Nucleotides. Part LXVII. The 2-cyanoethyl and (2-cyanoethoxy)carbonyl group for base protection in nucleoside and nucleotide chemistry

Article abstract of DOI:10.1002/1522-2675(20001220)83:12<3198::AID-HLCA3198>3.0.CO;2-Q

The amino functions of the common 2′-deoxyribo- and ribonucleosides were blocked by the (2-cyanoethoxy) carbonyl group on treatment with 2-cyanoethyl carbonochloridate (5) or 1-[(2-cyanoethoxy)carbonyl] -3-methyl-1H-imidazolium chloride (6) leading to 7, 18.8, 19.9, and 20. In 2′-deoxyguanosine, the amide group was additionally blocked at the O° position by the 2-cyanoethyl (→ 27) and 2-(4-nitrophenyl)ethyl group (→ 31, 32). Comparative kinetic studies regarding the cleavage of the ce/ceoc and npe/npeoc group by β-elimination revealed valuable information about the case and sequential deprotection of the various blocking groups at different sites of the nucleobases. Besides the 5′-O-(dimethoxytrityl)-protected 3′-(2-cyanoethyl diisopropylphosphoramidites) 38 and 39 of N⁴-[(2-cyanoethoxy)carbonyl]-2′-deoxycytidine and N⁶-[(2-cyanoethoxy) carbonyl]-2′-deoxyadenosine, respectively, the N²-[(2-cyanoethoxy)carbonyl]-2′-deoxy-O°- [2-(4-nitrophenyl) ethyl]guanosine analog 40 is recommended as building block for oligo-2′-deoxyribonucleotide synthesis.

Full text of DOI:10.1002/1522-2675(20001220)83:12<3198::AID-HLCA3198>3.0.CO;2-Q

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Helvetica Chimica Acta ± Vol. 83 (2000)
Nucleotides
Part LXVII1)
The 2-Cyanoethyl and (2-Cyanoethoxy)carbonyl Group for Base Protection
in Nucleoside and Nucleotide Chemistry
by Claudia Merk, Tilman Reiner, Evgeny Kvasyuk, and Wolfgang Pfleiderer*
Fakultät für Chemie, Universität Konstanz, Postfach 5560, D-78434 Konstanz
The amino functions of the common 2'-deoxyribo- and ribonucleosides were blocked by the (2-
cyanoethoxy)carbonyl group on treatment with 2-cyanoethyl carbonochloridate (5) or 1-[(2-cyanoethoxy)car-
bonyl]-3-methyl-1H-imidazolium chloride (6) leading to 7, 18, 8, 19, 9, and 20. In 2'-deoxyguanosine, the amide
group was additionally blocked at the O6 position by the 2-cyanoethyl ( ! 27) and 2-(4-nitrophenyl)ethyl group
( ! 31, 32). Comparative kinetic studies regarding the cleavage of the ce/ceoc and npe/npeoc group by
b-elimination revealed valuable information about the ease and sequential deprotection of the various blocking
groups at different sites of the nucleobases. Besides the 5'-O-(dimethoxytrityl)-protected 3'-(2-cyanoethyl
diisopropylphosphoramidites) 38 and 39 of N4-[(2-cyanoethoxy)carbonyl]-2'-deoxycytidine and N6-[(2-cyano-
ethoxy)carbonyl]-2'-deoxyadenosine, respectively, the N2-[(2-cyanoethoxy)carbonyl]-2'-deoxy-O6-[2-(4-nitro-
phenyl)ethyl]guanosine analog 40 is recommended as building block for oligo-2'-deoxyribonucleotide syn-
thesis.
Introduction. ± The development of the 2-(4-nitrophenyl)ethyl (npe) and [2-(4-
nitrophenyl)ethoxy]carbonyl (npeoc) groups as versatile protective groups for
nucleobases [2], phosphate [3][4], and phosphite functions [5][6] broadened the
strategy of nucleoside and nucleotide protection in oligonucleotide synthesis univer-
sally. The great advantange of these groups is their high stability under acidic and mild
basic hydrolytic conditions as, for example, in the presence of ammonia and amines in
MeOH, dioxane, or H2O, whereas their cleavage can easily and quantitatively be
achieved by 1,8-diazabicyclo[5.4.0]undecene (DBU) in aprotic solvents by a b-elimi-
nation process.
The 2-cyanoethyl (ce) group [7][8] is the most common blocking group for the
phosphate and phosphite moiety of nucleotides, but strangely enough, the ce and its
corresponding (2-cyanoethoxy)carbonyl (ceoc) group have not been applied for base
protection in analogy to the npe/npeoc couple. Several years ago, we [9][10] developed
the ce/ceoc strategy as an alternative approach for the synthesis of special
oligonucleotides. Details about the nucleobase protection by the ce and ceoc group
will now be reported since N-{[(2-cyanoethoxy)carbonyl]oxy}succinimide has recently
been described as a new reagent for protection of amino groups in oligonucleotides
[11]. We protected the amino group in cytidine, adenosine, and guanosine as well as in
the corresponding 2'-deoxynucleosides with the ceoc group and utilized the ce group
1
)
Part LXVI: [1].

Helvetica Chimica Acta ± Vol. 83 (2000)
3199
for blocking of the O6-function in 2'-deoxyguanosine in order to study its stability
regarding DBU cleavage in comparison to the npe and 2-(phenylsulfonyl)ethyl (pse)
group.
Synthesis. ± The (2-cyanoethoxy)carbonylation reactions were achieved either by 2-
cyanoethyl carbonochloridate (5), which was prepared by an improved synthesis
according to Kondratenko and Khaskin [12] from 3-hydroxypropanenitrile and
phosgene, or by 1-[(2-cyanoethoxy)carbonyl]-3-methyl-1H-imidazolium chloride (6),
resulting from 5 and 1-methyl-1H-imidazole, in 94% yield. Cytidine (1), adenosine (2),
and guanosine (3) were acylated by 5 after transient protection [13] at the sugar moiety
by trimethylsilylation to give, after workup, N4-[2-(cyanoethoxy)carbonyl]cytidine (7)
in 68%, N6-[(2-cyanoethoxy)carbonyl]adenosine (8) in 80%, and N2-[(2-cyanoethoxy)-
carbonyl]guanosine (9) in 54% yield (Scheme). Also 2',3',5'-tri-O-acetyladenosine (4)
was treated with a mixture of 5 and 1-methyl-1H-imidazole in MeCN to give 2',3',5'-tri-
O-acetyl-N6-[(2-cyanoethoxy)carbonyl]adenosine (10) in 90% isolated yield. Mono-
methoxytritylation of 7 and 8 worked also very well yielding the corresponding 5'-O-
(monomethoxytrityl) derivatives 11 and 12.
In the 2'-deoxyribonucleoside series, transient protection of the sugar moiety of 2'-
deoxycytidine (13) was not necessary due to the high nucleophilicity of its amino group,
forming, under otherwise analogous conditions, N4-[(2-cyanoethoxy)carbonyl]-2'-
deoxycytidine (18) in a highly selective reaction in 79% yield. The 2'-deoxyadenosine
(14) reacted, after transient silylation, to N6-[(2-cyanoethoxy)carbonyl]-2'-deoxyaden-
osine (19), whereas 2'-deoxyguanosine (15) expectedly caused problems for solubility
reasons, to give only 19% of N2-[2-(cyanoethoxy)carbonyl]-2'-deoxyguanosine (20),
and starting from 3',5'-bis-O-[(tert-butyl)dimethylsilyl)]-2'-deoxyguanosine (16), to
form 21 in 22% yield. Monotritylation of 18 and 19 gave the 5'-protected derivatives 22
and 23, respectively.
The direct introduction of the 2-cyanoethyl group onto O6 of 15 by a Mitsunobu
reaction starting from 16 or from its N2-isobutyryl- and N2-(dimethoxytrityl) derivative
was unsuccessful because elimination of acrylonitrile from the intermediary (2-
cyanoethoxy)triphenylphosphonium ion is much faster than substitution at the O6-
position by alkylation. Also an attempted alkylation with 3-iodopropanenitrile did not
proceed in the anticipated manner. Finally, the method of Jones and co-workers
[14][15] was applied, converting 16 by 2,4,6-triisopropylbenzenesulfonyl chloride in the
presence of Et3N and N,N-dimethylpyridin-4-amine (DMAP) to 3',5'-bis-O-[(tert-
butyl)dimethylsilyl]-2'-deoxy-O6-[(2,4,6-triisopropylphenyl)sulfonyl]guanosine (24),
which reacted with 3-hydroxypropanenitrile in presence of Et3N in a DABCO-
catalyzed reaction to 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-O6-(2-cyanoethyl)-2'-deox-
yguanosine (25) in 63% yield (DABCO ˆ 1,4-diazabicyclo[2.2.2]octane). Transforma-
tion of 25 into its N2-[(2-cyanoethoxy)carbonyl] derivative 27 proceeded well in a
stepwise reaction first with chlorotrimethylsilane and then with 2-cyanoethyl carbon-
ochloridate (5) in pyridine to give 27 in 92% yield. In a second, less effective two-step
approach, 27 was prepared from 24 by (2-cyanoethoxy)carbonylation to give 3',5'-bis-
O-[(tert-butyl)dimethylsilyl]-N2-[(2-cyanethoxy)carbonyl]-2'-deoxy-O6-[(2,4,6-triiso-
propylbenzene)sulfonyl]guanosine (26) in 17% yield, which was first treated with
Me3N and then with 2-hydroxypropanenitrile and DBU for 10 min, yielding 53% of 27.

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Scheme

Helvetica Chimica Acta ± Vol. 83 (2000)
3201
The known [14] O6-(2-cyanoethyl)-protected 2'-deoxyguanosine derivative 28 was prepared by a slightly
modified procedure in an improved yield of 79% (see Exper. Part), whereas the corresponding O6-[2-(4-
nitrophenyl)ethyl] derivative 29 was obtained from the N2-isobutyryl derivative of 16 with PPh3 and 2-(4-
nitrophenyl)ethanol in the presence of ethyl azodicarboxylate in 51% yield.
The combination of the npe and ceoc group was achieved by transient protection of
2'-deoxy-O6-[2-(4-nitrophenyl)ethyl]guanosine (30) [2] by trimethylsilylation and
subsequent reaction with 5 in pyridine/CH2Cl2, yielding 75% of N2-[(2-cyanoeth-
oxy)carbonyl]-2'-deoxy-O6-[2-(4-nitrophenyl)ethyl]guanosine (31). Its 5'-O-dimethoxy-
trityl derivative 32 resulted from treatment with dimethoxytrityl chloride in pyridine/
CH2Cl2 in 81% yield. A third blocking-group strategy was directed towards the
synthesis of N2-[(2-cyanoethoxy)carbonyl)]-2'-deoxy-O6-[2-(phenylsulfonyl)ethyl]-
guanosine (36) starting from 3',5'-di-O-acetyl-2'-deoxy-O6-[2-(phenylthio)ethyl]gua-
nosine (33) [16] prepared in a Mitsunobu reaction from 17 with 2-(phenylthio)ethanol.
Deacetylation by ammonia converted 33 into 2'-deoxy-O6-[2-(phenylthio)ethyl]gua-
nosine (34), which reacted at the thioether function by 3-chloroperbenzoic acid
oxidation to the corresponding sulfone 35, in 86% yield. Finally, 35 was acylated by 2-
cyanoethyl carbonochloridate (5) in a similar manner as 30 under transient
trimethylsilyl protection to give 85% of 36. The dimethoxytritylation of the latter
worked also well under standard conditions, forming N2-[(2-cyanoethoxy)carbonyl]-2'-
deoxy-5'-O-(dimethoxytrityl)-O6-[2-(phenylsulfonyl)ethyl]guanosine (37) in 73% iso-
lated yield.
New monomeric building blocks for oligo-2'-deoxyribonucleotide synthesis via the
phosphoramidite approach were prepared from the 2'-deoxy-5'-O-(dimethoxytrityl)ri-
bonucleosides 22, 23, and 32 by 2-cyanoethyl tetraisopropylphosphorodiamidite and
1H-tetrazole, leading in very good yields to the corresponding 3'-(2-cyanoethyl
diisopropylphosphoramidites) 38 ± 40.
Cleavage Studies. ± In oligonucleotide synthesis, the final cleavage of the various
protecting groups is of crucial importance to get clean products in reasonable times. To
get a more quantitative understanding of the cleavage rates of the npe/npeoc vs. the ce/
ceoc blocking groups at the various sites of the common nucleosides, we started some
comparative studies, focusing on the b-elimination reactions. The cleavages were done
at room temperature with 0.5m DBU in MeCN or for solubility reasons in MeCN/DMF
1:1 and with 15 mol-equiv. of the appropriate nucleoside. Expectedly, there were small
differences in the cleavage rates comparing analogous derivatives of the 2'-deoxyribo-
and the ribo-series, as e.g. 11 vs. 22 and 12 vs. 23, respectively, showing a somewhat

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higher stability of the latter compounds (see Table). Furthermore, small differences
were also noticed on changing the solvent from MeCN to MeCN/DMF 1:1 or by
various substituents at the sugar moiety, but a drastic increase in the cleavage rates was
observed in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a tetrasub-
stituted guanidine derivative instead of DBU representing a cyclic amidine. A
comparison of the (2-cyanoethoxy)carbonyl (ceoc) vs. the [2-(4-nitrophenyl)ethoxy]-
carbonyl (npeoc) group revealed a substantial acceleration of the b-elimination process
with the ceoc-protected compounds.
Table. Half-lives of Protected Nucleosides
Unsubstituted
nucleoside
Substituentsa) at
t1/2 [min]
Solvent
Base
N2-
N4
N6-
O6-
3'-O
5'-O
dC (see 22)
C (see 11)
ceoc
ceoc
(MeO)2Tr
MeOTr
5
7
2.5
42
50
65
3.5
13
27
2
96
MeCN
DBU
DBU
TBD
DBU
DBU
DBU
TBD
DBU
DBU
TBD
DBU
DBU
DBU
TBD
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DMF/MeCN
DMF/MeCN
MeCN
DMF/MeCN
MeCN
DMF/MeCN
DMF/MeCN
DMF/MeCN
DMF/MeCN
MeCN
DMF/MeCN
MeCN
DMF/MeCN
DMF/MeCN
MeCN
DMF/MeCN
MeCN
DMF/MeOH
DMF/MeCN
MeCN
dC
C
npeoc
npeoc
(MeO)2Tr
MeOTr
dA (see 23)
A (see 12)
ceoc
ceoc
(MeO)2Tr
MeOTr
dA
dA
npeoc
npeoc
(MeO)2Tr
MeOTr
97
111
7
A
npeoc
MeOTr
tmbs
94
126
5700
5000
27
32
0.5
17
12
156
180
dG (see 27)
G (see 9)
dG (see 31)
dG (see 32)
dG (see 28)
dG
G
dG
dG (see 37)
ceoc
ceoc
ceoc
ceoc
ibu
ibu
ibu
npeoc
ceoc
ce
tmbs
npe
npe
ce
npe
npe
npe
pse
(MeO)2Tr
tbds
tbds
MeOTr
(MeO)2Tr
(MeO)2Tr
tbds
tbds
DMF/MeCN
DMF/MeCN
DMF/MeCN
DMF/MeCN
a) ceˆ 2-cyanoethyl, ceocˆ 2-(cyanoethoxy)carbonyl, npe ˆ 2-(4-nitrophenyl)ethyl, npeoc ˆ [2-(4-nitrophenyl)-
ethoxy]carbonyl, tbds ˆ (tert-butyl)dimethylsilyl, MeOTrˆ monomethoxytrityl, (MeO)2Tr ˆ dimethoxytrityl,
pse ˆ 2-(phenylsulfonyl)ethyl.
The most problematic nucleosides are always seen in the dG and G series, since
their specific protection is most difficult and the removal of the protecting groups
affords, in general, more severe reaction conditions. Also under our standard ceoc
cleavage conditions, the presence of the N2-ceoc group increased the t1/2 of the
protected dG or G nucleosides to more than 8 ± 9 h (see 9 and 27), since that anion
formation at the nucleobase counteracts the b-elimination process for electronic
reasons. On the other hand, it can also be seen from 28 that the O6-ce group is very
labile, it showed the fastest cleavage rate of all protecting groups discussed. This result

Helvetica Chimica Acta ± Vol. 83 (2000)
3203
limits the combination of the ce and ceoc group in 27, since the preferential deblocking
of the O6-substituent leads to a stable, partially protected guanosine intermediate.
From these findings, it can be concluded that the unified use of b-elimination protecting
groups in the guanosine series requires a relatively stable O6-substituent, such as the
npe group combined with the N2-ceoc function. N2-[(2-Cyanoethoxy)carbonyl]-2'-
deoxy-O6-2-[(4-nitrophenyl)ethyl]guanosine (31) and its 5'-O-(dimethoxytrityl)deri-
vative 32 seem to have the anticipated properties, showing a t1/2 of 27 and 32 min,
respectively, which demonstrates the cleavage of the N2- prior to that of the O6-
substituent, as seen in the HPLC (Fig.). The introduction of the O6-2-(phenyl-
sulfonyl)ethyl (pse) group in 37 is no alternative to npe since its cleavage was also very
fast and succeeded the N2-ceoc group.
Figure. Time-dependent cleavage of the N2-ceoc and O6-npe protecting groups of 32 by DBU in DMF/MeCN at
room temperature
Experimental Part
General. TLC: precoated silica-gel thin-layer sheets 60 F 254 from Merck. Prep. column chromatography:
silica gel 0.04 mm from Baker. HPLC: Merck-Hitachi L6200 and L6200-A, column RP-18 LiChrospher (Merck,
125  4 mm, 5 mm), flow rate 1 ml/min, mobile phase 0.1m AcONH4/MeCN. FC ˆ flash chromatography. UV/
VIS: Perkin-Elmer Lambda-15; lmax in nm (lg e). 1H-NMR: Bruker AC-250; d in ppm rel. to SiMe4. 31P-NMR:
JEOL-400; d in ppm rel. to H3PO4.
1. 2-Cyanoethyl Carbonochloridate (ceoc-Cl; 5) [12]. Phosgene (14.85 g, 150 mmol) was condensed at ꢀ508
into a flask and then diluted with THF (34 ml). A soln. of 3-hydroxypropanenitrile (6.82 g, 96 mmol) in THF
(140 ml) was added dropwise within 1.5 h under N2. The soln. was stirred for 1.5 h at ꢀ308 and another 3 h at r.t.
The excess phosgene together with the THF was condensed off under high vacuum into a cooling trap. The
colorless viscous product still contained ca. 5% of 3-hydroxypropanenitrile. Because of the similar boiling
points of educt and product, a further purification was not possible, and 5 containing 5% of educt was used in the
following: 10.61 g (83%) of crude 5. Colorless viscous liquid. B.p. 80 ± 828/0.2 Torr ([12]: 110 ± 1128/11 Torr).
1H-NMR (CDCl3): 2.85 (t, CH2CN); 4.52 (t, CH2O).
2. 1-[(2-Cyanoethoxy)carbonyl]-3-methyl-1H-imidazolium Chloride (6). To an ice-cold soln. of 5 (4.67 g,
35 mmol) in CH2Cl2 (30 ml), 1-methyl-1H-imidazole (2.87 g, 35 mmol) in CH2Cl2 (2.5 ml) was added within

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Helvetica Chimica Acta ± Vol. 83 (2000)
5 min. The mixture was stirred for 10 min at 08 and 3 h at r.t. The colorless precipitate was filtered off, washed
with CH2Cl2 (3Â) and dried in vacuo: 7.1 1 g (94%) of colorless powder. M.p. 1018. 1H-NMR ((D6)DMSO): 3.12
(t, OCH2CH2CN); 3.95 (s, MeN); 4.7 (t, 2 H, OCH2CH2CN); 7.95 (s, HꢀC(5)); 8.15 (s, HꢀC(4)); 10.1
(s, HꢀC(2)). Anal. calc. for C8H10ClN3O3 (215.6): C 44.56, H 4.67, N 19.49; found: C 43.99, H 4.75, N 19.05.
3. N4-[(2-Cyanoethoxy)carbonyl]cytidine (7). A mixture of cytidine (1; 0.487 g, 2 mmol) and a few crystals
of ammonium sulfate was heated with hexamethyldisilazane (HMDS; 5 ml) in dry dioxane (5 ml) for 3 h under
reflux. After evaporation, the residue was treated with toluene (50 ml), the mixture filtered, and the filtrate
evaporated. The solid was dissolved in dry CH2Cl2 (40 ml), then 6 (0.56 g, 2.6 mmol) was added and stirred at r.t.
for 24 h. After evaporation, MeOH (30 ml) was added, the mixture stirred for 24 h, and then the soln.
concentrated to a smaller volume. The product started to crystallize and was collected after standing in the ice-
box overnight. Washing with cold MeOH (10 ml) and Et2O and drying under high vacuum yielded colorless
crystals: 0.579 g (85%) of 7. M.p. 156 ± 1578. UV (MeOH): 211 (4.31), 239 (4.15), 293 (3.87). 1H-NMR
((D6)DMSO): 10.90 (s, NH); 8.41 (d, HꢀC(6)); 6.98 (d, HꢀC(5)); 5.75 (d, HꢀC(1')); 5.40 (d, OHꢀC(2')); 5.16
(t, OHꢀC(5')); 5.04 (d, OHꢀC(3')); 4.28 (t, OCH2CH2CN); 3.96 ± 3.89 (m, HꢀC(2'), HꢀC(3'), HꢀC(4'));
3.74 ± 3.53 (m, 2 HꢀC(5')); 2.92 (t, OCH2CH2CN). Anal. calc. for C13H16N4O7 (340.3): C 45.88, H 4.74, N 16.46;
found: C 45.65, H 4.82, 16.28.
4. N4-2-[(Cyanoethoxy)carbonyl]-5'-O-(4-methoxytrityl)cytidine (11). Compound 7 (0.5 g, 1.47 mmol) was
co-evaporated with dry pyridine (3 Â 5 ml), the residue suspended in dry pyridine (7 ml), monomethoxytrityl
chloride (0.543 g, 1.76 mmol) added, and the mixture stirred at r.t. for 24 h. After evaporation and co-
evaporation with toluene (3 Â 12 ml), the residue was dissolved in CHCl3 and then extracted with phosphate
buffer pH 7 (2 Â 50 ml) and H2O (50 ml). The org. phase was dried (Na2SO4), evaporated, and purified by CC
(silica gel (50 g, 3 Â 16 cm), 0 ± 5% MeOH/CHCl3). The main fraction yielded, on evaporation and drying under
high vacuum, a colorless solid: 0.686 g (76%) of 11. UV (MeOH): 205 (4.76), 234 (4.39), 294 (3.89). 1H-NMR
((D6)DMSO): 10.91 (s, NH); 8.25 (d, HꢀC(6)); 7.40 ± 7.23 (m, 12 arom. H); 6.90 (d, 2 H o to MeO); 6.78
(d, HꢀC(5)); 5.74 (s, HꢀC(1')); 5.64 (d, OHꢀC(2')); 5.11 (d, OHꢀC(3')); 4.28 (t, OCH2CH2CN); 4.14
(m, HꢀC(2')); 4.01 (m, HꢀC(3'), HꢀC(4')); 3.74 (s, MeO); 2.91 (t, OCH2CH2CN). Anal. calc. for
C33H32N4O8 ´ 0.35 CHCl3 (654.4): C 61.21, H 4.98, N 8.56; found: C 61.24, H 5.09, N 9.14.
5. N4-[(2-Cyanoethoxy)carbonyl]-2'-deoxycytidine (18). To a suspension of 2'-deoxycytidine hydrochloride
(13 ´ HCl; 2.64 g, 10 mmol) and 6 (2.59 g, 12 mmol) in DMF (100 ml), iPr2NEt (1.7 ml, 10 mmol) was added and
shaken for ca. 3 min to give a homogeneous soln. After stirring for 3 h at r.t., the solvent was distilled off under
high vacuum at 308 bath temp. The residue was purified by FC (silica gel, 0 ± 20% MeOH/CH2Cl2 (2 l)). The
i
main fraction gave a solid, which was recrystallized from PrOH: 2.57 g (79%) of 18. Colorless crystals. TLC
(CHCl3/MeOH 4 :1): Rf 0.44. M.p. 150 ± 1518. UV (MeOH): 294 (3.89). 1H-NMR ((D6)DMSO): 2.0
(m, HꢀC(2')); 2.25 (m, 1 HꢀC(2')); 2.9 (t, OCH2CH2CN); 3.60 (m, 2 HꢀC(5')); 3.88 (m, HꢀC(4')); 4.2
(m, HꢀC(3')); 4.3 (t, OCH2CH2CN); 5.11 (t, OHꢀC(5')); 5.28 (d, OHꢀC(3')); 6.08 (t, HꢀC(1')); 7.0
(d, HꢀC(5)); 8.33 (d, HꢀC(6)); 10.9 (s, NHCOO). Anal. calc. for C13H16N4O6 (324.3): C 48.15, H 4.97,
N 17.28; found: C 48.18, H 5.03, N 16.79.
6. N4-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)cytidine (22). A soln. of 18 (2.57 g,
7.93 mmol) in pyridine (10 ml) was co-evaporated twice and the residue then dissolved in pyridine (30 ml).
Dimethoxytrityl chloride (2.95 g, 8.7 mmol) was added and stirred for 16 h. The reaction was quenched with
MeOH (10 ml), the mixture evaporated, and the residue dissolved in CH2Cl2 (200 ml). After washing with sat.
NaHCO3 soln. (2 Â 200 ml), the org. phase was dried (Na2SO4) and evaporated. The residue was recrystallized
from AcOEt. From the filtrate, a second crop was obtained by FC (silica-gel (20 g), toluene/AcOEt/MeOH
1:2 :0 ! 5 :5 :1): 4.15 g (84%) of 15. Colorless crystals. TLC (CHCl3/MeOH 9 :1): Rf 0.34. UV (MeOH): 283
(3.95), 294 (sh, 3.92). 1H-NMR ((D6)DMSO): 2.15 (m, 1 HꢀC(2')); 2.35 (m, 1 HꢀC(2')); 2.95
(t, OCH2CH2CN); 3.25 (m, 2 HꢀC(5')); 3.75 (s, 2 MeO); 3.95 (m, HꢀC(4')); 4.21 (m, HꢀC(3')); 4.3
(t, OCH2CH2CN); 5.35 (d, OHꢀC(3')); 6.1 (t, HꢀC(1')); 6.9 (d, HꢀC(5)); (d, 4 H o to MeO); 7.2 ± 7.4
(m, 9 arom. H); 8.15 (d, HꢀC(6)); 10.9 (s, NHCOO). Anal. calc. for C34H34N4O8 ´ H2O (644.7): C 63.35, H 5.63,
N 8.69; found: C 63.81, H 5.58, N 8.52.
7. N6-[(2-Cyanoethoxy)carbonyl]adenosine (8). Adenosine (2; 1.106 g, 4 mmol) and a few crystals of
ammonium sulfate were heated with HMDS (16 ml) in dry dioxane (15 ml) for 6 h under reflux. The mixture
was evaporated, the resulting syrup treated with dry toluene, the undissolved material filtered off, and then the
filtrate evaporated. The residue was dissolved in dry CH2Cl2 (80 ml), 6 (1.08 g, 5 mmol) was added and stirred at
r.t. for 20 h. After evaporation, the residue was treated with EtOH (60 ml) and MeOH (10 ml) by stirring to give
a colorless precipitate. The solid was washed with EtOH and Et2O and dried under vacuum: 1.165 g (80%) of 8.
M.p. 158 ± 1618. UV (MeOH): 209 (4.45), 266 (4.26), 273 (sh, 4.15). 1H-NMR ((D6)DMSO): 10.82 (s, NH); 8.70

Helvetica Chimica Acta ± Vol. 83 (2000)
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(s, HꢀC(2)); 8.65 (s, HꢀC(8)); 5.99 (d, HꢀC(1')); 5.54 (br. s, OHꢀC(2')); 5.21 (br. s, OHꢀC(3'), OHꢀC(5'));
4.60 (t, HꢀC(2')); 4.31 (t, OCH2CH2CN); 4.16 (m, HꢀC(3')); 3.95 (m, HꢀC(4')); 3.85 ± 3.52 (m, 2 HꢀC(5'));
2.94 (t, OCH2CH2CN). Anal. calc. for C14H16N6O6 (364.3): C 46.15, H 4.42, N 23.06; found: C 46.05, H 4.57,
N 22.78.
8. 2',3',5'-Tri-O-acetyl-N6-[(2-cyanoethoxy)carbonyl]adenosine (10). To a cold suspension of 2',3',5'-tri-O-
acetyladenosine (4; 0.197 g, 0.5 mmol) in 1m 1-methyl-1H-imidazole in MeCN (2 ml), 5 (0.35 g, 2.3 mmol) was
added under stirring and ice-cooling. After 30 min, the mixture was warmed to r.t., and stirring continued until
the starting material had disappeared. The mixture was diluted with CHCl3 and extracted twice with H2O
(25 ml). The org. phase was dried (Na2SO4) and evaporated, and the residue purified by CC (silica gel (20 g,
40 Â 2 cm), CHCl3/MeOH 95 :5, then 4 :1): 0.225 g (90%) of 10. Colorless solid foam. UV (MeOH): 209 (4.39),
254 (sh, 4.13), 265 (4.22), 272 (sh, 4.12). 1H-NMR (CDCl3): 9.30 (s, NHCOO); 8.81 (s, HꢀC(2)); 8.25
(s, HꢀC(8)); 6.25 (d, HꢀC(1')); 5.97 (m, HꢀC(2')); 5.7 (m, HꢀC(3')); 4.55 ± 4.335 (m, 2 HꢀC(5'), HꢀC(4'),
OCH2CH2CN); 2.85 (t, OCH2CH2CN); 2.15 (3s, Ac). Anal. calc. for C20H22N6O9 ´ 0.5 H2O (499.4): C 48.10,
H 4.64, N 16.83; found: C 48.02, H 4.62, N 16.64.
9. N6-[(2-Cyanoethoxy)carbonyl]-5'-O-(4-methoxytrityl)adenosine (12). N6-[(2-Cyanoethoxy)carbonyl]-
adenosine (8; 0.5 g, 1.37 mmol) was co-evaporated with dry pyridine (3 Â 10 ml), the residue dissolved in the
same solvent (5 ml), and then monomethoxytrityl chloride (0.5 g, 1.64 mmol) added. After stirring at r.t. for
24 h, the reaction was quenched by MeOH (1 ml) and stirring for 25 min. The soln. was evaporated and co-
evaporated with toluene (2 Â 10 ml) and CHCl3 (3 Â 10 ml). The residue was dissolved in CHCl3 and extracted
with phosphate buffer pH 7 (2 Â 50 ml) and H2O (50 ml). The org. phase was dried (Na2SO4) and evaporated
and the residue purified by CC (silica gel (50 g, 16 Â 3 cm), 0 ± 3% MeOH/CHCl3: 0.751 g (86%) of 12.
Colorless foam: UV (MeOH): 206 (4.80), 232 (sh, 4.25), 266 (4.26), 274 (sh, 4.14). 1H-NMR ((D6)DMSO):
10.83 (s, NH); 8.58 (s, HꢀC(2), HꢀC(8)); 7.37 ± 7.19 (m, 12 arom. H); 6.84 (d, 2 H o to MeO); 6.02
(d, HꢀC(1')); 5.61 (d, OHꢀC(2')); 5.27 (d, OHꢀC(3')); 4.74 (m, HꢀC(2')); 4.31 (m, HꢀC(3'),
OCH2CH2CN); 4.10 (m, HꢀC(4')); 3.72 (s, MeO); 3.23 (br. s, 2 HꢀC(5')); 2.92 (t, OCH2CH2CN). Anal. calc.
for C34H32N6O7 ´ 0.4 CHCl3 (684.4): C 60.36, H 4.77, N 12.27; found: C 60.25, H 4.89, N 12.12.
10. N6-[(2-Cyanoethoxy)carbonyl]-2'-deoxyadenosine (19). A soln. of 2'-deoxyadenosine (14; 0.50 g,
2 mmol) in dioxane (8 ml) was treated with HMDS (8 ml) and some crystals of (NH4)2SO4 by heating under
reflux for 5 h. The mixture was evaporated and the residue co-evaporated with toluene (5 ml) and then
redissolved in CH2Cl2 (40 ml). Then 6 (0.538 g, 2.5 mmol) was added and the resulting suspension stirred for
20 h at r.t. The precipitate was filtered off and washed with CH2Cl2 (3 Â 5 ml), the combined org. soln.
evaporated, and the residue redissolved in H2O/MeOH 1:1 (20 ml). After stirring for 6 h, the solvent was
distilled off and the residue co-evaporated with toluene (2 Â 5 ml), MeOH (2 Â 5 ml), and CH2Cl2 (2 Â 5 ml).
The resulting solid was purified by FC (silica gel (15 g), 0 ± 10% MeOH/CH2Cl2 (800 ml)): 0.57 g (82%) of 19.
Colorless foam. TLC (CHCl3/MeOH 4 :1): Rf 0.57. UV (MeOH): 266 (4.20), 273 (sh, 4.11). 1H-NMR
((D6)DMSO): 2.30 (m, 1 HꢀC(2')); 2.75 (m, 1 HꢀC(2')); 2.95 (t, OCH2CH2CN); 3.55 (m, 2 HꢀC(5')); 3.88
(m, HꢀC(4')); 4.30 (t, OCH2CH2CN); 4.42 (m, HꢀC(3')); 5.01 (t, OHꢀC(5')); 5.35 (d, OHꢀC(3')); 6.45
(t, HꢀC(1')); 8.65 (s, HꢀC(2)); 8.67 (s, HꢀC(8)); 10.8 (s, NHCOO). Anal. calc. for C14H16N6O5 ´ H2O (366.3):
C 45.90, H 4.95, N 22.94; found: C 45.63, H 4.62, N 22.51.
11. N6-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)adenosine (23). A soln. of 19 (3.8 g,
10.9 mmol) in dry pyridine (15 ml) was evaporated, the residue dissolved in pyridine (40 ml), 4,4'-
dimethoxytrityl chloride (4 g, 12 mmol) added, and the mixture stirred for 16 h. The soln. was reduced to 1/3
of its volume, diluted with CH2Cl2 (300 ml), and washed with sat. NaHCO3 soln. (2 Â 300 ml). The org. phase
was dried (Na2SO4), evaporated, and co-evaporated with toluene (2 Â 15 ml), MeOH (2 Â 15 ml), and CH2Cl2
(2 Â 15 ml). The residue was purified by FC (silica-gel (70 g), AcOEt/toluene 3 :1 ! AcOEt/toluene/MeOH
40 :2 :1 (1800 ml)): 6.5 g (92%) of 23. Colorless solid foam. TLC (CHCl3/MeOH 9 :1): Rf 0.41. UV (MeOH):
267 (4.23), 274 (sh, 4.14). 1H-NMR ((D6)DMSO): 2.37 (m, 1 HꢀC(2')); 2.90 (m, 1 HꢀC(2')); 2.93
(t, OCH2CH2CN); 3.17 (m, 2 HꢀC(5')); 3.69 (s, 2 MeO); 4.01 (m, HꢀC(4')); 4.31 (t, OCH2CH2CN); 4.50
(m, HꢀC(3')); 5.42 (d, OHꢀC(3')); 6.46 (t, HꢀC(1')); 6.77 (m, 4 H o to MeO); 7.1 ± 7.35 (m, 9 arom. H); 8.56
(s, HꢀC(2), HꢀC(8)); 10.81 (s, NHCOO). Anal. calc. for C34H34N6O7 (650.7): C 64.61, H 5.27, N 12.92; found:
C 64.82, H 5.48, N 12.14.
12. N2-[(2-Cyanoethoxy)carbonyl]guanosine (9). A mixture of dry guanosine (3) (1.4 g, 5 mmol) and
Me3SiCl (5 ml) in pyridine (40 ml) was stirred at r.t. for 4 h. Then, a soln. of 5 (1.33 g, 10 mmol) in CH2Cl2
(10 ml) was added. The resulting mixture was stirred at r.t. for 20 h and the precipitate filtered off. The soln. was
evaporated and co-evaporated with toluene (30 ml). The residue was dissolved in MeOH (70 ml), the soln.
stirred at r.t. for 18 h and evaporated, and the residue purified by CC (silica gel (3.5 Â 5 cm), CH2Cl2/MeOH

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Helvetica Chimica Acta ± Vol. 83 (2000)
50 :1 ! 9 :1) and finally crystallized from 90% EtOH: 1.02 g (54%) of 9. M.p. 1908. UV (MeOH): 256 (4.23),
277 (sh, 4.04). 1H-NMR ((D6)DMSO): 11.62, 11.31 (2s, NH); 8.23 (s, HꢀC(8)); 5.77 (d, HꢀC(1')); 5.46
(d, OHꢀC(2')); 5.16 (d, OHꢀC(3')); 4.97 (br. s, OHꢀC(5')); 4.47 (t, OCH2CH2ON); 4.12 (m, HꢀC(3')); 3.87
(m, HꢀC(4')); 3.57 (m, 2 HꢀC(5')); 2.96 (t, OCH2CH2CN). Anal. calc. for C14H16N6O7 (380.3): C 44.21, H 4.24,
N 22.09; found: C 44.01, H 4.34, N 21.67.
13. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxyguanosine (20). Dry 2'-deoxyguanosine (15; 0.2 g, 0.75 mmol)
was twice co-evaporated with dry pyridine (5 ml). The residue was suspended in dry pyridine (1.5 ml), Me3SiCl
(0.5 ml, 4 mmol) added, the mixture stirred for 90 min, and then a soln. of 5 (0.14 g, 1.05 mmol) in CH2Cl2
(1.5 ml) dropwise added. Stirring was continued at r.t. for 7 h, the solid pyridinium chloride filtered off and
washed twice with CH2Cl2 (2.5 ml), and then the filtrate evaporated. The residue was dissolved in MeOH
(5 ml), the soln. stirred for 20 min and evaporated, the solid dissolved in AcOEt (15 ml), the soln. washed with
H2O (15 ml), the aq. phase re-extracted with AcOEt (3 Â 15 ml), the combined org. phase dried (Na2SO4) and
evaporated, and the residue purified by CC (silica gel (8 g; 2.5 cm), CH2Cl2/MeOH 9 :1): 51 mg (19%). TLC
(CHCl3/MeOH 6 :1): Rf 0.21. 1H-NMR ((D6)DMSO): 11.58 (s, NH); 11.30 (s, NH); 8.21 (s, HꢀC(8)); 6.17
(ꢀtꢁ, HꢀC(1')); 5.25 (d, OHꢀC(3')); 4.92 (t, OHꢀC(5')); 4.38 (m, HꢀC(3'), OCH2CH2CN); 3.80
(m, HꢀC(4')); 3.55 (m, 2 HꢀC(5')); 2.95 (t, OCH2CH2CN); 2.65 (m, 1 HꢀC(2')); 2.25 (m, 1 HꢀC(2')).
14. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-N2-[(2-cyanoethoxy)carbonyl]-2'-deoxyguanosine (21). A mix-
ture of 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxyguanosine (16) [17] (0.14 g, 0.28 mmol) in dry pyridine
(2 ml) was evaporated twice. The residue was dissolved in CH2Cl2 (3 ml), and then Et3N (0.16 ml, 1.15 mmol)
and Me3SiCl (0.1 ml, 0.8 mmol) were added. After stirring for 15 min, the mixture was evaporated and co-
evaporated twice with dry pyridine (2 ml), the residue dissolved in CH2Cl2 (2 ml) and pyridine (2 ml), and 5
(38 mg, 0.29 mmol) added. The mixture was stirred at r.t. for 15 h and then evaporated, the residue dissolved in
CH2Cl2 (10 ml), the soln. extracted with H2O (5 ml), the aq. phase re-extracted with CH2Cl2 (3Â), the
combined org. phase dried (Na2SO4) and evaporated, and the residue purified by CC (silica gel (6 g), 0.1 ± 6%
MeOH/CH2Cl2): 29 mg (22%) of 21. Colorless solid foam. Rf (CHCl3/MeOH 9 :1) 0.22. 1H-NMR ((D6)DMSO):
11.55 (s, NH); 11.40 (s, NH); 8.20 (s, HꢀC(8)); 6.21 (ꢀtꢁ, HꢀC(1')); 4.50 (m, HꢀC(3')); 4.43 (t, OCH2CH2CN);
3.81 (m, HꢀC(4')); 3.65 (m, 2 HꢀC(5')); 2.95 (t, OCH2CH2CN); 2.75 (m, 1 HꢀC(2')); 2.40 (m, 1 HꢀC(2'));
0.88 (s, 1 tBu); 0.85 (s, tBu); 0.10 (s, Me2Si); 0.00 (s, Me2Si).
15. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-O6-(2-cyanoethyl)-2'-deoxy-N2-isobutyrylguanosine (28) [14].
15.1. To a soln. of 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxy-N2-isobutyryl-O6-[(2,4,6-triisopropylphenyl)-
sulfonyl]guanosine [14] (1.27 g, 1.46 mmol) and 3-hydroxypropanenitrile (1.35 ml, 19.8 mmol) in dry CH2Cl2
(25 ml) under N2, Me3N (1.27 g, 21.5 mmol) was added under cooling. After stirring for 10 min, DBU (0.45 g,
0.445 mmol) was added, and, after another 10 min, the reaction was quenched by diluting with CH2Cl2 (25 ml)
and extraction with sat. NH4Cl soln. (2 Â 25 ml) and H2O (25 ml). The org. phase was dried (Na2SO4) and
evaporated, and the residue purified by CC (silica gel (20 g; 5 Â 2.5 cm), CH2Cl2): 0.71 g (79%) of 28. Yellowish
solid foam.
15.2. As described in 15.1, with small amounts of DABCO (3 mg) instead of DBU, and with Et3N instead of
Me3N, and with stirring at r.t. for 28 h. Workup yielded 64% of 28. UV (MeOH): 219 (sh, 4.32), 268 (4.22), 279
(4.06). 1H-NMR ((D6)DMSO): 10.45 (s, NH); 8.41 (s, HꢀC(8)); 6.30 (ꢀtꢁ, HꢀC(1')); 4.70 (m, HꢀC(3'),
OCH2CN2CN); 3.81 (m, 2 HꢀC(5')); 3.75 (m, HꢀC(4')); 3.20 (t, OCH2CH2CN); 2.95 (sept., Me2CH); 2.80
(m, 1 HꢀC(2')); 2.21 (m, 1 HꢀC(2')); 1.08 (d, Me2CH); 0.90 (s, 1 tBu); 0.82 (s, 1 tBu); 0.12 (s, Me2Si); ꢀ0.02
(s, 1 Me2Si). Anal. calc. for C29H50N6O5Si2 (618.9): C 56.27, H 8.14, N 13.58; found: C 56.32, H 8.21, N 13.42.
16. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxy-N2-isobutyryl-O6-[2-(4-nitrophenyl)ethyl]guanosine
(29). After co-evaporation of 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxy-N2-isobutyrylguanosine (0.5 g,
0.88 mmol) with dry dioxane (2 Â 8 ml), the residue was dissolved in the same solvent (18 ml). Triphenylphos-
phine (0.37 g, 1.41 mmol) and 2-(4-nitrophenyl)ethanol (0.222 g, 1.32 mmol) were added. After stirring for
15 min at r.t., ethyl azodicarboxylate (ˆdiethyl diazenedicarboxylate; 0.25 g, 1.42 mmol) was added, the
mixture stirred for 3 h, and again some ethyl azodicarboxylate (30 ml) added to complete the reaction within 1 h.
The soln. was evaporated, the residue dissolved in CH2Cl2 (3.5 ml) and the soln. cooled overnight. The
precipitate of diethyl hydrazine-1,2-dicarboxylate was washed with CH2Cl2, the combined filtrate evaporated
and the residue again treated with CH2Cl2 to give a second crop of diethyl hydrazine-1,2-dicarboxylate.
Purification by CC (silica gel (18 g; 6 Â 2.5 cm), 0 ± 12% AcOEt/toluene) yielded a crude product, which still
contained some diethyl hydrazine-1,2-dicarboxylate: 0.322 g (51%). A pure anal. sample of 29 was obtained by
prep. TLC (silica gel, toluene/AcOEt 3 :1). UV (MeOH): 206 (4.38), 217 (sh, 4.46), 269 (4.42), 281 (4.29).
1H-NMR ((D6)DMSO): 10.38 (s, NH); 8.35 (s, HꢀC(8)); 8.15 (d, 2 H o to NO2); 7.65 (d, 2 H m to NO2); 6.30
(ꢀtꢁ, HꢀC(1')); 4.81 (t, ArCH2CH2O); 4.7 (m, HꢀC(3')); 3.79 (m, 2 HꢀC(5')); 3.68 (m, HꢀC(4')); 3.32

Helvetica Chimica Acta ± Vol. 83 (2000)
3207
(t, ArCH2CH2O); 3.00 (sept., Me2CH); 2.79 (m, 1 HꢀC(2')); 2.25 (m, 1 HꢀC(2')); 1.10 (d, Me2CH); 0.91
(s, 1 tBu); 0.82 (s, 1 tBu); 0.12 (s, Me2Si); ꢀ0.05 (s, 1 Me2Si). Anal. calc. for C34H54N6O7Si2 (715.0): C 57.11,
H 7.61, N 11.75; found: C 56.54, H 7.51, N 11.54.
17. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxy-O6-[(2,4,6-triisopropylphenyl)sulfonyl]guanosine (24).
A soln. of 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxyguanosine (16; 1 g, 2.02 mmol) in dry pyridine (8 ml)
was twice co-evaporated and then dissolved in CH2Cl2 (15 ml) and pyridine (15 ml). After addition of 2,4,6-
triisopropylbenzenesulfonyl chloride (1.27 g, 4.2 mmol), DMAP (25 mg, 0.2 mmol), and Et3N (1.5 ml,
10.6 mmol), the soln. was stirred for 14 h at 508 to give a colorless precipitate of (Et3NH)Cl. The soln. was
diluted with CH2Cl2 (30 ml) and washed with H2O (3 Â 20 ml), the aq. phase extracted with CH2Cl2 (20 ml), and
then the combined org. phases dried (Na2SO4) and evaporated. Purification by FC (silica-gel (30 g), toluene/
AcOEt 10 :1) gave 0.907 g (59%) of 24. Lightly orange foam. TLC (CHCl3/MeOH 49 :1): Rf 0.41. UV
1
(MeOH): 291 (3.98), 252 (sh, 3.93), 207 (4.72). H-NMR ((D6)DMSO): ꢀ0.05 (s, 1 Me2Si); 0.00 (s, 1 Me2Si);
0.80 (s, 1 tBu); 0.84 (s, 1 tBu); 1.20 (d, 3 Me2CH); 2.25 (m, 1 HꢀC(2')); 2.75 (m, 1 HꢀC(2')); 2.95 (sept.,
1 Me2CH); 3.65 (m, 2 HꢀC(5')); 3.81 (m, HꢀC(4')); 4.1 (sept., 2 Me2CH o to SO2); 4.5 (m, HꢀC(3')); 6.20
(t, HꢀC(1')); 6.65 (s, NH2); 7.33 (s, 2 arom. H); 8.20 (s, HꢀC(8)). Anal. calc. for C37H63N5O6SSi2 (762.2):
C 58.31, H 8.33, N 9.19; found: C 58.22, H 8.44, N 9.17.
18. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-O6-(2-cyanoethyl)-2'-deoxyguanosine (25). After co-evaporation
of 24 (0.2 g, 0.26 mmol) with CH2Cl2 (2 Â 5 ml), the residue was dissolved in CH2Cl2 (8 ml), and after addition
of Et3N (0.3 ml, 2.11 mmol), 3-hydroxypropanenitrile (0.1 ml, 1.42 mmol), and a cat. amount of DABCO, the
mixture was stirred for 48 h at r.t. The soln. was diluted with CH2Cl2 (10 ml), washed twice with sat. NH4Cl soln.,
the org. phase dried (Na2SO4) and evaporated, and the resulting solid purified by FC (silica gel (8 g), CH2Cl2
(300 ml), CH2Cl2/MeOH 49 :1 (200 ml)): 90 mg (63%) of 29. Yellowish foam. TLC (CHCl3/MeOH 25 :1):
Rf 0.34. UV (MeOH): 283 (3.98), 247 (4.00). 1H-NMR ((D6)DMSO): 0.05 (s, 1 Me2Si); 0.14 (s, 1 Me2Si); 0.86
(s, tBu); 0.88 (s, tBu); 2.25 (m, 1 HꢀC(2')); 2.75 (m, 1 HꢀC(2')); 3.1 (t, OCH2CH2CN); 3.7 (m, 2 HꢀC(5'));
3.85 (m, HꢀC(4')); 4.52 (m, HꢀC(3')); 4.58 (t, OCH2CH2CN); 6.20 (t, HꢀC(1')); 6.55 (s, NH2); 8.12
(s, HꢀC(8)). Anal. calc. for C25H44N6O4Si2 (548.8): C 54.72, H 8.08, N 15.31; found: C 54.20, H 8.08, N 14.76.
19. 2'-Deoxy-O6-[2-(phenylthio)ethyl]guanosine (34). A soln. of 3',5'-bis-O-acetyl-2'-deoxy-O6-[2-(phenyl-
thio)ethyl]guanosine (33) [16] (3.23 g, 5.66 mmol; contaminated with 16% of triphenyl phosphine) in dioxane
(100 ml), MeOH (100 ml), and conc. ammonia (50 ml) was stirred for 20 h at r.t. The solvents were distilled off,
and the residue was co-evaporated with toluene (2 Â 20 ml), MeOH (2 Â 20 ml), and CH2Cl2 (20 ml). The solid
was purified by FC (silica gel (60 g), 0 ± 5% MeOH/CH2Cl2 (1500 ml)): 2.03 g (89%) of 34. Yellowish foam.
TLC (CHCl3/MeOH 9 :1): Rf 0.42. UV (MeOH): 281 (4.08), 250 (4.26). 1H-NMR ((D6)DMSO): 2.20
(m, 1 HꢀC(2')); 2.61 (m, 1 HꢀC(2')); 3.45 (t, OCH2CH2S); 3.52 (m, 2 HꢀC(5')); 3.82 (m, HꢀC(4')); 4.38
(m, HꢀC(3')); 4.55 (t, OCH2CH2S); 5.00 (t, OHꢀC(5')); 5.28 (d, OHꢀC(3')); 6.18 (t, HꢀC(1')); 6.45 (s, NH2);
7.18 (m, H p to SCH2); 7.32 (m, 2 H o to SCH2); 7.45 (m, 2 H m to SCH2); 8.10 (s, HꢀC(8)). Anal. calc. for
C18H21N5O4S ´ H2O (421.5): C 51.30, H 5.5, N 16.62; found: C 51.88, H 5.55, N 17.10.
20. 2'-Deoxy-O6-[2-(phenylsulfonyl)ethyl]guanosine (35). A soln. of 34 (42 mg, 0.104 mmol) in CH2Cl2
(5 ml) was treated with 55% 3-chloroperbenzoic acid (65 mg, 0.208 mmol) at 08 for 1 h. The soln. was
concentrated in vacuo to 1/5 of its volume and then the product isolated by FC (silica gel (4 g), CH2Cl2 (200 ml),
CH2Cl2/MeOH 98 :2 (100 ml)): 41 mg (91%) of 35. Colorless foam. TLC (CHCl3/MeOH 9 :1): Rf 0.38. UV
(MeOH): 283 (3.93), 272 (4.40). 1H-NMR ((D6)DMSO): 2.2 (m, 1 HꢀC(2')); 2.55 (m, 1 HꢀC(2')); 3.55
(m, 2 HꢀC(5')); 3.80 (m, HꢀC(4')); 3.95 (t, OCH2CH2SO2); 4.35 (m, HꢀC(3')); 4.65 (t, OCH2CH2SO2); 4.95
(t, OHꢀC(5')); 5.25 (d, OHꢀC(3')); 6.18 (t, HꢀC(1')); 6.43 (s, NH2); 7.55 ± 7.70 (m, 3 arom. H); 7.90
(m, 2 arom. H); 8.05 (s, HꢀC(8)). Anal. calc. for C18H21N5O6S ´ 0.75 H2O (448.9): C 48.16, H 5.05, N 15.59;
found: C 48.26, H 4.92, N 15.01.
21. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-N2-2-[(cyanoethoxy)carbonyl]-2'-deoxy-O6-[(2,4,6-triisopropyl-
phenyl)sulfonyl]guanosine (26). A soln. of 3',5'-bis-O-[(tert-butyl)dimethylsilyl]-2'-deoxy-O6-[(2,4,6-triisopro-
pylphenyl)sulfonyl]guanosine (24; 0.3 g, 0.394 mmol) in dry pyridine (2 Â 3 ml) was co-evaporated and then
dissolved in a mixture of CH2Cl2 (3 ml) and pyridine (3 ml). Me3SiCl (0.27 ml, 2.16 mmol) was added, the soln.
stirred for 30 min, then 5 (158 mg, 1.18 mmol) added, and the resulting suspension stirred for 4 h at 408. The
reaction was quenched by the addition of H2O (4 ml) followed by CH2Cl2 (10 ml), the mixture stirred for 5 min
and then extracted twice with H2O, the org. phase dried (Na2SO4) and evaporated, and the residue co-
evaporated with toluene (2 Â 5 ml), MeOH (2 Â 5 ml), and CH2Cl2 (2 Â 5 ml). The resulting solid was purified
by FC (silica-gel (8 g), 10 ± 30% AcOEt/toluene (400 ml)): 50 mg (17%) of 26. TLC (CHCl3/MeOH 49 :1):
Rf 0.32. UV (MeOH): 278 (4.10), 238 (sh, 4.25). 1H-NMR ((D6)DMSO): ꢀ0.05 (s, 1 Me2Si); 0.10 (s, 1 Me2Si);
0.80 (s, 1 tBu); 0.9 (s, tBu); 1.2 (d, 3 Me2CH); 2.35 (m, 1 HꢀC(2')); 2.95 ± 3.05 (m, 1 HꢀC(2'), OCH2CH2CN,

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Helvetica Chimica Acta ± Vol. 83 (2000)
1 Me2CH); 3.65 ± 3.90 (m, 2 HꢀC(5'), HꢀC(4')); 4.15 (sept., 2 Me2CH); 4.25 (t, OCH2CH2CN); 4.65
(m, HꢀC(3')); 6.35 (t, HꢀC(1')); 7.35 (s, 2 arom. H); 8.55 (s, HꢀC(8)); 10.68 (s, NHꢀC(2)). Anal. calc. for
C41H66N6O8SSi2 ´ H2O (877.3): C 56.14, H 7.81, N 9.57; found: C 56.40, H 7.83, N 8.84.
22. 3',5'-Bis-O-[(tert-butyl)dimethylsilyl]-N2-2-[(cyanoethoxy)carbonyl]-O6-(2-cyanoethyl)-2'deoxyguano-
sine (27). 22.1. Into a soln. of 26 (80 mg, 0.09 mmol) in CH2Cl2 (3 ml), Me3N (130 mg, 0.106 mmol) was
condensed at 08. After stirring for 10 min at 08, 3-hydroxypropanenitrile (0.166 g, 2.34 mmol) and DBU (44 mg,
0.29 mmol) were added, and the mixture was stirred for max. 10 min at 08 and then diluted with CH2Cl2 (4 ml).
The soln. was washed with NH4Cl soln. (3Â), the org. phase dried (Na2SO4) and evaporated, and the residue
purified by FC (silica gel (4 g), CH2Cl2 (200 ml), CH2Cl2/MeOH 49 :1 (200 ml)): 31 mg (53%) of 27. Yellowish
foam.
22.2. After co-evaporation of 25 (0.15 g, 0.27 mmol) with dry pyridine (2 Â 2 ml), the residue was dissolved
in pyridine (1.5 ml) and CH2Cl2 (3 ml). Me3SiCl (88 mg, 0.9 mmol) in CH2Cl2 (0.5 ml) was added, and the soln.
was stirred for 30 min at r.t. Then 5 (57 m, 0.425 mmol) was added and stirring continued for another 4.5 h. The
reaction was quenched by the addition of MeOH (8 ml), the mixture stirred for 15 min and then evaporated,
and the residue treated with H2O (10 ml) and AcOEt (10 ml). The aq. phase was extracted with AcOEt (3 Â
10 ml), the combined org. phase dried (Na2SO4) and evaporated, and the resulting solid purified by FC (silica-
gel (6 g), CH2Cl2 (400 ml), then CH2Cl2/MeOH 400 :1 (200 ml)): 0.161 g (92%) of 27, identical with the product
from 22.1. Yellowish foam. TLC (CHCl3/MeOH 9 :1): Rf 0.68. UV (MeOH): 268 (4.13), 277 (sh, 3.96).
1H-NMR ((D6)DMSO): 0.05 (s, 1 Me2Si); 0.1 (s, 1 Me2Si); 0.85 (s, 1 tBu); 0.9 (s, 1 tBu); 2.30 (m, 1 HꢀC(2')); 3.0
(m, 1 HꢀC(2')); 2.95 (t, OCH2CH2CN); 3.15 (t, OCH2CH2CN); 3.60 (m, HꢀC(4')); 3.81 (m, 2 HꢀC(5')); 4.28
(t, OCH2CH2CN); 4.70 (m, HꢀC(3')), OCH2CH2CN); 6.35 (t, HꢀC(1')); 8.45 (s, HꢀC(8)); 10.62
(s, NHꢀC(2)). Anal. calc. for C29H47N7O6Si2 (645.9): C 53.93, H 7.33, N 15.18; found: C 53.82, H 7.15, N 14.83.
23. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-O6-[2-(4-nitrophenyl)ethyl]guanosine (31). A soln. of 2'-
deoxy-O6-[2-(4-nitrophenyl)ethyl]guanosine (30) [2] (0.90 g, 2.16 mmol) in dry pyridine (2 Â 5 ml) was co-
evaporated and then the residue dissolved in a mixture of dry pyridine (8.5 ml) and CH2Cl2 (11 ml). After
addition of Me3SiCl (1.6 ml, 12.8 mmol) and stirring for 20 min, a precipitate of pyridinium chloride was formed.
To this mixture, 5 (0.414 g, 3.1 mmol) in CH2Cl2 (2 ml) was added, and stirring was continued at r.t. for 4 h. The
reaction was quenched by addition of MeOH (20 ml) to hydrolyze the Me3Si groups. After stirring for 20 min
and evaporation, the residue was treated with H2O (40 ml) and AcOEt (40 ml). The aq. phase was extracted
with AcOEt (3Â) and the combined org. phase evaporated and co-evaporated with toluene (2 Â 15 ml), MeOH
(2 Â 15 ml), and CH2Cl2 (3 Â 15 ml). The resulting residue was suspended in CH2Cl2, then the insoluble material
filtered off and dried: 0.836 g (75%) of 31. Yellowish powder. TLC (CHCl3/MeOH 9 :1): Rf 0.37. UV (MeOH):
267 (4.38), 275 (sh, 4.32). 1H-NMR ((D6)DMSO): 2.25 (m, 1 HꢀC(2')); 2.75 (m, 1 HꢀC(2')); 2.95
(t, OCH2CH2CN); 3.32 (t, ArCH2CH2O); 3.65 (m, 2 HꢀC(5')); 3.85 (m, HꢀC(4')); 4.30 (t, CH2O)); 4.4
(m, HꢀC(3')); 4.75 (t, CH2O); 4.88 (t, OHꢀC(5')); 5.32 (d, OHꢀC(3')); 6.32 (t, HꢀC(1')); 7.65 (d, 2 H m to
NO2); 8.20 (d, 2 H o to NO2); 8.42 (s, HꢀC(8)); 10.65 (s, NHꢀC(2)). Anal. calc. for C22H23N7O8 (513.5):
C 51.46, H 4.51, N 19.10; found: C 51.52, H 4.66, N 18.66.
24. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)-O6-[2-(4-nitrophenyl)ethyl]guano-
sine (32). Compound 31 (217 mg, 0.41 mmol) was co-evaporated twice with dry pyridine (2 Â 2.5 ml) and then
dissolved in a mixture of pyridine (0.5 ml) and CH2Cl2 (1.5 ml). Dimethoxytrityl chloride (172 mg, 0.51 mmol)
was added and the soln. stirred for 2 h at r.t. After addition of MeOH (0.5 ml), the mixture was evaporated and
the residue redissolved in CH2Cl2 (10 ml). The soln. was extracted with sat. NaHCO3 soln. (2 Â 5 ml), the org.
phase evaporated and co-evaporated with toluene (2 Â 5 ml), MeOH (2 Â 5 ml), and CH2Cl2 (2 Â 5 ml), and the
residue purified by FC (silica gel (8 g), CH2Cl2 (600 ml)): 278 mg (81%) of 32. Yellowish solid foam. TLC
(CHCl3/MeOH 9 :1): Rf 0.55. UV (MeOH): 269 (4.41), 277 (sh, 4.34). 1H-NMR ((D6)DMSO): 2.35
(m, 1 HꢀC(2')); 2.90 (m, 1 HꢀC(2')); 2.95 (t, OCH2CH2CN); 3.15 (m, 2 HꢀC(5')); 3.30 (t, ArCH2CH2O);
3.70 (s, 2 MeO); 3.95 (m, HꢀC(4')); 4.31 (t, OCH2CH2CN); 4.55 (m, HꢀC(3')); 4.75 (t, ArCH2CH2O); 5.3
(d, OHꢀC(3')); 6.35 (t, HꢀC(1')); 6.75 (m, 4 H o to MeO); 7.15 ± 7.3 (m, 9 arom. H); 7.65 (d, 2 H m to NO2);
8.18 (d, 2 H o to NO2); 8.3 (s, HꢀC(8)); 10.48 (s, NHꢀC(2)). Anal. calc. for C43H41N7O10 (815.8): C 63.31,
H 5.07, N 12.02; found: C 63.18, H 5.09, N 11.99.
25. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-O6-[2-(phenylsulfonyl)ethyl]guanosine (36). After co-evapo-
ration of 35 (0.25 g, 0.57 mmol) in dry pyridine (3 Â 2 ml), the residue was dissolved in a mixture of dry pyridine
(2 ml) and CH2Cl2 (2 ml). Then Me3SiCl (0.385 ml, 3.08 mmol) was added and stirred for 20 min to give a
colorless precipitate of pyridinium chloride. To this suspension, 5 (0.115 g, 0.86 mmol) in CH2Cl2 (1 ml) was
added, then the mixture stirred for 6 h, the reaction quenched by the addition of MeOH (15 ml), and the
mixture evaporated after stirring for 20 min. The residue was treated with H2O (20 ml) and AcOEt (20 ml), the

Helvetica Chimica Acta ± Vol. 83 (2000)
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aq. phase extracted with AcOEt (3 Â 20 ml), the combined org. phase evaporated and co-evaporated with
toluene (2 Â 10 ml), MeOH (2 Â 20 ml), and CH2Cl2 (2 Â 20 ml), and the residue purified by FC (silica gel (5 g)
0 ± 5% MeOH in CH2Cl2 (700 ml)): 0.26 g (85%) of 36. Colorless foam. TLC (CHCl3/MeOH 9 :1): Rf 0.32. UV
(MeOH): 265 (4.15), 277 (sh, 4.0). 1H-NMR ((D6)DMSO): 2.30 (m, 1 HꢀC(2')); 2.70 (m, 1 HꢀC(2')); 2.95
(t, OCH2CH2CN); 3.55 (m, 2 HꢀC(5')); 3.85 (m, HꢀC(4')); 4.15 (t, SO2CH2CH2O); 4.30 (t, OCH2CH2CN);
4.40 (m, HꢀC(3')); 4.72 (t, SO2CH2CH2O); 4.90 (t, OHꢀC(5')); 5.31 (d, OHꢀC(3')); 6.28 (t, HꢀC(1')); 7.45 ±
7.60 (m, 2 H m and 1 H p to SO2); 7.85 (m, 2 H o to SO2); 8.45 (s, HꢀC(8)); 10.55 (s, NHꢀC(2)). Anal. calc. for
C22H24N6O8S ´ 0.5 H2O (541.5): C 48.80, H 4.28, N 15.52; found: C 48.99, H 4.52, N 15.35.
26. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)-O6-[2-(phenylsulfonyl)ethyl]gua-
nosine (37). After co-evaporation of 36 (0.25 g, 0.47 mmol) in dry pyridine (2 Â 2 ml), the residue was
dissolved in pyridine (2 ml) and CH2Cl2 (1.7 ml). Then dimethoxytrityl chloride (0.192 g, 0.57 mmol) was added
and stirred for 50 min at r.t. The reaction was quenched with MeOH (0.5 ml), the mixture evaporated, and the
residue redissolved in AcOEt (7.5 ml). The soln. was washed with sat. NaHCO3 soln. (2 Â 5 ml), dried
(Na2SO4), evaporated, and co-evaporated with toluene (2 Â 5 ml), MeOH (2 Â 5 ml), and CH2Cl2 (2 Â 5 ml),
and the residue purified by FC (silica-gel (8 g), CH2Cl2 (250 ml), then CH2Cl2/MeOH 49 :1 (200 ml)): 0.29 g
(73%) of 37. Yellowish foam. TLC (CHCl3/MeOH 9 :1): Rf 0.5. UV (MeOH): 267 (4.20), 281 (sh, 3.95).
1H-NMR ((D6)DMSO): 2.32 (m, 1 HꢀC(2')); 2.83 (m, 1 HꢀC(2')); 2.97 (t, OCH2CH2CN); 3.12
(m, 1 HꢀC(5')); 3.29 (m, 1 HꢀC(5')); 3.69 (s, 2 MeO); 3.98 (m, HꢀC(4')); 4.12 (t, SO2CH2CH2O); 4.28
(t, OCH2CH2CN); 4.53 (m, HꢀC(3')); 4.73 (t, SO2CH2CH2O); 5.30 (d, OHꢀC(3')); 6.30 (t, HꢀC(1')); 6.75
(m, 4 H o to MeO); 7.10 ± 7.33 (m, 9 arom. H); 7.42 ± 7.60 (m, 2 H m and 1 H p to SO2); 7.88 (m, 2 H o to SO2);
8.27 (s, HꢀC(8)); 10.47 (s, NHꢀC(2)). Anal. calc. for C43H42N6O10S ´ 0.5 H2O (843.8): C 61.20, H 5.02, N 9.96;
found: C 61.17, H 5.05, N 9.85.
27. N4-2-[(Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)cytidine 3'-(2-Cyanoethyl Diisopro-
pylphosphoramidite) (38). A soln. of 22 (1 g, 1.59 mmol) in dry CH2Cl2 (10 ml) was evaporated and dissolved in
CH2Cl2 (10 ml). Then 1H-tetrazole (54 mg, 0.775 mmol) and 2-cyanoethyl tetraisopropylphosphorodiamidite
(980 mg, 3.25 mmol) were added and stirred for 1 h under N2. After dilution with CH2Cl2 (40 ml) and washing
with 5% NaHCO3 soln. (40 ml), the aq. phase was extracted with CH2Cl2 (2 Â 40 ml), the combined org. phase
dried (Na2SO4) and evaporated, and the resulting solid purified by FC (silica gel (17 g), toluene/AcOEt 1 :1).
The product fractions (>150 ml) containing both diastereoisomers were evaporated and co-evaporated with
CH2Cl2 (3Â): 1.06 g (83%) of 38. Solid foam. TLC (CHCl3/MeOH 9 :1): Rf 0.64, 0.69 (2 diastereoisomers). UV
1
(MeOH): 282 (4.09), 303 (sh, 3.93), 235 (sh, 4.60), 206 (sh, 4.89). H-NMR (CDCl3): 1.29 (d, 2 Me2CH); 2.34
(m, 1 HꢀC(2')); 2.48 (t, CH2CN); 2.67 (m, 1 HꢀC(2')); 2.80 (m, 4 H, CH2O, CH2CN); 3.39 ± 3.71 (m, 2 Me2CH,
2 HꢀC(5')); 3.80 (s, 2 MeO); 4.21 (m, HꢀC(4')); 4.41 (t, CH2O); 4.68 (m, HꢀC(3')); 6.27 (t, HꢀC(1')); 6.85
(m, 4 H o to MeO); 7.23 ± 7.45 (m, 7 arom. H, HꢀC(5), HꢀC(6)); 8.35 (s, NHꢀC(4)). 31P-NMR (CDCl3):
149.96; 149.45. Anal. calc. for C43H51N6O9P (826.9): C 62.46, H 6.22, N 10.16; found: C 61.86, H 6.79, N 10.20.
28. N6-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)adenosine 3'-(2-Cyanoethyl Diiso-
propylphosphoramidite) (39). A soln. of 23 (1.5 g, 2.3 mmol) in CH2Cl2 (10 ml) was evaporated and the residue
dissolved in CH2Cl2 (12 ml). Then 1H-tetrazole (80.7 mg, 1.15 mmol) and 2-cyanoethyl tetraisopropylphos-
phorodiamidite (1.38 g, 4.58 mmol) were added and stirred under N2 at r.t. for 1 h. The mixture was diluted with
CH2Cl2 (50 ml) and washed with 5% NaHCO3 soln. (50 ml), the aq. phase extracted with CH2Cl2 (2 Â 50 ml),
the combined org. phase dried (Na2SO4) and evaporated, and the residue purified by FC (silica gel (30 g),
toluene/AcOEt 2 :3 (600 ml)). The combined fractions of both diastereoisomers were evaporated and co-
evaporated with CH2Cl2: 1.70 g (87%) of 39. Colorless foam. TLC (CHCl3/MeOH 25 :1): Rf 0.16, 0.19. UV
(MeOH): 267 (4.35), 276 (4.22). 1H-NMR (CDCl3): 1.20 (d, 2 Me2CH); 2.47 (t, CH2CN); 2.62 (t, CH2CN); 2.81
(t, CH2O); 2.60 ± 2.95 (m, 2 HꢀC(2')); 3.30 ± 3.80 (m, 2 Me2CH, 2 HꢀC(5')); 3.78 (s, 2 MeO); 4.32
(m, HꢀC(4')); 4.47 (t, CH2O); 4.80 (m, HꢀC(3')); 6.48 (t, HꢀC(1')); 6.78 (d, 4 H o to MeO); 7.15 ± 7.45
(m, 9 arom. H); 8.19 (s, HꢀC(8)); 8.38 (s, HꢀC(2)); 8.71 (s, NHꢀC(6)). 31P-NMR (CDCl3): 149.544; 149.420.
Anal. calc. for C44H51N8O8P (850.9): C 62.11, H 6.04, N 13.17; found: C 62.05, H 6.28, N 12.44.
29. N2-[(2-Cyanoethoxy)carbonyl]-2'-deoxy-5'-O-(4,4'-dimethoxytrityl)-O6-[2-(4-nitrophenyl)ethyl]guano-
sine 3'-(2-Cyanoethyl Diisopropylphosphoramidite) (40). A soln. of 32 (0.80 g, 0.98 mmol) in CH2Cl2 (3 ml) was
evaporated and the residue dissolved in CH2Cl2 (5 ml). Then, 1H-tetrazole (34.5 mg, 0.49 mmol) and 2-
cyanoethyl tetraisopropylphosphorodiamidite (591 mg, 1.95 mmol) were added. After stirring at r.t. for 1 h, the
soln. was diluted with CH2Cl2 (20 ml) and washed with 5% NaHCO3 soln. (20 ml). The aq. phase was extracted
with CH2Cl2 (2 Â 20 ml), the combined org. phase dried (Na2SO4) and evaporated, and the resulting residue
purified by FC (silica gel (18 g), toluene/AcOEt 2 :3 (400 ml)): 0.836 g (84%) of 40. Yellowish solid foam. TLC
1
(toluene/AcOEt 1 :1): Rf 0.28, 0.40. UV (MeOH): 269 (4.38), 236 (sh, 4.42, 206 (sh, 4.83). H-NMR (CDCl3):

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Helvetica Chimica Acta ± Vol. 83 (2000)
1.17 (d, 2 Me2CH); 2.60 ± 2.95 (m, 2 HꢀC(2')); 2.64 (t, CH2CN); 2.76 (t, CH2CN); 3.33 (t, ArCH2, CH2O);
3.50 ± 3.90 (m, 2 Me2CH, 2 HꢀC(5')); 3.79 (s, 2 MeO); 4.38 (m, HꢀC(4')); 4.39 (t, CH2O); 4.84
(t, CH2OꢀC(6), HꢀC(3')); 6.38 (t, HꢀC(1')); 6.77 (d, 4 H o to MeO); 7.15 ± 7.40 (m, 9 arom. H); 7.55 (d, 2 H
m to NO2); 8.18 (d, 2 H o to NO2); 7.99 (s, HꢀC(8)). 31P-NMR (CDCl3): 149.45; 149.31. Anal. calc. for
C52H58N9O11P (1016.1): C 61.47, H 5.75, N 12.41; found: C 61.05, H 6.44, N 11.92.
30. Kinetics. An exact amount of nucleoside was weighed into a screw-cap glass vial (5 ml) and mixed with a
15-fold molar excess of 0.5m DBU/MeCN. This soln. was stirred at 22 ± 238. Samples were taken with a 25-ml
Eppendorf pipette and quenched by the addition to 0.25m AcOH (0.5 ml). The samples were stored in a
refrigerator until the measurement was performed. The nucleosides 2'-deoxy-5'-O-(4,4'-dimethoxytrityl)-N4-{[2-
(4-nitrophenyl)ethoxy]carbonyl}cytidine (Cnpeoc), 2'-deoxy-5'-O-(4,4'-dimethoxytrityl)-N6-{[2-(4-nitrophenylethyl)-
ethoxy]carbonyl}adenosine (Anpeoc) and 2'-deoxy-5'-O-(4,4'-dimethoxytrityl) -O6-[2-(4-nitrophenyl)ethyl]-N2-
{[2-(4-nitrophenyl)ethoxy]carbonyl}guanosine (Gnpe,npeoc) used for comparison with the ce/ceoc-substituted
nucleosides were prepared as described in [2].
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Received July 21, 2000