The ribofuranose oxygens of the three adenosine residues of the 5'-monophosphate of the 2-5A core [adenylyl(2'----5')adenylyl(2'----5')adenosine] were replaced by methylenes through the synthesis of an aristeromycin [9-[(1R,2S,3R,4R)-2,3-dihydroxy-4-(hydroxymethyl)cyclopentyl]adenosine] analogue. In the synthetic approach, the chlorophosphite triester procedure was employed together with the use of dimethoxytrityl and tert-butyldimethylsilyl protecting groups. The final product 14 was bound to the 2-5A-dependent endonuclease of mouse, rabbit, or human cells 100-300 times less effectively than parent p5'A2'p5'A2'p5'A. In extracts of human Daudi cells where the monophosphate p5'A2'p5'A2'p5'A was able to effect ribosomal RNA cleavage at 2 X 10(-7) M, 14 required a concentration of 2 X 10(-5) M to bring about discernible rRNA cleavage.