9-Vinylguanine: An easy access to aza-analogs of 2′,3′-dideoxyguanosine

Article abstract of DOI:10.1016/S0040-4020(01)00284-8

9-Vinylguanine, obtained for the first time and fully characterised by X-ray analysis, allows access to aza-analogues of 2′,3′-dideoxynucleosides through cycloaddition processes.

Full text of DOI:10.1016/S0040-4020(01)00284-8

TETRAHEDRON
Pergamon
Tetrahedron 57 52001) 4035±4038
9-Vinylguanine: an easy access to aza-analogs
of 20,30-dideoxyguanosine
Renato Dalpozzo, Antonio De Nino, Loredana Maiuolo, Antonio Procopio, Giovanni De Munno
and Giovanni Sindonap
Á
Dipartimento di Chimica, Universita della Calabria, ponte Bucci, 87030 Arcavacata di Rende ꢀCS), Italy
Received 26 October 2000; revised 8February 2001; accepted 1 March 2001
AbstractÐ9-Vinylguanine, obtained for the ®rst time and fully characterised by X-ray analysis, allows access to aza-analogues of 20,30-
dideoxynucleosides through cycloaddition processes. q 2001 Elsevier Science Ltd. All rights reserved.
In the last years there has been much effort to develop
ef®cient treatment against HIV infection. Good results
were obtained by using a combination of agents that inhibit
different steps in the HIV life cycle 5reverse transcriptase
and protease inhibitors). In several clinical trials 5CPCRA,
DELTA, etc.) this strategy showed a considerable delay in
progression of the disease and prolonged survival in patients
with advanced HIV.1
On the other hand the direct N-vinylation of guanine gives
approximately equal amounts of the N-7 and N-9-vinyl
products.11,12
Since 2-N-acetyl-6-O-diphenylcarbamoylguanine was
successful in giving highly regioselective coupling with
acetylated pentafuranoses and a-halo ethers,13 we decided
to use the same substrate in vinyl exchange between vinyl
acetate and a nitrogen heterocyclic compound using vinyl
acetate in a valuable one-step preparation of N-vinylhetero-
cycles 5Scheme 1).
Modi®ed nucleosides 5AZT, ddC, etc¼) have, very often,
shown good antiviral properties pushing the interest towards
other related systems like, for example, isoxazolidinyl
nucleosides.2 Synthesis of 40-aza analogues of 20,30-
dideoxynucleosides was proposed as a potential way to
reach this aim.3,4 We have already reported a valuable
strategy for the preparation of isoxazolidine nucleosides of
thymine, adenine and cytosine, and we found that the 40-
aza-20,30-dideoxy-erythrofuranosyl derivative of thymine
acts against HIV `in vitro' test.3 Considering the antiviral
activities showed by other guanine analogues in similar
homologous series [i.e. acyclovir, DMPG, Carbovir,
etc.],5±7 the synthesis of deoxyguanosine derivatives of
isoxazolidinyl nucleosides seems quite interesting.
The reaction sequence gives product 4 without detecting any
N-7 contaminant as shown by the absence of any other
signals in the 1H NMR spectrum. These data are in accord-
ance with the previous observations that the signals of H8
and NH2 for the N-9 isomer are respectively shifted up®eld
The key intermediate to take advantage of using the method
we previously described3 is certainly 9-vinylguanine,
already isolated and characterised as a radiation-induced
degradation product of 20-deoxyguanosine,8 but never
synthesised before. In fact the methods proposed, so far,
consist of relatively low yielding, multistep procedures,
which only give a precursor of the desired substrate.9±11
Moreover attempts to use the direct alkylation of guanine
with ethylene oxide resulted in the 7-alkyl derivative.10,12
Keywords: vinylation; purines; cycloadditions; antivirals.
Corresponding author. Tel.: 139-0984492046; fax: 139-0984492044;
e-mail: sindona@unical.it
p
Scheme 1.
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020501)00284-8

4036
R. Dalpozzo et al. / Tetrahedron 57 ꢀ2001)4035±4038
Figure 1.
Scheme 2.
and down®eld relative to the signals of H8 and NH2 signals
or the N-7 isomer.8,14
1.Experimental
1.1. General
The structure of 3 was proven by X-ray analysis 5Fig. 1).15
Silica gel-pre-coated plates were used for TLC and Kiesel-
gel 60 H without gypsum was used for short-column
chromatography. HPLC system: HP 1100 5Hewlett±
Packard), sample loop: 2.0 ml, column 250£4.6 mm Jupiter
C18 10 m Phenomenex; detection: UV 230 nm, Hewlett±
Packard 5Variable Wavelength). Solvent 5A): 0.1% TFA
aq.; solvent 5B) MeOH; gradient: A 100% for 5 min, 10%
B in A in 2 min., 30% B in A 5 min.; and after 5 min, 100%
A in 5 min.; ¯ow 3.0 ml min21. IR spectra were determined
as ®lm on KBr on PE Paragon 1000 PC FT-IR Spectrometer.
Finally the tritylated isomer 5 was devised in order to
exploit a cycloaddition process leading to a fully acid-labile
protected azanucleoside and afforded 8 after complete
deblocking with tri¯uoroacetic acid, according to Scheme 2.
The presented method allows the easy synthesis of
9-vinylguanine which is now available for the preparation
of nucleoside derivatives obtained using their vinyl
synthons.

R. Dalpozzo et al. / Tetrahedron 57 ꢀ2001)4035±4038
4037
Melting points were obtained on a Ko¯er apparatus. NMR
spectra were obtained on WM-300 Bruker spectrometer
with tetramethylsilane as internal standard and DMSO-d6
or CDCl3 as solvents, J values are given in Hz. Structural
assignments have been made after extensive decoupling
experiments. FAB-Mass spectra were obtained on a
VG-micromass ZAB 2F mass spectrometer, from a 2 mm3
m-nitrobenzyl alcohol or glycerol solution of sample by
using the standard gun operated with neutral Xenon beam
of 8keV and a neutral current of 10 mA.
10 min and then the suspension was warmed to 608C for 1 h.
When the reaction was completed 5TLC: CHCl3/MeOH 9:1
v/v), the solvent was removed under reduced pressure. The
residue was suspended in CH2Cl2 and stirred for 20 min.
Then it was ®ltered and washed with CH2Cl2. A white
solid was obtained 51.01 g, 95%); mp.3008C; nmax 5KBr)
3317, 3165, 1698, 998, 892; dH 5DMSO-d6): 5.05 5d, 1H; 20-
CHcis, Jcisˆ9.39), 5.87 5d, 1H; 20-CHtrans, Jtransˆ16.12), 6.70
5brs, 2H; 2-NH2), 7.085dd, 1H; 1 0-CH, Jtransˆ16.12,
Jcisˆ9.39), 8.12 5s, 1H; 8-CH), 10.74 5brs, 1H; 1-NH);
FAB-MS 51), Gly m/z: 270 [M1gly1H]1 59%), 178
[M1H]1 5100%), 152 533%); anal. calcd. for C7H7N5O C
47.46, H 3.98, N 39.53 found C 47.40, H 4.02, N 39.57.
1.1.1. 2-'N-Acetyl)-6-'O-diphenylcarbamoyl) guanine '2).
Guanine 510.0 g; 66.2 mmol) and dry N,N-dimethylaceta-
mide 585 mL) were placed in a 250 mL ¯ask ®tted with a
re¯ux condenser under dry nitrogen. Acetic anhydride
520 mL; 211.6 mmol) was added to this suspension. The
solution was warmed to 1608C. When the reaction was
completed, it was cooled and the white solid obtained
52,9-diacetylguanine) was ®ltered and washed with EtOH.
1.1.4. 2-'N-Trityl)-9-vinylguanine '5).4 51.5 g, 8.4 mmol)
was suspended in dry pyridine 5100 mL). Trityl chloride
53.7 g, 13.0 mmol) was added to this suspension. The reac-
tion solution was warmed to 908C until TLC 5CHCl3/MeOH
90:10 v/v) showed no starting material. After stirring for 5 h
the solution was allowed to cool, 6 mL of water was added
and the mixture was stirred for 20 min. The solvents were
removed under reduced pressure and the crude product was
coevaporated with toluene and then with ethanol, providing
a solid residue. Aqueous NaHCO3 55% w/w, 25 mL) was
added and aqueous layer was extracted with CHCl3
53£20 mL). The organic layer was dried by sodium sulphate
and concentrated in vacuo. The crude product was puri®ed
by ¯ash chromatography. A white solid was obtained with
yield of 75% 52.63 g); mp 278±2808C; nmax 5KBr) 3220,
1706, 989, 900; dH 5DMSO- d6): 4.65 5d, 1H, 20-CHcis,
To a suspension of 2,9-diacetylguanine 510 g; 42.5 mmol)),
DIPEA 515 mL) and dry pyridine 5200 mL), diphenylcarba-
moyl chloride 510.88 g, 46.9 mmol) was added. The reac-
tion was stirred for 4 h 5TLC: CH2Cl2/MeOH 9:1 v/v). Then
H2O 520 mL) was added and the mixture was stirred for
10 min. The solvents were removed in vacuo. The residue
was placed in a solution of H2O and EtOH 51:1, v/v) and
heated to 708C. After 3.5 h the suspension was cooled and
®ltered. A white solid was obtained 523.36 g, 91%); mp
153±1558C; nmax 5KBr) 3387, 1769, 1596, 1470; dH
0
5DMSO-d6): 2.185s, 3H; CH CO), 7.26±7.56 5m, 10H;
3
Jcisˆ9.21); 5.285d, 1H, 2 -CHtrans, Jtransˆ16.11); 6.56 5dd,
N5C6H5)2), 8.46 5s, 1H; 8-CH); FAB-MS 51), NBA m/z:
411 [M1Na]1 523%), 389 [M1H]1 529%), 196
[CONPh2]1 5100%); anal. calcd. for C20H16N6O3 C 61.85,
H 4.15, N 21.64 found C 61.79, H 4.20, N 21.69.
1H, 10-CH, Jtransˆ16.11, Jcisˆ9.21); 7.20±7.34 5m, 15H,
ArH); 7.78 5s, 1H, 2-NH); 7.89 51H, s, 8-CH); 10.71 51H,
brs, 1-NH); FAB-MS 51), m-NBA, m/z: 442 [M1Na]1
561%), 420 [M1H]1 5100%), 342 [M1H-C6H6]1 58%);
anal. calcd. for C26H21N5O C 74.44, H 5.05, N 16.70
found C 74.45, H 5.08, N 16.68.
1.1.2.
2-'N-Acetyl)-6-'O-diphenylcarbamoyl)-9-vinyl-
guanine '3). A solution of sulfuric acid 50.25 mL) in
AcOEt 512 mL) was added to a suspension of Hg5OAc)2
50.49 g; 1.5 mmol) in vinyl acetate 548mL; 1.5 mmol).
After two minutes, a clear solution appeared. Then, 2-5N-
acetyl)-6-5O-diphenylcarbamoyl) guanine 56.0 g; 15.0
mmol) in DMF 580 mL) was added. As a polymerisation
inhibitor hydroquinone 50.1 g) was added. The mixture
was re¯uxed for 6 h to allow the reaction to be ®nished.
Then aqueous NaHCO3 55% w/w) was added up to neutra-
lisation. A white solid was formed, ®ltered; the brown
®ltrate extracted with Et2O 53£50 mL), the organic layer
dried over sodium sulphate and concentrated in vacuo.
The crude product was puri®ed by ¯ash chromatography
with CHCl3/MeOH 597.5:2.5 v/v). A yellow solid was
obtained 53.72 g, 60%); mp 187±1888C; nmax 5KBr) 3210,
1734, 1001, 906; dH 5CDCl3): 2.53 5s, 3H; CH3), 5.19 5dd,
1.1.5. 40-Aza-40-'N-trityl)-20,30-dideoxy-2-'N-trityl) guano-
sine '6).5 51.0 g, 2.3 mmol) was added under dry nitrogen
to a suspension of N-tritylhydroxylamine 51.6 g, 5.7 mmol)
and paraformaldehyde 50.21 g, 6.90 mmol) in dry toluene
560 mL). Some crystals of hydroquinone were added as a
polymerisation inhibitor. The mixture was stirred and
heated under re¯ux for 5 h 5TLC: CH2Cl2/MeOH 95: 5
v/v). Then the solvent was removed under reduced pressure
and the residue was coevaporated with ethanol to remove
traces of toluene. The crude product was puri®ed by short
column chromatography. Yield: 74% 51.20 g); mp 189±
1908C; nmax 5KBr) 3322, 3036, 1684, 742, 702; dH
5CDCl3): 1.25±1.30 5m, 1H, 30-CH); 1.42±1.62 5m, 1H,
0
20-CH); 2.74±2.95 5m, 1H, 20-CH); 3.10±3.285m, 1H, 3 -
CH); 5.11±5.31 5m, 1H, 10-CH); 6.90±7.685m, 31H, ArH 1
2-NH); 7.95 5s, 1H, 8-CH); 11.72 5brs, 1H, 1-NH); FAB-MS
51), m-NBA m/z: 729 [M1Na]1 548%), 707 [M1H]1
5100%), 486 [M-Tr1Na]1 536%), 464 [M-Tr1H]1 512%);
anal. calcd. for C46H38N6O2 C 78.16, H 5.42, N 11.89 found
C 78.25, H 5.39, N 11.86.
1H; 20-CHcis, Jcisˆ9.3, Jgemˆ1.8), 5.91 5dd, 1H; 20-CHtrans
,
Jtransˆ16.0, Jgemˆ1.8), 7.10 5dd, 1H; 10-CH, Jtransˆ16.0,
Jcisˆ9.3), 7.20±7.55 [m, 10H; N5C6H5)2],), 8.12 5s, 1H; 8-
CH), 8.25 5brs, 1H; 2-NH); FAB-MS 51), NBA m/z: 437
[M1Na]1 54%), 415 [M1H]1 515%), 196 [CONPh2]1
5100%); anal. calcd. for C22H18N6O3 C 63.76, H 4.38, N
20.28found C 63.83, H 4.36, N 20.27.
1.1.6. 40-Aza-20,30-dideoxy-2-'N-trityl) guanosine '7). To
6 50.50 g, 0.7 mmol) tri¯uoroacetic acid 510 mL, 5%
dichloromethane solution) and tri¯uoroethanol 51 mL)
were added at room temperature for 5 min. The reaction
1.1.3. 9-Vinylguanine '4). In a solution of 3 52.5 g;
6.0 mmol) and MeOH 560 mL) ammonia was bubbled for

4038
R. Dalpozzo et al. / Tetrahedron 57 ꢀ2001)4035±4038
T. C.; Reichman, R. C.; Hooper, C.; Corey, L. N. Engl. J.
Med. 1996, 334, 1011±1017. 5b) Katzenstein, D. A.; Hammer,
S. M.; Hughes, M. D.; Gundacker, H.; Jackson, J. B.; Fiscus,
S.; Rasheed, S.; Elbeik, T.; Reichman, R.; Japour, A.;
Merigan, T. C.; Hirsch, M. S. N. Engl. J. Med. 1996, 335,
1091±1098. 5c) Saravolatz, L. D.; Winslow, D. L.; Collins,
G.; Hodges, J. S.; Pettinelli, C.; Stein, D. S.; Markowitz, N.;
Reves, R.; Loveless, M. O.; Crane, L.; Thompson, M.;
Abrams, D. N. Engl. J. Med. 1996, 335, 1099±1106.
5d) Corey, L. N. Engl. J. Med. 1996, 335, 1142±1143.
2. Pan, S.; Amankulor, N. M.; Zhao, K. Tetrahedron 1998, 54,
6587±6604.
was stirred until TLC 5CHCl3/MeOH 8:2 v/v) showed no
starting material. Aqueous NaHCO3 55% w/w, 5 mL) was
added to solution that was extracted with CHCl3 53£15 mL).
The organic layer was dried over sodium sulfate and the
solvents were removed under reduced pressure. The residue
was puri®ed by ¯ash chromatography. Yield: 85% 50.28 g);
mp 230±2328C; nmax 5KBr) 3320, 3202, 1702, 740, 700; dH
5DMSO-d6): 1.91±2.52 5m, 4H, 30-CH 01 20-CH); 5.68±5.77
5brs, 1H, 40-NH); 5.75±5.985m, 1H, 1 -CH); 6.95±7.50 5m,
15H, ArH); 7.70 5s, 1H, 8-CH); 7.80 5s, 1H, 2-NH); 10.73
5brs, 1H, 1-NH); FAB-MS 51), m-NBA, m/z: 48 7
[M1Na]1 529%), 464 [M1H]1 573%), 394 530%), 316
510%), 243 5100%), 165 563%), 152 543%); anal. calcd.
for C27H24N6O2 C 69.81, H 5.21, N 18.09 found C 69.89,
H 5.22, N 18.04.
3. 5a) Leggio, A.; Liguori, A.; Procopio, A.; Siciliano, C.;
Sindona, G. Tetrahedron Lett. 1996, 37, 1277±1280.
5b) Giglio, G.; Napoli, A.; Leggio, A.; Liguori, A.; Procopio,
A.; Siciliano, C.; Sindona, G. Synth. Commun. 1996, 26,
4211±4217.
1.1.7. 40-Aza-20,30-dideoxyguanosine '8).7
50.50 g;
1.0 mmol) was treated with a 15% dichloromethane solution
of tri¯uoroacetic acid 510 mL) and tri¯uoroethanol 51 mL)
at room temperature for 10 min. When the reaction was
®nished 5CHCl3/MeOH 8:2 v/v), some drops of Et3N were
added to the solution and evaporated to dryness under
reduced pressure. The crude product was puri®ed by
preparative HPLC 5gradient MeOH/H2O). A white solid
was obtained 50.11 g, 50%); mp 287±2898C; nmax 5KBr)
3302, 3155, 1683; dH 5DMSO-d6): 2.64±2.82 5m, 1H, 20-
CH); 2.83±2.94 5m, 1H, 20-CH); 3.35±3.47 5m, 1H, 30-CH);
3.61±3.80 5m, 1H, 30-CH); 6.18±6.60 5brs, 1H, 40-NH); 6.47
5dd, 1H, 10-CH, Jtransˆ7.3, Jcisˆ3.6); 6.81 5brs, 1H, 2-NH2);
8.13 5s, 1H, 8-CH); 11.08 5s, 1H, 1-NH). FAB-MS 51), m-
NBA, m/z: 245 [M1Na]1 520%), 223 [M1H]1 56%), 152
518%), 72 5100%). Anal. calcd. for C8H10N6O2 C 43.24, H
4.54, N 37.82 found C 43.22, H 4.51, N 37.85.
4. 5a) Leggio, A.; Liguori, A.; Procopio, A.; Siciliano, C.;
Sindona, G. Nucleosides Nucleotides 1997, 16, 1515±1518.
5b) Colacino, E.; Converso, A.; De Nino, A.; Leggio, A.;
Liguori, A.; Maiuolo, L.; Napoli, A.; Procopio, A.; Siciliano,
C.; Sindona, G. Nucleosides Nucleotides 1999, 18, 581±583.
5. Gundersen, L. L.; Benneche, T.; Undheim, K. Tetrahedron
Lett. 1992, 33, 1085±1088. Ogilvie, K. K.; Cheriyan, U. O.;
Radatus, B. K.; Smith, K. O.; Galloway, K. S.; Kennell, W. L.
Can. J. Chem. 1982, 60, 3005±3010.
6. 5a) Martin, J. C.; Dvorak, C. A.; Smee, D. F.; Matthews, T. R.;
Verheyden, J. P. H. J. Med. Chem. 1983, 26, 759±761.
5b) Martin, J. C.; Smee, D. F.; Verheyden, J. P. H. J. Org.
Chem. 1985, 50, 755±759.
7. Lin, T. S.; Liu, M. C. Tetrahedron Lett. 1984, 25, 905±906.
8. Gromova, M.; Nardin, R.; Cadet J. Chem. Soc., Perkin Trans.
2 1998, 1365±1374.
9. 5a) Yashima, E.; Tajima, T.; Mayauchi, N. Biopolymers 1992,
32, 811±817. 5b) Yashima, E.; Suheiro, N.; Akashi, M.;
Miyauchi, N. Chem. Lett. 1990, 1113±1116.
1.2. X-Ray crystal structure analysis
10. Kondo, K.; Iwasaki, H.; Ueda, N.; Takemoto, K.; Imoto, M.
Makromolekulare Chemie 1969, 125, 298±301.
11. Pitha J. Org. Chem. 1975, 40, 3296±3298.
Brucker R3m/V automatic diffractometer, Mo-Ka radiation,
Ê
lˆ0.71073 A, graphite monochromator, 295 K, Lorentz-
polarization corrections. Data collection, solution and
re®nement: v22u, standard methods and subsequent
Fourier recycling, SHELXTL-PLUS computer program.
C22H18N6O3, triclinic, space group P, aˆ8.635 52),
bˆ14.629 53), cˆ17.779 52), aˆ66.77 52), bˆ83.73 52),
12. Kaye, H.; Chang, S. H. Tetrahedron 1970, 26, 1369±1376.
13. Zou, R.; Robins, M. J. Can. J. Chem. 1987, 65, 1436±1437.
14. Kjellberg, J.; Johansson, N. G. Tetrahedron 1986, 42, 6541±
6544.
23
Ê 3
15. Two independent molecules [N51) and N51a)] constitute the
asymmetric unit in the crystallographic cell of compound 3,
which is more suitable for X-ray analysis than 4. These mol-
ecules are joined together by means of hydrogen bonds in
which the N512) and O514a), O514) and N512a) atoms are
involved in such a way as to constitute a supramolecular
entity. These entities are held in the crystal by Van der
Waals forces. Both ®rst and second molecules show similar
C±C and C±N bond distances and differ solely for the fact that
C515) is near to N51) atom in the ®rst whereas the correspond-
ing C515a) is near N53a) in the second one.The value of the
dihedral angles formed by C54)±C55) N57)±C58)±N59) and
N59)±C510)±C511), C54a)±C55a) N57a)±C58a)±N59a) and
N59a)±C510a)±C511a) of 4.9 and 4.28 respectively, ensures
that the vinyl moiety is nearly coplanar with the imidazole
portion of the purine ring. The establishment of an extended
p system lowers the LUMO of the exocyclic double bond thus
favouring the cycloaddition approach.
gˆ82.5 52)8, Uˆ2042.0 58) A , Zˆ4, Dcˆ1.348g cm
,
crystal size 0.52£0.48£0.45 mm. 7968re¯ections measured
in the range 3,2u,508, 7242 unique and 5344 assumed as
observed with I.3s5I); Rˆ0.053 and Rwˆ0.059. Atomic
coordinates, bond lengths and angles, and thermal para-
meters have been deposited at the Cambridge Crystallo-
graphic Data Centre 5CCDC) as supplementary
publication number CCDC 153438. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: 144-
50)1223-336033 or e-mail: deposit@CCDC.cam.ac.uk].
References
1. 5a) Collier, A. C.; Coombs, R. W.; Schoenfeld, D. A.; Bassett,
R. L.; Timpone, J.; Baruch, A.; Jones, M.; Facey, K.;
Whitacre, C.; McAuliffe, V. J.; Friedman, H. M.; Merigan,