Nucleoside homodimerisation by cross metathesis

Article abstract of DOI:10.1016/S0040-4039(00)02292-9

The cross metathesis of 3′-allylic analogues of thymidine, 2′-deoxyuridine and 2′-deoxycytidine is used to obtain nucleoside dimers, in which an unsaturated hydrocarbon chain links the 3′ positions of the sugar moieties. The biological activity on HIV inf

Full text of DOI:10.1016/S0040-4039(00)02292-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1491–1493
Nucleoside homodimerisation by cross metathesis
Nathalie Batoux,a Rachida Benhaddou-Zerrouki,a Philippe Bressolier,a Robert Granet,a
Ge´raldine Laumont,b Anne-Marie Aubertinb and Pierre Krausza,*
aLaboratoire de Chimie des Substances Naturelles, Universite´ de Limoges, 123 Av. Albert Thomas,
87060 Limoges Cedex, France
bUnite´ INSERM 74, Institut de Virologie, Faculte´ de Me´decine, 3 rue Koeberle´, 67000 Strasbourg, France
Received 12 December 2000; revised 14 December 2000
Abstract—The cross metathesis of 3%-allylic analogues of thymidine, 2%-deoxyuridine and 2%-deoxycytidine is used to obtain
nucleoside dimers, in which an unsaturated hydrocarbon chain links the 3% positions of the sugar moieties. The biological activity
on HIV infected cells was also evaluated. © 2001 Elsevier Science Ltd. All rights reserved.
Metathesis is receiving a renewal of interest since the
discovery of new and efficient catalysts such as the
ruthenium carbene complexes recently introduced by
Grubbs and co-workers.1 It can be used for many
synthetic applications: ring opening polymerization,1,2
cross-metathesis between identical or different termi-
nal olefins3–5 and ring-closing metathesis6 which is the
most used application of this technique. Thanks to
this progress, we have planned the synthesis of
nucleoside dimers, analogues of dinucleotides. It is
now well known that nucleoside and oligonucleotide
analogues7 possess therapeutic activities in particular
against retro-viruses. With the purpose of getting new
active molecules, we describe herein the synthesis
of dinucleosides linked by an olefinic chain between
the positions 3% of their glycosidic moiety, using a cross
metathesis reaction. Such
a
hydrocarbon chain
should enhance the lipophilic character of the
oligonucleotide analogues and help penetration of
these compounds into cells. To the best of our knowl-
edge, metathesis has not been applied to nucleosides
yet.
For the choice of a catalyst, we decided to use a
ruthenium carbene complex developed by Grubbs and
co-workers,1 benzylidenebis(tricyclohexylphosphine)di-
chloro-ruthenium(IV) (1), for its high reactivity, stabil-
ity and remarkable functional group tolerance.
Scheme 1. (i) TBDPSCl (1.2 equiv.), DMAP (0.02 equiv.), dry pyridine; (ii) PhOC(S)Cl (1.1 equiv.), DMAP (2 equiv.), dry
acetonitrile; (iii) Bu3SnCH2CHꢀCH2 (4 equiv.), dry toluene.
* Corresponding author. Fax: 33 (0) 5.55.45.72.02; e-mail: krausz@unilim.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02292-9

1492
N. Batoux et al. / Tetrahedron Letters 42 (2001) 1491–1493
Scheme 2.
exhibits a significant activity in the presence of a large
variety of functional groups. Here, nucleosides contain
pyrimidine bases with amido groups, the hydroxyl moi-
ety of the sugar parts is protected by a silyl-ether. The
metathesis is effected in dry dichloromethane at reflux
with 10–20% mol. catalyst (1) for 6 h (a longer reaction
time does not improve the yield). The yield is about
45% with both thymidine and the 2%-deoxyuridine
dimer. The metathesis reaction has also been attempted
using the 5%-hydroxyl unprotected compound in the
case of the thymidine analogue: the expected product
(7a) is obtained with a lower yield (15%).
The allylic nucleosides (5a,b) (Scheme 1) are obtained
in three steps starting from the corresponding 2%-
deoxynucleosides. The first one is the selective protec-
tion of the 5%-hydroxy group of 2%-deoxynucleoside
(2a,b), using t-butyldiphenylsilyl chloride (1.1 equiv.) in
dry pyridine in presence of DMAP (0.02 equiv.) giving
the 5%-selectively protected product (3a,b) in high yields
(90–95%). Next, the 3%-hydroxy moiety is changed to
the corresponding phenylthionocarbonate (4a,b) by
reaction with phenylchlorothionocarbonate (1.1 equiv.)
in dry acetonitrile with DMAP (2 equiv.).8 The 3%-allyl-
5%-t-butyldiphenylsilyl-2%,3%-dideoxy nucleoside9 (5a,b) is
obtained after a free radical reaction with allyl-
tributyltin (4 equiv.) in dry toluene with AIBN as
radical initiator.
The case of the allylcytidine metathesis is not so
straightforward: without protecting the amino group,
metathesis does not occur with any significant conver-
sion. A convenient acetylation of the amino moiety12
(5c) is led under microwave irradiation in a domestic
oven, with acetic anhydride (2 equiv.) in DMF, in 2
min. After acetylation, metathesis is realized in 20 h
and the (Z/E) isomers have been partially isolated by
preparative TLC, in 15% global yield.
This strategy is not efficient to prepare the allylcytidine
analogue:10 the yields of the two last steps are very low
(less than 10%), even if the silyl-ether protection system
is modified by a selective benzoylation on N-4 and O-5%
position. Finally for the obtention of the cytidine
derivative, we decided to use the allyluridine (5b)
already synthesized and change the C-4 carbonyl group
into an amino moiety in two steps:11 firstly, the car-
bonyl moiety is converted into a thiocarbonyl with
Lawesson’s reagent (0.75 equiv.) in dry 1,2-dichloro-
ethane (90%); secondly, this thiouridine is reacted with
methanolic ammonia to give the expected 3%-allyl-5%-t-
butyldiphenylsilyl-2%,3%-dideoxycytidine (50%).
In each case, the dimeric final products are obtained
after deprotection of the 5%-hydroxyl group with t-butyl-
ammonium fluoride (2.5 equiv.) in THF in about 70%
yield.
The separation of the Z and E isomers of the thymidine
and uridine dimers is not as easy as in the case of the
cytidine dimer: the separation of the isomeric mixture
can be carried out only after deprotection of the 5%-
hydroxyl group, by HPLC on a reverse phase Prep-
Nova Pack C18 column (WATERS). The Z/E ratio so
determined is about 55:45 for the uridine and thymidine
The second step consists of the self cross metathesis
reaction (Scheme 2).† Grubbs’s ruthenium catalyst (1)
† Typical procedure for metathesis reaction: 211 mg (0.43 mmol) of
5b dissolved in 3.5 mL of dry dichloromethane are introduced in a
round bottom flask, under argon atmosphere, 49 mg (0.06 mmol) of
1 dissolved in 1 mL of dry dichloromethane are added slowly.
Under stirring, the purple reaction mixture is heated to 35°C for 6
h. The solvent of resulting dark solution is removed under reduced
pressure. The residue is purified by preparative TLC (CHCl3/EtOH
95/5) to give 93 mg of 6b (45%) as a foam.
1
products. In the NMR spectra, H chemical shifts do
not allow the identification of the Z and E isomers but
13C chemical shifts of the carbon atom close to the
double bond (C-a) are different for the Z and E iso-
mers.13 The upper value (la=35.85 ppm) is assigned to
the E isomer (eluting last) and the lower one (la=30.33

N. Batoux et al. / Tetrahedron Letters 42 (2001) 1491–1493
1493
ppm) to the Z isomer in the case of 2%-deoxyuridine
References
dimer (7b).‡
1. Preparation of catalyst: Schwab, P.; Grubbs, R. H.;
Ziller, J. W. J. Am. Chem. Soc., 1996, 118, 100. For
recent reviews about metathesis: (a) Fu¨rstner, A.; Angew.
Chem., Int. Ed., 2000, 39, 3012; (b) Grubbs, R. H.;
Chang, S. Tetrahedron 1998, 54, 4413; (c) Armstrong, S.
K. J. Chem. Soc., Perkin Trans. 1 1998, 371; (d) Schuster,
M.; Blechert, S. In: Transition Metals for Organic Synthe-
sis; Beller, M.; Bolm, C., Ed.; 1998; p. 275; (e) Schuster,
M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36,
2036.
Trying to explain the moderate yields obtained for
metathesis dimerisation of nucleosides, compared to
those obtained for example with sugar derivatives, we
can notice that metathesis with N containing products
affords lower yields.6 This could be due to an inhibition
of the catalyst by nitrogen atoms14 that are present in
pyrimidines and especially in cytosine. However, trying
the metathesis reaction with an excess of catalyst does
not improve the yield.
2. Lynn, D. M.; Mohr, B.; Grubbs, R. H. J. Am. Chem.
Soc. 1998, 120, 1627.
The biological activities of thymidine (7a, mix of both
isomers) and uridine (7b, E isomer) dimers were evalu-
ated on CEM-SS cells infected by HIV-1 LAI virus and
on MT4 cells infected by HIV-1 IIIB according to
standardised protocols.15 These compounds do not
show any activity against these viruses and are not
cytotoxic (IC50>10−1 mg/mL).
3. (a) O’Leary, D. J.; Blackwell, H. E.; Washenfelder, R. A.;
Grubbs, R. H. Tetrahedron Lett. 1998, 39, 7427; (b)
Feng, J.; Schuster, M.; Blechert, S. Synlett 1997, 129.
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A.; Ackermann, L. Chem. Commun. 1999, 95; (b) Scholl,
M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetra-
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Leeuwenburgh, M. A.; Overkleeft, H. S.; van der Marel,
G. A.; van Boom, J. H. Synlett 1997, 1263.
In summary, this paper presents for the first time an
application of cross coupling olefin metathesis for the
synthesis of dimeric nucleoside analogues. Others ana-
logues are currently under investigation in our
laboratory.
Acknowledgements
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Tetrahedron Lett. 1999, 40, 1787; (b) Wendeborn, S.;
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The authors are grateful to the Conseil Re´gional du
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for his help in the preparation of this manuscript.
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‡ Structure of all products have been checked by NMR (Bruker
1
DPX-400, 400 MHz for H, 100 MHz for 13C) and mass spectrome-
try. Selected NMR values for final compound of dimeric uridine
(7b) in CD3OD: E isomer: 1H: 8.11 (d, 2H, J=8.1 Hz, H-5), 6.01
(dd, 2H, J=6.3, 3.6 Hz, H-1%), 5.65 (d, 2H, J=8.1 Hz, H-6), 5.53
(m, 2H, CHꢀCH), 3.87 (dd, 2H, J=12.2, 2.3 Hz, H-5%a), 3.74 (m,
2H, H-4%), 3.68 (dd, 2H, J=12.2, 3.6 Hz, H-5%b), 2.30ꢀ2.05 (m, 10
H, H-2%, H-2%%, H-3%, CH2-CHꢀ). 13C: 166.50 (C-4), 152.28 (C-2),
142.73 (C-5), 130.97 (CHꢀCH), 101.90 (C-6), 87.95 (C-4%), 86.83
(C-1%), 62.46 (C-5%), 39.90 (C-2%), 38.21 (C-3%), 35.85 (C6 H2-CHꢀ). Z
isomer: 1H: 8.12 (d, 2H, J=8.1 Hz, H-5), 6.04 (dd, 2H, J=6.4, 3.6
Hz; H-1%), 5.65 (d, 2H, J=8.1 Hz, H-6), 5.53 (m, 2H, CHꢀCH), 3.89
(dd, 2H, J=12.2, 2.4 Hz, H-5%a), 3.77 (m, 2H, H-4%), 3.71 (dd, 2H,
J=12.2, 3.6 Hz, H-5%b), 2.34ꢀ2.05 (m, 10 H, H-2%, H-2%%, H-3%,
CH2
(CHꢀCH), 101.92 (C-6), 88.01 (C-4%), 86.78 (C-1%), 62.46 (C-5%),
39.93 (C-2%), 38.44 (C-3%), 30.33 (CH2-CHꢀ).
6
-CHꢀ). 13C: 166.52 (C-4), 152.28 (C-2), 142.72 (C-5), 129.75
6
.