Synthesis of the C8-deoxyguanosine adduct of the food mutagen IQ.

Article abstract of DOI:10.1021/ol006968h

[structure: see text] The C8-2'-deoxyguanosine adduct of the food mutagen 2-amino-3-methylimadazo[4,5-f]-quinoline (IQ) has been synthesized. The key step is a palladium-catalyzed N-arylation of a suitably protected 8-bromo-2'-deoxyguanosine derivative.

Full text of DOI:10.1021/ol006968h

ORGANIC
LETTERS
2001
Vol. 3, No. 4
565-568
Synthesis of the C8-Deoxyguanosine
Adduct of the Food Mutagen IQ
Zhiwei Wang and Carmelo J. Rizzo*
Department of Chemistry, Vanderbilt UniVersity, VU Station B 351822,
NashVille, Tennessee 37235-1822
c.j.rizzo@Vanderbilt.edu
Received December 6, 2000
ABSTRACT
The C8-2′-deoxyguanosine adduct of the food mutagen 2-amino-3-methylimadazo[4,5-f]-quinoline (IQ) has been synthesized. The key step is
a palladium-catalyzed N-arylation of a suitably protected 8-bromo-2′-deoxygunaosine derivative.
Covalent modification of DNA by electrophiles is the initial
step in chemical carcinogenesis.1 If these modifications are
not repaired, they compromise the fidelity of DNA replica-
tion, leading to mutations and possibly cancer. Many such
electrophiles are generated only after metabolic activation
of a procarcinogen. Examples of such procarcinogens include
polycyclic aromatic hydrocarbons, vinyl chloride, and aryl-
amines. To properly study the mutagenic effects, structure,
and repair of these lesions, strategies for the site-specific
incorporation of DNA-carcinogen adducts into oligonucle-
otides must be developed. The adducted nucleosides are of
value as potential building blocks for modified oligonucle-
otides as well as for analytical standards.
carcinogenic species is an arylnitrenium ion generated by
cytochrome P450 oxidation to the corresponding hydroxyl-
amine, followed by esterification and solvolysis. The pre-
dominant site of reaction is the C8-position of deoxygua-
nosine, although N2-adducts have also been isolated as minor
products (Figure 2).
Oligonucleotides containing a site-specific C8-dG adduct
A growing number of mutagenic compounds from cooked
meats have been identified.2 These compounds are believed
to arise from the pyrolysis of amino acids and proteins. One
class, shown in Figure 1, possesses a common 2-amino-3-
methylimidazole subunit fused to a heteroaromatic ring
system. These compounds are highly mutagenic in the Ames
Salmonella test system. The most potent food mutagens are
IQ (2) and MeIQ (3), which are 15 and 24 times more
mutagenic than aflatoxin b1, respectively.1b The ultimate
(1) Garner, R. C. Mutat. Res. 1998, 402, 67.
(2) (a) Schut, H. A. J.; Snyderwine, E. G. Carcinogenesis 1999, 20, 353.
(b) Sugimura, T. Mutat. Res. 1997, 376, 211.
Figure 1. Aminoimidazoazaarene (AIA) food mutagens.
10.1021/ol006968h CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/20/2001

Scheme 1
Figure 2. C8- and N2-deoxyguanosine AIA adducts.
of 2-aminofluorene (AF) and its N-acetyl analogue have been
prepared, and their structures and mutagenic effects are well
studied.3 Recently, PhIP (9) has been site-specifically
incorporated into oligonucleotides.4 These oligonucleotides
were prepared by a biomimetic approach in which N-acetoxy-
PhIP was reacted with oligonucleotides containing a single
guanosine; Johnson has previously commented on the
drawbacks to this synthetic appraoch.3c Overall, the mutage-
nicity of the C8-PhIP adduct was similar to that of the
corresponding AF adduct; however, the mutagenic frequency
of PhIP was up to nine times higher depending on sequence.
A computational study of an IQ adduct has been recently
reported and suggests some structural differences from the
corresponding AF adduct.5 We report here the synthesis of
the C8-2′-deoxyguanosine adduct of the food mutagen IQ
(2). The synthesis features a Buchwald-Hartwig6 palladium-
catalyzed N-arylation of a suitably protected 8-bromo-2′-
deoxyguanosine derivative with IQ as the key reaction.
The Buchwald-Hartwig reaction has been used recently
for the preparation of nucleoside-carcinogen adducts by
Lakshman7 and later Johnson,8a for the preparation of N6-
2′-deoxyadensosine derivatives. Hopkins and Sigurdsson9 and
Johnson4b-d synthesized N2-dG-N2-dG and N2-dG-N6-dA
nitrous acid cross-links as well as other N2-dG aryl deriva-
tives via an N-arylation reaction. It is worth noting that
Johnson’s N-arylation approach involved coupling of the
exocyclic amino groups of a suitably protected dA and dG
derivative with bromoarenes, while Lakshman and Hopkins
and Sigurdsson employed the corresponding bromopurine
with arylamines. To our knowledge, the synthesis of C8-dG
adducts of arylamine using a palladium-catalyzed N-arylation
reaction has not yet been reported.
Buchwald and others have reported the N-arylation of
amides.10 Thus, conventional amide protecting groups for
N2 of dG would be unsatisfactory. Initially, we employed a
dimethyl formamidine group which is commonly used for
N2-dG protection (Scheme 1). The O6-position was protected
as a benzyl ether. Attempted palladium-catalyzed N-arylation
of 13 with model substrate 14 gave only transfer of the N2-
protecting group to the amino group of 14. No N-arylation
of the C8-position was observed. The transfer of the
formamidine group to other amines has been previously
reported.11
We next examined the tetramethyldisilylazacyclopentane
(STABASE) group, a base-stable protecting group for
primary amines developed by Magnus.12 The substrate for
the N-arylation reaction (17) was readily prepared from
8-bromo-2′-deoxyguanosine according to Scheme 2. Buch-
wald-Hartwig reaction of 17 with model amine 14 under
the conditions shown in Scheme 1 gave the desired product
in 32% yield. We found that the prolonged reaction time
led to decomposition of 17. Other mild bases such Cs2CO3
or K3PO4 gave lower yields. The optimal conditions for the
desired reaction involved increasing the catalyst loading to
10 mol % and using lithium hexamethyldisilazide as the base
(Scheme 2). Under these conditions a 68% yield of the
desired product (18) could be obtained in just 20 min. These
conditions were also satisfactory for the coupling of IQ (2)
with 17 to give 19 in 68% yield (Scheme 3). Treatment of
(3) (a) Zhou, Y.; Romano, L. J. Biochemistry 1993, 32, 14043. (b)
Shibutani, S.; Gentles, R. G.; Iden, C. R.; Johnson, F. J. Am. Chem. Soc.
1990, 112, 5667. (c) Johnson, F.; Huang, C.-Y.; Yu, P.-L. EnViron. Healh
Perspect. 1994, 102 Supp. 6, 143. (d) Patel, D. J.; Mao, B.; Gu, Z.; Hingerty,
B. E.; Gorin, A.; Basu, A. K.; Broyde, S. Chem. Res. Toxicol. 1998, 11,
391.
(4) Shibutani, S.; Fernandes, A.; Suzuki, N.; Zhou, L.; Johnson, F.;
Grollman, A. P. J. Biol. Chem. 1999, 274, 27433.
(5) Wu, X.; Shapiro, R.; Broyde, S. Chem. Res. Toxicol. 1999, 12, 895.
(6) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Acc. Chem. Res. 1998, 31,
852. (c) Hartwig, J. F. Angew. Chem., Int. Ed.1998, 37, 2046. (d) Yang, B.
H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125.
(7) Lakshman, M. K.; Keeler, J. C.; Hilmer, J. H.; Martin, J. Q. J. Am.
Chem. Soc. 1999, 121, 6090.
(8) (a) De Riccardis, F.; Bonala, R. R.; Johnson, F. J. Am. Chem. Soc.
1999, 121, 10453. (b) Bonala, R. R.; Yu, P.-L.; Johnson, F. Tetrahedron
Lett. 1999, 40, 597. (c) De Riccardis, F.; Johnson, F. Org. Lett. 2000, 2,
293. (d) Bonala, R. R.; Shishkina, I. G.; Johnson, F. Tetrahedron Lett. 2000,
41, 7281.
(10) Yin J.; Buchwald, S. L. Org. Lett. 2000, 2, 1101 and references
therein.
(11) (a) Vincent, S.; Mons, S.; Lebeau, L.; Mioskowski, C. Tetrahedron
Lett. 1997, 38, 7527. (b) Theisen, P.; McCollum, C.; Andrus, A. Nucleosides
Nucleotides 1993, 12, 1033.
(12) Djuric, S.; Venit, J.; Magnus, P. Tetrahedron Lett. 1981, 22, 1787.
(13) (a) Louie, J.; Hartwig, J. F.; Fry, A. J. J. Am. Chem. Soc. 1997,
119, 11695. (b) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609.
(14) Kelly, T. A.; McNeil, D. W. Tetrahedron Lett. 1994, 35, 9003.
(9) (a) Harwood, E. A.; Hopkins, P. B.; Sigurdsson, S. Th. J. Org. Chem.
2000, 65, 2959. (b) Harwood, E. A.; Sigurdsson, S. Th.; Edfeldt, N. B. F.;
Reid, B. R.; Hopkins, P. B. J. Am. Chem. Soc. 1999, 121, 5081.
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Org. Lett., Vol. 3, No. 4, 2001

Scheme 2
Scheme 4
that LiHMDS is generating an appreciable concentration of
the corresponding lithium amide of 14 or 2 which is the
reactive substrate.
To examine the generality of this approach for the
synthesis of C8-dG arylamine adducts, the N-arylation of
17 with benzylamine, 4-aminobiphenyl, 2-aminofluorene, and
2-naphthylamine was attempted. However, the optimal
conditions for the Buchwald-Hartwig reaction of 17 with
14 and 2 shown Schemes 2 and 3 gave largely decomposition
with these simple arylamines; less than 10% of the desired
product was observed. We concluded that the STABASE
group was not satisfactory for the N-arylation of other
amines. Protection of the N2-position as a bis-BOC derivative
(20) improved the results.14 The bis-BOC group is sensitive
to strong base. When the coupling was attempted with
LiHMDS or sodium tert-butoxide, the desired product could
be obtained in 30-40% yields as a mixture of di-BOC and
mono-BOC protected products. The optimal conditions for
N-arylation of 20 are shown in Scheme 4, providing the C8-
arylamine products (21) in 50-60% yields (Table 1). These
conditions were as described by Lakshman for the N-
arylation of 6-bromopurine. Comparable yields were obtained
when 2-dicyclohexylphosphino-2′-(N,N-dimethylamino)bi-
phenyl and BINAP were used as the catalyst.
19 with flouride followed by hydrogenolysis of the O6-benzyl
group gave the desired C8-IQ adduct of dG (1) in 66%
overall yield. The synthesis of 1 required six steps from
commercially available 8-bromo-2′-deoxyguanosine and
proceeded in 32% overall yield. The key to the successful
coupling of 17 with 14 or 2 is the use of lithium
hexamethyldisilazide (LiHMDS) as the base, which is much
stronger than is typically used for the N-arylation reaction.
Hartwig has reported the use of lithium amides or amines
with LiHMDS in the cross-coupling with bromo-
arenes in high yield and short reaction times.13 It is possible
Scheme 3
In conclusion, we have demonstrated the feasibility of the
Buchwald-Hartwig palladium-catalyzed N-arylation reaction
Table 1. N-Arylation of 20 with Mutagenic Arylamines
Org. Lett., Vol. 3, No. 4, 2001
567

for the synthesis of C8-dG amine adducts. Using this method,
we synthesized the C8-dG adduct of the food mutagen IQ.
In the process, we introduced the use of the STABASE and
bis-BOC protecting groups for N2 of dG. This strategy
appears to be general and should be applicable to the
synthesis of other C8-dG food mutagen adducts. Work on
the conversion of 1 into a phosphoramidite reagent suitable
for solid-phase oligonucleotide synthesis as well as the
synthesis of other C8-adducts of food mutagens is currently
underway.
Acknowledgment. This work was supported by Grant
RPG-96-061-04-CDD from the American Cancer Society.
Mr. C. Eric Elmquist is gratefully acknowledged for the
preparation of IQ.
Supporting Information Available: Experimental pro-
cedures for the preparation of 1 and copies of all 1H and 13
C
NMR spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL006968H
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Org. Lett., Vol. 3, No. 4, 2001