C. Chatgilialoglu et al. / Tetrahedron Letters 47 (2006) 711–714
713
Formation of product 3a from 1 and 2a requires TEA;
the reaction in the absence of base led to total decompo-
sition of 8-Br-dAdo within 2 h. The principal product
obtained was 8-bromoadenine together with sulfide 4a
being formed in only 15% yield; compound 3a was com-
pletely absent. Importantly, 8-bromoadenine was found
to result from simple ribose hydrolysis as determined by
a blank reaction lacking both thiol and TEA.
2-mercaptoethanol (2a) at 100 ꢁC in the presence of
TEA. This procedure has been successfully extended to
the synthesis of 8-thio-20-deoxyadenosine derivative
(3c/3c0).
Acknowledgements
Work supported in part by the European Commu-
nityÕs Human Potential Programme under contract
MCRTN-CT-2003-505086 [CLUXTOXDNA] is grate-
fully acknowledged.
These findings indicated that 8-oxo-dAdo 3a was
formed through the intermediacy of sulfide 4a under
basic conditions. In turn, sulfide 4a can be easily
accounted for through a nucleophilic aromatic substitu-
tion of the bromide ion by the thiolate ion (Scheme 2).
In principle, the formation of 3a from 4a could be
explained through two different reaction pathways: (path
A) intermolecular aromatic substitution by the water
solvent (or hydroxy ion) with displacement of the 2-
hydroxythiolate ion, or (path B) 1,4-sulfur-to-oxygen
migration of the C8 carbon atom, possibly occurring
in a two-step process through an initial nucleophilic
addition of the b-oxygen to the C8–N double bond with
the intermediacy of the spiro-compound 5. The resulting
thiolate ion 6 could afford the oxo-derivative 3a through
intramolecular nucleophilic aromatic substitution by the
b-sulfur atom at the a-carbon atom, with displacement
of a thiirane molecule (Scheme 2).
References and notes
1. For reviews, see: Dizdaroglu, M.; Jaruga, P.; Birincioglu,
M.; Rodriguez, H. Free Radical Biol. Med. 2002, 32, 1102–
1115; Cadet, J.; Douki, T.; Gasparutto, D.; Ravanat, J.-L.
Mutation Res. 2003, 531, 5–23.
2. Bodepudi, V.; Shibutani, S.; Johnson, F. Chem. Res.
Toxicol. 1992, 5, 608–617.
3. Sekine, M.; Okada, K.; Seio, K.; Kakeya, H.; Osada,
H.; Obata, T.; Sasaki, T. J. Org. Chem. 2004, 69, 314–326.
4. Postigo, A.; Ferreri, C.; Navacchia, M. L.; Chatgilialoglu,
C. Synlett 2005, 2854–2856.
5. HPLC analyses were performed in all cases on a Zorbax
MS C18 column (4.6 · 150 mm, 5 lm) with a linear
gradient H2O/acetonitrile from 100:0 to 50:50 at a flow
rate 0.4 ml/min, detection at k = 260 nm.
Significant support of the proposed pathway B has been
obtained. First, formation of product 3a from 1 and 2a
in 18O water did not lead to isotopic incorporation. This
excludes the possibility that water is the source of the C8
moiety.7 This observation was supported by the reaction
of 1 with butane thiol 2b, which led to quantitative for-
mation of sulfide 4b.8
6. Compound 4a was obtained as an almost pure sample by
silica gel column chromatography by elution with ethyl
acetate/methanol. 1H NMR (400 MHz, DMSO-d6), d 8.02
(1H, s; H2), 7.20 (2H, br s), 6.20 (1H, dd, J1 = 8.4,
J2 = 6.0 Hz; H10), 5.40 (1H, br s; OH), 5.30 (1H, br s;
OH), 4.40 (1H, m; H30), 3.86 (1H, dt, Jd = 2.4,
Jt = 4.4 Hz; H40), 3.67 (2H, t, J = 6.4 Hz; O–CH2) 3.63
(1H, dd, J1 = 12.0, J2 = 4.4 Hz; H50), 3.47 (1H, dd,
J1 = 12.0, J2 = 4.4 Hz; H500), 3.37 (2H, ABX2 system,
JAB = 14.0, JAX = JBX = 6.4 Hz; inner line separation
2.0 Hz; S-CH2), 3.04 (1H, ddd, J1 = 13.5, J2 = 8.4 Hz;
J3 = 6.0 Hz; H20), 2.10 (1H, ddd, J1 = 13.5, J2 = 6.0 Hz;
J3 = 2.0 Hz; H2, 00). 13C NMR (75 MHz, DMSO-d6), d
35.52 (CH2), 38.05 (CH2), 60.17 (CH2), 62.77 (CH2), 71.87
(CH), 85.49 (CH), 88.73 (CH), 120.00 (q), 149.27 (q),
151.16(q), 151.89 (CH), 154.80 (q). MS ES(+) 328 (M+1),
MS2 212].
7. In order to investigate the possible role of the solvent in
the formation of 3a the reaction of 1 with 2a and TEA was
also carried out in methanol as a solvent. However, the
reaction in methanol was found much slower. After 7 h
heating at 100 ꢁC HPLC-MS analysis showed the presence
of starting material 1 in 20% yield, together with sulfide 4a
as the major product (50% yield), small amounts of 8-oxo
derivative 3a (3%) and significant amounts (27%) of 8-
bromoadenine.
8. Compound 4b was obtained as an almost pure sample by
silica gel column chromatography by elution with ethyl
acetate/methanol. 1H NMR (400 MHz, DMSO-d6), d 8.00
(1H, s; H2), 7.00 (2H, br s), 6.20 (1H, dd, J1 = 8.4,
J2 = 6.0 Hz; H10), 5.4 (1H, br s; OH), 5.3 (1H, br s; OH),
4.40 (1H, m; H30), 3.87 (1H, dt, Jd = 2.0 , Jt = 4.5 Hz;
H40), 3.62 (1H, dd, J1 = 12.5, J2 = 4.5 Hz; H50), 3.48 (1H,
dd, J1 = 12.5, J2 = 4.5 Hz; H500), 3.28 (2H, ABX2 system,
JAB = 11.2, JAX = JBX = 7.2 Hz; inner line separation
3.2 Hz; S-CH2), 3.00 (1H, ddd, J1 = 13.5, J2 = 8.4 Hz;
J3 = 5.5 Hz; H20), 2.10 (1H, ddd, J1 = 13.5, J2 = 6.0 Hz;
J3 = 2.0 Hz; H200), 1.66 (2H, quintuplet, J = 7.2 Hz;
Further evidence supporting the mechanism B came
from the reaction with 1,2-ethanedithiol (2c). After 2 h
reaction time the mixture was treated with 10% HClaq
and extracted with ethyl acetate. Solvent evaporation
gave a tautomeric 3c/3c0 mixture9,10 in a 20:80 ratio,
1
as indicated by HPLC-MS and H NMR analysis, in
75% overall yield (Scheme 2). The 3c/3c0 ratio changed
from 20:80 to 80:20 by subsequent silica gel column
chromatography by elution with ethyl acetate/methanol.
HPLC-MS analysis of the reaction mixture at different
reaction times gave no evidence of a possible intermedi-
ate 4c, thus indicating the fact that the intramolecular
nucleophilic aromatic substitution leading to 3c/3c0 is
a fast process. This result was not surprising, since the
sulfur anion is expected to be a better leaving group that
the oxygen anion. The formation of 3c/3c0 in fairly good
yields appears to be of interest from a synthetic point of
view. In fact, the one-pot reaction herein reported can
represent a convenient synthetic method alternative to
the two-step synthesis from 8-Br-dAdo recently reported
in the literature.11
In conclusion, we have reported a new facile synthetic
methodology for the preparation of 8-oxo-adenine
derivatives 3a, R = H or OH, in very high yield (93–
95%) through a one-pot reaction in water solution of
8-bromo-(20-deoxy)adenosine (1, R = H or OH) with