A novel class of modified nucleosides: Synthesis of alkylidene isoxazolidinyl nucleosides containing thymine

Article abstract of DOI:10.1002/ejoc.200600817

The synthesis of hitherto unknown 1-[3-(hydroxymethyl)-2-methyl-4- methyleneisoxazolidin-5-yl]thymine (5), 1-{(Z)-[3-(hydroxymethyl)-2- methylisoxazolidin-4-ylidene]methyl}-thymine (6), and 1-{(E)-[3-(hydroxymethyl)- 2-methylisoxazohdin-5-ylidene]methyl}thymine (7) is described. The first compound can be regarded as a N,O-analogue of Entecavir and DMDC, which contains thymine as a nucleobase. The key allenic nucleobase was prepared in good yields starting from 2,3-dibromopropene. The subsequent 1,3-dipolar cyclo-addition proceeded with good site selectivity to give 5 and regioisomeric nucleosides 6 and 7. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600817

FULL PAPER
DOI: 10.1002/ejoc.200600817
A Novel Class of Modified Nucleosides: Synthesis of Alkylidene Isoxazolidinyl
Nucleosides Containing Thymine
Anna Piperno,*[a] Antonio Rescifina,[b] Antonino Corsaro,[b] Maria A. Chiacchio,[b]
Antonio Procopio,[c] and Roberto Romeo*[a]
Keywords: Dipolar cycloaddition / Nucleosides / Nitrogen heterocycles / Oxygen heterocycles
The synthesis of hitherto unknown 1-[3-(hydroxymethyl)-2-
methyl-4-methyleneisoxazolidin-5-yl]thymine (5), 1-{(Z)-[3-
(hydroxymethyl)-2-methylisoxazolidin-4-ylidene]methyl}-
thymine (6), and 1-{(E)-[3-(hydroxymethyl)-2-methylisoxaz-
olidin-5-ylidene]methyl}thymine (7) is described. The first
compound can be regarded as a N,O-analogue of Entecavir
and DMDC, which contains thymine as a nucleobase. The
key allenic nucleobase was prepared in good yields starting
from 2,3-dibromopropene. The subsequent 1,3-dipolar cyclo-
addition proceeded with good site selectivity to give 5 and
regioisomeric nucleosides 6 and 7.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
structure–activity relationship (SAR) studies led chemists
to design a large variety of modifications of nucleosides that
either affect only their base or sugar part or both of these
moieties together.[7]
Introduction
Drug discovery for antiviral chemotherapy during the
last 30 years has provided effective treatments for numerous
viral diseases. In particular, the fight against AIDS has
prompted the development of new classes of antiviral drugs,
which have provided considerable decrease in HIV-associ-
ated mortality as well as improvement in the quality of life
of AIDS patients.
Three categories of antiretroviral agents in clinical use
are nucleoside reverse transcriptase inhibitors (NRTI),[1]
such as zidovudine and stavudine (d4T), protease inhibi-
tors,[2] and nonnucleoside reverse transcriptase inhibitors
(NNRTI).[3] Although new, promising targets are being
identified, nucleoside analogues remain the cornerstone of
antiviral therapy.[4] Currently, seven of the sixteen drugs
available for the treatment of AIDS (AZT, ddC, ddI, d4T,
3TC, abacavir, and tenofovir)[5] belong to the NRTI cat-
egory. Among NRTIs for AIDS, 3TC is also the drug of
choice for the treatment of hepatitis B virus infection.[6]
For many years, the design of modified nucleosides has
been a focal point of research in medicinal chemistry. Now-
adays, among a plethora of synthetic analogues, a number
of compounds have emerged that exhibit a broad spectrum
of biological activity as the result of their interaction with
various pathogen-specific enzyme systems. In this regard,
Heterocyclic nucleosides[8] have gained considerable at-
tention during the last years. In this context, our research
group has disclosed a new series of extremely modified nu-
cleosides featuring, as a spacer unit, an isoxazolidine system
instead of the sugar moiety. These new N,O-nucleoside ana-
logues have shown interesting physiological activities. For
instance, (–)-ADFU (1) is characterized by low cytotoxicity
and, noteworthy, has been shown to specifically induce re-
markable levels of apoptosis on lymphoid and monocytoid
cells. Phosphonated N,O-nucleosides 2 are proven to be po-
tential antiviral agents (Figure 1).[9] Recently, we have re-
ported the synthesis of azanucleosides; some of which are
good anti-HCV inhibitors.
[a] Dipartimento Farmaco-Chimico, Università di Messina,
Via SS. Annunziata, Messina 98168, Italy
Fax: +39-090-356230
E-mail: apiperno@pharma.unime.it
[b] Dipartimento di Scienze Chimiche, Università di Catania,
Viale Andrea Doria 6, Catania 95125, Italy
[c] Dipartimento di Scienze Farmaco-Biologiche, Università della
Magna Graecia,
Complesso Ninì Barbieri, 88021 Roccelletta di Borgia (Cz),
Italy
Figure 1. Nucleoside analogues.
Eur. J. Org. Chem. 2007, 1517–1521
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A. Piperno et al.
FULL PAPER
As part of our drug discovery program, and following performed under microwave irradiation: the reaction time
our interest in isoxazolidinyl nucleosides, we report in this was reduced to 4 h and the yields increased to 55%.
article the synthesis of 1-[3-(hydroxymethyl)-2-methyl-4-
The structure of the obtained adducts was assigned on
methyleneisoxazolidin-5-yl]thymine (5) (Scheme 1), the first the basis of spectroscopic measurements. Thus, the 1H
member of the new series of alkylidene isoxazolidinyl nucleo- NMR spectrum of compound 10 shows the diagnostic reso-
sides. Compound 5 can be considered as a thymine-con- nance of the exocyclic methylene protons at C4 as two dou-
taining isoxazolidine analogue of Entecavir (BMS-200475) blets centered at δ = 5.39 and 5.21 ppm (J = 1.2 Hz); the
(3),[4] which shows potent and selective activity against proton at C5 resonates as a singlet at δ = 5.55 ppm, whereas
HBV, and of DMDC (4),[10] which is a synthetic analogue H3 gives rise to a singlet at δ = 4.43 ppm.
of deoxycitidine that potently inhibits the growth of various
human tumor cells both in vitro and in vivo.
For compound 11, the diagnostic resonance of H6 is at δ
= 6.69 ppm, whereas H3 gives rise to a singlet at δ =
4.85 ppm and the methylene protons at C5 resonate as two
doublets at δ = 4.75 and 4.80 ppm (J = 4.8 Hz). Compound
12 shows the resonance of two methylene protons at C4 as
two distinct doublets of doublets at δ = 2.85 and 3.62 ppm,
whereas H3 resonates as a doublet of doublets at δ =
3.45 ppm.
The assigned stereochemistry of the obtained adducts is
supported by NOE measurements. In compound 10, the
lack of NOE for H3 when H5 is irradiated is indicative of a
trans relationship between these protons.
For compound 11, the (Z)-configuration of the double
bond was confirmed on the basis of the NOE observed for
H3 when H6 is irradiated. In 12 on the contrary, no NOE
is observed for H6 when the methylene protons at C4 are
irradiated.
The cycloaddition process shows good site selectivity: at-
tack of the dipole to the unsubstituted double bond of 9
is the preferential reaction course, which gives rise to the
formation of 11 and 12 (the ratio of 11/12 versus 10 is 3:1):
furthermore, good regioselectivity is also observed with a
7:1 ratio between 11 and 12.
The cycloaddition process was also rationalized by an in
silico study with PM3 semiempirical level calculations. To
perform this study, we have taken into consideration the
eight possible transition states arising from the addition of
the nitrone in both the (E)- and (Z)-configurations to each
of the two double bonds (DB1 and DB2) of allene 9 in
either an endo or exo fashion, respectively (Scheme 2). For
TS1–4, the endo and exo approaches are in accord to the
classical nomenclature for Diels–Alder reactions,[11]
whereas for TS5–8, the terminology refers to the thymine
moiety that is oriented forward (towards) or backward
(away), respectively, from the nitrone functionality along
Scheme 1. a) Silylated thymine, TMSTf, CH3CN; b) NaOH, diox-
ane/H2O; c) NaBH4, dioxane, H2O.
The designed synthetic methodology allows the simulta-
neous preparation of two other isomeric analogues 6 and 7
(Scheme 1) with extensively modified carbohydrate units.
Results and Discussion
The synthetic strategy of the present study, which is the approach path.
based on 1,3-dipolar cycloaddition methodology, is de-
The calculated enthalpy for the formation of all TSs to-
picted in Scheme 1. The key step of the process is the syn- gether with the product percentages that were calculated in
thesis of the hitherto unknown 1-(propa-1,2-dienyl)thymine accordance with the Boltzmann distribution equation are
(9), which was prepared from commercial 2,3-dibromopro- reported in Table 1. These data point out a 10/11/12 ratio
pene by reaction with thymine in the presence of bis(tri- of 2.9:7.9:1 that is in good agreement with the experimental
methylsilyl)acetamide. Obtained 1-(2-propenyl)thymine 8 one. Moreover, the calculation results showed that com-
(95% yield) was then treated with NaOH in dioxane/water pounds 10–12 originate from an (E)-exo TS approach.
1:1 and heated at reflux for 1 h to give target compound 9
The synthetic scheme was completed by reduction of the
in good yield (85%). Subsequent 1,3-dipolar cycloaddition ethoxycarbonyl group: treatment with NaBH4 afforded nu-
with C-ethoxycarbonyl-N-methylnitrone heated at reflux in cleosides 5, 6, and 7 in 92% yield.
THF for 4 d afforded a mixture of compounds 10, 11, and
We have devised a slightly different reaction route which
12 (global yield 30%) in a 2.6:7:1 ratio. Better results were allows easy access to minor compound 7, as the exclusive
obtained when the 1,3-dipolar cycloaddition process was adduct. In this way, the cycloaddition reaction of C-ethoxy-
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 1517–1521

Synthesis of Alkylidene Isoxazolidinyl Nucleosides Containing Thymine
FULL PAPER
Scheme 2. Combinations of all possible transition states involved in cycloaddition.
Table 1. PM3 formation enthalpies for transition states 1–6a,b.
Transition state
∆Hf
% Calculated Product (Compound)
TS1a
TS1b
TS2a
TS2b
TS3a
TS3b
TS4a
TS4b
TS5a
TS5b
TS6a
TS6b
TS7a
TS7b
TS8a
TS8b
–64.62
–63.29
–66.73
–61.50
–63.23
–59.42
–64.36
–60.17
–66.08
–63.78
–63.46
–62.35
–67.31
–65.19
–59.91
–61.39
0.70
0.08
24.26
0.00
0.07
0.00
0.46
0.00
8.15
0.17
0.10
0.02
64.16
1.83
0.00
0.00
P1
P2 (10)
P3
P4
P5 (12)
P6
P7 (11)
P8
Scheme 3. a) THF, microwave at 300 W, 80 °C, 3 h; b) NaBH4,
MeOH; c) Et3N.
carbonyl-N-methylnitrone with 1-(2-propenyl)thymine 8 led
to the formation of derivative 13 as the only obtained ad-
duct, in racemic form, which was subsequently reduced to
nucleoside 7 (global yield 52%) (Scheme 3).
Experimental Section
General Section: Melting points were determined with a Kofler ap-
paratus and are uncorrected. Elemental analyses were performed
with a Perkin–Elmer elemental analyzer. NMR spectra were re-
corded with a Varian instrument at 300 or 500 MHz (1H NMR)
and at 75 or 125 MHz (13C NMR) with the use of deuteriochloro-
form or deuterated methanol as the solvent; chemical shifts are
given in ppm from TMS as the internal standard. Thin-layer chro-
matographic separations were performed on Merck silica gel 60-
F254 precoated aluminium plates. Preparative separations were per-
formed by flash chromatography by using Merck silica gel 0.035–
0.070 mm, or by centrifugally accelerated radial thin-layer
chromatography (PCAR-TLC) with a Chromatotron Model 7924
T (Harrison Research, Palo Alto, CA, USA); the rotors (2 or 4 mm
layer thickness) were coated with silica gel Merck grade type 7749,
Conclusions
On the basis of the consideration that introduction of a
rigid structural element into the nucleoside structure can
lead to effective antiviral agents,[12] the synthesis of alkylid-
ene isoxazolidinyl nucleosides, hitherto not described in lit-
erature, has been designed.
Additional in vitro tests as well as studies of structure–
activity relationships are in progress and will be reported at
a later date.
Eur. J. Org. Chem. 2007, 1517–1521
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A. Piperno et al.
FULL PAPER
TLC grade, with binder and fluorescence indicator (Aldrich
34,644–6), and the eluting solvents were delivered by a pump at a
flow-rate of 1.5–3.5 mLmin–1.
C13H17N3O5 295.1168; found 295.1173. Second eluted compound
was ethyl (3RS,4Z)-2-methyl-4-{[5-methyl-2,4-dioxo-3,4-dihy-
dropyrimidin-1(2H)-yl]methylene}isoxazolidine-3-carboxylate (11).
Yield: 150.16 mg, (36.32 %), white foam. 1H NMR (300 MHz,
The identification of samples from different experiments was se-
cured by mixed mps and superimposable NMR spectra.
3
3
CDCl3, 27 °C): δ = 1.30 (t, J = 7.2 Hz, 3 H), 1.92 (d, J = 0.9 Hz,
3
3
3 H,), 3.21 (s, 3 H, N–CH3), 4.24 (q, J = 7.2 Hz, 2 H), 4.75 (d, J
Preparation of Allylthymine 8: A mixture of thymine (0.05 mol,
6.3 g) and N,O-bis(trimethylsilyl)acetamide (0.1 mol, 24 mL) in an-
hydrous acetonitrile (30 mL) was stirred at room temperature until
the solution clarified. 2,3-Dibromoprop-1-ene (0.55 mol, 56 mL)
and TMSTf (0.005 mol, 0.98 mL) were then added, and the solu-
tion was heated at 80 °C for 12 h. The solvent was removed, and
the residue was washed with CHCl3. The organic layers were dried
with magnesium sulfate and evaporated under reduced pressure to
afford the crude product which was purified by flash chromatog-
raphy (CHCl3/MeOH 99:1). Yield: 1.63 g (95%), white foam. 1H
NMR (300 MHz, CDCl3, 27 °C): δ = 1.32 (d, 3J = 1.2 Hz, 3 H),
3
= 4.8 Hz, 1 H, 5a-H), 4.80 (d, J = 4.8 Hz, 1 H, 5b-H), 4.85 (s, 1
3
H, 3-H), 6.69 (s, 1 H, 6-H), 7.05 (q, J = 0.9 Hz, 1 H), 8.05 (br. s,
1 H, N–H) ppm. 13C NMR (75 MHz, CDCl3, 27 °C): δ = 14.2,
15.8, 39.2, 62.2, 70.0, 79.7, 111.3, 117.7, 118.0, 138.4, 146.9, 164.0,
173.1 ppm. C13H17N3O5 (295.30): calcd. C 52.88, H 5.80, N 14.23;
found C 52.83, H 5.81, N 14.27. HRMS: calcd. for C13H17N3O5
295.1168; found 295.1164. Third eluted compound was ethyl
(3RS,5E)-2-methyl-5-{[5-methyl-2,4-dioxo-3,4-dihydropyrimidin-
1(2H)-yl]methylene}isoxazolidine-3-carboxylate (12). Yield:
21.45 mg (5.19 %), white foam. 1H NMR (300 MHz, CDCl3,
3
3
27 °C): δ = 1.32 (t, J = 7.2 Hz, 3 H), 1.92 (d, J = 0.9 Hz, 3 H),
3
3
4.79 (s, 2 H), 5.71(d, J = 1.5 Hz, 1 H) 5.90 (d, J = 1.5 Hz, 1 H),
3
3
2.85 (dd, J = 7.2 Hz, J = 16.5 Hz, 1 H, 4a-H), 3.23 (s, 3 H, N–
CH3), 3.45 (dd, J = 7.2 Hz, J = 7.5 Hz, 1 H, 3-H), 3.62 (dd, J
7.05 (q, J = 1.2 Hz, 1 H), 9.95 (br. s, 1 H, N–H) ppm. 13C NMR
3
3
3
3
(75 MHz, CDCl3, 27 °C): δ = 16.0, 54.7, 110.0, 119.5, 120.0, 138.0,
150.8, 163.6 ppm. C8H9BrN2O2 (245.08): calcd. C 39.21, H 3.70,
N 11.43; found C 39.10, H 3.68, N 11.46. HRMS: calcd. for
C8H9BrN2O2 243.9847; found 243.9850.
3
3
= 7.5 Hz, J = 16.5 Hz,1 H, 4b-H), 4.24 (q, J = 7.2 Hz, 2 H), 6.65
(s, 1 H, 6-H), 7.12 (q, 3J = 0.9 Hz, 1 H), 8.10 (br. s, 1 H, N–H)
ppm. 13C NMR (75 MHz, CDCl3, 27 °C): δ = 14.1, 15.7, 35.2, 43.2,
61.2, 72.0, 90.7, 110.3, 133.5, 146.9, 160.8, 162.7, 161.7, 173.1 ppm.
C13H17N3O5 (295.30): calcd. C 52.88, H 5.80, N 14.23; found C
52.75, H 5.78, N 14.20. HRMS: calcd. for C13H17N3O5 295.1168;
found: 295.1167.
Preparation of Allene 9: A solution of 8 (0.01 mL, 2.45 g) in diox-
ane (50 mL) was treated with 1  NaOH (50 mL); and the reaction
mixture was heated at reflux for 1 h. After cooling to room tem-
perature, the mixture was evaporated under reduced pressure, and
the crude product was purified by PCAR-TLC (hexane/ethyl ace-
tate/Et3N 5:4.1:0.1; 1.5 mL/min). Yield: 1.55 g (85%), white foam.
1H NMR (300 MHz, CDCl3, 27 °C): δ = 1.95 (d, 3J = 0.9 Hz, 3
Reaction of C-Ethoxycarbonyl-N-methylnitrone with Allylthymine 8:
The first eluted product was ethyl (3SR,5RS)-5-bromo-2-methyl-5-
{[5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl]methyl}-
isoxazolidine-3-carboxylate (13b). Yield: 95.93 mg (17%), brown
foam. 1H NMR (500 MHz, CDCl3, 27 °C): δ = 1.29 (t, 3J = 7.2 Hz,
3
3
H), 5.56 (d, J = 6.6 Hz, 2 H), 7.02 (q, J = 0.9 Hz, 1 H), 7.23 (t,
3J = 6.6 Hz, 1 H), 8.95 (br. s, 1 H, N–H) ppm. 13C NMR (75 MHz,
CDCl3, 27 °C): δ = 12.4, 90.2, 96.8, 112.0, 135.6, 138.2, 149.5,
163.7 ppm. C8H8N2O2 (164.16): calcd. C 58.53, H 4.91, N 17.06;
found C 58.42, H 4.89, N 17.10. HRMS: calcd. for C8H8N2O2
164.0586; found 164.0589.
3
3 H), 1.92 (d, J = 0.9 Hz, 3 H), 2.89 (s, 3 H, N–CH3), 3.71 (m, 1
3
H, 4a-H), 4.32 (m, 1 H, 4b-H), 4.33 (q, J = 7.2 Hz, 2 H), 4.78 (d,
3J = 18.5 Hz, 1 H, 6a-H), 4.91 (m, 1 H, 3-H), 5.16 (d, 3J = 18.5 Hz,
1 H, 6b-H), 6.88 (q, 3J = 0.9 Hz, 1 H), 8.80 (br. s, 1 H, N–H) ppm.
13C NMR (125 MHz, CDCl3, 27 °C): δ = 12.7, 14.1, 44.9, 53.0,
54.1, 62.3, 71.4, 82.9, 110.8, 140.6, 150.7, 158.7, 168.1 ppm.
C13H18BrN3O5 (376.21): calcd. C 41.50, H 4.82, N 11.17; found C
41.38, H 4.85, N 11.20. HRMS: calcd. for C13H18BrN3O5 375.0430;
found 375.0433. Second eluted compound was ethyl (3SR,5SR)-5-
bromo-2-methyl-5-{[5-methyl-2,4-dioxo-3,4-dihydropyrimidin-
1(2H)-yl]methyl}isoxazolidine-3-carboxylate (13a). Yield: 73.36 mg
Preparation of Compounds 10–13. Method A: Compound
9
(2.5 mmol, 410 mg) was added to a solution of C-ethoxycarbonyl-
N-methylnitrone (5.1 mmol, 668 mg) in dry THF (20 mL), and the
solution was heated at reflux for 4 d. The reaction mixture was
then evaporated under reduced pressure and purified by PCAR-
TLC (cyclohexane/ethyl acetate 4:1, 3.5 mL/min) to afford com-
pounds 10, 11, and 12, with a global yield of 30% and a 2.6:7:1
ratio. Method B: A solution of C-ethoxycarbonyl-N-methylnitrone
(2.6 mmol, 334 mg) and 9 (1.4 mmol, 205 mg) or 8 (1.5 mmol,
395 mg) in THF (8 mL) was irradiated under microwave conditions
at 100 W and 80 °C for 4 h. The reaction mixture was evaporated
under reduced pressure and purified as in Method A to give com-
pounds 10, 11, and 12, with a global yield of 55% (2.6:7:1 ratio)
and compounds 13a,b with a global yield of 30% (1.3:1 ratio),
respectively.
1
(13%), brown foam. H NMR (500 MHz, CDCl3, 27 °C): δ = 1.27
3
3
(t, J = 7.2 Hz, 3 H), 1.90 (d, J = 0.9 Hz, 3 H), 2.88 (s, 3 H, N–
CH3), 3.65 (m, 1 H, 4a-H), 4.01 (m, 1 H, 4b-H), 4.31 (q, 3J =
7.2 Hz, 2 H), 4.83 (d, J = 18.1 Hz, 1 H, 6a-H), 4.88 (m, 1 H, 3-
3
3
3
H), 5.22 (d, J = 18.1 Hz, 1 H, 6b-H), 6.81 (q, J = 0.9 Hz, 1 H),
8.92 (br. s, 1 H, N–H) ppm. 13C NMR (75 MHz, CDCl3, 27 °C): δ
= 13.3, 14.7, 45.1, 53.3, 53.9, 64.7, 73.6, 84.5, 112.3, 141.5, 152.4,
159.4, 167.9 ppm. C13H18BrN3O5 (376.21): calcd. C 41.50, H 4.82,
N 11.17; found C 41.62, H 4.83, N 11.14. HRMS: calcd. for
C13H18BrN3O5 375.0430; found 375.0425.
Reaction of C-Ethoxycarbonyl-N-methylnitrone with Allene 9: The
first eluted product was ethyl (3RS,5RS)-2-methyl-5-[5-methyl-2,4-
dioxo-3,4-dihydropyrimidin-1(2H)-yl]-4-methyleneisoxazolidine-3-
carboxylate (10). Yield: 55.77 mg, (13.49%), white foam. 1H NMR
Preparation of Nucleosides 5–7. General Procedure: NaBH4
(0.1 mmol, 3.8 mg) was added to a solution of isoxazolidine 10–12
(0.1 mmol) in dioxane/H2O (5 mL), and the mixture was stirred for
2 h. The solvent was then removed, and the residue was subjected
to column chromatography (chloroform/methanol 95:5) to afford
nucleosides 5–7, respectively, in 92% yield.
3
(300 MHz, CDCl3, 27 °C): δ = 1.35 (t, J = 7.2 Hz, 3 H), 1.90 (d,
3J = 0.9 Hz, 3 H), 3.25 (s, 3 H, N–CH3), 4.24 (q, 3J = 7.2 Hz, 2
H), 4.43 (s, 1 H, 3-H), 5.21 (d, 3J = 1.2 Hz, 1 H), 5.39 (d, 3J =
1.2 Hz, 1 H), 5.55 (s, 1 H, 5-H), 7.05 (q, 3J = 0.9 Hz, 1 H), 8.05
(br. s, 1 H, N–H) ppm. 13C NMR (75 MHz, CDCl3, 27 °C): δ =
14.1, 15.5, 43.2, 62.2, 75.0, 93.7, 111.0, 111.5, 137.4, 148.5, 150.9,
162.9, 169.0 ppm. C13H17N3O5 (295.30): calcd. C 52.88, H 5.80, N
14.23; found C 52.95, H 5.83, N 14.19. HRMS: calcd. for
In the case of isoxazolidines 13, the crude mixture after the re-
duction reaction was dissolved in THF (10 mL) and Et3N
(0.1 mmol, 0.014 mL) and heated at reflux for 2 h. The solvent was
then removed and the residue was purified as reported above to
give 7 in 52% yield.
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Synthesis of Alkylidene Isoxazolidinyl Nucleosides Containing Thymine
FULL PAPER
Reaction of 10 with NaBH4: 1-[(3RS,5RS)-3-(Hydroxymethyl)-2-
NMR (75 MHz, CDCl3, 27 °C): δ = 15.6, 43.8, 47.1, 60.6, 69.7,
90.9, 110.9, 134.9, 146.9, 160.6, 163.9 ppm. C11H15N3O4 (253.26):
calcd. C 52.17, H 5.97, N 16.59; found C 51.98, H 5.98, N 16.57.
methyl-4-methyleneisoxazolidin-5-yl]-5-methylpyrimidine-2,4-
(1H,3H)dione (5). Yield: 23.52 mg (92%), white foam. 1H NMR
3
(300 MHz, CDCl3, 27 °C): δ = 1.90 (d, J = 0.9 Hz, 3 H), 2.95 (s, HRMS: calcd. for C11H15N3O4 253.1063; found 253.1066.
3 H, N–CH3), 3.93 (t, 3J = 5.9 Hz, 1 H, 3-H), 3.95 (dd, 3J = 5.9 Hz,
3
3
3J = 10.8 Hz, 1 H), 4.04 (dd, J = 5.9 Hz, J = 10.8 Hz, 1 H), 5.15
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Chem. 2006, 14, 955–959.
3
3
(d, J = 1.2 Hz, 1 H), 5.30 (d, J = 1.2 Hz, 1 H), 5.95 (s, 1 H, 5-
H), 7.05 (q, 3J = 0.9 Hz, 1 H), 8.05 (br. s, 1 H, N–H) ppm. 13C
NMR (75 MHz, CDCl3, 27 °C): δ = 15.1, 43.1, 57.5, 72.2, 95.7,
110.0, 110.5, 137.4, 148.5, 150.9, 163.8 ppm. C11H15N3O4 (253.26):
calcd. C 52.17, H 5.97, N 16.59; found C 52.05, H 5.95, N 16.62.
HRMS: calcd. for C11H15N3O4 253.1063; found 253.1065.
Reaction of 11 with NaBH4: 1-{(Z)-[(3RS)-3-(Hydroxymethyl)-2-
methylisoxazolidin-4-ylidene]methyl}-5-methylpyrimidine-2,4-
(1H,3H)dione (6). Yield: 23.40 mg (92%), white foam. 1H NMR
3
(300 MHz, CDCl3, 27 °C): δ = 1.95 (d, J = 0.9 Hz, 3 H), 2.98 (s,
3 H, N–CH3), 3.24 (t, 3J = 4.8 Hz, 1 H, 3-H), 3.92 (dd, 3J = 4.8 Hz,
3
3
3J = 10.7 Hz, 1 H), 3.98 (dd, J = 4.8 Hz, J = 10.7 Hz, 1 H), 4.25
3
3
(d, J = 6.3 Hz, 1 H, 5a-H), 4.80 (d, J = 6.3 Hz, 1 H, 5b-H), 7.09
(s, 1 H), 7.55 (q, 3J = 0.9 Hz, 1 H), 9.05 (br. s, 1 H, N–H) ppm.
13C NMR (75 MHz, CDCl3, 27 °C): δ = 15.6, 43.2, 57.8, 70.6, 74.7,
110.9, 117.3, 117.7, 119.0, 134.9, 146.9, 164.0 ppm. C11H15N3O4
(253.26): calcd. C 52.17, H 5.97, N 16.59; found C 52.32, H 5.95,
N 16.55. HRMS: calcd. for C11H15N3O4 253.1063; found 253.1060.
Reaction of 12 with NaBH4: 1-{(E)-[(3RS)-3-(Hydroxymethyl)-2-
methylisoxazolidin-5-ylidene]methyl}-5-methylpyrimidine-2,4-
(1H,3H)dione (7). Yield: 23.32 mg (92%), white foam. 1H NMR
(300 MHz, CDCl3, 27 °C): δ = 1.93 (d, 3J = 0.9 Hz, 3 H), 2.05 (dd,
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202; b) X. Chen, E. R. Kern, J. C. Drach, E. Gullen, Y.-C.
Cheng, J. Zemlicka, J. Med. Chem. 2003, 46, 1531–1537.
Received: September 19, 2006
3
3
3
3J = 3.8 Hz, J = 9.5 Hz, 1 H, 4a-H), 2.30 (dd, J = 4.8 Hz, J =
3
9.5 Hz, 1 H, 4b-H), 2.68 (s, 3 H, N–CH3), 2.98 (ddd, J = 3.8 Hz,
3J = 4.8 Hz, 3J = 6.5 Hz, 1 H, 3-H), 3.65 (dd, 3J = 6.5 Hz, 3J =
3
3
10.4 Hz, 1 H), 3.87 (dd, J = 6.5 Hz, J = 10.4 Hz, 1 H), 7.09 (s, 1
H), 7.59 (q, 3J = 0.9 Hz, 1 H), 9.95 (br. s, 1 H, N–H) ppm. 13C
Published Online: February 2, 2007
Eur. J. Org. Chem. 2007, 1517–1521
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1521