An efficient process for synthesis of 2′-O-methyl and 3′-O-methyl guanosine from 2-aminoadenosine using diazomethane and the catalyst stannous chloride

Article abstract of DOI:10.1080/15257770500544529

An improved strategy for the selective synthesis of 2′- O -methyl and 3′- O -methyl guanosine from 2-aminoadenosine is reported by using the catalyst stannous chloride. The regioselectivity of the 2′ and 3′- O -alkylation was achieved by optimizing the addition, timing, and concentration of the catalysts and diazomethane during the methylation reaction. An efficient and selective alkylation at 2′-OH of 2-aminoadenosine was achieved by mixing a stoichiometric amount of stannous chloride at room temperature in DMF. The reaction mixture was stirred at 50°C for 1 min and immediately followed by addition of diazomethane. The resulting 2′- O -methyl 2-aminoadenosine was treated with the enzyme adenosine deaminase, which resulted in an efficient conversion to the desired 2′- O -methylguanosine (98% yield). The product was isolated by crystallization. In contrast, the methylation at 3′-OH of 2-aminoadenosine was achieved by mixing a stoichiometric amount of stannous chloride in DMF and stirring at 50°C for 15 min, followed by addition of diazomethane. The resulting mixture containing 3′- O -methyl-2- aminoadenosine in 90% yield and 2′- O -methyl-2-aminoadenosine in 10% yield was treated with the enzyme adenosine deaminase, which preferentially deaminated only 3′- O -methyl-2-aminoadenosine, resulting in the production of 3′- O -methylguanosine in 88% yield. Due to the extremely low solubility 3′- O -methylguanosine, the compound precipitated and was isolated by centrifugation. This synthetic route obviates the chromatographic purification. Selective monomethylation is achieved by using the unprotected ribonucleoside. As a result, the method described herein represents a significant improvement over the current synthetic approach by providing superior product yield and economy, a much more facile purification of 2′,3′- O -methylated isomers, and eliminating the need for protected ribonucleosides reagents. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15257770500544529

This article was downloaded by: [University of Arizona]
On: 03 January 2013, At: 19:44
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Nucleosides, Nucleotides and Nucleic
Acids
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lncn20
An Efficient Process for Synthesis
of 2′-O-methyl and 3′-O-methyl
Guanosine from 2-aminoadenosine Using
Diazomethane and the Catalyst Stannous
Chloride
Anilkumar R. Kore a , Gaurang Parmar a & Srinu Reddy a
a Bioorganic Chemistry Division, Ambion, Inc., Austin, Texas, USA
Version of record first published: 23 Aug 2006.
To cite this article: Anilkumar R. Kore , Gaurang Parmar & Srinu Reddy (2006): An Efficient Process
for Synthesis of 2′-O-methyl and 3′-O-methyl Guanosine from 2-aminoadenosine Using Diazomethane
and the Catalyst Stannous Chloride, Nucleosides, Nucleotides and Nucleic Acids, 25:3, 307-314
To link to this article: http://dx.doi.org/10.1080/15257770500544529
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae, and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

Nucleosides, Nucleotides, and Nucleic Acids, 25:307–314, 2006
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770500544529
AN EFFICIENT PROCESS FOR SYNTHESIS OF 2ꢀ-O-METHYL AND
3ꢀ-O-METHYL GUANOSINE FROM 2-AMINOADENOSINE USING
DIAZOMETHANE AND THE CATALYST STANNOUS CHLORIDE
Anilkumar R. Kore, Gaurang Parmar, and Srinu Reddy
Ambion, Inc.,
2
Bioorganic Chemistry Division, Austin, Texas, USA
An improved strategy for the selective synthesis of 2 ꢁ-O-methyl and 3 ꢁ-O-methyl guanosine from
2
2-aminoadenosine is reported by using the catalyst stannous chloride. The regioselectivity of the 2 ꢁ and
3 ꢁ-O-alkylation was achieved by optimizing the addition, timing, and concentration of the catalysts
and diazomethane during the methylation reaction. An efficient and selective alkylation at 2 ꢁ-OH
of 2-aminoadenosine was achieved by mixing a stoichiometric amount of stannous chloride at room
temperature in DMF. The reaction mixture was stirred at 50 ◦C for 1 min and immediately followed by
addition of diazomethane. The resulting 2 ꢁ-O-methyl 2-aminoadenosine was treated with the enzyme
adenosine deaminase, which resulted in an efficient conversion to the desired 2 ꢁ-O-methylguanosine
(98% yield). The product was isolated by crystallization. In contrast, the methylation at 3 ꢁ-OH of
2-aminoadenosine was achieved by mixing a stoichiometric amount of stannous chloride in DMF
and stirring at 50 ◦C for 15 min, followed by addition of diazomethane. The resulting mixture
containing 3 ꢁ-O-methyl-2-aminoadenosine in 90% yield and 2 ꢁ-O-methyl-2-aminoadenosine in
10% yield was treated with the enzyme adenosine deaminase, which preferentially deaminated
only 3 ꢁ-O-methyl-2-aminoadenosine, resulting in the production of 3 ꢁ-O-methylguanosine in 88%
yield. Due to the extremely low solubility 3 ꢁ-O-methylguanosine, the compound precipitated and was
isolated by centrifugation. This synthetic route obviates the chromatographic purification. Selective
monomethylation is achieved by using the unprotected ribonucleoside. As a result, the method described
herein represents a significant improvement over the current synthetic approach by providing superior
product yield and economy, a much more facile purification of 2 ꢁ,3 ꢁ-O-methylated isomers, and
eliminating the need for protected ribonucleosides reagents.
Keywords Nucleoside Analogues; Adenosine Deaminase; Diazomethane; Catalyst
INTRODUCTION
2ꢁ-O-Methylribonucleosides are widely distributed in RNA[1] and have
been the focus of considerable synthetic efforts since the late 1950s. These
Received 18 October 2005; accepted 29 December 2005.
The financial support from National Institute of Health (SBIR Phase II: R44GM070156-02) is grate-
fully acknowledged. We are thankful to Prof. Dr. Fritz Eckstein (Max-Planck Institut fu¨r Experimentelle
Medizin, Go¨ttingen, Germany) for commenting and critical reading of this manuscript. We also thank
Zhongting Hu and Gary Latham for helpful discussion.
Address correspondence to Anilkumar Kore, Ambion, Inc., Bioorganic Chemistry Division, 2130
Woodward St., Austin, TX, 78744. E-mail: akore@ambion.com
307

308
A. R. Kore et al.
synthetic efforts were further stimulated by recent applications of 2ꢁ-O-
methyloligoribonucleotides in studying pre-mRNA splicing[2] and the struc-
ture of splicesomes[3] as well as in the preparation of nuclease resis-
tant hammerhead ribozymes.[4] 2ꢁ-O-Alkyl substituted oligonucleotides have
also emerged as a second-generation antisense construct with improved
hybridization properties and favorable nuclease resistance and pharmaco-
logical profiles.[5] Recently, 2ꢁ-O-methylguanosine has been used to synthe-
size anti-reverse cap analogs with superior translational properties.[6] 3ꢁ-O-
Methylguanosine was used to synthesize a chain-terminating dinucleotide
mRNA cap analog[7] and was also reported as an anti-reverse cap analog[8]
that has shown better translation efficiency over unmodified cap analogs.
A survey of the literature revealed a variety of problems encountered
in the preparation and isolation of 2ꢁ and 3ꢁ-O-methylribonucleosides.[9−13]
Yields were often very low and the presence of the 3ꢁ-O-methyl-isomer compli-
cated the purification, usually achieved by anion-exchange chromatography
on Dowex. Robins et al.[14] markedly improved synthesis by demonstrating
the utility of using one equivalent of diazomethane and the catalyst stan-
nouschloridetomono-methylationthe2ꢁ,3ꢁ-diolmoiety. Yieldswererelatively
high except in case of guanosine. However, the mixture of the 2ꢁ-O-methyl
and 3ꢁ-O-methyl isomers synthesized through this approach necessitates la-
borious ion-exchange chromatography to separate the isomers. In partic-
ular, the recent advances in protecting group methodology using the 3ꢁ,
5ꢁ-O-(tetraisopropyldisiloxane-1,3-diyl)[15] and methylene-bis (diisopropyl-
[16]
silylchloride) MDPSCl2
protecting group have now eliminated the iso-
mer problem. In a subsequent publication by Robins et al.[17] a 51% yield of
2ꢁ-O-methyl-2-aminoadenosine starting from 2,6-diaminopurine riboside and
a 47% yield of the 3ꢁ-O-methyl-2-aminoadenosine were obtained. Conversion
to the 2ꢁ and 3ꢁ-O-methylguanosine was accomplished with adenosine deam-
inase. In light of the growing interest in oligonucleotide-based therapeu-
tics, and a key synthon in synthesis of anti-reverse cap analogs, it is increas-
ingly important to develop a cost effective synthesis of 2ꢁ-O-methyl and 3ꢁ-O-
methylguanosine. This guanosine derivatives are essential reagents for the
production of modified RNA that is indispensable for many biological assays.
Indeed, the alkylations of the 2ꢁ and 3ꢁ-OH groups offer several advantages
including an improved binding affinity to the target RNA, enhanced nucle-
ase resistance and chemical stability against depurination, improved phar-
macokinetics, and decreased toxicity. In this communication, we report an
efficient synthesis of 2ꢁ, and 3ꢁ-O -methylguanosine from 2-aminoadenosine,
which eliminates the problematic purification and the need for protection
of the 3ꢁ, 5ꢁ-OH groups of the nucleosides.
RESULTS AND DISCUSSION
The biological value of 2ꢁ and 3ꢁ-O-alkylated nucleosides has sparked a
considerable effort directed toward the development of an efficient synthetic

Synthesis of 2 ꢁ-O-Methyl and 3 ꢁ-O-Methyl Guanosine
309
approach. Due to its insolubility in most organic solvents, the direct methy-
lation of unprotected guanosine is not convenient. Therefore, we looked
for an alternative strategy for the selective alkylations of the 2ꢁ and 3ꢁ-OH
group in unprotected 2-aminoadenosine by using diazomethane as an alky-
lating agent and stannous chloride as a catalyst and subsequent enzymatic
deamination of the products by the enzyme adenosine deaminase to the 2ꢁ
and 3ꢁ-O-methylguanosines. The same strategy has been reported by Robins
et al.[17] Following their synthetic strategy, we discovered that parameters
such as temperature, the concentration of catalyst and addition of the alkylat-
ing agent to the reaction mixture significantly impacts the reaction selectivity
and yield. This observation encouraged us to re-investigate the reaction with
2-aminoadenosine. Under our experimental conditions, an efficient and se-
lective alkylation at 2ꢁ-OH of 2-aminoadenosine was achieved by mixing a sto-
ichiometric amount of stannous chloride at room temperature in DMF. After
stirring the reaction mixture at 50◦C for 1 min, diazomethane was added all
at once. In contrast, in the reported method,[17] the catalyst in hot DMF and
diazomethane was added drop wise over 3–4 h. The methylation at 3ꢁ-OH of
2-aminoadenosine was achieved by mixing a stoichiometric amount of stan-
nous chloride in DMF and the reaction mixture was stirred at 50◦C for 15 min,
followed by at once addition of diazomethane. The resulting mixture of 3ꢁ-O-
methyl nucleoside (90%) and 2ꢁ-O-methyl nucleoside (10%) was treated with
the enzyme adenosine deaminase, which under experimental conditions re-
sult in preferential deamination of the 3ꢁ moiety within 36 h (Scheme 1).
Deamination of the 2ꢁ group occurs only after extremely prolonged
incubations with a vast excess of adenosine deaminase. Although it is dif-
ficult to speculate its exact mechanism in the reaction, we strongly be-
lieve that the high yield and specificity are attributed to the acidity and
SCHEME 1 Selective synthesis of 2ꢁ, and 3ꢁ-O-methylguanosine. (i) Addition of SnCl2·2H2O to com-
pound (1) in DMF at room temperature, followed by immediate addition of diazomethane at 50◦C and
stirring overnight. (ii) Addition of SnCl2·2H2O to compound (1) at 50◦C in DMF stirring for at least
15 min, followed by addition of diazomethane and stirring overnight.

310
A. R. Kore et al.
pKa value of 2ꢁ and 3ꢁ hydroxyl group of nucleoside in the presence of
catalyst and diazomethane. It is also known that nucleosides that con-
tain an acidic proton in the base can be monomethylated on the sugar
by raising the concentration of catalyst and addition of dilute solution of
diazomethane.[14]
Preparation of Diazomethane Stock Solution
0.2 M Diazomethane was prepared by a reported procedure[14] with
a slight change in protocol. To an ice-cold mixture of 100 mL of 40%
aqueous KOH and 150 mL of 1,2-dimethoxyethane (glyme) was slowly
added (7.5 g; 72.75 mmol) of N-nitroso-N-methylurea with vigorous stir-
ring. Stirring was continued for an additional 40 min at 0◦C and the
phases were allowed to separate. The upper organic layer was decanted
and dried over KOH pellets for at least 15 min. KOH pellets were changed
while drying. The dried solution was filtered before use. Standardization
of the diazomethane solution using 0.2 N benzoic acid in glyme and back
titration with 0.1 N NaOH gave an average value of 0.2 N CH2N2 in
glyme.
Special Precautions for Diazomethane
Diazomethane is an explosive, insidious poison and a strong irritant,
which may cause delayed hypersensitivity and cancer.[18] Concentrated so-
lutions may explode violently, especially if impurities are present. Rough
surfaces, such as ground glass, may also cause explosions.[19] Therefore,
it should be prepared by experienced chemists with proper glassware. A
Diazald kit with clear-seal joints from Sigma-Aldrich is quite useful, and it is
specially designed for the safe preparation of diazomethane.[20]
Cautions: Because of the highly toxic and explosive nature of dia-
zomethane, all reactions involving its preparation and use should be carried
out in an efficient chemical fume hood and behind a safety shield. Avoid use
of PVC tubing or other plastic tubings with Diazald kits.[20]
Synthesis of 2,6-Diamino-9-(2-O-methyl-β-D-ribofuranosyl) purine (2).
In a typical reaction, 2-aminoadenosine (1) (2.5 g; 8.84 mmol) and
SnCl2·2H2O (0.115 g; 0.51 mmol) in 170 mL of N,N-dimethylformamide
were stirred at room temperature for 10 min. The mixture was then heated to
50◦C and 130 mL of 0.2 M diazomethane solution was then added all at once.
The resulting yellow solution became cloudy and stirring at 50◦C was con-
tinued overnight. Complete disappearance of (1) with formation of a single
product was observed by tlc and analytical HPLC. The bright yellow color of
the reaction mixture became faintly yellow after overnight stirring. This was a

Synthesis of 2 ꢁ-O-Methyl and 3 ꢁ-O-Methyl Guanosine
311
good indication of the selective 2ꢁ-O-methylation; as for the 3ꢁ-O-methylation
(described below), the bright yellow color became cloudy and colorless so-
lution. The mixture was evaporated and the residue dissolved in water and
evaporated four times with 150 mL water to get 2,6-Diamino-9-(2-O-methyl-
β-D-ribofuranosy) purine (2) as pure white powder. The resulting solid was
recrystallized from water and then from MeOH to give (2.4 g; 8.1 mmol) in
(96%) yield, mp 121–122◦C, UV max at 280, 256, and 216 nm. The product
1
was identical to an authentic sample by HPLC, UV, and H NMR.[14] This
product (2) was used for enzymatic deamination.
2ꢁ-O-Methylguanosine 3.
A solution of adenosine deaminase (calf
spleen, Worthington Biochemical Corporation, Lakewood, NJ, USA.)
(0.0064 g; 111 U) in 0.1 M Tris, pH 7.5 (15 mL), 0.1 M sodium phosphate, pH
7.4 (0.75 mL), and DMSO (3.63 mL) was added to a solution of compound
(2) (2.4 g; 8.1 mmol) in 0.1 M Tris, pH 7.5 (22 mL), 0.1 M sodium phosphate,
pH 7.4 (1.5 mL), and DMSO (5.54 mL). The cloudy white mixture was stirred
at room temperature for 36 h. During the deamination time, compound (3)
was precipitated, which was centrifuged for 30 min at 5000 rpm. The product
was analysed by HPLC. (Figures 1a and b) It was recrystallized from water
and then from MeOH to give (2.3 g; 7.73 mmol) (92%) of compound (3),
mp 233–235◦C. (Lit. mp, 234–236◦C).[14] The compound (3) was identical
to an authentic sample by HPLC, UV, and 1H NMR.[14]
Synthesis of 2,6-Diamino-9-(3-O-methyl- β-D-ribofuranosyl) Purine 4.
In a typical reaction, compound (1) (2.5 g; 8.84 mmol) and SnCl2·2H2O
(0.115 g; 0.51 mmol) in 170 mL of N,N-dimethylformamide were stirred at
50◦C for 15 min and 130 mL of 0.2 M diazomethane solution was added
all at once. The reaction mixture was stirred overnight at 50◦C. Complete
disappearance of compound (1) with formation of two products was observed
by tlc and analytical HPLC. The starting bright yellow color of the reaction
mixture became a cloudy and colorless solution. HPLC analysis of the crude
reaction mixture showed that 90% of the 3ꢁ derivative and 10% of the 2ꢁ
derivative had been formed. The mixture was evaporated and the residue
dissolved in water and evaporated four times with 150 mL water to get the
mixture of 3ꢁ (4) and 2ꢁ isomer (5) as a white powder. The resulting solid was
recrystallized from water and then from MeOH to give (2.4 g; 8.1 mmol) in
96% yield. The mixture of compounds (4) and (5) were directly used for the
enzymatic deamination by adenosine deamination.
3ꢁ-O-Methylguanosine 6.
A crude reaction mixture of compounds (4)
and (5) of (2.4 g; 8.1 mmol) was treated identically to the above-described
condition to obtain (2.25 g; 7.56 mmol) 94% of recrystallized (6), mp 263◦C.
(Lit. mp, 258◦C).[14] Under experimental conditions, the 3ꢁ is preferentially
deaminated within 36 h and that only after longer incubations with more

312
A. R. Kore et al.
FIGURE 1 (a) Anion-exchange HPLC analysis of crude reaction mixture of (3). HPLC Column used:
Hypersil SAX, 5 µm, 250 × 4.6 mm. Using a gradient system from 0 to 100% of buffer B over a period
of 16 min. Buffer A was 5 mM (NH4)H2PO4, pH 2.8, and buffer B was 750 mM (NH4)H2PO4, pH 3.7.
(b) After enzymatic deamination UV spectra of (3) in water.
enzyme also deaminated the 2ꢁ isomer. (Figures 2a and b) Compounds (2)
and (4) have a strong UV max at 280 and 256 nm, while after enzymatic
deamination compounds (3) and (6) have a typical guanosine UV absorption
at 253 nm and a sholder at 270 nm (Figure 2b). The product (6) was identical
to an authentic sample by HPLC, UV, and 1H NMR.[14]

Synthesis of 2 ꢁ-O-Methyl and 3 ꢁ-O-Methyl Guanosine
313
FIGURE 2 (a) Anion-exchange HPLC analysis of crude reaction mixture of (4) and (5). HPLC conditions
were same as in Figure 1a. (b) After enzymatic deamination UV spectra of (6) and undigested (5) in water.
CONCLUSION
In conclusion, we have developed a versatile and efficient synthetic
route enabling the selective preparation of 2ꢁ-O-methylguanosine and 3ꢁ-
O-methylguanosine, which have a considerable value in biological assays.
This synthetic route obviates the chromatographic purification necessary for

314
A. R. Kore et al.
previously published methods and requires no protection of the starting nu-
cleoside unprotected ribonucleoside, resulting in improved product yields
and a simpler and more cost-effective procedure.
REFERENCES
1. Hall, R.H. The Modified Nucleosides in Nucleic Acids; Columbia University Press: New York, 1971.
2. Lamond, A.I.; Sproat, B.S.; Ryder, U.; Hamm, J. Probing the structure and function of U2 snRNP
with antisense oligonucleotides made of 2ꢁ-O Me RNA. Cell 1989, 58, 383–390.
3. Blencowe, B.J.; Sproat, B.S.; Ryder, U.; Barbino, S.; Lamond, A.I. Antisense probing of the human
U4/U6 snRNP with biotinylated 2ꢁ-O Me RNA oligonucleotides. Cell 1989, 59, 531–539.
4. Beigelman, L.; McSwiggen, J.A.; Draper, K.G.; Gonzalez, C.; Jensen, K.; Karpeisky, A.M., Modak, A.S.;
Matulic-Adamic, J.; DiRenzo, A.B.; Haeberli, P.; Sweedler, D.; Tracz, D.; Grimm, S.; Wincott, F.E.;
Thackray, V.G.; Usman, N. Chemical modification of hammerhead ribozymes, catalytic activity and
nuclease resistance. J. Biol. Chem. 1995, 70, 25702–25708.
5. Altman, K.H.; Dean, N.M.; Fabbro, D.; Frier, S.M.; Geiger, T.; Haner, R.; Husken, D.; Martin, P.;
Monia, B.P.; Muler, M.; Natt, F.; Nicklin, P.; Phillips, J.; Pieles, U.; Sasmor, H.; Moser, H.H. Second
generation of antisense oligonucleotides: From nuclease resistance to biological efficacy in animals.
Chimia 1995, 50, 168–176.
6. Jemielity, J.; Fowler, T.; Zuberek, J.; Stepinski, J.; Lewdorowicz, M.; Niedzwiecka, A.; Stolarski, R.;
Darzynkiewicz, E.; Rhoads, R.E. Novel anti reverse cap analogs with superior translational properties,
RNA 2003, 9, 1108–1122.
7. Peng, Z.; Sharma, V.; Singleton, S.; Gershon, P. Synthesis and application of a chain-terminating
dinucleotide mRNA cap analog. Org. Lett. 2002, 4, 161–164.
8. Stepinski, J.; Waddell, C.; Stolarski, R.; Darzynkiewicz, E.; Rhoads, R.E. Synthesis and properties of
mRNAs containing the novel anti-reverse cap analogs 7-methyl (3ꢁ-O-methyl) GpppG and 7-methyl
(3ꢁ-deoxy) GpppG, RNA 2001, 7, 1486–1495.
9. Broom, A.D.; Robins, K.K. The direct preparation of 2ꢁ-O-methyladenosine from adenosine. J. Am.
Chem. Soc. 1965, 87, 1145–1146.
10. Khwaja, T.A.; Robins, K.K. Purine nucleosides. XVI. Synthesis of the naturally occurring 2ꢁ-O-
methylpurine ribonucleosides and related derivatives. J. Am. Chem. Soc. 1966, 88, 3640–3643.
11. Martin, D.M.G.; Reese, C.B.; Stephenson, G.F. The action of diazomethane on ribonucleosides.
Preparation of ribonucleoside 2ꢁ-and 3ꢁ-methyl ethers. Biochemistry 1968, 7, 1406–1412.
12. Gin, J.B.; Dekker, C.A. The preparation and properties of O-methylated adenosine derivatives.
Biochemistry 1968, 7, 1413–1420.
13. Robins, M.J.; Naik, S.R. Nucleic acid related compounds. 1. Methylation and transformation of
4-methoxy-2-pyrimidinone 1-β-D-ribofuranoside into 2ꢁ-O-methyl nucleoside components of ribonu-
cleic acid, their analogs, and derivatives. Biochemistry 1971, 10, 3591–3597.
14. Robins, M.J.; Naik, S.R.; Lee, A.S.K. Nucleic acid related compounds. The facile and high-yield stan-
nous chloride catalyzed monomethylation of the cis-glycol system of nucleosides by diazomethane.
J. Org. Chem. 1974, 39, 1891.
15. Markiewicz, W.T. The 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl (TIPDS) group for the simultaneous
protection of the 3ꢁ and 5ꢁ hydroxyl groups of nucleotides. J. Chem. Res. (S) 1979, 24.
16. Chow, S.; Wen, K.; Sanghvi, Y.S.; Theodorakis, E.A. MDPSCL2: A new protecting group for chemos-
elective synthesis of 2ꢁ-O-alkylated guanosine. Nucleo. Nucleot. Nucleic. Acids 2003, 22, 583–587.
17. Robins, M.J.; Hansske, F.; Bernier, S.E. Nucleic acid related compounds. Synthesis of the 2ꢁ-O-methyl
and 3ꢁ-O-methyl ethers of guanosine and 2-aminoadenosine and correlation of O ꢁ-methylnucleoside
13C nmr spectral shifts. Can. J. Chem. 1981, 59, 3360.
18. NIOSH/OSHA. Occupation health guidelines for chemical hazards, U.S. Department of Health and
Human Services. NIOSH. 1981, 81–123.
19. Merck Index, 10th ed; Merck & Co, Inc: Whitehouse Station, NJ, 1983.
20. Howard, B.T. The preparation and reaction of diazomethane. Aldrichim. Acta. 1983, 16, 3–10.