Bioorganic & Medicinal Chemistry Letters 19 (2009) 4210–4212
Bioorganic & Medicinal Chemistry Letters
Synthesis of 9-b-D-arabinofuranosylguanine by combined use of two whole
cell biocatalysts
Rosario Médici a, Adolfo M. Iribarren a,b, Elizabeth S. Lewkowicz a,
*
a Biotransformations Laboratory, Universidad Nacional de Quilmes. R.S. Peña 352, 1876 Bernal, Buenos Aires, Argentina
b INGEBI, CONICET, Vuelta de Obligado 2490, 1428 Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Unlike the preparation of other purine nucleosides, transglycosylation from a pyrimidine nucleoside and
guanine is difficult because of the low solubility of this base. Thus, another strategy, based on the coupled
action of two whole cell biocatalyzed reactions, transglycosylation and deamination, was used. Enterobacter
gergoviae and Arthrobacter oxydans were employed to synthesize 9-b-D-arabinofuranosylguanine (AraG), an
efficient anti leukemic drug.
Received 8 April 2009
Revised 22 May 2009
Accepted 27 May 2009
Available online 30 May 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Arabinofuranosylguanine
Nucleoside phosphorylases
Adenosine deaminase
Whole cell biocatalysts
Guanosine analogues are frequently used as antiviral and anti-
cancer agents and as building blocks for oligonucleotide therapeu-
tics.1 AraG as well as their prodrugs, the 6-modified analogues, are
powerful anti leukemic agents.2
Chemical syntheses of guanine nucleosides have been achieved
stereoselectively, but using expensive and polluting reagents and
several steps including protection and deprotection procedures.3
Alternatively, the synthesis of purine nucleosides by enzymatic
transglycosylation between a pyrimidine nucleoside and a purine
base, catalyzed by nucleoside phosphorylases or microorganisms
that contain them, has attracted considerable attention4,5 since
the reaction is regio- and stereoselective and affords reduced by-
products content.
Unlike other purine bases, the poor solubility of guanine is a
drawback to carry out microbial transglycosylations in the pH
range in which the enzymes are active. Some strategies were re-
ported to overcome this disadvantage including the use of more
soluble guanine derivatives like glyoxal-guanine6 and guanosine7
or immobilized biocatalysts stable in high pH media where guan-
ine is more soluble.8,9
A good alternative for the synthesis of guanine nucleosides con-
sists in the combination of transglycosylation and deamination
reactions. Adenosine deaminase (ADA) catalyzes the hydrolysis of
adenosine to inosine and ammonia. It is commercially available
from mammalian sources (calf spleen or calf intestine) but, even
though the use of easily over-expressed bacterial enzymes is con-
sidered more attractive,10 this approach has not been extensively
studied. Several purine nucleoside analogues are substrates of
ADA with different degrees of efficacy.11 Thus, employing soluble
2-amino-6-substituted purine bases and 20-deoxyuridine (dU) or
thymidine, the corresponding 20-deoxynucleosides were prepared
by transglycosylation using bacterial whole cells or isolated en-
zymes. Afterwards, the sequential hydrolytic action of ADA affor-
ded 20-deoxyguanosine (dG).10,12,13
One of our first attempts to obtain guanosine derivatives, based
on a previously reported strategy7 that employed guanosine as a
more soluble base source, consisted in the use of Escherichia coli
BL21, an efficient whole cell biocatalyst widely studied in our
group14 to perform microbial transglycosylation reactions.
Employing dU as sugar donor, dG was obtained in 85% yield after
3.5 h reaction. The same strategy was applied to the preparation
of 9-b-D-arabinofuranosylguanine (AraG) starting from arabinofur-
anosyluracil (AraU) and guanosine. However, the yield was poor
probably as consequence of the lower hydrolysis rate of AraU com-
pared to that of guanosine and the consumption of the resulting
guanine by other metabolic pathways. To avoid this problem,
pulses of guanosine were added every 4 h. but only 17% of AraG
(2.5 mM) was achieved after 12 h.
Recently, the use of whole cells of Arthrobacter oxydans as ADA
source has been reported by our group.15 This bacterium showed
high ADA activity and was utilized as biocatalyst for the preparation
of guanosine and dG from the corresponding 2,6-diaminopurine
analogues. However, thereaction timemust be rigorouslycontrolled
since the products are substrates of endogenous PNP. In particular,
purine arabinosides are poor substrates of the A. oxydans PNP;
* Corresponding author. Tel.: +54 11 43657100; fax: +54 11 43657132.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.05.106