A Universal Biocatalyst for the Preparation of Base- and Sugar-Modified Nucleosides via an Enzymatic Transglycosylation

The 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 K-phosphate buffer (10 mM; pH 7.0) in the presence of the biocatalyst at 65° 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 60° for 30 h led to the formation of 1-(β-D-ribofuranosyl)-1,2,4-triazole-3-carboxamide (8; ribavirin) in 90–92% yield (67–70% isolated yield) (Scheme 2).