The phosphorylated inositol diphosphates, including the diphosphoinositol pentakisphosphate regioisomers, play critical roles in signal transduction and cellular regulation. In particular, the IP(7) isomer 5-PP-Ins(1,2,3,4,6)P(5) is implicated in a nonenzymatic phosphate transfer converting a protein serine phosphate residue to a serine diphosphate. A scalable, practical new synthesis of 5-PP-Ins(1,2,3,4,6)P(5) is described that also allows access to a variety of IP(7) and IP(8) regioisomers. The identity of the synthetic 5-PP-Ins(1,2,3,4,6)P(5) was validated using IP6K1 to catalyze the conversion of IP(7) + ADP to ATP + IP(6).