6-alkynylpurines bearing electronacceptor substituents: Preparation, reactivity in cycloaddition reactions and cytostatic activity

Article abstract of DOI:10.1135/cccc2011176

While direct Sonogashira coupling of 6-halopurines with methyl propiolate and with propargyl aldehyde was not successful, the corresponding orthoester and propargyl aldehyde diethylacetal reacted smoothly. Such prepared orthoester was then converted to the desired methylester by methanolysis, the acetal was too stable to be hydrolyzed. The obtained 6-ethynylpurines, bearing orthoester, acetal, methoxycarbonyl and for comparison also the phenyl substituent on the ethynyl group, were subjected to the cycloaddition reaction with cyclopentadiene, diazomethane and phenylazide. Electron deficient alkynylpurines were considerably more reactive in this reaction compared to the not activated phenylethynyl derivative. The prepared alkynylpurines exhibited medium cytostatic activity (IC₅₀ = 2.6-15 μM), while the cycloadducts were inactive.

Full text of DOI:10.1135/cccc2011176

6-Alkynylpurines Bearing Electronacceptor Substituents
1487
6-ALKYNYLPURINES BEARING ELECTRONACCEPTOR SUBSTITUENTS:
PREPARATION, REACTIVITY IN CYCLOADDITION REACTIONS AND
CYTOSTATIC ACTIVITY
b
c
d
Martin KŘOVÁČEKa1, Hana DVOŘÁKOVÁ , Ivan VOTRUBA , Ivana CÍSAŘOVÁ and
a2,
Dalimil DVOŘÁK
*
a
Department of Organic Chemistry, Institute of Chemical Technology, Prague, Technická 5,
1
2
166 28 Prague 6, Czech Republic; e-mail: krovacek@gmail.com, dalimil.dvorak@vscht.cz
Laboratory of NMR Spectroscopy, Institute of Chemical Technology, Prague, Technická 5,
166 28 Prague 6, Czech Republic; e-mail: hana.dvorakova@vscht.cz
b
c Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i.,
Flemingovo nám. 2, 166 10 Prague 6, Czech Republic; e-mail: votruba@uochb.cas.cz
d
Department of Inorganic Chemistry, Charles University in Prague, Hlavova 2030,
128 40 Prague 2, Czech Republic; e-mail: cisarova@natur.cuni.cz
Received September 22, 2011
Accepted October 6, 2011
Published online December 16, 2011
Dedicated to the 75th anniversary of Professor Antonín Holý birthday.
While direct Sonogashira coupling of 6-halopurines with methyl propiolate and with
propargyl aldehyde was not successful, the corresponding orthoester and propargyl aldehyde
diethylacetal reacted smoothly. Such prepared orthoester was then converted to the desired
methylester by methanolysis, the acetal was too stable to be hydrolyzed. The obtained
6-ethynylpurines, bearing orthoester, acetal, methoxycarbonyl and for comparison also
the phenyl substituent on the ethynyl group, were subjected to the cycloaddition reaction
with cyclopentadiene, diazomethane and phenylazide. Electron deficient alkynylpurines
were considerably more reactive in this reaction compared to the not activated phenyl-
ethynyl derivative. The prepared alkynylpurines exhibited medium cytostatic activity (IC50
2.6–15 µM), while the cycloadducts were inactive.
=
Keywords: Alkynylpurines; Cytotoxicity; Cycloaddition reactions.
Purine bases bearing C-6 substituent exhibit wide range of biological ef-
fects, mainly cytostatic1, antiviral2 and antimycobacterial3. Bis(purin-6-yl)-
acetylenes and diacetylenes designed as covalent analogues of DNA
base-pairs display significant cytostatic activity in vitro4 and high cyto-
toxicity was found in the case of 6-alkynyl-2-oxopurines5 and 6-alkynyl
purines6. This high cytotoxicity was ascribed6 to the ability of 6-alkynyl-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507
© 2011 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2011176

1488
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
purines to undergo easily conjugate additions of wide range of O, N
and S-nucleophiles7. Therefore, we assumed, that the introduction of the
ethynyl moiety bearing electron withdrawing substituent may further en-
hance the cytotoxicity. For that reason we attempted to synthesize purine
derivatives substituted in the position 6 with ethynyl group bearing ester
and aldehyde groups in conjugation with the triple bond.
However, all attempts to prepare derivatives of the propiolic acid bearing
9-protected purin-6-yl group at the position 3 using the Sonogashira cou-
pling of the ethyl propiolate or the propiolic acid itself with 9-protected
6-iodopurines 1 have failed. Also attempts to prepare these compounds by
the Negishi coupling, reaction of the lithium salt of the 6-ethynylpurine
with CO2 or by an oxidation of the 3-(purin-6-yl)propargyl alcohol have
failed.
Therefore, a different strategy consisting in the protection of the
carboxylic and aldehyde functionalities was used. Thus, diethyl acetal of
propargylaldehyde (2a) and the orthoester 2b were used for the coupling
reactions. These alkynes were reacted with 6-iodopurine (1a), 9-benzyl-
6-iodopurine (1b), with the derivatives with removable protecting group at
the position 9 (6-iodo-9-tetrahydropyranylpurine (1c) and 9-Boc-6-iodo-
purine (1d)) and with acetylated riboside 1e (Scheme 1). After some experi-
mentation, it was found that the best results can be obtained using
PdCl2(PPh3)2 (method A) or Pd(dba)2 in the presence of tris(2-furyl)phos-
phine (method B).
Z
I
N
N
N
N
N
Pd-cat., CuI, Et3N
DMF
N
Z
+
N
N
2
R
R
1
3
1
a
b
c
d
e
2
Z
a
b
AcO
O
O
R
H
CH2C6H5 THP
Boc
CH(OC2H5)2
CH3
O
O
AcO
OAc
3
a
b
c
d
e
f
g
h
i
j
AcO
AcO
O
O
R
H
CH2C6H5 THP
Boc
H
CH2C6H5 THP
Boc
CH3
AcO
OAc
AcO
OAc
O
Z
CH(OC2H5)2
O
O
SCHEME 1
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

6-Alkynylpurines Bearing Electronacceptor Substituents
1489
The unsubstituted 6-iodopurine (1a) reacted only with propargylaldehyde
diethylacetal (2a) in low yield (Table I, entry 1), while no reaction with the
orthoester 2b (Table I, entry 7) was observed. On the opposite, all the
9-substituted 6-iodopurines gave the desired 6-alkynylpurines 3. Generally,
the method B using Pd(dba)2 in the presence of tris(2-furyl)phosphine gave
somewhat better yields compared to the method A. This is especially the
case of 9-Boc derivative 1d (Table I, entries 10, 11), where the yield of the
desired 3i using method A was only 6%, while the method B furnished the
same product in 86% (Table I, entries 10, 11). In this case, the very low
yield of 3i using the method A is most probably due to the thermal instabil-
ity of the Boc derivative 1d in prolonged reaction time. 9-Unsubstituted
orthoester 3f, which is not accessible by direct coupling of 6-iodopurine
(1a) with 2b, was obtained in 69% yield by the thermolysis of the 9-Boc de-
TABLE I
The Sonogashira reaction of 1 with alkynes 2
Entry
Purine 1
Alkyne 2
Methoda
Reaction time, h
3, Yield, %
1
2
1a
1b
1c
1d
1e
1e
1a
1b
1c
1d
1d
1e
1e
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
A
8
4
3a, 32c
3b, 84
3c, 84
3d, 77
3e, 50
3e, 73
3f, 0
A
3
A
Bb
12
12
4
4
5
A
6
B
3
7
A, B
A
24
12
12
12
17
15
3
8
3g, 75
3h, 79
3i, 6
9
A
10
11
12
13
A
Bb
3i, 72
3j, 50
3j, 68
A
B
a
Method A: A mixture of 6-iodopurine 1, alkyne (1.5 equiv.), PdCl2(PPh3)2 (2 mole %), CuI
(5 mole %), triethylamine (3 equiv.) and DMF (2 ml per mmol of iodopurine) was stirred at
65 °C; Method B: A mixture of 6-iodopurine 1, alkyne (1.5 equiv.), Pd(dba)2 (2 mole %),
tris(2-furyl)phospine (4 mole %), CuI (5 mole %), triethylamine (3 equiv.) and DMF (2 ml
b
per mmol of iodopurine) was stirred at 50 °C. The reaction was performed at a laboratory
c
temperature due to the thermal instability of the Boc group. The compound was alterna-
tively prepared in 58% yield by acidolysis of 3c.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1490
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
rivative 3i in DMF. The orthoester group remains untouched under these
conditions.
The removing of the protective groups was addressed next. All attempts
of the hydrolysis of the propargyl acetal 3b (CF3COOH/H2O/DCM, TsOH/
H2O/acetone, LiBF4/CH3CN/H2O) were not successful, also no reaction or
full decomposition of the starting 3b was observed.
On the opposite, the conversion of the orthoester 3g to the correspond-
ing methyl ester 3k proceeded smoothly. The best results were obtained in
a two-step process. At first the orthoester 3g was converted in THF–
methanol–H2O mixture under acid catalysis to the monoester of 1,1,1-tris-
(hydroxymethyl)ethane which was then without isolation converted to the
methylester 3k by methanolysis giving the desired compound in 82% iso-
lated yield. Also the removal of the acetoxy groups of the ribosides 3e and
3j by methanolysis gave the corresponding nucleosides 3l and 3m in 90%
and 78% yield, respectively.
CH3
O
O
N
O
CH(OC2H5)2
CO2CH3
C6H5
N
N
N
N
N
N
N
HO
HO
N
N
N
O
N
O
N
N
N
N
HO
OH
HO
OH
CH2C6H5
CH2C6H5
3k
3l
3m
3n
Reactivity of the title 6-alkynylpurines was studied on the reaction with
cyclopentadiene and 1,3-dipoles – diazomethane and phenylazide. From
the alkynylpurines the acetal 3b, the orthoester 3g and the methylester 3k
were used as the representative examples together with the unactivated
9-benzyl-6-(phenylethynyl)purine (3n) which was used for the comparison
(Scheme 2).
Cycloadition reaction of alkynylpurines 3b, 3k, 3d and 3n with cyclo-
pentadiene was carried out in dichloromethane in a sealed glass tube at
60 °C (Table II, entries 1–4). While the reactions of the ester 3k and the
orthoester 3g were finished within hours in quantitative yield, the acetal
3b reacted repeatedly incompletely even after overnight heating, giving the
expected product 4b in 56% yield. The phenylethynyl derivative 3n did not
react with cyclopentadiene under these conditions.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

6-Alkynylpurines Bearing Electronacceptor Substituents
1491
Z
or
N
5A/5B
6A/6B
4
or
or
N
H2C
or
N N
+
N
N
CH2C6H5
C6H5
N N N
3
C6H5
N
N
N
N
N
NH
N
N
N
HN
Z
C6H5
Z
Z
Z
Z
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
4
5A
5B
6A
6B
3, 4, 5, 6
Z
b
k
g
n
O
CH3
CH(OC2H5)2
CO2CH3
C6H5
O
O
SCHEME 2
The reaction with diazomethane was accomplished in THF at room tem-
perature (Table II, entries 5–8). The alkynes 3b, 3k and 3d were consumed
during several hours giving the expected products 5k, 5b and 5g in practi-
cally quantitative yields. In contrast, the less activated 3n gave only low
yield of the 5n after prolonged reaction time (Table II, entry 8). In the reac-
tion with diazomethane formation of two regioisomers of the product is
possible. This was really observed in the reaction of the ester derivative 3k,
where the mixture of regioisomers 5kA and 5kB in approximately 3:1 ratio
was obtained. The structure assignment was done by NMR. The minor iso-
mer in contrast to the major one showed NOE between the ester methyl
group and the CH of the pyrazole ring. Therefore, the major isomer was as-
signed as 5kA and the minor as 5kB. The other alkynes gave single isomers
of the product 5bB, 5gB and 5nB, respectively. The structure of the 5bB was
unambiguously determined using HMBC after transformation to the corre-
sponding dimethylacetal 7 to avoid presence of diastereotopic CH2 groups.
The structure of 5gB was obtained after transformation to the correspond-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1492
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
TABLE II
Cycloaddition reactions of the purinylalkynes 3 with the cyclopentadiene, diazomethane
and phenylazide
Entry
Alkyne, Z
Reagent
Solvent, temp., °C Time, h
Product, Yield,%
1
2
3
4
3k
3b
3g
3n
CH2Cl2, 60
CH2Cl2, 60
CH2Cl2, 60
CH2Cl2, 60
1
16
4
4k, 100
4b, 56
4g, 99
a
16
–
5
3k
CH2N2
THF, r.t.
1
5kA, 61
5kB, 18
5bB 100
5gB, 100
5nB, 27b
6
7
8
3b
3g
3n
THF, r.t.
THF, r.t.
THF, r.t.
3
3
72
9
10
11
12
3k
3b
3g
3n
C6H5N3
DMF, 100
DMF, 100
DMF, 100
DMF, 100
2
28
8
6kA, 30
6kB, 55
6bA, 27
6bB, 67
6gA, 26
6gB, 38
6nA, 40
6nB, 40
48
a
b
No reaction; 66% of the starting material was recovered.
ing methylester by methanolysis. The resulting compound appeared to be
identical with 5kB obtained previously.
The structure assignment of 5nB by NMR was complicated by very low
solubility of this compound in CDCl3 and by formation of mixture of
tautomers in DMSO (not observed in CDCl3). Therefore this compound was
N-methylated by CH3I in DMSO in the presence of K2CO3 giving approxi-
mately 1:1 mixture of two regioisomers 8a and 8b. The structure of these
compounds was then unambiguously determined using HMBC and NOE
(Scheme 3).
The cycloaddition of the above alkynylpurines with phenylazide required
somewhat harsher conditions compared to the previous reactions (Table II,
entries 9–12). This reaction was done in DMF at 100 °C. Again the methyl-
ester 3k was the most reactive and full conversion of the starting material
was observed within 2 h. The acetal 3b and the orthoester 3g required 8 h
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

6-Alkynylpurines Bearing Electronacceptor Substituents
1493
NOE
HMBCH
N
H
H3C
N
N
HN
Ph
N
C6H5
N
N
N
H3C
HMBCN
C6H5
N
C6H5
N
CH3I, K2CO3
DMSO
N
CH2N2
+
N
N
N
N
N
N
N
N
N
N
CH2Ph
5nB
8a
8b
SCHEME 3
at these conditions to achieve full conversion and with 3n even after 48 h
the conversion was only about 80%. All alkynes furnished chromatog-
raphically separable mixtures of both possible regioisomers. Their structure
was assigned on the basis of NOE experiments. The NOE effect was ob-
served between the triazole phenyl group and the ester methyl group in the
case of 6kA, between the phenyl group and the purine H-2 in the case of
6kB, between the phenyl group and CH of the acetal moiety of 6bA, and be-
tween the phenyl group and the CH2 moiety of the orthoester in 6gA. This
approach was not successful with 6n, however the structure of 6nA was
assigned using X-ray analysis (Fig. 1).
FIG. 1
X-ray structure of 6nA
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1494
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
The representative examples of the alkynes 3 and the cycloadducts 5 and
6 were tested on their in vitro inhibition of the cell growth in the following
cell cultures: mouse leukemia L1210 cells (ATCC CCL 219), human pro-
myelocytic leukemia HL 60 cells (ATCC CCL 240), human cervix carcinoma
HeLa S3 cells (ATCC CCL 2.2), and human T lymphoblastoid CCRF-CEM
cell line (ATCC CCL 119). The results in Table III show that the ethynyl de-
rivatives 3 exhibited a medium cytostatic activity in these assays (IC50
=
2.4–15 µM) which is lower than that previously reported for phenylethynyl-
purines6 and bis(purinyl)ethynes4. In contrast, the cycloadducts 5 and 6
were entirely inactive.
TABLE III
Cytostatic activity of compounds 3, 5 and 6
a
IC50, µM
Compound
L1210
HL60
HeLa S3
CCRF-CEM
3a
5.0 ( 0.20)
6.5 ( 0.39)
13.7 ( 0.92)
7.3 ( 0.48)
8.6 ( 0.56)
>12
2.9 ( 0.21)
4.7 ( 0.22)
4.4 ( 0.25)
3.7 ( 0.21)
5.7 ( 0.33)
>15
10 ( 0.6)
15.4 ( 1.2)
8.0 ( 0.6)
9.4 ( 0.56)
15 ( 1.1)
>15
7.5 ( 0.61)
5.4 ( 0.35)
2.6 ( 0.15)
3.4 ( 0.21)
7.2 ( 0.42)
12 ( 0.8)
15 ( 1.4)
2.4 ( 0.15)
na
3b
3cb
3g
3h
3lb
3mb
12 ( 1.1)
4.8 ( 0.29)
nac
10.8 ( 0.8)
4.7 ( 0.32)
na
4.0 ( 0.2)
15 ( 1.1)
na
3k
5bB
5gB
5kA
6bA
6bB
na
na
na
na
na
na
>20
na
na
na
na
na
>12
na
na
na
a
b
Values are means of four experiments, standard deviation is given in parentheses; the so-
c
lution is not stable in time (THP derivative and the nucleosides); na, not active (inhibition
of cell growth at 10 µM was lower than 30%).
In conclusion, while the attempts to accomplish the Sonogashira cou-
pling of methyl propiolate or propargylaldehyde with 9-protected-6-halo-
purines have not been successful, the coupling of the compounds with
protected carbonyl group – the acetal 2a and the orthoester 2b proceeds
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

6-Alkynylpurines Bearing Electronacceptor Substituents
1495
smoothly. The orthoesters can be easily converted to the corresponding
methylesters, while the acetals 3a–3d are too stable to be hydrolyzed. The
resulting electron-deficient alkynes are more reactive towards cycloaddition
reactions compared to the non-activated 6-phenylethynyl purines. Accord-
ing to our expectations, the compound 3b, bearing the carbonyl group in
the conjugation with the triple bond, is the most reactive, but both the
orthoester 3g and acetal 3b are still considerably more reactive compared to
the 6-phenylethynylpurine 3n. The ethynylpurines showed moderate cyto-
toxicity, in contrast with their cycloadducts 5 and 6 which were completely
inactive.
EXPERIMENTAL
Unless stated otherwise, all reaction were performed in flame-dried flasks under an argon
atmosphere. NMR spectra were measured on a Varian Gemini 300 (1H, 300.07 MHz; 13C,
75.46 MHz), a Bruker AMX3 400 (1H, 400.13 MHz; 13C, 100.62 MHz) or a Bruker DRX 500
Avance (1H, 500.13 MHz; 13C, 125.77 MHz) spectrometer at 298 K. Unambiguous assign-
ment of the NMR signals is based on 13C{1H}, 13C APT, COSY, HMQC and 13C HMBC spec-
tra. Chemical shifts are given in ppm (δ-scale), coupling constants (J) in Hz. IR spectra (ν,
cm–1) were recorded on Nicolet 740 FT-IR. High resolution mass spectra were recorded on
LTQ Orbitrap Velos spectrometer (Thermo Scientific). Melting points were measured on a
Koffler block and are uncorrected. The solvents were dried and degassed by standard proce-
dures; silica gel (ICN SiliTech, 32-63) was used for column chromatography. 6-Iodopurine8,
9-benzyl-6-iodopurine9, 6-iodo-9-(tetrahydropyran-2-yl)purine10, 9-(O,O,O-triacetyl-β-D-ribo-
furanosyl)-6-iodopurine11 and 9-benzyl-6-phenylethynylpurine12 were prepared via proce-
dures described in literature. Cyclopentadiene was freshly prepared from dicyclopentadiene
by cracking13. Other compounds were purchased.
Cytostatic activity tests were performed according to literature procedure1b
.
9-(tert-Butyloxycarbonyl)-6-iodopurine (1d)
To a suspension of thoroughly powdered 6-iodopurine (12.3 g, 50 mmol) in THF (100 ml),
ditertbutyldicarbonate (10.9 g, 1 equiv.) and triethylamine (7 ml, 1 equiv.) were added. The
resulting mixture was stirred at the room temperature until the solid was completely dis-
solved. Then the solvent was evaporated and the residue was dissolved in a minimum
amount of dichloromethane, the solution was filtered through silica gel, which was subse-
quently eluted with a mixture of ethyl acetate and hexane (2:1). The eluate was dried with
Na2SO4 and evaporated to dryness in vacuum. The crystallization (ethyl acetate–hexane, 1:2)
yielded 1d (15.58 g, 90%) as white crystals; m.p. > 100 °C (dec.). 1H NMR (300 MHz,
CDCl3): 1.66 (s, 9 H, CH3), 8.51 (s, 1 H, H-8), 8.72 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3):
28.1 (CH3), 88.2 (C(CH3)3), 123.1 (C-5), 139.5 (C-6), 143.4 (C-8), 145.8 (C=O), 147.6 (C-4),
154.1 (C-2). IR (ATR): 3119 (w), 3010 (w), 2975 (m), 2934 (w), 1917 (w), 1829 (w), 1784 (m),
1756 (s), 1581 (m), 1549 (s), 1488 (w), 1435 (m), 1372 (m), 1344 (m), 1293 (m), 1256 (m),
1218 (m), 1204 (m), 1164 (s), 1088 (w), 963 (w), 914 (m), 838 (m), 822 (m), 789 (w), 764 (m),
662 (w). HRMS (EI): m/z [M]+ calculated for C10H11IN4O2: 345.9927; found: 345.9937.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1496
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
Preparation of 6-Alkynylpurines. General Procedure
Method A: A mixture of 6-iodopurine 1, alkyne (1.5 equiv.), PdCl2(PPh3)2 (2 mole %), CuI
(5 mole %), triethylamine (3 equiv.) and DMF (2 ml per mmol of iodopurine) was stirred at
65 °C for the amount of time given in Table I. After the removal of volatiles in vacuo, the
product was isolated using column chromatography on silica gel.
Method B: A mixture of 6-iodopurine 1, alkyne (1.5 equiv.), Pd(dba)2 (2 mole %),
tris(2-furyl)phospine (4 mole %), CuI (5 mole %), triethylamine (3 equiv.) and DMF (2 ml
per mmol of iodopurine) was stirred at 50 °C for the amount of time given in Table I. After
the removal of volatiles, the product was isolated using column chromatography on silica
gel.
6-(3,3-Diethoxyprop-1-yn-1-yl)purine (3a)
Method A, starting from 6-iodopurine (1a; 246 mg, 1.0 mmol) afforded after column chro-
matography (ethyl acetate) 3a (79 mg, 32%) as a white crystalline solid; m.p. 118–122 °C.
Alternatively, 3a was prepared in higher yield via acidolysis of 3c. 6-(3,3-Diethoxyprop-
1-yn-1-yl)-9-(tetrahydropyran-2-yl)purine (3c; 55 mg, 0.17 mmol) was dissolved in ethanol
(4 ml), anhydrous CuCl2 (2.9 mg, 13 mole %) was added and the reaction mixture was
stirred at 50 °C for 4 h. The solvent was then evaporated and the residue was column
chromatographed (ethyl acetate) to obtain 24 mg (58%) of white crystalline 3a (m.p.
118–122 °C) and 8 mg (15%) of the starting material. 1H NMR (300 MHz, CDCl3): 1.25 (t,
J = 7.0, 6 H, CH3), 3.65–3.90 (m, 4 H, CH2), 5.58 (s, 1 H, CH-C≡C), 8.53 (s, 1 H, H-8), 9.04
(s, 1 H, H-2). 13C NMR (75 MHz, CDCl3): 15.2 (CH2CH3), 61.8 (CH2CH3), 79.5 (C≡C), 91.7
(CH-C≡C), 93.4 (C≡C), 132.3 (CqPur), 138.7 (CqPur), 146.2 (CHPur), 152.7 (CHPur), 154.4
(CqPur). IR (CHCl3): 3434 (w), 3111 (w), 3061 (w), 2935 (w), 2891 (w), 2827 (w), 2717 (w),
1583 (s), 1477 (w), 1443 (w), 1428 (w), 1392 (m), 1353 (w), 1324 (s), 1299 (w), 1119 (s),
1083 (m), 1051 (s), 1014 (w), 924 (w), 892 (w), 860 (w), 828 (w), 639 (w), 606 (w). HRMS
(EI): m/z [M]+ calculated for C10H9N4O (M – C2H5O): 201.0776; found: 201.0779.
9-Benzyl-6-(3,3-diethoxyprop-1-yn-1-yl)purine (3b)
Method A, starting from 9-benzyl-6-iodopurine (1b; 1.01 g, 3.0 mmol), column chromatog-
raphy (ethyl acetate–hexane, 2:1) afforded 848 mg (84%) of 3b as a brown solid. Crystalliza-
tion (ethyl acetate–hexane) yielded 736 mg (73%) of light brown needles; m.p. 77–78 °C.
1H NMR (500 MHz, CDCl3): 1.27 (t, J = 7.0, 6 H, CH3), 3.70–3.90 (m, 4 H, 2 × CH2-CH3),
5.45 (s, 2 H, CH2-Ph), 5.62 (s, 1 H, CH-C≡C), 7.28–7.37 (m, 5 H, Ph), 8.09 (s, 1 H, H-8), 8.97
(s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 15.0 (CH3), 47.4 (CH2-Ph), 61.4 (CH2-CH3), 79.6
(C≡C), 91.6 (C≡C-CH), 93.0 (C≡C), 127.8 (CHPh), 128.7 (CHPh), 129.2 (CHPh), 134.6 (CqPh),
134.8 (C-5), 140.8 (C-6), 145.4 (C-8), 151.8 (C-4), 152.7 (C-2). IR (CHCl3): 3068 (w), 2983 (w),
2935 (w), 2897 (w), 1582 (s), 1497 (w), 1444 (w), 1404 (w), 1354 (w), 1328 (m), 1297 (w),
1149 (w), 1114 (m), 1101 (m), 1048 (m), 1017 (w), 948 (w), 894 (w), 669 (w), 645 (w). HRMS
(EI): m/z [M]+ calculated for C17H15N4O (M – CH3CH2O): 291.1246; found: 291.1249.
6-(3,3-Diethoxyprop-1-yn-1-yl)-9-(tetrahydropyran-2-yl)purine (3c)
Method A, starting from 9-(tetrahydropyran-2-yl)-6-iodopurine (1c; 672 mg, 2.0 mmol) af-
forded after column chromatography (ethyl acetate–hexane, 2:1) 598 mg (84%) of 3c as a
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6-Alkynylpurines Bearing Electronacceptor Substituents
1497
light brown viscous liquid. 1H NMR (500 MHz, CDCl3): 1.29 (t, J = 7.0, 6 H, CH3), 1.65–1.86
(m, 3 H, 2 × H-5′ + H-4′), 2.03–2.21 (m, 3 H, 2 × H-3′ + H-4′), 3.70–3.94 (m, 5 H, 2 ×
CH2CH3 + H-6′), 4.20 (m, 1 H, H-6′), 5.64 (s, 1 H, CH-C≡C), 5.81 (m, 1 H, H-2′), 8.35 (s, 1 H,
H-8), 8.92 (s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 15.1 (CH3), 22.7 (C-4′), 24.8 (C-5′),
31.8 (C-3′), 61.3 (CH2CH3), 68.8 (C-6′), 79.5 (C≡C), 82.1 (C-2′), 91.6 (CH-C≡C), 92.9 (C≡C),
134.7 (C-5), 140.7 (C-6), 143.4 (C-8), 150.9 (C-4), 152.5 (C-2). IR (CHCl3): 2982 (w), 2951 (w),
2898 (w), 2869 (w), 1583 (s), 1492 (w), 1443 (w), 1403 (w), 1354 (w), 1326 (m), 1290 (w),
1267 (w), 1186 (w), 1160 (w), 1089 (m), 1047 (s), 1017 (w), 994 (w), 949 (w), 912 (w),
878 (w), 823 (w), 642 (w). HRMS (EI): m/z [M]+ calculated for C17H22N4O3: 330.1692; found:
330.1691.
6-(3,3-Diethoxyprop-1-yn-1-yl-9-(tert-butyloxycarbonyl)purine (3d)
Method B (reaction performed at a laboratory temperature), starting from 9-(tert-butyloxy-
carbonyl)-6-iodopurine (1d; 346 mg, 1.0 mmol) gave after column chromatography (ethyl
acetate–hexane, 1:1) 266 mg (77%) of 3d as a pale yellow viscous liquid. 1H NMR (300 MHz,
CDCl3): 1.25 (t, 3 H, J = 7.0, CH2CH3), 1.68 (s, 9 H, C(CH3)3), 3.65–3.75 (m, 2 H, CH2),
3.80–3.91 (m, 2 H, CH2), 5.59 (s, 1 H, CH-C≡C), 8.52 (s, 1 H, H-8), 9.05 (s, 1 H, H-2).
13C NMR (75 MHz, CDCl3): 15.3 (CH2CH3), 28.1 (C(CH3)3), 61.6 (CH2CH3), 79.2 (C≡C), 87.9
(C(CH3)3), 91.7 (CH-C≡C), 94.0 (C≡C), 135.6 (C-5), 141.8 (C-6), 144.3 (C-8), 145.8 (C=O),
151.3 (C-4), 154.7 (C-2). IR (ATR): 3123 (w), 3061 (w), 2979 (m), 2933 (w), 2889 (w), 2242 (w),
1778 (m), 1758 (s), 1579 (s), 1496 (w), 1478 (w), 1440 (m), 1389 (m), 1372 (m), 1342 (m),
1327 (m), 1294 (s), 1262 (m), 1230 (m), 1196 (m), 1165 (s), 1148 (m), 1114 (s), 1053 (m),
1017 (m), 941 (w), 894 (w), 839 (m), 806 (w), 769 (w), 728 (w), 664 (w). HRMS (EI): m/z [M]+
calculated for C15H17N4O3 (M – C2H5O): 301.1301; found: 301.1286.
9-(2′,3′,5′-Tri-O-acetyl-β-D-ribofuranosyl)-6-(3,3-diethoxyprop-1-yn-1-yl)purine (3e)
Method A, starting from 9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)-6-iodopurine (1e; 504 mg,
1.0 mmol) afforded after column chromatography (ethyl acetate–hexane, 2:1) 252 mg (50%)
of 3e as a brown viscous liquid. Method B, starting from 9-(2′,3′,5′-tri-O-acetyl-β-D-ribo-
furanosyl)-6-iodopurine (1e; 252 mg, 0.5 mmol) afforded after column chromatography
(ethyl acetate–hexane, 2:1) 184 mg (73%) of 3e as a light brown viscous liquid. 1H NMR
(300 MHz, CDCl3): 1.26 (t, 6 H, CH2CH3), 2.06 (s, 3 H, COCH3), 2.10 (s, 3 H, COCH3), 2.14
(s, 3 H, COCH3), 3.65–3.92 (m, 4 H, 2 × CH2CH3), 4.33–4.49 (m, 3 H, 2 × H-5′ + H-4′), 5.60
(s, 1 H, CH-C≡C), 5.64 (t, J = 5.3, 1 H, H-3′), 5.93 (t, J = 5.6, 1 H, H-2′), 6.22 (d, J = 5.3, 1 H,
H-1′), 8.27 (s, 1 H, H-8), 8.93 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3): 15.3 (CH2CH3), 20.6
(COCH3), 20.7 (COCH3), 20.9 (COCH3), 61.6 (CH2CH3), 63.1 (C-5′), 70.7 (C-3′), 73.2 (C-2′),
79.4 (C≡C), 80.7 (C-4′), 86.7 (C-1′), 91.7 (CH(OEt)2), 93.8 (C≡C), 135.4 (C-5), 141.3 (C-6),
144.3 (C-8), 151.4 (C-4), 153.0 (C-2), 169.6 (C=O), 169.8 (C=O), 170.5 (C=O). IR (ATR):
2978 (w), 2935 (w), 2890 (w), 1752 (m), 1582 (m), 1495 (w), 1442 (w), 1402 (w), 1373 (w),
1333 (w), 1242 (s), 1164 (w), 1104 (m), 1056 (m), 1018 (w), 899 (w), 823 (w), 806 (w),
732 (w). HRMS (EI): m/z [M]+ calculated for C21H23N4O8 (M – C2H5O): 459.1516; found:
459.1507.
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1498
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
1-(2-(Purin-6-yl)ethynyl)-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane (3f)
A solution of 1-(2-(9-(tert-butyloxycarbonyl)purin-6-yl)ethynyl)-4-methyl-2,6,7-trioxabicyclo-
[2.2.2]octane (3i; 90 mg, 0.24 mmol) in 1.5 ml DMF was heated at 100 °C for 1 h. After
removal of DMF, the residue was column chromatographed (ethyl acetate–methanol, 20:1),
yielding 45 mg (69%) of a white powder, m.p. > 340 °C (dec.). 1H NMR (500 MHz, DMSO,
373 K): 0.83 (s, 3 H, CH3), 4.01 (s, 6 H, 3 × OCH2), 8.58 (s, 1 H, H-8), 8.88 (s, 1 H, H-2).
13C NMR (125 MHz, DMSO, 373 K): 13.0 (CH3), 29.3 (C-CH3), 72.1 (OCH2), 75.5 (C≡C),
88.7 (C≡C), 101.2 (C(OR)3), 131.4 (C-5), 135.7 (C-6), 146.2 (C-8), 151.5 (C-2), 153.5 (C-4). IR
(ATR): 3101 (w), 3063 (w), 2957 (m), 2883 (m), 2803 (m), 2691 (m), 2539 (w), 1818 (w),
1723 (w), 1596 (s), 1583 (s), 1465 (w), 1455 (w), 1395 (m), 1335 (s), 1319 (s), 1272 (w), 1246 (m),
1234 (m), 1192 (m), 1151 (w), 1112 (w), 1076 (m), 1044 (w), 1025 (m), 1000 (s), 939 (w),
921 (w), 867 (w), 807 (w), 766 (w), 724 (w). HRMS (EI): m/z [M]+ calculated for C13H12N4O3:
295.0807; found: 295.0817.
1-(2-(9-Benzylpurin-6-yl)ethynyl)-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane (3g)
Method A, starting from 9-benzyl-6-iodopurine (1b; 672 mg, 2.0 mmol) afforded after col-
umn chromatography (ethyl acetate–hexane, 2:1) 544 mg (75%) of 3g as a light brown solid.
Crystallization (ethyl acetate–hexane) yielded 500 mg (69%) of white crystals; m.p. 194–208 °C.
Another crystallization from chloroform yielded an analytic sample with m.p. 214–215 °C.
1H NMR (300 MHz, CDCl3): 0.85 (s, 3 H, CH3), 4.03 (s, 6 H, 3 × OCH2), 5.44 (s, 2 H,
CH2-Ph), 7.27–7.36 (m, 5 H, Ph), 8.09 (s, 1 H, H-8), 8.95 (s, 1 H, H-2). 13C NMR (300 MHz,
CDCl3): 14.4 (CH3), 30.4 (C-CH3), 47.4 (CH2Ph), 73.1 (OCH2), 75.9 (C≡C), 90.0 (C≡C), 102.4
Ph
(C(OR)3), 127.8 (CHPh), 128.6 (CHPh), 129.2 (CHPh), 134.8 + 135.0 (Cq + C-5), 140.0 (C-6),
145.6 (C-8), 151.8 (C-4), 152.6 (C-2). IR (CHCl3): 3067 (w), 2969 (w), 2945 (w), 2885 (w),
1581 (s), 1497 (w), 1456 (w), 1444 (w), 1405 (w), 1395 (w), 1338 (s), 1327 (s), 1272 (w),
1148 (w), 1135 (w), 1078 (w), 1051 (m), 995 (s), 889 (w), 869 (w), 698 (w), 640 (w). HRMS
(EI): m/z [M]+ calculated for C20H18N4O3: 362.1379; found: 362.1363.
1-(2-(9-(Tetrahydropyran-2-yl)purin-6-yl)ethynyl)-4-methyl-
2,6,7-trioxabicyclo[2.2.2]octane (3h)
Method A, starting from 9-(tetrahydropyran-2-yl)-6-iodopurine (1c; 447 mg, 1.35 mmol)
gave after column chromatography (ethyl acetate–hexane, 2:1) 382 mg (79%) of 3h as pale
yellow crystals; m.p. 223.5–225 °C. 1H NMR (500 MHz, CDCl3): 0.84 (s, 3 H, CH3), 1.62–1.85
(m, 3 H, H-4′ + 2 × H-5′), 2.02–2.17 (m, 3 H, 2 × H-3′ + H-4′), 3.76 (t, J = 10.5, 1 H, H-6′),
4.01 (s, 6 H, 3 × OCH2), 4.15 (d, J = 11.0, 1 H, H-6′), 5.76 (d, J = 10.0, 1 H, H-2′), 8.30
(s, 1 H, H-8), 8.91 (s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 14.4 (CH3), 22.6 (C-4′),
24.7 (C-5′), 30.4 (C-CH3), 31.7 (C-3′), 68.8 (C-6′), 73.1 (OCH2), 75.8 (C≡C), 82.1 (C-2′),
89.9 (C≡C), 102.2 (C(OR)3), 135.2 (C-5), 139.9 (C-6), 143.6 (C-8), 150.9 (C-4), 152.4 (C-2).
IR (CHCl3): 2949 (w), 2883 (w), 1731 (w), 1582 (s), 1520 (w), 1491 (w), 1469 (w), 1442 (w),
1408 (w), 1397 (w), 1338 (s), 1323 (m), 1304 (w), 1275 (w), 1155 (w), 1088 (m), 1048 (s),
996 (s), 927 (w), 914 (w), 873 (w), 848 (w), 642 (w), 627 (w). HRMS (EI): m/z [M]+ calculated
for C18H20N4O4: 356.1485; found: 356.1500.
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6-Alkynylpurines Bearing Electronacceptor Substituents
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1-(2-(9-(tert-Butyloxycarbonyl)purin-6-yl)ethynyl)-4-methyl-
2,6,7-trioxabicyclo[2.2.2]octane (3i)
Method A, starting from 9-(tert-butyloxycarbonyl)-6-iodopurine (1d; 692 mg, 2.0 mmol) af-
forded after column chromatography (ethyl acetate–hexane, 2:1) 296 mg (60%) of
9-H-6-iodopurine and 47 mg (6%) of 3i as a pale yellow solid; m.p. > 100 °C (dec.). Method B,
reaction performed at laboratory temperature, starting from 9-(tert-butyloxycarbonyl)-6-iodo-
purine (1d; 346 mg, 1.0 mmol) afforded after column chromatography (ethyl acetate–
hexane, 2:1) 268 mg (72%) of 3i as white crystals; m.p. > 100 °C (dec.). 1H NMR (300 MHz,
CDCl3): 0.81 (s, 3 H, CH3), 1.64 (s, 9 H, C(CH3)3), 3.98 (s, 6 H, 3 × OCH2), 8.50 (s, 1 H,
H-8), 9.01 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3): 14.6 (CH3), 28.1 (C(CH3)3), 30.6
(C-CH3), 73.4 (CH2), 75.5 (C≡C), 87.8 (C(CH3)3), 90.8 (C≡C), 102.5 (C(OR)3), 135.9 (C-5),
141.0 (C-6), 144.6 (C-8), 145.7 (C=O), 151.3 (C-4), 154.5 (C-2). IR (ATR): 3478 (w), 3156 (w),
3124 (m), 3077 (s), 2992 (m), 2970 (m), 2937 (m), 2879 (s), 2686 (w), 2583 (w), 2444 (w),
1960 (w), 1796 (w), 1738 (s), 1706 (w), 1630 (w), 1575 (s), 1499 (m), 1459 (m), 1440 (m),
1384 (m), 1335 (s), 1309 (s), 1290 (m), 1262 (m), 1243 (m), 1190 (m), 1145 (m), 1130 (s),
1100 (m), 1041 (s), 974 (s), 893 (m), 869 (w), 840 (m), 806 (m), 770 (s), 752 (m), 721 (m),
667 (m). HRMS (EI): m/z [M]+ calculated for C17H20N4O3 (M – CO2): 328.1535; found:
328.1527.
9-(2′,3′,5′-Tri-O-acetyl-β-D-ribofuranosyl)-6-(2-(4-methyl-
2,6,7-trioxabicyclo[2.2.2]octan-1-yl)ethynyl)purine (3j)
Method A, starting from 9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)-6-iodopurine (1e; 378 mg,
0.75 mmol) gave after column chromatography (ethyl acetate–hexane, 2:1) 199 mg (50%) of
3j as a pale yellow amorphous solid. Method B, starting from 9-(2′,3′,5′-tri-O-acetyl-β-D-ribo-
furanosyl)-6-iodopurine (1e; 252 mg, 0.5 mmol) gave after column chromatography
(ethyl acetate–hexane, 2:1) 180 mg (68%) of 3j as a pale yellow amorphous solid. 1H NMR
(500 MHz, CDCl3): 0.85 (s, 3 H, CH3), 2.06 (s, 3 H, COCH3), 2.10 (s, 3 H, COCH3), 2.14 (s,
3 H, COCH3), 4.03 (s, 6 H, 3 × OCH2), 4.35–4.46 (m, 3 H, 2 × H-5′ + H-4′), 5.65 (t, J = 4.9,
1 H, H-3′), 5.94 (t, J = 5.4, 1 H, H-2′), 6.22 (d, J = 5.0, 1 H, H-1′), 8.25 (s, 1 H, H-8), 8.94 (s,
1 H, H-2). 13C NMR (CDCl3): 14.4 (CH3), 20.3 (COCH3), 20.5 (COCH3), 20.7 (COCH3), 30.4
(C-CH3), 62.9 (C-5′), 70.5 (C-3′), 73.0 (C-2′), 73.3 (OCH2), 75.6 (C≡C), 80.5 (C-4′), 86.4
(C-1′), 90.4 (C≡C), 102.4 (C(OR)3), 135.7 (C-5), 140.6 (C-6), 144.1 (C-8), 151.2 (C-4), 152.7
(C-2), 169.3 (C=O), 169.5 (C=O), 170.3 (C=O). IR (CHCl3): 2969 (w), 2947 (w), 2885 (w),
1751 (s), 1582 (m), 1493 (w), 1444 (w), 1409 (w), 1394 (w), 1374 (w), 1340 (m), 1142 (w),
1098 (w), 1050 (s), 999 (s), 924 (w), 894 (w), 823 (w), 638 (w). HRMS (EI): m/z [M]+ calcu-
lated for C24H26N4O10: 530.1649; found: 530.1649.
9-Benzyl-6-(2-methoxycarbonylethynyl)purine (3k)
A mixture of 1-(2-(9-benzylpurin-6-yl)ethynyl)-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane (3g;
200 mg, 0.138 mmol), TsOH (20 mg), THF (20 ml), methanol (20 ml) and water (0.2 ml) was
stirred at laboratory temperature for 3 h. The solution was then filtered through 5 g of an-
hydrous MgSO4 for the removal of water, the dissicant was washed with methanol (5 ml)
and to the combined filtrates it was added MeONa (2 ml of 0.1 M solution in MeOH). The
mixture was stirred at r.t. until the methanolysis of the intermediate ester was complete ac-
cording to TLC (ca. 1 h). After removal of the volatiles, the residue was column chromatog-
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1500
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
raphed on silica gel (ethyl acetate–hexane, 1:1). It was obtained 126 mg (78%) of 3k as a
white solid; m.p. 101–105 °C. 1H NMR (500 MHz, CDCl3): 3.88 (s, 3 H, CH3), 5.47 (s, 2 H,
CH2-Ph), 7.32–7.40 (m, 5 H, Ph), 8.16 (s, 1 H, H-8), 9.03 (s, 1 H, H-2). 13C NMR (125 MHz,
CDCl3): 47.6 (CH2), 53.1 (CH3), 78.7 (C≡C-CO), 86.0 (C≡C-CO), 127.9 (CHPh), 128.9 (CHPh),
129.2 (CHPh), 134.5 (CqPh), 135.1 (C-5), 138.6 (C-6), 146.2 (C-8), 152.0 (C-4), 152.7 (C-2),
153.2 (C=O). IR (CHCl3): 2956 (w), 1720 (s), 1581 (s), 1497 (w), 1455 (w), 1440 (m),
1403 (w), 1330 (m), 1303 (m), 1261 (m), 1149 (w), 1134 (w), 1080 (w), 967 (w), 889 (w),
848 (w), 646 (w). HRMS (EI): m/z [M]+ calculated for C16H12N4O2: 292.0960; found:
292.0964.
9-(β-D-Ribofuranosyl)-6-(3,3-diethoxyprop-1-yn-1-yl)purine (3l)
To the solution of 9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)-6-(3,3-diethoxyprop-1-yn-1-yl)-
purine (3e; 89 mg, 0.18 mmol) in dry methanol (10 ml), 0.01 M sodium methoxide in
methanol (1.8 ml, 0.1 equiv.) was added. After 24-h stirring at the laboratory temperature,
1
M AcOH in methanol (0.1 ml) was added and the solution was evaporated to dryness.
Column chromatography (ethyl acetate–methanol, 9:1) afforded 60 mg (90%) of 3l as a pale
yellow amorphous solid. 1H NMR (500 MHz, DMSO): 1.21 (t, J = 7.1, 6 H, CH3), 3.56–3.80
(m, 6 H, 2 × H-5′ + 2 × CH2CH3), 3.99 (m, 1 H, H-4′), 4.20 (m, 1 H, H-3′), 4.61 (m, 1 H,
H-2′), 5.08 (t, J = 5.5, 1 H, OH-5′), 5.23 (d, J = 5.1, 1 H, OH-3′), 5.54 (d, J = 5.8, 1 H, OH-2′),
5.72 (s, 1 H, CH(OEt)2), 6.05 (d, J = 5.3, 1 H, H-1′), 8.93 (s, 1 H, H-8), 8.95 (s, 1 H, H-2).
13C NMR (125 MHz, DMSO): 15.1 (CH3), 60.9 (CH2CH3), 61.1 (C-5′), 70.2 (C-3′), 73.9 (C-2′),
79.4 (C≡C), 85.7 (C-4′), 87.9 (C-1′), 91.0 (CH-C≡C), 92.6 (C≡C), 134.8 (C-5), 139.1 (C-6),
146.2 (C-8), 151.5 (C-4), 152.2 (C-2). IR (ATR): 3312 (m), 2976 (w), 2930 (w), 1585 (s),
1493 (w), 1445 (w), 1399 (w), 1332 (m), 1210 (m), 1090 (s), 1048 (s), 901 (w), 865 (w),
804 (w), 732 (w). HRMS (EI): m/z [M]+ calculated for C17H22N4NaO6 (MNa+): 401.1437;
found: 401.1456.
9-(β-D-Ribofuranosyl)-6-(2-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)-
ethynyl)purine (3m)
To the solution of 9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)- 6-(2-(4-methyl-2,6,7-trioxa-
bicyclo[2.2.2]octan-1-yl)ethynyl)purine (3j; 30 mg, 0.056 mmol) in dry methanol (3 ml),
0.01 M sodium methoxide in methanol (0.56 ml, 0.1 equiv.) was added. After 24-h stirring at
the laboratory temperature, 1 M AcOH in methanol (0.1 ml) was added and the solution was
evaporated to dryness. Column chromatography (ethyl acetate–methanol, 9:1) afforded
18 mg (78%) of 3m as white crystalline solid; m.p. 183–186 °C. 1H NMR (500 MHz, DMSO):
0.82 (s, 3 H, CH3), 3.55–3.61 (m, 1 H, H-5′), 3.66–3.72 (m, 1 H, H-5′), 3.99 (m, 1 H, H-4′),
4.01 (s, 6 H, OCH2), 4.19 (m, 1 H, H-3′), 4.6 (m, 1 H, H-2′), 5.08 (t, J = 5.5, 1 H, OH-5′), 5.23
(d, J = 5.1, 1 H, OH-3′), 5.54 (d, J = 5.8, 1 H, OH-2′), 6.04 (d, J = 5.4, H-1′), 8.95 (s, 1 H, H-8),
8.96 (s, 1 H, H-2). 13C NMR (125 MHz, DMSO): 13.5 (CH3), 29.8 (C-CH3), 61.0 (C-5′), 70.1
(C-3′), 72.4 (OCH2), 73.8 (C-2′), 75.5 (C≡C), 85.7 (C-4′), 87.8 (C-1′), 89.5 (C≡C), 101.6
(C(OR)3), 134.9 (C-5), 138.1 (C-6), 146.4 (C-8), 151.6 (C-4), 152.1 (C-2). IR (ATR): 3393 (m),
2925 (w), 2884 (w), 1584 (s), 1492 (w), 1442 (w), 1399 (m), 1334 (s), 1304 (m), 1245 (m),
1208 (m), 1190 (w), 1081 (w), 1043 (m), 988 (s), 867 (w), 725 (w). HRMS (EI): m/z [M]+
calculated for C18H20N4NaO7 (MNa+): 427.1230; found: 427.1243.
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6-Alkynylpurines Bearing Electronacceptor Substituents
1501
Diels–Alder Reaction of 6-Alkynylpurines with Cyclopentadiene. General Procedure
6-Alkynylpurine was dissolved in DCM (0.5 ml per 0.1 mmol of alkyne) and cyclo-
pentadiene (same volume) was added. The mixture was placed in a sealed glass tube and
heated at 60 °C for the period of time given in Table II. Volatiles were evaporated and the
residue was column chromatographed.
9-Benzyl-6-(3-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-dien-2-yl)purine (4k)
The reaction of compound 3k (20 mg, 0.07 mmol) with cyclopentadiene, after column chro-
matography (ethyl acetate–hexane, 1:1), afforded 25 mg (~100%) of 4k as a colorless oil.
1H NMR (300 MHz, CDCl3): 2.21 (m, 1 H, H-7′), 2.52 (m, 1 H, H-7′), 3.60 (s, 3 H, CH3), 4.11
(m, 1 H, H-3′ or H-6′), 4.34 (m, 1 H, H-3′ or H-6′), 5.43 (s, 2 H, CH2Ph), 7.04 (m, 2 H, H-4′ +
H-5′), 7.25–7.36 (m, 5 H, Ph), 8.00 (s, 1 H, H-8), 8.98 (s, 1 H, H-2). 13C NMR (75 MHz,
CDCl3): 47.5 (CH2Ph), 51.8, 54.0, 56.3, 72.9, 128.1 (CHPh), 128.8 (CHPh), 129.4 (CHPh),
131.4, 135.3, 142.9, 143.5, 144.5, 150.2, 151.9, 152.6, 153.6, 157.7, 166.5. IR (ATR):
3060 (w), 2998 (w), 2946 (w), 2871 (w), 1717 (s), 1622 (w), 1581 (s), 1498 (m), 1438 (m),
1405 (m), 1326 (s), 1294 (m), 1241 (s), 1215 (m), 1197 (m), 1153 (m), 1098 (m), 1048 (s),
1020 (w), 868 (w), 949 (w), 876 (w), 823 (w), 774 (w), 727 (m), 698 (w). HRMS (EI): m/z [M]+
calculated for C21H18N4O2: 358.1430; found: 358.1429.
9-Benzyl-6-(3-(diethoxymethyl)bicyclo[2.2.1]hepta-2,5-dien-2-yl)purine (4b)
The reaction of compound 3b (50 mg, 0.15 mmol) with cyclopentadiene, after column chro-
matography (ethyl acetate–hexane, 1:1), afforded 34 mg (56%) of 4b as a pale yellow viscous
liquid. Also, 16 mg (32%) of the starting material was recovered. 1H NMR (500 MHz,
CDCl3): 1.00 (t, J = 7.0, 3 H, CH3), 1.22 (t, J = 7.0, 3 H, CH3), 2.03 (d, J = 6.5, 1 H, H-7′),
2.18 (d, J = 6.5, 1 H, H-7′), 3.12 (m, 1 H, OCH2), 3.31 (m, 1 H, OCH2), 3.64 (m, 1 H, OCH2),
3.79 (m, 1 H, OCH2), 3.95 (s, 1 H, H-4′), 4.71 (s, 1 H, H-1′), 5.37 (d, J = 6.4, 2 H, CH2Ph),
6.24 (s, 1 H, CH(OEt)2), 6.83 (m, 1 H, H-5′ or H-6′), 6.93 (m, 1 H, H-5′ or H-6′), 7.22–7.32
(m, 5 H, Ph), 7.94 (s, 1 H, H-8), 8.83 (s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 15.1 (CH3),
15.4 (CH3), 47.2 (CH2Ph), 51.4 (C-4′), 53.6 (C-1′), 62.2 (CH2CH3), 62.9 (CH2CH3), 70.4
(C-7′), 98.0 (CH(OEt)2), 127.8 (CHPh), 128.5 (CHPh), 129.1 (CHPh), 130.6 (C-5), 135.2 (CqPh),
142.4 (C-5′ or C-6′), 142.8 (C-5′ or C-6′), 143.4 (C-8), 147.2 (C-2′ or C-3′), 152.2 (C-2), 152.3
(C-4), 153.6 (C-6), 160.9 (C-2′ or C-3′). IR (ATR): 3064 (w), 2973 (m), 2931 (m), 2870 (w),
1690 (w), 1625 (w), 1578 (s), 1566 (s), 1496 (m), 1448 (m), 1403 (m), 1372 (m), 1320 (s),
1247 (w), 1214 (m), 1137 (m), 1103 (w), 1058 (m), 996 (w), 888 (w), 812 (w), 770 (w),
728 (w), 698 (w). HRMS (ESI): m/z [M + Na]+ calculated for C24H26N4NaO2: 425.1953; found:
425.1964.
9-Benzyl-6-(3-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octane-1-yl)-
bicyclo[2.2.1]hepta-2,5-dien-2-yl)purine (4g)
The reaction of compound 3g (34 mg, 0.094 mmol) with cyclopentadiene, after column
chromatography (ethyl acetate–hexane, 1:1), afforded 40 mg (~100%) of 4g as a pale yellow
viscous oil. The product is prone to acid hydrolysis. Therefore, both TLC and column chro-
matography require non-acidic sorbent (alumina or Et3N pre-treated silica). Also, the CDCl3
for NMR must not be acidic, otherwise the compound degrades rapidly. 1H NMR (300 MHz,
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1502
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
CDCl3): 0.71 (s, 3 H, CH3), 2.04 (m, 1 H, H-7′), 2.43 (m, 1 H, H-7′), 3.77 (s, 6 H, 3 × OCH2),
3.94 (m, 1 H, H-1′ or H-4′), 4.03 (m, 1 H, H-1′ or H-4′), 5.42 (s, 2 H, CH2Ph), 7.01 (m, 1 H,
H-5′ or H-6′), 7.07 (m, 1 H, H-5′ or H-6′), 7.24–7.36 (m, 5 H, Ph), 7.98 (s, 1 H, H-8), 8.94 (s,
1 H, H-2). 13C NMR (75 MHz, CDCl3): 14.8 (CH3), 30.6 (C-CH3), 47.3 (CH2Ph), 52.9 (C-1′ or
C-4′), 56.9 (C-1′ or C-4′), 72.0 (C-7′), 72.7 (OCH2), 107.3 (C(OR)3), 127.9 (CHPh), 128.7
(CHPh), 129.3 (CHPh), 132.0 (C-5), 135.7 (CqPh), 143.3 (C-5′ or C-6′), 143.4 (C-5′ or C-6′),
143.8 (C-8), 148.4 (C-2′ or C-3′), 151.5 + 152.2 + 153.3 (C-2 + C-4 + C-6), 156.4 (C-2′ or
C-3′). IR (ATR): 2964 (w), 2933 (w), 2873 (w), 1663 (w), 1578 (m), 1497 (w), 1455 (w),
1395 (w), 1363 (w), 1348 (w), 1325 (m), 1293 (m), 1252 (w), 1214 (m), 1195 (w), 1148 (w),
1097 (m), 1036 (m), 969 (m), 747 (s), 723 (s), 698 (m). HRMS (EI): m/z [M]+ calculated for
C
25H24N4O3: 428.1848; found: 428.1835.
Reaction of 6-Alkynylpurines with Diazomethane. General Procedure
To a solution of 6-alkynylpurine in THF (1 ml per 0.1 mmol of alkyne) was added diazome-
thane (2 equiv., as 0.5 M solution in Et2O) and the mixture was stirred at r.t. for the period
of time given in Table II. Volatiles were evaporated and the residue was column chromatog-
raphed (unless stated otherwise).
9-Benzyl-6-(3-(methoxycarbonyl)pyrazol-4-yl)purine (5kA) and
9-Benzyl-6-(4-(methoxycarbonyl)pyrazol-3-yl)purine (5kB)
The reaction of compound 3k (208 mg, 0.71 mmol) with diazomethane, after column chro-
matography (ethyl acetate), afforded 217 mg (92%) of a white solid. The product turned out
to be a 3:1 mixture of regioisomers 5kA and 5kB. Repetitive chromatography on chromato-
tron (EtOAc–MeOH, 40:1) finally lead to a complete resolution of the regioisomers. There
were obtained 145 mg (61%) of 5kA and 42 mg (18%) of 5kB. The correct identities were as-
signed using NOE experiments.
5kA (higher RF). 1H NMR (300 MHz, CDCl3): 3.84 (s, 3 H, OCH3), 5.47 (s, 2 H, CH2),
7.30–7.40 (m, 5 H, Ph), 8.08 (s, 1 H, H-8), 8.34 (s, 1 H, CHpyr), 9.03 (s, 1 H, H-2), 12.95 (br s,
1 H, NH). 13C NMR (75 MHz, CDCl3): 47.4 (CH2Ph), 52.4 (CH3), 118.6, 128.0 (CHPh), 128.7
(CHPh), 129.2 (CHPh), 131.4, 135.0, 135.6, 139.5, 144.3, 150.1, 151.6, 152.6, 162.1 (C=O).
IR (ATR): 3137 (w), 2951 (w), 1726 (w), 1596 (m), 1496 (w), 1455 (w), 1402 (w), 1362 (w),
1324 (w), 1271 (w), 1215 (w), 1133 (w), 1067 (w), 945 (w), 915 (w), 851 (w), 808 (w), 726 (s),
696 (m). HRMS (EI): m/z [M]+ calculated for C17H14N6O2: 334.1178; found: 334.1180.
5kB (lower RF). 1H NMR (300 MHz, CDCl3): 3.83 (s, 3 H, CH3), 5.48 (s, 2 H, CH2), 7.35
(m, 5 H, Ph), 8.11 (s, 1 H, H-8 or CHpyr), 8.13 (s, 1 H, H-8 or CHpyr), 9.14 (s, 1 H, H-2),
12.42 (br s, 1 H, NH). 13C NMR (75 MHz, CDCl3): 47.8 (CH2Ph), 52.0 (CH3), 114.7, 128.2
(CHPh), 129.0 (CHPh), 129.5 (CHPh), 131.2, 135.0, 139.6, 144.4, 145.3, 147.2, 152.4, 153.2,
163.6 (C=O). IR (ATR): 2949 (w), 2923 (w), 1719 (s), 1597 (s), 1585 (s), 1496 (m), 1454 (m),
1436 (m), 1399 (m), 1363 (m), 1329 (s), 1282 (m), 1266 (s), 1210 (s), 1198 (s), 1135 (m),
1103 (w), 1074 (s), 1028 (w), 953 (m), 936 (m), 922 (m), 784 (m), 754 (m), 731 (s), 693 (m).
HRMS (EI): m/z [M]+ calculated for C17H14N6O2: 334.1178; found: 334.1183.
9-Benzyl-6-(4-(diethoxymethyl)pyrazol-3-yl)purine (5bB)
The reaction of compound 3b (34 mg, 0.1 mmol) with diazomethane, without chromatog-
raphy, afforded 38 mg (100%) of 5bB as a pale yellow powder, pure according to 1H and
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6-Alkynylpurines Bearing Electronacceptor Substituents
1503
13C NMR. 1H NMR (300 MHz, CDCl3): 1.24 (t, J = 7.3, 6 H, CH3), 3.65–3.82 (m, 4 H,
CH2CH3), 5.48 (s, 2 H, CH2Ph), 6.67 (s, 1 H, CH(OEt)2), 7.28–7.40 (m, 5 H, Ph), 7.92 (s, 1 H,
CHpyr), 8.08 (s, 1 H, H-8), 9.05 (s, 1 H, H-2), 12.75 (br s, 1 H, NH). 13C NMR (75 MHz,
CDCl3): 15.5 (CH3), 47.6 (CH2Ph), 62.2 (CH2CH3), 96.7 (CH(OEt)2), 122.6, 128.1 (CHPh),
129.0 (CHPh), 129.5 (CHPh), 129.8, 135.1, 137.2, 138.7, 144.7 (C-8), 147.0, 152.0, 153.2
(C-2). IR (ATR): 3158 (m), 2974 (m), 1659 (w), 1588 (s), 1550 (m), 1517 (w), 1485 (w),
1453 (m), 1368 (w), 1324 (m), 1214 (w), 1115 (m), 1059 (m), 955 (w), 820 (w), 728 (m),
696 (m). HRMS (EI): m/z [M]+ calculated for C20H22N6NaO2 (MNa+): 401.1702; found:
401.1712.
The identity of the compound was assigned using HMBC after previous transformation to
a corresponding dimethyl acetal 7.
Compound 5bB (20 mg, 0.05 mmol) and TsOH (3 mg) was dissolved in methanol (2 ml)
and the solution was stirred at room temperature for 1 h. Triethylamine (5 mg) was then
added and the reaction mixture was evaporated to dryness. The crude product was purified
by column chromatography (DCM–methanol, 20:1). There were obtained 16 mg (86%) of
white amorphous 7. 1H NMR (500 MHz, CDCl3): 3.45 (s, 6 H, CH3), 5.48 (s, 2 H, CH2), 6.59
(s, 1 H, CH(OMe)2), 7.29–7.40 (m, 5 H, Ph), 7.91 (s, 1 H, CHpyr), 8.09 (s, 1 H, H-8), 9.07 (s,
1 H, H-2), 12.65 (br s, 1 H, NH). 13C NMR (125 MHz, CDCl3): 47.4 (CH2Ph), 53.2 (CH3),
98.0 (CH(OMe)2), 121.1 (C-5′), 127.9 (CHPh), 128.8 (CHPh), 129.2 (CHPh), 129.5 (C-5), 134.8
(CqPh), 136.7 (C-4′), 138.9 (C-3′), 144.5 (C-8), 146.3 (C-6), 151.9 (C-4), 153.2 (C-2). IR (ATR):
3166 (s), 3029 (w), 2938 (w), 2831 (w), 1751 (w), 1671 (w), 1588 (s), 1546 (w), 1517 (w),
1485 (w), 1454 (w), 1438 (w), 1367 (w), 1324 (m), 1281 (w), 1217 (m), 1198 (w), 1167 (w),
1075 (s), 966 (s), 930 (w), 885 (m), 827 (m), 809 (m), 738 (w), 732 (s), 696 (m). HRMS (EI):
calculated for C18H18N6NaO2 (MNa+): 373.1389; found: 373.1376.
9-Benzyl-6-(4-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octane-1-yl)pyrazol-3-yl)purine (5gB)
The reaction of compound 3g (36 mg, 0.1 mmol) with diazomethane, without chromatog-
raphy, afforded 40 mg (100%) of 5gB as a white amorphous solid. The compound exists in
a DMSO solution as a mixture of two tautomers A and B in approximately 6:5 ratio (accord-
ing to 1H NMR). The structure of the 5gB was confirmed after methanolysis by comparison
of the NMR spectra of obtained 5kB with the spectra of authentic sample of 5kB. 1H NMR
(300 MHz, DMSO): 0.69 (m, 3 H, CH3), 3.75 (m, 6 H, OCH2), 5.51 (m, 2 H, CH2Ph), 7.35 (m,
5 H, Ph), 7.6 (s, CHB), 7.84 (s, CHA), 8.62 (s, CHA), 8.78 (s, CHB), 8.89 (s, CHA), 8.97 (s,
CHB), 13.16 (bs, NHA), 13.32 (bs, NHB). IR (ATR): 3118 (m), 2927 (m), 2882 (m), 1762 (w),
1601 (s), 1565 (m), 1526 (m), 1456 (w), 1434 (w), 1395 (m), 1364 (m), 1329 (m), 1250 (w),
1193 (m), 1148 (w), 1074 (s), 1048 (w), 1005 (s), 922 (m), 885 (w), 834 (w), 725 (m),
695 (m), 677 (w). HRMS (EI): m/z [M]+ calculated for C21H20N6NaO3 (MNa+): 427.1495;
found: 427.1501.
9-Benzyl-6-(4-phenylpyrazol-3-yl)purine (5nB)
The reaction of compound 3n (62 mg, 0.2 mmol) with diazomethane, after column chroma-
tography (DCM–MeOH, 20:1), afforded 41 mg (66%) of the starting material and 19 mg
(27%) of white amorphous 5nB. The compound exists in a DMSO solution as a mixture of
two tautomers in ca. 1:1 ratio (according to 1H NMR). The identity of the compound was
confirmed after N-methylation and identification of two obtained N-methyl derivatives 8a
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1504
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
and 8b. 1H NMR (500 MHz, DMSO): 5.51 (s, 2 H, CH2), 7.28–7.45 (m, 8 H, Ph), 7.63–7.68
(m, 2 H, Ph), 8.7 (m, CH), 13.45 (s, NH), 13.53 (s, NH). IR (ATR): 3110 (m), 2906 (m),
1594 (s), 1490 (m), 1459 (m), 1443 (m), 1401 (m), 1342 (s), 1327 (s), 1249 (m), 1216 (m),
1142 (m), 1067 (w), 912 (w), 848 (w), 772 (w), 730 (m), 698 (m). HRMS (EI): m/z [M]+ calcu-
lated for for C21H16N6Na (MNa+): 375.1334; found: 375.1327.
9-Benzyl-6-(4-phenyl-1-methylpyrazol-5-yl)purine (8a) and
9-Benzyl-6-(4-phenyl-1-methylpyrazol-3-yl)purine (8b)
To a solution of compound 5nB (10 mg, 0.028 mmol) in DMSO (0.5 ml) was added
iodomethane (40 mg, 10 equiv.) and anhydrous K2CO3 (46 mg, 12 equiv.) and the suspen-
sion was stirred at r.t. for 17 h. It was then dilluted with EtOAc (20 ml) and washed with
5% NaCl (3 × 20 ml). The organic layer was dried with MgSO4 and evaporated to dryness.
There were obtained 10 mg (~100%) of a pale yellow wax. According to 1H NMR, the mix-
ture contained 8a and 8b in a 7:6 ratio. Column chromatography (EtOAc–hexane, 2:1) af-
forded 4 mg (~40%) of pure 8a (RF 0.5) and 4 mg (~40%) of pure 8b (RF 0.37), both isomers
as colourless amorphous solids. The assignment of the corresponding structures was possible
using 1H(13C) HMBC and 1H NOE experiments.
8a (higher RF). 1H NMR (500 MHz, CDCl3): 3.81 (s, 3 H, NCH3), 5.42 (s, 2 H, CH2Ph),
7.26–7.36 (m, 6 H, Ph), 7.41–7.47 (m, 4 H, Ph), 7.98 (s, 1 H, H-8), 8.62 (s, 1 H, H-2), 8.78 (s,
1 H, H-3′). 13C NMR (125 MHz, CDCl3): 37.4 (NCH3), 47.1 (CH2Ph), 116.4 (C-4′), 127.8
(CHPh), 128.3 (CHPh), 128.5 (CHPh), 128.8 (CHPh), 129.1 (CHPh), 130.3 (CHPh), 130.4 (CqPh),
130.6 (C-5), 135.4 (CqPh), 141.8 (C-3′), 143.2 (C-8), 143.6 (C-5′), 151.3 (C-4 or C-6), 151.5
(C-4 or C-6), 152.5 (C-2). IR (ATR): 2921 (s), 2851 (m), 1581 (m), 1486 (m), 1465 (m),
1451 (m), 1435 (m), 1382 (m), 1325 (m), 1255 (w), 1204 (m), 1182 (w), 1072 (w), 966 (w),
915 (w), 850 (m), 809 (w), 773 (w), 729 (m), 693 (w), 659 (w). HRMS (ES): m/z [M]+ calcu-
lated for C22H18N6Na (MNa+): 389.1491; found: 389.1489
8b (lower RF). 1H NMR (500 MHz, CDCl3): 4.05 (s, 3 H, CH3), 5.46 (s, 2 H, CH2),
7.31–7.40 (m, 8 H, Ph), 7.73 (m, 2 H, Ph), 8.00 (s, 1 H, H-8), 8.67 (s, 1 H, H-5′), 8.81 (s, 1 H,
H-2). 13C NMR (125 MHz, CDCl3): 39.1 (CH3), 47.2 (CH2), 115.0 (C-4′), 127.8 (CHPh), 127.9
(CHPh), 128.0 (CHPh), 128.6 (CHPh), 129.2 (CHPh), 129.3 (CHPh), 130.1 (C-5), 133.6 (CqPh),
135.3 (CqPh), 135.5 (C-5′), 143.3 (C-8), 151.4 (C-4 or C-6), 151.5 (C-4 or C-6), 151.9 (C-3′),
152.7 (C-2). IR (ATR): 2922 (s), 2852 (s), 1580 (s), 1539 (m), 1495 (m), 1454 (m), 1395 (m),
1326 (m), 1249 (m), 1183 (m), 1134 (w), 1074 (w), 915 (w), 849 (w), 808 (w), 772 (w),
724 (m), 695 (m), 644 (w). HRMS (ES): m/z [M]+ calculated for C22H18N6Na (MNa+):
389.1491; found: 389.1501.
Reaction of 6-Alkynylpurines with Phenyl Azide. General Procedure
To a solution of 6-alkynylpurine in DMF (2 ml per mmol of alkyne) was added phenyl azide
(10 equiv.) and the mixture was stirred at 100 °C for the period of time given in Table II.
The reaction mixture was subsequently dilluted with ethyl acetate (triple volume) and
washed 3× with equal volume of 5% brine. The organic layer was dried with MgSO4 and
after the evaporation of volatiles the residue was column chromatographed.
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6-Alkynylpurines Bearing Electronacceptor Substituents
1505
9-Benzyl-6-(3-phenyl-4-methoxycarbonyl-1,2,3-triazol-5-yl)purine (6kA) and
9-Benzyl-6-(1-phenyl-5-methoxycarbonyl-1,2,3-triazol-4-yl)purine (6kB)
The reaction of compound 3k (47 mg, 0.16 mmol) with phenyl azide, after column chroma-
tography (DCM–methanol, 40:1), afforded 56 mg (85%) of the mixture of 6kA and 6kB.
Chromatography on chromatotron (DCM–methanol, 40:1) yielded 20 mg (30%) of 6kA and
36 mg (55%) of 6kB as colourless amorphous solids.
6kA (higher RF). 1H NMR (500 MHz, CDCl3): 3.81 (s, 3 H, CH3), 5.53 (s, 2 H, CH2),
7.32–7.42 (m, 5 H, Ph), 7.58 (m, 3 H, Ph), 7.65 (m, 2 H, Ph), 8.19 (s, 1 H, H-8), 9.11 (s, 1 H,
H-2). 13C NMR (125 MHz, CDCl3): 47.4 (CH2), 53.2 (CH3), 125.0 (CHPh), 127.8 (CHPh), 128.7
(CHPh), 129.2 (CHPh), 129.4 (CHPh), 130.2 (CHPh), 130.3 (C-5′), 131.0 (C-5), 135.0 (CqPh-Bn),
136.1 (CqPh-triaz.), 143.8 (C-4′), 145.4 (C-8), 147.1 (C-6), 152.6 (C-4), 152.7 (C-2), 160.3
(C=O). IR (ATR): 3064 (w), 3033 (w), 2993 (w), 2952 (w), 1740 (s), 1595 (s), 1547 (w), 1498 (s),
1455 (m), 1403 (w), 1376 (w), 1328 (s), 1285 (m), 1244 (m), 1221 (m), 1202 (m), 1170 (m),
1077 (w), 1007 (w), 955 (w), 923 (w), 859 (w), 810 (w), 760 (m), 728 (m), 694 (m), 645 (m),
605 (w), 554 (w). HRMS (ES): m/z [M]+ calculated for C22H17N7NaO2 (MNa+): 434.1341;
found: 434.1342.
6kB (lower RF). 1H NMR (500 MHz, CDCl3): 3.86 (s, 3 H, CH3), 5.47 (s, 2 H, CH2),
7.33–7.44 (m, 10 H, Ph), 8.07 (s, 1 H, H-8), 9.04 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3):
47.7 (CH2), 52.3 (CH3), 125.1 (CHPh), 128.2 (CHPh), 128.9 (CHPh), 129.2 (CHPh), 129.3
(CHPh), 129.8 (CHPh), 133.3 (C-5), 134.4 (CqPh-Bn), 135.3 (C-5′), 135.8 (CqPh-triaz.), 139.3
(C-4′), 145.2 (C-6), 146.0 (C-8), 152.2 (C-4), 152.4 (C-2), 160.9 (C=O). IR (ATR): 3065 (w),
2952 (w), 2849 (w), 1730 (m), 1592 (m), 1558 (m), 1498 (s), 1453 (m), 1400 (w), 1367 (w),
1328 (s), 1213 (s), 1169 (m), 1085 (m), 1005 (m), 920 (m), 818 (m), 791 (m), 751 (s), 727 (s),
692 (s), 643 (m), 566 (w), 532 (w), 510 (w). HRMS (ES): m/z [M]+ calculated for
C
22H17N7NaO2 (MNa+): 434.1341; found: 434.1349.
9-Benzyl-6-(1-phenyl-4-(diethoxymethyl)-1,2,3-triazol-5-yl)purine (6bA) and
9-Benzyl-6-(1-phenyl-5-(diethoxymethyl)-1,2,3-triazol-4-yl)purine (6bB)
The reaction of compound 3b (101 mg, 0.3 mmol) with phenyl azide, after column chroma-
tography (DCM–methanol. 80:1), afforded 128 mg (94%) of the mixture of 6bA and 6bB.
Chromatography on chromatotron (DCM–methanol, 80:1) yielded 37 mg (27%) of 6kA and
91 mg (67%) of 6kB as colourless amorphous solids.
6bA (lower RF). 1H NMR (500 MHz, CDCl3): 1.05 (t, J = 7.0, 6 H, CH3), 3.48–3.55 (m, 2 H,
CH2CH3), 3.67–3.73 (m, 2 H, CH2CH3), 5.53 (s, 2 H, CH2Ph), 6.78 (s, 1 H, CH(OEt)2),
7.32–7.40 (m, 5 H, Ph), 7.48–7.53 (m, 3 H, Ph), 7.74–7.78 (m, 2 H, Ph), 8.26 (s, 1 H, H-8),
9.05 (s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 14.8 (CH3), 47.3 (CH2Ph), 63.7 (OCH2), 95.4
(CH(OEt)2), 126.2 (CHPh), 127.7 (CHPh), 128.4 (CHPh), 128.6 (CHPh), 129.1 (CHPh), 129.3
(CHPh), 130.7 (C-5), 135.1, 136.6, 137.9, 141.8, 145.4 (C-8), 148.9 (C-6), 152.1 (C-2), 152.6
(C-4). IR (ATR): 3063 (w), 2976 (m), 2928 (m), 1717 (w), 1596 (s), 1546 (m), 1500 (s), 1454 (m),
1404 (w), 1380 (m), 1326 (s), 1257 (m), 1210 (m), 1191 (m), 1117 (s), 1062 (s), 1028 (m),
931 (m), 863 (w), 811 (w), 764 (m), 728 (m), 693 (m). HRMS (ESI): m/z [M + Na]+ calculated
for C25H25N7NaO2: 478.1967; found: 478.1980.
6bB (higher RF). 1H NMR (300 MHz, CDCl3): 0.91 (t, J = 7.2, 6 H, CH3), 3.44–3.66 (m, 4 H,
CH2CH3), 5.43 (s, 2 H, CH2Ph), 6.06 (s, 1 H, CH), 7.22–7.38 (m, 10 H, Ph), 8.04 (s, 1 H,
H-8), 8.93 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3): 15.1 (CH3), 47.7 (CH2Ph), 61.3 (OCH2),
96.2 (CH(OEt)2), 125.2 (CHPh), 128.3 (CHPh), 129.0 (CHPh), 129.3 (CHPh), 129.39 (CHPh),
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

1506
Křováček, Dvořáková, Votruba, Císařová, Dvořák:
129.44 (CHPh), 130.6, 133.2, 134.9, 136.9, 145.7, 146.5, 146.8, 152.2, 152.5. IR (ATR):
3394 (w), 3379 (m), 3062 (w), 3030 (w), 2927 (w), 2858 (w), 1693 (w), 1600 (s), 1572 (s),
1543 (m), 1502 (m), 1454 (m), 1448 (m), 1376 (w), 1328 (m), 1288 (w), 1227 (m), 1188 (m),
1146 (m), 1106 (m), 1079 (m), 1026 (m), 991 (w), 941 (m), 821 (w), 791 (w), 759 (w), 733 (m),
694 (w). HRMS (ESI): m/z [M + Na]+ calculated for C25H25N7NaO2: 478.1967; found:
478.1969.
9-Benzyl-6-(1-phenyl-5-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octane-1-yl)-
1,2,3-triazol-4-yl)purine (6gA) and 9-Benzyl-6-(1-phenyl-4-(4-methyl-
2,6,7-trioxabicyclo[2.2.2]octane-1-yl)-1,2,3-triazol-5-yl)purine (6gB)
The reaction of compound 3g (72 mg, 0.2 mmol) with phenyl azide, after column chroma-
tography (DCM–ethyl acetate, 1:1), afforded 8 mg (11%) of the starting material and 78 mg
(82%) of a mixture of two regioisomers 6gA and 6gB. Repeated chromatography on chro-
matotron (hexane–ethyl acetate, 2:1 with 3% MeOH and 2% Et3N) yielded 25 mg (26%) of
6gA and 36 mg (38%) of 6gB as colourless waxes, prone to acid-catalyzed hydrolysis.
6gA (higher RF). 1H NMR (500 MHz, CDCl3): 0.73 (s, 3 H, CH3), 3.77 (s, 6 H, OCH2), 5.51
(s, 2 H, CH2Ph), 7.33–7.40 (m, 5 H, PhBn), 7.47–7.52 (m, 3 H, Phtriaz), 7.69–7.73 (m, 2 H,
Phtriaz), 8.18 (br s, 1 H, H-8), 9.14 (s, 1 H, H-2). 13C NMR (125 MHz, CDCl3): 14.14 (CH3),
30.5 (C-CH3), 47.3 (CH2Ph), 72.6 (OCH2), 105.1 (C(OR)3), 126.0 (CHPh), 127.8 (CHPh), 128.3
(CHPh), 128.6 (CHPh), 129.1 (CHPh), 129.2 (CHPh), 132.3 (C-5), 135.1 (CqPh-Bn), 138.1
(CqPh-triaz.), 141.6 (C-4′ or C-5′), 145.1 (C-8), 149.6 (C-6), 152.0 (C-4), 152.3 (C-2). IR (ATR):
3063 (w), 2966 (w), 2935 (w), 2881 (w), 1595 (m), 1583 (m), 1563 (m), 1498 (m), 1469 (w),
1456 (m), 1396 (w), 1376 (w), 1350 (w), 1328 (m), 1304 (w), 1238 (w), 1209 (w), 1192 (m),
1167 (m), 1064 (m), 1047 (m), 1022 (s), 998 (s), 923 (w), 854 (w), 814 (w), 755 (s), 729 (m),
693 (m). HRMS (ESI+): m/z [M + Na]+ calculated for C26H23N7NaO3: 504.1760; found:
504.1752.
6gB (lower RF). 1H NMR (500 MHz, CDCl3): 0.80 (s, 3 H, CH3), 3.93 (s, 6 H, OCH2), 5.44
(s, 2 H, CH2Ph), 7.24–7.40 (m, 10 H, 2 × Ph), 7.97 (s, 1 H, H-8), 9.06 (s, 1 H, H-2). 13C NMR
(125 MHz, CDCl3): 14.4 (CH3), 30.6 (C-CH3), 47.4 (CH2Ph), 72.9 (OCH2), 105.5 (C(OR)3),
125.1 (CHPh), 127.8 (CHPh), 128.7 (CHPh), 128.9 (CHPh), 129.1 (CHPh), 129.2 (CHPh), 130.3
(C-4′ or C-5′), 133.4 (C-5), 134.8 (CqPh-Bn), 136.4 (CqPh-triaz), 144.2 (C-4′ or C-5′), 145.3 (C-8),
146.9 (C-6), 151.9 (C-4), 152.2 (C-2). IR (ATR): 3065 (w), 2933 (w), 2879 (w), 1617 (w),
1585 (m), 1568 (m), 1499 (m), 1456 (w), 1431 (w), 1399 (m), 1351 (w), 1329 (m), 1226 (m),
1195 (m), 1093 (w), 1075 (m), 1047 (m), 1016 (s), 994 (s), 922 (w), 850 (w), 813 (w), 753 (s),
728 (m), 693 (m). HRMS (ESI+): m/z [M + Na]+ calculated for C26H23N7NaO3: 504.1760;
found: 504.1745.
9-Benzyl-6-(1,5-diphenyl-1,2,3-triazol-4-yl)purine (6nA) and
9-Benzyl-6-(1,4-diphenyl-1,2,3-triazol-5-yl)purine (6nB)
The reaction of compound 3n (93 mg, 0.3 mmol) with phenyl azide, after column chroma-
tography (DCM–ethyl acetate, 4:1), afforded 18 mg (19%) of the starting material and
103 mg (80%) of the mixture of regioisomers 6nA and 6nB. Separation on chromatotron
(DCM–ethyl acetate, 4:1) yielded 52 mg (40%) of 6nA as a white powder and 51 mg (40%) of
6nB as a colourless wax. Crystalization of 6nA (ethyl acetate) afforded colourless crystals;
m.p. 192.2–192.5 °C.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

6-Alkynylpurines Bearing Electronacceptor Substituents
1507
6nA (lower RF). 1H NMR (300 MHz, CDCl3): 5.46 (s, 2 H, CH2), 7.21–7.40 (m, 15 H, 3 ×
Ph), 8.12 (s, 1 H, H-8), 8.85 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3): 47.5 (CH2Ph), 125.8,
127.0, 128.0, 128.5, 128.8, 129.3, 129.5, 129.6, 130.9, 131.6, 135.4, 136.5, 138.3, 141.6,
145.5, 149.2, 152.6. IR (ATR): 3380 (w), 3062 (w), 2920 (w), 2850 (w), 1591 (s), 1548 (m),
1496 (s), 1453 (m), 1400 (m), 1325 (s), 1216 (w), 1189 (m), 1149 (w), 1069 (w), 996 (w),
923 (m), 857 (m), 812 (w), 749 (s), 727 (s), 694 (s), 664 (m), 645 (m), 609 (m), 554 (w).
HRMS (EI+): calculated for C26H19N7: 429.1702; found: 429.1713.
6nB (higher RF). 1H NMR (300 MHz, CDCl3): 5.40 (s, 2 H, CH2), 7.20–7.41 (m, 13 H, Ph),
7.64–7.70 (m, 2 H, Ph), 7.87 (s, 1 H, H-8), 9.07 (s, 1 H, H-2). 13C NMR (75 MHz, CDCl3):
47.7 (CH2Ph), 125.1, 127.9, 128.1, 128.6, 128.7, 128.8, 129.1, 129.27, 129.32, 129.5, 130.6,
133.3, 134.7, 137.0, 145.9, 147.4, 147.7, 152.5, 153.0. IR (ATR): 3064 (w), 3006 (w), 1599 (m),
1553 (w), 1499 (m), 1456 (w), 1400 (w), 1366 (w), 1328 (m), 1250 (w), 1211 (w), 1159 (w),
1145 (w), 1073 (w), 1001 (m), 921 (w), 851 (w), 813 (w), 754 (s), 726 (m), 696 (s), 644 (w),
572 (w), 517 (w). HRMS (EI+): calculated for C26H19N7: 429.1702; found: 429.1706.
This project was supported by the Research Centre “The Structure and Synthetic Applications of
Transition Metal Complexes-LC06070” of the Ministry of Education, Youth and Sports of the Czech
Republic and by the Czech Science Foundation (Grant No. 203/09/1552).
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Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 12, pp. 1487–1507

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