On the basis of a bioisosteric rationale, 4'-thionucleoside analogues of IB-MECA (N(6)-(3-Iodo-benzyl)-9-(5'-methylaminocarbonyl-beta-d-ribofuranosyl)adenine), which is a potent and selective A(3) adenosine receptor (AR) agonist, were synthesized from d-gulonic acid gamma-lactone. The 4'-thio analogue (5h) of IB-MECA showed extremely high binding affinity (K(i)=0.25 nM) at the human A(3)AR and was more potent than IB-MECA (K(i)=1.4 nM). Bulky substituents at the 5'-uronamide position, such as cyclohexyl and 2-methylbenzyl, in this series of 2-H nucleoside derivatives were tolerated in A(3)AR binding, although small alkyl analogues were more potent.