Synthesis of 8-(1,2,3-triazol-1-yl)-7-deazapurine nucleosides by azide–alkyne click reactions and direct C[sbnd]H bond functionalization

Article abstract of DOI:10.1016/j.tetlet.2016.08.053

Treatment of toyocamycin or sangivamycin with 1,3-dibromo-5,5-dimethylhydantoin in MeOH (rt/30?min) gave 8-bromotoyocamycin and 8-bromosangivamycin in good yields. Nucleophilic aromatic substitution of 8-bromotoyocamycin with sodium azide provided novel 8

Full text of DOI:10.1016/j.tetlet.2016.08.053

Tetrahedron Letters 57 (2016) 4364–4367
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 8-(1,2,3-triazol-1-yl)-7-deazapurine nucleosides by
azide–alkyne click reactions and direct CAH bond functionalization
Sam Kavoosi, Ramanjaneyulu Rayala, Brenna Walsh, Maria Barrios, Walter G. Gonzalez,
⇑
Jaroslava Miksovska, Logesh Mathivathanan, Raphael G. Raptis, Stanislaw F. Wnuk
Florida International University, Department of Chemistry and Biochemistry, Miami, FL 33199, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2016
Revised 13 August 2016
Accepted 17 August 2016
Available online 19 August 2016
Treatment of toyocamycin or sangivamycin with 1,3-dibromo-5,5-dimethylhydantoin in MeOH
(rt/30 min) gave 8-bromotoyocamycin and 8-bromosangivamycin in good yields. Nucleophilic aromatic
substitution of 8-bromotoyocamycin with sodium azide provided novel 8-azidotoyocamycin. Strain pro-
moted click reactions of the latter with cyclooctynes resulted in the formation of the 1,2,3-triazole prod-
ucts. Iodine-mediated direct C8AH bond functionalization of tubercidin with benzotriazoles in the
presence of tert-butyl hydroperoxide gave the corresponding 8-benzotriazolyltubercidin derivatives.
The 8-(1,2,3-triazol-1-yl)-7-deazapurine derivatives showed moderate quantum yields and a large
Stokes shifts of ꢀ100 nm.
Keywords:
Azido nucleosides
Click chemistry
Deazapurine nucleosides
Direct CAH functionalization
Fluorescence
Ó 2016 Elsevier Ltd. All rights reserved.
Triazoles
Synthetic transformations, biological activities, and physico-
chemical properties of 7-deazapurine derivatives (mainly tuber-
cidin, toyocamycin, and sangivamycin antibiotics) have been
extensively studied1–9 and are subject of the excellent reviews.10–
protocol involves strain-promoted click chemistry between 8-
azido-7-deazapurine nucleosides and cyclooctynes whereas the
second approach explores direct C8AH bond functionalization of
7-deazapurine nucleosides and coupling with benzotriazoles.
The synthesis of the 8-azido-7-deazapurines (3a–c) was
attempted from 8-halo-7-deaza substrates. Thus, bromination of
20,30,50-tri-O-acetyltubercidin 1a using recently reported protocols
for the C-8 bromination of purine nucleosides,34 employing 1,3-
dibromo-5,5-dimethylhydantoin (DBH, 0.65 equiv) in CH2Cl2 at
0 °C for 40 min, provided 7-bromo 2a (32%) in addition to 7,8-
dibromo 2b (10%, TLC; Scheme 1). Deacetylation of 2a and 2b with
methanolic ammonia gave 7-bromotubercidin 2c (80%) and 7,8-
dibromotubercidin 2d (57%). Treatment of tubercidin 1b with N-
bromosuccinimide (NBS, 2 equiv) in the presence of KOAc in DMF
yielded 8-bromotubercidin35 2e (34%), whereas attempted bromi-
nation of 1b with DBH in DMF gave a complex reaction mixture.
The 8-bromination of toyocamycin 1c with 0.75 equiv of DBH in
MeOH (30 min.) proceeded efficiently to give after crystallization
2f (85%). Treatment of 1c with NBS (1.1 equiv) in DMF at rt for
2.5 h also yielded 2f (45%) after column chromatography. Replace-
ment of DMF with MeOH gave 2f (64%) after crystallization,
although the reaction took 4 h for completion. The 8-bromotoy-
ocamycin (2f) has been previously prepared either by coupling of
the corresponding 8-bromo-7-deazapurine with the ribose precur-
sor36 or by bromination of toyocamycin 1c with Br2/H2O (61%).37
12
In recent years 7-deazapurine nucleosides have been explored
as substrates for 1,4-regioselective copper-catalyzed azide/alkyne
cycloaddition (CuAAC), which resulted in developing fluorogenic
DNA probes.13–16 These studies have mainly concentrated on the
application of 7-alkynyl derivatives of 7-deazapurines and 7-
deazanucleosides for the preparation of the fluorescent 1,2,3-tria-
zol-4-yl probes.4,17,18 Studies with 7-azido-7-deazapurine nucleo-
sides as well as 8-azido- or 8-alkynyl-7-deazapurine nucleosides
have received much less attention due to the lack of availability
of the convenient synthetic protocols for their synthesis.17,19 This
is despite the fact that azidonucleosides20–22 including 8-azido as
well as 8-alkynyl purine23 nucleosides have been used as conve-
nient substrates for CuAAC and strain-promoted azide–alkyne
cycloaddition (SPAAC).24–26 This approach is a rapidly growing
field with various applications, including in vivo imaging, of
azido/alkyne-modified nucleoside/nucleotides26,27 or DNA/RNA
fragment click adducts.28–33
Herein, we report two independent methods for the synthesis of
novel 8-triazol-1-yl derivatives of 7-deazapurine nucleosides. One
⇑
Corresponding author. Tel.: +1 305 348 6195; fax: +1 305 348 3772.
E-mail address: wnuk@fiu.edu (S.F. Wnuk).
http://dx.doi.org/10.1016/j.tetlet.2016.08.053
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

S. Kavoosi et al. / Tetrahedron Letters 57 (2016) 4364–4367
4365
Scheme 1. Synthesis of 8-bromo- and 8-azido-7-deazapurine nucleosides.
Thus the DBH/MeOH combination is the most efficient for synthe-
sizing 2f in terms of yield, reaction time, and product isolation.
Treatment of sangivamycin 1d with DBH (1.1 equiv) in MeOH
(rt, 30 min) yielded 2g (63%), whereas bromination of 1d with
NBS (1.5 equiv) in DMF for 1.5 h gave 2g in 40% yield in addition
to unidentified byproducts. The synthesis of 8-bromosangivamy-
cin 2g is usually achieved by the hydrolysis of 8-bromotoyoca-
mycin 2f with a concentrated NH4OH and 50% H2O2 solution.36
Attempts to synthesize 8-azidotubercidin 3a from 8-bromotu-
bercidin 2e [NaN3 (1–5 equiv)/DMF/rt–153 °C; NaN3 (3 equiv)/
TsOH (3 equiv)/EtOH/reflux] were unsuccessful. The higher elec-
unchanged starting material under various reaction conditions.
These results are in sharp contrast to 7-halo-7-deazapurines that
undergo smooth Sonogashira cross-coupling providing 7-alkynyl
analogs, which after further modifications provide derivatives with
interesting photophysical and biological properties.4,17
Since syntheses of 8-azidotubercidin 3a for the click reactions
were unsuccessful, the possibility to prepare 8-triazolyl adducts
of tubercidin 1b, using a direct C8AH activation of the 7-deaza-
purine ring, was explored next. Direct CAH fuctionalization of
purines and purine nucleosides has recently gained much atten-
tion.19,40–43 The examples include regioselective Pd-catalyzed
direct C8AH arylation of the 6-phenyl-7-deazapurines with aryl
halides to provide 8-arylated products albeit in low to moderate
yields.44 A regioselective direct CAH amination of the 7-deazapuri-
nes to give access to the 8-amino-,45 or 7-amino-7-deazapurine
analogs has also been reported.46 We found that treatment of
tubercidin 1b with benzotriazole (2 equiv) in the presence of I2
(0.4 equiv) and tert-butylhydroperoxide (TBHP; 5–6 M/decane,
2 equiv)47 in anhydrous DMF at 35 °C for 96 h showed ꢀ30% con-
version (TLC) to 8-(benzotriazol-1-yl)tubercidin 10 which was iso-
lated in 18% yield after column chromatography and HPLC
(Scheme 3). Increasing the amount of iodine from catalytic to sto-
ichiometric (3 equiv) afforded 10 in 35% yield after 16 h. 1H NMR
data showed that the 7.60 ppm doublet for H7 in 1b collapsed to
a singlet at 7.04 ppm in 10.
tron density of
p-excessive pyrrole ring most probably prevented
the nucleophilic substitution reaction. However, treatment of 8-
bromotoyocamycin 2f with NaN3 in DMF at room temperature
for 16 h produced 8-azidotoyocamycin 3b in 46% isolated yield
(70%, based on TLC and 1H NMR of the crude reaction mixture).
Although this azidation reaction was light- and heat-sensitive
and required the reaction, purification, and characterization to be
carried out in the dark, the isolated 8-azido product 3b was stable
for days when stored in refrigerator at 4 °C.23,38 The presence of the
EWG on the 7-position of the pyrrole ring in toyocamycin, as com-
pared to tubercidin, could be the reason for successful substitution
of bromide by azide in 2f. Treatment of 8-bromosangivamycin 2g
with NaN3 in DMF (as well as in DMSO or MeOH) at rt or 70 °C
failed to produce the desired 8-azidosangivamycin 3c.
SPAAC reaction of 8-azidotoyocamycin 3b with symmetrically
fused cyclopropyl cyclooctyne 4 (OCT) in an aqueous solution of
acetonitrile (ACN) at ambient temperature (4 h) gave the 8-
(1,2,3-triazol-1-yl) product 5 as a mixture of two isomers (60%,
Scheme 2). Analogous reaction (4 h) of 3b with a strain modulated
dibenzylcyclooctyne 6 (DBCO) produced triazole 7 as a mixture of
isomers (47%) after RP-HPLC purification. The resulting triazolyl
products have ‘light-up’ (inherited) fluorescent properties (Table 1),
due to increased conjugation in the 7-deazapurine ring, and can
therefore be used for fluorescent imaging in cancer cells, as
recently reported with 8-triazolyl purine and 5-triazolyl pyrim-
idine nucleosides.26,27 CuAAC reaction of 8-azido 3b with pheny-
lacetylene in aqueous MeOH (4 days/rt) gave 8-(4-phenyl-1H-
The C8 regioselectivity for the oxidative cross-coupling product
10 was established by an X-ray crystal structure determination
(Fig. 1).48 It is noteworthy that the 7-deazapurine ring and the ben-
zotriazole ring are twisted to each other with an angle of 67.9°
between the planes, which does not favor intermolecular
p-p
interactions. The glycosyl torsion angle C4–N9–C10–O40 is 59.8°
(syn conformation) and the furanose pseudorotation angle49 is
162.9° (2E conformation). The C30–C40–C50–O50 torsion angle is
54.1° and is in g+/gg range.
Treatment of 1b with 5-methylbenzotriazole [I2 (0.2 equiv)/
TBHP/DMF/rt/5 days] produced a 3:2 mixture of 5-methyl- and
6-methylbenzotriazol-1-yl adducts 11a/11b (6%). Reaction with
stoichiometric amount of iodine (3 equiv) resulted in the efficient
conversion to 11a/11b (48%; ꢀ80% based on TLC after 15 min),
showing generality of the iodine-mediated direct activation of
tubercidin ring at C8. Analogous treatment of 1b with 5-chloroben-
zotriazole (3 equiv) produced 5/6-chlorobenzotriazol-1-yl adducts
12a/12b (1:1, 21%; ꢀ60% conversion on TLC). Because 7-deaza-
purine is structurally similar to indole, the coupling of 1b with
benzotriazole might have occurred via initial iodination of the pyr-
role ring of 7-deazapurine followed by trapping of the resulting
iodonium, or 7-iodo iminium intermediates, with nucleophilic tri-
azole. Subsequent elimination of HI, as proposed for the iodine-cat-
alyzed direct activation of indoles with azoles mediated by TBHP,47
1,2,3-triazol-1-yl)toyocamycin
purification.
9
(65%) after silica column
Our attempts to prepare 8-alkynyl-7-dezapurine derivatives,
which could serve as substrates for the synthesis of the underde-
veloped 8-(1,2,3-triazol-4-yl) adducts via click reactions with orga-
noazides were unsuccessful. Thus, treatment of 8-bromotubercidin
(2e), 8-bromotoyocamycin (2f), 8-bromo-20,30,50-tri-O-acetyltoy-
ocamycin,39 or 8-iodotoyocamycin [prepared by treatment of the
silylated toyocamycin with LDA (5 equiv) and iodine (I2, 1.5 equiv)
in THF (16%)] with trimethylsilylacetylene (TMSA) in the presence
of TEA and (PPh3)2PdCl2/CuI in anhydrous DMF showed mainly

4366
S. Kavoosi et al. / Tetrahedron Letters 57 (2016) 4364–4367
Scheme 2. Click reactions between 8-azidotoyocamycin and cyclooctynes 4, 6 and phenylacetylene 8.
Table 1
Photophysical data for 8-(1,2,3-triazol-1-yl)-7-deazapurine nucleosides
5
7
9
10
11a
12a
emax (Mꢁ1 cmꢁ1
)
12,200
287
295
380
85
12,600
290
299
434
135
0.5
16,000
295
301
412
111
0.4
16,700
278
321
416
95
17,100
278
322
419
97
17,700
276
343
480
137
0.8
kmax (abs)b (nm)
b
kmax (exc) (nm)
kmax (emi)b (nm)
Stokes shift (nm)
b
/
(%)
1.4
0.2
0.7
F
a
As mixture of 5- and 6-isomers.
In MeOH.
b
NH2
NH2
N
N
N
N
N
X
N
N
N
I2
N
O
N
O
N
+
HO
HO
N
H
TBHP
X'
X
X = H, CH3, Cl
HO
1b
HO
10
11a
OH
OH
X = H, X' = H
Figure 1. X-ray crystal structure of 10 showing atom labeling. Solvent molecules
X = CH3, X' = H
and H-atoms are omitted for clarity.
11b X = H, X' = CH3
X = Cl, X' = H
12b X = H, X' = Cl
12a
The photophysical properties of 8-(1,2,3-triazol-1-yl)-7-deaza-
purine adducts 5, 7, 9, and 10–12 were determined in MeOH. All
triazol derivatives have similar absorption and excitation spectra
with an absorption maximum between 276 nm and 295 nm and
comparable extinction coefficient values (Table 1 and Fig. S2).
Moderate quantum yields and a large Stokes shifts of ꢀ100 nm
observed here are analogous to the fluorescence properties of ali-
phatic and benzylic derivatives of 1-(purin-8-yl)triazoles.23 The
emission maximum of triazol 5 (380 nm) is blue shifted compared
to other derivatives (412–480 nm), likely due to the absence of an
aromatic substituent (Fig. 2). Emission spectrum of chloro substi-
tuted benzotriazole 12 is red-shifted compared to benzotriazoles
Scheme 3. Iodine-mediated oxidative cross-coupling of tubercidin with
benzotriazole.
should yield products 10–12. Subjection of the sangivamycin 1c
and toyocamycin 1b to direct C8AH functionalization with benzo-
triazole (rt to 80 °C/48 h) gave unchanged substrate or resulted in
rather complex reaction mixture. These results reiterate the litera-
ture report that the iodine catalyzed CAH activation of indoles
with azoles works rather sluggishly with electron deficient indole
substrates.47

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Acknowledgments
We would like to express our gratitude to Dr. Morris J. Robins
for generous donation of 7-deazapurine substrates, NSF-REU
(Grant No: CHE1156886 to FIU) for support to BW (James Madison
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.08.
053.
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