Bicyclic nucleosides; stereoselective dihydroxylation and 2'-deoxygenation.

Article abstract of DOI:10.1039/b210439c

A series of polyhydroxylated bicyclic nucleoside derivatives is approached applying stereoselective dihydroxylation reactions. Three out of four isomeric and protected products were obtained after the stereoselectivity of dihydroxylation has been completely inverted comparing a bicyclic nucleoside with a tricyclic furanose substrate. A corresponding 2'-deoxynucleoside derivative has been obtained after an optimized deoxygenation procedure.

Full text of DOI:10.1039/b210439c

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Bicyclic nucleosides; stereoselective dihydroxylation and
2Ј-deoxygenation†
Jacob Ravn, Morten Freitag and Poul Nielsen*
Nucleic Acid Center‡, Department of Chemistry, University of Southern Denmark,
5230 Odense M, Denmark. E-mail: pon@chem.sdu.dk
Received 30th October 2002, Accepted 16th January 2003
First published as an Advance Article on the web 11th February 2003
A series of polyhydroxylated bicyclic nucleoside derivatives is approached applying stereoselective dihydroxylation
reactions. Three out of four isomeric and protected products were obtained after the stereoselectivity of
dihydroxylation has been completely inverted comparing a bicyclic nucleoside with a tricyclic furanose substrate. A
corresponding 2Ј-deoxynucleoside derivative has been obtained after an optimized deoxygenation procedure.
Introduction
The design of conformationally restricted nucleosides is a very
important approach towards potentially antiviral agents1 and
monomers in conformationally restricted oligonucleotides for
potential antisense therapeutic and diagnostic purposes.2 Prime
examples of such nucleic acid analogues with the carbohydrate
moieties strongly restricted in bicyclic systems3 are the pioneer-
ing bicycloDNA4 and LNA (locked nucleic acids).5 In LNA,
the nucleosides are locked in N-type conformations6 by an
oxymethylene linkage between the C2Ј and the C4Ј positions,5
whereas in bicycloDNA, 2Ј-deoxynucleosides are restricted
towards S-type conformations by an ethylene linkage between
the C3Ј and the C5Ј positions.4 In a previous paper, we reported
the synthesis of bicyclic nucleoside analogues which are ribo-
configured analogues of the bicycloDNA monomers con-
taining an unsaturated C3Ј–C5Ј linkage.7 These nucleosides
were constructed via a convergent synthetic strategy applying a
Ring-Closing Metathesis (RCM) reaction as the key step.7An
intriguing advantage of these unsaturated bicyclic nucleoside
Scheme 1 Reagents and conditions: i, Ref. 7, 4 steps, 75%; ii, Ref. 8, 2
analogues is the opportunity for further derivatisation of the
double bond. In this paper we report the results obtained with
2Ј-deoxygenation as well as dihydroxylation of these nucleo-
sides and of a corresponding furanose derivative.
steps, 82%; iii, Ref. 8, OsO4, NMO, THF, H2O, 50 ЊC, 83%.
Table 1 1H NMR data of the hydroxylated bicyclic nucleosidesa
4c
5
11
18
Results
H1Ј
H2Ј
H4Ј
H5Ј
H6Ј
H7Ј
OH2Ј
OH6Ј
OH7Ј
3JH1ЈH2Ј
3JH4ЈH5Ј
3JH5ЈH6Ј
3JH6ЈH7Ј
5.98
4.29
4.29
3.98
4.13
4.09
5.98
5.05
4.91
5.7
5.97
3.96
4.42
3.89
4.22
4.09
5.78
5.13
5.05
8.8
5.92
4.65
4.38
3.77
4.12
4.12
5.41
5.31
5.24
8.3
6.17
1.82/2.72
4.37
3.92
4.19
3.98
—
5.13
5.03
5.3/9.6
7.1
In the original construction of our bicyclic nucleosides, the tri-
cyclic furanose derivative 1 (Scheme 1) was synthesised from
-glucose.7 Subsequent standard transformations afforded the
bicyclic ribo-configured nucleoside derivative 2 (Scheme 1),7
which was converted in two easy steps to its arabino-configured
epimer 3.8 En route to a tricyclic nucleoside analogue, 3 was
treated with osmium tetraoxide and dihydroxylated to give
4 in a high yield but after a long reaction time (six days)
(Scheme 1).8 Obviously, this dihydroxylation reaction has pro-
ceeded exclusively from the less hindered convex face of the
bicyclic system. The configuration of 4 has been proven
indirectly, as a tricyclic nucleoside obtained from 4 in two
simple conversions was examined by X-ray crystallography.8
6.9
7.7
4.2
5.5
6.6/7.5
NDb
3.8
8.4
4.1
NDb
a All data obtained at 300 MHz in DMSO-d6. Chemical shifts in ppm
relative to TMS and coupling constants in Hz. Numbering follows
standard nucleoside rules continued in the carbocyclic ring as depicted
in Scheme 1 for 4. b Not determined due to spectral overlap. c Ref. 8.
1
For further comparison, however, the H NMR data of 4 are
given in Table 1 and discussed below.
In our program for the construction of functionalised con-
formationally restricted nucleoside analogues, the ribo-con-
figured bicyclic nucleoside 2 was also treated with osmium
tetraoxide using the same conditions as used with 3 (Scheme 2).
However, the reaction was slow even at elevated temperature
(50 ЊC) affording only the product 5 in a low 22% yield as
well as a bis-dihydroxylated product 6 in 23% yield. When the
† Electronic supplementary information (ESI) available: 13C NMR
spectra for compounds 7, 8 and 17 as well as H NMR data spectra
for compounds 5, 11 and 18. See http://www.rsc.org/suppdata/ob/b2/
b210439c/
‡ Nucleic Acid Center is funded by the Danish National Research
Foundation for studies on nucleic acid chemical biology.
1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 8 1 1 – 8 1 6
811

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hydroxylated nucleoside derivative 11 in 62% yield from 9.
To our surprise, 11 was not identical to 5 and after an NMR-
spectroscopic analysis of the products (vide infra), the con-
figuration of 11 was proven to be a result of a dihydroxylation
from the more hindered concave face of the substrate 1. How-
ever, it was first attempted to prepare the arabino-configured
isomer of 11 through the same anhydro approach as used in the
conversion of 2 to 3. Thus, this conversion might in the case of
a product identical to 4 disprove the determined configuration
of 5. On the other hand, it would complete the set of four
isomeric nucleoside derivatives. Thus, 11 was protected as its
acetonide 12 and converted to a methanesulfonic ester 13.
However, standard basic treatment afforded not the target 14
but a complex mixture of compounds including 12. This
might be explained by the acetonide sterically preventing the
nucleobase from forming the anhydro intermediate. Finally
and in order to fulfil a more comprehensive NMR analysis
of the different isomeric products, also 4 was converted to its
acetonide 15 (Scheme 3).
Scheme 2 Reagents and conditions: i, OsO4, NMO, THF, H2O, rt,
33% 5 and 22% 6.
reaction was accomplished at room temperature for nine days,
the yield of 5 was elevated to 33% in addition to 22% of 6
and recovery of 10% of 2. Even though a dihydroxylation of
a thymine moiety by nearly similar reaction conditions has
been demonstrated before,9 the formation of 6 was unexpected.
Thus, other thymidine analogues containing unsaturated sub-
stituents have been used in dihydroxylation reactions to give
vicinal diols without notably affecting the nucleobase10 as has
been exemplified also in the preparation of 4.
In order to develop a better strategy towards the tri-
hydroxylated bicyclic thymidine derivative 5, we decided to
perform the dihydroxylation reaction before the nucleobase
coupling reaction, as outlined in Scheme 3. Thus, 1 was treated
with osmium tetraoxide to give the diol 7 in a high 86% yield
and after a much shorter reaction time, only four hours. The
exact configuration of 7 was not elucidated at this stage, and
7 was protected as the diester 8 and converted to the diastereo-
meric mixture of tetraacetates 9 in 60% overall yield. These
were coupled to thymine using standard Vorbrüggen-type con-
ditions to give exclusively the β-configured bicyclic nucleoside
10 which after a standard transesterification afforded the tri-
In Table 1 are given all 1H NMR data for the three isomeric
trihydroxylated bicyclic nucleosides 4, 5 and 11. Several obser-
vations confirm the determination of the three configurations.
Thus, the large coupling constant 3JH5ЈH6Ј = 7.7 Hz observed for
5 suggests the trans positioning of H5Ј and H6Ј whereas the
3
smaller JH5ЈH6Ј = 3.8 Hz observed for 11 suggests a smaller
torsion angle and a cis positioning of these two hydrogen
atoms. Simple modelling suggests that the cyclopentane
rings of nucleosides 4 and 5 adopt envelope conformations in
which the C6Ј takes an endo-position (i.e. pointing towards the
furanose ring), as this conformation would allow favourable
gauche interactions between all oxygen substituents. This is the
same conformation observed in both the parent bicycloDNA
thymidine monomer4 and its ribo-configured analogue.7 In both
4 and 5 this results in a perfect anti positioning of H5Ј and
H6Ј in full agreement with the observed coupling constants.
3
3
Furthermore, the JH4ЈH5Ј and JH5ЈH6Јcoupling constants for 4
are more like the ones observed for 5 than the ones observed for
11 (Table 1), and the configuration of 4 has been exclusively
determined before (vide supra).8 For both 5 and 11 the large
3JH1ЈH2Јcoupling constants confirm the nucleosides to be β-ribo-
configured and restricted in S-type conformations.
The large difference in chemical shift for H2Ј observed
between the three isomers can also be explained. Thus, the H2Ј
for 11 is in close proximity to the hydroxyl functionalities of
C6Ј and C7Ј and hereby deshielded explaining the high shift at
4.65 ppm observed for 11. The H2Ј of 4 is likewise deshielded
by the C3Ј-oxygen explaining the slightly higher shift observed
in this case compared to H2Ј of 5. Finally, the chemical shifts
of the hydroxyl groups also confirm the determinations of con-
figuration as higher shifts are observed for the hydroxyl protons
of C6Ј and C7Ј in 11 as these are in closer proximity to the C5Ј-
and C4Ј-oxygens as well as the nucleobase. NOE difference
spectroscopy has also been applied. Thus, a mutual contact was
observed in 12 between one of the methyl groups of the aceto-
nide and the H6 of the nucleobase. A similar contact was not
observed for 15, and the different configurations of C6Ј and C7Ј
in 11 compared to 4 are hereby further confirmed.
In order to obtain bicyclic nucleoside derivatives more
suitable for oligodeoxynucleotide sequences we decided to
study also the 2Ј-deoxygenated bicyclic nucleoside analogue as
a substrate for a dihydroxylation reaction. We have earlier
reported a preliminary deoxygenation strategy,7 which is
hereby optimised (Scheme 4). Thus, 2 was converted to an
imidazolethiocarboxylate derivative 16 in 84% yield, and
subsequently, a careful treatment with standard Barton–
McCombee conditions afforded the product 17 in 61% yield as
well as 32% of the starting alcohol material 2, i.e. a combined
yield over the two steps of 74% based on the recovery of
starting material. When 17 was treated with osmium tetraoxide,
the expected product 18 was obtained in a moderate 34% yield
Scheme 3 Reagents and conditions: i, OsO4, NMO, THF, H2O, 60 ЊC,
86%; ii, Ac2O, pyridine, 76%; iii, 80% AcOH; iv, Ac2O, pyridine, 79%
(2 steps); v, thymine, N,O-bis(trimethylsilyl)acetamide, TMS–OTf,
CH3CN, 65%; vi, NaOCH3, CH3OH, 95%; vii, (CH3O)2C(CH3)2,
p-TsOH, acetone, 66% 12, 70% 15; viii, MsCl, pyridine, 80%; ix, NaOH,
EtOH, H2O, 0%.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 8 1 1 – 8 1 6
812

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excludes the possibility of a Cieplak effect from this C–H bond
and, thereby, only dihydroxylation from the less hindered face
is observed. The same conformation seems very likely for both
17 and 3, as C2Ј-endo conformations should be even more
favoured due to the lack of, or opposite direction of, the O2Ј–
O4Ј gauche effect, respectively. A simple steric hindrance from
the nucleobase, however, cannot be excluded as the explanation
for the preferred stereoselectivity of the dihydroxylations on 2,
3 and 17. Nevertheless, the reason why dihydroxylation of the
nucleobase is seen with 2, to a lesser degree with 17 and not at
all with 3 might be found in smaller conformational and stereo-
electronic differences.
Conclusions
Three out of the four possible diastereomeric trihydroxylated
3Ј,5Ј-di-O-protected bicyclic nucleoside derivatives have been
obtained in addition to one of the two possible 2Ј-deoxygenated
analogues. We expect these nucleosides to be readily depro-
tected and to be used as conformationally restricted nucleoside
models in structure–activity studies on nucleoside/nucleotide
accepting enzymes and receptors or as building blocks in the
development of DNA and RNA recognising nucleic acid
analogues.
Scheme 4 Reagents and conditions: i, (Im)2CS, CH3CN, toluene, 85%;
ii, AIBN, Bu3SnH, CH3CN, 61%; iii, OsO4, NMO, THF, H2O, 34%; iv,
H2, Pd(OH)2/C, EtOH, 75%. Im = N-imidazolyl.
in addition to 17% starting material and trace amounts of the
bis-dihydroxylated product. The 1H NMR data of 18 (Table 1),
especially the coupling constants 3JH4ЈH5Ј and 3JH5ЈH6Ј, confirmed
the configuration of this compound and thereby the dihydroxy-
lation from the convex face of the bicyclic system. Further-
more, NOE difference spectra revealed strong mutual contacts
between H2Љ and H6Ј/H7Ј.
As a last outcome of the significant optimisation of the
2Ј-deoxygenation strategy, the synthesis of the bicycloDNA
thymidine monomer should now be comparable to the parent
synthesis developed by Tarköy and Leumann.4a Thus, com-
pound 17 was readily deprotected and hydrogenated to give this
target compound 19 in 74% yield (Scheme 4) and the present
RCM-based synthetic strategy7 afforded hereby 19 in 16 steps
and 20% overall yield from diacetone--glucose avoiding
troublesome separations of isomers, whereas in the original
strategy,4a 19 was obtained in 11 steps but only 6% overall yield
from another simple starting material and after separation of
isomers.
Experimental
All commercial reagents were used as supplied. All reactions
were performed under an atmosphere of nitrogen. Column
chromatography was carried out on glass columns using Silica
gel 60 (0.040–0.063 mm). NMR spectra were obtained on a
Varian Gemini 2000 spectrometer. FAB mass spectra were
recorded in positive ion mode on a Kratos MS50TC spectro-
meter, and MALDI mass spectra were recorded on an Ionspec
Ultima Fourier Transform mass spectrometer. Assignments of
NMR spectra when given are based on 2D spectra and follow
standard carbohydrate and nucleoside style; i.e. the carbon
atom next to a nucleobase is assigned C-1Ј; the numbering
continues in the additional ring following C-5 (or C-5Ј) to C-6
(C-6Ј) and C-7 (C-7Ј) as depicted in Scheme 1. However,
compound names for bi- and tricyclic compounds are given
according to the von Baeyer nomenclature.
(1R,3R,4R,5R,6R,7R,8R)-5,8-Dibenzyloxy-4,6,7-trihydroxy-3-
(thymin-1-yl)-2-oxabicyclo[3.3.0]octane 5
Discussion
To a solution of 2 (125 mg, 0.27 mmol) in THF (4 cm3) and
H2O (4 cm3) was added N-methylmorpholine N-oxide (145 mg,
1.24 mmol) and osmium tetraoxide (2.5% in t-BuOH, 0.4 cm3,
0.046 mmol) and the mixture was stirred at room temperature
for 9 days. An aqueous solution of Na2S2O5 (20%, 10 cm3) was
added and the solvent was partly evaporated under reduced
pressure. The mixture was extracted with ethyl acetate (3 ×
20 cm3) and the organic phase was washed with brine, dried
(MgSO4) and evaporated under reduced pressure. The residue
was purified by silica gel column chromatography (3–8%
MeOH in CH2Cl2) to give the product 5 (44 mg, 33%) as well
as the by-product 6 (32 mg, 22%) and the starting material 2
(13 mg, 10%). 5: δH (300 MHz; DMSO-d6; Me4Si) 1.62 (3H, s,
CH3), 3.89 (1H, dd, J 7.7, 6.9, H-5Ј), 3.96 (1H, dd, J 5.4, 8.8,
H-2Ј), 4.09 (1H, dd, J 4.3, 4.2, H-7Ј), 4.22 (1H, m, H-6Ј), 4.42
(1H, d, J 6.9, H-4Ј), 4.49 (1H, d, J 11.7, CH2Ph), 4.59 (1H, d,
J 11.7, CH2Ph), 4.90 (1H, d, J 11.6, CH2Ph), 5.05 (1H, d, J 4.3,
OH-7Ј), 5.09 (1H, d, J 11.6, CH2Ph), 5.13 (1H, d, J 6.6, OH-6Ј),
5.78 (1H, d, J 5.4, OH-2Ј), 5.97 (1H, d, J 8.8, H-1Ј), 7.40–7.25
(10H, m, Ph), 7.54 (1H, s, H-6), 11.41 (1H, s, NH); m/z (FAB)
519 (M ϩ Na), 497 (M ϩ H). 6: δH (300 MHz; DMSO-d6;
Me4Si) 1.13 (3H, s, CH3), 3.85 (1H, t, J 7.7, H-5Ј), 3.96 (1H, dd,
J 5.8, 8.5, H-2Ј), 4.01 (1H, m, H-7Ј), 4.08 (1H, m, H-6Ј), 4.31
(1H, d, J 6.6, H-4Ј), 4.45 (1H, d, J 12.1, CH2Ph), 4.62 (1H, d,
The dihydroxylation results described herein deserves some dis-
cussion. Several factors determine the stereospecific outcome
of dihydroxylations such as steric or stereoelectronic factors
in the substrates e.g. from allylic alkoxy groups.11 It has been
demonstrated that the conformational behaviour of cyclo-
pentene substrates strongly influences the stereoselectivity of
dihydroxylations.12 Thus, dihydroxylation from the more
hindered face cis to the 3- and 5-substituents of cyclopentenes
has been observed12,13 and explained by the Cieplak effect,14 i.e.
stabilisation of the more hindered transition state by electron
donation from an antiperiplanar C–H bond to the antibonding
orbital of the incipient bond.12 Thus, if 1 adopts a conform-
ation allowing the H5 to be pseudoaxial, i.e. a 4-endo-con-
formation of the cyclopentene corresponding to a C3-endo
conformation of the furanose moiety, the Cieplak activity
explains the observed exclusive dihydroxylation from the more
hindered face of the double bond. This conformation seems
reasonable though not to be proved from the experimental
coupling constants.7 On the other hand, the bicyclic nucleoside
2 (or rather its debenzylated analogue) has been demonstrated
from molecular modelling to adopt the opposite C2Ј-endo con-
formation,7 with the cyclopentene ring in a 4Ј-exo conformation
and hereby the H5Ј in a pseudoequatorial position. This
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 8 1 1 – 8 1 6
813

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J 12.1, CH2Ph), 4.76 (1H, d, J 4.9, OH-6), 4.89 (1H, d, J 11.8,
CH2Ph), 5.02 (1H, d, J 3.0, OH-7Ј), 5.05 (1H, d, J 4.7, OH-6Ј),
5.06 (1H, d, J 11.8, CH2Ph), 5.47 (1H, d, J 5.8, OH-2Ј), 5.52
(1H, s, OH-5), 5.81 (1H, d, J 8.5, H-1Ј), 6.43 (1H, d, J 4.9, H-6),
7.21–7.44 (10H, m, Ph), 10.37 (1H, s, NH).
without further purification in the next step: m/z (FAB) 497
(M Ϫ OAc).
(1R,3R,4R,5R,6S,7S,8R)-4,6,7-Triacetyloxy-5,8-dibenzyloxy-
3-(thymin-1-yl)-2-oxabicyclo[3.3.0]octane 10
A mixture of 9 (115 mg, 0.207 mmol) and thymine (78 mg,
0.619 mmol) was dried and dissolved in anhydrous MeCN
(5 cm3). The mixture was treated with N,O-bis(trimethylsilyl)-
acetamide (0.25 cm3, 1.02 mmol) and stirred under reflux for
15 min. After cooling of the mixture to 0 ЊC, trimethylsilyl
triflate (0.1 cm3, 0.55 mmol) was added dropwise and the
solution was stirred at 50 ЊC for 16 h. The reaction mixture was
quenched with an ice-cold saturated aqueous solution of
NaHCO3 (10 cm3) and extracted with CH2Cl2 (3 × 10 cm3). The
combined organic phases were dried (MgSO4) and evaporated
under reduced pressure. The residue was purified by silica gel
column chromatography (1–2% MeOH in CH2Cl2) to give the
product 10 (84 mg, 65%) as a white solid material which was
used without further purification in the next step: δH (300 MHz;
CDCl3; Me4Si) 1.73 (1H, d, J 1.2, CH3), 2.05 (3H, s, COCH3),
2.06 (3H, s, COCH3), 2.17 (3H, s, COCH3), 4.09 (1H, dt, J 5.7,
4.6, H-5Ј), 4.61 (1H, d, J 5.7, H-4Ј), 4.72–4.61 (3H, m, CH2Ph),
4.81 (1H, d, J 11.2, CH2Ph), 5.44 (1H, d, J 5.1, H-7Ј), 5.62 (1H,
dd, J 5.1, 4.6, H-6Ј), 5.97 (1H, d, J 8.3, H-2Ј), 6.39 (1H, d, J 8.3,
H-1Ј), 7.35–7.24 (10H, m, CH2Ph), 7.58 (1H, d, J 1.2, H-6), 9.24
(1H, br s, NH); m/z (FAB) 623 (M ϩ H).
(1R,3R,7R,8R,9S,10S,11R)-8,11-Dibenzyloxy-9,10-dihydroxy-
5,5-dimethyl-2,4,6-trioxatricyclo[6.3.0.03.7]undecane 7
To a solution of 1 (650 mg, 1.65 mmol) in THF (15 cm3) and
H2O (15 cm3) was added N-methylmorpholine N-oxide (386
mg, 3.29 mmol) and osmium tetraoxide (2,5% in t-BuOH,
0.3 cm3, 0.035 mmol) and the mixture was stirred at 60 ЊC for
4 h. After cooling to room temperature an aqueous solution of
Na2S2O5 (20%, 30 cm3) was added and the solvent was partly
evaporated under reduced pressure. The mixture was extracted
with ethyl acetate (3 × 40 cm3) and the organic phase was
washed with brine, dried (MgSO4) and evaporated under
reduced pressure. The residue was purified by silica gel column
chromatography (0–2% MeOH in CH2Cl2) to give the product 7
(606 mg, 86%) as a white solid material: δH (300 MHz; CDCl3;
Me4Si) 1.40 (3H, s, CH3), 1.59 (3H, s, CH3), 3.12 (1H, d, J 6.6,
OH-7), 3.33 (1H, d, J 7.0, OH-6), 3.97 (1H, dd, J 5.3, 3.7, H-5),
4.21 (1H, dd, J 6.6, 6.2, H-7), 4.31 (1H, m, H-6), 4.57 (1H, d,
J 3.7, H-4), 4.58 (1H, d, J 10.8, CH2Ph), 4.65 (1H, d, J 11.8,
CH2Ph), 4.71 (1H, d, J 11.8, CH2Ph), 4.78 (1H, d, J 10.8,
CH2Ph), 5.07 (1H, d, J 3.9, H-2), 5.82 (1H, d, J 3.9, H-1),
7.40–7.24 (10H, m, Ph); δC (75 MHz; CDCl3; Me4Si) 137.8,
137.1 (2 × Ph), 128.5, 128.3, 128.1, 128.0, 127.7, 127.7 (Ph),
113.2 (C(CH3)2), 107.1 (C-1), 94.4 (C-3), 85.2, 79.2, 77.0, 74.9,
72.0, 71.4, 67.5 (C-2, C-4, C-5, C-6, C-7, 2 × CH2Ph), 27.2, 27.0
(2 × CH3); m/z (FAB) 451 (M ϩ Na), 429 (M ϩ H).
(1R,3R,4R,5R,6S,7S,8R)-5,8-Dibenzyloxy-4,6,7-trihydroxy-3-
(thymin-1-yl)-2-oxabicyclo[3.3.0]octane 11
To a solution of 10 (70 mg, 0.113 mmol) in anhydrous methanol
(3 cm3) was added sodium methoxide (55 mg, 1.02 mmol)
and the mixture was stirred at room temperature for 16 h. The
reaction mixture was neutralised with aqueous HCl and
extracted with CH2Cl2 (3 × 10 cm3). The combined extracts
were washed with a saturated aqueous solution of NaHCO3
(2 × 10 cm3) and then dried (MgSO4). The solvent was evapor-
ated under reduced pressure and the residue was purified by
silica gel column chromatography (3–7% MeOH in CH2Cl2)
to give the product 11 (53 mg, 95%) as a white solid material:
δH (300 MHz; DMSO-d6; Me4Si) 1.71 (1H, d, J 1.1, CH3), 3.77
(1H, dd, J 5.5, 3.8, H-5Ј), 4.12 (2H, m, H-6Ј, H-7Ј), 4.38 (1H, d,
J 5.5, H-4Ј), 4.58 (2H, s, CH2Ph), 4.65 (1H, dd, J 8.3, 6.6, H-2Ј),
4.78 (1H, d, J 11.9, CH2Ph), 4.96 (1H, d, J 11.9, CH2Ph), 5.24
(1H, d, J 5.5, OH-7Ј), 5.31 (1H, d, J 2.5, OH-6Ј), 5.41 (1H, d,
J 6.6, OH-2Ј), 5.92 (1H, d, J 8.3, H-1Ј), 7.24–7.35 (10H, m, Ph),
7.75 (1H, d, J 1.1, H-6), 11.34 (1H, br s, NH); δC (75 MHz;
DMSO-d6; Me4Si) 163.6 (C-4), 150.9 (C-2), 139.5, 138.3, 137.1
(C-6, 2 × Ph), 128.7, 128.5, 128.4, 128.1, 128.0, 127.7 (2 × Ph),
109.4 (C-5), 89.4, 87.6, 83.1, 75.8, 74.1, 73.2, 73.1, 70.3, 66.9
(C-1Ј, C-2Ј, C-3Ј, C-4Ј, C-5Ј, C-6Ј, C-7Ј, 2 × CH2Ph), 12.3
(CH3).
(1R,3R,7R,8R,9S,10S,11R)-9,10-Diacetyloxy-8,11-dibenzy-
loxy-5,5-dimethyl-2,4,6-trioxatricyclo[6.3.0.03.7]undecane 8
To a solution of 7 (303 mg, 0.707 mmol) in anhydrous pyridine
(15 cm3) was added acetic anhydride (5 cm3) and the mixture
was stirred at room temperature for 16 h. The mixture was
diluted with H2O (15 cm3) and extracted with CH2Cl2 (3 ×
30 cm3). The organic phase was dried (MgSO4) and evaporated
under reduced pressure. The residue was purified by silica gel
column chromatography (1% MeOH in CH2Cl2) to give the
product 8 (275 mg, 76%) as a colourless oil: δH (300 MHz;
CDCl3; Me4Si) 1.48 (3H, s, CH3), 1.58 (3H, s, CH3), 2.13 (3H, s,
COCH3), 2.15 (3H, s, COCH3), 3.97 (1H, t, J 4.6, H-5), 4.33
(1H, d, J 10.4, CH2Ph), 4.62 (2H, s, CH2Ph), 4.63 (1H, d, J 4.6,
H-4), 4.69 (1H, d, J 10.4, CH2Ph), 5.02 (1H, d, J 4.0, H-2), 5.43
(1H, d, J 4.9, H-7), 5.60 (1H, m, H-6), 6.12 (1H, d, J 4.0, H-1),
7.40–7.26 (10H, m, Ph); δC (75 MHz; CDCl3; Me4Si) 169.8,
168.9 (C᎐O), 137.3, 137.2 (2 × Ph), 128.5, 128.3, 128.0, 127.9,
᎐
127.9, 127.7 (Ph), 113.9 (C(CH3)2), 108.5 (C-1), 90.8, 86.5, 80.6,
74.6, 74.6, 71.9, 71.7, 71.4, 67.9 (C-2, C-3, C-4, C-5, C-6, C-7,
2 × CH2Ph), 27.9, 27.3 (2 × CH3), 20.7, 20.6 (COCH3);
m/z (FAB) 512 (M ϩ H), 453 (M Ϫ OAc).
(1R,2S,6S,7R,8R,10R,11R)-1,7-Dibenzyloxy-4,4-dimethyl-
11-methylsulfonyloxy-10-(thymin-1-yl)-3,5,9-trioxatricyclo-
[6.3.0.02.6]undecane 13
(3RS,1R,4R,5R,6S,7S,8R)-3,4,6,7-Tetraacetyloxy-5,8-dibenzy-
loxy-2-oxabicyclo[3.3.0]octane 9
Compound 11 (66 mg, 0.133 mmol) was dissolved in acetone
(4 cm3) and 2,2-dimethoxypropane (0.4 cm3, 3.25 mmol), and
p-TsOHؒH2O (5 mg, 0.026 mmol) were added. The mixture was
stirred at room temperature for 16 h and the solvent was evap-
orated under reduced pressure. The residue was redissolved
in CH2Cl2 and washed with a saturated aqueous solution of
NaHCO3 (2 × 5 cm3) and then dried (MgSO4). The solvent was
removed under reduced pressure and the residue was purified
by silica gel column chromatography (1–2% MeOH in CH2Cl2)
to give the intermediate product 12 (47 mg, 66%) as a colourless
oil. The compound 12 (45 mg, 0.084 mmol) was dissolved
in anhydrous pyridine (2 cm3) and methanesulfonyl chloride
(0.05 cm3, 0.65 mmol) was added dropwise at 0 ЊC. The reaction
Compound 8 (234 mg, 0.457 mmol) was dissolved in 80% acetic
acid (10 cm3) and stirred at 90 ЊC for 16 h. The solvent was
evaporated under reduced pressure and the residue was co-
evaporated with anhydrous ethanol (3 × 10 cm3), toluene (3 ×
10 cm3) and pyridine (10 cm3). The crude intermediate was
redissolved in anhydrous pyridine (5 cm3) and acetic anhydride
(1 cm3) was added. After stirring for 16 h the mixture was
diluted with H2O (10 cm3) and extracted with CH2Cl2 (3 ×
10 cm3). The organic phase was dried (MgSO4) and evaporated
under reduced pressure. The residue was purified by silica gel
column chromatography (0–1% MeOH in CH2Cl2) to give the
product 9 (200 mg, 79%) as a colourless oil which was used
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 8 1 1 – 8 1 6
814

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mixture was stirred for 16 h at room temperature, quenched
with H2O (3 cm3) and extracted with CH2Cl2 (3 × 10 cm3). The
combined extracts were washed with a saturated aqueous solu-
tion of NaHCO3 (2 × 10 cm3) and then dried (MgSO4). The
solvent was evaporated under reduced pressure and the residue
was co-evaporated with toluene (2 × 10 cm3) and then purified
by silica gel column chromatography (1–2% MeOH in CH2Cl2)
to give the product 13 (41 mg, 62%) as a colourless oil which
was used without further purification in the next step: δH (300
MHz; CDCl3; Me4Si) 1.40 (1H, s, CH3), 1.64 (1H, s, CH3), 1.70
(1H, s, CH3), 3.00 (1H, s, SO2CH3), 3.98 (1H, t, J 5.5, H-5Ј),
4.54 (1H, d, J 5.5, H-4Ј), 4.70–4.93 (6H, m, H-6Ј, H-7Ј, 2 ×
CH2Ph), 5.63 (1H, d, J 7.8, H-2Ј), 6.48 (1H, d, J 7.8, H-1Ј),
7.24–7.47 (10H, m, Ph), 7.74 (1H, s, H-6), 8.42 (1H, s, NH); m/z
(FAB) 615 (M ϩ H).
reflux at 80 ЊC for 22 h and then cooled. The solvent was evap-
orated under reduced pressure and the residue was purified by
silica gel column chromatography (0–2% MeOH in CH2Cl2)
to give the product 17 (220 mg, 61%) as a white solid as well
as the starting alcohol 2 (119 mg, 32%). 17: mp 132–134 ЊC;
δH (300 MHz; CDCl3; Me4Si) 1.58 (3H, s, CH3), 2.17 (1H, dd,
J 7.1, 13.6, H-2Јb), 2.80 (1H, dd, J 6.2, 13.6, H-2Јa), 4.37 (1H, d,
J 11.3, CH2Ph), 4.44 (1H, d, J 11.3, CH2Ph), 4.57 (1H, d, J 11.2,
CH2Ph), 4.64 (2H, br s, H-4Ј, H-5Ј), 4.77 (1H, d, J 11.2,
CH2Ph), 6.04 (2H, br s, H-6Ј, H-7Ј), 6.48 (1H, t, J 6.4, H-1Ј),
7.28–7.41 (10H, m, 2 × Ph), 7.73 (1H, s, H-6), 9.18 (1H, br s,
NH), δC (75 MHz; CDCl3; Me4Si) 164.0 (C-4), 150.4 (C-2),
137.8, 137.6 (2 × Ph), 136.3 (C-6), 135.0, 134.5 (C-6Ј, C-7Ј),
128.5, 128.4, 128.0, 127.8, 127.3 (2 × Ph), 110.5 (C-5), 96.7
(C-3Ј), 88.0 (C-1Ј), 84.0, 81.0 (C-4Ј, C-5Ј), 72.6 (CH2Ph),
67.1 (CH2Ph), 44.1 (C-2Ј), 12.1 (CH3); m/z (MALDI) 469
(M ϩ Naϩ).
(1R,2R,6R,7R,8R,10R,11S )-1,7-Dibenzyloxy-4,4-dimethyl-11-
hydroxy-10-(thymin-1-yl)-3,5,9-trioxatricyclo[6.3.0.02.6]-
undecane 15
(1R,3R,5R,6R,7R,8R)-5,8-Dibenzyloxy-6,7-dihydroxy-3-
(thymin-1-yl)-2-oxabicyclo[3.3.0]octane 18
Same procedure as for the synthesis of 12 using 4 (32 mg,
0.0645 mmol), 2,2-dimethoxypropane (0.3 cm3, 0.24 mmol),
p-TsOHؒH2O (5 mg, 0,026 mmol) and acetone (4 cm3). Purifi-
cation by silica gel column chromatography (1–2% MeOH
in CH2Cl2) gave the product 15 (24 mg, 70%) as a colourless oil:
δH (300 MHz; CDCl3; Me4Si) 1.40 (1H, s, CH3), 1.62 (1H, s,
CH3), 1.72 (1H, s, CH3), 4.03 (1H, dd, J 8.1, 2.6, H-5Ј), 4.44
(1H, d, J 8.1, H-4Ј), 4.57–4.80 (7H, m, H-2Ј, H-6Ј, H-7Ј, 2 ×
CH2Ph), 5.27 (1H, br s, OH-2Ј), 6.12 (1H, d, J 2.6, H-1Ј), 7.27–
7.48 (10H, m, Ph), 7.78 (1H, s, H-6), 11.21 (1H, br s, NH); m/z
(FAB) 537 (M ϩ H).
Compound 7 (70 mg, 0.16 mmol) was dissolved in a 1 : 1 mix-
ture of THF and water (3 cm3) and added N-methylmorpholine
N-oxide (19 mg, 0.19 mmol) and osmium tetraoxide (2.5% in
t-BuOH, 79 µL, 6.3 µmol) and the reaction mixture was stirred
at 50 ЊC for 3 h. An aqueous solution of Na2S2O5 (5%, 1 cm3)
was added and the mixture was stirred for another 30 min. The
solvent was partly evaporated under reduced pressure and the
residue was extracted with EtOAc (4 × 5 cm3). The combined
organic phases were dried (Na2SO4) and then evaporated under
reduced pressure. The residue was purified by silica gel column
chrmatography (1–2% MeOH in CH2Cl2) to give the product 18
(26 mg, 34%) as a white solid as well as starting material 17
(12 mg, 17%). 18: mp 182–183 ЊC; δH (300 MHz; DMSO-d6;
Me4Si) 1.53 (3H, s, CH3), 1.82 (1H, dd, J 9.6, 14.3, H-2Јb), 2.71
(1H, dd, J 5.3, 14.3, H-2Јa), 3.92 (1H, dd, J 7.1, 8.4, H-5Ј), 3.98
(1H, dd, J 4.1, 4.7, H-7Ј), 4.19 (1H, m, H-6Ј), 4.37 (1H, d, J 7.1,
H-4Ј), 4.51 (1H, d, J 1.7, CH2Ph), 4.59 (1H, d, J 11.7, CH2Ph),
4.63 (1H, d, J 11.7, CH2Ph), 4.70 (1H, d, J 11.7, CH2Ph), 5.03
(1H, d, J 4.7, OH-7Ј), 5.13 (1H, d, J 7.0, OH-6Ј), 6.17 (1H, dd,
J 5.3, 9.6, H-1Ј), 7.25–7.42 (10H, m, 2 × Ph), 7.70 (1H, s, H-6),
11.35 (1H, br s, NH); δC (75 MHz; DMSO-d6; Me4Si) 163.6
(C-4), 150.3 (C-2), 138.9, 138.3 (2 × Ph), 135.7 (C-6), 128.1,
128.1, 127.7, 127.4, 127.2 (2 × Ph), 109.5 (C-5), 90.4 (C-3Ј), 84.7
(C-1Ј), 83.3 (C-4Ј), 80.7 (C-5Ј), 75.0 (C-6Ј), 71.3 (C-7Ј), 71.0
(CH2Ph), 65.7 (CH2Ph), 38.5 (C-2Ј), 12.0 (CH3), HR MALDI
FT-MS m/z 503.1771. Calc. 503.1788 (M ϩ Naϩ).
(1R,3R,4R,5R,8R)-5,8-Dibenzyloxy-4-(N-imidazolylthiocarb-
onyloxy)-3-(thymin-1-yl)-2-oxabicyclo[3.3.0]oct-6-ene 16
The bicyclic nucleoside 2 (529 mg, 1.14 mmol) was coevapo-
rated with anhydrous CH3CN (2 × 5 cm3) and dissolved in a
1 : 1 mixture of CH3CN and toluene (8 cm3). Thio-
carbonyldiimidazole (407 mg, 2.28 mmol) was added, and the
mixture was stirred under at 80 ЊC for 3 h. The mixture was
cooled and the solvent evaporated under reduced pressure. The
residue was dissolved in 15 cm3 DCM and washed with a 5%
aqueous solution of HCl (2 × 15 cm3) and water until neutrality
of the aqueous phase (4 × 15 cm3) and then washed with brine
(2 × 30 cm3). The organic phase was dried (Na2SO4) and con-
centrated under redused pressure. The residue was purified by
silica gel column chromatography (0–2% MeOH in CH2Cl2) to
give the product 16 (549 mg, 84%) as a white solid material
which was used without further purification in the next step: mp
78–80 ЊC; δH (300 MHz; CDCl3; Me4Si) 1.59 (3H, s, CH3), 4.37
(1H, d, J 11.7, CH2Ph), 4.46 (1H, d, J 11.7, CH2Ph), 4.60–4.66
(2H, m, CH2Ph, H-5Ј), 4.74–4.78 (2H, m, CH2Ph, H-4Ј), 5.78
(1H, d, J 6.3, H-2Ј), 6.22 (1H, d, J 6.2, H-7Ј), 6.27 (1H, d, J 6.2,
(1R,3R,5R,8R)-5,8-Dihydroxy-3-(thymin-1-yl)-2-oxabicyclo-
[3.3.0]octane 194a
Compound 17 (29 mg, 0.066 mmol) was dissolved in anhydrous
methanol (1 cm3) and added Pd(OH)2–C (16 mg). The reaction
mixture was bubbled with H2 for 10 min and then stirred under
an atmosphere of H2 for 20 h. The mixture was filtered through
a layer of celite and then evaporated under reduced pressure.
The residue was purified by silica gel column chromatography
(2–5% MeOH in CH2Cl2) to give the product 19 (14 mg,
75%) as a white solid: All data were in accordance with the
literature.4a
H-6Ј), 6.65 (1H, d, J 6.3, H-1Ј), 7.02 (1H, s, C᎐CH(Im)), 7.13–
᎐
7.40 (10H, m, 2 × Ph), 7.59 (1H, s), 7.62 (1H, s) (C᎐CH(Im),
᎐
H-6), 8.35 (1H, s, N᎐CH(Im)), 8.38 (1H, br s, NH); δ (75 MHz;
᎐
C
CDCl ; Me Si) 183.2 (C᎐S), 163.1 (C-4), 150.2 (C-2), 137.4,
᎐
3
4
137.3, 134.9, 133.5, 131.2, 128.7, 128.4, 128.3, 128.2, 127.9,
127.9 (2 × Ph, C-6, C-6Ј, C-7Ј, C(Im)), 126.8 (C(Im)), 118.1
(C(Im)), 111.9 (C-5Ј), 93.6 (C-3Ј), 88.6, 84.8, 83.8, 80.1,
72.6, 67.2 (2 × CH2Ph, C-1Ј, C-2Ј, C-4Ј, C-5Ј), 12.1 (CH3);
m/z (MALDI) 595 (M ϩ Naϩ).
References
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(1R,3R,5R,8R)-5,8-Dibenzyloxy-3-(thymin-1-yl)-2-oxabicyclo-
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The thiocarboxylate 16 (460 mg, 0.80 mmol) was coevaporated
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