Azide?tetrazole equilibrium of C-6 azidopurine nucleosides and their ligation reactions with alkynes

Article abstract of DOI:10.1021/jo902342z

Facile syntheses of C-6 azidopurine ribonucleosides and 2′-deoxyribonucleosides have been developed. For silyl- and acetyl-protected as well as unprotected nucleosides, access to the azido derivatives could be readily attained via displacement of BtO^? from the O⁶-(benzotriazol-1-yl)inosine nucleosides by azide anion. Use of diphenylphosphoryl azide/DBU as a simple route to the acetyl-protected azido nucleosides was also evaluated, but this proved to be inferior. Since these azido nucleosides can exist in an azidetetrazole equilibrium, the effect of solvent polarity on this equilibrium was investigated. Subsequently, a detailed analysis of Cu-mediated azide?alkyne (click) ligation was undertaken. Biphasic CH₂Cl₂/H₂O medium proved to be best for the ligation reactions, suppressing the undesired azide reduction that was competing. Interestingly, although the tetrazolyl isomer predominates (ca. 80%) in CD₂Cl₂ and in CD₂Cl₂/D ₂O, the Cu-catalyzed click reactions proceed smoothly with the silyl-protected ribo- and 2′-deoxyribonucleosides, leading to the C-6 triazolyl products in good to excellent yields. Thus, depletion of the azido form from the reaction mixture shifts the azidetetrazole equilibrium, eventually resulting in complete consumption of azide and tetrazole. In several cases, major and minor azide?alkyne ligation products were observed, and characterization data are provided for both. In order to confirm the regiochemistry leading to the major isomer, one product was crystallized and evaluated by X-ray crystallography. The Cu-catalyzed azide?alkyne ligation is clearly efficient and significantly superior to thermal reactions, which were slow. Biological evaluation showed low cytotoxicities for the agents, suggesting their usefulness as biological probes.

Full text of DOI:10.1021/jo902342z

pubs.acs.org/joc
Azide-Tetrazole Equilibrium of C-6 Azidopurine Nucleosides and Their
Ligation Reactions with Alkynes
Mahesh K. Lakshman,*,† Manish K. Singh,† Damon Parrish,‡ Raghavan Balachandran,§
and Billy W. Day§,
†Department of Chemistry, The City College and The City University of New York, 160 Convent Avenue,
New York, New York 10031, ‡Naval Research Laboratory, Code 6030, 4555 Overlook Avenue, Washington,
D.C. 20375, §Department of Pharmaceutical Sciences, and Department of Chemistry,
University of Pittsburgh, Pennsylvania 15213
lakshman@sci.ccny.cuny.edu
Received November 1, 2009
Facile syntheses of C-6 azidopurine ribonucleosides and 20-deoxyribonucleosides have been developed. For
silyl- and acetyl-protected as well as unprotected nucleosides, access to the azido derivatives could be readily
attained via displacement of BtO- from the O6-(benzotriazol-1-yl)inosine nucleosides by azide anion. Use of
diphenylphosphoryl azide/DBU as a simple route to the acetyl-protected azido nucleosides was also
evaluated, but this proved to be inferior. Since these azido nucleosides can exist in an azide tetrazole
3
equilibrium, the effect of solvent polarity on this equilibrium was investigated. Subsequently, a detailed
analysis of Cu-mediated azide-alkyne (“click”) ligation was undertaken. Biphasic CH2Cl2/H2O medium
proved to be best for the ligation reactions, suppressing the undesired azide reduction that was competing.
Interestingly, although the tetrazolyl isomer predominates (ca. 80%) in CD2Cl2 andinCD2Cl2/D2O, the Cu-
catalyzed click reactions proceed smoothly with the silyl-protected ribo- and 20-deoxyribonucleosides,
leading to the C-6 triazolyl products in good to excellent yields. Thus, depletion of the azido form from the
reaction mixture shifts the azide tetrazole equilibrium, eventually resulting in complete consumption of azide
3
and tetrazole. In several cases, major and minor azide-alkyne ligation products were observed, and
characterization data are provided for both. In order to confirm the regiochemistry leading to the major
isomer, one product was crystallized and evaluated by X-ray crystallography. The Cu-catalyzed azide-
alkyne ligation is clearly efficient and significantly superior to thermal reactions, which were slow. Biological
evaluation showed low cytotoxicities for the agents, suggesting their usefulness as biological probes.
Introduction
transformations.1-7 In 2002, the use of CuI-mediated azide-
alkyne ligation was reported to proceed in a facile and regiose-
lective manner, producing the 1,4-disubstituted triazoles in
The Huisgen azide-alkyne cycloaddition reaction, leading to
the formation of triazoles, is one of the most atom-economical
(5) Lwowski, W. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.;
John Wiley & Sons: New York, 1984; Vol. 1, Chapter 5, pp 559-651.
(6) Fan, W.-Q.; Katritzky, A. R. In Comprehensive Heterocyclic Chem-
istry II; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Elsevier
Science: New York, 1996; pp 1-126.
(7) Bastide, J.; Henri-Rousseau, O. In The Chemistry Of The Carbon-
Carbon Triple Bond; Patai, S., Ed.; Interscience Publishers: London, 1978;
pp 447-522.
*To whom correspondence should be addressed. Tel: (212) 650-7835. Fax:
(212) 650-6107.
(1) Michael, A. J. Prakt. Chem. 1893, 48, 94–95.
(2) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1963, 2, 565–598.
(3) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1963, 2, 633–645.
(4) Huisgen, R.; Szeimies, G.; Moebius, L. Chem. Ber. 1967, 100, 2494–
2507.
DOI: 10.1021/jo902342z
r
Published on Web 03/18/2010
J. Org. Chem. 2010, 75, 2461–2473 2461
2010 American Chemical Society

Lakshman et al.
JOCArticle
preference to the 1,5-isomers.8 Soon after, a convenient meth-
od involving the use of CuII was discovered that was more
reliable and superior than that utilizing CuI.9 This method has
become highly popular and has impacted areas such as poly-
mer, materials, carbohydrate, and biological chemistry.
describes a facile approach to 6-azidopurine nucleosides, their
behavior in solution, and their ability to partner in Cu-catalyzed
ligation reactions with alkynes.
Results and Discussion
Nucleosides are central to a variety of biological processes,
and the ability to modify nucleosides via facile approaches is of
high importance for developing novel applications in biochem-
istry, biology, and medicine. The nucleoside scaffold has pro-
ven to be highly versatile, modification of which has resulted in a
wide range of pharmacologically important entities. For exam-
ple, modification of the adenosine core has the potential to yield
new compounds such as those possessing activity at the adeno-
sine receptors.10,11 Azide-alkyne click reactions have found a
variety of applications in nucleoside chemistry recently,12 and
the importance of click reactions in the field of nucleosides has
recently been reviewed.13 However, in the current literature
azidopurine nucleoside derivatives have received very little
attention as reactive partners in these reactions. To our knowl-
edge, there are only two reports of azide-alkyne ligation
reactions wherein C-2 azidopurine nucleoside analogues have
been utilized.14,15 Since there could be differences in reactivity at
the various positions of purine nucleosides and because the
azide-alkyne ligation reactions of 6-azidopurine nucleosides
could be a powerful tool for biomolecular modification, we
became interested in this class of reactions. The present paper
Synthesis of the 6-Azidopurine Nucleoside Derivatives. At
the beginning of our studies, the first question was whether a
reliable method to synthesize 6-azidopurine ribo- and 20-
deoxyribonucleosides could be found. Three methods have
been reported in the literature for the synthesis of 6-azido-9-β-
D-ribofuranosylpurine. The oldest method involves conversion
of 6-chloro-9-β-D-ribofuranosylpurine to an unstable 6-hydra-
zino derivative followed by treatment with nitrous acid.16 The
acidic conditions are obviously not compatible with the labile
20-deoxyribonucleosides. In a second method, 6-methylsulfo-
nyl-9-β-D-ribofuranosylpurine was converted to the azidopur-
ine nucleoside.17 It has been noted in these reports that dis-
placement of chloride from 6-chloro-9-β-D-ribofuranosylpur-
ine by azide ion results in decomposition.16,17 Consistent with
these observations, more recent syntheses of 6-azido-9-β-D-
ribofuranosylpurine18 and 6-azido-9-β-D-arabinofuranosyl-
purine19 by replacement of chloride with azide gave low 21%
and 38.4% yields of the respective azidopurine nucleoside analo-
gues. Given these relatively unsatisfactory methods, we opted to
develop a new synthesis of 6-azido-9-(2-deoxy-β-D-ribofuranosyl)-
purine as well as 6-azido-9-β-D-ribofuranosylpurine.
We have recently reported facile, high-yielding syntheses
of O6-(benzotriazol-1-yl)inosine and 20-deoxyinosine de-
rivatives.20-22 We reasoned that these compounds could be
convenient precursors to the 6-azidopurine nucleoside deri-
vatives (Scheme 1).
(8) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67,
3057–3064.
(9) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew.
Chem., Int. Ed. 2002, 41, 2596–2599.
ꢀ
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Helguera, A. M. Curr. Med. Chem. 2006, 13, 2253–2266. (b) Akkari, R.;
ꢀ
As shown in Scheme 1, silylated 20-deoxyinosine 1a and
inosine 1b as well as the unprotected nucleosides 1e and 1f
were converted to the O6-(benzotriazol-1-yl) derivatives via
known procedures.20 Using similar methodology, the pre-
viously undescribed acetate-protected compounds 2c and 2d
were prepared as well. Table 1 summarizes the conditions
used for the conversion of 2a-f to azido nucleosides 3a-f.
Although reactions of 2a and 2b could be conducted in DMF
as solvent, competing formation of small amounts of the N6,
N6-dimethylamino nucleoside derivative prompted the use of
DMSO. These reactions proceeded smoothly and in high
yields. Reactions of the unprotected derivatives 2e and 2f were
somewhat problematic considering solubility as well as ease of
product isolation. In both cases, polymer-supported azide23
proved to be useful for the conversion. With the deoxy deriva-
tive 2e, reasonable reaction was attained in water. We have
demonstrated that O6-(benzotriazoly-1-yl) nucleoside analo-
gues are relatively stable toward hydroxylic solvents such as
water and alcohol under neutral conditions.20,24 Thus, the
€
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247–264. (d) Joshi, B. V.; Jacobson, K. A. Curr. Top. Med. Chem. 2005, 5,
1275–1295.
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Hutchinson, S. A.; Scammells, P. J. Bioorg. Med. Chem. 2008, 16, 1861–1873.
(b) Cappellacci, L.; Franchetti, P.; Vita, P.; Petrelli, R.; Lavecchia, A.; Costa,
B.; Spinetti, F.; Martini, C.; Klotz, K.-N.; Grifantini, M. Bioorg. Med. Chem.
2008, 16, 336–353. (c) Ashton, T. D.; Aumann, K. M.; Baker, S. P.; Schiesser,
C. H.; Scammells, P. J. Bioorg. Med. Chem. Lett. 2007, 17, 6779–6784. (d)
Adachi, H.; Palaniappan, K. K.; Ivanov, A. A.; Bergman, N.; Gao, Z.-G.;
Jacobson, K. A. J. Med. Chem. 2007, 50, 1810–1827. (e) Volpini, R.; Ben, D.
D.; Lambertucci, C.; Taffi, S.; Vittori, S.; Klotz, K.-N.; Cristalli, G. J. Med.
Chem. 2007, 50, 1222–1230.
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Chem. Soc. 2009, 131, 3958–3964. (b) Wojtczak, B. A.; Andrysiak, A.;
Gruner, B.; Lesnikowski, Z. J. Chem.;Eur. J. 2008, 14, 10675–10682. (c)
Lolk, L.; Pøhlsgaard, J.; Jepsen, A. S.; Hansen, L. H.; Nielsen, H.; Steffan-
sen, S. I.; Sparving, L.; Nielsen, A. B.; Vester, B.; Nielsen, P. J. Med. Chem.
2008, 51, 4957–4967. (d) Isobe, H.; Fujino, T.; Yamazaki, N.; Guillot-
Nieckowski, M.; Nakamura, E. Org. Lett. 2008, 10, 3729–3732. (e) Nakane,
M.; Ichikawa, S.; Matsuda, A. J. Org. Chem. 2008, 73, 1842–1851. (f) Seela,
F.; Sirivolu, V. R. Org. Biomol. Chem. 2008, 6, 1674–1687. (g) Jatsch, A.;
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Kopyshev, A.; Mena-Osteritz, E.; Bauerle, P. Org. Lett. 2008, 10, 961–964.
(h) Jin, X.; Ding, H.; Yang, R.; Xiao, Q.; Ju, Y. Synthesis 2008, 865–870. (i)
Seela, F.; Sirivolu, V. R.; Chittepu, P. Bioconjugate Chem 2008, 19, 211–224.
(j) Jin, X.; Yang, R.; Jin, P.; Xiao, Q.; Ju, Y. Synthesis 2007, 2967–2972. (k)
Seela, F.; Sirivolu, V. R. Nucleosides Nucleotides Nucleic Acids 2007, 26, 597–
601. (l) Seela, F.; Sirivolu, V. R. Helv. Chim. Acta 2007, 90, 535–552. (m)
Oyelere, A. K.; Chen, P. C.; Yao, L. P.; Boguslavsky, N. J. Org. Chem. 2006,
71, 9791–9796. (n) Gierlich, J.; Burley, G. A.; Gramlich, P. M. E.; Hammond,
D. M.; Carell, T. Org. Lett. 2006, 8, 3639–3642. (o) O’Mahony, G.; Ehrman,
E.; Grøtli, M. Tetrahedron Lett. 2005, 46, 6745–6748.
(16) Johnson, J. A., Jr.; Thomas, H. J.; Schaeffer, H. J. J. Am. Chem. Soc.
1958, 80, 699–702.
(17) Wetzel, R.; Eckstein, F. J. Org. Chem. 1975, 40, 658–660.
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(18) Frieden, M.; Avino, A.; Eritja, R. Nucleosides Nucleotides Nucleic
Acids 2003, 22, 193–202.
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J.-P.; Boudinot, F. D.; Schinazi, R. F.; Chu, C. K. J. Med. Chem. 1996,
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2462 J. Org. Chem. Vol. 75, No. 8, 2010

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SCHEME 1. Synthesis of C-6 Azidopurine Nucleoside Analogues from the O6-(Benzotriazol-1-yl)inosine Derivatives
only upon protonation with trifluoroacetic acid.25,26 Similarly,
in the solid state, 6-azidopurine ribonucleoside exists as the
tetrazolyl isomer lacking an IR absorption at 2000-2200 cm-1
corresponding to the azido tautomer.16,18 Since the subsequent
stage of our work entailed the development of azide-alkyne
ligation chemistry, we wanted to evaluate the influence solvent
polarity elicits on the equilibrium involving the azido nucleo-
sides and the corresponding tetrazolo[5,1-i]purinyl isomers.
For this analysis, 1H NMR spectroscopy appeared to be opti-
mal since the H-2 and H-8 resonances of the azido and tetra-
zolyl isomers are well resolved and diagnostic. In the tetrazolyl
forms of 6-azidopurine, its N9- and N7-benzyl derivatives, H-2
appears at ∼9.7-10 ppm, whereas H-8 appears at ∼8.6-9.4
ppm.26 On the other hand, in the azido isomers of the same
purine derivatives, the line separation between the H-2 and H-8
resonances is much smaller, with H-2 appearing at ∼9.1-9.2
ppm and the H-8 at ∼8.8-9.1 ppm (see Table S1 in the
Supporting Information).26 Thus, H-2 of the tetrazolyl isomer
appears farther downfield compared to its H-8 resonance as
well as the H-2 and H-8 resonances of the azido tautomer.26 On
the basis of these as well as NOESY data from 3b (in acetone-
d6) and3d (inCDCl3) showing correlation between H-10 and H-
8, we determined the chemical shifts of purinyl H-2, H-8 and the
saccharide H-10 in the azido (A) and tetrazolyl (T) forms of
compounds 3a-f . These results are shown in Table 2.
TABLE 1. Conditions for the Conversion of O6-(Benzotriazol-1-yl)-
inosine Derivatives 2a-f to the Azido Nucleosides 3a-f
product:
entry
substrate
conditions
yielda (%)
1
2
3
4
5
2a
2b
2c
2d
2e
NaN3, DMSO, 40 °C, 2 h
NaN3, DMSO, 40 °C, 3.5 h
NaN3, DMSO, rt, 1 h
3a: 99
3b: 96
3c: 95
3d: 75
3e: 59
NaN3, DMSO, rt, 1 h
polymer-supported N3
H2O, 50 °C, 3.5 h
-
,
-
6
2f
polymer-supported N3
DMF, 50 °C, 5 h
,
3f: 70
aYields of isolated and purified products.
modest yield in this case may not be due to hydrolysis. With the
ribose derivative 2f, DMF proved to be optimal for workup
and isolation reasons. In principle, cleavage of protecting
groups in 3a-d should also readily yield the unprotected
compounds 3e,f.
Next, we considered a more direct access to protected 6-
azidopurine nucleosides by conversion of the amide carbonyl
to a leaving group in situ. For this, we decided to use
diphenylphosphoryl azide [(PhO)2P(O)N3, DPPA] in con-
junction with a base. Mechanistically, deprotonation of the
amide and reaction with DPPA would produce a diphenyl
phosphate intermediate that could undergo subsequent
SNAr reaction with azide anion (Scheme 2).
For determining the proportion of the azido and tetrazolyl
forms in 3a-f, the H-2 and H-8 resonances were related to
the H-10 resonances of the two isomers. The integrals of the
H-10 resonances were then used to determine the ratios of the
two isomeric forms. Several solvents were selected with varying
dielectric constants (ε), and the results are shown in Table 3.
With this analysis, it became clear that generally the
proportion of the tetrazolyl isomer increases with increasing
solvent dielectric constant. This is consistent with prior
results on 6-azidopurine, where lower solvent dielectric
constant has been implicated in tetrazole destabilization.25
In fact, in chloroform, about 40-50% of the tetrazolyl form
is present (the triacetates 3c and 3d show a slightly higher
proportion of the azido form in chloroform). In THF as well
as in dichloromethane, which are close in dielectric constant,
the tetrazolyl form is major. Finally, in acetone and DMSO
the tetrazolyl form predominates. For unprotected 3e, where
analysis was possible in THF, a greater proportion of the
azido isomer is observed in THF than in DMSO. The ratio of
these isomers in 3e is also consistent with data for the
other azidopurine nucleoside derivatives shown in Table 3.
On the basis of this rationale, 1c and 1d were exposed to
DPPA and DBU in THF at 0 °C followed by warming to
60 °C. Azido nucleosides 3c and 3d were produced in yields of
67% and 41%, respectively, after purification. Interestingly,
a similar reaction of silylated derivative 1a led to only a 36%
isolated yield of 3a, perhaps indicating some influence of the
saccharide protecting group on the efficiency of the SNAr
reaction. Thus, displacement reactions on O6-(benzotriazol-
1-yl) derivatives 2a-f by azide offer generally good access to
C-6 azidopurine nucleosides.
Azide-Tetrazole Equilibium of the 6-Azidopurine Nucleo-
side Derivatives. It is known that both 6-azidopurine as well
as the corresponding ribonucleoside can exist in the alternate
tetrazolyl forms.16,25,26 In fact, in DMSO-d6, 6-azidopurine
as well as its N7- and N9-benzyl derivatives exist exclusively
as the tetrazolyl isomers, and the azido forms are observable
(25) Temple, C., Jr.; Thorpe, M. C.; Coburn, W. C., Jr.; Montgomery,
J. A. J. Org. Chem. 1966, 31, 935–938.
(26) Temple, C., Jr.; Kussner, C. L.; Montgomery, J. A. J. Org. Chem.
1966, 31, 2210–2215.
J. Org. Chem. Vol. 75, No. 8, 2010 2463

Lakshman et al.
JOCArticle
SCHEME 2. Synthesis of 3c and 3d via Reaction of 1c and 1d with (PhO)2P(O)N3/DBU
TABLE 2. Chemical Shifts (δ ppm) of Purinyl H-2, H-8 and the Sugar
H-10 in the Azido and Tetrazolyl Isomers of 3a-f
azido form (A)
tetrazolyl form (T)
FIGURE 1. Other copper catalysts tested for the azide-alkyne
ligation.
compd
solvent
H-2
H-8
H-10
H-2
H-8
H-10
3a
CDCl3
THF-d8
CD2Cl2
acetone-d6 8.65 8.51
DMSO-d6
CDCl3
THF-d8
CD2Cl2
acetone-d6 8.68 8.59
DMSO-d6
CDCl3
THF-d8
CD2Cl2
acetone-d6 8.68 8.53
DMSO-d6
CDCl3
THF-d8
CD2Cl2
acetone-d6 8.70 8.53
DMSO-d6
8.63 8.33
8.58 8.36
8.63 8.33
6.47
6.45
6.47
6.54
9.51
9.78
9.51
9.91
10.14
9.49
9.82
9.52
9.97
10.19
9.53
9.81
9.54
9.95
10.16
9.54
9.83
9.56
9.97
10.19
9.78
10.13
10.15
8.57
8.54
8.57
8.69
8.82
8.66
8.66
8.67
8.78
8.88
8.40
8.52
8.42
8.71
8.87
8.38
8.52
8.41
8.71
8.88
8.68
8.88
8.93
6.61
6.60
6.61
6.70
6.57
6.25
6.24
6.25
6.33
6.17
6.59
6.62
6.60
6.73
6.61
6.33
6.41
6.36
6.52
6.46
6.64
6.57
6.15
speculated that some of the lower yielding azide-alkyne
ligations involving a C-2 azidopurine nucleoside could be
due to the presence of the tetrazolyl tautomer.13
Thus, we began screening conditions for the Cu-catalyzed
azide-alkyne ligation8,9 reaction between 3a and phenylacety-
lene. At the outset, the CuSO4/Na ascorbate system in t-BuOH/
H2O was chosen.9 However, under these conditions, an in-
complete reaction was observed after 3.5 h at room tempera-
ture, with only 26% product formation. Importantly, 11%
reduction of the azide to the amine was observed (entry 1 in
Table 4), and the resulting disilyl 20-deoxyadenosine was iso-
lated and identified. This clearly points to the facile reducibility
of the azide functionality in 3a, which contrasts with the
reactivity of simpler azides where such reduction is generally
not observed under the click reaction conditions.
3b
3c
3d
8.65 8.41
8.60 8.46
8.64 8.40
6.10
6.07
6.09
6.17
8.67 8.18
8.60 8.35
8.65 8.19
6.47
6.48
6.48
6.57
8.68 8.15
8.62 8.35
8.67 8.16
6.21
6.27
6.22
6.36
On the basis of the results obtained above, we screened
azide-alkyne ligation conditions extensively (Table 4), and
in subsequent reactions TLC analysis was conducted to
include detection (UV visualization) of the azide reduction.
Replacing t-BuOH with THF or toluene did not provide any
significant improvement (entries 2 and 3). Several other Cu
catalysts shown in Figure 1 were also evaluated, and Table 4
lists some representative results.
3ea
3fa
THF-d8
DMSO-d6
DMSO-d6
8.57 8.46
6.48
aSolubility constraints precluded assessment in other solvents.
It stands to reason, therefore, that only the tetrazolyl form of
azidopurine nucleoside 3f could be detected in the solid state
and as a solution in DMSO.16,18,28 These observations then
led to the obvious question: Do the azide-tetrazole mixtures
undergo efficient ligation (“click”) reactions with alkynes? The
answer to this is not obvious in the context of a recent report
showing 2-azido-3-phenylpyridine to exist as the 8-
phenyltetrazolo[1,5-a]pyridine and being recalcitrant to-
ward the Staudinger reaction with PPh3.29 Also, it has been
From these experiments two important features emerged.
Use of a biphasic CH2Cl2/H2O system30 was important for
successful reaction. Second, and more significantly, despite a
major proportion of the tetrazolyl tautomer in this solvent
system, reactions with phenylacetylene proceeded smoothly.
1
To illustrate this point, H NMR data for 3a and 3b were
obtained in CD2Cl2/10% D2O. In this solvent, the amounts of
the azido and tetrazolyl forms of 3a were 18.4% and 81.6%,
respectively, whereas for 3b they were 21% and 79%, respec-
tively. These ratios are very similar to the data obtained in
(27) Dielectric constants were obtained from: Riddick, J. A.; Bunger, W.
B.; Sakano, T. K. Techniques of Chemistry, Vol. 2. Organic Solvents: Physical
Properties and Methods of Purification, 4th ed; John Wiley & Sons, Inc.:
New York, 1986.
ꢀ
(28) Mathe, C.; Lioux, T.; Gosselin, G. Nucleosides Nucleotides Nucleic
Acids 2003, 22, 605–609.
(29) Laha, J. K.; Cuny, G. D. Synthesis 2008, 4002–4006.
(30) Lee, B.-Y.; Park, S. R.; Jeon, H. B.; Kim, K. S. Tetrahedron Lett.
2006, 47, 5105–5109.
2464 J. Org. Chem. Vol. 75, No. 8, 2010

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JOCArticle
TABLE 3. Percentage Populations of the Azido (A) and Tetrazolyl (T) Isomers of 3a-f in Various Solventsa
CDCl3, ε = 4.8
THF-d8, ε = 7.6
CD2Cl2, ε = 8.9
acetone-d6, ε = 20.6
DMSO-d6, ε = 46.5
compd
% A
% T
% A
% T
% A
% T
% A
% T
% A
% T
3a
3b
3c
3d
3e
3f
46.0
47.6
56.9
61.9
ndb
54.0
52.4
43.1
38.1
ndb
14.8
16.5
14.1
16.9
15.1
ndb
85.2
83.5
85.9
83.1
84.9
ndb
20.8
19.7
27.5
31.4
ndb
79.2
80.3
72.5
68.6
ndb
7.5
8.2
6.2
7.6
ndb
ndb
92.5
91.8
93.8
92.4
ndb
0.1
0.1
0.9
1.9
0.1
0.9
99.9
99.9
99.1
98.1
99.9
99.1
ndb
ndb
ndb
ndb
ndb
aThe dielectric constants (ε) of the corresponding protio solvents are reported (ref 27). bSolubility constraints precluded assessment.
TABLE 4. Evaluation of Conditions for the Reaction of 3a with Phenylacetylene
entry
catalyst
solvent
temp (°C), time (h)
resultsa,b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CuSO4/Na ascorbate
CuSO4/Na ascorbate
CuSO4/Na ascorbate
CuSO4/Na ascorbate
CuSO4/Na ascorbate
Cu(Phen)(PPh3)Br
Cu(PPh3)3Br
CuIMes
CuIPr
CuCl
CuTC
1:1 t-BuOH/H2O rt, 3.5
1:1 THF/H2O
incomplete reaction: 3a, 63%; 4a, 26%; reduction, 11%
incomplete reaction: 3a, 46%; 4a, 30%; reduction, 24%
incomplete reaction: 3a, 67%; 4a, 22%; reduction, 11%
4a, 78% yieldc
4a, 81% yield
4a, 46% yield
4a, 73% yield; an uncharacterized byproduct was also formed
incomplete reaction: 3a, 60%; 4a, 29%; reduction, 11%
no reaction, only 3a present
4a, 76% yield
4a, 63% yield
rt, 6
rt, 6
1:1 toluene/H2O
1:1 CHCl3/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
1:1 CH2Cl2/H2O
rt, 2
rt, 3.5
rt, 24
rt, 15
rt, 20
rt, 24
rt, 5
rt, 6
Cu/C and 2 molar equiv of Et3N 1,4-dioxane
Cu/C
Cu/C
rt, 15, then 60, 1.5
rt, 23
rt, 2.5
4a, 51%; reduction, 38%
no reaction, only 3a present
no reaction, only 3a present
1,4-dioxane
1:1 CH2Cl2/H2O
aIn reactions that proceeded to completion, yield reported is that of isolated and purified 4a. bIn some cases, the reduction product 30,50-bis-O-(tert-
butyldimethylsilyl)-20-deoxyadenosine was isolated and characterized. In other cases, this product was detected by TLC (UV visualization) and the
amount formed was estimated by 1H NMR analyses of the crude reaction mixtures. cBy TLC, reaction was less clean compared to entry 5.
CD2Cl2 alone (Table 3), indicating that the azide/tetrazole
ratio is not substantially altered in the presence of water.
Once conditions leading to successful click reactions were
realized, attention was focused on the issue of regiochemis-
try. In reactions of simpler systems, only 1,4-disubstituted
triazoles are formed in the Cu-catalyzed azide-alkyne liga-
tion reactions.9 We wanted to assess whether this was also
the case with these nucleoside substrates, which can demon-
strate complex reactivities. Thus, a reaction of 3a and 4-
ethynyltoluene was conducted under the optimized condi-
tions. In addition to a major product, a minor, more polar
material was observed by TLC (spectral data could not be
obtained on the minor product as it was formed to a very low
extent). It proved difficult, however, to ascertain the exact
regioisomeric structure of the major product. Therefore, we
attempted structure evaluation by X-ray analysis. Gratify-
ingly, the major product from the reaction of 3a and 4-
ethynyltoluene could be crystallized as long needles from
hexanes. The X-ray structure (Figure 2) clearly indicated this
to be the anticipated 1,4-disubstituted triazole derivative
(a pdb file as well as ORTEP are included in the Supporting
Information). The crystal structure shows that the purine
and the triazole rings are nearly coplanar, with a syn-
conformation of the purine around the glycosidic bond,
and a 30-endo sugar ring pucker.
In order to compare the catalyzed reaction with the
uncatalyzed process, two different reactions were conducted.
In one case, a mixture of 3a and 4-ethynyltoluene was heated
at 85 °C in 1:3 MeOH-H2O. This reaction, which was
incomplete at 36 h, gave 48% of the 1,4-disubstituted
J. Org. Chem. Vol. 75, No. 8, 2010 2465

Lakshman et al.
JOCArticle
FIGURE 2. X-ray structure of the click reaction product of 3a and 4-ethynyltoluene.
triazole, 5% of the 1,5-isomer, and 18% of recovered 3a. A
second reaction between 3a and 4-ethynyltoluene was con-
ducted in toluene at 65 °C. This reaction was incomplete at
120 h and showed ∼10% 3a, ∼80% of the 1,4-disubstituted
triazole, as well as ∼10% of the 1,5-isomer (estimates based
upon 1H NMR analysis of the crude reaction mixture). These
results clearly indicate that the rate of the ligation reaction
benefits significantly from Cu catalysis.
The next stage involved an analysis of the generality of the
azide-alkyne ligation reaction using the optimized condi-
tions (CuSO4/Na ascorbate in CH2Cl2/H2O). A range of
alkynes underwent successful ligation with 3a and 3b in good
yields and within reasonably short reaction times (Table 5).
In many cases, particularly with the ribosides, additional
minor products could be isolated. From the NMR as well as
the HRMS data, these minor compounds are quite possibly
the 1,5-disubstituted azide-alkyne ligation products. Where
possible, characterization data for the minor products are
presented in the Experimental Section as well.
These results also indicate a point of departure in the reac-
tivity of nucleoside azides in comparison to those of simpler
azides. From a mechanistic consideration31 (Scheme 3), it
appears that a major and a minor process are in operation,
leading to the two regioisomeric products. Since no product
formation was observed by simply stirring 4-ethynyltoluene
and 3b in CH2Cl2/H2O at room temperature for 48 h, it is likely
that the minor 1,5-disubstituted products also arise via a
catalyzed process (after 48 h at 100 °C, 32% of 3b, 16% of
the major and 2.5% of the minor cycloaddition products were
isolated from this reaction). Although it is difficult to rationa-
lize why the reaction partitions through the minor pathway, it is
possible that presence of the subsitutent (R) and the nucleoside
(Nuc) on vicinal carbons is not completely sterically disfavored.
We were curious to evaluate the azide-alkyne ligation
reactions of 4-azidoquinazoline, which is indicated in the
literature to exist as the corresponding tetrazolo[1,5-c]quina-
zoline.32 Several attempts were directed at the reaction of this
compound with phenylacetylene (Scheme 4), and these in-
cluded the reaction conditions used herein, CuSO4/Na as-
corbate in t-BuOH/H2O at room temperature and at 50 °C,
as well as a reaction in the presence of excess alkyne in
refluxing toluene. No product formation was observed under
any of these conditions. These results are also indicative of
the markedly different reactivity 6-azidopurine nucleoside
derivatives possess in comparison to this simpler model.
Finally, it was of interest to determine the biological activities
of these new triazolyl nucleoside analogues. Ribo- and 20-deo-
xyribonucleosides are typically of low toxicity to human cells.
Compounds 4a-g and 5a-g were desilylated to yield the free
nucleosides (Scheme 5). The desilylated compounds were tested
for antiproliferative activity (range of concentrations tested was
1-100 μM, 72 h continuous exposure) against a panel of hu-
man cancer cell lines: wild-type p53þ/þ and p53-/- HCT116
colon carcinoma cells and paclitaxel sensitive (1A9) and resis-
tant (1A9/PTX10 and 1A9/PTX22) ovarian carcinoma cells.
Only weak antiproliferative or cytotoxic actions were noted for
compounds 6e, 6f, 6g, 7d, and 7f (Table S2 in the Supporting
Information). All these new agents are presently being tested
for inhibitory activities against a panel of kinases, phospha-
tases, polymerases, and reverse transcriptases, and the results
will be reported in due course.
Conclusions
We have demonstrated that O6-(benzotriazol-1-yl)inosine
and 20-deoxyinosine derivatives are excellent substrates for
conversion to the C-6 azidopurine nucleoside derivatives in a
simple operation using azide anion. The azido nucleosides
exist in equilibrium with the tetrazolyl forms, and increasing
solvent dielectric constant generally increases the proportion
of the tetrazolyl tautomer. Despite the azide tetrazole equi-
3
librium, successful azide-alkyne click reactions can be
accomplished using CuSO4/Na ascorbate. However, the
biphasic CH2Cl2/H2O solvent system is required for success-
ful reaction, where the competing reduction of the azide to
the amine is suppressed. In contrast to click reactions of
simpler organic azides, those of the C-6 azido nucleoside
analogues are less regioselective, and minor amounts of the
1,5-disubstituted triazoles were isolated and characterized in
some cases. By X-ray crystallographic analysis, the structure of
(32) Wan, Z.-K.; Wacharasindhu, S.; Levins, C. G.; Lin, M.; Tabei, K.;
Mansour, T. S. J. Org. Chem. 2007, 72, 10194–10210.
(31) Ahlquist, M.; Fokin, V. V. Organometallics 2007, 26, 4389–4391.
2466 J. Org. Chem. Vol. 75, No. 8, 2010

Lakshman et al.
JOCArticle
TABLE 5. Azide-Alkyne Click Reactions of Nucleosides 3a and 3b
aYields of isolated and purified products. bThe isomeric 1,5-disubstituted triazole was also isolated as a minor byproduct.
SCHEME 3. Plausible Mechanistic Pathways Leading to the
Regioisomeric Products
SCHEME 4. Attempted Ligation of Tetrazolo[1,5-c]quinazo-
line with Phenylacetylene
SCHEME 5. Desilylation of the Click Products
the major product from a reaction between 3a and 4-ethynyl-
toluene was shown to be the 1,4-disubstituted triazole. To our
knowledge, this is the first report describing a simple, unifying
synthesis of C-6 azidopurine 20-deoxyribo- and ribonucleo-
sides, their behavior in solution, and their use in click reactions
with alkynes. In contrast to the successful reactions of the
6-azidopurine nucleosides, azide-alkyne ligation reactions of
tetrazolo[1,5-c]quinazoline were unsuccessful. This demon-
strates markedly different behavior of the 6-azidopurine nu-
cleosides in comparison to the simpler heterocycle.
J. Org. Chem. Vol. 75, No. 8, 2010 2467

Lakshman et al.
JOCArticle
Experimental Section
with a stirring bar were placed O6-(benzotriazol-1-yl)-20,30,50-
tri-O-(tert-butyldimethylsilyl)inosine (2b) (2.55 g, 3.51 mmol)
and NaN3 (684 mg, 10.53 mmol) in anhydrous DMSO (17.5
mL). The reaction mixture was flushed with nitrogen gas and
stirred under a nitrogen balloon at 40 °C for 3.5 h. The reaction
mixture was transferred to a separatory funnel and partitioned
between EtOAc and a 1:1 mixture of water-brine. The organic
layer was washed with 1:1 water-brine mixture (4ꢀ), then with
water (3ꢀ), and finally with brine. The organic layer was dried
over Na2SO4 and concentrated under reduced pressure. Filtra-
tion of the crude material through a silica gel plug using 20%
EtOAc in hexanes afforded 3b as a white foam (2.14 g, 96%
yield). Rf (silica gel, 20% EtOAc in hexanes) = 0.46. 1H NMR
(500 MHz, DMSO-d6): δ 10.19 (s, 1H, Ar-H), 8.88 (s, 1H, Ar-H),
6.17 (d, 1H, H-10, J = 5.5 Hz), 4.79 (t, 1H, H-20, J = 4.7 Hz),
4.37 (m, 1H, H-30) 4.10-4.06 (m, 1H, H-40), 4.02 (dd, 1H, H-50, J
= 5.6, 11.0 Hz), 3.80 (dd, 1H, H-50, J = 3.0, 11.0 Hz), 0.93, 0.91,
and 0.74 (3s, 27H, t-Bu), 0.14, 0.11, -0.05, and -0.28 (4s, 18H,
Si-CH3). 13C NMR (125 MHz, DMSO-d6): δ 145.3, 142.5, 141.8,
136.2, 120.3, 87.8, 85.3, 75.2, 71.6, 62.1, 25.8, 25.7, 25.4, 18.0,
17.7, 17.4, -4.6, -4.9, -5.4, -5.5, -5.6. HRMS (ESI): calcd for
C28H53N7O4Si3Na [M þ Na]þ 658.3359, found 658.3340.
6-Azido-9-(2-deoxy-3,5-di-O-acetyl-β-D-ribofuranosyl)pu-
rine (3c). From the O6-(Benzotriazol-1-yl)-20-deoxyinosine Dia-
cetate. In a clean, dry vial equipped with a stirring bar were
placed O6-(benzotriazol-1-yl)-20-deoxyinosine diacetate 2c (249
mg, 0.550 mmol) and NaN3 (107 mg, 1.65 mmol) in DMSO (2.5
mL). The mixture was flushed with nitrogen gas and stirred at
room temperature for 1 h at which time TLC indicated the
reaction to be complete. The mixture was diluted with EtOAc
and transferred to a separatory funnel. The mixture was ex-
tracted with 1:1 water-brine (3ꢀ), water (3ꢀ), and finally once
with brine. The organic layer was dried over Na2SO4 and
evaporated under reduced pressure. Chromatography of the
crude material on a silica gel column using 40% EtOAc in
hexanes afforded 3c as a white foamy material (195 mg, 95%
yield).
From 20-Deoxyinosine Diacetate. In a clean, dry reaction vial
equipped with a stirring bar were placed 20-deoxyinosine-30,50-
diacetate (1c) (500 mg, 1.487 mmol) and THF (3.0 mL). The
mixture was cooled with stirring to 0 °C in an ice bath. DPPA
(0.48 mL, 2.23 mmol) and DBU (0.34 mL, 2.23 mmol) were
added. The nitrogen gas-flushed mixture was allowed to stir at
ice bath temperature for 5 min and then at room temperature for
10 min. Finally, the mixture was stirred in a 60 °C sandbath for
1 h. Another aliquot of DPPA (0.48 mL, 2.23 mmol) and DBU
(0.34 mL, 2.23 mmol) were added, and the reaction was con-
tinued for an additional 1 h. The mixture was evaporated to
dryness, and the residue was dissolved in CH2Cl2 and washed
with water followed by brine. Chromatographic purification on
a silica gel column using 2% MeOH/50% EtOAc/48% hexanes
afforded 3c as a yellow foam (362 mg, 67%). Rf (silica gel, 80%
EtOAc in hexanes) = 0.26. 1H NMR (500 MHz, DMSO-d6): δ
10.16 (s, 1H, Ar-H), 8.87 (s, 1H, Ar-H), 6.61 (t, 1H, H-10, J = 6.8
Hz), 5.50-5.41 (m, 1H, H-30), 4.37-4.29 (m, 2H, H-40, H-50),
4.24 (dd, 1H, H-50, J = 5.5, 11.4 Hz), 3.15 (app quint, 1H, H-20,
Japp ≈ 7.1 Hz), 2.74-2.66 (m, 1H, H-20), 2.11 and 2.01 (2s, 6H,
OCOCH3). 13C NMR (125 MHz, DMSO-d6): δ 170.1, 170.0,
145.4, 142.8, 141.6, 136.1, 120.6, 84.4, 82.1, 74.0, 63.4, 36.1, 20.8,
20.5. HRMS (ESI): calcd for C14H15N7O5Na [M þ Na]þ
384.1027, found 384.1027.
Please see the Supporting Information for general experi-
mental methods.
30,50-Di-O-acetyl-O6-(benzotriazol-1-yl)-20-deoxyinosine (2c).
In a clean, dry 100 mL round-bottomed flask equipped with a
stirring bar were placed 20-deoxyinosine 30,50-diacetate 1c (300
mg, 0.893 mmol) and BOP (790 mg, 1.79 mmol) in anhydrous
CH2Cl2 (8.0 mL). To this stirred mixture was added (i-Pr)2NEt
(0.31 mL, 1.79 mmol), and the stirring was continued at room
temperature for 22 h. The mixture was evaporated to dryness,
and the residue was redissolved in EtOAc. The mixture was
washed with water (3ꢀ) and then with brine. The organic layer
was dried over Na2SO4 and concentrated under reduced pres-
sure. The crude product was purified by column chromatography
on silica gel using 30% EtOAc in hexanes to provide 2c as a white
foamy solid (382 mg, 94% yield). Rf (silica gel, 10% EtOAc in
hexanes) = 0.70. 1H NMR (500 MHz, CDCl3): δ 8.44 (s, 1H, Ar-
H), 8.33 (s, 1H, Ar-H), 8.14 (d, 1H, Ar-H, J = 8.3 Hz), 7.58-7.41
(m, 3H, Ar-H), 6.52 (t, 1H, H-10, J = 6.5 Hz), 5.46 (br s, 1H, H-
30), 4.44-4.36 (m, 3H, 1H-40, 2H-50), 3.00 (app quint, 1H, H-20,
Japp ≈ 7.1 Hz), 2.71 (ddd, 1H, H-20, J = 2.6, 6.0, 14.1 Hz), 2.15
and 2.10 (2s, 6H, OCOCH3). 13C NMR (125 MHz, CDCl3):
δ 170.5, 170.4, 159.4, 153.7, 151.9, 143.7, 143.3, 129.0, 125.1,
120.8, 108.8, 85.5, 83.1, 74.4, 63.8, 37.9, 21.1, 21.0. HRMS (ESI):
calcd for C20H19N7O6Na [M þ Na]þ 476.1295, found 476.1292.
20,30,50-Tri-O-acetyl-O6-(benzotriazol-1-yl)inosine (2d). This
compound was prepared by the procedure described for 2c using
1d (300 g, 0.761 mmol), BOP (673 mg, 1.52 mmol), anhydrous
CH2Cl2 (8.0 mL), and (i-Pr)2NEt (0.26 mL, 1.52 mmol). The crude
product was purified by column chromatography on silica gel
using 40% EtOAc in hexanes to provide 2d as a white foamy solid
(353 mg, 91% yield). Rf (silica gel, 20% EtOAc in hexanes) = 0.32.
1H NMR (500 MHz, CDCl3):δ8.45 (s, 1H, Ar-H), 8.30 (s, 1H, Ar-
H), 8.14 (d, 1H, Ar-H, J = 8.3 Hz), 7.57-7.46 (m, 3H, Ar-H), 6.27
(d, 1H, H-10, J = 5.1 Hz), 5.96 (t, 1H, H-20, J = 5.4 Hz), 5.60 (t,
1H, H-30, J = 5.1 Hz), 4.51-4.38 (m, 3H, H-40, 2H-50), 2.16, 2.14,
and 2.10 (3s, 9H, OCOCH3). 13C NMR (125 MHz, CDCl3): δ
170.4, 169.7, 169.5, 159.5, 153.8, 152.1, 143.6, 143.5, 129.0, 125.1,
120.7, 120.3, 108.8, 87.1, 80.7, 73.3, 70.6, 63.0, 53.6, 20.9, 20.7, 20.5.
HRMS (ESI): calcd for C22H21N7O8Na [M þ Na]þ 534.1349,
found 534.1351.
6-Azido-9-[2-deoxy-3,5-di-O-(tert-butyldimethylsilyl)-β-D-ribo-
furanosyl]purine (3a). In a clean, dry 50 mL round-bottomed
flask equipped with a stirring bar were placed O6-(benzotria-
zol-1-yl)-20,30-di-O-(tert-butyldimethylsilyl)-20-deoxyinosine (2a)
(1.036 g, 1.73 mmol) and NaN3 (326 mg, 5.0 mmol) in anhydrous
DMSO (8.6 mL). The reaction mixture was flushed with nitrogen
gas and stirred under a nitrogen balloon at 40 °C for 2 h. The
reaction mixture was transferred to a separatory funnel and
partitioned between EtOAc and a 1:1 mixture of water-brine.
The organic layer was washed with 1:1 water -brine mixture (4ꢀ),
then with water (3ꢀ), and finally with brine. The organic layer was
dried over Na2SO4 and evaporated under reduced pressure.
Filtration of the crude material through a silica gel plug using
20% EtOAc in hexanes solution afforded 3a as a clear gum (871
mg, 99%). Rf (silica gel, 1% MeOH in CH2Cl2) = 0.12. 1H NMR
(500 MHz, DMSO-d6): δ 10.14 (s, 1H, Ar-H), 8.82 (s, 1H, Ar-H),
6.57 (t, 1H, H-10, J = 6.5 Hz), 4.64 (m, 1H, H-30), 3.91 (m, 1H, H-
40), 3.82 (dd, 1H, H-50, J = 5.2, 11.0 Hz), 3.70 (dd, 1H, H-50, J =
4.0, 11.0 Hz), 2.88 (app quint, 1H, H-20, Japp ≈ 6.0 Hz), 2.46 (m,
1H, H-20), 0.89 and 0.82 (2s, 18H, t-Bu), 0.11, 0.02, and -0.001 (3s,
12H, Si-CH3). 13C NMR (125 MHz, DMSO-d6): δ 145.3, 142.5,
141.5, 135.9, 120.4, 87.3, 84.2, 71.5, 62.3, 39.9, 25.7, 25.6, 17.9, 17.6,
-4.8, -5.0, -5.6. HRMS (ESI): calcd for C22H39N7O3Si2Na
[M þ Na]þ 528.2545, found 528.2534.
6-Azido-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)purine (3d). From
the O6-(Benzotriazol-1-yl)inosine Triacetate. In a clean, dry vial
equipped with a stirring bar were placed O6-(benzotriazol-1-yl)-
inosine triacetate 2d (281 mg, 0.550 mmol) and NaN3 (107 mg,
1.65 mmol) in DMSO (2.5 mL). The mixture was flushed with
nitrogen gas and stirred at room temperature for 1 h at which time
TLC indicated the reaction to be complete. The mixture was diluted
6-Azido-9-[2,3,5-tri-O-(tert-butyldimethylsilyl)-β-D-ribofura-
nosyl]purine (3b). In a clean, dry round-bottomed flask equipped
2468 J. Org. Chem. Vol. 75, No. 8, 2010

Lakshman et al.
JOCArticle
with EtOAc and transferred to a separatory funnel. The mixture was
extracted with 1:1 water-brine (3ꢀ), water (3ꢀ), and finally once
with brine. The organic layer was dried over Na2SO4 and evaporated
under reduced pressure. Chromatography of the crude material on a
silica gel column using 40% EtOAc in hexanes afforded 3d as a white
foamy material (173 mg, 75% yield).
equipped with a stirring bar were placed the azido nucleoside 3a
(240 mg, 0.474 mmol), CH2Cl2 (1.6 mL), and phenylacetylene
(104 μL, 0.949 mmol). An aqueous solution of sodium ascorbate
(0.047 mmol, 0.95 mL of freshly prepared 0.05 M solution) was
added followed by an aqueous solution of CuSO4 (24 μmol, 0.60
mL of freshly prepared 0.04 M solution). The mixture was
stirred at room temperature for 4 h, at which time TLC indicated
the reaction to be complete. The reaction mixture was diluted
with EtOAc and washed with water. The organic layer was dried
over Na2SO4 and concentrated under reduced pressure. Chro-
matographic purification on a silica gel column using 20%
EtOAc in hexanes afforded 4a as an off-white foam (222 mg,
From Inosine Triacetate. In a clean, dry reaction vial equipped
with a stirring bar were placed inosine-20,30,50-triacetate (1d)
(600 mg, 1.521 mmol) and THF (3.6 mL). The mixture was
cooled with stirring to 0 °C in an ice bath. DPPA (0.49 mL, 2.28
mmol) and DBU (0.35 mL, 2.28 mmol) were added. The
nitrogen gas-flushed mixture was allowed to stir at the ice bath
temperature for 5 min and then at room temperature for 10 min.
Finally, the mixture was stirred in a 60 °C sandbath for 1 h.
Another aliquot of DPPA (0.49 mL, 2.28 mmol) and DBU (0.35
mL, 2.28 mmol) were added, and the reaction was continued for
anadditional 1 h. The mixture was evaporated to dryness, and the
residue was dissolved in CH2Cl2 and washed with water followed
by brine. Chromatographic purification on a silica gel column
using 60% EtOAc in hexanes afforded 3d as a yellow foam (266
mg, 41%yield). Rf (silica gel, 80% EtOAc inhexanes) =0.35. 1H
NMR (500 MHz, DMSO-d6): δ 10.19 (s, 1H, Ar-H), 8.88 (s, 1H,
Ar-H), 6.46 (d, 1H, H-10, J = 5.0 Hz), 5.98 (t, 1H, H-20, J = 5.4
Hz), 5.63 (t, 1H, H-30, J = 5.3 Hz), 4.47 (br s, 1H, H-40), 4.43 (dd,
1H, H-50, J = 2.9, 12.0 Hz), 4.30 (dd, 1H, H-50, J = 5.3, 12.0 Hz),
2.14, 2.05, and 2.04 (3s, 3H, OCOCH3). 13C NMR (125 MHz,
DMSO-d6): δ 170.0, 169.4, 169.2, 145.4, 143.3, 141.3, 136.3,
120.8, 86.4, 79.8, 72.6, 69.8, 62.7, 20.5, 20.3, 20.2. HRMS (ESI):
calcd for C16H17N7O7Na [M þ Na]þ 442.1082, found 442.1077.
6-Azido-9-(2-deoxy-β-D-ribofuranosyl)purine (3e). In a clean,
dry reaction vial equipped with a stirring bar were placed
O6-(benzotriazol-1-yl)-20-deoxyinosine 2e (150 mg, 0.406 mmol)
and polymer-supported azide23 (530 mg, 0.406 mmol). Water
(3.75 mL) was added, the reaction mixture was flushed with
nitrogen gas and allowed to stir in a 50 °C sandbath for 3.5 h.
The reaction mixture was filtered and evaporated to dryness
under reduced pressure. The residue was washed with Et2O to
afford 3e as a white powder (66 mg, 59% yield). Rf (silica gel,
1
77% yield). Rf (silica gel, 20% EtOAc in hexanes) = 0.18. H
NMR (500 MHz, CDCl3): δ 9.35 (s, 1H, Ar-H), 8.96 (s, 1H, Ar-
H), 8.59 (s, 1H, Ar-H), 8.02 (d, 2H, Ar-H, J = 7.2 Hz), 7.49 (t,
2H, Ar-H, J = 7.6 Hz), 7.40 (t, 1H, Ar-H, J = 7.3 Hz), 6.61 (t,
1H, H-10, J = 6.4 Hz), 4.65 (m, 1H, H-30), 4.09 (q, 1H, H-40, J =
3.2 Hz), 3.91 (dd, 1H, H-50, J = 3.8, 11.3 Hz), 3.81 (dd, 1H, H-50,
J = 2.9, 11.3 Hz), 2.68 (app quint, 1H, H-20, Japp ≈ 6.4 Hz), 2.54
(ddd, 1H, H-20, J = 3.8, 6.1, 13.1 Hz), 0.93 and 0.92 (2s, 18H, t-
Bu), 0.13 and 0.11 (2s, 12H, Si-CH3). 13C NMR (125 MHz,
CDCl3): δ 154.0, 152.3, 148.5, 144.9, 144.7, 130.1, 129.1, 128.9,
126.4, 123.4, 120.1, 88.5, 85.2, 72.1, 62.9, 41.8, 26.2, 25.9, 18.6,
18.2, -4.4, -4.6, -5.1, -5.2. HRMS (ESI): calcd for C30H45-
N7O3Si2Na [M þ Na]þ 630.3015, found 630.2999.
6-[4-(4-Methylphenyl)-1,2,3-triazol-1-yl]-9-[2-deoxy-3,5-di-
O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (4b). Syn-
thesized from 3a (195 mg, 0.385 mmol) and 4-ethynyltoluene
(98 μL, 0.773 mmol). Chromatography of the crude reaction
mixture on a silica gel column using 10% EtOAc in hexanes
yielded 4b as a white foamy solid (209 mg, 87% yield). Rf (silica
gel, 20% EtOAc in hexanes) = 0.22. 1H NMR (500 MHz,
CDCl3): δ 9.30 (s, 1H, Ar-H), 8.95 (s, 1H, Ar-H), 8.58 (s, 1H, Ar-
H), 7.90 (d, 1H, Ar-H, J = 8.0 Hz), 7.29 (d, 2H, Ar-H, J = 8.0
Hz), 6.61 (t, 1H, H-10, J = 6.3 Hz), 4.65 (m, 1H, H-30), 4.08 (q,
1H, H-40, J = 3.2 Hz), 3.91 (dd, 1H, H-50, J = 3.9, 11.2 Hz), 3.81
(dd, 1H, H-50, J = 2.9, 11.2 Hz), 2.68 (app quint, 1H, H-20,
Japp ≈ 6.4 Hz), 2.54 (ddd, 1H, H-20, J = 3.8, 6.1, 13.1 Hz), 2.41
(s, 3H, CH3), 0.93 and 0.92 (2s, 18H, t-Bu), 0.12 and 0.11 (2s,
12H, Si-CH3). 13C NMR (125 MHz, CDCl3): δ 154.0, 152.4,
148.5, 145.0, 144.7, 138.8, 129.8, 127.2, 126.3, 123.4, 119.7, 88.5,
85.2, 72.1, 63.0, 41.8, 26.2, 26.0, 21.6, 18.6, 18.2, -4.4, -4.6,
-5.1, -5.2. HRMS (ESI): calcd for C31H47N7O3Si2Na [M þ
Na]þ 644.3171, found 644.3154.
6-[4-(4-Methoxyphenyl)-1,2,3-triazol-1-yl]-9-[2-deoxy-3,5-
di-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (4c). Syn-
thesized from 3a (204 mg, 0.400 mmol) and 4-ethynylanisole
(102.5 μL, 0.807 mmol). Chromatography of the crude reaction
mixture a silica gel column using 10% EtOAc in hexanes yielded
4c as a white foamy solid (231 mg, 90% yield). Rf (silica gel, 20%
EtOAc in hexanes) = 0.43. 1H NMR (500 MHz, CDCl3): δ 9.25
(s, 1H, Ar-H), 8.95 (s, 1H, Ar-H), 8.58 (s, 1H, Ar-H), 7.94 (d, 2H,
Ar-H, J = 8.8 Hz), 7.01 (d, 2H, Ar-H, J = 8.8 Hz), 6.61(t, 1H, H-
10, J = 6.4 Hz), 4.65 (m, 1H, H-30), 4.08 (q, 1H, H-40, J = 3.2 Hz),
3.91 (dd, 1H, H-50, J = 3.8, 11.3 Hz), 3.87 (s, 3H, OCH3), 3.81
1
10% MeOH in CH2Cl2) = 0.2. H NMR (500 MHz, DMSO-
d6): δ 10.13 (s, 1H, Ar-H), 8.88 (s, 1H, Ar-H), 6.57 (t, 1H, H-10,
J = 6.5 Hz), 5.40 (d, 1H, OH, J = 4.3 Hz), 4.98 (t, 1H, OH J =
5.4 Hz), 4.46 (m, 1H, H-30), 3.93 (app q, 1H, H-40, Japp ≈ 4.1 Hz),
3.68-3.61 (m, 1H, H-50), 3.59-3.53 (m, 1H, H-50), 2.75 (app
quint, 1H, H-20, Japp ≈ 6.5 Hz), 2.44 (ddd, 1H, H-20, J = 3.9, 6.3,
13.3 Hz). 13C NMR (125 MHz, DMSO-d6): δ 145.4, 142.5,
141.6, 135.9, 120.5, 88.2, 84.4, 70.3, 61.3, 39.8. HRMS (ESI):
calcd for C10H11N7O3Na [M þ Na]þ 300.0816, found 300.0818.
6-Azido-9-(β-D-ribofuranosyl)purine (3f). In a clean, dry reac-
tion vial equipped with
a stirring bar were placed
O6-(benzotriazol-1-yl)inosine 2f (150 mg, 0.389 mmol) and
polymer-supported azide23 (530 mg, 0.406 mmol). DMF (3.9
mL) was added, and the reaction mixture was flushed with
nitrogen gas and allowed to stir in a 50 °C sandbath for 5 h. The
reaction mixture was filtered, and the DMF was coevaporated
with toluene. The residue was washed with Et2O to afford 3f as a
white powder (80 mg, 70% yield). Rf (silica gel, 10% MeOH in
(dd, 1H, H-50, J = 2.9, 11.3 Hz), 2.67 (app quint, 1H, H-20, Japp
≈
1
CH2Cl2) = 0.19. H NMR (500 MHz, DMSO-d6): δ 10.15 (s,
6.4 Hz), 2.54 (ddd, 1H, H-20, J = 3.8, 6.1, 13.1 Hz), 0.93 and 0.92
(2s, 18H, t-Bu), 0.12 and 0.11 (2s, 12H, Si-CH3). 13C NMR (125
MHz, CDCl3): δ 160.2, 154.0, 152.3, 148.3, 145.0, 144.6, 127.8,
123.3, 122.7, 119.1, 114.5, 88.5, 85.1, 72.1, 62.9, 55.6, 41.8, 26.2,
25.9, 18.6, 18.2, -4.4, -4.6, -5.1, -5.2. HRMS (ESI): calcd for
C31H47N7O4Si2Na [M þ Na]þ 660.3120, found 660.3105.
1H, Ar-H), 8.93 (s, 1H, Ar-H), 6.15 (d, 1H, H-10, J = 5.1 Hz),
5.63 (d, 1H, OH, J = 5.6 Hz), 5.30 (d, 1H, OH, J = 5.2 Hz), 5.10
(t, 1H, OH, J = 5.3 Hz), 4.57 (q, 1H, H-20, J = 5.2 Hz), 4.20 (q,
1H, H-30, J = 4.6 Hz), 4.02 (m, 1H, H-40), 3.76-3.68 (m, 1H, H-
50), 3.65-3.56 (m, 1H, H-50). 13C NMR (125 MHz, DMSO-d6):
δ 145.4, 142.6, 141.9, 136.1, 120.2, 88.3, 85.7, 74.5, 70.0, 60.9.
HRMS (ESI): calcd for C10H11N7O4Na [M þ Na]þ 316.0765,
found 316.0765.
6-[4-(4-Fluorophenyl)-1,2,3-triazol-1-yl]-9-[2-deoxy-3,5-di-
O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (4d). Syn-
thesized from 3a (210 mg, 0.415 mmol) and 1-ethynyl-4-fluor-
obenzene (95 μL, 0.830 mmol). Chromatography of the crude
reaction mixture on a silica gel column using 20% EtOAc in
hexanes yielded 4d as a yellow, crystalline solid (240 mg, 92%
Typical Procedure for Azide-Alkyne Ligation: Synthesis of
6-(4-Phenyl-1,2,3-triazol-1-yl)-9-[2-deoxy-3,5-di-O-(tert-butyl-
dimethylsilyl)-β-D-ribofuranosyl]purine (4a). In a clean, dry vial
J. Org. Chem. Vol. 75, No. 8, 2010 2469

Lakshman et al.
JOCArticle
yield). Rf (silica gel, 40% EtOAc in hexanes) = 0.36. 1H NMR
(500 MHz, CDCl3): δ 9.30 (s, 1H, Ar-H), 8.96 (s, 1H, Ar-H), 8.60
(s, 1H, Ar-H), 7.99 (dd, 2H, Ar-H, JH,H = 8.6 Hz, JF,H = 5.4
Hz), 7.17 (t, 2H, Ar-H, JH,H = JF,H = 8.6 Hz), 6.61 (t, 1H, H-10,
J = 6.3 Hz), 4.65 (m, 1H, H-30), 4.08 (q, 1H, H-40, J = 3.2 Hz),
3.91 (dd, 1H, H-50, J = 3.7, 11.2 Hz), 3.81 (dd, 1H, H-50, J = 2.9,
11.2 Hz), 2.68 (app quint, 1H, H-20, Japp ≈ 6.4 Hz), 2.54 (ddd,
1H, H-20, J = 3.8, 6.0, 13.0 Hz), 0.93 and 0.92 (2s, 18H, t-Bu),
0.12 and 0.11 (2s, 12H, Si-CH3). 13C NMR (125 MHz, CDCl3): δ
6-[4-(1-Butyl)-1,2,3-triazol-1-yl]-9-[2-deoxy-3,5-di-O-(tert-
butyldimethylsilyl)-β-D-ribofuranosyl]purine (4g). Synthesized
from 3a (190 mg, 0.375 mmol) and 1-hexyne (82.5 μL, 0.751
mmol). Chromatography of the crude reaction mixture on a
silica gel column using 20% EtOAc in hexanes yielded 4g as a
white solid (166 mg, 75% yield). Rf (silica gel, 20% EtOAc in
1
hexanes) = 0.16. H NMR (500 MHz, CDCl3): δ 8.91 (s, 1H,
Ar-H), 8.81 (s, 1H, Ar-H), 8.53 (s, 1H, Ar-H), 6.58 (t, 1H, H-10,
J = 6.3 Hz), 4.64 (m, 1H, H-30), 4.07 (m, 1H, H-40), 3.89 (dd, 1H,
H-50, J = 3.9, 11.2 Hz), 3.79 (dd, 1H, H-50, J = 2.9, 11.2 Hz),
2.87 (t, 2H, butyl-CH2, J = 7.8 Hz), 2.66 (app quint, 1H, H-20,
Japp ≈ 6.4 Hz), 2.50 (ddd, 1H, H-20, J = 3.8, 5.9, 13.0 Hz), 1.76
(quint, 2H, butyl-CH2, J = 7.6 Hz), 1.44 (sextet, 2H, butyl-CH2,
J = 7.4 Hz), 0.95 (t, 3H, butyl-CH3, J = 7.3 Hz), 0.92 and 0.90
(2s, 18H, t-Bu), 0.11 and 0.09 (2s, 12H, Si-CH3). 13C NMR (125
MHz, CDCl3): δ 153.9, 152.3, 149.4, 145.1, 144.5, 123.3, 121.4,
88.5, 85.1, 72.1, 62.9, 41.7, 31.5, 26.2, 25.9, 25.5, 22.5, 18.6, 18.2,
14.0, -4.5, -4.6, -5.2, -5.3. HRMS (ESI): calcd for
C28H49N7O3Si2Na [M þ Na]þ 610.3328, found 610.3328.
6-(4-Phenyl-1,2,3-triazol-1-yl)-9-[2,3,5-tri-O-(tert-butyldime-
thylsilyl)-β-D-ribofuranosyl]purine (5a). Synthesized from 3b
(200 mg, 0.314 mmol) and phenylacetylene (69.0 μL, 0.628
mmol). Chromatography of the crude reaction mixture on a
silica gel column using 20% EtOAc in hexanes yielded 5a as a
brown crystalline solid (210 mg, 90% yield). Rf (silica gel, 20%
acetone in hexanes) = 0.49. 1H NMR (500 MHz, CDCl3): δ 9.34
(s, 1H, Ar-H), 8.95 (s, 1H, Ar-H), 8.64 (s, 1H, Ar-H), 8.0 (d, 2H,
Ar-H, J = 7.5 Hz), 7.45 (t, 2H, Ar-H, J = 7.6 Hz), 7.36 (t, 1H,
Ar-H, J = 7.4 Hz), 6.2 (d, 1H, H-10, J = 5.1 Hz), 4.65 (t, 1H, H-
20, J = 4.6 Hz), 4.33 (t, 1H, H-30, J = 3.9 Hz), 4.18 (q, 1H, H-40,
J = 3.0 Hz), 4.04 (dd, 1H, H-50, J = 3.5, 11.3 Hz), 3.82 (dd, 1H,
H-50, J = 2.3, 11.3 Hz), 0.97, 0.94, and 0.79 (3s, 27H, t-Bu), 0.17,
0.16, 0.11, 0.10, -0.02, and -0.23 (6s, 18H, Si-CH3). 13C NMR
(125 MHz, CDCl3): δ 154.3, 152.3, 148.4, 144.9, 130.0, 129.0,
128.7, 126.7, 123.2, 120.1, 88.6, 86.0, 76.5, 72.1, 62.6, 26.2, 25.9,
25.7, 18.7, 18.2, 18.0, -4.2, -4.4, -4.5, -4.8, -5.2. HRMS
(ESI): calcd C36H59N7O4Si3Na [M þ Na]þ 760.3829, found
760.3826.
1
163.0 (d, JC,F = 248.1 Hz), 153.8, 152.1, 147.4, 144.6, 144.5,
128.0 (d, 3JC,F = 8.2 Hz), 126.0 (d, 4JC,F = 3.2 Hz), 123.1, 119.6,
115.9 (d, 2JC,F = 21.8 Hz), 88.3, 84.9, 71.9, 62.7, 41.6, 25.9, 25.6,
18.4, 18.0, -4.6, -4.8, -5.3, -5.5. HRMS (ESI): calcd for
C30H44FN7O3Si2Na [M þ Na]þ 648.2920, found 648.2911.
6-[4-(N-Phthalimidomethyl)-1,2,3-triazol-1-yl]-9-[2-deoxy-
3,5-di-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (4e).
Synthesized from 3a (190 mg, 0.376 mmol) and N-propar-
gylphthalimide (139 mg, 0.751 mmol). Chromatography of the
crude reaction mixture on a silica gel column using 10% EtOAc
in hexanes yielded 4e as an off-white, foamy solid (246 mg, 95%
yield). Rf (silica gel, 20% EtOAc in hexanes) = 0.15. 1H NMR
(500 MHz, CDCl3): δ 9.10 (s, 1H, Ar-H), 8.90 (s, 1H, Ar-H), 8.55
(s, 1H, Ar-H), 7.88 (dd, 2H, Ar-H, J = 3.1, 5.4 Hz), 7.73 (dd, 2H,
Ar-H, J = 3.0, 5.4 Hz), 6.57 (t, 1H, H-10, J = 6.4 Hz), 5.16 (s,
2H, NCH2), 4.64 (m, 1H, H-30), 4.06 (q, 1H, H-40, J = 3.3 Hz),
3.89 (dd, 1H, H-50, J = 3.9, 11.2 Hz), 3.79 (dd, 1H, H-50, J = 3.1,
11.2 Hz), 2.64 (app quint, 1H, H-20, Japp ≈ 6.4 Hz), 2.52 (ddd,
1H, H-20, J = 3.8, 6.0, 13.0 Hz), 0.92 and 0.90 (2s, 18H, t-Bu),
0.11, 0.092, and 0.088 (3s, 12H, Si-CH3). 13C NMR (125 MHz,
CDCl3): δ 167.8, 154.1, 152.2, 144.8, 143.6, 134.3, 132.3, 123.7,
123.6, 123.5, 88.5, 85.1, 72.1, 62.9, 41.8, 33.3, 26.2, 25.9, 18.6,
18.2, -4.5, -4.6, -5.2, -5.3. HRMS (ESI): calcd for C33H46-
N8O5Si2Na [M þ Na]þ 713.3022, found 713.2999.
6-(4-Ferrocenyl-1,2,3-triazol-1-yl)-9-[2-deoxy-3,5-di-O-(tert-
butyldimethylsilyl)-β-D-ribofuranosyl]purine (4f). Synthesized
from 3a (209 mg, 0.413 mmol) and ethynylferrocene (173 mg,
0.820 mmol). Chromatography of the crude reaction mixture on
a silica gel column using 15% acetone in hexanes yielded 4f as a
brown, foamy solid (247 mg, 83% yield). Rf (silica gel, 15%
acetone in hexanes) = 0.22. 1H NMR (500 MHz, CDCl3): δ 8.99
(s, 1H, Ar-H), 8.94 (s, 1H, Ar-H), 8.58 (s, 1H, Ar-H), 6.60 (t, 1H,
H-10, J = 6.3 Hz), 4.88 (br t, 2H, ferrocenyl-H), 4.65 (m, 1H, H-
30), 4.36 (t, 2H, ferrocenyl-H, J = 1.8 Hz), 4.12 (s, 5H, ferro-
cenyl-H), 4.08 (q, 1H, H-40, J = 3.2 Hz), 3.91 (dd, 1H, H-50, J =
3.9, 11.2 Hz), 3.81 (dd, 1H, H-50, J = 2.9, 11.2 Hz), 2.67 (app
quint, 1H, H-20, Japp ≈ 6.4 Hz), 2.54 (ddd, 1H, H-20, J = 3.9, 6.0,
13.0 Hz), 0.93 and 0.92 (2s, 18H, t-Bu), 0.12 and 0.11 (2s, 12H,
Si-CH3). 13C NMR (125 MHz, CDCl3): δ 154.0, 152.3, 147.9,
144.9, 144.5, 123.3, 118.8, 88.5, 85.1, 74.6, 72.1, 69.9, 69.2, 67.3,
62.9, 41.8, 26.2, 25.9, 18.7, 18.2, -4.4, -4.6, -5.1, -5.2. HRMS
(ESI): calcd for C34H49FeN7O3Si2Na [M þ Na]þ 738.2677,
found 738.2677.
Minor Isomer: 6-(5-Ferrocenyl-1,2,3-triazol-1-yl)-9-[2-deoxy-
3,5-di-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (4f0).
Obtained 18.4 mg (6% yield) of a reddish-brown solid. Rf
(silica gel, 15% acetone in hexanes) = 0.14. 1H NMR (500
MHz, CDCl3): δ 8.97 (s, 1H, Ar-H), 8.55 (s, 1H, Ar-H), 7.89 (s,
1H, Ar-H), 6.61 (t, 1H, H-10, J = 6.5 Hz), 4.64 (m, 1H, H-30),
4.58 (m, 2H, ferrocenyl-H), 4.27 (m, 2H, ferrocenyl-H), 4.07
(q, 1H, H-40, J = 3.1 Hz), 3.99 (s, 5H, ferrocenyl-H), 3.88 (dd,
1H, H-50, J = 3.9, 11.2 Hz), 3.80 (dd, 1H, H-50, J = 2.9, 11.2
Hz), 2.64 (app quint, 1H, H-20, Japp ≈ 6.4 Hz), 2.52 (ddd, 1H,
H-20, J = 3.7, 6.0, 13.0 Hz), 0.93 and 0.90 (2s, 18H, t-Bu), 0.12,
0.09, and 0.08 (3s, 12H, Si-CH3). 13C NMR (125 MHz, CDCl3):
δ 154.1, 151.9, 147.1, 145.5, 138.9, 133.3, 128.6, 88.5, 85.1, 72.1,
70.6, 70.0, 69.6, 69.5, 63.0, 41.9, 26.2, 26.0, 18.7, 18.2, -4.4,
-4.6, -5.1, -5.2. HRMS (ESI): calcd for C34H49FeN7O3-
Si2Na [M þ Na]þ 738.2677, found 738.2655.
Minor Isomer: 6-(5-Phenyl-1,2,3-triazol-1-yl)-9-[2,3,5-tri-
O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (5a0). Ob-
tained 4.4 mg (2% yield) of a brown solid. Rf (silica gel, 20%
acetone in hexanes) = 0.19. 1H NMR (500 MHz, CDCl3): δ 8.82
(s, 1H, Ar-H), 8.55 (s, 1H, Ar-H), 7.93 (s, 1H, Ar-H), 7.36-7.28
(m, 5H, Ar-H), 6.19 (d, 1H, H-10, J = 5.1 Hz), 4.60 (t, 1H, H-20,
J = 4.7 Hz), 4.31 (t, 1H, H-30, J = 3.8 Hz), 4.17 (m, 1H, H-40),
4.02 (dd, 1H, H-50, J = 3.7, 11.5 Hz), 3.81 (dd, 1H, H-50, J = 2.4,
11.5 Hz), 0.95, 0.94, and 0.79 (3s, 27H, t-Bu), 0.13, 0.11, 0.10,
-0.02, and -0.25 (5s, 18H, Si-CH3). 13C NMR (125 MHz,
CDCl3): δ 154.5, 152.1, 146.9, 145.7, 139.5, 133.9, 129.4, 128.8,
127.7, 127.1, 88.7, 86.1, 76.5, 72.1, 62.7, 26.3, 26.0, 25.9, 18.8,
18.3, 18.0, -4.2, -4.4, -4.5, -4.8, -5.1. HRMS (ESI): calcd
C36H59N7O4Si3Na [M þ Na]þ 760.3829, found 760.3829.
6-[4-(4-Methylphenyl)-1,2,3-triazol-1-yl]-9-[2,3,5-tri-O-(tert-
butyldimethylsilyl)-β-D-ribofuranosyl]purine (5b). Synthesized
from 3b (200 mg, 0.314 mmol) and 4-ethynyltoluene (79.5 μL,
0.629 mmol). Chromatography of the crude reaction mixture on
a silica gel column using 10% acetone in hexanes yielded 5b as a
light-yellow solid (214 mg, 90% yield). Rf (silica gel, 10%
1
acetone in hexanes) = 0.29. H NMR (500 MHz, CDCl3): δ
9.31 (s, 1H, Ar-H), 8.96 (s, 1H, Ar-H), 8.67 (s, 1H, Ar-H), 7.90
(d, 2H, Ar-H, J = 8.0 Hz), 7.29 (d, 2H, Ar-H, J = 8.0 Hz), 6.22
(d, 1H, H-10, J = 5.2 Hz), 4.66 (t, 1H, H-20, J = 4.6 Hz), 4.33 (t,
1H, H-30, J = 3.9 Hz), 4.19 (q, 1H, H-40, J = 3.1 Hz), 4.05 (dd,
1H, H-50, J = 3.5, 11.4 Hz), 3.83 (dd, 1H, H-50, J = 2.5, 11.4
Hz), 2.41 (s, 3H, CH3), 0.98, 0.95, and 0.80 (3s, 27H, t-Bu), 0.18,
0.17, 0.12, 0.11, -0.01, and -0.22 (6s, 18H, Si-CH3). 13C NMR
(125 MHz, CDCl3): δ 154.4, 152.4, 148.6, 145.1, 145.0, 138.8,
129.8, 127.3, 126.4, 123.3, 119.7, 88.7, 86.1, 76.5, 72.2, 62.7, 26.3,
2470 J. Org. Chem. Vol. 75, No. 8, 2010

Lakshman et al.
JOCArticle
26.1, 25.8, 21.5, 18.8, 18.3, 18.0, -4.2, -4.4, -4.5, -4.8, -5.1.
HRMS (ESI): calcd for C37H61N7O4Si3Na [M þ Na]þ 774.3985,
found 774.3986.
t-Bu), 0.13, 0.11, 0.10, -0.02, and -0.25 (5s, 18H, Si-CH3). 13
C
NMR (125 MHz, CDCl3): δ 163.4 (d, 1JC,F = 250.3 Hz), 154.6,
3
152.1, 146.5, 145.7, 138.6, 133.9, 130.8 (d, JC,F = 8.6 Hz),
6-[4-(4-Methoxyphenyl)-1,2,3-triazol-1-yl]-9-[2,3,5-tri-O-(tert-
butyldimethylsilyl)-β-D-ribofuranosyl]purine (5c). Synthesized
from 3b (200 mg, 0.314 mmol) and 4-ethynylanisole (81.3 μL,
0.628 mmol). Chromatography of the crude reaction mixture on
a silica gel column using 20% EtOAc in hexanes yielded 5c as a
white, foamy solid (228 mg, 94% yield). Rf (silica gel, 20%
EtOAc in hexanes) = 0.29. 1H NMR (500 MHz, CDCl3): δ 9.27
(s, 1H, Ar-H), 8.95 (s, 1H, Ar-H), 8.63 (s, 1H, Ar-H), 7.94 (d, 2H,
Ar-H, J = 8.8 Hz), 7.01 (d, 2H, Ar-H, J = 8.8 Hz), 6.22 (d, 1H,
H-10, J = 5.8 Hz), 4.66 (t, 1H, H-20, J = 4.7 Hz), 4.33 (t, 1H, H-
30, J = 3.9 Hz), 4.19 (q, 1H, H-40, J = 3.0 Hz), 4.05 (dd, 1H, H-
50, J = 3.6, 11.5 Hz), 3.87 (s, 3H, OCH3), 3.83 (dd, 1H, H-50, J =
2.4, 11.5 Hz), 0.98, 0.95, and 0.80 (3s, 27H, t-Bu), 0.18, 0.17,
0.12, 0.11, -0.01, and -0.23 (6s, 18H, Si-CH3). 13C NMR (125
MHz, CDCl3): δ 160.2, 154.4, 152.4, 148.4, 145.0, 144.9, 127.8,
123.3, 122.8, 119.2, 114.5, 88.7, 86.0, 76.5, 72.1, 62.7, 55.6, 26.3,
26.0, 25.8, 18.8, 18.3, 18.0, -4.2, -4.4, -4.5, -4.8, -5.1. HRMS
(ESI): calcd for C37H61N7O5Si3Na [M þ Na]þ 790.3934, found
790.3933.
Minor isomer: 6-[5-(4-Methoxyphenyl)-1,2,3-triazol-1-yl]-
9-[2,3,5-tri-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine
(5c0). Obtained 9.1 mg (3% yield) of a clear, gummy material. Rf
(silica gel, 20% EtOAc in hexanes) = 0.10. 1H NMR (500 MHz,
CDCl3): δ 8.84 (s, 1H, Ar-H), 8.55 (s, 1H, Ar-H), 7.88 (s, 1H, Ar-
H), 7.22 (d, 2H, Ar-H, J = 8.8 Hz), 6.83 (d, 2H, Ar-H, J = 8.8
Hz), 6.19 (d, 1H, H-10, J = 5.1 Hz), 4.60 (t, 1H, H-20, J = 4.7
Hz), 4.32 (t, 1H, H-30, J = 3.9 Hz), 4.17 (m, 1H, H-40), 4.02 (dd,
1H, H-50, J = 3.7, 11.5 Hz), 3.81 (merged with OCH3 resonance,
1H, H-50), 3.80 (s, 3H, OCH3), 0.95, 0.94, and 0.79 (3s, 27H, t-
Bu), 0.13, 0.11, 0.10, -0.02, and -0.25 (5s, 18H, Si-CH3). 13C
NMR (125 MHz, CDCl3): δ 160.5, 154.5, 152.1, 146.9, 145.7,
139.4, 133.4, 130.2, 127.9, 119.3, 114.3, 88.7, 86.0, 76.5, 72.1,
62.7, 55.5, 26.3, 26.0, 25.8, 18.8, 18.3, 18.0, -4.2, -4.4, -4.5,
-4.8, -5.1. HRMS (ESI): calcd for C37H61N7O5Si3Na [M þ
Na]þ 790.3934, found 790.3944.
127.6, 123.3, 116.0 (2JC,F = 22.0 Hz), 88.7, 86.1, 76.6, 72.1, 62.7,
26.3, 26.0, 25.8, 18.8, 18.3, 18.0, -4.2, -4.4, -4.5, -4.8, -5.1.
HRMS (ESI): calcd for C36H58FN7O4Si3Na [M þ Na]þ
778.3734, found 778.3739.
6-[4-(N-Phthalimidomethyl)-1,2,3-triazol-1-yl]-9-[2,3,5-tri-
O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (5e). Syn-
thesized from 3b (200 mg, 0.314 mmol) and N-propargylphtha-
limide (0.132 g, 0.628 mmol). Chromatography of the crude
reaction mixture on a silica gel column using 15% EtOAc in
hexanes yielded 5e as an off-white, foamy solid (196 mg, 76%
yield). Rf (silica gel, 25% EtOAc in hexanes) = 0.22. 1H NMR
(500 MHz, CDCl3): δ 9.11 (s, 1H, Ar-H), 8.90 (s, 1H, Ar-H), 8.61
(s, 1H, Ar-H), 7.88 (dd, 2H, Ar-H, J = 2.9, 5.4 Hz), 7.72 (dd, 2H,
Ar-H, J = 2.9, 5.4 Hz), 6.19 (d, 1H, H-10, J = 4.9 Hz), 5.16 (s,
2H, NCH2), 4.63 (t, 1H, H-20, J = 4.3 Hz), 4.31 (m, 1H, H-30),
4.17 (m, 1H, H-40), 4.03 (dd, 1H, H-50, J = 3.4, 11.7 Hz), 3.81
(dd, 1H, H-50, J = 2.4, 11.7 Hz), 0.96, 0.94, and 0.78 (3s, 27H, t-
Bu), 0.16, 0.15, 0.10, -0.03, and -0.25 (5s, 18H, Si-CH3). 13C
NMR (125 MHz, CDCl3): δ 167.6, 154.2, 152.1, 145.0, 144.6,
143.4, 134.1, 132.1, 123.5, 123.4, 123.2, 88.5, 85.8, 76.2, 71.9,
62.4, 33.1, 26.1, 25.8, 25.6, 18.5, 18.1, 17.8, -4.4, -4.6, -4.7,
-5.0, -5.3. HRMS (ESI): calcd for C39H60N8O6Si3Na [M þ
Na]þ 843.3836, found 843.3833.
Minor Isomer: 6-[5-(N-Phthalimidomethyl)-1,2,3-triazol-1-
yl]-9-[2,3,5-tri-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]-
purine (5e0). Obtained 8.5 mg (3% yield) of an off-white, gummy
material. Rf (silica gel, 25% EtOAc in hexanes) = 0.10. 1H
NMR (500 MHz, CDCl3): δ 8.95 (s, 1H, Ar-H), 8.66 (s, 1H, Ar-
H), 7.86 (dd, 2H, Ar-H, J = 3.1, 5.4 Hz), 7.75 (dd, 2H, Ar-H,
J = 3.4, 5.4 Hz), 7.69 (s, 1H, Ar-H), 6.26 (d, 1H, H-10, J = 5.1
Hz), 5.55 (s, 2H, NCH2), 4.64 (t, 1H, H-20, J = 4.6 Hz), 4.34 (t,
1H, H-30, J = 3.9 Hz), 4.21-4.17 (m, 1H, H-40), 4.04 (dd, 1H, H-
50, J = 3.4, 11.5 Hz), 3.83 (dd, 1H, H-50, J = 2.2, 11.5 Hz), 0.96,
0.95, and 0.82 (3s, 27H, t-Bu), 0.16, 0.15, 0.12, 0.001, and -0.19
(5s, 18H, Si-CH3). 13C NMR (125 MHz, CDCl3): δ 167.5, 154.7,
151.7, 146.6, 145.5, 135.2, 134.6, 134.5, 132.0, 125.7, 123.9, 88.6,
86.0, 76.6, 72.1, 62.7, 33.3, 26.4, 26.1, 25.9, 18.8, 18.3, 18.1, -4.1,
-4.4, -4.5, -4.7, -5.1. HRMS (ESI): calcd for C39H60N8O6-
Si3Na [M þ Na]þ 843.3836, found 843.3830.
6-[4-(4-Fluorophenyl)-1,2,3-triazol-1-yl]-9-[2,3,5-tri-O-(tert-
butyldimethylsilyl)-β-D-ribofuranosyl]purine (5d). Synthesized
from 3b (200 mg, 0.314 mmol) and 1-ethynyl-4-fluorobenzene
(72.0 μL, 0.629 mmol). Chromatography of the crude reaction
mixture on a silica gel column using 10% EtOAc in hexanes
yielded 5d as a light-yellow, foamy solid (194 mg, 82% yield).
Rf (silica gel, 20% EtOAc in hexanes) = 0.41. 1H NMR (500
MHz, CDCl3): δ 9.32 (s, 1H, Ar-H), 8.96 (s, 1H, Ar-H), 8.64 (s,
1H, Ar-H), 7.99 (dd, 2H, Ar-H, JH,H = 8.6 Hz, JF,H = 5.3 Hz),
7.17 (t, 2H, Ar-H, JH,H = JF,H = 8.6 Hz), 6.23 (d, 1H, H-10,
J = 5.1 Hz), 4.66 (t, 1H, H-20, J = 4.6 Hz), 4.33 (t, 1H, H-30, J =
3.9 Hz), 4.19 (m, 1H, H-40), 4.05 (dd, 1H, H-50, J = 3.3, 11.3 Hz),
3.84 (dd, 1H, H-50, J = 2.2, 11.3 Hz), 0.98, 0.95, and 0.80 (3s,
27H, t-Bu), 0.18, 0.17, 0.11, -0.013, and -0.23 (5s, 18H, Si-CH3).
6-(4-Ferrocenyl-1,2,3-triazol-1-yl)-9-[2,3,5-bis-O-(tert-butyldi-
methylsilyl)-β-D-ribofuranosyl]purine (5f). Synthesized from 3b
(200 mg, 0.314 mmol) and ethynylferrocene (0.132 g, 0.628
mmol). Chromatography of the crude reaction mixture on a
silica gel column using 5% acetone in hexanes yielded 5f as a
reddish-brown, foamy solid (189 mg, 71% yield). Rf (silica gel,
10% acetone in hexanes) = 0.18. 1H NMR (500 MHz, CDCl3): δ
8.99 (s, 1H, Ar-H), 8.94 (s, 1H, Ar-H), 8.64 (s, 1H, Ar-H), 6.21 (d,
1H, H-10, J = 5.1 Hz), 4.88 (br s, 2H, ferrocenyl-H), 4.65 (t, 1H,
H-20, J = 4.5 Hz), 4.36 (br s, 2H, ferrocenyl-H), 4.33 (t, 1H, H-30,
J = 4.0 Hz), 4.19 (m, 1H, H-40), 4.12 (s, 5H, ferrocenyl-H), 4.05
(dd, 1H, H-50, J = 3.3, 11.5 Hz), 3.83 (dd, 1H, H-50, J = 2.0, 11.5
Hz), 0.98, 0.94, and 0.81 (3s, 27H, t-Bu), 0.18, 0.16, 0.11, 0.107,
-0.01, and -0.20 (6s, 18H, Si-CH3). 13C NMR (125 MHz,
CDCl3): δ 154.4, 152.4, 147.9, 145.0, 144.9, 123.3, 118.8, 88.7,
85.9, 76.4, 74.7, 72.1, 69.9, 69.2, 67.3, 62.7, 26.3, 26.0, 25.9, 18.8,
18.3, 18.0, -4.2, -4.4, -4.5, -4.7, -5.1. HRMS (ESI): calcd for
C40H63FeN7O4Si3Na [M þ Na]þ 868.3491, found 868.3487.
Minor Isomer: 6-(5-Ferrocenyl-1,2,3-triazol-1-yl)-9-[2,3,5-bis-
O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (5f0). Ob-
tained 28.3 mg (10% yield) of a brown solid. Rf (silica gel,
40% EtOAc in hexanes) = 0.41. 1H NMR (500 MHz, CDCl3): δ
8.98 (s, 1H, Ar-H), 8.60 (s, 1H, Ar-H), 7.89 (s, 1H, Ar-H), 6.22
(d, 1H, H-10, J = 5.1 Hz), 4.60 (t, 1H, H-20, J = 4.6 Hz), 4.41 (m,
2H, ferrocenyl-H), 4.32 (t, 1H, H-30, J = 4.0 Hz), 4.25 (m, 2H,
ferrocenyl-H), 4.18 (q, 1H, H-40, J = 3.1 Hz), 4.02 (dd, 1H, H-50,
13C NMR (125 MHz, CDCl3): δ 163.2 (d, JC,F = 248.1 Hz),
1
154.5, 152.4, 147.6, 145.1, 144.9, 128.2 (d, 3JC,F = 8.2 Hz), 126.3
(d, 4JC,F = 5.1 Hz), 123.3, 119.9, 116.1 (d, 2JC,F = 21.8 Hz), 88.7,
86.1, 76.5, 72.1, 62.7, 26.3, 26.1, 25.8, 18.8, 18.3, 18.1, -4.2, -4.4,
-4.5, -4.8, -5.1. HRMS (ESI): calcd for C36H58FN7O4Si3Na
[M þ Na]þ 778.3734, found 778.3721.
Minor Isomer: 6-[5-(4-Fluorophenyl)-1,2,3-triazol-1-yl]-9-
[2,3,5-tri-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]purine (5d0).
Obtained 2.9 mg (1% yield) of a clear, gummy material. Rf
(silica gel, 20% EtOAc in hexanes) = 0.08. 1H NMR (500 MHz,
CDCl3): δ 8.83 (s, 1H, Ar-H), 8.56 (s, 1H, Ar-H), 7.91 (s, 1H, Ar-
H), 7.29 (dd, 2H, Ar-H, JH,H = 8.7 Hz, JF,H = 5.2 Hz), 7.01 (t,
2H, Ar-H, JH,H = JF,H = 8.7 Hz), 6.19 (d, 1H, H-10, J = 5.1
Hz), 4.58 (t, 1H, H-20, J = 4.6 Hz), 4.31 (t, 1H, H-30, J = 3.9 Hz),
4.19 (m, 1H, H-40), 4.02 (dd, 1H, H-50, J = 3.4, 11.5 Hz), 3.81
(dd, 1H, H-50, J = 2.2, 11.5 Hz), 0.95, 0.94, and 0.79 (3s, 27H,
J. Org. Chem. Vol. 75, No. 8, 2010 2471

Lakshman et al.
JOCArticle
J = 3.4, 11.5 Hz), 3.99 (s, 5H, ferrocenyl-H), 3.81 (dd, 1H, H-50,
J = 2.4, 11.5 Hz), 0.943, 0.939, and 0.81 (3s, 27 H, t-Bu), 0.14,
0.13, 0.10, -0.005, and -0.20 (5s, 18H, Si-CH3). 13C NMR (125
MHz, CDCl3): δ 154.4, 152.0, 147.1, 145.7, 138.9, 133.3, 128.5,
88.7, 85.9, 76.6, 72.0, 70.0, 69.5, 69.4, 62.6, 53.6, 26.3, 26.0, 25.9,
18.8, 18.3, 18.0, -4.2, -4.4, -4.5, -4.7, -5.1. HRMS(ESI): calcd
for C40H63FeN7O4Si3Na [M þ Na]þ 868.3491, found 868.3489.
6-[4-(1-Butyl)-1,2,3-triazol-1-yl]-9-[2,3,5-bis-O-(tert-butyldime-
thylsilyl)-β-D-ribofuranosyl]purine (5g). Synthesized from 3b
(200 mg, 0.314 mmol) and 1-hexyne (72.2 μL, 0.628 mmol).
Chromatography of the crude reaction mixture on a silica gel
column using 5% acetone in hexanes yielded 5g as a yellow,
foamy solid (184 mg, 81% yield). Rf (silica gel, 10% acetone in
with Et3N 3HF (85 μL, 0.525 mmol) and chromatographic
purification as for 6a afforded 6c as white powder (50.8 mg,
3
1
83% yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.32. H
NMR (500 MHz, DMSO-d6): δ 9.50 (s, 1H, Ar-H), 9.02 (s, 1H,
Ar-H), 9.01 (s, 1H, Ar-H), 7.99 (d, 2H, Ar-H, J = 8.8Hz), 7.08 (d,
2H, Ar-H, J =8.8Hz), 6.57 (t, 1H, H-10, J =6.6Hz), 4.48 (m, 1H,
H-30), 3.93 (app q, 1H, H-40, Japp ≈ 4.0 Hz), 3.66 (dd, 1H, H-50,
J = 4.6, 11.7 Hz), 3.57 (dd, 1H, H-50, J = 4.5, 11.7 Hz), 2.82 (app
quint, 1H, H-20, Japp ≈ 6.5 Hz), 2.43 (ddd, 1H, H-20, J = 3.9, 6.3,
13.5 Hz). 13C NMR (125 MHz, acetone-d6 þ 15 drops of DMSO-
d6): δ 160.9, 155.0, 152.6, 148.2, 147.0, 145.4, 128.1, 124.3, 123.5,
120.4, 115.3, 89.6, 85.7, 71.8, 62.6, 55.9, 41.3. HRMS (ESI): calcd
for C19H20N7O4 [M þ H]þ 410.1571, found 410.1570.
1
hexanes) = 0.22. H NMR (500 MHz, CDCl3): δ 8.92 (s, 1H,
6-[4-(4-Fluorophenyl)-1,2,3-triazol-1-yl]-9-(2-deoxy-β-D-ribo-
furanosyl)purine (6d). Desilylation of 4d (93.9 mg, 0.150 mmol)
Ar-H), 8.85 (s, 1H, Ar-H), 8.59 (s, 1H, Ar-H), 6.20 (d, 1H, H-10,
J = 5.1 Hz), 4.65 (t, 1H, H-20, J = 4.6 Hz), 4.32 (t, 1H, H-30, J =
3.9 Hz), 4.18 (q, 1H, H-40, J = 3.0 Hz), 4.03 (dd, 1H, H-50, J =
3.7, 11.3 Hz), 3.82 (dd, 1H, H-50, J = 2.5, 11.3 Hz), 2.88 (t, 2H,
butyl-CH2, J = 7.7 Hz), 1.77 (quint, 2H, butyl-CH2, J = 7.6 Hz),
1.45 (sextet, 2H, butyl-CH2, J = 7.4 Hz), 0.97, 0.94, and 0.79
(overlapping 3s and t, 30H, t-Bu and butyl-CH3), 0.17, 0.16, 0.11,
0.10, -0.02, and -0.24 (6s, 18H, Si-CH3). 13C NMR (125 MHz,
CDCl3): δ 154.4, 152.4, 149.4, 145.2, 144.8, 123.3, 121.4, 88.7,
86.1, 76.4, 72.2, 62.7, 31.6, 26.3, 26.1, 25.8, 25.5, 22.5, 18.8, 18.3,
18.1, 14.0, -4.2, -4.4, -4.5, -4.8, -5.1. HRMS (ESI): calcd for
C34H63N7O4Si3Na [M þ Na]þ 740.4142, found 740.4139.
with Et3N 3HF (85 μL, 0.525 mmol) and chromatographic
purification as for 6a afforded 6d as white solid (50.8 mg, 83%
3
1
yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.37. H NMR
(500 MHz, DMSO-d6): δ 9.62 (s, 1H, Ar-H), 9.04 (s, 1H, Ar-H),
9.03 (s, 1H, Ar-H), 8.13 (dd, 2H, Ar-H, JH,H = 8.6 Hz, JF,H = 5.5
Hz), 7.37 (t, 2H, Ar-H, JH,H = JF,H = 8.6 Hz), 6.57 (t, 1H, H-10,
J = 6.3 Hz), 5.39 (br s, 1H, OH), 5.01 (br s, 1H, OH), 4.49 (m, 1H,
H-30), 3.94 (app q, 1H, H-40, Japp ≈ 4.1 Hz), 3.66 (dd, 1H, H-50,
J = 4.5, 11.4 Hz), 3.57 (dd, 1H, H-50, J = 4.2, 11.4 Hz), 2.83 (app
quint, 1H, H-20, Japp ≈ 6.5 Hz), 2.43 (ddd, 1H, H-20, J = 3.7, 6.3,
13.3 Hz). 13C NMR (125 MHz, acetone-d6 þ 10 drops of DMSO-
d6): δ 163.4 (d, 1JC,F = 245.7 Hz), 155.2, 152.6, 147.3, 147.2, 145.5,
129.0 (d, 3JC,F = 8.3 Hz), 127.6 (d, 4JC,F = 3.2 Hz), 124.5, 121.4,
116.8 (2JC,F = 21.9 Hz), 89.7, 85.9, 72.0, 62.7, 41.5. HRMS (ESI):
calcd for C18H17FN7O3 [M þ H]þ 398.1371, found 398.1386.
6-[4-(N-Phthalimidomethyl)-1,2,3-triazol-1-yl]-9-(2-deoxy-β-
D-ribofuranosyl)purine (6e). Desilylation of 4e (112.9 mg, 0.163
General Method for Desilylation of the Click Products. Syn-
thesis of 6-(4-Phenyl-1,2,3-triazol-1-yl]-9-(2-deoxy-β-D-ribofura-
nosyl)purine (6a). In a clean, dry plastic vial equipped with a
stirring bar was placed the nucleoside derivative 4a (91.2 mg,
0.150 mmol) in anhydrous THF (2.0 mL). Et3N 3HF (85 μL,
3
0.525 mmol) was added to the stirring mixture at room tem-
perature. After 34 h at room temperature, TLC indicated
complete consumption of the starting material at which time
the mixture was evaporated to dryness under a stream of nitro-
gen gas. Chromatographic purification on a silica gel column
using 3-5% MeOH in hexanes afforded 6a as white powder (54.6
mg, 96% yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.30. 1H
mmol) with Et3N 3HF (85 μL, 0.525 mmol) and chromato-
graphic purification as for 6a afforded 6e as white solid (64.5 mg,
3
1
85% yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.27. H
NMR (500 MHz, DMSO-d6): δ 9.22 (s, 1H, Ar-H), 9.00 (s, 1H,
Ar-H), 8.98 (s, 1H, Ar-H), 7.92 (m, 2H, Ar-H), 7.87 (m, 2H, Ar-
H), 6.54 (t, 1H, H-10, J = 6.6 Hz), 5.03 (s, 2H, NCH2), 4.46 (m,
1H, H-30), 3.92 (app q, 1H, H-40, Japp ≈ 4.0 Hz), 3.64 (dd, 1H, H-
50, J = 4.5, 11.8 Hz), 3.55 (dd, 1H, H-50, J = 4.5, 11.8 Hz), 2.79
(app quint, 1H, H-20, Japp ≈ 6.5 Hz), 2.41 (ddd, 1H, H-20, J = 3.9,
6.3, 13.3 Hz). 13C NMR (125 MHz, acetone-d6 þ 15 drops of
DMSO-d6): δ 168.2, 154.8, 152.4, 147.0, 144.9, 144.2, 135.2, 132.7,
124.4, 123.9, 89.3, 85.4, 71.5, 62.4, 41.0, 33.5. HRMS (ESI): calcd
for C21H19N8O5 [M þ H]þ 463.1473, found 463.1476.
NMR (500 MHz, DMSO-d6):
δ 9.62 (s, 1H, Ar-H),
9.04 (s, 1H, Ar-H), 9.03 (s, 1H, Ar-H), 8.07 (d, 2H, Ar-H, J =
7.3 Hz), 7.53 (t, 2H, Ar-H, J = 7.6 Hz), 7.43 (t, 1H, Ar-H, J =
7.4 Hz), 6.57 (t, 1H, H-10, J = 6.5 Hz), 5.40 (d, 1H, OH,
J = 4.2 Hz), 5.01 (t, 1H, OH, J = 5.5 Hz), 4.49 (m, 1H, H-30),
3.94 (app q, 1H, H-40, Japp ≈ 3.6 Hz), 3.66 (m, 1H, H-50), 3.07 (m,
1H, H-50), 2.83 (m, 1H, H-20), 2.43 (m, 1H, H-20, superimposed
with solvent). 13C NMR (125 MHz, acetone-d6 þ 4 drops of
DMSO-d6): δ 155.2, 152.7, 148.5, 147.2, 145.6, 131.2, 130.0,
129.6, 126.9, 124.5, 121.6, 89.8, 86.0, 72.1, 62.9, 41.7. HRMS
(ESI): calcd for C18H18N7O3 [M þ H]þ 380.1466, found
380.1465.
6-(4-Ferrocenyl-1,2,3-triazol-1-yl)-9-(2-deoxy-β-D-ribofurano-
syl)purine (6f). Desilylation of 4f (107.3 mg, 0.150 mmol) with
Et3N 3HF (85 μL, 0.525 mmol) and chromatographic purifica-
tion as for 6a afforded 6f as an orange solid (51.8 mg, 71% yield).
3
1
Rf (silica gel, 10% MeOH in CH2Cl2) = 0.26. H NMR (500
MHz, DMSO-d6): δ 9.23 (s, 1H, Ar-H), 9.02 (s, 1H, Ar-H), 9.01
(s, 1H, Ar-H), 6.57 (t, 1H, H-10, J = 6.6 Hz), 4.95 (br t, 2H,
ferrocenyl-H, J = 1.5 Hz), 4.48 (m, 1H, H-30), 4.40 (br t, 2H,
ferrocenyl-H, J = 1.5 Hz), 4.10 (s, 5H, ferrocenyl-H), 3.94 (app
q, 1H, H-40, Japp ≈ 4.3 Hz), 3.66 (dd, 1H, H-50, J = 4.6, 11.7 Hz),
3.57 (dd, 1H, H-50, J = 4.4, 11.7 Hz), 2.81 (app quint, 1H, H-20,
Japp ≈ 6.5 Hz), 2.43 (ddd, 1H, H-20, J = 4.0, 6.3 13.3 Hz). 13C
NMR (125 MHz, acetone-d6 þ 10 drops of DMSO-d6): δ 155.3,
152.8, 148.2, 147.1, 145.8, 124.5, 120.4, 90.0, 86.2, 76.1, 72.3,
70.6, 70.0, 68.0, 63.0, 41.9. HRMS (ESI): calcd for C22H21Fe-
N7O3Na [M þ Na]þ 510.0948, found 510.0948.
6-[4-(4-Methylphenyl)-1,2,3-triazol-1-yl]-9-(2-deoxy-β-D-ribo-
furanosyl)purine (6b). Desilylation of 4b (93.3 mg, 0.150 mmol)
with Et3N 3HF (0.15 mL, 0.901 mmol) and chromatographic
3
purification as for 6a afforded 6b as white powder (43.9 mg, 74%
yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.32. 1H NMR
(500 MHz, DMSO-d6): δ 9.55 (s, 1H, Ar-H), 9.03 (s, 1H, Ar-H),
9.02 (s, 1H, Ar-H), 7.95 (d, 2H, Ar-H, J = 7.8 Hz), 7.33 (d, 2H,
Ar-H, J = 7.8 Hz), 6.57 (t, 1H, H-10, J = 6.6 Hz), 5.40 (d, 1H,
OH, J = 4.1 Hz), 5.01 (t, 1H, OH, J = 5.2 Hz), 4.49 (br m, 1H,
H-30), 3.93 (app q, 1H, H-40, Japp = 3.8 Hz), 3.66 (m, 1H, H-50),
3.57 (m, 1H, H-50), 2.83 (app quint, 1H, H-20, Japp ≈ 6.5 Hz),
2.43 (ddd, 1H, H-20, J = 4.5, 6.5, 13.1 Hz). 13C NMR (125 MHz,
acetone-d6 þ 15 drops of DMSO-d6): δ 154.8, 152.3, 148.0,
146.9, 145.1, 138.9, 130.2, 127.9, 126.4, 124.0, 120.8, 89.3, 85.3,
71.5, 62.3, 40.7, 21.3. HRMS (ESI): calcd for C19H20N7O3 [M þ
H]þ 394.1622, found 394.1619.
6-[4-(1-Butyl)-1,2,3-triazol-1-yl]-9-(2-deoxy-β-D-ribofurano-
syl)purine (6g). Desilylation of 4g (88.2 mg, 0.150 mmol) with
Et3N 3HF (85 μL, 0.525 mmol) and chromatographic purifica-
tion as for 6a afforded 6g as a clear, gummy material (38.0 mg,
3
1
70% yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.35. H
6-[4-(4-Methoxyphenyl)-1,2,3-triazol-1-yl]-9-(2-deoxy-β-D-ribo-
furanosyl)purine (6c). Desilylation of 4c (95.7 mg, 0.150 mmol)
NMR (500 MHz, DMSO-d6): δ 8.98 (s, 1H, Ar-H), 8.97 (s, 1H,
Ar-H), 8.96 (s, 1H, Ar-H), 6.55 (t, 1H, H-10, J = 6.6 Hz), 5.39
2472 J. Org. Chem. Vol. 75, No. 8, 2010

Lakshman et al.
JOCArticle
2
(d, 1H, OH, J = 4.4 Hz), 4.99 (t, 1H, OH, J = 5.5 Hz), 4.47 (m,
1H, H-30), 3.93(appq, 1H, H-40, Japp ≈4.0Hz), 3.65(m, 1H, H-50),
3.56 (m, 1H, H-50), 2.84-2.76 (m, 3H, H-20 and butyl-CH2), 2.42
(ddd, 1H, H-20, J = 3.9, 6.3, 13.3 Hz), 1.64 (quint, 2H, butyl-CH2,
J = 7.5 Hz), 1.38 (sextet, 2H, butyl-CH2, J = 7.4 Hz), 0.93 (t, 3H,
butyl-CH3, J = 7.3 Hz). 13C NMR (125 MHz, acetone-d6 þ 15
drops of DMSO-d6): δ 154.7, 152.4, 149.0, 146.7, 145.3, 123.8,
122.5, 89.3, 85.4, 71.5, 62.4, 41.0, 31.8, 25.4, 22.6, 14.1. HRMS
(ESI): calcd for C16H22N7O3 [M þ H]þ 360.1779, found 360.1791.
6-(4-Phenyl-1,2,3-triazol-1-yl)-9-(β-D-ribofuranosyl)purine (7a).
121.2, 116.7 (d, JC,F = 21.8 Hz), 90.4, 87.7, 76.5, 71.8, 62.5.
HRMS (ESI): calcd for C18H17FN7O4 [M þ H]þ 414.1321,
found 414.1319.
6-[4-(N-Phthalimidomethyl)-1,2,3-triazol-1-yl]-9-(β-D-ribofura-
nosyl)purine (7e). Desilylation of 5e (115.0 mg, 0.15 mmol) with
Et3N 3HF (0.12 mL, 0.75 mmol) and chromatographic purifi-
cation as for 7a afforded 7e as a white solid (61.7 mg, 85% yield). Rf
(silica gel, 10% MeOH in CH2Cl2) = 0.26. 1H NMR (500 MHz,
DMSO-d6): δ 9.22 (s, 1H, Ar-H), 9.05 (s, 1H, Ar-H), 8.99 (s, 1H,
Ar-H), 7.94-7.91 (m, 2H, Ar-H), 7.89-7.86 (m, 2H, Ar-H), 6.12
(d, 1H, H-10, J = 5.1 Hz), 5.03 (s, 2H, NCH2), 4.60 (t, 1H, H-20,
J = 5.2 Hz), 4.21 (t, 1H, H-30, J = 4.3 Hz), 4.01 (app q, 1H, H-40,
Japp ≈ 3.8 Hz), 3.72 (dd, 1H, H-50, J = 3.6, 12.0 Hz), 3.60 (dd, 1H,
H-50, J= 3.8, 12.0 Hz). 13C NMR (125 MHz, THF-d8 þ 4 drops of
DMSO-d6): δ 168.3, 155.5, 152.7, 147.3, 145.7, 144.5, 135.2, 133.5,
124.6, 124.5, 124.1, 90.2, 87.6, 76.5, 71.8, 62.5, 33.8. HRMS (ESI):
calcd for C21H19N8O6 [M þ H]þ 479.1422, found 479.1432.
6-(4-Ferrocenyl-1,2,3-triazol-1-yl)-9-(β-D-ribofuranosyl)pu-
rine (7f). Desilylation of 5f (126.9 mg, 0.15 mmol) with
3
Desilylation of 5a (110.7 mg, 0.150 mmol) with Et3N 3HF (0.12
3
mL, 0.75 mmol) and chromatographic purification on a silica gel
column using 3-7% MeOH in hexanes afforded 7a as a white,
fluffy solid (57.3 mg, 96% yield). Rf (silica gel, 10% MeOH in
CH2Cl2) = 0.28. 1H NMR (500 MHz, DMSO-d6): δ 9.62 (s, 1H,
Ar-H), 9.09 (s, 1H, Ar-H), 9.05 (s, 1H, Ar-H), 8.05 (d, 2H, Ar-H,
J = 7.3 Hz), 7.53 (t, 2H, Ar-H, J = 7.5 Hz), 7.43 (t, 1H, Ar-H, J =
7.6 Hz) 6.15 (d, 1H, H-10, J = 4.9 Hz), 5.62 (d, 1H, OH, J = 5.6
Hz), 5.28 (d, 1H, OH, J = 5.1 Hz), 5.14 (t, 1H, OH, J = 5.1 Hz),
4.65 (m, 1H, H-20), 4.24 (m, 1H, H-30), 4.03 (m, 1H, H-40), 3.74 (m,
1H, H-50), 3.62 (m, 1H, H-50). 13C NMR (125 MHz, THF-d8 þ 4
drops of DMSO-d6): δ 155.5, 152.7, 148.5, 147.2, 146.0, 131.6,
129.8, 129.3, 126.9, 124.7, 121.2, 90.4, 87.7, 76.5, 71.9, 62.6. HRMS
(ESI): calcd for C18H18N7O4 [M þ H]þ 396.1415, found 396.1416.
6-[4-(4-Methylphenyl)-1,2,3-triazol-1-yl]-9-(β-D-ribofurano-
syl)purine (7b). Desilylation of 5b (112.8 mg, 0.15 mmol) with
Et3N 3HF (0.12 mL, 0.75 mmol) and chromatographic purifi-
3
cation as for 7a afforded 7f as an orange solid (73.4 mg, 97%
yield). Rf (silica gel, 10% MeOH in CH2Cl2) = 0.28. 1H NMR
(500 MHz, DMSO-d6): δ 9.23 (s, 1H, Ar-H), 9.07 (s, 1H, Ar-H),
9.02 (s, 1H, Ar-H), 6.14 (d, 1H, H-10, J = 5.1 Hz), 4.95 (br t, 2H,
ferrocenyl-H), 4.64 (t, 1H, H-20, J = 4.9 Hz), 4.41 (br t, 2H,
ferrocenyl-H), 4.23 (t, 1H, H-30, J = 4.4 Hz), 4.10 (s, 5H,
ferrocenyl-H), 4.03 (app q, 1H, H-40, Japp ≈ 3.9 Hz), 3.74 (dd,
1H, H-50, J = 3.9, 11.8 Hz), 3.62 (dd, 1H, H-50, J = 3.9, 11.8
Hz). 13C NMR (125 MHz, THF-d8 þ 4 drops of DMSO-d6): δ
155.5, 152.7, 148.0, 147.1, 145.8, 124.5, 120.0, 90.4, 87.6, 76.5,
76.3, 71.8, 70.5, 69.7, 62.5. HRMS (ESI): calcd for C22H22Fe-
N7O4 [M þ H]þ 504.1077, found 504.1060.
Et3N 3HF (0.12 mL, 0.75 mmol) and chromatographic purifi-
cation as for 7a afforded 7b as a white solid (59.2 mg, 96% yield).
3
1
Rf (silica gel, 10% MeOH in CH2Cl2) = 0.28. H NMR (500
MHz, DMSO-d6): δ 9.56 (s, 1H, Ar-H), 9.08 (s, 1H, Ar-H), 9.04
(s, 1H, Ar-H), 7.96 (d, 2H, Ar-H, J = 7.8 Hz), 7.33 (d, 2H, Ar-H,
J = 7.8 Hz), 6.15 (d, 1H, H-10, J = 4.9 Hz), 5.62 (d, 1H, OH, J =
5.8 Hz), 5.28 (d, 1H, OH, J = 5.1 Hz), 5.14 (t, 1H, OH, J = 5.3
Hz), 4.65 (m, 1H, H-20), 4.24 (m, 1H, H-30), 4.02 (m, 1H, H-40),
3.74 (m, 1H, H-50), 3.62 (m, 1H, H-50), 2.37 (s, 3H, CH3). 13C
NMR (125 MHz, THF-d8 þ 4 drops of DMSO-d6): δ 155.5,
152.7, 148.6, 147.1, 145.9, 139.0, 130.4, 128.8, 126.8, 124.7,
120.7, 90.5, 87.7, 76.4, 71.9, 62.6, 21.5. HRMS (ESI): calcd for
C19H20N7O4 [M þ H]þ 410.1571, found 410.1572.
6-[4-(1-Butyl)-1,2,3-triazol-1-yl]-9-(β-D-ribofuranosyl)purine
(7g). Desilylation of 5g (107.7 mg, 0.15 mmol) with Et3N 3HF
3
(0.12 mL, 0.75 mmol) and chromatographic purification as for 7a
afforded 7g as a white solid (53.5 mg, 95% yield). Rf (silica gel,
10% MeOH in CH2Cl2) = 0.28. 1H NMR (500 MHz, DMSO-d6):
δ9.03(s, 1H, Ar-H), 8.99(s, 1H, Ar-H), 8.97(s, 1H, Ar-H), 6.12(d,
1H, H-10, J = 5.1 Hz), 5.60 (br s, 1H, OH), 5.27 (br s, 1H, OH),
5.12 (br s, 1H, OH), 4.63 (t, 1H, H-20, J = 4.7 Hz), 4.22 (t, 1H, H-
30, J = 4.2 Hz), 4.01 (app q, 1H, H-40, Japp ≈ 3.8 Hz), 3.72 (dd, 1H,
H-50, J = 3.4, 11.7 Hz), 3.61(dd, 1H, H-50, J = 3.2, 11.7 Hz), 2.80
(t, 2H, butyl-CH2, J = 7.6 Hz), 1.70 (quint, 2H, butyl-CH2, J =
7.5 Hz), 1.39 (sextet, 2H, butyl-CH2, J = 7.4 Hz), 0.94 (t, 3H,
butyl-CH3, J = 7.3 Hz). 13C NMR (125 MHz, THF-d8 þ 4 drops
of DMSO-d6): δ 155.3, 152.7, 149.2, 146.9, 146.0, 124.4, 122.4,
90.4, 87.7, 76.4, 71.9, 62.6, 32.4, 26.1, 23.3, 14.4. HRMS (ESI):
calcd for C16H22N7O4 [M þ H]þ 376.1728, found 376.1731.
6-[4-(4-Methoxyphenyl)-1,2,3-triazol-1-yl]-9-(β-D-ribofurano-
syl)purine (7c). Desilylation of 5c (115.2 mg, 0.150 mmol) with
Et3N 3HF (0.12 mL, 0.75 mmol) and chromatographic purifi-
3
cation as for 7a afforded 7c as a white solid (51.3 mg, 81% yield).
Rf (silica gel, 10% MeOH in CH2Cl2) = 0.30. H NMR (500
1
MHz, DMSO-d6): δ 9.51 (s, 1H, Ar-H), 9.07 (s, 1H, Ar-H), 9.04
(s, 1H, Ar-H), 8.01 (d, 2H, Ar-H, J = 8.8 Hz), 7.08 (d, 2H, Ar-H,
J = 8.8 Hz), 6.15 (d, 1H, H-10, J = 5.4 Hz), 4.65 (t, 1H, H-20, J =
5.0 Hz), 4.23 (t, 1H, H-30, J = 4.5 Hz), 4.03 (app q, 1H, H-40,
Japp ≈ 4.9 Hz), 3.83 (s, 3H, OCH3), 3.74 (dd, 1H, H-50, J = 3.9,
11.7 Hz), 3.62 (dd, 1H, H-50, J = 3.9, 11.7 Hz). 13C NMR (125
MHz, THF-d8 þ 4 drops of DMSO-d6): δ 161.2, 155.5, 152.7,
148.5, 147.2, 145.9, 128.2, 124.6, 124.0, 120.1, 115.3, 90.3, 87.6,
76.5, 71.8, 62.5, 55.8. HRMS (ESI): calcd for C19H20N7O5 [M þ
H]þ 426.1520, found 426.1521.
Acknowledgment. This work was supported by NSF Grant
No. CHE-0640417 and PSC CUNY awards to M.K.L. Infra-
structural support at CCNY was provided by NIH RCMI Grant
No. G12 RR03060. We thank Dr. Amit Kumar for results on the
ligation reactions of tetrazolo[1,5-c]quinazoline, Dr. Padmanava
Pradhan (CCNY NMR facility manager) for assistance with
some NMR experiments, and Dr. Cliff Soll (Hunter College) for
HRMS analysis of new compounds. X-ray crystallographic
support was made possible by NIDA under the Interagency
Agreement No. Y1 DA6005. Cell lines were generous gifts from
Prof. Bert Vogelstein and Thomas Hamilton.
6-[4-(Fluorophenyl)-1,2,3-triazol-1-yl]-9-(β-D-ribofuranosyl)-
purine (7d). Desilylation of 5d (113.4 mg, 0.15 mmol) with
Et3N 3HF (0.12 mL, 0.75 mmol) and chromatographic purifi-
3
cation as for 7a afforded 7d as a white solid (44.0 mg, 71% yield).
Rf (silica gel, 10% MeOH in CH2Cl2) = 0.26. H NMR (500
1
MHz, DMSO-d6): δ 9.63 (s, 1H, Ar-H), 9.08 (s, 1H, Ar-H), 9.05
(s, 1H, Ar-H), 8.13 (dd, 2H, Ar-H, JH,H = 8.9 Hz, JF,H = 5.5
Hz), 7.37 (t, 2H, Ar-H, JH,H = JF,H = 8.9 Hz), 6.15 (d, 1H, H-10,
J = 5.1 Hz), 5.62 (d, 1H, OH, J = 5.8 Hz), 5.28 (d, 1H, OH, J =
5.3 Hz), 5.14 (t, 1H, OH, J = 5.3 Hz), 4.65 (m, 1H, H-20), 4.24
(m, 1H, H-30), 4.03 (m, 1H, H-40), 3.74 (m, 1H, H-50), 3.62 (m,
1H, H-50). 13C NMR (125 MHz, THF-d8 þ 4 drops of DMSO-
Supporting Information Available: General experimental
details, materials, and methods for antiproliferative tests and
the results, ORTEP of 4b, copies of 1H and 13C NMR spectra of
2c, 2d, 3a-f, 4a-g, 4f0, 5a-g, 5a0, 5c0-f0, 6a-6g, and 7a-7g.
This information is available free of charge via the Internet at
http://pubs.acs.org.
1
d6): δ 163.7 (d, JC,F = 246.3 Hz), 155.6, 152.7, 147.6, 147.3,
145.8, 128.9 (d, 3JC,F = 8.2 Hz), 128.0 (d, 4JC,F = 3.2 Hz), 124.7,
J. Org. Chem. Vol. 75, No. 8, 2010 2473