.93 mL,
8.0 mmol) in 1,2-dichloroethane (10 mL) was added to the whole
mixture. After being stirred for 1 h at 85 ꢀC, the reaction was
quenched by addition of ice. The reaction mixture was partitioned
between EtOAc and H2O, the organic layer was washed with satd
aqueous NaHCO3 and brine, dried (Na2SO4) and concentrated in
vacuo. The residue was purified by a silica gel column, eluted with
hexane/EtOAc (4/1e1/1), to give a mixture of 9 and 10 (845 mg, 64%
nylsilyl-4-seleno-b-D
-ribofranosyl)-9H-purine (6)21
To a suspension of 6-chloropurine (618 mg, 4.0 mmol) in 1,2-
dichloroethane (20 mL) were added 2,6-lutidine (0.93 mL,
8.0 mmol) and TMSOTf (2.9 mL, 16 mmol), and the mixture was
stirred at room temperature until giving a clear solution. The
resulting solution containing silylated 6-chloropurine was added to
another solution of 3 (951 mg, 2.0 mmol) in 1,2-dichloroethane
(20 mL) containing iodosylbenzene (528 mg, 2.4 mmol) drop-
wisely. Then, a solution of 2,6-lutidine (0.93 mL, 8.0 mmol) in 1,2-
dichloroethane (10 mL) was added to the whole mixture. After
being stirred for 2.5 h at 85 ꢀC, the reaction was quenched by ad-
dition of ice. The reaction mixture was partitioned between EtOAc
and H2O, the organic layer was washed with satd aqueous NaHCO3

K. Ishii et al. / Tetrahedron 72 (2016) 6589e6594
6593
as a white foam). To the solution of the mixture of 9 and 10 in
toluene (10 mL) was added TMSOTf (120 L, 0.64 mmol), and the
reaction mixture was stirred for 1 h at 90 ꢀC. After being cooled to
ꢀC, the reaction was quenched by addition of satd aqueous
129.95, 127.83, 127.76, 123.60, 112.37, 90.42, 85.28, 65.86, 56.43,
51.20, 27.75, 26.80, 25.23, 19.21.
m
0
4.6. 9-(2,3-O-Isopropylidene-5-O-tert-butyldiphenylsilyl-4-
seleno-b-D
-ribofranosyl)guanine (13)21
NaHCO3. The reaction mixture was partitioned between EtOAc and
H2O, the organic layer was washed with satd aqueous NaHCO3 and
brine, dried (Na2SO4) and concentrated in vacuo. The residue was
purified by a silica gel column, eluted with hexane/EtOAc (4/1e1/1),
to give 10 (817 mg, 62% in 2 steps as a pale yellow foam).
A solution of 12 (644 mg, 1.0 mmol) in methanolic ammonia
(saturated at 0 ꢀC, 25 mL) was heated at 150 ꢀC for 4 h in a steel
container. After being cooled to room temperature, the reaction
mixture was concentrated in vacuo. Then, the residue was purified
by a silica gel column, eluted with MeOH in CHCl3 (1%e5%), to give
Analytical data of 10: ESI-LRMS m/z 685 (MNaþ); ESI-HRMS
calcd for C29H32Cl2N4O3NaSeSi 685.0684, found 685.0687; 1H
13 (478 mg, 77% as a white solid): 1H NMR (CDCl3)
d 12.04 (1H, br s,
NMR (CDCl3)
d 8.24 (1H, s), 7.67e7.62 (4H, m), 7.45e7.30 (6H, m),
6.26 (1H, d, J¼3.8 Hz), 5.01 (1H, dd, J¼3.8 and 5.5 Hz), 4.94 (1H, dd,
J¼2.8 and 5.5 Hz), 4.15e4.10 (2H, m), 3.99 (1H, dd, J¼10.3 and
12.8 Hz), 1.61 (3H, s), 1.30 (3H, s), 1.07 (9H, s); 13C NMR (CDCl3)
exchangeable with D2O), 7.67e7.64 (5H, m), 7.45e7.35 (6H, m), 6.09
(1H, d, J¼3.6 Hz), 5.98 (2H, br s), 5.00 (1H, dd, J¼3.6 and 5.5 Hz),
4.89 (1H, dd, J¼3.0 and 5.5 Hz), 4.11e4.01 (2H, m), 3.93 (1H, dd,
J¼6.3 and 9.0 Hz), 1.59 (3H, s), 1.30 (3H, s), 1.07 (9H, s).
d
153.05, 152.42, 152.09, 144.92, 135.59, 135.54, 132.86, 132.70,
131.38, 129.97, 127.82, 127.75, 112.48, 90.13, 85.35, 65.80, 57.06,
51.55, 27.73, 26.82, 25.19, 19.23.
Acknowledgements
Analytical data of 9: ESI-LRMS m/z 685 (MNaþ); ESI-HRMS calcd
for C29H32Cl2N4O3NaSeSi 685.0684, found 685.0683; 1H NMR
This work was supported by Grant-in-Aid for Scientific Research
(B) (Grant Number; 24390027 and H15H04656). N.T and S.Y. thank
the research program for development of intelligent Tokushima
artificial exosome (iTEX) from the Tokushima University. We would
like to thank Mr. S. Kitaike (Center for Instrumental Analysis,
Tokushima University) for technical supports of analytical data.
(CDCl3)
d 8.66 (1H, s), 7.67e7.64 (4H, m), 7.48e7.37 (6H, m), 6.64
(1H, J¼3.5 Hz), 4.82 (1H, dd, J¼3.0 and 5.5 Hz), 4.74 (1H, dd, J¼3.5
and 5.5 Hz), 4.10 (1H, ddd, J¼3.0, 7.3 and 7.3), 3.96 (1H, dd, J¼7.3 and
11.0 Hz), 3.91 (1H, dd, J¼7.3 and 11.0 Hz), 1.59 (3H, s), 1.28 (3H, s),
1.09 (9H, s); 13C NMR (CDCl3)
d 163.90, 153.49, 149.52, 143.91,
135.58, 135.54, 132.47, 130.16, 130.12, 127.95, 121.57, 112.58, 91.32,
85.11, 65.73, 59.54, 51.22, 27.47, 26.86, 24.99, 19.25.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2016.08.071.
4.4. 6-Amino-2-chloro-9-(2,3-O-isopropylidene-5-O-tert-bu-
tyldiphenylsilyl-4-seleno-b-D-ribofranosyl)-9H-purine (11)
References and notes
A solution of 10 (2.72 g, 4.1 mmol) in methanolic ammonia
(saturated at 0 ꢀC, 100 mL) was heated at 80 ꢀC for 11 h in a steel
container. After being cooled to room temperature, the reaction
mixture was concentrated in vacuo. Then, the residue was purified
by a silica gel column, eluted with hexane/EtOAc (2/1e1/2), to give
11 (2.51 g, 95% as a white foam): ESI-LRMS m/z 666 (MNaþ); ESI-
HRMS calcd for C29H34ClN5O3NaSeSi 666.1182, found 666.1179; 1H
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d 7.89 (1H, s), 7.69e7.63 (4H, m), 7.45e7.33 (6H, m),
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d 156.30, 154.11, 150.57, 139.93, 135.59, 135.55, 133.04,
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