Synthesis and properties of oligodeoxynucleotide analogs with bis(methylene) sulfone bridges

Article abstract of DOI:10.1002/hlca.200390244

A convergent, solution-phase synthesis was developed for the bis(methylene) sulfone-bridged oligodeoxynucleotide analogs (SNA) 5′-d(HOCH ₂-Tso₂Tso₂Tso₂Cso ₂Tso₂Tso₂Tso₂T-CH₂SO ₃)-3′ (35b) and 5′-d(HOCH₂-Tso ₂Tso₂Tso₂Tso₂Tso₂Tso ₂Tso₂T-CH₂SO₂)-3′ (34c) (SO₂ corresponds to CH₂SO₂CH₂ instead of OP(=O)(O^-)(O). In these, the phosphodiester linkages are replaced by non-ionic bis(methylene) sulfone linkers. The general strategy involved convergent coupling of 3′,5′-bishomo-β -D-deoxyribonucleotide analogs functionalized at the 6′-end (=CH ₂-C(5′)) as bromides or mesylates and at the CH ₂-C(3′) position as thiols, with the resulting thioether being oxidized to the corresponding sulfone. A single charge was introduced at the terminal CH₂-C(3′) position of the octamers to increase their solubility in water. During the synthesis, it became apparent that the key intermediates generated secondary structures through either folding or aggregation in a variety of solvents. This generated unusual reactivity and was unique for very similar structures. For example, although the dimeric thiol d(BzOCH₂-Tso₂C¹⁰C-CH₂SH) (14b) was a well-behaved synthetic intermediate, the tetrameric thiol d(TrOCH ₂-Tso₂Tso₂Tso₂C-CH₂SH) derived from the corresponding thioacetate was rapidly converted to a disulfide by very small amounts of oxidant (28 → 29, Scheme 6), while the analogous tetrameric thiol d(BzOCH₂-Tso₂TsTso₂T-CH ₂SH) (26), differing only by a single heterocycle, was oxidized much more slowly (Bz = PhCO, Tr = Ph₃C, to = 2-MeC₆H ₄CO (at N⁴ of dc)). The sequence-dependent reactivity, well known in many classes of natural products (including polypeptides), is not prominent in natural oligonucleotides. These results are discussed in light of the proposal that the repeating negative charge in nucleic acids is key to their ability to serve as genetic molecules, in particular, their capability to support Darwinian evolution. The ability of 5′-d(HOCH₂- Tso₂TSo₂TSo₂Cso₂TSo ₂TSo₂TSo₂T-CH₂SO₃ ^-)-3′ (35b) to bind as a third strand to duplex DNA was also examined. No triple-helix-forming propensity was detected in this molecule.

Full text of DOI:10.1002/hlca.200390244

Helvetica Chimica Acta Vol. 86(2003)
2959
Synthesis and Properties of Oligodeoxynucleotide Analogs with
Bis(methylene) Sulfone Bridges
by Bernd Eschgf‰ller1), J¸rgen G. Schmidt2), Marcel Kˆnig, and Steven A. Benner*
Departments of Chemistry, and Anatomy and Cell Biology, University of Florida, Gainesville, FL 32611, USA
A convergent, solution-phase synthesis was developed for the bis(methylene) sulfone-bridged oligode-
oxynucleotide analogs (SNA) 5'-d(HOCH2-Tso2Tso2Tso2Cso2Tso2Tso2Tso2T-CH2SOÀ3 )-3' (35b) and 5'-
d(HOCH2-Tso2Tso2Tso2Tso2Tso2Tso2Tso2T-CH2SOÀ3 )-3' (34c) (SO2 corresponds to CH2SO2CH2 instead of
OP(ˆO)(OÀ)(O). In these, the phosphodiester linkages are replaced by non-ionic bis(methylene) sulfone
linkers. The general strategy involved convergent coupling of 3',5'-bishomo-b-d-deoxyribonucleotide analogs
functionalized at the 6'-end (ˆCH2ÀC(5')) as bromides or mesylates and at the CH2ÀC(3') position as thiols,
with the resulting thioether being oxidized to the corresponding sulfone. A single charge was introduced at the
terminal CH2ÀC(3') position of the octamers to increase their solubility in water. During the synthesis, it
became apparent that the key intermediates generated secondary structures through either folding or
aggregation in a variety of solvents. This generated unusual reactivity and was unique for very similar structures.
For example, although the dimeric thiol d(BzOCH2-Tso2C-CH2SH) (14b) was a well-behaved synthetic
intermediate, the tetrameric thiol d(TrOCH2-Tso2Tso2Tso2toC-CH2SH) derived from the corresponding
thioacetate was rapidly converted to a disulfide by very small amounts of oxidant (28 ! 29, Scheme 6), while
the analogous tetrameric thiol d(BzOCH2-Tso2TsTso2T-CH2SH) (26), differing only by a single heterocycle, was
oxidized much more slowly (Bz ˆ PhCO, Trˆ Ph3C, to ˆ 2-MeC6H4CO (at N4 of dc)). The sequence-dependent
reactivity, well known in many classes of natural products (including polypeptides), is not prominent in natural
oligonucleotides. These results are discussed in light of the proposal that the repeating negative charge in nucleic
acids is key to their ability to serve as genetic molecules, in particular, their capability to support Darwinian
evolution. The ability of 5'-d(HOCH2-Tso2Tso2Tso2Cso2Tso2Tso2Tso2T-CH2SOÀ3 )-3' (35b) to bind as a third
strand to duplex DNA was also examined. No triple-helix-forming propensity was detected in this molecule.
Introduction. A considerable amount of work has been devoted over the past few
years to analyze the −chemical etiology× of nucleic acids [1]. To this end, effort has been
devoted towards chemical synthesis of alternative forms of nucleic acids. This work has
inquired what structures might support the rule-based molecular recognition required
for genetics, and how genetic molecules might appear if they emerged elsewhere in the
cosmos, independent of life on Earth [2][3]. Considerable progress has been made in
understanding the degree to which the heterocyclic nucleobases can tolerate
modification [4 8], and an entirely new genetic alphabet has been created, shown to
expand the number of amino acids that can be encoded in proteins [9], and
incorporated into commercial diagnostics and drug-discovery products [10]. Likewise,
the sugar units of DNA have been modified in DNA analogs [11 14], as have the
phosphate linkages [15][16]. Some strikingly attractive analogs of DNA have emerged
that appear to support rule-based molecular recognition quite well [17][18].
1
)
)
Present address: Noxxon Pharma AG, Gustav-Mayer-Allee 25, D-13355, Berlin.
Present address: Los Alamos National Laboratory, Bioscience Division, B-3/MS E 529, Los Alamos, NM
87545, USA.
2

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The interaction between the phosphate, sugar, and nucleobase pieces in DNA is, of
course, a key to the etiology of DNA, as it is with other classes of molecules. The
emerging view holds that the backbone plays a more-important role in the process of
molecular recognition than was explicitly incorporated into the Watson-Crick model
[19][20]. In part, this view was based on oligonucleotide analogs in which the bridging
phosphodiesters were replaced by non-ionic groups, including phosphonates [21],
amide backbones [21], and bis(methylene) sulfones [22][23]. The last are sulfone-
linked oligonucleotide analogs (SNAs) that are isoelectronic, largely isosteric, chemi-
cally stable, and contain no stereocenters. A radiolabeled tetrameric rSNA showed
remarkably good bioavailability in a mouse study [24], and a short sulfone-linked RNA
dinucleotide analog formed a Watson-Crick duplex in a crystal [25]. Longer sulfone-
linked RNA analogs displayed rich conformational properties, however, far broader
than those allowed by simple Watson-Crick rules [26].
While the conformation of non-ionic analogs of nucleic acids has proven to be
remarkably complex, certain areas remain where non-ionic oligonucleotide analogs
might be useful. For example, triple helices have a high charge density; they may be
more easily formed by non-ionic nucleic acid analogs. Further, the degree to which
physical and chemical properties of large molecules are changed by small changes in
sequences, first suggested in an SNA matrix, needs to be further explored. Given the
availability of substantial amounts of building blocks (see the preceding paper [27]), we
have explored in detail the synthesis of octameric dSNAs and tested their molecular-
recognition properties. The details of the synthesis provide insights into the change in
reactivity as one proceeds from small molecules to large molecules.
Results. Synthesis of SNAs. The general strategy for the synthesis of SNAs in
solution involves the formation of a thioether via the reaction of a thiolate of one unit
with a C-atom carrying a leaving group at the other unit. The resulting thioether is then
oxidized to yield the sulfone. The synthesis is convergent, with monomers yielding
dimers, dimers yielding tetramers, and tetramers yielding octamers [28].
Two leaving groups at the 6'-position (ˆCH2ÀC(5')), mesylate and bromide, were
examined in the formation of dinucleotide analogs. In trial runs, 6'-O-trityl(Tr)-
protected thiol 1 and 2b carrying a 6'-mexyloxy group were coupled in degassed DMF
in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) at room temperature to
yield the dinucleotide analog 3b in 80% yield after chromatography (Scheme 1). The
analogous reaction in THFas solvent gave 3b in 72% yield. With 6'-bromo derivative 4b
as the electrophile in the presence of Cs2CO3 as the base in degassed DMF at room
temperature, 3b was obtained in 83% yield after chromatography. Similar results were
achieved in coupling reactions of mesylate 2a and thiol 1 in the presence of Cs2CO3 in
DMF at 458 (82% yield of 3a), as well as of bromide 4a and thiol 1 in the presence of
Cs2CO3 in THF at 458 (84% yield of 3a). Modestly higher yields were obtained when
Cs2CO3 was used as base, even though Cs2CO3 did not dissolve completely in the
organic solvents [29]. Coupling was slower in THF than in DMF. This became
especially noticeable for the coupling of dimeric and tetrameric SNAs to give tetramers
and octamers, respectively. Richert performed analogous reactions in MeCN/H2O
mixtures, which enabled the complete solubilization of Cs2CO3 but led to up to six times
longer reaction times [30]. Following optimization, conditions for coupling 6'-O-

Helvetica Chimica Acta Vol. 86(2003)
2961
Scheme 1
MsO
B
O
RO
T
TBDPSO
2a B = T
b B = to
O
C
RO
T
S
O
base, solvent
B
O
HS
Br
TBDPSO
B
1 R = Tr
5 R = Bz
O
3a R = Tr, B = T
b R = Tr, B = toC
6 R = Bz, B = T
TBDPSO
4a B = T
b B = to
C
RO
RO
T
T
O
O
Oxone
Oxone
SO2
SO
B
B
O
O
TBDPSO
TBDPSO
8a R = Tr, B = T
b R = Tr, B = toC
7a R = Tr, B = T
b R = Tr, B = toC
c R = Bz, B = T
c R = Bz, B = T
TBDPS ˆ (t-Bu)Ph2Si, to ˆ 2-MeC6H4CO, Trˆ Ph3C
benzoyl(Bz)-protected thiol 5 and bromide 4a with Cs2CO3 in degassed DMF at 508
over 4 h generated 6 in 99% yield.
Oxone, a 2 KHSO4 ¥ K2SO4 ¥ KHSO4 triple salt, converts sulfides to sulfones
selectively [31]. To avoid detritylation and depurination, oxidations of sulfides 3a,b
and 6 were performed in THF/MeOH/H2O mixtures buffered with NaOAc. Under
these conditions, the sulfides were oxidized within few minutes to the mixtures 7a c of
diastereoisomeric sulfoxides, which were not separated (Scheme 1). Subsequent
oxidation converted the sulfoxides 7a c to the corresponding sulfones 8a c. Under
optimized oxidation conditions, 8b and 8c were obtained in essentially quantitative
yields, and 8a in 96% yield.
The Tr group was removed from dimers 8a and 8b in view of their transformation to
become electrophiles for the next cycle of coupling. Initially, TsOH in MeOH was used
to deprotect the dimer, e.g., 8b; the reaction mixture was neutralized with NaHCO3
before direct chromatography of the crude product to yield 9b in 91% yield

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Helvetica Chimica Acta Vol. 86(2003)
(Scheme 2). This method generated a small amount of by-product after prolonged
reaction times. Cleavage with mild Lewis acids was, therefore, examined. For example,
8b was dissolved in CH2Cl2 and treated with 5 equiv. of ZnCl2 ¥ Et2O solution
(Scheme 2). The resulting turbid, yellow suspension was filtered through a layer of
silica gel after 10 min, and 9b was obtained in quantitative yield after chromatography.
Likewise, 8a was deprotected with 10 equiv. of ZnCl2 ¥ Et2O solution to give 9a in 99%
yield.
Scheme 2
HO
RO
T
T
O
O
ZnCl2 · Et2O
SO2
CH2Cl2 or
SO2
B
B
TsOH, MeOH
O
O
TBDPSO
9a B = T
b B = toC
TBDPSO
MsCl
C5H5N
8a R = Tr, B = T
b R = Tr, B = toC
c R = Bz, B = T
CBr4
PPh3
C2H4Cl2
for 8a, for 8c,
Bu4NF THF
Br
MsO
RO
T
T
T
O
O
O
SO2
SO2
SO2
toC
B
T
O
O
O
TBDPSO
TBDPSO
HO
12a R = Tr
b R = Bz
11a B = T
b B = toC
10
TBDPS ˆ (t-Bu)Ph2Si, to ˆ 2-MeC6H4CO, Trˆ Ph3C
For the next step of a convergent synthesis of SNAs, a leaving group was required at
the 6'-end of the dimers. Preliminary work suggested that better coupling yields might
be obtained the electrophile were a mesylate rather than a bromide. But mesylation of
9b in pyridine generated 10 in only 47% yield (Scheme 2). However, bromination of 9b
in 1,2-dichloroethane with CBr4 and PPh3 first at 08 and then at room temperature
(75 min) gave, after workup and chromatography, 11b in 86% yield. Compound 9a was
similarly converted to bromide 11a in 97% yield. Removal of the (t-Bu)Ph2Si group at
OCH2ÀC(3') was also necessary to support convergent synthesis of SNAs. This proved

Helvetica Chimica Acta Vol. 86(2003)
2963
to be readily done with Bu4NF in THF. Under these conditions, compounds 8a and 8c
gave 12a (93%) and 12c (91%), respectively, after chromatography. Loss of the benzoyl
group in the deprotection of 8c was not detected.
The Mitsunobu reaction was used to introduce the S-atom into the dimers to
generate the nucleophile for higher-order couplings. Thioacetates 13a,b were prepared
from 12a,b with Ph3P, diisopropyl azodicarboxylate (DIAD), and thioacetic acid in
anh. THF in 91 and 99% yield, respectively, after chromatography (Scheme 3).
Thioacetate 13a was then converted to thiol 14a, either by treatment with NaBH4 in
degassed MeOH or by ammonolysis in degassed MeOH in quantitative yield.
Treatment of 13b with NaBH4 led to the loss of the 6'-O-benzoyl group; therefore,
ammonolysis was used to generate 14b, in quantitative yield. Disulfide was observed
neither during the reaction steps nor during the purification, a result that was to
become significant as the convergent synthesis proceeded.
Scheme 3
RO
RO
T
T
O
O
PPh3, DIAD
AcSH, THF
for 13a, NaBH4,MeOH
for 13b, NH3, MeOH
SO2
SO2
T
T
O
O
AcS
HO
12a R = Tr
b R = Bz
13a R = Tr
b R = Bz
R
RO
T
T
O
O
for 14b
DMTCl
Et3N, THF
SO2
SO2
T
T
O
O
DMT-S
HS
15 R = BzO
16 R = OH
17 R = Br
14a R = Tr
b R = Bz
NaOH in MeOH
CBr , PPh ,
C2H4Cl2
4
3
Tr ˆ Ph3C, DIAD ˆ diisopropyl azodicarboxylate, DMTˆ (MeO)2Tr ˆ (4-MeOC6H4)2PhC
These optimized conditions for preparing dimeric SNAs became less satisfactory in
the case of longer-SNA synthesis. In previous work, Huang synthesized an all-sulfide-
linked octamer and attempted to oxidize the oligomer in a single step at the end of the
synthesis [28]. The oxidation was very slow and gave only poor yields; this was
attributed to the poor solubility of the intermediate, but the observation could not then
be examined in greater detail due to lack of sufficient starting material [28].

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Helvetica Chimica Acta Vol. 86(2003)
Aggregation also appeared to be more severe in oligomers with sulfide-containing
linkers than with sulfoxide- or sulfone-containing linkers. For this reason, Richert
oxidized thioethers immediately after every coupling step with Oxone [30]. Bl‰ttler and
Kˆnig also chose this strategy for the synthesis of their dSNAs and rSNAs, respectively
[32][33]. One way to manage solubility in longer oligonucleotide analogs is to
introduce an anionic group at the end of the oligomer. Huang, e.g., treated a
(MeO)2TrSCH2ÀC(3') thioether with Oxone to generate
a
terminal
KOSO2CH2ÀC(3') group, and found its solubility to be enhanced. Kˆnig introduced
such a sulfonate group at the 6'-position by analogous oxidation of a TrSCH2ÀC(5')
thioether [33]. Thus, a (MeO)2Tr thioether group was introduced strategically at an
early stage in the present synthesis, with the intent of oxidizing it to a sulfonate with
Oxone at the end of the synthesis. Accordingly, the mercapto group CH2SH at C(3') of
14b was protected with (MeO)2TrCl and Et3N in anh. THF to give 15 in 99% yield after
chromatography (Scheme 3). The overall yield for the three steps from 12b to 15 was
98%. The benzoyl protection of 15 was removed with 2m NaOH in MeOH to give 16
(98%), which was treated with PPh3 and CBr4 in 1,2-dichloroethane to rapidly yield 6'-
bromo derivative 17 (97%). A trace of (MeO2)Tr-cleavage product was observed under
these conditions.
To couple dimers to give tetramers, Cs2CO3 and DMF were used under a set of
optimized conditions. Coupling 14a and 11a at room temperature gave tetramer 19a
(85%), and coupling 14a and 11b at 458 gave tetramer 19b (81%) (Scheme 4). Coupling
14b and 17 at 458 gave tetramer 18 (92%) (Scheme 5). In general, the coupling at 40
508 seemed preferable. The tetrameric monosulfide derivatives 19a and 19b were
oxidized with Oxone in MeOH/THF 2 :1 in the presence of NaOAc to give the
tetrameric trisulfone derivatives 20a (92%) and 20b (90%) (Scheme 4). The thioether
linkage in 18 could not be selectively oxidized to the corresponding sulfone without also
oxidatively cleaving the (MeO)2Tr thioether at the 3'-end. Compound 18, therefore,
entered subsequent coupling reactions as the mixed sulfoxide-sulfide derivative. The 6'-
O-trityl group of 20a was cleaved with ZnCl2 ¥ Et2O solution in CH2Cl2 within 10 min to
yield 21 in quantitative yield after chromatography (Scheme 4). Removal of the 6'-O-
benzoyl group of 18 was achieved by hydrolysis with 2m NaOH in MeOH/THF 3 :1 for
30 min to yield 22 in 96% yield after chromatography (Scheme 5).
As the SNA intermediates became longer, issues relating to purification were
revisited. For monomers and oligomers, flash chromatography (FC) on silica gel with a
stepwise gradient 0% ! 20% MeOH/CH2Cl2 sufficed; for dimers and tetramers,
addition of 0.25% H2O gave improved separations (cf. Sect. 9 in the Exper. Part). For
3',5'-deprotected tetramers, MeOH (2.5 5%) and H2O (0.25%) in the starting solvent
were essential to obtain good separation. With pure CH2Cl2, the SNAs eluted from the
silica gel only in a broad band.
The 6'-OH compounds 21 and 22 were activated by introduction of a bromo
substituent as leaving group (Scheme 5). The small scale of the bromination reaction
was problematic, given that traces of H2O would consume the reagents, while excess
reagent generated side reactions. This problem was circumvented by addition of a
secondary educt by Richert, who added greater than stoichiometric amounts of a 6'-OH
uridine analog to his tetramer to enhance the absolute alcohol concentration in the
bromination reaction (turnover rate 80%, yielding 60% brominated monomer and

Helvetica Chimica Acta Vol. 86(2003)
2965
Scheme 4
Br
TrO
T
O
T
O
RO
SO2
T
B
O
O
SO2
11a B = T
b B = to
T
TBDPSO
SO2
O
S
C
T
O
Cs2CO3, DMF
T
HS
for 14a
O
14a R = Tr
b R = Bz
SO2
B
O
TBDPSO
19a B = T
b B = to
C
HO
TrO
T
T
O
O
SO2
SO2
T
T
O
O
Oxone, NaOAc
SO2
ZnCl2 · Et2O
for 20a
SO2
T
T
O
O
SO2
SO2
20a B = T
b B = toC
T
B
O
O
TBDPSO
TBDPSO
21
Tr ˆ Ph3C, to ˆ 2-MeC6H4 CO, TBDPS ˆ (t-Bu)Ph2Si
40% brominated tetramer [30]). Following this strategy, tetramer 21 and monomer 25
(compound 11 of the preceding paper [27]) were brominated with PPh3 and CBr4 in 1,2-
dichloroethane/MeCN 4 :1 (Scheme 5). TLC Monitoring showed the complete turn-
over of starting material after 90 min. By-products were not detected. The crude
products were purified by FC (silica gel) to give tetramer 24 in 91% yield and
monomeric bromide 4a in 96% yield. The 6'-OH compound 22 was treated with PPh3
and CBr4 in 1,2-dichloroethane/MeCN 4 :1 to yield the corresponding bromo derivative

2966
Helvetica Chimica Acta Vol. 86(2003)
Scheme 5
BzO
T
O
BzO
Br
T
T
SO2
O
O
T
SO2
O
SO2
T
O
T
S
O
DMT-S 17
10% CF3COOH
5% HSCH2CH2SH
+
T
O
Cs2CO3
S
14b
2% (i-Pr)3SiH
1% H2O
T
DMF
SO2
O
18
T
O
SO2
HS
T
O
26
DMTS
2M NaOH
MeOH/THF 3:1
HO
Br
T
T
O
O
O
H3C
O
SO2
NH
N
SO2
HO
O
T
T
O
O
TBDPSO
25
S
X
CBr4, PPh3
T
CBr4
PPh3
T
C2H4Cl2/
MeCN 4:1
O
O
21
C2H4Cl2/
MeCN 4:1
SO2
SO2
4a
T
T
O
O
DMTS
22
R
23 R = DMT-S, X = S
24 R = TBDPSO, X = SO2
DMTˆ (MeO)2Tr ˆ (4-MeOC6H4)2PhC, TBDPS ˆ (t-Bu)Ph2Si
23 in 74% yield (95% based on recovered starting material); as with dimer 16, small
amounts of detritylation product were observed (Scheme 5).
Huang described the cleavage of (MeO)2Tr thioethers with silver nitrate in H2O/
MeOH/THF and treatment of the remaining silver thiolate with dithioerythrol (DTE)

Helvetica Chimica Acta Vol. 86(2003)
2967
to obtain the deprotected thiol [28]. This reaction was generally low yielding (< 50%).
Huang was not able to use acidic cleavage due to the presence of a (MeO)2Tr ether at
the 6'-end of his building blocks [28]. The use of the 6'-O-benzoyl protecting group in
this work (see 18) makes acidic cleavage strategically acceptable. Thus, the
(MeO)2TrÀS bond was cleaved in tetrameric 18 to yield the corresponding thiol 26
with 10% CF3COOH, 5% ethane-1,2-dithiol, 2% triisopropylsilane [34], and 1% H2O
as scavengers in CH2Cl2 for 5 min (Scheme 5). Starting material 18 (6% yield) and thiol
26 (80% yield; 85% based on recovered starting material) were obtained after
chromatography. The formation of the disulfide of 26 was not problematic.
A second nucleophilic tetramer was created by the removal of the silyl protecting
group from 20b (Scheme 6). Buffering the Bu4NF solution with AcOH to pH 5 6
prevented the loss of the N4-(o-toluoyl) protecting group [35]. The (t-Bu)Ph2Si group
of 20b was, thus, cleaved in THF within 2 h to give 27 in 93% yield. Compound 27 with
a CH2OH group at C(3') was converted to the corresponding thioacetate on a small
scale with a secondary educt to increase the overall concentration, in analogy to
Richert×s mixed bromination procedure [30]. Reaction with PPh3, diisopropyl
azodicarboxylate (DIAD), and thioacetic acid gave 28 (94% yield) as a colorless
foam after chromatography.
Treatment of 28 with either ammonia or 2m NaOH in pyridine/EtOH generated
disulfide 29 (Scheme 6) as the sole product, even though all solutions were degassed,
first with Ar (1 h), and then by repeated freeze-thaw cycles. No thiol was detectable by
TLC at any stage during the reaction. Even though thiols in basic media are known to
be sensitive to oxidation, the result was surprising, as the expected thiol was structurally
quite similar to the tetrameric thiol 26 that was prepared in over 80% yield by closely
analogous procedures without yielding disulfide. To further investigate the sequence-
dependence of disulfide formation, tetramer thiol 26 was dissolved in degassed MeOH,
and treated either with gaseous ammonia or 2m NaOH. In both cases, the
corresponding disulfide could not be detected. The only difference between 26 and
30 was that the latter had an N4-(o-toluoyl)cytosine replacing a thymine moiety at the
analogous position in the sequence, and the 6'-O-protecting groups were benzoyl and
trityl respectively. In natural DNA, such differences would not change reactivity
dramatically; here they did.
Disulfide 29 was reduced to the corresponding thiol 30 with PBu3 in degassed THF/
H2O [33][36 38] ( Scheme 6). Attempts to purify 30 by chromatography on silica gel
yielded disulfide 29 as the single product, the oxidation presumably occurring during
the purification step. Efforts to obtain the stable thiol from 29 were pursued further.
Ekathiol, a resin carrying immobilized dithiothreitol, reduces disulfides in aqueous or
organic solvents under neutral, mildly acidic, or mildly basic conditions when present in
stoichiometric excess. Purification is achieved by simply separating the solution from
the resin. Disulfide 29 and 10 equiv. of Ekathiol resin were suspended in degassed THF
and shaken for 4 h at room temperature. After filtration and evaporation at 08, 30 was
isolated in essentially quantitative yield.
The instability of 30 with respect to the formation of disulfide proved an obstacle to
the synthesis of octamers from tetramers. The first attempts to couple the tetramers 24
and 30 in Cs2CO3 in DMF at room temperature followed by oxidation with Oxone
generated only tetrameric sulfonate 31 resulting from oxidation of disulfide 29

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Helvetica Chimica Acta Vol. 86(2003)
Scheme 6
TrO
TrO
T
T
O
O
SO2
SO2
T
T
O
O
PPh3, DIAD
Bu4NF
THF
NH3, MeOH or
2M NaOH in
20b
SO2
AcSH
SO2
EtOH/pyridine
T
T
O
O
SO2
SO2
toC
toC
O
O
AcS
HO
28
27
TrO
TrO
TrO
T
O
T
T
O
O
SO2
SO2
SO2
T
PBu3,
THF/H2O
or
O
T
T
O
O
SO2
Ekathiol resin
SO2
SO2
T
O
T
T
O
O
SO2
SO2
toC
SO2
toC
O
toC
O
O
R
29
30 R = SH
S
S
–
31 R = SO3
1. PBu3
2. Cs2CO3,
DMF/ H2O
24
octamer synthesis
32a
Tr ˆ Ph3C, to ˆ 2-MeC6H4CO, DIAD ˆ diisopropyl azodicarboxylate
32a

Helvetica Chimica Acta Vol. 86(2003)
2969
(Scheme 6). A similar result was achieved when disulfide 29 was reduced with Ekathiol
resin in degassed DMF, even when the solution was passed with filtration directly under
Ar into a mixture of 24 and Cs2CO3, further degassed with multiple freeze-pump
cycles, and stirred at 458 overnight. Huang, for d(UsUsUsUsUsUsUsU) [28], and
Richert, for r(Aso2Uso2Gso2GsUso2Cso2Aso2U) [30], had also observed that the
coupling of the functionalized tetrameric SNA to yield the corresponding octamers
proved to be more difficult than the coupling of smaller fragments. Huang performed
his coupling reaction with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) in THF/MeOH
10 :1, in 20 30% yield. The addition of MeOH was necessary in this case because the
precursor tetrameric thiolate precipitated in THF. Richert reported very long reaction
times with considerable formation of by-products in the coupling reaction when MeCN/
H2O or THF was used as solvent. Disulfide was detected as well, and the overall
coupling yield was only 50 70%.
In the next coupling attempt (Scheme 7), disulfide 29 was dissolved in degassed
DMF/H2O 5 :1, PBu3 was added, and the mixture was stirred for 2 h. The solution
containing thiol 30 was transferred via capillary under Ar pressure to a solution of
bromide 24 and Cs2CO3 in DMF/H2O. The reaction mixture was again degassed with
multiple freeze-pump cycles and stirred for 6h at 45 8. Crude 32a was directly oxidized
with Oxone and NaOAc in MeOH/THF/H2O 4 :2 :1 for 12 h to the corresponding
sulfone 33a. 33a was analyzed by HPLC and the molecular mass of the octamer was
confirmed by MALDI-TOF MS. As previously reported for other octameric SNAs by
Huang and Richert, the HPLC trace for 33a changed depending on the amount of
injected SNA, resulting in either a single peak for low concentrations or a twin peak for
higher concentrations.
To prepare the analogous octamer carrying a CH2SOÀ3 group at the 3'-end, disulfide
29 was again reduced with PBu3 in DMF/H2O 5 :1 prior to the coupling and the resulting
30 directly added via capillary transfer to bromide 23 and Cs2CO3 (Scheme 7). After
multiple freeze-pump cycles, the mixture was stirred at 458 overnight. The crude product
32b was immediately oxidized with Oxone and NaOAc. TLC Monitoring showed the
complete oxidation of the terminal (MeO)2Tr thioether to sulfonate after 1 h. HPLC
Analysis and MALDI-TOF MS showed the formation of the singly charged octamer 33b.
In contrast to the synthesis of 32a and 32b, octamers containing only thymine as the
nucleobase could be prepared without in situ reduction of the disulfide. Thiol 26,
bromide 23, and Cs2CO3 were mixed in degassed DMF at 458 for 7 h and worked up as
the octamers above. No disulfide formation was observed. Crude 32c was first
debenzoylated at the 6'-end with 2m NaOH in MeOH/THF/H2O to give 33c
(Scheme 7). The starting material was surprisingly difficult to dissolve in aqueous
base, even though the thymidine moieties should be deprotonated. The product
mixture was neutralized, and desalted with SepPak-C18 cartridges. Oxidation of 33c
with Oxone also proved to be difficult; after 12 h at room temperature, MALDI-TOF
MS showed that the octamer was not completely oxidized, requiring the oxidation
procedure to be extended by treatment with additional reagent for 24 h. The fully
oxidized, singly charged, and deprotected all-dT octamer 34c was then purified by
HPLC (Scheme 8).
Octamer 33a was detritylated with 2.2m ZnCl2 ¥ Et2O solution in CH2Cl2 to give 34a
(Scheme 8). HPLC Analysis and MALDI-TOF MS showed that ca. 80% was

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Helvetica Chimica Acta Vol. 86(2003)
Scheme 7
Br
RO
T
T
O
O
SO2
SO2
T
T
O
X2
Cs2CO3, PBu3
DMF/H2O
O
X1
PBu3
29
+
DMF/H2O
T
T
O
O
SO2
SO2
T
B
O
O
R
HS
23 R = DMTS, X2 = S
24 R = TBDPSO, X2 = SO2
26 R = Bz, B = T, X1 = S
30 R = Tr, B = toC, X1 = SO2
RO
T
O
32a (from 30 + 24) B = toC, R = Tr,
X1 = X2 = SO2, R' = TBDPSO
32b (from 30 + 23) B = toC, R = Tr,
X1 = SO2, X2 = S, R' = DMTS
32c (from 26 + 23) B = T, R = Bz,
X1 = X2 = S, R' = DMTS
SO2
Oxone, NaOAc
Oxone, NaOAc
2M NaOH
33a
33b
33c
T
O
X1
T
O
SO2
B
See Scheme 8 for structures of 33a - c
O
33a B = toC, R = Tr, X = SO2, R' = TBDPSO
33b B = toC, R = Tr, X = SO2, R' = SO3
–
S
T
33c B = T, R = H, X = S, R' = DMTS
O
32a - c
SO2
T
O
X2
T
O
SO2
T
O
R'
to ˆ 2-MeC6H4CO, DMTˆ (MeO)2Tr, TBDPS ˆ (t-Bu)Ph2Si

Helvetica Chimica Acta Vol. 86(2003)
2971
Scheme 8
RO
T
O
SO2
T
O
X
T
O
general octamer structure
for 33a Ð c, 34a Ð c, and 35a, b
SO2
B
O
33a B = toC, R = Tr, X = SO2, R' = TBDPSO
X
T
–
b B = toC, R = Tr, X = SO2, R' = SO3
O
SO2
c B = T, R = H, X = S, R' = DMTS
T
O
X
T
O
SO2
T
O
R'
34a B = toC, R = H,
ZnCl2 · Et2O
2M NaOH
35a B = C, R = H,
33a
33b
33c
X = SO2, R' = TBDPSO
X = SO2, R' = OH
ZnCl2 · Et2O
34b B = toC, R = H,
2M NaOH
35b B = C, R = H
–
–
X = SO2, R' = SO3
X = SO2, R' = SO3
Oxone, NaOAc
–
34c B = T, R = H, X = SO2, R' = SO3
to ˆ 2-MeC6H4CO, Trˆ Ph3C, TBDPS ˆ (t-Bu)Ph2Si, DMTˆ (MeO)2Tr
detritylated and 20% remained protected, even after prolonged and repeated
treatment under the acidic conditions. Crude 34a was then treated with 2m NaOH in
MeOH/THF/H2O at 458 for 18 h to give 35a. The deprotection was monitored by

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Helvetica Chimica Acta Vol. 86(2003)
HPLC; no considerable by-products were detected. The mixture was buffered with
acetate and desalted thrice with SepPak-C18 cartridges followed by purification by HPLC.
Octamer 33b was deprotected via 34b as described for 33a, or treated first with 2m
NaOH in MeOH/THF/H2O at 408 for 20 h, followed by detritylation with TsOH ¥ H2O
in MeOH to yield 35b. The trityl cleavage with TsOH ¥ H2O was monitored by HPLC.
After ca. 3 h, the formation of a less polar by-product was detected, and after 5.5 h, ca.
90% octamer was deprotected, 5% remained protected, and 5% by-product was
formed. The reaction mixture was neutralized and desalted five times with a SepPak-
C18 cartridge. In test reactions, elongated reaction times resulted in up to 50% by-
product formation. The crude deprotected, singly charged octamer 35b was purified by
HPLC.
Fig. 1 shows an HPLC profile of purified, deprotected all-dToctamer sulfonate 34c.
The molecules 34c and 35a,b were further characterized by mass spectrometry. For
molecules of this size at the boundary between classical organic and biomolecules,
standard analytical methods fail to provide data having the same precision as obtained
for precursors to the octamer (see Sect. 9 in the Exper. Part). Nevertheless, the samples
were sufficiently well characterized to permit them to serve as the starting point for
biophysics-type experiments.
Fig. 1. HPLC Profile of purified, deprotected all-d octamer sulfonate 34c (absorbance at l 260 nm vs. time)
Melting Studies with SNAs. We first asked whether SNAs could form duplexes
similar to those formed by standard DNA. Several different aqueous buffers were used
for the pertinent analyses (Table 1). The structures of the studied SNAs and DNAs are
given in Table 2.
Table 1. Buffers for Melting-Curve Experiments
NaCl
MgCl2
Tris ¥ HCl
pH
Buffer 1
Buffer 2
Buffer 3
Buffer 4
1m
1m
50 mm
50 mm
100 mm
10 mm
10 mm
10 mm
10 mm
7.0
5.0
5.0
7.0
0.15m

Helvetica Chimica Acta Vol. 86(2003)
2973
Table 2. Sequences of the SNAs and DNAs for the Melting Curves
Sequencea)
Backbone
35a
35b
34c
5'-d(HOCH2-Tso2Tso2Tso2Cso2Tso2Tso2Tso2T-CH2OH)-3'
5'-d(HOCH2-Tso2Tso2Tso2Cso2Tso2Tso2Tso2T-CH2SO3À)-3'
5'-d(HOCH2-Tso2Tso2Tso2Tso2Tso2Tso2Tso2T-CH2SO3À)-3'
5'-d(HO-GCGAAAAGAAACGC-OH)-3'
5'-d(HO-GCGTTTTCTTTCGC-OH)-3'
5'-d(HO-GCGAAAGAAAACGC-OH)-3'
5'-d(HO-GCGTTTTTTTTCGC-OH)-3'
5'-d(HO-GCGAAAAAAAACGC-OH)-3'
5'-d(HO-TTTCTTTT-OH)-3'
SNA
SNA
SNA
DNA1
DNA2
DNA3
DNA4
DNA5
DNA6
DNA7
DNA8
DNA9
DNA
DNA
DNA
DNA
DNA
DNA
DNA
DNA
DNA
5'-d(HO-AAAAGAAA-OH)-3'
5'-d(HO-TTTTTTTT-OH)-3'
5'-d(HO-AAAAAAAA-OH)-3'
a) so2 in 35a,b and 34c corresponds to the linker CH2SO2CH2 instead of the implied standard linker
OP(ˆO)(OÀ )O in DNA1 DNA9.
First, as a control, evidence for self-folding of the DNA single strands used in this
work was sought by examining the UVabsorbance (l 260 nm) of DNA2 and DNA3 as a
function of temperature (Fig. 2). These −melting curves× for DNA2 showed no evidence
of self-folding, while the curve for DNA3 did, slightly. The hyperchromicity normally
associated with unfolding was small, only 12 14% between 10 and 808C, and occurred
slowly over the entire temperature interval.
To determine whether an SNA could bind to a complementary DNA, the octameric
SNA 35a was mixed with its complementary oligomeric DNA DNA1 (1 mm/oligomer,
18/min). No melting transition above those assigned to the single-strand melting was
observed (Fig. 3,a) in all buffers between 0 and 908. In contrast, the DNA ¥ DNA pair
from DNA6 and DNA7 showed a classical melting transition (Fig. 3,b). These results
suggest that no duplex forms between this SNA sequence and its complementary DNA
under conditions where DNA ¥ DNA duplexes are formed.
Next, evidence for triple-helix formation was sought. DNA Octamers having the
sequences d(TTTTTTTT) and d(TTTCTTTT) are well known to bind to duplex DNA
in the major groove, with thymine binding by Hoogsteen base pairing to adenine in the
triplex, and protonated cytosine binding by Hoogsteen base pairing to guanine [39].
Reasoning that the absence of charge on the SNA might make it an especially good
binder as a third helix, the SNAs 35a, 35b, and 34c were mixed with the DNA duplex
formed between sequences DNA2 and DNA3, and melting transitions were sought.
The DNA sequences had a GCG moiety at the 5'-end and a CGC at the 3'-end to
enhance the DNA duplex stability and ensure tight hybridization at the ends of the
DNA strands. The study concentrations of 3 mm/oligomer and a lower heating (0.58/
min) rate were chosen in consideration of the observation that association-dissociation
rates of triple helices are more than 100 times lower than those of double helices [40].
Here, the 8mer duplex DNA6 ¥ DNA7 melted at 188 (buffer 4, see Table 1) (Fig. 3,b),
while the 14mer duplex DNA2 ¥ DNA3 melted at 508 (buffer 3, see Table 1) and 628
(buffers 1 and 2, see Table 1), respectively. As is frequently observed, the melting
temperature of the natural duplex increases at the higher salt concentrations of

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Helvetica Chimica Acta Vol. 86(2003)
Fig. 2. Melting curves of the single-stranded DNA oligomers (absorbance at l 260 nm vs. T in 8, in buffer 4),
seeking self-folding: a) DNA2, showing no significant hyperchromicity, implying no self-folding; b) DNA3,
showing only modest hyperchromicity, implying little self-folding
buffers 1 and 2, whereas the pH difference between buffer 1 and 2 does not seem to
have an effect on duplex melting.
Triple-helix formation could be detected from melting curves in mixtures of three
DNA strands. For example, melting of the triplex formed by DNA2 ‡ DNA3 ‡ DNA6
(in buffer 1) occurred at ca. 48, the duplex melted at 628 (Fig. 4,a). Accurate
determination of the triplex Tm was difficult because the melting process was not
complete at À108, the temperature at which the aqueous buffers started to freeze. No
UV transition at all was observed that could be assigned to the melting of a triple
joining SNA 35a,b or 34c with DNA2 ¥ DNA3 (Fig. 4,b d) or DNA4 ¥ DNA5,

Helvetica Chimica Acta Vol. 86(2003)
2975
Fig. 3. Seeking SNA ¥ DNA duplex structure by Means of melting curves (absorbance at l 260 nm vs. T in 8, in
buffer 4): a) melting curves of SNA 35a and DNA1, showing slight hyperchromicity, i.e., no evidence for duplex
formation, b) melting of DNA6 ‡ DNA7, showing the classical melting curve, i.e., evidence of duplex formation
however. The only hyperchromicity observed was that for the melting of the DNA
duplex and melting of the single-stranded SNA. Therefore, it must be concluded that
the SNAs examined here do not form duplexes or triplexes with natural DNA
oligonucleotides that are more stable than those formed by DNA itself.

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Helvetica Chimica Acta Vol. 86(2003)
Fig. 4. Seeking the ability of SNA to bind as a third strand to duplex DNA by means of melting curves
(absorbance at l 260 nm vs. T in 8): a) control containing the duplex DNA2 ¥ DNA3 with DNA6 as a third strand;
b) duplex DNA2 ¥ DNA3 in the presence of SNA 35a; c) duplex DNA2 ¥ DNA3 in the presence of SNA 35b; d)
duplex DNA2 ¥ DNA3 in the presence of SNA 34c. Each panel has two curves, representing experiments in
buffers 1 and 2.

Helvetica Chimica Acta Vol. 86(2003)
2977
Fig. 4. (cont.)
Discussion. One of the most-characteristic features of nucleic acids is that their
reactivity does not change with small (or even large!) changes in chemical composition.
Changing a DNA sequence from d(TTTTTTTT) to d(TTTCTTTT), for example, has
essentially no impact on physical behavior, chemical reactivity, or molecular
recognition, which continues to follow the Watson-Crick rules (A pairs with T, G
pairs with C). Indeed, the physical, reactivity, and molecular-recognition properties of
DNA remain rule-based for virtually any DNA sequence, of which there are 65536 for
an octamer, and far more for longer oligonucleotides.
The insensitivity of the physical behavior of a DNA molecule with respect to
changes in nucleobase sequence is, of course, central to its functioning as a genetic
molecule. Life as we know it is no more (and no less) than a special type of chemistry,
one that has joined a property common in organic molecules (the ability to undergo
spontaneous chemical transformation) with an uncommon property (the ability to
direct the synthesis of copies of itself) in a way that allows changes in molecular
structure arising from spontaneous transformation to themselves be copied. Any
chemical system having this combination is expected to undergo natural selection,
evolving in structure to replicate faster through more-efficient use of available
molecular resources and energy. This will generate life, which may be defined as a self-
sustaining chemical system capable of undergoing Darwinian evolution [41].
To support Darwinian evolution, a biopolymer must be able to change its structure
without changing its overall physical properties, at least not to the level that such
changes will disrupt whatever processes that are essential to its replication. This is
arguably the most-fundamental etiological principle behind the DNA structure.
As these and other results suggest, the ability of DNA to retain its overall physical
properties, even as it changes its sequence, arises from its repeating anion backbone.
This polyelectrolyte character has been discussed elsewhere from this perspective [20].
The work reported here contains one striking example of how changes in molecular
structure that would be considered insignificant in the context of a DNA create quite

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Helvetica Chimica Acta Vol. 86(2003)
significant changes in the behavior of the SNA: the susceptibility of thiolated tetramers
to disulfide formation (see Scheme 6, 28 ! 29 (via 30) vs. stable 26). Were SNAs
incorporated into living systems, the idiosyncratic behavioral changes that are a
consequence of small structural changes would create as many problems in their
biosynthesis as they created for us in their abiological synthesis.
From this, we can conclude that SNAs are not likely to be alternative genetic
substances. This conclusion is made despite the observation that short SNAs form
canonical Watson-Crick base pairs [25]. Similar behavior is observed with PNA,
another non-ionic DNA analog, whose practical application is hampered by limited
solubility in aqueous systems and pronounced self-organization [42]. In PNA, addition
of a negative charge to one end of the molecule diminishes aggregation and improves
solubility. This was also observed in the SNA analogs, where adding a CH2SOÀ3 group at
the 3'-end diminished aggregation and improved solubility in these species. Never-
theless, it remains to be explained why the Watson-Crick-type molecular interactions
survive in PNA molecules past the dinucleotide level of oligomerization.
Experimental Part
1. General. See [27].
2. Monomers. 1-{3'-{{[(tert-Butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-6'-O-(methylsulfonyl)-b-d-
erythro-hexofuranosyl}thymine (2a). The 1-{3'-{{[(tert-Butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-b-d-
erythro-hexofuranosyl}thymine (compound 11 in the preceding paper [27]; 278 mg, 0.546mmol) was co-
evaporated 3 Â with pyridine and dissolved in pyridine/CH2Cl2 4 :1 (5 ml). The soln. was cooled to 08 and
methanesulfonyl chloride (65 ml, 0.84 mmol) was added dropwise. The mixture was allowed to warm to r.t.,
stirred for 3 h, and hydrolyzed with 1.6% H2SO4 soln. (2 ml). CH2Cl2 (50 ml) was added, the soln. washed with
1.6% H2SO4 soln. (3 Â 10 ml) and brine (3 Â 10 ml), dried (Na2SO4), and evaporated, and the crude product
chromatographed (silica gel, AcOEt/petroleum ether 1:1, 3 :1): 2a (299 mg, 93%). Colorless foam. UV
1
(MeCN): 203 (26700), 266 (8900). H-NMR (CDCl3, 300 MHz): 1.07 (s, t-Bu); 1.96( s, MeÀC(5)); 1.99 2.08
(m, 2 HÀC(5')); 2.17 2.36( m, HÀC(3'), 2 HÀC(2')); 2.99 (s, MeSO2); 3.63 3.73 (m, CH2ÀC(3')); 3.98
(dt, J ˆ 2.6 , 8.8, HÀC(4')); 4.27 4.37 (m, 1 HÀC(6')); 4.38 4.47 (m, 1 HÀC(6')); 6.05 (dd, J ˆ 4.1, 7.1,
HÀC(1')); 7.14 (2s, HÀC(6)); 7.39 7.47 (m, 6H, Ph 2Si); 7.62 7.66 (m, 4 H, Ph2Si); 9.26(br., NH). 13C-NMR
(CDCl3, 75 MHz): 12.62 (q, MeÀC(5)); 19.61 (s, Me3C); 26.84 (q, Me3C); 34.34 (t, C(5')); 34.69 (t, C(2')); 37.31
(q, MeSO2); 45.05 (d, C(3')); 63.43 (t, CH2ÀC(3')); 66.92 (t, C(6')); 78.79 (d, C(4')); 84.94 (d, C(1')); 111.21
(s, C(5)); 127.90, 130.01 (2d, Ph2Si); 132.80 (s, Ph2Si); 135.20 (d, C(6)); 135.55 (d, Ph2Si); 150.20 (d, C(2));
‡
‡
‡
163.74 (s, C(4)). ESI-MS (pos.): 1194.8 ([2M ‡ Na] ), 1172.7 ([2M ‡ H] ), 609.2 ([M ‡ Na] ), 586.9 ([M ‡
‡
H] ).
1-{6'-Bromo-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-b-d-erythro-hexofuranosyl}thy-
mine (4a). The 1-{3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-b-d-erythro-hexofuranosyl}thy-
mine (compound 11 in the preceding paper [27]; 50 mg, 98 mmol) and PPh3 (51 mg, 196 mmol) were co-
evaporated 3 Â with toluene, dried overnight under high vacuum at r.t., and then dissolved in 1,2-
dichloroethane/MeCN 4 :1 (10 ml). A soln. of CBr2 (65 mg, 196 mmol) in 1,2-dichloroethane (2 ml) was added,
and the mixture was stirred for 1.5 h at r.t. MeOH (1 ml) was added to quench the reaction. The solvents were
evaporated and the residue submitted to FC (silica gel, CH2Cl2/AcOEt 1:1): 4a (54 mg, 97%). Colorless foam.
UV (MeCN): 215 (17000), 265 (10400). 1H-NMR (CDCl3, 300 MHz): 1.08 (s, t-Bu); 1.95 (s, MeÀC(5)); 1.98
2.09 (m, 1 HÀC(5')); 2.10 2.23 (m, 1 HÀC(5'), HÀC(3')); 2.25 2.38 (m, 2 HÀC(2')); 3.41 3.59
(m, 2 HÀC(6')); 3.67 (d, J ˆ 4.0, CH2ÀC(3')); 4.03 (dt, J ˆ 2.5, 9.0, HÀC(4')); 6.06 (dd, J ˆ 4.4, 7.1, HÀC(1'));
7.11, 7.12 (2s, HÀC(6)); 7.26 7.44 (m, 6H, Ph 2Si); 7.64 7.66 (m, 4 H, Ph2Si); 9.17 (br., NH). 13C-NMR (CDCl3,
75 MHz): 12.70 (q, MeÀC(5)); 19.15 (s, Me3C); 26.83 (q, Me3C); 29.48 (t, C(6')); 34.87 (t, C(5')); 38.07
(t, C(2')); 44.87 (d, C(3')); 63.40 (t, CH2ÀC(3')); 80.63 (d, C(4')); 87.89 (d, C(1')); 110.89 (s, C(5)); 127.82, 129.92
(2d, Ph2Si); 132.85 (s, Ph2Si); 135.20 (d, C(6)); 135.55 (d, Ph2Si); 150.22 (d, C(2)); 163.76 (s, C(4)). FAB-MS
‡
‡
‡
(NOBA; pos.): 573 (24, [M1 ‡ H] ), 572 (10, [M2 ‡ 2 H] ), 571 (25, [M2 ‡ H] ), 559 (14), 515 (23), 513 (21), 492
‡
‡
(37, [M À Br ‡ H] ), 491 (100, [M À Br] ), 433 (17), 289 (20), 287 (22), 269 (32), 263 (20), 261 (29), 251 (22),

Helvetica Chimica Acta Vol. 86(2003)
2979
247 (31), 244 (21), 243 (74), 239 (44), 237 (18), 235 (60), 233 (19), 229 (18), 227 (35), 225 (34), 223 (24), 213
(28), 211 (19), 209 (21), 207 (22), 203 (24), 201 (31), 200 (33), 190, 189, 136, 135.
1-{3'-{{[(tert-Butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-6'-O-(methylsulfonyl)-b-d-erythro-hexofur-
anosyl}-N4-(o-toluoyl)cytosine (2b). As described for 2a, with 1-{3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-
2',3',5'-trideoxy-b-d-erythro-hexofuranosyl}-N4-(o-toluoyl)cytosine (compound 14a in the preceding paper [27];
380 mg, 0.62 mmol), pyridine (3Â), pyridine/CH2Cl2 1:2 (6ml), and methanesulfonyl chloride (67 ml,
0.86mmol). Chromatography (silica gel, AcOEt) yielded 2b (321 mg, 95%). Colorless foam. UV (MeCN):
253 (19700), 309 (8600). 1H-NMR (CDCl3, 300 MHz): 1.08 (s, t-Bu); 1.98 2.15 (m, 2 HÀC(5')); 2.16 2.23
(m, HÀC(3')); 2.28 2.40 (m, 1 HÀC(2')); 2.46 2.58 ( m, 1 HÀC(2')); 2.52 (s, MeÀC(5)); 3.02 (s, MeSO2);
3.65 3.74 (m, CH2ÀC(3')); 4.13 (dt, J ˆ 2.0, 9.0, HÀC(4')); 4.35 4.50 (m, 2 HÀC(6')); 6.06 (dd, J ˆ 3.0, 7.0,
HÀC(1')); 7.26 7.32 ( m, 2 H, to); 7.37 7.46( m, 7 H, Ph2Si, HÀC(5)); 7.48 7.53 (m, 1 H, to); 7.55 7.60
(m, 1 H, to); 7.60 7.67 (m, 4 H, Ph2Si); 7.95 (d, J ˆ 7.0, HÀC(6)); 8.34 (br., NH). 13C-NMR (CDCl3, 75 MHz):
19.23 (s, Me3C); 20.12 (q, Me (to)); 26.04 (q, Me3C); 36.16 (t, C(5')); 37.18 (t, C(2')); 37.53 (q, MeSO2); 44.61
(d, C(3')); 62.67 (t, C(6')); 62.92 (t, CH2ÀC(3)); 83.12 (d, C(4')); 87.65 (d, C(1')); 95.95 (d, C(5)); 126.17, 126.98
(2d, to); 127.87, 129.94 (2d, Ph2Si); 131.57, 131.80 (2d, to); 132.83 (s, Ph2Si); 134.15 (s, Cipso (to)); 135.55
‡
(d, Ph2Si); 137.45 (s, Co (to)); 144.04 (d, C(6)); 162.10 (s, CˆO). FAB-MS (NOBA; pos.): 1380 (27, 2M ), 692
‡
‡
‡
(18, [M ‡ 2 H] ), 691 (42, [M ‡ H] ); 690 (100, M ), 632 (28), 594 (13), 365 (15), 277, 230
‡
([MeC6H4CONHC4H3N2O ‡ H] ).
1-{6'-Bromo-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-b-d-erythro-hexofuranosyl}-N4-
(o-toluoyl)cytosine (4b). As described for 4a, with 1-{3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-
trideoxy-b-d-erythro-hexofuranosyl}-N4-(o-toluoyl)cytosine (compound 14a in the preceding paper [27];
50 mg, 82 mmol), PPh3 (43 mg, 163 mmol), toluene (3Â), 1,2-dichloroethane (10 ml), and CBr4 (49 mg,
147 mmol) in 1,2-dichloroethane (2 ml) (added at 08). The mixture was allowed to warm to r.t. and stirred for 2 h.
The soln. was poured into sat. NaHCO3 soln. (15 ml) containing ice (10 g). CH2Cl2 (60 ml) was added, the aq.
phase extracted with CH2Cl2 (4 Â 20 ml), the combined org. phase dried (MgSO4) and evaporated (308 water-
bath temp.), and the residue submitted to FC (silica gel, CH2Cl2 (100 ml), CH2Cl2/AcOEt 3 :1 (200 ml): 4b
1
(41 mg, 74%). Colorless foam. UV (MeCN): 254 (15400), 308 (7500). H-NMR (CDCl3, 300 MHz): 1.06( s, t-
Bu); 1.98 2.28 (m, 2 HÀC(5'), HÀC(3')); 2.28 2.42 (m, 1 HÀC(2')); 2.43 2.61 (m, 1HÀC(2')); 2.52 (s, Me
(to)); 3.48 3.67 (m, 2 HÀC(6')); 3.68 3.74 (d, CH2ÀC(3')); 4.19 (dt, HÀC(4')); 6.04 (dd, HÀC(1')); 7.25 7.31
(m, 2 H, to); 7.36 7.52 ( m, 7 H, Ph2Si, to); 7.52 7.61 (m, 3 H, to, HÀC(5)); 7.61 7.70 (m, 4 H, Ph2Si, to); 7.94
(d, J ˆ 7.0, HÀC(6)); 8.38 (br., NH). 13C-NMR (CDCl3, 75 MHz): 19.18 (s, Me3C); 20.14 (q, Me (to)); 26.84
(q, Me3C); 29.41 (t, C(6')); 36.18 (t, C(5')); 38.09 (t, C(2')); 44.16( d, C(3')); 62.90 (t, CH2ÀC(3')); 82.04
(d, C(4')); 87.46( d, C(1')); 95.77 (d, C(5)); 126.19, 126.91 (2d, to); 127.86, 129.95 (2d, Ph2Si); 131.60, 131.83 (2d,
to); 132.82 (s, Ph2Si); 134.16( s, Cipso (to)); 135.55 (d, Ph2Si); 137.48 (s, Co (to)); 143.72 (d, C(6)); 162.03
‡
‡
‡
(s, CˆO). FAB-MS (NOBA; pos.): 677 (38, [M1 ‡ 2 H] ), 676 (50, [M1 ‡ H] ), 675 (77, [M2 ‡ H] ), 674 (95,
‡
‡
‡
[M2 ‡ H] ), 663 (25), 618 (23, M1 À (t-Bu) ‡ H] ), 616 (23, [M2 À (t-Bu) ‡ H] ), 573 (35), 572 (30), 571 (50),
561 (20), 515 (70), 513 (63), 448 (21), 447 (62), 446 (23), 445 (66), 389 (55), 387 (47), 383 (25), 369 (25), 365
(29), 341 (25), 339 (32), 337 (24), 335 (21), 329 (23), 327 (32), 326(20), 325 (50), 323 (20), 319 (29), 317 (35),
‡
316(29), 315 (34), 313 (22), 230 ([MeC H4CONHC4H3N2O ‡ H ]), 190, 189, 136, 135.
6
3. Dimers: Coupling and Oxidation. 5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylyl-
methylenethiomethylene-(3' ! 5')-3'-{{[(tert-butyl)diphenylsilyl]oxy}methy l}-2',3',5'-trideoxy-N4-(o-toluoyl)-
cytidine (d(TrOCH2-TstoC-CH2OTBDPS); 3b). Method 1: Thiol 1 (159 mg, 0.30 mmol) and mesylate 2b
(173 mg, 0.25 mmol) were dried overnight under high vacuum and dissolved in degassed DMF (5 ml, 1 h Ar).
The soln. was degassed a second time by performing three freeze-pump cycles. The soln. was cooled to 08, and
DBU (49 ml, 0.33 mmol) was added. The soln. was allowed to warm to r.t., stirred overnight, and cooled again to
08. AcOH was added until pH 6was reached. The solvent was evaporated at max. 40 8 and the remaining crude
product chromatographed (silica gel, AcOEt/CH2Cl2 3 :1): 3b (224 mg, 80%). Colorless foam.
Method 2: Thiol 1 (178 mg, 337 mmol), bromide 4b (220 mg, 319 mmol), and Cs2CO3 (189 mg, 933 mmol)
were dissolved in degassed DMF (5 ml, 1 h Ar). The soln. was degassed a second time by performing three
freeze-pump cycles. The mixture was stirred overnight at r.t. Acetate buffer (3m AcOH/1m AcONa; 470 ml) was
added and the mixture evaporated at max. 408. The crude product was chromatographed (silica gel, 0 5%
MeOH/CH2Cl2): 3b (298 mg, 83%). Colorless foam. UV (MeCN): 256(21400), 308 (7500). 1H-NMR (CDCl3,
500 MHz): 1.06( s, t-Bu); 1.88 (2s, MeÀC(5)(T)); 1.85 1.92 (m, 1 HÀC(5')(T), 1 HÀC(5')(C)); 1.97 2.10
(m, 1 HÀC(5')(T), HÀC(3')(C), 1 HÀC(5')(C)); 2.13 2.19 (m, 1 HÀC(2')(C), 1 HÀC(2')(T), HÀC(3')(T));
2.21 2.27 (m, 1 HÀC(2')(T)); 2.46 2.52 ( m, 1 HÀC(2')(C), 1 HÀC(6')(C)); 2.52 (s, Me (to)); 2.62 2.68
(m, 1 HÀC(6')(C), 1 H of CH2ÀC(3')(T)); 2.73 2.78 (m, 1 H of CH2ÀC(3')(T)); 3.30 (t, J ˆ 6.5,

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Helvetica Chimica Acta Vol. 86(2003)
2 HÀC(6')(T)); 3.64 3.71 (m, CH2ÀC(3')(C)); 3.87 3.90 (m, HÀC(4')(T)); 4.07 4.11 (dd, J ˆ 2.9, 8.0,
HÀC(4')(C)); 5.98 (dd, J ˆ 3.9, 6.8, HÀC(1')(T)); 6.04 (dd, J ˆ 2.9, 6.8, HÀC(1')(C)); 7.10 (2s, HÀC(6)(T));
7.20 7.24 (m, 3 H, Tr); 7.27 7.30 (m, 8 H, Tr, to); 7.37 7.45 (m, 13 arom. H); 7.51 (d, J ˆ 7.4, HÀC(5)(C));
7.55 7.56( m, 1 H, to); 7.61 7.64 (m, 4 H, Ph2Si); 7.94 (d, J ˆ 7.4, HÀC(6)(C)); 8.30 (br., 1 NH); 8.43 (br.,
1 NH). 13C-NMR (CDCl3, 125 MHz): 12.72 (q, MeÀC(5)(T)); 19.24 (s, Me3C); 20.18 (q, Me (to)); 26.90
(q, Me3C); 29.70 (t, C(5')(T)); 30.18 (t, C(5')(C)); 34.75 (t, CH2ÀC(3')(T)); 35.06( t, C(6')(C)); 36.37
(t, C(2')(C)); 38.49 (t, C(2')(T)); 43.14 (d, C(3')(T)); 44.42 (d, C(3')(C)); 60.65 (t, C(6')(T)); 63.31
(t, CH2ÀC(3')(C)); 82.16( d, C(4')(T)); 82.84 (d, C(4')(C)); 84.90 (d, C(1')(T)); 86.90 (s, Tr); 87.43
(d, C(1')(C)); 95.84 (d, C(5)(C)); 110.56( s, C(5)(T)); 126.20, 126.99 (2d, to); 127.07 (d, Tr); 127.83, 127.89
(2d, Ph2Si); 128.63, 128.85 (2d, Tr); 129.97, 129.99 (d, Ph2Si); 131.64, 131.86 (2d, to); 132.94 (s, Ph2Si); 134.06
(s, Cipso (to)); 135.13 (d, C(6)(T)); 135.56 (d, Ph2Si); 137.55 (s, C2 (to)); 143.79 (s, Tr); 144.06( s, C(6)(C)); 149.96
(s, C(2)(T)); 155.02 (d, C(2)(C)); 162.09 (s, CˆO); 163.40 (s, C(4)(T)); 168.60 (s, C(4)(C). MALDI-TOF (A ˆ
dimer in CH2Cl2, B ˆ 0.1m 2,5-DHB (2,5-dihydroxybenzoic acid) in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1 :1):
‡
‡
1163.0 ([M ‡ K] ), 1145.7 ([M ‡ Na] ).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenethiomethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(TrOCH2-TsT-CH2OTBDPS); 3a). Meth-
od 1: Thiol 1 (264 mg, 0.50 mmol), mesylate 2a (270 mg, 0.46mmol), and Cs 2CO3 (450 mg, 1.38 mmol) were
dissolved in degassed DMF (15 ml, 1 h Ar). The soln. was degassed a second time by performing three freeze-
pump cycles. The mixture was warmed to 458 and stirred for 4 h. Acetate buffer (3m AcOH/1m AcONa, 690 ml)
was added, and the solvent was evaporated at max. 408. The crude product was purified by FC (silica gel,
CH2Cl2/AcOEt 3 :1, 1 :1): 3a (383 mg, 82%). Colorless foam.
Method 2: Thiol 1 (50 mg, 95 mmol), bromide 4a (50 mg, 87 mmol), and Cs2CO3 (114 mg, 350 mmol) were
dissolved in degassed THF (10 ml). The soln. was degassed a second time by performing three freeze-pump
cycles. The mixture was warmed to 458 and stirred for 4 h. Acetate buffer (3m AcOH/1m AcONa; 175 ml) was
added, and the solvent was evaporated at max. 408. The crude product was chromatographed (silica gel, CH2Cl2/
1
AcOEt 3 :1, 1:1): 3a (75 mg, 84%). Colorless foam. UV (MeCN): 266 (21200). H-NMR (CDCl3, 300 MHz):
1.06( s, t-Bu); 1.91, 193 (2s, 6H, Me ÀC(5)(T)); 1.81 2.06( m, 2 HÀC(5')(T1), 2 HÀC(5')(T2), HÀC(3')(T2));
2.07 2.38 (m, 1 HÀC(2')(T1), 1 HÀC(2')(T2), HÀC(3')(T1)); 2.44 2.75 (m, 2 HÀC(6')(T2),
CH2ÀC(3')(T1)); 3.29 (t, J ˆ 6.0, 2 HÀC(6')(T1)); 3.68 (t, J ˆ 4.8, CH2ÀC(3')(T2)); 3.83 3.98
(m, HÀC(4')(T1), HÀC(4')(T2)); 5.96 6.08 ( m, HÀC(1')(T1), HÀC(1')(T2)); 7.10, 7.13 (2s, 2 H,
HÀC(6)(T)); 7.21 7.33 (m, 9 H, Tr); 7.36 7.49 ( m, 12 H, 6 H of Ph2Si, 6 H of Tr); 7.60 7.69 (m, 4 H, Ph2Si);
8.79 (br., 1 NH); 8.81 (br., 1 NH). 13C-NMR (CDCl3, 125 MHz)3): 12.73, 12.77 (2q, MeÀC(5)(T)); 19.24
(s, Me3C); 26.91 (q, Me3C); 30.00 (t, C(5')(T1), C(5')(T2)); 34.69 (t, CH2ÀC(3')(T1)); 35.06, 35.08 (2t,
C(6')(T2), C(2')(T2)); 38.49 (t, C(2')(T1)); 43.09 (d, C(3')(T1)); 45.13 (d, C(3')(T2)); 60.66 (t, C(6')(T1));
63.77 (t, CH2ÀC(3')(T2)); 81.38 (d, C(4')(T1)); 82.14 (d, C(4')(T2)); 84.83, 84.84 (2d, C(1')(T2)); 86.90 (s, Tr);
110.62, 110.90 (2s, C(5)(T)); 127.06( d, Tr); 127.82, 127.88, 127.89, 128.63 (4d, Ph2Si, Tr); 129.98, 130.00 (2d,
Ph2Si); 132.95, 132.99 (2s, Ph2Si); 135.08, 135.24 (2d, C(6)(T)); 135.55, 135.58 (2d, Ph2Si); 144.07 (s, Tr); 150.15,
‡
150.19 (s, C(2)(T)); 163.60, 163.63 (2s, C(4)(T)). ESI-MS (pos.): 1041.3 ([M ‡ Na] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenethiomethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(BzOCH2-TsT-CH2OTBDPS); 6). Thiol 5
(123 mg, 314 mmol), bromide 4a (175 mg, 305 mmol), and Cs2CO3 (399 mg, 1.22 mmol) were dissolved in
degassed DMF (10 ml, 1 h Ar). The soln. was degassed a second time by performing three freeze-pump cycles.
The mixture was warmed to 508 and stirred for 4 h. Acetate buffer (3m AcOH/1m AcONa, 610 ml) was added,
and the solvent was evaporated at max. 408. The crude product was purified by FC (silica gel, CH2Cl2/MeOH
100 :0, 40 :1, 20 :1, 10 :1): 6 (267 mg, 99%). Colorless foam. 1H-NMR (CDCl3, 300 MHz): 1.06( s, 9 t-Bu); 1.92,
1.93 (2s, 6H, Me ÀC(5)(T)); 1.84 2.34 (m, 2 HÀC(5')(T1), 2 HÀC(5')(T2), HÀC(3')(T2), 2 HÀC(2')(T1),
2 HÀC(2')(T2), HÀC(3')(T1)); 2.54 2.78 (m, 2 HÀC(6')(T2), CH2ÀC(3')(T1)); 3.68 (d, J ˆ 4.7,
CH2ÀC(3')(T2)); 3.90 3.98 (m, HÀC(4')(T1), HÀC(4')(T2)); 4.39 4.47 (m, 1 HÀC(6')(T1)); 4.54 4.62
(m, 1 HÀC(6')(T1)); 6.04 6.09 (m, HÀC(1')(T1), HÀC(1')(T2)); 7.17, 7.24 (2s, 2 H, HÀC(6)(T)); 7.36 7.47
(m, 2 H of Bz, 6 H of Ph2Si); 7.53 7.58 (t, J ˆ 7.4, 1 H, Bz); 7.63 7.65 (m, 4 H, Ph2Si); 8.02 8.04 (m, 2 H, Bz);
10.22 (br., 2 NH).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-N4-(o-toluoyl)cytidine
(d(TrOCH2-Tso2toC-
3
)
Spectrum kindly provided by Daniel Hutter [43].

Helvetica Chimica Acta Vol. 86(2003)
2981
CH2OTBDPS); 8b). To a soln. of 3b (39 mg, 35 mmol) in MeOH/THF 4 :1 (10 ml), Oxone (107 mg, 175 mmol)
and NaOAc (49 mg, 600 mmol) in deionized H2O (0.6ml) were added with vigorous stirring ( ! opaque soln.).
After 5 min, TLC showed the complete conversion to two diastereoisomeric sulfoxides, which were further
oxidized to 8b after 2 h. Excess Oxone was reduced with sat. Na2S2O3 soln. (5 ml). The soln. was concentrated to
50% of the original volume. CH2Cl2 (40 ml) was added, the aq. phase extracted with CH2Cl2 (2 Â 20 ml), the
combined org. phase washed with brine (2 Â 20 ml), which was reextracted with CH2Cl2 (2 Â 20 ml), and the
combined org. phase dried (MgSO4) and evaporated. Filtration through a layer of silica gel (CH2Cl2/MeOH
10 :1) yielded 8b (40 mg, quant.). Colorless foam. UV (MeCN): 205 (70200), 255 (19200), 309 (5400). 1H-NMR
(CDCl3 , 500 MHz): 1.07 (s, t-Bu); 1.90 (s, MeÀC(5)); 1.86 2.04 ( m, 2 HÀC(5')(T)); 2.10 2.24
(m, HÀC(3')(T), 2 HÀC(5')(C)); 2.33 2.61 (m, 2 HÀC(2')(C), 2 HÀC(2')(T), HÀC(3')(T)); 2.52 (s, Me
(to)); 2.87 3.05 (m, 1 HÀC(6')(C), 1 H of CH2ÀC(3')(T)); 3.13 3.25 (m, 1 HÀC(6')(C), 1 H of
CH2ÀC(3')(T)); 3.29 3.33 (m, 2 HÀC(6')(T)); 3.65 3.76 (m, CH2ÀC(3')); 3.82 3.89 (m, HÀC(4')(T));
3.95 4.01 (dt, HÀC(4')(C)); 6.03 6.05 (m, HÀC(1')(T), HÀC(1')(C)); 7.04 (s, HÀC(6)(T)); 7.20 7.31
(m, 9 H of Tr, 2 H of to); 7.38 7.47 (m, 6 H of Tr, 6 H of PhSi, HÀC(5)(C)); 7.49 7.51 (m, 1 H, to); 7.57
2
7.64 ( m, 1 H of to, 4 H of Ph2Si); 7.86( d, J ˆ 7.4, HÀC(6)(C)); 8.20 (br., 1 NH); 8.42 (br., 1 NH). 13C-NMR
(CDCl3, 125 MHz): 12.87 (q, MeÀC(5)(T)); 19.21 (s, Me3C); 20.13 (q, Me(to)); 26.89 (q, Me3C); 33.12
(t, C(5')(T)); 35.17 (t, C(5')(C)); 35.71 (d, C(3')(T)); 36.37 (t, C(2')(C)); 38.09 (t, C(2')(T)); 44.83
(d, C(3')(C)); 50.98 (t, CH2ÀC(3')(T)); 54.51 (t, C(6')(C)); 60.40 (t, C(6')(T)); 63.31 (t, CH2ÀC(3')(C));
82.16( d, C(4')(T)); 82.84 (d, C(4')(C)); 84.72 (d, C(1')(T)); 86.90 (s, Tr); 87.56 ( d, C(1')(C)); 95.84
(d, C(5)(C)); 110.56( s, C(5)(T)); 126.17, 126.99 (2d, to); 127.13 (d, Tr); 127.88, 127.89 (2d, Ph2Si); 128.55,
128.85 (2d, Tr); 130.01; 130.05 (2d, Ph2Si); 131.66, 131.83 (2d, to); 132.77 (s, Ph2Si); 134.08 (s, Cipso(to)); 135.14
(d, C(6)(T)); 135.52 (d, Ph2Si); 137.56( s, C2(to)); 143.86( s, Tr); 144.10 (d, C(6)(C)); 149.99 (s, C(2)(T)); 155.07
(d, C(2)(C)); 162.08 (s, CˆO); 163.43 (s, C(4)(T)); 168.62 (s, C(4)(C)). FAB-MS (NOBA; pos.): 1176([ M ‡
‡
‡
Na] ), 1154 (M ), 289 (3), 259 (2), 245 (2), 244 (22), 243 (100), 231 (3), 230 (17). MALDI-TOF MS (A ˆ
dimer in CH2Cl2, B ˆ 0.1m CCA (a-cyano-4-hydroxycinnamic acid) in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B
‡
1:1): 1177.4 ([M ‡ Na] ).
‡
‡
‡
Corresponding sulfoxide. FAB-MS (NOBA; pos.): 1162 ([M ‡ Na] ), 1139 ([M ‡ H] ), 1138 (M ), 290
(3), 289 (7), 273 (2), 259 (3), 245 (3), 244 (21), 243 (100), 242 (2), 230 (8).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(TrOCH2-Tso2T-CH2OTBDPS); 8a). As de-
scribed for 8b, with 3a (354 mg, 347 mmol), MeOH/THF 6:1 (70 ml), Oxone (850 mg, 1.39 mmol), NaOAc
(370 mg, 4.50 mmol), and deionized H2O (15 ml). Workup with sat. Na2S2O3 soln. (15 ml) and CH2Cl2 (60 ml):
8a (350 mg, 96%). Colorless foam. UV (MeCN): 265 (23700). 1H-NMR (CDCl3, 300 MHz): 1.07 (s, t-Bu); 1.89,
1.93 (2s, 6H, Me ÀC(5)(T)); 1.86 2.04 ( m, 2 HÀC(5')(T2)); 2.05 2.18 (m, 1 HÀC(2')(T2), HÀC(3')(T2));
2.21 2.39 (m, 1 HÀC(2')(T1), 1 HÀC(2')(T2), 2 HÀC(5')(T1)); 2.40 2.62 (m, HÀC(2')(T1), HÀC(3')(T1));
2.83 2.91 (m, 1 HÀC(6')(T2)); 2.92 3.02 (m, 1 H of CH2ÀC(3)(T2)); 3.07 3.12 (m, 1 HÀC(6')(T2)); 3.12
3.21 (m, 1 H of CH2ÀC(3')(T2)); 3.32 (t, J ˆ 6.0, 2 HÀC(6')(T1)); 3.75 3.83 (m, CH2ÀC(3')(T2)); 3.80 3.91
(m, HÀC(4')(T1), HÀC(4')(T2)); 5.95 6.03 (m, HÀC(1')(T1), HÀC(1')(T2)); 7.03, 7.05 (2s, 2 H,
HÀC(6)(T)); 7.21 7.34 (m, 9 H, Tr); 7.38 7.50 (m, 6 H of Ph2Si, 6 H of Tr); 7.61 7.68 (m, 4 H, Ph2Si); 8.42
(br., 1 NH); 8.56(br., 1 NH). 13C-NMR (CDCl3, 125 MHz): 12.63, 12.70 (2q, MeÀC(5)(T)); 19.23 (s, Me3C);
26.61 (t, C(5')(T1)); 26.93 (q, Me3C); 34.17, 34.25 (2t, C(5')(T2), C(2')(T2)); 36.61 (d, C(3')(T1)); 38.41
(t, C(2')(T1)); 45.34 (d, C(3')(T2)); 51.05 (t, CH2ÀC(3')(T1)); 55.00 (t, C(6')(T2)); 60.44 (t, C(6')(T1)); 63.58
(t, CH2ÀC(3')(T2)); 80.64 (d, C(4')(T1)); 81.59 (d, C(4')(T2)); 84.87 (d, C(1')(T1)); 85.69 (d, C(1')(T2)); 87.05
(s, Tr); 111.14, 111.36(2 s, C(5)(T5)); 127.14 (d, Tr); 127.90, 127.94, 127.96, 128.59 (4d, Ph2Si, Tr); 130.06, 130.08
(2d, Ph2Si); 132.86, 132.88 (2s, Ph2Si); 135.10 (d, C(6)(T)); 135.57 (d, Ph2Si); 135.69 (d, C(6)(T)); 143.92 (s, Tr);
150.25, 150.27 (2s, C(2)(T)); 163.57, 163.65 (2s, C(4)(T)). MALDI-TOF MS (A ˆ dimer in CH2Cl2, B ˆ 0.1m
‡
‡
CCA in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1 :1): 1097 ([M ‡ 2Na À H] ), 1090 ([M ‡ K À H] ), 1074 ([M ‡
‡
Na À H] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' !
5')-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(BzOCH2-Tso2T-CH2OTBDPS); 8c).
Method 1: As described for 8b, with 6 (270 mg, 306 mmol), MeOH/THF 6:1 (60 ml), Oxone (752 mg,
1.22 mmol), NaOAc (550 mg, 4.04 mmol), and deionized H2O (10 ml) (conversion to sulfoxides within 2 min).
Workup with sat. Na2S2O3 soln. (20 ml) and CH2Cl2 (50 ml), then extraction with CH2Cl2 (3 Â 20 ml) (no
washing with brine): 8c (279 mg, quant.). Colorless foam.
Method 2: Dimer 9a (95 mg, 117 mmol) was co-evaporated 3 Â with pyridine and dissolved in pyridine
(15 ml). N,N-Dimethylpyridin-4-amine (DMAP; 0.5 mg) was added, the soln. cooled to 08, and benzoyl chloride

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Helvetica Chimica Acta Vol. 86(2003)
(27.2 ml, 234 mmol) slowly added dropwise over 5 min. The soln. was allowed to warm to r.t. and stirred for 5 h.
After cooling to 08, the reaction was terminated by the slow addition of sat. NaHCO3 soln. (5 ml). The mixture
was concentrated to ca. 10 ml, CH2Cl2 (20 ml) and deionized H2O (5 ml) were added, and the aq. phase was
extracted with CH2Cl2 (2 Â 25 ml, 2 Â 15 ml). The org. phases were washed with 5% HCl soln. (2 Â 10 ml), sat.
NaHCO3 soln. (1 Â 15 ml), and brine (1 Â 10 ml) and the aq. phases reextracted with CH2Cl2 (1 Â 20 ml). The
combined org. phase was evaporated. FC (silica gel, CH2Cl2/MeOH 100 :0, 20 :1, 10 :1) yielded 8c (76mg, 71%).
Colorless foam. The products of the two methods were identical according to TLC and 1H-NMR. UV (MeCN):
1
219 (27000), 265 (17200). H-NMR ((D6)DMSO, 300 MHz): 1.00 (s, t-Bu); 1.78, 1.80 (2s, 6H, Me ÀC(5)(T));
2.00 2.17 (m, 2 HÀC(5')(T1), 2 HÀC(5')(T2)); 2.20 2.34 (m, HÀC(3')(T2), 1 HÀC(2')(T1),
2 HÀC(2')(T2));
2.39 2.50
(m, 1 HÀC(2')(T1));
2.62 2.73
(m, HÀC(3')(T1));
2.18 3.36
(m, 2 HÀC(6')(T2), 1 H of CH2ÀC(3')(T1)); 3.45 3.53 (m, 1 H of CH2ÀC(3')(T1), partly under DMSO);
3.65 3.72 (m, CH2ÀC(3')(T2)); 3.80 3.91 (m, HÀC(4')(T1), HÀC(4')(T2)); 4.28 4.37 (m, 1 HÀC(6')(T1));
4.39 4.49 (m, 1 HÀC(6')(T1)); 6.03 6.08 (m, 2 HÀC(1')(T1), HÀC(1')(T2)); 7.40 7.55 (m, 2 HÀC(6)(T),
2 H of Bz, 6H of Ph 2Si); 7.60 7.68 (m, 1 H of Bz, 4 H of Ph2Si); 7.96( d, J ˆ 7.4, 2 H, Bz); 11.28 (br., 2 NH).
13C-NMR (CDCl3, 75 MHz): 12.50 (2q, MeÀC(5)(T)); 19.11 (s, Me3C); 26.80 (q, Me3C); 32.69 (t, C(5')(T1));
34.04 (t, C(5')(T2)); 36.44 (d, C(3')(T2)); 37.89 (2t, C(2')(T)); 45.19 (d, C(3')(T1)); 50.99 (t, CH2ÀC(3')(T1));
54.63 (t, C(6')(T2)); 61.64 (t, C(6')(T1)); 63.51 (t, CH2ÀC(3')(T2)); 80.61 (d, C(4')(T2)); 80.98 (d, C(4')(T1));
84.95 (d, C(1')(T2)); 85.83 (d, C(1')(T1)); 111.15, 111.30 (2s, C(5)(T)); 127.82 (d, Ph2Si); 128.34, 129.46
(2d, Bz); 129.93 (d, Ph2Si); 129.94 (s, Bz); 132.76( s, Ph2Si); 133.06( d, Bz); 135.17 (d, C(6)(T)); 135.43
(d, Ph2Si); 135.88 (d, C(6)(T)); 150.42 (2s, C(2)(T)); 163.91 (2s, C(4)(T)); 166.36 (s, CˆO). ESI-MS (pos.):
‡
‡
‡
‡
957.2 ([M À H ‡ 2Na] ), 935.3 ([M ‡ Na] ), 809.3 ([M À T¥ H ‡ Na] ); 683.3 (M À 2T¥ H ‡ Na ).
4. Functionalization of the Dimers. 5'-Deoxy-3'-de(phosphinicooxy)-5'-(hydroxymethyl)thymidylylmethyl-
enesulfonylmethylene-(3' ! 5')-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-N4-(o-toluoyl)cyti-
dine (d(HOCH2- Tso2toC-CH2OTBDPS); 9b). Method 1: TsOH (20 mg, 0.1 mmol) was dissolved in MeOH
(10 ml) and a soln. of 8b (200 mg, 173 mmol) in THF (4 ml) was added. The mixture was stirred overnight at r.t.
Sat. NaHCO3 soln. was added until pH 8 was reached. The mixture was filtered and evaporated. FC of the crude
product (silica gel, CH2Cl2/MeOH 20 :1) yielded 9b (144 mg, 91%). Colorless foam.
Method 2: To a soln. of 8b (177 mg, 153 mmol) in CH2Cl2 (3 ml), 2.2m ZnCl2 ¥ Et2O in CH2Cl2 (348 ml,
765 mmol) was added ( ! yellow soln., and after ca. 250 ml, yellow precipitate). The deprotection was complete
after 10 min. The suspension was filtered through a layer of silica gel and the filtrate evaporated. FC (silica gel,
CH2Cl2/MeOH 100 :0, 40 :1, 20 :1, 10 :1) yielded 9b (140 mg, quant.). Colorless foam. UV (MeCN): 256
(20000); 309 (6500). 1H-NMR (CDCl3, 300 MHz): 1.07 (s, t-Bu); 1.92, 1.93 (2s, MeÀC(5)(T)); 1.89 1.97
(m, 1 HÀC(5')(T)); 1.99 2.05 (m, 1 HÀC(5')(T)); 2.13 2.30 (m, HÀC(3')(C), 2 HÀC(5')(C)); 2.32 2.54
(m, 2 HÀC(2')(C), 2 HÀC(2')(T)); 2.51 (s, Me(to)); 2.70 2.79 (m, HÀC(3')(T)); 2.97 (dd, J ˆ 9.8, 13.7,
1 HÀC(6')(C)); 3.10 (ddd, J ˆ 5.3, 10.5, 13.7, 1 HÀC(6')(C)); 3.29 3.37 (m, CH2ÀC(3')(T)); 3.67 3.76
(m, 2 HÀC(6')(T)); 3.77 3.92 (m, CH2ÀC(3')(C), HÀC(4')(T)); 4.00 (dd, J ˆ 2.8, 9.0, HÀC(4')(C)); 5.97
(dd, J ˆ 3.6, 7.1, HÀC(1')(T)); 6.05 (dd, J ˆ 4.2, 7.7, HÀC(1')(C)); 7.12, 7.13 (2s, HÀC(6)(T)); 7.27 7.30 (m, 2 H,
to); 7.36 7.43 ( m, 6H, Ph 2Si); 7.45 (d, J ˆ 7.3, HÀC(5)(C)); 7.50 7.52 (d, J ˆ 7.9, 1 H, to); 7.58 7.59 (m, 1 H,
to); 7.62 7.65 (m, 4 H, Ph2Si); 7.84 (d, J ˆ 7.4, HÀC(6)(C)); 8.49 (br., NH); 8.62 (br., NH). 13C-NMR (CDCl3,
75 MHz): 12.33 (q, MeÀC(5)(T)); 18.98 (s, Me3C); 19.77 (q, Me(to)); 26.67 (q, Me3C); 35.47 (t, C(5')(C));
35.54 (d, C(3')(C)); 35.58 (t, C(5')(T)); 36.16 (t, C(2')(C)); 37.89 (t, C(2')(T)); 44.53 (d, C(3')(T)); 50.86
(t, CH2ÀC(3')(T)); 54.29 (t, C(6')(C)); 58.66 (t, C(6')(T)); 63.01 (t, CH2ÀC(3')(C)); 81.49 (d, C(4')(T)); 82.05
(d, C(4')(C)); 84.60 (d, C(1')(C)); 87.50 (d, C(1')(T)); 96.95 (d, C(5)(C)); 111.07 (s, C(5)(T)); 125.69, 127.19
(2d, to); 127.74 (d, Ph2Si); 129.85 (d, Ph2Si); 131.14, 131.24 (2d, to); 132.61 (s, Ph2Si); 134.09 (s, Cipso (to)); 135.47
(d, C(6)(T)); 135.56 (d, Ph2Si); 137.04 (s, Co(to)); 143.99 (d, C(6)(C)); 150.44 (s, C(2)(T)); 155.63 (d, C(2)(C));
‡
163.73 (s, CˆO); 164.11 (s, C(4)(T)); 169.53 (s, C(4)(C)). FAB-MS (NOBA; pos.): 945 (1, [M ‡ Na] ), 914 (2,
‡
‡
[M ‡ H] ), 913 (7, M ), 301 (13), 243 (11), 231 (17), 230 (100), 199 (15), 197 (14), 183 (5), 165 (7), 163 (5), 155
(6), 154 (14), 149 (6), 139 (10), 138 (9), 137 (19), 136 (19), 135 (37), 128 (5), 127 (32), 125 (6), 123 (11), 121
(10), 119 (47), 111 (7), 109 (33), 107 (14), 105 (11).
5'-Deoxy-3'-de(phosphinicooxy)-5'-(hydroxymethyl)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(HOCH2-Tso2T-CH2OTBDPS); 9a). As de-
scribed for 9b (Method 2), with 8a (75 mg, 72 mmol), CH2Cl2 (1.5 ml), and 2.2m ZnCl2 ¥ Et2O in CH2Cl2 (326 ml,
717 mmol): 9a (58 mg, 99%). Colorless foam. 1H-NMR (CDCl3, 300 MHz): 1.08 (s, t-Bu); 1.93 (2s, 6 H,
MeÀC(5)(T)); 1.84 2.05 (m, 2 HÀC(5')(T1)); 2.08 2.21 (m, HÀC(5')(T2)); 2.22 2.40 (m, 1 HÀC(5')(T2),
HÀC(3')(T2),
1 HÀC(2')(T1),
2 HÀC(2')(T2));
2.43 2.54
(m, 1 HÀC(2')(T1));
2.71 2.80
(m, HÀC(3')(T1)); 2.94 3.12 (m, 1 H of CH2ÀC(3')(T1), 1 HÀC(6')(T2)); 3.22 3.32 (m, 1 H of

Helvetica Chimica Acta Vol. 86(2003)
2983
CH2ÀC(3')(T1), 1 HÀC(6')(T2)); 3.68 3.72 (m, 2 HÀC(6')(T1)); 3.74 3.91 (m, CH2ÀC(3')(T2),
HÀC(4')(T1), HÀC(4')(T2)); 5.85 5.88 (m, HÀC(1')(T2)); 6.03 (dd, J ˆ 4.1, 7.4, HÀC(1')(T1)); 7.04, 7.12
(2s, 2 H, HÀC(6)(T)); 7.39 7.48 (m, 6H, Ph 2Si); 7.62 7.65 (m, 4 H, Ph2Si); 9.15 (br., 1 NH); 9.36(br., 1 NH).
13C-NMR (CDCl3, 75 MHz): 12.47, 12.56(2 q, MeÀC(5)(T)); 19.16( s, Me3C); 26.83 (q, Me3C); 33.73
(t, C(5')(T1)); 35.57 (t, C(5')(T2)); 36.36 (d, C(3')(T2)); 38.00 (2t, C(2')(T)); 45.25 (d, C(3')(T1)); 50.75
(t, CH2ÀC(3')(T1)); 54.66 (t, C(6')(T2)); 59.03 (t, C(6')(T1)); 63.52 (t, CH2ÀC(3')(T2)); 80.63 (d, C(4')(T2));
81.83 (d, C(4')(T1)); 85.08 (d, C(1')(T2)); 86.62 (d, C(1')(T1)); 111.24, 111.34 (2s, C(5)(T)); 127.87, 129.99
(2d, Ph2Si); 132.80 (s, Ph2Si); 135.50 (d, Ph2Si); 136.47 (2d, C(6)(T)); 150.36, 150.44 (2s, C(2)(T)); 163.83,
‡
‡
164.04 (2s, C(4)(T)). FAB-MS (NOBA; pos.): 810 (12, [M ‡ H] ), 809 (25, M ), 327 (10), 289 (13), 281 (18),
221 (14), 207 (22), 156(10), 155 (32), 154 (88), 153 (11), 152 (14), 148 (12), 147 (52), 139 (24), 138 (42), 137
(73), 136(100), 133 (14) (only m/z > 130 were considered).
5'-(Bromomethyl)-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-N4-(o-toluoyl)cytidine
(d(BrCH2-Tso2toC-
CH2OTBDPS); 11b). Compound 9b (76mg, 83 mmol) and PPh3 (59 mg, 225 mmol) were dried overnight
under high vacuum at r.t. and dissolved in 1,2-dichloroethane (6ml). The soln. was cooled to 0 8, and CBr4
(75 mg, 225 mmol) in 1,2-dichloroethane (1.5 ml) was added. The mixture was allowed to warm to r.t. and stirred
for 75 min. The soln. was poured into sat. NaHCO3 soln. (15 ml) containing ice (10 g). CH2Cl2 (60 ml) was
added, the aq. phase extracted with CH2Cl2 (3 Â 20 ml), and combined org. phase dried (MgSO4) and
evaporated (308 water-bath temp.). FC (silica gel, CH2Cl2/MeOH 100 :0, 40 :1, 20 :1) yielded 11b (70 mg, 86%).
Colorless foam. UV (MeCN): 256(17600), 309 (5700). 1H-NMR (CDCl3, 300 MHz): 1.05 (s, t-Bu); 1.92
(s, MeÀC(5)(T)); 2.06 2.28 ( m, 2 HÀC(5')(T), HÀC(3')(C), 2 HÀC(5')(C)); 2.30 2.55 (m, 2 HÀC(2')(C),
2 HÀC(2')(T)); 2.50 (s, Me(to)); 2.65 2.73 (m, HÀC(3')(T)); 2.98 (dd, J ˆ 9.3, 13.5, 1 HÀC(6')(C)); 3.06
3.16( m, 1 HÀC(6')(C), 1 H of CH2ÀC(3')(T)); 3.24 3.35 (m, 1 H of CH2ÀC(3')(T)); 3.44 3.61
(m, 2 HÀC(6')(T)); 3.64 3.76 (m, CH2ÀC(3')(C)); 3.82 3.87 (dt, HÀC(4')(T)); 3.96 4.03 ( m, HÀC(4')(C));
6.00 (dd, J ˆ 3.5, 7.0, HÀC(1')(T)); 6.05 (dd, J ˆ 4.2, 7.5, HÀC(1')(C)); 7.03, 7.04 (2s, HÀC(6)(T)); 7.25 7.28
(m, 2 H, to); 7.37 7.47 (m, 7 H, Ph2Si, HÀC(5)(C)); 7.53 (dd, J ˆ 1.4, 7.4, 1 H, to); 7.59 7.68 (m, 5 H, to, Ph2Si);
7.85 (d, J ˆ 7.4, H ÀC(6)(C)); 8.36 (br., 1 NH); 8.57 (br., 1 NH). 13C-NMR (CDCl3, 75 MHz): 12.33
(q, MeÀC(5)(T)); 18.98 (s, Me3C); 19.77 (q, Me (to)); 26 .6 7q, (Me3C); 35.47 (t, C(5')(C)); 35.54
(d, C(3')(C)); 35.58 (t, C(5')(T)); 36.16 (t, C(2')(C)); 37.89 (t, C(2')(T)); 44.53 (d, C(3')(T)); 50.86
(t, CH2ÀC(3')(T)); 54.29 (t, C(6')(C)); 58.66 (t, C(6')(T)); 63.01 (t, CH2ÀC(3')(C)); 81.49 (d, C(4')(T));
82.05 (d, C(4')(C)); 84.60 (d, C(1')(C)); 87.50 (d, C(1')(T)); 96.95 (d, C(5)(C)); 111.07 (s, C(5)(5)); 125.69,
127.19 (2d, to); 127.74 (d, Ph2Si); 129.85 (d, Ph2Si); 131.14, 131.24 (d, to); 132.61 (s, Ph2Si); 134.09 (s, Cipso(to));
135.47 (d, C(6)(T)); 135.56 (d, Ph2Si); 137.04 (s, Co(to)); 143.99 (d, C(6)(C)); 150.44 (s, C(2)(T)); 155.63
(d, C(2)(C)); 163.73 (s, CˆO); 164.11 (s, C(4)(T)); 169.53 (s, C(4)(C)). FAB-MS (NOBA; pos.): 977 (26, [M ‡
‡
‡
‡
2 H] ), 976(37, [ M1 ‡ H] ), 974 (51, [M2 ‡ H] ), 613 (16), 292 (12), 291 (15), 290 (20), 289 (44), 273 (10), 230
(26), 167 (14), 166 (16), 165 (16), 154.
5'-(Bromomethyl)-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymidine (d(BrCH2-Tso2T-CH2OTBDPS); 11a). Com-
pound 9a (60 mg, 74 mmol) and PPh3 (39 mg, 148 mmol) were dried overnight under high vacuum at r.t. and
dissolved in 1,2-dichloroethane (4 ml). CBr4 (49 mg, 148 mmol) in 1,2-dichloroethane (1 ml) was added, and the
mixture was stirred for 60 min. The soln. was poured into sat. NaHCO3 soln. (15 ml) containing ice (10 g).
CH2Cl2 (60 ml) was added, the aq. phase extracted with CH2Cl2 (4 Â 20 ml), and the combined org. phase dried
(MgSO4) and evaporated (308 water-bath temp.). FC (silica gel, CH2Cl2/MeOH 100 :0, 40 :1, 20 :1, 10 :1)
1
yielded 11a (63 mg, 97%). Colorless foam. UV (MeCN): 265 (14400). H-NMR (CDCl3, 300 MHz): 1.09 (s, t-
Bu); 1.94 (2s, 6H, Me ÀC(5)(T)); 2.05 2.41 (m, 4 HÀC(5'), HÀC(3')(T2), 1 HÀC(2')(T1), 2 HÀC(2')(T2));
2.46 2.55 ( m, HÀC(2')(T1)); 2.68 2.75 (m, HÀC(3')(T1)); 2.97 3.15 (m, 1 H of CH2ÀC(3')(T1),
2 HÀC(6')(T2)); 3.22 3.31 (m, 1 H of CH2ÀC(3')(T1)); 3.48 3.62 (m, 2 HÀC(6')(T1)); 3.82 3.95
(m, HÀC(4')(T1), HÀC(4')(T2)); 5.92 5.96( m, HÀC(1')(T2)); 6.03 6.09 (dd, HÀC(1')(T1)); 7.06(2 s, 2 H,
HÀC(6)(T)); 7.39 7.50 (m, 6H, Ph 2Si); 7.61 7.66 (m, 4 H, Ph2Si); 9.08 (br., 1 NH); 9.17 (br., 1 NH). 13C-NMR
(CDCl3, 75 MHz): 12.61, 13.24 (2q, MeÀC(5)(T)); 19.34 (s, Me3C); 26.94 (q, Me3C); 29.72 (t, C(6')(T1)); 30.98
(t, C(2')(T1)); 33.31 (t, C(5')(T1)); 34.84 (t, C(5')(T2)); 38.02 (t, C(2')(T2)); 38.44 (d, C(3')(T2)); 46.28
(d, C(3')(T1)); 48.94 (t, CH2ÀC(3')(T1)); 52.88 (t, C(6')(T2)); 64.80 (t, CH2ÀC(3')(T2)); 80.83 (d, C(4')(T2));
83.95 (d, C(4')(T1)); 84.78 (d, C(1')(T2)); 92.81 (d, C(1')(T1)); 110.23, 111.84 (2s, C(5)(T)); 127.94, 130.13
(2d, Ph2Si); 132.80 (s, Ph2Si); 135.62 (d, Ph2Si); 137.12 (2d, C(6)(T)); 150.11, 151.23 (2s, C(2)(T)); 163.82
‡
‡
(2s, C(4)(T)). FAB-MS (NOBA; pos.): 873 (14, [M1 ‡ H] ), 871 (14, [M2 ‡ H] ), 669 (12), 667 (11), 491 (15),
489 (13), 366 (14), 365 (72), 364 (14), 363 (68), 289 (17), 287 (16), 279 (21), 269 (15), 265 (11), 262 (11), 257

2984
Helvetica Chimica Acta Vol. 86(2003)
(11), 251 (10), 247 (20), 243 (13), 239 (23), 237 (11), 235 (13), 233 (19), 229 (11), 227 (21), 225 (15), 223 (10),
217 (10), 199 (93), 189 (100), 183 (36), 165 (45), 154, 135.
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-
3',5'-dideoxy-3'-(hydroxymethyl)thymidine (d(TrOCH2-Tso2T-CH2OH); 12a). Compound 8a (70 mg, 67 mmol)
was dissolved in THF (10 ml) at r.t., and 1m Bu4NF in THF (0.25 ml, 0.25 mmol) was added ( ! immediately
yellow). The mixture was stirred for 2 h at r.t. Me3SiOMe (0.23 ml, 165 mmol) was added, stirring continued for
10 min, and the mixture filtered through a layer of silica gel (CH2Cl2/MeOH 20 :1). The filtrate was evaporated
and the residue chromatographed (silica gel, CH2Cl2/MeOH 20 :0, 10 :1): 12a (50 mg, 93%). Colorless foam.
UV (MeCN): 203 (43100), 265 (13200). 1H-NMR (CDCl3, 300 MHz): 1.88, 1.91 (2s, MeÀC(5)(T)); 1.83 2.04
(m, 2 HÀC(5')(T1)); 2.09 2.27 (m, 2 HÀC(5')(T2)); 2.30 2.50 (m, HÀC(3')(T2), 1 HÀC(2')(T1),
1 HÀC(2')(T2)); 2.51 2.64 (m, 1 HÀC(2')(T1)); 2.80 2.89 (m, HÀC(3')(T1)); 3.00 (dd, J ˆ 9.7, 14.2, 1 H of
CH2ÀC(3')(T1)); 3.09 3.23 (m, 1 H of CH2ÀC(3')(T1), 2 HÀC(6')(T2)); 3.30 (t, J ˆ 6.0, 2 HÀC(6')(T1));
3.60 3.68 (m, 1 H of CH2ÀC(3')(T2)); 3.70 3.77 (m, 1 H of CH2ÀC(3')(T2)); 3.83 3.96( m, HÀC(4')(T1),
HÀC(4')(T2)); 5.93 6.00 (m, HÀC(1')(T1), HÀC(1')(T2)); 7.08, 7.14 (2s, 2 H, HÀC(6)(T)); 7.20 7.32
(m, 9 H, Tr); 7.40 7.45 (m, 6H, Tr); 9.4 (br., 2 NH). 13C-NMR (CDCl3, 75 MHz): 12.61, 12.70 (2q,
MeÀC(5)(T)); 34.17 (t, C(5')(T1)); 34.41 (t, C(5')(T1)); 34.41 (t, C(5')(T2)); 36.79 (d, C(3')(T2)); 38.39
(2t, C(2')(T1)); 44.99 (d, C(3')(T1)); 50.63 (t, CH2ÀC(3')(T1)); 54.97 (t, C(6')(T2)); 60.46 (t, C(6')(T1)); 62.63
(t, CH2ÀC(3')(T2)); 80.93 (d, C(4')(T2)); 81.78 (d, C(4')(T1)); 85.56( m, C(1')(T2)); 85.76( d, C(1')(T1)); 87.03
(s, Tr); 110.99, 111.34 (2s, C(5)(T)); 127.15, 127.92, 128.58 (3d, Tr); 135.49, 135.94 (2d, C(6)(T)); 143.91 (s, Tr);
‡
150.40 (s, C(2)(T)); 163.78 (s, C(4)(T)). FAB-MS (NOBA; pos.): 814 (19, [M ‡ H] ), 766 (20), 664 (39), 663
(23), 648 (24), 622 (20), 614 (38), 613 (100), 597 (21), 596 (31), 595 (22), 576 (29), 566 (21), 552 (23), 535 (31),
530 (31), 289, 243, 207, 155, 153 (only m/z > 150 were considered). MALDI-TOF MS (0.1m 2,5-DHB in MeCN/
‡
EtOH/H2O 50 :45 :5): 835.5 ([M ‡ Na] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphin icooxy)thymidylylmethylenesulfonylmethylene-(3' !
5')-3',5'-dideoxy-3'-(hydroxymethyl)thymidine (d(BzOCH2-Tso2T-CH2OH); 12b). As described for 12a, with
8c (275 mg, 301 mmol), THF (10 ml), and 1m Bu4NF in THF (0.92 ml, 0.25 mmol) for 1.5 h, then with Me3SiOMe
(104 ml, 753 mmol). Chromatography (silica gel, CH2Cl2/MeOH 20 :0, 10 :1, 5 :1) yielded 12b (185 mg, 91%).
Colorless foam. 1H-NMR (CDCl3/MeOH 4 :1, 300 MHz): 1.89, 1.90 (2s, 6H, Me ÀC(5)(T)); 2.03 2.51
(m, 2 HÀC(5')(T1), 2 HÀC(5')(T2), 2 HÀC(2')(T1), 2 HÀC(2')(T2), HÀC(3')(T2)); 2.64 2.69
(m, HÀC(3')(T1)); 3.09 3.35 (m, CH2ÀC(3')(T1), 2 HÀC(6')(T2)); 3.56 3.62 ( m, 1 H of CH2ÀC(3')(T2));
3.62 3.70 (m, 1 H of CH2ÀC(3')(T2)); 3.83 3.96( m, HÀC(4')(T1), HÀC(4')(T2)); 4.38 4.49
(m, 1 HÀC(6')(T1)); 4.51 4.61 (m, 1 HÀC(6')(T1)); 5.96 6.02 ( m, HÀC(1')(T2)); 6.04 6.09
(m, HÀC(1')(T1)); 7.22, 7.23 (2s, 2 H, HÀC(6)(T)); 7.39 7.46 (m, 2 H, Bz); 7.53 7.60 (m, 1 H, Bz); 7.98
8.02 (m, 2 H, Bz). 13C-NMR (CDCl3/MeOH 4 :1, 75 MHz): 11.99 (2q, MeÀC(5)(T)); 26.59; 32.26
(t, C(5')(T1)); 34.16( t, C(5')(T2)); 36.17 (d, C(3')(T2)); 37.57 (2t, C(2')(T)); 44.89 (d, C(3')(T1)); 50.66
(t, CH2ÀC(3')(T1)); 54.01 (t, C(6')(T2)); 61.61 (2t, C(6')(T1), CH2ÀC(3')(T2)); 80.59, 80.80 (2d, C(4')(T1));
84.84 (2d, C(1')(T1)); 110.90 (2s, C(5)(T)); 128.16, 129.22 (2d, Bz); 129.46( s, Bz); 133.98 (d, Bz); 135.39, 135.64
(2d, C(6)(T)); 150.39, 150.44 (2s, C(2)(T)); 164.31 (2s, C(4)(T)); 166.53 (s, CˆO).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-
[(acetylthio)methyl]-3',5'-dideoxythymidine (d(TrOCH2-Tso2T-CH2SAc); 13a). PPh3 (77 mg, 295 mmol) was
dried under high vacuum at 458 for 3 h and was dissolved in THF (1 ml). The soln. was cooled to 08, and DIAD
(42 ml, 216 mmol) was added dropwise. The soln. was stirred for 30 min at 08. A white precipitate was formed
after 10 min. Thioacetic acid (15.4 ml, 216 mmol) and 12a (80 mg, 98 mmol; dried overnight under high vacuum at
r.t.) were dissolved separately in THF (each 1 ml) and alternately added dropwise, beginning with thioacetic
acid. The mixture was allowed to warm to r.t., stirred for 2 h, and quenched with Et3N/MeOH 2 :1 (1 ml). The
soln. was evaporated and the crude residue chromatographed (silica gel, CH2Cl2/MeOH 100 :0, 40 :1, 20 :1): 13a
(78 mg, 91%). Colorless foam. UV (MeCN): 203 (48300), 265 (13400). 1H-NMR (CDCl3, 300 MHz): 1.90, 1.92
(2s,
6
H, MeÀC(5)(T)); 1.85 2.04 (m, 1 HÀC(5')(T1)); 2.36( s, Ac); 2.06 2.53 (m, 1 HÀC(5')(T1),
2 HÀC(5')(T2), HÀC(3')(T2), 2 HÀC(2')(T1), 2 HÀC(2')(T2)); 2.58 2.67 (m, HÀC(3')(T1)); 2.92 3.11
(m, 1 H of CH2ÀC(3')(T1), CH2ÀC(3')(T2), 1 HÀC(6')(T2)); 3.16 3.27 ( m, 1 H of CH2ÀC(3')(T1),
1 HÀC(6')(T2)); 3.32 (t, J ˆ 6.0, 2 HÀC(6')(T1)); 3.68 3.74 (m, HÀC(4')(T2)); 3.83 3.96( m, HÀC(4')(T1));
5.96 6.00 ( dd, HÀC(1')(T2)); 6.00 6.05 (dd, HÀC(1')(T1)); 7.05, 7.06(2 s, 2 HÀC(6)(T)); 7.22 7.34 (m, 9 H,
Tr); 7.40 7.45 (m, 6H, Tr); 8.92 (br., 1 NH); 9.05 (br., 1 NH). 13C-NMR (CDCl3, 75 MHz): 12.69 (2q,
MeÀC(5)(T)); 25.95 (t, CH2ÀC(3')(T2)); 29.97 (t, C(5')(T2)); 30.65 (q, MeCO); 34.13 (t, C(5')(T1)); 36.69
(d, C(3')(T2)); 37.10, 38.38 (2t, C(2')(T1)); 42.85 (d, C(3')(T1)); 50.69 (t, CH2ÀC(3')(T1)); 55.01 (t, C(6')(T2));
60.41 (t, C(6')(T1)); 81.61 (d, C(4')(T2)); 81.84 (d, C(4')(T1)); 85.07 (d, C(1')(T2)); 85.62 (d, C(1')(T1)); 87.00

Helvetica Chimica Acta Vol. 86(2003)
2985
(s, Tr); 111.07, 111.39 (2s, C(5)(T)); 127.11, 127.87, 128.55 (3d, Tr); 135.18, 135.75 (2d, C(6)(T)); 143.88 (s, Tr);
150.24 (s, C(2)(T)); 163.62 (s, C(4)(T)); 195.17 (s, MeCO). MALDI-TOF MS (A ˆ dimer in CH2Cl2, B ˆ 0.1m
‡
2,5-DHB in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1:1): 895 ([M ‡ H ‡ Na] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' !
5')-3'-[(acetylthio)methyl]-3',5'-dideoxythymidine (d(BzOCH2-Tso2T-CH2SAc); 13b). As described for 13a,
with PPh3 (212 mg, 809 mmol), THF (5 ml), DIAD (115 ml, 593 mmol), thioacetic acid (42.3 ml, 593 mmol), 12b
(182 mg, 270 mmol; dried overnight under high vacuum at r.t.), and THF (each 1 ml); reaction for 1.5 h and
quenching with Et3N/MeOH 2 :1 (2 ml). Chromatography (silica gel, CH2Cl2/MeOH 100 :0, 40 :1, 20 :1, 10 :1)
yielded 13b (195 mg, 99%). Colorless foam. UV (MeCN): 221 (17300), 265 (16400). 1H-NMR ((D6)DMSO,
300 MHz): 1.78 (s, 6 H, MeÀC(5)(T)); 2.03 2.43 (m, 2 HÀC(5')(T1), 2 HÀC(5')(T2), 2 HÀC(2')(T1),
2 HÀC(2')(T2), HÀC(3')(T2)); 2.35 (s, Ac); 2.62 2.67 (m, HÀC(3')(T1)); 2.93 (dd, J ˆ 5.4, 13.6, 1 H of
CH2ÀC(3')(T2)); 3.06( dd, J ˆ 7.5, 13.6, 1 H of CH2ÀC(3')(T2)); 3.21 3.32 (m, 1 H of CH2ÀC(3')(T1),
2 HÀC(6')(T2)); 3.21 3.32 (m, 1 H of CH2ÀC(3')(T1)); 3.56 3.62 ( m, 1 H of CH2ÀC(3')(T2)); 3.62 3.70
(dt, HÀC(4')(T2)); 3.80 3.87 (dt, HÀC(4')(T1)); 4.28 4.37 (m, 1 HÀC(6')(T1)); 4.40 4.48
(m, 1 HÀC(6')(T1)); 6.00 6.07 (m, HÀC(1')(T2), HÀC(1')(T1)); 7.44, 7.47 (2s, 2 H, HÀC(6)(T)); 7.50 7.53
(m, 2 H, Bz); 7.62 7.67 (m, 1 H, Bz); 7.95 7.97 (m, 2 H, Bz); 11.30 (br., 1 NH). 13C-NMR (CDCl3, 75 MHz):
12.45 (2q, MeÀC(5)(T)); 26.00 (t, CH2ÀC(3')(T2)); 29.87 (t, C(5')(T2)); 30.50 (q, MeCO); 32.60
(t, C(5')(T1)); 36.49 (t, C(2')(T1)); 36.88 (d, C(3')(T2)); 37.78 (t, C(2')(T2)); 42.73 (d, C(3')(T1)); 50.58
(t, CH2ÀC(3')(T1)); 54.55 (t, C(6')(T2)); 61.71 (t, C(6')(T1)); 81.00, 81.77 (2d, C(4')(T1)); 85.12, 85.37
(2d, C(1')(T1)); 111.12, 111.14 (2s, C(5)(T)); 128.31, 129.41 (2d, Bz); 129.75 (s, Bz); 133.03 (d, Bz); 135.34,
135.89 (2d, C(6)(T)); 150.45, 150.52 (2s, C(2)(T)); 163.96 (2s, C(4)(T)); 166.32 (s, CˆO); 195.22 (s, MeCO).
ESI-MS (neg.): 767.1 ([M ‡ Cl]À), 737.1, 731.1 ([M À H]À), 688.9 ([M À Ac]À), 611.2 ([M À Bz]À).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-
3',5'-dideoxy-3'-(mercaptomethyl)thymidine (d(TrOCH2-Tso2T-CH2SH); 14a). Method 1: NaBH4 (6.2 mg,
164 mmol) was dissolved in degassed MeOH (2 ml, 1 h Ar), a soln. of NaOMe (0.5 mg) in degassed MeOH
(0.5 ml, 1 h Ar) was added, and the soln. was cooled to 08. Then 13a (57 mg, 65 mmol) in degassed MeOH/THF
1:1 (4 ml, 1 h Ar) was slowly added dropwise. The mixture was allowed to warm to r.t., stirred for 4 h, and
cooled again to 08. AcOH was added until pH 5 was reached. The soln. was concentrated to 25% of the original
volume, CH2Cl2 (20 ml) was added, and the soln. was filtered through alox B and silica gel. The filtrate was
evaporated and the residue chromatographed (silica gel, CH2Cl2/MeOH 100 :0, 40 :1, 20 :1, 10 :1): 14a (53 mg,
98%). Colorless foam.
Method 2: A soln. of 13a (70 mg, 566 mmol) in degassed MeOH (2 ml, 1 h Ar) was cooled to 08. Ammonia
was bubbled through the soln. for 15 min, and stirring was continued for 2 h. The mixture was carefully
evaporated at a water-bath temp. of 08. The acetamide was removed under high vacuum at r.t. overnight: 14a
1
(300 mg, quant.). Colorless foam. UV (MeCN): 208 (37500), 265 (16800). H-NMR (CDCl3, 300 MHz): 1.47
(t, J ˆ 16.6, SH); 1.88, 1.92 (2s, MeÀC(5)(T)); 1.80 2.02 (m, 2 HÀC(5')(T1)); 2.08 2.19 (m, 1 HÀC(5')(T2));
2.22 2.49 (m, 1 HÀC(5')(T2), HÀC(3')(T2), 2 HÀC(2')(T1), 2 HÀC(2')(T2)); 2.53 2.72 (m, HÀC(3')(T1),
1 H of CH2ÀC(3')(T2)); 2.90 3.13 (m, 1 H of CH2ÀC(3')(T1), 1 HÀC(6')(T2)); 3.13 3.26( m, 1 H of
CH2ÀC(3')(T1), 1 HÀC(6')(T2)); 3.31 (t, J ˆ 6.0, 2 HÀC(6')(T1)); 3.77 3.85 (m, HÀC(4')(T2)); 3.86 3.92
(m, HÀC(4')(T1)); 5.97 6.04 (m, HÀC(1')(T2), HÀC(1')(T1)); 7.06, 7.08 (2s, 2 H, HÀC(6)(T)); 7.21 7.32
(m, 9 H, Tr); 7.40 7.45 (m, 6H, Tr); 9.21 (br., 1 NH); 9.31 (br., 1 NH). 13C-NMR (CDCl3, 75 MHz): 12.60,
12.68 (2q, MeÀC(5)(T)); 26.01 (t, CH2ÀC(3')(T2)); 34.13 (t, C(5')(T1)); 36.67 (d, C(3')(T2)); 36.90
(t, C(5')(T2)); 38.39 (t, C(2')(T1)); 45.74 (d, C(3')(T1)); 50.54 (t, CH2ÀC(3')(T1)); 50.67 (t, C(6')(T2)); 55.17
(t, C(2')(T2)); 60.40 (t, C(6')(T1)); 81.61 (d, C(4')(T2)); 81.66 (d, C(4')(T1)); 85.14 (d, C(1')(T2)); 85.76
(d, C(1')(T1)); 87.00 (s, Tr); 1101.04, 111.44 (2s, C(5)(T)); 127.11, 127.88, 128.53 (3d, Tr); 135.22, 135.78
(2d, C(6)(T)); 143.85 (s, Tr); 150.29 (s, C(2)(T)); 163.65 (s, C(4)(T)); MALDI-TOF MS (A ˆ dimer in CH2Cl2,
‡
‡
B ˆ 0.1m 2,5-DHB in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1 :1): 1681.3 ([2M ‡ Na] ), 852.5 ([M ‡ Na] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' !
5')-3',5'-dideoxy-3'-(mercaptomethyl)thymidine (d(BzOCH2-Tso2T-CH2SH); 14b). As described for 14a
(Method 2), with 13b (25 mg, 34 mmol), MeOH (3 ml, 1 h Ar), and ammonia (95 min): 14b). (300 mg, quant.).
Colorless foam. UV (MeCN): 265 (16100). 1H-NMR (CDCl3, 300 MHz): 1.92 (s, 6H, Me ÀC(5)(T)); 2.01 2.45
(m, 2 HÀC(5')(T1), 2 HÀC(5')(T2), 1 HÀC(2')(T1), 2 HÀC(2')(T2), HÀC(3')(T2)); 2.46 2.82
(m, 1 HÀC(2')(T1), HÀC(3')(T1), CH2ÀC(3')(T2)); 3.03 3.37 (m, CH2ÀC(3')(T1), 2 HÀC(6')(T2));
3.78 3.86( m, HÀC(4')(T2)); 3.87 3.96( m, HÀC(4')(T1)); 4.40 4.49 (m, HÀC(6')(T1)); 4.54 4.63
(m, HÀC(6')(T1)); 5.92 5.99 (m, HÀC(1')(T2)); 6.02 6.09 (m, HÀC(1')(T1)); 7.07/7.18 (2s, 2 H,
HÀC(6)(T)); 7.41 7.48 (m, 2 H, Bz); 7.56 7.61 ( m, 1 H, Bz); 8.01 8.05 (m, 2 H, Bz); 9.58 (br., 1 NH).

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Helvetica Chimica Acta Vol. 86(2003)
13C-NMR (CDCl3, 75 MHz): 12.56(2 q, MeÀC(5)(T)); 26.16 (t, CH2ÀC(3')(T2)); 32.76(2 t, C(5')); 36.75
(d, C(3')(T2)); 37.52, 37.97 (2t, C(2')); 45.72 (d, C(3')(T1)); 50.61 (t, CH2ÀC(3')(T1)); 55.00 (t, C(6')(T2));
61.70 (t, C(6')(T1)); 81.14 (d, C(4')(T2)); 81.69 (d, C(4')(T1)); 85.50 (d, C(1')(T2)); 86.11 (d, C(1')(T1)); 111.36
(2s, C(5)(T)); 128.45 (d, Cm(Bz)); 129.54 (d, Co(Bz)); 129.84 (s, Cipso (Bz)); 133.17 (d, Co(Bz)); 135.40, 136.14
(2d, C(6)(T)); 150.39 (2s, C(2)(T)); 163.76 (2s, C(4)(T)); 166.46 (s, PhCO).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-
3',5'-dideoxy-3'-{[(4,4'-dimethoxytrityl)thio]methyl}thymidine (d(BzOCH2-Tso2T-CH2STr(OMe)); 15). Com-
pound 14b (110 mg, 159 mmol) and (MeO)2TrCl (108 mg, 318 mmol) were dried overnight under high vacuum at
r.t. and then dissolved in THF (10 ml). Et3N (0.5 ml) was added (clear red ! turbid yellow soln.). The mixture
was stirred for 20 min at r.t. and then quenched with MeOH (1 ml) and evaporated. The residue was
chromatographed (silica gel (column loaded with 1% Et3N), CH2Cl2/MeOH 100 :0, 40 :1, 20 :1): 15 (157 mg,
99%). Colorless foam. UV (MeCN): 209 (52600), 228 (34400), 265 (17100). 1H-NMR (CDCl3, 300 MHz): 1.89
(s, 6 H, MeÀC(5)(T)); 1.75 1.99 (m, 2 HÀC(5')(T2)); 2.00 2.19 (m, 2 HÀC(5')(T1), HÀC(3')(T2),
1 HÀC(2')(T2)); 2.22 2.41 (m, 1 HÀC(2')(T1), CH2ÀC(3')(T2), 1 HÀC(2')(T2)); 2.44 2.55
(m, 1 HÀC(2')(T1)); 2.66 2.78 (m, HÀC(3')(T1)); 2.96 3.10 ( m, 1 H of CH2ÀC(3')(T1), 1 HÀC(6')(T2));
3.14 3.27 (m, 1 H of CH2ÀC(3')(T1), 1 HÀC(6')(T2)); 3.46 3.54 ( dt, HÀC(4')(T2)); 3.78 (s, 2 MeO); 3.84
3.93 (m, HÀC(4')(T1)); 4.38 4.47 (m, 1 HÀC(6')(T1)); 4.52 4.61 (m, 1 HÀC(6')(T1)); 5.86( dd, J ˆ 4.2, 6.8,
HÀC(1')(T2)); 6.06 (dd, J ˆ 4.4, 6.6, bHÀC(1')(T1)); 6.80 6.83 (m, J ˆ 8.8, 4 H, Hm of Ar2PhC); 6.92, 7.15 (2s,
2 H, HÀC(6)(T)); 7.18 7.24 (m, 1 H, (MeO)2Tr); 7.26 7.32 ( m, 6H, (MeO) 2Tr); 7.36 7.44 ( m, 4 H, Ho of
Ar2PhC, 2 H of Bz); 7.51 7.56( m, 1 H, Bz); 8.01 (d, J ˆ 7.1, 2 H, Bz); 9.83 (br., 1 NH). 13C-NMR (CDCl3,
75 MHz): 12.50, 12.53 (2q, MeÀC(5)(T)); 25.88 (t, CH2ÀC(3')(T2)); 32.67, 33.43 (2t, C(5')(T)); 36.51
(d, C(3')(T2)); 37.43, 37.89 (2t, C(2')(T)); 42.61 (d, C(3')(T1)); 50.61 (t, CH2ÀC(3')(T1)); 54.81
(t, C(6')(T2)); 55.17 (q, MeO); 6 1.6 5 t(, C(6')(T1)); 66.27 (s, Ar2PhC); 81.03 (d, C(4')(T2)); 82.03
(d, C(4')(T1)); 85.15 (2d, C(1')(T)); 111.20, 111.25 (2s, C(5)(T)); 113.78 (d, Cm(Ar2PhC)); 126.70 (d, Cp
(Ar2PhC)); 127.93 (d, Co (Ar2PhC)); 128.35 (d, Cm(Bz)); 129.19 (d, Cm (Ar2PhC)); 129.46( d, Co(Bz)); 129.78
(s, Cipso(Bz)); 130.52 (d, Co(Ar2PhC)); 133.07 (d, Cp(Bz)); 135.23, 135.64 (2d, C(6)(T)); 136.52 (s, Cip-
so(Ar2PhC)); 144.85 (s, Cipso(Ar2PhC)); 150.40, 150.46(2 s, C(2)(T)); 158.08 (s, Cp(Ar2PhC)); 163.85
(2s, C(4)(T)); 166.35 (s, PhCO). ESI-MS (neg.): 1027.4 ([M ‡ C]À), 991.6([ M À H]À), 948.0
5'-Deoxy-3'-de(phosphinicooxy)-5'-(hydroxymethyl)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3',5'-
dideoxy-3'-{[(4,4'-dimethoxytrityl)thio]methyl}thymidine (d(HOCH2-Tso2T-CH2STr(OMe)2); 16). To a suspen-
sion of 15 (157 mg, 158 mmol) in MeOH (10 ml), 2m NaOH (1.0 ml) was added slowly ( ! soln.). After 30 min,
the mixture was neutralized with acetate buffer (3m AcOH/1m AcONa 3 :1; 1.0 ml) and immediately poured
into sat. NaHCO3 soln. (15 ml) containing ice (10 g). CH2Cl2 (60 ml) was added and the org. phase washed with
brine (2 Â 15 ml). The aq. phases were reextracted with CH2Cl2 (3 Â 20 ml). The combined org. phase was dried
(MgSO4) and evaporated. FC (silica gel (column loaded with 1% Et3N), CH2Cl2/MeOH 100 :0, 20 :1, 10 :1, 5 :1)
yielded 16 (138 mg, 98%). Colorless foam. UV (MeCN): 235 (211000), 264 (19000). 1H-NMR (CDCl3,
500 MHz): 1.82 1.91 (m, 2 HÀC(5')(T2)); 1.91, 1.92 (2s, 6H, Me ÀC(5)(T)); 1.97 2.12 (m, 2 HÀC(5')(T1),
1 HÀC(2')(T2)); 2.19 2.28 (m, 1 HÀC(2')(T2), 1 H of CH2ÀC(3')(T2)); 2.29 2.37 (m, HÀC(3')(T2),
1 HÀC(2)(T2)); 2.22 2.41 (m, 1 HÀC(2')(T1), CH2ÀC(3')(T2), 1 HÀC(2')(T2)); 2.44 2.55 (m, 1 H of
CH2ÀC(3')(T1), 1 HÀC(2')(T1)); 2.46 2.52 ( m, 1 HÀC(2')(T1)); 2.70 2.78 (m, HÀC(3')(T1)); 2.97 (dd, J ˆ
10.2, 13.8, 1 H of CH2ÀC(3')(T1)); 3.01 3.06( m, 1 HÀC(6')(T2)); 3.15 3.21 (m, 1 HÀC(6')(T2)); 3.27
(dd, J ˆ 3.5, 13.7, 1 H of CH2ÀC(3')(T1)); 3.51 3.56( dt, HÀC(4')(T2)); 3.74 3.79 (m, 1 HÀC(6')(T1)); 3.79
(s, 2 MeO); 3.82 3.88 (m, 1 HÀC(6')(T1), HÀC(4')(T1)); 5.80 (dd, J ˆ 4.2, 7.6, HÀC(1')(T2)); 6.00 (dd, J ˆ
4.1, 7.6, HÀC(1')(T1)); 6.80 6.84 (m, J ˆ 9.0, 4 H, Hm of Ar2PhC); 6.91, 6.92 (2s, 1 H, HÀC(6)(T)); 7.12 (2s,
1 H, HÀC(6)(T)); 7.19 7.23 (m, 1 H, (MeO)2Tr); 7.26 7.31 ( m, 6H, (MeO) 2Tr); 7.37 7.39 (m, 2 H, Ho of
Ar2PhC). 13C-NMR (CDCl3, 75 MHz): 12.55 (2q, MeÀC(5)(T)); 26.03 (t, CH2ÀC(3')(T2)); 33.45, 35.59
(2t, C(5')(T)); 36.39 (d, C(3')(T2)); 37.32, 37.97 (2t, C(2')(T)); 42.66 (d, C(3')(T1)); 50.40 (t, CH2ÀC(3')(T1));
54.65 (t, C(6')(T2)); 55.18 (q, MeO); 58.93 (t, C(6')(T1)); 66.26 (s, Ar2PhC); 81.82, 82.13 (2d, C(4')(T)); 84.99,
85.29 (2d, C(1')(T)); 111.20 (2s, C(5)(T)); 113.17 (d, Cm(Ar2PhC)); 126.71 (s, Cp(Ar2PhC)); 127.92
(d, Co(Ar2PhC)); 129.19 (d, Cm(Ar2PhC)); 130.53 (d, Co(Ar2PhC)); 135.53, 135.80 (2d, C(6)(T)); 136.54
(s, Cipso(Ar2PhC)); 144.86( s, Cipso(Ar2PhC)); 150.55, 150.64 (2s, C(2)(T)); 158.07 (s, Co(Ar2PhC)); 164.02
‡
‡
‡
(2s, C(4)(T)). ESI-MS (pos.): 911.0 ([M ‡ Na] ), 865.3, 785.9 ([M À T‡ Na] ), 303 (Tr ).
5'-(Bromomethyl)-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3',5'-
dideoxy-3'-{[(4,4'-dimethoxytrityl)thio]methyl}thymidine (d(BrCH2-Tso2T-CH2STr(OMe)2); 17). Compound 16
(138 mg, 155 mmol) and PPh3 (79 mg, 300 mmol) were dried overnight under high vacuum at 458 and then
dissolved in 1,2-dichloroethane (10 ml). Then CBr4 (80 mg, 240 mmol) in 1,2-dichloroethane (1 ml) was added

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slowly. After the addition of ca. 1.2 equiv. of CBr4 the soln. turned slightly orange because of negligible
detritylation. The mixture was stirred for 60 min at r.t., quenched with Et3N/MeOH 2 :1 (3 ml), and evaporated
(308 water-bath temperature). FC (silica gel (column loaded with 1% Et3N), CH2Cl2/MeOH 100 :0, 20 :1, 10 :1,
5 :1) yielded 17 (143 mg, 97%). Colorless foam. UV (MeCN): 207 (49700), 265 (17300). 1H-NMR (CDCl3,
500 MHz): 1.81 1.91 (m, 2 HÀC(5')(T2)); 1.94 (2s, 6H, Me ÀC(5)(T)); 1.98 2.15 (m, 2 HÀC(2')(T2)); 2.18
2.30 (m, 2 HÀC(5')(T1), 1 H of CH2ÀC(3')(T2), HÀC(3')(T2)); 2.33 2.39 (m, 1 H of CH2ÀC(3')(T2),
1 HÀC(2')(T1)); 2.50 2.55 (m, 1 HÀC(2')(T1)); 2.70 2.73 (m, HÀC(3')(T1)); 2.98 (dd, J ˆ 9.8, 13.7, 1 H,
CH2ÀC(3')(T1)); 2.99 3.05 (m, 1 HÀC(6')(T2)); 3.10 (dd, J ˆ 3.5, 13.7, 1 H, CH2ÀC(3')(T1)); 3.13 3.19
(m, 1 HÀC(6')(T2)); 3.49 3.61 (m, 2 HÀC(6')(T1), HÀC(4')(T2)); 3.80 (s, 2 MeO); 3.87 (dd, J ˆ 3.4, 8.3,
HÀC(4')(T1)); 5.83 (dd, J ˆ 4.4, 7.8, HÀC(1')(T2)); 6.00 (dd, J ˆ 3.9, 7.8, HÀC(1')(T1)); 6.81 6.84 (m, J ˆ 9.8,
4 H, Hm of Ar2PhC); 6.92 (2s, 1 H, HÀC(6)(T)); 7.04 (2s, 1 H, HÀC(6)(T)); 7.21 7.24 (m, 1 H, (MeO)2Tr);
7.28 7.31 (m, 6H, (MeO) 2Tr); 7.39 (d, J ˆ 7.3, 2 H, Ho of Ar2PhC); 8.33 (br., 1 NH); 8.42 (br., 1 NH). 13C-NMR
(CDCl3, 75 MHz): 12.53 (2q, MeÀC(5)(T)); 25.90 (t, CH2ÀC(3')(T2)); 29.38 (t, C(6')(T1)); 33.39, 36.01
(2t, C(5')(T)); 36.45 (d, C(3')(T2)); 37.43, 37.83 (2t, C(2')(T)); 42.54 (d, C(3')(T1)); 50.53 (t, CH2ÀC(3')(T1));
54.61 (t, C(6')(T2)); 55.14 (q, MeO); 66.22 (s, Ar2PhC); 81.45 (d, C(4')(T2)); 82.05 (d, C(4')(T1)); 85.17
(2d, C(1')(T)); 111.14, 111.25 (2s, C(5)(T)); 113.14 (d, Cm(Ar2PhC)); 126.66 (s, Cp(Ar2PhO)); 127.79
(d, Co(Ar2PhC)); 129.15 (d, Cm(Ar2PhC)); 130.48 (d, Co(Ar2PhC)); 135.41 (2d, C(6)(T)); 136.48 (s, Cip-
so(Ar2PhC)); 144.80 (s, Cipso(Ar2PhC)); 150.37, 150.39 (2s, C(2)(T)); 158.04 (s, Cp(Ar2 PhC)); 163.75, 163.83
(2s, C(4)(T)). ESI-MS (neg.): 1031.7 ([M ‡ Br]À), 987.0 ([M ‡ Cl]À), 950.9, 948.9 (MÀ), 905.1 ([M À Br‡
Cl]À).
5. Tetramers: Coupling and Oxidation. 5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylyl-
methylenesulfonylmethylene-(3' ! 5')-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenethiomethylene-
(3' ! 5')-5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-3'-{{[(tert-butyl)di-
phenylsilyl]oxy}methyl}-2',3',5'-trideoxy-N4-(o-toluoyl)cytidine (d(TrOCH2-Tso2TsTso2toC-CH2OTBDPS);
19b). Thiol 14a (40 mg, 48 mmol), bromide 11b (44 mg, 45 mmol), and Cs2CO3 (64 mg, 180 mmol) were
dissolved in degassed DMF (3 ml, 1 h Ar). The soln. was degassed a second time by performing three freeze-
pump cycles. The mixture was warmed to 458 and stirred for 5 h. Acetate buffer (3m AcOH/1m AcONa; 90 ml)
was added, and the solvent was evaporated at max. 408. The crude product was chromatographed (silica gel,
CH2Cl2/MeOH 100 :0, 40 :1, 20 :1, 10 :1): 19b (63 mg, 81%). Colorless foam. 1H-NMR ((D6)DMSO, 500 MHz):
1.00 (s, t-Bu); 1.75, 1.77, 1.78 (6s, 9 H, MeÀC(5)(T)); 1.83 1.94 (m, 2 H, HÀC(5')); 1.96 2.41 ( m, 17 H,
6 HÀC(5'), 8 HÀC(2'), 3 HÀC(3')); 2.36( s, Me(to)); 2.50 2.54 (m, partly under DMSO, 1 H, HÀC(3'));
2.55 2.62 (m, 2 H, CH2SCH2); 2.67 2.74 (m, 2 H, CH2SCH2); 3.02 3.08 (m, 2 H, CH2SO2CH2); 3.18 3.37 (m,
partly under HDO, 6H, CH 2SO2CH2, 2 HÀC(6')(T1)); 3.40 3.48 (m, 2 H, CH2SO2CH2); 3.65 3.73 (m, 4 H,
2 HÀC(4'), CH2ÀC(3')(C)); 3.73 3.79 (dt, 1 H, HÀC(4')); 3.99 4.04 (dt, 1 H, HÀC(4')); 5.97 6.00 (dd, 1 H,
HÀC(1')); 6.00 6.04 (m, 3 H, HÀC(1')); 7.20, 7.21, 7.22 (3s, 3 HÀC(6)(T)); 7.24 7.31 (m, 8 arom. H); 7.33
7.40 (m, 9 arom. H); 7.41 7.48 (m, 9 H); 7.60 7.64 (m, 4 H, Ph2Si); 8.15 (d, J ˆ 7.4, HÀC(6)(C)); 11.18, 11.25
(2 br., 4 H, NH). 13C-NMR ((D6)DMSO, 125 MHz): 12.05, 12.09 (2q, MeÀC(5)(T)); 18.73 (s, Me3C); 19.46
(q, Me(to)); 25.44, 26.24 (2t, C(5')); 26.62 (q, Me3C); 28.75 (t, CH2SCH2); 32.68, 33.02, 33.44 (3t, C(5'),
CH2SCH2); 35.09 (t, C(5')); 35.75, 36.24 (2d, C(3')); 36.63, 36.81, 36.89 (3t, C(2')); 42.02 (d, C(3')(T1)); 43.91
(d, C(3')(C)); 49.52, 49.80 (2t, C(6')(T2), C(6')(T4)); 53.69 (t, CH2ÀC(3')(T1), CH2ÀC(3')(T3)); 60.65
(t, C(6')(T1)); 63.72 (t, CH2ÀC(3')(T4)); 80.35, 81.74, 81.83, 81.89 (4d, C(4')); 83.31, 83.62, 83.68 (3d, C(1'));
86.04 (s, Tr); 86.11 (d, C(1')(C)); 96.06 (d, C(5)(C)); 109.62, 109.76, 109.88 (2s, C(5)(T)); 125.53 (d, to); 126.93
(d, Tr); 127.81 (d, Tr); 127.95 (d, Ph2Si); 128.16( d, Tr); 129.91 (d, Ph2Si); 130.52, 130.63 (2d, to); 132.60, 132.68
(2s, Ph2Si); 135.07, 135.08 (2d, Ph2Si); 135.17, 135.86(2 s, Co(to)); 135.98, 136.12, 136.19 (3d, C(6)(T)); 143.87
(s, Tr); 145.14 (d, C(6)(C)); 150.28, 150.31, 150.37 (3s, C(2)(T)); 154.36( d, C(2)(C)); 162.64 (s, CˆO); 163.67
(s, C(4)(T)); 169.50 (s, C(4)(C)). MALDI-TOF MS (A ˆ dimer in CH2Cl2/MeOH 10 :1, B ˆ 0.1m CCA in
‡
MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1:1): 1744.6([ M ‡ Na] ).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-5'-
deoxy-3'-de(phosphinicooxy)thymidylylmethylenethiomethylene-(3' ! 5')-5'-deoxy-3'-de(phosphinicooxy)thy-
midylylmethylenesulfonylmethylene-(3' ! 5')-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-3',5'-dideoxythymi-
dine (d(TrOCH2-Tso2TsTso2T-CH2OTBDPS); 19a). As described for 19b, with 14a (32 mg, 38.6 mmol), 11a
(31 mg, 35.6 mmol), Cs2CO3 (35 mg, 107 mmol), DMF (2 ml, 1 h Ar) (stirring of r.t. overnight), and acetate
buffer (3m AcOH/1m AcONa; 54 ml): 19a (49 mg, 85%). Colorless foam. UV (CH2Cl2/MeOH 20 :1): 229, 266.
1H-NMR (CDCl3, 500 MHz): 1.06( s, t-Bu); 1.84 2.05 (m, 5 H); 1.89, 1.89, 1.90, 1.90 (4s, 12 H, MeÀC(5)(T));
2.06 2.17 ( m, 2 H); 2.18 2.40 (m, 10 H); 2.42 2.51 (m, 2 H); 2.54 2.70 (m, 4 H, CH2SCH2); 2.72 2.80
(m, 1 H, HÀC(3')(T)); 2.99 3.26( m, 8 H, CH2SO2CH2); 3.27 3.31 (m, 2 H, HÀC(6')(T1)); 3.65 3.91

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(m, 6H, H ÀC(4'), CH2ÀC(3')(T4')); 5.87 5.91 (dd, 1 H, HÀC(1')); 5.92 5.96( m, 2 H, HÀC(1')); 6.04
(dd, J ˆ 4.5, 7.3, 1 H, HÀC(1')); 7.03, 7.07, 7.08, 7.11 (4s, 4 H, HÀC(6)(T)); 7.19 7.29 (m, 9 H, Tr); 7.37 7.47
(m, 12 H, 6 H of Ph2Si, 6 H of Tr); 7.60 7.64 (m, 4 H, Ph2Si); 9.53, 9.61, 9.68, 9.82 (4 br., 4 NH). 13C-NMR
(CDCl3, 125 MHz): 12.56, 12.59, 12.69 (3q, MeÀC(5)(T)); 19.24 (s, Me3C); 26.03 (t); 26.94 (q, Me3C); 29.54,
33.47, 33.72, 34.19, 34.54 (5t, C(5'), CH2ÀC(3')(T2), C(6')(T3)); 36.30, 36.74 (2d, C(3')(T2), C(3')(T3)); 37.33,
37.47, 37.76, 38.43 (4t, C(2')); 42.82 (d, C(3')(T1)); 45.38 (d, C(3')(T4)); 50.38, 51.02 (2t, C(6')(T2), C(6')(T4));
54.73, 55.12 (2t, CH2ÀC(3')(T1), CH2ÀC(3')(T3)); 60.52 (t, C(6')(T1)); 63.65 (t, CH2ÀC(3')(T4)); 80.79, 81.85,
82.06, 82.46 (4d, C(4')); 84.82, 85.58, 85.89, 86.83 (4d, C(1')); 87.00 (s, Tr); 110.83, 111.20, 111.35, 111.53
(s, C(5)(T)); 127.13 (d, Tr); 127.90, 127.93, 127.95, 127.99 (d, Ph2Si); 128.59 (d, Tr); 130.03, 130.05 (2d, Ph2Si);
132.82, 132.95 (2s, Ph2Si); 135.44, 135.53 (2d, C(6)(T)); 135.57 (d, Ph2Si); 136.13, 136.77 (2d, C(6)(T)); 143.95
(s, Tr); 150.50, 150.58, 150.61 (3s, C(2)(T)); 163.78, 163.81, 163.95, 164.03 (4s, C(4)(T)). MALDI-TOF-MS
‡
(A ˆ dimer in CH2Cl2, B ˆ 0.1m CCA in MeCN/EtOH/H2O 50 :45 :5, C ˆ A/B 1:1): 1642.5 ([M ‡ Na] ).
5'-[(Benzoyloxy)methyl]-5'-deoxy-3'de(phosphinicooxy)thymidylylmethylenesulfonylmethylene-(3' ! 5')-
5'-deoxy-3'-de(phosphinicooxy)thymidylylmethylenethiomethylene-(3' ! 5')-5'-deoxy-3-de(phosphinicooxy)-
thymidylylmethylenesulfonylmethylene-(3' ! 5')-3',5'-dideoxy-3'-{[(4,4'-dimethoxytrityl)thio]methyl}thymidine
(d(BzOCH2-Tso2TsTso2T-CH2STr(OMe)2); 18). As described for 19b, with 14b (78 mg, 113 mmol), 17 (114 mg,
120 mmol), Cs2CO3 (147 mg, 452 mmol), DMF (10 ml, 1 h Ar), (stirring at 458 for 4.5 h), and acetate buffer (3m
AcOH/1m AcONa; 226 ml). Chromatography (silica gel (column loaded with 1% Et3N), CH2Cl2/MeOH 100 :0,
40 :1, 20 :1, 15 :1, 5 :1) yielded 18 (162 mg, 92%). Colorless foam. UV (CH2Cl2/CH3OH 20 :1): 228 (56600), 267
(51400). 1H-NMR (CDCl3, 500 MHz): 1.84 1.93 (m, 2 H); 1.89, 1.89, 1.90, 1.90 (4s, 12 H, MeÀC(5)(T)); 1.98
2.06( m, 3 H); 2.08 2.17 (m, 3 H); 2.17 2.41 (m, 11 H); 2.43 2.48 (m, 1 H); 2.49 2.68 (m, 4 H); 2.72 2.79
(m, 2 H); 3.03 3.32 (m, 8 H, CH2SO2CH2); 3.52 3.57 (m, 1 H, HÀC(4')); 3.74 3.85 (m, 2 H, HÀC(4')); 3.79
(s, 2 MeO); 3.88 3.92 (dt, 1 H, HÀC(4')); 4.40 4.46( m, 1 H, HÀC(6')(T)); 4.53 4.59 (m, 1 H,
HÀC(6')(T1)); 5.81 (dd, J ˆ 4.4, 7.8, 1 H, HÀC(1')); 5.87 (dd, J ˆ 4.6, 7.0, 1 H, HÀC(1')); 5.98 (dd, J ˆ 4.4,
7.3, 1 H, HÀC(1')); 6.01 (dd, J ˆ 4.4, 7.3, 1 H, HÀC(1')); 6.81 6.84 (m, J ˆ 8.8, 4 H, Hm of Ar2PhC); 6.91, 7.06,
7.09, 7.19 (4s, 4 H, HÀC(6)(T)); 7.20 7.25 (m, 1 H, (MeO)2Tr); 7.26 7.31 ( m, 6H, (MeO) 2 Tr); 7.37 7.43
(m, 4 H, Ho of Ar2PhC, 2 H of Bz); 7.53 7.56( m, 1 H, Bz); 7.99 8.01 (m, 2 H, Bz). 13C-NMR (CDCl3/CD3OD
4 :1, 75 MHz): 12.05 (q, MeÀC(5)(T)); 25.86(2 t, CH2ÀC(3')(T4), CH2ÀC(3')(T4)); 29.09 (t, C(6')(T3)); 32.37,
33.14, 33.99 (3t, C(5')(T)); 35.80, 36.21 (2d, C(3')(T)); 37.43, 37.59 (2t, C(2')(T)); 42.41 (d, C(3')(T)); 50.38
(t, CH2ÀC(3')(T)); 54.37 (t, C(6')(T)); 54.92 (s, MeO); 61.64 (t, C(6')(T1)); 66.04 (s, Ar2PhC); 80.85, 81.90,
81.93 (3d, C(4')(T)); 84.58, 84.62, 85.03, 85.07 (4d, C(1')(T)); 110.89 (s, C(5)(T)); 112.97 (d, Cm(Ar2PhC) );
126.50 (s, Cp(Ar2PhC)); 127.70 (d, Co(Ar2PhC)); 128.18 (d, Cm(Bz)); 129.01 (d, Cm(Ar2PhC)); 129.22
(d, Co(Bz)); 129.46( s, Cipso(Bz)); 130.34 (d, Co(Ar2PhC)); 133.01 (d, Cp(Bz)); 135.49, 135.64 (2d, C(6)(T));
136.40 (s, Cipso(Ar2PhC)); 144.64 (s, Cipso(Ar2PhC)); 150.36( s, C(2)(T)); 157.89 (s, Cp(Ar2PhC)); 164.26
(s, C(4)(T)); 166.54 (s, PhCO). MALDI-TOF MS (A ˆ dimer in CH2Cl2/MeOH 10 :1, B ˆ 0.5m 2,4,6-THA in
EtOH/0.1m DAC (diammonium citrate) in H2O 2 :1, C ˆ A/B 3 :1): 1561.1 (MÀ).
5'-Deoxy-3'-de(phosphinicooxy)-5'-[(trityloxy)methyl]thymidylylmethylenesulfonylmethylene-(3' ! 5')-5'-
deoxy-3'-de(phosphinicooxy)thymidylylmethylenethiomethylene-(3' ! 5')-5'-deoxy-3'-de(phosphinicooxy)thy-
midylylmethylenesulfonylmethylene-(3' ! 5')-3'-{{[(tert-butyl)diphenylsilyl]oxy}methyl}-2',3',5'-trideoxy-N4-
(o-toluoyl)cytidine (d(TrOCH2-Tso2Tso2Tso2toC-CH2OTBDPS); 20b). As described for 8b, with 19b (63 mg,
37 mmol), MeOH/THF 2 :1 (24 ml), Oxone (90 mg, 146 mmol), NaOAc ¥ 3 H2O (65 mg, 480 mmol), and
deionized H2O (4 ml) ( ! opaque suspension; conversion to sulfoxides within 5 min, then to sulfone within
2.5 h). Workup with sat. Na2S2O3 soln. (3 ml) and CH2Cl2 (40 ml), then extraction with CH2Cl2 (4 Â 15 ml) (no
washing with brine): 20b (58 mg, 90%). Colorless foam. UV (CH2Cl2/MeOH 20 :1): 229, 260, 308. 1H-NMR
((D6)DMSO, 500 MHz): 1.00 (s, t-Bu); 1.76, 1.78, 1.79 (6s, 9 H, MeÀC(5)(T)); 1.84 1.92 (m, 1 H, HÀC(5'));
1.99 2.18 (m, 5 H, HÀC(5')); 2.20 2.38 (m, 11 H, 8 HÀC(2'), 3 HÀC(5')); 2.36( s, Me (to)); 2.49 2.56
(m, partly under DMSO, 1 H, HÀC(3')); 2.63 2.73 (m, 2 H, HÀC(3')); 3.02 3.08 (m, 2 H, CH2SO2CH2);
3.16 3.39 ( m, partly under HDO, 9 H, CH2SO2CH2, 2 HÀC(6')(T1)); 3.42 3.48 (m, 1 H, CH2ÀC(3')); 3.50
3.56( m, 2 H, CH2ÀC(3')); 3.67 3.74 (m, 4 H, 2 HÀC(4'), CH2ÀC(3')(C)); 3.75 3.80 (dt, 1 H, HÀC(4'));
4.00 4.05 (dt, 1 H, HÀC(4')); 5.97 6.00 (dd, 1 H, HÀC(1')); 6.01 6.07 (m, 3 H, HÀC(1')); 7.21, 7.23, 7.24 (3s,
3 H, HÀC(6)(T)); 7.25 7.32 (m, 8 arom. H); 7.33 7.40 (m, 9 arom. H); 7.41 7.50 (m, 9 H); 7.60 7.63 (m, 4 H,
Ph2Si); 8.15 (d, J ˆ 7.5, HÀC(6)(C)); 11.25 (br., 4 H, NH). 13C-NMR ((D6)DMSO, 125 MHz): 12.03, 12.04,
12.10 (3q, MeÀC(5)(T)); 18.74 (s, Me3C); 19.46( q, Me (to)); 24.43, 26.12 (t, C(5')); 26.65 (q, Me3C); 33.00,
35.06(2 t, C(5')); 35.85, 36.24 (2d, C(3')); 36.62, 36.67, 36.81 (3t, C(2')); 43.94 (d, C(3')(C)); 49.54, 49.62, 49.82
(3t, C(6')(T2), C(6')(T3), C(6')(T4)); 53.62, 54.87 (t, CH2ÀC(3')(T1), CH2ÀC(3')(T3)); 60.65 (t, C(6')(T1));
63.71 (t, CH2ÀC(3')(T4)); 80.36, 81.30, 81.73 (3d, C(4')); 83.62, 83.68 (2d, C(1')); 86.04 (s, Tr); 86 .10