Stereoselective synthesis of new bicyclic N,O-iso-homonucleoside analogues

Article abstract of DOI:10.1016/S0040-4020(03)00772-5

The synthesis of two new bicyclic nucleoside analogues is reported. These compounds are iso-homonucleoside and are synthesised through a 1,3-dipolar cycloaddition of an enantiopure cyclic nitrone to protected allyl acohol and subsequent introduction of thymine by a Mitsunobu reaction.

Full text of DOI:10.1016/S0040-4020(03)00772-5

Tetrahedron 59 (2003) 5231–5240
Stereoselective synthesis of new bicyclic
N,O-iso-homonucleoside analogues
†
Barbara Richichi,a Stefano Cicchi,a Ugo Chiacchio,c Giovanni Romeob
,
,
*
and Alberto Brandia
,
*
aDipartimento di Chimica Organica ‘Ugo Schiff’, Universita’ di Firenze, Polo Scientifico, Via della Lastruccia 13, I-50019 Sesto Fiorentino,
Firenze, Italy
bDipartimento Farmaco-Chimico, Universita’ di Messina, Viale Annunziata, Messina I-98100, Italy
cDipartimento di Scienze Chimiche, Universita’ di Catania, Viale A. Doria 6, Catania I-95125, Italy
Received 3 March 2003; revised 23 April 2003; accepted 16 May 2003
Abstract—The synthesis of two new bicyclic nucleoside analogues is reported. These compounds are iso-homonucleoside and are
synthesised through a 1,3-dipolar cycloaddition of an enantiopure cyclic nitrone to protected allyl acohol and subsequent introduction of
thymine by a Mitsunobu reaction. q 2003 Elsevier Science Ltd. All rights reserved.
Nucleoside analogues display a wide range of biological
activities and have attracted particular attention as anti-
cancer and antiviral agents.1 Accordingly, the synthesis of
new modified nucleosides represents an important and
increasing area of interest for organic chemists in the search
for compounds with improved biological properties. Since
hydrogen bond interactions of heterocyclic bases are
fundamental for the biological activity of nucleoside
analogues, any variation of the base moiety should preserve
such intramolecular forces.2 As a consequence, only minor
variations of bases are found in biologically active nucleo-
side analogues.3 The most notable modification is found in
C-nucleosides, in which the typical C–N glycosidic bond
has been replaced by a non-hydrolizable C–C bond.4 On the
contrary extensive modifications have been made to the
sugar moiety,5 which has been substituted by acyclic,6
heterocyclic2 and carbocyclic moieties.7
demanded by the enzyme and a particular sugar confor-
mation proves troublesome due to the flexibility of the furan
ring. Such flexibility is responsible for significant differ-
ences between the conformation in solid state and in
solution.10 For this reason, conformationally locked nucleo-
side analogues were synthesised with the double aim to
develop more efficient antiviral drugs and to obtain a deeper
insight into the interaction between the substrate and the
enzyme.
Some authors have proposed examples of bicyclic nucleo-
side analogues characterised by a restricted conformational
freedom: typical examples are the 20,30-dideoxy-2030-a-
methylenecytidine 111 and bicarbocyclic nucleoside 2.12
The biological activity of nucleoside analogues is often
modulated by the nature and the spatial disposition of
specific functional groups on the sugar moiety.8 Such
functional groups exert a profound effect on the confor-
mation and puckering of the ribose ring,9 or any of the
several different carbocyclic or heterocyclic rings present in
the analogues, and are able to control the outcome of the
reactions between these prodrugs and the specific enzymes.
The correlation between the conformational preference
Keywords: nucleoside analogues; 1,3-dipolar cycloaddition; Mitsunobu
reaction.
*
Corresponding authors. Tel.: þ390554573485; fax: þ390554573531;
e-mail: alberto.brandi@unifi.it
`
On leave from the Universita di Messina
†
Chart 1.
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00772-5

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B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
under certain conditions, allows a strict control on the
several stereocenters formed in the reaction. These
characteristics make this approach particularly useful for
the synthesis of a class of substrates where a proper
relative disposition of substituents is a primary requisite.
Recent results using enantiopure five membered cyclic
nitrones demonstrate their versatility in the control of the
configuration of the different stereogenic centers produced
in the 1,3-dipolar cycloaddition.19 In this paper we
report our first results on the stereoselective synthesis of
bicyclic N,O-iso-homonucleosides bearing a thymine group
with the aim of demonstrating the feasibility of the
approach.
Scheme 1. Two possible retrosynthetic pathways for compound 6.
These nucleosides were recently found to have moderate to
significant antiviral activity against HIV through inhibition
of HIV reverse transcriptase.
The first and more direct approach was the one depicted by
the retrosynthetic arrow A (Scheme 1) in which a properly
substituted nitrone, 8, undergoes a 1,3-dipolar cycloaddition
with allylic alcohol 9. A known procedure20 was modified in
order to insert a pyrimidine base, namely thymine, in the
precursor, dimesylate 12, of the desired nitrone 8a.
However, several unsuccessful experiment according to
the Mitsunobu21 reaction between 12 and the nucleobase,
performed under different conditions, showed that this
procedure was not practicable (Scheme 2).
These findings triggered further studies on byciclic nucleo-
side analogues bearing the CH2OH group and the base on
the same ring or on different rings, as compounds 3–513,14
(Chart 1). These considerations prompted us to exploit
bicyclic isoxazolidines as useful scaffolds for the synthesis
of new conformationally restricted nucleoside analogues,
where the sugar unit has been replaced by an isoxazolidine
ring. Simple N,O-nucleosides have already been described
as nucleoside analogues which are able to inhibit the reverse
transcriptase.15
Thus, we turned our attention onto the retrosynthetic
pathway B (Scheme 1) which proceeds through the insertion
of the nucleobase directly onto the cycloadduct 10 derived
from the cycloaddition of nitrone 11 to allylic alcohol 9
(R¼H).
The cycloaddition reaction of nitrone 1322 with allylic
alcohol, protected as tetrahydropyranyl ether, 1423
proceeded smoothly and quantitatively afforded three
different diastereomers 15a–c in the relative ratio 5:1.7:1,
respectively (Scheme 3). Structural assignments to all the
adducts isolated have been demonstrated, after deprotection
(Scheme 3) of the primary hydroxyl function, on the basis of
a n.O.e. analysis, as shown in Figure 1, as well as by
comparison with previous reported results.22,24 Compound
16a was assigned as derived from an exo–anti approach of
the dipolarophile to the nitrone, while 16c was assigned as
deriving from an endo–syn approach, on the basis of a
In this paper we have turned our attention to the design of a
synthetic approach to bicyclic nucleosides 6 (Scheme 1),
which can be regarded as N,O-iso-homonucleosides,
strictly correlated to iso-homonucleosides with general
structure 7, where the isoxazolidine ring mimics the
sugar moiety and the nucleobase is linked to the 30
position16 through a carbon bridge.17 The presence of the
second five membered ring fused to the isoxazolidine
moiety induces a restricted conformational mobility which
might result in a more reliable structure-reactivity
relationship.
The 1,3-dipolar cycloaddition reaction of nitrones is a
powerful tool for the synthesis of isoxazolidines18 and,
Scheme 2.
Scheme 3. (i) Refluxing toluene, 8 h, quantitative; (ii) Amberlyst 15, MeOH, 70% 16a, 90% 16b, 84% 16c.

B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
5233
Figure 1. n.O.e. for compounds 16a–c, 25 and 35.
n.O.e. between H4 and H2 which lacks in 16b (endo–anti)
(Fig. 1).
Deprotection of the secondary hydroxyl function in the
major product 15a led to isoxazolidine 17, which is the
synthetic precursor of nucleoside analogues (Scheme 4). On
the basis of our previous experience in the synthesis of
thymine and uracil substituted pyrrolidine 20,25 we decided
to carry out the nucleosidation reaction under Mitsunobu
condition. A comparison of thymine 18 and its N 3-protected
derivative 19 as nucleophile in the reaction was also
examined (Chart 2).26 In the synthesis of compounds 20, the
free base 18 revealed more efficient than 19, avoiding the
formation of O-alkylated byproducts, which often are
produced in the reactions of 19 (Scheme 5).27 The use of
19 afforded a mixture of compounds 23 and 24, with the O-
alkylated derivative 24 as the major component. The
temperature proved to be a crucial parameter, since decrease
of the reaction temperature from 110 to 808C resulted in an
increase of the relative ratio of 23/24 from 1:2.3 to 1:1.9
(Table 1, entries 1 and 2). The best results were obtained
Scheme 4. (i) NaOH, MeOH, 1 h, 93% yield.
Chart 2.
Scheme 5. (i) 18, PPh3, DEAD, dioxane; (ii) 19, PPh3, DEAD, dioxane.

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B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
Table 1.
The reactivity of reagents 18 and 19 were strikingly
different. Protected thymine 19 afforded mainly O-alkylated
products while thymine (18), even at high temperature,
afforded a better ratio of the N-alkylated derivative 21,
which became the major product at lower temperature.
While in the reaction with simple hydroxypyrrolidine no
O-alkylated derivatives where detected, in this case
O-alkylation is a side reaction and, apparently, the first
Entry
Reagents
21/22 23/24 Yield (%)
1
2
3
4
5
19/P(Ph)3/DEAD 4 h, 1108C, 4 h
19/P(Ph)3/DEAD 4 h, 808C, 4 h
18/P(Ph)3/DEAD 4 h, 1108C, 4 h 1: 1.1
18/P(Ph)3/DEAD 4 h, 508C, 4 h
18/P(Ph)3/DEAD 4 h, 408C, 4 h
1:2.3
1:1.9
22 (23)
20 (23)
21 (21)
42 (21)
42 (21)
2.3:1
3.3:1
Scheme 6. (i) MeOH, Amberlyst 15, yield 90% (25), yield 52% (26).
using the free base, 18, which afforded, at 1108C, a mixture
of compounds 21 and 22 in a 1:1 ratio, optimizable at a 3.3:1
ratio by performing the reaction at 408C (entry 5).
Compounds 21–24 were separated by chromatography
and their structure confirmed by comparison of both 1H and
13C NMR data with literature precedents.25 The O 2,O 4
dialkylated derivative 22 w0as identified on the basis of the
chemical shift27a of the H-4 and H-400 signals of 22 (d 5.60,
5.37, respectively) which were appreciably shifted down-
field with respect to H-4 of 21 (d 5.28 ppm) and H-3 of 20a
and b (5.11, 5.18 pp0m, respe0c0 tively).25 Furthermore, the
chemical shifts of C-4 and C-4 in 22 (d 76.1, 2C) were also
shifted downfield with respect to C-4 of 21 (d 57.6 ppm) and
C-3 of 20a and b (53.5, 53.1 ppm, respectively).25 The same
analysis was applied to compound 24 (H-4 d 5.43 ppm, C-4
d 84.0 ppm). All compounds were mixtures of epimers at
the tetrahydropyranyl stereocenter. Compounds 21 and 22
were deprotected to afford the desired final compound 25 and
the bis-O-alkylated derivative 26 (Scheme 6). Their structures
were confirmed on the basis of n.O.e. measurements.
O-alkylated derivative is reactive enough to react with
another molecule of isoxazolidine to afford compound 22.
No traces of N 3-alkylated product could be detected in the
crude reaction mixture.
Compound 17 was also the starting material for the
synthesis of another potential antiviral agent epimeric at
the carbon atom bearing the thymine ring. A Mitsunobu
reaction with benzoic acid afforded, in good yield, benzoate
27 with inversion of configuration at C-4 (Scheme 7). The
inversion of configuration was demonstrated by a compari-
son of 28, obtained by deprotection of the primary hydroxyl
function of 27, with 16a. The n.O.e. present in 16a between
H-4 and H-2 is now absent (compare with Figure 1) and in
the 1H NMR spectrum the proton H-3a (3.90 ppm) showed
two cis coupling constant (J¼7.6, 7.0 Hz) and one trans
(J¼1.6 Hz) in agreement with the cis relationship between
H-4 and H-3a. The hydrolysis of the ester group afforded 29
which was the substrate for a new Mitsunobu reaction with
thymine Scheme 8). The reaction of thymine, under the
For compound 25 the NOESY spectrum evidenced the
presence of a n.O.e. effect between H-6 of thymine ring
(resonating at 7.24 ppm) and H-4 of the bicyclic moiety
(Fig. 1). The n.O.e. between the H-6-thymine and H-2
confirmed the inversion of configuration at C-4 during the
Mitsunobu reaction, supported also by the large coupling
constant between H-3a and H-4 (8.1 Hz). The presence of
these n.O.e. effects confirms also the assignment of the
regiochemistry, as observed in similar systems.27b The
deprotection of compounds 22 afforded compound 26
whose complex spectrum did not allowed a complete
n.O.e. analysis, but showed a correlation between the H-6 of
the thymine ring and the H-3 and H-5 of both the
isoxazolidinyl rings. The absence of any correlation
between H-6 of the thymine and H-4 of the isoxazolidines
ring, confirmed, with the lowfield chemical shift in the 13C
NMR spectrum, the bis-O-alkylation (76.8 and 76.2 ppm).
Scheme 7. (i) Benzoic acid, DEAD, P(Ph)3, 87% yield; (ii) MeOH,
Amberlyst 15, 89% yield; (iii) MeOH, NaOH, 83%.

B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
5235
Scheme 8. (i) 19, PPh3, DEAD; (ii) 18, PPh3, DEAD.
same conditions used for 17, afforded an inseparable
mixture of three compounds in a 1.6:1:1.5 ratio which
were tentatively assigned structures 30–32, respectively.
The formation of compound 31, where the pyrimidine
proton resonates at lower field respect to N1-alkylated
derivatives (0.2 ppm) and did not show any n.O.e. with H-4,
was unprecedented in this kind of reactions and make the
use of free thymine synthetically unsuitable.
Although it appears necessary to tune the reaction
conditions to obtain satisfactory yields of compounds 25
and 35, the general strategy was feasible and straight-
forward for the preparation of a new class of bicyclic
nucleosides analogues. The use of protected thymine 19 and
thymine (18) was complementary allowing a synthetically
useful yield of the desired product. This result, together with
the use of other dipolarophiles, will afford access to other
nucleoside analogues with a different spatial arrangement of
the pharmacophores, allowing a tailored synthesis for new
potential antiviral and anticancer drugs.
This time the use of N 3-benzoylated thymine 19 proved
feasible even at 408C affording derivatives 33 and 34 in
1.2:1 ratio, respectively. Derivative 33 was isolated from the
reaction mixture in 42% yield.
1. Experimental
1.1. General
Finally compound 33, after purification, was deprotected to
afford the desired compound 35, whose n.O.e. data (Fig. 1)
confirmed the expected structure (Scheme 9). Thus, the
presence of a correlation between H-4 and H-2 of the
bicyclic system and between thymine-H-6 with H-4,
together with the absence of a correlation between
thymine-H-6-and H-2, demonstrated the inversion of
configuration at C4.
All reaction were carried out under nitrogen. Rf values refer
to TLC carried out on 0.25 mm silica gel plates (Merck
F254), with the same eluent as indicated for the column
chromatography separation. Microanalyses were carried out
with a Perkin–Elmer 240 analyzer. 1H and 13C NMR
spectra were recorded at 200 and 50 MHz, respectively,
with a Varian Gemini, 400 and 100 MHz, respectively, with
a Bruker UXNMR; the chemical shifts for 1H and 13C NMR
spectra are given in ppm from TMS. IR spectra were
recorded with a Perkin–Elmer 881 spectrophotometer.
Optical rotation measurements were carried out with a
Jasco DIP-370 polarimeter. Nitrone 13 was synthesised
according to Ref. 22.
Two new different nucleoside analogues, 25 and 35 can be
synthesised through this 1,3-dipolar cycloaddition-
Mitsunobu reaction approach.
1.2. Cycloaddition of nitrone 13 to 2-allyloxy-tetrahydro-
pyran (14)
A solution of 14 (16.7 mmol, 2.08 g) and nitrone 13
(6.8 mmol, 1.4 g) in toluene (3 mL) was stirred under reflux
for 8 h. The solution was then concentrated and the crude
mixture purified by flash column chromatography to afford
Scheme 9. (i) MeOH, Amberlyst 15, 2 h, 508C; (ii) MeOH/NH3 4:1, 2 h,
room temperature 56%.

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B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
cycloadducts 15a–c in a 5:1.7:1 ratio calculated by 1H
NMR signals integration in the crude mixture.
1.2.4. (2R,3aR,4S)-2-(Tetrahydro-pyran-20-yloxy-
methyl)-hexahydro-pyrrolo[1,2]isoxazol-4-ol (17). To a
solution of NaOH (343 mg, 8.58 mmol) in MeOH (10 mL)
was added a solution of 15a (426 mg, 1.22 mmol) in MeOH
(3 mL). The mixture was stirred for 1 h at room temperature
and then concentrated under reduced pressure. The residue
was dissolved in water (10 mL) and the solution was
extracted with CH2Cl2 (2£20 mL). The organic phase was
dried with Na2SO4 and concentrated to give 279 mg (93%
yield) of compound 17 as a colorless oil.
1.2.1. (2R,3aR,4S)-Benzoic acid 2-(tetrahydro-pyran-20-
yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl
ester (15a). Colorless oil, 1.54 g, 65% yield; Rf (pet. ether/
AcOEt 2:3)¼0.54; 1H NMR (CDCl3): d 8.00 (d, 2H,
J¼7.4 Hz), 7.56 (m, 1H), 7.43 (m, 2H), 5.22 (m, 1H, 4-H),
4.63 (s, 1H, THP), 4.34 (m, 1H, 2-H), 3.92–3.84 (m, 1H, 3a-
H), 3.85–3.38 (m, 2H, CH2OTHP), 3.73–3.52 (m, 2H,
THP), 3.40–3.30 (m, 2H, 6-H), 2.60–2.30 (m, 3H, 3-5-H),
2.00 (dddd, 1H, J¼9.8, 9.4, 3.2, 2.8 Hz, 5-H), 1.86–1.5
(m, 6H, THP); 13C NMR (CDCl3): d 163.1 (s, 1C, CvO),
133.0 (d, 1C), 129.7 (s, 1C), 128.4 (d, 2C), 128.3 (d, 2C),
99.0–98.8 (d, 1C, THP), 81.2–80.9 (d, 1C, 4-C), 76.7–76.3
(d, 1C, 2-C), 71.9 (d, 1C, 3a-C), 68.7–68.0 (t, 1C,
CH2OTHP), 62.2–62.0 (t, 1C, THP), 55.5–55.4 (t, 1C,
6-C), 37.4–37.2 (t, 1C, 3-C), 30.9 (t, 1C, 5-C), 30.4 (t, 1C,
THP), 25.3 (t, 1C, THP), 19.4 (t, 1C, THP); IR (CDCl3): n
2946, 2872, 1713, 1201, 1177 cm21; MS (m/z): [Mþ1]
348, 264, 247, 225, 158, 142. Anal. calcd for C19H25NO5:
C, 65.69; H, 7.25; N, 4.03. Found C, 65.81; H, 7.36; N,
4.25.
1
Rf (CH2Cl2/MeOH 9:1)¼0.61; H NMR (CDCl3): d 4.60
(m, 1H, THP), 4.26–4.16 (m, 1H, 2-H), 4.1 (m, 1H, 4-H),
3.85 (m, 1H, 3a-H), 3.75–3.63 (m, 2H, CH2OTHP), 3.57–
3.30 (m, 4H, 6-H, THP, OH), 3.23–3.12 (m, 1H, 6-H),
2.36–2.01 (m, 3H, 3-5H), 1.80–1.46 (m, 7H, 5H, THP); 13C
NMR (CDCl3): d 99.1–98.9 (d, 1C, THP), 77.4–77.2 (d,
1C, 2-C), 76.4–76.1 (d, 1C, 4-C), 73.28–73.21 (d, 1C, 3a-
C), 68.6–68.0 (t, 1C, CH2OTHP), 62.29–62.18 (t, 1C,
THP), 55.16 (t, 1C, 6-C), 37.1–36.9 (t, 1C, 3-C), 33.6 (t, 1C,
5-C), 30.4 (t, 1C, THP), 25.3 (t, 1C, THP), 19.4 (t, 1C,
THP); IR (CDCl3) n 3607, 2945, 2866, 1118 cm21; MS
(m/z): 243, 225, 158, 143, 129, 122. Anal. calcd for
C12H21NO4 C, 59.24; N, 5.76; H, 8.70. Found C, 58.96; N,
5.59; H 9.02.
1.2.2. (2S,3aR,4S)-Benzoic acid 2-(tetrahydro-pyran-20-
yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl
ester (15b). Colorless oil, 521 mg, 22% yield; Rf
(AcOEt)¼0.62; 1H NMR (CDCl3): d 7.97 (d, 2H,
J¼7.4 Hz), 7.54 (m, 1H), 7.40 (m, 2H), 5.20 (t, 1H,
J¼6.0 Hz, 4-H), 4.66 (s, 1H, THP), 4.16 (m, 1H, 2-H),
3.93–3.75 (m, 3H, 3a-H, CH2OTHP), 3.66–3.36 (m, 3H,
6-HþTHP), 3.25 (ddd, J¼13.1, 12.0, 5.7 Hz, 1H, 6-H),
2.75–2.43 (m, 2H, 3-5-H), 2.20 (m, 1H, 5-H), 1.93 (dd, 1H,
J¼13.8, 5.6 Hz, 3-H), 1.82–1.50 (m, 6H, THP); 13C NMR
(CDCl3): d 166.2 (s, 1C, CvO), 133.0 (d, 1C), 129.6 (s,
1C), 129.4 (d, 2C), 128.3 (d, 2C), 98.6 (d, 1C, THP), 82.4 (d,
1C, 4-C), 76.6 (d, 1C, 2-C), 73.2 (d, 1C, 3a-C), 66.3 (t, 1C,
CH2OTHP), 62.0 (t, 1C, THP), 55.2 (t, 1C, 6-C), 37.1 (t, 1C,
3-C), 30.4 (2C, 5-CþTHP), 25.3 (t, 1C, THP), 19.2 (t, 1C,
THP); IR (CDCl3): n 2944, 2872, 1713 cm21; MS (m/
z): 347, 263, 247, 225, 140, 111. Anal. calcd for
C19H25NO5: C 65.69, H 7.25, N 4.03. Found C 65.67, H
7.57, N 4.21.
1.3. Mitsunobu reaction of 17 with 18
DEAD (537 mg, 3.1 mmol) was added over 2 h to a
suspension of PPh3 (808 mg, 3.1 mmol), 18 (153 mg,
1.21 mmol), and 17 (250 mg, 1.03 mmol) in dry dioxane
(4 mL) at 408C. After stirring for 2 h at 408C the solvent was
removed under reduced pressure. The yellow residue was
purified by flash chromatography on silica gel to afford 21
(150 mg, 0.43 mmol) and 22 (120 mg, 0.21 mmol).
1.3.1. 5-Methyl-1-[(2R,3aR,4R) 2-(tetrahydro-pyran-2-
yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl]-
1H-pyrimidine-2,4-dione (21). Pale yellow oil, Rf
1
(CH2Cl2/MeOH 95:5)¼0.51; H NMR (CDCl3): d 7.20 (s,
1H, CH-Thymine), 5.28 (m, 1H, 4-H), 4.63 (m, 1H, THP),
4.20–4.10 (m, 2H, 2-3a-H), 3.94–3.44 (m, 4H, THP,
CH2OTHP), 3.30 (dd, 1H, J¼10.3, 5.1 Hz, 6-H), 3.10 (m,
1H, 6-H), 2.33–2.18 (m, 3H, 3-5H), 1.95 (s, 3H, CH3-
Thymine), 1.92–1.53 (m, 7H, 5H, THP); 13C NMR
(CDCl3): d 163.5 (s, 1C, CvO), 151.0 (s, 1C, CvO),
137.2 (d, 1C, CH-Thymine), 111.2 (s, 1C, C-Thymine),
99.5–99.1 (t, 1C, THP), 76.8–76.4 (d, 1C, 2-C), 69.2–69.0
(t, 1C, CH2OTHP), 68.0 (d, 1C, 3a-C), 62.6–62.5 (t, 1C,
THP), 57.6–57.5 (d, 1C, 4-C), 52.9 (t, 1C, 6-C), 33.6–33.4
(t, 1C, 3-C), 30.5 (t, 1C, 5-C), 29.5–29.3 (t, 1C, THP), 25.4
(t, 1C, THP), 19.6–19.5 (t, 1C, THP), 12.6 (q, 1C, CH3-
Thymine); IR (CDCl3): n 3392, 2941, 1268, 2862, 1686,
1467 cm21; MS (m/z): 351, 236, 225, 142, 126, 111. Anal.
calcd for C17H25N3O5: C, 58.11; N, 11.96; H, 7.17. Found
C, 58.13; N, 11.95; H, 6.89.
1.2.3. (2S,3aS,4S)-Benzoic acid 2-(tetrahydro-pyran-20-
yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl
ester (15c). Colorless oil, 308 mg, 13% yield; Rf (AcOEt/
MeOH 3:1)¼0.62; 1H NMR (CDCl3): d 8.00 (d, 2H,
J¼7.0 Hz), 7.62 (m, 1H), 7.43 (m, 2H), 5.47 (q, 1H,
J¼5.8 Hz, 4-H), 4.60 (s, 1H, THP), 4.38 (m, 1H, 2-H),
3.97–3.90 (m, 1H, 3a-H), 3.85–3.40 (m, 2H, CH2OTHP),
3.74–3.50 (m, 2H, THP), 3.43–3.33 (m, 1H, 6-H), 3.24–
3.15 (m, 1H, 6-H), 2.42–1.98 (m, 4H, 3-5-H), 1.80–1.50
(m, 6H, THP); 13C NMR (CDCl3): d 165.6 (s, 1C, CvO),
133.4 (d, 1C), 129.6 (s, 1C), 129.3 (d, 2C), 128.3 (d, 2C),
99.1–98.6 (d, 1C, THP), 76.6 (d, 1C, 4-C), 74.8–74.7 (d,
1C, 2-C), 69.1 (d, 1C, 3a-C), 68.1–67.6 (t, 1C, CH2OTHP),
62.3–61.9 (t, 1C, THP), 53.3 (t, 1C, 6-C), 33.1–33.0 (t, 1C,
3-C), 31.5–31.4 (t, 1C, 5-C), 30.4–30.3 (t, 1C, THP), 25.3
(t, 1C, THP), 19.5–19.2 (t, 1C, THP); IR (CDCl3): n 2944,
2872, 1715 cm21; MS (m/z): 347, 262, 247, 232, 207, 140,
124. Anal. calcd for C19H25NO5C, 65.69; H, 7.25; N, 4.03.
Found C, 65.72; H, 7.40; N, 4.29.
1.3.2. 5-Methyl-2,4-bis-[(2R,3aR,4R)-(200R,3a00R,400R)-2-
200-(tetrahydro-pyrany-2-yloxymethyl)-hexahydro-pyr-
rolo[1,2-b]isoxazol-4yl]-pyrimidine (22). Colorless oil, Rf
1
(CH2Cl2/MeOH 95:5)¼0.47; H NMR (CDCl3): d 8.04 (s,
1H, CH-Thymine), 5.60 (q, 1H, J¼6.4 Hz, 4-H), 5.37 (q, 1H,
J¼5.8 Hz, 400-H), 4.65 (s, 2H, THP), 4.20 (m, 2H, 2-200-H),

B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
5237
4.10–3.96 (m, 2H, 3a-3a00-H), 3.92–3.72 (m, 4H, CH2-
OTHP), 3.54–3.46 (m, 4H, THP), 3.42–3.04 (m, 2H, 6-600-
H), 3.27–3.15 (m, 2H, 6-600-H), 2.45–2.24 (m, 6H), 2.10 (s,
3H, CH3-Thymine), 2.05–1.97 (m, 2H), 1.94–1.50 (m, 12H,
THP); 13C NMR (CDCl3): d 168.3 (s, 1C, C–O-Thymine),
162.9 (s, 1C, C–O-Thymine), 157.9 (d, 1C, CH-Thymine),
111.2 (s, 1C, C-Thymine), 99.3–98.9 (d, 2C, THP), 77.2-
76.7 (d, 2C, 2-200-C), 76.1 (d, 2C, 4-400-C), 69.1–68.8 (t, 1C,
CH2OTHP), 68.1–67.9 (t, 1C, CH2OTHP), 67.6- 67.3 (d,
2C, 3a-3a00-C), 62.1 (t, 2C, THP), 53.2–53.1 (t, 2C, 6-600-C),
33.3–33.1 (t, 1C, 3-C), 32.9–32.5 (t, 1C, 300-C), 31.7 (t, 2C,
5-500-C), 30.4 (t, 2C, THP), 25.3 (t, 2C, THP), 19.3 (t, 2C,
THP), 11.8 (q, 1C, CH3-Thymine); IR (CDCl3): n 2927,
2857, 1661, 1602, 1422 cm21; FAB-MS (m/z) calcd
576.3159. Found 577.3230 [Mþþ1]. Anal. calcd for
C29H44N4O8: C, 60.40; H, 7.69; N, 9.72. Found C, 60.05;
H, 7.68; N, 9.94.
1.4.3. (2R,3aR,4R)-Benzoic acid 2-(tetrahydro-pyran-20-
yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl
ester (27). A solution of 17 (279 mg, 1.15 mmol), PPh3
(902 mg, 3.44 mmol), and benzoic acid (168 mg,
1.37 mmol), in dry THF (6 mL), was stirred for 20 min at
08C under N2. DEAD (600 mg, 0.53 mL, 3.44 mmol) was
added dropwise and the mixture was stirred, for 1 h at 08C,
and at room temperature for 15 h. The solvent was removed
under reduced pressure. The yellow residue was purified by
flash chromatography on silica gel to afford 27 (345 mg,
0.9 mmol) as colorless oil in 87% of yield; Rf (CH2Cl2/
MeOH 95:5)¼0.41; 1H NMR (CDCl3): d 8.00 (d, 2H,
J¼6.8 Hz), 7.54 (m, 1H), 7.40 (m, 2H), 5.40 (q, 1H,
J¼5.8 Hz, 4-H), 4.60 (s, 1H, THP), 4.42 (m, 1H, 2-H),
3.80–3.10 (m, 2H, HCHOTHP, 3a-H), 3.70–3.60 (m, 1H,
THP), 3.54–3.30 (m, 3H, HCHOTHP, THP, 6-H), 3.23–
3.10 (m, 1H, 6-H), 2.34 (ddd, 1H, J¼12.4, 7.0, 1.8 Hz, 5-H),
2.26–2.12 (m, 2H, 3-H), 2.10–1.90 (m, 1H, 5-H), 1.80–
1.30 (m, 6H, THP); 13C NMR (CDCl3): d 165.6 (s, 1C,
CvO), 132.0 (d, 1C), 129.5 (s, 1C), 129.3 (d, 2C), 128.2 (d,
2C), 99.0–98.6 (d, 1C, THP), 76.9 (d, 1C, 4-C), 74.7 (d, 1C,
2-C), 69.3 (t, 1C, CH2OTHP), 67.5 (d, 1C, 3a-C), 62.2–61.8
(t, 1C, THP), 53.0 (t, 1C, 6-C), 33.0–32.9 (t, 1C, 3-C),
31.4–31.2 (t, 1C, 5-C), 30.2–30.1 (t, 1C, THP), 25.4 (t, 1C,
THP), 19.3–19.1 (t, 1C, THP); IR (CDCl3): n 3020, 2946,
2874, 1718, 1216, 1178 cm21; MS (m/z): 347, 263, 245,
225, 140. Anal. calcd for C19H25NO5: C, 65.69; H, 7.26; N,
4.03. Found C, 65.67; H, 7.20; N, 3.73.
1.4. Mitsunobu reaction of 17 and 19
DEAD (323 mg, 1.86 mmol) was added over 2 h to a
suspension of PPh3 (487 mg, 1.86 mmol), 19 (156 mg,
0.68 mmol), and 17 (150 mg, 0.62 mmol) in dry dioxane
(4 mL) at 808C. After stirring for 2 h at 808C the solvent was
removed under reduced pressure. The yellow residue was
purified by flash chromatography on silica gel (eluent
petroleum ether/ethyl acetate 1:9) to give 23 (54 mg,
0.11 mmol, 20%) and 24 (102 mg, 0.22 mmol, 40%).
1.4.1. N 1-[(2R,3aR,4R)-2-(Tetrahydro-pyran-2-yloxy-
methyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl]-N 3-
(benzoyl)-thymine (23). Yellow oil, H NMR (CDCl3): d
1.4.4. (2R,3aR,4R)-2-(Tetrahydro-pyran-2-yloxy-
methyl)-hexahydro-pyrrolo[1,2]isoxazol-4-ol (29). To a
solution of NaOH (230 mg, 5.75 mmol) in MeOH (7 mL)
was added a solution of 27 (287 mg, 0.83 mmol) in MeOH
(2 mL). The mixture was stirred for 1 h at room temperature
and then the solvent was evaporated. The residue was
dissolved in water and the solution was extracted by
CH2Cl2. The organic phase was dried with Na2SO4 and
concentrated to give 166 mg (0.68 mmol, 83% yield) of the
desired compound 29 as a colorless oil.
1
8.00 (d, 2H, J¼7.6 Hz), 7.60 (m, 1H), 7.53 (m, 2H), 7.35 (s,
1H, CH-Thymine), 5.22 (m, 1H, 4-H), 4.64 (m, 1H, THP),
4.15 (m, 1H, 2-H), 3.94–3.46 (m, 5H, 3a-H, THP,
CH2OTHP), 3.30 (m, 1H, 6-H), 3.00 (m, 1H, 6-H), 2.36–
3.0 (m, 3H, 3-5-H), 1.97 (s, 3H, CH3-Thymine), 1.86–1.24
(m, 7-H, 5-H, THP); 13C NMR (CDCl3): d 168.2 (s, 1C,
CvO), 158.3 (s, 1C, CvO), 137.2 (d, 1C, CH-Thymine),
135.4 (d, 1C), 131.1 (s, 1C), 130.3 (d, 2C), 128.5 (d, 2C),
110.2 (s, 1C, C-Thymine), 100.1 (t, 1C, THP), 76.6 (d, 1C,
2-C), 69.2 (t, 1C, CH2OTHP), 67.9 (d, 1C, 3a-C), 62.54–
62.44 (t, 1C, THP), 57.88–57.64 (d, 1C, 4-C), 52.79–52.72
(t, 1C, 6-C), 33.67–33.46 (t, 1C, 3-C), 30.4 (t, 1C, 5-C),
29.4 (t, 1C, THP), 25.3 (t, 1C, THP), 19.6 (t, 1C, THP), 12.6
(q, 1C, CH3-Thymine).
1
Rf (CH2Cl2/MeOH 95:5)¼0.33; H NMR (CDCl3): d 4.60
(m, 1H, THP), 4.34–4.21 (m, 2H, 2-4-H), 3.88–3.80 (m,
1H, 3a-H), 3.82–3.42 (m, 4H, CH2OTHP, THP), 3.32–3.08
(m, 3H, 6-H, OH), 2.54 (m, 1H, 5-H), 2.05–1.73 (m, 3H,
3-H, 5-H), 1.68–1.48 (m, 6H, THP); 13C NMR (CDCl3): d
98.9–98.6 (d, 1C, THP), 77.1–76.7 (d, 1C, 4-C), 72.1–71.8
(d, 1C, 2-C), 68.9 (d, 1C, 3a-C), 68.7 (t, 1C, CH2OTHP),
62.1–62.0 (t, 1C, THP), 53.0 (t, 1C, 6-C), 34.3–34.2 (t, 1C,
3-C), 32.2 (t, 1C, 5-C), 30.3 (t, 1C, THP), 25.3 (t, 1C, THP),
1.4.2. O 2-[(2R,3aR,4R)-2-(Tetrahydro-pyran-2-yloxy-
methyl)-hexahydro-pyrrolo[1,2-b]isoxazol-4-yl]-N 3-
1
(benzoyl)-thymine (24). Yellow oil, H NMR (CDCl3): d
19.3 (t, 1C, THP); IR (CDCl3): n 3619, 2945, 1216 cm21
;
8.42 (s, 1H, CH-Thymine), 7.90 (d, 2H, J¼7.6 Hz), 7.62
(m, 1H), 7.46 (m, 2H), 5.43 (q, 1H, J¼4.6 Hz, 4-H), 4.64
(s, 1H, THP), 4.40 (m, 1H, 2-H), 4.24–3.20 (m, 7H, 3a-6-H,
CH2OTHP, THP), 2.39–2.00 (m, 2-H, 3-H), 1.99 (s,
3H, CH3-Thymine), 1.80–1.30 (m, 8H, 5-H, THP); 13C
NMR (CDCl3): d 162.4 (s, 1C, CvO), 137.7 (d, 1C,
CH-Thymine), 135.2 (d, 1C), 131.2 (s, 1C), 130.2 (d,
2C), 129.4 (d, 2C), 112.0 (s, 1C, C-Thymine), 98.9 (t, 1C,
THP), 84.3 (d, 1C, 4-C), 76.3 (d, 1C, 2-C), 68.7 (t, 1C,
CH2OTHP), 67.5 (d, 1C, 3a-C), 62.1 (t, 1C, THP), 53.4 (t,
1C, 6-C), 32.9 (t, 1C, 3-C), 30.8 (t, 1C, 5-C), 29.5 (t, 1C,
THP), 25.2 (t, 1C, THP), 19.2 (t, 1C, THP), 12.3 (q, 1C,
CH3-Thymine).
MS (m/z): 243, 158, 140, 129. Anal. calcd for C12H21NO4:
C, 59.24; N, 5.76; H, 8.70. Found C, 59.28; N, 5.56; H,
8.77.
1.5. Mitsunobu reaction of 29 and 19
DEAD (324 mg, 0.29 mL, 1.86 mmol) was added over 1 h
to a suspension of PPh3 (488 mg, 1.86 mmol), 19 (171 mg,
0.74 mmol), and 29 (151 mg, 0.62 mmol) in dry dioxane
(5 mL) at 408C. After stirring for 3 h at 408C the solvent
was removed under reduced pressure. 1H NMR
spectrum revealed the presence of compounds 33 and 34
in a 1.2:1 ratio. The yellow residue was purified by flash

5238
B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
chromatography on silica gel to give 33 (130 mg,
0.28 mmol) and 34 (55 mg, 0.156 mmol).
2.53 (m, 1H, 3-H), 2.50–2.40 (m, 1H, 5-H), 2.46 (dd, 1H,
J¼7.2, 2.8 Hz, 3-H), 2.00 (ddd, 1H, J¼12.0, 5.6, 2.0 Hz, 5-
H); 13C NMR (CDCl3): d 166.1 (s, 1C, CvO), 133.1 (d,
1C), 129.7 (s, 1C), 129.5 (d, 2C), 128.3 (d, 2C), 81.5 (d, 1C,
4-C), 77.9 (d, 1C, 2-C), 72.7 (d, 1C, 3a-C), 64.6 (t, 1C,
CH2OH), 55.3 (t, 1C, 6-C), 36.3 (t, 1C, 3-C), 30.8 (t, 1C, 5-
C); MS (m/z): 263, 245, 159, 140. Anal. calcd for
C14H17NO4 C, 63.87; H, 6.51; N, 5.32. Found C, 63.72;
H, 6.40; N, 5.29.
1.5.1. 3-Benzoyl-5-methyl-1-[(2R,3aR,4S)-2-(tetrahydro-
pyran-2-yloxymethyl)-hexahydro-pyrrolo[1,2-b]isoxa-
zol-4-yl]-1H-pyrimidin-2,4-dione (33). Yellow oil, yield:
1
46%; Rf (AcOEt)¼0.36; H NMR (CDCl3): d 7.90 (d, 2H,
J¼7.4 Hz), 7.65 (m, 1H), 7.5 (m, 2H), 7.36 (d, 1H,
J¼0.2 Hz, CH-Thymine), 5.07 (m, 1H, 4-H), 4.60 (m, 1H,
THP), 4.50 (m, 1H, 2-H), 3.84–3.70 (m, 2H, CH2OTHP),
3.6 (m, 1H, 3a-H), 3.56–3.47 (m, 3H, THP, 6-H), 3.15 (m,
1H, 6-H), 2.60–2.37 (m, 2H, 3-5-H), 2.3 (m, 2H, 3-5-H),
1.98 (s, 3H, CH3-Thymine), 1.93–1.54 (m, 6H, THP); 13C
NMR (CDCl3): d 168.0 (s, 1C, CvO), 155.0 (s, 1C, CvO),
136.1 (d, 1C, CH-Thymine), 135.3 (d, 1C), 131.2 (s, 1C),
130.4 (d, 2C), 129.1 (d, 2C), 112.5 (s, 1C, C-Thymine), 99.0
(d, 1C, THP), 76.3 (d, 1C, 2-C), 69.7–69.6 (d, 1C, 3a-C),
68.5–67.8 (t, 1C, CH2OTHP), 62.2 (t, 1C, THP), 58.9–58.1
(d, 1C, 4-C), 53.8–53.4 (t, 1C, 6-C), 36.6 (t, 1C, 3-C), 36.2
(t, 1C, 5-C), 30.3–30.1 (t, 1C, THP), 25 (t, 1C, THP), 19 (t,
1C, THP), 12.6 (q, 1C, CH3-Thymine); IR (CDCl3): n 3020,
2942, 1729, 1700, 1659, 1250 cm21; HRMS: calcd
455.2056. Found 456.2131 [Mþþ1]; MS (m/z): 456
[Mþþ1], 372, 312, 232.
1.6.2. (2S,3aR,4S)-Benzoic acid 2-hydroxymethyl-hexa-
hydro-pyrrolo[1,2-b]isoxazol-4-yl ester (16b). Colorless
oil, yield: 90%; Rf (AcOEt)¼0.31; [a]2D5¼210.2 (c¼0.49,
1
CHCl3); H NMR (CDCl3): d 8.00 (d, 2H, J¼7.3 Hz), 7.6
(m, 1H), 7.44 (m, 2H), 5.20 (d, 1H, J¼6.0 Hz, 4-H), 4.10
(m, 1H, 2-H), 3.85–3.60 (m, 2H, CH2OH), 3.83–3.80 (m,
1H, 3a-H), 3.50 (dd, 1H, J¼13.6, 6.0 Hz, 6-H), 3.28 (ddd,
1H, J¼13.5, 11.8, 7.0 Hz, 6-H), 3.00 (bs, 1H, OH), 2.67
(ddd, 1H, J¼12.5, 8.8, 6.2 Hz, 3-H), 2.53 (m, 1H, 5-H), 2.12
(m, 1H, 3-H), 1.99 (dt, 1H, J¼13.6, 6.0 Hz, 5-H); 13C
NMR (CDCl3): d 166.2 (s, 1C, CvO), 133.1 (d, 1C), 129.8
(s, 1C), 129.6 (d, 2C), 128.3 (d, 2C), 82.2 (d, 1C, 4-C), 78.3
(d, 1C, 2-C), 73.25 (d, 1C, 3a-C), 62.4 (t, 1C, CH2OH),
55.3 (t, 1C, 6-C), 36.5 (t, 1C, 3-C), 30.3 (t, 1C, 5-C); MS
(m/z): 263, 246, 231, 158, 123. Anal. calcd for C14H17NO4
C, 63.87; H, 6.51; N, 5.32. Found C, 63.94; H, 6.68; N,
5.42.
1.5.2. 3-Benzoyl-5-methyl-2-[(2R,3aR,4S)-2-(tetrahydro-
pyran-2-yloxymethyl)-hexahydro-pyrrolo[1,2]isoxazol-
4-yloxy]-3H-pyrimidin-4-one (34). Pale yellow oil, yield:
1
25%; Rf (AcOEt)¼0.16; H NMR (CDCl3): d 7.50 (s, 1H,
1.6.3. (2S,3aS,4S)-Benzoic acid 2-hydroxymethyl-hexa-
hydro-pyrrolo[1,2-b]isoxazol-4-yl ester (16c). Colorless
oil, yield: 84%; Rf (AcOEt) 0.16; [a]2D5¼þ100.0 (c¼0.61,
CH-Thymine), 5.20 (m, 1H, 4-H), 4.60 (s, 1H, THP), 4.30
(m, 1H, 2-H), 3.80–3.63 (m, 3H, 3aH, CH2OTHP), 3.50–
3.20 (m, 4H, 6-H, THP), 2.50–2.30 (m, 4H, 3-5-H), 1.96 (s,
3H, CH3-Thymine), 1.80–1.40 (m, 6H, THP); 13C NMR
(CDCl3): d 164.7 (s, 1C, CvO), 154.8 (s, 1C, CvO), 150.8
(d, 1C, CH-Thymine), 117.7 (s, 1C, C-Thymine), 99.1–98.8
(d, 1C, THP), 84.1–83.8 (d, 1C, 4-C), 76.6 (d, 1C, 2-C),
71.6 (d, 1C, 3a-C), 68.8–68.1 (t, 1C, CH2OTHP), 62.3–
62.1 (t, 1C, THP), 55.3 (t, 1C, 6-C), 37.4 (t, 1C, 3-C), 30.8–
30.39 (t, 1C, 5-C), 29.6 (t, 1C, THP), 25.3 (t, 1C, THP),
19.45–19.36 (t, 1C, THP), 12.3 (q, 1C, CH3-Thymine); IR
(CDCl3): n 3686, 2942, 1728, 1676, 1272, 1250 cm21; MS
(m/z): 351, 337, 311, 233, 235.
1
CHCl3); H NMR (CDCl3): d 8.00 (d, 2H, J¼7.5 Hz), 7.6
(m, 1H), 7.5 (m, 2H), 5.52 (q, 1H, J¼6.8 Hz, 4-H), 4.33
(dddd, 1H, J¼7.2, 6.4, 6.0, 2.8 Hz, 2-H), 3.93 (ddd, 1H,
J¼7.6, 6.5, 2.0 Hz, 3a-H), 3.72–3.60 (m, 2H, CH2OH), 3.46
(ddd, 1H, J¼13.2, 6.8, 6.0 Hz, 1H, 6-H), 3.2 (ddd, 1H,
J¼13.2, 7.2, 6.4 Hz, 6-H), 2.78 (bs, 1H, OH), 2.38 (ddd, 1H,
J¼13.2, 7.2, 2.0 Hz, 3-H), 2.33–2.22 (m, 3H, 3H, 5H); 13C
NMR (CDCl3): d 165.8 (s, 1C, CvO), 133.3 (s, 1C), 129.7
(d, 1C), 129.4 (d, 2C), 128.5 (d, 2C), 78.5 (d, 1C, 4-C), 74.9
(d, 1C, 2-C), 68.4 (d, 1C, 3a-C), 64.5 (t, 1C, CH2OH), 53.3
(t, 1C, 6-C), 31.7 (t, 1C, 3-C), 31.6 (t, 1C, 5-C); MS (m/z):
263, 158, 141, 113. Anal. calcd for C14H17NO4: C, 63.87; H,
6.51; N, 5.32. Found C, 63.60; H, 6.35; N, 5.50.
1.6. General procedure for the deprotection of THP
group using Amberlyst 15
1.6.4. (2R,3aR,4R)-1-(2-Hydroxymethyl-hexahydro-pyr-
rolo[1,2-b]isoxazol-4-yl)-5-methyl-1H-pyrimidine-2,4-
dione (25). 45 mg, pale yellow oil, yield: 90%; [a]2D5¼þ65.2
A solution of the substrate (0.3 mmol) in MeOH (3 mL),
was added with Amberlyst 15 (500 mg). The suspension
was stirred at 508C for 2 h. The solution was filtered off and
the resin was washed with methanol. Finally the resin was
eluted with a solution of MeOH/NH3 4:1. The solution was
concentrated under reduced pressure and the crude mixture
was purified by flash chromatography to afford the
corresponding alcohol.
1
(c¼0.09, CHCl3); Rf (CH2Cl2/MeOH 95:5)¼0.47; H NMR
(CDCl3): d 9.70 (bs, 1H, NH), 7.24 (s, 1H, CH-Thymine), 5.28
(q, 1H, J¼8.1 Hz, 4-H), 4.04 (m, 1H, 2-H), 3.9 (t, 1H,
J¼8.1 Hz, 3a-H), 3.66–3.06(m, 3H, 6-H, CH2OH), 3.09(ddd,
1H, J¼14.5, 12.0, 8.1 Hz, 6-H,), 3.00 (bs,1H, OH), 2.34–2.23
(m, 3H, 3-H, 5-H), 1.96–1.9 (m, 1H, 3-H), 1.93 (s, 3H, CH3-
Thymine); 13C NMR (CDCl3): d 163.6 (s, 1C, CvO), 151.7 (s,
1C, CvO), 137.1 (d, 1C, CH-Thymine), 111 (s, 1C, C-
Thymine), 77.9 (d, 1C, 2-C), 68.2 (d, 1C, 3a-C), 65.3 (t, 1C,
CH2OH), 57.4 (d, 1C, 4-C), 52.6 (t, 1C, 6-C), 32 (t, 1C, 3-C),
29.2 (t, 1C, 5-C), 12.6 (q, 1C, CH3-Thymine); IR (CDCl3): n
3691, 3556, 2926, 2857, 1704, 1686, 1600 cm21; MS-FAB
(m/z): calcd 267.1219. Found 268.1202 [Mþþ1]. Anal. calcd
for C12H17N3O4: C, 53.92; H, 6.41; N, 15.72. Found C, 54.15;
H, 6.07; N, 15.62.
1.6.1. (2R,3aR,4S)-Benzoic acid 2-hydroxymethyl-hexa-
hydro-pyrrolo[1,2-b]isoxazol-4-yl ester (16a). Colorless
oil, yield: 70%; Rf (AcOEt)¼0.39; [a]2D5¼245.9 (c¼0.50,
CHCl3); 1H NMR (CDCl3): d 8.00 (d, 2H, J¼7.6 Hz), 7.60
(m, 1H), 7.44 (m, 2H), 5.20 (d, 1H, J¼7.2 Hz, 4-H), 4.26
(m, 1H, 2-H), 3.84 (m, 1H, 3a-H), 3.72–3.57 (m, 2H, CH2-
OH), 3.50 (ddd, 1H, J¼14.0, 6.8, 2.0 Hz, 6-H), 3.33 (ddd,
1H, J¼14.0 10.1, 6.4 Hz, 6-H), 3.02 (bs, 1H, OH), 2.60–

B. Richichi et al. / Tetrahedron 59 (2003) 5231–5240
5239
1.6.5. 5-Methyl-2,4-bis-[(2R,3aR,4R)-(2R0,3aR0,40R)-2-20-
(hydroxymethyl)-hexahydro-pyrrolo[1,2-b]isoxazol-
4yl]-pyrimidine (26). Pale yellow oil, 18 mg, yield: 52%;
[a]2D5¼215.9 (c¼0.36, CHCl3); Rf (AcOEt/MeOH
C, 53.92; H, 6.41; N, 15.72. Found C, 54.05; H, 6.18; N,
15.95.
1
1:1)¼0.20; H NMR (CDCl3): d 8.00 (d, 1H, J¼0.7 Hz,
Acknowledgements
CH-Thymine), 5.57 (q, 1H, J¼6.6 Hz, 4-H), 5.38 (q,0 1H,
J¼6.5 Hz, 40-H), 4.35 (m, 1H, 2-H), 4.30 (m, 1H, 2 -H),
4.06–3.97 (m, 2H, 3a-3a0-H), 3.76–3.65 (m, 4H, CH2OH),
3.40 (ddd, 2H, J¼13.4, 13.0, 7.1 Hz, 6-60-H), 3.17 (ddd, 2H,
J¼13.5, 13.2,07.3 Hz, 6-60-H), 2.76 (bs, 2H, OH), 2.39–2.10
(m, 8H, 3-5-3 -50-H), 2.08 (d, 3H, J¼0.9 Hz, CH3-Thymine);
13C NMR (CDCl3): d 168.2 (s, 1C, C–O-Thymine), 162.7 (s,
1C, C–O-Thymine), 157.8 (d, 1C, CH-Thymine), 111.2 (s,
1C, C-Thymin0e), 78.5 (d, 2C, 2-20-C), 76.84 (d, 1C, 4-C),
76.2 (d, 1C, 4 -C), 68.2 (d, 1C, 3a-C), 67.9 (d, 1C, 3a0-C),
64.4 (t, 1C, CH2OH), 64.1 (t, 1C, CH2OH), 53.6 (t, 2C, 6-60-
C), 31.6 (t, 2C, 3-30-C), 30.9 (t, 2C, 5-50-C), 11.9 (q, 1C,
CH3-Thymine); IR (CDCl3): n 3690, 2945, 2871, 1602,
1572, 1421 cm21; MS (m/z): 409 [Mþþ1], 268, 142. Anal.
calcd for C19H28N4O6 C, 55.87; H, 6.91; N, 13.72. Found C,
55.54; H, 7.09; N, 14.04.
We thank MIUR (Ministero per l’Istruzione l’Universita’e
Ricerca) for financial support (COFIN 2002).
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