4 '',6 ''-Didehydro-cADPcR (3), an unsaturated carbocyclic ribose analog of a Ca2+-mobilizing second messenger cyclic ADP-ribose (cADPR), was designed and successfully synthesized using a key intramolecular condensation reaction forming the 18-membered pyrophosphate ring structure with a S-phenyl phosphorothioate-type substrate. Biological evaluation showed that 4 '',6 ''-didehydro-cADPcR is a potent Ca2+-mobilizing agent in T cells. (C) 2008 Elsevier Ltd. All rights reserved.