β-Heteroarylethyl Groups - New Phosphate-Protecting Groups for Phosphotriester Chemistry

Article abstract of DOI:10.1002/1522-2675(20001108)83:11<3053::AID-HLCA3053>3.0.CO;2-X

The β-heteroaryl-substituted ethanols 6-10 were synthesized and, together with pyridine-2-ethanols and pyridine-4-ethanols, were tested as a new type of phosphate-protecting groups in the synthesis of oligonucleotides by the phosphotriester approach. The synthesis of 5'-O-(monomethoxytrityl)thymidine 3'-(β-heteroarylethyl 2,5-dichlorophenyl phosphates) 13-17 and 21 provided useful monomeric building blocks in which the various blocking groups could be removed selectively by acid (MeOTr), oximate (2,5-dichlorophenyl phosphate), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (heteroarylethyl phosphate) treatment. The new, fully blocked dimers 38-41, with β-heteroarylethyl protecting groups in the phosphate moiety, were synthesized. The β-heteroarylethyl groups show a broad range of stability towards base treatment in aprotic solvents depending upon the activation of the H-C(β) atoms by the heterocyclic moiety.

Full text of DOI:10.1002/1522-2675(20001108)83:11<3053::AID-HLCA3053>3.0.CO;2-X

Helvetica Chimica Acta ± Vol. 83 32000)
3053
Nucleotides
Part LXVI1)
b-Heteroarylethyl Groups ± New Phosphate-Protecting Groups for Phosphotriester
Chemistry
by Tilman Reiner, Evgeny Kvasyuk, and Wolfgang Pfleiderer*
Fakultät für Chemie, Universität Konstanz, Postfach 5560, D-78434 Konstanz
The b-heteroaryl-substituted ethanols 6 ± 10 were synthesized and, together with pyridine-2-ethanols and
pyridine-4-ethanols, were tested as a new type of phosphate-protecting groups in the synthesis of oligo-
nucleotides by the phosphotriester approach. The synthesis of 5'-O-3monomethoxytrityl)thymidine 3'-3b-
heteroarylethyl 2,5-dichlorophenyl phosphates) 13 ± 17 and 21 provided useful monomeric building blocks in
which the various blocking groups could be removed selectively by acid 3MeOTr), oximate 32,5-dichlorophenyl
phosphate), and 1,8-diazabicyclo[5.4.0]undec-7-ene 3DBU) 3heteroarylethyl phosphate) treatment. The new,
fully blocked dimers 38 ± 41, with b-heteroarylethyl protecting groups in the phosphate moiety, were
synthesized. The b-heteroarylethyl groups show a broad range of stability towards base treatment in aprotic
solvents depending upon the activation of the HÀC3b) atoms by the heterocyclic moiety.
1. Introduction. ± The use of b-eliminating phosphate-protecting groups can be
regarded as an important improvement in phosphotriester chemistry [2] since cleavage
of the phosphotri- to the phosphodiester stage will proceed without nucleophilic attack
at the P-atom, avoiding a potential breakdown of the internucleotidic linkage. Among
those protecting groups are the b-cyanoethyl 3ce) group [3], the methyl-substituted b-
cyanoethyl groups [4], and the b-3alkylsulfonyl)- and b-3arylsulfonyl)ethyl groups [5].
Since these blocking groups are relatively sensitive to base treatment and eliminate
already with ammonia and amines, a more stable type was found in the 2-34-
nitrophenyl)ethyl 3npe) group [2a], which is inert towards amines but will be cleaved
by 1,8-diazabicyclo[5.4.0]undec-7-ene 3DBU) as a much stronger base in a clean b-
eliminating process. The npe group has proven its advantages in a series of
oligonucleotide syntheses [6], which could be extended to a unified blocking-group
strategy by introduction of the 2-34-nitrophenyl)ethoxycarbonyl 3npeoc) group for
base protection [7] and the 2-32,4-dinitrophenyl)ethoxycarbonyl 3dnpeoc) [8], the 2-34-
nitrophenyl)ethylsulfonyl 3npes) [9], and the 2-3dansylethoxycarbonyl) 3dnseoc) [10]
groups for sugar-OH blocking. To broaden the spectrum of b-eliminating protecting
groups for oligonucleotide chemistry, the use of N-containing heteroaryl residues has
been tested as activators in b-elimination reactions [11]. Since an sp2-hybridized ring N-
atom reveals a similar activation power as a NO2 group, the pyridinyl, pyrimidinyl, and
pyrazinyl moieties were chosen as new entities in these investigations.
1
)
Part LXV: [1].

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Helvetica Chimica Acta ± Vol. 83 32000)
2. Syntheses. ± Various 5-substituted pyrimidine-2-ethanols were synthesized in
analogy to 5-phenylpyrimidine-2-ethanol [12] from 3-hydroxypropanamidine hydro-
chloride [13] and the 2-substituted 3-3dimethylamino)acrylaldehydes in a base-
catalyzed 3 ‡ 3 cyclocondensation reaction 3Scheme 1); the pyrimidine derivatives
6 ± 9 were obtained in moderate to good yields without optimizing the reaction
conditions. The 3-3dimethylamino)-2-phenyl- 32) and 2-chloro-3-3dimethylamino)ac-
rylaldehyde 33) were prepared according to Arnold [14], and the 3-3dimethylamino)-2-
34-nitrophenyl)- 34) and 3-3dimethylamino)-2-34-methoxyphenyl)acrylaldehyde 35)
resulted from an analogous Vilsmeier reaction with the corresponding AcOH
derivatives.
Scheme 1
The synthesis of pyrazine-2-ethanol 310) was achieved from 2-methylpyrazine and
formaldehyde in a hydroxymethylation reaction [15] in relatively low yield, and 2,2-
diphenylethanol 311) resulted from the reaction of phenyloxirane with phenylmagne-
sium bromide [16] according to literature procedures.
The syntheses of the new modified thymidine phosphotriesters were performed
from 5'-O-3monomethoxytrityl)thymidine 312) by the triazolide method [2a][17] with
2,5-dichlorophenyl phosphorodichloridate [18] and 1,2,4-1H-triazole in pyridine to give
first the intermediate 3'-phosphodiester triazolides, which reacted subsequently with
the b-substituted ethanols to the corresponding 5'-O-3monomethoxytrityl)thymidine
3'-phosphotriesters 13 ± 20 in 56 ± 94% yield, respectively 3Scheme 2).
Detritylation of 13 ± 18 was achieved in the usual manner by treatment with p-
toluenesulfonic acid to give 22 ± 27 in high yields. The 2,5-dichlorophenyl group could
also be cleaved selectively by the oximate method [19] with p-nitrobenzaldehyde
oxime/Et3N in dioxane/H2O, and the resulting phosphodiesters 28 ± 35 were isolated by
silica-gel column chromatography as their triethyl ammonium salts. Finally, the
chemical stability of the b-heteroarylethyl protecting groups was established to be
sufficient for the formation of internucleotidic bonds by the normal phosphotriester
approach. The phosphodiesters 28, 32, 34, and 35 were coupled with 3'-O-acetyl-

Helvetica Chimica Acta ± Vol. 83 32000)
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Scheme 2

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thymidine 337) to form, under 2,4,6-triisopropylbenzenesulfonyl chloride 3TPS-Cl)/1-
methyl-1H-imidazole activation [20], the dinucleoside phosphotriesters 38 ± 41 in 67,
68, 77, and 74% yields, respectively. The corresponding dithymidinyl npe phospho-
triester 42 was prepared according to [21] for comparative studies.
3. Stability of the Phosphate-Protecting Groups. ± The phosphotriesters 13 ± 21
were treated with various bases under aprotic conditions to study their stabilities
towards 0.5m DBU or 1,5-diazabicyclo[4.3.0]non-5-ene 3DBN) in MeCN and pyridine,
respectively, as well as towards Et3N/solvent 1:1. The removal of the base-labile
protecting groups was analyzed quantitatively by reversed-phase HPLC, in comparison
to the npe phosphotriester 21 as the standard 3see Figure). The reactions were
monitored by aliquots quenched with dilute AcOH at distinct time intervals. The
arithmetical evaluation of the data obtained showed that all reactions obey a pseudo-
first-order rate law. The HPLCs established that the cleavage of the protecting groups
occurred by a b-elimination reaction, as shown by the detection of the corresponding
styrene derivatives.
Various conclusions can be drawn from the kinetic data 3Table and Fig.) showing
that the b-elimination process is fastest in MeCN with DBU as base. DBN does not
show much dependence on the solvent, and Et3N, even in very high concentration, is
much too weak for this type of cleavage reaction.
The activation of the H-atoms at C3b) by the NO2 group in the npe residue is
superseded by the 2-3pyrimidin-2-yl)ethyl group, which can be tuned in its stability by
electron-attracting and -donating groups in the usual manner. Therefore, the 2-[5-34-
nitrophenyl)pyrimidin-2-yl]ethyl residue is the most-labile protecting group in this
series, whereas its structural analog, the 2-[5-3methoxyphenyl)pyrimidin-2-yl]ethyl
group meets the stability of the npe residue. The 2-3pyrazin-2-yl)ethyl residue in 17 is
much more stable than the pyrimidinylethyl groups since the second ring N-atom
located in meta position to the ethyl side-chain activates only by the inductive effect of
this heteroatom. The benzhydryl group on 18 is absolutely stable under the applied
reaction conditions and is, therefore, useless in approaches towards the synthesis of
oligonucleotides.
In the series of the 2-3pyridinyl)ethyl deriatives 19, 20, 40, and 41, the 2-3pyridin-2-
yl)ethyl blocking group in 20 and 41 is more stable than the 2-3pyridin-4-yl)ethyl group
in triesters 19 and 40. In the reaction of the phosphotriesters 19 and 20 with 0.5m DBU/
MeCN solution, we observed also the diester 36 as a by-product. We assume that the
formation of this phosphodiester took place because of the presence of H2O in the
starting materials. This assumption was confirmed by the special experiments in which
the triesters 19 and 20 were treated with 0.5m DBU solution in MeCN/H2O 9 :1; the
diester 36 was the only product of these reactions. Therefore, the use of 2-3pyridin-4-
yl)ethyl and 2-3pyridin-2-yl)ethyl blocking groups in the synthesis of oligonucleotides is
problematic and can not be recommended.
Experimental Part
General. TLC: Precoated silica-gel thin-layer sheets F 1500 LS254 from Schleicher & Schüll. Prep. TLC:
silica gel Merck60 PF 254. Column chromatography 3CC): silica gel Merck 60 30.063 ± 0.2 mesh). HPLC for
compounds 13 ± 17, 21, 38, and 42: Spectra Physics, SP8000 B; column RP 18 3LiChrosorb 250 Â 4.6 mm, 7 mm,

Helvetica Chimica Acta ± Vol. 83 32000)
3057
Table. Kinetic Data of Phosphotriester Cleavage by b-Elimination
a
Base/Solvent
t1/2
t1
)
Retention time tR [s]
triester
styrene
diester
86
13
14
15
16
0.5m DBU/MeCN
0.5m DBU/pyridine
0.5m DBU/MeCN
0.5m DBN/pyridine
Et3N/MeCN 1 : 1
0.5m DBU/MeCN
0.5m DBU/pyridine
0.5m DBN/MeCN
0.5m DBN/pyridine
Et3N/MeCN 1 : 1
0.5m DBU/MeCN
0.5m DBU/pyridine
0.5m DBN/MeCN
0.5m DBN/pyridine
Et3N/MeCN 1 : 1
0.5m DBU/MeCN
0.5m DBU/pyridine
0.5m DBN/MeCN
0.5m DBN/pyridine
Et3N/MeCN 1 : 1
0.5m DBU/MeCN
0.5m DBU/MeCN
0.5m DBU/MeCN
0.5m DBU/MeCN
0.5m DBU/pyridine
0.5m DBN/MeCN
0.5m DBN/pyridine
Et3N/MeCN 1 : 1
Et3N/pyridine 1 : 1
0.5m DBU/MeCN
0.5m DBU/MeCN
0.5m DBU/MeCN
0.5m DBU/MeCN
1.1 min
2.6 min
4.2 min
4.2 min
20 h
0.4 min
0.5 min
1.5 min
1.7 min
6.1 h
0.15 min
0.17 min
0.41 min
0.41 min
2.73 h
2.8 min
6.2 min
7.4 min
11.2 min
34 h
18.8 min
1.16 h
8.25 h
2.8 min
2.8 min
6.6 min
5.7 min
31 h
53 h
4 min
6.25 h
114 h
13 min
24 min
42 min
44 min
194 h
370, 392
215
195
190
215
4 min
5 min
330, 345
320, 342
352, 372
90
90
90
14 min
15 min
58 h
1.6 min
1.6 min
3.8 min
3.8 min
20 h
27 min
54 min
70 min
107 min
334 h
152 min
10 h
80 h
26 min
28 min
61 min
52 min
276 h
17
19
20
21
1250
1020
190
700b)
863
863
87
1118, 1130
1161, 1175
312, 330
490 h
38 min
55 h
38
40
41
42
411, 435
860
87
367c)
728
874
443
728
5.4 min
51 min
175
59d)
a) Calculated for 0.5%. b) LiChrosphere 100-RP18; gradient: MeCN/0.1m 3Et3NH)OAc 3pH 7) 100 : 0 ! 1 : 1 in
7 min and 1 : 1 ! 0 : 100 in 15 min. c) Mobile phase: H2O/MeCN 1 : 1. d) Mobile phase: H2O/MeCN 1 : 3.
Merck); flow rate 1.3 ml/min; elution: MeOH/H2O 89 :11. HPLC for compounds 19, 20, 40, and 41: Merck-
Hitachi, L-6200-Intelligent pump, D-2000 chromatointegrator, detection at 260 nm 3Uvicon 730 SLC, Fa.
Kontron); column RP 18 3LiChrospher 125  4 mm, 5 mm, Merck 50943); flow rate 1 ml/min: A ˆ 0.1m aq.
3Et3NH)OAc buffer 3pH 7.0), B ˆ 0.1m aq. 3Et3NH)OAc buffer 3pH 7.0)/MeCN 1:1, C ˆ MeCN; gradient: A in
3 min, from A to B in 7 min, from B to C in 15 min. M.p.: Büchi apparatus, model Dr. Tottoli; no corrections.
UV/VIS: Kontron, Uvikon 820, and Perkin-Elmer, Lambda 15; lmax in nm 3log e). 1H-NMR: Bruker WM 250; d
in ppm rel. toSiMe 4 as internal standard.
1. 3-8Dimethylamino)-2-84-nitrophenyl)prop-2-enal 34). Toa cold mixture 30 8) of POCl3 363 g) and of DMF
353 ml), 34-nitrophenyl)acetic acid 325 g, 138 mmol) was added and then heated in an oil bath to 908 for 7 h with
stirring. After cooling, the mixture was poured slowly on ice and then neutralized by solid Na2CO3. Toluene
3300 ml) was added and the mixture heated on a boiling-water bath for 30 min. The aq. phase was additionally
extracted twice with CH2Cl2 32 l) and then combined with the toluene extract. After drying 3Na2SO4) and
evaporation, the remaining DMF was distilled off under vacuum, and the resulting residue was recrystallized

3058
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Figure. Kinetics of npe-cleavage in 21 by base-catalyzed b-elimination
from CHCl3 with charcoal: 21.5 g 371%) of 4. Yellowish crystals. M.p. 105 ± 1108. UV 3MeOH): 287 34.44), 349
1
33.70). H-NMR 3CDCl3): 9.1 32s, CHO); 8.17 ± 8.34 32d, 2 H o toNO 2); 7.30 ± 7.55 32d, 2 H m toNO 2); 7.00
3br. s, CH); 2.4 ± 3.5 3m, Me2N). Anal. calc. for C11H12N2O3 3220.2): C 59.99, H 5.49, N 12.72; found: C 60.11,
H 5.54, N 12.93.
2. 3-8Dimethylamino)-2-84-methoxyphenyl)prop-2-enal 35). Analogously to Exper. 1, with POCl3 355 g),
DMF 346 ml), and 34-methoxyphenyl)acetic acid 320 g, 0.12 mol) for 5 h. Extraction with CH2Cl2 32.5 l) gave a
red-brown sirup. On treatment with Et2O and stirring, a solid resulted, which was dried in a vacuum desiccator:
13.8 g 354%) of 5. Deliquescent crystals. The crude material was used for the condensation reaction. UV
3MeOH): 221 33.92), 250 33.58), 314 34.32).
3. 2-85-Substituted Pyrimidine-2)-ethanols: General Procedure. To a soln. of Na 31.1 g, 48 mmol) in abs.
EtOH 390 ml), 3-hydroxypropanamidine hydrochloride [13] 38.3 g, 67 mmol) was added and stirred for 30 min.
The precipitate of NaCl was filtered off, then 2-substituted 3-3dimethylamino)prop-2-enal 348 mmol) added to
the filtrate, and the mixture heated under reflux for 6 h. After evaporation, the residue was dissolved in CHCl3
and the soln. washed with Na2SO4 soln. and H2O, dried 3Na2SO4), and again evaporated.
5-Chloropyrimidine-2-ethanol 37). From 2-chloro-3-3dimethylamino)prop-2-enal 33) 35.32 g, 47 mmol)
[14]. The resulting solid was recrystallized from AcOEt 321 ml) and hexane 32 ml): 2.8 g 338%) of 7. Brownish
crystals. M.p. 688. UV 3MeOH): 257 3sh, 3.34), 262 33.42), 294 32.71). 1H-NMR 3CDCl3): 8.65 3s, HÀC34),
HÀC36)); 4.1 3m, CH2CH2O); 3.46 3s, OH); 3.22 3t, CH2CH2O). Anal. calc. for C6H7ClN2O 3158.6): C 45.45,
H 4.44, N 17.66; found: C 45.36, H 4.26, N 17.53.
5-84-Nitrophenyl)pyrimidine-2-ethanol 38). From 4 310.1 g, 46 mmol). Recrystallization from CHCl3
350 ml) and EtOH 350 ml) with charcoal gave 8.5g 376%) of 8. Yellow crystals. M.p. 176 ± 1788. UV 3MeOH):
213 34.13), 288 34.24). 1H-NMR 3CDCl3): 8.90 3s, HÀC34), HÀC36)); 8.45 3m, 2 H o toNO 2); 7.75 3m, 2 H m to
NO2); 4.10 3m, 2 H, CH2 CH2O); 3.65 3br. s, OH); 3.30 3t, CH2CH2O). Anal. calc. for C12H13N3O3 3245.2): C
58.77, H 4.52, N 17.13; found: C 58.47, H 4.47, N 17.04.
5-84-Methoxyphenyl)pyrimidine-2-ethanol 39). From 5 310 g, 48 mmol). The resulting dark red residue was
purified by CC 310 Â 4.5 cm, CHCl3) and recrystallized from AcOEt 320 ml) and pentane 31 ml): 11.9 g 364%) of
9. Colorless crystals. M.p. 100 ± 1028. UV 3MeOH): 207 34.25), 271 34.33). 1H-NMR 3CHCl3): 8.85 3s, HÀC34),
HÀC36)); 7.5 3m, 2 H o toMeO); 7.05 3 s, 2 H m toMeO); 4.12 3 m, CH2CH2O, OH); 3.90 3s, MeO); 3.27
3t, CH2CH2O). Anal. calc. for C13H14N2O2 3230.2): C 67.81, H 6.13, N 12.17; found: C 68.06, H 6.47, N 12.22.
4. 5'-O-84-Methoxytrityl)thymidine 3'-Phosphotriester: General Procedure. A soln. of 1,2,4-1H-triazole
30.414 g, 6 mmol) in abs. pyridine 34.5 ml) was treated with 2,5-dichlorophenyl phosphorodichloridate 30.732 g,
2.6 mmol) and stirred at 08 for 30 min. Then a soln. of 5'-O-3monomethoxytrityl)thymidine 312; 1.03 g, 2 mmol)
in abs. pyridine 310 ml) was added dropwise. Stirring was continued for 1 h. Finally, the 2-substituted ethanol

Helvetica Chimica Acta ± Vol. 83 32000)
3059
derivative 34 mmol) was added and the mixture stirred for 1 d. To protect unreacted 12, the mixture was treated
with Ac2O 31.5 ml) for 2 h, evaporated, and co-evaporated with toluene 33 Â 15 ml). The residue was dissolved
in CHCl3, the soln. washed with H2O, dried 3Na2SO4), then concentrated to a small volume, and submitted to
CC 3silica gel, 20 Â 3 cm, 0 ± 5% MeOH/CHCl3). The product was co-evaporated with CH2Cl2: colorless
amorphous solid, which was dried under vacuum at r.t.
5'-O-8Monomethoxytrityl)thymidine 3'-[2,5-Dichlorophenyl 2-85-Phenylpyrimidin-2-yl)ethyl Phosphate]
313). With 5-phenylpyrimidine-2-ethanol 36) [24] 30.8 g): 1.68 g 391%) of 13. Colorless amorphous foam. UV
3MeOH): 228 3sh, 4.58), 249 34.45). 1H-NMR 3CDCl3); 8.85 32s, 2 H 3pyr)); 8.2 3s, HÀN33)); 7.65
3m, 21 arom. H); 6.82 3d, 2 H o toMeO); 6.46 3 m, HÀC31')); 5.35 3m,HÀC33')); 4.77 3m, CH2CH2O); 4.32
3m, HÀC34')); 3.78 3s, MeO); 3.56 ± 3.33 3m, CH2CH2O, 2 HÀC35')); 2.70 3m, 1 HÀC32')); 2.40
3m, 1 HÀC32')); 1.35 3s, Me). Anal. calc. for C48H43Cl2N4O9P 3926.3): C 62.22, H 4.68, N 6.05; found: C 61.85,
H 4.74, N 5.92.
5'-O-8Monomethoxytrityl)thymidine 3'-[2-85-Chloropyrimidin-2-yl)ethyl 2,5-Dichlorophenyl Phosphate]
314). With 7 30.634 g): 1.55 g 388%) of 14. Colorless foam. UV 3MeOH): 227 3sh, 4.42), 264 34.10), 281
3sh, 3.79). 1H-NMR 3CDCl3): 8.58 3d, 2 H 3pyr)); 8.34 3s, HÀN33)); 7.56 3s, HÀC36)); 7.43 ± 7.08 3m, 15 ar-
om. H); 6.82 3m, 2 H o toMeO); 6.45 3 m, HÀC31')); 5.32 3m, HÀC33')); 4.70 3m, CH2CH2O); 4.30
3m, HÀC34')); 3.78 3s, MeO); 3.60 ± 3.30 3m, CH2CH2O, 2 HÀC35')); 2.70 3m, 1 HÀC32')); 2.40
3m, 1 HÀC32')); 1.37 3s, Me)). Anal. calc. for C42H38Cl3N4O9P 3880.0): C 57.32, H 4.35, N 6.37; found: C 57.48,
H 4.35, H 6.22.
5'-O-8Monomethoxytrityl)thymidine 3'-{2,5-Dichlorophenyl 2-[5-84-Nitrophenyl)pyrimidin-2-yl]ethyl
Phosphate} 315). With 8 30.98 g): 1.08 g 356%) of 15. Colorless foam. UV 3MeOH): 223 3sh, 4.55), 273
34.35), 279 3sh, 4.34), 304 3sh, 4.18). 1H-NMR 3CDCl3): 8.90 3s, 2 H 3pyr)); 8.35 3m, 2 H o toNO 2); 8.20
3d, NH); 7.73 3m, 2 H m toNO 2); 7.55 3d, HÀC36)); 7.5 ± 7.0 3m, 15 arom. H); 6.80 3d, 2 H m toMeO); 6.47
3m, HÀC31')); 5.37 3m, HÀC33')); 4.80 3m, CH2CH2O); 4.32 3m, HÀC34')); 3.80 3s, MeO); 3.55 3m, CH2CH2O,
2 HÀC35')); 2.75 3m, 1 HÀC32')); 2.40 3m, 1 HÀC32')); 1.40 3s, Me). Anal. calc. for C48H42Cl2N5O11P 3966.7):
C 59.63, H 4.37, N 7.24; found: C 59.33, H 4.61, N 7.22.
5'-O-8Monomethoxytrityl)thymidine 3'-{2,5-Dichlorophenyl 2-[5-84-Methoxyphenyl)pyrimidin-2-yl]ethyl
Phosphate} 316). With 9 30.92 g): 1.29 g 368%) of 16. Colorless foam. UV 3MeOH): 270 34.43), 280
3sh, 4.37). 1H-NMR 3CDCl3): 8.78 3d, 2 H 3pyr)); 8.30 3s, HÀN33)); 7.53 3s, HÀC36)); 7.45 3m, 2 H o toMeO);
7.39 ± 7.09 3m, 15 arom. H); 7.00 3m, 2 H m toMeO); 6.79 3 m, 2 H m toMeO 3MeOTr)); 6.43 3 m, HÀC31'));
5.32 3m, HÀC33')); 4.75 3m, CH2CH2O); 4.32 ± 4.24 3m, HÀC34')); 3.84 3s, MeO); 3.75 3s, MeO); 3.51 ± 3.33
3m, CH2CH2O, 2 HÀC35')); 2.65 3m, 1 HÀC32')); 2.40 3m, 1 HÀC32')); 1.43 3s, Me). Anal. calc. for
C49H45Cl2N4O10P 3951.8): C 61.83, H 4.72, N 5.89; found: C 61.76, H 4.65, N 5.85.
5'-O-8Monomethoxytrityl)thymidine 3'-[2,5-Dichlorophenyl 2-8Pyrazin-2-yl)ethyl Phosphate] 317). With
pyrazine-2-ethanol 310) [15] 30.5 g): 1.15 g 368%) of 17. Colorless foam. UV 3MeOH): 227 3sh, 4.43), 258
3sh, 4.18), 265 34.24), 281 33.81). 1H-NMR 3CDCl3): 8.5 ± 8.3 3m, HÀC35)3Pyr), HÀC36)3pyr), HÀN33)); 8.05
3s, HÀC33)3pyr)); 7.50 3s, HÀC36)); 7.4 ± 7.0 3m, 15 arom. H); 6.80 3d, 2 H m toMeO); 6.43 3 m,HÀC31')); 5.27
3m, HÀC33')); 4.63 3m, CH2CH2O); 4.25 3m, HÀC34')); 3.78 3s, MeO); 3.45 3m, CH2CH2O); 3.16
3dd, 2 HÀC35')); 2.60 3m, 1 HÀC32')); 2.38 3m, 1 HÀC32')); 1.35 3s, Me). Anal. calc. for C42H39Cl2N4O9P
3845.7): C 59.65, H 4.65, N 6.63; found: C 59.66, H 4.66, N 6.55.
5'-O-8Monomethoxytrityl)thymidine 3'-82,5-Dichlorophenyl 2,2-Diphenylethyl Phosphate) 318). With 2,2-
diphenylethanol 311) [16] 30.79 g): 1.73 g 394%) of 18. Colorless amorphous solid. UV 3MeOH): 230 3sh, 4.42),
259 3sh, 4.03), 265 34.06), 280 3sh, 3.85). 1H-NMR 3CDCl3): 8.45 3s, HÀN33)); 7.50 3s, HÀC36)); 7.45 ± 7.0
3m, 25 arom. H); 6.83 3d, 2 H m toMeO); 6.40 3 m, HÀC31')); 5.17 3m, HÀC33')); 4.70 3m, CHCH2O); 4.38
3t, CHCH2O); 4.10 3m, HÀC34')); 3.80 3s, MeO); 3.33 3m, 2 HÀC35')); 2.50 3m, 1 HÀC32')); 2.20
3m, 1 HÀC32')); 1.52 3s, Me). Anal. calc. for C50H45Cl2N2O9P 3919.8): C 65.29, H 4.93, N 3.04; found: C 64.71,
H 4.98, N 2.93.
5'-O-8Monomethoxytrityl)thymidine 3'-[2,5-Dichlorophenyl 2-8Pyridin-4-yl)ethyl Phosphate] 319). With
pyridine-4-ethanol 3200 mg, 1.6 mmol): 470 mg 354%) of 19. Solid foam. UV 3MeOH): 263 34.09), 228
3sh, 4.42). 1H-NMR 33D6)DMSO): 11.41 3s, NH); 8.40 3d, 2 HÀC32)3py), HÀC36)3py)); 7.62, 7.58 32s,
HÀC36)3Cl2C6H3)); 7.47 3s, HÀC36)3T)); 7.40 ± 6.82 3m, 18 aro m. H, HÀC33)3py), HÀC35)3py)); 6.20
3dd, HÀC31')); 5.14 3m, HÀC33')); 4.43 3m, POCH2CH2); 4.13 3m, HÀC34')); 3.70 3s, MeO); 3.20
3m, 2HÀC35')); 2.93 3t, POCH2CH2); 2.43 3m, 2 HÀC32')); 1.48 3s, Me). Anal. calc. for C43H40Cl2N3O9P ´
H2O 3862.7): C 59.86, H 4.90, N 4.87; found: C 60.45, H 5.01, N 4.52.
5'-O-8Monomethoxytrityl)thymidine [2,5-Dichlorophenyl 2-8Pyridin-2-yl)ethyl Phosphate] 320). With
pyridine-2-ethanol 3200 mg, 1.6 mmol): 640 mg 374%) of 20. Solid foam. UV 3MeOH): 261 34.10), 228

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Helvetica Chimica Acta ± Vol. 83 32000)
3sh, 4.40). 1H-NMR 33D6)DMSO): 11.40 3s, NH); 8.43 3m, HÀC36)3py)); 7.68 3m, HÀC35)3py)); 7.59
3m, HÀC36)3Cl2C6H3)); 7.47 3s, HÀC36)3T)); 7.37 ± 6.85 3m, 18 arom. H, 2 HÀC33)3py), HÀC34)3py)); 6.19
3dd, HÀC31')); 5.14 3m, HÀC33')); 4.53 3m, POCH2CH2); 4.14 3m, HÀC34')); 3.70 3s, MeO); 3.22
3m, 2 HÀC35')); 3.07 3t, POCH2CH2); 2.43 3m, 2 HÀC32')); 1.48 3s, Me3T)). Anal. calc. for C43H40Cl2N3O9P ´
H2O 3862.7): C 59.86, H 4.90, N 4.87; found: C 60.27, H 4.97, N 4.57.
5. Thymidine 3'-Phosphotriesters 22 ± 27: General Procedure. The 5'-O-3monomethoxytrityl)thymidine 3'-
phosphotriester 13 ± 18 30.35 mmol) was dissolved in a soln. of 1% TsOH in CHCl3/MeOH 4 :1 and stirred at r.t.
for 1.5 h. After dilution with CHCl3 3100 ml) and extraction with phosphate buffer 3pH 7; 2 Â 35 mmol)
followed by H2O, the org. layer was dried 3Na2SO4), evaporated to a small volume, and purified by prep. TLC
3silica gel, 40 Â 20 Â 0.2 cm, CHCl3/MeOH 19 :1). The product band was eluted with CHCl3/MeOH 4 :1, the
eluate evaporated, the residue redissolved in CH2Cl2, and the soln. filtered through cotton, and again
evaporated: colorless amorphous foam.
Thymidine 3'-[2,5-Dichlorophenyl 2-85-Phenylpyrimidin-2-yl)ethyl Phosphate] 322). From 13 30.324 g):
1
0.209 g 392%) of 20. UV 3MeOH): 223 34.34). 228 34.32), 254 34.35). H-NMR 3CDCl3): 8.88 3d, 2 H 3pyr));
8.20 3br. s, HÀN33)); 7.65 ± 7.06 3m, 9 arom. H); 6.13 3ꢀtꢁ, HÀC31')); 5.25 3m, HÀC33')); 4.80 3m, CH2CH2O);
4.21 3m, HÀC34')); 3.87 3br. s, 2 HÀC35')); 3.43 3t, CH2CH2O); 3.37 3br. s, OH); 2.50 3m, 2 HÀC32')); 1.89
3s, Me). Anal. calc. for C28H27Cl2N4O8P 3649.4): C 51.78, H 4.19, N 8.63; found: C 51.61, H 4.21, N 8.45.
Thymidine 3'-[2,5-Dichlorophenyl 2-85-Chloropyrimidin-2-yl)ethyl Phosphate] 323). From 14 30.308 g):
0.172 g 381%) of 23. UV 3MeOH): 210 3sh, 4.45), 263 34.10), 268 3sh, 4.08), 280 3sh, 3.79). 1H-NMR 3CDCl3):
9.05 3s, HÀN33)); 8.65 3s, 2 H 3pyr)); 7.45 3s, HÀC36)); 7.4 ± 7.1 3m, 3 arom. H); 6.20 3ꢀtꢁ, HÀC31')); 5.28
3m, HÀC33')); 4.76 3m, CH2CH2O); 4.23 3m, HÀC34')); 3.87 3m, 2 HÀC35')); 3.40 3t, CH2CH2O); 3.23
3m,OH); 2.50 3m, 2 HÀC32')); 1.90 3s, Me). Anal. calc. for C22H22Cl3N4O8P ´ 1.5 H2O 3634.7): C 42.84, H 3.76,
N 9.08; found: C 42.95, H 3.98, N 9.14.
Thymidine 3'-{2,5-Dichlorophenyl 2-[5-84-Nitrophenyl)pyrimidin-2-yl]ethyl Phosphate} 324). From 15
1
30.34 g): 0.177 g 373%) of 24. UV 3MeOH): 218 3sh, 4.46), 273 34.37), 280 3sh, 4.35), 302 3sh, 4.19). H-NMR
3CDCl3): 11.3 3s, HÀN33)); 9.20 3s, 2 H3pyr)); 8.35 3d, 2 H o toNO 2); 8.15 3d, 2 H m toNO 2); 7.70 ± 7.25
3m, HÀC36), 3 arom. H); 6.15 3m, HÀC31')); 5.25 3t, OH); 5.10 3m, HÀC33')); 4.75 3m, CH2CH2O); 4.05
3m, HÀC34')); 3.57 3m, 2 HÀC35')); 3.40 3t, CH2CH2O); 2.3 3m, 2 HÀC32')); 1.75 3s, Me). Anal. calc. for
C28H26Cl2N5O10P ´ 0.5 H2O 3702.4): C 47.81, H 3.86, N 9.96; found: C 47.89, H 3.68, N 9.92.
Thymidine 3'-{2,5-Dichlorophenyl 2-[5-84-Methoxyphenyl)pyrimidin-2-yl]ethyl Phosphate} 325). From 16
30.333 g): 0.2 g 385%) of 25. UV 3MeOH): 223 3sh, 4.38), 267 34.43). 1H-NMR 3CDCl3): 8.85 3m, HÀN33), 2 H,
3pyr)); 7.55 ± 7.30 3m, HÀC36), 4 arom. H); 7.05 3d, 2 H o toMeO); 6.17 3ꢀ tꢁ, HÀC31')); 5.30 3m, HÀC33')); 4.80
3m, CH2CH2O); 4.22 3m, HÀC34')); 3.87 3m, MeO, 2 HÀC35')); 3.56 3m, OH); 3.43 3t, CH2CH2O); 2.52
3m, 2 HÀC32')); 1.90 3s, Me). Anal. calc. for C29H29Cl2N4O9P 3679.4): C 51.26, H 4.30, N 8.24; found: C 50.95,
H 4.26, N 8.07.
Thymidine 3'-[2,5-Dichlorophenyl 2-8Pyrazin-2-yl)ethyl Phosphate] 326). From17 30.296 g): 0.182 g 391%)
of 26. UV 3MeOH): 221 3sh, 4.23), 264 34.24), 270 3sh, 4.19), 307 33.02). 1H-NMR 3CDCl3): 8.66 3m, HÀN33),
HÀC35)3pyr), HÀC36)3pyr)); 8.45 3s, HÀC33)3pyr)); 7.43 3s, HÀC36)); 7.36 ± 7.10 3m, 3 arom. H); 6.15 3ꢀtꢁ,
HÀC31')); 5.24 3m, HÀC33')); 4.69 3m, CH2CH2O); 4.20 3m, HÀC34')); 3.82 3m, 2 HÀC35')); 3.25
3t, CH2CH2O); 2.90 3m, OH); 2.49 3m, 2 HÀC32')); 1.82 3s, Me). Anal. calc. for C22H23Cl2N4O8P 3573.3):
C 46.08, H 4.04, N 9.77; found: C 45.98, H 3.98, N 9.67.
Thymidine 3'-[2,5-Dichlorophenyl 2,2-Diphenylethyl Phosphate] 327). From 18 30.322 g): 0.206 g 391%) of
27. UV 3MeOH): 218 3sh, 4.43), 264 34.04). 1H-NMR 3CDCl3): 8.33 3s, HÀN33)); 7.45 ± 7.07 3m, HÀC36),
12 arom. H); 6.08 3dd, HÀC31')); 5.08 3m, HÀC33')); 4.70 3m, CHCH2O); 4.42 3t, CHCH2); 4.05
3m, HÀC34')); 3.70 3m, 2 HÀC35')); 2.56 3br. s, OH); 2.32 3m, 2 HÀC32')); 1.80 3s, Me). Anal. calc. for
C30H29Cl2N2O8P 3647.4): C 55.65, H 4.51, N 4.32; found: C 55.44, H 5.01, N 4.19.
6. 5'-O-8Monomethoxytrityl)thymidine 3'-82-Heteroarylethyl Triethylammonium Phosphates) 28 ± 35: Gen-
eral Procedure. At r.t. 4-nitrobenzaldehyde oxime 30.83 g, 5 mmol) was dissolved with stirring in dioxane
310 ml), Et3N 310 ml), and H2O 310 ml). After 10 min, the 5'-O-3monomethoxytrityl)thymidine 3'-phospho-
triester 13 ± 20 30.5 mmol) was added, and stirring was continued for 2.5 h. The mixture was then evaporated, co-
evaporated with pyridine 34 Â 10 ml) and toluene 34 Â 10 ml), and then submitted toCC 3silica gel, 15 Â 2.5 cm,
CHCl3/MeOH 95 :5 3 ! excess oxime and 4-nitrobenzonitrile), then CHCl3/MeOH/Et3N 100 :10 :3 3 ! prod-
uct)). The product was co-evaporated with CH2Cl2, taken up in CH2Cl2, filtered through cotton, and then
evaporated again: colorless foam, which was dried at 408 high vacuum.
5'-O-8Monomethoxytrityl)thymidine 3'-[2-85-Phenylpyrimidin-2-yl)ethyl Triethylammonium Phosphate
‡
328). From 13 30.463 g): 0.435 g 398%) of 28. 1H-NMR 3CDCl3): 12.2 3br. s, Et3NH ); 8.80 3s, 2 H 3pyr));

Helvetica Chimica Acta ± Vol. 83 32000)
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8.37 3s, HÀN33)); 7.60 3s, HÀC36)); 7.55 ± 7.12 3m, 17 arom. H); 6.80 3d, 2 H o toMeO); 6.48 3 dd, HÀC31'));
5.07 3m, HÀC33')); 4.45 ± 4.33 3m, CH2O, HÀC34')); 3.78 3s, MeO); 3.45 3dd, 2 HÀC35')); 3.32 3t, CH2CH2O);
3.05 3q, 3 MeCH2N); 2.68 3m, 1 HÀC32')); 2.35 3m, 1 HÀC32')); 1.34 3m, 3 MeCH2N, Me) .
5'-O-8Monomethoxytrityl)thymidine 3'-[2-85-Chloropyrimidin-2-yl)ethyl Triethylammonium Phosphate]
‡
329). From 14 30.44 g): 0.287 g 370%) of 29. 1H-NMR 3CDCl3): 12.5 3br. s, Et3NH ); 8.40 3br. s, HÀN33),
2 H3pyr)); 7.61 3s, HÀC36)); 7.55 ± 7.22 3m, 16 arom. H); 6.80 3d, 2 H o toMeO); 6.51 3 m, HÀC31')); 5.03
3m, HÀC33')); 4.35 3m, CH2O, HÀC34')); 3.79 3s, MeO); 3.45 3m, 2 HÀC35')); 3.25 3t, CH2CH2O); 3.05
3m, 3 MeCH2N); 2.70 3m, 1 HÀC32')); 2.35 3m, 1 HÀC32')); 1.30 3m, 3 MeCH2N, Me) .
5'-O-8Monomethoxytrityl)thymidine 3'-{2-[5-84-Nitrophenyl)pyrimidin-2-yl]ethyl Triethylammonium
1
‡
Phosphate} 330). From 15 30.483 g): 0.41 g 390%) of 30. H-NMR 3CDCl3): 12.6 3br. s, Et3NH ); 8.90 3s, 2 H
3pyr)); 8.64 3br. s, HÀN33)); 8.33 3d, 2 H o toNO 2); 7.8 ± 7.5 3m, HÀC36), 2 H m toNO 2); 7.42 ± 7.20
3m, 14 arom. H); 6.82 3d, 2 H o toMeO); 6.51 3 m, HÀC31')); 5.15 3m, HÀC33')); 4.39 3m, CH2O, HÀC34'));
3.80 3s, MeO); 3.70 ± 3.30 3m, CH2CH2O, 2 HÀC35')); 3.09 3m, 3 MeCH2N); 2.75 3m, 1 HÀC32')); 2.40
3m, 1 HÀC32')); 1.40 3m, 3 MeCH2N, Me) .
5'-O-8Monomethoxytrityl)thymidine 3'-{2-[5-84-Methoxyphenyl)pyrimidin-2-yl]ethyl Triethylammonium
‡
Phosphate} 331). From 16 30.476 g): 0.393 g 388%) of 31. 1H-NMR 3CDCl3): 12.2 3br. s, Et3NH ); 8.77
3s, 2 H 3pyr)); 8.58 3br. s, HÀN33)); 7.71 3s, HÀC36)); 7.45 ± 7.15 3m, 16 H, H m toMeO, arom. H); 7.00 3 d, 2 H o
toMeO); 6.81 3 d, 2 H o toMeO 3MeOTr)); 6.51 3 dd, HÀC31')); 5.06 3m, HÀC33')); 4.39 3m, CH2O, HÀC34'));
3.87 3s, MeO); 3.77 3s, MeO); 3.45 3m, 2 HÀC35')); 3.29 3t, CH2CH2O); 3.10 3q, 3 MeCH2N); 2.70
3m, 1 HÀC32')); 2.35 3m, HÀC32')); 1.3 3m, 3 MeCH2N, Me).
5'-O-8Monomethoxytrityl)thymidine 3'-[2-8Pyrazin-2-yl)ethyl Triethylammonium Phosphate] 332). From
17 30.422 g): 0.372 g 393%) of 32. UV 3MeOH): 205 34.75), 230 3sh, 4.17), 265 34.14), 310 32.96). 1H-NMR
‡
3CDCl3): 12.2 3br. s, Et3NH ); 8.55 ± 8.35 3m, HÀN33), 2 H 3pyr)); 8.25 3s, 1 H 3pyr)); 7.61 3s, HÀC36)); 7.45 ±
7.17 3m, 12 arom. H); 6.83 3d, 2 H o toMeO); 6.45 3 m, HÀC31')); 4.98 3m, HÀC33')); 4.33 ± 4.18 3m, CH2CH2O,
HÀC34')); 3.78 3s, MeO); 3.40 3dd, 2 HÀC35')); 3.05 3m, CH2CH2O; 3 MeCH2N); 2.65 3m, 1 HÀC32')); 2.34
3m, 1 HÀC32')); 1.36 3m, 3 MeCH2N, Me) .
5'-O-8Monomethoxytrityl)thymidine 3'-[2,2-Diphenylethyl Triethylammonium Phosphate] 333). From 18
30.46 g): 0.346 g 379%) of 33. 1H-NMR 3CDCl3): 12.2 3br. s, Et3NH); 8.17 3s, HÀN33)); 7.54 3s, HÀC36)); 7.4 ±
7.05 3m, 22 arom. H); 6.81 3d, 2 H o toMeO); 6.40 3 m, HÀC31')); 4.91 3m, HÀC33')); 4.41 3t, CHCH2O); 4.31
3d, CHCH2O); 4.17 3m, HÀC34')); 3.78 3s, MeO); 3.30 3dd, 2 HÀC35')); 2.83 3m, 3 MeCH2N); 2.48
3m, 1 HÀC35')); 2.15 3m, 1 HÀC35')); 1.31 3s, Me); 1.15 3m, 3 MeCH2N).
5'-O-8Monomethoxytrityl)thymidine 3'-[2-8Pyridin-4-yl)ethyl Triethylammonium Phosphate] 334). From
20 3410 mg, 0.46 mmol): 320 mg 388%) of 34. Solid foam. UV 3MeOH): 262 34.04), 230 3sh, 4.21). 1H-NMR
‡
33D6)DMSO): 11.37 3s, HÀN33)); 10.58 3br. s, Et3NH ); 8.33 3d, HÀC33)3py), HÀC35)3py)); 7.46
3s, HÀC36)3T)); 7.38 ± 6.87 3m, 14 aro m. H, HÀC32)3py), HÀC36)3py)); 6.17 3dd, HÀC31')); 4.60
3m, HÀC33')); 4.05 3m, HÀC34')); 3.82 3m, POCH2CH2); 3.72 3s, MeO); 3.17 3m, 2 HÀC35')); 2.94
3q, 3 MeCH2N); 2.74 3t, POCH2CH2); 2.19 3m, 2 HÀC32')); 1.36 3s, Me); 1.11 3t, 3 MeCH2N). Anal. calc. for
C43H53N4O9P ´ H2O 3818.9): C 63.07, H 6.76, N 6.84; found: C 63.15, H 6.76, N 6.25.
5'-O-8Monomethoxytrityl)thymidine 3'-[2-8Pyridin-2-yl)ethyl Triethylammonium Phosphate] 335). From
21 3570 mg, 0.66 mmol): 470 mg 389%) of 35. Solid foam. UV 3MeOH): 267 3sh, 4.11), 261 34.12), 230
‡
3sh, 4.23). 1H-NMR 33D6)DMSO): 11.35 3s, HÀN33)); 10.77 3br. s, Et3NH ); 8.41 3d, HÀC36)3py)); 7.62 3s,
HÀC35)3T)); 7.46 3s, HÀC36)3T)); 7.37 ± 6.86 3m, 14 arom. H, HÀC33)3py), HÀC34)3py)); 6.17
3dd, HÀC31')); 4.61 3m, HÀC33')); 4.06 3m, HÀC34')); 3.93 3m, POCH2CH2); 3.70 3s, MeO); 3.18
3m, 2 HÀC35')); 2.92 3m, POCH2CH2, 3 MeCH2N); 2.24 3m, 2 HÀC32')); 1.36 3s, Me3T)); 1.11 3t, 3 MeCH2N).
Anal. calc. for C43H53N4O9P ´ 1.5 H2O 3827.91): C 62.38, H 6.81, N 6.76; found: C 62.44, H 6.64, N 6.40.
7. 5'-O-8Monomethoxytrityl)thymidylyl-{3'-{OP-[2-85-phenylpyrimidin-2-yl)ethyl]}-5'}-3'-O-acetylthymi-
dine 338). A mixture of 28 30.281 g, 0.32 mmol) and 3'-O-acetylthymidine 337) 30.07 g, 0.25 mmol) was twice
co-evaporated with abs. pyridine. The residue was dissolved in abs. pyridine 32.5 ml), 1-methyl-1H-imidazole
30.15 ml, 1.9 mmol) and TPS-Cl 30.194 g, 0.64 mmol) were added, and the mixture was stirred at r.t. for 24 h. The
soln. was then treated with H2O 32.5 ml) and evaporated, the residue dissolved in CHCl3 340 ml), the soln.
washed twice with ice-water, dried 3Na2SO4) and evaporated, and the residue co-evaporated with toluene,
dissolved in a little CHCl3, and submitted toprep. TLC 3silica gel, 2 plates 40 Â 20 Â 0.2 cm, CHCl3/MeOH
95 :5). The main band was eluted with CHCl3/MeOH 4 :1 and the eluate rechromatographed by the same
procedure. The resulting foam was dissolved in dioxane 38 ml) and lyophilized: 0.175 g 367%) of 38. Amorphous
solid. UV 3MeOH): 236 34.45), 257 34.47). 1H-NMR 3CDCl3): 8.86, 8.83 32s, 2 H 3pyr)); 8.67, 8.57 32s, 2 NH);
7.6 ± 7.1 3 m, 19 arom. H); 6.83 3d, 2 H o toMeO); 6.46 ± 6.27 3 m, 2 H, HÀC31')); 5.30 3m, 2 H, HÀC33'));

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Helvetica Chimica Acta ± Vol. 83 32000)
4.72 ± 4.52 3m, OCH2CH2); 4.36 ± 4.05 3m, 4 H, 2 HÀC34'), 2 HÀC35')); 3.78 3s, MeO); 3.6 ± 3.27 3m, 4 H,
OCH2CH2, 2 HÀC35')); 2.67 ± 2.12 3m, 4 H, 2 HÀC32')); 2.10 3s, AcO); 1.87 3s, 3 H, MeÀC35)); 1.40 3s, 3 H,
MeÀC35)). Anal. calc. for C54H55N6O15P ´ H2O 31077.0): C 60.21, H 5.34, N 7.80; found: C 60.02, H 5.45, N 7.54.
8. 5'-O-8Monomethoxytrityl)thymidine-{3'-{OP-[2-8pyrazin-2-yl)ethyl]}-5'}-3'-O-acetylthymidine 339).
Analogously to Exper. 7 from 32 30.206 g, 0.24 mmol), 37 356 mg, 0.2 mmol), pyridine 32 ml), 1-methyl-1H-
imidazole 30.11 ml, 0.14 mmol), and TPS-Cl 30.15 g, 0.48 mmol): 0.134 g 368%) of 39. Amorphous powder. UV
3MeOH): 230 3sh, 4.28), 265 34.39), 272 3sh, 4.34), 309 32.86). 1H-NMR 3CDCl3): 8.80 3br. s, 2 NH); 8.54 ± 8.30
3m, 3 H3pyr)); 7.56, 7.52 32s, 2 H, HÀC36)); 7.42 ± 7.20 3m, 12 arom. H); 6.85 3d, 2 H o toMeO); 6.44 ± 6.26
3m, 2 H, HÀC31')); 5.27 ± 5.08 3m, 2 H, HÀC33')); 4.56 ± 4.35 3m, OCH2CH2); 4.30 ± 4.04 3m, 4 H, 2 HÀC34'),
2 HÀC35')); 3.78 3s, MeO); 3.42 3dd, 2 HÀC35')); 3.24 ± 3.05 3m, OCH2CH2); 2.65 ± 2.12 3m, 4 H, 2 HÀC32'));
2.10 32s, AcO); 1.87 32s, 3 H, MeÀC35)); 1.40 32s, 3 H, MeÀC35)). Anal. calc. for C48H51N6O15P ´ H2O 31001.0):
C 57.93, H 5.34, N 8.40; found: C 57.83, H 5.55, N 8.27.
9. 5'-O-8Monomethoxytrityl)thymidine-{3'-{OP-[2-8pyridin-4-yl)ethyl]}-5'}-3'-O-acetylthymidine 340). A
mixture of 34 3310 mg, 0.387 mmol) and 37 399 mg, 0.35 mmol) was co-evaporated with pyridine 34 Â 10 ml).
The residue was dissolved in pyridine 35 ml), 1-methyl-1H-imidazole 3190 mg, 2.32 mmol) and TPS-Cl 3234 mg,
0.774 mmol) were added, and the mixture was stirred at r.t. for 24 h. The soln. was then treated with H2O
30.5 ml) and evaporated. The residue was dissolved in AcOEt 3200 ml), washed with phosphate buffer pH 7
350 ml) and H2O 350 ml), the org. layer dried 3Na2SO4) and evaporated, and the residue co-evaporated with
toluene and purified by CC 3silica gel, 3.5 Â 12 cm, CH2Cl2, then CH2Cl2/MeOH 50 :1 and 20 :1): 270 mg 377%)
of 40. Solid foam. UV 3MeOH): 263 34.27), 232 3sh, 4.29). 1H-NMR 33D6)DMSO): 11.39, 11.36 32s, 2 HÀN33));
8.41, 8.39 32d, HÀC32)3py), HÀC36)3py)); 7.45 3s, 2HÀC36)3T)); 7.37 ± 6.88 3m, 16 arom. H), HÀC33)3py),
HÀC35)3py)); 6.15 3m,HÀC31')); 5.12, 4.97 32m, 2 HÀC33')); 4.21 3m, POCH2); 4.16 3m, 2 HÀC34'),
MeOTrOCH2); 3.71 3s, MeO); 3.20 3m, 2HÀC35')); 2.87 3m, POCH2CH2); 2.40, 2.24 32m, 4 HÀC32')); 2.04,
2.03 32s, 2 AcO 32 diastereoisomers)); 1.70, 1.69, 1.44, 1.43 34s, 2 Me 3T) 32 diastereoisomers)). Anal. calc. for
C49H52N5O14P ´ 2 H2O 31002.0): C 58.73, H 5.63, N 6.98; found: C 58.67, H 5.64, N 6.87.
10. 5'-O-8Monomethoxytrityl)thymidine-{3'-{OP-[2-8pyridin-2-yl)ethyl]}-5'}-3'-O-acetylthymidine 341). A
mixture of 35 3440 mg, 0.55 mmol) and 37 3128 mg, 0.45 mmol) was co-evaporated with pyridine 34 Â 10 ml).
The residue was dissolved in pyridine 34 ml), 1H-tetrazole 3230 mg, 3.3 mmol) and 2,4,6-triisopropylbenzene-
sulfonyl chloride 3333 mg, 1.1 mmol) were added, and the mixture was stirred at r.t. for 20 h. The soln. was
diluted with CHCl3 3100 ml) and washed with phosphate buffer pH 7 350 ml) and H2O 350 ml). The org. layer
was dried 3Na2SO4), evaporated, and co-evaporated with toluene 320 ml). The residue was purified by CC
3silica gel, 3.5 Â 10 cm, CHCl3, then CHCl3/MeOH 30 :1): 326 mg 374%) of 41. UV 3MeOH): 261 34.43), 232
3sh, 4.39). 1H-NMR 33D6)DMSO): 11.39, 11.36 32s, 2 HÀN33)); 8.42 3d, HÀC36)3py)); 7.63 3dd, HÀC35)3py));
7.46 3s, 2 HÀC36)3T)); 7.37 ± 6.86 3m, 16 H, arom. H, HÀC33)3py), HÀC34)3py)); 6.13 3dd, HÀC31')); 5.11,
4.97 32m, 2 HÀC33')); 4.33 3m, POCH2CH2); 4.07 3m, 2 HÀC34'), MeOTrOCH2); 3.71 3s, MeO); 3.20
3m, 2 HÀC35')); 3.01 3m, POCH2CH2); 2.40, 2.24 32m, 4 HÀC32')); 2.04, 2.03 32s, 2 AcO); 1.68, 1.42 32s,
2 Me 3T)). Anal. calc. for C49H52N5O14P ´ 2 H2O 31002.0): C 58.73, H 5.63, N 6.98; found: C 58.78, H 5.66, N 6.61.
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Received June 15, 2000