Stereoselective synthesis of L-isoxazolidinyl thymidine from N-benzyl-1,2-di-O-isopropylidene-D-glyceraldehyde nitrone (BIGN)

Article abstract of DOI:10.1016/S0957-4166(00)00094-X

A synthetic approach to L-isoxazolidinyl nucleosides is demonstrated by the stereoselective conversion of N-benzyl-1,2-di-O-isopropylidene-D-glyceraldehyde nitrone (BIGN) into cis and trans L-isoxazolidinyl thymidine. The methodology consists of the 1,3-dipolar cycloaddition of BIGN with either vinyl acetate or a vinyl base to give key intermediates that are easily transformed into the target compound. The experimental results of the cycloaddition reactions can be qualitatively explained by theoretical ab initio calculations. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00094-X

Tetrahedron: Asymmetry 11 (2000) 1543±1554
Stereoselective synthesis of l-isoxazolidinyl thymidine
from N-benzyl-1,2-di-O-isopropylidene-d-glyceraldehyde
nitrone (BIGN)
Pedro Merino,* Eva M. del Alamo, Santiago Franco, Francisco L. Merchan,
Ana Simon and Tomas Tejero
Â
Departamento de Quõmica Organica, ICMA, Facultad de Ciencias, Universidad de Zaragoza, E-50009 Aragon, Spain
Â
Received 1 February 2000; accepted 9 March 2000
Abstract
A synthetic approach to l-isoxazolidinyl nucleosides is demonstrated by the stereoselective conversion of
N-benzyl-1,2-di-O-isopropylidene-d-glyceraldehyde nitrone (BIGN) into cis and trans l-isoxazolidinyl
thymidine. The methodology consists of the 1,3-dipolar cycloaddition of BIGN with either vinyl acetate or
a vinyl base to give key intermediates that are easily transformed into the target compound. The experi-
mental results of the cycloaddition reactions can be qualitatively explained by theoretical ab initio calcu-
lations. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
Nucleoside analogues in which the furanose ring has been replaced by a di€erent carbo- or
heterocyclic ring have attracted special interest by virtue of their biological action mainly as
antiviral agents.1 In particular, isoxazolidinyl nucleosides 1 are emerging as an important class of
compounds which are interesting to synthesize in order to investigate their pharmacological
activities.2
* Corresponding author. Fax: +34 976 762075; e-mail: pmerino@posta.unizar.es
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00094-X

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In connection with our synthetic studies on the utility of chiral nitrones for the synthesis of
biologically interesting nitrogenated compounds,3 we have recently devised a new synthetic route
to isoxazolidinyl nucleosides of type 1 via nucleophilic addition of enolates to a-alkoxy-4 and a-
aminonitrones.5 By this route simple analogues 1a and more complex products such as a-amino
acid isoxazolidinyl nucleosides 1b have become accessible.
Previously reported synthetic approaches to enantiomerically pure isoxazolidinyl nucleosides 1
included Michael addition of N-methyl hydroxylamine to unsaturated esters6 and lactones,7
and 1,3-dipolar cycloaddition reactions of nitrones with vinyl acetate.8 By the ®rst route Zhao
and co-workers described the synthesis of both d- and l-analogues of type 1a; by the second one,
Chiacchio and co-workers have recently reported the synthesis of compounds 1c and 1d.
Since we had achieved4 the synthesis of d-isoxazolidinyl thymidine 3 starting from BIGN 2
(Scheme 1) Ð via nucleophilic addition of sodium enolate Ð and it has also been reported9 that
1,3-dipolar cycloaddition of electron-rich alkenes to chiral a-alkoxy nitrones gave preferentially
anti adducts,10 we realized that application of Chiacchio's methodology to nitrone 2 could serve
as a strategy for the synthesis of enantiomeric11 l-isoxazolidinyl nucleosides like 4. This has prompted
us to publish our work on the stereodivergent synthesis of such compounds and this paper
describes the synthesis of l-isoxazolidinyl thymidine 4 as well as its trans-isomer. We also report
the ®rst stereoselective 1,3-dipolar cycloaddition of a vinyl base to form homochiral isoxazolidinyl
nucleosides.12
Scheme 1.
2. Results and discussion
The nitrone 2 is readily available from 2,3-O-isopropylidene-d-glyceraldehyde and it can be
prepared in multigram scale as described.13 The cycloaddition of 2 with vinyl acetate took place
without solvent to give a mixture of four stereoisomeric isoxazolidines 5±8 in an isomer ratio of
ca. 5:6:7:8=7:2:1:1 and a combined yield of 94% (Scheme 2).
Scheme 2.

P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
1545
The crude mixture was puri®ed by preparative centrifugally accelerated, radial, TLC14 and the
four cycloadducts 5±8 could be obtained in pure form with the only exception being 8, which was
contaminated by 8% of 7 judged by integration of H-5 hydrogens (isoxazolidine ring numbering)
1
in the H NMR spectrum. The cis/trans stereochemistry of the adducts was readily deduced by
means of NOE measurements (Fig. 1). Thus, for cis compounds 5 and 7 irradiation of H-4a
produced strong enhancements of both H-3 (10±12%) and H-5 (8±10.5%) along with a much
larger enhancement of H-4b (20±22%, not indicated in Fig. 1). Irradiation of H-5 produced
enhancement of H-4a (9±11%) and a much smaller enhancement of H-4b (2±2.5%, not indicated
in Fig. 1). Irradiation of H-3 only produced enhancement of H-4a (9±10%). For trans com-
pounds 6 and 8, irradiation of H-3 produced a strong enhancement of only H-4a (8±10%) and
irradiation of H-5 produced enhancement of only H-4b (8±9%). In addition, irradiation of H-4a
and H-4b in the same experiment produced enhancements of H-3 (9±10%) and H-5 (9±11%).
Figure 1. Selected NOEs observed for compounds 5±8 (Zobs given as percent of Zmax
)
The absolute con®gurations were established by comparison of adducts 7 and 8 with the same
compounds previously prepared by us through the nucleophilic addition route.4 Furthermore,
conversion of these compounds into known isoxazolidinyl thymidine 3 and transformation of
adducts 5/6 into enantiomeric 4 also support the assigned stereochemistry (see below).
A mixture of anti adducts 5/6 was transformed into isoxazolidinyl nucleoside analogues as
depicted in Scheme 3.15 The condensation of a mixture of 5 and 6 with silylated thymine,16 using
the glycosylation methodology developed by Vorbruggen,17 resulted in a nucleoside product
consisting of cis-10 and trans-11 isomers (3:1) which were isolated in 80% combined yield. The
anomers could be separated by ¯ash chromatography, the trans-isomer showing the lower Rf.
Scheme 3.
1
The anomeric con®guration of 10 and 11 was assigned by H NMR and NOE experiments in
which the irradiation of H-4a (d 2.30) in 10 increased the proton signals of H-5 (d 6.05) and H-3
by 14% and 5%, respectively, indicating that the relative con®guration of 11 must be cis. Similarly,
upon irradiation of the H-5 proton of 11 the signal for H-4b (d 2.49) was enhanced by 8%. NOE
was also observed as a 6% increase between H-3 and H-4a; however, H-4b was not a€ected.

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P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
Deprotection of the separated anomers was achieved by using catalytic p-TosOH in MeOH
(Scheme 4). The resulting 1,2-diols were treated sequentially with sodium periodate and sodium
borohydride. Puri®cation by column chromatography on silica gel led to the isolation of
isoxazolidinyl nucleosides 4 and 14 in high yields (66% and 48% overall yields from 10 and 11,
respectively). The structure of compound 4 was assigned as l-isoxazolidinyl thymidine with a
speci®c rotation of [ꢀ]D=^6.9 (c 1.10, MeOH). This assignment was based on the independent
synthesis of its antipode 3 with [ꢀ]D=+6.1 (c 0.80, MeOH), prepared from the nucleophilic
addition of methyl acetate enolates to 2.4 Similarly, d-isoxazolidinyl thymidine 3 was produced
from a mixture of minor adducts 7 and 8 as we had described previously.4 The synthesis of 3
provided additional proof for the structure of cycloadducts 7 and 8 and as a consequence for that
of major ones 5 and 6.
Scheme 4. Reaction conditions: (i) p-TosOH, MeOH, re¯ux. (ii) NaIO4, MeOH±H2O, 0ꢀC. (iii) NaBH4, MeOH, 0ꢀC
The direct 1,3-dipolar cycloaddition between vinyl thymine12d 15 and 2 was also examined
(Scheme 5). As has been illustrated by earlier reports concerning racemic compounds,12 this
approach should be a highly promising reaction for the straightforward synthesis of a variety of
isoxazolidinyl nucleosides. However, to the best of our knowledge, chiral versions of that reaction
have not yet been investigated. The reaction between 2 and 15 in re¯uxing toluene proceeded
smoothly to give an 8:2:1 mixture of three adducts 10, 11 and 16 in 88% combined yield.
Scheme 5. Reaction conditions: (i) toluene, re¯ux. (ii) p-TosOH, MeOH, 60ꢀC. (iii) NaIO4, MeOH±H2O, 0ꢀC. (iv)
NaBH4, MeOH, 0ꢀC

P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
1547
The three stereoisomers could be obtained in a pure state by means of column chromatography
and con®guration of each isomer was determined by comparison with the same compounds pre-
viously prepared from the corresponding isoxazolidines 5±8 (see above).
The obtained cycloadducts were transformed into the corresponding isoxazolidinyl nucleosides
by using the same conditions described above. Using the results of this transformation, the
overall yield of l-isoxazolidinyl thymidine 4 from BIGN 2 was 42.2% (four steps).
2.1. Theoretical calculations
The observed di€erence in selectivity in the cycloaddition reaction between BIGN 2 and vinyl
acetate is well reproduced by calculations of the transition states. We attempted an explanation of
the selectivity on the basis of both semiempirical18 and ab initio19 theoretical calculations. For the
purpose of comparison all fully optimized structures (reactants, transition states and products)
were calculated at PM3 and HF/3-21G* levels without any geometry restriction.
The more stable conformation was chosen for nitrone (after exhaustive exploration of the
potential energy surface) and alkene (s-cis conformation). The optimized geometries of the nitrone
2 and vinyl acetate showed the expected bond lengths and angles. The sum of the calculated heats
of formation of nitrone and alkene was considered to be the heat of formation of the reactants.
The TSs for the four di€erent approaches of vinyl acetate to 2 were located by the calculation
of a reaction path pro®le starting from optimized geometries of the corresponding ®nal adducts,
followed by an optimization of the TS with respect to all structural variables. The TSs were
characterized through the calculation of the force constant matrix by ensuring that they had one
and only one imaginary harmonic vibrational frequency corresponding to the formation of new
Table 1
Total electronic energies,a electronic activation energiesb and relative Gibbs energiesc of reactants, transition
states and products for cycloadditions of BIGN 2 with vinyl acetate

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P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
bonds. Both sides of the reaction path were also investigated starting from each transition state
by using the internal reaction coordinate (IRC) procedure. In all cases, it could be veri®ed that
the TSs proceed from the reactants (nitrone+alkene) and give rise to the products (isoxazolidines).
Single point calculations at HF/6-31G*//HF/3-21G and B3LYP/6-31G*//HF/3-21G levels have
been carried out in order to check the energetic results. These results are summarized in Table 1
and the TSs illustrated in Fig. 2.y The calculated bond lengths for the TSs and products are also
given in Table 2.
Figure 2. Optimized transition states (HF/3-21G) for the cycloaddition of BIGN with vinyl acetate. Forming bond
lengths are given in A. Selected hydrogens have been omitted for clarity
Whereas the most stable transition state (TS exo-Si) is predicted to be the same by ab initio
methods, irrespective of the theory level used, semiempirical methods (PM3) do not predict
correctly the observed selectivity. For this reason, PM3 values were not considered further. The
extent of bond formation in the transition states will be approximately in inverse proportion to
the lengths of the incipient bonds. As expected for 1,3-dipolar cycloadditions all TSs were asyn-
chronous, the newly created C±O bond being formed to a greater extent than the C±C bond.
y
Full thermodynamic data and optimized geometries of all stationary points are available from the authors.

P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
1549
Table 2
HF/3-21G* Calculated properties of transition structures and products
In the discussion on the selectivity derived from the relative values of the Gibbs energies of
activation (ÁGs), it is clear that only the estimation of the major adduct 5, coming from TS exo-Si,
is always correct. Experimental ®ndings on the presence (and absence in the case of cycloaddition
of 2 with 15) of minor products are only reproduced in the HF/3-21G calculations. However, the
close values (di€erences lower than 1 kcal mol^1) calculated for TS endo-Si and TS exo-Re in
both HF/6-31G* and B3LYP/6-31G* single point calculations suggest a satisfying congruence
between trends in the computed Gibbs energies and the experimental results. So, it appears that
the calculated energy di€erences by ab initio methods are qualitatively correct and account for
the stereoselectivities observed for the reaction.
3. Conclusions
In summary, two stereoselective approaches to l-isoxazolidinyl nucleosides, based on 1,3-
dipolar cycloaddition chemistry of nitrone 2, are described. The major stereoisomers of the key
cycloaddition reactions arise from an exo transition state, as is well documented for cyclo-
addition of a-alkoxy nitrones with electron-rich alkenes,9 and from reaction on the more steric-
ally accessible Si face of the nitrone. Theoretical ab initio calculations correctly predicted the
preferential formation of the major adducts since the lowest calculated TS energy is for the exo
approach of the dipolarophile to the Si face of the nitrone 2. The methodology is complementary
to the previously reported approach for the synthesis of d-isoxazolidinyl nucleosides and makes
nitrone 2 a suitable starting material for the stereodivergent synthesis of isoxazolidinyl nucleo-
sides by two routes that are potentially amenable to the preparation of multi-gram quantities.
Investigation of isoxazolidinyl nucleosides with di€erent substitutions on the 40- and 50- positions
as well as biological evaluations of the synthesized compounds are in progress and will be
reported elsewhere.

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4. Experimental
4.1. General remarks
The reaction ¯asks and other glass equipment were heated in an oven at 130ꢀC overnight and
assembled in a stream of Ar. All reactions were monitored by TLC on silica gel 60 F254; the
positions of the spots were detected with 254 nm UV light or by spraying with one of the following
staining systems: 50% methanolic sulfuric acid, 5% ethanolic phosphomolybdic acid and iodine.
Preparative column chromatography was performed on columns of silica gel (60±240 mesh) and
with solvents that were distilled prior to use. Preparative centrifugally accelerated radial thin-layer
chromatography (PCAR-TLC) was performed with a Chromatotron1 Model 7924 T (Harrison
Research, Palo Alto, CA, USA); the rotors (1 or 2 mm layer thickness) were coated with silica gel
Merck grade type 7749, TLC grade, with binder and ¯uorescence indicator (Aldrich 34,644±6)
and the eluting solvents were delivered by the pump at a ¯ow-rate of 0.5±1.5 mL min^1. Melting
1
points are uncorrected. H and 13C NMR spectra were recorded on a Varian Unity or on a
Bruker 300 instrument in CDCl3 at 55ꢀC. Chemical shifts are reported in ppm (d) relative to
CHCl3 (d=7.26) in CDCl3. Optical rotations were taken at 25ꢀC on a Perkin±Elmer 241 polar-
imeter. Elemental analysis was performed on a Perkin±Elmer 240B microanalyzer. Nitrone 213
and vinyl thymine12d were prepared according to the reported procedures. Vinyl acetate 2 was
purchased (Aldrich) and distilled prior to use.
4.2. Cycloaddition of BIGN 2 with vinyl acetate 3
BIGN 2 (0.710 g, 3 mmol) was dissolved in vinyl acetate 3 (12.92 g, 150 mmol) and the result-
ing mixture was stirred at re¯ux under an Ar atmosphere until no more nitrone was observed by
TLC (12 h). The mixture was evaporated under reduced pressure and the products ratio was
1
established by H NMR analysis of the crude product. The residue was puri®ed by preparative,
centrifugally accelerated, radial, thin-layer chromatography (Chromatotron1) using an 80:20
mixture of hexane:EtOAc as an eluent.
4.2.1. (3S,5R)-5-Acetoxy-2-benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidine 5
1
(0.580 g, 60%); Oil; Rf (hexane:EtOAc, 80:20)=0.30; [ꢀ]D ^82.1 (c 0.68, CHCl3); H NMR
(CDCl3) ꢁ 1.31 (s, 3H), 1.34 (s, 3H), 2.06 (s, 3H), 2.43 (ddd, 1H, J=1.0, 3.3, 13.8 Hz), 2.53 (ddd,
1H, J=5.7, 8.6, 13.8 Hz), 3.19 (dt, 1H, J=3.3, 8.6 Hz), 3.44 (dd, 1H, J=5.7, 8.5 Hz), 3.95 (d, 1H,
J=13.2 Hz), 3.97 (dd, 1H, J=6.1, 8.5 Hz), 4.07 (d, 1H, J=13.2 Hz), 4.23 (dt, 1H, J=5.9, 8.3 Hz),
6.46 (dd, 1H, J=1.0, 4.9 Hz), 7.30±7.38 (m, 5H); 13C NMR (CDCl3) ꢁ 21.3, 25.2, 26.8, 37.3, 63.2,
64.3, 67.6, 75.7, 96.5, 109.2, 127.9, 128.5, 129.6, 135.9, 170.1. Anal. calcd for C17H23NO5: C,
63.54; H, 7.21; N, 4.36. Found: C, 63.28; H, 7.40; N, 4.19.
4.2.2. (3S,5S)-5-Acetoxy-2-benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidine 6
1
(0.164 g, 17%); Oil; Rf (hexane:EtOAc, 80:20)=0.35; [ꢀ]D +97.7 (c 0.59, CHCl3); H NMR
(CDCl3) ꢁ 1.29 (s, 3H), 1.34 (s, 3H), 2.10 (s, 3H), 2.63 (ddd, 1H, J=2.6, 7.4, 14.1 Hz), 2.70 (ddd,
1H, J=4.7, 5.7, 14.1 Hz), 3.34 (dt, 1H, J=4.7, 7.4 Hz), 3.40 (m, 1H), 3.94 (m, 2H), 4.00 (d, 1H,
J=12.8 Hz), 4.29 (d, 1H, J=12.8 Hz), 6.45 (dd, 1H, J=2.6, 5.7 Hz), 7.25±7.40 (m, 5H); 13C NMR
(CDCl3) ꢁ 21.4, 25.2, 26.8, 38.3, 64.2, 65.5, 67.7, 76.6, 98.8, 109.3, 127.8, 128.5, 129.1, 136.6, 169.9.
Anal. calcd for C17H23NO5: C, 63.54; H, 7.21; N, 4.36. Found: C, 63.71; H, 7.35; N, 4.47.

P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
1551
4.2.3. (3R,5S)-5-Acetoxy-2-benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidine 7
1
(0.087 g, 9%); Oil; Rf (hexane:EtOAc, 80:20)=0.23; [ꢀ]D +137.2 (c 0.39, CHCl3); H NMR
(CDCl3) ꢁ 1.32 (s, 3H), 1.40 (s, 3H), 2.00 (ddd, 1H, J=2.0, 7.9, 13.8 Hz), 2.04 (s, 3H), 2.60 (ddd,
1H, J=6.4, 8.9, 13.8 Hz), 3.10 (dt, 1H, J=7.9, 8.9 Hz), 3.70 (dd, 1H, J=7.0, 8.3 Hz), 3.99 (dd,
1H, J=5.6, 8.3 Hz), 4.00 (d, 1H, J=14.1 Hz), 4.17 (q, 1H, J=6.8 Hz), 4.42 (d, 1H, J=14.1 Hz),
6.24 (dd, 1H, J=2.0, 6.4 Hz), 7.24±7.40 (m, 5H); 13C NMR (CDCl3) ꢁ 21.3, 25.2, 26.6, 39.1, 61.4,
66.0, 66.8, 76.7, 94.8, 110.0, 127.3, 128.1, 129.4, 136.5, 170.4. Anal. calcd for C17H23NO5: C,
63.54; H, 7.21; N, 4.36. Found: C, 63.62; H, 7.08; N, 4.25.
4.2.4. (3R,5R)-5-Acetoxy-2-benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidine 8
1
(0.086 g, 9%); Oil; Rf (hexane:EtOAc, 80:20)=0.20; H NMR (CDCl3) ꢁ 1.30 (s, 3H), 1.38 (s,
3H), 2.03 (s, 3H), 2.30±2.40 (m, 2H), 3.42 (dt, 1H, J=6.8, 8.4 Hz), 3.68 (dd, 1H, J=6.0, 8.2 Hz),
4.00 (dd, 1H, J=6.5, 8.2 Hz), 4.06 (q, 1H, J=6.5 Hz), 4.21 (ABq, 2H, J=13.4 Hz), 6.30 (dd, 1H,
J=1.2, 4.0 Hz), 7.25±7.50 (m, 5H); 13C NMR (CDCl3) ꢁ 21.4, 26.1, 26.6, 38.0, 64.5, 64.7, 66.3,
76.9, 97.2, 109.7, 127.5, 128.3, 129.4, 136.9, 169.9. Compound 8 was shown to contaminate 8% of
1
the stereoisomer 7 judged by integration of H-5 hydrogens in the H NMR spectrum of the
products. Nevertheless, some diastereomeric enrichment could be accomplished by repeated
chromatography.
4.3. 1-{(3S,5R)-2-Benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidin-5-yl}thymine 10 and
1-{(3S,5S)-2-benzyl-3-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidin-5-yl}thymine 11
A 7:2 mixture of 5 and 6 (0.6 g, 1.87 mmol) was dissolved in CH3CN (35 mL) and treated
sequentially with bis(trimethylsilyl)thymine (0.810 g, 3 mmol) and TMSOTf (0.58 mL, 3 mmol).
The resulting mixture was stirred at ambient temperature for 4 h at which time saturated
aqueous NaHCO3 (50 mL) was added. The reaction mixture was diluted with EtOAc (50 mL)
and the organic layer separated. The aqueous layer was extracted with EtOAc (2Â50 mL). The
combined organic extracts were washed with brine, dried (MgSO4) and concentrated under
reduced pressure. The crude product was puri®ed by column chromatography (EtOAc) on silica
gel.
Eluted ®rst was 10 (0.435, 60%): oil; Rf (CHCl3:MeOH, 98:2)=0.39; [ꢀ]D ^21.8 (c 0.35,
1
CHCl3); H NMR (CDCl3) ꢁ 1.33 (s, 3H), 1.40 (s, 3H), 1.81 (d, 3H, J=1.0 Hz), 2.27 (ddd, 1H,
J=3.3, 7.9, 13.8 Hz), 2.98 (ddd, 1H, J=7.8, 8.5, 13.8 Hz), 3.20 (dt, 1H, J=3.5, 8.5 Hz), 3.70 (dd,
1H, J=6.9, 8.1 Hz), 3.90 (d, 1H, J=14.0 Hz), 4.04 (dd, 1H, J=6.7, 8.1 Hz), 4.25 (dt, 1H, J=3.5,
6.8 Hz), 4.31 (d, 1H, J=14.0 Hz), 6.05 (dd, 1H, 3.3, 7.8 Hz), 7.30±7.39 (m, 5H), 7.49 (q, 1H,
J=1.0 Hz), 8.40 (bs, 1H); 13C NMR (CDCl3) ꢁ 12.5, 24.8, 26.1, 39.3, 61.7, 65.6, 66.3, 74.6, 82.9,
109.8, 109.9, 127.9, 128.6, 128.9, 136.5, 136.6, 150.4, 163.7. Anal. calcd for C20H25N3O5: C, 62.00;
H, 6.50; N, 10.85. Found: C, 61.83; H, 6.48; N, 11.00.
Eluted second was 11 (0.145 g, 20%): oil; Rf (CHCl3:MeOH, 98:2)=0.33; [ꢀ]D ^13.6 (c 0.60,
1
CHCl3); H NMR (CDCl3) ꢁ 1.35 (s, 3H), 1.42 (s, 3H), 1.79 (d, 3H, J=1.1 Hz), 2.45 (ddd, 1H,
J=5.4, 7.7, 13.4 Hz), 2.97 (ddd, 1H, J=3.8, 7.2, 13.4 Hz), 3.40 (m, 1H), 3.65 (dd, 1H, J=6.5, 8.3
Hz), 4.08 (dd, 1H, J=6.6, 8.3 Hz), 4.13 (ABq, 2H, J=13.7 Hz), 4.28 (q, 1H, J=6.3 Hz), 6.02 (dd,
1H, J=5.4, 7.2 Hz), 7.10 (q, 1H, J=1.1 Hz), 7.30±7.38 (m, 5H), 8.30 (bs, 1H); 13C NMR (CDCl3)
ꢁ 12.5, 25.0, 26.5, 37.5, 62.9, 65.2, 67.3, 74.1, 85.0, 109.8 (2C), 128.0, 128.6, 129.0, 135.5, 136.1,
150.5, 162.0. Anal. calcd for C20H25N3O5: C, 62.00; H, 6.50; N, 10.85. Found: C, 61.92; H, 6.63;
N, 10.65.

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P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
4.4. Cycloaddition of BIGN 2 with vinyl thymine 15
To a solution of BIGN 2 (0.235 g, 1 mmol) in toluene (50 mL) was added vinyl thymine (0.761
g, 5 mmol) and the resulting mixture was stirred at re¯ux under an Ar atmosphere until no more
nitrone was observed by TLC (15 h). The mixture was evaporated under reduced pressure and the
products ratio was established by 1H NMR analysis of the crude product. The residue was puri®ed
by preparative, centrifugally accelerated, radial, thin-layer chromatography (Chromatotron1) using
a 98:2 mixture of chloroform:methanol as an eluent. Eluted ®rst was compound 10 (0.248 g, 64%);
eluted second was compound 11 (62 mg, 16%) and eluted third was compound 16 (31 mg, 8%).
4.4.1. 1-{(3R,5S)-2-Benzyl-3-[(1S)-2,2-dimethyl-1,3-dioxolan-4-yl]isoxazolidin-5-yl}thymine 16
1
Oil; Rf (CHCl3:MeOH, 98:2)=0.29; [ꢀ]D ^36.2 (c 1.11, CHCl3); H NMR (CDCl3) ꢁ 1.38 (s,
3H), 1.45 (s, 3H), 1.78 (d, 3H, J=1.2 Hz), 2.35 (ddd, 1H, J=3.8, 7.6, 13.8 Hz), 2.52 (ddd, 1H,
J=6.2, 7.6, 13.8 Hz), 3.40 (q, 1H, J=7.6 Hz), 3.68 (dd, 1H, J=6.2, 8.1 Hz), 4.06 (dd, 1H, J=6.4,
8.1 Hz), 4.21 (d, 1H, J=14.2 Hz), 4.30 (q, 1H, J=7.1 Hz), 4.40 (d, 1H, J=14.2 Hz), 5.98 (dd, 1H,
J=3.8, 7.6 Hz), 7.01 (q, 1H, J=1.2 Hz), 7.36±7.42 (m, 5H), 8.21 (bs, 1H); 13C NMR (CDCl3) ꢁ
12.4, 25.1, 26.5, 38.9, 60.5, 65.9, 67.8, 75.3, 86.1, 109.8, 109.9, 127.6, 128.3, 129.5, 135.4, 136.6,
151.0, 161.8. Anal. calcd for C20H25N3O5: C, 62.00; H, 6.50; N, 10.85. Found: C, 61.73; H, 6.38;
N, 10.99.
4.5. 1-[(3S,5R)-2-Benzyl-3-(hydroxymethyl)isoxazolidin-5-yl]thymine 4 (l-isoxazolidinyl thymidine)
To a solution of 10 (0.4 g, 1.03 mmol) in MeOH (60 mL) was added p-TosOH (43 mg, 0.25
mmol) and the resulting solution was heated under re¯ux for 4 h. The reaction mixture was
cooled at ambient temperature and neutralized with Amberlite IRA-400. The mixture was ®ltered
and the ®ltrate evaporated under reduced pressure. The crude diol 12 (1H NMR (acetone-
d6+D2O) ꢁ 1.80 (bs, 3H), 2.47 (ddd, 1H, J=5.8, 8.2, 13.6 Hz), 2.65 (ddd, 1H, J=3.2, 8.4, 13.6
Hz), 2.90 (q, 1H, J=8.3 Hz), 3.60 (m, 2H), 3.79 (d, 1H, J=14.4 Hz), 3.90 (m, 1H), 4.10 (d, 1H,
J=14.4 Hz), 6.14 (dd, 1H, J=5.8, 8.4 Hz), 7.20±7.40 (m, 6H)) was taken up into a 1:1 mixture of
MeOH:H2O (30 mL), cooled at 0ꢀC and treated with NaIO4 (0.214 g, 1 mmol). The resulting
suspension was stirred at 0ꢀC for 1 h and then ®ltered. The ®ltrate was maintained at 0ꢀC and
treated with solid NaBH4 (0.117 g, 3 mmol). The mixture was stirred at 0ꢀC for an additional
hour, at which time it was concentrated under reduced pressure. The residue was puri®ed by
column chromatography (EtOAc) on silica gel to give pure 4 (0.216 g, 66%) as a foam; Rf
1
(EtOAc)=0.37; [ꢀ]D ^6.9 (c 1.10, MeOH); H NMR (CDCl3) ꢁ 1.74 (d, 3H, J=0.8 Hz), 2.29
(ddd, 1H, J=3.4, 8.5, 13.6 Hz), 2.53 (bs, 1H), 2.98 (dt, 1H, J=7.7, 13.7 Hz), 3.12 (ddt, 1H, J=3.7,
4.8, 8.5 Hz), 3.68 (m, 1H), 3.78 (bd, 1H, J=11.8 Hz), 3.84 (d, 1H, J=13.9 Hz), 4.31 (d, 1H,
J=13.9 Hz), 5.92 (dd, 1H, J=3.4, 7.4 Hz), 7.23±7.40 (m, 6H), 9.37 (bs, 1H); 13C NMR (CDCl3)
ꢁ 12.4, 40.7, 61.5, 61.6, 66.3, 83.3, 109.7, 127.8, 128.5, 128.9, 136.0, 136.7, 150.5, 164.2. Anal.
calcd for C16H19N3O4: C, 60.56; H, 6.03; N, 13.24. Found: C, 60.72; H, 6.21; N, 13.05.
4.6. 1-[(3S,5S)-2-Benzyl-3-(hydroxymethyl)isoxazolidin-5-yl]thymine 14
The method described above to convert 10 to 4 was applied to 11 (0.1 g, 0.258 mmol) to give,
after column chromatography (EtOAc), pure 14 (39 mg, 48%) as an oil; Rf (EtOAc)=0.25; [ꢀ]D
^38.8 (c 0.89, MeOH); 1H NMR (CDCl3) ꢁ 1.79 (d, 3H, J=1.2 Hz), 1.87 (bs, 1H), 2.44 (ddd, 1H,

P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
1553
J=4.6, 7.9, 13.7 Hz), 2.80 (ddd, 1H, J=5.3, 7.6, 13.7 Hz), 3.45 (m, 1H), 3.74 (m, 2H), 4.19
(s, 2H), 6.00 (dd, 1H, J=4.6, 7.6 Hz), 7.08 (q, 1H, J=1.2 Hz), 7.30±7.40 (m, 5H), 8.15 (bs, 1H);
13C NMR (CDCl3) ꢁ 12.3, 38.6, 59.9, 61.6, 64.7, 84.0, 110.7, 127.9, 128.6, 129.1, 135.7, 136.3,
150.0, 163.3. Anal. calcd for C16H19N3O4: C, 60.56; H, 6.03; N, 13.24. Found: C, 60.37; H, 6.19;
N, 13.46.
4.7. 1-[(3R,5S)-2-Benzyl-3-(hydroxymethyl)isoxazolidin-5-yl]thymine 3 (d-isoxazolidinyl thymidine)
The method described above to convert 10 to 4 was applied to 16 (30 mg, 0.077 mmol) to give,
after column chromatography (EtOAc), pure 3 (13 mg, 55%) as an oil; Rf (EtOAc)=0.37; [ꢀ]D
1
+6.1 (c 0.8, MeOH); H NMR (CDCl3) ꢁ 1.70 (bs, 1H), 1.77 (d, 3H, J=1.2 Hz), 2.29 (ddd, 1H,
J=3.6, 8.5, 13.7 Hz), 2.99 (dt, 1H, J=7.4, 13.7 Hz), 3.15 (ddt, 1H, J=3.6, 5.2, 8.6 Hz), 3.68 (dd,
1H, J=5.0, 11.7 Hz), 3.79 (dd, 1H, J=3.3, 11.7 Hz), 3.92 (d, 1H, J=13.9 Hz), 4.32 (d, 1H,
J=13.9 Hz), 5.99 (dd, 1H, J=3.6, 7.4 Hz), 7.23±7.50 (m, 6H), 8.47 (bs, 1H); 13C NMR (CDCl3) ꢁ
12.4, 40.7, 61.5, 61.6, 66.3, 83.3, 109.7, 127.8, 128.5, 128.9, 136.0, 136.7, 150.5, 164.2. Anal. calcd
for C16H19N3O4: C, 60.56; H, 6.03; N, 13.24. Found: C, 60.61; H, 5.88; N, 13.37.
Acknowledgements
The authors gratefully acknowledge the ®nancial support by the Direccion General de Ensen-
anza Superior (Project PB97-1014, MEC, Madrid, Spain).
References
1. (a) Nguyen-Ba, N.; Brown, W. L.; Chan, L.; Lee, N.; Brasili, L.; La¯eur, D.; Zacharie, B. Chem. Commun. 1999,
1245. (b) Lin, J.-S.; Kira, T.; Gullen, E.; Choi, Y.; Qu, F.; Chu, C. K.; Cheng, Y.-C. J. Med. Chem. 1999, 42, 2212.
(c) Westwood, N. B.; Walker, R. T. Tetrahedron 1998, 54, 13391. (d) Mansour, T. S.; Evans, C. A.; Charron, M.;
Korba, B. E. Bioorg. Med. Chem. Lett. 1997, 7, 303. (e) Perry, C. M.; Faulds, D. Drugs 1997, 53, 657. (f) Deng, L.;
Scharer, O. D.; Verdine, G. L. J. Am. Chem. Soc. 1997, 119, 7865. (g) Dyson, M. R.; Coe, P. L.; Walker, R. T.
J. Med. Chem. 1991, 34, 2782. (h) Peterson, M. L.; Vince, R. J. Med. Chem. 1991, 34, 2787.
2. For a review, see: Pan, S.; Amankulor, N. M.; Zhao, K. Tetrahedron 1998, 54, 6587.
3. For recent accounts, see: (a) Merino, P.; Franco, S.; Merchan, F. L.; Tejero, T. Synlett 2000, 442. (b) Merino, P.;
Franco, S.; Merchan, F. L.; Tejero, T. In Recent Research Developments in Synthetic Organic Chemistry; Pandalai,
S. G., Ed.; Transworld Research Network: Trivandrum, India, 1998; Vol. 1, p. 109.
4. Merino, P.; Franco, S.; Garces, N.; Merchan, F. L.; Tejero, T. Chem. Commun. 1998, 493.
5. Merino, P.; Franco, S.; Merchan, F. L.; Tejero, T. Tetrahedron Lett. 1998, 39, 6411.
6. Xiang, Y.; Gi, H.-J.; Niu, D.; Schinazi, R. F.; Zhao, K. J. Org. Chem. 1997, 62, 7430. This approach has also been
exempli®ed in our laboratory in synthesis of isoxazolidin-5-ones, useful precursors to a number of analogues of
type 1b. See: Merino, P.; Franco, S.; Merchan, F. L.; Tejero, T. Tetrahedron: Asymmetry 1998, 9, 3945.
7. (a) Xiang, Y.; Chen, J.; Schinazi, R. F.; Zhao, K. Tetrahedron Lett. 1995, 36, 7193. (b) Xiang, Y.; Gong, Y.; Zhao,
K. Tetrahedron Lett. 1996, 37, 4877.
8. Chiacchio, U.; Corsaro, A.; Gumina, G.; Resci®na, A.; Iannazzo, D.; Piperno, A.; Romeo, G.; Romeo, R. J. Org.
Chem. 1999, 64, 9322. For 1,3-dipolar cycloadditions leading to racemic compounds, see: Li, P.; Gi, H.-J.; Sun, L.;
Zhao, K. J. Org. Chem. 1998, 63, 366. Chiacchio, U.; Gumina, G.; Resci®na, A.; Romeo, R.; Uccella, N.;
Casuscelli, F.; Piperno, A.; Romeo, G. Tetrahedron 1996, 52, 8889.
9. (a) DeShong, P.; Li, W.; Kennington Jr., J. W.; Ammon, H. L.; Leginus, J. M. J. Org. Chem. 1991, 56, 1364. (b)
DeShong, P.; Dicken, C. M.; Leginus, J. M.; Whittle, R. R. J. Am. Chem. Soc. 1984, 106, 5598. (c) Xu, Z.;
Johannes, C. W.; La, D. S.; Ho®lena, G. E.; Hoveyda, A. H. Tetrahedron 1997, 53, 16377.

1554
P. Merino et al. / Tetrahedron: Asymmetry 11 (2000) 1543±1554
10. In this work we use the syn/anti nomenclature according to Masamune convention, see: Masamune, S.; Ali, S. A.;
Snitman, D. L.; Garvey, D. S. Angew. Chem., Int. Ed. Engl. 1980, 19, 557 and Masamune, S.; Kaiho, T.; Garvey,
D. S. J. Am. Chem. Soc. 1982, 104, 5521.
11. The importance of l-nucleoside analogues has been pointed out; see: (a) Furman, P. A.; Wilson, J. E.; Reardon,
J. E.; Painter, G. R. Antiviral Chemistry & Chemotherapy 1995, 6, 345. (b) Nair, V.; Jahnke, T. S. Antimicrob.
Agents Chemother. 1995, 39, 1017. (c) Wang, P.; Gullen, B.; Newton, M. G.; Cheng, Y.-C.; Schinazi, R. F.; Chu,
C. K. J. Med. Chem. 1999, 42, 3390. (d) Wang, P.; Agrofoglio, L. A.; Newton, M. G.; Chu, C. K. J. Org. Chem.
1999, 64, 4173. (e) Varaprasad, C. V.; Averett, D. A.; Ramasamy, K. S.; Wu, J. Tetrahedron 1999, 55, 13345.
12. The 1,3-dipolar cycloaddition between a nitrone and a vinyl base has been applied to the synthesis of
isoxazolidinyl nucleosides, but there was until now no example of the synthesis of homochiral compounds by
such an approach; see: (a) Leggio, A.; Liguori, A.; Procopio, Siciliano, C.; Sindona, G. Tetrahedron Lett. 1996, 37,
1277. (b) Leggio, A.; Liguori, A.; Procopio, A.; Siciliano, C.; Sindona, G. Nucleosides and Nucleotides 1997, 16,
1515. (c) Leggio, A.; Liguori, A.; Maiuolo, L.; Napoli, A.; Procopio, A.; Siciliano, C.; Sindona, G. J. Chem. Soc.,
Perkin Trans. 1 1997, 3097. (d) Adams, D. R.; Boyd, A. S. F.; Ferguson, R.; Grierson, D. S.; Monneret, C.
Nucleosides and Nucleotides 1998, 17, 1053. (e) Colacino, E.; Converso, A.; De Nino, A.; Leggio, A.; Liguori, A.;
Maiuolo, L.; Napoli, A.; Procopio, A.; Siciliano, C.; Sindona, G. Nucleosides and Nucleotides 1999, 18, 581.
13. Merino, P.; Franco, S.; Merchan, F. L.; Tejero, T. Tetrahedron: Asymmetry 1997, 8, 3489.
14. Desai, H. K.; Joshi, B. S.; Panu, A. M.; Pelletier, S. W. J. Chromatogr. 1985, 322, 223. Separation was carried out
in a Chromatotron1, Harrison Research, Palo Alto, CA, USA.
15. For synthetic purposes it is possible to use the mixture of cis/trans anti isomers for the N-glycosylation step.
16. Niahimura, T.; Iwai, T. Chem. Pharm. Bull. 1964, 12, 352.
17. Vorbruggen, H.; Krolikiewicz, K.; Bennua, B. Chem. Ber. 1981, 114, 1234.
18. Semiempirical geometry optimizations were carried out at the PM3 level of theory as implemented in
WINMOPAC 2.0, Fujitsu Limited, 1998. For simplicity the N-benzyl group of the nitrone 2 was replaced by a
methyl group.
19. Ab initio calculations were performed using Gaussian 98, Revision A.3, M. Frisch, J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery Jr., J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi,
J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Cli€ord, S.; Ochterski, J.; Petersson,
G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, USA, 1998.