Synthesis of 2',3'-dideoxy-2',3'-didehydro nucleosides via a serendipitous route.

Article abstract of DOI:10.1081/NCN-100002532

This paper describes a "green" synthesis of 2',3'-unsaturated 2',3'-dideoxynucleosides via an electrochemical reaction. Using this approach d4T, d4U, ddA and ddI can be synthesized in high yields.

Full text of DOI:10.1081/NCN-100002532

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SYNTHESIS OF 2′,3′-DIDEOXY-2′,3′-DIDEHYDRO
NUCLEOSIDES VIA A SERENDIPITOUS ROUTE
Z. Guo a , Y. S. Sanghvi b , L. E. Brammer Jr. c & T. Hudlicky c
a Isis Pharmaceuticals, Inc. , 2292 Faraday Avenue, Carlsbad, California, 92008, U.S.A.
b Isis Pharmaceuticals, Inc. , 2292 Faraday Avenue, Carlsbad, California, 92008, U.S.A.
c University of Florida , Gainesville, Florida, 32604, U.S.A.
Published online: 07 Feb 2007.
To cite this article: Z. Guo , Y. S. Sanghvi , L. E. Brammer Jr. & T. Hudlicky (2001) SYNTHESIS OF 2′,3′-DIDEOXY-2′,3′-
DIDEHYDRO NUCLEOSIDES VIA A SERENDIPITOUS ROUTE, Nucleosides, Nucleotides and Nucleic Acids, 20:4-7, 1263-1266,
DOI: 10.1081/NCN-100002532
To link to this article: http://dx.doi.org/10.1081/NCN-100002532
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NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS, 20(4–7), 1263–1266 (2001)
SYNTHESIS OF 2ꢀ,3ꢀ-DIDEOXY-2ꢀ,3ꢀ-DIDEHYDRO
NUCLEOSIDES VIA A SERENDIPITOUS ROUTE
Z. Guo,1 Y. S. Sanghvi,1,∗ L. E. Brammer, Jr., 2 and T. Hudlicky2
1Isis Pharmaceuticals, Inc., 2292 Faraday Avenue, Carlsbad,
California 92008
2University of Florida, Gainesville, Florida 32604
ABSTRACT
This paper describes a “green” synthesis of 2ꢀ,3ꢀ-unsaturated 2ꢀ,3ꢀ-dideoxynucl-
eosides via an electrochemical reaction. Using this approach d4T, d4U, ddA
and ddI can be synthesized in high yields.
INTRODUCTION
With the growing success of clinical programs and potential market demand
for oligonucleotide-based drugs, it is important that all raw materials for synthe-
sis of these drugs are easily accessible at affordable cost (1). For the assembly of
oligonucleotides, 2ꢀ-deoxynucleosides (dNs) are the key building-blocks (2). Cur-
rently all dNs are in limited supply and remain expensive due to their isolation from
natural sources such as fish milt (3). Therefore, we have focused our attention on
methods development for the synthesis of dNs, particularly pyrimidine dNs.
RESULTS AND DISCUSSION
Although thymidine can be synthesized (4) via direct glycosylation proce-
dure (Scheme 1), the concomitant formation of α-anomer decreases the overall
∗Corresponding author.
1263
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Copyright 2001 by Marcel Dekker, Inc.
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Scheme 1. Synthesis of thymidine via glycosylation procedure.
yield and increases the cost of desired β-anomer. Therefore, we elected to use
ribonucleosides as starting material for synthesis of dNs due to their availability
in bulk at low cost from bacterial fermentation and preformed glycosidic linkage
(Scheme 2).
The Mattocks reaction of uridine analog (1) has been reported to provide
2ꢀ-α-bromo-3ꢀ-acetoxy nucleoside (2) due to anchimeric assistance of oxygen at
the 2-position of the pyrimidine ring (5). Subsequent reduction of the halide with
tin reagents furnished dNs (3, Scheme 2). Scale-up of tin mediated reduction is
not practical due to the toxic nature of the reagent. In order to implement “green”
chemistry in our manufacturing processes, we chose an electrochemical method
that offers a versatile and nonstoichiometric alternative to metal-based reduction of
the halide.
Treatment of acetyl bromide with uridine and 5-methyluridine in acetonitrile
gave quantitative yields of 2ꢀ-bromo-3ꢀ,5ꢀ-diacetyluridine (2, B = U) and 2ꢀ-bromo-
3ꢀ,5ꢀ-diacetyl-5-methyluridine (2, B = T), respectively. Electrochemical reduction
(6) of 2 in methanol did not furnish the expected 2ꢀ-deoxy nucleoside derivatives
3. Interestingly, both reactions gave 2ꢀ,3ꢀ-unsaturated 2ꢀ,3ꢀ-dideoxynucleosides 6
via elimination of the 3ꢀ-acetoxy group (5, Scheme 3). Subsequent treatment with
base resulted in formation of d4T and d4U. All attempts to reduce 2ꢀ-α-bromo-3ꢀ-
acetoxy nucleosides 2 via electrochemical methods have resulted in formation of
elimination products only.
Scheme 2. Synthesis of thymidine via 2ꢀ-deoxygenation procedure.

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2ꢀ ,3ꢀ -DIDEOXY-2ꢀ ,3ꢀ-DIDEHYDRO NUCLEOSIDES
1265
Scheme 3. Serendipitous synthesis of d4T.
Scheme 4. Green synthesis of ddI.
The concept of electrochemical reduction was further extended towards syn-
thesis of ddI (9). The 3ꢀ-bromo-3ꢀ-deoxy nucleoside 7 was conveniently synthesized
from inosine following a literature protocol (5). Electrochemical reduction of 7 in
MeOH under similar conditions furnished >80% yield of 2ꢀ,3ꢀ-unsaturated nucle-
oside 8 (Scheme 4). Transformation of 8 to 9 has been reported in the literature
(5).
SUMMARY
Interestingly literature search revealed that electrochemical reductions of nu-
cleosides have been tried (7). However the yields of 2ꢀ,3ꢀ-unsaturated nucleosides
were somewhat lower than what we report herein (8). It seems that choice of solvent
plays a major role during electrochemical reactions and we found that MeOH gave
best results. Furthermore, we believe that this is a first “green” synthesis of d4T
and may have potential for further scale-up. In conclusion, we have failed to ac-
complish our original goal of synthesis of 2ꢀ-deoxynucleosides by electrochemical
reduction.

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ACKNOWLEDGMENTS
We are thankful to Dr. Douglas L. Cole for his support and encouragement
throughout this project.
REFERENCES AND NOTES
1. Sanghvi, Y.S., Andrade, M., Deshmukh, R.R., Holmberg, L., Scozzari, A.N. and Cole,
D.L. In Manual of Antisense Methodology, G. Hartmann and S. Endres (eds.), Kluwer
Academic Publishers 1999, pp 3–23.
2. Sanghvi, Y. S. DNA with Altered Backbones in Antisense Applications In Comprehen-
sive Natrual Products Chemistry, Editors-in-Chief, D.H.R Barton and K. Nakanishi,
in Volume 7: DNA and Aspects of Molecular Biology E.T. Kool (ed.), Pergamon Press,
258–311 (1999).
3. The bulk price for 2ꢀ-deoxynucleosides isolated from fish milt is around $1,000/Kg.
4. Freskos, J.N. and Senaratne, P.A. US Patent 4,914,233, April 3, 1990.
5. Shiragami, H., Amino, Y., Honda, Y., Arai, M., Tanaka, Y., Iwagami, H., Yukawa, T.
and Izawa, K. Nucleosides Nucleotides 1996, 15, 31–45.
6. Cell assembly for electrolysis: The cell was home built using a glass beaker fitted with a
compartment (fritted disk at the bottom) containing a platinum plate (2 × 2 cm, 0.1 mm
thick) as anode and mercury cathode (pool at the bottom). The top was covered with
a plate with an inlet and outlet for argon, a thermometer and an Ag/Ag+ (1 mm silver
wire immersed in 0.1 M AgNO3 in acetonitrile) reference electrode. Cyclic and linear
sweep voltammatries were conducted on the electrolysis solutions prior to the electrol-
ysis to determine the optimum reduction potentials. Electrochemical reductions were
performed in methanol or acetonitrile with tetraethylammonium p-toluenesulfonate
(0.2 M) as electrolyte. The concentration of the electrolyte was maintained in a 5-fold
excess relative to the electroactive species. The nucleosidic substrate (5 mmol) was
added and the stirred solution was electrolysed under argon at an initial constant poten-
tial of -1.9 V for 1 h. The progress of the reaction was followed by TLC and most of the
starting material was consumed within 1 h. Upon completion of the reaction, the solu-
tion was concentrated under vacuum and the residue purified by short silica gel column
chromatography. The yield of the isolated product was always >80%. The deprotected
(NH4OH) products were identical (TLC, UV, NMR) with an authentic sample.
7. Adachi, T., Iwaski, T., Inoue, I. and Miyoshi, M. J. Org. Chem. 1979, 44, 1404–1409.
8. Johansen, O. Marcuccio, S.M. and Mau, A.W.-H. Aust. J. Chem. 1994, 47, 1843–1850.

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