antiviral or antitumor agents.4 Some of the promising
candidates are the 5-fluorinated analogue of 3TC (emtricit-
abine, 2),5 â-L-ddC (zalcitabine, 3),6 â-L-Fd4C (6),7 and â-L-
FddC (4).8
which resulted in enzymatic kinetic resolution of â-D- and
L-configured dideoxynucleosides,12 and (ii) the separation of
D/L-adenosine via deamination reaction using an unknown
cell culture PS-264.13
Favorable features of L-nucleosides include lower toxicity,
while maintaining an antiviral activity comparable to and
sometimes greater than their D-counterparts, and higher
metabolic stability. For example, 1-(â-D-2-fluoro-2-deoxyara-
bino-furanosyl)-5-methyluracil (FMAU) showed promising
anti-HBV activity, but its development was interrupted in
phase I clinical trials due to severe neurological toxicity.
Whereas, L-FMAU (clevudine, 7) is reported to be nontoxic
and more active.9 The dioxolane analogue L-OddC (troxa-
citabine, 5)10 is the first L-nucleoside showing antitumor
activity, and unlike its D-form, it is not metabolized by
cytidine deaminase, which can prolong its effect as a drug.
Furthermore, â-L-2′-deoxynucleosides have been described
to inhibit the replication of HBV and woodchuck hepatitis
viruses (WHV).4b,c In addition, the L-series of nucleosides
are also of interest as precursors to nuclease-stable L-
oligonucleotides.11 The tremendous therapeutic potential of
L-nucleosides has stimulated interest in their synthesis.
Formation of a mixture of D- and L-nucleosides is a common
occurrence during the synthesis of these compounds. This
results in a challenging separation of the racemic mixtures.
Surprisingly, the separation of D/L-nucleosides remains an
under-explored area of research. Two relevant examples in
the literature are (i) a pig liver esterase-mediated hydrolysis,
Although enzymatic-catalyzed reactions are becoming
standard procedures for the preparation of enantiomerically
pure compounds, they have not been fully exploited as a
separation tool in the racemic mixture of nucleosides.14 Due
to the increased therapeutic applications of L-nucleosides,
there is a need for a separation method that permits easy
isolation of D-nucleosides from L-nucleosides. Recently, we
have described the enzyme-catalyzed regioselective acyla-
tion15 of unprotected â-D-nucleosides and the hydrolysis of
acylated â-D-nucleosides. Herein, we report an extension of
our studies in this area demonstrating for the first time a
regioselective enzymatic acylation of L-nucleosides and its
application in the resolution of D/L-mixtures. Enzymatic
hydrolysis of acylated L-nucleosides is also reported.
For the acylation of L-nucleosides, we chose the levulinyl
group due to its ease of cleavage under neutral conditions
and the potential use of these levulinyl protected nucleosides
in the solution-phase synthesis of oligonucleotides.16 Ac-
etonoxime levulinate was selected as the acylating agent since
oxime esters have proven to be excellent reagents toward
the regioselective enzymatic acylation of nucleosides.15 The
selective 5′-O-acylation of L-2′-deoxynucleosides 8a-d17 was
accomplished using CAL-B at 30 °C (Scheme 1). Thus,
(3) (a) Chang, C.-N.; Doong, S.-L.; Zhou, J. H.; Beach, J. W.; Jeong, L.
S.; Chu, C. K.; Tasi, C.-H.; Cheng, Y.-C. J. Biol. Chem. 1992, 267, 13939-
13942. (b) Doong, S.-L.; Tasi, C.-H.; Schinazi, R. F.; Liotta, D. C.; Cheng,
Y.-C. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 8495-8499.
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Sommadossi, J.-P. In Recent Advances in Nucleosides; Chu, C. K., Ed.;
Elsevier: Amsterdam, 2002; pp 417-432. (c) Lee, K.; Chu, C. K.
Antimicrob. Agents Chemother. 2001, 45, 138-144. (d) Standing, D. N.;
Bridges, E. G.; Placidi, L.; Faraj, A.; Loi, A. G.; Pierra, C.; Dukhan, D.;
Gosselin, G.; Imbach, J.-L.; Herna´ndez, B.; Juodawlkis, A.; Tennant, B.;
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Scheme 1 a
a See Table 1 for details.
(5) (a) Furman, P. A.; Davis, M.; Liotta, D. C.; Paff, M.; Frick, L. W.;
Nelson, D. J.; Dornsife, R. E.; Wyrster, J. A.; Wilson, L. J.; Fyfe, J. A.;
Tuttle, J. V.; Miller, W. H.; Condreay, L.; Averett, D. R.; Schinazi, R. F.;
Painter, G. R. Antimicrob. Agents Chemother. 1992, 36, 2686-2692. (b)
Schinazi, R. F.; McMillan, A.; Cannon, D.; Mathis, R.; Lloyd, R. M.; Peck,
A.; Sommadossi, J.-P.; St. Clair, M.; Wilson, J.; Furman, P. A.; Painter, G.
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entries 1-4 in Table 1 indicate exclusively the formation of
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