Dedicated to Prof. Dr. Wolfgang Pfleiderer on the occasion of his 75th birthdayThe E. coli BMT-4D/1A cells have been selected according to Munch-Petersen et al. They carry two regulatory mutations (cytR and deoR) and are able to synthesize constitutively nucleoside-catabolizing enzymes, e.g., cells that possess high UPase and PNPase activities. The cells have been cross-linked by glutaraldehyde to afford a biocatalyst that retained high UPase and PNPase activities and was comfortable for repeated use. An incubation of 2'-deoxyguanosine (1) and 2-chloroadenine (2) (molar ratio 3 : 1) in Kphosphate buffer (10 mm ; pH 7.0) in the presence of the biocatalyst at 658 for 7 h resulted in quantitative transformation of 2 into 2-chloro-2'-deoxyadenosine (4; cladribine) that was isolated in 81% yield (Scheme 1). Similarly, the reaction of guanosine (5) and 1,2,4-triazole-3-carboxamide (6) (molar ratio 1 : 1) in K-phosphate buffer (10 mm ; pH 7.0) in the presence of the biocatalyst at 608 for 30 h led to the formation of 1-(b-dribofuranosyl)-1,2,4-triazole-3-carboxamide (8; ribavirin) in 90 ± 92% yield (67 ± 70% isolated yield) (Scheme 2).Introduction. ± Nucleoside analogues are a mainstream in the treatment of many viral infections and play a significant role in the chemotherapy of different malignancies [1 ± 4]. Most of them are produced by chemical methods employing the natural nucleosides as starting compounds, or by a convergent approach via the condensation of the carbohydrate precursor and heterocyclic base. The latter strategy suffers from many drawbacks, most serious of which are the low stereospecificity of the glycosidic-bond formation and ambiguous regiospecificity, especially in the case of purine bases. As a result, chemical schemes of nucleoside-analogue preparation are rather complicated and accompanied by a tedious isolation of the individual desired compounds [5].Considerable progress in the preparation of nucleoside analogues under consideration was achieved by combination of chemical methods and biochemical transformations (for reviews, see [6]). The enzymatic equilibrium-transfer reaction of the sugar moiety of one nucleoside to the heterocyclic base was discovered by Krenitsky in the 60×s [7] [8]. It was established that this equilibrium reaction is catalyzed by purine nucleoside phosphorylase (PNPase) in the case of purine nucleosides and purine bases, and by uridine (thymidine) phosphorylases (UPase or TPase) in the case of pyrimidine nucleosides and pyrimidines. More than a decade later, it was shown that this reaction can be used for the preparation of base-and sugar-modified nucleosides [9 ± 13].

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