Palladium-catalyzed synthesis of carcinogenic polycyclic aromatic hydrocarbon epoxide-nucleoside adducts: The first amination of a chloro nucleoside

Article abstract of DOI:10.1021/ol027084w

(Matrix presented) Pd-catalyzed coupling of the axially constrained, less reactive benzo[a]pyrene bay-region amino benzoates, derived from the tetrahydro and diol epoxides, with C-6 and C-2 halopurine deoxynucleosides offers an efficient approach to the s

Full text of DOI:10.1021/ol027084w

ORGANIC
LETTERS
2003
Vol. 5, No. 1
39-42
Palladium-Catalyzed Synthesis of
Carcinogenic Polycyclic Aromatic
Hydrocarbon Epoxide-Nucleoside
Adducts: The First Amination of a
Chloro Nucleoside1
Mahesh K. Lakshman* and Padmaja Gunda
Department of Chemistry, City College of CUNY, 138th Street at ConVent AVenue,
New York, New York 10031-9198
lakshman@sci.ccny.cuny.edu
Received October 11, 2002
ABSTRACT
Pd-catalyzed coupling of the axially constrained, less reactive benzo[a]pyrene bay-region amino benzoates, derived from the tetrahydro and
diol epoxides, with C-6 and C-2 halopurine deoxynucleosides offers an efficient approach to the synthesis of the corresponding nucleoside−
epoxide adducts. Also reported are the first examples involving the coupling of a 6-chloropurine deoxynucleoside with these amines, a reaction
that is difficult by direct halide displacement. Certain mechanistic aspects of this metal-catalyzed C−N bond formation are also discussed.
Polycyclic aromatic hydrocarbons (PAHs) are widely present
in the environment, and several members of this class of
compounds undergo metabolic activation to four isomeric
diol epoxides.2 These electrophiles alkylate cellular DNA
producing covalent lesions,2,3 which is considered to be the
first step in the multistep cascade leading to tumorigenesis.
DNA alkylation by diol epoxides occurs by protonation and
ring opening of the oxirane, followed by the formation of a
C-N bond between the amino groups of the nucleobases
and the benzylic carbon of the PAH. Metabolism and DNA
binding of any PAH results in the formation of two sets of
8 isomeric deoxyadenosine and deoxyguanosine adducts. For
detailed studies into PAH-induced carcinogenesis substantial
effort by us and others has led to the development of
techniques for the site-specific modification of DNA by
stereochemically defined PAH diol epoxide-nucleoside
adducts.4 The most generally applicable method involves the
coupling of an amino derivative of a hydrocarbon with an
electrophilic nucleoside. Due to conformational factors, bay-
region amines of PAHs are not very nucleophilic and the
(3) (a) Sims, P.; Grover, P. L.; Swaisland, A.; Pal, K.; Hewer, A. Nature
1974, 252, 326-327. (b) Weinstein, I. B.; Jeffrey, A. M.; Jennette, K. W.;
Blobstein, S. H.; Harvey, R. G.; Harris, C.; Autrup, H.; Kasai, H.; Nakanishi,
K. Science 1976, 193, 592-594. (c) Meehan, T.; Straub, K. Nature 1979,
277, 410-412. (d) Jerina, D. M.; Chadha, A.; Cheh, A. M.; Schurdak, M.
E.; Wood, A. W.; Sayer, J. M. In Biological ReactiVe Intermediates IV,
Molecular and Cellular Effects and Their Impact on Human Health; Witmer,
C. M., Snyder, R., Jollow, D. J., Kalf, G. F., Kocsis, J. J., Sipes, I. G.,
Eds.; Plenum Press: New York, 1990; pp 533-553.
(1) Gunda, P.; Lakshman, M. K. Abstracts of Papers; 224th National
Meeting of the American Chemical Society, Boston, MA; American
Chemical Society: Washington, DC, 2002; ORGN 465.
(2) Dipple, A. In DNA Adducts: Identification and Biological Signifi-
cance; Hemminki, K., Dipple, A., Shuker, D. E. G., Kadlubar, F. F.,
Segerba¨ck, D., Bartsch, H., Eds.; IARC Scientific Publication No. 125;
IARC: Lyon, France, 1994; pp 107-129.
10.1021/ol027084w CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/13/2002

Scheme 1
highly reactive fluoro or triflate bearing nucleosides are
studies on DNA containing the tetrahydro and diol epoxides
will be useful in understanding what, if any influence the
two additional hydroxyl groups have on the local DNA
structure that is ultimately reflected in the biological
response. Amine (()-2 and nucleosides 4-7 required for
these studies were prepared on the basis of known methods
(see the Supporting Information).
Our earlier studies7a,12 provided a reasonable starting point
for determining whether (()-2 and the halo nucleosides could
be coupled by Pd catalysts. Using the catalytic system Pd2-
(dba)3/2-(dicyclohexylphosphino)-2′-(N,N-dimethylamino)-
1,1′-biphenyl (10:30 mol %) and Cs2CO3 (1.4 molar equiv)
the cross-coupling of 4 with (()-2 [4:(()-2 1:1.1 molar
equiv] was conducted in PhMe at 80 °C. In contrast to
arylamination reactions7a this reaction was slow reaching
completion in 34 h and in low yield (25% of the N6-
deoxyadenosine adducts, Scheme 1). Replacement of Cs2-
CO3 with K3PO4 (1.5 molar equiv), under otherwise identical
conditions, also resulted in a slow reaction and low yield
(35 h, 27%). Changing the supporting ligand for Pd was then
considered. Use of the Pd2(dba)3/(()-BINAP/Cs2CO3 system
required for the synthesis of the nucleoside adducts.4a,c,d,5,6
Recently, we7 and others8-10 have developed methods for
Pd-catalyzed C-N bond formation of nucleosides based upon
new catalysis methods.11 Since the bromo nucleosides
required for these reactions are simpler to prepare than the
fluoro analogues, we reasoned that Pd-mediated C-N bond
formation would offer a unique and more facile entry to the
biologically important PAH epoxide-nucleoside adducts.
This letter discloses our preliminary results on the use of Pd
catalysis as well as an analysis of certain mechanistic aspects
of the reactions leading to these adducts.
Although diol epoxides (A and B, Figure 1) are the
metabolically activated forms of the carcinogen benzo[a]-
pyrene (BaP), the tetrahydroepoxide (()-1 (Figure 1) was
(4) For examples see the following and references therein: (a) Chaturvedi,
S.; Lakshman, M. K. Carcinogenesis 1996, 17, 2747-2752. (b) Steinbre-
cher, T.; Becker, A.; Stezowski, J. J.; Oesch, F.; Seidel, A. Tetrahedron
Lett. 1993, 34, 1773-1774. (c) Kim, S. J.; Stone, M. P.; Harris, C. M.;
Harris, T. M. J. Am. Chem. Soc. 1992, 114, 5480-5481. (d) Cooper, M.
D.; Hodge, R. P.; Tamura, P. J.; Wilkinson, A. S.; Harris, C. M.; Harris, T.
M. Tetrahedron Lett. 2000, 41, 3555-3558. (e) Cosman, M.; Ibanez, V.;
Geacintov, N. E.; Harvey, R. G. Carcinogenesis 1990, 11, 1667-1672.
(5) Lakshman, M.; Lehr, R. E. Tetrahedron Lett. 1990, 31, 1547-1550.
(6) Kim, S. J.; Harris, C. M.; Jung, K.-Y.; Koreeda, M.; Harris, T. M.
Tetrahedron Lett. 1991, 32, 6073-6076.
(7) (a) Lakshman, M. K.; Keeler, J. C.; Hilmer, J. H.; Martin, J. Q. J.
Am. Chem. Soc. 1999, 121, 6090-6091. (b) Reviewed in: Lakshman, M.
K. J. Organomet. Chem. 2002, 653, 234-251.
(8) (a) De Riccardis, F.; Bonala, F.; Johnson, F. J. Am. Chem. Soc. 1999,
121, 10453-10460. (b) De Riccardis, F.; Johnson, F. Org. Lett. 2000, 2,
293-295. (c) Bonala, R.; Shishkina, I. G.; Johnson, F. Tetrahedron Lett.
2000, 41, 7281-7284.
(9) (a) Harwood, E. A.; Hopkins, P. B.; Sigurdsson, S. T. J. Org. Chem.
2000, 65, 2959-2964. (b) Harwood, E. A.; Sigurdsson, S. T.; Edfeldt, N.
B. F.; Reid, B. R.; Hopkins, P. B. J. Am. Chem. Soc. 1999, 121, 5081-
5082.
(10) (a) Wang, Z.; Rizzo, C. J. Org. Lett. 2001, 3, 565-568. (b)
Schoffers, E.; Olsen, P. D.; Means, J. C. Org. Lett. 2001, 3, 4221-4223.
(c) Meier, C.; Gra¨sl, S. Synlett 2002, 802-804.
Figure 1. Structures of the BaP diol and tetrahydro epoxides, the
amino benzoates derived by a trans ring opening of these as well
as halo nucleosides utilized in this study.
(11) For recent review articles see the following: (a) Hartwig, J. F. Synlett
1997, 329-340. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-
2067. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805-818. (d) Hartwig, J. F. Acc. Chem. Res. 1998,
31, 852-860. (e) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999,
576, 125-146.
(12) Lakshman, M. K.; Hilmer, J. H.; Martin, J. Q.; Keeler, J. C.; Dinh,
Y. Q. V.; Ngassa, F. N.; Russon, L. M. J. Am. Chem. Soc. 2001, 123, 7779-
7787.
chosen as the experimental prototype for several reasons.
(a) Epoxide (()-1 is simpler to prepare compared to the diol
epoxides. (b) In contrast to the four chiral centers in A and
B, (()-1 has only two but this model is representative of
both A and B. This feature simplifies assessment of chiral
integrity. (c) Since B is carcinogenic but not 1, comparative
40
Org. Lett., Vol. 5, No. 1, 2003

Scheme 2
led to improved coupling yield and reaction time (60%, 5
example of Pd-catalyzed C-N bond formation inVolVing
chloro nucleoside 5 and also the first efficient synthesis of
bay region amine adducts from this substrate.
h). Next Pd(OAc)2-based catalytic systems were tested. These
reactions were conducted in toluene at 80 °C, with 10:30
mol % of Pd(OAc)2:ligand in each case. A summary of these
experiments is as follows. (a) 2-(Dicyclohexylphosphino)-
1,1′-biphenyl, Cs2CO3 led to complete consumption of 4
within 23 h but an insignificant amount of product was
observed. (b) 2-(Dicyclohexylphosphino)-2′-(N,N-dimethyl-
amino)-1,1′-biphenyl, Cs2CO3 led to no appreciable product
formation. (c) (()-BINAP and K3PO4 resulted in a 73% yield
of product in 12 h. (d) (()-BINAP and Cs2CO3 produced a
5 h reaction with a product yield of 80%. (e) 1,1′-Bis-
(diphenylphosphino)ferrocene with Cs2CO3 produced a 10
h reaction time and a 44% product yield. These catalyst
optimization experiments showed that the combination of
Pd(OAc)2/(()-BINAP/Cs2CO3 in toluene at 80 °C gave the
best results. The diastereomeric N6-deoxyadenosine adducts
could not be easily resolved, but they could be readily
characterized as a mixture.
Next we turned to the investigation of Pd-mediated C-N
bond formation using nucleoside 5. Despite the fact that
chloroaromatics and chloroheterocycles have been utilized
for C-N bond formation,13 the extension to chloro nucleo-
sides is not necessarily obvious. We have recently shown
that 5 is superior to 4 for C-C cross coupling,12 but the
utility of 5 for metal-catalyzed C-N bond formation is
currently unknown. Coupling of 5 with (()-2 using the
optimized catalytic system described above was very inter-
esting. This reaction was complete in 5.5 h affording the
N6-deoxyadenosine adducts in 90% yield. Thus, it appears
that chloro nucleoside 5 may be superior and at the least
comparable to the bromo analogue 4 in its reactivity. It is
noteworthy that bay region amines such as (()-2 react poorly
with 5 via the SNAr mechanism.5,6 This represents the first
At this point it was important to address some critical
questions. The first, more obvious, was a determination that
the reactions of 4 and 5 were mediated by Pd and that the
reactions did not proceed by the well-known SNAr displace-
ment (Scheme 2). In control experiments with 4 and 5 the
reactions were conducted under the optimized conditions
except that Pd(OAc)2 was omitted from the reaction mixtures.
In these cases, no product formation was observed even after
24 h, clearly indicating the catalytic role of Pd. This implied
that intermediate I (Scheme 2) was not responsible for
product formation. The second and perhaps more important
question was with regard to the chiral integrity in the
reaction. As described above, since a catalytic role of Pd
had been established, the putative intermediate II leads to
the N6 adduct by a reductive-elimination and regeneration
of the Pd(0) catalyst. Intermediate II can also potentially
undergo â-hydride elimination and such a process has been
known to cause loss of chirality in aryl aminations.14 In the
present case, a â-hydride elimination would yield a highly
conjugated 10-imino BaP derivative. In fact, facile oxidation
at the C-10 position of tetrahydro BaP is known, such as
the mild DDQ-mediated oxidation to the ketone as well as
the benzylic acetate.15 If the â-hydride elimination from
intermediate II was reversible, then loss of stereochemical
integrity at C-10 was possible. This could result in adducts
with cis and trans stereochemistry. Although bis-coordinating
ligands such as (()-BINAP have been shown to prevent
attrition of chirality,14 whether loss of stereochemistry was
a problem in the present case had to be unequivocally
addressed. Fluoride displacement from 6 by (()-2 should
occur via intermediate I without erosion of the relative
stereochemistry. Thus, the N6-deoxyadenosine mixture was
synthesized via the SNAr displacement.4a Comparison of the
NMR data of the product from this reaction to those isolated
(13) See for example: (a) Old, D. W.; Wolfe, J. P.; Buchwald, S. L. J.
Am. Chem. Soc. 1998, 120, 9722-9723. (b) Wolfe, J. P.; Buchwald, S. L.
Angew. Chem., Int. Ed. 1999, 38, 2413-2416. (c) Hartwig, J. F.; Kawatsura,
M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-Roman, L. M. J. Org. Chem.
1999, 64, 5575-5580. (d) Bei, X.; Guram. A. S.; Turner, H. W.; Weinberg,
W. H. Tetrahedron Lett. 1999, 40, 1237-1240. (e) Wolfe, J. P.; Tomori,
H.; Sadighi, J. P.; Yin, J.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1158-
1174. (f) Kosˇmrlj, J.; Maes, B. U. W.; Lemie`re, G. L. F.; Haemers, A.
Synlett 2000, 1581-1584. (g) Hong, Y.; Tanoury, G. J.; Wilkinson, H. S.;
Bakale, R. P.; Wald, S. A.; Senanayake, C. H. Tetrahedron Lett. 1997, 38,
5607-5610.
(14) Wagaw, S.; Rennels, R. A.; Buchwald, S. L. J. Am. Chem. Soc.
1997, 119, 8451-8458.
(15) (a) Fu, P. P.; Clark, J. D.; Huang, A. Y. J. Chem. Res. (S) 1982,
121. (b) Lehr, R. E.; Kole, P. L.; Tschappat, K. D. Tetrahedron Lett. 1986,
27, 1649-1652. (c) Lee, H.; Harvey, R. G. J. Org. Chem. 1988, 53, 4587-
4589.
Org. Lett., Vol. 5, No. 1, 2003
41

from the Pd-mediated reactions of 4 and 5 revealed these
were identical. This clearly indicated that no detectable loss
of stereochemistry had occurred in the metal-catalyzed
reaction, and that â-hydride elimination was perhaps not
competitive with reductive-elimination. Another factor also
became clear in this comparison. Although dimeric products
are known to form in Pd-catalyzed aminations of nucleosides,
particularly at the N6,8a,16 such a product was not obvious
in the present case.
of BaP diol epoxides.18 Thus, the more mobile diastereomer
on silica gel was assigned a 10S absolute configuration at
the point of attachment of the hydrocarbon to the nucleoside
and in the less mobile isomer this was 10R.
Finally, the method was evaluated for synthesis of diol
epoxide adducts. In a single unoptimized reaction (()-3,
derived from the carcinogen (()-B,19 was coupled with 5
using the optimized catalytic system. This reaction was
complete within 4 h and produced a 71% yield of the N6
-
At this point we were interested in testing the method for
the synthesis of the N2-deoxyguanosine adducts, for which
compound 7 was a suitable substrate. Thus, we tested the
coupling of (()-2 with 7 using the optimized conditions [Pd-
(OAc)2/(()-BINAP/Cs2CO3 in toluene at 80 °C]. This
reaction was complete within 3.5 h producing the O6-benzyl-
N2-deoxyguanosine adducts in 79% yield. This yield is
significantly better than that reported for the synthesis of
similar N2-deoxyguanosine adducts by fluoride displace-
ment.17 Debenzylation of the adduct mixture (1:1 THF-
MeOH, 1 atm of H2, 5 h at room temperature) yielded the
N2-deoxyguanosine adducts (Scheme 1) in 97% yield.
Whereas the O6-benzyl derivatives were not separated on
TLC, a clear separation of the two diastereomers was evident
after debenzylation. Partial resolution of the diastereomers
was therefore performed by chromatography. Chirality
assignment to the two diastereomers was done by CD
diol epoxide adducts. For comparison, this adduct mixture
was also synthesized from a reaction of (()-3 with 6, and
the products from the two methods proved identical.
This study demonstrates that Pd-catalyzed C-N bond
formation between the axially constrained PAH bay-region
amines and halo nucleosides offers a facile approach to the
biologically important PAH epoxide-nucleoside adducts. It
is also shown, for the first time, that chloro nucleosides are
good partners in these reactions, and this metal-mediated
synthesis of such adducts appears superior to conventional
ones.20 These adducts can be converted to phosphoramidites
for DNA assembly. Alternatively, Pd-mediated amination of
DNA oligomers containing 4, 5, and 7 may provide a facile
method for site-specific modification. These and other related
aspects are being studied in our laboratories.
Acknowledgment. This work was funded by NCI grant
1R15 CA094224-01 and a PSC-CUNY 33 award. Purchase
of a CD spectrometer was funded by NSF CHE-0130787.
We thank Dr. Herman Yeh (NIDDK-NIH) for assistance with
some high-field NMR data. Mr. John Carriker and Dr.
Amanda Jenkins (Jasco, Inc.) are thanked for the CD data.
Y. Dinh performed some preliminary experiments.
Supporting Information Available: Synthetic details and
1H NMR spectra of the adducts. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL027084W
(16) Lakshman, M. K.; Ngassa, F. N.; Bae, S.; Buchanan, D. G.; Hahn,
H.-G.; Mah, H. Unpublished observations.
(17) Zajc, B.; Lakshman, M. K.; Sayer, J. M.; Jerina, D. M. Tetrahedron
Lett. 1992, 33, 3409-3412.
(18) Cheng, S. C.; Hilton, B. D.; Roman, J. M.; Dipple, A. Chem. Res.
Toxicol. 1989, 2, 334-340.
Figure 2. CD spectra of the diastereomeric N2-deoxyguanosine
adducts in MeOH, normalized to 1 OD at λmax
.
(19) (a) Lakshman, M. K.; Chaturvedi, S.; Lehr, R. E. Synth. Commun.
1994, 24, 2983-2988. (b), Lakshman, M.; Nadkarni, D. V.; Lehr, R. E. J.
Org. Chem. 1990, 55, 4892-4897.
(20) Coupling of (()-3 and the 6-fluoro nucleoside was complete in 18
h at 85 °C (Chaturvedi, S. Ph.D. Dissertation, University of Oklahoma,
Norman, OK, 1993), comparable to our experience.
analysis (Figure 2) and comparison to the CD spectra of
deoxyguanosine adducts arising from a trans ring opening
42
Org. Lett., Vol. 5, No. 1, 2003