Fig. 3: Synthesis of Ca²⁺-sensing-redox compounds.

a Synthetic routes for BAPDiT^ox, BAPDiT^red, and BAPDiS^ox. Starting material 1 was prepared in 51% overall yield through 3 steps as described previously (see Supplementary Information). Reaction conditions: (i) NaH and 2-fluoronitrobenzene in dimethylformamide (DMF) at 120 °C for 19 h; (ii) Zn and acetic acid (AcOH) in tetrahydrofuran (THF)/methanol (MeOH) at 25 °C for 17 h; (iii) NaI, proton sponge, and methyl bromoacetate in CH₃CN for 16 h under the reflux condition; (iv) H₂ and Pd/C in MeOH at 25 °C for 22 h; (v) KOH in THF/MeOH at 25 °C for 2 h; (vi) MsCl in pyridine at 0 °C for 2 h; (vii) KSAc in DMF at 60 °C for 2 h; (viii) KOH in THF/MeOH at 25 °C for 3 h, and then H₂O₂ at 25 °C for 0.5 h; (ix) KOH in THF/MeOH at 25 °C for 3 h, and then TCEP at 25 °C for 1 h; (x) (PMBSe)₂ and NaBH₄ in THF/EtOH for 2.5 h under the reflux condition; (xi) I₂ in MeOH/H₂O at 25 °C for 2 h, and then KOH in THF/MeOH. b ¹H nuclear magnetic resonance (NMR) spectra of compound 5a as a chain-opened model and BAPDiT^ox as a chain-closed model in the presence or absence of Ca²⁺ at pH 7.5 and 25 °C. Reaction conditions: for bottom panels, CaCl₂ (97 μmol) was added to solution of compound 5a (9.7 μmol) in 100 mM Tris-HCl buffer solution at pH 7.5 (400 μL); for top panels, CaCl₂ (90 μmol) was added to solution of BAPDiT^ox (9.0 μmol) in 100 mM Tris-HCl buffer solution at pH 7.5 (400 μL). ¹H NMR (500 MHz) spectra were recorded at 25 °C using a double sample tube, which comprised of an inner and outer tube including D₂O and the sample solution, respectively.