Straightforward Synthesis of the Poly(ADP-ribose) Branched Core Structure.

Poly(ADP-ribosyl)ation is a post-translational modification that produces poly(ADP-ribose) with a branched structure every 20-50 units; such branching structure has been previously suggested to be involved in regulating chromatin remodeling. To elucidate its detailed functions, we developed a straightforward method for the synthesis of the poly(ADP-ribose) branched core structure, α-d-ribofuranosyl-(1‴ → 2″)-α-d-ribofuranosyl-(1″ → 2')-adenosine 5',5'',5‴-trisphosphate 1 , from 6-chloropurine ribofuranoside 4 in 10 steps and 6.1% overall yield. The structure poses synthetic challenges for constructing iterative α-1,2- cis -glycosidic bonds in the presence of a purine base and the installation of three phosphate groups at primary hydroxyl groups. Iterative glycosidic bonds were formed by α-1,2- cis -selective ribofuranosylation using 2- O -(2-naphthylmethyl)-protected thioglycoside donor 6 and a thiophilic bismuth promoter. After the construction of diribofuranosyl adenosine 5 had been constructed, it was chemo- and regioselectively phosphorylated at a later stage. Subsequent deprotection provided the synthetic target 1 .