Synthesis of 7-hydroxy(phenyl)ethylguanines by alkylation of 2-amino-6-chloropurine with allyl-protected bromohydrins

Article abstract of DOI:10.1021/ol0272803

(Matrix presented) Protecting the hydroxyl group in both 2-bromo-2-phenylethanol and 2-bromo-1-phenylethanol enhanced the alkylation of 2-amino-6-chloropurine to give corresponding 7- and 9-alkylated products. Subsequent hydrolysis and deprotection led to

Full text of DOI:10.1021/ol0272803

ORGANIC
LETTERS
2003
Vol. 5, No. 5
637-639
Synthesis of
7-Hydroxy(phenyl)ethylguanines by
Alkylation of 2-Amino-6-chloropurine
with Allyl-Protected Bromohydrins
,†
Jan Nova´k,† Igor Linhart,* Hana Dvorˇa´kova´,‡ and Vladislav Kubelka‡
Department of Organic Chemistry and Central Laboratories, Institute of Chemical
Technology, Prague, Technicka´ 1905, CZ-166 28 Prague, Czech Republic
linharti@Vscht.cz
Received November 14, 2002
ABSTRACT
Protecting the hydroxyl group in both 2-bromo-2-phenylethanol and 2-bromo-1-phenylethanol enhanced the alkylation of 2-amino-6-chloropurine
to give corresponding 7- and 9-alkylated products. Subsequent hydrolysis and deprotection led to 7- and 9-hydroxy(phenyl)ethylguanines.
7-Hydroxy(phenyl)ethylguanines are major guanine adducts formed by interaction of styrene 7,8-oxide with DNA.
Styrene 7,8-oxide (1), an electrophilic metabolic intermediate
of styrene, reacts with nucleophilic centers in nucleic acids,
the main site of the attack being position 7 at guanine.
7-Hydroxy(phenyl)ethylguanine derivatives formed by this
process were found in DNA of animals and humans exposed
to styrene and can be therefore used as markers of exposure
for early detection of damage to DNA.1 7-Alkylguanines and
3-alkyladenines are the main types of DNA adducts excreted
in urine.2 The reaction of alkylating agents such as oxirane
1 with nucleotides proceeds with low regioselectivity and
low conversion of the nucleotide. The resulting nucleotide
adducts are isolated from the reaction mixture using HPLC.3
The aim of the present study was to develop a synthetic
procedure for 7-hydroxy(phenyl)ethylguanines derived from
1. Oxirane 1 itself is not a suitable alkylating reagent due to
its low reactivity and lack of selectivity in alkylation
reactions.3 Therefore, we searched for a suitable synthetic
equivalent. Two types of DNA adducts with respect to the
site of attack on the oxirane ring are formed. Under acidic
conditions, the attack on the R-carbon predominates, leading
to 2-hydroxy-1-phenylethyl derivatives (R-adducts), whereas
under basic conditions, the attack on the â-carbon predomi-
nates, yielding 2-hydroxy-2-phenylethyl derivatives (â-
adducts). Therefore, 2-bromo-2-phenylethanol (2a) and
2-bromo-1-phenylethanol (2b) are possible synthetic equiva-
lents of 1, which should yield R- and â-adducts, respectively.
In fact, bromohydrin 2b alkylated 2-amino-6-chloropurine
(3) to yield a 6:1 mixture of 2-amino-6-chloro-9-(2-hydroxy-
† Department of Organic Chemistry.
‡ Central Laboratories.
(1) (a) Monographs on the EValuation of Carcinogenic Risks to Humans;
International Agency for Research on Cancer: Lyon, France, 1994; Vol.
60. (b) Hemminki, K.; Koskinen, M.; Rajaniemi, H.; Zhao, Ch. Regul.
Toxicol. Pharmacol. 2000, 32, 246.
(3) (a) Vodicka, P.; Stetina, R.; Kumar, R.; Plna; K., Hemminki, K.
Carcinogenesis 1996, 17, 801. (b) Koskinen, M.; Schweda, E. K. H.;
Hemminki, K. J. Chem. Soc., Perkin Trans. 2 1999, 2441. (c) Koskinen,
M.; Vodicˇka, P.; Hemminiki, K. Chem.-Biol. Interact. 2000, 124, 13.
(2) Shuker, D. E.; Farmer, P. B. Chem. Res. Toxicol. 1992, 5, 450.
10.1021/ol0272803 CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/12/2003

2-phenylethyl)purine (4b) and 2-amino-6-chloro-7-(2-hy-
droxy-2-phenylethyl)purine (5b) (Scheme 1). However, the
corresponding guanine derivative by alkaline hydrolysis (3
h of reflux with 1 M NaOH). The protective group was then
removed reductively using a palladium complex catalyst5 to
yield 7-(2-hydroxy-1-phenylethyl)guanine (10a) as shown in
Scheme 2. Regioisomeric 7-(2-hydroxy-2-phenylethyl)gua-
Scheme 1
Scheme 2a
regioisomeric bromohydrin 2a failed to alkylate 3 under the
same conditions, i.e., in DMF solution with potassium
carbonate as a base. In the presence of base, 2a underwent
dehydrobromination and subsequent condensation reactions
leading to a mixture of products and leaving purine 3
unreacted. Therefore, a protection of the hydroxyl group in
2a was apparently needed to prevent these condensation
reactions. We used several protective groups such as acetyl,
tetrahydropyranyl, and allyl to protect hydroxyl function in
2a. Whereas both the acetyl and tetrahydropyranyl groups
were insufficiently stable under the conditions of alkylation,
so that attempted alkylations resulted in complicated product
mixtures, the allyl group effectively prevented formation of
byproducts. Moreover, when attached to the vicinal hydroxyl,
the allyl group facilitates loss of the leaving group. Participa-
tion of allylic π-electrons can effectively stabilize both the
intermediary carbocation in SN1 reaction and the transition
state of SN2 reaction, so that nucleophilic substitution on both
benzylic (in 2a) and phenylethylic carbons (in 2b) is
facilitated. Bromide (6a,b) or tosylate (7b) was used as a
leaving group. Alkylation reactions were carried out in DMF
solution using potassium carbonate as a base.4 Alkylating
reagents used are shown in Figure 1. Substitution on both
a Reaction conditions: (a) K2CO3/DMF, 6a, 60 °C. (b) (i)
Aqueous NaOH; (ii) (PPh3)4Pd, PMHS, ZnCl2, DMF.
nine (10b) was prepared by an analogous procedure using
6b or 7b (Scheme 3). Bromide 6b showed lower reactivity
Scheme 3a
Figure 1. Structures of styrene-7,8-oxide and its synthetic equiva-
lents used as alkylating reagents. In 2a/b, X ) Br and R ) H; in
6a/b, X ) Br and R ) CH2-CHdCH2; and in 7b, X ) OTs and
R ) CH2-CHdCH2.
a Reaction conditions: (a) K2CO3/DMF, 6b or 7b, 80 °C. (b) (i)
Aqueous NaOH; (ii) (PPh3)4Pd, PMHS, ZnCl2, DMF.
than its regioisomer 6a with a benzyl bromide moiety, so
complete conversion of 3 to alkylated products 8b and 9b
could not be achieved. Unlike bromide 6b, tosylate 7b gave
complete conversion of 3 in the alkylation reaction and a
higher proportion of the desired regioisomer 9b.
The results of the alkylation reaction are summarized in
Table 1.
imidazol ring nitrogens occurred, yielding a mixture of 7-
and 9-alkylated products with the 9-substituted purine
predominating. So, the reaction of 3 with bromide 6a yielded
a mixture of 9-(2-allyloxy-1-phenylethyl)-2-amino-6-chloro-
purine (8a) and 7-(2-allyloxy-1-phenylethyl)-2-amino-6-
chloropurine (9a). The isomer 9a was converted to the
638
Org. Lett., Vol. 5, No. 5, 2003

alkylated product as compared to the alkylation with
unprotected bromohydrin 2b. In all cases, the 9-substituted
products were the major ones, while the 7-substituted isomers
required were the minor ones. Nevertheless, the procedure
described seems to be a promising way for preparation of
7-hydroxy(phenyl)ethylguanines as analytical standards for
research on DNA adducts derived from styrene and for early
detection of damage to DNA.
Table 1. Alkylation of 2-Amino-6-chloropurine (3)
yield in %
product ratio
(9-N:7-N)
reagent
9-N
7-N
2b
6a
6b
7b
4b:5b ) 6:1
8a :9a ) 3:1
8b:9b ) 4:1
8b:9b ) 7:2
48
62
51
62
8
22
13
21
Acknowledgment. The authors are grateful to the Min-
istry of Education of the Czech Republic for the financial
support of this study by Grants 562/2002 and 223100001.
Protection of the hydroxyl group in the alkylating reagents
improved both the reaction rate and the yield of the products
(Table 1). It also increased the proportion of desired 7-N-
Supporting Information Available: Detailed description
of experimental procedures for preparation of alyklating
reagents 6a,b and compounds 8a, 9a, 10a, 8b, 9b, and 10b
and spectra of newly prepared compounds (UV, NMR, MS).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(4) An equimolar mixture of 3 and K2CO3 with 2 molar equiv of the
alkylating agent (6a, 6b, or 7b) was stirred in dry DMF at 60-80 °C under
an atmosphere of dry nitrogen for 3-4 h. The solvent was removed by
evaporation in a vacuum, and the residue was separated by column
chromatography on silica gel (50:1 chloroform-methanol). The two formed
products were characterized by NMR, MS, and UV spectra and identified
as corresponding 7-N- and 9-N-derivatives of 3. The position of the
substituent was determined by HMQC and HMBC two-dimensional NMR
experiments. Alkylation of 3 by unprotected 2b was performed in a similar
way at 110 °C overnight.
OL0272803
(5) Chandrasekhar, S.; Raji, R. Ch.; Jagadeeshwar, R. R. Tetrahedron
2001, 57, 3435.
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