Abstract
Dexamethasone phosphate in aqueous solution is susceptible to reversible bisulfite addition to form an A-ring-substituted sulfonic acid salt, sodium 16α-methyl-9α-fluorohydrocortisone-1-sulfonate-21-phosphate. Addition of bisulfite and dissociation of adduct proceed at comparable rates in neutral solution. The addition is a second-order reaction in which sulfite ion and monoanionic dexamethasone phosphate are the major participants; that is, a long-range effect of the steroid phosphate group on the A ring is observed. The adduct dissociation is a second-order general-base-catalyzed reaction in which dianionic adduct is more reactive than trianionic adduct. A large isotope effect was observed for dissociations of protonated and C2-deuterated adducts. Strong dependence of equilibrium steroid composition on temperature was correlated with the temperature dependence of the ionization constant of water.
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