Scheme 2.
Synthesis of di- and tri-sulfonaphthyl tyrosine sulfate mimetics. Reagents and conditions: (a) (1) HBF4, H2O, 0 °C, 1 h; (2) NaNO2, H2O, 0 °C, 10 min; (b) NaOAc, DMF/H2O (1:1), 0 °C, 10 min; (c) thiosemicarbazide, AcOH, 70 °C, 4 h; (d) n-Bu4NI(cat), DMF.