Total synthesis of cyclic ADP-carbocyclic-ribose, a stable mimic of Ca2+-mobilizing second messenger cyclic ADP-ribose

The synthesis of cyclic ADP-carbocyclic-ribose (cADPcR, 4) designed as a st able mimic of cyclic ADP-ribose (cADPR, 1), a Ca2+-mobilizing second messen ger, was achieved using as the key step a condensation reaction with the ph enylthiophosphate-type substrate 14 to form an intramolecular pyrophosphate linkage. The N-1-carbocyclic-ribosyladenosine derivative 16 was prepared v ia the condensation between the imidazole nucleoside derivative 17, prepare d from AICA-riboside (19), and the readilv available optically active carbo cyclic amine 18. Compound 16 was then converted to the corresponding 5 " -p hosphoryl-5 ' phenylthiophosphate derivatives 14. Treatment of 14 with AgNO 3 in the presence of molecular sieves (3 A) in pyridine at room temperature gave the desired cyclization product 32 in 93% yield, and subsequent acidi c treatment provided the target cADPcR (4), This represents a general metho d for synthesizing biologically important cyclic nucleotides of this type. H-1 NMR analysis of cADPcR suggested that its conformation in aqueous mediu m is similar to that of cADPR. cADPcR. unlike cADPR. was stable under neutr al and acidic conditions, where under basic conditions. it formed the Dimro th-rearranged N-6-cyclized product 34. cADPcR was also stable in rat brain membrane homogenate which has cADPR degradation activity. Furthermore, cADP cR was resistant to the hydrolysis by CD38 cADPR hydrolase. while cADPR was rapidly hydrolyzed under the same conditions. When cADPcR was injected int o sea urchin eggs, it caused a significant release of Ca2+ in the cells, an effect considerably stronger than that of cADPR. Thus, cADPcR was identifi ed as a stable mimic of cADPR.