Scheme 1. Synthesis of phosphonoacetate ADPR analogue 1. Reagents and conditions: (a) NaOH, water, EtOH, 20 °C (97%); (b) DCC, DMAP (cat.), THF, 20 °C (82%); (c) TMSBr, pyridine, 20 °C, 16 h (46%); (d) DCC, pyridine, 120 °C, 16 h (57%); (e) aqueous TFA 75%, 2 h, 20 °C (36%).