Modified nucleosides. A general and diastereoselective approach to N,O-psiconucleosides

Article abstract of DOI:10.1016/S0040-4020(01)01161-9

An efficient reaction route towards the new class of N,O-psiconucleosides has been designed, based on the 1,3-dipolar cycloaddition of C-[(tert-butyldiphenylsilyl)oxy]methyl-N-methyl nitrone with ethyl 2-acetyloxyacrylate, followed by nucleosidation. The process has been successfully applied to all pyrimidine and purine nucleobases.

Full text of DOI:10.1016/S0040-4020(01)01161-9

TETRAHEDRON
Pergamon
Tetrahedron 58 (2002) 581±587
Modi®ed nucleosides. A general and diastereoselective approach
to N,O-psiconucleosides
a
Daniela Iannazzo, Anna Piperno, Venerando Pistara, Antonio Resci®na and Roberto Romeo
a
b
b
a,p
Á
a
Á
Dipartimento Farmaco-Chimico, Universita di Messina, Via SS. Annunziata, Messina 98168, Italy
Á
Dipartimento di Scienze Chimiche, Universita di Catania, Viale Andrea Doria 6, Catania 95125, Italy
b
Received 23 October 2001; accepted 22 November 2001
AbstractÐAn ef®cient reaction route towards the new class of N,O-psiconucleosides has been designed, based on the 1,3-dipolar cyclo-
addition of C-[(tert-butyldiphenylsilyl)oxy]methyl-N-methyl nitrone with ethyl 2-acetyloxyacrylate, followed by nucleosidation. The
process has been successfully applied to all pyrimidine and purine nucleobases. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
silylated bases and ®nally reduction of ester groups with
NaBH4 to give a-and b-nucleosides. When performed
under thermodynamic control conditions, the reaction
leads exclusively to the corresponding b-nucleosides, in
the case of pyrimidine derivatives, and to a-nucleosides in
the case of purine derivatives6 (Fig. 1).
Modi®ed nucleosides have received great attention over the
last decade largely due to their apparent promise as antiviral
and antitumor agents.1 In this context, a great deal of interest
has been recently devoted towards the synthesis of systems
in which the furanose moiety is replaced by alternative
carbo- or heterocyclic rings. Since these compounds are
analogs of natural compounds, they utilize the biochemical
machinery of the host cells and act at the catalytic site of
HIV reverse transcriptase by terminating DNA synthesis.2
In this context, relevant importance assume N,O-modi®ed
nucleosides, and considerable progress has been made since
the pioneering work of Tronchet and coll.3
Figure 1.
The designed reaction route appears to be versatile and of
general application but suffers from a serious limitation in
the dif®cult reduction of ester groups. In the present paper
we report a successful implementation of the synthetic
strategy achieved by the use of the novel nitrone 2, contain-
ing a protected alcoholic group in a with respect to the
nitrone functionality: in this way different functionaliza-
Our efforts in these area have been devoted towards the
development of new synthetic schemes for the construction
of a series of N,O-modi®ed nucleosides, some of which have
shown promising anti-AIDS activity in vitro.4 The synthetic
strategy we privileged moves from the potentialities offered
by 1,3-dipolar cycloaddition processes for the synthesis of
stereochemically de®ned heterocyclic systems.
0
0
tions at C3 and C5 could be achieved. The approach has
been successfully extended to all purine and pyrimidine
nucleobases.
In a previous paper,5 we have reported the ®rst synthetic
approach towards the till now unreported class of N,O-
psiconucleosides in which the heterocyclic ring is branched
at the anomeric position: by this route N,O-psiconucleosides
1, carrying a hydroxymethyl group at the anomeric position
and at C4 have become accessible. The synthetic scheme
develops in only three steps: cycloaddition of a C-alkoxy-
carbonylnitrone with enol esters, nucleosidation with
2. Results and discussion
The synthetic scheme is based on the 1,3-dipolar cyclo-
addition of C-a-silyloxymethyl-N-methyl nitrone 2 with
ethyl 2-acetyloxyacrilate 3, followed by nucleosidation
performed with silylated purine and pyrimidine bases.
Keywords: psiconucleosides; modi®ed nucleosides; 1,3-dipolar cyclo-
addition; PM3 calculation.
p
Nitrone 2 has been prepared in good yields starting from
mannitol; thus, d-mannitol 4 (1 mmol) has been reacted
Corresponding author. Tel.: 139-90-356-230; fax: 139-90-676-6562;
e-mail: romeog@unime.it
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)01161-9

582
D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
Scheme 1. (a) t-Butyldiphenylsilylchloride (TBDPSCI), imidazole, DMF; (b) Pb(AcO)4, benzene; (c) CH3NHOH, CH2Cl2.
Scheme 2.
with t-butyl-diphenylsilylchloride (2 mmol) in the presence
of imidazole (4.4 mmol) in anhydrous DMF to give the 1,6-
di-O-tert-butyldiphenylsilyl-d-mannitol. The subsequent
treatment with lead tetracetate in benzene afforded aldehyde
5 which has been reacted with N-methyl hydroxylamine in
toluene, in the presence of triethylamine, to give, after
column chromatography separation (chloroform/methanol
98:2 as eluant), the expected nitrone 2 (70% global yield
starting from mannitol) (Scheme 1).
at C5 (d, 2.07 ppm). These data support a cis relationship
between H4b, H3 and the acetyl group at C5 and between the
CH2OR group at C3 and the CO2Et group at C5.
As a consequence, the con®guration of compound 7 can be
assigned as the alternative one showing the group at C3 and
the acetyl moiety at C5 in a cis relationship.
The stereochemical outcome obtained in the cycloaddition
process can be explained by considering that nitrone 2 has
1
been shown by H NMR and NOE data to be the expected
The cycloaddition reaction between 2 and the enol acetate 3,
in anhydrous ether at room temperature for 24 h, has been
found to proceed with a moderate stereoselectivity, afford-
ing a mixture of epimeric isoxazolidines 6 and 7 in a relative
ratio 2.5:1 (global yield 85%) (Scheme 2). The crude
mixture was puri®ed by ¯ash chromatography (cyclo-
hexane/diethyl ether 9:1 as eluant) followed by HPLC and
two cycloadducts 6 and 7 were obtained in pure form.
Z-isomer: thus, the major product 6 could be formed by the
Z-nitrone reacting in an endo mode, according to the results
reported for similar a-alkoxyalkylnitrones.7 PM3 calcula-
tions afford a theoretical support to experimental data: the
Z-endo transition state leading to 6 is about 0.52 kcal mol21
more stable than Z-exo transition state leading to Z stereo-
isomer 7. This value is in satisfactory agreement with the
observed E/Z ratio.
The structure of the obtained adducts has been assigned on
the basis of H NMR data and con®rmed by NOE experi-
1
In the previously reported cycloaddition reaction of
C-alkoxycarbonyl-N-methyl nitrone 8 with the same
dipolarophile 3, a better diastereoselectivity had been
previously obtained: in this case, isoxazolidines 9 and 10
are obtained in a relative ratio 8.6:1 (Scheme 3). The
different results can be rationalized taking into account
that nitrone 8 exists as a 4:1 mixture of E and Z stereo-
isomers with the more reactive E-isomer predominating:8
thus, as con®rmed by PM3 calculations, compounds 9 and
10 originate from the E form through an exo transition state.
ments. Thus, the main product 6 shows the resonance of H3
as a multiplet at 3.08 ppm, while H4 protons give rise to two
doublets of doublets centered at 2.36 and 3.08 ppm; the
methylene group of the substituent at C3 resonates as two
doublets of doublets at 3.74 and 3.75 ppm. In compound 7,
H3 resonates at 3.26 ppm, while H4 protons collapse to a
multiplet at 2.69 ppm; the CH2OR group at C3 gives rise
to resonances centered at 3.70 and 3.79 ppm. The stereo-
chemistry of the adducts was readily deduced by means of
NOE measurements. Thus, for compound 6, irradiation of
H4a (dd, 2.36 ppm) produced strong enhancements for the
CH2OR group (dd, 3.74 and 3.75 ppm; 11%) and H4b (dd,
3.08 ppm; 20%); conversely, when the resonance at 3.08
(H3 and H4b) was irradiated, a positive NOE effect was
observed for H4a and the methyl group of the acetyl moiety
Two cycloadducts were independently coupled with all
the silylated nucleobases, according to the VorbruÈggen
glycosylation methodology. The condensation with silyl-
ated thymine, N-acetylcytosine, 5-¯uorouracil, adenine
and 2-N-acetyl-6-O-diphenylcarbamylguanine, has been
Scheme 3.

D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
583
Scheme 4.
performed in acetonitrile, at 508C, in the presence of
0.4 equiv. of TMSOTf as catalyst: a mixture of a-and
b-nucleoside 11±15 has been obtained. (Scheme 4). Pure
anomers were isolated by ¯ash chromatography and the
relative con®guration was assigned on the basis of 1H
NMR and NOE experiments. In particular, for
0
b-compounds 11b and 13b, irradiation of H4 a (the up®eld
0
resonance of methylene protons at C4 ) increased the proton
0
signal of H4 b and the vinylic proton in the base moiety at C6;
0
conversely, when H4 b was irradiated, a positive NOE effect
are not in a close proximity to induce any steric effect upon
the incoming group at C5.6
The obtained anomeric distribution depends on the attack-
ing nucleobase. When pyrimidine nucleobases were used,
the b-anomers clearly predominate with thymine and
5-¯uorouracil, while, in the case of N-acetylcytosine, the
a-nucleoside was the exclusive product. By contrast, with
purine bases, both anomers are obtained in signi®cative
amounts. Evidently, the attack on the intermediate oxonium
ion from either a-or b-side is possible, and hence the
product distribution is sensitive to structural changes of
the reactants.9
0
is detected for H4 a and H3 . These data unambiguously point
0
0
to a b-con®guration indicating that H3 and the pyrimidine
0
base at C5 are in a trans relationship. Furthermore, the
a-con®guration for anomers 11a±13a is supported by the
0
strong NOE observed for H6 when irradiating H3 .
PM3 data gave a rationalization to the experimental results
(Table 1).
The stereochemistry of purine derivatives 14 and 15 is also
derived from NOE measurements. In fact, diagnostic NOE
effects have been detected by irradiation of the methyl group
on the isoxazolidine nitrogen atom: the assignment of the
a-con®guration to anomers 14a and 15a receives support
The anomeric ratio is insensitive to the adopted experi-
mental conditions. In fact, silylated thymine, chosen as
model compound, was coupled with the mixture of
isoxazolidines 6 and 7 at different temperatures and using
different amounts of catalyst: no difference was observed in
the product distribution.
0
from the enhancements observed for H3 , H9 and the down-
0
®eld resonance of methylene protons at C4 . Consequently
the b-con®guration may be con®dently assigned to 14b and
15b, where the irradiation of N-CH3 induced a positive NOE
Finally, the synthetic scheme has been completed by
removal of the protecting group and reduction of the estereal
0
function at C5 . Thus, 11a,b and 12a, as selected models,
0
effect on the up®eld resonance of methylene protons at C4
and on the methylene protons of CH2OSi± group.
0
furnished C5 -branched nucleosides 16a,b and 17a by treat-
The ratio between a-and b-nucleosides did not change if the
nucleosidation reaction was performed starting from the
crude mixture of isoxazolidines without any preventive
separation. These results show that the coupling reaction
of isoxazolidines with silylated bases occurs without
selectivity with respect to the anomeric center. As
previously reported, this is due to the formation of an inter-
mediate oxonium ion, which would not be expected to have
any signi®cant facial bias since the substituents at C3 and C4
ment with TBAF in THF. The reduction with NaBH4 of 11b,
12a and 14a led to the target psiconucleosides 18b, 19a and
Table 1. PM3 calculated heat formation for compounds 12±16
Psiconucleoside DHfa a-Anomer DHf b-Anomer Calculated a/b ratio
11
12
13
14
15
2261.49
2241.57
2292.51
132.51
2261.69
2240.25
2292.70
2131.18
2183.41
1:1.4
9.3:1
1:1.3
9.4:1
2:1
2183.85
a
kcal mol21
.
Scheme 5.

584
D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
20a, with a yield of 70% after column chromatography
puri®cation on neutral alumina (Scheme 5).
1361, 1113, 998, 823 cm21. Exact mass calculated for
C19H25NO2Si: 327.1654. Found: 327.1649.
4.1.2. Preparation of isoxazolidines 6and 7. A solution of
nitrone 2 (3.80 g, 17.5 mmol) ethyl 2-acetyloxyacrilate 3
(2.85 g, 18 mmol) in dry ether (100 mL) was stirred at
room temperature for 24 h. The reaction mixture was evapo-
rated and the residue puri®ed by ¯ash chromatography
(cyclohexane/ethyl ether 8:2) to afford a mixture of
isoxazolidines 6 and 7 (85% global yield), that were further
separated by HPLC (n-hexane/2-propanol 96:4). The ®rst
eluted product was ethyl (3RS,5SR)-5-(acetyloxy)-3-
({[tert-butyl(diphenyl)silyl]oxy}methyl)-2-methylisoxa-
zolidine-5-carboxylate 6 (tR 11.0 min, 60.7%) as a colorless
oil; dH (500 MHz, CDCl3) 1.05 (s, 9H), 1.28 (t, 3H,
Jˆ7.5 Hz), 2.07 (s, 3H), 2.36 (dd, 1H, H4a, Jˆ8.5,
13.0 Hz), 2.93 (s, 3H, N-CH3), 3.08 (m, 2H, H3 and H4b),
3. Conclusion
In summary, we have designed an ef®cient reaction route
towards the new class of N,O-psiconucleosides. The process
has been easily extended to all pyrimidine and purine
derivatives. The enantioselective synthesis of these
compounds is actually in progress as well as their clinical
evaluation.
4. Experimental
4.1. General
0
0
,
3.74 (dd, 1H, H3 a, Jˆ4.5, 10.1 Hz), 3.75 (dd, 1H, H3 b
Jˆ7.5, 10.1 Hz), 4.25 (q, 2H, Jˆ7.5 Hz), 7.4±7.6 (m, 6H),
7.65 (d, 4H, Jˆ8.0 Hz); dC (125 MHz, CDCl3) 13.20, 13.90,
26.72, 29.60, 44.00, 44.60, 62.50, 63.75, 69.75, 103.50,
127.81, 129.90, 135.59, 140.00, 156.40, 163.00. IR (neat)
3072, 3050, 2959, 2931, 2858, 1756, 1746, 1589, 1428,
1370, 1142, 1097, 1013, 824, 752 cm21. Exact mass calcu-
lated for C26H35NO6Si: 485.2233. Found: 485.2232.
Melting points were measured on a Ko¯er apparatus and are
uncorrected. Elemental analyses were performed with a
Perkin±Elmer elemental analyzer. H NMR spectra were
1
measured on a 500 MHz Varian Unity Inova instrument in
CDCl3 as solvent. Chemical shifts are in ppm (d) from TMS
as internal standard. NOE difference spectra were obtained
by subtracting alternatively right-off-resonance free induc-
tion decays (FIDS) from right-on-resonance-induced FIDS.
Merck silica gel 60H was used for preparative short-column
chromatography. Preparative HPLC was performed with a
The second eluted product was ethyl (3RS,5RS)-5-(acetyl-
oxy)-3-({[tert-butyl(diphenyl)silyl]oxy}methyl)-2-methyl-
isoxazolidine-5-carboxylate 7 (tR 11.9 min, 24.3%) as a
colorless oil; dH (500 MHz, CDCl3) 1.05 (s, 9H), 1.27 (t,
3H, Jˆ7.0 Hz), 2.12 (s, 3H), 2.65 (dd, 1H, H4a, Jˆ9.5,
13.0 Hz), 2.70 (dd, 1H, H4b, Jˆ9.5, 13.0 Hz), 2.93 (s, 3H,
Ê
microsorb silica DYNAMAX-100 A (21£250 mm) column.
4.1.1. Preparation of Z-nitrone 2. To a suspension of 4
(2.00 g, 11 mmol) and imidazole (3.30 g, 44 mol) in dry
DMF (50 mL) at 08C, a solution of tert-butyldiphenyl-
silylchloride (5.8 mL, 22 mmol) in dry DMF (15 mL) was
added dropwise. The reaction mixture was stirred at 08C for
1 h and then at 258C for 24 h. At the end of this time the
clear solution was extracted with dichloromethane/water;
the combined organic layers were dried over Na2SO4 and
evaporated under reduced pressure to afford 1,6-di-O-tert-
butyldiphenylsilyloxy-d-mannitol as a colorless oil. Lead
tetraacetate (14.60 g, 33 mmol) was then added to a stirred
solution of the above protected mannitol (7.25 g, 11 mmol)
in benzene (100 mL). The reaction mixture was stirred at
room temperature for 3 h and then ®ltered. The ®ltrate was
evaporated under reduced pressure to afford an oil, identi-
®ed as diphenyl-tert-butylsilyloxyacetaldehyde 5;10 dH
(500 MHz, CDCl3) 1.47 (s, 9H), 4.26 (s, 2H), 7.37±7.50
(m, 6H); 7.77 (d, 4H, Jˆ7.5 Hz), 9.78 (s, 1H, CHO).
0
0
N-CH3), 3.26 (m, 1H, H3 ), 3.70 (dd, 1H, H3 a, Jˆ5.5,
0
10.5 Hz), 3.79 (dd, 1H, H3 b, Jˆ4.5, 10.5 Hz), 4.24 (q, 2H,
Jˆ7.0 Hz), 7.26±7.43 (m, 6H), 7.65 (d, 4H, Jˆ8.0 Hz); dC
(125 MHz, CDCl3) 13.90, 19.16, 21.12, 26.76, 43.13, 44.50,
62.40, 63.50, 69.75, 103.45, 127.78, 129.85, 135.54, 135.99,
139.41, 156.35, 162.50. IR (neat) 3071, 3051, 2959, 2927,
2855, 1754, 1741, 1463, 1428, 1370, 1261, 1112, 1014, 824,
750 cm21
. Exact mass calculated for C26H35NO6Si:
485.2233. Found: 485.2233.
4.2. Preparation of N,O-psiconucleosides 12±16Ð
general procedure
A suspension of bases (2.0 mmol) in dry acetonitrile
(70 mL) was treated with bis(trimethylsilyl)acetamide
(0.61 g, 3.0 mmol) and stirred at room temperature until a
clear solution was obtained (0.5±6 h). A solution of isoxa-
zolidine 6 or 7 (0.55 g, 1.0 mmol) in dry acetonitrile (5 mL)
and TMSOTf (0.056 g, 0.25 mmol) were then added and the
resulting mixture was stirred at 45±508C for 6 h. After this
time, the mixture was neutralized by addition of aqueous 5%
sodium bicarbonate, and then concentrated in vacuo. The
aqueous layer was extracted with ethyl acetate (5£10 mL)
and the combined organic layers were dried over sodium
sulfate, ®ltered, and evaporated to dryness. The residue was
puri®ed by ¯ash chromatography (cyclohexane/ethyl
acetate 1:1 or chloroform/methanol 95:5) to give compound
12±15.
To a solution of crude aldehyde 5 in toluene, N-methyl
hydroxylamine hydrochloride (1.80 g, 22 mmol) and
triethylamine (3 mL, 22 mmol) were added and the mixture
was maintained under stirring for 3 h at 258C. After ®ltra-
tion, the solvent was evaporated at reduced pressure and the
residue was subjected to silica gel column chromatography
(chloroform/methanol 98:2) to give (Z)-C-[(tert-butyl-
diphenylsilyl)oxy]methyl-N-methyl nitrone
2
(3.34 g,
70%) as a colorless oil, dH (500 MHz, CDCl3) 1.1 (s, 9H),
3.60 (s, 3H, N-CH3), 4.65 (d, 2H, Jˆ4.0 Hz), 6.86 (t, 1H,
Jˆ4.0 Hz), 7.31±7.40 (m, 6H); 7.68 (d,4H, Jˆ7.0 Hz); dC
(125 MHz, CDCl3) 19.06, 26.75, 51.78, 60.58, 127.83,
129.93, 132.69, 135.46, 141.02. IR (neat) 3264, 3071,
3049, 2958, 2931, 2857, 1619, 1589, 1472, 1428, 1391,
4.2.1. Reaction of isoxazolidine 6or 7 with thymine.
First eluted product was (30RS,50RS)-1-[30-({[tert-butyl-

D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
585
(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-methyl-
10,20-isoxazolidin-50-yl]thymine 11b (40.65%) as a sticky
oil; dH (500 MHz, CDCl3) 1.02 (s, 9H), 1.25 (t, 3H,
6.0, 7.9, 9.5 Hz), 3.57 (dd, 1H, H4 b, Jˆ7.9, 14.1 Hz), 3.65
0
00
00
(dd, 1H, H3 a, Jˆ6.0, 11.2 Hz), 3.68 (dd, 1H, H3 b, Jˆ4.0,
11.2 Hz), 4.23 (dq, 1H, Jˆ7.2, 10.8 Hz), 4.27 (dq, 1H,
Jˆ7.2, 10.8 Hz), 7.35±7.45 (m, 6H), 7.63 (d, 4H,
Jˆ8.0 Hz), 7.79 (d, 1H, H6, Jˆ6.4 Hz), 8.01 (d, 1H, NH,
Jˆ4.0 Hz); dC (125 MHz, CDCl3) 13.83, 19.11, 26.70,
44.53, 45.24, 63.06, 63.29, 70.16, 93.01, 123.98, 127.83,
129.95, 130.00, 132.56, 135.46, 135.50, 135.54, 138.36,
141.49, 157.04, 157.40, 164.63. IR (KBr) 3440, 3208,
3103, 2981, 2930, 2859, 1759, 1699, 1663, 1471, 1302,
1185, 1010, 850, 750 cm21. Exact mass calculated for
C28H34FN3O6Si: 555.2201. Found: 555.2200.
0
Jˆ7.1 Hz), 1.95 (d, 3H, Jˆ1.3 Hz), 2.37 (dd, 1H, H4 a,
Jˆ9.7, 13.9 Hz), 2.91 (s, 3H, N-CH3), 3.10 (dddd, 1H,
0
0
H3 , Jˆ4.0, 5.9, 7.7, 9.7 Hz), 3.59 (dd, 1H, H4 b, Jˆ7.7,
00
13.9 Hz), 3.65 (dd, 1H, H3 a, Jˆ5.9, 11.2 Hz), 3.75 (dd,
00
1H, H3 b, Jˆ4.0, 11.2 Hz), 4.21 (dq, 1H, Jˆ7.1, 13.1 Hz),
4.25 (dq, 1H, Jˆ7.1, 13.1 Hz), 7.40±7.54 (m, 6H), 7.55 (q,
1H, H6, Jˆ1.3 Hz), 7.64 (d, 4H, Jˆ8.0 Hz), 8.86 (bs, 1H,
NH); dC (125 MHz, CDCl3) 12.70, 13.83, 19.08, 26.65,
44.75, 45.30, 62.99, 63.07, 70.27, 92.80, 109.17, 127.77,
129.87, 129.93, 132.62, 134.93, 135.46, 135.51, 150.38,
165.25, 164.30. IR (neat) 3204, 3101, 2983, 2927, 2854,
1760, 1698, 1659, 1469, 1294, 1192, 1020, 863,
799 cm21. Exact mass calculated for C29H37N3O6Si:
551.2451. Found: 551.2453.
Second eluted compound was (30RS,50SR)-1-[30-({[tert-
butyl(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-
methyl-10,20-isoxazolidin-50-yl]5-¯uorouridine 13a (8%) as
a white solid: mp 145±148; dH (500 MHz, CDCl3) 1.05 (s,
0
9H), 1.24 (t, 3H, Jˆ7.0 Hz), 2.78 (dd, 1H, H4 a, Jˆ7.5,
Second eluted compound was (30RS,50SR)-1-[30-({[tert-
butyl(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-
methyl-10,20-isoxazolidin-50-yl]thymine 11a (19.35%) as a
sticky oil; dH (500 MHz, CDCl3) 1.05 (s, 9H), 1.20 (t, 3H,
0
14.7 Hz), 2.82 (m, 1H, H3 ), 2.98 (s, 3H, N-CH3), 3.29
0
00
(dd, 1H, H4 b, Jˆ7.2, 14.7 Hz), 3.74 (m, 1H, H3 a,), 3.89
00
(m, 1H, H3 b), 4.25 (dq, 1H, Jˆ7.0, 11.1 Hz), 4.29 (dq,
1H, Jˆ7.0, 11.1 Hz), 7.35±7.45 (m, 6H), 7.65 (d, 4H,
Jˆ8.0 Hz), 7.80 (d, 1H, H6, Jˆ6.4 Hz), 8.85 (bs, 1H,
NH); dC (125 MHz, CDCl3) 13.84, 19.05, 25.68, 43.90,
44.10, 63.06, 63.29, 68.05, 93.20, 123.00, 127.83, 129.95,
130.00, 135.46, 135.50, 135.54, 141.49, 157.04, 157.40,
164.63. IR (KBr) 3430, 3200, 3080, 2990, 2945, 2863,
0
Jˆ7.1 Hz), 1.97 (d, 3H, Jˆ1.3 Hz), 2.75 (dd, 1H, H4 a,
0
Jˆ6.7, 14.1 Hz), 2.95 (s, 3H, N-CH3), 2.98 (m, 1H, H3 ),
0
00
3.33 (dd, 1H, H4 b, Jˆ7.7, 14.1 Hz), 3.73 (m, 1H, H3 a), 3.91
00
(m, 1H, H3 b), 4.17 (dq, 1H, Jˆ7.1, 10.8 Hz), 4.20 (dq, 1H,
Jˆ7.1, 10.8 Hz), 7.37±7.45 (m, 6H), 7.61 (q, 1H, H6,
Jˆ1.3 Hz), 7.64 (d, 4H, Jˆ8.0 Hz), 8.84 (bs, 1H, NH); dC
(125 MHz, CDCl3) 12.74, 13.78, 19.08, 26.73, 29.65, 36.44,
42.89, 62.97, 70.01, 93.24, 109.43, 127.79, 129.86, 129.81,
132.80, 134.93, 135.47, 135.51, 150.25, 164.07, 165.65. IR
(neat) 3210, 3090, 2987, 2937, 2850, 1763, 1700, 1655,
1460, 1299, 1191, 1020, 865, 800 cm21. Exact mass calcu-
lated for C29H37N3O6Si: 551.2451. Found: 551.2449.
1761, 1707, 1650, 1310, 1200, 1020, 860, 800 cm21
.
Exact mass calculated for C28H34FN3O6Si: 555.2201.
Found: 555.2204.
4.2.4. Reaction of isoxazolidine 6or 7 with adenine. First
eluted product was (30SR,50)-1-[30-({[tert-butyl(diphenyl)-
silyl]oxy}methyl)-50-ethoxycarbonyl-20-methyl-10,20-isoxa-
zolidin-50-yl]adenine 14a (45%) as a sticky oil; dH
(500 MHz, CDCl3) 1.02 (s, 9H), 1.17 (t, 3H, Jˆ7.2 Hz),
4.2.2. Reaction of isoxazolidine 6or 7 with 4- N-acetyl-
cytosine.
(30RS,50SR)-1-[30-({[ter-Butyl(diphenyl)silyl]-
0 0
2.95 (s, 3H, N-CH3), 3.18 (m, 2H, H4 a and H3 ), 3.81 (m,
oxy}methyl)-50-etoxycarbonyl-20-methyl-10,20-isoxazolidin-
50-yl]4-N-acetylcytosine 12a (70%), white solid: mp 62±
658C; dH (500 MHz, CDCl3) 0.99 (s, 9H), 1.20 (t, 3H,
3H, H4 b and H3 ), 4.17 (q, 2H, Jˆ7.2 Hz), 5.85 (bs, 2H,
NH2), 7.35±7.45 (m, 6H), 7.60 (d, 4H, Jˆ8.0 Hz), 8.16 (s,
1H, H3), 8.28 (s, 1H, H9); dC (125 MHz, CDCl3) 13.76,
19.07, 26.56, 41.55, 45.45, 63.19, 63.75, 68.36, 92.43,
120.37, 127.73, 129.83, 123.70, 132.80, 135.39, 135.46,
135.51, 149.22, 152.81, 155.68, 166.26. IR (neat) 3303,
3153, 3072, 2959, 2929, 2857, 1757, 1674, 1604, 1470,
1205, 1112, 824, 799 cm21. Exact mass calculated for
C29H36N6O4Si: 560.2567. Found: 560.2566.
0
00
0
Jˆ7.1 Hz), 2.23 (s, 3H), 2.28 (dd, 1H, H4 a, Jˆ9.9,
0
14.1 Hz), 2.91 (s, 3H, N-CH3), 3.12 (dddd, 1H, H3 Jˆ3.5,
00
6.6, 7.9, 9.9 Hz), 3.60 (dd, 1H, H3 a, Jˆ6.6, 11.2 Hz), 3.64
00
0
(dd, 1H, H3 b, Jˆ3.5, 11.2 Hz), 3.70 (dd, 1H, H4 b, Jˆ7.9,
14.1 Hz), 4.18 (dq, 1H, Jˆ7.1, 13.2 Hz), 4.20 (dq, 1H,
Jˆ7.1, 13.2 Hz), 7.30±7.40 (m, 10H); 7.46 (d, 1H, H5,
Jˆ7.7 Hz), 8.12 (d, 1H, H6, Jˆ7.7 Hz), 10.37 (bs, 1H,
NH); dC (125 MHz, CDCl3) 13.76, 19.06, 24.69, 26.67,
44.26, 45.30, 62.86, 63.20, 70.27, 93.54, 95.99, 127.60,
127.76, 127.78, 129.83, 129.90, 132.57, 132.73, 134.77,
135.42, 135.46, 143.93, 155.13, 163.56, 164.83, 171.16.
IR (KBr) 3441, 3130, 3050, 2983, 1781, 1700, 1612,
1503, 1464, 1314, 1219, 1092, 813, 781 cm21. Exact
mass calculated for C30H38N4O6Si: 578.2560. Found:
578.2558.
Second eluted compound was (30RS,50RS)-1-[30-({[tert-
butyl(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-
methyl-10,20-isoxazolidin-50-yl]adenine 14b (15%) as a
sticky oil; dH (500 MHz, CDCl3) 1.04 (s, 9H), 1.26 (t, 3H,
0
Jˆ7.2 Hz), 2.98 (s, 3H, N-CH3), 3.00 (dd, 1H, H4 a, Jˆ5.7,
0
0
13.2 Hz), 3.13 (m, 1H, H3 ), 3.47 (dd, 1H, H4 b, Jˆ9.6,
00
13.2 Hz), 3.83 (m, 2H, H3 ), 4.11 (dq, 1H, Jˆ7.2,
11.3 Hz), 4.17 (dq, 1H, Jˆ7.2, 11.3 Hz), 5.81 (bs, 2H,
NH2), 7.35±7.45 (m, 6H), 7.70 (d, 4H, Jˆ8.0 Hz), 8.17 (s,
1H, H3), 8.27 (s, 1H, H9); dC (125 MHz, CDCl3) 13.80,
19.15, 26.61, 41.60, 44.50, 63.25, 63.27, 70.38, 92.42,
120.37, 127.73, 129.83, 123.70, 132.80, 135.39, 135.46,
135.51, 149.22, 152.81, 155.40, 166.05. IR (neat) 3300,
3155, 3070, 2961, 2933, 2855, 1759, 1670, 1609, 1475,
1198, 1110, 830, 800 cm21. Exact mass calculated for
C29H36N6O4Si: 560.2567. Found: 560.2565.
4.2.3. Reaction of isoxazolidine 6or 7 with 5-¯uorouracil.
First eluted product was (30RS,50RS)-1-[30-({[tert-butyl-
(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-methyl-
10,20-isoxazolidin-50-yl]5-¯uorouridine 13b (72%) as a
white solid: mp 136±1398C; dH (500 MHz, CDCl3) 1.03
0 0
(s, 9H), 1.26 (t, 3H, Jˆ7.2 Hz), 2.41 (dd, 1H, H4 a, Jˆ9.5,
14.1 Hz), 2.90 (s, 3H, N-CH3), 3.12 (dddd, 1H, H3 , Jˆ4.0,

586
D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
4.2.5. Reaction of isoxazolidine 6or 7 with 2- N-acetyl-6-
O-diphenylcarbamoylguanine. First eluted product was
(30RS,50SR)-1-[30-({[tert-butyl(diphenyl)silyl]oxy}methyl)-
50-ethoxycarbonyl-20-methyl-10,20-isoxazolidin-50-yl]2-N-
acetyl-6-O-diphenylcarbamylguanine 15a (30%) as a white
solid: mp 130±1338C; dH (500 MHz, CDCl3) 0.95 (s, 9H),
4.3.2. Reaction of 11b with TBAF. (30RS,50RS)-1-[50-
Ethoxycarbonyl-30-hydroxymethyl-20-methyl-10,20-isoxa-
zolidin-50-yl]thymine 16b (85%), sticky oil; dH (500 MHz,
CDCl3) 1.25 (t, 3H, Jˆ7.2 Hz), 1.93 (d, 3H, Jˆ1.1 Hz), 2.57
0
(dd, 1H, H4 a, Jˆ9.5, 13.9 Hz), 2.92 (s, 3H, N-CH3), 3.12
0
0
,
(dddd, 1H, H3 , Jˆ3.6, 5.1, 7.5, 9.5 Hz), 3.65 (dd, 1H, H4 b
0
00
1.16 (t, 3H, Jˆ7.1 Hz), 2.49 (s, 3H), 2.80 (dd, 1H, H4 a,
Jˆ7.5, 13.9 Hz), 3.66 (dd, 1H, H3 a, Jˆ5.1, 12.0 Hz), 3.75
0
00
Jˆ9.0, 13.7 Hz), 2.96 (s, 3H, N-CH3), 3.21 (m, 1H, H3 ),
(dd, 1H, H3 b, Jˆ3.6, 12.0 Hz), 4.21 (q, 2H, Jˆ7.2 Hz), 7.61
0
00
3.50 (dd, 1H, H4 b, Jˆ8.1, 13.7 Hz), 3.62 (m, 2H, H3 ), 4.15
(dq, 1H, Jˆ7.1, 11.2 Hz), 4.21 (dq, 1H, Jˆ7.1, 11.2 Hz),
7.24±7.71 (m, 20H), 7.90 (bs, 1H, NH), 8.26 (s, 1H, H8); dC
(125 MHz, CDCl3) 13.83, 19.01, 25.12, 26.59, 43.45, 45.33,
63.40, 68.13, 70.30, 91.99, 121.42, 127.79, 129.17, 129.94,
132.51, 132.61, 135.38, 135.47, 141.72, 144.28, 150.29,
151.76, 153.85, 155.97, 165.28, 171.22. IR (KBr) 3300,
3071, 3043, 2960, 2927, 2855, 1752, 1700, 1619, 1588,
1492, 1290, 1186, 1112, 701 cm21. Exact mass calculated
for C44H47N7O7Si: 813.3306. Found: 813.3304.
(q, 1H, H6, Jˆ1.1 Hz), 9.90 (bs, 1H, NH); dC (125 MHz,
CDCl3) 12.50, 12.57, 44.31, 44.74, 60.57, 62.63, 70.18,
92.87, 109.21, 134.92, 150.61, 164.53, 165.34. IR (neat)
3443, 3200, 2980, 2947, 2850, 1760, 1695, 1660, 1470,
1295, 1190, 1020, 865, 750 cm21. Exact mass calculated
for C13H19N3O6: 313.1273. Found: 313.1272.
4.3.3. Reaction of 12a with TBAF. (30RS,50SR)-1-[50-
Ethoxycarbonyl-30-hydroxymethyl-20-methyl-10,20-isoxa-
zolidin-50-yl]-N-acetylcytosine 17a (86%), sticky oil; dH
(500 MHz, CDCl3) 1.21 (t, 3H, Jˆ7.1 Hz), 2.21 (s, 3H),
Second eluted compound was (30RS,50RS)-1-[30-({[tert-
butyl(diphenyl)silyl]oxy}methyl)-50-ethoxycarbonyl-20-
methyl-10,20-isoxazolidin-50-yl]2-N-acetyl-6-O-diphenyl-
carbamylguanine 15b (30%) as a white solid: mp 125±
1278C; dH (500 MHz, CDCl3) 1.04 (s, 9H), 1.16 (t, 3H,
Jˆ7.0 Hz), 2.49 (s, 3H), 2.97 (s, 3H, N-CH3), 3.12 (m,
0
2.61 (dd, 1H, H4 a, Jˆ9.5, 14.1 Hz), 2.90 (s, 3H, N-CH3),
0
3.09 (dddd, 1H, H3 , Jˆ3.4, 4.5, 7.9, 9.5 Hz), 3.62 (dd, 1H,
00
00
H3 a, Jˆ4.5, 12.1 Hz), 3.69 (dd, 1H, H3 b, Jˆ3.4, 12.1 Hz),
0
3.74 (dd, 1H, H4 b), 4.12 (q, 2H, Jˆ7.1 Hz), 7.45 (d, 1H, H5,
Jˆ7.5 Hz), 8.12 (d, 1H, H6, Jˆ7.5 Hz), 9.99 (bs, 1H, NH);
dC (125 MHz, CDCl3) 13.76, 24.73, 43.69, 44.65, 60.27,
65.54, 70.38, 93.74, 96.05, 115.99, 143.79, 149.66,
155.52, 163.21, 170.96. IR (neat) 3418, 3341, 3230, 3071,
2958, 2929, 2856, 1717, 1700, 1655, 1603, 1488, 1275,
1112, 1068, 823, 795 cm21. Exact mass calculated for
C14H20N4O6: 340.1383. Found: 340.1384.
0
0
1H, H3 ), 3.22 (dd, 1H, H4 a, Jˆ9.7, 3.4 Hz), 3.51 (m, 1H,
0
00
H4 b), 3.78 (dd, 1H, H3 a, Jˆ4.4, 10.8 Hz), 3.85 (dd, 1H,
00
H3 a, Jˆ7.0, 10.8 Hz), 4.14 (dq, 1H, Jˆ7.0, 11.2 Hz), 4.19
(dq, 1H, Jˆ7.0, 11.2 Hz), 7.25±7.70 (m, 20H), 7.98 (bs, 1H,
NH), 8.29 (s, 1H, H8); dC (125 MHz, CDCl3) 13.83, 19.09,
25.13, 26.72, 38.68, 41.46, 63.42, 68.11, 68.23, 92.54,
121.64, 126.94, 127.81, 129.20, 129.93, 132.60, 132.70,
135.47, 135.52, 141.65, 145.02, 150.32, 151.93, 153.71,
156.29, 165.74, 170.94. IR (KBr) 3300, 3070, 3050, 2970,
2930, 2850, 1750, 1705, 1620, 1590, 1490, 1290, 1185,
1117, 700 cm21. Exact mass calculated for C44H47N7O7Si:
813.3306. Found: 813.3307.
4.4. Preparation of N,O-psiconucleosides 18b, 19a and
20aÐgeneral procedure
To a solution of nucleosides 11b, 12a and 14a (1 mmol) in
methanol (20 mL), at 08C, NaBH4 (0.060 g, 1.5 mmol) was
added and the mixture was stirred for 3 h. At the end of this
time, the solvent was removed and the residue was subjected
to column chromatography on neutral alumina (chloroform/
methanol 95:5).
0
4.3. Preparation of N,O-C5 -branched nucleosides 16a,b
and 17aÐgeneral procedure
4.4.1. Reaction of 11b with NABH4. Already reported.5
To a solution of nucleosides 11a,b and 12a (1 mmol) in dry
THF (20 mL), TBAF (1.05 mL, 1.1 mmol, 1 M solution in
THF) was added, and the mixture was stirred at room
temperature for 1.5 h. At the end of this time, the solvent
was removed and the residue was subjected to silica gel
column chromatography (chloroform/methanol 95:5).
4.4.2. Reaction of 12a with NaBH4. (30RS,50SR)-1-[30-
({[tert-Butyl(diphenyl)silyl]oxy}methyl)-50-hydroxymethyl-
20-methyl-10,20-isoxazolidin-50-yl]-N-acetylcytosine 19a
(80%), sticky oil; dH (500 MHz, CDCl3) 1.10 (s, 9H), 2.09
0
(s, 3H), 2.40 (dd, 1H, H4 a, Jˆ9.3, 13.5 Hz), 2.59 (dd, 1H,
0
H4 b, Jˆ7.5, 13.5 Hz), 2.80 (s, 3H, N-CH3), 3.81 (m, 1H,
4.3.1. Reaction of 11a with TBAF. (30RS,50SR)-1-[50-
Ethoxycarbonyl-30-hydroxymethyl-20-methyl-10,20-isoxa-
zolidin-50-yl]thymine 16a (80%), sticky oil; dH (500 MHz,
CDCl3) 1.26 (t, 3H, Jˆ7.2 Hz), 1.97 (d, 3H, Jˆ1.3 Hz), 1.98
0
H3 ), 3.50 (m, 2H, H3 ), 3.52±3.60 (bs, 2H, NH and OH),
00
00
00
,
3.68 (d, 1H, H5 a, Jˆ11.8 Hz), 4.15 (d, 1H, H5 b
Jˆ11.8 Hz), 5.60 (d, 1H, H5 or H6 Jˆ6.9 Hz), 7.30±7.40
(m, 10H), 7.70 (d, 1H, H5 or H6 Jˆ6.9 Hz); dC (125 MHz,
CDCl3) 19.03, 26.70, 42.17, 45.30, 64.29, 64.40, 70.06,
93.60, 96.65, 127.70, 127.63, 129.74, 132.80, 132.91,
135.43, 135.49, 142.15, 156.29, 165.83. IR (neat) 3440,
3200, 3030, 3015, 2980, 2950, 1698, 1665, 1475, 1100,
0
0
(m, 1H, H4 a), 2.92 (m, 1H, H3 ), 2.95 (s, 3H, N-CH3), 3.49
0
00
(dd, 1H, H4 b, Jˆ5.1, 14.4 Hz), 3.71 (m, 1H, H3 a), 3.84 (m,
00
1H, H3 b), 4.22 (dq, 1H, Jˆ7.2, 10.8 Hz), 4.26 (dq, 1H,
Jˆ7.2, 10.8 Hz), 7.61 (q, 1H, H6, Jˆ1.3 Hz), 8.65 (bs, 1H,
NH); dC (125 MHz, CDCl3) 12.73, 14.10, 44.32, 44.80,
61.16, 63.27, 71.10, 92.90, 109.64, 135.01, 150.32,
163.91, 165.97. IR (neat) 3440, 3210, 3090, 2987, 2937,
2850, 1763, 1700, 1655, 1460, 1299, 1191, 1020, 865,
1010, 850, 750 cm21
.
C28H36N4O5Si: 536.2455. Found: 536.2458.
Exact mass calculated for
4.4.3. Reaction of 14a with NaBH4. (30RS,50SR)-1-[30-
({[tert-Butyl(diphenyl)silyl]oxy}methyl)-50-hydroxymethyl-
20-methyl-10,20-isoxazolidin-50-yl]adenine 20a (65%), sticky
800 cm21
313.1273. Found: 313.1271.
.
Exact mass calculated for C13H19N3O6:

D. Iannazzo et al. / Tetrahedron 58 (2002) 581±587
587
0
oil; dH (500 MHz, CDCl3) 1.03 (s, 9H), 2.68 (dd, 1H, H4 a,
Chim. Ther. 1992, 27, 555±560. (b) Tronchet, J. M. J.;
Iznaden, M.; Barbalat-Rey, F.; Komaromi, I.; Dolatshahi,
N.; Bernardinelli, G. Nucleosides Nucleotides 1995, 14,
1737±1758.
0
Jˆ8.1, 12.3 Hz), 2.96 (s, 3H, N-CH3), 3.02 (m, 2H, H4 b and
0
00
00
H3 ), 3.62 (m, 2H, H3 ), 4.10 (d, 1H, H5 a, Jˆ12.1 Hz), 4.19
00
(d, 1H, H5 b, Jˆ12.1 Hz), 5.12 (bs, 1H, OH), 6.06 (bs, 2H,
4. (a) Chiacchio, U.; Gumina, G.; Resci®na, A.; Romeo, R.;
Uccella, N.; Piperno, A.; Romeo, G. Tetrahedron 1996, 52,
8889±8898. (b) Chiacchio, U.; Corsaro, A.; Gumina, G.;
Iannazzo, D.; Resci®na, A.; Piperno, A.; Romeo, G.;
Romeo, R. J. Org. Chem. 1999, 64, 9321±9327. (c) Chiacchio,
U.; Corsaro, A.; Resci®na, A.; Romeo, G. Tetrahedron:
Asymmetry 2000, 11, 2045±2048. (d) Gi, H. J.; Xiang, Y.;
Schinazi, R. F.; Zhao, K. J. Org. Chem. 1997, 62, 88±92.
(e) Tronchet, J. M. J.; Zsely, M.; Brenas, L.; Lassout, O.;
Grand, E.; Seuret, P.; Grigorov, M.; Rivara-Minten, E.;
Geoffroy, M. Nucleosides Nucleotides 1999, 18, 1077±1078.
(f) Leggio, A.; Liguori, A.; Procopio, A.; Siciliano, C.;
Sindona, G. Tetrahedron Lett. 1996, 37, 1277±1280.
(g) Leggio, A.; Liguori, A.; Procopio, A.; Sindona, G.
Nucleosides Nucleotides 1997, 16, 1515±1518.
NH2), 6.60 (d, 4H, Jˆ7.5 Hz), 7.25±7.35 (m, 6H), 8.16 (s,
1H, H9), 8.23 (s, 1H, H3); dC (125 MHz, CDCl3), 18.23,
26.60, 42.27, 45.16, 63.63, 65.60, 70.17, 95.37, 120.10,
127.63, 129.48, 129.79, 129.83, 132.70, 132.79, 134.82,
135.42, 135.49, 148.35, 152.0, 155.24. IR (neat) 3420±
3340, 3220, 3060, 3030, 2960, 2925, 1705, 1645, 1600,
1480, 1105, 1050, 850, 790 cm21. Exact mass calculated
for C27H34N6O3Si: 518.2461. Found: 518.2459.
Acknowledgements
This work was partially supported by M.U.R.S.T and Italian
C.N.R.
5. Chiacchio, U.; Corsaro, A.; Iannazzo, D.; Resci®na, A.;
Piperno, A.; Romeo, R.; Romeo, G. Tetrahedron Lett. 2001,
1777±1780.
References
1. (a) Kuritzkes, D. HIV Clinical Management, Vol. 13;
Medscape Inc, Hillsboro, 1999. (b) Schinazi, R. F.; Mead,
J. R.; Feorino, P. M. AIDS RES. Hum. Retroviruses 1992, 8,
963±970. (c) Bonnet, P.; Robins, R. K. J. Med. Chem. 1995,
36, 635±641. (d) Chiacchio, U.; Iannazzo, D.; Resci®na, A.;
Romeo, G. J. Org. Chem. 1999, 64, 28±36. (e) Caputo, R.;
Guaragna, A.; Palumbo, G.; Pedatella, S. Eur. J. Org. Chem.
1999, 1455±1458.
Á
6. Chiacchio, U.; Corsaro, A.; Pistara, V.; Resci®na, A.;
Iannazzo, D.; Piperno, A.; Romeo, G.; Romeo, R.; Grassi,
G. Eur. J. Org. Chem., submitted for publication.
Â
7. Herrera, R.; Nagarajan, A.; Morales, M. A.; Mendez, F.;
 Â
Jimenez-Vazquez, H. A. L.; Zepeda, G.; Tamariz, J. J. Org.
Chem. 2001, 66, 1252±1263.
8. (a) DeShong, P.; Dicken, C. M.; Staib, R. R.; Freyer, A. J.;
Weireb, S. M. J. Org. Chem. 1982, 47, 4397±4403.
(b) Kametani, T.; Chu, S.-D.; Honda, T. J. Chem. Soc., Perkin
Trans. 1 1988, 1593±1598.
2. Tan, X.; Boudinot, F. D.; Chu, C. K.; Egron, D.; Perigaud, C.;
Gosselin, G.; Imbach, G. L. Antiviral Chem. Chemoth. 2000,
11, 203±212.
9. Wilson, L. J.; Hager, M. W.; El Katten, Y. A.; Liotta, D. C.
Synthesis 1995, 1465±1479.
10. Jenn, T.; Heissler, D. Tetrahedron 1998, 54, 107±118.
3. (a) Tronchet, J. M. J.; Iznaden, M.; Barlat-Rey, F.; Dhimane,
H.; Ricca, A.; Balzarini, J.; De Clercq, E. Eur. J. Med. Chem.