Synthesis of pyrene and benzo[a]pyrene adducts at the exocyclic amino groups of 2′-deoxyadenosine and 2′-deoxyguanosine by a palladium-mediated C - N bond-formation strategy

Article abstract of DOI:10.1021/jo030113b

Single-electron oxidation of the carcinogenic hydrocarbon benzo[a]pyrene (BaP) is thought to result in a radical cation intermediate and this species has been proposed to cause alkylation at the nitrogens of the purine nucleobases. Although several differ

Full text of DOI:10.1021/jo030113b

Syn th esis of P yr en e a n d Ben zo[a ]p yr en e Ad d u cts a t th e Exocyclic
Am in o Gr ou p s of 2′-Deoxya d en osin e a n d 2′-Deoxygu a n osin e by a
P a lla d iu m -Med ia ted C-N Bon d -F or m a tion Str a tegy
Mahesh K. Lakshman,*,§ Felix N. Ngassa,† Suyeal Bae,‡ Dennis G. Buchanan,§
Hoh-Gyu Hahn, and Heduck Mah*,‡
Department of Chemistry, City College of CUNY, 138th Street at Convent Avenue,
New York, New York 10031-9198, Department of Chemistry, University of North Dakota,
Grand Forks, North Dakota 58202-9024, Department of Chemistry, Kyonggi University,
Suwon 442-760, Korea, and Organic Chemistry Laboratory, Korea Institute of Science and Technology,
P.O. Box 131, Cheongryang, Seoul 136-791, Korea
lakshman@sci.ccny.cuny.edu; hdma@kuic.kyonggi.ac.kr
Received April 2, 2003
Single-electron oxidation of the carcinogenic hydrocarbon benzo[a]pyrene (BaP) is thought to result
in a radical cation intermediate and this species has been proposed to cause alkylation at the
nitrogens of the purine nucleobases. Although several different nucleoside adducts have been isolated
as arising from this mode of metabolic activation, there are no selective, total syntheses of the
stable exocyclic amino group adducts formed by the single-electron oxidation of any hydrocarbon
with the purine 2′-deoxynucleosides to date. In this paper we disclose the synthesis of the model
adducts N6-(1-pyrenyl)-2′-deoxyadenosine and N2-(1-pyrenyl)-2′-deoxyguanosine as well as the first
synthesis of the carcinogen-linked nucleoside derivatives N6-(6-benzo[a]pyrenyl)-2′-deoxyadenosine
and N2-(6-benzo[a]pyrenyl)-2′-deoxyguanosine via a palladium-mediated C-N bond formation. Two
different coupling strategies were attempted: coupling of an aryl bromide with a suitably protected
nucleoside and the coupling of an arylamine with a suitable halonucleoside. The former had
somewhat limited applicability in that only N6-(1-pyrenyl)-2′-deoxyadenosine was prepared by this
method; on the other hand, the latter was more general. However, there are noteworthy differences
in the amination reactions at the C-6 and C-2 positions. Reactions at the C-6 resulted in the
competing formation of a 1:2 amine-nucleoside adduct in addition to the desired monoaryl
nucleoside. Such a dimer formation was not observed at the C-2. The C-2 adducts, however, displayed
an interesting conformational behavior.
In tr od u ction
diol epoxides (referred to as the monooxygenation path-
way).2 These electrophiles, in the presence of DNA
undergo epoxide bond-scission leading to benzylic car-
bocations that are then trapped by the amino groups of
adenine and guanine within DNA.3 In the second pro-
posed metabolic activation, BaP undergoes loss of a single
By virtue of their environmental prevalence, several
polycyclic aromatic hydrocarbons (PAHs) pose a health
threat to humans.1 Benzo[a]pyrene (BaP), an alternant,
bay-region containing hydrocarbon, is one such contami-
nant that is known to be a potent carcinogen and has
attracted research interest for several decades. In mam-
malian systems, two possible modes of metabolic activa-
tion have been proposed for this hydrocarbon as respon-
sible sources for its tumorigenicity (Scheme 1). In the
first, BaP is converted by the combined actions of
cytochrome P450 and epoxide hydrolase to four isomeric
(2) Reviewed in: (a) Thakker, D. R.; Levin, W.; Wood, A. W.; Conney,
A. H.; Yagi, H.; J erina, D. M. In Drug Stereochemistry, Analytical
Methods and Pharmacology; Wainer, I. W., Drayer, D. E., Eds.; Marcel
Dekker: New York, 1988; pp 271-296. (b) J erina, D. M.; Sayer, J . M.;
Agarwal, S. K.; Yagi, H.; Levin, W.; Wood, A. W.; Conney, A. H.; Pruess-
Schwartz, D.; Baird, W. M.; Pigott, M. A.; Dipple, A. In Biological
Reactive Intermediates III; Kocsis, J . J ., J ollow, D. J ., Witmer, C. M.,
Nelson, J . O., Snyder, R., Eds.; Plenum Press: New York, 1986; pp
11-30. (c) J erina, D. M.; Sayer, J . M.; Yagi, H.; Croisy-Delcey, M.;
Ittah, Y.; Thakker, D. R.; Wood, A. W.; Chang, R. L.; Levin, W.; Conney,
A. H. In Biological Reactive Intermediates II, Part A; Snyder, R., Parke,
D. V., Kocsis, J . J ., J ollow, D. J ., Gibson, C. G., Witmer, C. M., Eds.;
Plenum Press: New York, 1986; pp 501-523.
(3) (a) Dipple, A. In DNA Adducts: Identification and Biological
Significance; Hemminki, K., Dipple, A., Shuker, D. E. G., Kadlubar,
F. F., Sagerba¨ck, D., Bartsch, H., Eds.; Scientific Publication No. 125;
International Agency for Research on Cancer: Lyon, 1994; pp 107-
129. (b) Szeliga, J .; Dipple, A. Chem. Res. Toxicol. 1998, 11, 1-11. (c)
J erina, D. M.; Chadha, A.; Cheh, A. M.; Schurdak, M. E.; Wood, A.
W.; Sayer, J . M. In Biological Reactive Intermediates IV; Witmer, C.
M., Snyder, R., J ollow, D. J ., Kalf, G. F., Kocsis, J . J ., Sipes, I. G.,
Eds.; Plenum Press: New York, 1991; pp 533-553.
* Corresponding authors. M.K.L.: phone (212) 650-7835, fax (212)
650-6107. H.M.: phone +82-31-249-9631, fax +82-31-249-9605
§ City College of CUNY.
† University of North Dakota.
‡ Kyonggi University.
Korea Institute of Science and Technology.
(1) PAH carcinogenesis has been reviewed in: (a) Polycyclic Aro-
matic Hydrocarbon Carcinogenesis: Structure-Activity Relationships;
Yang, S. K., Silverman, B. D., Eds.; CRC Press: Boca Raton, FL, 1988;
Vols. I and II. (b) Polycyclic Aromatic Hydrocarbons: Chemistry and
Carcinogenicity; Harvey, R. G., Ed.; Cambridge University Press:
Cambridge, UK, 1991. (c) Polycyclic Hydrocarbons and Carcinogenesis;
Harvey, R. G., Ed.; ACS Symp. Ser. 283: Washington, DC, 1985.
10.1021/jo030113b CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/20/2003
6020
J . Org. Chem. 2003, 68, 6020-6030

Synthesis of Pyrene and Benzo[a]pyrene Adducts
SCHEME 1
electron resulting in a radical-cation that is again trapped
by the nitrogens of the DNA bases.4 It has been postu-
lated that a variety of enzymatic systems cause single-
electron oxidation of BaP and that a variety of nucleoside
and predominantly purine adducts are formed by such
enzymatic activation. For instance, activation of BaP by
horseradish peroxidase has yielded N-7 adducts of ad-
enine and guanine, an N-3 adduct of adenine, as well as
a C-8 adduct of guanine where the BaP is linked to the
nucleobase via its C-6.5 Microsomal activation also
produces much of the same adducts (except for the N-3
adenine adduct) as well as the adducts from the diol
epoxide activation pathway.5 Interestingly, cytochrome
P450, which is largely implicated in the oxidative me-
tabolism of BaP leading to diol epoxides, has also been
found to produce single-electron oxidation of BaP. Thus,
the N-7 guanine adduct of BaP has been isolated from
metabolic activation by this enzyme,5 and this adduct has
also been detected in the urine and feces of rats treated
with BaP.6 In addition, several of the aforementioned
adducts have been found in the mouse skin and rat
mammary gland and are formed in vitro by rat liver
microsomal activation.5,7-9 Formation of single-electron
oxidation products from BaP has been ascribed to its low
ionization potential,10 and in every single adduct the BaP
moiety is appended to the nucleobase solely at its C-6
position. This is consistent with the formation of a
carbocationic center at this position.11
single electron-oxidation pathway is the following. In the
former stable nucleoside adducts are produced involving
the exocyclic amino groups of the nucleobases, whereas
in the latter most of the identified adducts are unstable
and undergo deglycosylation leading to isolation of the
BaP-nucleobase conjugates. Thus, in the case of the diol
epoxides the covalent DNA lesions have been deemed
responsible for the terminal tumorigenic event. On the
other hand, in the case of single-electron oxidation, the
abasic sites resulting from alkylation-depurination are
considered important.5 For instance, c-Harvey-ras muta-
tions in mouse skin papillomas have been related to
replication at abasic sites in DNA.12 On the basis of these
factors substantial effort from several research groups
has been directed toward the synthesis of nucleoside
adducts of the various BaP metabolites. The diol epoxide-
nucleoside adducts are not only markers for comparisons
with metabolism studies, but several have also been used
in DNA assembly for producing site-specifically modified
DNA oligomers that are critical for structural, biological,
and biochemical studies.13 On the other hand, most of
the effort on the synthesis of single-electron oxidation
adducts (largely BaP-nucleobase adducts) has been for
the generation of markers for metabolism studies.5,10,14
It is these studies that drew our attention to the single-
electron oxidative metabolism of BaP. Several electro-
chemical oxidation methods that have been used to
generate the nucleobase adducts in several instances also
yield nucleoside adducts. Two specific cases that pertain
to BaP and its higher angular ring analogue dibenzo-
[a,l]pyrene (DB[a,l]P) are as follows. Anodic oxidation of
the former in the presence of 2′-deoxyguanosine yielded
in addition to the C-8 and N-7 guanine adducts three
nucleoside adducts where BaP was linked via its C-6 to
the C-8, N-3, and the exocyclic amino group of the
nucleobase.15a Similarly, anodic oxidation of DB[a,l]P in
A primary difference in the DNA adducts resulting
from the monooxygenation metabolism of BaP and the
(4) (a) Cavalieri, E.; Rogan, E. In Polycyclic Aromatic Hydrocarbons
and Carcinogenesis; Harvey, R. G., Ed.; ACS Symp. Ser. No. 283:
Washington, DC, 1985; pp 290-305. (b) Cavalieri, E.; Rogan. E.
Environ. Health Perspect. 1985, 64, 69-84.
(5) Chen, L.; Devanesan, P. D.; Higginbotham, S.; Ariese, F.;
J ankowiak, R.; Small, G. J .; Rogan, E. G.; Cavalieri, E. L. Chem. Res.
Toxicol. 1996, 9, 897-903.
(6) Rogan, E. G.; RamaKrishna, N. V. S.; Higginbotham, S.; Cava-
lieri, E. L.; J eong, H.; J ankowiak, R.; Small, G. J . Chem. Res. Toxicol.
1990, 3, 441-444.
(12) Chakravarti, D.; Pelling, J . C.; Cavalieri, E. L.; Rogan, E. G.
Proc. Natl. Acad. Sci. 1995, 92, 10422-10426.
(7) Rogan, E. G.; Devanesan, P. D.; RamaKrishna, N. V. S.;
Higginbotham, S.; Padmavathi, N. S.; Chapman, K.; Cavalieri, E. L.;
J eong, H.; J ankowiak, R.; Small, G. J . Chem. Res. Toxicol. 1993, 6,
356-363.
(8) Todorovic, R.; Ariese, F.; Devanesan, P.; J ankowiak, R.; Small,
G. J .; Rogan, E.; Cavalieri, E. Chem. Res. Toxicol. 1997, 10, 941-947.
(9) Devanesan, P. D.; RamaKrishna, N. V. S.; Todorovic, R.; Rogan,
E. G.; Cavalieri, E. L.; J eong, H.; J ankowiak, R.; Small, G. J . Chem.
Res. Toxicol. 1992, 5, 302-309.
(10) Rogan, E. G.; Cavalieri, E. L.; Tibbels, S. R.; Cremonesi, P.;
Warner, C. D.; Nagel, D. L.; Tomer, K. B.; Cerny, R. L.; Gross, M. L.
J . Am. Chem. Soc. 1988, 110, 4023-4029.
(11) (a) Rochlitz, J . Tetrahedron 1967, 23, 3043-3048. (b) J eftic, L.;
Adams, R. N. J . Am. Chem. Soc. 1970, 92, 1332-1337. (c) J ohnson,
M. D.; Calvin, M. Nature 1973, 241, 271-272.
(13) For some examples on the site-specific modification of DNA by
diol epoxides, please see: (a) Lakshman, M. K.; Chaturvedi, S.
Carcinogenesis 1996, 17, 2747-2752. (b) Kim, S. J .; Stone, M. P.;
Harris, C. M.; Harris, T. M. J . Am. Chem. Soc. 1992, 114, 5480-5481.
(c) Steinbrecher, T.; Becker, A.; Stezowski, J . J .; Oesch, F.; Seidel, A.
Tetrahedron Lett. 1993, 34, 1773-1774. (d) Lee, H.; Luna, E.; Hinz,
M.; Stezowski, J . J .; Kiselyov, A. S.; Harvey, R. G. J . Org. Chem. 1995,
60, 5604-5613. (e) Mao, B.; Xu, J .; Li, B.; Margulis, L. A.; Smirnov,
S.; Ya, N. Q.; Coutney, S.; Geacintov, N. E. Carcinogenesis 1995, 16,
357-365. (f) DeCorte, B. L.; Tsarouhtsis, D.; Kuchimanchi, S.; Cooper,
M. D.; Horton, P.; Harris, C. M.; Harris. T. M. Chem. Res. Toxicol.
1996, 9, 630-637. (g) Kroth, H.; Yagi, H.; Seidel, A.; J erina, D. M. J .
Org. Chem. 2000, 65, 5558-5564.
(14) Hanson, A. A.; Rogan, E. G.; Cavalieri, E. L. Chem. Res. Toxicol.
1998, 11, 1201-1208.
J . Org. Chem, Vol. 68, No. 15, 2003 6021

Lakshman et al.
the presence of 2′-deoxyadenosine yielded the N-1 and
N-7 adducts of adenine where DB[a,l]P was linked to the
nucleobase via the C-10 position (this position corre-
sponds to the C-6 of BaP).15b In addition, a 2′-deoxyad-
enosine adduct was also isolated to an extent of 26%, with
the C-10 position of DB[a,l]P linked to the exocyclic
amino group of the purine.15b
On the basis of these observations, and despite the
controversial nature of the single-electron-oxidation me-
tabolism, we reasoned that it was perhaps likely that
minor stable nucleoside adducts of hydrocarbons such as
BaP produced by this pathway may not have been
isolated. However, synthesis of these adducts would
contribute not only to the database of markers for
metabolic studies, but these could be readily used for site-
specific modification of DNA. Thus, this raises the
possibility for side-by-side comparisons of diol epoxide
and single-electron-oxidation adducts of any single hy-
drocarbon within specific DNA contexts. Such studies
could be instructive about the relative importance of
stable adducts arising from both metabolic pathways in
the overall scheme of tumorigenesis. In addition, our
syntheses also constitute a continued evaluation of Pd-
mediated C-N bond formation within the domain of
nucleoside modification leading to complex, biologically
important paradigms.
F IGURE 1. Two possible disconnection modes for the as-
sembly of the benzo[a]pyrene 2′-deoxyadenosine and 2′-de-
oxyguanosine adducts.
For such nucleoside modification two alternative ap-
proaches have been reported; (a) coupling of a suitable
halo nucleoside with an amino group donor,19-25 and (b)
coupling of a suitably protected nucleoside with an aryl
halide or triflate. Although method b has found ap-
plicability with only o-nitro aryl halides and triflates,26
we were curious whether extended aromatic systems
would find applicability through this route. Therefore,
in the present study we anticipated evaluation of both
approaches and Figure 1 shows disconnections (a) and
(b) representing the two.
On the basis of known chemistry, it was difficult to
determine a priori which of the two approaches would
prove satisfactory, and for the sake of simplicity initial
optimization experiments with 1-bromopyrene and 1-ami-
nopyrene (1 and 2, Figure 2) were planned. It was
plausible that results obtained from the pyrene models
would direct the experimentation with the B[a]P deriva-
tives 3 and 4. The commercial availability of 1 and 2
further justified their use.27 In contrast, compounds 3 and
4 required syntheses as follows; 6-bromo B[a]P 3 was
synthesized by bromination of B[a]P28 and 6-amino B[a]P
4 was prepared by nitration of B[a]P followed by reduc-
tion.29 Syntheses of the nucleoside precursors for testing
the two approaches shown in Figure 1 are also convenient
as 6 is derived from 519,30 and 8 from 7.20,21 With all of
the requisite starting materials at hand (Figure 2), the
next stage was the evaluation of coupling partners as well
as catalytic systems required to accomplish the synthe-
ses.
Resu lts a n d Discu ssion
Over the recent years Pd-catalyzed C-N bond forma-
tion has become an effective method for the synthesis of
a large assortment of compounds,16,17 and this method
has begun to find applicability for previously unknown
functionalization of nucleosides as well.18 Thus, the
syntheses of 2′-deoxyadenosine and 2′-deoxyguanosine
analogues that contain modifications at the exocyclic
amino groups (N6 and N2, respectively) as well as the
C-8 aminated purine nucleosides have been achieved.19-26
(15) (a) Ramakrishna, N. V. S.; Gao, F.; Padmavathi, N. S.; Cava-
lieri, E. L.; Rogan, E. G.; Cerny, R. L.; Gross, M. L. Chem. Res. Toxicol.
1992, 5, 293-302. (b) Li, K.-M.; Byun, J .; Gross, M. L.; Zamzow, D.;
J ankowiak, R.; Rogan, E. G.; Cavalieri, E. L. Chem. Res. Toxicol. 1999,
12, 749-757.
(16) Reviewed in: (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. 1999,
31, 805-818. (d) Hartwig, J . F. Acc. Chem. Res. 1999, 31, 852-860.
(e) Frost, C. G.; Mendonca, P. J . Chem. Soc., Perkin Trans. 1 1998,
2615-2623. (f) Yang, B. H.; Buchwald, S. L. J . Organomet. Chem. 1999,
576, 125-146. (g) Hartwig, J . F. In Modern Amination Methods; Ricci,
A., Ed.; Wiley-VCH: New York, 2000; pp 195-262. (h) Prim, D.;
Campagne, J .-M.; J oseph, D.; Andrioletti, B. Tetrahedron 2002, 58,
2041-2075.
Syn th esis of N6-(1-P yr en yl)- 3′,5′-bis-O-(ter t-bu t-
yld im eth ylsilyl)-2′-d eoxya d en osin e: (a ) P d -Med i-
a ted Cou p lin g of 1-Br om op yr en e (1) w ith 3′,5′-Bis-
O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxya d en osin e (5).
The commercial availability of 1 and the simple one-step
(17) For some examples, see: (a) Louie, J .; Hartwig, J . F. Tetrahe-
dron Lett. 1995, 36, 3609-3612. (b) Guram, A. S.; Rennels, R. A.;
Buchwald, S. L. Angew. Chem., Int. Ed. Engl. 1995, 34, 1348-1350.
(c) Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 7217-
7218. (d) Wolfe, J . P.; Wagaw, S.; Buchwald, S. L. J . Am. Chem. Soc.
1996, 118, 7215-7216. (e) Hamann, B. C.; Hartwig, J . F. J . Am. Chem.
Soc. 1998, 120, 7369-7370. (f) Yamamoto, T.; Nishiyama, M.; Koie,
Y. Tetrahedron Lett. 1998, 39, 2367-2370. (g) Bei, X.; Guram, A. S.;
Turner, H. W.; Weinberg, W. H. Tetrahedron Lett. 1999, 40, 1237-
1240. (h) Hartwig, J . F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy,
K. H.; Alcazar-Roman, L. M. J . Org. Chem. 1999, 64, 5575-5580. (i)
Grasa, G. A.; Viciu, M. S.; Huang, J .; Nolan, S. P. J . Org. Chem. 2001,
66, 7729-7737.
(21) Harwood, E. A.; Hopkins, P. B.; Sigurdsson, S. T. J . Org. Chem.
2000, 65, 2959-2964.
(22) Bonala, R.; Shishkina, I. G.; J ohnson, F. Tetrahedron Lett. 2000,
41, 7281-7284.
(23) Wang, Z.; Rizzo, C. J . Org. Lett. 2001, 3, 565-568.
(24) Schoffers, E.; Olsen, P. D.; Means, J . C. Org. Lett. 2001, 3,
4221-4223.
(25) Meier, C.; Gra¨sl, S. Synlett 2002, 802-804.
(26) De Riccardis, F.; Bonala, R. R.; J ohnson, F. J . Am. Chem. Soc.
1999, 121, 10453-10460.
(27) 1-Bromopyrene and 1-aminopyrene are commercially available
from one or more sources.
(18) Reviewed in: Lakshman, M. K. J . Organomet. Chem. 2002, 653,
234-251.
(28) Dewhurst, F.; Kitchen, D. A. J . Chem. Soc., Perkin Trans. 1
1972, 710-712.
(19) Lakshman, M. K.; Keeler, J . C.; Hilmer, J . H.; Martin, J . Q. J .
Am. Chem. Soc. 1999, 121, 6090-6091.
(20) Harwood, E. A.; Sigurdsson, S. T.; Edfeldt, N. B. F.; Reid, B.
R.; Hopkins, P. B. J . Am. Chem. Soc. 1999, 121, 5081-5082.
(29) Fieser, L. F.; Hershberg, E. B. J . Am. Chem. Soc. 1939, 61,
1565-1574.
(30) In ref 19 the use of CBr4 for the synthesis of the bromo
nucleoside is an error, CHBr3 is the correct reagent.
6022 J . Org. Chem., Vol. 68, No. 15, 2003

Synthesis of Pyrene and Benzo[a]pyrene Adducts
TABLE 1. Op tim iza tion Exp er im en ts for th e Syn th esis of
N6-(1-P yr en yl)-3′,5′-bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxya d en osin e (9)
entry
X
Y
Pd species
liganda
base
solvent
time (h), temp (°C)
resultb,c
1
2
3
Br
Br
Br
NH2 Pd(OAc)2
NH2 Pd(OAc)2
NH2 Pd2(dba)3
L-1
L-1
L-2
Cs2CO3
Cs2CO3
K3PO4
PhMe
PhMe
PhMe
16.5, 90
4, 90
18, 90
81% yield, relatively pure product
89% yield, clean product
after 18 h more catalyst was added
and reaction continued for another 2 h;
41% yield, impure product and
pyrene byproduct were observed
after 18 h more catalyst was added
and reaction continued for another 2 h;
24% yield, impure product and
pyrene byproduct were observed
no reaction
no reaction
no reaction
no reaction
52% yield, clean product
4
Br
NH2 Pd2(dba)3
L-2
t-BuONa
PhMe
18, 90
5
6
7
8
9
Br
Br
Br
Br
NH2 Pd(OAc)2
NH2 Pd(OAc)2
NH2 Pd(OAc)2
NH2 Pd(OAc)2
Pd2(dba)3
Pd(OAc)2
NH2 Pd(OAc)2
L-3
L-3
L-4
L-4
L-2
L-1
L-1
t-BuONa
K3PO4
t-BuONa
K3PO4
K3PO4
Cs2CO3
Cs2CO3
PhMe
PhMe
PhMe
PhMe
1,2-DME
1,2-DME
PhMe
4.5, rt then 16, 90
20, 90
21, 90
21, 90
2, 80
NH2 Br
NH2 Br
OTf
10
11
1, 80
24, 90
73% yield, somewhat impure product
substantial unreacted starting materials
a
Ligands: L-1 ) (()-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl; L-2 ) 2-(dicyclohexylphosphino)-2′-(N,N-dimethylamino)-1,1′-biphenyl;
L-3 ) 2-(dicyclohexylphosphino)biphenyl; L-4 ) 2-(di-tert-butylphosphino)biphenyl. Reactions were monitored by TLC. c In the cases
where yields are cited, yield refers to that of product isolated after chromatography.
b
use of the Pd2(dba)3/L-2 system with either K3PO4 or
t-BuO-Na+ resulted in low yield and the formation of the
reduction product pyrene (entries 3 and 4). Consistent
with our previous observations31 on aryl amination at the
C-6 position, use of L-3 and L-4 is essentially ineffective.
(b) P d -Med ia ted Cou p lin g of 1-Am in op yr en e (2)
w it h
6-Br om o-9-[2-d eoxy-3,5-b is-O-(ter t-b u t yld i-
m eth ylsilyl)-â-D-er yth r op en tofu r a n osyl]p u r in e (6).
On the basis of our previous report,19 commercially
available 1-aminopyrene was coupled with bromo nucleo-
side 6. The results of two optimization experiments are
shown in entries 9 and 10 of Table 1. Using conditions
we had previously described, the combination of Pd2-
(dba)3/L-2/K3PO4 in 1,2-DME at 80 °C afforded a modest
52% yield of 9 in 2 h. On the other hand, use of Pd(OAc)2/
L-1/Cs2CO3 in 1,2-DME produced an improved yield but
a decrease in product quality.
(c) P d -Med ia ted Cou p lin g of 1-P yr en yl Tr ifla te
w ith 3′,5′-Bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-
a d en osin e (5). In a single experiment (entry 11 in Table
1) the coupling of 1-pyrenyl triflate32 with protected 2′-
deoxyadenosine 5 was attempted utilizing the catalytic
system that had provided positive results above. After
24 h at 90 °C substantial amounts of starting materials
were present, indicating that even triflates of polyaro-
matic phenols are poor coupling partners in C-N bond
F IGURE 2. The hydrocarbon and nucleoside coupling com-
ponents used in the synthesis of the pyrene and benzo[a]-
pyrene nucleoside adducts.
silylation of 2′-deoxyadenosine led to initial experimenta-
tion with these two substrates. To determine an ap-
propriate catalytic system several parameters needed to
be varied. These were the Pd species, the ligand, the base,
and the solvent. The results of this optimization are
shown in entries 1-8 of Table 1. It is clear from this table
that the use of Pd(OAc)2/L-1/Cs2CO3 in toluene at 90 °C
resulted in an effective reaction leading to the N6-(1-
pyrenyl) adduct 9 within 4 h (entry 2). On the other hand,
(31) 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.
(32) 1-Pyrenyl triflate was prepared by reaction of 1-hydroxypyrene
with triflic anhydride in CH2Cl2 in the presence of 2,6-lutidine at -30
to -35 °C.
J . Org. Chem, Vol. 68, No. 15, 2003 6023

Lakshman et al.
TABLE 2. Op tim iza tion Exp er im en ts for th e Syn th esis of
N6-(6-Ben zo[a ]p yr en yl)-3′,5′-bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxya d en osin e (10)
time (h),
temp (°C)
entry
X
Y
Pd species liganda
base
solvent
PhMe
1,2-DME
1,4-dioxane
resultb,c
insignificant reaction
insignificant reaction
1
2
3
4
5
Br
NH2
NH2
NH2
NH2
NH2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
L-1
L-1
L-1
L-1
L-1
L-1
L-2
L-1
L-1
L-1
Cs2CO3
Cs2CO3
Cs2CO3
24, 100
24, 80
43, 100
43, 100
72, 100
16, 80
18, 80
18, 80
5.5, 100
21, 100
Br
Br
Br
Br
insignificant reaction (∼12% product isolated)
t-BuONa 1,4-dioxane
no reaction
Cs2CO3
Cs2CO3
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
PhMe-THF (1:1)
PhMe
1,2-DME
1,2-DME
1,4-dioxane
PhMe-THF (1:1)
insignificant reaction
6
7
8
9
NH2 Br
NH2 Br
NH2 Br
NH2 Br
NH2 Br
%10:%11 ) 20.3:25.7; total 46%
%10:%11 ) 38.6:10.3; total 48.9%
%10:%11 ) 29.5:22; total 51.5%
%10:%11 ) 51.3:15; total 66.3%
%10:%11 ) 15.9:24.8; total 40.7%
10
a
Ligands: L-1 ) (()-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl; L-2 ) 2-(dicyclohexylphosphino)-2′-(N,N-dimethylamino)-1,1′-biphenyl;
L-3 ) 2-(dicyclohexylphosphino)biphenyl; L-4 ) 2-(di-tert-butylphosphino)biphenyl. Reactions were monitored by TLC. c In the cases
where yields are cited, yield refers to that of product isolated after chromatography.
b
formation. No further experimentation was conducted
with this substrate.
Syn th esis of N6-(6-Ben zo[a ]p yr en yl)-3′,5′-bis-O-
(ter t-bu tyld im eth ylsilyl)-2′-d eoxya d en osin e: (a ) P d -
Med ia ted Cou p lin g of 6-Br om oben zo[a ]p yr en e (3)
w ith 3′,5′-Bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-
a d en osin e (5). On the basis of the result that the N6-
(1-pyrenyl) adduct 9 could be synthesized from 1-bro-
mopyrene and 5, we anticipated that a similar approach
using 6-bromo B[a]P would provide the simplest access
to the N6-(6-benzo[a]pyrenyl) adduct as well. Further-
more, the required 6-bromo B[a]P was available in one
step from B[a]P.28 Thus, the coupling of 3 with 5 was
F IGURE 3. Structure of the dimer, benzo[a]pyrene dinucleo-
attempted with the Pd(OAc)2/L-1/Cs2CO3 combination
side adduct.
that had proven successful in the reaction of 1-bromopy-
rene. Surprisingly insignificant reaction was observed.
with 6 was conducted in toluene at 80 °C. This reaction
This led to the evaluation of slightly modified conditions,
led to consumption of starting materials and yielded a
the results of which are summarized in entries 1-5 of
surprising result: the formation of two products with
Table 2. Considering that solubility of 6-bromo B[a]P
different fluorescent properties under UV light (365 nm).
could be a factor leading to inefficient coupling, solvents
The more mobile product on silica gel had a greenish
fluorescence whereas the less mobile one was bluish.
such as 1,2-DME, 1,4-dioxane, and toluene-THF com-
binations were tested. None proved satisfactory and not
These two products were separated and analyzed. The
much product formation was observed even after pro-
longed reaction times in these cases. Thus, coupling of
1H NMR data of the more mobile product clearly indicated
a ratio of one B[a]P unit to a single nucleoside moiety,
6-bromo B[a]P (3) with the 2′-deoxyadenosine derivative
whereas in the less mobile product there were two
5 does not appear to be a viable route to the N6 adduct,
which contrasts with the good result obtained with
nucleoside units to a single B[a]P. Mass spectral analysis
also confirmed these structures. The dimeric compound
1-bromopyrene.
(11, shown in Figure 3) showed sharp aromatic and
(b) P d -Med ia ted Cou p lin g of 6-Am in oben zo[a ]-
p yr en e (4) w ith 6-Br om o-9-[2-d eoxy-3,5-bis-O-(ter t-
bu tyld im eth ylsilyl)-â-D-er yth r op en tofu r a n osyl]p u -
r in e (6). The failure of method a above in yielding the
aliphatic resonances. On the other hand, the aromatic
signals in compound 10 were sharp and the aliphatic
resonances were broadened (at ambient temperature in
CDCl3). It must be noted that formation of such dimeric
requisite adduct, therefore, prompted a test of the ap-
products by further reaction of an initially formed species
proach involving coupling of 6-amino B[a]P 4, available
has been reported in Pd-catalyzed aminations of nucleo-
in two steps from B[a]P (nitration and reduction),29 with
sides.26,33 However, this is the first case in nucleoside aryl
bromo nucleoside 6. With use of the catalytic system Pd-
(OAc)2/L-1/Cs2CO3, which yielded the best result for the
synthesis of the N6-(1-pyrenyl) adduct, the coupling of 4
(33) De Riccardis, F.; J ohnson, F. Org. Lett. 2000, 3, 293-295.
6024 J . Org. Chem., Vol. 68, No. 15, 2003

Synthesis of Pyrene and Benzo[a]pyrene Adducts
F IGURE 5. Structures of the N6-(1-pyrenyl)- and N6-(6-benzo-
[a]pyrenyl)-2′-deoxyadenosine and the triacetate of the latter.
F ea tu r es of Th eir NMR Sp ectr a . Small portions of the
synthetically derived 9 and 10 were subjected to desily-
F IGURE 4. UV spectra of the pyrene (green), benzo[a]pyrene
(blue), and dimeric benzo[a]pyrene (red) adducts in MeOH.
1
lation with n-Bu4N+F- in THF. The H NMR spectra of
the products 12 and 13 at room temperature in CDCl3
show similarities, the most striking of which is the
appearance of a doublet and triplet pattern for the
diastereotopic H-5′ protons. Such a resonance pattern has
been reported for purine 2′-deoxynucleosides in CDCl3
and has been attributed to a possible intramolecular
hydrogen bond between the 5′-hydroxyl proton and the
N3 of the base.35 However, the H-2′ resonances of 13 are
somewhat broader compared to those of 12, perhaps an
influence of the added angular ring in the former. For
additional characterization purposes, compound 13 (Fig-
ure 5) was acetylated with Ac2O/pyridine/DMAP. Inter-
estingly, this reaction yielded the N,O,O-triacetate 14
rather than the O,O-diacetate. This result is different
compared to that from similar acetylation reactions of
B[a]P diol epoxide adducts at the N6 position of 2′-
deoxyadenosine where no N-acetylation is normally
observed.36
Syn th esis of N2-(1-P yr en yl)-O6-ben zyl-3′,5′-bis-O-
(ter t-bu tyld im eth ylsilyl)-2′-d eoxygu a n osin e: (a ) P d -
Med ia ted Cou p lin g of 1-Br om op yr en e (1) w ith O6-
Ben zyl-3′,5′-bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-
gu a n osin e (7). The coupling of 1 with 7 was conducted
in a fashion similar to the reactions of 1 with 5. Here
again, reaction progress was observed in most cases
(entries 1-7 of Table 3) resulting in good yields of 15 in
some (Table 3, entries 1 and 3). However, in the higher
yielding reactions product 15 was contaminated with
inseparable impurities. In the few cases where pure 15
was isolated the yields of the product were not high
(Table 3, entries 4, 5, and 7). Thus, the coupling of
1-bromopyrene with the O6-protected 2′-deoxyguanosine
analogue does not appear to be a practical method for
the synthesis of the N2-(1-pyrenyl)-2′-deoxyguanosine
adduct.
amination that a 2:1 adduct of nucleoside to arylamine
has been observed.
Since this reaction had yielded two products of which
only one was desired, a study was undertaken to evaluate
whether the ratio of 10:11 could be altered by changing
the catalytic system, base, or solvent. The results of these
experiments are shown in entries 6-10 of Table 2. From
this analysis it appeared that use of Pd(OAc)2/L-1/Cs2-
CO3 in 1,4-dioxane at 100 °C provided the best ratio of
10 to 11 in a moderately good yield (10:11 ) 51.3:15,
overall 66.3%).
While this work was in progress, Ve´liz and Beal
reported that direct displacement of bromide from the
C-6 position of acetyl (but not TBDMS)-protected purine
nucleosides by arylamines offers a facile approach to C-6
amino aryl nucleosides.34 This method seemed to offer
the possibility that dimer formation could be suppressed,
a factor that is important while considering scale-up.
Their method utilizes 6 molar equiv of the arylamine in
an alcohol medium, MeOH or EtOH. Whereas such large
excesses can be utilized with commercially available
arylamines, in the present case this was not possible.
Thus, in our attempt, 3 molar equiv of 4 was allowed to
react with 6-bromo-9-[2-deoxy-3,5-bis-O-acetyl-â-D-eryth-
ropentofuranosyl]purine34 in MeOH at 65 °C over a 24-h
period. Careful purification of the mixture returned
mainly the bromo nucleoside as the major nucleoside
material with no significant proportion of adduct-like
compound. Perhaps solubility of the arylamine in MeOH
is a factor, but no further attempts were made to utilize
this method.
Som e fea tu r es of th e UV Sp ectr a of 9, 10, a n d 11.
The UV spectra of compounds 9-11 are shown in Figure
4. As anticipated, the pyrene adduct 9 (shown in green)
has substantially blue-shifted absorbances compared
with the B[a]P analogue 10 (shown in blue). However,
the dimeric nucleoside 11 (shown in red) and the mono-
adduct 10 show some interesting differences. Although
the absorption bands of 10 and 11 in the 250-270 and
280-310 nm regions are very similar, the bands in the
350-410 nm region of 11 shown substantial bathochro-
mic shifts compared to those of 10.
(b) P d -Med ia ted Cou p lin g of 1-Am in op yr en e (2)
w it h
2-Br om o-O6-b en zyl-3′,5′-b is-O-(ter t-b u t yld i-
m eth ylsilyl)-2′-d eoxyin osin e (8). Since coupling of the
aryl halide with the amino group of the protected
nucleoside did not seem to be the best route, the coupling
(35) Lakshman, M. K.; Lehr, R. E. Nucleosides Nucleotides 1992,
11, 1039-1046.
(36) (a) Lakshman, M. K.; Sayer, J . M.; J erina, D. M. J . Am. Chem.
Soc. 1991, 113, 6589-6594. (b) Lakshman, M. K.; Sayer, J . M.; J erina,
D. M. J . Org. Chem. 1992, 57, 3438-3443. (c) Lakshman, M. K.; Sayer,
J . M.; Yagi, H.; J erina, D. M. J . Org. Chem. 1992, 57, 4585-4590.
Syn th esis of N6-(1-P yr en yl) a n d N6-(6-Ben zo[a ]-
p yr en yl)-2′-d eoxya d en osin e (12 a n d 13) a n d Som e
(34) Ve´liz, E. A.; Beal, P. A. J . Org. Chem. 2001, 66, 8592-8598.
J . Org. Chem, Vol. 68, No. 15, 2003 6025

Lakshman et al.
TABLE 3. Op tim iza tion Exp er im en ts for th e Syn th esis of
N2-(1-P yr en yl)-O6-ben zyl-3′,5′-bis-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxygu a n osin e (15)
entry
X
Y
Pd species
liganda
base
solvent
time (h), temp (°C)
resultb,c
1
2
3
4
Br
NH2
NH2
NH2
NH2
Pd(OAc)2
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
L-1
L-2
L-2
L-3
Cs2CO3
K3PO4
t-BuONa
t-BuONa
PhMe
PhMe
PhMe
PhMe
3.5, 90
19, 90
19, 90
24, 90
86% yield, impure product
37% yield, impure product
70% yield, impure product
19% yield, relatively pure product
but incomplete reaction
Br
Br
Br
5
6
7
Br
Br
Br
NH2
NH2
NH2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
L-3
L-4
L-4
K3PO4
PhMe
PhMe
PhMe
24, 90
24, 90
24, 90
28% yield, relatively pure product
but incomplete reaction
31% yield, impure product and
incomplete reaction
23% yield, relatively pure product
but incomplete reaction
t-BuONa
K3PO4
8
9
NH2
NH2
Br
Br
Pd(OAc)2
Pd2(dba)3
L-1
L-2
Cs2CO3
K3PO4
1,2-DME
1,2-DME
18, 80
18, 80
80% yield, clean product
75% yield, clean product
a
Ligands: L-1 ) (()-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl; L-2 ) 2-(dicyclohexylphosphino)-2′-(N,N-dimethylamino)-1,1′-biphenyl;
L-3 ) 2-(dicyclohexylphosphino)biphenyl; L-4 ) 2-(di-tert-butylphosphino)biphenyl. Reactions were monitored by TLC. c In the cases
where yields are cited, yield refers to that of product isolated after chromatography.
b
SCHEME 2a
a
Reagents and conditions: (i) 4, Pd(OAc)2, L-1, Cs2CO3, 1,4-dioxane, 90 °C; (ii) n-Bu4N+F-, THF, rt; (iii) CH2Cl2, (CH3CO)2O, rt; (iv)
10% Pd-C, H2, 1:1 THF-MeOH, rt; (v) 2, Pd(OAc)2, L-1, Cs2CO3, 1,2-DME, 80 °C.
of 2 with bromo nucleoside 8 was attempted. The two
different catalytic systems that were tested (entries 8 and
9 of Table 3) were comparably effective in yielding
product 15 of good purity. Thus, for 2′-deoxyguanosine
modification the experiments indicated that coupling of
the amino hydrocarbon with a halo nucleoside was
substantially better than reactions with the opposite
coupling partners.
Syn th esis of N2-(6-Ben zo[a ]p yr en yl)-O6-ben zyl-
3′,5′-b is-O-(t er t -b u t yld im e t h ylsilyl)-2′-d e oxygu a -
n osin e: P d -Med ia ted Cou p lin g of 6-Am in oben zo-
[a ]p yr en e (4) w ith 2-Br om o-O6-ben zyl-3′,5′-bis-O-
(ter t-b u t yld im et h ylsilyl)-2′-d eoxyin osin e (8). The
reaction leading to N2-(6-benzo[a]pyrenyl)-O6-benzyl-3′,5′-
bis-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine (16) was
carefully considered in light of the results obtained in
the coupling of bromo nucleoside 6 with 4. Since Pd-
(OAc)2/L-1/Cs2CO3 in 1,4-dioxane had provided the best
result for the synthesis of the 2′-deoxyadenosine adduct,
this was the combination chosen in the present case. With
use of this catalyst-solvent system, coupling of 4 with 8
proceeded to completion smoothly, within 5 h at 90 °C,
to yield the desired adduct 16 in ca. 88% yield (Scheme
2). Interesting to note, in contrast to the synthesis of the
adenine adduct no dimeric adduct was detected in this
case.
F ea tu r es of th e NMR Sp ectr a of N2-(1-P yr en yl)-
a n d N2-(6-Ben zo[a ]p yr en yl)-O6-b en zyl-3′,5′-b is-O-
(ter t-bu tyld im eth ylsilyl)-2′-d eoxygu a n osin e (15 a n d
16), a s Well a s th e Cor r esp on d in g Dep r otected 2′-
Deoxygu a n osin e Der iva tives. Adducted nucleoside
derivatives of 2′-deoxyguanosine are often more polar
6026 J . Org. Chem., Vol. 68, No. 15, 2003

Synthesis of Pyrene and Benzo[a]pyrene Adducts
F IGURE 6. (A) 1H NMR spectrum of N2-(6-benzo[a]pyrenyl)-O6-benzyl-2′-deoxyguanosine (17) in DMSO-d6 at 80 °C. (B) Partial
1H NMR spectrum of N2-(6-benzo[a]pyrenyl)-3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine (20) in DMSO-d6 at 80 °C (not
shown are the tert-butyldimethylsilyl resonances).
compared to the 2′-deoxyadenosine analogues, and this
led us to consider characterization of the 2′-deoxygua-
nosine adducts containing protecting groups on the
carbohydrate moiety. Initial effort was directed at the
generation of the N2-(1-pyrenyl)-3′,5′-bis-O-(tert-butyldim-
ethylsilyl)-2′-deoxyguanosine (21 in Scheme 2). This was
achieved by catalytic hydrogenolysis of the O6-benzyl
protecting group. Whereas the 1H NMR spectrum (in
CDCl3 at room temperature) of compound 15 showed
excellent resolution of all signals, in the debenzylated
product 21 the aliphatic resonances showed substantial
broadening. FAB HRMS of 21 showed an expected
696.3427 (M+ + H) signal.
In contrast to the pyrene adduct 15 the benzo[a]pyrene
derivative 16 showed sharp signals for the aromatic
protons but the sugar resonances were very broad
signals, barely discernible from the baseline, in CDCl3
at ambient. Increasing the temperature to 40 °C produced
an increased intensity of the sugar resonances but the
signals were still very broad. Changing the solvent to
DMSO-d6 produced results similar to those with CDCl3
at both room temperature and 60 °C. Compound 16 was
subjected to debenzylation, but significant signal broad-
ening was observed in 20 (in CDCl3 at room tempera-
ture). With a view to improving the signal resolution for
product characterization, other derivatives were selected.
Desilylation of 16 yielded the dihydroxy derivative 17;
however, this compound and its diacetate 18 both showed
well-resolved aromatic resonances but very broad sugar
signals along the baseline, in CDCl3 at room temperature.
The triacetate derivative (22 in the Supporting Informa-
tion) was prepared from 17 (Ac2O, pyridine, DMAP) and
this compound exhibited an interesting property. Whereas
this triacetate showed extremely well resolved signals for
both the aromatic and the sugar protons, duplicity of
some signals was observed, and the phenomenon per-
sisted in DMSO-d6 at 80 °C (Figure 14 in the Supporting
Information). This led us to consider the possibility that
the significant signal broadening that was observed in
adducted 2′-deoxyguanosine derivatives was a result of
slow dynamic molecular motion at room temperature and
that acetylation of the N2 froze out certain conformers.
This is consistent with two other observations: the signal
broadening in 21 (though not as pronounced as in the
B[a]P adducts) and the signal broadening observed in the
N6 B[a]P adduct 13 compared to the pyrene analogue
12 (in the case of 13 the H-2′ resonances showed
substantial broadening). Therefore, we decided to reana-
lyze the 1H NMR spectra of 16-20 in DMSO-d6 at an
elevated temperature (Figures 8-12 in the Supporting
Information). At 80 °C 16 clearly showed sharp aromatic
resonances; however, the aromatic resonances of the
benzyl moiety as well as the benzylic methylene and
remaining sugar protons were broad but discernible. The
dihydroxy derivative 17 shows many of the same features
but the two hydroxyl protons were also visible and were
assigned by deuterium exchange. Figure 6A shows the
1H NMR spectrum of 17. Diacetate 18 also showed the
same pattern described for 16 and 17. Among the three
derivatives 17 shows the best identifiable features of the
glycosidic portion of the N2 adduct in the 1H NMR
spectrum.
1
Two other derivatives were analyzed by H NMR in
DMSO-d6 at 80 °C, and these were compounds 19 and
20 where the O6 protection had been removed. In both
cases the glycosidic protons were again clearly distin-
guishable although signal broadening was still observed.
Among the two 20 appears to be slightly better and
splitting for the 2′ protons can be observed (Figures 6B
and 12 in the Supporting Information). From Figure 6A,B
some interesting observations can be made. In these N2
aryl derivatives the BaP residue is not subject to a slow
J . Org. Chem, Vol. 68, No. 15, 2003 6027

Lakshman et al.
dynamic exchange. On the other hand, the benzyl moiety
of 17 and the carbohydrate protons of 17 and 20 are
influenced even at 80 °C. What is notable is that despite
the absence of the bulky tert-butyldimethylsilyl groups
in 17 the carbohydrate resonances are broad. It also
appears that removal of the O6 benzyl moiety from 16
produces slightly improved signal resolution in 20, which
still retains the tert-butyldimethylsilyl groups. Therefore,
the silyl groups alone are not contributors to the signal
broadening observed in these cases. HRMS data for all
compounds in the 2′-deoxyguanosine series were consis-
tent with the calculated results.
Thus, in comparison to the N2-(1-pyrenyl)-2′-deoxygua-
nosine derivatives the N2-(6-benzo[a]pyrenyl)-2′-deoxy-
guanosine analogues display some unique conformational
properties. This seems to stem from the additional
angular ring on the pyrene moiety at N2 leading to a
substantial decrease in the conformational interchange
rate and significant signal broadening of the sugar
resonances. Whereas this type of pattern is also evident
in the N6 adducts, the effect is far less pronounced
perhaps due to a greater separation between the large
hydrocarbon and the glycosidic portion.
Exp er im en ta l Section
For the 2′-deoxyadenosine adducts, thin-layer chromatog-
raphy was performed on 250-µm silica plates and routine
column chromatographic purifications were performed on 200-
300 mesh silica gel. Separations of 10 and 11 were performed
on 230-400 mesh silica gel. For the 2′-deoxyguanosine series
all chromatographic purifications were performed on 230-400
mesh silica gel. NMR spectra were recorded at 500 or 600 MHz
in deacidified CDCl3 (deacidification was performed by perco-
lating CDCl3 through a bed of solid NaHCO3 and basic
alumina) or DMSO-d6 as indicated. Chemical shifts (δ) are
reported in ppm and coupling constants (J ) are reported in
Hz.
N6-(1-P yr en yl)- 3′,5′-bis-O-(ter t-bu tyld im eth ylsilyl)-2′-
d eoxya d en osin e (9). Meth od A: Cou p lin g of 1-Br om o-
p yr en e (1) w ith P r otected 2′-Deoxya d en osin e 5. Into an
oven-dried screw-cap vial equipped with a stirring bar were
placed Pd(OAc)2 (1.5 mg, 6.68 µmol) and L-1 (6.3 mg, 10.1
µmol). Toluene (0.68 mL) was added, the vial was flushed with
N2 gas, and the mixture was stirred at room temperature for
ca. 5 min. To this mixture were added 1-bromopyrene (1, 19.1
mg, 67.9 µmol) and nucleoside 5 (49.0 mg, 0.102 mmol)
followed by Cs2CO3 (31.0 mg, 95.1 µmol). The vial was again
flushed with N2 gas, sealed with a Teflon-lined cap, and heated
in a sand bath that was maintained at 92-94 °C. The reaction
was monitored by TLC and judged to be complete in 4 h at
which time the mixture was cooled, diluted with EtOAc, and
extracted twice with water. The organic layer was dried over
Na2SO4 and evaporated to dryness. The crude product was
loaded onto a silica gel column packed in CH2Cl2 and eluted
with this solvent followed by 5% acetone in CH2Cl2. The N6
Con clu sion s
In this report we have clearly demonstrated the
assembly of biologically important, putative N6 2′-deoxy-
adenosine and N2 2′-deoxyguanosine adducts arising
from the single-electron oxidation of B[a]P. For this
purpose Pd catalysis offers a convenient route. It is of
interest to note that reactions of simpler models are
perhaps not representative of more elaborate reactants.
For example, coupling of 1-bromopyrene with protected
2′-deoxyadenosine was quite successful and moderately
so with 2′-deoxyguanosine; however, the chemistry does
not readily apply to 6-bromo B[a]P. On the other hand,
coupling of 1-aminopyrene and 6-amino B[a]P to bromo
nucleosides produces the desired nucleoside adducts. But
here again lies a difference; no dimeric adduct was
observed with 1-aminopyrene but with 6-amino B[a]P an
unusual dimeric adduct was obtained in addition to the
desired product. Between reactions at the 6 and 2
positions of the purine, dimeric adduct formation seems
to occur at the former by a second arylation of the initially
formed 2′-deoxyadenosine adduct, but this does not
appear to be the case with the 2′-deoxyguanosine ana-
logue. Also, in comparing the pyrene and the B[a]P
adducts the latter compounds, perhaps due to slower
conformational exchange, show signal broadening in their
1H NMR spectra. This feature is quite pronounced in the
case of the N2 2′-deoxyguanosine adducts. Compound 13
has been incorporated into codon 61 of the human N-ras
sequence (5′-CGGACAAGAAG), and studies on this
modified DNA oligomer are currently in progress. Com-
bined with our recently reported Pd-catalyzed synthesis
of PAH epoxide adducts at the exocyclic amino groups of
2′-deoxyadenosine and 2′-deoxyguanosine,37 this ap-
proach offers convenient access to PAH-nucleoside ad-
ducts formed via both the diol epoxide and the single-
electron-oxidation metabolic activation pathways.
1
adduct 9 was isolated as a yellow foam (41.5 mg, 89.9%). H
NMR (500 MHz, CDCl3): 8.62 (d, 1H, J ) 8.3); 8.50 (s, 1H);
8.28-8.25 (m, 3H); 8.21-8.18 (m, 3H); 8.13-8.00 (m, 4H); 6.54
(t, 1H, J ) 6.5); 4.67 (m, 1H); 4.06 (m, 1H); 3.93 (dd, 1H, J )
4.1, 11.1); 3.82 (dd, 1H, J ) 3.0, 11.1); 2.71 (app quint, 1H, J
∼ 6.5); 2.50 (ddd, 1H, J ) 4.1, 6.1, 12.9); 0.96 and 0.95 (2s,
18H, t-Bu); 0.14 (s, 12H, SiCH3). HRMS calcd for C38H50N5O3-
Si2 (M+ + H) 680.3452, found 680.3454. UV-vis λmax (MeOH):
238, 275, and 341 nm.
Meth od B: Cou p lin g of 1-Am in op yr en e (2) w ith Br o-
m o Nu cleosid e 6. Into an oven-dried screw-cap vial equipped
with a stirring bar were placed Pd(OAc)2 (1.6 mg, 7.13 µmol),
L-1 (13.2 mg, 21.2 µmol), and Cs2CO3 (34.6 mg, 0.106 mmol).
1-Aminopyrene (2, 23.0 mg, 0.106 mmol) and bromo nucleoside
6 (38.5 mg, 70.8 µmol) were added followed by 1,2-DME (0.7
mL). The vial was flushed with N2 gas, sealed with a Teflon-
lined cap, and heated in a sand bath that was maintained at
82-84 °C. The reaction was monitored by TLC and judged to
be complete in 1 h at which time the mixture was cooled,
diluted with EtOAc, and extracted with water. The aqueous
layer was back extracted with EtOAc twice, and the organic
layers were combined, dried over Na2SO4 and evaporated to
dryness. The crude product was loaded onto a silica gel column
packed in CH2Cl2 and eluted with this solvent to remove excess
amine and another nonpolar material. Subsequent elution with
5% acetone-CH2Cl2 yielded the N6 adduct 9, isolated as a
1
greenish-yellow foam (35.0 mg, 73%). The H NMR spectrum
of this material was identical with that reported in Method A
above, except that some minor impurities were also evident.
N6-(6-Ben zo[a ]p yr en yl)- 3′,5′-bis-O-(ter t-bu tyld im eth -
ylsilyl)-2′-d eoxya d en osin e (10). Into an oven-dried screw-
cap vial equipped with a stirring bar were placed Pd(OAc)2
(2.1 mg, 9.35 µmol) and L-1 (17.18 mg, 27.6 µmol) in 1,4-
dioxane (1 mL). The vial was flushed with N2 gas and the
mixture was stirred at room temperature for ca. 6 min.
6-Aminobenzo[a]pyrene (4, 49.2 mg, 0.184 mmol), the bromo
nucleoside 6 (50.0 mg, 91.9 µmol), Cs2CO3 (44.94 mg, 0.138
mmol), and 1,4-dioxane (0.84 mL) were added, and the vial
was again flushed with N2 gas, sealed with a Teflon-lined cap,
and heated in a sand bath that was maintained at 100-102
°C. After 5.5 h, at which time the reaction was judged to be
(37) Lakshman, M. K.; Gunda, P. Org. Lett. 2003, 5, 39-42.
6028 J . Org. Chem., Vol. 68, No. 15, 2003

Synthesis of Pyrene and Benzo[a]pyrene Adducts
complete by TLC, the mixture was cooled, diluted with EtOAc,
and washed with water. The organic layer was dried over Na2-
SO4 and evaporated to dryness. The crude product was loaded
onto a silica gel column (230-400 mesh) packed in CH2Cl2.
The column was eluted with CH2Cl2 followed by 2% acetone-
CH2Cl2. Three fractions were collected. The fastest eluting was
residual 6-aminobenzo[a]pyrene, followed by the desired mono-
adduct 10 and then the bis-adduct 11. Mono-adduct 10 was
obtained as a yellowish-brown solid (34.5 mg, 51.3%) whereas
the bis-adduct 11 was obtained as a yellow solid (16.4 mg, 15%)
after addition and evaporation of anhydrous pentane.
addition and evaporation of anhydrous hexane to the material
obtained by chromatography (49.0 mg, 72%). 1H NMR (600
MHz, CDCl3): 9.10 (d, 1H, J ) 8.4); 9.07 (d, 1H, J ) 8.4); 8.38
(br, 1H); 8.34 (d, 1H, J ) 9.6); 8.25 (d, 1H, J ) 7.2); 8.16 (s,
1H); 8.13 (s, 1H); 8.10 (br, 1H); 8.08 (d, 1H, J ) 7.2); 7.99-
7.96 (m, 2H); 7.94 (d, 1H, J ) 9.0); 7.84 (td, 1H, J ) 1.2, 7.8);
7.73 (t, 1H, J ) 7.2); 6.54 (d, 1H, J ) 11.0); 6.41 (dd, 1H, J )
5.4, 9.0); 4.81 (br s, 1H); 4.24 (s, 1H); 3.96 (d, 1H, J ) 13.2);
3.77 (app t, 1H, J ∼ 13.2); 3.18 (app septet, 1H); 2.36 (app dd,
1H, J ) 5.4, 13.8); 1.89 (br s, 1H). HRMS calcd for C30H24N5O3
(M+ + H) 502.1879, found 502.1897.
N2-(1-P yr en yl)-O6-ben zyl-3′,5′-bis-O-(ter t-bu tyld im eth -
ylsilyl)-2′-d eoxygu a n osin e (15). Meth od A: Cou p lin g of
1-Br om op yr en e (1) w ith P r otected 2′-Deoxygu a n osin e
7. Into a dry reaction flask equipped with a stirring bar were
placed Pd(OAc)2 (1.5 mg, 6.68 µmol), L-1 (6.4 mg, 10.3 µmol),
and anhydrous toluene (0.68 mL). The flask was flushed with
N2 gas and stirred for 5 min at room temperature. The
protected 2′-deoxyguanosine 7 (59.8 mg, 0.102 mmol), 1-bro-
mopyrene (1, 19.1 mg, 0.068 mmol), and Cs2CO3 (31.0 mg, 95.1
µmol) were added to the reaction flask, which was again
flushed with N2 gas, sealed, and stirred at 90° C for 3.5 h.
The reaction mixture was then cooled to room temperature,
diluted with EtOAc (20 mL), and washed with water. The
organic layer was dried over anhydrous MgSO4 and evaporated
to dryness. The crude product was purified on a silica gel
column with 20% EtOAc-n-hexane to afford the desired
adduct 15 as a brownish-yellow, oily product (46 mg, 86%).
The 1H NMR spectrum of this material was identical with that
reported in Method B below except that some impurities were
also evident.
A larger scale reaction with 6 (101.7 mg, 0.187 mmol) and
6-aminobenzo[a]pyrene (4, 100 mg, 0.374 mmol) yielded the
mono-adduct 10 in 49% yield and the bis-adduct in 19% yield.
(In some reactions these compounds have been isolated as
brown solids and in others as brown foams with a gray metallic
sheen.)
1
Compound 10 H NMR (600 MHz, CDCl3): 9.10 (d, 1H, J )
8.4); 9.07 (d, 1H, J ) 9.6); 8.42 (d, 1H, J ) 8.4); 8.33 (d
superimposed on br signal, 2H, J ) 9.6); 8.23 (d, 1H, J ) 7.8);
8.22 (br s, 1H); 8.14 (d, 1H, J ) 9.0); 8.06 (d, 1H, J ) 7.2);
7.96 (m, 2H); 7.88 (d, 1H, J ) 9.0); 7.83 (t, 1H, J ) 7.2); 7.72
(t, 1H, J ) 7.2); 6.52 (br, 1H); 4.66 (br, 1H); 4.04 (br, 1H); 3.91
(br, 1H); 3.81 (br d, 1H, J ) 9.0); 2.75 (br m, 1H); 2.48 (br m,
1H); 0.91 (br s, 18H, t-Bu); 0.11 (br s, 12H, SiMe3). HRMS calcd
for C42H52N5O3Si2 (M+ + H) 730.3609, found 730.3607. UV-
vis λmax (MeOH): 256, 265, 287, 299, 372, and 392 nm.
Compound 11 1H NMR (600 MHz, CDCl3): 9.10 (m, 2H);
8.50 (s, 1H); 8.49 (s, 1H); 8.46 (d, 1H, J ) 9.0); 8.34 (d, 1H, J
) 9.0); 8.22 (d, 1H, J ) 7.8); 8.19 (d, 1H, J ) 9.6); 8.00 (d, 1H,
J ) 6.6); 7.93 (t, 1H, J ) 7.2); 7.87 (s, 2H); 7.78 (d, 1H, J )
9.0); 7.75 (t, 1H, J ) 7.8); 7.59 (t, 1H, J ) 7.2); 6.42 (2 closely
spaced t, 2H, J ) 6.6 and 6.6); 4.56 (m, 2H); 3.96 (m, 2H);
3.75 (dd, 2H, J ) 5.4, 10.8); 3.69 (dd, 2H, J ) 3.6, 10.8); 2.67
(app quint, 2H, J ∼ 6.3); 2.39-2.36 (m, 2H); 0.88, 0.87, and
0.77 (3s, 36H, t-Bu); 0.065, -0.03, -0.038, -0.062, and -0.077
(5s, 24H, SiMe3). HRMS calcd for C64H90N9O6Si4 (M+ + H)
1192.6091, found 1192.6066. UV-vis λmax (MeOH): 255, 265,
288, 300, 377, and 398 nm.
Meth od B: Cou p lin g of 1-Am in op yr en e (2) w ith Br o-
m o Nu cleosid e 8. Into a dry flask equipped with a stirring
bar were placed 1-aminopyrene (2, 21.8 mg, 0.10 mmol), bromo
nucleoside 8 (43.4 mg, 0.067 mmol), L-1 (12.5 mg, 20.07 µmol),
Pd(OAc)2 (1.5 mg, 6.68 µmol), and Cs2CO3 (32.7 mg, 0.10
mmol). Anhydrous 1,2-dimethoxyethane (0.7 mL) was added
and the flask was flushed with N2 gas. The flask was sealed
and the mixture was stirred at 80 ° C for 18 h. The flask was
cooled to room temperature, diluted with EtOAc (20 mL), and
washed with water. The organic layer was dried over anhy-
drous MgSO4 and evaporated to dryness. The crude product
was purified by flash chromatography on silica gel with CH2-
Cl2 and then 20% EtOAc-n-hexane to afford the adduct 15 as
a greenish-yellow oil (42 mg, 80%). 1H NMR (500 MHz,
CDCl3): 8.55 (d, 1H, J ) 8.3); 8.26 (d, 1H, J ) 9.2); 8.19-8.16
(m, 3H); 8.11-8.00 and 7.75 (m and one br s, 6H); 7.33-7.32
N6-(1-P yr en yl)-2′-deoxyaden osin e (12). The disilyl pyrene
adduct 9 (37 mg, 54.4 µmol) was dissolved in THF (0.55 mL)
to produce a 0.1 M solution that was cooled to 0 °C in an ice
bath. To this was added a 1 M solution of n-Bu4N+F- in THF
(0.12 mL, 2.2 molar equiv) and the mixture was stirred at 0
°C for 90 min at which time the reaction was complete. The
mixture was diluted with EtOAc, and washed three times with
water. The organic layer was dried over Na2SO4, filtered, and
evaporated to dryness. The resulting solid was chromato-
graphed on a silica gel column packed in 5% MeOH-CH2Cl2,
using the same solvent. The desilylated adduct 12 was
obtained as an off-white powder (10.1 mg, 41%). 1H NMR (600
MHz, CDCl3): 8.52 (d, 1H, J ) 8.4); 8.41 (s, 1H); 8.27 (br s,
1H); 8.24-8.22 (m, 2H); 8.19 (d, 1H, J ) 7.8); 8.17 (d, 1H, J )
7.8); 8.10 (d, 1H, J ) 9.0); 8.06 (ABquart, 2H, J ) 9.6); 8.00 (t,
1H, J ) 7.8); 7.93 (s, 1H); 6.52 (d, 1H, J ) 12.0); 6.38 (dd, 1H,
J ) 6.0); 4.82 (app d, 1H, J ) 4.8); 4.24 (s, 1H); 3.99 (d, 1H, J
) 13.2); 3.80 (app t, 1H, J ∼ 11.7); 3.16 (ddd, 1H, J ) 5.4,
10.2, 15.0); 2.43 (app dd, 1H, J ) 5.4, 13.2); 1.93 (br s, 1H).
(m, 2H); 7.26-7.22 (m, 3H); 6.41 (t, 1H, J ) 6.4); 5.52 (ABquart
,
2H, J ) 12.3); 4.57 (m, 1H); 4.01 (app q, 1H, J ∼ 3.6); 3.82
(dd, 1H, J ) 4.2, 11.1); 3.78 (dd, 1H, J ) 3.3, 11.1);
2.60 (app quint, 1H, J ∼ 6.5); 2.42 (ddd, 1H, J ) 3.9, 6.1,
13.1); 0.93 (s, 18H, t-Bu), 0.11 and 0.098 (2s, 12H, SiMe3);
HRMS calcd for C45H56N5O4Si2 (M+ + H) 786.3871, found
786.3884.
N 2-(6-Be n zo[a ]p yr e n yl)-O6-b e n zyl-3′,5′-b is-O-(t er t -
bu tyld im eth ylsilyl)-2′-d eoxygu a n osin e (16). The 2-bromo
nucleoside 8 (430 mg, 0.66 mmol), 6-aminobenzo[a]pyrene (4,
183 mg, 0.69 mmol), Pd(OAc)2 (14.8 mg, 66.0 µmol), L-1 (124
mg, 0.2 mmol), and Cs2CO3 (322 mg, 0.99 mmol) were placed
in a dry flask equipped with a magnetic stirring bar. Freshly
distilled 1,4-dioxane (12 mL) was added and the mixture was
stirred under N2 gas at 90 °C for 5 h. Upon completion of the
reaction as judged by TLC, the mixture was cooled to room
temperature and then evaporated under reduced pressure to
produce a dark-brown gummy solid. This material was directly
purified by flash chromatography on a silica gel column with
CH2Cl2 and 1:1 n-hexanes-EtOAc sequentially, to provide the
protected adduct derivative 16 as a greenish-yellow foam
(487.8 mg, 88.5%). 1H NMR (600 MHz, DMSO-d6 at 80 °C):
9.52 (s, 1H); 9.24 (d, 1H, J ) 8.4); 9.23 (d, 1H, J ) 9.0); 8.48
(d, 1H, J ) 8.4); 8.42 (d, 1H, J ) 8.4); 8.33 (d, 1H, J ) 7.8);
8.19 (d, 1H, J ) 9.0); 8.16 (d, 1H, J ) 7.2); 8.03 (t, 1H, J )
HRMS calcd for
452.1716.
C
26H22N5O3 (M+ + H) 452.1723, found
N6-(6-Ben zo[a ]p yr en yl)-2′-d eoxya d en osin e (13). The
disilyl benzo[a]pyrene adduct 10 (98.9 mg, 0.135 mmol) was
dissolved in THF (1.36 mL) to produce a 0.1 M solution that
was cooled to 0 °C in an ice bath. To this was added a 1 M
solution of n-Bu4N+F- in THF (0.3 mL, 2.2 molar equiv) and
the mixture was stirred at 0 °C for 3 h at which time the
reaction was complete. The mixture was diluted with EtOAc,
and washed three times with water. The organic layer was
dried over Na2SO4, filtered, and evaporated to dryness. The
resulting solid was chromatographed on a silica gel column
packed in 5% MeOH-CH2Cl2, using the same solvent. The
desilylated adduct 13 was obtained as a yellow solid upon
J . Org. Chem, Vol. 68, No. 15, 2003 6029

Lakshman et al.
7.8); 7.99 (s, 1H); 7.97 (d, 1H, J ) 9.0); 7.88 (t, 1H, J ) 7.8);
7.78 (t, 1H, J ) 7.8); 7.10 and 6.97 (broad signals, 5H); 6.07
(br, 1H); 5.10 (br, 2H); 4.16 (m, 1H); 3.69 (br, 1H); 3.48 (br,
2H); 2.62 (br, 1H); 2.02 (br, 1H); 0.80 and 0.75 (2s, 18H, t-Bu);
-0.076 and -0.086 (2s, 12H, SiMe3). HRMS calcd for
was then stirred for 18 h under a hydrogen atmosphere
(balloon). Upon completion of the reaction, the mixture was
filtered though a plug of Celite and then evaporated to dryness.
Purification of the crude material by flash chromatography
on a silica gel column with 5% MeOH-CH2Cl2 afforded the
desired compound 21 as a brownish-white solid (10.9 mg,
25.7%). 1H NMR (600 MHz, CDCl3): 12.66 (br, 1H); 10.32 (br,
1H); 8.38 (d, 1H, J ) 9.0); 8.24 (d, 1H, J ) 7.8); 8.10 (d, 1H, J
) 7.2); 8.07-7.96 (m, 5H); 7.91 (t, 1H, J ) 7.5); 7.47 (br, 1H);
5.78 (br m, 1H); 3.98 (br s, 1H); 3.66 (br s, 1H); 3.37 (br d, 1H,
J ) 8.4); 3.25 (br m, 1H); 2.18 (br m, 1H); 1.91 (m, 1H); 0.67,
0.66 (2s, 18H, t-Bu); -0.22, -0.27 (2s, 12H, SiMe3). HRMS
calcd for C38H50N5O4Si2 (M+ + H) 696.3401, found 696.3427.
C
49H58N5O4Si2 (M+ + H) 836.4027, found 836.4037.
N2-(6-Ben zo[a ]p yr en yl)-O6-ben zyl-2′-d eoxygu a n osin e
(17). A ∼0.015 M solution of the O6-benzyl-3′,5′-bis-O-(tert-
butyldimethylsilyl) nucleoside 16 (232.8 mg, 0.278 mmol) in
THF (19 mL) was prepared. To this was added a 1 M solution
of n-Bu4N+F- in THF (0.61 mL, 2.2 molar equiv) and the
mixture was stirred at room temperature. Upon completion
of the reaction in 2 h the solvent was removed under reduced
pressure. The residue was purified by flash chromatography
on a silica gel column with 5% MeOH-CH2Cl2 to afford the
O6-benzyl nucleoside 17 as a yellow, foamy solid (168 mg, 99%).
1H NMR (600 MHz, DMSO-d6 at 80 °C): 9.49 (s, 1H); 9.26 (d,
1H, J ) 8.4); 9.25 (d, 1H, J ) 9.0); 8.48 (d, 1H, J ) 8.4); 8.43
(d, 1H, J ) 9.0); 8.34 (d, 1H, J ) 7.2); 8.21 (d, 1H, J ) 10.2);
8.18 (d, 1H, J ) 7.2); 8.08 (s, 1H); 8.04 (t, 1H, J ) 8.4); 7.99
(d, 1H, J ) 9.6); 7.91 (t, 1H, J ) 8.4); 7.80 (t, 1H, J ) 7.8);
7.02-6.78 (3 br signals, 5H); 6.16 (br, 1H); 4.95 (br, 2H); 4.86
(br, 1H); 4.45 (br, 1H); 4.18 (br, 1H); 3.75 (br, 1H); 3.38 (br,
2H); 2.55 (br, 1H); 2.16 (br, 1H). HRMS calcd for C37H30N5O4
(M+ + H) 608.2298, found 608.2300.
Ack n ow led gm en t. S.B. and H.M. are grateful to the
Kyonggi University for partial financial support of this
work (year 2000). M.K.L. is grateful for support from
the NIH (NCI 1R15 CA094224-01) and a PSC-CUNY
33 award. We wish to thank Dr. Seo J ung J u at Korea
Basic Science Institute for assistance with mass spectral
data, Dr. Surendra Chaturvedi (PrimeSyn Lab Inc., NJ )
for assistance with DNA synthesis, and Dr. Mark Hail
(Enovatia, NJ ) for mass spectral analysis of the ad-
ducted oligomer.
N2-(6-Ben zo[a ]p yr en yl)-3′,5′-bis-O-(ter t-bu tyld im eth yl-
silyl)-2′-d eoxygu a n osin e (20). To a solution of the O6-benzyl-
3′,5′-bis-O-(tert-butyldimethylsilyl) nucleoside 16 (50.9 mg, 61
µmol) in 1:1 THF-MeOH (1.7 mL) was added 10% Pd-C (5
mg). The flask was evacuated and flushed with hydrogen, and
this procedure was repeated three times. The reaction mixture
was then stirred for 24 h under a hydrogen atmosphere
(balloon). The mixture was filtered and evaporated under
reduced pressure. Flash chromatographic purification of the
crude material on a silica gel column with ethyl acetate and
5% MeOH-THF sequentially afforded the desired disilyl
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures for N6,3′,5′-triacetyl-N6-(6-benzo[a]pyrenyl)-2′-deoxyad-
enosine (14), N2-(6-benzo[a]pyrenyl)-O6-benzyl-3′,5′-diacetyl-
2′-deoxyguanosine (18), N2-(6-benzo[a]pyrenyl)-3′,5′-diacetyl-
2′-deoxyguanosine (19), and N2-(6-benzo[a]pyrenyl)- N2,3′,5′-
triacetyl-O6-benzyl-2′-deoxyguanosine (22) as well as proton
NMR spectra for 9-22. This material is available free of charge
via the Internet at http://pubs.acs.org.
Note Ad d ed in P r oof
1
adduct 20 as a yellowish-white powder (41.6 mg, 91.5%). H
After this manuscript was accepted we became aware
of a communication (Chakraborti, D.; Colis, L.; Schneider,
R.; Basu, A. K. Org. Lett., submitted) wherein the Pd-
mediated coupling of bromo and bromo nitro pyrenes with
O6-benzyl-3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxygua-
nosine as well as amino and amino nitro pyrenes with
2-bromo-O6-benzyl-3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-
deoxyinosine was studied. Despite some experimental
differences, for bromopyrene the results paralleled our
observations, whereas for the nitropyrenes, coupling of
the halo aromatic with protected deoxyguanosine af-
forded good yields, indicating that even a remote nitro
group was adequate for efficient coupling.
NMR (600 MHz, DMSO-d6 at 80 °C): 10.80 (br, 1H); 9.24 (d,
1H, J ) 8.4); 9.23 (d, 1H, J ) 9.0); 9.13 (br, 1H); 8.46 (d, 1H,
J ) 9.0); 8.45 (d, 1H, J ) 8.4); 8.36 (d, 1H, J ) 7.8); 8.19 (d,
1H, J ) 7.2); 8.17 (d, 1H, J ) 9.0); 8.06 (d, 1H, J ) 7.2); 8.05
(d, 1H, J ) 9.0); 7.89 (t, 1H, J ) 7.8); 7.84 (t, 1H, J ) 7.8);
7.74 (s, 1H); 5.64 (br, 1H); 3.74 (br, 1H); 3.46 (br, 1H); 3.16 (br
m, 1H); 3.09 (br m, 1H); 2.26-2.30 (app quint, 1H); 1.68 (m,
1H); 0.73 and 0.54 (2s, 18H, t-Bu); -0.18, -0.19, -0.30, and
-0.36 (4s, 12H, SiMe3). HRMS calcd for C42H52N5O4Si2 (M+
+
H) 746.3558, found 746.3566.
N2-(1-P yr en yl)-3′,5′-b is-O-(ter t-b u t yld im et h ylsilyl)-2′-
d eoxygu a n osin e (21). To a solution of the O6-benzyl-3′,5′-
bis-O-(tert-butyldimethylsilyl) nucleoside 15 (47.9 mg, 0.061
mmol) in 1:1 THF-MeOH (3 mL) was added 10% Pd-C (25
mg). The flask was evacuated and flushed with hydrogen, and
this procedure was repeated three times. The reaction mixture
J O030113B
6030 J . Org. Chem., Vol. 68, No. 15, 2003