Copper-catalyzed synthesis of purine-fused polycyclics

Article abstract of DOI:10.1021/ol301848v

A novel protocol for a Cu-catalyzed direct C_(sp2)-H activation/intramolecular amination reaction of 6-anilinopurine nucleosides has been developed. This approach provides a new access to a variety of multiheterocyclic compounds from purine comp

Full text of DOI:10.1021/ol301848v

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Copper-Catalyzed Synthesis
of Purine-Fused Polycyclics
Gui-Rong Qu,† Lei Liang,† Hong-Ying Niu,‡ Wei-Hao Rao,† Hai-Ming Guo,*,† and
John S. Fossey*,§
College of Chemistry and Environmental Science, Henan Normal University,
Xinxiang 453007, Henan, China, School of Chemistry and Chemical Engineering, Henan
Institute of Science and Technology, Xinxiang 453003, China, and School of Chemistry,
University of Birmingham, Edgbaston, Birmingham, West Midlands, B15 2TT, U.K.
guohm518@hotmail.com; j.s.fossey@bham.ac.uk
Received July 18, 2012
ABSTRACT
A novel protocol for a Cu-catalyzed direct C(sp2)ÀH activation/intramolecular amination reaction of 6-anilinopurine nucleosides has been
developed. This approach provides a new access to a variety of multiheterocyclic compounds from purine compounds via Cu-catalyzed
intramolecular NÀH bond tautomerism which are endowed with fluorescence.
Purine nucleoside analogues are an important class of
nitrogen-containing heterocycle that exhibit great therapeu-
tic potential,1 possessing anti-HCV,2 antimicrobacterial,3
and antineoplastic4 activities. Although investigations into
the modification of purine nucleosides have attracted in-
creasing attention,5 intramolecular CÀH bond amination
reactions on purine rings remain challenging due to their
inherent reduced activity in metal-catalyzed transforma-
tions as a result of nitrogen coordination deactivating the
catalyst. Moreover, it is known that the glycosidic bond is
fragile and is often cleaved under harsh conditions. To the
best of our knowledge, there are no reports on Cu-catalyzed
CÀH activaton/intramolecular amination reactions as a
basis for the synthesis of purine nucleosides.
Employing CÀH bond activation to obtain nitrogen-
containing heterocycles with good biological activities has
attracted extensive interest.6x The construction of nitro-
gen-containing heterocycles via the formation of CÀN
bonds through transition-metal-catalyzed CÀH activation/
amination reactions employing palladium,7 rhodium,8 and
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† Henan Normal University.
‡ Henan Institute of Science and Technology.
§ University of Birmingham.
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r
10.1021/ol301848v
XXXX American Chemical Society

ruthenium9 for example have shown certain advantages
in some cases over more established procedures, such
as BuchwaldÀHartwig,10 Ullmann, and Goldberg coupl-
ings.11 Such direct CÀH activaton/amination reactions
require fewer steps and proceed under milder conditions
in comparison to the more classical coupling reactions.12
However, construction of these heterocycles via an intra-
molecular CÀN bond formation strategy remains challen-
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activation/intramolecular amination for carbazole synthe-
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the construction of such hetereocycles.14
Table 1. Optimization of Reaction Conditionsa
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catalyst
oxidant
additive yield/
b
entry (5 mol %)
(1.5 equiv)
solvent
PhMe
(1.5 equiv)
%
1c Cu(OTf)2 PhI(OAc)2
2c Cu(OTf)2 PhI(OAc)2
3c Cu(OTf)2 PhI(OAc)2
4c Cu(OTf)2 PhI(OAc)2
5c Cu(OTf)2 PhI(OAc)2
6c Cu(OTf)2 PhI(OAc)2
;
38
PhMe
K2CO3
TFA
48
52
PhMe
PhMe
NaOAc 55
DMF
;
;
;
;
;
;
;
;
;
;
30
AcOH
52
€
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7
8
9
Cu(OTf)2 PhI(OAc)2
Cu(OAc)2 PhI(OAc)2
AcOH/Ac2O
AcOH/Ac2O
AcOH/Ac2O
AcOH/Ac2O
AcOH/Ac2O
88
82
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10 CuBr
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11 Cu(OTf)2 oxone
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n.d.d
12 Cu(OTf)2 Fe2(NO3)3 9H2O AcOH/Ac2O
3
13 Cu(OTf)2 BQ
AcOH/Ac2O
AcOH/Ac2O
82
14 Cu(OTf)2 K2S2O8
81
a Reaction conditions: 1a (0.3 mmol), oxidant (0.45 mmol), AcOH
(1 mL), Ac2O (1 mL), 80 °C for 2 h. b Isolated yield. c solvent (2 mL),
120 °C for 24 h. d n.d. = not detected.
Herein, we report a copper-catalyzed intramolecular
CÀH bond amination reaction of purine and itsderivatives
related to our previous work.15 By employing PhI(OAc)2
as a simple oxidant, a useful and facile alternative process
for the synthesis of purine-fused polycyclics is provided
that might be useful in medicinal studies.
€
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B
Org. Lett., Vol. XX, No. XX, XXXX

The direct CÀH activation/intramolecular amination of
6-anilinopurine nucleoside 1a using conditions optimized
by Gaunt,7k albeit for a different reaction, led to no
reaction. However the knowledge that copper could cata-
lyze the carbazole formation reactions of Gaunt prompted
us to further investigate conditions for our proposed
reaction. Upon changing the copper source to Cu(OTf)2
(5 mol %) along with PhI(OAc)2 (1.5 equiv) as the oxi-
dant in toluene at 120 °C for 48 h, cyclized product 2a
was formed, but only in 38% yield (unreacted 1a, 40%)
(Table 1, entry 1). Upon varying the additives and chan-
ging the solvent, improved yields were possible (Table 1,
entries 2À7). Interestingly, it was found that the presence
of Ac2O significantly improved the reaction rate, and
starting material 1a was consumed within 2 h. Further
investigation revealed that a 1:1 mixture of acetic acid and
acetic anhydride was the optimal solvent. To our delight,
under these conditions, 1a readily cyclized to give com-
pound 2a in high yield (88%, Table 1, entry 7). Subse-
quently, several copper sources were also probed as
Table 3. Electronic and Steric Effectsa
Table 2. Effect of N9-Substitutenta
a Unless otherwise stated, all reactions were carried out with 1
(0.3 mmol), catalyst (5 mol %), oxidant (0.45 mmol), AcOH (1 mL),
Ac2O (1 mL) at 80 °C for 2 h. b Isolated yield. c Reaction conditions: 1
(0.3mmol), catalyst(5mol%), oxidant(0.45mmol), AcOH(1mL), at80°C
for 1 h, then 1 mL of Ac2O was added and stirred for an additional 1 h.
a The reactions were carried out with 1 (0.3 mmol), catalyst (5 mol %),
oxidant (0.45 mmol), AcOH (1 mL), Ac2O (1 mL) at 80 °C for 2 h.
b Isolated yield.
Org. Lett., Vol. XX, No. XX, XXXX
C

catalysts for the present reaction. Cu(II) exhibited better
catalytic activity than Cu(I), and Cu(OTf)2 won out as the
copper source of choice (Table 1, entries 7À10). With
Cu(OTf)2 as the catalyst, a range of oxidants were tried
(Table 1, entries 11À14). The yield observed with PhI-
(OAc)2 as oxidant was higher than any other oxidant tried.
As such the optimal conditions were determined to be 5
mol % of Cu(OTf)2, 1.5 equiv of PhI(OAc)2 in a mixture of
AcOH and Ac2O (1:1) as the solvent at 80 °C for 2 h.
With optimized conditions in hand, the scope of this intra-
molecular amination reaction was next tested. As shown in
Table 2, a series of N9-substituted substrates, including
alkyl, benzyl, allyl, etc., afforded the corresponding cyclized
products in good to excellent yields (2bÀ2h). Thus it may be
concluded that the N9-substituents have little effect on the
course of the reaction. Notably, the tolerance of the reaction
to the presence of double bonds offers an ideal opportunity
for further synthetic manipulation (2e).
results, a plausible catalytic cycle was outlined in Scheme 1.
Coordination of Cu(OTf)2 with substrate A, followed by
an electrophilic substitution process, yields a Cu(II) inter-
mediate C. Reductive elimination delivers Cu0, which can
be reoxidize to regenerate Cu(OTf)2 to close the CuII/Cu0
catalytic cycle.
Scheme 1. Plausible Catalytic Cycle
To explore the electronic and steric effects of substitu-
ents on the aniline ring, the reactions of 1iÀ1o were
investigated. Each reaction proceeded smoothly to afford
the corresponding product 2iÀ2o in moderate to high yield
(Table 3, entries 1À8). Among these reactions substrates
with electron-withdrawing groups gave higher yields than
those with electron-donating groups. Steric effects may
also play a role since further studies showed that the
substrates with substiuents at the ortho position of the
aniline ring were less reactive than those with a para
substituent (entries 2À3, 5À6). Particularly noteworthy is
the reaction of 6-(o-chloroaniline)-purine-nucleoside 1m
which gave exclusively product 2m, demonstrating that the
present catalytic system exploiting a CÀH activation/
amination reaction was more readily executed than tradi-
tional Ullman-type couplings (entry 6). Although opti-
mized conditions generally worked well compound 1p did
not cyclize under these conditions. Various cyclization
attempts unveiled conditions where it was possible to
obtain compound 2p by adjusting the acetic anhydride
addition time, but the yield was still lower than that with
other substrates (entry 9).
It was envisaged thatthe nucleoside derivatives prepared
in this study might be useful as fluorescent tags in biolo-
gical scenarios. For the investigation of fluorescence prop-
erties, see the Supporting Information.
In conclusion, we have developed an intramolecular
CÀH activation/amination reaction of purine nucleosides
to synthesize a series of multiheterocyclic compounds,
which are of great importance in medicinal chemistry. To
the best of our knowledge, this is the first example that uses
an intramolecular CÀH activation/amination reaction for
the synthesis of purine nucleosides, which offers facile
alternative access to some useful multifused ring purine-
heterocyclic compounds. The fluorescence of these purine
nucleoside compounds will be helpful for the study of their
medicinal relevance. Further studies into the mechanism
and the role of acetic anhydride are currently planned.
Based on the previous studies of intramolecular CÀH
Acknowledgment. We are grateful for financial support
from the National Natural Science Foundation of China
(Grant Nos. 20802016, 21072047), the Program for New
Century Excellent Talents in University of Ministry of
Education (No. NCET-09-0122), Excellent Youth Founda-
tion of Henan Scientific Committee (No. 114100510012),
PIRT (2012IRTSTHN006), PCSIRT (IRT1061), and the
Excellent Youth Program of Henan Normal University. J.S.F.
thanks Henan Normal University for a visiting professorship
and the University of Birmingham for support.
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Supporting Information Available. Typical experimen-
tal procedures, characterization of compounds, com-
pound spectroscopic information, and the investigation
of fluorescence properties. This material is available free
of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX