Synthesis of fluorobenzene and benzimidazole nucleic-acid analogues and their influence on stability of RNA duplexes

Article abstract of DOI:10.1002/1522-2675(20000809)83:8<1791::AID-HLCA1791>3.0.CO;2-K

Six different ribonucleoside phosphoramidites with fluorobenzenes or fluorobenzimidazoles as base analogues, one abasic site, and inosine were synthesized and incorporated into oligoribonucleotides. The oligomers were investigated by means of UV and CD spectroscopy to assess the contribution of H-bonding, base stacking, and solvation to the stability of the RNA duplex. CD Spectra show that the incorporation of modified nucleosides does not lead to changes in the structure of RNA. The T(m) differences determined are based on changes in base stacking and solvation. Individual contributions of base stacking and solvation of the modified nucleosides could be determined. In fluorobenzene·fluorobenzimidazole-modified base pairs, a duplex-stabilizing force was found that points to a weak F ··· H H-bond.

Full text of DOI:10.1002/1522-2675(20000809)83:8<1791::AID-HLCA1791>3.0.CO;2-K

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Synthesis of Fluorobenzene and Benzimidazole Nucleic-Acid Analogues
and Their Influence on Stability of RNA Duplexes
by Jörg Parsch and Joachim W. Engels*
Institut für Organische Chemie, Johann Wolfgang Goethe-Universität, Marie-Curie-Strasse 11,
D-60439 Frankfurt am Main
(e-mail: Joachim.Engels@chemie.uni-frankfurt.de)
Dedicated to Prof. Dr. Albert Eschenmoser on the occasion of his 75th birthday
Six different ribonucleoside phosphoramidites with fluorobenzenes or fluorobenzimidazoles as base
analogues, one abasic site, and inosine were synthesized and incorporated into oligoribonucleotides. The
oligomers were investigated by means of UVand CD spectroscopy to assess the contribution of H-bonding, base
stacking, and solvation to the stability of the RNA duplex. CD Spectra show that the incorporation of modified
nucleosides does not lead to changes in the structure of RNA. The Tm differences determined are based on
changes in base stacking and solvation. Individual contributions of base stacking and solvation of the modified
nucleosides could be determined. In fluorobenzene ´ fluorobenzimidazole-modified base pairs, a duplex-
stabilizing force was found that points to a weak F ´´´ H H-bond.
1. Introduction. ± RNA shows a great variety of secondary structures, like double
helices, hairpin loops, bulges, and internal loops. All these secondary structures are
stabilized mainly by interactions of the bases with their neighbours [1][2]. The most
important interaction in the stability of RNA is H-bonding. In addition, recent studies
of RNA helices [3] suggest that stacking of the aromatic bases plays a significant role in
the stability of duplex RNA. The origins of base-stacking stability [4] in nucleic acids
are still discussed controversially, and there are relatively little experimental data that
can help to settle the question of the origin of this phenomenon.
In natural RNA, bases are limited to the four predominant structures U, C, A, and
G, so the number of compounds that can be used to investigate the parameters of base
pairing and base stacking is limited. To address this problem, we decided to synthesize
some new nucleic-acid analogues in which the nucleobases are replaced by fluoro-
benzimidazoles or fluorobenzenes. Thus, we prepared eight protected phosphorami-
dites, six of them with base modifications, one abasic site, and one derived from inosine
(Fig. 1), i.e., the monomer building blocks 1 for inosine 2, for 1'-deoxy-1'-(4-fluoro-1H-
benzimidazol-1-yl)-b-d-ribofuranose 3 for 1'-deoxy-1'-(4,6-difluoro-1H-benzimidazol-
1-yl)-b-d-ribofuranose, 4 for 1'-deoxy-1'-(4-fluorophenyl)-b-d-ribofuranose, 5 for
1'-deoxy-1'-(3-fluorophenyl)-b-d-ribofuranose, 6 for 1'-deoxy-1'-(2-fluorophenyl)-b-
d-ribofuranose, 7 for 1'-deoxy-1'-(2,4-difluorophenyl)-b-d-ribofuranose, and 8 for
1-deoxy-ribofuranose incorporation into oligoribonucleotides.
The parent nucleoside of 2 (i.e., 13) is an isostere of the natural inosine, and that of
7 (i.e., 31) is isosteric to uridine. The aromatic-ring moiety of 1 ± 7 was designed to be
the closest possible steric mimic of the natural nucleobases avoiding the presence of

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Fig. 1. Monomer building blocks 1 ± 8 for the incorporation into RNA oligonucleotides bysolid-phase synthesis
hydrophilic O- or N-containing groups [5]. The best isosteric replacement of the CˆO
functionality is the CÀF group, because of nearly identical bond lengths [6]. In the
parent nucleosides of 4, 5, and 6 (i.e. 28, 29, and 30, resp.), the natural bases are
substituted by monofluorobenzenes [7]. These compounds should show the influence
of F ´´´ H H-bonding in duplex RNA. The F-atom has been introduced in all three possible
positions on the benzene ring so that the influence of the F-position can be investigated
as well. It is very interesting to investigate the influence of the F ´´´ H H-bonding
because, in its crystal structure, 28 shows a very short CÀF ´´´ HÀC distance of 230 pm
[8]. This is significantly shorter than the sum of the van der Waals radii of the F- and

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H-atoms. In contrast, Schweitzer and Kool [9] have shown that 1',2'-dideoxy-1'-(2,4-difluoro-
tolyl)-b-d-ribofuranose, an isostere of the natural DNA base thymidine, develops no
H-bonds to other natural bases or base analogues in duplex DNA. To evaluate the
contribution of base-stacking effects to the stability of duplex RNA and the influence of
the F-atom, we also synthesized the phosphoramidite 8 of the parent abasic site 20.
2. Results and Discussion. ± 2.1. Chemical Syntheses. The synthesis of 1'-deoxy-1'-
(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (13) was performed by the glyco-
sylation procedure of Vorbrüggen [10] (Scheme 1). Refluxing 4-fluoro-1H-benzimid-
azole (9) [11] with N,O-bis(trimethylsilyl)acetamide and subsequent reaction of the
persilylated base with 1,2,3,5-tetra-O-acetyl-b-d-ribofuranose (10) in the presence of
the Lewis acid trimethylsilyl trifluoromethanesulfonate afforded the desired 2',3',5'-tri-
O-acetyl-1'-deoxy-1'-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (12) in 65%
yield. The undesired N3-isomer 11 was obtained in only 8% yield. In the case of 4,6-
difluoro-1H-benzimidazole (14) [12], the desired 2',3',5'-tri-O-acetyl-1'-deoxy-1'-(4,6-
difluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (16) was obtained in 67% yield,
besides 11% of the corresponding N3-isomer 15. Deprotection of the acetylated
nucleosides 12 and 16 in methanolic ammonia furnished 1'-deoxy-1'-(4-fluoro-1H-
benzimidazol-1-yl)-b-d-ribofuranose (13) and 1'-deoxy-1'-(4,6-difluoro-1H-benzimid-
azol-1-yl)-b-d-ribofuranose (17) in 89 and 94% yield, respectively.
Scheme 1. Synthesis of 1'-Deoxy-1'-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (13) and 1'-Deoxy-1'-
(4,6-difluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (17)
BSA ˆ N,O-bis(trimethylsilyl)acetamide, TMSOTf ˆ trimethylsilyl trifluoromethanesulfonate
The synthesis of 1-deoxy-d-ribofuranose (ˆ1,4-anhydro-d-ribitol; 20) was per-
formed according to Purdyet al . [13] (Scheme 2). Thus, 2,3,5-tri-O-benzylribofuranose
18 [14] was dehydroxylated to 19 (84%) by treatment with triethylsilane and BF3 ´
OEt2. Subsequent deprotection with 20% Pd(OH)2/C in the presence of cyclohexene as
the hydrogen donor [15] in EtOH gave 20 in 95% yield.

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Scheme 2. Synthesis of 1'-Deoxy-d-ribofuranose 20
The C-glycosylated nucleosides 24 ± 27 were all obtained in a similar way [16]
(Scheme 3). We describe the synthesis of nucleoside 24 as an example. Lithiation of 1-
bromo-4-fluorobenzene 22 with BuLi in THFat À788 followed by addition of 2,3,5-tri-
O-benzyl-d-ribono-1,4-lactone (21) [17] gave the intermediate lactol 23, which was
directly dehydroxylated with triethylsilane and BF3 ´ OEt2 to afford stereoselectively 24
in 75% yield. Interestingly, all monofluorobenzenes were glycosylated in THF as the
solvent of choice leading to the highest yield. In the case of the glycosylation of
difluorobenzene in THF, the protected nuceloside 27 was obtained in only 31% yield
[18]; by changing the solvent to Et2O, the yield increased to 84%. Deprotection of the
benzylated nucleoside 24 with 20% Pd(OH)2/C in the presence of cyclohexene in
EtOH yielded 28 (99%). Similarly, 29 ± 31 were obtained.
Scheme 3. Synthesis of Unprotected C-Nucleosides 28 ± 31
The conversions of the unprotected nucleosides 13, 17, and 28 ± 31 or the
unprotected sugar 20 to the corresponding phosphoramidites 2 ± 8 are very similar.
In the following, only the synthesis of phosphoramidite 4 is described (Scheme 4; for all
other phosphoramidites, see Exper. Part). The 5'-OH function of 28 was protected with
4,4'-dimethoxytrityl chloride ((MeO)2TrCl) in dry pyridine and Et3N to afford the 5'-O-
(4,4'-dimethoxytrityl) derivative 32 in 79% yield. To protect the 2'-OH function, 32 in
THF/pyridine 1:1 was treated with AgNO3 and a 1m (tert-butyl)dimethylsilyl chloride
(tBuMe2SiCl) ( ! 33 in 43% yield). In all fluorinated compounds, the tBuMe2Si at C(2')

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tended to move to the 3'-position in polar solvents, resulting often in low yields of the
desired 2'-protected nucleosides. The final phosphitylation of 33 gave 4 in 67% yield.
To prevent the isomerization of the silyl protecting group, sym-collidine and 1-methyl-
1H-imidazole were used instead of iPr2EtN [19].
Scheme 4. Synthesis of Phosphoramidites 2 ± 8
The inosine phosphoramidite 1 was synthesized according to literature procedures
[20].
2.2. Oligonucleotide Synthesis. The RNA oligomers were synthesized on an
Eppendorf-D300 ‡ synthesizer by phosphoramidite chemistry, with a coupling time
for the modified monomers of 12 min [21]. The fully protected dodecamers were
cleaved from the controlled-pore-glass (CPG) support with 32% aqueous NH3 solution
at 558 overnight. The 2'-silyl groups were deprotected with Et3N ´ 3 HF within 24 h at
room temperature [22]. The crude RNA oligomer was precipitated with BuOH at
À208, and the fully deprotected RNA was purified by means of anion-exchange HPLC
(NucleoPac-PA-100). The pure oligomer was subsequently desalted (Sephadex-G25).
All oligoribonucleotides were characterized by MALDI-TOF-MS, and the masses
obtained were in good agreement with the calculated ones.
2.3. UV Melting Curves. UV Melting profiles of the RNA duplexes were recorded in
a phosphate buffer containing NaCl (140 mmol, pH 7.0) at oligonucleotide concen-
trations of 2 mm for each strand at wavelengths of 260 and 274 nm [23]. Each melting
curve was determined twice. The temperature range was 0 ± 708 with a heating rate of
0.58/min. A lower heating rate of 0.28/min led to identical results. The thermodynamic

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data were extracted from the melting curves by means of a two-state model for the
transition from duplex to single strands [24].
2.4. Thermodynamic Data. The modified nucleosides were tested in a defined
RNA sequence. This A,U-rich sequence was chosen in order to be able to
compare our results with the those of Schweitzer and Kool [9], who incorporated
modified nucleosides in the corresponding DNA. In the 12-mer oligoribonucleotides
(5'-CUU UUC XUU CUU-3' paired with 3'-GAA AAG YAA GAA-5'), only one
position was modified, marked as X and Y, respectively. First, we measured only RNA
duplexes containing natural bases (Table 1, Entry1 ). The wobble base pair U ´ G shows
the highest Tm (38.68). This is 0.88 higher than the natural U ´ A base pair (Tm 37.88).
The U ´ C and U ´ U mismatches show nearly the same stability (Tm 30.48 and 30.18).
Table 1. Synthesized
Non-
or
Mono-Modified
Duplex
RNA
(5'-CUU UUC XUU CUU-3'
paired
with
3'-GAA AAG YAA GAA-5') and Their Thermodynamic Properties. For X, see Scheme 4. DH0, T´ DS0, and DG0, in kcal/mol
(T 298 K). Errors: Tm, Æ 0.28; DH0 and T´ DS0, Æ5%; DG0, Æ2%.
Entry X
Yˆ A
Yˆ C
Yˆ G
Yˆ U
Tm [8] DH0 T´ DS0 DG0 Tm [8] DH0 T´ DS0 DG0 Tm [8] DH0 T´ DS0 DG0 Tm [8] DH0 T´ DS0 DG0
1
2
3
4
5
6
7
U
37.8 87.8 75.9 11.9 30.4 84.5 74.8 9.8 38.6 83.0 71.1 11.9 30.1 89.5 79.8
9.7
9.1
8.4
8.4
8.7
31 (R ˆ 2,4-F2C6H3) 27.4 88.6 79.6 9.0 27.3 84.8 75.9 8.9 27.6 83.6 74.6 9.0 27.9 91.2 82.1
28 (R ˆ 4-FÀC6H4) 23.8 81.6 73.7 7.9 24.1 83.0 75.0 8.0 24.2 80.2 72.2 8.0 25.6 85.8 77.4
29 (R ˆ 3-FÀC6H4) 24.7 79.3 71.1 8.2 25.0 82.6 74.4 8.2 25.0 80.0 71.8 8.2 25.7 84.1 75.7
30 (R ˆ 2-FÀC6H4) 27.3 89.9 81.0 8.9 25.1 83.3 75.0 8.3 27.4 91.2 82.2 9.0 26.5 87.1 78.4
I
31.2 91.8 81.7 10.1 41.7 97.9 84.5 13.4 31.7 89.7 79.5 10.2 34.2 91.1 80.1 11.0
28.0 86.7 77.6 9.1 27.5 81.8 72.9 8.9 28.7 84.1 74.8 9.3 28.5 85.8 76.6
13 (R ˆ 4-fluoro-
9.2
9.5
6.6
benzimidazol-1-yl)
8
9
17 (R ˆ 4,6-difluoro- 28.4 81.4 72.2 9.2 28.7 81.5 72.3 9.2 29.4 84.8 75.3 9.5 29.3 85.3 75.8
benzimidazol-1-yl)
20 (R ˆ H; abasic
20.6 67.2 60.0 7.2 18.6 51.3 44.6 6.7 20.9 65.1 57.8 7.3 18.2 58.9 52.3
site)
In a second series, we measured the fluorobenzene nucleosides 28 ± 31 paired
against natural bases (Table 1, Entries 2 ± 5). In these cases, all Tm values are lower than
those for the natural bases. Possible explanations for these findings are: i) there are no
H-bonds between the modified and the natural bases and ii) the modified bases are less
solvated by H2O molecules. The absence of differences in Tm values by pairing for
example 2,4-difluorobenzene against a purine or a pyrimidine indicates that there are
no H-bonds; Table 1, Entry2 , shows that all Tm values are nearly identical (27.3 ± 27.98).
As for the 2,4-difluorobenzene nucleoside 31, which is isosteric to uridine, we found a
new universal base [25] that paired with all natural bases without energy discrim-
ination. An explanation for our second hypothesis is that the destabilization of the
RNA duplex arises from the cost of desolvation of the H-bond donors or acceptors of
the natural bases during formation of the corresponding modified-natural base pair [9].
So, we have two destabilizing effects that lower the stability of the RNA duplexes by ca.
8 ± 148.
Interestingly, when an F-atom is in the 2-position of the benzene ring, the duplex is
2 ± 38 more stable than with a H-atom at the same position. So there seems to be an
interaction between this F-atom and a further atom. Possibly this F-atom can form a H-
bond to the H-atoms at C(5') of its own sugar moiety. This requires the benzene ring to

Helvetica Chimica Acta ± Vol. 83 (2000)
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take up a syn-conformation. A weak interaction between the F-atom and a H-atom at
C(5') observed in the corresponding crystal structures [7a,b] supports this hypothesis.
The fluorobenzimidazole-modified nucleosides 13 and 17 were compared to inosine
(I) (Table 1, Entries 6 ± 8). Both modified nucleosides show a destabilization of the
duplex of ca. 4 ± 148. The destabilization between an inosine mismatch base pair and a
fluorobenzimidazole ´ natural base pair is lower than in the case of the fluorobenzenes
(Table 1, Entries 2 ± 5). All RNA duplexes with the 4,6-difluorobenzimidazole-
modified nucleoside are by 0.4 ± 1.28 more stable than the ones with the 4-
fluorobenzimidazole (Table 1, Entries 7 and 8).
Table 2 shows the results of pairing modified nucleosides aginst each other. We
paired the 2,4-difluorobenzene-modified nucleoside 31 (uridine analogue) and the two
fluorobenzimidazole-modified nucleosides 13 and 17 against the fluorobenzene-
modified nucleosides 28 ± 31 and the abasic site 20. The RNA duplexes with the 4,6-
difluorobenzimidazole nucleoside 17 are even ca. 18 more stable than those with the
4-fluorobenzimidazole nucleoside 13 (cf. the last two columns in Table 2).
Table 2. Synthesized Doubly Modified Duplex RNA (5'-CUU UUC XUU CUU-3' paired with 3'-GAA AAG YAA -
GAA-5') and Their Thermodynamic Properties. For X and Y, see Scheme 4. DH0, T´ DS0, and DG0 in kcal/mol (Tˆ
298 K). Errors: Tm Æ0.28; DH0 and T´ DS0, Æ5%; DG0, Æ2%.
Entry
X
Yˆ 31
Yˆ 13
Yˆ 17
(R ˆ 2,4-F2C6H3)
(R ˆ 4-fluoro-1H-
benzimidazol-1-yl)
(R ˆ 4,6-difluoro-1H-
benzimidazol-1-yl)
Tm [8] DH0 T´ DS0 DG0 Tm [8] DH0 T´ DS0 DG0 Tm [8] DH0 T´ DS0 DG0
1
2
3
4
5
31 (R ˆ 2,4-F2C6H3)
28 (R ˆ 4-FÀC6H4)
29 (R ˆ 3-FÀC6H4)
30 (R ˆ 2-FÀC6H4)
32.5
29.9
31.3
31.9
82.4 72.2
84.1 74.5
77.5 67.7
83.5 73.4
59.7 52.0
10.2 33.5
88.1 77.4
82.2 72.4
76.8 67.2
86.9 76.4
58.1 49.8
10.7 34.6
94.4 83.2
85.0 75.0
81.8 71.8
87.6 76.9
55.1 44.6
11.2
10.0
10.0
10.7
8.5
9.6
9.8
30.6
30.3
9.8
9.6
31.3
31.4
10.1 32.8
7.7 25.3
10.5 33.6
8.3 26.3
20 (R ˆ H; abasic site) 22.6
What is the individual contribution of base stacking, solvation, and H-bonding to
the stability of duplex RNA? Calculating the incorporation of the 2,4-difluorobenzene
against the abasic site (Tm 22.68, Table 2, Entry5 ) gives a 4.48 (1.1 kcal/mol) more stable
duplex RNA than the one with a uridine ´ abasic site base pair (Tm 18.28; Table 1,
Entry9 ). This shows the contribution of stacking of the 2,4-difluorobenzene nucleoside
compared with uridine. A U ´ U base pair in duplex RNA (Tm 30.18; Table 1, Entry1 ) is
2.28 more stable than a U ´ 2,4-difluorobenzene base pair (Tm 27.98; Table 1, Entry2 ).
The incorporation of one 2,4-difluorobenzene stabilizes the duplex by ca. 4.48 (1.1 kcal/
mol) by stronger stacking, but the U ´ 2,4-difluorobenzene base pair is 2.28 less stable
than a U ´ U base pair. Thus, the contribution of solvation is 6.68 (1.7 kcal/mol) in
destabilization of the RNA duplex per base pair. A RNA duplex with a 2,4-
difluorobenzene ´ 2,4-difluorobenzene base pair should be 2.28 more stable (‡2 ´ 4.48
(2 ´ 1.1 kcal/mol; stronger stacking), À6.68 (À1.7 kcal/mol; less solvation)) than a U ´ U
base pair. In our measurement, a U ´ U base pair has a Tm of 30.18 (9.7 kcal/mol;
Table 1, Entry1 ) and a 2,4-difluorobenzene ´ 2,4-difluorobenzene base pair a Tm of
32.58 (10.2 kcal/mol; Table 2, Entry1 ), confirming our calculations. In the same
comparison for 4-fluorobenzimidazole and 4,6-difluorobenzimidazole, the destabiliza-
tion of solvation is 6.38 (1.6 kcal/mol) and 6.58 (1.5 kcal/mol), respectively. The

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stabilization of the duplex incorporating the 4-fluorobenzimidazole or 4,6-difluoro-
benzimidazole nucleoside by stacking effects are ca. 4.48 (1.0 kcal/mol) and 5.48
(1.2 kcal/mol) with regard to guanosine, respectively. A U ´ 4-fluorobenzimidazole base
pair shows a Tm of 28.5 (9.2 kcal/mol; Table 1, Entry7 ) and a 2,4-difluorobenzene ´ 4-
fluorobenzimidazole base pair a Tm of 33.58 (10.7 kcal/mol; Table 2, Entry1 ). The
exchange of uridine by the 2,4-difluorobenzene nucleoside stabilizes the duplex by 4.48
(1.1 kcal/mol). The exchange of the second natural base by the 2,4-difluorobenzene
nucleoside adds no further energy of solvation to the duplex RNA. So the
corresponding RNA duplex with the 2,4-difluorobenzene ´ 4-fluorobenzimidazole base
pair is 0.68 (0.4 kcal/mol) more stable than calculated. A similar result was obtained for
the 2,4-difluorobenzene ´ 4,6-difluorobenzimidazole base pair. The corresponding
RNA duplex with the 2,4-difluorobenzene ´ 4,6-difluorobenzimidazole base pair is
0.98 (0.6 kcal/mol) more stable than calculated. Thus, there seems to be another
stabilizing force that increases Tm. It may be possible that this increase of Tm results
from a weak F ´´´ H H-bond between the modified nucleosides. The existence of such
F ´´´ H H-bonds in this class of molecules has been shown in the crystal structure of 28
with a F ´´´ H distance of 230 pm [7a]. This H-bond would be one of the first F ´´´ H
H-bonds of so-called ꢀorganic fluorineꢁ [26] in aqueous solution. Further investigations
to characterize this effect are under way.
2.5. CD Spectra. CD Spectra of RNA duplexes were recorded at 315 ± 200 nm with
oligonucleotide concentration of 2 mm of each strand in a phosphate buffer containing
NaCl (140 mmol, pH 7). The measurement was performed at 108 to ensure that only
duplex RNA was present.
The CD spectra of a RNA duplex with only one modified base shows a typical curve
for an A-type helix (Fig. 2). There is a strong maximum at ca. 270 nm, a weak minimum
at ca. 245 nm, a weak maximum at ca. 225 nm and a strong minimum at ca. 210 nm.
Fig. 2. CD Spectra of dodecamer RNA duplex with one inosine ´ cytosine (Ð), one 4-fluorobenzimidazole ´
cytosine (- ´ - ´ ), or one 4,6-difluorobenzimidazole ´ cytosine (- - -) base pair

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Fig. 2 shows the CD spectra of three different RNA duplexes with the base pairs
inosine´ cytosine, 4-fluorobenzimidazole´ cytosine, and 4,6-difluorobenzimidazole´ cyt-
osine. In comparison with the modified RNA, the unmodified RNA inosine ´ cytosine
shows a small shift of ca. 2 nm for the strong maximum to 268 nm. The intensity at
270 nm corresponds to the number of paired and unpaired bases as well as to the extent
of base-stacking interactions [27]. At this maximum, intensity differences are only
small, so that the effect of missing H-bonds is compensated by stronger stacking
interactions of the modified bases. The CD spectra of duplex RNA modified with
fluorobenzenes have, in comparison with a U ´ A base pair, no significant differences.
Fig. 3 shows the CD spectra of duplex RNAwith one base pair of modified nucleosides.
All spectra are almost identical. Only in the region of 270 nm are small differences
observed that could be explained by different stacking abilities of the individual bases.
Fig. 3. CD Spectra of dodecamer RNA duplex with one 2,4-difluorobenzene ´ 4,6-difluorobenzimidazole (Ð),
one 4-fluorobenzene ´ 4,6-difluorobenzimidazole (- - -), one 3-fluorobenzene ´ 4,6-difluorobenzimidazole (´ ´´´ ),
or one 2-fluorobenzene ´ 4,6-difluorobenzimidazole (- ´ - ´ ) base pair
All CD spectra indicate that the structure of duplex RNA is not disturbed by
incorporation of one of our modified nucleic-acid analogues. A base pair of modified
nucleic-acid analogues does not alter the A-type RNA structure. Thus, all differences
determined by UV measurements are a consequence of changes in stacking, solvation,
or the ability to form H-bonds, and not of structural changes of the RNA duplex.
We thank Thorsten Strube for supporting synthesis and purification of oligonucleotides and Dr. Gerd
Dürner for prep. HPLC purification of tBuMe2Si-protected nucleosides. J.P. wishes to thank the Graduiertenkol-
leg Chemische und Biologische Synthese von Wirkstoffen for a Ph.D. fellowship.
Experimental Part
General. The fluorobenzenes and the fluoroacetanilides were obtained from Lancaster. The anh. solvents,
e.g. THF, CH2Cl2, pyridine, or Et2O were obtained from Fluka and used without further purification. Dry MeCN
(H2O <30 ppm) for the phosphitylation reaction was purchased from Perseptive Biosystems. Flash column

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chromatography (FC): silica gel 60 (40 ± 63 mm) from Merck. TLC: silica gel 60 F254 plates from Merck. HPLC:
anion-exchange column NucleoPac PA-100 from Dionex; desalting with a Sephadex-G25 column from
Pharmacia. UV/Melting profiles: Varian-Cary-1 UV/VIS spectrophotometer, Cary temperature controller, 10-
mm cuvette. CD-Spectra: Jasco-710 spectropolarimeter. NMR: Bruker-AM250 and Bruker-WH270 (1H,13C)
and Bruker-AMX400 (1H,13C,31P) spectrometers; d in ppm, J in Hz; for convenience, primed locants are given to
the monosaccharide moiety and unprimed ones to the ꢀnucleobaseꢁ moiety. MS: PerSeptive Biosystems MALDI-
TOF spectrometer Voyager DE; ES Iˆ electrospray ionization.
2'-O-[(tert-Butyl)dimethylsilyl]-5'-O-(4,4'-dimethoxytrityl)inosine 3'-(2-Cyanoethyl Diisopropylphosphor-
amidite) (1). Synthesis according to [19].
2,3,5-Tri-O-benzyl-b-d-ribofuranose (18). Synthesis according to [14].
2,3,5-Tri-O-benzyl-d-ribono-1,4-lactone (21). Synthesis according to [16].
4-Fluoro-1H-benzimidazole (9). Synthesis according to [11].
4,6-Difluoro-1H-benzimidazole (14). Synthesis according to [12].
2,3,5-Tri-O-acetyl-1-deoxy-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (12). To a suspension of 9
(3.2 g, 23.5 mmol) in MeCN (80 ml), N,O-bis(trimethylsilyl)acetamide (8.8 ml, 36 mmol) was added and heated
under reflux for 15 min. After cooling to r.t., 1,2,3,5-tetra-O-acetyl-b-d-ribofuranose (10) (7.6 g, 23.9 mmol) in
MeCN (35 ml) and trimethylsilyl trifluoromethanesulfonate (5.4 ml, 29.8 mmol) were added and heated under
reflux for 2.5 h. The mixture was treated with 5% NaHCO3 soln. and extracted with CH2Cl2, the org. phase dried
(MgSO4) and evaported, and the residue purified by FC (CH2Cl2/MeOH 98 :2): 12 (6.2 g, 65.7%). White foam.
TLC (CH2Cl2/MeOH 98 :2): Rf 0.41. 1H-NMR (250 MHz, (D6)DMSO): 8.54 (s, HÀC(2)); 7.60 (d, J ˆ 7.8,
HÀC(7)); 7.30 (dt, J ˆ 4.9, 8.1, HÀC(6)); 7.10 (dd, J ˆ 7.8, 11.0, HÀC(5)); 6.34 (d, J ˆ 6.2, HÀC(1')); 5.66 (t, J ˆ
6.3, HÀC(2')); 5.43 (dd, J ˆ 4.6, 6.3, HÀC(3')); 4.42 (m, HÀC(4')); 4.37 (m, 2 HÀC(5')); 2.13, 2.07, 2.03 (3s,
Me). 13C-NMR (62.9 MHz, (D6)DMSO): 170.01, 169.52, 169.22 (CˆO); 143.04 (C(2)); 135.37 (d, J ˆ 8.7, C(6));
132.24 (d, J ˆ 14.3, C(5)); 123.94 (d, J ˆ 7.2, C(8)); 108.02 (d, J ˆ 18.8, C(9)); 107.87 (C(7)); 86.47 (C(1')); 79.47
‡
(C(4')); 71.75 (C(2')); 69.53 (C(3')); 62.94 (C(5')); 20.50, 20.36, 20.15 (Me). ESI-MS: 395.1 ([M ‡ H] ).
1-Deoxy-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (13). A soln. of 12 (5.46 g, 13.8 mmol) in
NH3-sat. MeOH (150 ml) was stirred for 20 h and then evaporated. The residue was purified by FC (CH2Cl2/
MeOH 4 :1): 13 (3.58 g, 96.4%). Yellow oil. TLC (CH2Cl2/MeOH 4 :1): Rf 0.42. 1H-NMR (250 MHz,
(D6)DMSO): 8.51 (s, HÀC(2)); 7.60 (d, J ˆ 8.2, HÀC(7)); 7.26 (dt, J ˆ 5.0, 8.0, HÀC(6)); 7.06 (dd, J ˆ 8.0, 11.0,
HÀC(5)); 5.89 (d, J ˆ 6.2, HÀC(1')); 5.51 (d, J ˆ 6.4, OHÀC(2')); 5.23 (d, J ˆ 4.7, OHÀC(3')); 5.14 (t, J ˆ 5.2,
OHÀC(5')); 4.36 (q, J ˆ 5.9, HÀC(2')); 4.13 (m, HÀC(3')); 3.99 (q, J ˆ 3.4, HÀC(4')); 3.64 (m, 2 HÀC(5')).
13C-NMR (67.9 MHz, (D6)DMSO): 142.81 (C(2)); 136.04 (d, J ˆ 10.1, C(6)); 132.46 (d, J ˆ 16.8, C(5)); 123.28
(d, J ˆ 7.3, C(8)); 108.05 (C(7)); 107.37 (d, J ˆ 17.5, C(9)); 88.89 (C(1')); 85.62 (C(4')); 73.77 (C(2')); 70.04
(C(3')); 61.13 (C(5')). 19F-NMR (254.2 MHz, (D6)DMSO): À128.92 (m, FÀC(4)). ESI-MS: 267.1 ([M À H]À).
1-Deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (44). To a soln. of
13 (1.08 g, 4 mmol) in anh. pyridine (20 ml) and Et3N (740 ml, 6 mmol), 4,4'-dimethoxytrityl chloride
((MeO)2TrCl; 1.44 g, 4.3 mmol) was added and the mixture stirred for 6.5 h (TLC monitoring) under Ar at r.t.
The reaction was quenched by addition of MeOH (3 ml). The mixture was evaporated, the residue dissolved in
CH2Cl2, the soln. extracted with 5% NaHCO3 soln., dried (MgSO4), evaporated, and co-evaporated twice with
toluene, and the crude product purified by FC (CH2Cl2/MeOH 95 :5); 44 (2.04 g, 89.1%). Yellow foam. TLC
(CH2Cl2/MeOH 9 :1): Rf 0.40. 1H-NMR (250 MHz, (D6)DMSO): 8.42 (s, HÀC(2)); 7.54 (d, J ˆ 9.0, HÀC(7));
7.39 ± 6.81 (m, 13 arom. H); 7.07 (m, HÀC(5), HÀC(6)); 5.94 (d, J ˆ 5.4, HÀC(1')); 5.66 (d, J ˆ 6.0,
OHÀC(2')); 5.30 (d, J ˆ 5.5, OHÀC(3')); 4.52 (q, J ˆ 5.6, HÀC(2')); 4.23 (q, J ˆ 5.1, HÀC(3')); 4.13
(m, HÀC(4')); 3.71 (s, 2 MeO); 3.24 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz, (D6)DMSO): 158.12 ((MeO)2Tr);
153.34 (d, J ˆ 250.2, C(4)); 144.76 ((MeO)2Tr); 142.65 (C(2)); 135.88 (d, J ˆ 8.6, C(6)); 135.42, 135.30
((MeO)2Tr); 132.29 (d, J ˆ 16.85, C(5)); 129.75, 127.86, 127.73, 126.75 ((MeO)2Tr); 123.33 (d, J ˆ 7.3, C(8));
113.20 ((MeO)2Tr); 108.38 (C(7)); 107.62 (d, J ˆ 17.5, C(9)); 89.18 (C(1')); 85.72 ((MeO)2Tr); 83.53 (C(4'));
73.25 (C(2')); 70.10 (C(3')); 63.59 (C(5')); 55.05 (MeO). ESI-MS: 569.4 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ri-
bofuranose (45). To a soln. of 44 (2.52 g, 4.4 mmol) in anh. THF/pyridine 1:1 (40 ml), AgNO3 (900 mg,
t
5.3 mmol) and 1m BuMe2SiCl in THF (6.2 ml, 6.2 mmol) were added and stirred for 20 h under Ar at r.t. The
reaction was quenched by addition of sat. aq. NaHCO3 soln. The suspension was filtered, the filtrate extracted
with CH2Cl2, the org. phase dried (MgSO4) and evaporated, and the residue co-evaporated twice with toluene
and purified by prep. HPLC (MN Nucleoprep 100-20 from Macherey-Nagel, hexane/dioxane 5 :2): faster-
migrating isomer 45 (1.23 g, 40.7%). Colourless foam. TLC (hexane/AcOEt 4 :1): Rf 0.26. 1H-NMR (270 MHz,
(D6)DMSO): 8.45 (s, HÀC(2)); 7.57 (d, J ˆ 7.6, HÀC(7)); 7.41 ± 6.83 (m, 13 arom. H); 6.98 (m, HÀC(5),

Helvetica Chimica Acta ± Vol. 83 (2000)
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HÀC(6)); 5.97 (d, J ˆ 6.3, HÀC(1')); 5.26 (d, J ˆ 5.3, OHÀC(3')); 4.60 (q, J ˆ 6.0, HÀC(2')); 4.35
(m, HÀC(3')); 4.09 (m, HÀC(4')); 3.72 (s, 2 MeO); 3.30 (m, 2 HÀC(5')); 0.82 (s, tBuSi); 0.07, 0.02 (2s, Me2Si).
13C-NMR (67.9 MHz, (D6)DMSO): 158.10 ((MeO)2Tr); 153.27 (d, J ˆ 250.9, C(4)); 144.47 ((MeO)2Tr); 142.78
(C(2)); 135.63 (d, J ˆ 8.4, C(6)); 135.21, 135.14 ((MeO)2Tr); 132.28 (d, J ˆ 17.3, C(5)); 129.62, 127.74, 127.60,
126.69 ((MeO)2Tr); 123.15 (d, J ˆ 7.1, C(8)); 113.13 ((MeO)2Tr); 108.36 (C(7)); 107.52 (d, J ˆ 17.3, C(9)); 89.07
(C(1')); 85.85 ((MeO)2Tr); 83.95 (C(4')); 72.64 (C(2')); 71.81 (C(3')); 62.99 (C(5')); 54.98 (MeO); 25.67
‡
(Me3CSi); 17.90 (Me3CSi); À4.56, À5.19 (Me2Si). ESI-MS: 685.5 ([M ‡ H] ).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ri-
bofuranose (46) was obtained from the reaction described above as the slower-migrating isomer (1.48 g, 49.0%).
Colourless foam. TLC (hexane/AcOEt 4 :1): Rf 0.26. 1H-NMR (270 MHz, (D6)DMSO): 8.46 (s, HÀC(2)); 7.56
(d, J ˆ 9.0, HÀC(7)); 7.38 ± 6.82 (m, 13 arom. H); 7.07 (m, HÀC(5), HÀC(6)); 5.92 (d, J ˆ 5.9, HÀC(1')); 5.53
(d, J ˆ 6.5, OHÀC(2')); 4.51 (q, J ˆ 6.0, HÀC(2')); 4.35 (m, HÀC(3')); 4.09 (m, HÀC(4')); 3.72 (2s, Me); 3.30
(m, 2 HÀC(5')); 0.82 (s, tBuSi); 0.07, 0.02 (2s, Me2Si). 13C-NMR (67.9 MHz, (D6)DMSO): 158.10 ((MeO)2Tr);
153.27 (d, J ˆ 250.9, C(4)); 144.47 ((MeO)2Tr); 142.78 (C(2)); 135.63 (d, J ˆ 8.4, C(6)); 135.21, 135.14
((MeO)2Tr); 132.28 (d, J ˆ 17.3, C(5)); 129.62, 127.74, 127.60, 126.69 ((MeO)2Tr); 123.15 (d, J ˆ 7.1, C( 8 )) ;
113.13 ((MeO)2Tr); 108.36 (C(7)); 107.52 (d, J ˆ 17.3, C(9)); 89.07 (C(1')); 85.85 ((MeO)2Tr); 83.95 (C(4'));
72.64 (C(2')); 71.81 (C(3')); 62.99 (C(5')); 54.98 (MeO); 25.67 (Me3CSi); 17.90 (Me3CSi); À4.56, À5.19
‡
(Me2Si). ESI-MS: 685.5 ([M ‡ H] ).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluoro-1H-benzimidazol-1-yl)-b-d-ri-
bofuranose 3-(2-Cyanoethyl Diisopropylphosphoramidite) (2). To a soln. of 45 (200 mg, 0.29 mmol) in anh.
MeCN (10 ml), collidine (ˆ2,4,6-trimethylpyridine; 380 ml, 2.9 mmol), 1-methyl-1H-imidazole (12 ml,
0.15 mmol), and 2-cyanoethyl diisopropylphosphoramidochloridite (96 ml, 0.43 mmol) were added and the
mixture stirred for 15 min at 08 and for 45 min at r.t. (TLC monitoring) under Ar. The reaction was quenched by
addition of sat. aq. NaHCO3 soln., the mixture extracted wtih CH2Cl2, the extract dried (MgSO4) and
evaporated, and the crude product purified by FC (CH2Cl2/MeOH 99 :1): 2 (135 mg, 52.3%; diastereoisomer
1
mixture). Colourless foam. TLC (CH2Cl2/MeOH 98 :2): Rf 0.33, 0.41. H-NMR (400 MHz, CDCl3): 8.12, 8.08
(2s, HÀC(2)); 7.49 ± 6.81 (m, 13 arom. H, HÀC(5), HÀC(6), HÀC(7)); 5.92, 5.84 (2d, J ˆ 7.7, 5.8, HÀC(1'));
4.75 (m, HÀC(2')); 4.40 (m, HÀC(3')); 4.32 (m, HÀC(4')); 3.80, 3.79 (2s, 2 MeO); 3.48 (m, 2 HÀC(5')); 2.66
(m, CH2O); 1.20 (m, 2 Me2CH); 0.75 (s, tBuSi); À0.12, À0.38 (2s, Me2Si). 31P-NMR (161.98 MHz, CDCl3):
‡
152.49, 149.32; ratio 1:2.7. ESI-MS: 885.6 ([M ‡ H] ).
2,3,5-Tri-O-acetyl-1-deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (16). As described
above for 12, with 4,6-difluoro-1H-benzimidazole (14; 3.1 g, 20 mmol) MeCN (80 ml), N,O-bis(trimethyl-
silyl)acetamide (7.4 ml, 30 mmol), 10 (6.4 g, 20 mmol), MeCN (35 ml), and trimethylsilyl trifluoromethanesul-
fonate (4.5 ml, 25 mmol): 16 (5.6 g, 67.8%). White foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.49. 1H-NMR
(250 MHz, (D6)DMSO): 8.56 (s, HÀC(2)); 7.57 (dd, J ˆ 9.0, 2.1, HÀC(7)); 7.19 (dt, J ˆ 10.5, 2.0, HÀC(5)); 6.33
(d, J ˆ 6.1, HÀC(1')); 5.64 (t, J ˆ 6.3, HÀC(2')); 5.43 (dd, J ˆ 6.3, 4.3, HÀC(3')); 4.40 (m, HÀC(4'),
2 HÀC(5')); 2.14, 2.08, 2.04 (3s, Me). 13C-NMR (63.9 MHz, (D6)DMSO): 170.00, 169.52, 169.26 (CˆO);
158.48 (dd, J ˆ 11.2, 238.9, C(4)); 152.67 (dd, J ˆ 15.1, 253.2, C(6)); 143.67 (C(2)); 134.67 (dd, J ˆ 5.5, 16.5,
C(9)); 128.94 (d, J ˆ 18.1, C(8)); 98.32 (dd, J ˆ 22.5, 7.5, C(5)); 95.09 (dd, J ˆ 28.4, 4.4, C(7)); 86.43 (C(1'));
‡
79.57 (C(4')); 71.69 (C(3')); 69.43 (C(2')); 62.95 (C(5')); 20.48, 20.38, 20.18 (CH3). ESI-MS: 413.0 ([M ‡ H] ).
1-Deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-b-d-ribofuranose (17). As described for 13, with 16 (5.59 g,
13.5 mmol) and NH3-sat. (150 ml): 17 (3.66 g, 94.2%). Yellow solid. TLC (CH2Cl2/MeOH 4 :1): Rf 0.64.
1H-NMR (250 MHz, (D6)DMSO): 8.52 (s, HÀC(2)); 7.65 (dd, J ˆ 9.1, 2.2, HÀC(7)); 7.13 (dt, J ˆ 2.2, 10.5,
HÀC(5)); 5.87 (d, J ˆ 6.4, HÀC(1')); 5.51 (d, J ˆ 6.5, OHÀC(2')); 5.25 (d, J ˆ 4.6, OHÀC(3')); 5.22 (t, J ˆ 5.0,
OHÀC(5')); 4.34 (q, J ˆ 6.3, HÀC(2')); 4.12 (m, HÀC(3')); 4.00 (q, J ˆ 3.1, HÀC(4')); 3.66 (m, 2 HÀC(5')).
13C-NMR (63.9 MHz, (D6)DMSO): 158.13 (dd, J ˆ 238.3, 11.3, C(4)); 152.53 (dd, J ˆ 252.7, 15.2, C(6)); 143.70
(C(2)); 134.95 (dd, J ˆ 10.7, 5.4, C(9)); 129.01 (d, J ˆ 17.7, C(8)); 97.66 (dd, J ˆ 22.1, 7.5, C(5)); 95.43 (d, J ˆ 28.4,
C(7)); 89.07 (C(1')); 85.83 (C(4')); 73.63 (C(2')); 70.04 (C(3')); 61.08 (C(5')). 19F-NMR (254.2 MHz,
(D6)DMSO): À116.40 (m, FÀC(6)); À125.25 (m, FÀC(4)). ESI-MS: 285.1 ([M À H]À).
1-Deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-5-O-(4,4'-dimethoxytrityl)-b-d-ribofuranose (47). As de-
scribed above for 44, with 17 (640 mg, 2.2 mmol), anh. pyridine (10 ml), Et3N (470 ml, 3.3 mmol), (MeO)2TrCl
(0.91 g, 2.6 mmol) (20 h): 47 (0.98 g, 74.8%). Yellow foam. TLC (CH2Cl2/MeOH 9 :1): Rf 0.52. 1H-NMR
(270 MHz, (D6)DMSO): 8.43 (s, HÀC(2)); 7.47 (dd, J ˆ 8.8, 1.9, HÀC(7)); 7.35 ± 6.81 (m, 13 arom. H,
HÀC(5)); 5.93 (d, J ˆ 5.1, HÀC(1')); 5.69 (d, J ˆ 5.9, OHÀC(2')); (d, J ˆ 5.5, OHÀC(3')); 4.49 (q, J ˆ 5.5,
HÀC(2')); 4.21 (q, J ˆ 5.2, HÀC(3')); 4.12 (m, HÀC(4')); 3.72 (s, 2 MeO); 3.23 (m, 2 HÀC(5')). 13C-NMR
(67.9 MHz, (D6)DMSO): 158.22 (dd, J ˆ 239.2, 11.3 Hz, C(4)); 158.04 ((MeO)2Tr); 152.57 (dd, J ˆ 253.3, 15.3,

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C(6)); 146.82 (C(2)); 144.69, 135.36, 135.31 ((MeO)2Tr); 135.13 (dd, J ˆ 10.8, 5.3, C(9)); 129.05 ((MeO)2Tr);
128.94 (d, J ˆ 15.5, C(8)); 127.75, 127.59, 126.66, 113.11 ((MeO)2Tr); 97.83 (dd, J ˆ 22.1, 7.3, C(5)); 95.17 (d, J ˆ
28.1, C(7)); 89.09 (C(1')); 85.62 ((MeO)2Tr); 83.50 (C(4')); 73.26 (C(2')); 69.90 (C(3')); 63.40 (C(5')); 54.95
(MeO). ESI-MS: 587.4 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-5-O-(4,4'-dimethoxytrityl)-b-
d-ribofuranose (48). As described above for 45, with 47 (2.53 g, 4.3 mmol). HPLC (MN Nucleoprep 100-20 from
Macherey-Nagel, hexane/AcOiPr 3 :2): slower-migrating isomer 48 (0.97 g, 32.1%). Colourless foam. TLC
(CH2Cl2/MeOH 99 :1): Rf 0.55. 1H-NMR (400 MHz, (D6)DMSO): 8.47 (s, HÀC(2)); 7.47 (dd, J ˆ 8.9, 2.0,
HÀC(7)); 7.39 ± 6.84 (m, 13 arom. H); 7.12 (dt, J ˆ 8.5, 2.0, HÀC(5)); 5.97 (d, J ˆ 6.3, HÀC(1')); 5.24 (d, J ˆ 5.5,
OHÀ(3')); 4.59 (t, J ˆ 5.8, HÀC(2')); 4.19 (m, HÀC(3')); 4.14 (m, HÀC(4')); 3.73 (s, 2 MeO); 3.31
(m, 2 HÀC(5')); 0.72 (s, tBuSi); À0.08, À0.24 (2s, Me2Si). 13C-NMR (100.6 MHz, (D6)DMSO): 158.08
(dd, J ˆ 250.9, 11.5, C(4)); 158.02 ((MeO)2Tr); 152.58 (dd, J ˆ 252.4, 15.4, C(6)); 144.63 ((MeO)2Tr); 143.52
(C(2)); 135.10, 135.05 ((MeO)2Tr); 134.45 (d, J ˆ 10.2, C(9)); 129.64 ((MeO)2Tr); 128.95 (d, J ˆ 16.7, C(8));
127.67, 127.43, 126.64, 113.07 ((MeO)2Tr); 97.91 (C(5)); 95.31 (d, J ˆ 32.6, C(7)); 88.56 (C(1')); 85.72
((MeO)2Tr); 84.23 (C(4')); 74.59 (C(2')); 69.67 (C(3')); 63.28 (C(5')); 54.88 (MeO); 25.31 (Me3CSi); 17.61
(Me3CSi); À5.16, À5.76 (Me2Si). ESI-MS: 701.5 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-5-O-(4,4'-dimethoxytriphenyl-
methyl)-b-d-ribofuranose (49) was obtained from the reaction described above as the faster-migrating isomer
(1.55 g, 51.3%). Colourless foam. TLC (CH2Cl2/MeOH 99 :1): Rf 0.55. 1H-NMR (400 MHz, (D6)DMSO): 8.47
(s, HÀC(2)); 7.48 (dd, J ˆ 8.7, 2.0, HÀC(7)); 7.36 ± 6.82 (m, 13 arom. H, HÀC(5)); 5.90 (d, J ˆ 5.6, HÀC(1'));
5.53 (d, J ˆ 6.4, OHÀC(2')); 4.50 (q, J ˆ 5.8, HÀC(2')); 4.32 (t, J ˆ 4.7, HÀC(3')); 4.07 (q, J ˆ 4.0, HÀC(4'));
3.72 (s, 2 MeO); 3.26 (m, 2 HÀC(5')); 0.81 (s, tBuSi); 0.06, À0.01 (2s, Me2Si). 13C-NMR (100.6 MHz,
(D6)DMSO): 158.20 (dd, J ˆ 239.5, 11.5, C(4)); 158.11 ((MeO)2Tr); 152.59 (dd, J ˆ 272.9, 15.0, C(6)); 144.55
((MeO)2Tr); 143.32 (C(2)); 135.22, 135.19 ((MeO)2Tr); 134.86 (d, J ˆ 10.7, C(9)); 129.64 ((MeO)2Tr); 129.04
(d, J ˆ 16.8, C(8)); 127.77, 127.57, 126.72, 113.13 ((MeO)2Tr); 97.87 (dd, J ˆ 20.6, 7.4, C(5)); 95.40 (d, J ˆ 27.9,
C(7)); 89.11 (C(1')); 85.86 ((MeO)2Tr); 84.00 (C(4')); 72.71 (C(2')); 71.65 (C(3')); 62.87 (C(5')); 54.97 ((MeO);
25.69 (Me3CSi); 17.93 (Me3CSi); À4.53, À5.19 (Me2Si). ESI-MS: 701.5 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-1-(4,6-difluoro-1H-benzimidazol-1-yl)-5-O-(4,4'-dimethoxytrityl)-b-
d-ribofuranose-3-(2-Cyanoethyl Diisopropylphosphoramidite) (3). As described above for 2, with 48 (200 mg,
0.29 mmol) (15 min at 08 and 25 min at r.t.): 3 (180 mg, 70.0%; diastereoisomer mixture). Colourless foam. TLC
(CH2Cl2/MeOH 98 :2): Rf 0.65. 1H-NMR (270 MHz, CDCl3): 8.12, 8.11 (2s, HÀC(2)); 7.46 ± 6.73 (m, 13
arom. H, HÀC(5), HÀC(7)); 5.85, 5.78 (2d, J ˆ 7.6, 7.2, HÀC(1')); 4.67 (m, HÀC(2')); 4.41 (m, HÀC(3')); 4.32
(m, HÀC(4')); 3.79, 3.78 (2s, 2 MeO); 3.52 (m, 2 HÀC(5')); CH2CN); 2.67 (m, CH2O); 1.19 (m, 2 Me2CH);
0.82, 0.76 (2s, tBuSi); À0.09, À0.10, À0.32, À0.34 (4s, Me2Si). 31P-NMR (161.98 MHz, CDCl3): 152.62, 149.33;
‡
ratio 1:2.7. ESI-MS: 903.6 ([M ‡ H] ).
2,3,5-Tri-O-benzyl-1-deoxy-d-ribofuranose (19). A soln. of 18 (1.0 g, 2.4 mmol) in MeCN (10 ml) was
treated at r.t. with Et3SiH (1.52 ml, 9.6 mmol) and BF3 ´ Et2O (0.6 ml, 4.8 mmol). The mixture was stirred for
1.5 h at r.t. under Ar, then quenched by addition of sat. aq. NaHCO3 soln., and extracted with CH2Cl2. The org.
phase was dried (MgSO4) and evaporated and the residue purified by FC (hexane/AcOEt 4 :1): 19 (850 mg,
88.4%). Colourless oil. TLC (CH2Cl2/MeOH 99 :1): Rf 0.49. 1H-NMR (250 MHz, (D6)DMSO): 7.29 ± 7.17
(m, 15 arom. H); 4.58 ± 4.41 (m, PhCH2); 4.09 (m, HÀC(4)); 3.89 (m, 2 HÀC(1), HÀC(2), HÀC(3)); 3.50
(m, 2 HÀC(5)). 13C-NMR (62.9 MHz, (D6)DMSO): 138.05, 137.88, 137.80, 128.25, 128.21, 128.18, 127.80, 127.72,
127.61, 127.49, 127.42 (arom. C); 80.35 (C(4)); 78.18 (C(1)); 76.42 (C(2)); 73.28, 72.05, 71.66 (PhCH2); 70.45
‡
(C(3)); 69.93 (C(5)). ESI-MS: 422.2 ([M ‡ NH4] ).
1-Deoxy-d-ribofuranose (ˆ1,4-Anhydro-d-ribitol; 20). To a soln. of 19 (3.9 g, 9.6 mmol) in EtOH (70 ml)
and cyclohexene (35 ml), 20% Pd(OH)2/C (800 mg) was added, and the suspension was stirred under reflux for
4 h (TLC monitoring). The catalyst was removed by filtration, the filtrate evaporated and the crude product
purified by FC (CH2Cl2/MeOH 9 :1): 20 (1.23 g, 95.3%). Colourless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.22.
1H-NMR (250 MHz, (D6)DMSO): 4.69 (m, OHÀC(2), OHÀC(3)); 4.57 (t, J ˆ 5.7, OHÀC(5)); 3.96 (m,
HÀC(4)); 3.83 (m, HÀC(2)); 3.74 (q, J ˆ 5.5, HÀC(3)); 3.52 (m, 2 HÀC(1), HÀC(5)); 3.35 (m, 1 HÀC(5)).
13C-NMR (62.9 MHz, (D6)DMSO): 83.20 (C(4)); 71.99 (C(1)); 71.55 (C(2)); 70.41 (C(3)); 61.76 (C(5)). ESI-
MS: 133.0 ([ M À H]À).
1-Deoxy-5-O-(4,4'-dimethoxytrityl)-d-ribofuranose (50). As described above for 44, with 20 (1.22 g,
9.1 mmol), pyridine (40 ml), Et3N (1.9 ml, 19 mmol), and (MeO)2TrCl (3.7 g, 10.9 mmol). FC (CH2Cl2/MeOH
98 :2) gave 50 (3.41 g, 85.9%). White foam. TLC (CH2Cl2/MeOH 98 :2): Rf 0.13. 1H-NMR (250 MHz,
(D6)DMSO): 7.42 ± 6.86 (m, 13 arom. H); 4.77 (d, J ˆ 4.5, OHÀC(2)); 4.73 (d, J ˆ 6.2, OHÀC(3)); 4.01

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(m, HÀC(2)); 3.94 (m, HÀC(1)); 3.77 (m, HÀC(3), HÀC(4)); 3.73 (s, 2 MeO); 3.60 (dd, J ˆ 8.9, 3.1, HÀC(1));
3.02 (m, 2 HÀC(5)). 13C-NMR (62.9 MHz, (D6)DMSO): 158.02, 145.06, 135.79, 129.70, 127.74, 126.58, 113.14,
85.15 ((MeO)2Tr); 80.97 (C(4)); 72.50 (C(1)); 72.24 (C(2)); 70.30 (C(3)); 64.45 (C(5)); 55.01 (MeO). ESI-MS:
435.2 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-d-ribofuranose (51). As described above
t
for 45, with 50 (3.2 g, 7.3 mmol), THF/pyridine 1:1 (60 ml), AgNO3 (1.5 g, 13.3 mmol), and 1m BuMe2SiCl in
THF (10.2 ml, 10.2 mmol). FC (CH2Cl2): slower-migrating isomer 51 (2.43 g, 60.3%). Colourless foam. TLC
1
(CH2Cl2): Rf 0.43. H-NMR (250 MHz, (D6)DMSO): 7.40 ± 6.83 (m, 13 arom. H); 4.49 (d, J ˆ 6.0, OHÀC(3));
4.19 (m, HÀC(2)); 3.96 (m, HÀC(1)); 3.77 (m, HÀC(3), HÀC(4)); 3.73 (s, 2 MeO); 3.58 (m, HÀC(1)); 3.00
(m, 2 HÀC(5)); 0.88 (s, tBuSi); 0.07, 0.06 (2s, Me2Si). 13C-NMR (100.6 MHz, (D6)DMSO): 158.05, 145.09,
135.75, 129.73, 127.78, 127.73, 127.42, 126.61, 113.16, 85.17 ((MeO)2Tr); 80.92 (C(4)); 73.56 (C(1)); 72.57 (C(2));
70.24 (C(3)); 64.33 (C(5)); 55.05 (MeO); 25.88 (Me3CSi); 18.11 (Me3CSi); À4.54, À4.87 (Me2Si). ESI-MS:
549.4 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-d-ribofuranose (52) was obtained from
the reaction described above as the faster-migrating isomer (1.1 g, 27.3%). Colourless foam. TLC (CH2Cl2):
Rf 0.64. 1H-NMR (250 MHz, (D6)DMSO): 7.42 ± 6.82 (m, 13 arom. H); 4.53 (d, J ˆ 6.0, OHÀC(2)); 4.17
(m, HÀC(2)); 3.95 (m, HÀC(1)); 3.89 (m, HÀC(3)); 3.79 (m, HÀC(4)); 3.72 (s, 2 MeO); 3.57 (m, HÀC(1));
3.02 (m, 2 HÀC(5)); 0.72 (s, tBuSi); À0.04, À0.15 (s, Me2Si). 13C-NMR (62.9 MHz, (D6)DMSO): 158.07, 144.94,
135.56, 129.68, 127.76, 127.65, 126.61, 113.82, 85.33 ((MeO)2Tr); 81.42 (C(4)); 73.36 (C(1)); 72.13 (C(2)); 71.77
(C(3)); 63.85 (C(5)); 25.61 (Me3CSi); 17.65 (Me3CSi); À4.71, À5.29 (Me2Si). ESI-MS: 550.7 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-d-ribofuranose 3-(2-Cyanoethyl Diiso-
propylphosphoramidite) (8). As described above for 2, with 51 (200 mg, 0.36 mmol), MeCN (11 ml), collidine
(480 ml, 3.7 mmol), 1-methyl-1H-imidazole (15 ml, 0.19 mmol), and 2-cyanoethyl diisopropylphosphoramido-
chloridite (122 ml, 0.54 mmol) (15 min. at 08 and 35 min. at r.t.): 8 (130 mg, 47.8%; diastereoisomer mixture).
Colourless foam. TLC (CH2Cl2/MeOH 99 :1): Rf 0.49, 0.55. 1H-NMR (270 MHz, CDCl3): 7.40 ± 6.72
(m, 13 arom. H); 4.36, 4.30 (2q, J ˆ 4.9, 5.3, HÀC(2)); 4.07 (m, 1 HÀC(1)); 4.00 (m, HÀC(3), HÀC(4));
3.72, 3.71 (2s, 2 MeO); 3.46 (m, CH2CN, 1 HÀC(1)); 3.28 (m, 2 HÀC(5)); 2.53 (m, CH2O); 1.05 (m, Me2CH);
0.85, 0.83 (2s, tBuSi); 0.05, 0.04, 0.02, 0.01 (4s, MeSi). 31P-NMR (161.98 MHz, CDCl3): 149.67, 149.15; ratio 1:1.3.
‡
ESI-MS: 751.1 ([M ‡ H] ).
2,3,5-Tri-O-benzyl-1-deoxy-1-(4-fluorophenyl)-b-d-ribofuranose (24). A soln. of 1-bromo-4-fluorobenzene
(22); 150 ml, 1.3 mmol) in anh. THF (80 ml) was treated under Ar at À788 within 10 min with 1.5m BuLi in
hexane (15.6 ml, 23.4 mmol). After 20 min at À788, a soln. of 21 (7.0 g, 16.7 mmol) in THF (50 ml) was added
over 30 min, and the mixture was stirred for an additional hour and then warmed within 2 h to À308 (TLC
control). The reaction was quenched by addition of H2O, the mixture extracted with Et2O, and the org. phase
dried (MgSO4) and evaporated to afford an oil. The residue was dissolved in CH2Cl2 (100 ml) and treated at
À788 with BF3 ´ Et2O (4.2 ml, 33.4 mmol) and Et3SiH (5.3 ml, 33.4 mmol). The mixture was stirred for 1 h at
À788 and then warmed overnight to 108. The reaction was quenched by addition of sat. aq. NaHCO3 soln., the
mixture extracted with CH2Cl2, the org. phase dried (MgSO4) and evaporated, and the residue purified by FC
(hexane/AcOEt 4 :1): 24 (6.31 g, 75.8%). Orange solid. TLC (hexane/AcOEt 4 :1): Rf 0.35. 1H-NMR
(250 MHz, (D6)DMSO): 7.44 ± 7.08 (m, 19 arom. H); 4.87 (d, J ˆ 6.8, HÀC(1')); 4.61 ± 4.42 (m, PhCH2); 4.24
(q, J ˆ 3.8, HÀC(4')); 4.07 (dd, J ˆ 4.8, 3.8, HÀC(3')); 3.87 (dd, J ˆ 5.0, 6.8, HÀC(2')); 3.64 (m,
2 HÀC(5')). 13C-NMR (67.9 MHz, (D6)DMSO): 138.19, 137.95 (arom. C); 136.63 (C(1)); 128.18, 128.11,
128.05, 127.74, 127.43, 127.36 (arom. C); 123.99 (C(5)); 114.95 (C(2)); 114.64 (C(6)); 83.41 (C(1')); 81.23 (C(4'));
81.08 (C(2')); 77.27 (C(3')); 72.41 (PhCH2); 71.10 (PhCH2); 70.96 (PhCH2); 70.27 (C(5')). ESI-MS: 516.2 ([M ‡
‡
H] ).
1-Deoxy-1-(4-fluorophenyl)-b-d-ribofuranose (28). As described above for 20, with 24 (3 g, 6 mmol),
EtOH (60 ml), cyclohexene (30 ml), and 20% Pd(OH)2/C (600 mg) (5 h): 28 (1.36 g, 99.5%). Colourless solid.
TLC (CH2Cl2/MeOH 9 :1): Rf 0.39. 1H-NMR (250 MHz, (D6)DMSO): 7.42 (m, 2 arom. H); 7.14 (m, 2 ar-
om. H); 4.97 (d, J ˆ 7.0, HÀC(1')); 4.91 (d, J ˆ 4.7, OHÀC(3')); 4.82 (t, J ˆ 5.5, OHÀC(5')); 4.56 (d, J ˆ 7.3,
OHÀC(2')); 3.89 (m, HÀC(3')); 3.81 (m, HÀC(4')); 3.66 (m, HÀC(2')); 3.54 (m, 2 HÀC(5')). 13C-NMR
(67.9 MHz, (D6)DMSO): 137.72 (C(1)); 128.06 (d, J ˆ 11.5, C(3)); 114.79 (C(2)); 114.47 (C(6)); 85.18 (C(1'));
82.21 (C(4')); 77.55 (C(2')); 71.35 (C(3')); 61.97 (C(5')). 19F-NMR (254.2 MHz, (D6)DMSO): À115.87 (sept. J ˆ
5.1, FÀC(4)). ESI-MS: 227.0 ([M À H]À). Anal. calc. for C11H13FO4 (228.21): C 57.89, H 5.74; found: C57.88,
H 5.84.
1-Deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluorophenyl)-b-d-ribofuranose (32). As described above for 44,
with 28 (1.37 g, 6 mmol), pyridine (30 ml), Et3N (1.25 ml, 9 mmol), and (MeO)2TrCl (2.44 g, 7.2 mmol) (4.5 h).

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FC (CH2Cl2/MeOH 98 :2): 32 (2.52 g, 79.1%). Yellow foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.37. 1H-NMR
(150 MHz, (D6)DMSO): 7.48 ± 6.87 (m, 17 arom. H); 5.13 (d, J ˆ 6.7, HÀC(1')); 5.01 (d, J ˆ 5.1, OHÀC(3'));
4.66 (d, J ˆ 6.6, OHÀC(2')); 3.98 (m, HÀC(4')); 3.89 (q, J ˆ 4.9, HÀC(3')); 3.74 (s, HÀC(2'), 2 MeO); 3.18
(m, 2 HÀC(5')). 13C-NMR (62.9 MHz, (D6)DMSO): 158.07 ((MeO)2Tr); 149.63 (C(4)); 144.95 ((MeO)2Tr);
137.44 (C(1)); 135.63, 129.76, 127.84, 127.78, 126.68 ((MeO)2Tr); 123.91 (C(5)); 115.03 (C(2)); 114.70 (C(6));
113.18, 85.40 ((MeO)2Tr); 83.19 (C(1')); 82.69 (C(4')); 77.60 (C(2')); 71.42 (C(3')); 64.14 (C(5')); 55.03 (MeO).
ESI-MS: 529.2 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluorophenyl)-b-d-ribofuranose (33).
As described above for 45, from 32 (530 mg, 1 mmol), THF/pyridine 1:1 (10 ml), AgNO3 (204 mg, 1.2 mmol),
t
and 1m BuMe2SiCl in THF (1.4 ml, 1.4 mmol). FC (CH2Cl2): slower-migrating isomer 33 (280 mg, 43.5%).
1
Colourless foam. TLC (CH2Cl2): Rf 0.22. H-NMR (400 MHz, (D6)DMSO): 7.49 ± 6.83 (m, 17 arom. H); 4.77
(d, J ˆ 4.4, OHÀC(3')); 4.67 (d, J ˆ 6.4, HÀC(1')); 4.00 (m, HÀC(4')); 3.93 (m, HÀC(2'), HÀC(3')); 3.73
(s, 2 MeO); 3.28 (m, 1 HÀC(5')); 3.15 (m, 1 HÀC(5')); 0.78 (s, tBuSi); À0.11, À0.17 (2s, Me2Si). 13C-NMR
(100.6 MHz, (D6)DMSO): 158.07 ((MeO)2Tr); 144.96 (C(4)); 144.75 ((MeO)2Tr); 137.22 (C(1)); 135.52, 135.44,
129.74, 129.71, 129.66, 128.88, 128.19, 127.89, 127.61 ((MeO)2Tr); 126.67 (C(3)); 114.94 (C(2)); 114.73 (C(6));
113.14, 85.57 ((MeO)2Tr); 83.50 (C(1')); 82.68 (C(4')); 79.59 (C(2')); 71.56 (C(3')); 63.83 (C(5')); 55.00 (MeO);
25.57 (Me3CSi); 17.85 (Me3CSi); À5.07, À5.30 (Me2Si). ESI-MS: 643.0 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluorophenyl)-b-d-ribofuranose (34)
was obtained from the reaction described above as the faster-migrating isomer (270 mg, 42.0%). Colourless foam.
TLC (CH2Cl2): Rf 0.42. 1H-NMR (400 MHz, (D6)DMSO): 7.49 ± 6.87 (m, 17 arom. H); 4.89 (d, J ˆ 7.2,
OHÀC(2')); 4.66 (d, J ˆ 6.7, HÀC(1')); 4.01 (m, HÀC(3')); 3.94 (m,HÀC(4')); 3.75 (m, HÀC(2')); 3.73
(s, 2 MeO); 3.29 (m, 1 HÀC(5')); 3.09 (m, 1 HÀC(5')); 0.79 (s, tBuSi); 0.01 ± 0.05 (2s, Me2Si). 13C-NMR
(100.6 MHz, (D6)DMSO): 158.07 ((MeO)2Tr); 144.96 (C(4)); 144.75 ((MeO)2Tr); 137.21 (C(1)); 135.51, 135.44,
129.74, 129.70, 129.66, 128.19, 127.77, 127.66 ((MeO)2Tr); 126.67 (C(3)); 114.94 (C(2)); 114.73 (C(6)); 113.14,
85.46 ((MeO)2/Tr); 83.93 (C(1')); 82.27 (C(4')); 77.27 (C(2')); 73.29 (C(3')); 63.49 (C(5')); 54.99 (MeO); 25.74
(Me3CSi); 17.95 (Me3CSi); À4.51, À5.09 (Me2Si). ESI-MS: 643.4 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(4-fluorophenyl)-b-d-ribofuranose 3-
(2-Cyanoethyl Diisopropylphosporamidite) (4). As described above for 2, with 33 (200 mg, 0.31 mmol), MeCN
(10 ml), collidine (440 ml, 3.1 mmol), 1-methyl-1H-imidazole (14 ml, 0.18 mmol), and 2-cyanoethyl diisopro-
pylphosphoramidochloridite (112 ml, 0.5 mmol) (15 min at 08 and 15 min at r.t.). FC (hexane/AcOEt 4 :1): 4
(176 mg, 67.2%; diastereoisomer mixture). Colourless foam. TLC (hexane/AcOEt 4 :1): Rf 0.42. 1H-NMR
(270 MHz, (D6)DMSO): 7.51 ± 6.81 (m, 17 arom. H); 4.75, 4.70 (2d, J ˆ 8.2, 8.1, HÀC(1')); 4.17 (m, HÀC(3'));
4.12 (m, HÀC(2')); 3.98 (m, HÀC(4')); 3.79, 3.78 (2s, 2 MeO); 3.54 (m, 1 HÀC(5'), CH2CN); 3.19
t
(m, 1 HÀC(5')); 2.66 (m, CH2O); 1.17 (m, 2 Me2CH); 0.80, 0.79 (2m, BuSi); À0.10, À0.12, À0.20, À0.28
‡
(4s, Me2Si). 31P-NMR (161.98 MHz, CDCl3): 151.37, 148.68; ratio 1:3.2. ESI-MS: 845.6 ([M ‡ H] ).
2,3,5-Tri-O-benzyl-1-deoxy-1-(3-fluorophenyl)-b-d-ribofuranose (25). As described above for 24, with 1-
bromo-3-fluorobenzene (395 ml, 3.6 mmol), THF (12 ml), 1.5m BuLi in hexane (2.25 ml, 3.4 mmol) 21 (1.0 g,
2.4 mmol), and THF (8 ml). Subsequent treatment in CH2Cl2 (15 ml) with BF3 ´ Et2O (600 ml, 4.8 mmol) and
Et3SiH (760 ml, 4.8 mmol): 25 (0.89 g, 74.8%). Orange solid. TLC (hexane/AcOEt 4 :1): Rf 0.26. 1H-NMR
(270 MHz, (D6)DMSO): 7.38 ± 7.05 (m, 19 arom. H); 4.92 (d, J ˆ 6.4, HÀC(1')); 4.50 (m, PhCH2); 4.26 (q, J ˆ
3.9, HÀC(4')); 4.08 (t, J ˆ 4.5, HÀC(3')); 3.91 (m, HÀC(2')); 3.65 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz,
(D6)DMSO): 162.14 (d, J ˆ 243.3, C(3)); 143.71 (d, J ˆ 7.0, C(1)); 138.21, 138.15, 137.97 (arom. C); 130.08
(d, J ˆ 8.4, C(5)); 128.22, 128.18, 128.13, 127.80, 127.53, 127.41, 127.34 (arom. C); 122.26 (d, J ˆ 2.6, C(6)); 114.30
(d, J ˆ 20.9, C(2)); 112.74 (d, J ˆ 22.2, C(4)); 83.37 (C(1')); 81.22 (C(4')); 81.17 (C(2')); 77.17 (C(3')); 72.42
‡
(PhCH2); 71.13 (PhCH2); 70.99 (PhCH2); 70.14 (C(5')). ESI-MS: 516.5 ([M ‡ NH4] ).
1-Deoxy-1-(3-fluorophenyl)-b-d-ribofuranose (29). As described above for 20, with 25 (0.86 g, 1.7 mmol),
EtOH (20 ml), cyclohexene (10 ml), and 20% Pd(OH)2/C (200 mg) (3 h): 29 (0.38 g, 96.7%). Colourless solid.
TLC (CH2Cl2/MeOH 9 :1): Rf 0.32. 1H-NMR (270 MHz, (D6)DMSO): 7.36, 7.23, 7.07 (m, arom. H); 5.03 (d, J ˆ
7.1, OH ÀC(3')); 4.94 (d, J ˆ 4.7, HÀC(1')); 4.85 (t, J ˆ 5.5, OHÀC(5')); 4.58 (d, J ˆ 7.2, OHÀC(2')); 3.90
(m, HÀC(3')); 3.82 (q, J ˆ 3.5, HÀC(4')); 3.65 (m, HÀC(2')); 3.54 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz,
(D6)DMSO): 162.11 (d, J ˆ 242.8, C(3)); 144.60 (d, J ˆ 7.1, C(1)); 129.84 (d, J ˆ 8.0, C(5)); 122.12 (d, J ˆ 2.2,
C(6)); 113.83 (d, J ˆ 20.9, C(2)); 112.57 (d, J ˆ 22.0, C(4)); 85.22 (C(1')); 82.16 (C(4')); 77.66 (C(2')); 71.33
(C(3')); 61.85 (C(5')). 19F-NMR (254.2 MHz, (D6)DMSO): À113.87 (m, FÀC(3)). ESI-MS: 227.1 ([M À H]À).
Anal. calc. for C11H13FO4 (228.21): C 57.89, H 4.74; found: C 57.94, H 5.81.
1-Deoxy-5-O-(4,4'-dimethoxytrityl)-1-(3-fluorophenyl)-b-d-ribofuranose (35). As described above for 44,
with 29 (600 mg, 2.6 mmol), with pyridine (20 ml), Et3N (560 ml, 4 mmol), and (MeO)2TrCl (1.06 g, 3.1 mmol)

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(4 h). FC (CH2Cl2/MeOH 98 :2): 35 (1.34 g, 96.4%). Yellow foam. TLC (CH2Cl2/MeOH 98 :2): Rf 0.24.
1H-NMR (250 MHz, (D6)DMSO): 7.45 ± 6.87 (m, 19 arom. H); 5.17 (d, J ˆ 6.7, OHÀC(3')); 5.00 (m, J ˆ 5.1,
HÀC(1')); 4.70 (d, J ˆ 6.4, OHÀC(2')); 4.01 (m, HÀC(4')); 3.89 (q, J ˆ 4.7, HÀC(3')); 3.79 (m, HÀC(2')); 3.74
(s, 2 MeO); 3.19 (m, 2 HÀC(5')). 13C-NMR (62.9 MHz, (D6)DMSO): 158.26 (d, J ˆ 248.8, C(3)); 158.03, 144.91
((MeO)2Tr); 144.38 (d, J ˆ 7.0, C(1)); 135.61, 135.55 ((MeO)2Tr); 129.93 (d, J ˆ 19.7, C(5)); 129.70, 127.75,
127.68, 126.62 ((MeO)2Tr); 123.85 (C(6)); 113.88 (d, J ˆ 13.0, C(2)); 113.13 ((MeO)2Tr); 112.32 (d, J ˆ 21.8,
C(4)); 85.37 ((MeO)2Tr); 83.17 (C(1')); 82.63 (C(4')); 77.57 (C(2')); 71.36 (C(3')); 64.10 (C(5')); 54.97 (MeO).
ESI-MS: 529.4 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytriphenylmethyl)-1-(3-fluorophenyl)-b-d-ribo-
furanose (36). As described above for 45, with 35 (1.32 g, 2.5 mmol), THF/pyridine 1:1 (20 ml), AgNO3
t
(508 mg, 3 mmol), and 1m BuMe2SiCl (3.5 ml, 3.5 mmol). FC (CH2Cl2): slower-migrating isomer 36 (530 mg,
33.1%). Colourless foam. TLC (CH2Cl2): Rf 0.36. 1H-NMR (400 MHz, (D6)DMSO): 7.45 ± 6.82 (m, 17 arom. H);
4.85 (d, J ˆ 5.2, OHÀC(3')); 4.70 (d, J ˆ 6.8, HÀC(1')); 4.00 (m, HÀC(4')); 3.92 (m, HÀC(2'), HÀC(3')); 3.73
(s, 2 MeO); 3.22 (m, 2 HÀC(5')); 0.78 (s, tBuSi); À0.10, À0.16 (s, Me2Si). 13C-NMR (62.9 MHz, (D6)DMSO):
163.50 (d, J ˆ 257.7, C(3)); 158.10, 145.05 ((MeO)2Tr); 144.83 (C(1)); 135.51, 135.41 ((MeO)2Tr); 130.10 (d, J ˆ
9.0, C(5)); 129.77, 127.81, 127.62 ((MeO)2Tr); 126.73 (C(6)); 113.77 (C(2)); 113.17 ((MeO)2Tr); 112.54 (C(4));
85.51 ((MeO)2Tr); 84.08 (C(1')); 82.27 (C(4')); 79.68 (C(2')); 71.65 (C(3')); 64.11 (C(5')); 55.02 (MeO); 25.63
(Me3CSi); 17.91 (Me3CSi); À4.94, À5.31 (Me2Si). ESI-MS: 643.4 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(3-fluorophenyl)-b-d-ribofuranose (37)
was obtained from the reaction described above as the faster-migrating isomer (680 mg, 42.5%). Colourless foam.
TLC (CH2Cl2): Rf 0.48. 1H-NMR (400 MHz, (D6)DMSO): 7.43 ± 6.85 (m, 17 arom. H); 5.00 (d, J ˆ 7.3,
OHÀC(2')); 4.68 (d, J ˆ 6.5, HÀC(1')); 3.97 (m, HÀC(3'), HÀC(4')); 3.77 (m, HÀC(2')); 3.73 (s, 2 MeO);
3.18 (m, 2 HÀC(5')); 0.78 (s, tBuSi); À0.01, À0.06 (s, Me2Si). 13C-NMR (62.9 MHz, (D6)DMSO): 162.22 (d, J ˆ
243.0, C(3)); 158.11, 144.83 ((MeO)2Tr); 144.60 (d, J ˆ 7.0, C(1)); 135.55, 135.44 ((MeO)2Tr); 130.09 (d, J ˆ 8.1,
C(5)); 129.70, 127.95, 127.67, 126.70 ((MeO)2Tr); 121.94 (C(6)); 114.05 (d, J ˆ 19.7, C(2)); 113.16 ((MeO)2Tr);
112.43 (d, J ˆ 18.0, C(4)); 85.61 ((MeO)2Tr); 83.57 (C(1')); 82.74 (C(4')); 77.39 (C(2')); 73.39 (C(3')); 63.50
(C(5')); 55.00 (MeO); 25.75 (Me3Si); 17.99 (Me3CSi); À4.51, À5.10 (Me2Si). ESI-MS: 643.4 ([M À H]À).
2'-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(3-fluorophenyl)-b-d-ribofuranose 3-
(2-Cyanoethyl Diisopropylphosphoramidite) (5). As described above for 2, with 36 (200 mg, 0.31 mmol), MeCN
(10 ml), collidine (440 ml, 3.1 mmol), 1-methyl-1H-imidazole (14 ml, 0.18 mmol), and 2-cyanoethyl diisopro-
pylphosphoramidochloridite (112 ml, 0.5 mmol) (15 min at 08 and 15 min at r.t.). FC (hexane/AcOEt 4 :1): 5
(170 mg, 64.9%; diastereoisomer mixture). Colourless foam. TLC (hexane/AcOEt 4 :1): Rf 0.31. 1H-NMR
(270 MHz, (D6)DMSO): 7.52 ± 6.81 (m, 17 arom. H); 4.77, 4.73 (2d, J ˆ 8.4, 7.2, HÀC(1')); 4.20 (m, HÀC(3'));
4.14 (m, HÀC(2')); 4.01 (m, HÀC(4')); 3.79, 3.78 (2s, 2 MeO); 3.52 (m, 1 HÀC(5'), CH2CN); 3.25
t
(m, 1 HÀC(5')); 2.64 (m, CH2O); 1.17 (m, 2 Me2CH); 0.87, 0.81 (2m, BuSi); À0.09, À0.11, À0.17, À0.27 (4s,
‡
Me2Si). 31P-NMR (161.98 MHz, CDCl3): 151.47, 148.69; ratio 1:3.7. ESI-MS: 845.6 ([M ‡ H] ).
2,3,5-Tri-O-benzyl-1-deoxy-1-(2-fluorophenyl)-b-d-ribofuranose (26). As described above for 24, with 1-
bromo-2-fluorobenzene (1.95 ml, 18 mmol), THF (50 ml), 1m BuLi in hexane (11.3 ml, 17 mmol), 21 (5.0 g,
12 mmol), and THF (25 ml). Subsequent treatment in CH2Cl2 (50 ml) with BF3 ´ Et2O (3.0 ml, 24 mmol) and
Et3SiH (3.8 ml, 24 mmol): 26 (4.76 g, 80.0%). Orange solid. TLC (hexane/AcOEt 4 :1): Rf 0.35. 1H-NMR
(270 MHz, (D6)DMSO): 7.57 (dt, J ˆ 7.6, 1.5, HÀC(5)); 7.39 ± 7.26 (m, 16 arom. H, HÀC(3)); 7.19
(m, HÀC(6)); 7.08 (dt, J ˆ 7.5, 0.8, HÀC(4)); 5.23 (d, J ˆ 3.8, HÀC(1')); 4.55 (m, PhCH2 benzyl); 4.23 (q, J ˆ
4.0, HÀC(4')); 4.06 (m, HÀC(2'), HÀC(3')); 3.69 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz, (D6)DMSO): 159.55
(d, J ˆ 245.3, C(2)); 138.21, 138.09 (arom. C); 129.55 (d, J ˆ 8.5, C(6)); 128.12 (C(4)); 128.08, 127.86, 127.66,
127.66, 127.48, 127.41 (arom. C); 127.23 (d, J ˆ 12.8, C(1)); 124.26 (d, J ˆ 3.0, C(5)); 115.05 (d, J ˆ 21.3, C(3));
82.13 (C(1')); 80.52 (C(4')); 76.88 (C(2')); 76.43 (C(3')); 72.37 (PhCH2); 71.02 (PhCH2); 69.76 (C(5')). ESI-MS:
‡
516.5 ([M ‡ NH4] ).
1-Deoxy-1-(2-fluorophenyl)-b-d-ribofuranose (30). As described above for 20, with 26 (4.76 g, 9.5 mmol),
EtOH (80 ml), cyclohexene (40 ml), and 20% Pd(OH)2/C (900 mg) (2 h): 30 (2.09 g, 95.9%). Colourless solid.
TLC (CH2Cl2/MeOH 9 :1): Rf 0.41. 1H-NMR (250 MHz, (D6)DMSO): 7.60 (dt, J ˆ 6.0, 1.8, HÀC(5)); 7.31
(m, HÀC(3)); 7.15 (m, HÀC(4), HÀC(6)); 5.02 (d, J ˆ 5.9, HÀC(1')); 4.88 (m, OHÀC(2'), OHÀC(3')); 4.81
(t, J ˆ 5.5, OHÀC(5')); 3.85 (m, HÀC(2'), HÀC(3'), HÀC(4')); 3.58 (m, 2 HÀC(5')). 13C-NMR (62.9 MHz,
(D6)DMSO): 159.97 (d, J ˆ 245.2, C(2)); 129.23 (d, J ˆ 8.2, C(6)); 128.44 (d, J ˆ 4.3, C(4)); 128.04 (d, J ˆ 12.9,
C(1)); 124.27 (d, J ˆ 3.3, C(5)); 115.00 (d, J ˆ 21.5, C(3)); 84.44 (C(1')); 77.58 (C(4')); 76.72 (C(2')); 71.00
(C(3')); 61.69 (C(5')). ESI-MS: 227.0 ([M À H]À).

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1-Deoxy-5-O-(4,4'-dimethoxytrityl)-1-(2-fluorophenyl)-b-d-ribofuranose (38). As described above for 44,
with 30 (1.81 g, 7.9 mmol), pyridine (50 ml), Et3N (1.2 ml, 12 mmol), and (MeO)2TrCl (3.22 g, 9.5 mmol) (3 h).
FC (CH2Cl2/MeOH 98 :2): 38 (4.11 g, 90.2%). Yellow foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.37. 1H-NMR
(250 MHz, (D6)DMSO): 7.57 (t, J ˆ 6.1, HÀC(5)); 7.46 ± 6.85 (m, 13 arom. H, HÀC(3), HÀC(4), HÀC(6));
5.18 (d, J ˆ 5.1, HÀC(1')); 4.97 (m, OHÀC(2'), OHÀC(3')); 3.98 (q, J ˆ 2.9, HÀC(4')); 3.90 (m, HÀC(2'),
HÀC(3')); 3.74 (s, 2 MeO); 3.22 (m, 2 HÀC(5')). 13C-NMR (62.9 MHz, (D6)DMSO): 159.74 (d, J ˆ 245.4,
C(2)); 158.08, 144.96, 135.67, 135.60, 129.76 ((MeO)2Tr); 129.32 (d, J ˆ 8.2, C(6)); 128.87 ((MeO)2Tr); 128.14
(d, J ˆ 4.6, C(4)); 127.90, 127.76, 126.64 ((MeO)2Tr); 125.29 (C(1)); 124.14 (C(5)); 115.13 (d, J ˆ 21.3, C(3));
113.14, 85.40 ((MeO)2Tr); 81.97 (C(1')); 78.61 (C(4')); 76.55 (C(2')); 70.93 (C(3')); 63.82 (C(5')); 55.00 (MeO).
ESI-MS: 529.0 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(2-fluorophenyl)-b-d-ribofuranose (39).
As described above for 45, with 38 (3.5 g, 6.6 mmol), THF/pyridine 1 :1 (50 ml), AgNO3 (1.35 g, 7.9 mmol),
t
and 1m BuMe2SiCl in THF (9.2 ml, 9.2 mmol). Prep. HPLC (MN Nucleoprep 100-20 from Macherey-Nagel,
hexane/AcOEt/CH2Cl2 47:3 :50): slower-migrating isomer 39 (1.33 g, 31.3%). Colourless foam. TLC (CH2Cl2):
Rf 0.30. 1H-NMR (250 MHz, (D6)DMSO): 7.62 (t, J ˆ 6.3, HÀC(5)); 7.47 ± 6.87 (m, 13 arom. H, HÀC(3),
HÀC(4), HÀC(6)); 5.01 (d, J ˆ 5.3, HÀC(1')); 4.84 (d, J ˆ 5.7, OHÀC(3')); 4.03 (m, HÀC(2'), HÀC(4')); 3.90
(m, HÀC(3')); 3.74 (s, 2 MeO); 3.23 (m, 2HÀC(5')); 0.80 (s, tBuSi); À0.06, À0.11 (s, Me2Si). 13C-NMR
(62.9 MHz, (D6)DMSO): 159.70 (d, J ˆ 245.1, C(2)); 158.08, 144.96, 135.51, 129.76 ((MeO)2Tr); 129.30 (d, J ˆ
8.1, C(6)); 128.14 (d, J ˆ 4.6, C(4)); 127.76, 127.67, 126.65 ((MeO)2Tr); 125.27 (C(1)); 124.15 (C(5)); 115.15
(d, J ˆ 21.4, C(3)); 113.13, 85.45 ((MeO)2Tr); 82.61 (C(1')); 79.00 (C(4')); 77.76 (C(2')); 70.76 (C(3')); 63.85
(C(5')); 54.99 (MeO); 25.56 (Me3CSi); 17.85 (Me3CSi); À4.89, À5.42 (Me2Si). ESI-MS: 643.6 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(2-fluorophenyl)-b-d-ribofuranose
(40) was obtained from the reaction described above as the faster-migrating isomer (1.26 g, 29.6%). Colourless
foam. TLC (CH2Cl2): Rf 0.30. 1H-NMR (250 MHz, (D6)DMSO): 7.63 (t, J ˆ 6.2, HÀC(5)); 7.45 ± 6.85
(m, 13 arom. H, HÀC(3), HÀC(4), HÀC(6)); 4.98 (m, HÀC(1'), OHÀC(2ꢁ)); 4.05 (t, J ˆ 5.2, HÀC(3'));
3.93 (m, HÀC(2'), HÀC(4')); 3.73 (s, 2 MeO); 3.19 (m, 2 HÀC(5')); 0.75 (s, tBuSi); À4.56, À5.19 (2s, Me2Si).
13C-NMR (62.9 MHz, (D6)DMSO): 159.72 (d, J ˆ 245.2, C(2)); 158.10, 144.86, 135.49 ((MeO)2Tr); 129.31
(d, J ˆ 8.3, C(6)); 128.15 (d, J ˆ 4.7, C(4)); 127.92, 127.75, 126.69 ((MeO)2Tr); 125.29 (C(1)); 124.19 (C(5));
115.19 (d, J ˆ 20.9, C(3)); 113.13, 85.57 ((MeO)2Tr); 82.18 (C(1')); 78.89 (C(4')); 76.15 (C(2')); 72.54 (C(3'));
63.20 (C(5')); 54.99 (MeO); 25.67 (Me3CSi); 17.86 (Me3CSi); À4.56, À5.19 (Me2Si). ESI-MS: 643.4 ([M À H]À).
2'-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-5-O-(4,4'-dimethoxytrityl)-1-(2-fluorophenyl)-b-d-ribofuranose 3-
(2-Cyanoethyl Diisopropylphosphoramidite) (6). As described above for 2, with 39 (200 mg, 0.31 mmol), MeCN
(10 ml), collidine (440 ml, 3.1 mmol), 1-methyl-1H-imidazole (14 ml, 0.18 mmol), and 2-cyanoethyl diisopro-
pylphosphoramidochloridite (112 ml, 0.5 mmol) (15 min at 08 and 1 h at r.t.). FC (hexane/AcOEt 4 :1): 6
(150 mg, 57.3%; diastereoisomer mixture). Colourless foam. TLC (hexane/AcOEt): Rf 0.29. 1H-NMR
(270 MHz, CDCl3): 7.73 ± 6.79 (m, 17 arom. H); 5.19, 5.17 (2d, J ˆ 7.8, HÀC(1')); 4.25 (m, HÀC(3')); 4.16
(m, HÀC(2')); 3.96 (m, HÀC(4')); 3.79, 3.78 (2s, 2 MeO); 3.65 (m, 1 HÀC(5'), CH2CN); 3.22 (m, 1HÀC(5'));
2.66 (m, CH2O); 1.14 (m, 2 Me2CH); 0.81, 0.80 (2m, tBuSi); À0.05; À0.09, À0.15, À0.19 (4s, Me2Si). 31P-NMR
‡
(161.98 MHz, CDCl3): 150.47, 148.83; ratio 1:2.7. ESI-MS: 845.6 ([M ‡ H] ).
2,3,5-Tri-O-benzyl-1-deoxy-1-(2,4-difluorophenyl)-b-d-ribofuranose (27). As described above for 24, with
1-bromo-2,4-difluorobenzene (150 ml, 1.3 mmol), anh. Et2O (10 ml), 1.5m BuLi in hexane (2.4 ml, 3.6 mmol), 21
(1.0 g, 2.4 mmol), and Et2O (5 ml). Subsequent treatment in CH2Cl2 (10 ml) with BF3 ´ Et2O (600 ml, 4.8 mmol)
and Et3SiH (760 ml, 4.8 mmol): 27 (1.04 g, 84.6%). Orange solid. TLC (hexane/AcOEt 4 :1): Rf 0.35. 1H-NMR
(270 MHz, (D6)DMSO): 7.59 (ꢀqꢁ, J ˆ 6.9, HÀC(6)); 7.29 (m, 16 arom. H, HÀC(3)); 6.93 (dt, J ˆ 8.4, 2.2,
HÀC(5)); 5.17 (d, J ˆ 4.5, HÀC(1')); 4.55 (m, PhCH2); 4.22 (q, J ˆ 3.9, HÀC(4')); 4.05 (m, HÀC(2'),
HÀC(3')); 3.67 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz, (D6)DMSO): 162.51 (dd, J ˆ 141.7, 12.6, C(4)); 158.87
(dd, J ˆ 143.7, 12.4, C(2)); 138.18, 138.10, 137.96 (arom. C); 129.36 (C(6)); 128.22, 128.09, 127.70, 127.50, 127.42,
127.11 (arom. C); 123.61 (C(1)); 111.30 (d, J ˆ 20.8, C(5)); 103.62 (t, J ˆ 25.7, C(3)); 82.10 (C(1')); 80.72 (C(4'));
‡
76.84 (C(2')); 75.96 (C(3')); 72.40 (PhCH2); 71.04 (PhCH2); 69.73 (C(5')). ESI-MS: 534.4 ([M ‡ NH4] ).
1-Deoxy-1-(2,4-difluorophenyl)-b-d-ribofuranose (31). As described above for 20 with 27 (6.5 g,
12.6 mmol), EtOH (100 ml), cyclohexene (50 ml) and 20% Pd(OH)2/C (1 g) (3 h): 31 (3.08 g, 99.3%).
Colourless solid. TLC (CH2Cl2/MeOH 9 :1): Rf 0.42. 1H-NMR (270 MHz, (D6)DMSO): 7.65 (ꢀqꢁ, J ˆ 6.9,
HÀC(6)); 7.18 (m, HÀC(3)); 7.07 (dt, J ˆ 8.6, 2.4, HÀC(5)); 5.04 (d, J ˆ 6.0, OHÀC(2')); 4.91 (d, J ˆ 5.0,
OHÀC(3')); 4.84 (m, HÀC(1'), OHÀC(5')); 3.85 (m, HÀC(2'), HÀC(3'), HÀC(4')); 3.57 (m, 2 HÀC(5')).
13C-NMR (67.9 MHz, (D6)DMSO): 162.58 (dd, J ˆ 109.2, 12.1, C(4)); 158.94 (dd, J ˆ 111.7, 12.1, C(2)); 129.67
(C(6)); 124.40 (dd, J ˆ 13.1, 3.5, C(1)); 114.74 (dd, J ˆ 20.9, 3.5, C(5)); 103.42 (t, J ˆ 25.9, C(3)); 84.59 (C(1'));

Helvetica Chimica Acta ± Vol. 83 (2000)
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77.08 (C(4')); 76.65 (C(2')); 70.94 (C(3')); 61.59 (C(5')). 19F-NMR (254.2 MHz, (D6)DMSO): À112.0
(quint., J ˆ 7.6, FÀC(4)); À114.74 (q, J ˆ 8.9, FÀC(2)). ESI-MS: 245.0 ([M À H]À). Anal. calc. for
C11H12F2O4 (246.2): C 53.66, H 4.91; found: C 53.65, H 4.94.
1-Deoxy-1-(2,4-difluorophenyl)-5-O-(4,4'-dimethoxytrityl)-b-d-ribofuranose (41). As described above for
44, with 31 (1 g, 4 mmol), pyridine (25 ml) Et3N (840 ml, 6 mmol), and (MeO)2TrCl (1.63 g, 4.8 mmol) (2.5 h).
FC (CH2Cl2/MeOH 98 :2): 41 (1.86 g, 83.4%). Yellow foam. TLC (CH2Cl2/MeOH 95 :5): Rf 0.48. 1H-NMR
(270 MHz, (D6)DMSO): 7.56 (ꢀqꢁ, J ˆ 6.8, HÀC(6)); 7.45 ± 6.86 (m, 13 arom. H, HÀC(3), HÀC(5)); 5.19
(d, J ˆ 5.4, OHÀC(2')); 4.98 (d, J ˆ 5.2, OHÀC(3')); 4.92 (d, J ˆ 4.3, HÀC(1')); 3.98 (q, J ˆ 5.2, HÀC(2')); 3.90
(m, HÀC(3'), HÀC(4')); 3.74 (s, 2 MeO); 3.20 (m, 2 HÀC(5')). 13C-NMR (67.9 MHz, (D6)DMSO): 161.71
(d, J ˆ 246.1, C(4)); 161.53 (d, J ˆ 246.4, C(2)); 158.04, 144.87, 135.63, 135.54, 129.71 ((MeO)2Tr); 129.11
(dd, J ˆ 9.8, 6.1, C(6)); 127.76, 127.72, 126.63 ((MeO)2Tr); 124.28 (dd, J ˆ 13.1, 3.2, C(1)); 113.13 ((MeO)2Tr);
111.12 (dd, J ˆ 20.8, 3.2, C(5)); 103.69 (t, J ˆ 25.9, C(3)); 85.40 ((MeO)2Tr); 82.25 (C(1')); 78.10 (C(4')); 76.36
(C(2')); 70.93 (C(3')); 63.77 (C(5')); 54.98 (MeO). ESI-MS: 547.2 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-1-(2,4-difluorophenyl)-5-O-(4,4'-dimethoxytrityl)-b-d-ribofuranose
(42). As described above for 45, with 41 (2.35 g, 4.3 mmol), anh. THF/pyridine 1:1 (40 ml), AgNO3 (870 mg,
5.2 mmol), and 1m tBuMe2SiCl (6 ml, 6 mmol). FC (CH2Cl2): slower-migrating isomer 42 (1.01 g, 35.5%).
Colourless foam. TLC (CH2Cl2): Rf 0.18. 1H-NMR (270 MHz, (D6)DMSO): 7.59 (q, J ˆ 6.7, HÀC(6)); 7.45 ±
6.82 (m, 15 arom. H, HÀC(3), HÀC(5)); 4.96 (d, J ˆ 5.7, OHÀC(3')); 4.84 (d, J ˆ 5.7, HÀC(1')); 4.02
(m, HÀC(3'), HÀC(4')); 3.93 (t, J ˆ 5.2, HÀC(2')); 3.74 (s, 2 MeO); 3.21 (m, 2 HÀC(5')); 0.79 (s, tBuSi);
À0.05, À0.11 (2s, Me2Si). 13C-NMR (67.9 MHz, (D6)DMSO): 161.69 (d, J ˆ 246.3, C(2)); 161.51 (d, J ˆ 246.5,
C(4)); 158.04, 144.87, 135.46, 135.41, 129.71, 129.68 ((MeO)2Tr); 129.07 (dd, J ˆ 11.6, 7.8, C(6)); 127.73, 127.62
((MeO)2Tr); 123.85 (dd, J ˆ 15.7, 3.4, C(1)); 113.12 ((MeO)2Tr); 111.12 (dd, J ˆ 20.8, 3.4, C(5)); 103.61 (t, J ˆ
26.1, C(3)); 85.45 ((MeO)2Tr); 82.94 (C(1')); 78.79 (C(4')); 77.20 (C(2')); 70.20 (C(3')); 63.51 (C(5')); 54.97
(MeO); 25.50 (Me3CSi); 17.78 (Me3CSi); À4.93, À5.47 (Me2Si). ESI-MS: 661.4 ([M À H]À).
3-O-[(tert-Butyl)dimethylsilyl]-1-deoxy-1-(2,4-difluorophenyl)-5-O-(4,4'-dimethoxytrityl)-b-d-ribofuranose
(43) was obtained from the reaction described above as the faster-migrating isomer (0.89 g, 31.4%). Colourless
foam. TLC (CH2Cl2): Rf 0.21. 1H-NMR (270 MHz, (D6)DMSO): 7.61 (q, J ˆ 8.2, HÀC(6)); 7.45 ± 6.84
(m, 15 arom. H, HÀC(3), HÀC(5)); 4.96 (d, J ˆ 6.6, OHÀC(2')); 4.83 (d, J ˆ 5.6, HÀC(1')); 4.02
(m, HÀC(3')); 3.94 (m, HÀC(2')); 3.73 (s, 2 MeO); 3.29 (m, HÀC(4')); 3.15 (m, 2 HÀC(5')); 0.76 (s, tBuSi);
À0.01, À0.08 (2s, Me2Si). 13C-NMR (67.9 MHz, (D6)DMSO): 161.65 (d, J ˆ 246.1, C(2)); 161.49 (d, J ˆ 246.3,
C(4)); 158.05, 144.67, 135.46, 135.42, 129.67 ((MeO)2Tr); 129.13 (dd, J ˆ 11.9, 8.0, C(6)); 127.71, 127.63, 126.63
((MeO)2Tr); 123.82 (dd, J ˆ 12.4, 3.9, C(1)); 113.11 ((MeO)2Tr); 111.30 (dd, J ˆ 24.6, 3.7, C(5)); 103.59 (t, J ˆ
26.7, C(3)); 85.46 ((MeO)2Tr); 82.52 (C(1')); 78.29 (C(4')); 75.94 (C(2')); 72.63 (C(3')); 63.21 (C(5')); 54.97
(MeO); 25.64 (Me3CSi); 17.82 (Me3CSi); À4.59, À5.21 (Me2Si). ESI-MS: 661.4 ([M À H]À).
2-O-[(tert-Butyl)dimethylsilyl)-1-deoxy-1-(2,4-difluorophenyl)-5-O-(4,4'-dimethoxytrityl)-b-d-ribofuranose
3-(2-Cyanoethyl Diisopropylphosphoramidite) (7). As described above for 2, with 42 (250 mg, 0.38 mmol),
MeCN (12 ml), collidine (500 ml, 3.8 mmol), 1-methyl-1H-imidazole (15 ml, 0.19 mmol), and 2-cyanoethyl
diisopropylphosphoramidochloridite (128 ml, 0.57 mmol) (15 min at 08 and 1 h at r.t.). FC (hexane/AcOEt 4 :1):
7 (202 mg, 62.1%, diastereoisomer mixture). Colourless foam. TLC (hexane/AcOEt 4 :1): Rf 0.36, 0.34.
1H-NMR (270 MHz, (D6)DMSO): 7.67 (m, HÀC(6)); 7.53 ± 6.77 (m, 13 arom. H, HÀC(3), HÀC(5)); 5.14, 5.12
(2d, J ˆ 7.7, 6.8 Hz, HÀC(1')); 4.23 (m, HÀC(2'), HÀC(3'), HÀC(4')); 3.80, 3.79 (2s, 2 MeO); 3.54
(m, 2 HÀC(5')); 1.17 (m, 2 Me2CH); 0.82 (s, tBuSi); À0.05, À0.17 (2s, Me2Si). 31P-NMR (161.98 MHz, CDCl3):
‡
150.70, 148.85; ratio 1:3.8. ESI-MS: 863.6 ([M ‡ H] ).
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Received April 27, 2000