Photochemistry of 6-azidopurine ribonucleoside in aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2012.02.103

In aqueous solution, 6-azido-9-β-d-ribofuranosylpurine undergoes an efficient photoinduced purine ring expansion to give a novel imidazole-fused 1,3,5-triazepinone nucleoside and partial conversion into adenosine. The structure of the major product was established as 1-(β-d-ribofuranosyl)-4, 6-dihydroimidazo[4,5-f][1,3,5]triazepin-5(1H)-one based on HRMS and NMR spectral data.

Full text of DOI:10.1016/j.tetlet.2012.02.103

Tetrahedron Letters 53 (2012) 2316–2318
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Tetrahedron Letters
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Photochemistry of 6-azidopurine ribonucleoside in aqueous solution
a
b
a
a
a,
⇑
´
´
Krzysztof Komodzinski , Joanna Nowak , Jolanta Lepczynska , Jan Milecki , Bohdan Skalski
a
´
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
b Centre of Advanced Technologies, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
In aqueous solution, 6-azido-9-b-D-ribofuranosylpurine undergoes an efficient photoinduced purine ring
Received 28 November 2011
Revised 12 February 2012
Accepted 23 February 2012
Available online 3 March 2012
expansion to give a novel imidazole-fused 1,3,5-triazepinone nucleoside and partial conversion into
adenosine. The structure of the major product was established as 1-(b- -ribofuranosyl)-4,6-dihydroimi-
dazo[4,5-f][1,3,5]triazepin-5(1H)-one based on HRMS and NMR spectral data.
D
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
6-Azidopurine
Photochemistry
Ring expansion
1,3,5-Triazepinone nucleoside
Azido derivatives of nucleobases have long been used as photo-
affinity labeling agents in the studies of nucleic acid–protein inter-
actions via a photocross-linking technique.1,2 So far, among purine
derivatives, 8-azidoadenosine has been the most frequently uti-
lized analogue, whereas the other azidopurines (8-azidoguanosine,
2-azidoadenosine and 2,6-diazidopurine) have been used to a
much lesser extent.1–3 Nearly 30 years ago, 6-azidopurine ribonu-
cleoside was proposed as a photoaffinity probe for ribosomal pep-
tidyltransferase.4 It has been shown that the 6-azidopurine moiety
is readily photolyzed by UV irradiation, however, to date, no re-
ports concerning the structure of the photoproducts formed, are
available.4 Apart from their use as photoaffinity reagents, azidonu-
cleosides, as with other aryl azides, are also good candidates for
creating ring-expanded, unsaturated azepine products via nitrogen
insertion.5 Indeed, an efficient photochemical ring expansion of 4-
azidouracil nucleosides to 1,3,5-triazepine-2,4-dione nucleosides
has been reported.6 In the above context, the photochemistry of
6-azidopurine nucleoside appeared worthy of more detailed inves-
tigation. Herein, we demonstrate that in aqueous solution, com-
Scheme 1. Azide-tetrazole tautomeric equilibrium of 6-azidopurine, 1a, in aqueous
solution and structures of photoproducts of its irradiation with near UV light under
aerobic conditions.
pound
1
undergoes efficient photochemical purine ring
expansion leading to the novel imidazole-fused 1,3,5-triazepinone
nucleoside 2 (Scheme 1).
(5 ꢀ 10ꢁ4 M) was irradiated under aerobic conditions in a 0.2 cm
UV cell with a high pressure mercury lamp (HBO 200) equipped
with a 313 nm interference filter. The progress of the reaction
was monitored by the disappearance of the absorption band in
the UV spectra of 1a,b (kmax ꢂ290 nm, Fig. 1) and by HPLC. The for-
mation of two photoproducts, 2 and 3, was observed by HPLC
(Fig. 2) with ca. 50% conversion of 1.
The synthesis of 6-azidopurine nucleoside 1 was carried out
according to a reported procedure.7 As indicated in Scheme 1, 6-
azidopurine nucleoside can exist in an azide-tetrazole equilibrium,
and in aqueous solution this equilibrium is shifted almost entirely
to the tetrazolyl isomer.8 Thus, an aqueous solution of 1a,b
The minor photoproduct 3 was identified as adenosine by com-
parison (HPLC, MS, UV) with an authentic sample. The molecular
⇑
Corresponding author. Tel.: +48 61 829 1351; fax: +48 61 829 1505.
E-mail address: bskalski@amu.edu.pl (B. Skalski).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.02.103

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K. Komodzinski et al. / Tetrahedron Letters 53 (2012) 2316–2318
Scheme 2. Synthesis of 20,30,50-tri-O-acetylated nucleoside 4 and numbering of
multiple ring system.
a 200 mL photoreactor with an immersed high pressure mercury
lamp equipped with a cut-off Pyrex filter (k >300 nm). The
photoproduct was isolated from the reaction mixture by semi-
preparative reversed-phase HPLC and subjected to acetylation with
acetic anhydride in anhydrous pyridine to give 20,30,50-tri-O-acety-
lated nucleoside 4 (Scheme 2).
The proposed structure of 4 was confirmed from HRMS and
NMR spectral data.9 Its high-resolution mass spectrum revealed a
molecular mass of 409.12335, which corresponded to the formula
Figure 1. Changes in the absorption spectrum of an aqueous solution of 1 during
irradiation at k >300 nm. The spectra were taken at 15 min increments.
C
16H19N5O8. Both the 1H and 13C NMR spectra showed typical sig-
nal patterns corresponding to the intact ribose part of the mole-
cule, which confirmed that the transformations occurred on the
aglycone. In the aromatic region of the 13C NMR spectrum of 4
there were five signals, two due to the methine carbons and three
from quaternary carbons (at 137.56, 131.23 ppm and 154.40,
128.40, 121.62 ppm, respectively). In the 1H MNR spectrum, apart
from the resonances due to the ribose unit, two C–H and two
exchangeable N–H signals were detected, at 7.27, 6.31 ppm, and
7.84, 6.76 ppm. The position of the latter two N–H signals shifted
considerably with temperature (up to 1 ppm over a range of
50 °C). The chemical shift of the carbon signal at 154.4 ppm sug-
gests that it was due to a urea C2 carbonyl. All the remaining car-
bon signals appeared within the range expected for the postulated
structure, but it was impossible to assign them on the basis of their
chemical shift alone. Several two-dimensional multiple resonance
experiments were performed (Fig. 3) and revealed correlations of
atoms corresponding to the following signals: 1H TOCSY revealed
the signals H(7.84), H(6.76) and H(6.31) (two N–H and one C–H)
as a one spin system, omitting the H(7.27) signal.
Heteronuclear carbon–proton correlation revealed one-bond
contacts between signals C(131.23) and H(7.27) as well as
C(137.56) and H(6.31). HMBC was detected between the following
signals: proton signal H(7.27) correlates with carbon signals
C(128.40), C(121.62) and C(86.54) (anomeric carbon atom); the
proton signal H(7.84) correlates with C(121.62); the anomeric
Figure 2. HPLC analysis of an aqueous solution of 1 before (dotted line) and after
30 min irradiation (solid line). The column was a Waters XBridge™ OST C18 2.5 lm
4.6 ꢀ 50 mm, eluted with acetonitrile/water using a linear gradient of 18–50% of
acetonitrile over 15 min at a flow rate of 1.0 mL/min. The inset shows the UV
absorption spectrum of the major photoproduct 2.
formula of the major photoproduct 2, C10H13N5O5, determined by
HR-LSIMS, indicated that its formation involved the loss of an N2
molecule and addition of H2O. Furthermore, the characteristic UV
spectrum of this photoproduct (Fig. 2, inset) exhibiting a low inten-
sity, broad absorption band centered at 358 nm indicated that a
major structural change occurred within the chromophore unit of
1 upon irradiation. These observations suggested the possible for-
mation of a purine ring expanded triazepine product 2 (Scheme 1)
via the generally accepted mechanism involving formation of an
intermediate nitrene, followed by its rearrangement into the se-
ven-membered ring carbodiimide and addition of
a water
molecule.5,6
To isolate and fully characterize the photoproduct 2 by NMR
spectroscopy, preparative scale irradiation of 1 was performed in
Figure 3. HMBC, NOEs and signal assignments for photoproduct 4.

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K. Komodzinski et al. / Tetrahedron Letters 53 (2012) 2316–2318
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proton H(5.73) shows correlation with C(131.23) and C(121.62);
proton H(6.31) correlates with carbon signals C(121.62) and
C(154.40). Heteronuclear nitrogen–proton correlations not only
provided clues as to the inter-atom relations, but also allowed
detection of the 15N resonance positions, that were too weak to
be detected directly. Five nitrogen signals appeared at 104, 125,
129.5, 171, and 239 ppm (relative to NH3 gas as a reference). One
bond (N–H) correlations were detected between N(104)–H(7.84)
and N(129.5)–H(6.76). Multiple bond correlations were present be-
tween H(7.84) with N(129.5); H(6.76) with N(125); H(7.27) with
both N(171) and N(239), and H(6.31) with N(129.5). The final set
of correlations was provided by selective NOESY which showed
contacts between H(7.84)–H(6.76); H(6.76)–H(6.31), and H(5.73,
anomeric)–H(7.27). All the above data agree very well with the
structure of the nucleoside 2 having the aglycone which contains
fused imidazole and 1,3,5-triazepin-2-one rings.
include MOL files and InChiKeys of the most important compounds
described in this article.
References and notes
1. Sylvers, L. A.; Wower, J. Bioconjugate Chem. 1993, 4, 411–418.
2. Meisenheimer, K. M.; Koch, T. H. Crit. Rev. Mol. Biol. 1997, 32, 101–140.
3. Meffert, R.; Dose, K.; Rathgeber, G.; Schafer, H. J. Methods Mol. Biol. 2001, 148,
323–335.
4. Quiggle, K.; Wejrowski, M. L.; Chladek, S. Biochemistry 1978, 17, 94–101.
5. Kantorowski, E. J.; Kurth, M. J. Tetrahedron 2000, 56, 4317–4353.
6. Peyrane, F.; Cesario, M.; Clivio, P. J. Org. Chem. 2006, 71, 1742–1745.
7. (a) Mathe, Ch.; Lioux, T.; Gosselin, G. Nucleosides Nucleotides Nucl. Acids 2003, 22,
605–609; (b) Johnson, J. A.; Thomas, H. J.; Schaeffer, H. J. J. Am. Chem. Soc. 1958,
80, 699–702.
8. (a) Temple, C.; Thorpe, M. C.; Coburn, W. C.; Montgomery, J. A. J. Org. Chem. 1966,
31, 935–938; (b) Temple, C.; Kussner, C. L.; Montgomery, J. A. J. Org. Chem. 1966,
31, 2210–2215; (c) Lioux, T.; Gosselin, G.; Mathe, Ch. Eur. J. Org. Chem. 2003,
3997–4002.
9. Compound 4: 1H NMR: (CDCl3) d 7.84 (s, 1H, H1), 7.27 (s, 1H, H9), 6.76 (s, 1H,
H3), 6.31 (s, 1H, H4), 5.73 (m, 1H, H10), 5.70 (m, 1H, H20), 5.48 (m, 1H, H30), 4.38
(m, 1H, H50), 4.34 (m, 1H, H40), 4.29 (m, 1H, H500), 2.12–2.10 (s, 9H, Ac). 13C NMR:
(CDCl3) d 170.31–169.16 (C@O-Ac), 154.40 (C2), 137.56 (C4), 131.23 (C9), 128.40
(C7), 121.62 (C6), 86.54 (C10), 79.67 (C40), 73.33 (C30), 70.30 (C20), 63.10 (C50),
20.87–20.55 (CH3-Ac). 15N NMR: (CDCl3) d 239.0 (N10), 171.0 (N8), 129.5 (N3),
In conclusion, the photochemistry of 6-azidopurine nucleosides
provides opportunities for the synthesis of potentially valuable
imidazole-fused 1,3,5-triazepinone nucleosides.
125.0 (N5), 104.0 (N1). HR MS calcd for
409.12347.
C16H19N5O8: 409.12335, found:
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.02.103. These data