Unexpected inversion of the 3':5'-regioselectivity was observed in the enzymatic methacryloylation, crotonylation and cinnamoylation of floxuridine (1.5:1, 2.3:1 and 4.4:1, respectively), where Thermomyces lanuginosus lipase preferentially catalyzed the acylation of 3'-hydroxyl rather than that of 5'-hydroxyl group. The possible reason might be the presence of a remote interaction between the unsaturated bond in the acyl group and the aromatic ring of amino acid residue Trp89 in the lid of the lipase.