Chemical syntheses of 2'-O-methoxy purine nucleosides

Several processes for the chemical synthesis of 2'-O-methoxy purine nucleosides are herein disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to the chemical synthesis of 2'-O-methyl, 
3'-O-methyl and 5'-O-methyl nucleosides. 
The following is a brief description of synthesis of methoxy nucleosides. 
This summary is not meant to be complete but is provided only for 
understanding of the invention that follows. This summary is not an 
admission that all of the work described below is prior art to the claimed 
invention. 
Sugar modifications, such as 2'-O-methyl, have been discovered in a variety 
of naturally occurring RNA (e.g., tRNA, mRNA, rRNA; reviewed by Hall, 1971 
The Modified Nucleosides in Nucleic Acids, Columbia University Press, New 
York; Limbach et al., 1994 Nucleic Acids Res. 22, 2183). In an attempt to 
understand the biological significance, structural and thermodynamic 
properties, and nuclease resistance of these sugar modifications in 
nucleic acids, several investigators have chemically synthesized 
nucleosides, nucleotides and phosphoramidites containing various sugar 
modifications and incorporated them into oligonucleotides. There are 
several reports in the literature describing the synthesis of 2'-O-methyl 
nucleosides, 2'-O-methyl nucleotides, 2'-O-methyl phosphoramidites and 
oligonucleotides containing 2'-O-methyl substitutions (Broom and Robins, 
1965 J. Am. Chem. Soc. 87, 1145; Martin et al., 1968 Biochemistry, 7, 
1406; Robins et al., 1974 J. Org. Chem. 39, 1891; Inoue et al., 1987 
Nucleic Acids Res. 15, 6131; Cotten et al., 1991 Nucleic Acids Res. 19, 
2629; Andrews et al., 1994 J. Heterocyclic Chem. 31, 765; Beigelman et 
al., 1995 Nucleosides & Nucleotides 14, 421; Sproat et al., 1990 Nucleic 
Acids Res. 18, 41). 
Broom and Robins, 1965 J. Am. Chem. Soc. 87, 1145 and Martin et al., 1968 
Biochemistry, 7, 1406, describe the synthesis of 2'-O-methyl 
ribonucleotides involving mono-methylation of a 2',3'-cis-diol system of a 
ribonucleoside with diazomethane. This procedure gives rise to a mixture 
of 2'- and 3'-O-methyl nucleosides in 20-40% combined yield. The two 
isomers are then separated by ion-exchange chromatography. 
Robins et al., 1974 J. Org. Chem. 39, 1891, describe the treatment of a 
methanolic solution of uridine with diazomethane (in glyme) in the 
presence of stannous chloride dihydrate (in methanol) to synthesize 
2'-O-methyluridine (58% yield). This reaction also yielded a significant 
fraction (28%) of 3'-O-methyluridine which is purified away from the 
2'-O-methyl form by chromatography. 
Inoue, Japanese Patent Publication No. 61291595 and Inoue et al., 1987 
Nucleic Acids Res. 15, 613, describe a process for the synthesis of 
2'-O-methyl ribonucleosides involving alkylation of 
3',5'-O-(tetraisopropyldisiloxane-1,3-diyl) (TIPDS)-ribonucleosides with 
methyl iodide. Inoue et al., state that (page 6133, second main 
paragraph): 
"Treatment of 3',5'-O-TIDPS-uridine (1) with benzoyl chloride . . . in 
N,N-dimethylacetamide in the presence of triethylamine . . . selectively 
gave the N.sup.3 -benzoylated derivative (2) in 70.5% yield. Then, 2 was 
treated with CH.sub.3 I . . . in benzene in the presence of Ag.sub.2 O . . 
. at 40.degree. C. overnight to give the N.sup.3 -benzoyl-2'-O-methyl 
derivative (3, 84.5%). Debenzoylation of 3 with dil. NH.sub.4 OH followed 
by removal of TIPDS group with 0.5N HCl afforded 2'-O-methyluridine . . . 
in 84% yield." 
Srivastava and Roy, U.S. Pat. No. 5,214,135, describe the synthesis of 
2'-O-methyl nucleosides using an approach similar to Inoue et al., supra, 
except that the reaction with methyl iodide/silver oxide was carried out 
at 25.degree. C. for 24-46 hr with an 80-86% yield. This reaction, similar 
to the one described by Inoue et al., supra, also gave rise to the 
3'-O-methyl isomer in 6-8% yield. 
Parmentier et al., 1994 Tetrahedron 50, 5361, describe a convergent 
synthesis of 2'-O-methyl uridine. This procedure uses a multi-step process 
involving--"a facile obtention of the 2'-O-methyl sugar synthon using 
totally selective and efficient methylation conditions; . . . a 
stereoselective high-yield condensation with an uracil derivative, 
yielding the desired .beta.-form with a satisfactory anomeric excess." 
(page 5361, fifth paragraph). 
Chanteloup and Thuong, 1994 Tetrahedron Letters 35, 877, describe synthesis 
of 2'-O-alkyl ribonucleosides using trichloroacetimidate D-ribofuranosides 
as ribosyl donors. They state in the abstract on page 877-- 
"Trichloroacetimidate-2-O-alkyl-3,5-O-TIPS-.beta.-D-ribofuranoside 
glycosylates silylated nucleobases in a fast high-yielding and 
stereoselective reaction promoted by trimethylsilyl 
trifluoromethanesulfonate. This method has been applied to the synthesis 
of 2'-O-alkyl ribonucleosides further transformed to building blocks ready 
for oligo(2'-O-alkyl)ribonucleotide construction." 
Beigelman et al., 1995 Nucleosides & Nucleotides 14, 421, describe three 
different approaches to the synthesis of 2'-O-methyl nucleosides. They 
state that-- 
Method 1: 
"Retrosynthetic analysis showed that 3-O-alkylated derivatives of 
1,2:5,6-di-O-isopropylidene(IP)-.alpha.-D-allofuranose (1) could be 
transformed to the related 2'-O-alkyl ribofuranosides by selective 
degradation of the C1-C2 bond with subsequent cyclization of the generated 
C2-formyl group to the C5--OH." (Page 421, third paragraph) 
Method 2: 
"The 3'-O-TBDMS-derivatives of protected ribonucleosides are byproducts 
obtained during the preparation of 2'-O-TBDMS derivatives--key building 
blocks in oligoribonucleotide synthesis. At the same time, 
3'-O-TBDMS-isomers could be useful starting compounds in the preparation 
of 2'-O-methyl-3'-O-phosphoramidites. We explored this possibility on 
cytidine derivative 14. Reaction of 3'-O-TBDMS-5'-O-DMT-N.sup.4 
-i-Bu-cytidine (14) with Ag.sub.2 O--CH.sub.3 I using a modified method of 
Ohtsuka et al. (supra) yielded 3'-O-TBDMS-5'-O-DMT-N.sup.4 
-i-Bu-2'-O-methyl cytidine (15) in 26% yield. The 2'-O-TBDMS isomer 16 was 
also obtained (22% yield) along with the starting 3'-O-isomer (18%). When 
2'-O-TBDMS-5'-O-DMT-N.sup.4 -i-Bu-cytidine (16) was subjected to the same 
reaction conditions, the same mixture of products was obtained. These 
results show that under the above reaction conditions migration of the 
TBDMS group accompanies the methylation reaction and methylation takes 
place selectively at the 2'-OH position." (Page 422, second full 
paragraph) 
Method 3: 
"Among different methods of indirect introduction of a methyl group, the 
use of 1-alkylthioalkyl intermediates seems to be the most promising. 
Although methods of synthesis of methylthiomethyl ethers of nucleosides 
and carbohydrates are well developed, their transformation into a methyl 
group sometimes requires additional steps. We were interested in the 
testing of more reactive methylthiophenyl ethers as precursors for methyl 
ethers. We found that methylthiophenyl ethers could be smoothly introduced 
by treating appropriately protected nucleosides or carbohydrates with 
PhSMe/Bz.sub.2 O.sub.2 in the presence of DMAP. Nucleoside 19 afforded 
methythiophenyl ether 20 in 65-70% yield, and .alpha.-ribofuranose 21 was 
transformed into .beta.-furanose 22 in 60% yield. Different attempts to 
radically (Bu.sub.3 SnH, Bz.sub.2 O.sub.2) reduce the thiophenyl group of 
furanose 22 were not successful, providing only starting material. 
However, under the same conditions, nucleoside 20 afforded 2'-O-Me 
derivative 24 in 70% yield. 
Haga et al., 1972 Carbohydrate Res. 21, 440 describe a "facile route" to 
the synthesis of 2- and 3-O-methyl-D-ribose from 3-O-methyl-D-allose. 
Nair et al., 1982, Synthesis 8, 670, describes modification of nucleic acid 
bases via radical intermediates. 
Leonard et al., 1992, Nucleosides & Nucleotides, 11, 1201, describe a 
method for the preparation of protected 2'-O-methylguanosine. This 
procedure is distinct from the one described in the instant invention. 
Wagner et al., 1991, Nucleic Acids Res., 19, 5965, describes a method for 
alkylation of ribonucleosides. 
The information disclosed in the references cited above are distinct from 
the presently claimed invention since they do not disclose and/or 
contemplate the processes for the synthesis of the methoxy nucleosides as 
claimed in the instant invention. 
SUMMARY OF THE INVENTION 
It has been postulated (Ueda, in Chemistry of Nucleosides and Nucleotides 
ed. L. B Townsend, v.1 Plenum Press 1988 pp.1-95) that protonation of the 
N.sub.3 atom of 2,2'-, 2,3' or 2,5'-anhydro pyrimidine nucleosides 
facilitates anhydro ring opening by different nucleophiles producing, in 
most cases, nucleoside analogs containing modifications in the 
carbohydrate portion of the nucleoside. Complexation of the N.sub.3 atom 
of the above-mentioned anhydro derivatives with Lewis acids e.g. 
B(OMe).sub.3 ! would provide the same effect directly or in the case of 
methanolysis, complexation of the MeOH with Lewis acids would acidify the 
related proton leading to potential protonation of the N.sub.3 atom of the 
above-mentioned anhydro derivatives. Applicant investigated methanolysis 
of 2,2'-, 2,3' or 2,5'-anhydro pyrimidine nucleosides in the presence of a 
Lewis acid, such as B(OMe).sub.3 and/or BF.sub.3.MeOH. The reaction 
involving a 2,2'-anhydro-1(.beta.-D-arabinofuranosyl) nucleoside, such as 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl) uracil or 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl) cytosine, with B(OMe).sub.3 
and/or BF.sub.3.MeOH, results in the production of 2'-O-methyl nucleosides 
with a yield of about 90-100%. 
By "Lewis Acid" is meant a substance that can accept an electron pair from 
a base. Examples of Lewis acids are, B(OCH.sub.3).sub.3, BF.sub.3, 
AlCl.sub.3, and SO.sub.3. 
In one aspect, the invention features a process for the synthesis of a 
2'-O-methyl adenosine nucleoside, comprising the step of contacting a 
solution of N.sup.4 -acetyl-5',3'-di-O-acetyl-2'-O-methyl cytidine with a 
Lewis acid under conditions suitable for the formation of said nucleoside. 
In another aspect, the invention features a process for the synthesis of 
2'-O-methyl guanosine nucleoside, comprising the steps of: a) methylating 
2amino-6-chloropurine riboside by contacting said 2-amino-6-chloropurine 
riboside with sodium hydride, dimethylformamide and methyl iodide under 
conditions suitable for the formation of 
2'-O-methyl-2-amino-6-chloropurine riboside; b) contacting said 
2'-O-methyl-2-amino-6-chloropurine riboside with 1,4-diazabicyclo(2.2.2) 
octane and water under conditions suitable for the formation of said 
2'-O-methyl guanosine nucleoside in a crude form; and c) purifying said 
2'-O-methyl guanosine nucleoside from said crude form. 
In another aspect, the invention features a process for the synthesis of 
2'-O-methyl adenosine nucleoside, comprising the steps of: a) methylating 
2amino-6-chloropurine riboside by contacting said 2-amino-6-chloropurine 
riboside with sodium hydride, dimethylformamide and methyl iodide under 
conditions suitable for the formation of 
2'-O-methyl-2-amino-6-chloropurine riboside; b) contacting said 
2'-O-methyl-2-amino-6-chloropurine riboside with acetic anhydride, 
4-dimethylaminopyridine and triethylamine under conditions suitable for 
the formation of 3',5'-di-O-acetyl-2'-O-methyl-6-chloro-2-aminopurine 
riboside; c) deaminating said 
3',5'-di-O-acetyl-2'-O-methyl-6chloro-2-aminopurine riboside with isoamyl 
nitrite and tetrahydrofuran to form 
3',5'-di-O-acetyl-2'-O-methyl-6-chloropurine; d) aminating said 
3',5'-di-O-acetyl-2'-O-methyl-6-chloropurine with ammonia to form 
2'-O-methyl adenosine nucleoside in a crude form; and e) purifying said 
2'-O-methyl adenosine nucleoside from said crude form. 
In yet another aspect, the invention features a process for the synthesis 
of 2'-O-methyl guanosine nucleoside, comprising the steps of: a) 
contacting 2,6-diaminopurine nucleoside with anhydrous pyridine and 
tetraisopropyl D-silyl chloride under conditions suitable for the 
formation of 
2,6-diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine; b) methylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine with anhydrous DMF and methyl iodide under conditions 
suitable for the formation of 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2'-Omethyl-.beta. 
-D-ribofuranosyl) purine; c) acylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine with anhydrous pyridine and isobutyryl chloride 
under conditions suitable for the formation of 2,6-Diamino-N.sup.2 
-isobutyryl-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2'-O-methyl-.beta 
.-D-ribofuranosyl) purine; d) deaminating and desilylating said 
2,6-Diamino-N.sup.2 
-isobutyryl-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine under conditions suitable for the formation of 
N.sup.2 -isobutyryl-2'-O-methyl guanosine nucleoside in a crude form; e) 
purifying said N.sup.2 -isobutyryl-2'-O-methyl guanosine nucleoside from 
said crude form; and f) deblocking said N.sup.2 -isobutyryl-2'-O-methyl 
guanosine nucleoside under suitable conditions to form said 2'-O-methyl 
guanosine nucleoside. 
In one aspect, the invention features a process for the synthesis of 
2'-O-methyl guanosine nucleoside, comprising the steps of: a) contacting 
2,6-diaminopurine nucleoside with anhydrous pyridine and tetraisopropyl 
D-silyl chloride (TIPSCI) under conditions suitable for the formation of 
2,6-Diamino-9(3',5'-O-tetraisopropyidisiloxane-(1,3-diyl)-.beta.-D-ribofur 
anosyl) purine; b) methylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine with anhydrous DMF and methyl iodide under conditions 
suitable for the formation of 
2,6Diamino-9-(3',5'-O-tetraisopropyldisiloxane-(1,3-diyl)-2'-O-methyl-.bet 
a.-D-ribofuranosyl) purine; c) acylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine with anhydrous pyridine and isopropylphenoxyacetyl 
chloride under conditions suitable for the formation of 
2,6-Diamino-N.sup.2 
-isopropylphenoxyacetyl-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2-O-m 
ethyl-.beta.-D-ribofuranosyl) purine; d) deaminating and desilylating said 
2,6-Diamino-N.sup.2 
-isopropylphenoxyacetyl-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-m 
ethyl-.beta.-D-ribofuranosyl) purine under conditions suitable for the 
formation of N.sup.2 -isopropylphenoxyacetyl-2'-O-methyl guanosine 
nucleoside in a crude form; e) purifying said N.sup.2 
-isopropylphenoxyacetyl-2'-O-methyl guanosine nucleoside from said crude 
form; and f) deblocking said N.sup.2 -isopropylphenoxyacetyl-2'-O-methyl 
guanosine nucleoside under suitable conditions to form said 2'-O-methyl 
guanosine nucleoside. 
In one aspect, the invention features a process for the synthesis of 
2'-O-methyl guanosine nucleoside, comprising the steps of: a) contacting 
guanosine with N,N-dimethylformamide dibenzyl acetal under conditions 
suitable for the formation of N1-benzyl guanosine; b) methylating said 
N1-benzyl guanosine by contacting said N1-benzyl guanosine with silver 
acetylacetonate, trimethylsulphonium hydroxide and dimethylformamide under 
conditions suitable for the formation of N1-benzyl-2'-O-methyl guanosine 
in a crude form; c) purifying said N1-benzyl-2'-O-methyl guanosine from 
said crude form; d) removing the N1-benzyl protection from said 
N1-benzyl-2'-O-methyl guanosine by contacting said N1-benzyl-2'-O-methyl 
guanosine with sodium naphthalene under conditions suitable for the 
formation of 2'-O-methyl guanosine nucleoside in a crude form; and e) 
purifying said 2'-O-methyl guanosine from said crude form. 
In yet another aspect, the invention features a process for the synthesis 
of 2'-O-methyl adenosine nucleoside, comprising the steps of: a) 
methylating adenosine by contacting said adenosine with dimethylformamide, 
silver acetylacetonate and trimethylsulphonium hydroxide under conditions 
suitable for the formation of 2'-O-methyl adenosine in a crude form; and 
b) purifying said 2'-O-methyl adenosine from said crude form. 
In one aspect, the invention also features a process for the synthesis of 
2'-O-methyl guanosine nucleoside, comprising the steps of: a) contacting 
guanosine with N,N-dimethylformamide dibenzyl acetal under conditions 
suitable for the formation of N1-benzyl guanosine; b) methylating said 
N1-benzyl guanosine by contacting said N1-benzyl guanosine with magnesium 
acetylacetonate, trimethylsulphonium hydroxide and dimethylformamide under 
conditions suitable for the formation of N1-benzyl-2'-O-methyl guanosine 
in a crude form; c) purifying said N1-benzyl-2'-O-methyl guanosine from 
said crude form; d) removing the N1-benzyl protection from said 
N1benzyl-2'-O-methyl guanosine by contacting said N1-benzyl-2'-O-methyl 
guanosine with sodium naphthalene under conditions suitable for the 
formation of 2'-O-methyl guanosine nucleoside in a crude form; and e) 
purifying said 2'-O-methyl guanosine nucleoside from said crude form. 
In one aspect, the invention features a process for the synthesis of 
2'-O-methyl adenosine nucleoside, comprising the steps of: a) methylating 
adenosine by contacting said adenosine with dimethylformamide, magnesium 
acetylacetonate and trimethylsulphonium hydroxide under conditions 
suitable for the formation of 2'-O-methyl adenosine in a crude form; and 
b) purifying said 2'-O-methyl adenosine from said crude form. 
In one aspect, the invention features a process for the synthesis of 
2'-O-methyl adenosine nucleoside, comprising the steps of: a) methylating 
adenosine by contacting said adenosine with dimethylformamide, strontium 
acetylacetonate and trimethylsulphonium hydroxide under conditions 
suitable for the formation of 2'-O-methyl adenosine in a crude form; and 
b) purifying said 2'-O-methyl adenosine from said crude form. 
In one aspect, the invention features a process for the synthesis of 
2'-O-methyl guanosine nucleoside, comprising the steps of: a) contacting 
2,6-diaminopurine nucleoside with anhydrous pyridine and TIPSCI under 
conditions suitable for the formation of 
2,6-diamino-9-(3',5'-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofu 
ranosyl) purine; b) methylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine by contacting said 
2,6-Diamino-9-(3',5'-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofu 
ranosyl) purine with anhydrous DMF and methyl iodide under conditions 
suitable for the formation of 
2,6Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2m-O-methyl-.beta. 
-D-ribofuranoyl) purine; c) acylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine with anhydrous pyridine and isobutyryl chloride 
under conditions suitable for the formation of 2,6-Diamino-N.sup.2 
-isobutyryl-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine; d) deaminating and desilylating said 
2,6-Diamino-N.sup.2 
-isobutyryl-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2'-O-methyl-.beta 
.-D-ribofuranosyl) purine under conditions suitable for the formation of 
N2-isobutyryl-2'-O-methyl guanosine nucleoside in a crude form; and e) 
purifying said N2-isobutyryl-2'-O-methyl guanosine nucleoside from said 
crude form. 
In yet another embodiment, the invention features a process for the 
synthesis of 2'-O-methyl guanosine nucleoside, comprising the steps of: a) 
contacting 2,6-diaminopurine nucleoside with anhydrous pyridine and TIPSCI 
under conditions suitable for the formation of 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine; b) methylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-.beta.-D-ribofura 
nosyl) purine with anhydrous DMF and methyl iodide under conditions 
suitable for the formation of 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine; c) acylating said 
2,6-Diamino-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-methyl-.beta. 
-D-ribofuranosyl) purine by contacting said 
2,6-Diamino-9-(3,5-O-tetraisopropyidisiloxane-(1,3-diyl)-2'-O-methyl-.beta 
.-D-ribofuranosyl) purine with anhydrous pyridine and 
isopropylphenoxyacetyl chloride under conditions suitable for the 
formation of 2,6-Diamino-N.sup.2 
-isopropylphenoxyacetyl-9-(3',5'-O-tetraisopropyldisiloxane-(1,3-diyl)-2'- 
O-methyl-.beta.-D-ribofuranosyl) purine; d) deaminating and desilylating 
said 2,6-Diamino-N.sup.2 
-isopropylphenoxyacetyl-9-(3,5-O-tetraisopropyldisiloxane-(1,3-diyl)-2-O-m 
ethyl-.beta.-D-ribofuranosyl) purine under conditions suitable for the 
formation of N.sup.2 -isopropylphenoxyacetyl-2'-O-methyl guanosine 
nucleoside in a crude form; and e) purifying said N.sup.2 
-isopropylphenoxyacetyl-2'-O-methyl guanosine nucleoside from said crude 
form. 
This invention features an improved and economical synthetic method for the 
preparation of 2'-O-methyl nucleosides in high yield. The method is not 
only cost efficient, but can be scaled up to several hundred gram 
quantities. The method generally utilizes inexpensive commercially 
available 2,2'-anhydro-1(.beta.-D-arabinofuranosyl) nucleoside, such as 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl)uracil or 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl)cytosine, as a starting material 
which is converted in a one or two step reaction sequence to 2'-O-methyl 
nucleosides with a yield of about 90-100%. 
The 2'-O-methyl or 3'-O-methyl nucleosides can be used for chemical 
synthesis of nucleotides, nucleotide-tri-phosphates and/or 
phosphoramidites as a building block for selective incorporation into 
oligonucleotides. These oligonucleotides can be used as an antisense 
molecule, 2-5A antisense chimera, triplex molecule or as an enzymatic 
nucleic acid molecule. The oligonucleotides can also be used as probes or 
primers for synthesis and/or sequencing of RNA or DNA. 
By "antisense" it is meant a non-enzymatic nucleic acid molecule that binds 
to target RNA by means of RNA-RNA or RNA-DNA or RNA-PNA (protein nucleic 
acid; Egholm et al., 1993 Nature 365, 566) interactions and alters the 
activity of the target RNA (for a review see Stein and Cheng, 1993 Science 
261, 1004). 
By "2-5A antisense chimera" it is meant, an antisense oligonucleotide 
containing a 5' phosphorylated 2'-5'-linked adenylate residues. These 
chimeras bind to target RNA in a sequence-specific manner and activate a 
cellular 2-5A-dependent ribonuclease which, in turn, cleaves the target 
RNA (Torrence et al., 1993 Proc. Nat. Acad. Sci. USA 90, 1300). 
By "triplex DNA" it is meant an oligonucleotide that can bind to a 
double-stranded DNA in a sequence-specific manner to form a triple-strand 
helix. Formation of such triple helix structure has been shown to inhibit 
transcription of the targeted gene (Duval-Valentin et aL, 1992 Proc. Natl. 
Acad. Sci. USA 89, 504). 
By "enzymatic nucleic acid" it is meant a nucleic acid molecule capable of 
catalyzing reactions including, but not limited to, site-specific cleavage 
and/or ligation of other nucleic acid molecules, cleavage of peptide and 
amide bonds, and trans-splicing. 
In preferred embodiments, the invention features a method for chemical 
synthesis of 2'-O-methyl or 3'-O-methyl nucleosides in which 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl) cytosine, 
2,2'-anhydro-1(.beta.-D-arabinofuranosyl) uracil, 
2,3'-anhydro-1(.beta.-D-arabinofuranosyl) uracil, or 
2,3'-anhydro-1(.beta.-D-arabinofuranosyl) cytosine is used as the starting 
material, and wherein said starting material is reacted with a Lewis acid. 
Another preferred embodiment of the invention features a method for 
chemical synthesis of 5'-O-methyl pyrimidine nucleoside in which 
2,5'-anhydro-1(.beta.-D-arabinofuranosyl) pyrimidine is used as the 
starting material and is reacted with a Lewis acid. 
In yet another preferred embodiment, the invention features novel processes 
for the synthesis of 2'-O-methyl purine nucleosides. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiments thereof, and from the 
claims.