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S. S. Bag et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2044–2050
stereoselective formation of b-glycosylated tetrazolyl nucleosides.
We envisioned that the steric bulk of the C-5 aromatic units of
tetrazoles and the stereoelectronic interactions with sugar might
influence the exclusive formation of N2-b-glycosylated nucleo-
sides. Moreover, comparatively less polar aprotic solvent such as
THF and larger K+ ion might also influence both regioselectivity
at N2-tetrazole as well as stereoselective glycosylation on Hoffer’s
chlorosugar. Our experimental results suggested the stereoselec-
tive and regioselective formation of N2-b-tetrazolyl donor/
acceptor aromatic nucleosides in all the cases with good yield
and no other side reaction. We also carried out theoretical calculation
(DFT) to support our experimental results and explained on the
basis of both steric and electronic factors operative in the SN2 tran-
Figure 2. Crystal packing (a) and molecular arrangement (b) of nucleoside 15a
[CCDC 1015970]. Crystal packing (c), molecular arrangement (d) and N. . .I bonded
molecular arrangement of nucleoside 17a [CCDC 1015969].
sition state of Hoffer’s
a-chlorosugar with 5-substituted tetrazole
nucleophiles. We believe that this work would have great potential
not only in regioselective alkylation of tetrazoles or stereoselective
glycosylation but surely draw attention to synthetic chemists.
For the creation of new class of donor/acceptor tetrazolyl
nucleosides we first synthesized 5-substituted aromatic tetrazoles.
Most of the reported route to 5-substituted tetrazoles suffers from
several disadvantages,6,7 we, therefore, employed a solvent free
eco-compatible strategy following a modified literature proto-
col.7c–e Thus, the aromatic tetrazoles (2A–L) were prepared via a
cycloaddition reaction of aryl nitriles (1A–L) with TMS-N3 under
solvent free condition at 85 °C using TBAF as an efficacious basic
activator (Scheme 1 and SI Sections 2 and 3). All the tetrazoles
except 2A, 2B and 2H are new and were characterized by 1H, 13C
and mass spectrometry.7e
The b-anomeric configuration and the N2-alkylation was fur-
ther confirmed from a X-ray single crystal structure analysis of
TzBB
TzFIB
bis-toluoylated tetrazolyl nucleosides,
B
(15a) and
B
Ac
Ac
(17a) (Fig. 2, SI Section 4). The X-ray diffraction analysis showed
that the orientation of the tetrazolyl bromobenzene ring of the
nucleoside 15a with respect to the sugar unit was locked via the
self-bonding interaction like N(tetrazole). . .Br(bromobenzene) (3.070 Å)
0
and Br(bromobenzene). . .O@C(5 -toluoyl) (3.189 Å). Overall the nucle-
oside 15a adopted
a
unidirectional layered structure
(Fig. 2a and b). On the other hand, the sugars of two consecutive
molecules pointed antiparallel and the nucleoside 17a adopted a
The synthesis of tetrazolyl aromatic nucleosides started with
layered structure held via p–p stacking and H-bonding like short
bis-toluoylated Hoffer’s
a
-chlorosugar, 6, prepared via our
interactions (Fig. 2c–e) with an inter-layer distance of 3.872 Å.
Next, we studied the sugar and N2-glycosidic conformation.
Sugar puckers in nucleosides TzBBBAc (15a) and TzFIBBAc (17a) were
South (S) (20-endo) and the tetrazole bases displayed anti-
reported procedure,2b,c which was made to undergo substitution
reaction with aromatic tetrazoles (2A–L, Scheme 1) and K2CO3 as
the base at room temperature in THF. The substitution of the
chloro- by 5-aryl-tetrazoles afforded N2-tetrazolyl-b-nucleosides
conformations with respect to the glycosyl torsion angle which
3
0
0
exclusively with very good yields (7a–18a, Scheme
Table 1). Most of the reported base mediated strategies for
N2-alkylation of tetrazoles offered mixture of both N2-/
1
and
was evidenced from a strong JH1 –H2 (7.2 for 15a and 7.0 for
17a Hz) coupling (SI, Section 5). Other modified nucleosides also
0
a
maintained a natural DNA-like C2 -endo sugar conformation as
N1-regiomers except only a few examples reported by Ovoskii
et al.6a,b,d Therefore, in the field of regioselective alkylation, our
result of exclusive formation of N2-glycosylated tetrazoles would
have great impact which we report for the first time and explain
on the basis of both steric as well as electronic effects. The bis-
toluoylated nucleosides (7a–18a) were then deprotected using
NaOMe in methanol to afford tetrazolyl donor/acceptor nucleo-
sides, 7b–17b, in very good yield (Scheme 1 and Table 1). The
pyridyl (18a) tetrazolyl nucleoside was not deprotected. All the
toluoyl protected and deprotected nucleosides were characterized
by NMR, mass, IR, melting temperature and in two cases by single
crystal X-ray analysis (SI Sections 2–4). Therefore, our experimen-
tal observations suggested the regioselective and stereoselective
formation of N2-tetrazolyl-b-nucleosides as the sole products
was revealed from the NOESY cross peak intensity and strong
JH1 –H2 coupling constants.9 A theoretical calculation based on
B3LYP/6-31G⁄ functional using G09 program package10a also sup-
ported the sugar conformations as S-locked nucleoside. A dihedral
scan for the glycosidic angle revealed that the adopted anti-glycosyl
conformation (f = ꢀ62°) of the tetrazole base was energetically
highly favorable compared to any other conformation (SI, Section 5)
which is probably due to the less repulsion between the lone pair
of tetrazole-N1 and O40 compared to the repulsion between tetra-
zoles-N3, N4 and O40. This repulsive force restrict the rotation of
the glycosyl bond and thus, fix the S-sugar conformation of
tetrazolyl nucleosides leading to the syn-disposition of the aryl
substituted tetrazolyl bases. Therefore, the tetrazolyl nucleosides
behaved the same way as the natural nucleosides. It is therefore
anticipated that the introduction into short oligonucleotide
sequence the tetrazolyl nucleoside with locked-S(anti) geometry
could fix the conformational state and would help understanding
the impact of conformational restrictions in DNA.9a,b
3
0
0
while no trace of
a-anomers or N1-tetrazolyl nucleosides were
observed. The b-stereoselectivity was also supported by the only
existing report of sodium salt glycosylation observed by
Ravankar.5c The tetrazolyl nucleosides reported herein were
found to be highly stable under strongly basic or acidic solution
or thermal condition. Therefore, during DNA synthesis the
It is a fact that, in addition to the a-/b-diastereomers, tetrazole
can in principle give rise to two regiomers N1- or N2-, upon forma-
tion of a glycosidic bond, the same was reported by Muller.8a
However, the electronic consideration based on Sadlej-
Sosnowska’s theoretical calculation11a and solvent polarity,11b,c in
combination with steric factors might have some bearing on our
repeated experimental observation on the regioselective
formation of N2-tetrazolyl-b-nucleosides. In aprotic solvents,
potassium salt of tetrazolate anion might exist as N1ꢀ. . .K+
bonded complex which essentially remained electronically either
as free anion or as ion pair. In this scenario, the aromaticity of the
tetrazole nucleosides would expected not to lead
scrambling or loss of the bases.
a/b-anomeric
The b-configuration of the nucleosides was next established via
TzPy
NOESY spectra of a representative tetrazolyl nucleoside,
B
Do
(10a) which shows a cross peak between H10–H20a and H10–H40.
In case of other tetrazolyl nucleosides, the signals of the sugar pro-
tons H10 and H30 give rise to intense cross-peaks to the signals of
H200 and H20, respectively, supporting the b-anomeric configura-
tions (SI Section 5).