Nucleosides and nucleotides. 192. Toward the total synthesis of cyclic ADP-carbocyclic-ribose. Formation of the intramolecular pyrophosphate linkage by a conformation-restriction strategy in a syn-form using a halogen substitution at the 8-position of the adenine ring

Efficient synthesis of exo-N-carbamoyl nucleosides: application to the synthesis of phosphoramidate prodrugs

An efficient protection protocol for the 6-exo-amino group of purine nucleosides with various chloroformates was developed utilizing N-methylimidazole (NMI). The reaction of an exo-N(6)-group of adenosine analogue 1 with alkyl/and aryl chloroformates under optimized conditions provided the N(6)-carbamoyl adenosines (2a-j) in good to excellent yields. The reaction of N(6)-Cbz-protected nucleosides (5a-c) with phenyl phosphoryl chloride (7) using t-BuMgCl followed by catalytic hydrogenation afforded the corresponding phosphoramidate pronucleotides (8a-c) in excellent yield.

Total synthesis of a cyclic adenosine 5'-diphosphate ribose receptor agonist

Stable cyclic adenosine 5′-diphosphate ribose (cADPR) analogues are chemical biology tools that can probe the Ca²⁺ release mechanism and structure–activity relationships of this emerging potent second messenger. However, analogues with an intact “northern” ribose have been inaccessible due to the difficulty of generating the sensitive N1-ribosyl link. We report the first total synthesis of the membrane permeant, hydrolytically stable, cADPR receptor agonist 8-Br-N1-cIDPR via regio- and stereoselective N1-ribosylation of protected 8-bromoinosine.

Synthesis of stable and cell-type selective analogues of cyclic ADP-ribose, a Ca(2+)-mobilizing second messenger. Structure--activity relationship of the N1-ribose moiety

We previously developed cyclic ADP-carbocyclic ribose (cADPcR, 2) as a stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. A series of the N1-ribose modified cADPcR analogues, designed as novel stable mimics of cADPR, which were the 2"-deoxy analogue 3, the 3"-deoxy analogue 4, the 3"-deoxy-2"-O-(methoxymethyl) analogue 5, the 3"-O-methyl analogue 6, the 2",3"-dideoxy analogue 7, and the 2",3"-dideoxydidehydro analogue 8, were successfully synthesized using the key intramolecular condensation reaction with phenylthiophosphate-type substrates. We investigated the conformations of these analogues and of cADPR and found that steric repulsion between both the adenine and N9-ribose moieties and between the adenine and N1-ribose moieties was a determinant of the conformation. The Ca(2+)-mobilizing effects were evaluated systematically using three different biological systems, i.e., sea urchin eggs, NG108-15 neuronal cells, and Jurkat T-lymphocytes. The relative potency of Ca(2+)-mobilization by these cADPR analogues varies depending on the cell-type used: e.g., 3"-deoxy-cADPcR (4) > cADPcR (2) > cADPR (1) in sea urchin eggs; cADPR (1) >> cADPcR (2) approximately 3"-deoxy-cADPcR (4) in T-cells; and cADPcR (2) > cADPR (1) > 3"-deoxy-cADPcR (4) in neuronal cells, respectively. These indicated that the target proteins and/or the mechanism of action of cADPR in sea urchin eggs, T-cells, and neuronal cells are different. Thus, this study represents an entry to cell-type selective cADPR analogues, which can be used as biological tools and/or novel drug leads.

Convergent synthesis and unexpected Ca(2+)-mobilizing activity of 8-substituted analogues of cyclic ADP-carbocyclic-ribose, a stable mimic of the Ca(2+)-mobilizing second messenger cyclic ADP-ribose

Cyclic ADP-carbocyclic-ribose (cADPcR, 2) is a biologically and chemically stable equivalent of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. In this study, a series of 8-substituted analogues of cADPcR, namely the 8-chloro analogue 6 (8-Cl-cADPcR), the 8-azido analogue 7 (8-N(3)-cADPcR), the 8-amino analogue 8 (8-NH(2)-cADPcR), and the 8-phenylthio analogue 9 (8-SPh-cADPcR), were designed as effective pharmacological tools for studies on cADPR-modulated Ca(2+) signaling pathways. These target compounds were synthesized by a convergent route via 8-Cl-cADPcR bisacetonide (14) as the common intermediate, in which a method for forming the intramolecular pyrophosphate linkage by activation of the phenylthiophosphate type substrate 15 with AgNO(3) to produce 14 was used as the key step. The carbocyclic analogues were tested for activity in the sea urchin egg homogenate system. Compounds were assessed for their calcium-mobilizing effects and their ability to cross-desensitize with calcium release induced by a normally maximal concentration of cADPR, as well as cADPR antagonism of cADPR-evoked calcium release. While cADPcR was 3-4 times more potent than cADPR, the 8-substituted analogues were less efficacious, with 8-SPh-cADPcR largely acting as a competitive antagonist. Most surprisingly, given that 8-N(3)-cADPR and 8-NH(2)-cADPR are known as potent antagonists, 8-N(3)-cADPcR and 8-NH(2)-cADPcR were full agonists, but ca. 80 and 2 times less potent than cADPR, respectively. These data contribute to developing structure-activity relationships for the interaction of cADPR with its receptor.

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 stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca2+-mobilizing second messenger, was achieved using as the key step a condensation reaction with the phenylthiophosphate-type substrate 14 to form an intramolecular pyrophosphate linkage. The N-1-carbocyclic-ribosyladenosine derivative 16 was prepared via the condensation between the imidazole nucleoside derivative 17, prepared from AICA-riboside (19), and the readily available optically active carbocyclic amine 18. Compound 16 was then converted to the corresponding 5' '-phosphoryl-5'-phenylthiophosphate derivatives 14. Treatment of 14 with AgNO3 in the presence of molecular sieves (3 A) in pyridine at room temperature gave the desired cyclization product 32 in 93% yield, and subsequent acidic treatment provided the target cADPcR (4). This represents a general method for synthesizing biologically important cyclic nucleotides of this type. 1H NMR analysis of cADPcR suggested that its conformation in aqueous medium is similar to that of cADPR. cADPcR, unlike cADPR, was stable under neutral and acidic conditions, where under basic conditions, it formed the Dimroth-rearranged N6-cyclized product 34. cADPcR was also stable in rat brain membrane homogenate which has cADPR degradation activity. Furthermore, cADPcR was resistant to the hydrolysis by CD38 cADPR hydrolase, while cADPR was rapidly hydrolyzed under the same conditions. When cADPcR was injected into sea urchin eggs, it caused a significant release of Ca2+ in the cells, an effect considerably stronger than that of cADPR. Thus, cADPcR was identified as a stable mimic of cADPR.

Intracellular Ca(2+)-mobilizing adenine nucleotides. Synthesis and biological activity of cyclic ADP-carbocyclic-ribose and C-glycosidic analog of adenophostin A

We designed novel Ca(2+)-mobilizing purine nucleotides, cyclic ADP-carbocyclicribose 4, and its inosine congener 5, and C-glycosidic adenophostin A 6. In the synthesis of cADPR analogs, the intramolecular condensation to form the pyrophosphate linkage should be the key step. We developed an efficient method for forming such an intramolecular pyrophosphate linkage by the activation of the phenylthiophosphate group with I2 or AgNO3. Using this method, we achieved to synthesize the target compounds 4 and 5. The synthesis of C-glycosidic analog 6 of adenophostin A was achieved using a temporary silicon-tethered radical coupling reaction for constructing (3'alpha, 1" alpha)-C-glycosidic structure as the key step.