Synthesis of biologically active n2-amine adducts of 2'-deoxyguanosine

Article abstract of DOI:10.1016/S0040-4039(00)01196-5

Several biologically important N²-adducts of 2'-deoxyguanosine (dG) that previously were difficultly accessible, have been synthesized directly by means of the Buchwald-Hartwig reaction. The reaction employed in each case involves the coupling

Full text of DOI:10.1016/S0040-4039(00)01196-5

Tetrahedron Letters 41 (2000) 7281±7284
Synthesis of biologically active N2-amine adducts of
20-deoxyguanosine
Radha R. Bonala, Irina G. Shishkina and Francis Johnson*
Department of Pharmacological Sciences, The State University of New York at Stony Brook, Stony Brook,
New York 11794-3400, USA
Received 12 June 2000; revised 5 July 2000; accepted 6 July 2000
Abstract
Several biologically important N2-adducts of 20-deoxyguanosine (dG) that previously were dicultly
accessible, have been synthesized directly by means of the Buchwald±Hartwig reaction. The reaction
employed in each case involves the coupling of 20-deoxy-2-bromoinosine with the appropriate amine.
Deprotection in all cases gave good yields of the desired 2-alkylated or -arylated deoxynucleoside. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: 20-deoxyguanosine; N2-adducts; Buchwald±Hartwig reaction.
The N2 position of deoxyguanosine residues in DNA appears to be one of the biologically most
important sites for modi®cation by mutagenic substances. In recent years a variety of methods for
the synthesis of such N2-substituted derivatives of the dG have been reported.1 4 Most of these
processes are either compound speci®c or limited in scope. In general, ¯exibility in terms of
product range is poor and overall yields are in most cases only modest. One apparent exception is
the method of Steinbrecher et al.2 who reported that the tri¯ate esters of hydroxypurines will
react with aliphatic and aryl amines to give the corresponding exocyclic N-alkyl or N-aryl
deoxynucleosides in good yield. However, our experience with this approach has been that the
tri¯ates are dicult to prepare and the yields are capricious.5 Recently, in searching for a method
that would be reliable and ecient, we examined6 the coupling of o-nitroaryl tri¯ates or bromides
with protected forms of 20-deoxyguanosine and 20-deoxyadenosine using the conditions of the
Buchwald±Hartwig reaction. This gave high yields of the 2-nitroaryl derivatives of dG and dA
arylated at the exocyclic amine group (Scheme 1; illustrated for dG only). However, it cannot be
used with alkyl or aralkyl halides.
Now we present an alternative approach which allows the synthesis of essentially any N2-(carbon-
coupled) derivative of 20-deoxyguanosine reproducibly and in excellent yield. The new method is
* Corresponding author. Tel: 631-632-8867; fax: 631-632-7394; e-mail: francis@pharm.sunysb.edu
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01196-5

7282
Scheme 1.
essentially the inverse of the above process in that a bromopurine is coupled to an amine. This is
similar both to the procedure that Hopkins7 and we ourselves8 used previously for the synthesis
of the cross-linked adducts of dG and to the work of Lakshman9 who coupled 6-bromo-20-
deoxynebularine with a series of aromatic amino compounds. The synthetic strategy (Scheme 2)
again involves a palladium-catalyzed amination reaction, this time between 2-bromo-O6-benzyl-
30,50-bis-O-tert-butyldimethylsilyl-20-deoxyinosine (1) and a series of aryl or alkyl amines (2a±2e)
in the presence of the complexing agent 2,20-bis(diphenylphosphino)-1,10-binaphthyl (BINAP)
and cesium carbonate, as the base10. The protected 2-bromo-20-deoxyinosine (1) was prepared as
previously described.8,11 The results of the coupling reactions are given in Table 1. All reactions
were carried out at 80ꢀC in toluene with the exception of 2e in which case THF was the solvent of
choice because of solubility problems with toluene.
Scheme 2.
Each of the products 3 was then converted by conventional chemistry in high yield to the
desired compound 4, known from previously published work. In all cases, except for that of 3e
(where reduction of the central ring of the anthracene group is facile), the benzyl group was
removed by catalytic hydrogenation over a 10% Pd/C catalyst. Compound 3e was debenzylated
by means of trimethylsilyl iodide in boiling THF.12 The tert-butyldimethylsilyl (TBS) protecting
groups were then easily removed by 1 M TBAF in THF, to give in each case the known reference
1
compound 4 in good to excellent yield (Table 1). In all cases the H NMR, 13C NMR and FAB
mass spectral data were identical to those previously reported. Compound 4a is an intermediate
that was used by our group to generate13 an acrolein adduct site-speci®cally in oligomeric DNA
whereas 4b14 was simply the pilot compound for the series. Both compound 4c and its 50-O-tri-
phosphate, powerful inhibitors of speci®c DNA polymerases, were available15 previously only by
total synthesis from guanine and 20-deoxyribose. The a-phenylglycidol adduct 4d is a known1
surrogate for the coupling of the aminotriol derivatives of the polycyclic aromatic hydrocarbons
such as the precarcinogen benzo[a]pyrene. Finally 4e, a hydrocarbon adduct of dG that has been

7283
Table 1
Reaction of amines with 2-bromo-20-deoxyinosine
incorporated into oligomeric DNA for biological studies, has been available4 by a four-step ele-
gant synthesis from 20-deoxyguanisine, but in only ꢁ12% overall yield.
The advantages of the method now reported for the synthesis of these N2-derivatives of dG are
its simplicity, its versatility and the excellent yields that are achievable. Further studies on the
application of the Buchwald±Hartwig reaction to the synthesis of other substituted nucleosides
are in progress.

7284
Acknowledgements
The authors thank Mr. Robert A. Rieger for obtaining the mass spectral data. This research
was supported by a grant (CA47995) from the National Cancer Institute, a division of the
National Institute of Health.
References
1. Harris, C. M.; Zhou, L.; Strand, E. A.; Harris, T. M. J. Am. Chem. Soc. 1991, 113, 4328±4329.
2. Steinbrecher, T.; Wameling, C.; Oesch, F.; Seidel, A. Angew. Chem. Int. Ed. Engl. 1993, 32, 404±406. Edwards, C.;
Boche, G.; Steinbrecher, T.; Scheer, S. J. Chem. Soc., Perkin Trans. 1 1997, 1887±1893.
3. Bonala, R. R.; Yu, P.-L.; Johnson, F. Tetrahedron Lett. 1999, 40, 597±598.
4. Casale, R.; McLaughlin, W. J. Am. Chem. Soc. 1990, 112, 5264±5271. Kasai, H.; Goto, M.; Ikeda, K.; Zama, M.;
Mizuno, Y.; Takemura, S.; Matsuura, S.; Sugimoto, T.; Goto, T. Biochemistry 1976, 15, 898±904.
5. See, however, a recent paper by Nechev et al. who have successfully used such a tri¯ate in a post-synthetic
coupling of aliphatic amines in DNA oligomers: Nechev, L. V.; Harris, C. M.; Harris, T. M. Chem. Res. Toxicol.
2000, 13, 421±429.
6. De Riccardis, F.; Bonala, R. R.; Johnson, F. J. Am. Chem. Soc. 1999, 45, 10453±10460.
7. Harwood, E. A.; Sigurdsson, S. T.; Edfeldt, N. B. F.; Reid, B. R.; Hopkins, P. B. J. Am. Chem. Soc. 1999, 121,
5081±5082.
8. De Riccardis, F.; Johnson, F. Org. Lett. 2000, 2, 293±295.
9. Lakshman, M. K.; Keeler, J. C.; Hilmer, J. H.; Martin, J. Q. J. Am. Chem. Soc. 1999, 121, 6090±6091.
10. Typical experimental procedure: An oven-dried reaction vial was charged with dry 1 (0.250 mmol), cesium
carbonate (0.350 mmol), palladium acetate (0.025 mmol), BINAP (0.037 mmol), the amine 2 (0.350 mmol), and
toluene (1 mL). The reaction mixture under nitrogen was stirred for 30 min at room temperature, heated at 80ꢀC
for 16 h, and then diluted with ethyl acetate. Centrifugation and concentration in a vacuum of the supernatant
liquid a€orded a residue, which was puri®ed by ¯ash chromatography (silica gel, hexane±ethyl acetate) to a€ord
1
the desired compound 3. H NMR, 13C NMR and FAB mass spectra were entirely consistent with the assigned
structures. Selected data are as follows: compound 3c: 1H NMR (300 MHz, CDCl3) ꢀ 7.97 (1H, s, H-8), 7.51 (4H,
m, C6H5CH2, -C6H4-NH), 7.38±7.29 (3H, m, C6H5CH2), 7.13 (2H, d, J=7.5 Hz, -C6H4-NH), 6.98 (1H, s, N-H),
6.40 (1H, t, J=6.6 Hz, H-10), 5.60 (2H, s, CH2-Ph), 4.58 (1H, m, H-30), 4.01 (1H, m, H-40), 3.80 (2H, m, H-50), 2.59
(2H, t, J=7.5 Hz, CH2C6H4), 2.53 (1H, m, H0-20), 2.43 (1H, m, H-20), 1.61 (2H, m, CH2CH2C6H4), 1.37 (2H, m,
CH2CH3), 0.93 (21H, m, (CH3)3C, CH3-CH2), 0.12 (6H, s, CH3Si), 0.09 (6H, s, CH3Si). 13C (62.9 MHz, CDCl3) ꢀ
^4.7 (Â2), ^4.6 (Â2), 14.0, 17.9, 18.2, 22.3, 25.8 (Â3), 26.0 (Â3), 33.8, 35.0, 41.4, 62.9, 68.1, 72.1, 83.9, 87.7, 116.1,
118.8, 119.0, 127.9 (Â2), 128.1, 128.3, 128.7 (Â2), 136.5, 136.7, 137.4, 137.9, 153.2, 156.0, 160.8. FABMS m/z 718
[M+1]+.
11. Robins, M. J.; Uznanski, B. Can. J. Chem. 1981, 59, 2608±2611.
12. Olah, G. A.; Narang, S. C.; Gupta, B. G. B.; Malhotra, R. J. Org. Chem. 1979, 44, 1247±1251.
13. Khullar, S.; Varaprasad, C. V.; Johnson, F. J. Med. Chem. 1999, 42, 947±950.
14. Moon, Ki-Y.; Moschel, R. C. Chem. Res. Toxicol. 1998, 11, 696±702.
15. Wright, G. E.; Dudycz, L. W. J. Med. Chem. 1984, 27, 175±181.