Synthesis of 9-halogenated 9-deazaguanine N7-(2′- deoxyribonucleosides)

Article abstract of DOI:10.1002/hlca.200490224

The syntheses of N-glycosylated 9-deazaguanine 1a as well as of its 9-bromo and 9-iodo derivatives 1b,c are described. The regioselective 9-halogenation with N-bromosuccinimide (NBS) and N-iodosuccinimide (NIS) was accomplished at the protected nucleobase

Full text of DOI:10.1002/hlca.200490224

Helvetica Chimica Acta Vol. 87 (2004)
2507
Synthesis of 9-Halogenated 9-Deazaguanine
N7-(2'-Deoxyribonucleosides)
by Frank Seela*, Khalil I. Shaikh, Thomas Wiglenda, and Peter Leonard
Laboratorium f¸r Organische und Bioorganische Chemie, Institut f¸r Chemie, Universit‰t Osnabr¸ck,
Barbarastrasse 7, D-49069 Osnabr¸ck
and Center for Nanotechnology (CeNTech), Gievenbecker Weg 11, D-48149 M¸nster
(phone: ‡49-(0)541-969-2791; fax: ‡49-(0)541-969-2370; e-mail: Frank.Seela@uni-osnabrueck.de)
The syntheses of N7-glycosylated 9-deazaguanine 1a as well as of its 9-bromo and 9-iodo derivatives 1b,c are
described. The regioselective 9-halogenation with N-bromosuccinimide (NBS) and N-iodosuccinimide (NIS)
was accomplished at the protected nucleobase 4a (2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-3-
[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one). Nucleobase-anion glycosylation of 4a c with 2-
deoxy-3,5-di-O-(p-toluoyl)-a-d-erythro-pentofuranosyl chloride (5) furnished the fully protected intermediates
6a c (Scheme 2). They were deprotected with 0.01m NaOMe yielding the sugar-deprotected derivatives 8a
c
(Scheme 3). At higher concentrations (0.1m NaOMe), also the pivaloyloxymethyl group was removed to give
7a c, while conc. aq. NH3 solution furnished the nucleosides 1a c. In D2O, the sugar conformation was always
biased towards S (67 61%).
Introduction.
In the following, the synthesis of the 9-deazaguanine N7-(2'-
deoxyribonucleosides) 1a c is reported (purine numbering is used throughout, except
for the Tables and the Exper. Part). The parent guanine 2'-deoxyribonucleoside 2 as
well as the adenine 2'-deoxyribonucleoside 3 have been shown to form stable base pairs
in duplex DNA [1][2] as well as in DNA triplexes [3 6]. Because of their unusual
glycosylation site, these nucleosides show altered base-pairing behavior compared to
the canonical DNA constituents dA and dG.
While compound 3 has already been incorporated into oligonucleotides [1],
compound 2 has been introduced in duplex and triplex DNA [2 6]. Triplexes
containing the 2'-deoxyguanosine derivative 2 show high third-strand-binding affinity
specific to the dG ¥ dC base pair. The base triplet formed between compound 2 and dG ¥
¹ 2004Verlag Helvetica Chimica Acta AG, Z¸rich

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Helvetica Chimica Acta Vol. 87 (2004)
‡
dC is structurally related to the dCH ¥ dG ¥ dC base triplet. Contrary to the latter being
stable only under acidic conditions, the 2 ¥ dG ¥ dC triplet is stable in neutral medium.
Experiments with halogenated derivatives of 7-deazapurine nucleosides [7 10] or
8-aza-7-deazapurine nucleosides have shown that halogen substituents stabilize the
DNA duplex structure [11][12]. As the equivalent position of the unusually
glycosylated nucleoside 2 is N(9), halogen substituents cannot be introduced in a
purine nucleoside. Therefore, 9-deazaguanine was chosen as nucleobase which is
expected to be accessible for halogenation at the 9-position. The 9-alkyl-substituted
derivatives of 9-deazaguanine show biological activity as purine nucleoside phosphor-
ylase inhibitors [13 15]. Concerning the duplex and triplex stability, the 9-position is
the only favorable site providing steric freedom when the nucleoside is a part of the
second or third strand of DNA. For compound 1a, a synthesis was already described
[16]. Since this route was found to be laborious for halogenated derivatives, we
now describe a protocol for the synthesis of the 9-deazaguanine N7-(2'-deoxyribo-
nucleoside) 1a that includes the 9-bromo- and 9-iodonucleosides 1b,c. It is sup-
posed that compounds 1b,c will stabilize triplex DNA compared to the nonhaloge-
nated 1a. Preliminary results of this work have been reported in a short communication
[17].
Results and Discussion. 1. Synthesis and Characterization. The N7-glycosylated 9-
deaza-2'-deoxyguanosine 1a was already synthesized by glycosylation of 2,6-dichloro-
9-deazaguanine followed by nucleophilic displacement with aqueous NaOH solution
and methanolic ammonia [16]. Later, a synthetic route was reported for the
preparation of the protected 9-deazaguanine base 4a [18]. Also an improved protocol
was described [19] which in our hands did not lead to the expected compound;
methylation of the heterocycle was observed at elevated temperatures. As compound
4a is fully protected and carries protecting groups suitable for the glycosylation reaction
as well as for oligonucleotide synthesis, it was used as starting material for the synthesis
of 9-deazapurine 2'-deoxyribonucleoside 1a and its 9-halogenated derivatives 1b,c.
From findings on 7-deazaguanine nucleosides, it was also anticipated that regioselective
halogenation at position 9 will be possible when the amino group of the base is
protected. Two synthetic routes were chosen for the synthesis of compounds 1b,c: i) the
halogenation of the nucleobase followed by glycosylation and ii) the halogenation of
the fully protected nucleoside.
On the first route, the fully protected key intermediate 4a was treated with an
excess of N-bromosuccinimide (NBS) or N-iodosuccinimide (NIS) in dichloroethane
at room temperature. The 9-bromo and 9-iodo derivatives 4b,c were formed in a
regioselective way in yields of 90 95% (Scheme 1 and Exper. Part). Neither formation
of another regioisomer nor of 7,8-dihalogenated derivatives were detected by TLC.
Then, the stereoselective nucleobase-anion glycosylation was applied to compound 4a
as well as to the 9-halogenated derivatives 4b,c. The reaction was performed with the
protected nucleobases dissolved in MeCN and 2-deoxy-3,5-di-O-(p-totuoyl)-a-d-
erythro-pentofuranosyl chloride (5); powdered KOH/TDA-1 (ˆtris[2-(2-methoxy-
ethoxy)ethyl]amine) was used to generate the nucleobase anion (Scheme 2). The fully
protected N7-b-d-nucleosides 6a c were obtained in a stereoselective way. They were

Helvetica Chimica Acta Vol. 87 (2004)
2509
Scheme 1
Scheme 2
purified by flash column chromatography (FC) yielding 6a (57%), 6b (75%), and 6c
(87%) (see Exper. Part).
The halogenation reaction was also performed with the fully protected nucleoside
6a. A suspension of 6a was treated with NBS or NIS in dichloroethane at room
temperature furnishing the halogenated nucleosides 6b,c in 58 and 24% yield,
respectively (see Scheme 2 and Exper. Part). As the yield of the halogenation was
low on the nucleoside level compared to that on the nucleobase, this route was
abandoned.
The deprotection of 6a c was carried out with 0.1m NaOMe/MeOH at room
temperature for 1 h resulting in the removal of the p-toluoyl groups protecting the
sugar moiety as well as of the pivaloyloxymethyl group at the nucleobase (Scheme 3).

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Scheme 3
The [(dimethylamino)methylidene]amino protecting groups are stable under these
conditions. Compounds 7a c were isolated in yields of 84% for 7a, 94% for 7b, and
87% for 7c. When the deprotection of 6a c was performed with 0.01m NaOMe/MeOH
at room temperature for 1 h, only the p-toluoyl groups were removed furnishing
nucleosides 8a c in good yields (72% for 1a, 68% for 1b, and 73% for 1c). Both
partially base-protected nucleosides 7a c as well as fully base-protected nucleosides
8a c can be used for further manipulation towards the synthesis of phosphoramidites.
To test the suitability of the protecting groups, their hydrolysis was studied. The
complete removal of all protecting groups of 7a c was accomplished in 25% aqueous
NH3 solution in a sealed vessel at 608 for 16 h furnishing the nucleosides 1a c in almost
quantitative yields (see Scheme 3 and Exper. Part).
The nucleosides described above as well as all synthetic intermediates were
characterized by 1H- and 13C-NMR spectra and elemental analyses (Table 1 and Exper.
Part). The position of halogenation and the configuration of the anomeric center were
1
confirmed by the H-NOE difference spectra of 1a c (Table 2 and discussion below
the Table). According to a calibration graph [20], compounds 1a c prefer the high-syn
conformation, which is reported for other 2'-deoxynucleosides [20][21]. Estimation of
syn- and anti-conformer populations according to [20] gave an anti-rotamer population
of 55% for 1a, 65% for 1b, and 75% for 1c. This is in line with earlier findings reported
for 7-deazapurine 2'-deoxyribonucleosides [22][23].

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Table 1. 13C-NMR Chemical Shifts [ppm] of 9-Deazapurine Nucleosides and Synthetic Precursors Measured in
(D6)DMSO at 238
a
C(2)a) C (4 ) ) C(4a)a) C(6)a) C(7)a) C(7a)a) Me MeN NˆCH C(1') C(2') C(3') C(4') C(5') CH2O CˆO
C(2)b) C(6)b) C(5)b) C(8)b) C(9)b) C(4)b) (Piv)
4a 152.9 154.0 113.4
b 153.6 153.8 113.4
c 153.4 153.6 113.7
6a 153.6 154.1 112.7
b 153.7 154.2 112.7
c 153.6 154.0 112.9
7a 154.4 155.0 113.8
b 154.7 155.0 113.8
c 154.7d) 154.8d) 114.0
8a 153.4 154.0 112.5
b 153.7d) 153.9d) 112.4
c 153.7d) 153.8d) 112.8
1a 150.8 154.2 111.6
b 151.5 153.8 111.4
c 151.3 153.8 111.7
128.2 101.7 144.0
26.6 34.2 156.6
26.6 34.4 156.7
26.7 34.5 156.7
26.7 34.4 156.9 85.4 38.3 74.9 81.1 64.2 65.3 176.7
26.6 34.5 156.8 85.6
26.6 34.5 156.8 85.6 38.2 74.9 81.4 64.1 65.4 176.6
34.4 157.0 85.3 41.0 70.7 87.1 61.8
34.4 157.0 85.5 41.1 70.4 87.3 61.5
34.5 157.0 85.6 41.2 70.5 87.3 61.6
26.7 34.4 156.8 85.2 41.0 70.6 87.2 61.7 65.4 176.7
26.6 34.5 156.8 85.5 41.1 70.3 87.3 61.4 65.3 176.6
26.7 34.6 156.9 85.7 41.3 70.4 87.4 61.5 65.5 176.7
85.3 40.9 70.6 87.0 61.8
654
65.4 176.6
65.5 176.7
176.6
127.7
132.1
88.8 141.2
57.6 144.6
129.3 103.3 145.7
c
126.5
131.8
90.9 142.7
60.2 146.0
)
74.8 81.4 64.1 65.4 176.6
127.3 102.5 146.8
126.4
130.8
89.9 143.7
59.1 147.0
128.5 102.7 145.2
127.5
132.0
89.9 142.2
59.2 145.7
127.4 101.6 148.1
126.3
130.7
88.9 145.0
57.7 148.4
85.5 41.0 70.3 87.2 61.5
85.6 41.1 70.4 87.2 61.6
a) Systematic numbering. b) Purine numbering. c) Overlapped with DMSO signal. d) Tentative.
Table 2. NOE Data of 9-Deaza-2'-deoxyguanosine (1a) and of the 9-Halogenated Derivatives 1b,ca)
Proton irradiated
NOE observed ([%])
1a
HÀC(6)
HÀC(1')
HÀC(7)
HÀC(1') (0), HÀC(2') (5)
HÀC(6) (1.1), HaÀC(2') (5.4), HÀC(4') (1.5)
HÀC(1') (0), HÀC(6) (5.7), HÀC(2') (0)
1b
1c
HÀC(6)
HÀC(1') (0.6), HÀC(2') (3.5), HÀC(3') (0.6), OHÀC(5') (0.5)
HÀC(1')
HÀC(6) (0), HaÀC(2') (5.6), HÀC(4') (1.6)
HÀC(6)
HÀC(1')
HÀC(1') (0), HÀC(2') (2.9)
HÀC(6) (0), HaÀC(2') (6.7), HÀC(4') (1.3)
a) Measured in (D6)DMSO at 258.
According to Table 1, the C(7) signals of 1b and 1c are shifted upfield compared to the nucleoside 1a due to
the halogen substituents attached (systematic numbering; (d (CH) 101.6, d (CBr) 88.9, and d (CI) 57.7 ppm). In
addition, a large 1J(C(6), HÀC(6)) coupling constant of 190.6 Hz and small 2J(C(7), HÀC(6)) and 3J(C(6),
HÀC(1')) coupling constants of 5.5 and 4.25 Hz, respectively, were obtained from the gated-decoupled
13C-NMR spectrum of 1c and suggest C(7) as the position of halogenation (systematic numbering). The b-d-
configuration of the anomeric center of 1a c was confirmed by the irradiation of HÀC(1') resulting in NOEs at
HÀC(4') (h ˆ 1.5% for 1a, h ˆ 1.6% for 1b, and h ˆ 1.3% for 1c).
The UV spectra of compounds 1b,c and, for comparison, of 1a measured in 0.1m
sodium phosphate buffer (pH 7.0) show three distinct maxima at 232, 262, and 287 nm
for 1a, 235, 272, and 297 nm for 1b, and 237, 276, and 299 nm for 1c. This indicates that
the 9-halogeno substituents induce a bathochromic shift. The nucleosides 1a c show
two pKa values between pH 3 and 11. The pKa values for compound 1a are 4.9 and 10.3,
while the halogenated derivatives show lower values, 3.5 and 9.6 for 1b and 3.5 and 9.7
for 1c.

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2. Conformational Analyses of the 9-Deazaguanine N7-Nucleosides 1a c. Con-
formational analysis of the sugar moiety of nucleosides 1a c was performed with the
program PSEUROT (version 6.3) [24][25]. Conformational changes of 9-substituted
9-deaza-2'-deoxyguanosine were studied on the basis of vicinal 1H,1H coupling
constants. Calculations were performed with pseudorotational starting parameters
recommended in the users manual of the program (Fmax ˆ 368 (both N and S)). The
input contained the following 1H,1H-coupling constants: J(1',2'), J(1',2''), J(2',3'),
J(2'',3'), and J(3',4') (see Table 3). During the iterations, either the puckering
parameters (P, Fmax) of the minor conformer (N) or the puckering amplitudes of
both conformers were constrained. In all cases, the root-mean-square (r.m.s.) values
were ꢀ 0.4Hz and the j DJmax j ꢀ 0.5 Hz. From the conformer population shown in
Table 3, some general trends can be deduced. The nonhalogenated compound 1a shows
a population of ca. 67% of the S-conformers; the conformation of the bromo and iodo
derivatives 1b,c are slightly shifted towards N (63 and 61% S, resp.). These values are
ca. 10 15% higher than those found for 2'-deoxyguanosine. The data demonstrate that
the higher the electron-withdrawing effect of the 9-substituent is, the more the N > S
equilibrium of the sugar moiety is biased towards the N-conformation [26][27]. This
suggests that 9-substituents on pyrrolo[3,2-d]pyrimidine 2'-deoxyribonucleosides have
a similar influence on the sugar puckering as electron-withdrawing 7-substituents in
pyrrolo[2,3-d]pyrimidine 2'-deoxyribonucleosides [28][29].
Table 3. N/S-Conformer Populations of the Sugar Moieties of 9-Deaza-2'-deoxyguanosine 1a and of the
Halogenated Derivatives 1b,c Measured in D2O at 298 K
3J(H,H) [Hz]a)
Conformation
1',2'
1',2''
2',3'
3.843.8433
6.55
6.13
2'',3'
3',4'
% N
% S
1a
1b
1c
6.85
6.63
6.65
6.546.75
6.41
6.29
67
4.00
4.07
4.16
4.24
37
39
63
61
a) 2' and 2'' represent the 2 HÀC(2').
In conclusion, it was shown that bromo and iodo substituents can be regioselectively
introduced at the 9-position of 9-deazapurine nucleosides. These compounds will be
used in duplex- and triplex-forming DNA. Studies on the formation of triplexes
containing 1a c (Fig., motif II) are under investigation; such triplex structures (see II)
are expected to be more stable than those formed by cytidine-containing oligonucleo-
tides (see motif I).
We thank Dr. H. Rosemeyer and Dr. Y. He for the NMR spectra. We also thank M. Dubiel, Eva-Maria
Becker, and E. Michalek for their kind help. We gratefully acknowledge financial support by the European
Community (Grant No.: QLRT-2001-00506, Flavitherapeutics).
Experimental Part
General. All chemicals were purchased from Aldrich, Sigma, or Fluka (Sigma-Aldrich Chemie GmbH,
Deisenhofen, Germany). Solvents were of laboratory grade. TLC: aluminium sheets, silica gel 60 F254, 0.2-mm
layer (VWR, Germany). Ion exchange: Serdolit AD-4 resin, 0.1 0.2 mm (Serva Electrophoresis GmbH,
Heidelberg, Germany). Column flash chromatography (FC): silica gel 60 (VWR, Germany) at 0.4bar; sample

Helvetica Chimica Acta Vol. 87 (2004)
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Figure. Triplex formation a) by cytidine-containing oligonucleotides (motif I) and b) by oligonucleotides
containing 1a c (motif II)
collection with an UltroRac-II fractions collector (LKB Instruments, Sweden). UV Spectra: U-3200
spectrometer (Hitachi, Tokyo, Japan); lmax (e) in nm. NMR Spectra: Avance-250 or AMX-500 spectrometers
(Bruker, Karlsruhe, Germany), at 250.13 MHz for 1H and 13C; d in ppm rel. to Me4Si as internal standard, J
values in Hz. Elemental analyses were performed by Mikroanalytisches Laboratorium Beller (Gˆttingen,
Germany).
7-Bromo-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-3-[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-
d]pyrimidin-4-one (4b). To a suspension of 2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-3-[(pivaloyl-
oxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (4a) [18] (1.0 g, 3.13 mmol) in 1,2-dichlorethane (50 ml) was
added NBS (725 mg, 4.07 mmol) under stirring at r.t. Stirring was continued for another 10 min, and the mixture
was poured into ice-water. The precipitate was filtered off in vacuo and washed with H2O. The residue was dried
and subjected to FC (silica gel, column 15 Â 3 cm, stepwise gradient CH2Cl2/MeOH 98 :2, 95 :5): 4b (1.12 g,
1
90%). Colorless solid. TLC (CH2Cl2/MeOH 95 :5): Rf 0.5. UV (MeOH): 261 (21800), 303 (15600). H-NMR
((D6)DMSO): 12.24( s, NH); 8.52 (s, NˆCH); 7.47 (s, HÀC(6)); 6.18 (s, CH2O); 3.16, 2.98 (2s, Me2N); 1.08
(s, 3 Me). Anal. calc. for C15H20BrN5O3 (398.26): C 45.24, H 5.06, N 17.59; found: C 45.36, H 5.02, N 17.50.
2-{[(Dimethylamino)methylidene]amino}-3,5-dihydro-7-iodo-3-[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-
d]pyrimidin-4-one (4c). As described for 4b, with 4a (1.0 g, 3.13 mmol) and NIS (855 mg, 3.80 mmol) in 1,2-
dichlorethane (50 ml): 4c (1.32 g, 95%). Colorless solid. TLC (CH2Cl2/MeOH 95 :5): Rf 0.5. UV (MeOH): 263
(20600), 305 (15000). 1H-NMR ((D6)DMSO): 12.27 (s, NH); 8.51 (s, NˆCH); 7.47 (s, HÀC(6)); 6.17
(s, CH2O); 3.16, 2.97 (2s, Me2N); 1.08 (s, 3 Me). Anal. calc. for C15H20IN5O3 (445.26): C 40.46, H 4.53, N
15.73; found: C 40.32, H 4.41, N 15.61.
5-[2-Deoxy-3,5-di-O-(p-toluoyl)-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-
3,5-dihydro-3-[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (6a). To a suspension of powdered
KOH (310 mg, 5.53 mmol) and TDA-1 (ˆtris[2-(2-methoxyethoxy)ethyl]amine; 70 ml, 0.22 mmol) in anh.
MeCN (15 ml) was added 4a (1 g, 3.13 mmol) while stirring at r.t. Stirring was continued for another 10 min, and
2-deoxy-3,5-di-O-(p-toluoyl)-a-d-erythro-pentofuranosyl chloride (5; 1.46 g, 3.76 mmol) [30][31] was added in
portions. After 30 min, insoluble material was filtered off, and the solvent was evaporated. The resulting foam
was applied to FC (silica gel, column 12 Â 4cm, stepwise gradient CH 2Cl2/MeOH 99 :1, 98 :2): 6a (1.19 gm,
1
57%). Colorless foam. TLC (CH2Cl2/MeOH 98 :2): Rf 0.3. UV (MeOH): 242 (33200), 307 (14600). H-NMR
((D6)DMSO): 8.56 (s, NˆCH); 7.94 7.86 ( m, 5 arom. H); 7.67 (d, J ˆ 3.1, HÀC(6)); 7.30 (m, 4arom. H); 7.05
(t, J ˆ 6.7, HÀC(1')); 6.23 (d, J ˆ 3.0, HÀC(7)); 6.17 (s, CH2O); 5.63 (m, HÀC(3')); 4.54 (m, 2 HÀC(5'),
HÀC(4')); 3.14, 2.96 (2s, Me2N); 2.64( m, 2 HÀC(2')); 2.39, 2.37 (2s, 2 Me); 1.08 (s, 3 Me). Anal. calc. for
C36H41N5O8 (671.74): C 64.37, H 6.15, N 10.43; found: C 64.40, H 6.20, N 10.40.
7-Bromo-5-[2-deoxy-3,5-di-O-(p-toluoyl)-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]-
amino}-3,5-dihydro-3-[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (6b). Method A: As described
for 6a, with powdered KOH (140 mg, 2.5 mmol), TDA-1 (46 ml, 0.14mmol), 4b (600 mg, 1.51 mmol), and 5
(970 mg, 2.50 mmol) in anh. MeCN (10 ml): 6b (850 mg, 75.1%). Colorless foam.

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Method B: To a suspension of 6a (400 mg, 0.60 mmol) in 1,2-dichlorethane (5 ml) was added NBS (116 mg,
0.65 mmol) under stirring at r.t. Stirring was continued for another 25 min, and the mixture was poured into ice-
cold H2O (100 ml). The precipitate was filtered off and dried under vacuum. The residue was applied to FC
(silica gel, column 12 Â 2 cm, stepwise gradient CH2Cl2/MeOH 99 :1, 98 :2): 6b (260 mg, 58%). Colorless foam.
TLC (CH2Cl2/MeOH 98 :2): Rf 0.4. UV (MeOH): 240 (48500), 311 (22600). 1H-NMR ((D6)DMSO): 8.53
(s, NˆCH); 7.93 7.86 (m, arom. H, HÀC(6)); 7.35 (m, arom. H); 7.04( t, J ˆ 6.7, HÀC(1')); 6.15 (s, CH2O); 5.64
(m, HÀC(3')); 4.54 (m, 2 HÀC(5'), HÀC(4')); 3.17, 2.98 (2s, Me2N); 2.75 (m, 2 HÀC(2'×)); 2.39, 2.37
(2s, 2 Me); 1.08 (s, 3 Me). Anal. calc. for C36H40BrN5O8 (750.64): C 57.60, H 5.37, N 9.33; found: C 58.00, H
5.42, N 9.19.
5-[2-Deoxy-3,5-di-O-(p-toluoyl)-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-
3,5-dihydro-7-iodo-3-[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (6c). Method A: As described
for 6a, with powdered KOH (390 mg, 6.95 mmol), TDA-1 (140 ml, 0.44 mmol), 4c (1 g, 2.25 mmol), and 5
(1.17 g, 3.0 mmol) in anh. MeCN (30 ml): 6c (1.55 g, 87%). Colorless foam.
Method B: As described for 6b, with 6a (400 mg, 0.56 mmol) and NIS (184 mg, 0.87 mmol) in CH2Cl2
(5 ml): 6b (115 mg, 24%). Colorless foam. TLC (CH2Cl2/MeOH 98 :2): Rf 0.44. UV (MeOH): 240 (38500), 265
(22200), 312 (16400). 1H-NMR ((D6)DMSO): 8.52 (s, NˆCH); 7.90 (m, arom. H, HÀC(6)); 7.34( m, arom. H);
7.03 (t, J ˆ 6.9, HÀC(1')); 6.15 (s, CH2O); 5.65 (m, HÀC(3')); 4.55 (m, 2 HÀC(5'), HÀC(4')); 3.17, 2.98
(2s, Me2N); 2.79 (m, HaÀC(2')); 2.67 (m, HbÀC(2')); 2.39, 2.37 (2s, 2 Me); 1.08 (s, 3 Me). Anal. calc. for
C36H40IN5O8 (797.64): C 54.21, H 5.05, N 8.78; found: C 54.24, H 4.95, N 8.75.
5-[2-Deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-4H-pyrro-
lo[3,2-d]pyrimidin-4-one (7a). A soln. of 6a (1.3 g, 1.94mmol) in 0.1 m NaOMe/MeOH (100 ml) was stirred for
1 h at r.t. Silica gel was added (5 g), and the solvent was evaporated. The mixture was applied to FC (silica gel,
column 14 Â 4cm, CH 2Cl2/MeOH 9 :1): 7a (520 mg, 84%). Colorless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.25.
UV (MeOH): 224(10600), 254(18700), 294(15900). 1H-NMR ((D6)DMSO): 11.13 (s, NH); 8.53 (s, NˆCH);
7.61 (d, J ˆ 3.1, HÀC(6)); 6.88 (t, J ˆ 6.8, HÀC(1')); 6.16 (d, J ˆ 3.1, HÀC(7)); 5.24( d, J ˆ 3.9, OHÀC(3')); 4.92
(t, J ˆ 5.4, OHÀC(5')); 4.28 (m, HÀC(3')); 3.78 (m, HÀC(4')); 3.52 (m, 2 HÀC(5')); 3.10, 2.98 (2s, Me2N); 2.18
(m, 2 HÀC(2')). Anal. calc. for C14H19N5O4 (321.33): C 52.33, H 5.96, N 21.79; found: C 52.67, H 5.70, N 21.39.
7-Bromo-5-[2-deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-
4H-pyrrolo[3,2-d]pyrimidin-4-one (7b). As described for 7a, with 6b (260 mg, 0.35 mmol) and 0.1m NaOMe/
MeOH (30 ml): 7b (130 mg, 94%). Colorless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.34. UV (MeOH): 226
1
(15500), 260 (25000), 297 (21700). H-NMR ((D6)DMSO): 11.37 (s, NH); 8.50 (s, NˆCH); 7.82 (s, HÀC(6));
6.88 (t, J ˆ 6.7, HÀC(1')); 5.25 (d, J ˆ 3.7, OHÀC(3')); 4.97 (t, J ˆ 5.2, OHÀC(5')); 4.27 (m, HÀC(3')); 3.77
(m, HÀC(4')); 3.52 (m, 2 HÀC(5')); 3.15, 3.00 (2s, Me2N); 2.24( m, 2 HÀC(2')). Anal. calc. for C14H18BrN5O4
(400.23): C 42.01, H 4.53, N 17.50; found: C 42.30, H 4.60, N 17.35.
5-[2-Deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-7-iodo-4H-
pyrrolo[3,2-d]pyrimidin-4-one (7c). As described for 7a, with 6c (1 g, 1.25 mmol) and 0.1m NaOMe/MeOH
(80 ml): 7c (485 mg, 87%). Colorless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.42. UV (MeOH): 229 (8600), 263
(17000), 301 (16100). 1H-NMR ((D6)DMSO): 11.34( s, NH); 8.50 (s, NˆCH); 7.81 (s, HÀC(6)); 6.88 (t, J ˆ 6.6,
6.9, HÀC(1')); 5.24( d, J ˆ 3.7, OHÀC(3')); 4.96 (t, J ˆ 5.2, OHÀC(5')); 4.28 (m, HÀC(3')); 3.78 (m, HÀC(4'));
3.52 (m, 2 HÀC(5')); 3.14, 3.01 (2s, Me2N); 2.25 (m, 2 HÀC(2')). Anal. calc. for C14H18IN5O4 (447.23): C 37.60,
H 4.06, N 15.66; found: C 37.68, H 4.10, N 15.28
5-[2-Deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-3-[(piva-
loyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (8a). A soln. of 6a (150 mg, 0.22 mmol) and 0.01m NaOMe/
MeOH (15 ml) was stirred for 1 h at r.t. Silica gel (1 g) was added, and the solvent was evaporated. The mixture
was applied to FC (silica gel, column 14 Â 2 cm, CH2Cl2/(CH3)2CO 98 :2): 8a (70 mg, 72%). Colorless solid. TLC
(CH2Cl2/MeOH 9 :1): Rf 0.5. UV (MeOH): 262 (20900), 309 (16700). 1H-NMR ((D6)DMSO): 8.56 (s, NˆCH);
7.72 (s, HÀC(6)); 6.89 (t, J ˆ 6.6, HÀC(1')); 6.20 (s, HÀC(7)), 6.18 (s, CH2O); 5.24( d, J ˆ 3.9, OHÀC(3')); 4.92
(t, J ˆ 5.3, OHÀC(5')); 4.29 (m, HÀC(3')); 3.79 (m, HÀC(4')); 3.51 (m, 2 HÀC(5')); 3.14, 2.97 (2s, Me2N); 2.27
(m, 2 HÀC(2')); 1.10 (s, 3 Me). Anal. calc. for C20H29N5O6 (435.47): C 55.16, H 6.71, N 16.08; found: C 54.78, H
6.36, N 16.10.
7-Bromo-5-[2-deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-3-
[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (8b). As described for 8a, with 6b (150 mg,
0.20 mmol) and 0.01m NaOMe/MeOH: 8b (70 mg, 68%). Colorless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.48.
UV (MeOH): 262 (20900), 309 (16700). 1H-NMR ((D6)DMSO): 8.52 (s, NˆCH); 7.91 (s, HÀC(6)); 6.87 (t, J ˆ
6.6, HÀC(1')); 6.16 (s, CH2O); 5.25 (d, J ˆ 3.9, OHÀC(3')); 4.99 (t, J ˆ 5.3, OHÀC(5')); 4.28 (m, HÀC(3'));

Helvetica Chimica Acta Vol. 87 (2004)
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3.79 (m, HÀC(4')); 3.53 (m, 2 HÀC(5')); 3.17, 2.98 (2s, Me2N); 2.31 2.23 (m, 2 HÀC(2')); 1.09 (s, 3 Me). Anal.
calc. for C20H28BrN5O6 (514.37): C 46.70, H 5.49, N 13.62; found: C 46.50, H 5.61, N 13.07.
5-[2-Deoxy-b-d-erythro-pentofuranosyl]-2-{[(dimethylamino)methylidene]amino}-3,5-dihydro-7-iodo-3-
[(pivaloyloxy)methyl]-4H-pyrrolo[3,2-d]pyrimidin-4-one (8c). As described for 8a, with 6c (500 mg,
0.63 mmol) and 0.01m NaOMe/MeOH (35 ml): 8c (256 mg, 73%). Colorless solid. TLC (CH2Cl2/MeOH 9 :1):
1
Rf 0.52. UV (MeOH): 266 (21100), 312 (18700). H-NMR ((D6)DMSO): 8.51 (s, NˆCH); 7.89 (s, HÀC(6));
6.86 (t, J ˆ 6.6, HÀC(1')); 6.15 (s, CH2O); 5.26 (d, J ˆ 3.9, OHÀC(3')); 4.99 (t, J ˆ 5.3, OHÀC(5')); 4.28
(m, HÀC(3')); 3.78 (m, HÀC(4')); 3.55 (m, 2 HÀC(5')); 3.16, 2.98 (2s, Me2N); 2.36 2.22 (m, 2 HÀC(2')); 1.08
(s, 3 Me3). Anal. calc. for C20H28IN5O6 (561.37): C 42.79, H 5.03, N 12.48; found: C 42.68, H 5.05, N 12.25.
2-Amino-5-(2-deoxy-b-d-erythro-pentofuranosyl)-3,5-dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one (1a).
Compound 7a (700 mg, 2.18 mmol) was stirred in 25% aq. ammonia (50 ml) at 608 for 12 h in a sealed vessel.
The solvent was evaporated, and the residue was dissolved in H2O (50 ml) and chromatographed (Serdolit AD-4
resin). The salt was washed off with distilled H2O for 2 h, and the product was collected with MeOH/H2O 1 :5:
1a (550 mg, 95%). Colorless solid. Crystallization from MeOH gave colorless crystals. M.p. 195 1968. TLC
(CH2Cl2/MeOH 8 :2): Rf 0.36. UV (0.1m sodium phosphate buffer pH 7.0): 232 (20000), 262 (8000), 287 (6200).
1H-NMR ((D6)DMSO): 10.60 (s, NH); 7.54( s, HÀC(6)); 6.79 (m, HÀC(1')); 6.01 (s, HÀC(7)); 5.83 (s, NH2);
5.21 (s, OHÀC(3')); 4.90 (s, OHÀC(5')); 4.27 (m, HÀC(3')); 3.70 (m, HÀC(4')); 3.47 (m, 2 HÀC(5')); 2.26
(m, HbÀC(2')); 2.15 (m, HaÀC(2')). Anal. calc. for C11H14N4O4 (266.25): C 49.62, H 5.30, N 21.04: found: C
49.65, H 5.27, N 20.87.
2-Amino-7-bromo-5-(2-deoxy-b-d-erythro-pentofuranosyl)-3,5-dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one
(1b). As described for 1a, with 7b (250 mg, 0.63 mmol) and 25% aq. ammonia (30 ml): 1b (195 mg, 90%).
M.p. > 2108. TLC (CH2Cl2/MeOH 8 :2): Rf 0.7. UV (0.1m sodium phosphate buffer pH 7.0): 235 (21000), 272
(7300), 297 (6800). 1H-NMR ((D6)DMSO): 10.74( s, NH); 7.54( s, HÀC(6)); 6.79 (m, HÀC(1')); 6.12 (s, NH2);
5.21 (d, J ˆ 3.3, OHÀC(3')); 4.93 (d, J ˆ 4.9, OHÀC(5')); 4.25 (m, HÀC(3')); 3.76 (m, HÀC(4')); 3.53
(m, 2 HÀC(5')); 2.19 (m, HbÀC(2')); 2.03 (m, HaÀC(2')). Anal. calc. for C11H13BrN4O4 (345.15): C 38.28, H
3.80, N 16.23; found: C 38.38, H 3.75, N 16.36.
2-Amino-5-(2-deoxy-b-d-erythro-pentofuranosyl)-3,5-dihydro-7-iodo-4H-pyrrolo[3,2-d]pyrimidin-4-one
(1c). As described for 1a, with 7c (220 mg, 0.49 mmol) and 25% aq. ammonia (30 ml): 1c (180 mg, 93%). M.p. >
2108. TLC (CH2Cl2/MeOH 8 :2): Rf 0.78. UV (0.1m sodium phosphate buffer pH 7.0): 237 (19700), 276 (7600),
299 (7000). 1H-NMR ((D6)DMSO): 10.74( s, NH); 7.54( s, HÀC(6)); 6.79 (m, HÀC(1')); 6.12 (s, NH2); 5.21
(d, J ˆ 3.3, OHÀC(3')); 4.93 (d, J ˆ 4.9, OHÀC(5')); 4.25 (m, HÀC(3')); 3.76 (m, HÀC(4')); 3.53
(m, 2 HÀC(5')); 2.26 (m, HbÀC(2')); 2.05 (m, HaÀC(2')). Anal. calc. for C11H13IN4O4 (392.15): C 33.69, H
3.34, N 14.29; found: 33.69, H 3.34, N 14.29.
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Received May 27, 2004