The substraterecognition of Candida antarctica lipase B (CAL-B) in the acylation of nucleosides was revealed through rational substrate engineering for the first time. CAL-B displayed lower activities and excellent 5-regioselectivities (94 to 99 %) in the acylation of ribonucleosides 1f–1j as compared to those in the acylation of 2-deoxynucleosides 1a– 1e. The excellent regioselectivities might be
comparative study on the substraterecognition was conducted successfully in Novozym 435-mediated acylation of various 2′- or 5-substituted nucleosides with acyl donors carrying different aliphatic chain lengths (C6, C10, and C14). The unexpected results revealed that the physicochemical property of the substituents (such as the size, hydrophobicity, and substitutional position) in nucleosides profoundly influenced